Surface & Interface Analysis Compendium

The Surface Science Society of Japan Editor Compendium of Surface and Interface Analysis Compendium of Surface and Interface Analysis The Surface Science Society of Japan Editor Compendium of Surface and Interface Analysis 123 Editor The Surface Science Society of Japan Tokyo Japan Additional material to this book can be downloaded from http://extras.springer.com. ISBN 978-981-10-6155-4 ISBN 978-981-10-6156-1 https://doi.org/10.1007/978-981-10-6156-1 (eBook) Library of Congress Control Number: 2017949165 © Springer Nature Singapore Pte Ltd. 2018 This work is subject to copyright. 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Neither the publisher nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Printed on acid-free paper This Springer imprint is published by Springer Nature The registered company is Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore List of the Editorial Staff Editor-in-Chief Manabu Kiguchi Vice Editor-in-Chief Takuya Masuda Hitoshi Abe Takahiro Kondo Kan Nakatsuji Toru Shimada Associate Editors Shuji Hasegawa Yoshikazu Homma Akiko N. Itakura Ryohei Kokawa Atsushi Kubo Fumihiko Matsui Tomonobu Nakayama Tetsuya Narushima Hidenori Noguchi Kazuyuki Sakamoto Kaoru Sasakawa Ryugo Tero v Preface Surfaces and interfaces are the places where the rotation/inversion symmetry of the crystal is broken and therefore the electronic and geometric structures significantly differ from those in bulk, leading to the unique electric, magnetic, catalytic and optical properties. In addition, various interesting processes such as adsorption/desorption, etching, deposition, corrosion, and electron transfer and catalytic reactions take place at the surfaces and interfaces. Since the electronic, geometric and molecular structures of surfaces and interfaces play crucial roles in those interfacial processes, it is important to understand such structures as well as the elemental composition. Despite the importance of surface and interface analysis, it is generally more difficult than bulk analysis because the very small number of atoms is the subject of investigations and the signals from the surface species are often buried within those from bulk. Therefore, tremendous effort has been dedicated to the development of analysis techniques which can extract the information of the surface species from those of bulk with a high sensitivity and selectivity. This book covers various surface analysis techniques to investigate the morphology, atomic structure, electronic structure and properties of the surfaces and interfaces. In each chapter, experts of the corresponding techniques briefly describe their principle, features and instrumentation together with a few examples of related works, so that readers can understand the capabilities of the techniques and requirements for the use in their own researches. The list of techniques summarized in this book is available from http://extras.springer.com. We hope that this book is useful to a wide range of scientists and students who study in this research field or start to do. vii viii Preface Finally, we thank Springer Publishing, especially Dr. Shin’ichi Koizumi, Ms. Risa Takizawa, for giving us an opportunity to edit such a book and all the authors for accepting to contribute to this book, and we hope that the readers will find this book both useful and delightful. Tokyo, Japan March 2017 Manabu Kiguchi Takuya Masuda Hitoshi Abe Kan Nakatsuji Takahiro Kondo Toru Shimada Contents 1 Acoustic Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Naohiro Hozumi 1 2 Action Spectroscopy with STM . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kenta Motobayashi 9 3 Ambient Pressure X-Ray Photoelectron Spectroscopy . . . . . . . . . . Hiroshi Kondoh 15 4 Angle-Resolved Ultraviolet Photoelectron Spectroscopy . . . . . . . . . Takafumi Sato 21 5 Atom Probe Field Ion Microscope . . . . . . . . . . . . . . . . . . . . . . . . . . Masahiko Tomitori 27 6 Atomic Force Microscope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Shintaro Fujii 33 7 Auger Electron Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fumihiko Matsui 39 8 Cathodoluminescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Takashi Sekiguchi 45 9 Conductive Atomic Force Microscopy . . . . . . . . . . . . . . . . . . . . . . . Risa Fuji 51 10 Differential Interference Contrast Microscopy/Phase-Contrast Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hiroshi Komatsu and Gen Sazaki 55 11 Dynamic Secondary Ion Mass Spectrometry . . . . . . . . . . . . . . . . . . Mitsuhiro Tomita 61 12 Elastic Recoil Detection Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . Daiichiro Sekiba 67 ix x Contents 13 Electrochemical Atomic Force Microscopy . . . . . . . . . . . . . . . . . . . Toru Utsunomiya, Yasuyuki Yokota and Ken-ichi Fukui 73 14 Electrochemical Infrared Spectroscopy . . . . . . . . . . . . . . . . . . . . . . Shen Ye 79 15 Electrochemical Scanning Tunneling Microscopy . . . . . . . . . . . . . . Tomoaki Nishino 87 16 Electrochemical Second Harmonic Generation . . . . . . . . . . . . . . . . Ichizo Yagi 91 17 Electrochemical Sum Frequency Generation . . . . . . . . . . . . . . . . . . Hidenori Noguchi 97 18 Electrochemical Surface X-Ray Scattering . . . . . . . . . . . . . . . . . . . 103 Toshihiro Kondo 19 Electrochemical Transmission Electron Microscopy . . . . . . . . . . . . 109 Yoshifumi Oshima 20 Electrochemical X-Ray Absorption Fine Structure . . . . . . . . . . . . . 113 Takuya Masuda 21 Electrochemical X-Ray Photoelectron Spectroscopy . . . . . . . . . . . . 119 Takuya Masuda 22 Electron Backscatter Diffraction. . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 Rika Yoda 23 Electron Energy-Loss Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . 133 Tadaaki Nagao 24 Electron Probe Microanalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 Hiroshi Sakamae 25 Electron-Stimulated Desorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 Naoya Miyauchi 26 Electron-Beam-Induced Current . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 Jun Chen and Takashi Sekiguchi 27 Ellipsometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 Toshihide Tsuru 28 Environmental SEM (Atmospheric SEM) . . . . . . . . . . . . . . . . . . . . 165 Yusuke Ominami 29 Environmental Transmission Electron Microscopy . . . . . . . . . . . . . 171 Tadahiro Kawasaki 30 Extended X-Ray Absorption Fine Structure . . . . . . . . . . . . . . . . . . 177 Hitoshi Abe Contents xi 31 Focused Ion Beam Scanning Electron Microscope . . . . . . . . . . . . . 181 Tetsuo Sakamoto 32 Force Curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 Akinori Kogure 33 Force Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193 Christina Puckert and Michael J. Higgins 34 Frequency-Modulation Atomic Force Microscopy . . . . . . . . . . . . . 201 Masayuki Abe 35 Gap-Mode Raman Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . 205 Katsuyoshi Ikeda 36 Glow Discharge Mass Spectrometry. . . . . . . . . . . . . . . . . . . . . . . . . 211 Takashi Saka 37 Glow Discharge Optical Emission Spectrometry . . . . . . . . . . . . . . . 219 Patrick Chapon, Sofia Gaiaschi and Kenichi Shimizu 38 Hard X-Ray Photoelectron Spectroscopy . . . . . . . . . . . . . . . . . . . . . 229 Akira Sekiyama 39 Helium Atom Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239 Takahiro Kondo 40 High-Resolution Elastic Recoil Detection Analysis . . . . . . . . . . . . . 247 Kaoru Nakajima 41 High-Resolution Electron Energy Loss Spectroscopy . . . . . . . . . . . 253 Hiroshi Okuyama 42 High-Resolution Rutherford Backscattering Spectrometry . . . . . . . 259 Kaoru Nakajima 43 High-Speed Atomic Force Microscopy . . . . . . . . . . . . . . . . . . . . . . . 263 Takayuki Uchihashi 44 Imaging Ellipsometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269 Akiko N. Itakura 45 Impact Collision Ion Scattering Spectroscopy . . . . . . . . . . . . . . . . . 275 Masakazu Aono and Mitsuhiro Katayama 46 Inelastic Electron Tunneling Spectroscopy . . . . . . . . . . . . . . . . . . . 283 Akitoshi Shiotari 47 Infrared External-Reflection Spectroscopy . . . . . . . . . . . . . . . . . . . 289 Takeshi Hasegawa 48 Infrared Reflection–Absorption Spectroscopy . . . . . . . . . . . . . . . . . 295 Jun Yoshinobu xii Contents 49 Interferometer Displacement Measurement . . . . . . . . . . . . . . . . . . . 301 Masaya Toda 50 Inverse Photoemission Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . 307 Kaname Kanai 51 Kelvin Probe Force Microscope . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313 Risa Fuji 52 Laser Ionization Secondary Neutral Mass Spectrometry . . . . . . . . 319 Tetsuo Sakamoto 53 Laser Photoelectron Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . 325 Ryuichi Arafune 54 Lateral Force Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331 Shiho Moriguchi 55 Liquid SPM/AFM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337 Akinori Kogure 56 Low-Energy Ion Scattering Spectroscopy . . . . . . . . . . . . . . . . . . . . 343 Kenji Umezawa 57 Low-Energy Electron Diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . 349 Yoshimi Horio 58 Low-Energy Electron Microscope . . . . . . . . . . . . . . . . . . . . . . . . . . 355 H. Hibino 59 Magnetic Force Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 361 Masato Hirade 60 Matrix-Assisted Laser Desorption/Ionization . . . . . . . . . . . . . . . . . . 365 Takaya Satoh 61 Medium-Energy Ion Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371 Tomoaki Nishimura 62 Micro-Raman Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375 Katsumasa Fujita 63 Microprobe Reflection High-Energy Electron Diffraction . . . . . . . . 381 Masakazu Ichikawa 64 Multiple-Probe Scanning Probe Microscope . . . . . . . . . . . . . . . . . . 387 Tomonobu Nakayama 65 Nanoscale Angle-Resolved Photoelectron Spectroscopy . . . . . . . . . 395 Koji Horiba 66 Nonlinear Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 401 Shoichi Yamaguchi Contents xiii 67 Nuclear Reaction Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405 Markus Wilde and Katsuyuki Fukutani 68 Optical Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413 Kazuya Kabayama and Ryugo Tero 69 Optical Second-Harmonic Generation Spectroscopy and Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 419 Khuat Thi Thu Hien and Goro Mizutani 70 Particle-Induced X-Ray Emission . . . . . . . . . . . . . . . . . . . . . . . . . . . 425 Koichiro Sera 71 Penning Ionization Electron Spectroscopy . . . . . . . . . . . . . . . . . . . . 435 Takuya Hosokai 72 Phase Mode SPM/AFM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 441 Hideo Nakajima 73 Photoelectron Diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 445 Fumihiko Matsui and Tomohiro Matsushita 74 Photoelectron Holography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 451 Tomohiro Matsushita and Fumihiko Matsui 75 Photoelectron Yield Spectroscopy. . . . . . . . . . . . . . . . . . . . . . . . . . . 457 Hisao Ishii 76 Photoemission Electron Microscope . . . . . . . . . . . . . . . . . . . . . . . . . 465 Toyohiko Kinoshita 77 Photoluminescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 471 Yuhei Miyauchi 78 Photon Emission from the Scanning Tunneling Microscope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477 Makoto Sakurai 79 Photo-Stimulated Desorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 487 Akihiko Ikeda and Katsuyuki Fukutani 80 Piezoresponse Force Microscope . . . . . . . . . . . . . . . . . . . . . . . . . . . 493 Masato Hirade 81 Positron-Annihilation-Induced Desorption Spectroscopy . . . . . . . . 497 Takayuki Tachibana and Yasuyuki Nagashima 82 p-Polarized Multiple-angle Incidence Resolution Spectrometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 501 Takeshi Hasegawa xiv Contents 83 Quartz Crystal Microbalance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 509 Yuji Teramura and Madoka Takai 84 Reflectance Difference Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . 521 Ken-ichi Shudo and Shin-ya Ohno 85 Reflection High-Energy Electron Diffraction . . . . . . . . . . . . . . . . . . 527 Yoshimi Horio 86 Resonant Inelastic X-Ray Scattering . . . . . . . . . . . . . . . . . . . . . . . . 531 Yoshihisa Harada 87 Rutherford Backscattering Spectrometry . . . . . . . . . . . . . . . . . . . . 539 Daiichiro Sekiba 88 Scanning Capacitance Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . 545 Nobuyuki Nakagiri 89 Scanning Electrochemical Microscopy . . . . . . . . . . . . . . . . . . . . . . . 551 Yasufumi Takahashi 90 Scanning Electron Microscope Energy Dispersive X-Ray Spectrometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 557 Masaki Morita 91 Scanning Electron Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 563 Yasuyuki Okano 92 Scanning Helium Ion Microscope . . . . . . . . . . . . . . . . . . . . . . . . . . . 571 Keiko Onishi and Daisuke Fujita 93 Scanning Near-Field Optical Microscopy/Near-Field Scanning Optical Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 577 Tetsuya Narushima 94 Scanning Probe Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 583 Ken Nakajima 95 Scanning Transmission Electron Microscopy . . . . . . . . . . . . . . . . . 587 Koji Kimoto 96 Scanning Transmission X-Ray Microscopy . . . . . . . . . . . . . . . . . . . 593 Yasuo Takeichi 97 Scanning Tunneling Microscopy. . . . . . . . . . . . . . . . . . . . . . . . . . . . 599 Yukio Hasegawa 98 Scanning Tunneling Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . 605 Keisuke Sagisaka 99 Soft X-Ray Absorption Fine Structure. . . . . . . . . . . . . . . . . . . . . . . 611 Kenta Amemiya Contents xv 100 Spectroscopic Ellipsometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 615 Takumi Moriyama 101 Spin- and Angle-Resolved Photoelectron Spectroscopy . . . . . . . . . . 623 Taichi Okuda 102 Spin-Polarized Scanning Electron Microscopy . . . . . . . . . . . . . . . . 631 Teruo Kohashi 103 Spin-Polarized Scanning Tunneling Microscopy . . . . . . . . . . . . . . . 637 Toyo Kazu Yamada 104 Spin-Resolved Photoemission Electron Microscopy. . . . . . . . . . . . . 643 Keiki Fukumoto 105 Super-Resolution Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 651 Kazuya Kabayama and Ryugo Tero 106 Surface Acoustic Wave . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 657 Shinya Sasaki 107 Surface Enhanced Raman Scattering . . . . . . . . . . . . . . . . . . . . . . . . 661 Katsuyoshi Ikeda 108 Surface Magneto-Optic Kerr Effect . . . . . . . . . . . . . . . . . . . . . . . . . 667 Takeshi Nakagawa 109 Surface Plasmon Resonance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 673 Kaoru Tamada 110 Surface Profilometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 679 Masahiro Tosa 111 Surface Sensitive Scanning Electron Microscopy . . . . . . . . . . . . . . 683 Yoshikazu Homma 112 Surface X-Ray Diffraction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 689 Etsuo Arakawa 113 Surface-Enhanced Infrared Absorption Spectroscopy . . . . . . . . . . 697 Masatoshi Osawa 114 Synchrotron Radiation Photoelectron Spectroscopy . . . . . . . . . . . . 707 Jun Fujii 115 Synchrotron Scanning Tunneling Microscope . . . . . . . . . . . . . . . . . 713 Toyoaki Eguchi 116 Thermal Desorption Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . 719 Shohei Ogura and Katsuyuki Fukutani 117 Time-of-Flight Secondary Ion Mass Spectrometry . . . . . . . . . . . . . 725 Satoka Aoyagi xvi Contents 118 Time-Resolved Photoelectron Spectroscopy . . . . . . . . . . . . . . . . . . . 733 Iwao Matsuda 119 Time-Resolved Photoemission Electron Microscopy . . . . . . . . . . . . 741 Atsushi Kubo 120 Time-Resolved Scanning Tunneling Microscopy . . . . . . . . . . . . . . . 749 Hidemi Shigekawa 121 Tip-Enhanced Raman Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . 755 Norihiko Hayazawa 122 Total Reflection X-Ray Fluorescence . . . . . . . . . . . . . . . . . . . . . . . . 763 Jun Kawai 123 Transmission Electron Diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . 769 Yoshio Matsui 124 Transmission Electron Microscope . . . . . . . . . . . . . . . . . . . . . . . . . . 775 Masanori Mitome 125 Ultraviolet Photoelectron Spectroscopy . . . . . . . . . . . . . . . . . . . . . . 783 Kenichi Ozawa 126 Ultraviolet–Visible Spectrophotometry . . . . . . . . . . . . . . . . . . . . . . . 791 Hiro Amekura 127 Vibrational Sum Frequency Generation Spectroscopy . . . . . . . . . . 801 Satoshi Nihonyanagi and Tahei Tahara 128 X-Ray Absorption Near Edge Structure . . . . . . . . . . . . . . . . . . . . . 809 Hitoshi Abe 129 X-Ray-Aided Noncontact Atomic Force Microscopy . . . . . . . . . . . . 815 Shushi Suzuki, Wang-Jae Chun, Masaharu Nomura and Kiyotaka Asakura 130 X-Ray Crystal Truncation Rod Scattering . . . . . . . . . . . . . . . . . . . 821 Tetsuroh Shirasawa 131 X-Ray Magnetic Circular Dichroism . . . . . . . . . . . . . . . . . . . . . . . . 827 Kenta Amemiya 132 X-Ray Photoelectron Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . 833 Makoto Nakamura 133 X-Ray Reflectivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 843 Wolfgang Voegeli 134 X-Ray Standing Wave Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 849 Akira Saito Chapter 1 Acoustic Microscopy Naohiro Hozumi Keywords Sound speed 1.1 1.1.1 Acoustic impedance Leaky surface wave Principles Observation of Soft Biological Matters Biological ultrasonic microscopy, also known as biological scanning acoustic microscope, provides quantitative acoustic parameters like sound speed and characteristic acoustic impedance that are relevant to elastic properties. As it needs no staining process, the observation can be performed rapidly without introducing any chemical or biological damage to the specimen. Figure 1.1 illustrates two types of acoustic microscopes for soft objects: sound speed mode and acoustic impedance mode. Either pulse or tone burst signal is supplied to the acoustic transducer to produce a strongly focused ultrasound. If the soft tissue can be treated as fluid-like, we may assume that only pressure wave (longitudinal wave) can propagates through the specimen. The sound speed of pffiffiffiffiffiffiffiffiffi a pressure wave is given as c ¼ K=q, where K is the elastic bulk modulus and q is the specific gravity. In order to measure the local sound speed, we compare the reflections from the front and rear surfaces of a tissue slice, which is placed on a glass substrate and dipped into a coupling medium (Fig. 1.1a). The sound speed c can be calculated as c ¼ d=Dt, where d is the thickness of the tissue and Dt is the time lag between the reflections from front and rear surfaces. In most cases, however, it is not easy to precisely measure d at the point where the sound beam is focused. Therefore, both the thickness and sound speed are simultaneously assessed by referring the sound speed of the coupling medium (mostly pure water), and the sound speed is obtained as N. Hozumi (&) Department of Electrical and Electronic Information Engineering, Toyohashi University of Technology, Toyohashi, Japan e-mail: hozumi@ee.tut.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_1 1 2 N. Hozumi Fig. 1.1 Two typical modes for observation of soft matters. a Sound speed mode using thin slice as a specimen. b Acoustic impedance mode for cross-sectional observation. The transducer is supplied with 50–500 MHz (or higher) in frequency depending on the required spatial resolution c¼ 1 / m c0 4pfm d 1 ; ð1:1Þ where c0 is the sound speed of the water, fm is one of the maximum points in the intensity spectrum, and /m is the corresponding phase angle (see Appendix for details). Direct reflection from the place where no specimen exists is taken for the reference signal c0. In some cases, it is required that the observation is performed without slicing the pffiffiffiffiffiffiffi tissue. In such a case, characteristic acoustic impedance Z ¼ Kq is alternatively determined. The acoustic impedance reflects the elasticity and therefore is basically equivalent to the sound speed. The acoustic impedance of the target specimen (Ztarget) is obtained as Ztarget ¼ 1A Zsub ; 1þA ð1:2Þ where A¼1 Starget Zsub Zref ; Sref Zsub þ Zref S represents the signal component at an arbitrary frequency, and subscripts target, sub and ref indicate the target specimen, substrate and reference material, respectively (See Appendix for details). In case water is used as the reference, its acoustic impedance is assumed to be 1.5 106 Ns/m3. 1 Acoustic Microscopy 3 Fig. 1.2 Schematic illustration of surface observation by means of leaky surface acoustic wave (LSAW). ‘ and z represent focal length and defocusing distance, D and R represent direct reflection and the reflection through LSAW, respectively. In addition to 2-D (x–y) scanning, scanning along z direction is needed to acquire V(z) signal 1.1.2 Observation of Solid Matters When an object is a solid material like semiconductor or metal, a strong leaky surface acoustic wave (LSAW) is excited when defocused acoustic beam is irradiated onto the surface. Propagation of LSAW is strongly affected by surface conditions; thus, the observation method making use of LSAW is very powerful to know physical properties of a surface. The basic concept is illustrated in Fig. 1.2. The focal point of the acoustic beam (focal length ‘) is set a bit under the surface (z). The directly reflected signal is detected at the beam component represented as D. LSAW is produced at the critical incident angle hR, and its beam component is represented as R. These two reflections (D and R) interfere; therefore, periodical change in the received signal (V(z)) is observed when degree of defocusing (z) is continuously varied. Speed of LSAW (cR) is obtained from V(z) as follows: ( c R ¼ c0 )1=2 c0 2 1 1 ; 2f Dz ð1:3Þ where c0 is the sound speed of the coupling medium (water is used most commonly), f is frequency of the ultrasound, and △z is the interval between minima in V(z) (See Appendix for details). cR is sensitive to surface morphology and physical properties. Therefore, the observation by means of cR is useful to detect very small defect on the surface, and to characterize the surface conditions and film thickness. 1.2 Features • Observe distribution of elasticity in the object quantitatively. • Biological specimens can be observed on the basis of acoustic properties without staining processes. 4 N. Hozumi • Surface condition of solid matters can be quantitatively characterized. 1.3 1.3.1 Applications Sound Speed Profile An example of two-dimensional sound speed profiles is shown in Fig. 1.3 [1]. Pulmonary structures such as alveoli, bronchial trees, blood vessels, and stromal connective tissues are recognized corresponding to those visualized with an optical microscope. The sound speed increases in the following order: normal alveoli, bronchial walls, congested alveoli, fibrosis with organizing pneumonia, cartilage, vascular smooth muscles, and blood. The mean speed through each component is significantly different from that through normal alveoli. 1.3.2 Acoustic Impedance Profiles Figure 1.4a shows cross-sectional acoustic impedance profile of the cerebellar cortex tissue of a rat at mature stage [2]. The granular layer (GL), which is composed of small neuronal cell bodies, molecular layer (ML), which is composed of Fig. 1.3 Sound speed profile of congested lung with dilated pulmonary vein and blood-filled alveoli. a The optical microscope image with hematoxylin and eosin (HE) staining (left) and b its corresponding sound speed image. The speed of sound is greater through vascular smooth muscle and blood-filled alveoli than through normal alveoli. 2.4 2.4 mm in field of view is indicated with 300 300 of lateral resolution. The images are reprinted from Ref. [1] with permission from Nature Publishing Group 1 Acoustic Microscopy 5 Fig. 1.4 Acoustic impedance profiles of biological tissue and cells. a Cerebellum tissue, rat. PL Purkinje layer, ML Molecular layer and GL Granular layer. b Glial cells cultured on film substrate. The images are reprinted from Refs. [2] and [3] with modification with permission from Elsevier elongated axon (neurite) called parallel fibers, and Purkinje layer (PL), which is composed of Purkinje cells as the origin of dendrites, are clearly distinguished. The correspondence with immunohistological observation is also obtained [2]. Figure 1.4b shows an acoustic impedance profile of glial cells cultured on a film substrate [3]. Nucleus is seen at the center of each cell. The nucleus has a round shape, and its acoustic impedance is as high as 1.6 MNs/m3. The nucleus is surrounded by the portion of which acoustic impedance is as high as 1.65–1.7 MNs/m3. This portion has the highest acoustic impedance in the cell. It is considered that the region close to the nucleus is filled with fibrous cytoskeleton, microtubules, which have higher density compared with the other part of the cell. Appendix Derivation of Eq. (1.1) Sound speed of the soft tissue can be assessed by either time domain or frequency domain analysis. An example of frequency domain analysis is as follows: The spectrum of the reflection from the object slice is normalized by the reference waveform. Assuming fm as one of the minimum and maximum points in the intensity spectrum, and /m as the corresponding phase angle, the phase difference between the two reflections at the minimum point is (2n − 1)p, giving 6 N. Hozumi 2pfm 2d ¼ /m þ ð2n 1Þp; c0 ð1:4Þ where d, c0, and n are the tissue thickness, sound speed of the water, and a nonnegative integer, respectively. The phase difference at the maximum point is 2np, giving 2pfm 2d ¼ /m þ 2np: c0 ð1:5Þ The phase angle /m can be expressed by 2pfm 2d 1 1 c0 c ¼ /m ; ð1:6Þ since /m is the phase difference between the wave passed through the distance 2d with sound speed c and that passed though the corresponding distance with sound speed c0. Equation (1.4) gives d¼ c0 f/ þ ð2n 1Þpg 4pfm m ð1:7Þ for the minimum point. For the maximum point, Eq. (1.5) gives d¼ c0 ð/ þ 2npÞ: 4pfm m ð1:8Þ Sound speed is finally calculated as c¼ 1 / m c0 4pfm d 1 ; which corresponds to Eq. (1.1). Derivation of Eq. (1.2) Hereafter, the signal component at an arbitrary frequency will be symbolized by S. Considering the reflection coefficient, the target signal Starget can be described as Starget ¼ Ztarget Zsub S0 ; Ztarget þ Zsub ð1:9Þ 1 Acoustic Microscopy 7 where S0 is the transmitted signal and Ztarget and Zsub are the acoustic impedances of the target and substrate, respectively. On the other hand, the reference signal can be described as Sref ¼ Zref Zsub S0 ; Zref þ Zsub ð1:10Þ where Zref is the acoustic impedance of the reference material. In case of using water as the reference, its acoustic impedance was assumed to be 1.5 106 Ns/m3. One can measure Starget and Zref; however, S0 cannot be directly measured. The acoustic impedance of the target is subsequently calculated as a solution of the simultaneous equations for Ztarget and S0, as S Ztarget ¼ 1 þ target S0 S 1 target S0 sub Zref 1 Starget ZZsub þ Zref ref S Zsub ¼ sub Zref 1 þ Starget ZZsub þ Zref ref S Zsub ; ð1:11Þ assuming that S0 is constant throughout the observation process. Derivation of Eq. (1.3) Phases of the two reflection signals (D and R) are represented as /D ¼ 2ð‘ zÞk0 ; /R ¼ 2 ‘ z k0 þ 2zkR tan hR ; cos hR ð1:12Þ where wave numbers k0 and kR are defined as k0 ¼ 2pf =c0 and kR ¼ 2pf =cR , respectively. Relative phase shift per unit distance of z is represented as /ðzÞ ¼ ð/D ðzÞ /R ðzÞÞ=z ¼ 2fk0 ð1 1= cos hR þ kR tan hR g: ð1:13Þ Two signals emphasize together when ð/D ðzÞ /R ðzÞÞ is 2p. Hence, periodical change appears on the V(z) curve. The interval Dz between minima can be represented by 2p 2pf cos hR 1 2pf sin hR ¼2 þ : Dz c0 cos hR cR cos hR ð1:14Þ Applying hR ¼ sin1 ðc0 =cR Þ (Snell’s law), 1 2f ð1 cos hR Þ ¼ ; Dz c0 thus ð1:15Þ 8 N. Hozumi cos hR ¼ 1 c0 : 2f Dz ð1:16Þ Speed of LSAW is determined as ( cR ¼ c0 = sin hR ¼ c0 c0 1 1 2f Dz 2 )1=2 ; which corresponds to Eq. (1.3). References 1. Miura, K., Yamamoto, S.: Pulmonary imaging with a scanning acoustic microscope discriminates speed-of-sound and shows structural characteristics of disease. Lab. Invest. 92, 1760–1765 (2012) 2. Gunawan, A.I., Hozumi, N., Yoshida, S., Saijo, Y., Kobayashi, K., Yamamoto, S.: Numerical analysis of ultrasound propagation and reflection intensity for biological acoustic impedance microscope. Ultrasonics 61, 79–87 (2015) 3. Gunawan, A.I., Hozumi, N., Takahashi, K., Yoshida, S., Saijo, Y., Kobayashi, K., Yamamoto, S.: Numerical analysis of acoustic impedance microscope utilizing acoustic lens transducer to examine cultured cells. Ultrasonics 63, 102–110 (2015) Chapter 2 Action Spectroscopy with STM Kenta Motobayashi Keywords Vibration spectroscopy Single molecule characterization Vibrationally induced reactions Inelastic electron tunneling Low temperature 2.1 Principle STM-AS is a spectroscopic method capable of vibrational analysis of individual adsorbates on surfaces [1]. It is well known that injection of tunneling electrons from an STM tip into adsorbates can excite their vibrational states via inelastic electron tunneling (IET) process, which is used as STM-IETS. Some of these excitations can induce motion and reactions of the adsorbates [2], such as lateral hopping, rotation, desorption, isomerization, and bond formation/scission (Fig. 2.1a). STM-AS measures the motion/reaction probability as a function of applied bias voltage showing remarkable increases near the bias voltages corresponding to the vibrational energies (Fig. 2.1c). Requiring the dynamics of adsorbates, STM-AS is complementary to STM-IETS which requires static behavior. STM-AS can also detect electronic excitations as well, but here the focus is on the more frequently employed usage of vibrational spectroscopy. In STM-AS, the reaction yield Y (reaction rate per electron) as a function of applied bias V is measured as follows. The STM tip is positioned over a target analyte at a fixed tunneling gap followed by feedback loop off. The tunneling electrons are then injected, and the time required for a single reaction event is read out from an I-t plot in which a sudden change in current takes place at the moment of reaction (Fig. 2.1b). Y(V) is statistically determined from multiple trials of this procedure. The Y–V plot can be fitted by the formula representing Y(V) to determine the vibrational energies as well as the vibrational broadening, rate constant, and reaction order (number of electrons required for the reaction) [3]. The vibrational K. Motobayashi (&) Department of Physical Science and Engineering, Graduate School of Engineering, Nagoya Institute of Technology, Nagoya, Japan e-mail: kmotobayashi@nitech.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_2 9 10 K. Motobayashi e- vibrational motion/ excitation reactions (c) motion/ reactions time reaction yield (b) tunneling current (a) Ω1 Ω2 sample bias Fig. 2.1 Schematics of (a) vibrationally induced motion/reactions of adsorbates under the STM tip, (b) tunneling current as a function of time indicating motion/reactions of the adsorbates, and (c) resulting reaction yield as a function of sample bias voltage in which increases in reaction yield indicate vibrational energies energies enable identification of the functional group and orientation of the adsorbate, and evaluation of the bond strength, in the same manner with other vibrational spectroscopies. Other parameters allow us to gain deeper insights into the microscopic elementary processes behind the reactions induced by IET. 2.2 • • • • • Features Vibrational modes (or electronic states) of a single molecule can be detected. Vibrational modes inducing dynamic motion of adsorbates can be detected. Selection rule is different from IR, Raman, and STM-IETS. Sufficiently low temperature for stabilizing adsorbates is required. Insights into elementary processes behind the dynamics can be obtained. 2.3 Instrumentation For stable measurement of STM-AS, thermal diffusion and desorption of adsorbates, and unintended additional adsorption of any species must be suppressed. Thermal drifting must also be suppressed so that the STM tip can be stably positioned over the analyte. Therefore, the STM system must be equipped with an UHV chamber and cooled to sufficiently low temperature (depending on the analyte). The tunneling current flowing at constant applied bias voltage is recorded by an STM controller or an external oscilloscope to detect the moment of reaction; no additional equipment is required for acquiring the spectra. 2 Action Spectroscopy with STM 2.4 11 Applications 2.4.1 Fundamental Structure of an Isolated Water Dimer on Pt(111) (a) 5Å Tunneling current (nA) The potential of STM-AS as a vibrational spectroscopy has been demonstrated in a structural study of a water dimer on Pt(111) [4]. This simplest building block of water clusters is a useful model for exploring the nature of the hydrogen bond (H-bond) and water–solid interactions. STM-AS is suitable for analyzing this mobile molecule even at low temperatures, and for obtaining cluster-size-specific vibrational information from mixture of differently sized clusters. Lateral hopping of a water dimer (Fig. 2.2a, b) was detected as a sudden decrease of tunneling current (Fig. 2.2c) at a fixed tunneling gap, enabling us to obtain the STM-AS (Fig. 2.2e). A theoretical fit represents the experimental data very well, which results in a reasonable and precise assignment of each vibrational signal (marked by arrows in Fig. 2.2e). In particular, the O–H stretch mode observed at 375 mV was absent in previously reported IRAS and HREELS studies, indicating the advantage of STM-AS in its sensitivity and selectivity. 6 (c) (e) 375: ν(OH)HB 335: δ(HOH)+δ(HOPt) 4 250: δ(HOH)+ν(OPt) 2 0 0 50 100 150 208: δ(HOH) Time (ms) (d) (b) 332: ν(OD)do δ(HOH) ν(OH)Pt ν(OPt) 5Å δ (HOPt) 313: ν(OD)Pt 272: ν(OD)HB H2O dimer D2O dimer Sample bias voltage (mV) Fig. 2.2 STM image of a H2O dimer on Pt(111) (a) and that recorded before and after an intentionally induced lateral hopping (b). (c) Tunneling current measured over a H2O dimer at fixed tunneling gap showing sudden decrease corresponding to lateral hopping. (d) A three-quarter view of the optimized “H-down” structure of a water dimer on Pt(111) obtained from a DFT calculation. (e) STM-AS of lateral hopping of H2O and D2O dimers on Pt(111). Red circles and blue squares represent the experimental results of STM-AS for H2O and D2O, respectively. Thick solid curves are best-fit spectra. Reprinted with the permission from Ref. [4]. Copyright 2014 American Chemical Society 12 K. Motobayashi The vibrational energies enable us to deduce the internal structure of the water dimer. Two DFT studies have concluded different optimized structures and different vibrational energies, and STM-AS shows good agreement in vibrational energies with one of the models. The structure of the water dimer observed by STM was thus concluded to be the so-called H-down model (Fig. 2.2d), where one of the water molecules interacts with the Pt substrate not through the oxygen lone pair but through an OH–Pt hydrogen bond. It was shown here that STM-AS has the capability of determining the internal structure of an isolated molecular cluster at the sub-nm scale, which is difficult even with simple STM imaging. 2.4.2 Dissociation Pathways of a Single Dimethyl Disulfide on Cu(111) The elementary process of S-S bond dissociation of dimethyl disulfide (DMDS, Fig. 2.3a, b) was revealed [5]. STM-AS (Fig. 2.3d) shows that the reaction is induced by excitation of the C–H stretch mode m(C–H), or the combination of m(C– H) and S–S stretch mode m(S–S). For the excitation of m(C–H), the reaction order N was found to be 2 (Fig. 2.3c), which is usually interpreted as the reaction induced by double-quanta excitation of m(C–H). However, the fitting analysis of STM-AS shows that this scenario cannot explain the STM-AS signal and the transition from N = 2 to N = 1 for the combination mode excitation. Instead, the bias dependence of N and Y can be consistently explained by assuming that the reaction is induced when one electron excites m(C–H) and the other excites m(S–S). Thus, the STM-AS measurement and fitting analysis reveal not only the vibrational modes that trigger -6 (a) (c) (b) DMDS DMDS-d6 2 Reaction yield (d)10 curve A (n1,1 = 2) curve B (n1,1 = n1,2 = 1) experimental -8 10 (a) 359: νCH -10 10 -10 396: 10 νCH + -12 νSS 10 1 -1000 300 400 500 Sample bias voltage (mV) 300 300 300 400 400 400 -500 500 500 500 600 600 600 Sample bias voltage (mV) Fig. 2.3 STM images of DMDS on Cu(111) before (a) and after (b) inducing the dissociation. (c) Reaction order N as a function of V for the DMDS dissociation determined by measuring the reaction rate as a function of tunneling current. (d) STM-AS for the DMDS dissociation. Red circles are experimental data. Curve B is a best fit of a fitting function Y(V); curve A with different parameters is shown for comparison. The inset is an STM-AS at negative sample bias. Reprinted with permission from Ref. [5]. Copyright 2014, American Institute of Physics 2 Action Spectroscopy with STM 13 the reaction, but also deeper insight into the mechanism of vibrationally induced reactions via IET, leading to finding a novel mechanism; specifically, a reaction is induced only when two vibrational modes are concurrently at excited states, whether excited by one or two electron(s). References 1. Kim, Y., Motobayashi, K., Frederiksen, T., Ueba, H., Kawai, M.: Action spectroscopy for single-molecule reactions—experiments and theory. Prog. Surf. Sci. 90, 85–143 (2015) 2. Stipe, B.C., Rezaei, M.A., Ho, W.: Inducing and viewing the rotational motion of a single molecule. Science 279, 1907–1909 (1998) 3. Motobayashi, K., Kim, Y., Ueba, H., Kawai, M.: Insight into action spectroscopy for single molecule motion and reactions through inelastic electron tunneling. Phys. Rev. Lett. 105, 076101/1-076101/4 (2010) 4. Motobayashi, K., Arnadottir, L., Matsumoto, C., Stuve, E.M., Jonsson, H., Kim, Y., Kawai, M.: Adsorption of water dimer on Platinum(111): identification of the -OHPt Hydrogen Bond. ACS Nano 8, 11583–11590 (2014) 5. Motobayashi, K., Kim, Y., Arafune, R., Ohara, M., Ueba, H., Kawai, M.: Dissociation pathways of a single dimethyl disulfide on Cu(111): reaction induced by simultaneous excitation of two vibrational modes. J. Chem. Phys. 140, 194705/1-194705/8 (2014) Chapter 3 Ambient Pressure X-Ray Photoelectron Spectroscopy Hiroshi Kondoh Keywords Element selective Ambient pressure 3.1 Chemical state Photoelectron spectroscopy Principle X-ray photoelectron spectroscopy (XPS) is one of most thoroughly used surface science techniques, which provides information on chemical states of both adsorbates and substrates on the basis of core-level shifts of photoelectrons excited primarily with X-rays. Since the photoelectrons are significantly attenuated by inelastic scattering with gas-phase species, the XPS measurements require high-vacuum conditions. In recent years, however, XPS measurements under near ambient pressure conditions have attracted much attention due to an increasing demand for understanding of surface phenomena under realistic conditions. To meet the demand, the ambient pressure (AP) XPS technique was developed by a combination of focused X-rays and differential pumping electrostatic lens system [1]. An example for AP-XPS system is shown in Fig. 3.1, where a focused X-ray beam is shined on a sample that is closely located to an aperture of the difference pumping system to reduce the attenuation effect on the photoelectrons by gas-phase species. A large part of emitted photoelectrons travel a short distance and enter the differential pumping system without inelastic scattering by the gas molecules. The photoelectrons further travel and reach an entrance of a hemispherical analyzer, while the gas-phase species are evacuated in the differential pumping system. With this kind of system, photoelectrons emitted from a sample in a high-pressure cell under near ambient pressure conditions can be detected and their kinetic energy is precisely analyzed, which yields XP spectra from samples under gas pressures typically up to several Torr. H. Kondoh (&) Department of Chemistry, Keio University, Yokohama, Japan e-mail: kondoh@chem.keio.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_3 15 16 H. Kondoh Fig. 3.1 Example for actual AP-XPS system 3.2 • • • • • Features Combination of short sample-aperture distance and differential pumping system. Photoelectron measurements under near ambient pressure conditions. Applicable to operando observation of catalytic reactions. Compatible to electrochemical setup and liquid-jet technique. Available for both synchrotron radiation and laboratory sources in VUV-SX and hard X-ray regions. 3.3 Instrumentation The key technology of AP-XPS is focusing of X-ray source on a narrow space of a short sample-aperture distance and differential pumping for electrostatic lenses as shown in Fig. 3.2. The distance d is typically 1 mm. To avoid inhomogeneous pressure distribution, the radius of hole of aperture R should be as small as less than d/2 [1], which means that R should be less than 500 lm. Therefore in order to use the X-ray source efficiently, the beam size should be of the order of several hundreds microns. In this sense, the synchrotron radiation (SR) has an advantage in efficient use of X-ray source. The shorter d and the smaller R enable to increase the upper limit of gas pressure; 100 Torr has been achieved with d = 200 lm and R = 50 lm at a kinetic energy of photoelectrons of 930 eV [2]. The photoelectron kinetic energy is another important factor to determine the pressure limit. If a higher photoelectron kinetic energy is used by photon source in a hard X-ray region, the pressure limit is significantly increased; recently, XP spectra were successfully measured using photon energy of ca. 8000 eV under the presence of N2 gas at 760 Torr. 3 Ambient Pressure X-Ray Photoelectron Spectroscopy 17 Fig. 3.2 Schematics for AP-XPS apparatus After the photoelectrons enter the differential pumping system, they are focused by electrostatic lenses to pass through the holes of the differential pumping walls efficiently, while the gas molecules are evacuated as schematically illustrated in Fig. 3.2. The electron lens system and the electron detector have been further modified to obtain spatial and temporal resolutions. Note that not only SR-based AP-XPS systems but also laboratory-source-based systems are commercially available [3]. Although the laboratory sources (21.2 eV for HeI, 1253.6 eV for Mg Ka, and 1486.6 eV for Al Ka) give relatively low cross section and low surface sensitivity, the AP-XPS systems using these laboratory sources will definitely contribute to expansion of the user community of photoelectron spectroscopy. 3.4 3.4.1 Applications Operando Observation of Chemical Reaction The high-pressure cell of AP-XPS is modified to apply this technique to a broad range of application studies such as operando observations of heterogeneous catalysts and fuel cells, XPS measurements of liquids and solid–liquid interfaces under electrochemical environments. Here, two application studies concerning operando observations of (1) an Ir catalyst for exhaust gas and (2) a polymer electrolyte fuel cell are introduced. • Ir catalyst for exhaust gas [4] Ir is known as a good catalyst for automobile exhaust gas, particularly NO reduction with a high N2 selectivity, although it is inactive at low temperatures 18 H. Kondoh below 250 °C. To understand the high N2 selectivity and the absence of low-temperature activity, operando observation was performed for Ir(111) with AP-XPS as shown in Fig. 3.3. From the XP spectra, the surface coverages under reaction conditions are deduced as a function of temperature together with catalytic activity monitored by N2 and CO2 mass intensity as shown in Fig. 3.4. Below 250 °C, the surface is dominated by CO, indicating of CO poisoning. Above 250 °C, the Ir surface is activated where NO dissociation takes place resulting in light-off of N + NO and N + N reactions. At this moment, the N + NO reaction does not yield N2O but N2, which gives rise to the high N2 selectivity. • Polymer electrolyte fuel cell [5] Ambient pressure hard X-ray photoemission spectroscopy (AP-HAXPES) was applied to in situ observation of Pt catalysts in the cathode of polymer electrolyte fuel cell under the presence of water vapor. Pt 3d XP spectra were successfully obtained, and potential-dependent oxidation and reduction of the Pt catalysts could be observed. This approach allows us to follow changes in chemical state at the Pt–water interfaces under working conditions as well as electrochemical control. Fig. 3.3 Operando observation with AP-XPS for NO reduction by CO on Ir(111) under 50 mTorr NO and 10 mTorr CO with increasing temperature. Reprinted with the permission from Ref. [4]. Copyright 2017 American Chemical Society 3 Ambient Pressure X-Ray Photoelectron Spectroscopy 19 Fig. 3.4 Catalytic activity monitored by N2 and CO2 mass intensity (upper) and coverages of surface species under NO reduction reaction conditions on Ir(111) (lower). Adapted with the permission from Ref. [4]. Copyright 2017 American Chemical Society References 1. Ogletree, D.F., Bluhm, H., Lebedev, G., Fadley, C.S., Hussain, Z., Salmeron, M.: A differentially pumped electrostatic lens system for photoemission studies in the millibar range. Rev. Sci. Instrum. 73, 3872–3877 (2002) 2. Kaya, S., Ogasawara, H., Näslund, L.-Å., Forsell, J.-O., Casalonguea, H.S., Miller, D.J., Nilsson, A.: Ambient-pressure photoelectron spectroscopy for heterogeneous catalysis and electrochemistry. Catal. Today 205, 101–105 (2013) 3. Tao, F.: Design of an in-house ambient pressure AP-XPS using a bench-top X-ray source and the surface chemistry of ceria under reaction conditions. Chem. Commun. 48, 3812–3814 (2012) 4. Ueda, K., Yoshida, M., Isegawa, K., Shirahata, N., Amemiya, K., Mase, K., Mun, B.S., Kondoh, H.: Operando observation of NO reduction by CO on Ir(111) surface using NAP-XPS and mass spectrometry: dominant reaction pathway to N2 formation under near realistic conditions. J. Phys. Chem. C. 121, 1763–1769 (2017) 5. Takagi, Y., Wang, H., Uemura, Y., Ikenaga, E., Sekizawa, O., Uruga, T., Ohashi, H., Senba, Y., Yumoto, H., Yamazaki, H., Goto, S., Tada, M., Iwasawa, Y., Yokoyama, T.: In situ study of an oxidation reaction on a Pt/C electrode by ambient pressure hard X-ray photoelectron spectroscopy. Appl. Phys. Lett. 105, 131602(1)–131602(5) (2014) Chapter 4 Angle-Resolved Ultraviolet Photoelectron Spectroscopy Takafumi Sato Keywords Photoemission 4.1 Band structure Fermi surface Layered materials Principle ARUPS enables us to determine experimentally the electronic band structure and Fermi surface of crystals. In ARUPS experiments, as shown in Fig. 4.1, vacuum ultraviolet (VUV) photons are irradiated onto the crystal surface, and the kinetic energy of photoelectrons emitted from the crystal surface is measured as a function of angle with respect to sample normal. Figure 4.2 shows the energy conservation relation diagram in photoemission process [1, 2]. (1) An electron, which was at first located on the occupied band (Ei), is excited to the otherwise empty band (Ef) by a VUV photon with the energy of hx. The energy conservation is as follows. hx ¼ Ef Ei : ð1Þ (2) The excited state is assumed to be a free electron state with the bottom of its energy dispersion at E0 with respect to the Fermi level (EF). Momentum parallel and perpendicular to the crystal surface of the photo-excited electron in the crystal are defined as k// and k? , respectively. Then, the energy of electron in the excited state in the crystal is described as follows. 2 2 Ef ¼ h2 ðk== þ k? Þ=2m E0 : ð2Þ T. Sato (&) Department of Physics, Tohoku University, Sendai, Japan e-mail: t-sato@arpes.phys.tohoku.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_4 21 22 T. Sato Fig. 4.1 Schematic view of ARUPS Fig. 4.2 Energy conservation diagram of the photoemission process (3) We define the kinetic energy of photoelectron emitted into vacuum as EK, then we have, Ef ¼ EK þ U; ð3Þ where U is the work function of the sample. (4) The momentum parallel to the crystal surface of photoelectron emitted into the vacuum is described as follows. K== ¼ pffiffiffiffiffiffiffiffiffiffiffiffi 2mEK sin h; where h is the polar angle of photoelectron (Fig. 4.1). ð4Þ 4 Angle-Resolved Ultraviolet Photoelectron Spectroscopy 23 (5) When the photoelectron is emitted from the crystal into vacuum through the surface, the momentum parallel to the surface is conserved. k== ¼ K== : ð5Þ Using formulae (1)–(5), we obtain the relationship between the energy and the momentum of the initial state in the crystal. hk== ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2mðEi þ hx UÞ sin h: By applying this formula to ARUPS data, one can map out the relationship between Ei and k== , namely the electronic band dispersion in the crystal. 4.2 Features • Energy band dispersion of crystal can be directly observed. • Fermi surface of metals can be determined. • Energy gap related to insulating and superconducting properties can be observed. • Many-body interactions responsible for the physical properties can be elucidated. 4.3 Instrumentation Figure 4.3 shows a schematic diagram of an ARUPS spectrometer. The apparatus consists of mainly four parts, (1) a large electrostatic hemispherical electron energy analyzer with the average diameter of about 40 cm, (2) a microwave-driven discharging lamp to produce high-intensity VUV light, (3) a sample preparation vacuum chamber where samples are prepared by cleaving, sputtering, evaporation, etc., and (4) an ultrahigh vacuum main chamber where the sample is irradiated by VUV light from the discharging lamp. In addition, the apparatus is equipped with a liquid-helium cryostat to cool down the sample, a sample transfer system, many vacuum pumps, etc. One can also use synchrotron radiation light to excite photoelectrons (see section Synchrotron Radiation Photoelectron Spectroscopy). The recent progress in the energy resolution in ARUPS experiments owes mainly to the improvement of electron energy analyzer. Figure 4.4 shows the schematic diagram to explain how the energy and the polar angle of photoelectrons are measured with the hemispherical analyzer. By using the two-dimensional detecting system with a multi-channel plate (MCP) and a CCD camera, we are able 24 T. Sato Fig. 4.3 High-resolution ARUPS spectrometer to measure simultaneously the energy and the polar angle of photoelectrons. This 2D-detection system together with a large size of hemispherical analyzer enables a rapid and precise ARUPS measurement to lead to the ultrahigh-resolution measurement. The ARUPS technique provides us rich information with which we can discuss the relationship between the electronic structure and the novel properties of materials such as superconductivity. 4.4 4.4.1 Applications Band Dispersion and Fermi Surface of 1T-VSe2 ARUPS technique is especially powerful for the layered materials due to the momentum conservation parallel to the surface. In layered materials, the energy dispersion perpendicular to the layer is negligibly small and therefore the obtained 4 Angle-Resolved Ultraviolet Photoelectron Spectroscopy Fig. 4.4 Hemispherical analyzer with two-dimensional detector Fig. 4.5 Experimentally determined band structure of layered compound 1T-VSe2 [3] 25 26 T. Sato Fig. 4.6 Fermi surface of 1T-VSe2 determined by ARUPS [3] band dispersions as a function of k// represent the band structure of the material. Figure 4.5 displays a set of ARUPS spectra of layered compound 1T-VSe2 and the experimental band dispersions obtained from these spectra [3]. Several characteristic bands from Se 4p and V 3d orbitals are clearly seen in the experimental band structure. One can find that the V 3d band reaches the Fermi level at midway between C and M points in the Brillouin zone, producing a metallic Fermi surface. The band structure calculation is also shown for comparison in Fig. 4.5. The good agreement between the experiment and the calculation indicates that ARUPS is able to map out the band structure of layered materials with high precision. Fermi surface is one of important physical concepts with which we discuss the electronic properties of materials. By measuring many photoemission spectra all over the Brillouin zone and plotting the spectral intensity at EF as a function of two-dimensional momentum, one can map out experimentally the two-dimensional Fermi surface. Figure 4.6 shows an example applied for 1T-VSe2, where the characteristic two-dimensional cylindrical Fermi surface is clearly mapped out by ARUPS [3]. References 1. Cardona, M., Ley, L.: Photoemission in Solids I and II. Springer, Heidelberg (1978) 2. Hüfner, S.: Photoelectron Spectroscopy: Principles and Applications. Springer, Heidelberg (2003) 3. Terashima, K., Sato, T., Komatsu, H., Takahashi, T., Maeda, N., Hayashi, K.: Phys. Rev. B 68, 155108 (2003) Chapter 5 Atom Probe Field Ion Microscope Masahiko Tomitori Keywords Mass spectroscopy Atomic resolution microscope Field ionization Field evaporation Field emission 5.1 Principle A high electric field of greater than 0.5 V/Å can be generated over the apex of a sharpened metallic needle (tip) with a radius of less than 100 nm, by application of a voltage of higher than a few kV to the tip. Suppose that gas atoms (or molecules) of He, Ne, and so on (imaging gas) are admitted to a vacuum chamber, in which the tip is placed and biased at the positive high voltage. The gas atoms are electrically polarized under the high electric field, and attracted to the tip apex owing to the non-uniform high electric field around the tip; some of them are field adsorbed onto the tip apex. The high electric field pulls down the potential barrier between the tip and the gas atom; the barrier usually confines the electrons in the gas atom to itself. When the polarized gas atom comes in the proximity of the tip apex, the electron can quantum mechanically tunnel from the atom to the tip under the high electric field, where the barrier width becomes less than one nanometer (Fig. 5.1). Consequently, the atom is ionized to be a positively charged gas ion. This is referred to as field ionization. The gas ion is exposed to the high electric field, and then the repulsive force acts for the gas ion to travel away from the tip (Fig. 5.2). The ion flies away, being accelerated under the field, on the trajectory almost along the line of electric force, which extends radially from the tip apex to an electrically grounded plate placed at the front of the tip. In field ion microscope (FIM) [1, 2], the plate is a phosphor screen, on which the ion-incident position emits light. Accordingly, an image of the ion arrivals, geometrically expanded projection along the line of electric force from the tip to the screen, can be observed; the bright spots M. Tomitori (&) School of Materials Science, Japan Advanced Institute of Science and Technology, Ishikawa, Japan e-mail: tomitori@jaist.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_5 27 28 M. Tomitori Fig. 5.1 Potential energy diagram for field ionization. E the electric field, x the distance from the tip, / the work function of the tip, I the ionization potential of the gas atom Fig. 5.2 Schematics of the FIM. A positive high voltage is applied to the tip. Imaging gas atoms of He are field-ionized under high electric fields on the screen correspond to events of field ionization of the gas atoms over the tip apex. The probability of field ionization sensitively increases on the surface atoms and steps of the tip apex, because the electric field slightly increases over those areas atomically protruded from the averaged curvature of the tip. This is the principle of FIM, capable of observing the atomic arrangements on the tip apex. By applying higher electric field, the protruded surface atoms on the tip apex can be ionized owing to the electric field permeation below the surface, which can make the ionized states of the tip atoms more stable than their neutral states in the electrostatic potential. Thereafter, the ionized tip atoms are evaporated, similarly to the ionized gas atoms in field ionization, referred to as field evaporation. The mass of individual tip atoms that field-evaporated by application of a fast-pulsed voltage is analyzed using a time-of-flight technique. This is the principle of atom probe (AP), which is usually used in combination with the FIM, i.e., as atom probe field ion microscope (APFIM). 5 Atom Probe Field Ion Microscope 5.2 29 Features • Atom-by-atom mass analysis and atom-resolved observation of the tip apex. • Magnification M of the FIM is approximately the ratio of the tip-screen distance R to the tip radius r; e.g., M * 106 = 10 cm/100 nm. • Layer-by-layer mass analysis is conducted by sequentially detecting the field-evaporated ions, because the field evaporation proceeds statically atom-by-atom and layer-by-layer, in order of the electric field strength over the surface atoms, which depends on their atomic protrusions from the averaged curvature of the tip. • In combination with a two-dimensional ion detector, three-dimensional mass analysis with atomic resolution is possible as the three-dimensional (3D) AP. • When a negative high voltage is applied to the tip, electrons are emitted from the tip apex through the quantum tunneling (i.e., field emission). The amount of field emission of electrons changes with the work functions and protrusions on the tip apex; the electron projection image can be observed similarly to the FIM, referred to as a field emission microscope (FEM). The combination of APFIM with FEM can reveal the electronic states on the tip apex. The addition of an energy analyzer for field-emitted electrons provides detailed information on the electronic states. 5.3 Instrumentation The tip and the phosphor screen are set face-to-face in a vacuum chamber equipped with a gas admission system. The tip is usually spot-welded to a tungsten loop for heating by passing current into the loop to clean the tip in the vacuum chamber, and it is biased at high voltages through vacuum electric feedthroughs. The tip can be cooled to suppress the thermal motion of the gas atoms for better resolution of the FIM image. For the AP mass analysis, the screen has a hole (probe hole) at its center, through which the field-evaporated tip atoms pass (Fig. 5.3). The direction of the tip can be mechanically tilted to change the analysis region of the tip apex. The trajectories of the field-ionized imaging gas ions to the screen are almost the same with those of the field-evaporated tip atoms. Thus, the atoms on the tip apex, which are imaged on the screen, can be targeted for the AP analysis by mechanically tilting the tip direction so as to fit the FIM spot of the targeted atom to the probe hole. In the AP analysis, the mass is measured atom-by-atom using the time-of-flight technique with a fast-pulsed high voltage or a fast laser pulse to promote the field evaporation of tip atoms, where the flight time from the tip to the ion detector is measured precisely; the flight time is shorter for the atoms with lower ratios of m:n, where m is the mass of the atom and n is the multiple of the elementary charge e for representing the electric charge of the ionized atom. 30 M. Tomitori Fig. 5.3 Schematic diagram of the APFIM setup Fig. 5.4 FIM images of a W tip of the [111]-orientation. a V = +2.8 kV. b After voltage pulse application. Imaging gas: Ne of 10−5 Torr. Temperature: 50 K. c Facet indices 5.4 5.4.1 Applications FIM Observation of a W Tip Figure 5.4 shows FIM images of an electrochemically etched [111]-oriented W tip. Bright spots correspond to W atoms, mostly located at the step edges of the facetted tip apex. The spot patterns reflect the symmetry of the body-centered cubic (bcc) structure of W. Three atoms on the (111) facet with the threefold symmetry of the bcc structure, in Fig. 5.4a, were field-evaporated, in Fig. 5.4b. 5.4.2 AP Analysis: Alloy of Pt–Ir Figure 5.5 shows the mass spectrum and plots of the cumulative number of the ions sequentially detected in the AP analysis for a Pt–Ir (10%) alloy tip. In the plots, the abscissa is the total number of the detected ions, and the ordinate is the number of respective detected atoms. The respective plots fot Pt and Ir show straight lines, indicating that the alloy is uniform. The ratio of It:Pr is calculated to be approximately 10.7%, in agreement with the specification of the supplied material. 5 Atom Probe Field Ion Microscope 31 Fig. 5.5 (a) Mass spectrum and (b) the cumulative numbers of the ionized tip atoms sequentially detected in the AP analysis for a Pt–10% Ir alloy tip Fig. 5.6 Cumulative numbers of the ions detected in pulsed-laser AP for an Al–GaAs tip 5.4.3 AP Analysis: The Interface Between Al and GaAs A GaAs tip with an Al overlayer deposited at low temperature was AP analyzed with laser pulses. The plots of the cumulative number of the ions in Fig. 5.6 show the abruptness of the interface between the Al overlayer and the substrate of GaAs. 32 M. Tomitori References 1. Tsong, T.T.: Atom-Probe Field Ion Microscopy: Field Ion Emission, and Surfaces and Interfaces at Atomic Resolution. Cambridge University Press, Cambridge (1990) 2. Miller, M.K., Smith, G.D.W.: Atom Probe Microanalysis: Principles and Applications to Materials Problems. Materials Research Society, Pittsburgh (1989) Chapter 6 Atomic Force Microscope Shintaro Fujii Keywords Surface topography Nonconducting surface Nano- or atomic-scale resolution Force spectroscopy 6.1 Principle AFM was developed to overcome a drawback of scanning tunneling microscopy (STM), which can only image conducting surfaces. The AFM has the advantage of imaging almost any type of flat surface, including polymers, ceramics, and biological samples. The AFM images topography of a sample surface by scanning a sharp tip with force sensor over a region of interest. The curvature radius of the tip is typically on the order of nanometers. A force between the tip and sample surface is used as a feedback signal. By using the feedback loop to control the height of the tip above the surface, the AFM can generate a topographic map of the surface features. 6.2 Features • AFM operates in various environmental conditions such as in air, liquid, and vacuum. • Nonconducting surface as well as conducting surfaces can be imaged with nanoor atomic-scale resolution. • Typical contact-mode AFM operates in a short-range repulsive force regime, where a tip and a sample are in mechanical contact. S. Fujii (&) Department of Chemistry, School of Science, Tokyo Institute of Technology, Tokyo, Japan e-mail: fujii.s.af@m.titech.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_6 33 34 S. Fujii • Along with mapping of surface topography, simultaneous tunneling current mapping is possible. • Single-molecule force spectroscopy with pico- or nano-Newton resolution can be routinely achieved. 6.3 Instrumentation For sensing force between an AFM tip and surface, a cantilever is typically used as force sensor. The most common cantilevers in use are micromachined silicon (Si) or silicon nitride (Si3N4) cantilever with integrated tips (Fig. 6.1a) and are commercially available. The spring constants of the cantilevers are in the range from 0.01 to 100 N/m. A typical AFM instrument consists of a sample holder mounted on a piezoelectric scanner, a cantilever tip, and a position-sensitive photodetector (PSD) for detecting a laser beam reflected off the end of the cantilever beam to provide cantilever deflection feedback signal (Fig. 6.1b). The deflection signal is measured by the difference in light intensities between the upper and lower photodetectors. In a contact mode of AFM operations, a mechanical contact of the AFM tip and the sample surface is made and the AFM scans the tip over the sample surface with the feedback loop that enables the Z scanner to maintain the tip position at a constant force (constant cantilever deflection) above the sample surface. As the tip scans the surface of the sample, the contour of the surface with the constant tip-surface force can be obtained as an AFM topographic image. In addition to the contact mode, AFM has a variety of operational modes (Table 6.1). Some of the operational modes are described in the following chapters. Fig. 6.1 a Scanning electron micrograph of a micromachined silicon cantilever with an integrated tip, which is placed on a test sample. Reprinted with permission from Ref [1]. b Schematic illustration of a typical AFM system 6 Atomic Force Microscope 35 Table 6.1 Operational modes of AFM Mode Subcategory Measured AFM signal Static mode Contact mode Dynamic mode Amplitude modulation (AM) mode Frequency modulation (FM) mode Deflection of a cantilever (normal force) Torsion of a cantilever (lateral force) Amplitude of a oscillating cantilever Frequency shift of a oscillating cantilever Fig. 6.2 a (Left) Atomically resolved image of KBr (001) in contact AFM mode. The small and large protrusions are attributed to K+and Br− ions, respectively. (Right) Surface structure of KBr (001). A lattice constant is 0.66 nm. The large circles represent the Br− ions (bare ion radius is 0.195 nm); the small circles represent the K− ions (bare ion radius is 0.133 nm). b Atomically resolved image of KBr (001) with a linear defect. The defect at the upper left of the image is indicated by an arrow. The red square corresponds to the surface area of 5 5 nm2. Reprinted with permission from Refs [2, 3] 6.4 6.4.1 Applications Atomic Resolution Imaging of an Insulating Surface in Contact Mode AFM can image nonconducting surfaces with atomic- or nano-scale resolution. Figure 6.2a shows the KBr (001) surface imaged in contact mode under ultrahigh vacuum (UHV) conditions at 4 K [2, 3]. The sample surface was prepared by cleaving KBr crystal in UHV along the (001) plane. A Si3N4 cantilever with a tungsten tip was used for the imaging. The spring constant of the cantilever was 0.37 N/m. Both ionic species are visible in the AFM image, because repulsive forces are used for imaging in the contact mode. On the basis of the observed size difference between the atoms, the small bumps are interpreted as K+ ions and the large bumps as Br− ions. Figure 6.2b shows another AFM image of the KBr (001) surface with a defect. At the upper left corner of the image, a linear atomic 36 S. Fujii defect was observed. This result indicates that true atomic resolution was achieved by the contact-mode AFM and neglects the possibility that the observed atomic periodicity was due to averaged forces between atoms on the surface and atoms on the tip. 6.4.2 Force Measurement of a Single-Molecule Junction AFM-based force spectroscopy was used to measure mechanical properties of single molecules. For example, interaction forces between single strands of DNA have been measured by covalently attaching DNA oligonucleotides to an AFM tip and surface [4]. The force spectroscopy can also measure rupture force of individual chemical bonds [5]. Figure 6.3a, b shows the force curve during breaking process of a single-molecule junction, where a 1,8-octanedithiol molecule binds to Au electrodes of an AFM cantilever tip and surface. Along with the force measurement, electronic conductance through the junction was simultaneously measured. While the conductance decreases in discrete steps, the force decreases in saw-tooth waves. The rupture force (i.e., the final step of the force curve, which is accompanied with a conductance drop to zero) is 1.5 nN. The rupture force is similar to the force required to break the Au–Au bond. Because the Au–S bond is stronger than the Au–Au bond, the Au–Au bond is responsible for the breakdown of the junction. In contrast, for the 4,4′-bipyridine (BPY) single-molecule junction, the rupture force is 0.8 nN (Fig. 6.3c), which is considerably smaller than the force required to break a Fig. 6.3 a Schematic representation of a single-molecule junction formed between a gold-coated cantilever tip and a gold substrate. b, c Simultaneously recorded conductance and force curves of (b) 1,8-ontancedithiol and d BPY junctions during stretching. The spring constant of the AFM cantilever was 36 N/m. Reprinted with permission from Ref. [5] 6 Atomic Force Microscope 37 Au–Au bond. This is because BPY binds to Au electrodes via N–Au bond, which is weaker than the S–Au bond. The rupture force of the single-molecule junction reveals the bonding nature of the molecule to the electrodes. References 1. Wolter, O., Bayer, Th, Greschner, J.: Micromachined silicon sensors for scanning force microscopy. J. Vac. Sci. Technol. B 9, 1353–1357 (1991) 2. Giessibl, F.J., Binnig, G.: Investigation of the (001) cleavage plane of potassium bromide with an atomic force microscope at 4.2 K in ultra-high vacuum. Ultramicroscopy 42–44, 281–289 (1992) 3. Giessibl, F.J.: Advances in atomic force microscopy. Rev. Mod. Phys. 75, 949–983 (2003) 4. Lee, G.U., Chrisey, L.A., Colton, R.J.: Direct measurement of the forces between complementary strands of DNA. Science 266, 771–773 (1994) 5. Xu, B., Xiao, X., Tao, N.J.: J. Am. Chem. Soc. 125, 16164–16165 (2003) Chapter 7 Auger Electron Spectroscopy Fumihiko Matsui Keywords Elemental composition 7.1 Depth analysis Microscopy Principle As each element carries unique values of core-level binding energies, EC, this has enabled the development of numerous elemental composition analysis methods based on core electron excitation. The excitation of a core level, C1, is accompanied by filling of the vacated core hole by an electron transition from a shallower bound state, C2. In particular cases, the excess energy released in this process can be transferred to another bound electron, C3, with a certain probability, leading to the subsequent emission of an Auger electron. The kinetic energy of an Auger electron is determined by the binding energies of the bound states involved: Ekin = EC1 − EC2 − EC3 − /; thus, it is dependent on the atomic number, Z, of the element involved, as shown in Fig. 7.1. Furthermore, the spectral features of Auger electrons are sensitive to chemical shifts in the core levels, as well as to the variations in the valence band structure. The limited inelastic mean free path of electrons with kinetic energies of less than a few keV [3] makes Auger electron a relatively sensitive surface probe, with resolutions in the order of a nanometer. Auger electron spectroscopy (AES) is thus widely used for solid surface composition analysis and characterization. This phenomenon is named after Pierre Auger [4], a pioneering atomic physicist; it is noteworthy that Lise Meitner also discovered this phenomenon independently [5] prior to Auger. Figure 7.2 illustrates the Auger electron emission process. Core electrons can be excited by X-rays, electrons, or ion beams, provided that they are of sufficient energy; note that Ekin is independent of the energy of the excitation source. To differentiate between particular Auger processes, the following nomenclature is used to refer to the subshells: “K” is for 1s, while “L1” is for 2s; when spin–orbital F. Matsui (&) Graduate School of Materials Science, Nara Institute of Science and Technology, Nara, Japan e-mail: matui@ms.naist.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_7 39 40 F. Matsui Fig. 7.1 The kinetic energy of Auger electrons for various elements. Solid symbols indicate the energy of primary components, while open ones represent less intense satellite components [1, 2]. Note that H and He do not produce Auger electrons Fig. 7.2 Schematic diagram of the Auger electron emission process. In practice, the work function of the analyzer, /ANA, is used instead of / for the sample surface, since the kinetic energy of Auger electron is measured from the vacuum level of the analyzer interactions are significant, the double degeneracy for the energy associated with core-hole generation for p, d, and f orbitals is removed, with “L3” and “L2” being used for 2p3/2 and 2p1/2, respectively. “V” is used when a valence electron is involved. As an example of the naming convention, KL1L2,3 stands for 1s excitation with the creation of two holes in 2s and 2p states. 7 Auger Electron Spectroscopy 41 Auger electron emission and X-ray fluorescence are complementary phenomena: Fluorescent X-rays possess a large probe depth and therefore provide bulk information, whereas Auger emission yields information from the surface region. In addition, the yields of Auger electrons depend on the atomic number, as well as on the types of core level involved. For example, the Auger transition probability, aK, for 1s (K shell) excitation can be approximately expressed as 1 aK ¼ 1 ð1 þ bK Z 4 Þ , where the fitting parameter, bK 1:12 106 . For Z values of less than 33, aK is larger than the corresponding transition probability for X-ray fluorescence, xK ¼ 1 aK . 7.2 • • • • Features Elemental composition and chemical environmental information. Electrons and X-rays can be used for excitation. Microscopic elemental imaging by scanning focused electron beams. Through combination with ion sputtering, depth composition analysis is possible. 7.3 Instrumentation AES measurement systems for laboratory use are equipped with an electron gun for the excitation source, an electron energy analyzer, and a sample manipulator in an ultra-high vacuum chamber. Typically, electron beams with kinetic energies in the range 3–5 keV are used: This is because the ionization cross section of a core level is largest when the electron beam energy is around three to five times the binding energy [6]. In the case of electronic excitation, the energies of backscattered electrons contribute to a strong background signal following the primary elastic electron peak. Conventionally, Auger electron spectra are presented in their derivative forms, as the background intensity in the measured spectra is much higher than in the case of X-ray excitation. Figure 7.3 shows an example of an AES measurement for an InSb surface passivated by sulfur, acquired using an electron gun and 4-grid low-energy electron diffraction (LEED) optics. The second derivative Auger electron spectra were measured using lock-in amplification to remove the background intensity. After annealing under ultra-high vacuum conditions, the intensities of the S MNN and C KLL signals were reduced, while the signals corresponding to In MNN and Sb MNN from the substrate increased. For high-sensitivity and high-energy-resolution measurements, cylindrical mirror analyzers (CMA) and concentric hemispherical analyzers (CHA) are used, respectively. For quantitative 42 F. Matsui Fig. 7.3 a Schematic diagram of a retarding-field and concentric-mirror analyzers. b The derivative Auger electron spectra from InSb(001) surface before and after the annealing treatment in the ultra-high vacuum condition analysis, refer to the ISO 18118, which describes the various effects that can alter the AES signal intensity [7]. 7.4 Applications X-ray Auger electron spectroscopy: Fig. 7.4a shows a two-dimensional measurement of Cu LMM Auger electron emission at the L3 and L3 absorption edges as a function of photon energy and photoelectron kinetic energy [8]: The diagonal lines where the kinetic energy increases with the photon energy correspond to the photoelectrons. By averaging the intensity of various photon energies at each kinetic energy, an X-ray-excited Auger electron spectrum can be obtained, as shown in Fig. 7.4b. The Cu L3M4,5M4,5 Auger electrons appear as horizontal lines with constant kinetic energies, starting at a photon energy of 933.1 eV in Fig. 7.4a. The peak with highest intensity, at a kinetic energy of 914 eV, and the second largest peak at 917 eV correspond to the two-hole final states of the 1G4 and 3F2,3,4 multiplets, respectively. For the further information about the fine structure of Auger electron spectra, refer to the review by Bambynek [9]. Depth profiling: The Auger electron probe depth can be altered by changing the polar angle of emission detection: For this purpose, a CHA with high angular resolution is adequate. Recently, various input lens/deflector combinations for CHAs have been commercialized, enabling investigations of thin films with thicknesses of a few atomic layers and surface segregation of alloys. Sample surface sputtering by noble gas ions is a destructive depth-profile analysis method, where the composition ratio obtained from AES analysis can be plotted as a function of sputtering time. As the sputtering efficiency differs for each element and material, calibration by other methods is necessary. 7 Auger Electron Spectroscopy 43 Fig. 7.4 a A two-dimensional intensity map of electrons from Cu surface. The abscissa and the ordinate are photon energy and photoelectron kinetic energy, respectively. b An X-ray-excited Auger electron spectrum is obtained by averaging the intensity for various photon energies at each kinetic energy. c An X-ray absorption spectrum is obtained by averaging the intensity for various kinetic energies of each photon energy Microscopy: Electron beams can be easily focused by electrostatic lenses. By adding an electron spectrometer to a scanning electron microscope system, the elemental composition of a particular point can be analyzed. As mentioned above, CHA gives access to depth information, and the sensitivity limit of AES is about 0.1–1% of surface monolayer adsorbates. Figure 7.5 shows the example of SEM observations combined with AES measurements: Scanning Auger microscopy (SAM) is achieved by mapping the intrinsic Auger electron peak intensity. As SAM image acquisition can take several hours, the region to be analyzed should be determined by SEM measurements prior to SAM imaging. 44 F. Matsui Fig. 7.5 SEM observation of a semiconductor device. The magnified array consists of Fe dots with a width of a few µm. C and O KLL Auger peaks are from the coating materials References 1. Physical Electronics, Inc. Handbook of Auger electron Spectroscopy, 3rd edn. (1996) 2. https://www.nist.gov 3. Tanuma, S.: Electron Attenuation Lengths in Surface Analysis by Auger and X-ray Photoelectron Spectroscopy. In: Briggs, D., Grant, J.T. (eds.) IM Publications and Surface Spectra Limited, pp. 259–294 (2003) 4. Auger, P.: Sur les rayons b secondaires produits dans un gaz par des rayons X. C. r. hebd, séances Acad. Sci. 177, 169–171 (1923) 5. Meitner, L.: Über die Entstehung der b-Strahl-Spektren radioaktiver Substanzen. Zeitschrift für Physik. 9, 131–144 (1922) 6. Casnati, E., Tartari, A., Baraldi, C.: An empirical approach to K-shell ionisation cross section by electrons. J. Phys. B 15, 155–167 (1982) 7. ISO 18118: Surface Chemical Analysis—Auger Electron Spectroscopy and X-ray Photoelectron Spectroscopy—Guide to the Use of Experimentally Determined Relative Sensitivity Factors for the Quantitative Analysis of Homogeneous Materials (2004) 8. Matsui, F., Maejima, N., Matsui, H., Nishikawa, H., Daimon, H., Matsushita, T., Muntwiler, M., Stania, R., Greber, T.: Circular dichroism in Cu resonant Auger electron diffraction. Z. Phys. Chem. 230, 519–535 (2016) 9. Bambynek, W., Crasemann, B., Fink, R.W., Freund, H.-U., Mark, H., Swift, C.D., Price, R.E., Venugopara Rao, P.: X-Ray fluorescence yields, Auger, and Coster-Kronig transition probabilities. Rev. Mod. Phys. 44, 716 (1972) Chapter 8 Cathodoluminescence Takashi Sekiguchi Keywords Luminescence 8.1 Spectra Defects Impurities Principle Cathodoluminescence is a light emission according to an electron beam injection. Using this phenomenon, we can characterize not only the nature of defects or impurities but also their distributions in the material. CL is useful for the characterization of semiconducting materials such as GaN, ZnO, III–V heterostructures, as well as fluorescent materials like rare-earth-doped oxides. CL can also be applied to the characterization of nanomaterials [1] and for the failure analysis of the optical devices such as light-emitting diodes (LEDs) or lasers. CL is related to direct transition of electrons in materials, such as excitonic emission, donor (D)- or accepter (A)-related transitions, D-A pair recombination. Low-temperature observation sometimes gives useful information for CL spectroscopy. 8.2 Features • Spectroscopy gives the energies of direct transition, which are related to the energy levels of defects or impurities. • CL monochromatic image gives the distribution of the defects or impurities. • CL can be combined with secondary electron imaging and EBIC observation. • Low energy electron beam excitation improves spatial resolution of CL. • Low-temperature observation typically improves CL intensity as well as sharpness of spectral feature. T. Sekiguchi (&) International Center for Material Nanoarchitechtonics, National Institute for Materials Science, Tsukuba, Japan e-mail: Sekiguchi.takashi@nims.go.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_8 45 46 8.3 T. Sekiguchi Instrumentation CL system is usually installed in a scanning electron microscope (SEM). As shown in Fig. 8.1, typical system is composed of light collecting unit, light detection unit as well as PC controller. Beam blanker is sometimes installed for lock-in amplification to suppress background signal of the charge-coupled device (CCD). Specimen cooling system is preferable for CL spectroscopy. Sometimes EBIC system is also attached on the specimen stage. The light collecting unit is composed of ellipsoidal mirror and optical fiber. Specimen is set at one focal point of ellipsoid so that CL emitted from the specimen is focused on another focal point, at which the end of optical fiber is fixed. The optical fiber is versatile to align the light collecting system in SEM chamber with detection system outside. Parabolic mirror is also used for light collection, but the off-axis light emitted from out focal point is difficult to collect. CL light is then guided into monochromator or optical filters to select the wavelength. The selected light is led to CCD or photomultiplier (PMT). The former is used for spectral detection (parallel), while latter for imaging (serial). Photon counting is preferable for PMT detection. Special consideration should be done for the detection of ultraviolet (UV) or infrared (IR) emissions to avoid unexpected absorption or low sensitivity. The spectra or images are recorded in the memory of personal computer (PC) to conserve quantitative information. Spectral line scan or two-dimensional mapping of CL spectra is preferable to analyze the detail of emission [2]. 8.4 Applications Nanostructures are suitable for CL because their geometry restricts the carrier diffusion to keep high spatial resolution. CL is also useful because it does not need any pretreatment like contact fabrication. Figure 8.2a, b shows the secondary Fig. 8.1 Block diagram of CL system 8 Cathodoluminescence 47 Fig. 8.2 ZnO nanotubes grown on sapphire. Secondary electron (SE) images (a, b), CL spectra at 300 K (c) and 10 K (d), SE and CL images at 300 K (e, f, g), and CL images at 10 K (h, i). Electron beam; 10 kV, 0.8 nA electron (SE) images of ZnO nanotubes grown on a sapphire substrate by MOCVD [3]. The nanotubes have hexagonal rod features of 1 lm in diameter and several lm long with the wall thickness of 150 nm. The CL spectrum at room temperature shown in (c) has a strong emission at 3.24 eV and a weak broad one around 2.1 eV. The former corresponds to band-edge emission and the latter to defect-related emission. These CL images are shown in Fig. 8.2f, g as well as SE image (e). The band-edge emission is localized at the bottom region of nanotube while defect-related emission is uniform. At 10 K, on the other hand, the band-edge emission becomes about 100 times stronger, and the peak splits into two sharp lines at 3.316 and 3.364 eV (Fig. 8.2d). These peaks are assigned as D-A pair recombination and neutral donor bound exciton recombination (D0X), whose images are shown in Fig. 8.2h, i. The D-A pair recombination looks more localized than D0X recombination, suggesting that acceptor species are nonuniformly distributed in ZnO nanotubes. Thus, low-temperature observation gives fruitful information of luminescence centers. GaN heterostructures are widely used for blue and/or white LEDs. They are typically grown on the sapphire substrates. The patterned substrate and GaN buffer layer are generally used to avoid high density of misfit dislocations. The cross-sectional CL observation of such structure is interesting to investigate the defect propagation. Figure 8.3 shows the cross-sectional CL images of a blue LED and line plot of spectra. The SE image (a) indicates the interface of sapphire and GaN layer. The schematic representation of this structure is shown in (b). Monochromatic CL image of 300 nm (c) is bright in the substrate region. CL at 360 nm (d) shows darker buffer GaN and brighter epitaxial GaN layers. The vertical 48 T. Sekiguchi Fig. 8.3 Cross-sectional CL observation of a GaN LED. a SE image and b schematics of this cross section. c, d, e Monochromatic CL images of 4.13, 3.44, and 2.88 eV, and f their combination. g Line spectra along yellow line in (d). SE and CL are taken at room temperature with an electron beam of 5 kV dark lines in epilayer are the dislocations, maybe propagated from the buffer layer. The 430 nm CL image (e) shows the bright line at the part of surface region, which is an InGaN/GaN active layer to emit blue light. These luminescence images are overlapped with the colors such as blue, green, and orange as shown in (f). The line scan of CL spectra along yellow line of image (d) is shown in (g). The analyses of peak positions and intensity distribution give more detailed information of this device structure. 8 Cathodoluminescence 49 References 1. Dierre, B., Yuan, X.L., Sekiguchi, T.: Low-energy cathodoluminescence microscopy for the characterization of nanostructures. Sci. Tech. Adv. Mat. 11, 043001 (2010) 2. Sekiguchi, T., Sumino, K.: Quantitative Electron Beam Tester for defects in semiconductors (CL/EBIC/SDLTS). Rev. Sci. Instrum. 66, 4277–4282 (1995) 3. Yuan, X.L., Dierre, B., Wang, J.B., Sekiguchi, T.: Spatial distribution of impurities in ZnO nanotubes characterized by cathodoluminescence. J. Nanosci. Nanotech. 7, 3323–3327 (2007) Chapter 9 Conductive Atomic Force Microscopy Risa Fuji Keywords Current image I/V measurement Bias voltage Contact mode 9.1 Electrical property Principle In CAFM mode as shown in Fig. 9.1, which is based on contact mode, a bias voltage is applied during scanning between the probe and the sample, the current that flows is detected, and images of the in-plane distribution are created at the same time as images of the shape. Moreover, this system can evaluate electrical properties of the sample surface by I/V measurement function which can measure the current between the probe and the sample while sweeping the sample bias voltage. 9.2 Features • Current between the probe and the sample can be measured. • Horizontal current of the sample surface can be measured by forming electrode to the sample surface. • Electrical properties of the sample surface can be evaluated by I/V measurement function. • Proper selection of the cantilever and measurement conditions is needed. • Consideration of the influence from adsorbed water in case of measurement in the air. R. Fuji (&) Global Application Development Center, Analytical & Measuring Instruments Division, Shimadzu Corporation, Kanagawa, Japan e-mail: r-fuji@shimadzu.co.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_9 51 52 R. Fuji Fig. 9.1 In CAFM mode, a bias voltage is applied during scanning between the probe and the sample 9.3 Instrumentation Figure 9.2 shows experimental setup for CAFM. The CAFM system consists of height data detection part and current data detection part. Cantilever holder for current mode is needed. Use carbon tape or other means to affix the sample to the center of the sample holder, keeping it as flat as possible. In current mode, a bias voltage is applied to the sample to measure the current, and thus, sufficient conductivity must be obtained. I/V measurement can be performed while the current image is being scanned. In I/V mode, the current flowing between the sample and the cantilever is measured when the sample bias voltage is changed. 9.4 9.4.1 Applications Observation of Carbon Resistor Figure 9.3 shows the topographic image and current distribution image of carbon resistor surface. The contact-resistance-increased region of a carbon resistor was observed. The shape of the surface-adhered substances and the conductivity distribution are obtained simultaneously. 9.4.2 Observation of P3HT/PCBM Blended Film Figure 9.4 shows the topographic image and current distribution image observed under dark (upper half) and under illumination (lower half). P3HT (regioregular poly(3-hexylthiophene)) and PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) are widely used semiconducting materials for polymer-based solar cells. The P3HT/PCBM blended film (1:1 wt ratio) was spin-coated onto a transparent electrode. To characterize the optoelectronic property at nanometer scale, the AFM 9 Conductive Atomic Force Microscopy 53 Fig. 9.2 Experimental setup for CAFM Fig. 9.3 Topographic image (left) and current distribution image (right) of carbon resistor surface imaging was performed for the blended film under the illumination of 532-nm light. As shown in current distribution image, the photocurrent (maximum 10 pA) was observed under illumination, while the topographic image collected simultaneously was not changed. The photocurrent image represents the charge-carrier generation in the P3HT/PCBM blended film. (Courtesy of Assistant Prof. Hiroaki BENTEN, Graduate School of Engineering, Kyoto University) 54 R. Fuji Fig. 9.4 Topographic image (left) and current distribution image (right) observed under dark (upper half) and under illumination (lower half) Reference 1. http://www.an.shimadzu.co.jp/surface/spm/sol/sp_index.htm Chapter 10 Differential Interference Contrast Microscopy/Phase-Contrast Microscopy Hiroshi Komatsu and Gen Sazaki Keywords Optical microscopy microtopography 10.1 Surface characterization Surface Principle A specimen that has an even magnitude of reflectivity all over the surface is called a phase object. When light is reflected at a stepped surface of a phase object (Fig. 10.1a), the plane wavefront of the incident light is deformed. If the height of the step is h, the amount of deformation is 2 h in its optical path. Hence, the reflected light keeps the information of the surface microtopography and shows a change in phase but no change in amplitude. Since the eye, a photographic film or an ordinary imaging sensor, is only sensitive to the amplitude of light wave, we need to convert the phase change into the amplitude change. In the case of the differential interference contrast microscopy (DIM), an incident-polarized beam is divided into two beams (sheared by a distance S) vibrating at right angles to each other: an ordinary ray and an extraordinary ray. Then, the beams are reflected back at the specimen surface and interfere with each other. If white light is used as a light source, interference colors appear according to the magnitude of the relative phase differences (retardations Δ0, Δ1, and Δ2) between the two beams, as shown in Fig. 10.1b. The phase-contract microscopy (PCM) utilizes the reflected waves from the higher and lower levels (S-wave and P-wave, respectively: Fig. 10.1c) of a specimen surface and provides contrast by utilizing their phase differences according to Abbe’s theory of image formation. The diffracted wave (D-wave), which is formed H. Komatsu Institute for Materials Research, Tohoku University, Sendai, Japan e-mail: fwkd4238@nifty.com G. Sazaki (&) Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan e-mail: sazaki@lowtem.hokudai.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_10 55 56 H. Komatsu and G. Sazaki Fig. 10.1 a Phase difference of the wavefront upon reflection at a stepped surface. b The principle of differential interference contrast microscopy. c A graphic explanation of the principle of phase-contrast microscopy. d Two types of contrasts in phase-contrast microscopy. These illustrations are reprinted from Ref. [1] with permission from Elsevier by the interference of the S- and P-waves, is retarded by approximately a quarter of a wavelength k with respect to the S- and P-waves. Then, by changing the phases of the S- and P-waves by ±k/4, we have the phase relations shown in Fig. 10.1d. The intensity of the P-wave becomes weaker (positive contrast) or stronger (negative contrast) than that of the S-wave, providing the different contrasts at the higher and lower levels. 10.2 Features • Both DIM and PCM can visualize a small height difference of several nm. Hence, their resolution in the vertical direction is an atomic/molecular level. However, their resolution in the lateral direction is strictly limited by a wavelength of the light: The best lateral resolution is about half of the wavelength of the light. • DIM shows a gradient of a phase as a gradient of interference colors. Hence, DIM is particularly suitable for the observation of curved or rugged surfaces. However, it is difficult to determine a sign of a slope (to judge whether a surface is concave or convex). • In contrast, PCM can judge whether a specimen surface is concave or convex from its contrast. But PCM is suitable for the observation of almost completely flat surfaces. In the case of rough or curved surfaces, too high white and dark halos mask detailed information. 10 Differential Interference Contrast Microscopy … 10.3 57 Instrumentation 10.3.1 DIM Figure 10.2a: The directions of a polarizer and an analyzer are perpendicular to each other. Then, between these polarizer and analyzer, a sample surface and a Nomarski prism are placed. The Nomarski prism divides an incident-polarized beam into an ordinary ray and an extraordinary ray. Two reflected light beams interfere with each other at the analyzer. 10.3.2 PCM Figure 10.2b: A phase plate is located at the rear focal plane of an objective lens. The phase plate decreases the intensities of the bright S- and P-waves (reflected at the sample surface) significantly and also changes their phases by ±k/4. When the intensities of the S-wave (or P-wave) and the D-wave are the same, we can obtain the highest contrast. Fig. 10.2 a Path of the rays in the differential interference contrast microscope of a Nomarski type. b Path of the rays in the phase-contrast microscope of an incident type. These illustrations are reprinted from Ref. [1] with permission from Elsevier 58 10.4 H. Komatsu and G. Sazaki Applications 10.4.1 Spiral Growth Steps on a Silicon Carbide (SiC) Crystal Figure 10.3a shows an example of a DIM image obtained from spiral growth steps on a SiC crystal. The thickness of spiral (concentric) steps is about 100 nm. Between these spiral steps, there are two-dimensional (2D) island structures (at the upper part of the image), whose thickness is about 30–50 nm (these structures are more clearly shown in Fig. 10.3b). In Fig. 10.3a, the differential interference contrast was adjusted as if the sample surface was illuminated by a light beam slanted from the upper-left to the lower-right direction. Deep trenches are also visualized clearly. Figure 10.3b shows an example of a PCM image (positive contrast) of the same sample surface as Fig. 10.3a. From the contrast of the halos around the spiral steps, we can conclude that the center of the spiral pattern is higher than periphery. When the height difference is higher than 200 nm, detailed structures cannot be visualized clearly since the contrast of the halos is too high. Then, the detailed structures of the deep trenches are unclear. As explained in this section, DIM and PCM are complementary to each other. Fig. 10.3 Spiral growth steps on a silicon carbide (SiC) crystal visualized by the differential interference contrast microscopy of a reflection type (a) and by the phase-contrast microscopy of a positive contrast type (b). The images were reprinted from Ref. [2] with permission from Kyoritsu Shuppan 10 Differential Interference Contrast Microscopy … 59 10.4.2 Single Molecular Step on Ice In this section, we introduce recent progress in DIM achieved by combining with laser confocal microscopy. After a laser was widely spread as a common light source, laser confocal microscopy was developed [3]. Its principle is shown in Fig. 10.4. Light (solid line) emitted from the sample plane passes through an objective lens, and then the light is concentrated at one point. An aperture located at this point can eliminate most of noise light (dotted line) emitted from the outside of the sample plane. Hence, this optical microscopy has a strong function of noise reduction. Also, this microscopy has a very thin focal depth. To obtain a 2D image, an illumination laser beam is scanned in the x- and y- (lateral) directions. By changing the height of the sample or the objective lens, one can obtain series of 2D images with different height and can construct three-dimensional (3D) images by computer processing. The combination of laser confocal microscopy and DIM (reflection type) takes both the advantages of the strong noise reduction function and 3D-like contrast. Figure 10.5 shows an example of images taken by this combined microscopy [4], demonstrating laterally growing steps on a basal face of an ice crystal under supersaturated water vapor. When steps of neighboring 2D islands coalesced, contrast of the steps always disappeared completely, as in the regions indicated by crossmarks in Fig. 10.5b, d. This result clearly indicates that the steps visualized by this optical microscopy were single (elementary) steps, which are 0.37 nm in thickness corresponding to the size of one water molecule. Fig. 10.4 Path of the rays in the laser confocal microscope of a reflection type 60 H. Komatsu and G. Sazaki Fig. 10.5 Surface morphology on the basal face of an ice crystal. The sequence of photomicrographs was taken by laser confocal microscopy combined with differential interference contrast microscopy: a 0 s, b 0.57 s, c 1.14 s, and d 1.72 s References 1. Komatsu, H.: Optical characterization of crystal surfaces. In: Kaldis, E., (ed.) Crystal Growth of Electronic Materials, pp. 359–370. Elsevier Science Publishers B. V., Amsterdam (1985) 2. Komatsu, H.: Optical microscope. In: The Surface Science Society of Japan (ed.) Hyomen Bunseki Zukan, pp. 74–75, Kyoritsu Shuppan Co., Ltd., Tokyo (1994) (in Japanese) 3. Takamatsu, T., Fujita, S.: Microscopic tomography by laser scanning microscopy and its 3-dimensional reconstruction. J. Microsc. 149, 167–174 (1988) 4. Sazaki, G., Zepeda, S., Nakatsubo, S., Yokoyama, E., Furukawa, Y.: Elementary steps at the surface of ice crystals visualized by advanced optical microscopy. Proc. Nat. Acad. Sci. USA. 107, 19702–19707 (2010) Chapter 11 Dynamic Secondary Ion Mass Spectrometry Mitsuhiro Tomita Keywords Primary ion 11.1 Secondary ion Sputtering Impurity Depth profile Principles Ion beam bombardment with energy of less than a few tens of kiloelectron volts (primary ion bombardment) onto the sample surface causes sputtering phenomena following cascade mixing in the near-surface of the sample. The sputtering induces secondary particle emission from the surface, instantaneously eroding it. Some of the emitted particles are positively or negatively ionized and are called “secondary ions.” Oxygen (O2+) and cesium (Cs+) ion beams are generally used as primary ions for increasing the yields of positively and negatively charged secondary ions, respectively. Dynamic secondary ion mass spectrometry (D-SIMS) detects the abundance of secondary ions with a mass spectrometer and measures depth profiles of elements near the sample surface in the depth direction while producing a sputtering crater by continuous ion bombardment onto the sample surface. Among surface analysis techniques, D-SIMS is one of the most sensitive methods of elemental analysis and can achieve a detection limit of parts per million (ppm) to parts per billion (ppb) for impurity analysis. Accordingly, D-SIMS is widely used for dopant analysis of semiconductor materials (e.g., boron and phosphorus in silicon). Standard reference materials whose impurity concentration is known are needed for quantitative analysis of impurity atoms. D-SIMS analyzes the mass of secondary ions (strictly, the mass-to-charge ratio) and therefore can measure any element (hydrogen to uranium) and can resolve different isotopes (e.g., 10B and 11B). Two-dimensional detection of the secondary ions in the depth direction enables three-dimensional measurement. Low-energy primary ion beam (<1 keV), which M. Tomita (&) Advanced LSI Technology Laboratory, Corporate Research & Development Center, Toshiba Corp., Kawasaki 212-8582, Japan e-mail: mitsuhiro.tomita@toshiba.co.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_11 61 62 M. Tomita suppress cascade mixing, is used to improve the depth resolution of D-SIMS measurements. 11.2 • • • • • • Features High-sensitive elemental analysis (detection limit of ppm to ppb) Analysis of any element (hydrogen to uranium) Isotope analysis Two- or three-dimensional analysis including the depth direction High depth resolution of a few nanometers to a few tens nanometers Quantitative analysis using standard reference materials 11.3 Instrumentation A D-SIMS instrument consists of three main components: a primary ion beam system (ion guns); a secondary ion detection system (mass spectrometer); and a vacuum system, which maintains a vacuum level of less than 10−7 Pa around measured samples in the vacuum chamber (see Fig. 11.1). The ion guns generally emit oxygen and cesium ion beams (O2+ and Cs+) with an energy of 0.15 to a few tens kiloelectron volts. An oxygen ion beam is used to increase positive ion yield, and a cesium ion beam is used to increase negative ion yield. The ion beam current must be stable to within a few percent to maintain a constant sputtering rate in the depth direction. An objective lens focuses the ion beam to a diameter 0.1–30 lmU with current of a few nanoamperes to a few microamperes. The ion beam is raster-scanned on the sample surface and creates square sputter craters (with side lengths of generally a few tens to a few hundreds of micrometers). An extraction lens introduces secondary ions emitted from the center of the sputter crater into the Fig. 11.1 Experimental setup of D-SIMS Primary ion gun (O2+) Mass spectrometer Primary ion gun Cs+ Sample Vacuum pump Vacuum chamber 11 Dynamic Secondary Ion Mass Spectrometry 63 mass spectrometer, which consists of three main parts: an energy analyzer, a mass analyzer, and a detector for secondary ions with target masses. Two types of mass analyzers are used for D-SIMS measurements, namely a quadrupole mass analyzer and a double-focusing mass analyzer. In each type, the secondary ions pass through an energy analyzer installed in front of or behind the mass analyzer. Mass-filtered secondary ions according to their mass-to-charge ratio are detected with an electron multiplier or a Faraday cup. The double-focusing mass analyzer has higher mass resolution of more than 10,000 (M/DM) and higher transmission for secondary ion filtering than the quadrupole mass analyzer. Thus, the double-focusing type is used when there is an interference ion with mass similar to that of the target secondary ion, for example, when measuring phosphorus in silicon (target ion, 31P: 30.9738 u; interference ion, 31(30Si1H): 30.9816 u). The quadrupole mass spectrometer, on the other hand, is used in SIMS instruments with a low-energy primary ion beam because the electrical field between the sample surface and extraction lens for secondary ions is weaker, enabling low-energy ion bombardment. The D-SIMS instrument may contain an electron gun to neutralize charge buildup on insulating sample as a result of primary ion bombardment. 11.4 Applications One of the most important applications of D-SIMS is depth profiling of dopants (electrically active impurities such as boron and phosphorous in silicon) for semiconductor technology. D-SIMS technologies have progressed in accordance with technologies for ion implantation in large-scale integration (LSI) fabrication. Depth profiles of ion implants can be measured down to the ppb level from the surface to micrometers deep. The dopant depth distribution has become shallower and shallower as the integration density of transistors in LSI has increased. Advanced LSI technology requires junction depths of 10 nm or less. It is necessary to measure such shallow dopant depth profiles, which have steep slopes, with high accuracy and precision. This article describes how boron depth profiles in silicon with shallow boron implant can be accurately obtained by D-SIMS analysis. To accomplish this in such a shallow region, attention should be paid to the depth resolution of D-SIMS measurements, the depth calibration method, and the concentration calibration method. D-SIMS measurement was performed using an oxygen ion (O2+) for the primary ion beam, because this generally provides high depth resolution and high boron ion yield. Depth resolution should be improved to obtain accurate depth profiles of very shallow boron implant, because the boron depth profiles are steep, with slopes of less than *2 nm/decade. In D-SIMS depth profiling, a measured depth profile can be expressed as a convolution of the true depth profile with the SIMS depth resolution function. Surface roughness by ion sputtering degrades depth resolution. However, if the sputtered surface is smooth, then the depth resolution function is 64 M. Tomita degraded mainly by cascade mixing, which is the random movement of sample atoms caused by energy deposited by the energetic primary ion beam. Oxygen ion bombardment at normal or near-normal incidence creates a smooth sputtered surface [1, 2]. The depth calibration method influences the accuracy and precision of the depth scale in D-SIMS depth profiles. In fact, the sputtering rate near the surface (from the surface to a few nanometers deep) is higher than that in the bulk (i.e., the steady-state sputtering rate) [1]. This variation in sputtering rate distorts the depth scale of shallow depth profiles when calibration is done using a uniform sputtering rate based on the sputtering time and crater depth. Also, shallow crater depths (<20 nm) that are measured with a profilometer usually have a large error, which decreases the precision of depth scale calibration. These problems are overcome by using a depth scale calibration method with multiple boron delta-doped reference materials. This calibration method was published as an ISO standard [3], in which the sputtered depth with shallow implants is calculated as Z ¼ L þ rt ð1Þ Here, Z is the sputtered depth, t is the sputtering time, r is the steady-state sputtering rate, and L is the shift distance of SIMS depth profiles toward the surface caused by the increased sputtering rate near the surface. In D-SIMS analysis, the impurity concentration is generally calibrated by using standard reference materials, in which a known dose of impurity ions is implanted. But for shallow depth profiling, it is preferable not to use ion-implanted reference materials for the following reasons. First, the secondary ion yield varies near the surface, possibly affecting the relative sensitivity factor (RSF; a factor used for concentration calibration of impurity atoms) calculated from the ion-implanted standard profile. Second, for ion-implanted reference materials prepared with a low implantation energy, the actual dose sometimes deviates from the design value. Third, if the ion-implantation energy is high, then the analysis time becomes long. For these reasons, a uniformly boron-doped (bulk-doped) material was used as a reference for boron concentration calibration, and the RSF of boron (11B) in silicon was calculated from an SIMS depth profile of the boron-doped sample [4, 5]. By using the information described above, depth profiles of shallow boron-implanted silicon samples can be measured by D-SIMS. Here, the sample used for the measurement was 11B-implanted silicon with an implantation energy of 200 eV and dose of 7.5 1014 cm−2, in which the implanted boron atoms were distributed within *20 nm of the surface. D-SIMS measurements were performed using oxygen primary ions with the energies of 200, 350, and 500 eV at incident angles of 30°–40°. These measurement conditions were expected to provide smooth sputtered surfaces during depth profiling [2]. Depth profiles of the boron-implanted samples are shown in Fig. 11.2. The depth scale of the profiles was calibrated using the steady-state sputtering rate and shift distance estimated from the depth profile of multiple boron delta-doped reference materials measured under the same conditions as those for the boron-implanted Dynamic Secondary Ion Mass Spectrometry 65 1022 3 Fig. 11.2 Depth profiles of a 11 B-implanted silicon sample measured with oxygen primary ions with the energies of 200, 350, and 500 eV B Concentration (atoms/cm ) 11 500 eV eV 200 eV 1021 500 eV 350 350 eV 200 eV 1020 1019 1018 1017 1016 0 5 10 15 20 Depth (nm) sample. The shift distances under 200, 350, and 500 eV bombardments were 1.0, 1.3, and 1.6 nm, respectively, resulting in a shift of the depth profiles. Boron concentration in Fig. 11.2 was calibrated using the RSF estimated from a boron bulk-doped sample. Measured boron depth profiles exhibited a steep decay when the primary oxygen ion beam energy was decreased. As discussed above, a lower energy oxygen ion beam provides higher depth resolution, and the depth profile measured with lower energy ions should therefore be accurate. References 1. Wittmaack, K.: Artifacts in low-energy depth profiling using oxygen primary ion beams: dependence on impact angle and oxygen flooding conditions. J. Vac. Sci. Technol., B 16, 2776–2785 (1998) 2. Jiang, Z.X., Lerma, J., Sieloff, D., Lee, J.J., Backer, S., Bagchi, S., Conner, J.: Ultrahigh resolution secondary ion mass spectrometry profiling with oblique O2+ beams below 200 eV. J. Vac. Sci. Technol., B 22, 630–635 (2004) 3. Surface chemical analysis—secondary-ion mass spectrometry—method for depth calibration for silicon using multiple delta-layer reference materials. ISO 23812 (2009) 4. Tomita, M., Hongo, C., Suzuki, M., Takenaka, M., Murakoshi, A.: Ultra-shallow depth profiling with secondary ion mass spectrometry. J. Vac. Sci. Technol., B 22, 317–322 (2004) 5. Surface chemical analysis—secondary-ion mass spectrometry—determination of relative sensitivity factors from ion-implanted reference materials. ISO 18114 (2003) Chapter 12 Elastic Recoil Detection Analysis Daiichiro Sekiba Keywords Rutherford scattering Light elements 12.1 Ion beam analysis Electrostatic accelerator Principle ERDA is a complementary method to RBS. While RBS is a way sensitive to heavy elements, ERDA is effective to quantify light elements, particularly for hydrogen. As well as RBS, ERDA is nondestructive and does not require the standard sample, because the differential cross section can be mathematically derived from the description of Rutherford scattering as follows. Readers can find the schematics and the definitions of variables in kinematics in Fig. 12.1 of the section of Rutherford backscattering spectrometry (RBS). 2 Z1 Z2 e2 ðM1 þ M2 Þ 1 rðE; /Þ ¼ : 2M2 E cos3 / The kinematic energy of recoil E2 can be written as follows. E2 ¼ 4M1 M2 cos2 / ðM1 þ M2 Þ2 E0 krecoil E0 : Here, / signifies the recoil angle with respect to the beam incident direction. The elemental composition can determine analytically from the differential cross section described as follows. D. Sekiba (&) Institute of Applied Physics, University of Tsukuba, Tandem Accelerator Complex, Tsukuba, Japan e-mail: sekiba@tac.tsukuba.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_12 67 68 D. Sekiba Fig. 12.1 Typical setup of ERDA for hydrogen detection with stopper foil. In this case, one can use the normal solid state detectors for both RBS and ERDA 12.2 • • • • Features Absolute quantification of light element; Depth profile with nm-scale depth resolution; Nondestructive; Sensitive to lattice relaxation by means of ion beam channeling. 12.3 Instrumentation 12.3.1 ERDA with Stopper Foil Figure 12.1 shows a typical setup of RBS/ERDA system. Usually, tandem-type (or sometimes single-end-type) electrostatic accelerator is used to create the incident beam as well as the simple RBS measurements. In the case shown in Fig. 12.1, the recoil angle / and backscattering angle h are set at 30° and 150°, respectively. As understood from this figure, in the most cases ERDA and RBS are taken simultaneously. The readers, however, should pay attention to the fact that these two methods are performed simultaneously but not pick up the coincident events. The readers also have to be careful of the difference between the technical terms of “scattering” and “recoil.” The setup in Fig. 12.1 is optimized to quantify hydrogen and/or heavy hydrogen in thin films. The material and thickness of the stopper foil in front of the ERDA detector are chosen considering the range of forward scattered He ion (or atom) and recoils of hydrogen and/or heavy hydrogen. Readers can find the details in the literatures [1, 2]. 12.3.2 ΔE-E Telescope ERDA The light elements, such as Li, C, N, O, and F, can be also good targets of absolute quantification with ERDA. When multiple light elements are included in a sample, the ability of particle identification is very effective in ERDA. ΔE-E telescope is a famous way of the particle identification. In this method, two detectors are used for measuring ΔE and the rest energy E. For the ΔE detector, there are some 12 Elastic Recoil Detection Analysis 69 Fig. 12.2 a ΔE-E telescope ERDA system for the separation of H and D included in amorphous carbon film. A thin SSD is used for the ΔE measurement. b ΔE-E telescope ERDA system with using gas ionization chamber for the ΔE measurement possibilities. For example, Ref. [3] reported the good performance of H and D (heavy hydrogen) separation by using thin SSD for the ΔE detector. The schematics of this setup are shown in Fig. 12.2a. On the other hand, for other light elements, the use of gas ionization chambers is one of the best solutions [4]. The setup of a gas ionization chamber is displayed in Fig. 12.2b. The Ar- or He-based gases are usually used for the gas ionization chambers. 12.3.3 TOF-E Telescope ERDA Recently, time-of-flight (TOF)-E telescope system is widely adopted as a particle identification method by many groups. When relatively lower energy heavy ions are employed as probe beams, TOF-E telescope system enables us to have better mass and energy resolutions compared with those of the ΔE-E telescope system. Figure 12.3 shows the schematics of TOF-E telescope ERDA. The two micro-channel plates (MCPs) detect the secondary electrons emitted from thin carbon foils, when recoils and forward scattered particle penetrate the carbon foils. These are used as start and stop trigger of TOF. Recently, gas ionization chamber is often used to measure the E (kinetic energy of recoils), because energy resolution has been improved than the conventional SSD [5]. Users have to pay attention to the detection efficiency and screening effect to use the TOF-E telescope ERDA. The detection efficiency is determined by the probability of secondary electron emission. Usually, lighter elements emit less secondary electrons. The screen effect becomes crucial when relatively low energy heavy ion is used as a probe beam. Due to the lower kinetic energy, the nucleus of the probe ion cannot be close to the nucleus of targets. In this case, inner shell electrons screen the positive coulomb potential of nucleus, and this induces the deviation from the Rutherford scattering. 70 D. Sekiba Fig. 12.3 Schematics of TOF-E telescope ERDA 12.4 Applications 12.4.1 Hydrogen and Deuteron (Heavy Hydrogen) Quantification on a-C:H Films Figure 12.4a, b shows the RBS and ERDA spectra taken on some hydrogenated amorphous carbon (a-C:H) film including deuterium (D), respectively. The typical stopper foil method displayed in Fig. 12.1 was employed. The concentrations of H and D are independently observed in the ERDA spectra, because the krecoil factors for H and D are different enough. This sensitivity for the isotope is also one of the advantages of ERDA for elemental quantification. From these results, authors could found out that the D concentration in a-C:H film is linearly controlled by changing the D2 partial pressure in the process gas during the physical vapor deposition (PVD) [6]. Fig. 12.4 a RBS and b ERDA spectra taken on the a-C:H (and D) films deposited by physical vapor deposition. The experimental geometry shown in Fig. 12.1 was used 12 Elastic Recoil Detection Analysis 71 Fig. 12.5 a ΔE-E plot taken on a TaO2N film deposited on the SrTiO3 substrate. The experimental geometry shown in Fig. 12.2b was used. b A simulation of the ΔE-E plot indicated in Fig. 12.5a by using SIMNRACF 12.4.2 ΔE-E Plot Taken on TaO2N Film Heavy ion ERDA, such as ΔE-E telescope ERDA and TOF-E telescope ERDA, is recognized as a useful tool on the quantification of multi-anion system, which is recent topic. Figure 12.5a shows the ΔE-E plot taken on a TaO2N film deposited on the SrTiO3 substrate, where one of three oxygen atoms is replaced by nitrogen in the film. Readers can see that light elements, such as O, N, and C, are clearly distinguished on the 2D plot. The slight amount of the C signals can be attributed to the surface contamination due to the not good enough vacuum pressure. Figure 12.5b shows the simulation of the same system by using SIMNRA. The behavior of the forward scattered particles and recoils from the both film and substrate is well reproduced by the simulation [7]. References 1. Doyle, B.-L., Peercy, P.-S.: Technique for profiling 1H with 2.5-MeV Van de Graaff accelerators. Appl. Phys. Lett. 34, 811–813 (1979) 2. Sekiba, D., Horikoshi, M., Abe, S., Ishii, S.: Mg segregation in Mg-rich Mg-Ni switchable mirror studied by Rutherford backscattering, elastic recoil detection analysis, and nuclear reaction analysis, J. Appl. Phys. 106, 114912/1-114912/5 (2009) 3. Wielunski, M., Mayer, M., Behrisch, R., Roth, J., Scherzer, B.-M.-U.: Simultaneous profiling of hydrogen and deuterium by 2.6 MeV 4He ERDA using a ΔE-E telescope detector. Nucl. Instr. Meth. Phys. Res. B 122, 113–120 (1997) 72 D. Sekiba 4. Harayama, I., Nagashima, K., Hirose, Y., Matsuzaki, H., Sekiba, D.: Development of DE-E telescope ERDA with 40 MeV 35Cl7+ beam at MALT in the University of Tokyo optimized for analysis of metal oxynitride thin films, Nucl. Instr. Meth. Phys. Res. B 384, 61–67 (2016) 5. Giangrandi, S., Sajavaara, T., Brijs, B., Arstila, K., Vantomme, A., Vandervorst, W.: Los-energy heavy-ion TOF-ERDA setup for quantitative depth profiling of thin films. Nucl. Instr. Meth. Phys. Res. B 266, 5144–5150 (2008) 6. Sekiba, D., Takemoto, N., Okada, M., Ishii, S., Sakurai, T., Akimoto, K.: Hydrogen isotope tracer experiment in a-C: H deposition: reactive RF magnetron sputtering with CH4 and D2. Diam. Relat. Mater. 27–28, 60–63 (2012) 7. For example, SIMNRA, (http://home.mpcdf.mpg.de/*mam/) Chapter 13 Electrochemical Atomic Force Microscopy Toru Utsunomiya, Yasuyuki Yokota and Ken-ichi Fukui Keywords Electrochemistry Electrode/electrolyte interfaces Solvation Force spectroscopy 13.1 In situ Principle Acronym: EC-AFM Atomic force microscopy (AFM) can image the surfaces of flat materials, irrespective of their conductivity. The sample is usually imaged in air, but can be in liquid environments and under vacuum, as described in other chapters (Chaps. 6, and 55). AFM has also been applied to the electrode/electrolyte interfaces, since its invention. One of the differences between electrochemical (EC-) AFM and AFM in liquids is that the sample potential with respect to the reference electrode is controlled by a potentiostat used for general electrochemical measurements. Figure 13.1 shows the schematic illustrations of the distribution of electrolyte ions close to the electrode/electrolyte interface as a function of the electrode potential within the electrochemical double layer region. Potential of zero charge (pzc) is a characteristic value of the electrode potential at which the electrode does not T. Utsunomiya (&) Department of Materials Science and Engineering, Graduate School of Engineering, Kyoto University, Yoshida-Honmachi, Sakyo-Ku, Kyoto 606-8501, Japan e-mail: utsunomiya.toru.5v@kyoto-u.ac.jp Y. Yokota Surface and Interface Science Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan K. Fukui Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka Osaka 560-8531, Japan © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_13 73 74 T. Utsunomiya et al. Water – Anion + Cation Potentiostat + – CE Electrolyte – + RE WE Electrode Electrochemical setup – + – – E < pzc + E = pzc + E > pzc Fig. 13.1 Schematic illustrations of an electrode/electrolyte interface at the electric double layer region. Potentiostat controls the electrochemical potential of the electrode, which affects the surface charge and the interfacial structures acquire an electrical charge when it comes into contact with the electrolyte. Microscopic structures of the interface are the key issue not only for academic interests, but also for industrial applications, such as battery and electrochemical deposition. Measurement principles and major parts of the EC-AFM apparatus are similar to the AFM in liquids. Early imaging studies using contact- or tapping-mode EC-AFM were thoroughly reviewed by Gewirth and Niece in 1997 [1]. In contrast to EC-STM (Chap. 15), AFM detects forces acting between the tip and sample as the feedback parameter. Therefore, AFM can be applied to in situ imaging of semiconductive and/or soft-materials on the electrode surface [2, 3]. The force curve measurements using contact-mode EC-AFM has been a useful technique to obtain the physicochemical properties at the electrode/electrolyte interfaces. The electrolyte for EC-AFM measurements is not limited to aqueous solution. Recently, Atkin and coworkers performed the force curve measurements at the ionic liquid/electrode interfaces, and showed the potential-dependent interfacial layering of the ionic liquid [4, 5]. However, so-called jump-in-contact problem is inevitable for contact-mode AFM, especially in aqueous solution. Since true atomic resolution images in liquid were taken by Fukuma et al. in 2005 [6], frequency modulation (FM-) AFM has become another promising tool for microscopic investigations of liquid/solid interfaces. The improved AFM with a potentiostat enables us to image the electrode/electrolyte interfaces, not only using contact- or amplitude modulation-mode, but also FM-mode. Furthermore, high sensitivity of the force better than 10 pN in liquids obtained by FM-AFM has also enabled us to characterize the liquid close to the electrode/electrolyte interface without jump-in-contact problems [7–9]. The operation principles of each AFM mode are described in other chapters (Chaps. 6, 34 and 55). The interfacial structuring of the liquid is the key factor for unveiling the electrochemical double layer structures. 13 Electrochemical Atomic Force Microscopy 13.2 75 Features • Atomic scale structures at the electrode surface can be determined under the electrochemical potential control. • The structuring of the interfacial liquids can be obtained using FM-mode. 13.3 Instrumentation The EC-AFM measurement system is a combination of the electrochemical system with an electrochemical cell and “in liquid” AFM, as shown in Fig. 13.2. Because the electrochemical setup is independent of the AFM apparatus, the electrochemical measurements, such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), can be performed in parallel with the EC-AFM operation. General cautions for preparing the EC-AFM setup are also similar to the EC-STM measurements, such as the choice of liquid cell materials, of the electrode potential, and cleaning processes, etc. Reliable connection of each electrode, reference electrode (RE), counter electrode (CE), and working electrode (WE), to the electrolyte solution is necessary for electrochemical measurements. So-called meniscus method, where liquid bridges the tip and the sample with its surface tension, may not be applicable. Careful cleaning of the EC-AFM cell is also important. Some contaminants can cause significant changes of the electrochemical responses, especially at single crystal electrode surfaces. For instances, the aqueous solutions should be prepared using ultrapure water and high-purity substances, such as analytical grade. The commercial RE for general measurements, such as Ag/AgCl, is not a good choice for the EC-AFM system, because of the limited space of the electrochemical cell at the AFM instruments, and of the unexpected halide contamination to the electrolyte. As for RE in aqueous solution, the oxidized Au wire or the flame-annealed Pt wire can be used as a quasi-reference Au/AuOx or a Pt electrode, respectively. Corrosion of these metals is very restricted, avoiding unexpected metal and halide contamination to the system. The calibration of the quasi-RE to other stable RE, such as Ag/AgCl, is important for discussing the results. For organic solvents and ionic liquids, Pt or other metal wires can be used as the RE and CE. The environmental control for removing the dissolved oxygen and residual water is important for elucidating the details of the interfaces. For cantilever-type EC-AFM, the Au-backside-coating probe is better than the Al-backside-coating probe. The backside metal coating enhances the reflectivity of the probe, and Al coating is more effective for reflecting the laser light than Au coating. However, Al coating can be corroded in acidic electrolytes and the dissolved Al ions can also be a contamination source for the system. The control of the 76 T. Utsunomiya et al. Fig. 13.2 Experimental setup for EC-AFM. A bipotentiostat can independently control the electrode potential of sample and tip with respect to the reference electrode. Applying an appropriate potential to the tip can avoid unexpected electrochemical reactions at the tip surface. It is noted that some commercial EC-AFM options do not include the bipotentiostat, and the tip potential is floating (without potential control) tip potential at an appropriate potential can reduce the possibilities of the unexpected electrochemical reactions at the tip surface. For cantilever-type EC-FM-AFM focusing on the atomic resolution images and solvation structures, the optics and electronics should be modified for significant reduction of the displacement sensor noise, as described in the literature [10]. The vibrational amplitude of the tip and the frequency shift are also the critical parameter for measurements. Generally, setting the amplitude less than the diameter of a solvent molecule is effective for detecting the solvation structures and for observing atomic resolution images. 13.4 Application 13.4.1 EC-FM-AFM Utilizing Iodine-Modified Au(111) Surface [9] Figure 13.3a shows a typical atom-resolved EC-FM-AFM image of iodine-modified (I-) Au(111) surface in 0.1 M HClO4 aqueous solution. The distance between the bright spots is more than 0.4 nm, and the pattern is distorted from the hexagonal lattice, indicating that the bright spots cannot be assigned to the Au 13 Electrochemical Atomic Force Microscopy 77 Fig. 13.3 a EC-FM-AFM image (Df = +1051 Hz, Ap-p = 0.25 nm) of an iodine-modified Au (111) surface in 0.1 M HClO4 at –0.8 V. b A typical Df-distance curve obtained at the 0.1 M HClO4/I-Au(111) interface at −0.8 V. Ap-p = 0.27 nm. Potential refers to Au/AuOx, and the tip potential was set to −0.8 V atoms of the Au(111) substrates, which are separated by 0.29 nm. The resemblance between EC-FM-AFM image and the previously obtained EC-STM images [11] strongly indicates that adsorbed iodine atoms on Au(111) were successfully imaged using EC-FM-AFM. Figure 13.3b shows a Df-distance curve obtained at the interface at −0.8 V versus Au/AuOx. The curve shows an oscillatory profile with some peaks. The period of each oscillation is about 0.4 nm, which is roughly equal to the size of water molecules as reported by another group [12]. Thus, oscillatory profiles indicate the presence of hydration layers at the interfaces. References 1. Gewirth, A.A., Niece, B.K.: Electrochemical applications of in situ scanning probe microscopy. Chem. Rev. 97, 1129–1162 (1997) 2. Masuda, T., Ikeda, K., Uosaki, K.: Potential-dependent adsorption/desorption behavior of perfluorosulfonated ionomer on a gold electrode surface studied by cyclic voltammetry, electrochemical quartz microbalance, and electrochemical atomic force microscopy. Langmuir 29, 2420–2426 (2013) 3. Utsunomiya, T., Yokota, Y., Enoki, T., Hirao, Y., Kubo, T., Fukui, K.: Voltammetric and in situ frequency modulation atomic force microscopic investigation of phenalenyl derivatives adsorbed on graphite surfaces. Carbon 77, 184–190 (2014) 4. Hayes, R., Borisenko, N., Tam, M.K., Howlett, P.C., Endres, F., Atkin, R.: Double layer structure of ionic liquids at the Au(111) electrode interface: an atomic force microscopy investigation. J. Phys. Chem. C 115, 6855–6863 (2011) 5. Hayes, R., Warr, G.G., Atkin, R.: Structure and nanostructure in ionic liquids. Chem. Rev. 115, 6357–6426 (2015) 6. Fukuma, T., Kobayashi, K., Matsushige, K., Yamada, H.: True atomic resolution in liquid by frequency-modulation atomic force microscopy. Appl. Phys. Lett. 87, 34101 (2005) 7. Negami, M., Ichii, T., Murase, K., Sugimura, H.: Visualization of ionic-liquid/solid interfaces by frequency modulation atomic force microscopy. ECS Trans. 50, 349–355 (2013) 78 T. Utsunomiya et al. 8. Utsunomiya, T., Yokota, Y., Enoki, T., Fukui, K.: Potential-dependent hydration structures at aqueous solution/graphite interfaces by electrochemical frequency modulation atomic force microscopy. Chem. Commun. 50, 15537–15540 (2014) 9. Utsunomiya, T., Tatsumi, S., Yokota, Y., Fukui, K.: Potential-dependent structures investigated at the perchloric acid solution/iodine modified Au(111) interface by electrochemical frequency-modulation atomic force microscopy. Phys. Chem. Chem. Phys. 17, 12616–12622 (2015) 10. Fukuma, T., Kimura, M., Kobayashi, K., Matsushige, K., Yamada, H.: Development of low noise cantilever deflection sensor for multienvironment frequency-modulation atomic force microscopy. Rev. Sci. Instrum. 76, 53704 (2005) 11. Batina, N., Yamada, T., Itaya, K.: Atomic level characterization of the iodine-modified Au (111) electrode surface in perchloric acid solution by in-situ STM and ex-Situ LEED. Langmuir 11, 4568–4576 (1995) 12. Asakawa, H., Yoshioka, S., Nishimura, K., Fukuma, T.: Spatial distribution of lipid headgroups and water molecules at membrane/water interfaces visualized by three-dimensional scanning force microscopy. ACS Nano 6, 9013–9020 (2012) Chapter 14 Electrochemical Infrared Spectroscopy Shen Ye Keywords Infrared spectroscopy Electrochemistry Electrode/solution interface Reaction products and intermediate Chemical structure 14.1 Principle As an application of infrared reflection absorption spectroscopy (IRAS) in the electrochemistry, electrochemical infrared spectroscopy (EC-IR) is becoming a routing technique to study the chemical structures on the electrode and solution interface during the electrochemical reactions. After a reflection on a highly reflective metal surface, the infrared radiation with s-polarization (electric field perpendicular to the incident plane) has a phase shift of approximately p for all incident angles [1]. As a result, the s-polarized electric field at the surface is practically zero, leading no interaction with the dipole of the molecules on the surface. The s-polarized radiation becomes active only when distance from the electrode surface becomes larger. On the other hand, the phase shift for the infrared radiation with p-polarization (electric field parallel to the incident plane) depends on the incident angle and changes to p/2 at a high incident angle close to 90° [1]. This causes a maximum electric field for the p-polarized light at the metal surface and decreases with the distance from the surface; thus, the p-polarized infrared radiation can strongly interact with the dipoles of molecules on the metal surface under certain conditions. This phenomenon is known as “surface selection rule” for the IRAS [2]. As IRAS is applied in the electrochemical system, a heavy problem appears due to the infrared attenuation by the electrolyte solution. To solve the problem, normally, the metal electrode is gently pressed onto an IR-transparent window leaving a very thin layer of electrolyte solution between the electrode and the window (see, Fig. 14.1a showing a typical structure for the EC-IR cell using S. Ye (&) Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan e-mail: ye.shen@tohoku.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_14 79 80 S. Ye external reflection configuration). The optimum incident angle can be estimated after the thin layer of solution is introduced between the window and electrode [3]. The external reflection configuration with a thin-layer cell for electrochemical study was firstly proposed and realized by Bewick et al. [4, 5] and is now widely used in in situ IR spectroscopic study on the surface of various electrodes, either single crystal electrodes or electrocatalytic materials [6–8]. The thin-layer structure of the electrolyte solution, however, can significantly affect the potential response, ohmic drop, and the mass migration with the bulk solution during the electrochemical processes. On the other hand, Osawa et al. observed that the IR absorption of the molecules adsorbed on the metal nanoparticle surfaces can be enhanced by the factor of approximately 101–103 due to the surface-enhanced infrared absorption (SEIRA) effect [9, 10]. Based on an electromagnetic mechanism, it is expected that an electric field induced on the metal nanoparticle is perpendicular to the surface and quickly decays with the distance. This leads high selectivity of infrared absorption on the surface of the metal nanoparticles. Based on the observation, Osawa et al. developed surface-enhanced infrared absorption spectroscopy with an ATR configuration (ATR-SEIRAS) in which metal nanoparticles deposited on a Si prism surface are employed as a working electrode (see Fig. 14.1b) [9, 10]. This method is applied for several metals such as platinum, palladium, gold, and copper. Especially, instead of vacuum evaporation, the chemical deposition method was successfully employed to prepare the SEIRA-active electrode. The reproducibility and quality of the electrode are largely improved. Not only enhancement forms the SEIRA effect, the ATR-SEPIAS setup (Fig. 14.1b) makes the direct contact of electrode surface with bulk solution, problems of potential response, and mass migration diffusion observed in the typical external IRAS setup (Fig. 14.1a) can be avoided. However, the method is mainly succeeded in limited kinds of metals on Si prism surface and one is hard to control surface structure of the metal nanoparticles. Fig. 14.1 Typical setup for the electrochemical infrared spectroscopy employed on electrode/solution interface: a infrared reflection absorption spectroscopy (IRAS) with external reflection configuration and b surface-enhanced infrared absorption spectroscopy with ATR configuration (ATR-SEIRA) 14 Electrochemical Infrared Spectroscopy 14.2 81 Features • Chemical structure of the adsorption species on the metal electrode surface can be obtained at a level of sub-monolayer during the electrochemical processes. • Surface selection rule on the metal surface works. • External reflection configuration with the thin-layer cell is widely used for various kinds of electrodes. • ATR-SEIRA is useful to study chemical structures on the thin-layer electrode of metal particles deposited on the Si prism surface. 14.3 Instrumentation At the early stage of the EC-IR study, a dispersive optical design named as electrochemically modulated infrared spectroscopy (EMIRS) was developed [5]. By modulating the electrode potential in frequency of typically 7–10 Hz, changes of IR reflection intensity were sensitively determined by a lock-in amplifier. EMIRS provided first successful probe on the electrode/solution interface with high S/N. However, the repetitive potential modulation prevents the investigation of irreversible electrochemical processes. On the other hand, such potential modulation is too fast for the configuration with a thin-layer electrolyte solution, inducing a high ohmic drop in potential and a high resistance in mass transfer. Furthermore, the dispersive infrared instrument with potential modulation is still too complicated for a normal electrochemical laboratory. This situation significantly changed as the Fourier transformed infrared spectrometer (FTIR) became commercially available. Now, a reliable FTIR instrument can be easily obtained with a reasonable price. By utilizing the multiplex advantage of the FTIR instrument, an IR spectrum in wide frequency region can be simultaneously obtained. The S/N ratio of the IR spectra can be improved by integrating more scans. To acquire the IR spectrum in the mid-IR region in a short period, mercury cadmium telluride (HgCdTe, MCT) detector cooled by liquid nitrogen is preferred than the deuterated triglycine sulfate (DTGS) detector which normally equipped to the instrument. Since the FTIR equipment can only obtain single beam spectra, one still needs to record a reference spectrum for the measurement. Only potential difference IR spectra can be obtained. Not only the potential for the sampling, but also the potential for the reference is important to select in order to obtain a meaningful IR spectrum. Depending on the way to select sample and reference potential, a number of procedures have been proposed, such as subtractively normalized interfacial Fourier transformed infrared spectroscopy (SNIFTIRS) and single-potential alteration infrared spectroscopy (SPAIRS). One can design their experiments based on their objectives. The results can be presented in the form of the normalized change of reflectance, ΔR/R, which is equal to (Rs − R0)/R0, where Rs and R0 are the reflectance at the sample and the 82 S. Ye reference potential, respectively. Under the notation, the downward and upward peaks can be attributed to the increase and decrease of the observed species at sample potential in comparison with that at reference potential. One may also show the IR intensity the form of −ΔR/R or absorbance. One should pay attention that the peak direction is opposite in the different notations. Design for the EC-IR cell is also a key point for the measurement. Typical setup for the EC-IR measurement in external IRAS and ATR-SEIRA is given in Fig. 14.1. In the case of external IRAS with the thin-layer configuration, a mirror-like metal electrode mounted in a KeF holder is gently pushed to the IR window a micrometer without rotation of the electrode, which may damage the electrode surface. The layer thickness of the electrolyte solution can be adjusted by monitoring spectral shape and intensity of the single beam from the FTIR instrument. In the mid-IR region (4000– 1100 cm−1), CaF2 window is a good choice. Typically, the incident angle for the IR beam is selected around 65° for the CaF2 window. The total internal reflection of IR beam on the IR window and electrolyte solution interface should be avoided. To carry out the EC-IR observation in the lower frequency region, other IR windows, such as ZnSe and BaF2, may be used. 14.4 Applications 14.4.1 Self-assembled Monolayer on the Electrode Surface The self-assembly method is now widely used to construct well-ordered molecular layers. Figure 14.2a shows a cyclic voltammogram (CV) of self-assembled monolayer of a quinone-terminated molecule, 2-(11-mercaptoundecy1) hydroquinone (abbreviated as H2QC11SH) on a gold electrode surface in 0.1 M HClO4 [11]. A pair of redox peaks, corresponding to the oxidation of hydroquinone (H2Q) to benzoquinone (Q) and the reduction of Q to H2Q, was observed with a large peak separation (0.58 V). Figure 14.2b shows an in situ EC-IR spectrum at 1.0 V for the monolayer obtained by using p- and s-polarized IR radiation (reference potential: 0 V). A number of well-defined bands were observed in the frequency region of 1700–1100 cm−1 in the p-polarization but no peak was observed in s-polarization. Based on the surface selection rule, the bands observed with p-polarization in Fig. 14.2b should be of the monolayer on the gold electrode surface. Even the IR signals were acquired from the monolayer, and the S/N ratio of the peaks in p-polarized spectrum is good. Two upward peaks at 1508 and 1456 cm−1 can be assigned to benzene ring stretch of the H2Q while that at 1206 cm−1 is assigned C–O stretching of H2Q. For the downward bands, a very strong band at 1660 cm−1 (C=O stretching of Q) was observed with two and two small bands at 1600 (C=C stretching of Q) and 1303 cm−1 (C–C stretching). Since the IR intensity is given by ΔR/R in Fig. 14.2b, the present in situ EC-IR results indicate that the formation of Q moieties in the self-assembled monolayer by oxidation of the H2Q moieties at 1.0 V and agree well with that expected from the reversible redox in CV (Fig. 14.1a). The 14 Electrochemical Infrared Spectroscopy 83 Fig. 14.2 a Cyclic voltammogram (0.1 V/s) of the H2QC11SH monolayer on a gold electrode in 0.1 M HClO4 solution. b In situ IRAS spectra of the H2QC11SH monolayer on a gold electrode obtained by p- and s-polarized IR. The sample and reference potentials were +1.0 V and 0 V, respectively time-resolved EC-IR measurements were further employed to study the rate of structural change during the redox process, and it was found that the reduction process was slower than the oxidation process (figures are not given) [11]. These results demonstrate that EC-IR observation is a powerful tool to investigate the structural changes of a monolayer on the electrode surface during the electrochemical reaction. 14.4.2 Reaction Intermediates During the Electrocatalytic Reactions As mentioned above, in addition to the EC-IR using external reflection configuration, the ATR-SEIRAS is also applied to study many electrocatalytic reactions, especially those on the noble metal electrode surfaces. For example, the electro-oxidation mechanism of small organic molecules has been extensively investigated by EC-IR. Figure 14.3a shows a CV of a Pt thin-film electrode in 0.1 M HClO4 containing 0.5 M methanol (MeOH), and the oxidation of MeOH occurs at E > 0.5 V with a current peak around 1.0 V in the positive-going sweep. While in the negative-going scan, the anodic current 84 S. Ye Fig. 14.3 a CV (5 mV/s) of Pt in 0.1 M HClO4 + 0.5 M MeOH. b ATR-SEIRA spectra of the Pt/solution interface in the range from 1200 to 2200 cm–1 during MeOH electro-oxidation on the Pt electrode increases from ca. 0.95 V with a peak at 0.9 V and then drops to nearly zero at E < 0.5 V. Figure 14.3b shows the in situ ATR-SEIRA spectra simultaneously recorded with the CV [12]. At 0.05 V, two bands are observed at 2060 and 1860 cm−1, which are attributed to CO molecules linearly and bridge-bonded to the Pt electrode surface, respectively. The intensities of these bands are almost constant in the potential range of 0.05–0.5 V and rapidly decrease at more positive potentials. Accompanying the decrease in the CO band intensities, a new band appears around 1320 cm−1, which is assigned to formate adsorbed on the Pt electrode surface. The intensity of this band increases with an increase of the potential and shows a maximum around 1.0 V, very similar to the potential dependence of the anodic current observed in the positive-going and negative-going sweeps. The correlation between the anodic current and IR band intensity implies that the formate should be included as a reactive intermediate in the non-CO pathway of the reaction mechanism for MeOH electro-oxidation [12]. The EC-IR observation provides very important information on the reaction intermediates during the electrocatalytic reactions. References 1. Greenler, R.G.: IR study of adsorbed molecules on metal surfaces by reflection techniques. J. Chem. Phys. 44, 310–315 (1966) 2. Suetaka, W.: Surface Infrared and Raman Spectroscopy, Methods and Applications. Plenum Press, New York (1995) 3. Seki, H., Kunimatsu, K., Golden, W.G.: A thin-layer electrochemical cell for infrared spectroscopic measurements of the electrode/electrolyte interface. Appl. Spectrosc. 39, 437–443 (1985) 4. Bewick, A., Kunimatsu, K., Robinson, J., Russell, J.W.: IR vibrational spectroscopy of species in the electrode-electrolyte solution interphase. J. Electroanal. Chem. 119, 175–185 (1981) 14 Electrochemical Infrared Spectroscopy 85 5. Bewick, A., Kunimatsu, K., Pons, B.S., Russell, J.W.: Electrochemically modulated infrared spectroscopy (EMIRS): experimental details. J. Electroanal. Chem. 160, 47–61 (1984) 6. Nart, F., Iwasita, T.: In situ FTIR as a tool for mechanistic studies. Fundamentals and applications. In: Bard, A.J., Stratmann, M., Calvo, E.J. (eds.) Encyclopedia of Electrochemistry, vol. 3, pp. 243–294, Interfacial Kinetics and Mass Transport. Wiley (2003) 7. Korzeniewski, C.: Recent advances in in-situ infrared spectroscopy and applications in single-crystal electrochemistry and electrocatalysis. Adv. Electrochem. Sci. Eng. 9, 233–268 (2006) 8. Zamlynny, V., Lipkowski, J.: Quantitative SNIFTIRS and PM IRRAS of organic molecules at electrode surfaces. Adv. Electrochem. Sci. Eng. 9, 315–376 (2006) 9. Osawa, M.: Dynamic processes in electrochemical reactions studied by surface-enhanced infrared absorption spectroscopy (SEIRAS). Bull. Chem. Soc. Jpn 70, 2861–2880 (1997) 10. Osawa, M.: In-situ surface-enhanced infrared spectroscopy of the electrode/solution interface. Adv. Electrochem. Sci. Eng. 9, 269–314 (2006) 11. Ye, S., Yashiro, A., Sato, Y., Uosaki, K.: Electrochemical in situ FT-IRRAS studies of a self-assembled monolayer of 2-(11-mercaptoundecyl)hydroquinone. J. Chem. Soc. Faraday Trans. 92, 3813–3822 (1996) 12. Chen, Y.X., Miki, A., Ye, S., Sakai, H., Osawa, M.: Formate, an active intermediate for direct oxidation of methanol on Pt electrode. J. Am. Chem. Soc. 125, 3680–3681 (2003) Chapter 15 Electrochemical Scanning Tunneling Microscopy Tomoaki Nishino Keywords Microscopy Single molecule Electrochemistry Biomolecule 15.1 In situ characterization Principle EC-STM directly visualizes atomic structures of solid surfaces and a variety of adsorbates thereon in electrolyte solutions. The working principle is based on electron tunneling, being the same as STM: the tunneling current flowing between the sample surface and atomically sharp tip is probed. Since the tunneling current strongly depends on the tip–sample distance in an exponential manner, the sample topography can be mapped out with the atomic resolution. The prominent feature of EC-STM is the independent control of electrochemical potential of the sample surface and the probe tip with respect to a common reference electrode, and this capability enables the surface imaging under electrochemical environment. 15.2 • • • • • Features The potential of the sample and tip can be independently controlled. Electrochemical processes can be observed in situ. The electrode potential can be utilized to control the sample property. Biomolecules can be imaged under physiological conditions. The probe tip requires careful insulation except for its apex. T. Nishino (&) Department of Chemistry, School of Science, Tokyo Institute of Technology, Tokyo, Japan e-mail: tnishino@chem.titech.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_15 87 88 15.3 T. Nishino Instrumentation In order to observe the surface topography, the tip is raster scanned over the sample surface by piezoelectric elements. The bias voltage is applied between the tip and the sample, and the resulting tunneling current in-between is measured by the EC-STM system. In usual cases, the tip–sample distance is adjusted during the scan by the feedback loop of the system so as to keep the tunneling current constant. The tip trajectory precisely reflects the topography of the sample surface owing to the exponential dependence of the tunneling current on the tip–sample distance. Alternatively, the tip is scanned over the sample surface at a constant height. In this mode of operation, the variation in the current is related to the geometry of the surface. The system described above is common to conventional STM. EC-STM is characterized by a four-electrode configuration controlled by a bipotentiostat (Fig. 15.1). Both the tip and sample serve as working electrodes, and their potentials are independently controlled by the bipotentiostat with respect to the reference electrode. The current measured by the tip could originate from not only tunneling but also electrochemical processes. The latter overwhelms the tunneling current, and consequently hampers the surface imaging. To avoid this, the EC-STM tip needs to be insulated, except for its apex, to reduce the area exposed to the electrolyte solution. As in a conventional electrochemical measurement, the counter electrode compensates for the current generated at the working electrode to enable accurate measurement of the working electrode potential. 15.4 Applications 15.4.1 Electrochemical Polymerization EC-STM was employed to reveal a single molecule process of electrochemical polymerization on a metal substrate [1]. A Au(111) substrate was immersed in an electrolyte solution containing monomer, 3-butoxy-4-methylthiophene, and iodine. Applying consecutive positive-voltage pulses of 1.4 V caused electrochemical growth of the polymer wire. The polymerization was visualized by EC-STM imaging. The EC-STM observation further revealed that the length and the density Fig. 15.1 Schematic illustration of EC-STM system. WE working electrode, RE reference electrode, CE counter electrode 15 Electrochemical Scanning Tunneling Microscopy 89 Fig. 15.2 EC-STM images of Au(111) substrate in solution of the 3-butoxy-4-methylthiophene, iodine and NBu4PF6, after applying a different number of voltage pulses (1.4 V, 150 ms) of a 5, b 13, c 15 times [1] of the polymer wire grow as the number of pulses increases. The maximum length of the wires was as long as 75 nm, corresponding to approximately 200 monomer repeat units. As seen in Fig. 15.2, the organized polythiophene wires aligned along the three specific directions, reflecting the three-fold symmetry of the iodine-covered Au(111) substrate. No well-organized polymerization occurs in the absence of iodine. It was proposed that iodine oxidizes the monomer to its trimer in the bulk solution, which acts as a nucleus for the propagation of the polymer wire on the surface. 15.4.2 Electrochemistry of a Single Molecule It has been demonstrated that the electrode potential can controllably modulate the electrical property of a single molecule based on the four-electrode configuration of EC-STM [2]. The sample molecule, denoted as 6V6, contains redox-active viologen (V2+) unit having two 6-mercaptohexyl (-(CH2)6SH) linkers. The two thiol groups in the linkers chemisorb to the Au tip and the substrate, and as a consequence, the molecule forms a bridge through the gap between the tip and the substrate. Under this circumstance, electron transfer takes place through the single molecule junction, and the conductance of the molecule can be measured. The V2+ moiety is known to be readily reduced to its radical cation V•+, and the effect of the redox states on the single molecule conductance was investigated. The single molecule conductance of 6V6 was measured at different potential of the substrate as shown in Fig. 15.3. It can be clearly seen that the conductance increases as the electrode potential decreases. The change in the conductance is attributed to the one-electron reduction from V2+ to V•+. The electrochemical reduction leads to increase in the electron density of the viologen unit and better energetic alignment between the LUMO of the viologen and the Fermi level of the Au substrate. 90 T. Nishino Fig. 15.3 Potential dependence of the single molecule conductance of 6V6 measured in a 0.1 M phosphate buffer solution. The potential was plotted as the overpotential with respect to the V2+/V•+ equilibrium potential [3] References 1. Sakaguchi, H., Matsumura, H., Gong, H.: Electrochemical epitaxial polymerization of single-molecular wires. Nat. Mater. 3, 551–557 (2004) 2. Haiss, W., van Zalinge, H., Higgins, S.J., Bethell, D., Höbenreich, H., Schiffrin, D.J., Nichols, R.J.: Redox state dependence of single molecule conductivity. J. Am. Chem. Soc. 125, 15294– 15295 (2003) 3. Nichols, R.J., Higgins, S.J.: Single molecule nanoelectrochemistry in electrical junctions. Acc. Chem. Res. 49, 2640–2648 (2016) Chapter 16 Electrochemical Second Harmonic Generation Ichizo Yagi Keywords Surface symmetry Electronic structure In situ measurement Ultrafast dynamics 16.1 Surface coverage Principle EC-SHG is an application of SHG to electrochemical interfaces. As has been described, SHG is a photon-in-photon-out process and can be applied to any kind of interfaces: liquid/solid, liquid/liquid, and solid/solid. The phenomenon is known as the conversion of two photons with the fundamental frequency, x, in one photon with the second harmonic (SH) frequency, 2x. Since the SH signal generate from both the several atomic or molecular layers at usual centrosymmetric electrode/electrolyte interfaces, the SH signal is inherently sensitive to interfacial parameters, such as charge density, adsorbate coverage, atomic orientation, and electronic structure around Fermi level. At electrochemical interfaces, electrolyte solution covering the electrode surfaces limits the application of electron spectroscopy, such as UPS and EELS. Monochromatic EC-SHG can be usually applied to monitoring electrochemical reaction in situ, but the modulation factors on the SHG signal cannot be easily attributed. For example, the charge density on electrode surfaces causes the change in the non-resonant SH intensity, and then, the coverage of anions adsorbed on the electrode surface. Electronic resonance (Fig. 16.1) sometimes causes the increase or decrease in the SH intensity. To understand the electronic resonance between the electronic transition at the electrode/electrolyte interfaces and fundamental or SH photons, one can change the frequency and obtain SHG spectrum [1]. Although the energetic positions of initial and final states of the electronic transition cannot be determined, the transition energy can be obtained, deriving interfacial electronic structure by referring UPS/IPS data for material surfaces in UHV. I. Yagi (&) Faculty of Environmental Earth Science, Hokkaido University, 060-0810 Sapporo, Japan e-mail: iyagi@ees.hokudai.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_16 91 92 I. Yagi Fig. 16.1 Resonant scheme for SHG spectroscopy 16.2 • • • • • Features Electronic and geometric changes at electrode surface can be detected. Rotational anisotropy measurement reflects surface symmetry [2]. Polarization combination can be selected to obtain proper information. Real-time and ultrafast measurements are possible. Tuning the wavelengths. 16.3 Instrumentation The measurement system [3] (Fig. 16.2) basically consists of pulse laser with ns-fs pulse duration and optical detection equipment. Using pulsed laser, the instantaneous electric field reaches the threshold for nonlinear optical effect without breakdown of samples. Although monochromatic laser is useful, tunable laser using optical parametric oscillator or amplifier is desirable to obtain SHG spectra. For relatively low repetition pulsed laser, boxcar integrator coupled with photomultiplier tube (PMT) or avalanche photodiode (APD) is used as detection equipment. For high repetition pulsed laser higher than 10 kHz, lock-in amplifier combined with PMT or APD can be synchronized with optical chopper or acoustic photo-modulator. Both input polarization and power can be controlled by the combination of polarizer and waveplate, such as double-Fresnel romb as s- or p-polarization. Output polarization is also selected by another polarizer. Spectroelectrochemical cells are the most important part in EC-SHG measurement, and incident angle can be kept by using two optical windows. The thickness of electrolyte solution is not limited. 16 Electrochemical Second Harmonic Generation 93 Fig. 16.2 Typical SHG measurement setup. In this case, optical parametric oscillator (OPO) excited by THG of Nd:YAG laser is used as tunable light source 16.4 Applications 16.4.1 Potential Dependent SHG-Rotational Anisotropy (SH-RA) SHG-rotational anisotropy (SH-RA) patterns can be obtained by plotting SH intensity as a function of azimuthal angle of well-ordered sample, such as single crystalline surfaces. Azimuthal angle u is defined as the difference between the crystalline axis parallel to the surface and incident plane, which is formed by the propagation vectors of incident and reflected optical beams. Thus, the SH-RA patterns can be obtained by measuring SH intensity with rotating the sample surface around the axis of the surface normal. Single crystalline metal electrodes are the most investigated surface by in situ SH-RA measurements. Surface reconstruction/ lifting at Au single crystalline electrodes, underpotential deposition (upd) of another metal atom on noble metal single crystalline electrodes, and hydrogen adsorption at Pt single crystalline electrodes had been investigated in 90s. Figure 16.3 shows an example of the change in the polar coordination plot of SH-RA pattern at Au(111) during Tellurium (Te) upd. At the positive potential (+750 mV vs. Ag|AgCl), bare Au(111) has (1 1) structure and threefold SH-RA pattern corresponding to C3V symmetry is observed at p-in/p-out polarization combination (Fig. 16.3a). For SH-RA measurement, fundamental beam of Nd:YAG laser with the wavelength of 1064 nm is incident and the reflected SH signal with the wavelength of 532 nm is detected [2]. The SH-RA pattern can be fitted as the following equation: 2 Ipp ð2xÞ ¼ aðpp1Þ þ dðpp3Þ cos ð3uÞ Ip ðxÞ2 ; 94 I. Yagi (b) (a) (c) (d) Potential / mV vs. Ag|AgCl Fig. 16.3 Polar plotted p-in/p-out SH-RA patterns of Au(111) electrodes at a +500, b +350, and c +150 mV in 0.1 M HClO4 aqueous solution containing 0.5 mM TeO2 [2]. Corresponding CV is shown in d with the sweep rate of 5 mV s−1 where I(2x) and I(x) are SH and fundamental intensities, a(∞) and d(3) represent the rotational constants composed of linear combinations of surface and bulk tensor elements. The superscripts of the constants indicate the associated azimuthal dependence. Thus, a(∞) and d(3) are associated with the isotropic and threefold contributions, respectively. The subscripts on the constants indicate their association to the input and output polarizations given by the first and second indices, respectively. The Fresnel factors are included in these constants. These functions imply that the p-polarized SH intensities, Ipp(2x), will have threefold dependence on the azimuthal rotation of the surface. The exact form depends on the magnitudes and phases of the tensor elements that are included in the constants. For Au(111) electrode, u = 0° corresponds to ½211 crystal direction. As shown in the cyclic voltammogram (CV) of Au(111) in 0.1 M HClO4 aqueous solution containing 0.5 mM TeO2 (Fig. 16.3d), the first upd starts at around +500 mV versus Ag|AgCl as cathodic current and terminates at +150 mV. After the first upd cathodic peak, 16 Electrochemical Second Harmonic Generation 95 (√3 √3)R30°-Te layer on Au(111) is formed, and the SH-RA pattern corresponding C3V symmetry with different phase angle between a(∞) and d(3). These results are not surprising because both Au(111) and (√3 √3)R30°-Te layer possess C3V symmetry including second and third atomic layers. At the potential of 350 mV, where the first upd current still continues to flow, the SH-RA pattern becomes isotropic and almost no contribution of d(3) is observed (Fig. 16.3b). The result indicates that the Te upd occurs randomly, and isotropic deposition of Te atoms occurs in the initial stage of the first upd process. The SH-RA pattern in Fig. 16.3b is reconstructed by extracting SH intensity at +350 mV from the results for potential dependent behaviors at fixed azimuthal angles, although Fig. 16.3a, c is obtained by rotating the surface at the fixed potentials. Thus, real-time and in situ monitoring the interfacial electronic and geometric change is possible by EC-SHG [2]. References 1. Yagi, I., Nakabayashi, S., Uosaki, K.: Excitation wavelength dependent three-wave mixing at a CO-covered platinum electrode. J. Phys. Chem. B 101, 7414–7421 (1997) 2. Yagi, I., Nakabayashi, S., Uosaki, K.: Real time monitoring of electrochemical deposition of tellurium on Au(111) electrode by optical second harmonic generation technique. Surf. Sci. 406, 1–8 (1998) 3. Awatani, T., Yagi, I., Noguchi, H., Uosaki, K.: Second harmonic generation study on electrochemical deposition of palladium on a polycrystalline gold electrode. J. Electroanal. Chem. 524–525, 183–184 (2002) Chapter 17 Electrochemical Sum Frequency Generation Hidenori Noguchi Keywords Electrochemistry Nonlinear optics 17.1 Solid/liquid interface In situ measurements Principle Since the surface is buried in liquid, electrochemical surface science is complicated. Therefore, one cannot benefit from the electron-based techniques mainly used in ultra-high vacuum (UHV) to study the structure and composition of surfaces since the mean free path of the electrons in dense media, i.e., water, is rather limited. Concerning the investigation of vibrational properties at the electrochemical interface, which is particularly relevant to identify adsorbed species as the vibrational spectrum of molecules is a real fingerprint, one has to use optical probe such as infrared reflection absorption spectroscopy (IRRAS), surface enhanced Raman spectroscopy (SERS), and IR-visible SFG spectroscopy. Sum frequency generation (SFG) is a second-ordered process involving two input laser fields, E1(x1) and E2(x2) that induce in a medium a nonlinear polarization: Pð2Þ ðxSFG Þ ¼ e0 vð2Þ: E1 ðx1 Þ E2 ðx2 Þ; ð1Þ where v(2) is the second-order nonlinear susceptibility tensor of the medium and xSFG = x1 + x2. Second harmonic generation (SHG) is a particular case of SFG with x1 = x2 = x and xSFG = 2x. SFG spectroscopy is based on the fact that second-ordered nonlinear optical processes are forbidden under the electric dipole approximation in the bulk of centrosymmetric media (v(2) = 0). At the interface between two media with inversion symmetry, the centrosymmetry is broken and three-wave mixing process such as SFG are allowed. Therefore, this technique is H. Noguchi (&) Global Research Center for Environment and Energy Based on Nanomaterials Science, National Institute for Materials Science, Namiki 1-1, Tsukuba 305-0044, Japan e-mail: noguchi.hidenori@nims.co.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_17 97 98 H. Noguchi inherently surface specific. This section will introduce IR-visible SFG vibrational spectroscopy in the frequency domain and time-resolved studies of vibrational dynamics. 17.2 • • • • Features Vibrational spectra of molecules adsorbed on electrode surface can be obtained. Submonolayer sensitivity can be achieved. Ambient condition is allowed. Extension to femtosecond time-resolved measurements is straightforward. 17.3 Instrumentation The output signal in a three-wave mixing experiment is proportional to 1/dt, where dt is the laser pulse width. Therefore, systems delivering short ns or ps or ultrashort fs laser pulses are preferred. In the case of IR-vis SFG spectroscopy, a tunable IR source is needed. Systems based on optical parametric generation/optical parametric amplification (OPG/OPA) generate signal IR pulses at a given repetition rate. Most of the table laser systems operate in 2–10 lm, which covers the most important molecular vibration. Two main schemes to detect the SFG signals are shown in Fig. 17.1. For ns and ps laser, the IR wavelength is scanned across the region of interest, and the SFG signals were recorded for each wavelength using spatial and spectral filters to remove the reflected input visible light. The resolution is determined by the monochromator as shown in Fig. 17.1a. For fs laser, the band width is of the order of *100 cm−1 and the SFG spectrum can be obtained without scanning the IR wavelength. As the nonlinear signal is self-dispersive, the SFG spectrum can be recorded in the focal plane of the detection optics (Fig. 17.1b). In this case, spectra resolution is given by the width of the visible beam (a few ps, *5 cm−1 usually). Fig. 17.1 Two main setup for IR-vis SFG spectroscopy. a Scheme for ps or ns laser pulse. The IR and visible pulse overlap spatially and temporally on the sample and the generated SFG photons are detected after spatial and spectral filtering. b Scheme for fs laser pulse. A short IR pulse with a broad spectral bandwidth including the region of interest spectrally narrowed visible pulse overlap on the sample. The SF signal is dispersed on a CCD, and the whole spectrum can be acquired at the same time 17 Electrochemical Sum Frequency Generation 99 Fig. 17.2 Scheme for spectro-electro electrochemical cell designed for SFG measurements. The cell is sealed by a prism which is transparent in the IR and visible region Spectro-electrochemical cell designed for SFG measurements is shown in Fig. 17.2. The design of this cell is quite similar to the one used in IRRAS measurements. The experimental difficulty is to avoid the strong IR absorption from the electrolyte solution. This problem can be solved by using a thin layer configuration in which the sample is slightly pressed against the optical window. The optical window is made from CaF2 allows to study up to wavelength of *8 lm. This IR wavelength range can be extended up to *12 lm by using BaF2. 17.4 Applications 17.4.1 Interfacial Water at Electrochemical Interface Interfacial water molecules play important roles in many physical, chemical, and biological processes. A molecular-level understanding of the structural arrangement of water molecules at electrode/electrolyte solution interface is one of the most important issues in electrochemistry. The presence of oriented water molecules induced by interaction between water dipoles and electrode and by the strong electric field within double layer has been proposed. Despite the numerous studies on the structure of water at metal electrode surfaces using various techniques such as surface enhanced Raman spectroscopy (SERS), surface infrared spectroscopy, 100 H. Noguchi Fig. 17.3 SFG spectra in the OH stretching region at Pt(left) and Au(right) electrodes at 0 mV (vs. Ag/AgCl (NaCl sat.)) in 0.1 M HClO4 solution surface enhanced infrared spectroscopy (SEIRAS), and X-ray diffraction, the exact nature of the structure of water at electrode/solution interface is still not fully understood. Figure 17.3 shows SFG spectra in OH stretching region (2800–3800 cm−1) obtained at the Au and Pt electrode in a 0.1 M HClO4 solution at 0 mV [vs. Ag/AgCl (NaCl sat.)]. The SFG spectrum obtained at Au electrode surface was dominated by a broad peak centered around 3500 cm−1, although SFG spectra obtained at Pt electrode surface showed two broad peaks at ca. 3200 cm−1 and ca. 3400 cm−1 [1, 2]. The present results suggest that water molecules at Au electrode/electrolyte solution interface are less ordered than those at Pt electrode/electrolyte solution interface. Although there is no direct spectroscopic evidence to show that the interactions between water molecules and Au and Pt are different, structure of water molecules in the first layer or in the vicinity of metal surface is very important to the whole interfacial water structure. 17.4.2 Molecular Dynamics at Electrochemical Interface The dynamics of interactions between molecules and a surface such as vibrational excitations, energy exchange, and relaxation are of fundamental importance in surface science. The time scale of these processes is in the p- to f- second regime. Recent development of short-pulse laser techniques has enabled direct observation of ultrafast surface dynamics not only to identify the surface species but also to probe the transient species generated by the pump pulse in real time. Although most of the SFG studies so far have been concerned with static structure of molecules at interfaces, a more important contribution of SFG spectroscopy should be its high time resolution, and time-resolved SFG (TR-SFG) is expected to be one of the most powerful methods for observing and identifying transient states of surface adsorbates. Figure 17.4a shows the time-resolved SFG spectra of adsorbed CO on Pt electrode surface at −80, 0, and 70 ps when the surface was irradiated by e pump (532 nm) pulse. At −80 ps, i.e., before pumping, the peak at 2055 cm−1 (mCO on-top) 17 Electrochemical Sum Frequency Generation 101 Fig. 17.4 a Time-resolved SFG spectra in the CO stretching region at delay times of −80, 0, and 70 ps. b Transient CO migration induced by intense pump pulse irradiation was observed. At 0 ps, in addition to the changes of the peak at 2055 cm−1, a new peak appeared at around 1980 cm−1 due to the CO adsorbed on a multi-bonded [3] or asymmetric bridge site [4]. Thus, the decrease of the peak at 2055 cm−1 and the appearance of the new peak at 1980 cm−1 were caused by reversible site migration of CO on the Pt surface the on-top site to a multi-bonded or asymmetric bridge site induced by intense pump pulse irradiation. Population of on-top CO instantly decreased, accompanied by an increase in multi-bonded CO due to the transient temperature jump at the surface and the initial state was recovered with on 100 ps, showing the transient reversible migration of CO molecules on the Pt surface under electrochemical conditions as shown in Fig. 17.4b [5]. References 1. Noguchi, H., Okada, T., Uosaki, K.: Molecular structure at electrode/electrolyte solution interfaces related to electrocatalysis. Faraday Disc. 140, 125–137 (2008) 2. Noguchi, H., Okada, T., Uosaki, K.: SFG study on potential dependent structure of water at Pt electrode/electrolyte solution interface. Electrochim. Acta 53, 6841–6844 (2008) 3. Peremans, A., Tadjeddine, A.: Spectroscopic investigation of electrochemical interfaces at overpotential by infrared-visible sum-frequency generation: Platinum in base and methanol-containing electrolyte. J. Electroanal. Chem. 395, 313–316 (1995) 4. Watanabe, S., Inukai, J., Ito, M.: Coverage and potential dependent CO adsorption on Pt(1111). (711) and (100) electrode surfaces studied by infrared reflection absorption spectroscopy. Surf. Sci. 293, 1–9 (1993) 5. Noguchi, H., Okada, T., Uosaki, K.: Photoinduced surface dynamics of CO adsorbed on a platinum electrode. J. Phys. Chem. B 110, 15055–15058 (2006) Chapter 18 Electrochemical Surface X-Ray Scattering Toshihiro Kondo Keywords Electrochemistry In situ structural analysis at electrode/electrolyte interfaces Surface X-ray diffraction Crystal truncation rod Single crystal electrode 18.1 Principle Electrochemical surface X-ray scattering (EC-SXS) is an application of surface X-ray scattering (SXS) technique, one of the surface analysis methods, to electrochemical interfaces, i.e., electrode/electrolyte solution interfaces [1–3]. Therefore, the basic experimental principle is the same as those described in Chaps 112 and 130. Because X-ray can transmit through the electrolyte solutions without any significant interactions, SXS is so-called in situ technique. SXS techniques have two experimental modes; one is crystal truncation rod (CTR) [1–4], which provides us the information of adsorbed species on the electrode surface, its coverage, layer distance between the adsorbate layer and outermost layer of the electrode surface, and its adsorbed site, and the other is surface X-ray diffraction (SXRD) [1–5], which provides us the two-dimensional lattice parameters both of the adsorbate layer and the surface layer of the electrode. Thus, combination of these two measurements allows us to know the three-dimensional interfacial atomic arrangements at the electrochemical interfaces in situ. In the CTR and SXRD measurements, the scattered X-ray intensity is measured along the normal and T. Kondo (&) Division of Chemistry, Graduate School of Humanities and Sciences, Ochanomizu University, Tokyo, Japan e-mail: kondo.toshihiro@ocha.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_18 103 104 T. Kondo Fig. 18.1 Schematic illustration of the (H, K, L) reciprocal space on the (111) surface. Bold perpendicular straight lines and red circles represent rods and Bragg’s points, respectively parallel, respectively, to the electrode surface in the reciprocal space (Fig. 18.1). Vector, of which direction is the normal to the surface in the reciprocal space, is so-called rod. 18.2 Features • EC-SXS, which has two experimental modes, such as CTR and SXRD, is one of the surface analysis tools. • EC-SXS provides us the three-dimensional structures at electrochemical interfaces with an atomic dimension in situ. • EC-SXS often requires a synchrotron radiation light as an X-ray source. 18.3 Instrumentation Since instrumentation in the SXS measurements was described in Chaps 112 and 130 as application to electrochemistry, in this section, electrochemical cells were explained. Several types of the electrochemical cells were employed for the in situ EC-SXS measurements (Fig. 18.2) [2]. Ultra-thin polymer membrane, such as Mylar® or Kapton®, with a thickness of several µm is generally used as an X-ray window. In order to minimize the X-ray absorption by the electrolyte solution and the scattered X-ray from the solution and window, the “thin-layer” configuration is usually used (Fig. 18.2a) [5]. In this case, the thickness of the electrolyte solution 18 Electrochemical Surface X-Ray Scattering 105 layer, in which X-rays go through, is kept at less than a few tens of µm. When the electrolyte thickness becomes too thin, however, a non-uniform potential distribution and high cell resistance are produced. Only when the potential is changed, therefore, the electrolyte thickness becomes thicker, and then the EC-SXS measurement is carried out when it becomes thinner again. Several “thick-layer” cells, such as a dome-shaped window [6], cylinder-shaped window [7], and a droplet type [8–10], have been devised (Fig. 18.2b–d). Recently, the cell that can control the electrolyte thickness by utilizing the flexibility of the polymer window has been reported [11]. 18.4 Applications 18.4.1 Single Crystal Electrode/Electrolyte Solution Interface [12] Platinum is one of the most important electrode materials not only for basic surface science but also for industrial applications as electrocatalysts. Thus, the structures of low index surfaces of single crystal platinum, especially Pt(111), have been extensively studied in electrochemical environments using a lot of techniques [1, Fig. 18.2 Schematic illustrations of EC-SXS cells [2]. a Thin-layer type [5], b thick-layer type with a dome-shaped window [6], c thick-layer type with a cylinder-shaped window [7], d thick-layer type with a droplet [8–10], and e tunable type between thin- and thick-layer types [11] 106 T. Kondo 13–15]. However, most of these studies concentrated on the limited potential region, CO adsorption/desorption, and/or electrochemical metal deposition including underpotential deposition (UPD). Using the in situ EC-SXS technique, potential-dependent surface structures of Pt(111) in acidic media, such as perchloric and sulfuric acids, were comprehensively investigated in the wide potential range between the UPD of hydrogen (0.05 V vs. RHE) and surface oxide formation (0.95 V) (Fig. 18.3) [12]. In both solutions, in the hydrogen UPD region (0.05 V–0.4 V), the interlayer spacing between first and second outermost Pt layers (d12) expands ca. 2% in bulk lattice spacing. When the electrode potential changes to positive in the double layer potential region in perchloric acid and in the SO42− (or HSO4−) adsorption region in sulfuric acid (0.4 V–0.6 V), this expansion is relaxed to the same spacing in bulk, which is associated with the adsorption of anions and/or water molecules. In the potential region more positive than 0.9 V, oxygen species, such as an adsorbed hydroxyl group (OHad), water molecule (H2O), and/or hydronium cation (H3O+), with a total coverage of 1 monolayer (ML) are adsorbed on the atop site of the Pt (111)-(1 1) surface with the (1 1) structure in perchloric acid and oxygen species, such as SO42− (or HSO4−), H2O, H3O+, and/or OHad, with a total coverage of 1 ML are coadsorbed on the atop site of the Pt(111)-(1 1) surface also with the (1 1) structure in sulfuric acid. Fig. 18.3 Cyclic voltammograms of the Pt(111) single crystal electrode measured in 0.1 M HClO4 (red line) and 0.05 M H2SO4 (blue line) with a scan rate of 20 mV s−1. Surrounding figures are schematic illustrations of the cross section and surface of the Pt(111) surface [12] 18 Electrochemical Surface X-Ray Scattering 107 Fig. 18.4 Cyclic voltammograms of the Ag UPD monolayer (upper) and bilayer (lower) formed on the Au(111) electrode surface, measured in 0.1 M KOH at a scan rate of 20 mV s−1 after transfer from the Ag UPD cell to the alkaline solution cell under air [16]. Insets: schematic illustrations of Ag layers on Au(111) 18.4.2 Electro-Deposited Metal Layers on the Single Crystal Electrode Surface [16] By using the in situ EC-SXS, it was proved that Ag UPD on Au(111) stably forms both monolayer and bilayer [17]. While the Ag UPD monolayer formed on Au(111) is unstable upon transfer to the acidic electrolyte solution, the Ag UPD bilayer on Au(111) is stable even after the transfer under the air. When the potential at the Ag UPD bilayer on Au(111) was cycled in the acidic solution containing Cl− ion, the Ag monolayer formed on the Au(111)-(1 1) surface with the (1 1) structure through the formation of AgCl monolayer with the (4 4) structure [18]. Sisson et al. applied these Ag UPD layers on Au(111) to the alkaline media [16]. While the Ag UPD bilayer on Au(111) is stable upon transfer to the alkaline electrolyte solution, the Ag UPD monolayer reorders to a partial bilayer structure upon or during the transfer process, as well as in the case in the acidic solution. Such partial Ag bilayer on Au(111) showed that a potential response for OH− adsorption that is similar to that of Ag single crystal electrode (Fig. 18.4). References 1. Uosaki, K.: In situ real-time monitoring of geometric, electronic, and molecular structures at solid/liquid interfaces. Jpn. J. Appl. Phys. Part1 54, 030102/1–030102/14 (2015) 2. Kondo, T., Masuda, M., Uosaki, K.: In situ SXS and XAFS measurements of electrochemical interface. In: Kumar, C.S.S.R. (eds.) X-ray and Neutron Techniques for Nanomaterials Characterization (Chap. 7), pp. 367–450. Springer, Berlin (2016) 3. Toney, M.F., McBreen, J.: In situ synchrotron X-ray techniques for determining atomic structure at electrode/electrolyte interfaces. Electrochem. Soc. Interface 2(Spring), 22–31 (1993) 4. Feidenhansl, R.: Surface-structure determination by modern X-ray physics. Surf. Sci. Rep. 10, 105–188 (1989) 108 T. Kondo 5. Wang, J., Ocko, B.M., Davenport, A.J., Isaacs, H.S.: In situ X-ray-diffraction and X-ray-reflectivity studies of the Au(111) electrolyte interface—reconstruction and anion adsorption. Phys. Rev. B 46, 10321–10338 (1992) 6. Scherb, G., Kazimirov, A., Zegenhagen, J., Lee, T.L., Bedzyk, M.J., Noguchi, H., Uosaki, K.: In situ X-ray standing-wave analysis of electrodeposited Cu monolayers on GaAs(001). Phys. Rev. B 58, 10800–10805 (1998) 7. Nagy, Z., You, H.: Applications of surface X-ray scattering to electrochemistry problems. Electrochim. Acta 47, 3037–3055 (2002) 8. Tamura, K., Wang, J.X., Adzic, R.R., Ocko, B.M.: Kinetics of monolayer Bi electrodeposition on Au(111): surface X-ray scattering and current transients. J. Phys. Chem. B 108, 1992–1998 (2004) 9. Ayyad, A.H., Stettner, J., Magnussen, O.M.: Electrocompression of the Au(111) surface layer during Au electrodeposition. Phys. Rev. Lett. 94, 066106/1-066106/4 (2005) 10. Krug, K., Stettner, J., Magnussen, O.M.: In situ surface X-ray diffraction studies of homoepitaxial electrochemical growth on Au(100). Phys. Rev. Lett. 96, 246101/1–246101/4 11. Kondo, T., Zegenhagen, J., Takakusagi, S., Uosaki, K.: In situ real-time study on potential induced structure change at Au(111) and Au(100) single crystal electrode/sulfuric acid solution interfaces by surface X-ray scattering. Surf. Sci. 631, 96–104 (2015) 12. Kondo, T., Masuda, T., Aoki, N., Uosaki, K.: Potential-dependent structures and potential-induced structure change at Pt(111) single-crystal electrode/sulfuric and perchloric acid interfaces in the potential region between hydrogen underpotential deposition and surface oxide formation by in situ surface X-ray scattering. J. Phys. Chem. C 120, 16118–16131 (2016) 13. Lucas, C.A., Cormack, M., Gallagher, M.E., Brownrigg, A., Thompson, P., Fowler, B., Gründer, Y., Roy, J., Stamenković, V., Marković, N.M.: From ultra-high vacuum to the electrochemical interface: x-ray scattering studies of model electrocatalyst. Faraday Discuss. 140, 41–58 (2008) 14. Tripkovic, D.V., Strmcnik, D., van der Vliet, D., Stamenković, V., Marković, N.M.: The role of anions in surface electrochemistry. Faraday Discuss. 140, 25–40 (2008) 15. Marković, N.M., Ross Jr., P.N.: Surface science studies of model fuel cell electrocatalysts. Surf. Sci. Rep. 45, 117–229 (2002) 16. Sisson, N., Gründer, Y., Lucas, C.A.: Structure and stability of underpotentially deposited Ag on Au(111) in alkaline electrolyte. J. Phys. Chem. C 120, 16100–16109 (2016) 17. Kondo, T., Morita, J., Okamura, M., Saito, T., Uosaki, K.: In situ structural study on underpotential deposition of Ag on Au(111) electrode using surface X-ray scattering technique. J. Electroanal. Chem. 532, 201–205 (2002) 18. Uosaki, K., Morita, J., Katsuzaki, T., Takakusagi, S., Tamura, K., Takahasi, M., Mizuki, J., Kondo, T.: In situ electrochemical, electrochemical quartz crystal microbalance, scanning tunneling microscopy, and surface X-ray scattering studies on Ag/AgCl reaction at the underpotentially deposited Ag bilayer on the Au(111) electrode surface. J. Phys. Chem. C 115, 12471–12482 (2011) Chapter 19 Electrochemical Transmission Electron Microscopy Yoshifumi Oshima Keywords TEM In-situ Cyclic voltammetry 19.1 Electrochemical reaction Structure Principle Electrochemical transmission electron microscopy (ECTEM) is one of in-situ TEM observation methods [1]. It enables us to visualize local structure and/or composition change at the interface between a solid electrode and a liquid electrolyte during the electrochemical process. Since the spatial resolution of TEM images is relatively high, local structural change, which occurs at a particular crystal facet, step or kink at the solid electrode surface in contact with the electrolyte, can be visualized. Also, compositional change at the interface can be obtained in the elementary mapping by energy dispersive X-ray spectroscopy (EDS) or electron energy loss spectroscopy (EELS). Those changes can be directly imaged with TEM in real time under the electrochemical condition, simultaneously with measuring the electrochemical current and potential. Since the rate of ion transfer between the electrode and solution species can be estimated by the current, electrochemically induced changes of local structure and elemental compositions can be quantitatively analyzed. ECTEM is similar to electrochemical scanning tunneling microscopy, but it has some advantages. For example, in the case of ECTEM, the image magnification can be changed more widely (from atomic scale to millimeter scale) and the electrochemical reaction at rough interface is observable. Y. Oshima (&) School of Materials Science, Japan Advanced Institute of Science and Technology, Nomi 923-1292, Japan e-mail: oshima@jaist.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_19 109 110 19.2 Y. Oshima Features • Interfaces between a solid electrode and a liquid electrolyte can be observed. • Structural changes can be detected during the electrochemical reactions. • Atomic diffusion process can be elucidated. 19.3 Instrumentation The sample for TEM observation is required to be kept at vacuum condition (around the order of 10−5 Pa) in the TEM column to avoid the scattering of electrons in media. It is necessary for observing an electrochemical reaction, which occurs at the interface between a solid electrode and a liquid electrolyte, to keep the liquid in a container, called electrochemical cell as shown in Fig. 19.1a [2]. It consists of two liquid tanks (source and drain) and the observation region, which is sandwiched by a pair of silicon nitride membranes (thickness 10–50 nm) (Fig. 19.1b). The observation region is filled with a liquid electrolyte by flowing the liquid from the source to the drain (Fig. 19.1c). The incident electron beam can pass through the observation region via the silicon nitride membranes. The TEM image is obtained by the scattering electrons at the observation region. Since the thickness of the observation region is around 100 nm, the spatial resolution must be reduced to about 1 nm due to multiply scattering effect. As concerned with electrical measurement, three terminals (WE: working electrode, RE: reference electrode and Fig. 19.1 a Photograph of electrochemical cell for TEM observation. b The body of the cell is constructed by three quartz glasses to make two liquid tanks and observation region. c Cross section of a pair of silicon nitride membranes supported by silicon substrates. It can be placed at the observation region of (a). d Schematic diagram of three electrodes geometry on the silicon nitride membranes. The square at the center corresponds to the observation region 19 Electrochemical Transmission Electron Microscopy 111 CE: counter electrode) are connected with a potentiostat or galvanostat (Fig. 19.1d). In this method, it is necessary to synchronize the electrical measurement with the TEM observation. If both acquisitions of TEM image and electrochemical measurement are achieved by the same computer, the time stamp of the computer can be useful to synchronize these data, for example. 19.4 Applications In this study, the electrochemical growth or dissolution of copper islands on gold film was observed directly [2]. The WE, RE and CE electrodes are Au, Au and Cu, respectively, and an aqueous solution of 0.2 M CuSO4 and 0.05 M H2SO4 is used as an electrolyte solution. TEM observation was performed at the accelerating voltage of 200 kV. Three consecutive CV measurements were achieved in the scan range from −0.2 to 0.2 V. The Cu atoms were observed to be deposited on the Au Fig. 19.2 (Top) A series of TEM images taken during 2nd and 3rd cycles of cyclic voltammetry. 0 V with a downward (upward) arrow corresponds to the TEM image taken in the process of decreasing (increasing) the potential. Cu islands are nucleated dominantly at the same sites as indicated by white circles. (Bottom) Bold lines indicate three consecutive cyclic voltammetry measured simultaneously with TEM observation. Dashed line indicates the cyclic voltammetry measure for a large cell 112 Y. Oshima film of WE when the potential was more negative than −0.05 V and be dissolved when the potential was more positive than 0.05 V. (The potential is a relative value with RE as a reference.) Comparing the nucleation sites of Cu islands at the 3rd cycle with those at the 2nd one, most of Cu islands were nucleated at the same sites with ones at 2nd cycle as indicated by white circles in Fig. 19.2. Cyclic voltammetry was measured using a potentiostat with a scan rate of 25 mV s−1 as shown in the bottom of Fig. 19.2. The number density and average size of the Cu islands and the deposition rate of Cu ions on the WE were estimated from the TEM and electrochemical current, respectively. We concluded that the Cu islands were grown three-dimensionally on the gold film in this experimental condition. During the observation, we reduced the beam current density as small as possible to avoid formation of H2 bubbles due to the electron beam irradiation. In this observation, bubbles were observed to be formed when the beam current density was above 1 10−15 A/nm2/sec. At the same time, the Cu islands were observed to be dissolved gradually. It suggests that the electrolyte becomes alkaline due to formation of H2 bubbles. The threshold current density depends on thickness and content of the electrolyte. References 1. Williamson, M.J., Tromp, R.M., Vereecken, P.M., Hull, R., Ross, F.M.: Dynamic microscopy of nanoscale cluster growth at the solid-liquid interface. Nat. Mater. 2, 532–536 (2003) 2. Oshima, Y., Tsuda, T., Kuwabata, S., Yasuda, H., Takayanagi, K.: Development of an electrochemical cell for in situ transmission electron microscopy observation. Microscopy 63, 481–486 (2014) Chapter 20 Electrochemical X-Ray Absorption Fine Structure Takuya Masuda Keywords Electrochemistry 20.1 Solid/liquid interfaces In situ X-ray Spectroscopy Principle X-ray absorption fine structure (XAFS) is a generic term for X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). Since XANES and EXAFS provide information about the electronic structure and local structure of X-ray absorbing atoms as described in Chap. 128 X-ray absorption near-edge structure and Chap. 30 extended X-ray absorption fine structure, in situ electrochemical (EC-) XAFS can clarify the mechanism of electrochemical reactions on the basis of the electronic structure of electrode surfaces and surface adsorbed species. Immediately after the pioneering works in the 1980s [1], EC-XAFS was rapidly utilized for a variety of electrochemical interfaces [2, 3]. Although XAFS is not inherently a surface-sensitive method, various tools and techniques have been developed to detect the surface species with a high sensitivity in the last several decades. Among the various measurement modes, the transmission and fluorescence modes are commonly utilized for EC-XAFS measurements. In the transmission mode (Fig. 20.1a) which is suitable for thin liquid and solid films, the intensities of X-rays before and after the transmission through the sample are monitored by ionization chambers as a function of the energy of the incident X-rays to obtain an absorption spectrum. In the fluorescence mode (Fig. 20.1b), the intensity of fluorescent X-rays is monitored typically by a silicon drift detector, as well as that of the incident X-rays by the ionization chamber, to obtain a quasi-absorption spectrum. The fluorescence mode is beneficial for samples with a low concentration and/or thick samples for X-ray transmission. T. Masuda (&) Research Center for Advanced Measurement and Characterization, National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan e-mail: MASUDA.Takuya@nims.go.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_20 113 114 T. Masuda Fig. 20.1 Schematic views of in situ EC-XAFS measurements in a transmission and b fluorescence modes [3] 20.2 Features • Electronic structure of electrode surfaces during electrochemical processes can be determined. • Local structure of electrode materials, such as surrounding species, coordination numbers and interatomic distances can be determined. • Transmission and fluorescence modes are available for electrochemical interfaces. 20.3 Instrumentation Figure 20.2 shows schematic illustrations of spectroelectrochemical cells for in situ EC-XAFS measurements. In the transmission mode, a thin layer liquid cell equipped with two X-ray windows, made of polythene, polyester or polyimide films, as shown in Fig. 20.2a [4], is used. A thin metal layer deposited on the X-ray window or a thin metal film/mesh inserted into the cell is used as a working electrode. Similar cells equipped with, at least, one X-ray window coated with a thin metal layer as shown in Fig. 20.2b [5] and various EC-SXS cells described in Chap. 18 Electrochemical surface X-ray scattering can be used for in situ EC-XAFS measurements in the fluorescence mode. As long as hard X-rays are used as a probe, EC-XAFS measurements using the above-mentioned cells with a relatively thick X-ray window (*10 lm) and a solution layer (*100 lm) can be performed in an ambient condition because hard X-rays can transmit through the gas and liquid phases without significant scattering 20 Electrochemical X-Ray Absorption Fine Structure 115 Fig. 20.2 Schematic illustrations of spectroelectrochemical cells for in situ EC-XAFS measurements in a the transmission [4] and b the fluorescence modes [5] and absorption. On the contrary, X-ray absorption spectroscopy (XAS) utilizing soft X-rays has to be performed in vacuum or a helium atmosphere because of the low transmittance of soft X-rays in air. In addition, a very thin film (*100 nm) is used as a X-ray window of the spectroelectrochemical cells [6]. 20.4 Applications 20.4.1 A Well-Defined Pt Layer on Rh(111) [7] Well-defined metal layers constructed on single crystal metal surfaces such as underpotentially deposited ultrathin metal layers on Au(111) surfaces [8–11] are one of the systems most studied by in situ EC-XAFS. Recently, Friebel et al. successfully determined the oxidation states and chemical bonding of Pt atoms at a well-defined Pt monolayer on a Rh(111) surface in an aqueous electrolyte solution [7]. Figure 20.3a shows the potential-dependent XANES spectra at the Pt L3 edge of the 1 ML Pt/Rh(111) electrode in a 0.01 M HClO4 solution at various potentials. As the potential goes more positive than 1.0 V versus RHE, a peak intensity at around 11,565 eV, the so-called white line, increases gradually. The white line at the Pt L3 edge corresponds to the electronic transition from 2p to 5d states, and thus its intensity reflects the vacancy of 5d states. Hence, the increase in the white line intensity shows the depletion of occupied 5d states due to the formation of Pt–O bonds. When the potential becomes more negative than 0.6 V to reduce the Pt oxide, the white line intensity gradually decreases and eventually recovers to the original value. The decrease in Pt–Pt and Pt–Rh bonds and increase in Pt–O bonds in the Fourier transforms of EXAFS oscillations (Fig. 20.3b) also indicate the formation of Pt oxide in the positive potential region. Fourier transforms at 0 and 1.6 V (Fig. 20.3c) are well-reproduced by the curve fittings based on a model of a two-dimensional Pt monolayer on a Rh(111) and a place-exchanged Pt oxide layer 116 T. Masuda Fig. 20.3 Potential-dependent a XANES spectra and b Fourier transforms of EXAFS oscillations at the Pt L3 edge for a Pt monolayer deposited on a Rh(111) surface measured in a 0.01 M HClO4 aqueous solution. c Fourier transforms and curve fittings corresponding to the Pt monolayer on the Rh(111) surface at 0.0 V and the Pt oxide layer on the Rh substrate at 1.6 V [7] on the Rh substrate, respectively, and structural parameters obtained from the curve fittings are in good agreement with those expected for corresponding structures. 20.4.2 Single Molecular Catalysts Confined Within Organic Molecular Layers [12, 13] Figure 20.4 shows XANES spectra and EXAFS oscillations for a Si(111) electrode modified with viologen moieties and Pt complexes, denoted as Pt-V++-Si(111), measured in the total-reflection fluorescence configuration [14] in air and in a 0.1 M Na2SO4 solution at various electrochemical potentials [12, 13]. Both the XANES spectrum (Fig. 20.4a) and EXAFS oscillation (Fig. 20.4b) of the Pt-V++-Si(111) measured in air are almost identical to those of a K2PtCl4 pellet measured as a reference, showing that PtCl42− is inserted into the molecular layer as a counter anion of the viologen moiety by immersing a viologen-modified Si(111) surface in an aqueous solution containing K2PtCl4. As the potential goes more negative for the hydrogen evolution reaction, the white line intensity increases in the XANES spectra (Fig. 20.4a) and the shape of the EXAFS oscillations changes significantly 20 Electrochemical X-Ray Absorption Fine Structure 117 Fig. 20.4 Pt L3 edge a XANES spectra, b EXAFS oscillations and c Fourier transforms for a Si (111) electrode modified with viologen moieties and Pt complexes, measured in air and at various electrochemical potentials in a 0.1 M Na2SO4 aqueous solution, together with references for K2PtCl4 pellet and Pt foil [12, 13]. d Schematic illustrations of the Si(111) surface modified with viologen moieties and Pt complexes, and the ligand exchange reaction of the Pt complexes induced by the potential applications (Fig. 20.4b). However, Fourier transforms of the EXAFS oscillations do not show any peak assignable to Pt–Pt bonds (Fig. 20.4c) although the Pt-V++-Si(111) is kept at the potential much more negative than the redox potential of PtCl42−, 0.51 V versus Ag/AgCl. On the basis of the curve fitting of the Fourier transforms, the increase in the white line intensity and change of the EXAFS oscillations are attributed to the ligand exchange reaction of PtCl42− with oxygenated species such as water molecules and hydroxyl groups as shown in (Fig. 20.4d). Thus, in situ EC-XAFS proves that Pt complexes inserted into the molecular layer act as a molecular catalyst for the hydrogen evolution reaction without being converted into nanoparticles. References 1. Abruña, H.D.: X-ray absorption spectroscopy in the study of electrochemical systems. In: Abruña, H.D. (ed.) Electrochemical Interfaces: Modern Techniques for In-situ Interface Characterization, pp. 1–54. VCH Publishers, New York (1991) 2. Kondo, T., Masuda, T., Uosaki, K.: In situ SXS and XAFS measurements of electrochemical interface. In: Kumar, C.S.S.R. (ed.) X-ray and Neutron Techniques for Nanomaterials Characterization, pp. 367–449. Springer, Berlin (2016) 118 T. Masuda 3. Masuda, T., Kondo, T., Uosaki, K.: Solid-liquid interfaces. In: Iwasawa, Y., Asakura, K., Tada, M. (eds.) XAFS Techniques for Catalysts, Nanomaterials, and Surfaces, pp. 505–525. Springer, Switzerland (2017) 4. Herron, M.E., Doyle, S.E., Pizzini, S., Roberts, K.J., Robinson, J., Hards, G., Walsh, F.C.: In situ studies of a dispersed platinum on carbon electrode using X-ray absorption-spectroscopy. J. Electroanal. Chem. 324, 243–258 (1992) 5. Gorlin, Y., Lassalle-Kaiser, B., Benck, J.D., Gul, S., Webb, S.M., Yachandra, V.K., Yano, J., Jaramillo, T.F.: In situ X-ray absorption spectroscopy investigation of a bifunctional manganese oxide catalyst with high activity for electrochemical water oxidation and oxygen reduction. J. Am. Chem. Soc. 135, 8525–8534 (2013) 6. Nagasaka, M., Yuzawa, H., Horigome, T., Kosugi, N.: In operando observation system for electrochemical reaction by soft X-ray absorption spectroscopy with potential modulation method. Rev. Sci. Instrum. 85, 104105/104101–104105/104107 (2014) 7. Friebel, D., Miller, D.J., O’Grady, C.P., Anniyev, T., Bargar, J., Bergmann, U., Ogasawara, H., Wikfeldt, K.T., Pettersson, L.G.M., Nilsson, A.: In situ X-ray probing reveals fingerprints of surface platinum oxide. Phys. Chem. Chem. Phys. 13, 262–266 (2011) 8. Blum, L., Abruña, H.D., White, J., Gordon, J.G., Borges, G.L., Samant, M.G., Melroy, O.R.: Study of underpotentially deposited copper on gold by fluorescence detected surface EXAFS. J. Chem. Phys. 85, 6732–6738 (1986) 9. White, J.H., Albarelli, M.J., Abruña, H.D., Blum, L., Melroy, O.R., Samant, M.G., Borges, G. L., Gordon, J.G.: Surface extended X-ray absorption fine-structure of underpotentially deposited silver on Au(111) electrodes. J. Phys. Chem. 92, 4432–4436 (1988) 10. Melroy, O.R., Samant, M.G., Borges, G.L., Gordon, J.G., Blum, L., White, J.H., Albarelli, M. J., Mcmillan, M., Abruña, H.D.: Inplane structure of underpotentially deposited copper on gold(111) determined by surface EXAFS. Langmuir 4, 728–732 (1988) 11. Tadjeddine, A., Guay, D., Ladouceur, M., Tourillon, G.: Electronic and structural characterization of underpotentially deposited submonolayers and monolayer of copper on gold (111) studied by in situ X-ray-absorption spectroscopy. Phys. Rev. Lett. 66, 2235–2238 (1991) 12. Masuda, T., Fukumitsu, H., Takakusagi, S., Chun, W.J., Kondo, T., Asakura, K., Uosaki, K.: Molecular catalysts confined on and within molecular layers formed on a Si(111) surface with direct Si–C bonds. Adv. Mater. 24, 268–272 (2012) 13. Masuda, T., Sun, Y., Fukumitsu, H., Uehara, H., Takakusagi, S., Chun, W.J., Kondo, T., Asakura, K., Uosaki, K.: Various active metal species incorporated within molecular layers on Si(111) electrodes for hydrogen evolution and CO2 reduction reactions. J. Phys. Chem. C 120, 16200–16210 (2016) 14. Chun, W.J., Asakura, K., Iwasawa, Y.: Polarization-dependent total-reflection fluorescence XAFS study of Mo oxides on a rutile TiO2(110) single crystal surface. J. Phys. Chem. B 102, 9006–9014 (1998) Chapter 21 Electrochemical X-Ray Photoelectron Spectroscopy Takuya Masuda Keywords Electrochemistry photoelectron spectroscopy 21.1 Solid/liquid interfaces In situ X-ray Principle Although X-ray photoelectron spectroscopy (XPS) requires a vacuum (Chap. 132 X-ray Photoelectron Spectroscopy), in situ electrochemical (EC-) XPS has been a long-standing dream not only for fundamental science, but also for a wide range of applications including fuel cells, rechargeable batteries, photocatalysts, and biological processes. To date, as shown in Fig. 21.1, three approaches enabling in situ EC-XPS have been successfully reported by using near-zero vapor pressure room temperature ionic liquids (RTILs) as an electrolyte solution [1], a near-ambient pressure (NAP-) XPS equipped with a differential pumping system [2, 3], and an environmental cell with an ultrathin membrane-type working electrode [4, 5]. 21.2 Features • Elemental composition at the solid/liquid interface can be determined under electrochemical potential control, as well as the oxidation state of each element. • Surfaces of an ionic liquid, an electrode and their boundary can be measured under electrochemical potential control by using an ionic liquid as an electrolyte solution in vacuum. T. Masuda (&) Research Center for Advanced Measurement and Characterization, National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan e-mail: MASUDA.Takuya@nims.go.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_21 119 120 T. Masuda Fig. 21.1 Schematic illustrations of in situ EC-XPS using a RTILs, b NAP-XPS, and c environmental cells with an ultrathin membrane-type working electrode • Electrode surfaces can be measured through a thin solution layer by utilizing a differential pumping system with an electrostatic lens. • Electrode surfaces in contact with a bulk solution can be measured through an ultrathin membrane-type working electrode which is also used as a separator between vacuum and the ambient. 21.3 Instrumentation An electrochemical cell consisting of RTILs as an electrolyte solution and working and reference/counter electrodes (Fig. 21.2a) can be directly introduced into a vacuum chamber. XPS measurements of the surfaces of the RTIL droplet and the working electrode can be taken under electrochemical potential control. In NAP-XPS, samples placed under a relatively low vacuum can be measured by utilizing a differential pumping system and electrostatic lens (as described in Chap. 3 Ambient Pressure X-ray Photoelectron Spectroscopy) [6, 7]. In this method, electrodes partially withdrawn from the electrochemical cell can be measured in the presence of a thin liquid layer under electrochemical potential control, by detecting photoelectrons emitted from the electrode surface through the liquid layer (Fig. 21.2b). The thickness of the liquid layer is limited to the range from several nanometers to several tens of nanometers by the inelastic mean free path of photoelectrons in the liquid layer and can be controlled by relative humidity and temperature. By using an environmental cell, in which an ultrathin membrane is used as a window for X-rays and photoelectrons, a separator between vacuum and liquid, and a working electrode for electrochemical reactions, XPS measurements of the electrode surface in the presence of a “filling” thick liquid layer can be performed 21 Electrochemical X-Ray Photoelectron Spectroscopy 121 Fig. 21.2 Experimental setups for in situ EC-XPS using a RTILs [8, 9], b NAP-XPS [3], and c environmental cells with an ultrathin membrane-type working electrode [4] by detecting photoelectrons emitted from the solid/liquid (membrane/liquid) interface through the membrane (Fig. 21.2c). 21.4 Applications 21.4.1 EC-XPS Utilizing RTILs [9, 10] Figure 21.3a shows a time course of XPS spectra in the Cu 2p region taken during the potentiostatic electrolysis of a Cu electrode at +1.8 V in (N-methylacetate)4-picolinium bis(trifluoromethylsulfonyl)imide [MAP][Tf2N] [9]. The surface of a [MAP][Tf2N] droplet at 2.2 mm from the electrode surface was measured in the snapshot mode. A peak due to the Cu 2p3/2 was observed from the third scan and the intensity gradually increased up to the eighth scan, suggesting that the Cu+ ions electrochemically dissolved from the Cu electrode in the ionic liquid and diffused to the analysis point. Figure 21.3b shows another example of the EC-XPS utilizing RTILs. In this report, the surface of a Ni mesh electrode was measured during the galvanostatic electrolysis at −1 10−5 A in N-butyl-N-methylpyrrolidinium bis (trifluoromethylsulfonyl)imide [C4mpyrr][NTf2] containing 0.1 M Rb[NTf2] [10]. A peak due to the Rb 3d clearly increased with time because of the electrochemical deposition of Rb on the Ni mesh electrode. Thus, the EC-XPS using RTILs is capable of chemical analysis and mass transport analysis during the electrochemical reactions. 122 T. Masuda Fig. 21.3 Time courses of XPS spectra in a the Cu 2p region at a surface of [MAP][Tf2N] droplet during the electrolysis of Cu electrode at +1.8 V (vs. Mo stub) [9] and b the Rb 3d region at a Ni mesh electrode during the electrolysis at −1 10−5 A in [C4mpyrr][NTf2] containing 0.1 M Rb [NTf2] [10] 21.4.2 EC-XPS Utilizing NAP-XPS [11] Figure 21.4a, b shows XPS spectra in the N 1s and O 1s regions of a Au electrode measured at various potentials in an aqueous solution of 1 M pyrazine plus 0.4 mM KOH in the configuration shown in Fig. 21.2b. In the N 1s region, two peaks corresponding to pyrazine molecules adsorbed on the electrode (PyESF) and solvated in the liquid phase (LPPy) were observed at 399.7 eV and a higher binding energy, respectively. In the O 1s region, three components attributed to water molecules in gas (GPW) and liquid phases (LPW) and adsorbed hydroxyl groups were observed. When the potential was made more negative (positive) from open circuit potential, ca. 150 mV, the peak positions for water molecules (LPW) and pyrazine in the liquid phase (LPPy) shifted to the higher (lower) binding energy although those for the adsorbed pyrazine (PyESF) and hydroxyl groups did not shift. In addition, the width of the peaks corresponding to the solution species such as LPW and LPPy significantly changed possibly due to the potential distribution across the electrical double layer (EDL). As shown in Fig. 21.4c and d, the minimum point of the double-layer capacitance coincides with that of the peak width at the potential of zero charge (PZC) at which the electrical charge on the electrode surface is zero. Thus, the EC-XPS using a NAP-XPS system enables to probe the potential distribution across the EDL although its very thin solution layer may cause a high cell resistance. 21 Electrochemical X-Ray Photoelectron Spectroscopy 123 Fig. 21.4 XPS spectra in a the N 1s and b the O 1s regions of a Au electrode measured at various potentials in an aqueous solution of 1 M pyrazine plus 0.4 mM KOH in the configuration shown in Fig. 21.2b [11]. c Double-layer capacitance obtained from electrochemical measurements by using both Gouy–Chapman (GC) and Gouy–Chapman–Stern (GCS) models and full-width at half-maximum (FWHM) of LPPy N 1s and LPW O 1s peaks as a function of the applied potential [11] 21.4.3 EC-XPS Utilizing an Environmental Cell [4] An environmental cell with an ultra-small volume was utilized for the ‘first’ in situ EC-XPS experiment in an ordinary solvent to prevent any possible damage caused by breakage of the ultrathin Si membrane-type working electrode used as a separator between vacuum and the ambient (Fig. 21.5a). In this configuration, anodic current started to flow from ca. 0.8 V only in the presence of water (Fig. 21.5b), implying the electrochemical growth of Si oxide. As shown in the EC-XPS spectra in Fig. 21.5c–d, not only doublet peaks at 100 and 99.5 eV corresponding to Si 2p1/2 and 2p3/2, respectively, but also a broad peak at 104.5 eV corresponding to SiO2 124 T. Masuda Fig. 21.5 a Schematic illustration and b I–V relationships of the environmental cell with (blue) and without water (red) [4]. c Photoelectron spectra in the Si 2p region of the Si membrane measured under various bias applications [4]. d Magnified image of (c) in the region between 107 and 102 eV, corresponding to SiO2. e Difference spectra at various conditions [4] was observed even without a bias application, suggesting the existence of native oxide. When a positive potential was applied to the Si membrane with respect to the Cu tape, the peak intensity due to the SiO2 increased gradually, confirming the electrochemical growth of SiO2 at the Si membrane/water interface. In addition, the ratio of Si 2p1/2 and 2p3/2 changed significantly, suggesting that strained Si atoms at the Si/Si oxide interface became closer to the analysis side, accompanying the electrochemical growth of SiO2, as clearly indicated in the difference spectra (Fig. 21.5e). On the basis of the inelastic mean free paths of the Si 2p photoelectrons in Si oxide and Si obtained using the TPP-2 M formula and the atom densities of Si in Si oxide and Si, the thickness change of SiO2 during the electrochemical growth was estimated in a sub-nanometer scale. The most difficult challenge of this technique is development of ultrathin films with very high conductivity and mechanical strength, which can be used as a membrane-type working electrode. References 1. Lovelock, K.R.J., Villar-Garcia, I.J., Maier, F., Steinruck, H.P., Licence, P.: Photoelectron spectroscopy of ionic liquid-based interfaces. Chem. Rev. 110, 5158–5190 (2010) 2. Axnanda, S., Crumlin, E.J., Mao, B.H., Rani, S., Chang, R., Karlsson, P.G., Edwards, M.O. M., Lundqvist, M., Moberg, R., Ross, P., Hussain, Z., Liu, Z.: Using “tender” X-ray ambient 21 Electrochemical X-Ray Photoelectron Spectroscopy 125 pressure X-ray photoelectron spectroscopy as a direct probe of solid-liquid interface. Sci. Rep. 5, 9788 (2015) 3. Karslioglu, O., Nemsak, S., Zegkinoglou, I., Shavorskiy, A., Hartl, M., Salmassi, F., Gullikson, E.M., Ng, M.L., Rameshan, C., Rude, B., Bianculli, D., Cordones, A.A., Axnanda, S., Crumlin, E.J., Ross, P.N., Schneider, C.M., Hussain, Z., Liu, Z., Fadley, C.S., Bluhm, H.: Aqueous solution/metal interfaces investigated in operando by photoelectron spectroscopy. Faraday Discuss. 180, 35–53 (2015) 4. Masuda, T., Yoshikawa, H., Noguchi, H., Kawasaki, T., Kobata, M., Kobayashi, K., Uosaki, K.: In situ X-ray photoelectron spectroscopy for electrochemical reactions in ordinary solvents. Appl. Phys. Lett. 103, 111605 (2013) 5. Velasco-Velez, J.J., Pfeifer, V., Havecker, M., Weatherup, R.S., Arrigo, R., Chuang, C.H., Stotz, E., Weinberg, G., Salmeron, M., Schlogl, R., Knop-Gericke, A.: Photoelectron spectroscopy at the graphene-liquid interface reveals the electronic structure of an electrodeposited cobalt/graphene electrocatalyst. Angew. Chem. Int. Ed. 54, 14554–14558 (2015) 6. Ogletree, D.F., Bluhm, H., Lebedev, G., Fadley, C.S., Hussain, Z., Salmeron, M.: A differentially pumped electrostatic lens system for photoemission studies in the millibar range. Rev. Sci. Instrum. 73, 3872–3877 (2002) 7. Salmeron, M., Schlogl, R.: Ambient pressure photoelectron spectroscopy: a new tool for surface science and nanotechnology. Surf. Sci. Rep. 63, 169–199 (2008) 8. Taylor, A.W., Qiu, F.L., Villar-Garcia, I.J., Licence, P.: Spectroelectrochemistry at ultrahigh vacuum: in situ monitoring of electrochemically generated species by X-ray photoelectron spectroscopy. Chem. Commun. 5817–5819 (2009) 9. Qiu, F.L., Taylor, A.W., Men, S., Villar-Garcia, I.J., Licence, P.: An ultra high vacuum-spectroelectrochemical study of the dissolution of copper in the ionic liquid (N-methylacetate)-4-picolinium bis(trifluoromethylsulfonyl)imide. Phys. Chem. Chem. Phys. 12, 1982–1990 (2010) 10. Wibowo, R., Aldous, L., Jacobs, R.M.J., Manan, N.S.A., Compton, R.G.: In situ electrochemical-X-ray photoelectron spectroscopy: rubidium metal deposition from an ionic liquid in competition with solvent breakdown. Chem. Phys. Lett. 517, 103–107 (2011) 11. Favaro, M., Jeong, B., Ross, P.N., Yano, J., Hussain, Z., Liu, Z., Crumlin, E.J.: Unravelling the electrochemical double layer by direct probing of the solid/liquid interface. Nat. Commun. 7, 12695 (2016) Chapter 22 Electron Backscatter Diffraction Rika Yoda Keywords Scanning electron microscopy Kikuchi pattern Crystal orientation 22.1 Electron backscatter diffraction Principle Electron backscatter diffraction patterns (EBSD patterns) can be used to determine the orientation of the crystal lattice. Principle of EBSD pattern is similar to Kikuchi pattern observed in transmission electron microscope (TEM). In the case of scanning electron microscope (SEM), when an electron beam enters a highly tilted crystalline material, it is inelastically scattered in all directions. Electrons from the source that are incident on a set of crystal planes at an angle that satisfies the Bragg equation (2dsinh = nk) are diffracted as shown in Fig. 22.1. For a family of atomic lattice planes, the electron backscatter diffraction results in a set of two cones of diffracted electrons. This figure is showing a 2D vector of the 3D conic projection. The Bragg angles are about 1°, so the cones of diffracted electrons are largely opened and nearly parallel to the diffracting plane. The intersection of these cones with the phosphor screen placed in front of the specimen leads to the formation of a pair of thin lines called Kikuchi band. The resulting EBSD pattern consists of many Kikuchi bands, each band corresponding to a family of diffracting planes. The angles between the bands are then calculated and compared to theoretical values for the material. This technique allows crystal orientations to be determined at very specific points in the microstructure. Examples of an EBSD pattern and an indexed pattern for austenitic steel are shown in Fig.22.2. The spatial resolution varies with the accelerating voltage, beam current, and spot size of the SEM along with the atomic number of the sample material and the strain state of the material [1–3]. R. Yoda (&) Material Solutions Division, Kobelco Research Institute, Inc, Kobe, Japan e-mail: yoda.rika@kki.kobelco.com © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_22 127 128 R. Yoda Fig. 22.1 Origin of Kikuchi lines from EBSD 22.2 Features • EBSD hardware is attached to a SEM, FIB-SEM, or EPMA. • Spatial resolution is about 10–50 nm with a field-emission-type SEM (FE-SEM). • Applied to all crystalline materials. • Many types of maps, histograms, and plots can be reconstructed from orientation measurement data by the use of analysis software, and these data also can be used for quantitative microstructural analysis such as the average grain size and the crystallographic texture [1–3]. 22.3 Instrumentation EBSD system consists of SEM, EBSD detector, and EBSD computer shown in Fig. 22.3. The specimen is placed highly tilted (usually at 70°) from the horizontal in the SEM to produce an optimum EBSD pattern signal. The detector unit consists of a phosphor screen and a low light camera. The camera is connected to a computer with a frame grabber which captures the EBSD pattern image. The EBSD 22 Electron Backscatter Diffraction 129 Fig. 22.2 Examples of EBSD pattern a EBSD pattern, b indexed EBSD pattern (Courtesy of Oxford Instruments plc) pattern is averaged and background-corrected and then digitized into the computer memory where it is automatically indexed. The following data are calculated and stored: three Euler angles defining the orientation of the crystal, (x, y) coordinates giving the position where the data were obtained on the specimen, a quality factor defining the sharpness of the EBSD pattern, a numerical value indicating the reliability of the solution, and an integer defining the phase of the material. This process is repeated to collect large amounts of crystallographic data from the specimen with the positioning of the electron beam on the specimen under computer control. The computer positions the beam sequentially on the points of a grid to cover the area of interest on the specimen allowing data to be collected without any user intervention except for the initial scan setup [1–3]. Fig. 22.3 Schematic of EBSD system 130 22.4 R. Yoda Applications 22.4.1 Inverse Pole Figure Map (IPF Map) Inverse pole figure map is one of the most fundamental analyzes. This map allows the crystallographic orientations to be quickly interpreted in terms of the sample coordinate system. Figure 22.4 shows IPF maps showing the crystal orientations parallel to (a) normal direction and (b) rolling direction for a steel sheet with high formability. These maps indicate that the material has <111> oriented microstructure parallel to the normal direction of the observed plane and <101> parallel to the rolling direction, respectively. 22.4.2 Pole Figure Orientations can be plotted as two-dimensional projections in pole figures. Such figures can be useful for simplifying the analysis of the orientation distribution. The example shown in Fig. 22.5 is a (111) pole figure for the same data in Fig. 22.4. 22.4.3 Grain Analysis Examples of grain analysis are shown in Fig. 22.6. Figure 22.6a shows the inverse pole figure map for austenitic stainless steel. Figure 22.6b, c, and d is representative examples of grain analysis for the same data as (a). In Fig. 22.6b, boundaries with greater than 5° misorientation are denoted by blue lines, while twin boundaries are denoted by red lines. Figure 22.6c shows the grain distribution. No two Fig. 22.4 Inverse pole figure maps showing the crystal orientations parallel to a the normal direction of the observed plane and b the rolling direction for a steel sheet with high formability 22 Electron Backscatter Diffraction 131 Fig. 22.5 Example of pole figure neighboring grains in the map have the same color. In EBSD method, the definition of a grain is very specific. It is constructed by collecting neighboring points of similar orientation. In this example, the tolerance angle is set at 5°, and twin boundaries were excluded from grain boundaries. Figure 22.6d shows the grain size histogram for the detected grains in (c). The average value also can be obtained. Fig. 22.6 Examples of grain analysis: a Inverse pole figure map for austenitic stainless steel, b boundary map showing grain boundaries and twin boundaries, c grain distribution map, and d grain size histogram 132 R. Yoda Fig. 22.7 Example of strain analysis using misorientation: a SEM micrograph around Rockwell indentation, b inverse pole figure map for EBSD measurement area, c change in local misorientation with the distance from indentation edge 22.4.4 Strain Analysis (Misorientation Analysis) An example of strain analysis is shown in Fig. 22.7. It was carried out for the deformed region around the Rockwell indentation shown in Fig. 22.7a. Figure 22.7b shows the inverse pole figure map for the EBSD measurement area. The color gradation within the grain is visible only near the indentation. It means that the orientations are initially uniform, but misorientation appears within the grain as the plastic deformation proceeds. Here, misorientation means the orientation difference between two measurement points. Figure 22.7c shows the misorientation analysis. The upper color map indicates the misorientation distribution called local misorientation which is the average misorientation between the measured point and neighboring points. It is shown that the local misorientation near the indentation is larger than other region. The profile shows the change in misorientation as a function of distance along an analysis line. References 1. ISO 24173 Microbeam analysis—Guideline for orientation measurement using electron backscatter diffraction (2009) 2. EDAX OIM manual (2013) 3. Oxford Instruments CHANNEL 5 manual (2009) Chapter 23 Electron Energy-Loss Spectroscopy Tadaaki Nagao Keywords Electron scattering Transmission electron microscopy Nanomaterials Electronic excitations Chemical composition Chemical mapping 23.1 Principle Fast electrons traveling with kinetic energy in the range from tens to hundreds of kilo-electron volts penetrate nanometer-scale objects and can probe both interior and surface portions of the nanomaterials. The traveling electrons interact with ion cores and electrons in the material and are scattered out from the sample with or without changing their kinetic energies. The former process is termed as elastic scattering which constitutes transmission electron diffraction (TED) pattern reflecting the atomic arrangements of the object. The latter is termed inelastic scattering that carries rather rich information of materials properties associated with their chemical compositions as well as dynamical and optical properties. The inelastically scattered electrons kick out core and valence electrons of the atoms, and some of the multiply scattered electrons are emitted out from the surface with low energy. These low-energy surface-emitted electrons can be used to image the surface morphologies of the material (scanning electron microscope (SEM)). Meantime, some of the incident electron beams excite core electrons of atoms, and the energy transfer from the incident beam to each atom takes place at their specific core-level energies. Thus, by measuring the transferred energies from the incident beams to the samples in the electron energy-loss spectra (EELS), we can analyze the chemical compositions of the materials. The advantages of EELS over other methods are its high energy resolution, excellent sensitivity for light elements, and atomic-scale special resolution that enable us to obtain chemical mapping with atomic resolution. Its high energy resolution enables us to analyze the chemical T. Nagao (&) International Center for Materials NanoArchitectonics, National Institute for Materials Science, Tsukuba, Japan e-mail: NAGAO.Tadaaki@nims.go.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_23 133 134 T. Nagao Fig. 23.1 a Electronic structure and electronic excitations measured by EELS. b Typical EELS. Lower energy-loss (low-loss) feature corresponds to electronic excitations in valence and conduction bands. Higher energy-loss (core-loss) feature corresponds to the excitations from core relevels to the higher states bonds and electronic band structure by measuring electronic excitations and plasmonic excitations in valence and conduction bands. Figure 23.1a shows a typical energy diagram indicating electronic excitations from valence bands and core levels. Figure 23.1b shows an example of an EELS. The highest sharp peak at zero-loss energy corresponds to the elastically scattered electrons. The smaller broad features on the right side of the elastic peak correspond to the loss peaks by plasmonic excitation and interband excitations in valence and conduction bands. These types of loss peaks typically reside below 20 eV and reflect the electronic and optical properties of the materials. The sharp peaks at higher loss energies correspond to the core-level excitations that contain chemical information of the material. 23.2 • • • • Features Electron energy analysis with energy resolution better than 0.1 eV. Spectroscopic measurement with atomic-scale spatial resolution (0.2 nm). Chemical analysis with spatial resolution from 0.2 nm to 1 µm. Light elements (carbon, boron) are detected by EELS which is complementary to the chemical analysis by EDS. 23 Electron Energy-Loss Spectroscopy 135 • High vacuum is required for instrumentation. • Information about electronic bands, chemical bonds, phase transitions, and optical properties is obtained. 23.3 Instrumentation Electron energy-loss spectrum is obtained by measuring the kinetic energy of the transmitted electrons by a scanning transmission electron microscope (STEM) equipped with an electron energy analyzer. Figure 23.2 shows typical setup of the TEM energy analysis system. The electron beam is generated by the electron gun, then passes the column, and penetrates the sample, and the scattered electrons enter into the post-column electron energy analyzer. The energy analyzer is composed of energy-filtering magnetic prism system where the electrons with different energies pass the trajectory with different radius in the magnetic sector. The electrons reaching different position in the radial direction hit the position-sensitive detectors and constitute energy-loss spectrum. Typical energy resolution for this type of systems is 0.1 eV. By combining a conventional post-column energy analyzer with an additional in-column energy filter (a monochromator) equipped before the sample, the energy resolution can reach below 0.01 eV [1]. 23.4 Applications 23.4.1 EELS from Graphite Figure 23.3 shows an example of EELS taken from graphite [1]. In the low-energy side, plasmonic excitation associated with the interband transitions in p bands is observed at 6 eV, and plasmons associated with the excitations in p and r bands are observed at around 30 eV. The excitations of K-shell (excitation of 1s electrons) give rise to sharp excitation to empty p and broader excitation to r antibonding orbitals above the Fermi level at 285.4 and 292.5 eV, respectively. The p peak as well as the p plasmon peak is often used to confirm the presence of graphitic structure in carbon-based materials, and its disappearance corresponds to the loss of p bonds such as seen for the case of amorphous diamond. 23.4.2 Visualization of Localized Surface Plasmon Modes in a Single Nanoparticle Electron beam-based spectroscopies, such as EELS and CL, have been recognized as powerful tool in nanophotonics field also because of their intrinsic high spatial 136 T. Nagao Fig. 23.2 Schematic illustration of an EELS analyzer equipped at the bottom of a transmission electron microscope resolution giving access to a mapping at the nanometer scale, far smaller than the wavelength of light. Figure 23.4 shows an example of EELS measurement of a single silver triangle nanoplatelet with nanometer-scale resolution [3]. The electron beam illumination at specific positions on the particle selectively excites different modes of localized surface plasmon resonance (LSPR) with different resonance energy. The nanoscale EELS mapping recorded at these three energies clearly visualizes three distinct modes: the lowest energy mode A (at 1.75 eV) localized at the tips, the medium energy mode B (at 2.70 eV) along the sides, and the high energy mode C (around 3.2 eV) roughly in the center. 23 Electron Energy-Loss Spectroscopy 137 Fig. 23.3 Energy-loss spectrum of graphite. Low-loss feature exhibits plasmon in p band (6 eV) and plasmons associated with p and r bands (30 eV). The K-edge shows peaks due to transitions of 1s electrons to p and r antibonding states. Reproduced from Ref. [2] Fig. 23.4 Localized surface plasmon (LSP) modes on a single silver triangular nanoparticle. (Upper left) The STEM image and profile. (Upper right) EELS recorded at three positions (a, c, and b) on the silver nanoparticle. (a)–(b) EELS mapping recorded at three LSPR energies at 1.75, 2.70, and 3.20 eV, respectively (reproduced from Ref. [3]) 138 T. Nagao References 1. Krivanek, O.L., Lovejoy, T.C., Dellby, N., Aoki, T., Carpenter, R.W., Rez, P., Soignard, E., Zhu, J., Batson, P.E., Lagos, M.J., Egerton, R.F., Crozier, P.A.: Vibrational spectroscopy in the electron microscope. Nature 514, 209–212 (2014) 2. Batson, P.E.: Local crystal anisotropy obtained in the small probe geometry. Micron 39, 648–652 (2008) 3. Nelayah, J., Kociak, M., Stéphan, O., Javier García de Abajo, F., Tencé, M., Henrard, L., Taverna, D., Pastoriza-Santos, I., Liz-Marzán, L. M., Colliex, C.: Mapping surface plasmons on a single metallic nanoparticle, Nature Physics 3, 348–353 (2007) Chapter 24 Electron Probe Microanalysis Hiroshi Sakamae Keywords Electron beam excitation Trace elements 24.1 Characteristic X-ray WDS Principle When an accelerated electron strikes a substance, it sometimes ejects the inner shell electron of the atom existing in the substance and creates a vacancy. This phenomenon is called electron beam excitation. At this time, one of the electrons of outer shells falls into the vacancy. This generates the X-ray photon which has the energy equal to the binding energy gap between two shells. This X-ray is called characteristic X-ray, and its energy or wavelength shows a specific value to the element. Figure 24.1 shows an example of these processes. By examining the energy or wavelength of the generated X-rays, it is possible to identify the elements existing in the electron beam irradiation point. Since the amount of generated characteristic X-rays correlates with the concentration of the element, the element composition at the beam irradiation point can be found from the type and amount of X-rays to be detected. Since the electron beam can be focused by using an electromagnetic lens and can be two-dimensionally scanned by electromagnetic deflector, by measuring the signal amount of X-rays synchronously with beam scanning, the two-dimensional element concentration distribution can be obtained in a minute region. H. Sakamae (&) Surface Analysis Business Unit, Analytical & Measuring Instruments Division, Shimadzu Corporation, Kyoto, Japan e-mail: sakamae@shimadzu.co.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_24 139 140 H. Sakamae Fig. 24.1 Example in which characteristic X-ray is generated as the electron of the L shell drops into the vacancy generated by the incident electron ejecting the electron of the K shell 24.2 Features • Composition analysis is possible for minute region up to about 0.1 lm. • A two-dimensional composition distribution can be obtained in a region with a side of several micrometers to the order of centimeters. • Heavier elements than Be can be analyzed with high quantitative accuracy.* • Trace elements can be analyzed, since the detection limits are on the order of 100 ppm for most elements.* * Features when using wavelength-dispersive X-ray spectrometer. 24.3 Instrumentation SEM + EDS, which is an electron microscope equipped with energy-dispersive X-ray spectrometer, is also included in the instruments using EPMA analysis method. But here we explain the instrumentation for the instrument which is aimed performing EPMA analysis, generally called electron probe microanalyzer. Figure 24.2 shows the instrumentation for electron probe microanalyzer. It consists of an electron optical system, a sample stage, more than one wavelength-dispersive X-ray spectrometer (WDS), an optical microscope, a secondary electron detector, a 24 Electron Probe Microanalysis 141 Fig. 24.2 Instrumentation for electron probe microanalyzer vacuum evacuation system that keeps the surroundings in a vacuum, various control devices, and a data processing device. The electron optical system consists of an electron gun, various lenses, scanning coils, etc., generates and accelerates an electron beam, then controls its current, focuses it, irradiates it on the sample, and scans it in two dimensions. By measuring the amount of secondary electrons in synchronization with beam scanning, you can observe the electron microscope image. The optical microscope is arranged coaxially with the beam, and its focal position is arranged to coincide with the analysis point of the instrument. In addition, the X-ray spectrometer is arranged so that X-rays generated from the analysis point can be spectrally separated appropriately. Therefore, by controlling the sample stage and observing the point of interest on the sample surface with an optical microscope and focusing the image, the center of the image becomes the analysis point, and X-rays can be measured under appropriate conditions. 24.4 Applications 24.4.1 Conduction Failure Analysis of Electrical Appliances EPMA is useful for migration and electrode corrosion assessment, which is a conduction failure factor in electrical appliances. In the example shown in 142 1mm Optical microscope image H. Sakamae Cu Cl Fig. 24.3 Distribution of Cu and Cl in the region where Cu migration occurred between printed circuit board patterns. These data show that Cl ion is deeply involved in this migration Fig. 24.3, it can be seen that migration of Cu occurs from the cathode side (−) to the anode side (+). Also, you can see that chlorine ion is deeply involved in this migration. Chapter 25 Electron-Stimulated Desorption Naoya Miyauchi Abstract Bonding states between adsorbates and a solid surface are electronically excited by irradiation of photons or electrons to the surface, and subsequently, as a result of the relaxation processes, adsorbates are released as ions or neutral particles having kinetic energy of a few eV. Such particle release is generally referred to as desorption-induced electronic transition (DIET). Keywords Electron-stimulated desorption transition Hydrogen Adsorbed atom 25.1 Desorption-induced electronic Principle Bonding states between adsorbates and a solid surface are electronically excited by irradiation of photons or electrons to the surface, and subsequently, as a result of the relaxation processes, adsorbates are released as ions or neutral particles having kinetic energy of a few eV. Such particle release is generally referred to as desorption-induced electronic transition (DIET). When electrons are used as an excitation source, it is also called electron-stimulated desorption (ESD). ESD is major surface analysis method. Figure 25.1 shows the basic principle of ion desorption. The mechanism can be explained well by the MGR model proposed by Menzel and Gomer, Redhead in 1964 [1, 2]. The bonding state of adsorbate is excited from the bonding orbital to the antibonding orbital by Franck–Condon transition, and subsequently, adsorbate is desorbed as ion. Antoniewicz proposed an extended MGR model, in which the ion produced by electronic excitation moves toward the surface and is neutralized. The atom desorbs along a repulsive potential curve as a neutral or an ion in some cases [3]. Besides, various theories such as Knotek–Feibelman model have been proposed [4]. N. Miyauchi (&) Department of Physics Faculty of Science, Toho University, Chiba, Japan e-mail: naoya@ph.sci.toho-u.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_25 143 144 N. Miyauchi Fig. 25.1 Potential diagram of DIET mechanism explained by MGR theory 25.2 Features • Surface sensitive. • Information of the adsorbed atoms on the surface and the adsorbed position with a lateral resolution of SEM level. • Metals, semiconductors, insulators, organic substances, etc., can be used for measurement. • Hydrogen on the surface can also be detected. • It is suitable for observing the electron excited state of the solid surface and adsorbed atoms, the dynamic process of the desorption mechanism. 25.3 Instrumentation Typical experimental setup is shown in Fig. 25.2. The ESD measurement system consists of an electron source and a mass analyzer in an ultra-high vacuum chamber. In order to identify ESD ions, a time-of-flight mass spectrometer (TOF-MS) or quadrupole mass spectrometer (QMS) is commonly used for mass analyzer. Since a signal intensity of mass-selected ion is very week, pulse counting method has been widely adopted for ion measurement. Ion signals detected by channeltron or microchannel plate are amplified by amplifiers and converted into pulse signals by a single channel analyzer. Mass spectra of desorbed ions are obtained by accumulation of ion number with respect to each time of flight measured by a time-to-amplitude converter (TAC). If accumulation time becomes long, it is necessary to keep constant electron current. A stable electron gun is recommended for the electron source. 25 Electron-Stimulated Desorption 145 Fig. 25.2 Typical experimental setup for ESD method. The electron source and the time-of-flight mass spectrometer are equipped in the ultra-high vacuum chamber. Reprinted from Ref. [5] Copyright 1996, with permission from Elsevier It is possible to know the distribution of adsorbed atoms on the surface, by synchronizing detection of adsorbed ions with incident position of electron [6]. 25.4 Applications 25.4.1 Angular Distribution of ESD Ions from the MgO (001) Surface [6] Cleaved MgO (001) has a multistep structure, and the yield and the angular distribution of desorbed ions depend on the step structure. The surface normal direction (h = 0°) of the (011) plane is the inclined direction of 45° in the normal direction of the (001) plane. Figure 25.3 shows the TOF spectra from MgO (011), which is characterized by step-rich structure. The intensity of the ESD ions has a maximum in the surface normal direction and decreases as the angle increases. Therefore, the ions from the step site are oriented in a direction inclined from the normal to the (001) surface. 146 Fig. 25.3 Angular distribution of O+ from MgO (011), i.e., the flight time (1024 ch. = 20 ls) Reprinted from ref [6] Copyright 1996, with permission from Elsevier Fig. 25.4 Dependence of amount of hydrogen adsorbed on W(001) and yield of H+. Reprinted from ref [7] Copyright 1986, with permission from Elsevier N. Miyauchi 25 Electron-Stimulated Desorption 147 25.4.2 Electron Excitation Desorption of Hydrogen Adsorbed on W(001) [7] When desorbed ions are measured as a function of the amount of hydrogen exposure, maximum values are obtained at the coverage of 1/4. The adsorption structure of hydrogen is c(2 2) structure from the result of LEED measurement. This drastic reduction in H+ yield over about 4 L indicates that the adsorption state of hydrogen has changed (Fig. 25.4). References 1. Menzel, D., Gomer, R.: Desorption from metal surfaces by low-energy electrons. J. Chem. Phys. 41, 3311–3328 (1964) 2. Redhead, P.A.: Interaction of slow electrons with chemisorbed oxygen. Can. J. Phys. 42, 886–905 (1964) 3. Antoniewicz, P.R.: Model for electron- and photon-stimulated desorption. Phys. Rev. B 21, 3811–3815 (1980) 4. Knotek, M.L., Feibelman, P.J.: Ion desorption by core-hole auger decay. Phys. Rev. Lett. 40, 964–967 (1978) 6. Gothoh, T., Fukunaga, Y., Takagi, S.: Observation of angle-, mass- and energy-resolved ESD ions from MgO(001) surface. Surf. Sci. 357–358, 690–692 (1995) 5. Takagi, S., Gothoh, T.: Observation of at titanium surface by desorbed ion images with scanning electron microscope. Surf. Sci. 287(288), 361–365 (1993) 7. Stechel, E.B., Knotek, M.L.: The effect of adsorbate-adsorbate interactions on stimulated desorption yields. Surf. Sci. 167, 297–312 (1986) Chapter 26 Electron-Beam-Induced Current Jun Chen and Takashi Sekiguchi Keywords Semiconductor 26.1 Dislocation Grain boundary Leakage Principle If we irradiate an electron beam onto a semiconductor, electrons and holes are generated within the volume where e-beam reaches. The generated carriers undergo diffusion and recombination to reach a new equilibrium. If there exists an electric field such as Schottky contact or p-n junction, the excess electrons and holes are collected by this field (Fig. 26.1). Since the carriers are created by e-beam injection, this signal is named as electron-beam-induced current (EBIC). If electrically active defects exist, the EBIC current is reduced because of the carrier recombination at such defects. Since EBIC current is generally 1000 times higher than the e-beam current, we can image electrical active defects in semiconducting materials. EBIC contrast is defined by C = (Ib −Id)/Ib, where Ib and Id are the EBIC currents at background and defect, respectively. This value represents the strength of carrier recombination activity. EBIC is also used for extracting material parameters such as diffusion length and recombination velocity. The early works of EBIC have been reviewed by Leamy in 1982 [1]. 26.2 Instrumentation EBIC observation is generally performed in a scanning electron microscope (SEM). An electrical circuit on the specimen can realize the EBIC detection. For the detailed analysis of semiconductor materials and devices, it is better to prepare J. Chen (&) T. Sekiguchi International Center for Material Nanoarchitectonics, National Institute for Materials Science, Tsukuba, Japan e-mail: CHEN.Jun@nims.go.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_26 149 150 J. Chen and T. Sekiguchi Fig. 26.1 EBIC principle advanced EBIC system for quantitative analysis [2]. Figure 26.2 shows the block diagram of such a system [3]. The important aspects are: (1) recording the absolute EBIC value; (2) cooling system; and (3) bias circuit. For recording the EBIC current at each pixel, two-dimensional EBIC signals are recorded in a memory of PC. The low-temperature operation is indispensable to image shallow-level defects. The bias circuit is necessary to monitor the leakage of MOSFET and device failure. Fig. 26.2 Multi-dimensional EBIC system with functional attachments including cooling stage, bias circuit, and digital data recording unit 26 Electron-Beam-Induced Current 26.3 151 Applications 26.3.1 Imaging of Extended Defects in Different Depth: Accelerating Voltage Dependence EBIC technique is useful for imaging extended defects in semiconductor materials. In the past, many works have been done on dislocations [4, 5]; stacking faults [6, 7]; and grain boundaries [8]. Figure 26.3 shows the EBIC result on the misfit dislocations in strained Si layer on SiGe virtual wafer [9]. The schematic of the specimen structure is shown in Fig. 26.3a. First, SiGe virtual substrate of few microns thick is grown on Si wafer. Then, thin strained Si layer of about 100 nm is grown on this SiGe layer. EBIC observations are done with 4 and 20 kV electron beams at 60 K. The electron ranges, namely the major of the observation depths, are 320 nm and 4.8 lm, respectively. The 4 kV EBIC image in Fig. 26.3b shows dark lines and dots with low density. The lines are the misfit dislocations lying along the interface of strained Si and SiGe substrate. The dots are threading dislocations penetrating the surface. The 20 kV EBIC image in Fig. 26.3c shows cross-hatch patterns lying along <110> directions. They are bundle of dislocations which exist in the SiGe substrate. Since the dislocation density is too high, each dislocation is not resolved but only their density variation is imaged as such pattern. 26.3.2 Energy Level of Grain Boundaries in Si: Temperature Dependence According to Shockley–Reed–Hall (SRH) theory [10], the recombination activity of defect levels exhibits temperature dependence due to the change in the occupation fraction caused by Fermi level position. Generally speaking, the shallow-level Fig. 26.3 Illustrated cross-sectional structure of a strained Si/SiGe multilayer (a), threading dislocations (TD) and misfit dislocations (MD) in strained Si (b), and misfit dislocations in graded SiGe (c) distinguished by EBIC at 4 and 20 kV, respectively 152 J. Chen and T. Sekiguchi Fig. 26.4 GBs in mc-Si: a secondary image; b EBSD image; c EBIC image at 300 K; and d EBIC image at 100 K defects show weak contrast at room temperature and strong contrast at low temperatures. Deep-level defects, on the other hand, show strong contrast at room temperature and become weaker at low temperatures. Figure 26.4 shows the EBIC result on the multicrystalline (mc) Si for solar cells [11]. The secondary electron image (a) and EBSD map (b) shows the geometry of grain boundaries (GBs) in this specimen. The black lines in (b) are the large angle (LA-)GBs indicated as R9 and R. The red lines are small angle (SA) GBs denoted as the misorientation angles. In the room temperature EBIC image shown in (c), only the latter SA-GBs are imaged as dark lines. The EBIC image at 100 K (d) shows dark lines of SA-GBs and dark spots in grains, which correspond to dislocations. LA-GBs are only faintly imaged. These results indicate that SA-GBs act as deep levels, dislocations as shallow levels, and LA-GBs are very weak for Fig. 26.5 Schematic cross section of gate region (a), SE image (b) and EBIC image of gate region (c) in a pMOS. Bias-dependent EBIC images of leakage sites (d) in an nMOS 26 Electron-Beam-Induced Current 153 recombination centers. If the mc-Si contain metallic impurities like Fe, the EBIC contrast of LA-GBs become high according to the impurity decoration. Thus, temperature-dependent study is very important to estimate energy levels of defects and to study impurity gettering of extended defects. 26.3.3 Device Characterization: Bias Dependence EBIC technique can be applied to the device characterization. Figure 26.5 shows the EBIC results of MOS device using high-k gate dielectric [12]. Since high-k dielectric is not so perfect as SiO2, this MOS shows the leakage behavior under bias voltages. The schematic (a) shows the cross section of the gate region. The SE image (b) shows the whole image of MOS structure with electrode. The EBIC image (c) shows the gate region (100 100 lm2) of a pMOS with uniform background. The EBIC images in (d) show bright spots in gate region (20 20 lm2) of an nMOS. This bright spots are leakage sites of high-k gate stack. The EBIC behavior of leakage sites depends on the bias voltage. Detail analysis of these data gives the information of leakage mechanism and device failure. References 1. Leamy, H.J.: Charge collection scanning electron microscopy. J. App. Phys. 53, R51–R80 (1982) 2. Sekiguchi, T., Sumino, K.: Quantitative electron beam tester for defects in semiconductors (CL/EBIC/SDLTS System). Rev. Sci. Instrum. 66, 4277–4282 (1995) 3. Chen, J., Yuan, X.L., Sekiguchi, T.: Advanced semiconductor diagnosis by multidimensional electron-beam-induced current technique. Scanning 30, 347–353 (2008) 4. Kittler, M., Seifert, W., Higgs, V.: Recombination activity of misfit dislocations in silicon. Phys. Stat. Sol. a 137, 327–335 (1993) 5. Kusanagi, S., Sekiguchi, T., Shen, B., Sumino, K.: Electrical activity of extended defects and gettering of metallic impurities in silicon. Mater. Sci. Technol. 11, 685–690 (1995) 6. Sekiguchi, T., Shen, B., Watanabe, T., Sumino, K.: EBIC study on the electrical activity of stacking faults in silicon. Mater. Sci. Eng., B 42, 235–239 (1996) 7. Chen, B., Chen, J., Sekiguchi, T., Ohyanagi, T., Matsuhata, H., Kinoshita, A., Okumura, H., Fabbri, F.: Electron-beam-induced current study of stacking faults and partial dislocations in 4H-SiC Schottky diode. Appl. Phys. Lett. 93, 033514/1–033514/3 (2008) 8. Wang, Z.J., Tsurekawa, S., Ikeda, K., Sekiguchi, T., Watanabe, T.: Relationship between electrical activity and grain boundary structural configuration in polycrystalline silicon. Interface Sci. 7, 197–205 (1999) 9. Yuan, X.L., Sekiguchi, T., Ri, S.G., Ito, S.: Detection of misfit dislocations at interface of strained Si/Si0.8Ge0.2 by electron-beam-induced current technique. Appl. Phys. Lett. 84, 3316–3318 (2004) 10. Shockley, W., Read, W.T.: statistics of the recombinations of holes and electrons. Phys. Rev. 87, 835–842 (1952) 154 J. Chen and T. Sekiguchi 11. Chen, J., Sekiguchi, T.: Carrier recombination activity and structural properties of small-angle grain boundaries in multicrystalline silicon. Jpn. J. Appl. Phys. 46, 6489–6497 (2007) 12. Chen, J., Sekiguchi, T., Fukata, N., Takase, M., Chikyo, T., Yamabe, K., Hasunuma, R., Akasaka, Y., Inumiya, S., Nara, Y., Yamada, K.: Observation of leakage sites in a hafnium silicon oxynitride gate dielectric of a MOSFET device by electron-beam-induced current. Appl. Phys. Lett. 89, 222104/1–222104/3 (2006) Chapter 27 Ellipsometry Toshihide Tsuru Keywords Polarization Reflection Complex refractive index Film thickness Complex amplitude reflectance 27.1 Principle In general, since the amplitude and phase are different between the p- and s-polarizations of the reflected light, the reflected light is elliptically polarized. Ellipsometry precisely measures the shape of reflection ellipse by irradiating totally polarized light to a planer bulk surface, a thin film, and a multilayer [1–3]. In order to obtain the optical properties such as complex refractive indices and film thicknesses, analysis using an appropriate optical model is necessary. Ellipsometry exhibits a high sensitivity to thickness and optical properties of thin films. Therefore, a carefully tuned ellipsometer is adequate to study ultrathin film characterization and/or fabrication for precise optics. Although many designs of ellipsometers are in use, these are generally classified into two types: a null instrument and a photometric instrument. In the measurement by the null ellipsometer, the null position is found by adjusting alternately the polarizer and the analyzer, where the azimuth of compensator is fixed. In the photometric instrument, the sinusoidally varying light intensity is recorded during the rotation of the polarization optics. 27.2 Features • Nondestructive and noncontact optical technique; • Finely tuned ellipsometer has a picometer thickness sensitivity [4] and can detect atomic step [5] or less thickness; T. Tsuru (&) Yamagata University, Yamagata, Japan e-mail: ttsuru@e.yamagata-u.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_27 155 156 T. Tsuru • In situ measurement in vacuum environment, atmospheric environment, and (in) solution is possible; • Not only for optical properties but also for composition, surface roughness, thickness (depth), crystallinity, concentration, conductivity, and other material properties can be evaluated. 27.3 Definition and Convention in Ellipsometry There are 512 combinations of parameters for describing the polarization in ellipsometry, which confuse researchers. Therefore, parameters in use of ellipsometry were unified in the 2nd International Conference on Ellipsometry at Nebraska [6] as shown in Table 27.1. The nomenclature is n k tanW D refractive index, extinction coefficient, relative amplitude attenuation, relative phase change. 27.4 Expression of Reflection Polarization 27.4.1 Reflection at the Planar Interface Between Two Media When the light of the wavelength k irradiates a sample at an angle of incidence / as illustrated in Fig. 27.1, the complex amplitude reflectances for the p- and s-components are expressed as: rp Erp e n 0 cos /0 n 1 cos / e ¼ ¼ rp exp idp ; Eip e n 0 cos /0 n 1 cos / þ e ð1Þ Table 27.1 Unified parameters in ellipsometry [6] Electric field a E ¼ E0 expfiðxt þ dÞg e Complex refractive index n ¼ n ik Complex relative amplitude attenuation q ¼ tan W expðiDÞ Relative phase difference D ¼ dp ds a E0, i, x, t, and d indicate amplitude of electric field, imaginary unit, time, angular frequency, and absolute phase 27 Ellipsometry 157 Fig. 27.1 Reflection and transmission of incident light at the planer interface rs Ers e n 1 cos /0 n 0 cos / e ¼ ¼ jrs jexpðids Þ: Eis e n 1 cos /0 n 0 cos / þ e ð2Þ Where ne0 and ne1 indicate the complex refractive indices of media 0 and 1, and /0 is the refraction angle. dp and ds give the phase shifts on reflection. rp and rs are called the Fresnel (reflection) coefficients. 27.4.2 Reflection by an Ambient-Film-Substrate System The important case in ellipsometry measurement is investigation of a polarized light reflected the film sandwiched between an ambient and a substrate media which are assumed optically homogeneous and isotropic, as shown in Fig. 27.2. If the Fresnel coefficients at the ambient-film and the film-substrate indicate r01 and r12, the complex amplitude reflectance R including the effects of multiple reflection and interference is given by Fig. 27.2 Reflection of a light at an incident angle / from a substrate covered by a parallel-plane film of thickness d. /’ is the angle of n1 , refraction in the film. f n0 , f and f n2 are the complex refractive indices of an ambient, the film, and a substrate, respectively 158 T. Tsuru R¼ r01 þ r12 expðidÞ : 1 þ r01 r12 expðidÞ ð3Þ Where the phase angle d is expressed as d¼ 4p 4pd ne1 d cos /0 ¼ k k qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ne1 2 ne0 2 sin2 /: ð4Þ The p- and s-polarizations of R can be obtained as Rp and Rs by substituting the p- and s-components of Fresnel coefficients of Eqs. (1) and (2). 27.4.3 Reflection by an Ambient-Multilayer-Substrate System When p- and s-polarized light of wavelength k is irradiated at an angle / on a multilayer composed of j-layers shown in Fig. 27.3, the amplitude reflectance Rj of the jth layer for both of p- and s-components is expressed as: rj þ R0j1 exp idj ; Rj ¼ 1 þ rj R0j1 exp idj dj ¼ 4pdj k qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi nej 2 ne0 2 sin2 /: ð5Þ ð6Þ Fig. 27.3 Calculation procedure of the amplitude reflectance of an ambient-multilayer-substrate system 27 Ellipsometry 159 Where rj is the Fresnel reflection coefficient at the ambient-jth layer interface and R’j is the resultant amplitude reflectance including the effect of multiple reflections in the layers beneath, and dj and nej are the thickness and the complex refractive index of the jth layer. Please note that the numbering order of layer number is reverse of Figs. 27.1 and 27.2. In the limiting case dj ! 0 in Eq. (5), the j-layer structure becomes the (j-1)-layer structure, which yields the following relation R0j1 ¼ Rj1 rj : 1 rj Rj1 ð7Þ 27.4.4 The Complex Relative Amplitude Attenuation and Ellipsometric Parameters Ellipsomerty can determine the ratio q of the complex amplitude reflectances for the p- and s-polarization as, q¼ rp Rp ¼ : rs Rs ð8Þ The ellipsometric parameters of the relative amplitude attenuation angle W and the relative phase change D are defined by q ¼ tan W expðiDÞ: ð9Þ From Eqs. (1), (2), (8), and (9), following equations can be obtained. rp ; ð10Þ D ¼ dp d s : ð11Þ tan W ¼ jrs j 27.4.5 Stokes Parameters The Stokes parameters S0, S1, S2, and S3 are four intensity-based parameters used to express the polarization state. The Stokes vector is the set of these Stokes parameters. Using amplitude of electric fields (Ex, Ey), ellipticity angle and azimuth (e, h), and the ellipsometric parameters (W, D) of reflected polarization indicated in Fig. 27.4, the Stokes parameters are described as: 160 T. Tsuru Fig. 27.4 Reflection ellipse in (Ex, Ey), (e, h), and (W, D) coordinate systems S0 ¼ Ex2 þ E 2 ; ð12Þ S1 ¼ Ex2 Ey2 ¼ S0 cos 2e cos 2h ¼ cos 2W; ð13Þ S2 ¼ 2Ex Ey cos d ¼ S0 cos 2e sin 2h ¼ sin 2W cos D; ð14Þ S3 ¼ 2Ex Ey sin d ¼ S0 sin e ¼ sin 2W sin D: ð15Þ When the right-handed ellipse is reflected, the ellipticity angle e [ 0. 27.4.6 Ellipsometric Growth Curves In the ellipsometry, the thickness and the complex refractive index of samples cannot be directly obtained. As mentioned above, the ellipsometer measures the complex relative amplitude attenuation q or the ellipsometric parameters W and D. Therefore, prior to the experiment, it is helpful and useful to know the q trajectories or the W-D trajectories of the films as the growth curves simulated by model calculation with expected film thickness and the complex refractive index. Figure 27.5a shows the theoretical q trajectories for a transparent and several absorbing monolayers on a Si substrate. Measured and simulated growth curves of Au deposition on a Si substrate are shown in Fig. 27.5b [7]. Figure 27.5c shows the theoretical growth curve of a Mo and Si monolayers and a Mo/Si multilayer composed of 40 periods on Si substrates. The interval of dotted marks for Mo and Si monolayers is distributed with 1 nm in thickness. In situ measurements of an ion beam depositions of a Mo/Si multilayer by using an automatic null ellipsometer is shown in Fig. 27.5d [4, 8]. 27 Ellipsometry 161 Fig. 27.5 a Theoretical ellipsometric growth curves of C: e n = 2.25−0.37i, Au: e n = 0.44−3.50i, n = 1.45 monolayers on a Si substrate: e n = 3.875 Ag: e n = 0.19−4.45i, Ru: e n = 3.95−5.34i, SiO2: e −0.0205i in the complex plane. The layer thickness of SiO2 is 280 nm, and the other is 500 nm. b The ellipsometric growth curves of Au monolayers. Black line indicates simulated curve. Blue line measured by an automatic null ellipsometer [4] shows fabrication of an ion beam sputtering. A wavelength of 632.8 nm and an angle of incidence of 70.00° for (a) and 70.81° for (b) are chosen for the calculations. c Theoretical ellipsometric growth curve of a Mo/Si multilayer composed of 40 periods, and Mo and Si monolayers of 100 nm thicknesses on a Si substrate. The optical constants and thickness of e n = 4.08−4.47i and 3 nm for Mo layers and e n = 4.81−1.02i and 4 nm for Si layers, respectively, were used for simulation. d The growth curve of 40 periods Mo/Si multilayer obtained by in situ ellipsometric measurement [8]. An irradiation wavelength and an incident angle are set to 632.8 nm and at 69.80° in (c) and (d). Inserted ellipses in (a), (b), and (c) show reflected polarization state. Red and blue ellipses indicate left-handed and right-handed ellipses When the thickness is zero, q represents the substrate in Figs. 27.5a, b, and c. q proceeds gradually toward the value of the bulk material as the thickness increases. Consequently, the trajectories depict spirals toward the bulk value. As shown in 162 T. Tsuru these figures, the direction and the length of the trajectories qualitatively correspond to the complex refractive index and the thickness, respectively. In case of a multilayer (Figs. 27.5c and d), starting from the substrate, the q trajectory of Mo appears to form a smooth line of which the direction and length are the same as for Mo monolayer. The direction is changed at the material switching point from Mo to Si. Such alternate movements of the Mo and Si segments continue up to the final layer. As the deposition proceeds, the multilayer is stacked beyond the penetration depth of the probing light and the bottom part of the multilayer stack ceases to contribute to q variation. At the final stage, the growth curve moves between two points as a closed loop, since the light probes the top part of the multilayer due to the lack of contribution of the reflected light from the substrate. 27.5 Instrumentation 27.5.1 Null Ellipsometry In principle, null ellipsometer [1, 4, 8] can easily realize high sensitivities to the thickness and the complex refractive index because careful calibrations such as nonlinearity of detector, fluctuations of light intensity are of less importance. A standard null ellipsometer has two optical arrangements. One is a PSCA null ellipsometer shown in Fig. 27.6 that consists of a monochromatic and collimated light, a linear polarizer (P), sample (S), a compensator (C), a linear analyzer (A), and a detector. The other is a PCSA ellipsometer. Null ellipsometry is based on finding a set of azimuthal angles for P, C, and A, where the detector signal is extinguished. Fig. 27.6 Optical configuration of the PSCA null ellipsometer. Polarization states at (I), (II), (III), n = 1.45, d = 50 nm) on a Si and (IV) are drawn, when nulling set (P, A) at zone 1 of SiO2 (e substrate at an incident angle of 75º is found 27 Ellipsometry 163 27.5.2 Four-Zone Averaging When the azimuth of a quarter wave plate (QWP) used as a compensator is fixed at ± p/4, the various sets of (P, C, A) at null conditions can be reduced to four sets. Note that P, C, and A denote the rotational azimuth angles of P, C, and A measured as positive counterclockwise from the p-direction when looking into the light. The zone relations [1] are summarized in Table 27.2. Using the four sets of nulling angles, the average of the ellipsometric parameter (W, D) can be obtained by W ¼ ðP3 P1 þ P4 P2 Þ=4; ð16Þ D ¼ ðA1 þ A3 A2 A4 Þ=2: ð17Þ The four-zone average can eliminate the azimuth angle errors and imperfection of the QWP. 27.6 Applications Since ellipsometry is a method to precisely measure the shape of reflection ellipse, the optical properties and/or layer thickness cannot be directly determined. Therefore, numerical analysis is necessary to obtain optical characteristics. Analysis methods of the data measured by the null ellipsometry are listed in Table 27.3. Table 27.2 Zone relations of the PSCA ellipsometry (P, A) and (W, D) Zone C 1 p4 0\P1 \p=2 p=4\A1 \3p=4 W1 ¼ P1 D1 ¼ 2A1 þ p=2 2 þ p4 0\P2 \p=2 5p=4\A2 \ p=4 W2 ¼ P2 D2 ¼ 2A2 p=2 3 p4 p=2\P3 \0 p=4\A3 \5p=4 W3 ¼ P3 D3 ¼ 2A3 p=2 4 þ p4 p=2\P4 \0 3p=4\A4 \p=4 W4 ¼ P4 D4 ¼ 2A4 þ p=2 164 T. Tsuru Table 27.3 Analysis method of data obtained by the null ellipsometry Measured sample Analysis method n and k of transparent or absorbing substrate n and d of transparent monolayer on a known substrate n, k, and d of absorbing monolayer on a known substrate n, k, and d of absorbing monolayer on an unknown substrate n, k, and d of a multilayer on a known substrate n and k can be numerically derived [9]. (Details are shown in a chapter of SE.) n and d can be numerically derived [1, 7]. Multiple combinations of (n, k, d) can be obtained [7]. Analysis is impossible. Multiple-angle-of-incidence ellipsometry should be adopted [1]. (n, k, d) can be obtained, when in situ ellipsometer is used [8]. References 1. Azzam, R.M.A., Bashara, N.M.: Ellipsometry and polarized light. North-Holland, Amsterdam (1987) 2. Tompkins, H.G., McGahan, W.A.: Spectroscopic ellipsometry and reflectometry: a user’s guide. Wiley, New York (1999) 3. Tompkins, H.G., Irene, E.A.: Handbook of ellipsometry. Willian Andrew, New York (2005) 4. Yamamoto, M., Hotta, Y., Sato, M.: A tracking ellipsometer of picometer sensitivity enabling 0.1% sputtering-rate monitoring of EUV nanometer multilayer fabrication. Thin solid films 433, 224–229 (2003) 5. Tosaka, A., Arakawa, I.: The comparative study of two dimensional condensation of Xe and Kr physisorbed on Ag(111) and Ag(100). Surf. Sci. 600, 1071–1076 (2006) 6. Muller, R.H.: Definitions and conventions in ellipsometry. Surf. Sci. 16, 14–33 (1969) 7. Yamamoto, M., Namioka, T.: In situ ellipsometric study of optical properties of ultrathin films. Appl. Opt. 31, 1612–1621 (1992) 8. Tsuru, T., Tsutou, T., Yamamoto, M.: Realtime layer-by-layer analysis for multilayer fabrication monitoring by an automatic null ellipsometer. Thin Solid Films 455–456, 705–709 (2004) 9. Saxena, A.N.: Change in the phase and amplitude of polarized light reflected from a film-covered surface and their relations with the film thickness. J. Opt. Soc. Am. 55, 1061– 1067 (1965) Chapter 28 Environmental SEM (Atmospheric SEM) Yusuke Ominami Keywords Environmental Atmospheric pressure 28.1 Variable pressure Low vacuum Principle Conventional SEM normally has vacuum environment between electron optic column and specimen chamber so that electron beam-emitted electron source travels to specimen without being scattered by gas molecular. The vacuum condition means that wet materials such as soft materials or biological tissues cannot be observed in their natural state in the SEM due to the fact that the structure of such hydrated materials cannot be maintained in vacuum environment. In order to observe the kind of specimens, SEMs which can be operated in low vacuum or atmospheric pressure (100–105 Pa) have been developed. The SEMs utilizing low vacuum state, which can control the pressure in the specimen chamber, are usually referred to as environmental SEM (ESEM), variable pressure SEM (VPSEM), or natural SEM (NSEM). These SEMs commonly have one or some aperture, which is sufficiently small to allow a pressure differential between electron optic column and the specimen chamber [1, 2]. Atmospheric SEM (ASEM) has a membrane separating the specimen chamber from vacuum region, resulting that the specimen chamber pressure can be controlled even up to 105 Pa [3]. In both ESEM and ASEM, the relaxed environment enables one to observe many materials in a stable manner without coating them with a conducting metal layer. Y. Ominami (&) Science Systems Product Division, Hitachi High-Technologies Corporation, Ibaraki, Japan e-mail: yusuke.ominami.ay@hitachi-hightech.com © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_28 165 166 28.2 Y. Ominami Features • Environmental condition utilizing desirable gas or liquid. • The relaxed conditions can maintain specimen structure in their original state without any preparation. • Liquid water is observable by means of temperature control or membrane separation. • In situ observation under cooling, heating, or gas condition. 28.3 Instrumentation In ESEM and ASEM, primary electrons collide with residual gases so that the beam arrived at specimen surface is broadened. The broadened beam size of electron beam depends on the gas path length. It shows that these SEMs require short gas path length as possible. Typical geometries of ESEM and ASEM are shown in Fig. 28.1. Conventional ESEM and the first-designed ASEM utilized differential evacuation, in which pressure-limiting apertures were placed inside and/or outside the objective lens [2]. The pressure-limiting aperture is placed inside objective lens. ESEMs with high-performance electron source such as Schottky emission or LaB6 electron source require two or more differential apertures with inter mediate pumps in order to restrict to a vacuum value. The specimen chamber pressure is controlled by means of a leak valve. The flow of gas into the chamber can be adjusted by varying the valve until the target value is achieved (100–103 Pa). For observing dehydrated materials, the specimen stage of ESEM normally has a cooling system, which can cool the specimen to near 0 °C. The arrangement makes it possible to control equilibrium phase diagram for water. While ESEM has higher resolution by means of cooling system, ASEM utilizing membrane separation can observe more actual state at room temperature. In ASEM, membrane allowing electron beam penetration is placed on or near the membrane in atmosphere between objective lens and specimen, so that only gas molecular in a gap between the membrane and specimen collide electron beam passing through the membrane. Electron scattering can be minimized by controlling the gap distance between membrane and specimen utilizing the specimen stage with Z-axis. Cooling system is not necessary for ASEM since the observed specimen can be in atmospheric pressure. Fully dehydrated soft material or biological specimens that are moist or wet can be observed without any tendency for them to dry and shrink. 28 Environmental SEM (Atmospheric SEM) (a) 167 (b) Electron source Vacuum pumps Electron source Vacuum pumps Apertures SEM Column Leak valve Thermo Controller SEM Column Detector Specimen Vacuum Cool stage Detector Membrane Atmosphere (103 ~ 105 Pa) Specimen Vacuum pump Low Vacuum (100 ~ 103 Pa) Vacuum pump Fig. 28.1 Typical geometries of a ESEM and b ASEM 28.4 Applications 28.4.1 Observation of Non-conductive Material in ESEM Figure 28.2 gives an example of imaging insulator fibers using ESEM. High-vacuum environment causes charge-up on observed specimen as shown in Fig. 28.2a. Metal coating on insulator surface keeps it from charge-up. Meanwhile, gas molecular introduced by controlling a leak valve in ESEM can relax charge-up on specimen surface without metal coating as shown in Fig. 28.2b. Observation of hydrated material in ESEM. Figure 28.3 shows hydrated superabsorbent polymer in 650 Pa cooled at −4 °C. The polymers expand by including water (purified water: 5 µL) as shown in Fig. 28.3b compared to dry condition (Fig. 28.3a). 28.4.2 Observation of Hydrated Material in ASEM Leafs of the Japanese radish were observed at under low vacuum (50 Pa) and atmospheric pressure (101 kPa) and at room temperature (Fig. 28.4). Epidermal cells and stomata on the leaf are clearly observed without any deformation in the atmospheric pressure as shown in Fig. 28.4b. 168 Y. Ominami (a) (b) ˩P ˩P Fig. 28.2 SEM images observed in a high vacuum and b low vacuum (80 Pa) (a) (b) 500μm 500μm Fig. 28.3 SEM images of polymers observed in a dry condition and b wet condition in 650 Pa cooled at −4°C (a) (b) 20μm 20μm Fig. 28.4 SEM images observed in a 50 Pa and b 101 kPa (atmospheric pressure) 28.4.3 Observation of Drying Process in ASEM Figure 28.5 shows images of sunscreen used to protect the skin against ultraviolet rays. Sunscreen contains silica, titanium, or other fine particles suspended in a fluid. 28 Environmental SEM (Atmospheric SEM) 169 0min After 5min After 15min 40μm 40μm 40μm Fig. 28.5 ASEM images of cosmetics on a substrate and regenerated cellulose fibers in the wet and dry conditions in 101 kPa (atmospheric pressure) and at room temperature Atmospheric pressure observations were made of the drying of sunscreen applied to a substrate. The images show how the large quantity of water or other liquid present at 30 s after application has largely evaporated after 5 min and is almost entirely dry after 15 min, leaving large numbers of residual fine particles adhering to the substrate. This behavior of fluid on a substrate could not be observed using conventional vacuum SEM. It is also common in the case of cosmetic, pharmaceutical, and other similar observations that drying of the substrate (such as skin) must be avoided as well as that of the liquid specimen. References 1. Danilatos, G.D., Robinson, V.N.E.: Principles of scanning electron microscopy at high specimen pressures. Scanning 2, 72–82 (1979) 2. Danilatos, G.D.: Design and construction of an atmospheric or environmental SEM (part 3). Scanning 7, 26–42 (1985) 3. Ominami, Y., et al.: A Novel Approach to Scanning Electron Microscopy at Ambient Atmospheric Pressure. Microscopy 64, 97–104 (2015) Chapter 29 Environmental Transmission Electron Microscopy Tadahiro Kawasaki Keywords In situ observation Gas Liquid 29.1 Differential pumping Sealing membrane Principle ETEM is a dynamic observation technique based on transmission electron microscopy (TEM). In normal TEMs, specimens are placed under a vacuum condition because electron scattering by residual gas molecules must be minimized. In contrast, ETEM enables to observe specimens exposed to gas or liquid environments. Therefore, ETEM can reveal nature of materials under the conditions in which they are formed or utilized, for example, catalysts in gas, battery electrodes in liquid electrolyte, and so on. ETEM has two types of system to keep specimens in gasses or liquids. One is an open-type with differential pumping system for gas experiments [1–3], and the other is a close-type for both gas and liquid flow [4–6]. The former is incorporated in TEM vacuum column where electron beam goes down (Fig. 29.1a). By adding orifices and pumping lines, gas molecules spreading from the specimen chamber can be mostly evacuated. In the latter type (Fig. 29.1b), a small space inside a specimen holder, called an environmental cell (E-cell), is separated from a vacuum with two thin membranes that permit electrons to pass through but forbidden gas/liquid molecules to leak out. These systems provide the T. Kawasaki (&) Nanostructures Research Laboratory, Japan Fine Ceramics Center, Nagoya 456-8587, Japan e-mail: t_kawasaki@jfcc.or.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_29 171 172 T. Kawasaki Fig. 29.1 Schematic illustrations of two types of ETEM; a open-type and b close-type special conditions only around the specimens but keep the other parts of the TEM vacuum. 29.2 Features • Materials in gas or liquid environments can be dynamically observed. • Observations in gas/liquid can be realized with the other actions on specimens such as heating, applying voltage, and so on. • This technique can be applied not only to TEM but also to the other electron microscopies, e.g., scanning electron microscopy (SEM), etc. • Atomic resolution is achievable by reducing thickness of gas/liquid layer and pressure of gasses. • Analysis techniques such as EELS (electron energy loss spectroscopy) and EDX (energy dispersive X-ray spectroscopy) can be combined. 29.3 Instrumentation In the open-type ETEM, the differential pumping system is assembled in a TEM vacuum system (Fig. 29.2a). A gas control system is connected to the specimen chamber to introduce gas inside the TEM, which consists of variable leak valves, mass flow meters, pressure gauges, etc. Achievable gas pressure around the specimen depends on performance of the differential pumping. Multistage differential pumping is needed for higher pressure, e.g., a several kPa. This ETEM system suits for various specimen holders such as double-tilting, heating, biasing, and so on. In the close-type (Fig. 29.2b), the high vacuum of TEM is maintained. Instead, the specimen holder must be dedicated; containing the E-cell space for specimens in which two membranes can be attached to pack gas/liquid, and pipes for inlet/outlet. A gas (or liquid) control system is connected to the specimen holder. The pressure inside the E-cell can be increased to that higher than ambient by using highly 29 Environmental Transmission Electron Microscopy 173 Fig. 29.2 Schematic illustrations of configurations of the a open- and b close-type ETEMs. Parts modified from the original TEMs are shown in red durable membranes even when the thickness is less than a few tens nanometers. Amorphous silicon nitride membranes fabricated with the microelectromechanical system (MEMS) technique are recently utilized, on which electrodes can be patterned for heating and biasing specimens. 29.4 Applications 29.4.1 In Situ Observation of Au/CeO2 Catalyst with Open-type ETEM Figure 29.3 shows an example of the open-type ETEM observations for a gold nanoparticulate catalyst supported on CeO2 [7]. These images were taken under three different environmental conditions; vacuum, 100 Pa of 1 vol.% CO/air, and 100 Pa of O2, by using Cs-corrected ETEM with an acceleration voltage of 300 kV. This result clearly shows with the atomic resolution that the morphology and surface structure of the gold particle are reversibly changed by the surrounding environments. The gold particle appeared to be well facetted in the reaction condition of CO/air, while in pure oxygen gas the particle changed to round shape. During CO oxidation, CO molecules absorbed on the gold surface stabilize the particle with polyhedral shape enclosed by the major facets of {111} and {100}. On the other hand, oxygen molecules dissociated into O atoms or active O-related species induce the formation of rounded surfaces of the gold. This result proves that structural analysis of catalysts should be done in situ during the catalytic reactions. 174 T. Kawasaki Fig. 29.3 Reversible change in the morphology and surface structure of a gold nanoparticle supported on CeO2 under different environments. Reprinted from ref. [7], copyright 2014, with permission from Elsevier 29.4.2 Visualization of Reaction Sites of Au/TiO2 Catalysts with Close-type ETEM Figure 29.4 shows in situ TEM images of a gold nanoparticulate catalyst supported on anatase TiO2 [8] as an example of the close-type ETEM observations. In order to reveal reaction sites of this gold catalyst, in situ analyses of catalytic reaction of propene epoxidation (selective oxidation of propene (C3H6) to propene oxide (PO; C3H6O)) are appropriate. Since the PO has a low vapor pressure of *5 104 Pa at room temperature, the PO remains in liquid form if the surrounding pressure is controlled to be more than this value. Here, the PO catalytic product might be observed in liquid form, but it is almost impossible to visualize gaseous products. By observing the PO formation, reaction sites on the catalyst surface can be determined directly. The close-type ETEM should be adopted for such relatively high-pressure condition close to 1 atm. The TEM images were taken under a reactant gas environment consisted of C3H6 (13%), O2 (18%), tiny amount of moisture (<0.1%), and N2 (69%; just for increasing pressure) with a conventional 200 kV-TEM. Before starting the reaction (Fig. 29.4a), surfaces of Au and TiO2 kept to be clean in a gas environment of O2, H2O, and N2 because of no propene. In contrast, by adding propene to a total pressure of 5 104 Pa, the PO product molecules started to be accumulated at the perimeter of interface between Au and TiO2 substrate, as indicated by arrowheads in Fig. 29.4b. The PO catalytic product disappeared in vacuum because the surrounding pressure became lower than its Fig. 29.4 In situ TEM images of Au/TiO2 observed during propene epoxidation reaction, a surrounding gas was O2, H2O, and N2 (not reacted), b propene gas was added at a total pressure of *5 104 Pa and the catalytic reaction started, c in vacuum after gas evacuation [8] 29 Environmental Transmission Electron Microscopy 175 vapor pressure (Fig. 29.4c). These results directly proved that the active site where the catalytic reaction occurs is the perimeter of Au/TiO2 interface, as schematically shown in Fig. 29.4d. References 1. Hashimoto, H.: A specimen treating device at high temperature for the electron microscope. J. Electron Microsc. 6, 8–11 (1958) 2. Gai, P.L.: In-situ environmental transmission electron microscopy. Nanocharacterization 3, 268–290 (2007) 3. Taheri, M.L., Stach, E.A., Arslan, I., Crozier, B.C., Kabius, P.A., LaGrange, T., Minor, A.M., Takeda, S., Tanase, M., Wargner, J.B.: Current status and future directions for in situ transmission electron microscopy. Ultramicroscopy 170, 86–95 (2016) 4. Haide, H.G.: Electron microscopic observation of specimens under controlled gas pressure. J. Cell Biol. 13, 147–152 (1962) 5. Fukushima, K., Ishikawa, A., Fukami, A.: Injection of liquid into environmental cell for in situ observations. J. Electron Microsc. 34, 47–51 (1985) 6. Creemer, J.F., Helveg, S., Hoveling, G.H., Ullmann, S., Molenbroek, A.M., Sarro, P.M., Zandbergen, H.W.: Atomic-scale electron microscopy at ambient pressure. Ultramicroscopy 108, 993–998 (2008) 7. Takeda, S., Kuwauchi, Y., Yoshida, H.: Environmental transmission electron microscopy for catalyst materials using a spherical aberration corrector. Ultramicroscopy 151, 178–190 (2015) 8. Kawasaki, T., Yoshida, K., Tanji, T.: Visualization of reaction site of functional materials by environmental TEM. AMTC Letters 5, 52–53 (2016) Chapter 30 Extended X-Ray Absorption Fine Structure Hitoshi Abe Keywords X-ray spectroscopy Synchrotron Element specific Local structure Bond length Coordination number 30.1 Principle Extended X-ray absorption fine structure (EXAFS) is a part of X-ray absorption spectroscopy methods to observe local structures of elements of interest. EXAFS spectra contain information typically on bond lengths, coordination numbers, and displacements of lattices (disorders of atoms) with element specificity [1, 2]. The physical quantity measured in EXAFS spectroscopy is the X-ray absorption coefficient l(E), which describes how much X-rays are absorbed by matters as a function of photon energy E of the incident X-ray. The coefficient is obtained by the same way as XANES. Quite simply, EXAFS is a higher and extended energy range above XANES. Energy ranges of generally from several tens electron volts (eV) from the absorption edges to several hundreds or more eV are called EXAFS ranges. Wave functions or absorption intensities of atoms that absorb X-ray to generate photoelectron waves get modulated by surrounding atoms. These surrounding atoms are often called scattering atoms. The modulations appear as EXAFS oscillations, k2(k), which depend on bond lengths and potentials of scattering atoms. The EXAFS oscillations are transformed via Fourier transforms (FTs). H. Abe (&) Institute of Materials Structure Science, High Energy Accelerator Research Organization, Tsukuba, Japan e-mail: hitoshi.abe@kek.jp H. Abe Department of Materials Structure Science, School of High Energy Accelerator Science, SOKENDAI (The Graduate University for Advanced Studies), Tsukuba, Japan © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_30 177 178 H. Abe Fig. 30.1 Typical flow from XAFS spectrum to FT. The XAFS spectrum is processed to the EXAFS oscillation by subtracting background and converting X-axis from photon energy to wave number of photoelectron. The EXAFS oscillation is transformed to the “RDF-like” function via Fourier transforms. Curve fitting analysis is applied to the FT to obtain physical properties such as a bond length The FTs show radial distribution function (RDF) “like” functions. A flow to obtain FTs from XAFS spectrum is shown in Fig. 30.1. Since they contain phase shifts of the related elements, their apparent peak positions are not true bond lengths. Curve fitting EXAFS analyses yield actual bond length and coordination numbers and other properties. 30.2 Features • X-ray absorption coefficients l(E) are measured with scanning incident X-ray photon energy. • Bond lengths, coordination numbers, and displacements of lattices (disorders of atoms) with element specificity can be obtained. • Long-range periodicity is not required to obtain bond lengths. Amorphous and nanoparticle materials are suitable objects to be investigated, which are not often suitable for diffraction measurements. • EXAFS measurements are compatible with in situ or operando environments. 30 Extended X-Ray Absorption Fine Structure 30.3 179 Instrumentation Equipment for EXAFS measurements basically consists of an X-ray source, monochromator, and detectors for I0 and I (or If) as the same method as XANES measurements. Some others optics are optionally utilized. X-rays pass through stainless steel tubes, and they form a long line, which is called a beamline. EXAFS measurements require an intense X-ray source and X-ray beams of tunable energy. An ideal X-ray source is synchrotron radiation, which is available at synchrotron radiation facilities Traditional X-ray tubes could be used for laboratory experiments, but not really ideal. Monochromators have functions to let X-ray of a desired energy pass from the “white” X-ray of synchrotron radiation and to scan the energy. Double-crystal monochromators are commonly used in hard X-ray absorption spectroscopy. Si(111) crystals are most widely installed in beamlines. The simplest and widely used X-ray detectors are ionization chambers, where gases are filled between a pair of electrodes. Electrons or ions of the gas generated by X-ray are captured at the electrodes, and the currents are measured, as they are proportional to the intensity of X-ray flux. Cryogenic coolers are sometimes used to suppress thermal vibrations of atoms in order to obtain EXAFS oscillations of quality with higher signal-to-noise ratio (S/N). 30.4 Applications 30.4.1 Bond Length Determination as Local Structure Analysis EXAFS analyses enable us to obtain local structure information such as bond lengths and coordination numbers. Absorption spectra, EXAFS oscillations, and FTs of Cu (blue) and CuO (red) are shown in Fig. 30.2. Fourier transformation was applied to the Cu EXAFS oscillation of k range of 2.7–15.0 Å−1. The r range of 1.6–2.8 Å of the Cu FT was fitted by the ideal Cu structure. The range was set over the peak structure, which could include the interaction of interest. For the CuO analysis, it was performed for the k range of 3.1–12.8 Å−1 and the r range of 1.0–1.9 Å. Curve fitting analyses of FTs are performed with theoretical EXAFS simulations on the related paths. Here, theoretical EXAFS spectra were calculated by the FEFF6 code [3], which is the most widely used package. The EXAFS data were analyzed by using the software set of ATHENA and ARTEMIS [4]. The nearest-neighbor distances of Cu–Cu in Cu metal and of Cu––O in CuO were obtained to be 2.54 and 1.95 Å, respectively. Structural parameters of unknown samples are often analyzed by using these values as standards. 180 H. Abe Fig. 30.2 a Normalized absorption spectra b EXAFS oscillations, and c their FTs of Cu and CuO. The FTs were fitted by the ideal structures, and the fitting curves are drawn as dashed lines References 1. Lee, P.A., Citrin, P.H., Eisenberger, P., Kincaid, B.M.: Rev. Mod. Phys. 53, 769 (1981) 2. Bunker, G.: Introduction to XAFS: a practical guide to X-ray absorption fine structure spectroscopy. Cambridge University Press, Cambridge (2010) 3. Zabinsky, S.I., Rehr, J.J., Ankudinov, A., Albers, R.C., Eller, M.J.: Phys. Rev. B 52, 2995 (1995) 4. Ravel, B., Newville, M.: J. Synchrotron Radiat. 12, 537 (2005) Chapter 31 Focused Ion Beam Scanning Electron Microscope Tetsuo Sakamoto Keywords Focused ion beam Liquid metal ion source Secondary electron Scanning microscope Ion beam machining Channeling contrast 31.1 Principle Focused ion beam is rather new type of ion beam. Its prominent feature is fine focusing ability around 100–10 nm. This is mainly due to an ion source used in FIB, named liquid metal ion source (LMIS). Liquid metal (typically gallium) is retained in a reservoir and then fed to a sharpened tungsten needle. When a positive high voltage relative is applied on the needle relative to an extraction electrode, “Taylor cone” consisting of liquid metal is built at the tip of the needle. Surface Coulomb force toward the outside of the cone exceeds surface tension, and atoms of the liquid metal are emitted as positive ions. The virtual source size of LMIS is said to be around 50 nm; therefore, it acts as very small source with a high brightness compared with other ion sources such as conventional gas plasma (Ar+, O2+, etc.) type. Figure 31.1 shows an FIB-induced secondary electron image of gold particles at a field of view of 10 µm. Consequently, FIB-SEM partly imitates electron beam-induced SEM (normally called “SEM”). 31.2 Features • High lateral resolution observation is realized around 100–10 nm. • Information depth of the image is shallow. In other words, it is surface sensitive. T. Sakamoto (&) Department of Applied Physics, School of Advanced Engineering, Kogakuin University, Tokyo, Japan e-mail: ct13087@ns.kogakuin.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_31 181 182 T. Sakamoto Fig. 31.1 FIB-induced secondary electron image of gold particles with a 10-µm field of view • Not only surface observation like a SEM, but also micro-machining can also be performed. Therefore, the interior of small solid sample is viewed with FIB-SEM. • Channeling contrast is strong compared with EB-SEM. This is useful for the observation of grain contrast of polycrystalline surfaces. • If a sampling probe is installed, thin film sectioning and sampling can be performed within the apparatus. This is useful for TEM sample preparation. • FIB-assisted deposition of tungsten or carbon is used for in situ circuit repair in an IC test chip. 31.3 Instrumentation Figure 31.2 illustrates a basic configuration of FIB-SEM apparatus. Basically, its layout is identical to that of EB-SEM except that an FIB column is used instead of an EB column. Vacuum is required like as EB-SEM at a range of 10−4 Pa or less. Measures to avoid vibration, type of vacuum pumps, air suspension between the floor and the surface table should be taken into account in the design. Apparatus shown in Fig. 31.2 seems to be uncommon if one needs only FIB-SEM function. For practical use, the integration of EB-SEM shown in Fig. 31.2 is useful, because the sample surface is consumed by the sputtering while FIB-SEM observation. Electron and ion dual-beam system is a solution for non-destructive observation using EB-SEM and micro-machining with FIB. 31 Focused Ion Beam Scanning Electron Microscope 183 Electron Beam Column Ga-FIB Column Secondary Electron Detector Sample Stage (x-y-z, tilt, rotation) Pump Fig. 31.2 Basic configuration of an FIB-SEM apparatus combined with EB-SEM simultaneous observation [1] 31.4 Applications 31.4.1 Difference in EB-SEM and FIB-SEM Images Both EB-SEM and FIB-SEM give us magnified image on solid surfaces. However, there is difference in the nature of images. One is surface sensitivity. Figure 31.3 shows secondary electron images taken with EB-SEM and FIB-SEM on the same field of view. In case of EB-SEM, the image features the shape, e.g., the edge. On the other hand, the small surface roughness also observed. This difference can be explained by that in escape depth of secondary electrons. Namely, in EB-SEM, primary electron penetrates deeper, and therefore, secondary electrons are escaped from deeper region. In other words, EB-SEM image is an averaged information between the topmost surface and beneath the surface. In contrast, FIB-SEM is much more surface sensitive. Primary ions collide with surface atoms, and they cannot 184 T. Sakamoto Fig. 31.3 Secondary electron images of an indium particle on an indium plate with a view width of 50 µm. (Left EB-SEM at 5 keV acceleration energy, Right FIB-SEM image at 30 keV) penetrate deeper region compared with primary electrons. Therefore, FIB-SEM image is mainly composed of secondary electrons emitted from vicinity of the surface. Recent years, low-energy primary electron gun has been developed (1 keVor less). Surface-sensitive SEM image can be acquired with also EB-SEM. 31.4.2 Micro-machining Micro-machining is a unique capability of FIB-SEM. Focused ion beam has a small beam spot (100–10 nm in diameter) and high current density greater than 10 A/cm2. This enables us to use an FIB as precise machining tool. Furthermore, combination of FIB-SEM and machining is useful for “machining at aimed position.” An example of micro-machining of a spherical particle is shown in Fig. 31.4. These micrographs were taken with integrated EB-SEM. If one needs to view the interior of a particle, it is easy by using FIB machining from the edge of the particle to just the half of it. The time required for machining depends on the volume to be processed. A particle with a few micrometers typically needs 10 min for sectioning. 31 Focused Ion Beam Scanning Electron Microscope 185 2.5 um Fig. 31.4 EB-SEM images before FIB cross-sectioning of a small particle and after the sectioning [2] 31.4.3 Grain Observation Using Channeling Contrast The last application of FIB-SEM is related to grain observation. Grain size and orientation play an important role in steel or ceramics sciences. EB-SEM can also view grains, but in FIB-SEM strong contrast depending on grain orientation is observed. Grain contrast is generated by “channeling effect” where ion beam incident angle is aligned parallel to the crystal axis or not. If the incident angle of the FIB is 45° from the surface normal, grain contrast changes while rotating the sample stage, because each grain has different crystal orientation. Figure 31.5 shows an example of grain contrast images while varying rotation angle, e.g., angle of incident with respect to each grain. If the brightness of a certain grain is plotted as a function of rotation angle, one can understand its crystal orientation. 186 T. Sakamoto Fig. 31.5 FIB-SEM images (field width = 100 µm) of indium surface at various azimuth angles. Red arrows indicate the orientation in the first image (top left) References 1. Sakamoto, T., Koizumi, M., Kawasaki, J., Yamaguchi, J.: Development of a high lateral resolution TOF-SIMS apparatus for single particle analysis. Appl. Surf. Sci. 255, 1617–1620 (2008) 2. Sakamoto, T., Morita, M., Takami, A., Hatakeyama, S., Murano, K., Ogawa, H., Ueda, K,: Individual analysis of aerosol particles using a high-resolution TOF-SIMS, presented at ASSAAQ13 (Atmospheric Sciences and Applications to Air Quality) Chapter 32 Force Curve Akinori Kogure Keywords Young’s modulus 32.1 JKR Hertz Adhesion force Principle The basis of the force curve measurement is the measurement performed at one point of the sample. As the distance of the probe changes relative to the sample, this distance can be plotted on the horizontal axis, as shown on the graph. Also, it is possible to calculate from the spring constant of the cantilever and plot this on the vertical axis as nN. When the probe and sample distance are far away, the force does not work; hence, the vertical axis is (a). When the cantilever touches the sample, it is (b). After that, the slope of the graph when the repulsive force acts reflects the hardness of the sample shown as (c). When a release curve is observed often a large attractive area can be seen. This is because the probe is caught by the adsorption layer on the sample surface shown as (d). From the approach curve and release curve, Young’s modulus can be calculated using JKR or Hertz. Therefore, by saving the data at each pixel, a mapping image can be constructed (Fig. 32.1). 32.2 Features • Measurement of local characteristics in nanometer size from pN to uN. • Young’s modulus calculation is possible. • Measurements can be obtained in the air, liquids and vacuums. A. Kogure (&) AMC Department Testing and Analysis Division, Shimadzu Techno-Research INC, Kanagawa, Japan e-mail: a_kogure00@shimadzu-techno.co.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_32 187 188 A. Kogure Fig. 32.1 Principle of force curve • It is possible to measure thin films of 100 nm or less. • Possible to acquire XY analysis images like height images, suction images and Young’s modulus (3D volume data). 32.3 Instrumentation The principle of the equipment is the same as an atmosphere—AFM, and it is composed of the following three: laser diode and photo detector; cantilever and holder; and scanner (Fig. 32.2). The AFM uses a cantilever with a needle-shaped tip to detect force. The tiny forces acting between the needle on the tip of the cantilever and the sample (atomic forces) cause the cantilever to vary in how it bends and vibrates. These variations are detected with high sensitivity using laser light reflected off the back side of the 32 Force Curve 189 cantilever. At the same time, either the cantilever or sample is precisely scanned and controlled three dimensionally by a scanner with a piezoelectric element. In general, the distance from the sample (Z-height) is fed back and controlled so that either the amount of bending (contact mode) or the amount of vibration is kept constant (dynamic mode), as the cantilever is scanning the sample surface (XY-plane) [1]. 32.3.1 Cantilever and Spring Constant The choice of the cantilever is important. In general, for soft samples such as biomaterials, the cantilever–spring constant should be around 0.1 N/m, for soft polymers 1 N/m and for hard polymers from 2 to 20 N/m. A colloidal probe may also be used to increase the contact area with the sample. 32.4 Applications 32.4.1 Measurement of HeLa Cells in Liquid Physical properties of cells are deeply related to function and cell morphology (Fig. 32.3). Observations of cells by SEM or TEM are common. However, there are few reports on the measurement of cells physical properties. By applying the force curve measurement to a cell, it is possible to get such information. The image on the right is a measurement of the physical properties of HeLa cells in liquid. Therefore, it is Fig. 32.2 Basic configuration of the scanning probe microscope 190 A. Kogure possible to measure the hardness of living cells. The force curve (1) shows glass and (2) and (3) indicate the force curve on the cells. (Data provided by Dr. Hirotaka James, Okano, Laboratory of Regenerative Medicine, The Jikei University School of Medicine). 32.4.2 Bacterial Spore Hardness Relationship between heat resistance and hardness of spores has been clarified by force curve measurement (Fig. 32.4). The figure shows the correlation between hardness, heat and UV tolerance. It has been shown that there is a positive correlation between hardness and tolerance. Force curve measurement is effective even for samples of 0.5–1 um size such as spores [2]. Fig. 32.3 HeLa cell, Young’s modulus and force curve 32 Force Curve 191 B.Subtilis Fig. 32.4 AFM image of spores. Correlation between the hardness of spores and their tolerance to heat or UV References 1. http://www.shimadzu.com/an/surface/spm/faq/index.html 2. Nakanishi, K., Kogure, A., Fujii, T., Kokawa, R., Deuchi, K.: Development of method for evaluating cell hardness and correlation between bacterial spore hardness and durability. J. Nanobiotechnol. 10(1), 22 (2012) Chapter 33 Force Spectroscopy Christina Puckert and Michael J. Higgins Keywords Atomic force microscopy 33.1 Force-distance curve Elastic property Principle Using atomic force microscopy (AFM) in force spectroscopy (FS) mode is a powerful tool to study the fundamental surface, physicochemical and biological forces between an AFM tip and a surface, molecule or living cell of interest. FS enables the detection of a wide range of forces from 10 pN up to several µN, while the use of an AFM tip with radius of 5–50 nm makes its accessible to probe single molecules. In contrast to AFM imaging, where the cantilever tip scans across the surface in the XY direction, FS measurements involve movement of the cantilever tip in the Z direction at a single XY position on the sample. Figure 33.1 shows the principle of FS and the evolution of a corresponding force-distance (F-D) curve that is obtained as the AFM tip interacts with the sample surface. The cantilever tip firstly approaches the sample, and no deflection of the cantilever is recorded because the tip does not interact with the surface (Fig. 33.1a, (i)). Further upon approach (red curve), the tip can detect fundamental forces such as electrostatic or van der Waals forces, causing deflection of the cantilever, prior to making contact with the sample surface (Fig. 33.1a, (ii)). As the tip continues to push into the surface, there is an upward deflection of the cantilever (Fig. 33.1a, (iii)) and this C. Puckert M.J. Higgins (&) ARC Centre of Excellence for Electromaterials Science, Intelligent Polymer Research Institute/AIIM Facility, Innovation Campus Squires Way, University of Wollongong, Wollongong, NSW 2500, Australia e-mail: mhiggins@uow.edu.au © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_33 193 194 C. Puckert and M.J. Higgins region of the curve provides information on the mechanical properties of the sample. Once the cantilever deflection has reached a pre-defined set point, the cantilever tip is retracted from the surface (blue curve) and an opposite downward deflection of the cantilever is recorded. If adhesion occurs between the tip and sample, the cantilever deflection will continue to decrease below the zero baseline (Fig. 33.1a, (iv)) until the tip “pulls off” from the surface (Fig. 33.1a, (v)), giving a quantitative measure of the adhesion force. The deflection then returns to the zero baseline, and the cantilever is retracted from the surface (Fig. 33.1a, (vi)). Figure 33.1 provides the basis of an F-D curve though applying FS for measuring single molecule and single live cell interactions has now become commonplace. Single-molecule force spectroscopy uses a ligand-functionalized tip via a flexible cross-linker such as PEG (Fig. 33.1b, (i)) that is brought into contact with a functionalized surface consisting of complementary receptors (Fig. 33.1b, (ii)). Upon retraction of the tip, the flexible cross-linker undergoes extension, causing a nonlinear downward deflection (Fig. 33.1b, (iii)), until unbinding of the ligand receptor proceeds (Fig. 33.1b, (iv)), and the cantilever deflects back to the zero baseline. For single-cell force spectroscopy, a living single cell such as a bacterium or mammalian cell is attached to the AFM cantilever (Fig. 33.1c, (i)), typically tipless, and similarly brought into contact with a functionalized surface. As the cell pushes into the surface, there is a nonlinear increase in the cantilever deflection associated with elastic properties and compression of the cell surface (Fig. 33.1c, (ii)). Upon retraction, there is a significant decrease in the deflection signal as the bulk of the cell detaches from the surface due to adhesion (Fig. 33.1c, (iii)), followed by smaller peaks and plateau forces (Fig. 33.1b, (iv)) that correspond to specific molecular-level interactions on the order of piconewton forces. Importantly, both approaches require the acquisition and analysis of > several hundred F-D curves to produce valid statistics. A critical aspect of FS is quantifying the forces by converting the cantilever deflection measured in volts by the photodiode into a force typically in nanonewtons (Fig. 33.1d). Firstly, a measurement of the sensitivity is required and achieved by performing an F-D curve on an infinitely hard surface, e.g., glass slide. In this case, given that the amount of cantilever deflection in volts is proportional to the distance travelled by the z-piezo (i.e., height of cantilever tip) in nanometers, then the inverse of the slope in the contact region is calculated as the sensitivity in nm/V and can be used to convert the deflection of the cantilever into units of a distance (Fig. 33.1d, middle). The sensitivity value is dependent on the type of cantilever, position of the cantilever in the holder, and laser position on the cantilever; thus, it must be measured before each experiment. To convert the cantilever deflection in nanometers into a force, the spring constant of the cantilever must be known (Fig. 33.1d, bottom) and can be obtained using a variety of spring constant 33 Force Spectroscopy 195 Fig. 33.1 a Schematic of a F-D curve during a basic FS experiment. b Schematic of a F-D curve during a single-molecule FS experiment when a molecule is attached to the tip. c Schematic of a F-D curve during a single-cell FS experiment with a single live cell as probe to quantify ligand-receptor binding. d Flowchart of converting the deflection signal (V) of the cantilever into force (nN) calibration methods. Two methods commonly implemented on commercial AFM systems include the thermal method [1] and Sader method [2], and it is recommended that more than one method is used to verify the spring constant value. Once the spring constant k is known, the force F can be calculated using simple Hooke’s 196 C. Puckert and M.J. Higgins law, F = k * d, where d is the deflection of the cantilever. Thus, the data can be converted into a true F-D curve (Fig. 33.1d, bottom). 33.2 Features • Measures forces in the range of 10–104 pN. • Measurements can be done in different media, including air and liquid. • Chemical functionalization of the AFM tip enables measurement of forces between a wide range of functional groups and molecules. • Biomolecular forces can be measured in physiological relevant conditions. 33.3 Instrumentation FS is implemented using standard AFM systems that typically operate using the setup shown in Fig. 33.2a. It employs an optical lever detection system whereby a laser is focused on the back of the cantilever and then reflected into a photodiode. Changes in the deflection of the cantilever are recorded by positional changes of the laser in the photodiode, which are given as a voltage signal. These changes in the cantilever deflection, due to the interaction forces, are recorded as the z-piezo adjusts the position of the cantilever tip relative to the sample surface during the FS measurement. Plotting the cantilever deflection versus the z-piezo distance gives the F-D curve. A key component of the instrumentation is the AFM cantilever tip that is fabricated from a silicon wafer and often made of monocrystalline silicon and Si3N4. Nowadays, there are many types of cantilevers (Fig. 33.2b, c) and their important features include (1) the dimensions of the cantilever that determine the spring constant and resonance frequency, (2) the tip radius and aspect ratio that influences the contact area and number of interacting molecules, and (3) tip coatings such as thin layers of gold and aluminum that improve the sensitivity value. In particular, silicon nitride tips are commonly used as they are more wear resistant and less prone to breaking compared to pure silicon, while gold layers can be applied to the cantilever tip to allow for easier functionalization such as using gold-thiol chemistry. A major development in FS over the past decade is chemical functionalization of the tip, leading to the attachment of specific functional groups, ligands, proteins, and living cells [3]. Various strategies for surface modification of the tips include the use of self-assemble monolayers, covalent coupling via 1-ethyl-3-(3dimethylaminopropyl) carbodiimide (EDC) and N-hydroxysuccinimide (NHS), biotinylated BSA, histidine tags, and polyethylene glycol (PEG) linkers (Fig. 33.2d) [3]. 33 Force Spectroscopy 197 Fig. 33.2 a Basic illustration of AFM principle, showing the laser deflecting the cantilever movement to the photodiode (Image reprinted from http://www.witec.de). b Range of cantilevers sizes in triangular and rectangular shape. c. SEM image of a tip on a cantilever (Images B-D reprinted from http://www.brukerafmprobes.com). Schematics of modified tips with biotinylated BSA and streptavidin, NTA-terminated alkanethiols, and PEG cross-linker for single-molecule FS studies. (Reprinted with permission from [3] Copyright (2006) American Chemical Society) Such coupling chemistries provide an approach for detecting chemical surface groups via binding forces and can be a very powerful tool for chemical and nanoscale spatial recognition at the level of single molecules. 33.4 Applications There are a significant number of examples of applications involving FS and too many to cover in this short chapter. Here, we briefly describe a few studies involving DNA interactions and living cells to highlight recent and interesting advances in FS. For a more comprehensive review of force spectroscopy, please see reviews [4, 5]. 198 C. Puckert and M.J. Higgins 33.4.1 Machine Learning for Analysis of Force Spectroscopy Data Single-molecule force spectroscopy experiments require acquisition and analysis of thousands of force curves to produce reliable statistics, particularly considering that a significantly smaller number of specific interactions are detected among a background of non-specific interactions. Thus, significant effort is required to sort and filter the data to select the force curves of interest. While specialized molecules can be used to imprint a characteristic “fingerprint” of the interaction (e.g., force tag) [6, 7], there is still manual labor involved in classifying the force curves. Therefore, Karatay et al. [7] have demonstrated an interesting approach by using machine learning algorithms to improve throughput and accuracy for analyzing the large data sets in FS experiments. To implement this, the AFM tip was functionalized with an azobenzene-modified DNA sequence that shows a difference in the binding force before or after exposure to UV light. This effect was designed for benchmarking the classification of the different force curves and assessing the efficacy of the machine learning. Figure 33.3a shows the functionalization of the AFM tips and configuration for the binding scheme, with a typical characteristic force curve and unbinding event shown in Fig. 33.3b. Prior to exposure with UV light, the azobenzene stabilizes binding between DNA sequences; however, this mechanism is perturbed upon UV exposure, thus reducing binding forces (Fig. 33.3c). For the Fig. 33.3 a Illustration of a gold-coated Si3N4 tip functionalized with DNA. b Schematic diagram of a F-D curve during DNA-SMFS experiment. c Histograms of unbinding forces showing significant reduction in binding strength after UV exposure. (Reprinted with permission from [7]. Copyright (2016) American Chemical Society). d F-D curve of a SCFS experiment. e Detachment forces of single Jurkat cells untreated and treated with a TNF are compared with different contact times represented as Box-Whisker plots. [Reproduced from (9). Copyright (2016, Nature Publishing Group)] 33 Force Spectroscopy 199 analysis, supervised and unsupervised machine learning algorithms showed very promising levels of accuracy rates, 95 and 80%, respectively, compared to those obtained by human researchers from the same laboratory. It is anticipated that further developments will drive automation and greatly speed up FS experiments. 33.4.2 Single-Cell Force Spectroscopy to Quantify Ligand-Receptor Binding In single-cell force spectroscopy (SCFS), a single live cell is attached to a tipless cantilever and brought into contact with a substrate to quantify binding between live cell receptors and recognition proteins adsorbed on a substrate [8]. The first step involves picking up a single cell with the cantilever, a process that has previously been facilitated by functionalization of the cantilever with the lectin-binding concanavalin-A and allowing the cell to establish adhesion to the cantilever for >5 min [8]. A typical F-D curve for a live cell interacting with a protein-functionalized surface is shown in Fig. 33.3d. As mentioned above, the curve consists of an initial large “pull-off” that reflects the single cell adhesion followed by interactions described by jumps and tethers (Fig. 33.3d) [9]. Jump interactions involve binding of receptors that have association with the internal cytoskeleton, while tethers oppositely involve those receptors with weak association, leading to the subsequent formation of a membrane tethers that can extend out from the cell for several microns [8]. The entire energy of adhesion can be given as the area under the curve (striped area) (Fig. 33.3d), and interesting SCFS parameters such as the contact time and speed can be modified to probe deeper into the mechanisms of cell adhesion. Importantly, blocking experiments whereby antibodies are introduced during the force curves are performed to confirm specific interactions and identify which receptors govern the cell adhesion. For example, Fig. 33.3e shows the contribution of untreated and treated Jurkat cells with tumor necrosis factor (TNF), a pro-inflammatory cytokine which mediates leukocyte adhesion by upregulating integrin ligands, contributing to the adhesion of a fibronectin-coated surface [9]. References 1. Butt, H.J., Jaschke, M.: Calculation of thermal noise in atomic force microscopy. Nanotechnology 6(1), 1 (1995) 2. Sader, J.E., Chon, J.W., Mulvaney, P.: Calibration of rectangular atomic force microscope cantilevers. Rev. Sci. Instrum. 70(10), 3967–3969 (1999) 3. Hinterdorfer, P., Dufrêne, Y.F.: Detection and localization of single molecular recognition events using atomic force microscopy. Nat. Methods 3(5), 347–355 (2006) 4. Hugel, T., Seitz, M.: The study of molecular interactions by AFM force spectroscopy. Macromol. Rapid Commun. 22(13), 989–1016 (2001) 200 C. Puckert and M.J. Higgins 5. Kienberger, F., Ebner, A., Gruber, H.J., Hinterdorfer, P.: Molecular recognition imaging and force spectroscopy of single biomolecules. Acc. Chem. Res. 39(1), 29–36 (2006) 6. Friddle, R.W., Noy, A., De Yoreo, J.J.: Interpreting the widespread nonlinear force spectra of intermolecular bonds. Proc. Natl. Acad. Sci. U.S.A. 109(34), 13573–13578 (2012) 7. Karatay, D.U., Zhang, J., Harrison, J.S., Ginger, D.S.: Classifying force spectroscopy of DNA pulling measurements using supervised and unsupervised machine learning methods. J. Chem. Inf. Model. 56(4), 621–629 (2016) 8. Friedrichs, J., Helenius, J., Muller, D.J.: Quantifying cellular adhesion to extracellular matrix components by single-cell force spectroscopy. Nat. Protoc. 5(7), 1353–1361 (2010) 9. Li, Q., Huth, Ss, Adam, D., Selhuber-Unkel, C.: Reinforecement of integrin-mediated T-Lymphocyte adhesion by TNF-induced inside-out Signaling. Sci. Rep. 6, 30452 (2016) Chapter 34 Frequency-Modulation Atomic Force Microscopy Masayuki Abe Abstract Frequency-Modulation atomic force microscopy (FM-AFM), one of the AFM modes, can image surface single atoms and molecules. Different from scanning tunneling microscopy (STM) that has also atomic resolution, FM-AFM can be operated on the insulator surface. Force spectroscopy using FM-AFM measures chemical bonding force between atoms of the AFM tip and sample surface. Keywords Force 34.1 Cantilever FM Atomic resolution Principle Frequency-modulation atomic force microscopy (FM-AFM) [1–3], also known as noncontact atomic force microscopy (NC-AFM), is an operational mode for AFM. As shown in Fig. 34.1, a cantilever with spring constant k is excited at its resonance frequency fr with constant amplitude A. The interaction force F acting between the atoms at the AFM tip apex and the atoms at the sample surface results in a shift in fr called the frequency shift Df . In FM-AFM, Df is measured and used to control the tip-sample distance Z. The relationship between F and Df can be expressed as [4] Z 1 fr u F ðZ Þ pffiffiffiffiffiffiffiffiffiffiffiffiffi du: Df ¼ p kA 1 1 u2 When A is small, the above expression can be simplified to Df ¼ fr dF ðZ Þ : 2k dZ M. Abe (&) Graduate School of Engineering Science, Center for Science and Technology Under Extreme Conditions, Osaka University, Osaka, Japan e-mail: abe@stec.es.osaka-u.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_34 201 202 M. Abe Fig. 34.1 Principle of frequency-modulation atomic force microscopy (FM-AFM). In FM-AFM, the shift in the cantilever resonance frequency (D f ) induced by the interaction force F is measured Although the relationship between F and Df is not simple, one can roughly say that an attractive (repulsive) force acts on the AFM tip if Df is negative (positive). 34.2 • • • • Features Atomic resolution imaging even on insulator surfaces and in liquid Various image contrasts depending on the species of on-top tip atom species Imaging molecular structure and intermolecular interaction Force distance curve (force spectroscopy). 34.3 Instrumentation Figure 34.2 shows a schematic illustration of a typical FM-AFM experimental setup. The deflection sensor detects cantilever movement; optical beam deflection is a commonly used deflection sensing method [5]. Recently, a method using quartz tuning forks has also been used due to its good frequency stability with time and changes in temperature [6]. The amplitude controller keeps the amplitude of the cantilever at a constant value and outputs the excitation signal to the piezoelectric material attached near the AFM cantilever. Df is measured by the FM demodulator and used to control the tip-sample distance Z. Topographic images are acquired by scanning the sample in the X-Y (horizontal) plane and recording the output of the Z feedback electronics. 34 Frequency-Modulation Atomic Force Microscopy Fig. 34.2 Experimental setup for FM-AFM 203 deflection sensor FM demodulator piezoelectric material AFM cantilever amplitude controller sample Z scanner X, Y 34.4 Z feedback Z X scan signal Y scan signal Applications 34.4.1 Atomic Resolution Imaging One of the remarkable features of the FM-AFM is atomic resolution imaging, which has been difficult to achieve using other AFM operational modes. Figure 34.3 shows FM-AFM images of (a) semiconductor (Si(100)), (b) insulator (KCl(100)), and (c) metal (Pb/Si(111)) surfaces obtained in an ultrahigh vacuum environment, respectively. Since FM-AFM measures the atomic force, the pair of atomic species at the AFM tip apex and at the sample surface is important. To obtain each image, the AFM tip apex is placed intentionally close to the surface to pickup surface atoms. Recently, at low temperature, individual molecular structure [7] and intermolecular bonds [8] and by controlling tip apex atoms and/or molecules were successfully obtained. Moreover, biological specimens such as lipid bilayers [9] have also been imaged in liquid environments. (a) Si(100)-(2x1) (b) KCl(100) (c) Pb/Si(111) Fig. 34.3 Atomic resolution images of FM-AFM operated in an ultrahigh vacuum environment at room temperature 204 M. Abe 34.4.2 Force Spectroscopy The Z-dependence of F ðZ Þ can be measured by scanning either the probe or the sample in the Z direction. This measurement is called force spectroscopy [10] and is carried out by measuring the Z-dependence of Df . There are algorithms for converting plots of Df versus Z into plots of F versus Z. By integrating F with Z, one can also obtain curves of the potential and lateral forces versus Z [11]. In order to obtain precise force spectroscopic data, the positioning of the tip relative to the sample is critical. Low temperature condition is preferable because of low thermal drift. Even at room temperature that has larger effect of the thermal drift, atom-tracking technique enables us to perform the force spectroscopy [12, 13]. References 1. Morita, S., Wiesendanger, R., Meyer, E. (eds.): Noncontact atomic force microscopy. Springer-Verlag (2002) 2. Morita, S., Giessibl, F.J., Wiesendanger, R.(eds.): Noncontact atomic force microscopy, vol. 2, Springer-Verlag (2009) 3. Morita, S. Giessibl, F.J., Meyer, E., Wiesendanger, R.(eds.): Noncontact atomic force microscopy, vol. 3, Springer-Verlag (2015) 4. Giessibl, F.J.: Forces and frequency shifts in atomic-resolution dynamic-force microscopy. Phys. Rev. B 56, 16010–16015 (1997) 5. Fukuma, T., Kimura, M., Kobayashi, K., Matsushige, K., Yamada, H.: Development of low noise cantilever deflection sensor for multienvironment frequency-modulation atomic force microscopy. Rev. Sci. Ins. 76, 053704-1/-8 (2005) 6. Giessibl, F.J.: Advances in atomic force microscopy. Rev. Mod. Phys. 75, 949–983 (2003) 7. Gross, L., Mohn, F., Moll, N., Liljeroth, P., Meyer, G.: The chemical structure of a molecule resolved by atomic force. Microscopy 325, 1110–1114 (2009) 8. Hämäläinen, S.K., Heijden, N., Lit, J., Hartog, S., Liljeroth, P., Swart, I.: Intermolecular contrast in atomic force microscopy images without intermolecular bonds. Phys. Rev.lett. 113 (18), 186102-1/-4 (2014) 9. Asakawa, H., Yoshioka, S., Nishimura, K., Fukuma, T.: Spatial distribution of lipid headgroups and water molecules at membrane/water interfaces visualized by three-dimensional scanning force. Microscopy 6, 9013–9020 (2012) 10. Lantz, M.A., Hug, H.J., Hoffmann, R., Schendel, P.J.A., Kappenberger, P., Martin, S., Baratoff, A., Güntherodt, H.-J.: Quantitative measurement of short-range chemical bonding forces. Science 291, 2580–2583 (2001) 11. Sugimoto, Y., Namikawa, T., Miki, K., Abe, M., Morita, S.: Vertical and lateral force mapping on the Si (111)−(77) surface by dynamic force microscopy. Phys. Rev. B 77(19), 195424-1/-9 (2008) 12. Abe, M., Sugimoto, Y., Custance, O., Morita, S.: Room-temperature reproducible spatial force spectroscopy using atom-tracking technique. App. Phys. Lett. 87(17), 173503-1/-9 (2005) 13. Abe, M., Sugimoto, Y., Namikawa, T., Morita, K., Oyabu, N., Morita, S.: Drift-compensated data acquisition performed at room temperature with frequency modulation atomic force microscopy. Appl. Phys. Lett. 90(20), 203103-1/-3 (2007) Chapter 35 Gap-Mode Raman Spectroscopy Katsuyoshi Ikeda Keywords Vibrational spectroscopy Gap-mode plasmon excitation Well-defined surface Metal nanoparticle 35.1 Principle Raman signal intensity can be enhanced near a metal surface through excitation of surface plasmon polaritons. This surface-enhanced Raman scattering (SERS) is widely utilized as a surface-sensitive spectroscopic method on a metal surface. According to the electromagnetic theory, significant signal enhancement in SERS is expected at a sharp edge of metal nanostructures or in a small gap between dimeric nanostructures, where electromagnetic fields are substantially localized. One of the important issues in SERS is, therefore, how to create such “hotspots” in a controlled manner. In the traditional SERS, electrochemically roughened metal surfaces have been frequently utilized as a SERS-active substrate. Owing to the recent advancements in nanofabrication, well-shaped nanodimers are now often utilized as SERS-hotspots. However, atomic surface morphology of these SERS-active structures is hardly controlled, which causes various problems in research fields of surface science. To investigate relation between atomic local structures and surface functionality such as catalytic activities, it is important to extend the application of SERS to atomically defined metal surfaces. Although direct excitation of surface plasmons is not allowed at a planar surface with atomically defined atomic arrangements, proximity of a metal nanoparticle (NP) to the surface can induce SERS activity through electromagnetic coupling between localized plasmons on the NP and surface plasmons on the substrate, as shown in Fig. 35.1 [1]. Since the excitation of these “gap-mode” plasmons is accompanied by significant confinement of electromagnetic fields into the nanogap, SERS signals can be K. Ikeda (&) Department of Physical Science and Engineering, Nagoya Institute of Technology, Nagoya 466-8555, Japan e-mail: kikeda@nitech.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_35 205 206 K. Ikeda Fig. 35.1 Schematic illustration of a metal nanosphere above a planar metal surface and energy diagram of electromagnetic coupling between particle plasmons and surface plasmons. The bonding plasmon corresponds to optically allowed gap-mode plasmon, which can induce SERS effect on the planar surface. Reprinted with the permission from Ref. [1]. Copyright 2009 American Chemical Society selectively obtained in between the substrate surface and NP. Significant advantages of this method are high reproducibility of SERS spectra and high sensitivity even on highly damping metal surfaces such as platinum group metals. 35.2 • • • • • Features Atomically defined metal surfaces can be utilized for SERS. SERS effect is obtained even on highly damping transition metal surfaces. SERS enhancement is highly reproducible and controllable. Surface selection rules are same as those for conventional SERS. The use of metal nanoparticles is required to measure SERS. 35.3 Instrumentation SERS measurement using sphere–plane-type-gap-mode plasmons can be conducted using a conventional Raman spectrometer consisting of an excitation laser, optical filters, and a CCD spectrometer. The resonance band of gap-mode plasmons must overlap with the excitation wavelength to obtain SERS effect. This can be tuned by changing the size of NPs and the gap distance. Generally, two different approaches are considered to fabricate the sphere–plane-type-nanogap structures on a metal surface. Silica-coated NPs may be directly deposited on a planar metal surface, which is called shell-isolated nanoparticle-enhanced Raman spectroscopy 35 Gap-Mode Raman Spectroscopy 207 (SHINERS) [2]. Alternatively, bare NPs may be physisorbed on top of molecular monolayers formed on a metal surface, which is particularly useful for studying metal–molecule interfaces [1]. The advantage of the latter is that the gap distance is better controlled by the monolayer thickness and aggregation of NPs can be avoided, resulting in creation of well-controlled SERS-hotspots on an atomically defined surface. 35.4 Applications 35.4.1 SERS Measurement on Single Crystalline Metal Surfaces Conventional SERS is measured on a nanostructured metal surface with undefined atomic surface arrangements. This is a major disadvantage in the surface-specific spectroscopy because atomic morphology of a metal surface significantly influences metal−molecule interactions. On the other hand, gap-mode SERS can apply to atomically defined surfaces. Figure 35.2 shows an example of gap-mode SERS spectra of molecular adsorbates with isocyanide anchor group, measured on Pt Fig. 35.2 Gap-mode SERS spectra of 4-chlorophenylisocyanide monolayers measured on Pt single crystal surfaces of various orientations Three arrows indicate stretching vibration modes of isocyanide anchor group with different geometries. Reprinted with the permission from Ref. [3]. Copyright 2014 American Chemical Society 208 K. Ikeda single crystal surfaces of various orientations [3]. In this case, there are three possible adsorption geometries at atop, bridge, and hollow sites. The stretching vibration peaks of the anchor in the spectra clearly show that the atop configuration is dominant on (100), (110), and (211). The (100) surface also shows the bridge adsorption. For the (111), the dominant adsorption is found at the hollow site. These differences in the dominant adsorption geometry can be explained by the contribution of surface atomic arrangements. For the atop adsorption, slight peak shift of the Raman band is also seen in the spectra, which is related to the difference of the surface electronic structure, which leads to different strengths of r-donation and p*back donation at the interface. 35.4.2 Local Observation of Atomic Surface Sites SERS can detect the small number of molecules due to its extraordinarily large enhancement factor. Moreover, the signal enhancement locally occurs in the hotspot. Therefore, if SERS-hotspots are selectively formed on specific local surface sites, nanoscale local information on molecule–metal interactions can be obtained using SERS. Figure 35.3 shows an example of such observation of atomic local surface sites using gap-mode SERS [3]. In this measurement, Pd clusters with Fig. 35.3 Gap-mode SERS observation of 4-chlorophenylisocyanide molecules adsorbed on two-dimensional Pd clusters formed on Au (111) using site-selective SERS-hotspot formation. Reprinted with the permission from Ref. [4]. Copyright 2015 American Chemical Society 35 Gap-Mode Raman Spectroscopy 209 monoatomic thickness were formed on defect sites of Au (111) using the underpotential deposition method. Since the isocyanide anchor is more strongly adsorbed on Pd than Au, the sphere–plane-type-SERS-hotspots can be selectively formed on Pd sites. When the size of Pd clusters is changed, the dominant adsorption geometry of the molecules is altered from atop to bridge as shown in the spectra. Moreover, the hollow configuration is found only at the very limited condition of Pd sizes. This result indicates that the molecular adsorption is indeed sensitive to the local surface structure at the atomic scale. References 1. Ikeda, K., Sato, J., Fujimoto, N., Hayazawa, N., Kawata, S., Uosaki, K.: Plasmonic enhancement of Raman scattering on non-SERS-active platinum substrates. J. Phys. Chem. C 113, 11816–11821 (2009) 2. Li, J.F., Huang, Y.F., Ding, Y., Yang, Z.L., Li, S.B., Zhou, Z.S., Fan, F.R., Zhang, W., Zhou, Z.Y., Wu, D.Y., Ren, B., Wang, Z.L., Tian, Z.Q.: Shell-isolated nanoparticle-enhanced Raman spectroscopy. Nature 464, 392–395 (2010) 3. Hu, J., Tanabe, M., Sato, J., Uosaki, K., Ikeda, K.: Effects of atomic geometry and electronic structure of platinum surfaces on molecular adsorbates studied by gap-mode SERS. J. Am. Chem. Soc. 136, 10299–10307 (2014) 4. Hu, J., Hoshi, N., Uosaki, K., Ikeda, K.: Vibrational spectroscopic observation of atomic-scale local surface sites using site-selective signal enhancement. Nano Lett. 15, 7982–7986 (2015) Chapter 36 Glow Discharge Mass Spectrometry Takashi Saka Keywords Mass spectrometry Glow discharge Ionization Wide dynamic range Quantitative analysis Relative sensitivity factor Metal Trace element 36.1 Principle Glow discharge mass spectrometry (GDMS) is a very sensitive technique to analyze elements in solid samples using glow discharge plasma. Samples serve as the cathode and discharge cells serve as the anode. A glow discharge can easily be obtained by applying a direct potential to a sample surrounded by an inert gas, which acts as the plasma support gas. The sample surface is irradiated with ions of the plasma support gas accelerated by an electric field in the cathode dark region, and neutral atoms are sputtered. These atoms diffuse into the plasma and are ionized mainly by the electron impact process ðM þ e ! M þ þ 2e Þ and Penning ionization process ðAr þ M ! M þ þ Ar þ e Þ, as shown in Fig. 36.1. Then, these ions are attracted to the anode and transported through a defining slit to a mass analyzer. Ions with only a given mass-to-charge ratio are transmitted through the mass analyzer, and the ion beam intensity is measured. The ion beam intensity of an analyzed element is normalized by that of the matrix element (or sometimes another reference element), and the elemental mass fraction is determined. As glow discharge plasmas are quite stable, a high reproducibility is expected. It is possible to analyze most elements in the periodic table, irrespectively of their fractions, from trace elements (*sub ppb level) to matrix elements, simultaneously. GDMS is usually applied to conductive materials such as metals and semiconductors in bulk. However, GDMS is also applicable to non-conductive materials by placing a metal mask containing an aperture on the surface of a non-conductive sample. The mask T. Saka (&) Department of Electrical and Electronic Engineering, College of Engineering, Daido University, Nagoya 457-8530, Japan e-mail: saka_takashi@tg.commufa.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_36 211 212 T. Saka Fig. 36.1 Schematic illustration of ionization processes in GDMS. M represents the sample atoms and Ar* is meta-stable Ar serves as the cathode. Conductive powder samples can also be measured by pressing the powder into an appropriate shape, as described later. In the case of non-conductive powders, the powders are first mixed with a conductive medium and then pressed into an appropriate shape prior to measurement. The technical specifications for GDMS operation have been reported as an ISO/TS and are thus available as a reference [1]. 36.2 Features • Direct analysis of solid sample is feasible without any special pre-treatment before measurement. • High stability and reproducibility are attained. • As the dynamic range is quite wide, both trace elements and matrix elements can be analyzed simultaneously in a single specimen. • Quantitative analyzes within approximately one order of magnitude are possible without any standard samples. • Isotope analysis is possible. • Elements are ionized with electrons or meta-stable gases, as a result of which the appearance of few multivalent ions can occur, and the effects due to interference are reduced. 36.3 Instrumentation A glow discharge mass spectrometer consists of a glow discharge cell and a mass analyzer. The sample, which serves as the cathode, is placed in the cell, and an inert gas plasma is maintained. For the mass analyzer, a double-focusing magnetic-sector 36 Glow Discharge Mass Spectrometry 213 instrument is conventionally employed to allow for high-resolution analysis. By increasing the magnitude of the magnetic field, ions with a larger mass-to-charge ratio can be transmitted through the analyzer, providing better mass separation. An electrostatic analyzer acts as an energy filter and transmits only ions with appropriate energies. A high resolving power of more than 4000 is commonly realized and is sufficient to overcome most interference effects. The glow discharge plasma is also used for glow discharge optical emission spectrometry (GD-OES). Another type of mass spectrometry that employs a plasma source is inductively coupled plasma mass spectrometry (ICP-MS). In ICP-MS, samples are dissolved and introduced into the plasma with the solvent. Therefore, it is impossible to analyze solids directly by ICP-MS. By contrast, GDMS does not allow the analysis of liquid samples. Double-focusing magnetic-sector instruments are also used in secondary ion mass spectrometry (SIMS). These plasmas and mass spectrometry techniques have been reported elsewhere. 36.4 Measurements 36.4.1 Discharge and Ionization The ion beam intensities observed strongly depend on both the plasma support gas and discharge conditions, which are determined by the current, potential, and gas pressure. Typically, instruments are operated under conditions of direct potential and direct current at 0.5–2 kV and 1–300 mA, respectively. The direct-current source may be operated under the constant current mode or constant potential mode. In both cases, the designed constant current or potential is achieved by adjusting the plasma support gas pressure. Ar is commonly used as the plasma support gas. However, helium is also used. The first ionization potential and potential energy in the meta-stable state of helium are high, which is conducive to measuring elements with high ionization energies such as N and O. 36.4.2 Samples Samples should preferably be prepared in the form of pins or disks. Holders suitable for these shapes are available. The use of disk-shaped samples is expected to yield higher ion beam intensities. In the case of pin-shaped samples, the dependence of ion beam intensities on sample geometry may be high, as shown in the next section. Thus, disk-shaped samples are much more desirable. 214 T. Saka 36.4.3 Mass Analyzes The mass fractions are determined by multiplying the ion beam ratios (IBRs) by relative sensitivity factors (RSFs). The RSFs are defined as follows: RSFX ¼ ðCx Abdx =CR AbdR Þ=ðIx =IR Þ; where C is the mass fraction, I is the ion beam intensity, Abd is the abundance of the isotope, and suffices X and R indicate that the respective quantities are attributed to the analyzed and reference (usually, the matrix) elements, respectively. On the other hand, sputtering phenomena involve individual atoms, and thus, it is sometimes convenient to use atomic fractions. For this purpose, relative ion yields (RIYs) are also used, defined as follows: RIYX ¼ ðIx =IR Þ Cx0 Abdx CR0 Abd ¼ ðMx =MR Þ=RSFX ; where M is relative atomic weight and C0 is atomic fraction, and all other notation is the same as in the equation for RSF. The mass fraction of an element X in a single-element matrix sample can be obtained as follows, by setting CR to unity: Cx ¼ ðIx =IR Þ RSFX ; where the reference element for RSFx is the matrix element. In cases where the RSFs for the matrix element are not available, it is possible to correct the IBRs using RSFs of a non-matrix element. GDMS instrument manufacturers usually provide RSF values, commonly in reference to Fe, but in principle, these values can be used for any matrix. The mass fraction of element X in a multi-element matrix sample can be described as follows: , Cx ¼ ðIx =Abdx Þ RSFX X ! ðIm =Abdm Þ RSFm ; m where m is an abundant element (typically with a mass fraction of more than 1%). The ion beam intensities and RSFs are affected by not only the discharge conditions, but also the sample geometry. Furthermore, RSFs may depend on the matrix elements and the fractions of the analyzed elements. Therefore, precise analyzes require the use of RSF values obtained using compositions similar to those of the analyzed samples. As an example of the dependence of RSFs on sample shape, the comparison of RSFs obtained using pin-shaped and disk-shaped samples prepared from the same bulk sample is shown in Fig. 36.2, the matrix element being Fe. It should be noticed that there exists a nearly linear relationship. Another example is shown in Fig. 36.3, where the dependence of the RSFs of pin-shaped samples on the sample length exposed to the plasma is plotted. 36 Glow Discharge Mass Spectrometry 215 Fig. 36.2 Comparison of RSFs for pin-shaped and disk-shaped samples prepared from the same block. The matrix element is Fe (Courtesy of Daido Bunseki Research) The ionization probabilities of the elements in the plasma strongly depend on their first ionization potentials. In general, for elements with smaller first ionization potentials, the ionization probabilities are expected to be higher and the RSFs tend to be smaller. The relationship between the first ionization potential and RSF is shown in Fig. 36.4 for an Fe matrix and Ar plasma support gas. As shown in Figs. 36.2, 36.3, and 36.4, the RSFs for most of the elements fall within approximately one order of magnitude, and thus, quantitative GDMS analysis in this range is possible without standard samples for calibration. This feature is quite convenient for surveying impurities in samples. When two ions have the nearly same mass-to-charge ratio, interference will occur, which seriously affects the analysis results. However, this problem can be Fig. 36.3 Dependence of ion beam ratios for pin-shaped samples with circular cross-section on the length exposed to plasma. a Si/Fe, b V/Fe. IBRs were measured using samples with different diameters. (●: 2.0 mm, ■: 2.5 mm, ▲: 3.0 mm). (Courtesy of Daido Bunseki Research) 216 T. Saka Fig. 36.4 Relationship between the first ionization potentials and RSFs. (Courtesy of Daido Bunseki Research) Fig. 36.5 Interference spectrum between 28Si+ (400 wtppm: atomic weight: 27.976927) and 56Fe2+ (*70 wt%: atomic weight: 55.93494). (Courtesy of Daido Bunseki Research) resolved in most cases using a magnetic-sector instrument with a resolving power of more than 4000. The resolving power is defined as DMM , where DM is the mass difference between two adjacent peaks and M is the mean mass of the two peaks or the mass of either peak. It is recommended to choose the most appropriate isotopes free of interferences and with high abundances. When Ar is used for the plasma support gas, an interference between Ca (atomic weight: 39.9625907) and Ar (atomic weight: 39.9623831) is expected, where M=DM 193,000. In the case of Ca, no abundant isotopes exist (the abundance of the second abundant isotope of 44 Ca is 2%). Therefore, for the analysis of trace Ca, it is recommended to employ a plasma support gas other than Ar. As an example, the interference spectrum between 28Si+ (atomic weight: 27.976927, mass fraction: 400 ppm) and 56Fe2+ (atomic weight: 55.93494, mass fraction: 70%) for a Nd magnet is shown in Fig. 36.5. Figure 36.5 also reveals that the appearance of multivalent ions is less probable, as the ionizations in GDMS consist of two independent processes, namely, sputtering and ionization, as shown in Fig. 36.1. 36 Glow Discharge Mass Spectrometry 217 Fig. 36.6 (a) Depth profile for SiC and (b) comparison between GDMS and SIMS. Reproduced from Seiji Akahori, The TRC News, 201607-04 [2]. (Courtesy of Toray Reseach Center) 36.5 Applications As GDMS is a relatively simple and stable method, high reproducibility can be expected, and the technique is widely applicable to trace element analysis of metals and semiconductors. It is indispensable for the detection of impurities in pure metals and alloys. However, a large area of the sample is exposed to the plasma, and atoms are sputtered from the whole exposed area. Therefore, it is difficult to measure impurity distribution, e.g., segregation on a flat surface. The depth profiles of the sample composition can be determined through repeated measurements. Sputtering rates will be relatively large and magnetic scanning will be necessary. Thus, the high depth resolutions achievable through SIMS are not feasible in GDMS. However, in SIMS, only a target element can be measured, while in GDMS, all elements can be measured simultaneously. Figure 36.6a shows an example of a depth profile of multiple-Al-implanted SiC. Depth profiles for all atoms included are measured by a single operation. Figure 36.6b compares the depth profiles of Al obtained by GDMS and SIMS. Owing to the high sputtering rate, GDMS cannot be used for the analysis of surfaces and thin films. References 1. ISO/TS 15338:2009 Surface chemical analysis-GD-MS-Introduction to use 2. Akahori, S.: The TRC News, 201607-04 (2016) Chapter 37 Glow Discharge Optical Emission Spectrometry Patrick Chapon, Sofia Gaiaschi and Kenichi Shimizu Keywords Plasma Elemental analysis Thin and thick films 37.1 Fast depth profiling Principle GDOES [1] is a spectrochemical technique that allows direct in-depth determination of major and trace elements. In GDOES, a pulsed radio frequency (RF) plasma source [2] is used to obtain the elemental profile of conductive, insulating, or hybrid materials from the first atomic layers down to more than 150 lm. There is no ultrahigh vacuum system. All elements can be measured. The GD plasma has a double role; firstly, it sputters a representative surface of the sample (typically 4 mm in diameter), and by optimizing the plasma conditions a flat crater bottom can be obtained; secondly it excites the sputtered atoms. As the sample is continuously sputtered, the collected light reflects the temporal evolution of the sputtered species; therefore, it is possible, using a fast simultaneous optical spectrometer and a built in interferometer, to obtain a depth-resolved quantitative elemental analysis, in best case with nanometric depth resolution [3]. Sputtering in GDOES is rapid: A typical sputtering rate is 10-150 nm s−1, i.e., 30–500 atomic layers per second, but since photomultiplier counting rates can be very fast, and integration times are typically 1 ms at high speed, the GDOES P. Chapon (&) S. Gaiaschi HORIBA France SAS, Avenue de la Vauve, Passage Jobin Yvon, 91120, CS 45002 Palaiseau, France e-mail: patrick.chapon@horiba.com K. Shimizu Keio University, 4-1-1 Hiyoshi, Yokohama 223-8521, Japan © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_37 219 220 P. Chapon et al. Fig. 37.1 Overlay of multiple GD depth profiles over a batch of industrial ZnAl coatings on steel (“Galvalume”). Control of the deposition uniformity instruments are routinely capable of making 1–10 measurements per element per atomic layer. Hence, GDOES is capable of true surface analysis [4] and its speed makes surface adsorption negligible. This speed and the absence of UHV change the way people approach surface analysis. With GDOES, one does not hesitate any longer to do multiple analyses on a batch of samples to test the uniformity of a deposition process or to compare good and bad samples as to track the origin of defects (Fig. 37.1). In addition, GDOES makes possible to rapidly access embedded layers that could then be complementary observed by SEM EDX, XPS [5], or micro-Raman. 37.2 • • • • • • • • • • • Features Acronym: GDOES; Technique: sputtering by plasma and spectrometric measurement; Results: elemental depth profiles (Fig. 37.2); Conductive and nonconductive layers and materials can be measured; All elements can be determined (including H, O, Li, N, etc.); Quantitative. layers composition and thickness readily available; Depth profile range: 1 nm–150 lm; Depth resolution: nm at best, sample dependant; Lateral resolution: spot size—4 mm typical; No UHV; Fast and easy to use. 37 Glow Discharge Optical Emission Spectrometry 221 Fig. 37.2 Range of applications of GDOES 37.3 Instrumentation As the name suggests, a GDOES instrument combines a GD plasma source (operated in pulsed RF mode [2]) and a high-resolution simultaneous optical spectrometer. A differential interferometer can also be added to the source for direct online measurement of the crater depth. 37.3.1 Source The geometry of the GD source is of hollow anode type. The two electrodes of the discharge are a grounded tube (the anode—typically 4 mm in diameter) and the sample to be analyzed (acting as the cathode). A gas flow is maintained in the discharge cavity to stabilize the pressure between 200 and 1400 Pa. As the volume of the GD plasma chamber is small, this corresponds to a very low flow— typically 0.2 l/mn during operation. Ar is the most commonly used plasma gas, but Ne is also of interest notably for F determination. However, diverse plasma gas mixtures have been used for various applications (notably Ar with O2 for ultrafast sputtering of polymers—HORIBA patent WO15166189A1). When a RF power (pulsed or not—typically of a few tens of Watt) is supplied to the sample, a plasma is created within the tubular anode. The pulsed operation 222 P. Chapon et al. allows minimizing the average power while keeping high instantaneous power for sensitivity. In this way, the thermal load is reduced, allowing the efficient analysis of fragile and heat sensitive materials. The applied RF conditions lead to an operation in the so-called abnormal discharge region where the plasma is homogeneous over the entire surface of the sample facing the tube. A negative potential, the self-bias, is established at the sample surface (on both conductive and nonconductive samples) due to the difference in size between the two electrodes (the inner part of the tube and the sample surface facing the anode). Plasma gas ions, created in the negative glow of the plasma, gain energy in the sheath (the cathode dark space) close to the sample surface. They are accelerated toward the sample, and they bombard the surface. This bombardment induces an erosion process called sputtering. The energy of the bombarding ions is quite low (typically 50 eV), but the plasma is very dense (1014 cm−3), and therefore the sputtering process is extremely efficient and rapid. The atoms sputtered from the surface of the sample diffuse through the dark region of the plasma into the negative glow. In this negative glow, the atoms are excited through collisions with high energy electrons, metastable atoms, and ions. The de-excitation of the excited species causes the emission of photons which are characteristic of the material. The two phenomena (sputtering and emission) are spatially separated and can be treated independently of one another, which is crucial for quantification. While the sputtering is material (or layer) dependant, the excitation only depends on the applied plasma conditions. Operating a GDOES instrument is straightforward [1]. Routinely, the sample is simply placed against a ceramic and a o’ring, and it is pressed (Fig. 37.3) in order to assure a primary vacuum. The ceramic assures that the spacing between the anode and the sample is kept between 0.1 and 0.2 mm, so that no plasma is possible directly between the annular surface of the anode and the sample surface. No UHV is used in GDOES, and because of the sweeping action of the plasma gas, the purity of the vacuum is essentially that of the high-purity gas used for the plasma discharge. Plasma cleaning strategies [6] can additionally be applied to clean the sample surface before analysis: Using soft conditions (typical power less than 3 W and pulsed mode), desorption of the surface takes place. This is revealed by a strong reduction of surface C and H contaminant signals during the measurement. The design of the source can be easily adapted when the samples to analyze are not flat—for instance, by designing curved ceramics matching the sample shape when rods or tubes have to be analyzed. Similarly, porous samples can be efficiently measured using dedicated holders that will assure the vacuum seal. This ease of use, which is generally considered as an advantage, might be a drawback in case of Li-based materials, where contact with air should be minimized or even prevented. For such applications, a transparent dedicated chamber has been designed allowing 37 Glow Discharge Optical Emission Spectrometry 223 Fig. 37.3 Standard GD source for flat samples (photograph and schematics) an easy sample handling and an efficient GD analysis, under Ar inert gas atmosphere [7]. 37.3.2 Optics The speed of GD sputtering requires the use of an ultrafast simultaneous high-resolution emission spectrometer (Fig. 37.4). The spectral range to cover is large (VUV, UV, visible, and near IR), going from H to D around 121 nm [8], O at 130 nm [9] to Li at 670 nm, and K at 766 nm. Moreover, as GD is a weak source of light, light collection and light throughput are crucial. The spectrometer that is normally used is a Paschen-Runge mounting type polychromator, based on a Rowland circle of large focal length (typically 50 cm) for optimal resolution and sensitivity, when coupled with a high groove density dedicated grating, with enhanced performance in the most critical VUV and UV domains. The IR extension is obtained with a second dedicated spectrometer and grating placed along the path of the zero order of the first grating. In this design, which minimizes the optical aberrations and does not use any beam splitter or fibers (sources of light loss), each exit slit selects a spectral line of interest. Atomic lines are usually the most intense and therefore the prime choice in GD. Behind each exit slit, a high dynamic range detector (photomultiplier tube— PMT) is mounted to record the light intensity of the selected wavelength. The signals from all PMTs are recorded simultaneously. 224 P. Chapon et al. Fig. 37.4 Principle of the simultaneous optical spectrometer (Paschen-Runge mounting) used in GDOES In addition to the polychromator, a tuneable Czerny-Turner-mounted monochromator—also with high dynamic range detection—can be used as an additional acquisition channel. This allows either to measure an additional element (spectroscopic line), which is not available on the polychromator, or to collect the entire emission spectrum of a bulk sample, or a thick layer. 37.3.3 Interferometer Finally, the introduction of a differential interferometer within the GD source (DIP system, from HORIBA, patent WO15166189A1) offers in situ direct measurement of the crater depth and therefore the determination of layer thickness and erosion rate. The sputtering rates in GDOES are material dependant, and when multilayers are measured, they change with each layer. Previously, the estimation of these sputtering rates—which is required for accurate depth quantification—was the result of calculations (prone to uncertainties, as they were notably relying on density calculations) or external measurements. With DIP, two laser beams, coming from the same laser source, are directed onto the sample to be analyzed. While one beam is reflected in the middle of the GD crater (the depth-sensing beam), the other one is reflected at the surface of the sample, close to the GD crater. A red laser diode (633 nm) is used as this wavelength does not interfere with the emission lines of the elements. If the material is reflective enough, the reflected laser beams are collected and their phase shift measured, giving direct access to the erosion rates, the layers thickness, and the crater depth (Fig. 37.5). 37 Glow Discharge Optical Emission Spectrometry 37.4 225 Applications 37.4.1 Photovoltaics (PV) There is a growing interest in the capabilities of GDOES for PV applications due to the fact that PV technologies evolve very rapidly, with a definitive trend toward thin films—the driving force of most research is cost reduction for which thin-film technologies are promising. Thin-films PV can be based on CIGS—quaternary CuInGaSe2, CdTe, Si, or organic materials. Hybrid configurations are also subject to intense research as, for instance perovskite solar cells. In parallel, investigations are made to replace bulk glass with on alternative substrates in order to create flexible structures. However, even if materials evolve and change rapidly, the need of a multielemental fast characterization technique is now well accepted. Indeed, the fabrication process involves a certain number of operations to be performed, and multiple key parameters need to be followed carefully as any deviation from the optimum conditions may induce major defects in the final product. Therefore, a rapid characterization technique assessing the quality of the fabrication process and providing an immediate feedback by measuring the depth distribution of major, traces, and contaminations is crucial. In many research centers worldwide, GDOES is now recognized as the metrological companion to solar cells fabrication (Fig. 37.6). Fig. 37.5 Principle of the differential interferometer Fig. 37.6 GDOES measurement of the absorber layer of a CIGS solar cell and view of the crater obtained (total analysis time: 2 min. Spot size: 4 mm) 226 P. Chapon et al. Stack of Mo/B4C/Si. Mo (3.2nm), B4C (0.3nm), Si (3.5nm) Fig. 37.7 GDOES measurement of ultrathin films 37.4.2 Ultrathin Films and Preparation for SEM For many industrial samples, in both metallurgy and automotive industry [10], the surface roughness and the grain structure of the material reduce the ultimate depth resolution, typically to about 10% of depth. However, for ideal samples, nanometric resolution has been observed. The following example (Fig. 37.7) illustrates the excellent depth resolution offered by pulsed RF GDOES. The sample is a X-ray mirror, featuring 60 stacks of Si/B4C/Mo multilayers deposited by magnetron sputtering [3]. Each stack is 7-nm thick, with the B4C layers being only 0.3 nm. Plasma sputtering is not a layer-by-layer process; therefore, GDOES suffers of sputter-induced effects, when materials are not amorphous [11]. However, a new application proposed by K. Shimizu [12] has proved to benefit from what is usually assumed to be a drawback. In his work, K. Shimizu has used the GD plasma to structure surfaces at the nanoscale, therefore preparing sample surfaces for SEM observation and EBSD measurements (Fig. 37.8). This is achieved not only by eliminating the residuals of the mechanical polishing step or by removing the native oxide layer, but mainly by revealing the grain structure and the phases within the material under investigation. Such structuring of the surface is obtained thanks to the preferential sputtering which is taking place over the large sputtered area. 37.4.3 Light Elements Focus The wide application range of GDOES is partly due to its capability to measure nearly all elements, including the lightest ones, from low levels of concentration in one layer up to 100% in another one. 37 Glow Discharge Optical Emission Spectrometry 227 Fig. 37.8 SEM observation of a sample prepared by GDOES Fig. 37.9 GDOES depth profile of a layered structure with H and D (from Ref. [8]) Depth profile of Cl in CdTe solar cells has been studied, as well as He penetration in tokamak walls. Na is a key element to follow in CIGS solar cells, multiple papers report the use of GDOES for nitruration (N) studies, and of course Li is the main component of the electrodes of Li batteries. The follow up of C is crucial for 228 P. Chapon et al. surface contamination, but C is also the major component of organic materials; in many domains, researches are focused on embedded layers below organic films. However, it is probably the measurement of O and H (and its isotope D) that is of special interest for multiple application domains—it is worth noting that many surface techniques cannot measure H (Fig. 37.9). Therefore, GDOES is a key technique for all studies on corrosion and oxidation processes, and of course for testing the efficiency of corrosion protection coatings or treatments. References 1. Nelis, T., Payling, R.: Practical guide to glow discharge optical emission spectrometry. Royal Society of Chemistry (2003) 2. Belenguer, Ph, Ganciu, M., Guillot, Ph, Nelis, Th: Pulsed glow discharges for analytical applications. Spectrochimica Acta Part B 64, 623–641 (2009) 3. Ber, B., Bábor, P., Brunkov, P.N., Chapon, P., Drozdov, M.N., Duda, R., Kazantsev, D., Polkovnikov, V., Yunin, P., Tolstogouzov, A.: Sputter depth profiling of Mo/B4C/Si and Mo/Si multilayer nanostructures: a round-robin characterization by different techniques. Thin Solid Films 540, 96–105 (2013) 4. Shimizu, K., Payling, R., Habazaki, H., Skeldon, P., Thompson, G.E.: Rf-GDOES depth profiling analysis of a monolayer of thiourea adsorbed on copper. J. Anal. At. Spectrom. 19, 1–5 (2004) 5. Mercier, D., Bouttemy, M., Vigneron, J., Chapon, P., Etcheberry, A.: GD-OES and XPS coupling: a new way for the chemical profiling of photovoltaic absorbers. Appl. Surf. Sci. 347, 799–807 (2015) 6. Molchan, I.S., Thompson, G.E., Skeldon, P., Trigoulet, N., Chapon, P., Tempez, A., Malherbe, J., Lobo Revilla, L., Bordel, N., Belenguer, Ph, Nelis, T., Zhari, A., Thérèse, L., Guillot, Ph, Ganciu, M., Michlert, J., Hohl, M.: The concept of plasma cleaning in glow discharge spectrometry. J. Anal. At. Spectrom. 24, 734–741 (2009) 7. Saito, Y., Rahman, M.K.: Investigation of positive electrodes after cycle testing of high power Li-ion battery cells IV. An approach to the power fading mechanisms by depth profile analysis of electrodes using glow discharge optical emission spectroscopy. J. Power Sources 174, 877–882 (2007) 8. Hatano, Y., Shi, J., Yoshida, N., Futagami, N., Oya, Y., Nakamura, H.: Measurement of deuterium and helium by glow-discharge optical emission spectroscopy for plasma–surface interaction studies. Fusion Eng. Des. 87, 1091–1094 (2012) 9. Warwick, M.E.A., Hyett, G., Ridley, I., Laffir, F.R., Olivero, C., Chapon, P., Binions, R.: Synthesis and energy modeling studies of titanium oxy-nitride films as energy efficient glazing. Sol. Energy Mater. Sol. Cells 118, 149–156 (2013) 10. Torok, T., Levai, G., Szabo, M., Pallosi, J.: Characterizing coatings of car body sheets by GDOES. In: Makhlouf, A. (eds.) High performance coatings for automotive and aerospace industries. Nova Science Publishers (2010) 11. Lanzutti, A., Marin, E., Lekka, M., Chapon, P., Fedrizzi, L.: RF GDOES analysis of composite metal/ceramic electroplated coatings with nano-to microceramic particles’size. Surf, Interface Anal (2011) 12. Shimizu, K., Mitani, T.: New horizons of applied scanning electron microscopy. Springer (2010) Chapter 38 Hard X-Ray Photoelectron Spectroscopy Akira Sekiyama Keywords Bulk/interface electronic structure Polarization Recoil effects 38.1 Synchrotron radiation Principle Hard X-ray photoelectron spectroscopy (HAXPES) is a kind of X-ray photoelectron spectroscopy (XPS), photoelectron spectroscopy by use of a higher-energy excitation (from synchrotron radiation, in most cases) than a conventional Al-Ka (hm = 1486.6 eV) or Mg-Ka (hm = 1253.6 eV) laboratory X-ray tube. Here, the words hard X-ray means the synchrotron radiation X-ray monochromatized by a double-crystal monochromator. Since single-crystalline Si is most widely used for the crystal monochromators in synchrotron radiation facilities, the excitation energy for HAXPES is normally hm > 2 keV (Remind that the longest wavelength for the monochromatic light using the Si crystal is 2d(Si(111)) = 6.27 Å). Since the photoelectron kinetic energy EK at HAXPES is much longer than that at conventional XPS and soft X-ray/VUV photoelectron spectroscopy, the photoelectron mean free path becomes longer (>50 Å at EK*8 keV) [1]. This allows us to probe bulk or buried interface electronic structures in solids using HAXPES as schematically shown in Fig. 38.1. In some cases, non-destructive analysis of materials is feasible by HAXPES owing to the longer photoelectron mean free path. In addition, light transmittance of air or gas is higher for hard X-ray than soft X-ray (attenuation coefficient of air is of the order of 10−1–10−2 cm−1 at hm*10 keV). Therefore, HAXPES is also utilized for (Near) ambient pressure XPS. Other characteristics of HAXPES at hm = 5–10 keV compared with the soft X-ray (hm = 0.5–1.5 keV) photoemission is that the photoionization cross sections for sand p-orbitals are comparable to those for d- and f-orbitals, and that orbital A. Sekiyama (&) Division of Materials Physics, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka, Japan e-mail: sekiyama@mp.es.osaka-u.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_38 229 230 A. Sekiyama Fig. 38.1 Schematic description for the bulk sensitivity of HAXPES compared with that of conventional photoelectron spectroscopy dependence of photoelectron angular distributions with respect to the light polarization [2–4]. 38.2 • • • • • Features Bulk and/or interface electronic structures of solids can be probed. UHV is not always required, leading to an affinity for ambient pressure XPS. Photoionization cross sections are comparable among the s, p, d, and f-orbitals. Orbital-resolved electronic structures are revealed by polarization dependence. Recoil effects are not negligible for light elements. 38.3 Instrumentation Instrumentation for HAXPES is similar to that for XPS. A high-energy version (up to EK*10 keV) of photoelectron spectrometer is commercially available. When hm > 4 keV, an (ultra-) high vacuum chamber with the spectrometer can be separated from the beamline by a Be window. Baking of the chamber is still mandatory even for HAXPES when the samples are cooled by liquid He or N2, although it is not necessary for room-temperature measurements. A relative energy resolution (DE/E) of the excitation light supplied by a crystal monochromator at a hard X-ray beamline depends on crystal quality, beam divergence, Bragg angle, and Miller indices of the reflection lattice plane. Single-crystalline silicon is widely used at the synchrotron radiation facilities since its perfect and large crystal is available. 38 Hard X-Ray Photoelectron Spectroscopy 231 A typical value of DE/E is *10−4 by the Si(111) reflection at high-brilliance synchrotron radiations, leading to DE*1 eV at hm*10 keV. To get a high-resolution excitation light, a post-monochromator called as a channel-cut crystal is employed while the photon energy cannot be easily changed without the modification of the beam path after the installation of the post-monochromator as schematically shown in Fig. 38.2. A Si(111) channel-cut crystal is often used for getting high energy resolutions since the photon energy can discretely be selected without changing the beam path from hm ≅ 6, 8, and 10 keV using the (333), (444), and (555) reflections, where DE*60, 40, and 20 meV at hm ≅ 6, 8, and 10 keV are available at low-emittance undulator radiations. The overall experimental resolution of *60 meV has so far been achieved at hm *8 keV [5, 6]. By using a diamond phase shifter (optional for HAXPES), the excitation-light polarization can be switched from the horizontally linear polarization from a planer undulator to vertically linear and left/right circular polarizations at hm = 5–10 keV as shown in Fig. 38.2, where the offset angles from the (111) or (220) Bragg reflections are of the order of tens arcsec [7, 8]. On the other hand, a laboratory HAXPES system with a monochromatic Cr-Ka (hm = 5.4 keV) line has recently been developed [9]. Undulator h ~7.9 keV h ~ 50 meV Horizontal Double-crystal monochromator Si(620) Channel-cut Vertical Diamond phase shifters Ellipsoidal K-B mirror e– Sample Analyzer Fig. 38.2 Example of experimental setup for HAXPES at a hard X-ray beamline at synchrotron radiation facilities. Channel-cut crystal and phase shifters are optional. Photograph of the Si(310) channel-cut crystal (for the (620) reflection at hm > 7 keV) is shown in the inset 232 38.4 A. Sekiyama Applications 38.4.1 Profiling the Interface Electronic State of Heterostructures Owing to the longer photoelectron mean free path for HAXPES than that for the conventional photoemission spectroscopy, one can imagine that the interface electronic states can be directly detected by HAXPES. Indeed, the conducting interface of LaAlO3/SrTiO3 hetrostructures has been studied by HAXPES at hm = 3 keV [10], where both LaAlO3 and SrTiO3 are insulators. It has been found that extra electrons are located at Ti sites in the interface from the Ti3+ signal in the 2p core-level photoemission spectra. Further, the layer thickness and carrier density have also successfully been obtained from the emission angle dependence of the Ti 2p core-level photoemission spectra with the photoelectron escape depth of 40 Å. 38.4.2 Probing the Bulk Electronic Structure of Strongly Correlated Systems In the strongly correlated electron systems in which the on-site Coulomb repulsion U is not negligible, the electronic structure is often determined as a function of U/W, where W denotes the bare bandwidth formed by the strongly correlated orbitals. Since W is reduced in the surface, the electronic states are often substantially different between the bulk and surface. In addition, the transition-metal 2p and rare-earth 3d core levels are located at the binding energies of 0.4–1.5 keV. When the conventional XPS is applied to the core-level photoemission of the strongly correlated electron systems, the surface contributions to the spectra would prevent from clarifying the bulk electronic states. HAXPES is quite useful for revealing the bulk electronic structure owing to its high bulk sensitivity. Figure 38.3 shows the comparison of the Yb2+ 4f photoemission spectra near EF between hm = 700 and 8180 eV for a Kondo semiconductor YbB12 [5, 6]. One can recognize that the surface Yb2+ 4f contributions are seen at the binding energy of *0.9 eV in the spectrum at hm = 700 eV, while such a surface contribution is negligible in the spectrum at hm = 8180 eV. Another example of the HAXPES for strongly correlated materials is referred. The high-resolution (*130 meV) temperature-dependent HAXPES for paradigmatic materials of a so-called “typical” Mott transition systems (V1–xCrx)2O3 at hm*8 keV has shown that U is unchanged across the transitions indicating that the so-called orbital-selective Mott transition scenario is the most plausible [11]. 38 Hard X-Ray Photoelectron Spectroscopy 233 Fig. 38.3 Comparison of the HAXPES spectrum near EF of YbB12 at hm = 8180 eV with the soft X-ray photoemission spectra at hm = 700 eV [5, 6] 38.4.3 Polarization-Dependent Valence-Band HAXPES Besides the bulk sensitivity, HAXPES at hm = 5–10 keV has unique characteristics as the comparable photoionization cross sections for the s and p states per electron with those for the d and f states, and strong orbital dependence of the photoelectron angular distribution with respect to the light polarization [2–4]. As an overall tendency, calculations on the basis of the photoelectron angular distribution parameters predict that the photoelectron intensity for the s state is strongly suppressed in the s-polarization configuration (s-pol., where the electric field of the incident light is perpendicular to the photoelectron detection direction) compared with the p-polarization configuration (p-pol., where the electric field is within the plane made of the photoelectron detection and incident light directions) while the d and f spectral weights are not suppressed very much even in the s-pol. Therefore, the extraction of the s contributions as well as that for the d and f contributions in the bulk valence band of solids becomes feasible by the linear polarizationdependent HAXPES. The polarization dependence of the valence-band HAXPES spectra for polycrystalline silver [12] is shown in Fig. 38.4. When the spectra are roughly normalized by the spectral weight of the binding energy of *4 eV in the so-called 4d band region, one recognizes that the spectral weight from the Fermi level (EF) to 3 eV is relatively reduced in the s-polarization with keeping the intensity ratio Is–pol/Ip–pol nearly independent of the binding energy in this region. The polarization-dependent valence-band HAXPES of silver tells us that the 4d bands are located far below EF and well separated from the conduction 5sp band 234 A. Sekiyama Fig. 38.4 Linear polarization-dependent valence-band HAXPES spectra of polycrystalline silver [12]. The inset shows the experimental geometry of the HAXPES measurements crossing EF. This is rather contradictory to the prediction from the band-structure calculations giving a more or less d-sp mixing. From the comparison of the result of the LDA+U-like calculation, the 4d electron correlation effects are responsible for the negligible d contribution to the conduction electrons in silver [12]. 38.4.4 Probing Localized Orbital Symmetry by Linear Dichroism in HAXPES For discussing the strongly correlated rare-earth 4f electronic states, the ionic picture is a good starting point since the effective on-site U is 6–10 eV. The 4fn multiplet levels of trivalent rare-earth ions determined by total orbital angular momentum L and spin S are split by the spin–orbit coupling (order of 0.2–1.3 eV depending on element) and further split by the crystalline electric field (CEF, of the order of several tens to hundreds K). The CEF-spit ground-state 4f charge distributions are deviated from the spherical symmetry. Since there is anisotropy in the Coulomb and exchange interactions between the 4f electrons and a created inner-core hole in the core-level photoemission process, the anisotropic 4f charge distributions can be detected by linear dichroism (LD) in angular distributions of the core-level photoemission (in other words, LD in angle-resolved core-level photoemission) measured at low temperatures. The polarization-dependent angle-resolved Yb3+ 3d5/2 core-level HAXPES spectra of YbCu2Si2 are shown in Fig. 38.5. There is a multiple-peak structure characteristic of ionic Yb3+ states yielding the 3d94f13 multiplet-split peaks in the photoemission final states in all the spectra. Clear LD defined by the difference in spectral weight between the s- and p-polarization configurations is seen in the spectra. For instance, in the spectra at h = 0°, the peak at 1527 eV is stronger in the 38 Hard X-Ray Photoelectron Spectroscopy 235 Fig. 38.5 Polarization-dependent angle-resolved [h = 0° and 60°, where h denotes the angle between the photoelectron detection and the [001] direction] Yb3+ 3d5/2 core-level HAXPES spectra of tetragonal YbCu2Si2. The inset illustration denotes the experimental configurations for HAXPES [13] s-polarization configuration (s-pol.) than in the p-polarization configuration (p-pol.), whereas the structure with the 1529.5 eV peak and 1530.5 eV shoulder is stronger in the p-pol. On the other hand, LD is reduced at h = 60° with keeping the same sign as that at the same binding energy with h = 0°. From the comparison between the experimental and theoretical ones on the basis of ionic calculations, the ground-state 4f wave function has successfully been determined as C27 i ¼ 0:36 5=2i þ 0:93j 3=2i for YbCu2Si2 [13]. Note that the atomically flat surface is not necessary for these angle-resolved measurements since the angle parameter is concerning the crystal axis in a real space, not in a reciprocal space. LD in the angle-resolved core-level HAXPES is also powerful for determining the 4f ground-state symmetry under the cubic symmetry. Actually the ground state of YbB12 has been determined as in the C8 symmetry by measuring the linear dichroism in the 3d core-level spectra along the [100], [110], and [111] directions, although LDs are much reduced compared with those in Fig. 38.5 [8, 14]. 236 A. Sekiyama Fig. 38.6 Hard and soft X-ray B 1s and Yb 4d core-level photoemission spectra lf Yb7/8Lu1/8B12 at 200 K with the energy resolution of *140 meV [15]. The Yb 4d spectra in the right panel are enlarged since the Yb 4d spectral weight is much weaker relative to the B 1s one at both excitations 38.4.5 Recoil Effects In the photoemission from a single free atom with mass M, the recoil energy ER ≅ (m/M) EK is given to the photoionized atom due to the energy and momentum conservation laws, where m is the electron mass. The resulting EK is thus slightly decreased by the amount ER. Since EK is much higher at HAXPES than that at the conventional XPS and soft X-ray photoelectron spectroscopy, the recoil effects can be seen mainly for light elements. Figure 38.6 shows the hard and soft X-ray B 1s and Yb 4d core-level spectra of Yb7/8Lu1/8B12 [15]. Although the shift of the peak binding energy in accord with the change in the excitation energy is negligible for the Yb 4d spectra, it is noticeable and as large as *300 meV for the B 1s spectra, which are comparable to ER of *400 meV. This contrasting difference reflects the difference in nucleus mass ≅ M. The peak broadening is also seen in the B 1s spectra. Although the recoil effects have been recognized in the photoemission spectra of core levels in gases [16], those in solids have been experimentally verified after the developments of HAXPES [17–20]. Note that the recoil effects are not only restricted in the core-level excitations but also seen in the valence-band excitations [18–20], which suggest that the photoemission process does take place in the vicinity of the nucleus even for nearly free electrons in solids. References 1. Tanuma, S., Powell, C.J., Penn, D.R.: Calculations of electron inelastic mean free path. IX. Data for 41 elemental solids over the 50 eV to 30 keV range. Surf. Interface Anal. 43, 689– 713 (2011) 2. Trzhaskovskaya, M.B., Nefedov, V.I., Yarzhemsky, V.G.: Photoelectron angular distribution parameters for elements Z = 1 to Z = 54 in the photoelectron energy range 100–5000 eV. At. Data Nucl. Data Table 77, 97–159 (2001) 38 Hard X-Ray Photoelectron Spectroscopy 237 3. Trzhaskovskaya, M.B., Nefedov, V.I., Yarzhemsky, V.G.: Photoelectron angular distribution parameters for elements Z = 55 to Z = 100 in the photoelectron energy range 100–5000 eV. At. Data Nucl. Data Table 82, 257–311 (2002) 4. Trzhaskovskaya, M.B., Nikulin, V.K., Nefedov, V.I., Yarzhemsky, V.G.: Non-dipole second order parameters of the photoelectron angular distribution for elements Z = 1–100 in the photoelectron energy range 1–10 keV. At. Data Nucl. Data Table 92, 245–304 (2006) 5. Yamaguchi, J., Sekiyama, A., Imada, S., Fujiwara, F., Yano, M., Miyamachi, T., Funabashi, G., Obara, M., Higashiya, A., Tamasaku, K., Yabashi, M., Ishikawa, T., Iga, F., Takabatake, T., Suga, S.: Kondo lattice effects and the collapse of lattice coherence in Yb1-xLuxB12 studied by hard X-ray photoelectron spectroscopy. Phys. Rev. B 79, 125121 (2009) 6. Sekiyama, A.: High-energy photoemission spectroscopy for investigating bulk electronic structures of strongly correlated systems. J. Electron Spectrosc. Relat. Phenom. 208, 100–104 (2016) 7. Suzuki, M., Kawamura, N., Mizumaki, M., Urata, A., Maruyama, H., Goto, S., Ishikawa, T.: Helicity-modulation technique using diffractive phase retarder for measurements of X-ray magnetic circular dichroism. Jpn. J. Appl. Phys. 37, L1488–L1490 (1998) 8. Fujiwara, H., Naimen, S., Higashiya, A., Kanai, Y., Yomosa, H., Yamagami, K., Kiss, T., Kadono, T., Imada, S., Yamasaki, A., Takase, K., Otsuka, S., Simizu, T., Shingubara, S., Suga, S., Yabashi, M., Tamasaku, K., Ishikawa, T., Sekiyama, A.: Polarized hard X-ray photoemission system with micro-positioning technique for probing ground state symmetry of strongly correlated materials. J. Synchrotron Rad. 23, 735–742 (2016) 9. Kobayashi, K., Kobata, M., Iwai, H.: Development of a laboratory system hard X-ray photoelectron spectroscopy and its applications. J. Electron Spectrosc. Relat. Phenom. 190, 210–221 (2013) 10. Sing, M., Berner, G., Goß, K., Muller, A., Ruff, A., Wetseherk, A., Thiel, S., Mannhart, J., Pauli, S.A., Schneider, C.W., Willmott, P.R., Gorgoi, M., Schäfers, F., Claessen, R.: Profiling the interface electron gas of LaAlO3/SrTiO3 heterostructures with hard X-ray photoelectron spectroscopy. Phys. Rev. Lett. 102, 176805 (2009) 11. Fujiwara, H., Sekiyama, A., Mo, S.-K., Allen, J.W., Yamaguchi, J., Funabashi, G., Imada, S., Metcalf, P., Higashiya, A., Yabashi, M., Tamasaku, K., Ishikawa, T., Suga, S.: Evidence for the constancy of U in the Mott transition V2O3. Phys. Rev. B 84, 075117 (2011) 12. Sekiyama, A., Yamaguchi, J., Higashiya, A., Obara, M., Sugiyama, H., Kimura, M.Y., Suga, S., Imada, S., Nekrasov, I.A., Yabashi, M., Tamasaku, K., Ishikawa, T.: The prominent 5d-orbital contribution to the conduction electrons in gold. New J. Phys. 12, 043045 (2010) 13. Mori, T., Kitayama, S., Kanai, Y., Naimen, S., Fujiwara, H., Higashiya, A., Tamasaku, K., Tanaka, A., Terashima, K., Imada, S., Yasui, A., Saitoh, Y., Yamagami, K., Yano, K., Matsumoto, T., Kiss, T., Yabashi, M., Ishikawa, T., Suga, S., Onuki, Y., Ebihara, T., Sekiyama, A.: Probing strongly correlated 4f-orbital symmetry of the ground state in Yb compounds by linear dichroism in core-level photoemission. J. Phys. Soc. Jpn. 83, 123702 (2014) 14. Kanai, Y., Mori, T., Naimen, S., Yamagami, K., Fujiwara, H., Higashiya, A., Kadono, T., Imada, S., Kiss, T., Tanaka, A., Tamasaku, K., Yabashi, M., Ishikawa, T., Iga, F., Sekiyama, A.: Evidence for C8 ground-state symmetry of cubic YbB12 probed by linear dichroism in core-level photoemission. J. Phys. Soc. Jpn. 84, 073705 (2015) 15. Suga, S., Sekiyama, A.: High energy photoelectron spectroscopy of correlated electron systems and recoil effects in photoelectron emission. Eur. Phys. J. Spec. Topic 169, 227–235 (2009) 16. Åsbrink, L.: The photoelectron spectrum of H2. Chem. Phys. Lett. 7, 549–552 (1970) 17. Takata, Y., Kayanuma, Y., Yabashi, M., Tamasaku, K., Nishino, Y., Miwa, D., Harada, Y., Horiba, K., Shin, S., Tanaka, S., Ikenaga, E., Kobayashi, K., Senba, Y., Ohashi, H., Ishikawa, T.: Recoil effects of photoelectrons in a solid. Phys Rev. B 75, 233404 (2007) 18. Takata, Y., Kayanuma, Y., Oshima, S., Tanaka, S., Yabashi, M., Tamasaku, K., Nishino, Y., Matsunami, M., Eguchi, R., Chainani, A., Oura, M., Takeuchi, T., Senba, Y., Ohashi, H., 238 A. Sekiyama Shin, S., Ishikawa, T.: Recoil effects of photoelectrons in the Fermi edge of simple metals. Phys. Rev. Lett. 101, 137601 (2008) 19. Suga, S., Sekiyama, A., Fujiwara, H., Nakatsu, Y., Miyamachi, T., Imada, S., Baltzer, P., Niitaka, S., Takagi, H., Yoshimura, K., Yabashi, M., Tamasaku, K., Higashiya, A., Ishikawa, T.: Do all nuclei recoil on photoemission in compounds? New J. Phys. 11, 073025 (2009) 20. Suga, S., Sekiyama, A.: Soft X-ray ARPES and Fermiology of strongly correlated electron systems and PES by hard X-ray and extremely low energy photons. J. Electron Spectrosc. Relat. Phenomen. 181, 48–55 (2010) Chapter 39 Helium Atom Scattering Takahiro Kondo Keywords He atom diffraction Phonon dispersion curve Vibration spectroscopy In situ and non-perturbed measurements 39.1 Principle HAS provides information about the outermost surface structure, phonon dispersion curve, and lattice dynamics of a material by measuring the angular intensity distribution and/or energy of helium beam scattered from the sample surface. Scattering of low energy He beam consists of elastic, quasi-elastic, and inelastic scatterings such as single-phonon and multi-phonon scatterings (scattering with annihilation/excitation of single- or multi-phonon scatterings of the solid surface). The dominant scattering channel is determined by the collision parameter of He with the surface such as surface temperature, incident translational energy of He, incident angle, and the property of the surface. HAS thus provides variety of information depending on the scattering channels. 39.2 Elastic Scattering [1, 2] Elastic scattering of low energy He beam has been mainly used to analyze the structure of the outermost surface of the solid. The de Broglie wavelength of He beam with translational energy of 20 meV corresponds to 0.1 nm which is an order of the atom–atom distance of the crystal surface. Thus, the outermost surface structure can be analyzed by elastic diffraction peaks of He beam scattered from the surface. That is, the locations of the diffraction peaks reveal the symmetry of the T. Kondo (&) Faculty of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan e-mail: takahiro@ims.tsukuba.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_39 239 240 T. Kondo two-dimensional space group that characterizes the observed outermost surface structure of the crystal. The width of the diffraction peaks reflects the size of the domain composed by ordered structure on the surface, and the detection limit size is determined by the energy spread of the beam. The elastic scattering is governed by two kinematic conditions—conservation of energy and the energy of the momentum component parallel to the crystal surface: Ef ¼ Ei ðk2i ¼ k2f ¼ k2f == þ k2f ? Þ kf ? ¼ ki? þ G Here, Ei and Ef are incident and scattered He beam energy, G is a reciprocal lattice vector of the crystal surface, ki and kf are the initial and final wave vectors of the helium atom, and // and ? represent the parallel and perpendicular component with respect to the surface. The Ewald sphere construction will determine the diffracted beams to be seen and the scattering angles at which they will appear. A characteristic diffraction pattern will appear in the angular intensity distribution of He beam scattered from the surface, which is determined by the periodicity of the surface, in a similar manner to that seen for Bragg scattering in electron and X-ray diffraction. Corrugation of the surface structure seen by He, i.e., corrugation of the potential between He and the surface, significantly affects the diffraction intensities of He. For example, distinct diffraction peaks can be observed from ionic-crystal surface such as LiF(001) owing to the large corrugation amplitude of the outermost electron cloud originated from the localized feature of the electronic structure, while very weak diffraction intensities except for the 0th order (specular) diffraction intensity can be observed for the close-packed single-crystal metal surface such as Pt(111) because of the flat outermost electron cloud reflecting the free-electron feature of metal surface. The advantage of the measurement is that the surface structure of the metal, semiconductor, insulator, and even molecular layer can be sensitively measured without any beam damage and perturbation of the surface owing to the neutral charge and low kinetic energy of the He beam (5–200 meV). Especially, superstructures of the surface even composed by small atoms such as hydrogen can be sensitively analyzed which is a sharp contrast to electron or X-ray diffractions. All of the obtained information reflects from the information of the outermost surface because He cannot penetrate the first layer of the surface. Rich surface information such as surface Debye temperature, morphological change, diffusion behavior of adspecies, and phase transition can also be derived by monitoring the intensity of the elastic scattering of He as a function of surface temperature. The sensitivity of the He for the detection of the morphological change is known to be extremely high owing to the large scattering cross section (c. a.100 Å2) of the low energy He atom toward adsorbed molecule or defect (the object which causes the modulation of the attractive potential of the surface). That is, the number of surface adspecies at coverage around 0.001 can be determined. The measurements focusing on the specularly reflected He beam were often called as the thermal energy He atom scattering (TEAS) [3]. 39 Helium Atom Scattering 39.3 241 Inelastic Scattering [1, 2, 4] Single-phonon scattering in the inelastic scattering component derived from the time-of-flight measurement (measurement of the He energy scattered from the surface) has been used to construct the complete surface phonon dispersion curves. The kinematics of the phonon annihilation or creation process are extremely simple—conservation of energy and momentum can be combined to yield an equation for the energy exchange DE and momentum exchange q during the collision process. This inelastic scattering process is described as a phonon of energy DE = ћx and wave vector q. The vibrational modes of the lattice can then be described by the dispersion relations x(q), which give the possible phonon frequencies x as a function of the phonon wave vector q. Time-of-flight measurements have also unveiled the vibrational energies of the adspecies such as that for the hindered translational mode of the adsorbed molecule on the surface toward parallel direction to the surface (Fig. 39.1). The vibrational energy of this mode is especially known to be quite difficult to measure with the typical vibrational spectroscopy methods such as infrared reflection absorption spectroscopy and high-resolution electron energy loss spectroscopy due to their selection rules in the measurement, but HAS can detect. 39.4 Features • Outermost ordered surface structure can be analyzed without beam damage and perturbations. • Phonon dispersion curve can be created. • Hindered translational mode (parallel to the surface) of the adsorbed molecule on the surface can be detected. • Surface Debye temperature, morphological change, diffusion behavior of adspecies, and phase transition can be analyzed. Fig. 39.1 Schematic of the hindered translational mode of CO parallel to the metal surface 242 39.5 T. Kondo Instrumentation Apparatus for HAS required at least five stainless-steel chambers as shown in Fig. 39.2. Each chamber should be independently pumped to ultrahigh vacuum. The supersonic helium atom beam, with a very narrow energy spread of less than 1 meV, can be created through free-jet expansion of helium at a pressure of *2 107 Pa into a low-vacuum chamber (*10−2 Pa at the steady state during beam operation) through a nozzle with small diameter of *5–10 lm. Depending on the system operating nozzle temperature range, typical helium atom energies produced can be 5–200 meV with an energy spread of ΔE/E ≅ 2% [1, 2, 5]. A conical aperture called the skimmer extracts the center portion of the helium beam. At this point, the atoms of the helium beam are moving with nearly uniform velocity as free-molecular flow (without collision with other He atoms) owing to the differential pumping systems. Chopper is responsible for creating the beam pulses which is used for the time-of-flight measurements or angular intensity distribution measurements. In time-of-flight measurements, time required to reach the detector from the chopper can be measured by using multi-channel scaler and trigger signal simultaneously generated with the pulse of beam at the chopper. The scattered He beam intensity from the surface can be analyzed by monitoring the scattered intensity toward the direction with the geometric condition of incident angle hi and scattering angle hf as shown in Fig. 39.2. To measure the angular intensity distributions, the sample is thus required to rotate toward polar and azimuthal directions with high angular resolution. These angular resolutions, size of the aperture at the entrance of the detector chamber, and the distance between sample and detector determine the angular resolution of the apparatus for the Fig. 39.2 Experimental setup for HAS 39 Helium Atom Scattering 243 measurement (typically 0.1° [1, 2, 5]). Also, such geometrical conditions and the energy spread of the incident He beam eventually determine the coherent length of the helium (maximum detectable periodicity of the ordered structure of the sample surface, typically 10–20 nm) [1, 2, 5]. Chopper after scattering from the sample is useful for the time-of-flight measurements because it directly shows the translational energy of He after the scattering, while the chopper prior to the collision with the surface can be useful to measure the angular intensity distribution owing to the separation of the background signal to the scattered signal [1, 2]. 39.6 Applications 39.6.1 He Diffraction Patterns from LiF(001) Figure 39.3 shows an example of diffraction measurements by HAS from LiF(001) surface. The incident angle was fixed at 0°, and the He signal scattered from the surface was detected by bolometer detector at the different scattering angles [6]. Thanks to the neutral charge of He, the diffraction patterns are well obtained even Fig. 39.3 Angular intensity distributions of He scattered from the LiF(001) surface at 80 K at the incident angle of 0° along the [110] and [100] azimuthal directions. Incident energy of He is set to be 63 meV. Reprinted from ref. [6], Copyright 1976, with permission from Elsevier 244 T. Kondo from insulator LiF(001) surface. Since the He diffraction intensity is significantly affected by the corrugation of the outermost electron cloud of the surface interacting with He, the diffraction intensity is quite different depending on the azimuthal directions even on the same LiF(001) surface. 39.6.2 Hindered Translational Mode of CO on Pt(111) Detected by HAS Figure 39.4 shows an example of time of flight by HAS from CO covered Pt(111) surface. Peaks at positive and negative energy transfer correspond to annihilation (energy gain for He) and excitation (energy loss for He) of the surface-specific modes such as phonon and molecular vibration. The two peaks to the right and left of the elastic peak (at 0 meV), which are both at jDEj ffi 6.0 meV, are attributed to the CO vibrational mode corresponding to a hindered translation of the upright molecule parallel to the surface. The additional peaks at larger energy transfer corresponding to −11.0 and +11.6 meV are attributed to the first overtone. In addition, another very weak structure has been consistently seen at +16.5 meV which can be ascribed to second overtone or bridge site hindered rotations of CO [7]. Fig. 39.4 A typical time-of-flight spectrum (converted to an energy transfer scale) measured with a CO covered Pt(111) surface at 300 K. The conditions are: exposure = 0.5 L, beam energy 32.93 meV, hi = 29°, hi + hf = 29°, and measuring time 7 h. The kinematic conditions are such that surface phonons are not expected. The numbers above the peaks indicate values of ħx in meV. Reprinted from ref. [7], Copyright 1986, with permission from Elsevier 39 Helium Atom Scattering 245 References 1. Heinz, K., Müller, K., Engel, T., Rieder, K.H.: Structural Studies of Surfaces. Springer, Berlin (1982) 2. Farias, D., Rieder, K.H.: Atomic beam diffraction from solid surfaces. Rep. Prog. Phys. 61, 1575 (1998) 3. Poelsema, B., Comsa, G. (eds.): Scattering of Thermal Energy Atoms from Disordered Surfaces. Springer-Verlag, Berlin, Springer Tracts in Modern Physics (1989) 4. Kress, W., de Wette, F.W.: Surface Phonons. Springer, Berlin (1991) 5. Kondo, T., Kato, H.S., Yamada, T., Yamamoto, S., Kawai, M.: Effect of the molecular structure on the gas-surface scattering studied by supersonic molecular beam. Eur. Phys. J. D 38, 129 (2006) 6. Boato, G., Cantini, P., Mattera, L.: A study of the (001)LiF surface at 80 K by means of diffractive scattering of He and Ne atoms at thermal energies. Surf. Sci. 55, 141–178 (1976) 7. Lahee, A.M., Toennies, J.P., Wöll, Ch.: Low energy adsorbate vibrational modes observed with inelastic helium atom scattering: CO on Pt(111). Surf. Sci. 177, 371–388 (1986) Chapter 40 High-Resolution Elastic Recoil Detection Analysis Kaoru Nakajima Keywords Ion scattering High depth resolution 40.1 Hydrogen Light element Depth profiling Principle HERDA is an advanced type of elastic recoil detection analysis (ERDA), depth resolution of which is improved. Figure 40.1 shows a schematic illustration of HERDA. In HERDA, as in a conventional ERDA, a well-collimated beam of monoenergetic He or heavier ions is incident on a solid sample. Ions of light elements elastically recoiled from the sample are energy-analyzed. Since both the incoming path of probe ions and the outgoing path of recoiled ions inside the sample are almost straight, the energy of a recoiled ion depends on the species of the recoil ion and the depth where the recoil event occurs. Therefore, the concentration and the depth profile of the light elements can be obtained nondestructively from the energy spectrum of the recoiled ions. HERDA allows highly quantitative analysis because the kinetics of elastic recoil is well-defined. In many cases, HERDA is used for hydrogen depth profiling, since many surface analysis techniques (XPS, AES, RBS) cannot detect hydrogen. Magnetic or electrostatic spectrometers are usually used to resolve the energy of the recoiled ions in HERDA [1–3]. The relative energy resolution DE/E of the spectrometer(s) is less than *0.1%, resulting in *1 nm or higher depth resolution. K. Nakajima (&) Department of Micro Engineering, Graduate School of Engineering, Kyoto University, Kyoto, Japan e-mail: nakajima.kaoru.4a@kyoto-u.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_40 247 248 K. Nakajima Fig. 40.1 Schematic illustration of HERDA 40.2 Features • Concentration of light elements including hydrogen in the near-surface region of a solid sample can be analyzed quantitatively. • Quasi-nondestructive depth profiling typically up to a few tens of nm in depth (depending on the energy of the probe ions). • Sub-nm depth resolution can be achieved at the surface. • Lateral spatial resolution is typically *1 mm. • Ultra-high vacuum (UHV) conditions are desired to minimize surface contamination of the sample. 40.3 Instrumentation The experimental equipment needed for HERDA have a lot in common with that for HRBS. In many cases, a HRBS apparatus can also be used for HERDA. As for a compact HERDA system, it consists of an ion accelerator which can produce 200– 500 keV He+ ions or heavier ions (C+, N+, O+, etc.), a UHV scattering chamber, a magnet spectrometer, and a detector. Figure 40.2 shows an example of an experimental setup of HERDA. The sample is mounted on a goniometer, which allows a precise translational and rotational motion of the sample, in the UHV scattering chamber. A well-collimated primary ion beam from the accelerator impinges on the sample. Ions recoiled from the sample are energy-analyzed with a sector magnetic spectrometer and detected with a one-dimensional position-sensitive detector (1D-PSD) placed on the focal plane of the spectrometer. Thus, an energy spectrum of the recoiled ions can be acquired without sweeping the magnetic field. Primary ions scattered from the sample will pass the magnetic spectrometer and be headed toward the detector if they have the same momentum-to-charge ratio as the recoiled ions to be analyzed. Usually, a thin foil is installed just in front of the detector to stop only the primary ions. Otherwise, an electrostatic deflector installed between magnetic spectrometer and the detector is often used to kick away the primary ions. It is preferable that the scattering chamber and the detector chamber are pumped to UHV using oil-free vacuum pumps to avoid surface contamination of the sample. The beam 40 High-Resolution Elastic Recoil Detection Analysis 249 Fig. 40.2 An experimental setup for HERDA current of the probe ions is typically *10 nA (*mm2 in beam size), and the acquisition time is 10–30 min. 40.4 Applications 40.4.1 Characterization of Subsurface Hydrogen in Diamond Films Figure 40.3 shows an example of HERDA spectrum for highly oriented diamond (HOD) films that had been treated by hydrogen/deuterium plasma before and after a conventional wet oxidation [4]. The films were irradiated by a collimated beam of 480 keV N+ ions at an incident angle of 70°. The H+/D+ ions recoiled at 30° with respect to the incident direction were analyzed by a sector magnetic spectrometer. The peak at 158.2 keV corresponds to deuterium at the surface. The spectrum for sample A revealed that the film surface was covered with deuterium (1.5 1015 cm−2) after an exposure to the deuterium plasma. After the wet oxidation, it was found that residual deuterium (3.8 1014 cm−2) was detected in the spectrum (sample B), while the surface termination changed from deuterium to oxygen. On the other hand, deuterium was not observed in sample C (annealed at 1000 °C before the wet oxidation). 40.4.2 Accumulation of Hydrogen Near a SiO2/Si(001) Interface Figure 40.4 shows another example of HERDA. SiO2 films (2.1 nm) grown on Si (001) by dry oxidation were ex situ prepared as samples for HERDA measurements. In HERDA, a beam of 400 keV C+ was incident on the sample in a UHV 250 Fig. 40.3 HERDA spectra obtained from HOD films treated by deuterium plasma before (sample A) and after conventional wet oxidation process (samples B and C). Sample C was annealed at 1000 °C for 1 h before the wet oxidation [4] Fig. 40.4 Change of depth profiles of hydrogen atoms in SiO2 (2.1 nm)/Si(001) during the irradiation of 400 keV C+ in pressure of 3 10−8 Torr. The times in the legend denote total irradiation times. The beam flux was about 5 1011 ions/(cm2 s) [5] K. Nakajima 40 High-Resolution Elastic Recoil Detection Analysis 251 scattering chamber at the angle of 20° from the sample surface. The protons recoiled at the angle of 25° were energy-analyzed with a 90° sector magnetic spectrometer and detected by a 1D-PSD. Figure 40.4 shows the change of the depth profile of hydrogen atoms during the ERDA measurements in a degraded vacuum condition (3 10−8 Torr). Whereas hydrogen density at the surface shows steep decrease at the beginning of ion beam irradiation in HERDA, hydrogen density near the SiO2/Si(001) interface increases with irradiation time, indicating that radiation damage induces hydrogen accumulation near a SiO2/Si(001) interface. References 1. Dollinger, G., Frey, C.M., Bergmaier, A., Faestermann, T.: Elastic recoil detection with single atomic layer depth resolution. Nucl. Instr. Meth. Phys. Res. B 136–138, 603–610 (1998) 2. Carstanjen, H.D.: Ion beam analysis with monolayer depth resolution. Nucl. Instr. Meth. Phys. Res. B 136, 1183–1190 (1998) 3. Kimura, K., Nakajima, K., Imura, H.: Hydrogen depth profiling with sub-nm resolution in high-resolution ERD. Nucl. Instr. Meth. Phys. Res. B 140, 397–401 (1998) 4. Hayashi, K., Kawakami, N., Ichihara, C., Kobashi, K.: Characterization of subsurface hydrogen in diamond films by high-resolution elastic recoil detection analysis. Phys. B 376– 377, 307–310 (2006) 5. Nakajima, K., Imaizumi, R., Suzuki, M., Kimura, K.: Accumulation of hydrogen near the interface between ultrathin SiO2 and Si(001) under ion irradiation in high-resolution elastic recoil detection. Nucl. Instr. Meth. Phys. Res. B 249, 425–428 (2006) Chapter 41 High-Resolution Electron Energy Loss Spectroscopy Hiroshi Okuyama Keywords Vibration spectroscopy Surface phonon 41.1 Electron scattering Molecular vibration Principle High-resolution electron energy loss spectroscopy (HREELS) is used to study normal vibrational modes of molecules on the surface or vibrations of the surface (surface phonon) under ultra-high vacuum conditions. The electrons incident on the surface can cause vibrational excitation of the adsorbed molecules. Consequently, they lose their kinetic energy by hv, which is the vibrational quantum of a molecule. Therefore, it is possible to conduct vibrational spectroscopy of molecules on the surface by observing the energy loss of the scattered electrons. The mechanism of vibrational excitation is mainly classified into two regimes: long-range (dipole) and short-range (impact) scatterings. In the former case, the electrons interact with spatially extended electric fields of the adsorbed molecules being scattered by the dynamic dipole moment induced by the vibrational excitation. Since the metallic surface screens the parallel component of the dipole moment, the normal modes that induce the dipole moment normal to the surface are allowed to be excited in the dipole-scattering regime. In other words, only the completely symmetric modes represented by A1, A′, and A are dipole-active. This is called the “surface selection rule” and is the same as the selection rule for infrared reflection absorption spectroscopy. In the latter case, the electrons interact with localized atomic potential (“collide” with the atom) leaving the molecules in an excited state. Thus, the vibrations polarized along the scattering plane are allowed to be excited by the impact scattering. These two mechanisms can be distinguished by measuring the angular dependence of the scattered electrons; dipole (long-range) scattering is observed nearly in the specular direction while impact (short-range) scattering H. Okuyama (&) Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto, Japan e-mail: hokuyama@kuchem.kyoto-u.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_41 253 254 H. Okuyama results in a relatively isotopic distribution. It is important to note that due to the low penetration depth of low-energy electrons, HREELS has very high surface sensitivity. This makes this technique particularly useful in surface science. 41.2 Features • Vibrational modes of molecules on the surface can be detected. • The structure of adsorbed molecules can be studied by applying the selection rule of electron scattering. 41.3 Instrumentation The operational design of HREELS has been undergoing improvements for more than 50 years [1–7] and its latest version has an energy resolution below 1 meV [6]. A first-generation apparatus is depicted in Fig. 41.1 [5]. The spectrometer is mainly composed of a filament, monochromator, analyzer, and electron multiplier. Electrical lenses are used to guide the electrons and optimize their paths. The electrons thermally emitted from the cathode are brought into the monochromator, and energy-selected electrons in an accelerated state are made incident on the sample surface. The scattered electrons are decelerated and then passed through the analyzer. The kinetic energy of the scattered electrons is determined by scanning the deceleration voltage. The multiplier allows the detection of low current because of which low signals from the molecular vibrations can be observed. The instrumental energy resolution of HREELS is determined by the pass energy of electrons through the monochromator and analyzer. Lower pass energy increases the energy resolution but simultaneously reduces the current. The high intensity of the energy loss peaks (current of the electrons) is required for sufficient signal-to-noise ratio to enable fast data acquisition. Therefore, it is important to ensure an optimal balance between the energy resolution and current depending on the objective of the study. 41 High-Resolution Electron Energy Loss Spectroscopy 255 Fig. 41.1 First-generation experimental setup for HREELS. Reprinted with permission from ref. [5]. Copyright 1982, American Institute of Physics 41.4 Applications 41.4.1 Ethylene Adsorption on Pd (110) Figure 41.2a shows an example of HREEL spectra for ethylene (C2H4 and C2D4) on Pd (110) recorded in the specular mode [8]. The modes observed here are mainly excited by the dipole-scattering mechanism. The primary electron energy was 4.7 eV, and the measurements were taken at 90 K. For comparison, Fig. 41.2b shows the spectra of ethylene on H-covered Pd (110). The coverage was approximately 0.1 ML (monolayer; 1 ML corresponds to the number of metal atoms on the Fig. 41.2 a HREEL spectra for C2H4 (lower) and C2D4 (upper) on Pd (110). b HREEL spectra for C2H4 (lower) and C2D4 (upper) on H-covered Pd (110) [8] 256 H. Okuyama metal surface) in both cases. The energy of each ethylene-derived peak is indicated in meV. The surface selection rule was applied to determine the adsorption structure of ethylene on the Pd surfaces [8]. The peaks corresponding to 39 and 46 meV in the case of C2H4 and 38 and 45 meV in the case of C2D4 in Fig. 41.2a were attributed to the symmetric and asymmetric Pd–C stretching modes, respectively, on the clean surface. The slight isotope shifts support this argument. The atomic displacements for these modes are depicted in Fig. 41.3a, b. Asymmetric stretching mode was observed in the dipole-scattering regime, which suggests that the C–C axis of the molecule was tilted with respect to the surface plane. This is based on the surface selection rule, which states that a completely symmetric mode can alone be dipole-active. On the other hand, the Pd–C stretching mode was observed at 38 meV for C2H4 and 37 meV for C2D4 in H-covered Pd (110) as shown in Fig. 41.2b. However, the asymmetric counterpart was absent here. This indicates that the C–C axis is flat on the H-covered surface. Likewise, the C–H2 wagging modes were observed at 112 and 137 meV in C2H4 and at 83 and 112 meV in C2D4 on the clean surface (Fig. 41.2a), which were associated with the symmetric and asymmetric C–H2 wagging motion, respectively (Fig. 41.3c, d). The latter was observed in the dipole-scattering regime because the tilt of the C–C axis caused this mode to be completely symmetric and thus dipole-active. On the other hand, the symmetric mode alone was observed at 113 meV in C2H4 and at 84 meV in C2D4 on the H-covered surface, which indicates again that the molecules are flat on the H-covered surface. As illustrated here, the structure (symmetry) of adsorbed molecules on metal surfaces can be studied by applying the surface selection rule to the HREEL spectra. It may be noted that the correct assignment of individual peaks by using various methods such as isotope shifts is essential for deducing such information from the spectra. Fig. 41.3 Approximate atomic displacements of vibrational modes (red arrows) for ethylene molecules on the Pd surface (side view) (a) symmetric and (b) asymmetric Pd–C stretching modes, and (c) symmetric and (d) asymmetric C–H2 wagging modes 41 High-Resolution Electron Energy Loss Spectroscopy 257 References 1. Ibach, H., Mills, D.L.: Electron Energy Loss Spectroscopy and Surface Vibrations. Academic Press, New York (1982) 2. Propst, F.M., Piper, T.C.: Detection of the vibrational states of gases adsorbed on tungsten by low-energy electron scattering. J. Vac. Sci. Technol. 4, 53–56 (1967) 3. Ibach, H.: Optical surface phonons in zinc oxide detected by slow-electron spectroscopy. Phys. Rev. Lett. 24, 1416–1418 (1970) 4. Andersson, S.: Surface vibrations of oxygen and sulphur in the p(22) and c(22) structures on Ni(100). Surf. Sci. 79, 385–393 (1979) 5. Nishijima, M., Masuda, S., Kobayashi, H., Onchi, M.: Apparatus for high-resolution vibrational electron energy-loss spectroscopy of solid surfaces. Rev. Sci. Instrum. 53, 790–796 (1982) 6. Ibach, H.: Electron energy loss spectroscopy with resolution below 1 meV. J. Electron Spectrosc. Relat. Phenom. 64(65), 819–823 (1993) 7. Nagao, T., Iizuka, Y., Umeuchi, M., Shimazaki, T., Nakajima, M., Oshima, C.: Construction of a high-resolution electron energy loss spectrometer. Rev. Sci. Instrum. 65, 515–516 (1994) 8. Okuyama, H., Ichihara, S., Kato, H., Yoshinobu, J., Kawai, M.: Molecular rearrangement induced by hydrogen co-adsorption: C2H4 on Pd(110). Chem. Phys. Lett. 310, 451–458 (1999) Chapter 42 High-Resolution Rutherford Backscattering Spectrometry Kaoru Nakajima Keywords Ion scattering Elemental composition profiling Sub-nm depth resolution 42.1 Non-destructive depth Principle High-resolution Rutherford backscattering spectrometry (HRBS) is an advanced type of Rutherford backscattering spectrometry (RBS), depth resolution of which is improved to sub-nm. Therefore, the principle of HRBS is the same as RBS and its characteristics are mostly common with RBS, for example, quantitative analysis of elemental composition, non-destructive depth profiling. Figure 42.1 shows a schematic illustration of HRBS. In HRBS, as in conventional RBS, a beam of monoenergetic He+ ions is incident on the solid sample, and the energy spectrum of He+ ions scattered from the sample at a given scattering angle is acquired. Since almost all scattered ion emerging from the sample undergo only a single large-angle scattering near the surface, the energy of the scattered ion contains information on the mass of the target atom that scattered the ion and the depth where the scattering occurs. This is why one can analyze the elemental contents and their depth profiles. In addition, highly quantitative elemental composition can be obtained because the differential cross section for elastic scattering into a given angle is approximately proportional to the square of the atomic number of the target atom. The improved depth resolution of HRBS is achieved by use of sub-MeV (typically 300–500 keV) He+ probe ions and a magnetic spectrometer analyzing the energy of the scattered ions. In this sense, HRBS is quite similar to medium energy ion scattering (MEIS), which achieves high depth resolution typically using medium energy protons or He ions as probe ions and an electrostatic energy analyzer. The energy resolution (DE/E) is totally about 10−3 in HRBS so that depth resolution of sub-nm can be K. Nakajima (&) Department of Micro Engineering, Graduate School of Engineering, Kyoto University, Kyoto, Japan e-mail: nakajima.kaoru.4a@kyoto-u.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_42 259 260 K. Nakajima Fig. 42.1 Schematic illustration of HRBS achieved in the near-surface region. A monolayer resolution can be achieved for a few topmost atomic layers, provided that the surface of the sample is atomically flat and a scattering geometry with grazing exit angle (typically 3°–5°) from the surface is adopted. The depth of information in HRBS is typically a few tens of nm, depending on the energy of the probe ions and the scattering geometry. 42.2 Features • Elemental composition in the near-surface region of a solid sample can be analyzed quantitatively. • Non-destructive depth profile to a depth of typically a few tens of nm can be obtained with *nm or higher depth resolution. • A few topmost atomic layers can be analyzed layer-by-layer. • Lateral spatial resolution is typically *1 mm. • Ultra-high vacuum (UHV) conditions are desired to minimize surface contamination of the sample. 42.3 Instrumentation Figure 42.2 shows an example of an experimental setup for HRBS. The typical HRBS system consists of an ion accelerator which can produce 300–500 keV He+ ion, a UHV scattering chamber, a magnetic spectrometer, and a detector. The Fig. 42.2 Experimental setup for HRBS 42 High-Resolution Rutherford Backscattering Spectrometry 261 sample is mounted on a goniometer, which allows a precise translational and rotational motion of the sample, in the UHV scattering chamber. A well-collimated beam of monoenergetic He+ ions with relative energy spread <0.1% is incident on the sample. He+ ions scattered by target atoms near the surface at a given angle are energy-analyzed with a sector magnetic spectrometer and detected with a one-dimensional position-sensitive detector (1D-PSD) placed on the focal plane of the spectrometer. Thus, an energy spectrum of the scattered ions over wide energy range (depending on the size of 1D-PSD) can be acquired without sweeping the magnetic field. It is preferable that the scattering chamber and the detector chamber are pumped to UHV using oil-free vacuum pumps to avoid surface contaminations. The beam current of the probe ions is typically several tens of nA (*mm2 in beam size) and the acquisition time is 10–20 min. 42.4 Applications 42.4.1 Layer-by-Layer Analysis of an Epitaxial Film Figure 42.3 shows an example of HRBS spectrum for a (111) surface of lead selenide (PbSe) grown epitaxially on a (111) surface of barium fluoride (BaF2) single crystal [1]. The probe ions were 300 keV He+ ions. The scattering angle and the exit angle with respect to the surface plane were 65° and 2.9°, respectively. The arrows indicate the calculated energies of He–Pb and He–Se elastic scattering, respectively. There are several peaks in the spectrum, which correspond to the individual atomic layers near the surface. The peaks at 292.6, 291, 288.8, 281.6, and 279.3 keV are those for the ions scattered from Pb atoms in the first, second, third molecular layers, and Se atoms in the first, second molecular layers, respectively. From the inelastic energy loss for each peak, it is found that the surface is terminated by a Pb atomic plane. It is also found that the areal atomic density of the topmost Pb atomic plane is about 40% of that of the bulk (111) atomic plane. Fig. 42.3 HRBS spectrum for a (111) surface of PbSe grown epitaxially on a (111) surface of BaF2 single crystal. The successive atomic layers near the surface are resolved as separated peaks [1] 262 K. Nakajima Fig. 42.4 (Left panel) HRBS spectrum of a HfO2/Si(001) sample for the incidence of 400 keV He+ ions. (Right panel) Depth profiles of Hf, Si, O, and Cl (5) atoms in HfO2/Si(001) derived from the observed HRBS spectrum. The solid curve shows twice the Hf concentration [2] 42.4.2 Interface Between a High-j Dielectric Film and a Si Substrate Figure 42.4 shows another example of HRBS analysis. An ultrathin HfO2 film *3 nm thick was prepared on p-type Si(001) by means of atomic-layer chemical vapor deposition (ALCVD) at 300 °C. The HfO2/Si(001) interface was observed ex situ with HRBS [2]. The probe ions were 400 keV He+ ions and the scattering angle was 50°. There is a prominent hafnium peak at *390 keV and an oxygen peak at *330 keV superimposed on a step with a leading edge at *350 keV corresponding to the Si substrate in the HRBS spectrum (left panel of Fig. 42.4). A small peak at *360 keV is attributed to Cl contamination, which may originate from the HfCl4 precursor. Depth profiles of Hf, Si, O, and Cl atoms derived from the observed HRBS spectrum are shown in the right panel of Fig. 42.4. A solid line shows twice the Hf concentration. An almost stoichiometric HfO2 film was formed by ALCVD. However, there are excess oxygen atoms in the interface region, showing formation of a thin SiOx (*1 nm) layer between the HfO2 film and the Si (001) substrate. References 1. Kimura, K., Mannami, M.: RBS with monolayer resolution. Nucl. Instr. Meth. B 113, 270–274 (1996) 2. Nakajima, K., Joumori, S., Suzuki, M., Kimura, K., Osipowicz, T., Tok, K.L., Zheng, J.Z., See, A., Zhang, B.C.: Characterization of HfO2/Si(001) interface with high-resolution Rutherford backscattering spectroscopy. Appl. Surf. Sci. 237, 416–420 (2004) Chapter 43 High-Speed Atomic Force Microscopy Takayuki Uchihashi Keywords Single-molecule imaging Molecular interaction 43.1 Protein Conformational dynamics Principle The basic principle of HS-AFM is similar to a conventional AFM in which force interactions between a sharp needle at the end of a cantilever and a solid surface is detected through the deflection of the cantilever [1]. Among various operation modes of AFM, HS-AFM employs so-called tapping mode. In the tapping-mode AFM, the cantilever is vertically oscillated at its first resonant frequency as shown in Fig. 43.1 [2]. The intermittent tip-sample contact can reduce both lateral and vertical force acting on the sample from the tip during the lateral scanning and thus prevent serious damage of fragile biological macromolecules weakly adsorbed on the solid substrate. The feedback control keeps the oscillation amplitude of the cantilever constant and the feedback signal constructs the topographic image of sample surface. For fast and nondestructive imaging with AFM, all components contained in the feedback loop; i.e., the cantilever, the optical-beam-deflection sensor, the amplitude detector, the PID feedback circuit, the piezoactuator-based scanner are optimized. 43.2 Features • Frame rate of HS-AFM imaging can achieve 12.5 fps. • Biological samples can be imaged in a physiological environment. T. Uchihashi (&) Department of Physics, Nagoya University, Aichi, Japan e-mail: uchihast@d.phys.nagoya-u.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_43 263 264 T. Uchihashi Fig. 43.1 Basic principle of tapping-mode AFM • Dynamics events of biological molecules such as conformational change and molecular interaction can be captured in real time. • Disturbance of physiological function of proteins is minimized. 43.3 Instrumentation Figure 43.2 shows the experimental setup of HS-AFM. The key components of HS-AFM are the cantilever and the scanner [3]. The cantilever used in HS-AFM should have a high-resonant frequency and a small spring constant to meet both fast and low-invasive imaging and thus inevitably decreases in its size. Most advanced small cantilever for HS-AFM with dimensions of *6 lm long, *2 lm wide, and *90 nm thick has the resonant frequencies *3.5 MHz in air and *1.2 MHz in water, a spring constant *0.2 N/m, and a quality factor *2 in water. The optical-beam-deflection method using an objective lens is used to detect the cantilever oscillation and the oscillation amplitude is detected by the Fourier detector which can measure the oscillation amplitude every cycle instead of a lock-in amplifier. The scanner needs high mechanical stability to suppress unwanted vibrations due to the fast raster scanning and hence is designed to be a simple and rigid structure based on a flexure stage. So far three types of scanners with different maximum scan ranges have been developed (Type-1: x = 1 lm, y = 4 lm, z = 1 lm; Type-2: x = 5 lm, y = 5 lm, z = 2 lm; and Type-3: x = 40 lm, y = 40 lm, z = 6 lm). The scanner of Type-1 is usually used for dynamic imaging of isolated proteins, while the latter two types are used for dynamic imaging of larger samples such as bacteria and eukaryotic cells. For quick motions of the zwithout mechanical excitations, the active Q-control circuit with mock z-scanners (LRC circuits) are implemented to damp mechanical vibrations. The X-scan signal for the fast lateral scanning is made though inverse transfer function compensation to damp unwanted vibrations at the turning point of triangle waves. 43 High-Speed Atomic Force Microscopy 265 Fig. 43.2 Schematic of experimental setup of HS-AFM 43.4 Applications Applications of HS-AFM covers a wide range of dynamic molecular events of biological systems such as structure dynamics, self-assembly processes, dynamic protein-protein interactions, diffusion processes. Also the HS-AFM can be applied to not only biological samples but also observing dynamic events on membrane/solid interface. Here two typical examples are illustrated. 43.4.1 Hand-Over-Hand Walking of Myosin V Myosin V is one of motor proteins associated to actin filament that works as transporter of intracellular cargos. Myosin V has two motor domains with actin binding and ATP hydrolysis sites and each motor domain is connected to extended neck regions which are dimerized by coiled-coil a-helix. Numerous studies using biophysical and biochemical techniques have been applied to elucidate the functional mechanism of myosin V. Finally, the hand-over-hand movement coupled with ATP hydrolysis has been proposed. Figure 43.3a shows successive images of myosin V walking along actin filament under the presence of ATP [4]. As a substrate, a lipid bilayer surface containing biotinylated lipid formed on mica is 266 T. Uchihashi Fig. 43.3 HS-AFM imaging of myosin V walking along actin filament. a Successive AFM images showing unidirectional processive movement of myosin V observed in 1 lM ATP (upper images, scale bar: 30 nm). Lower schematic shows two-headed bound myosin V on actin filament. b Successive AFM images capturing the hand-over-hand movement of myosin V (scale bar: 50 nm). The swinging lever is marked by thin white lines. All images were taken at a 146.7 ms/frame. c Schematic of myosin V motion suppressed by streptavidin molecules used. Partially biotinylated actin filament is anchored on the lipid surface through streptavidin-biotin binding while the myosin V is inert to the lipid surface. Further if excessive amounts of streptavidin are fixed on the biotinylated lipid, the streptavidin molecules can work as diffusion barrier of the myosin V and then slow down the motion of myosin V (Fig. 43.3b, c). The slow-motion pictures clearly show the hand-over hand fashion of the myosin V in which the detached trailing head rotates around the neck-neck junction, moves ahead and attaches the actin. 43.4.2 Crystal Dynamics of Annexin V Annexin V is a soluble protein that binds to negatively charged phospholipids and known to form two-dimensional crystal with a honeycomb structure on the lipid by assembly of the trimeric oligomer. The ‘holes’ of the honeycomb structure tend to be occupied with a relatively mobile trimer, which undergoes a more relaxed interaction with its surrounding cage than a molecule forming part of the honeycomb lattice. Figure 43.4a shows successive AFM images of the two-dimensional crystal of 43 High-Speed Atomic Force Microscopy 267 Fig. 43.4 Dynamics of annexin V crystal imaged by HS-AFM. a Binding dissociation dynamics of the trimer weakly trapped in the center hole of the honeycomb structure (imaging rate: 0.5 s/frame, scan area: 150 150 nm2). b Rotational diffusion of the trimer trapped in a lattice cage (imaging rate: 0.2 s/frame, scan area :50 50 nm2) annexin V demonstrating dynamic dissociation and association of trimers at the center hole of the honeycomb structure. Also, the rotational diffusion of a center trimer weakly bound to the surrounding cage can be captured. The center trimer encircled in Fig. 43.4b rotates counterclockwise with a 60° step indicating that the central trimer can assume two stable positions with identical association energy. References 1. Binnig, G., Quate, C.F., Gerber, C.: Atomic force microscope. Phys. Rev. Lett. 56, 930–933 (1986) 2. Zhomg, Q., Iniss, D., Kjoller, K., Elings, V.: Fractured polymer/silica fiber surface studied by tapping mode atomic force microscopy. Surf. Sci. 290, L688–L692 (1993) 3. Ando, T., Uchihashi, T., Fukuma, T.: High-speed atomic force microscopy for nano-visualization of dynamic biomolecular processes. Prog. Surf. Sci. 83, 337–437 (2008) 4. Kodera, N., Yamamoto, D., Ishikawa, R., Ando, T.: Video imaging of walking myosin V by high-speed atomic force microscopy. Nature 468, 72–76 (2010) Chapter 44 Imaging Ellipsometry Akiko N. Itakura Keywords Optical constant Polarization change Thickness mapping Surface uniformity 44.1 Imaging Principle The measured signals in imaging ellipsometry are the change in polarization as the incident radiation interacts with the material structure of interest at each point on the surface. Imaging ellipsometry, which combines the power of ellipsometry with microscopy, enhanced spatial resolution of imaging ellipsometers, expands ellipsometry into new areas of microanalysis, microelectronics, and bio-analytics. The polarization change is quantified by the amplitude ratio, W, and the phase difference, D with p-component of the complex amplitude reflectance of the reflected light (rp) and s-component (rs). If a sample is the multilayer film, rp and rs are given as a function of the refractive index (~n = n − ik) of the film and substrate, film thickness (d), incident angle (U), and the wavelength (k). These are referred to as Fresnel coefficient [1, 2]. Details of parameters and physics properties are shown in the chapter of ellipsometry. There are three ways to measure the polarization state of each point on the surface: moving the sample stage, moving the position of the probe incident light beam, and expanding beam on the surface. There are cases of the combination of these ways. A.N. Itakura (&) Surface Physics and Characterization Group, National Institute for Materials Science, Tsukuba, Japan e-mail: itakura.akiko@nims.go.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_44 269 270 44.2 A.N. Itakura Features • Imaging ellipsometry can be used to characterize composition, roughness, thickness (depth), crystalline nature, concentration, electrical conductivity, and other material properties. • Imaging ellipsometry can detect distributions of layer thicknesses, microstructure, composition, layer optical properties, and their uniformity. • The thicknesses, microstructure, composition, layer optical properties, and their uniformity of thin films can be measured during deposition in situ by an imaging ellipsometry. 44.3 Instrumentation 44.3.1 Single-Spot Ellipsometry There are 3 experiment types to get a map taken by imaging ellipsometry. Two types are by moving single-spot beam on a sample with stage driving and with beam scanning. The spatial resolution of the measurement is determined by the beam diameter of the spotlight and the motor driving mechanism of a stage. For example, the special limit under 0.1 mm was done with 0.1 mm x–y control of motor drive of the sample stage, a diameter of 0.8 mm spotlight of incident beam, and a pinhole before the analyzer arm (Five Lab Co., Ltd, MARY-102). The primary problem with mapping probe of single-spot spectroscopic ellipsometry is their reduced utility due to the long measurement time, when a large area is measured. The other one type is by expanded beam to a line or a large area. It is developed a high-speed measurement method to monitor the thin-film photovoltaic (PV) process in-line over large areas. 44.3.2 Expanded Beam Ellipsometry Another type of imaging ellipsometer needs expanded beam, an objective lens, and a spatially resolving detector, as CCD camera. Ellipsometry determines angle-of-incidence-dependent relative amplitude ratios and phase difference shifts upon specular reflection of light from a planar surface. Thus, collimated light beams are conventionally used with a well-defined angle of incidence at the reflecting surface. In a setting of Fig. 44.1, the sample is illuminated by “divergent beam” of light with diverse angles of incidence at every point of the sample [3]. Precise “angle-selection” is performed on the detector side by a pinhole camera. The angular resolution of this type of camera is dependent on the diameter of the 44 Imaging Ellipsometry 271 pinhole. In the case of a single wavelength measurement, this instrument solution is appropriate for measuring a large area sample with possible non-uniform properties such that the pinhole camera selects a single angle of incidence corresponding to each sample point, but that angle varies across the sample. By repeating the measurement with light of various wavelengths, multi-angle, multi-wavelength (but not continuous spectroscopic), ellipsometry data can be acquired. Figure 44.1 shows one setting-up example of imaging ellipsometer of beam expanding, for a large area measurement, rolling sample under deposition process. By confining the number of measured sample points to a narrow range along a line only, we can generate continuous spectroscopic data at each point along the line image using white light illumination with a spectral dispersion (grating) after the pinhole. This instrument produces spatial information in one dimension of the CCD array simultaneously with spectroscopic information in the perpendicular direction of the array. Both ellipsometric approaches, multi-angle/multi-wavelength and continuous spectroscopic, can be used, for example, in the analysis of product moving along a coating line. Fig. 44.1 Light from the light source expanded at rectangular aperture through film polarizer and illuminated on sample surface, which is in deposition chamber and under deposition process. Expanded reflected beam from sample surface refocused by spherical mirror and goes through compensator, analyzer, correcting mirror, pinhole, and into detection system, which has dispersion optics and CCD camera, reflected at spherical mirror (6) sample, (7) cylindrical mirror, corrected beam, (8) analyzer, (9) pinhole, (10) correction-dispersion optics, (11) CCD detector array 272 44.4 A.N. Itakura Applications Non-destructive analysis tools are needed at all stages of thin-film photovoltaic (PV) development and on production lines. In thin-film PV, layer thicknesses, microstructure, composition, layer optical properties, and their uniformity (because each elementary cell is connected electrically in series within a big panel) serve as an important point in the reliability and the evaluation. The dielectric functions of each component material can cover all variants of the material during the coating process. The imaging ellipsometer (J.A. Woollam Co., M-2000DI) succeeds to measure the different types of PV layers (anti-reflective coating, transparent conductive oxide (TCO), multi-diode-structure, absorber, and window layers), showing the existing dielectric function databases for the thin-film components of CdTe, CuInGaSe2, thin Si, and TCO layers. Off-line point-by-point mapping can be effective for characterization of non-uniformities in full-scale PV panels in developing laboratories, while the online mode is slow, when only 15 points can be obtained (within 1 min) as a 120-cm-long panel moves by the mapping station. The instrumentation was developed to provide a line image of spectroscopic ellipsometry (wavelength = 350–1000 nm) data. The magnetron-sputtered polycrystalline CdS and CdTe thin films were modeled by dielectric functions taken from real-time spectroscopic ellipsometry measurement and the model contained surface roughness and interface layers, too. Reference 3 shows the thickness of the CdTe layer and the “total thickness” (all layers in the models, including the surface roughness and interface layers). Figure 44.2 shows the thickness maps with the point-by-point mapping measurements (left) and imaging/mapping with the expanded beam instrument (right) for a SLG/Mo/CdTe/CdS sample. The total thickness is the sum of the thicknesses including the surface roughness and interface layers for each spatial point. If the lateral thickness change (inhomogeneity of the sample) is well over 1 nm in 1 mm distance, then the expanded beam ellipsometer does not measure the same thing as single-spot ellipsometers. In the reference measurement, which is the case of 80-nm-thick SiO2 film, the inhomogeneity was 6 nm/6 cm = 0.1 nm/mm, so the inhomogeneity within the expanded beam spectroscopic ellipsometry’s elementary area (30 cm) is 0.5 nm. In the case of the CdTe/CdS sample, the inhomogeneity is 300 nm/4 cm = 7.5 nm/mm, so the inhomogeneity within the expanded beam spectroscopic ellipsometry’s elementary area is 40 nm. However, the difference between the mapping results is below 1% in all cases. It was built a 30 and a 60 cm width expanded beam ellipsometer, the speed of which will be increased. Then, 1800 points can be mapped in a 1-min traverse of a 60 120 cm PV panel or flexible substrate. The thin films can be measured during deposition in situ and in real time [3]. The next example is a mapping of refractive index. Photoinduced diffraction grating formation in amorphous As2S3 thin films has been studied using imaging ellipsometry [4]. The Imaging Spectroscopic Ellipsometer (ACCURION, Nanofilm-EP3SE) was applied to obtain mapping of diffraction gratings in 44 Imaging Ellipsometry 273 Fig. 44.2 Thickness mapping with the point-by-point mapping measurements (left) and imaging/mapping with the expanded beam instrument (right) for a SLG/Mo/CdTe/CdS sample Fig. 44.3 Refractive index map, profile, and real refractive index 3-D image of recorded diffraction gratings: a underexposed; b proper exposed; c overexposed 274 A.N. Itakura amorphous As2S3 thin films based on different photoinduced phenomena, namely photodarkening and photoinduced changes of refractive index. A thin As2S3 film (about 1.0 lm) was deposited on an optical glass substrate at room temperature at a deposition rate of 100 Å/s. Monitoring of the As2S3 film thickness was carried out during the evaporation by an interference technique at a wavelength of 940 nm. Figure 44.3 a–c shows the topography of the refractive index pattern of areas with underexposed, properly exposed, and overexposed gratings, obtained by imaging ellipsometry. Imaging ellipsometry can be used to measure multiple parameters simultaneously. It was shown that refractive index of the gratings was investigated exactly and quantitatively by imaging ellipsometry. It was determined that the maximum change in refractive index was 0.035 ± 0.0002 when measured at room temperature (20 °C) for diffraction gratings recorded in As2S3 films. The magnitude of achievable index change is dependent on the exposure time and condition. It is shown that underexposure led to the formation of a sinusoidal profile of the refractive index. The proper exposure produced results close to the cycloidal profile. Overexposure led to the same cycloidal profile but with reduced amplitude of refractive index variation in comparison with that obtained under proper exposure. References 1. Azzam, R.M.A., Bashara, N.M.: Ellipsometry and Polarized Light. North Holland, Amsterdam (1987) 2. Tompkins, H.G., Irene, E.A.: Handbook of Ellipsometre. Willian An drew, New York (2005) 3. Fried, M., Juhasz, G., Major, C., Petrik, P., Polgar, O., Horvath, Z., Nutsch, A.: Thin Solid Films 519, 2730–2736 (2011) 4. Röling, C., Thiesen, P., Meshalkin, A., Achimova, E., Abashkin, V., Prisacar, A., Triduh, G.: J. Non-Cryst. Solids 365, 93–98 (2013) Chapter 45 Impact Collision Ion Scattering Spectroscopy Masakazu Aono and Mitsuhiro Katayama Keywords Ion scattering spectroscopy (ISS) Structure Composition Subsurface 45.1 Coaxial ICISS (CAICISS) Principle Ion scattering spectroscopy (ISS) [1] is a real-space method that enables simultaneous analysis of the composition and structure of solid surfaces by utilizing elastic scattering of ions at the surfaces. In ISS, a low-energy ion beam impinges on the specimen surface at a given angle of incidence and the energy spectrum of the scattered ions is measured. If the ion and target atom masses (m, M) and the scattering angle H are given, the scattered-ion energy E (incident-ion energy, E0) can be unambiguously determined by h i2 E ¼ E0 ½m=ðm þ MÞ 2 cos H þ ðM 2 =m2 sin2 HÞ1=2 : ð1Þ Different target atoms accordingly yield different scattered-ion energies, thus enabling the composition analysis of the solid surfaces. If the angle of ion incidence or scattered-ion detection is scanned and the change in the energy spectrum is measured, the surface atomic arrangement can also be analyzed by utilizing two “classical” effects: the shadowing effect, which prevents scattering by an atom located behind another atom from the perspective of the incident ion; and the blocking effect, which prevents detection of an ion scattered by an atom nearer to the detector. By virtue of the use of low-energy ions (on the order of keV) in ISS, M. Aono (&) International Center for Material Nanoarchitectonics (MANA), National Institute for Materials Science, Tsukuba, Japan e-mail: AONO.Masakazu@nims.go.jp M. Katayama Graduate School of Engineering, Osaka University, Suita, Osaka, Japan e-mail: katayama@nmc.eei.eng.osaka-u.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_45 275 276 M. Aono and M. Katayama the radius of the shadow cone (a cone-shaped shadow that forms behind a surface atom and cannot be entered by incident ions) is as large as an atomic radius, and therefore highly sensitive measurements for the outermost surface layer are possible. The large scattering cross section of ions and the capability for rapid measurement further enhances the potential for real-time observation of dynamic changes in surface structures. If low-energy noble-gas ions are used, the probability of their neutralization during scattering from individual surface atoms tends to be high and is strongly dependent on the electronic state of those atoms. Impact collision ion scattering spectroscopy (ICISS) [2] is designed for further realization of the potentialities of ISS through specialization by taking an experimental scattering angle at 180°, in which the observed ions are those that collide head-on with the surface atoms. In effect, it enables the center of each surface atom to be “seen” via perfect backscattering of ions, and facilitates quantitative analysis of the atom arrangement. The ideal in ICISS is complete achievement of an experimental scattering angle of 180°, which is attained in coaxial impact collision ion scattering spectroscopy (CAICISS) [3, 4]. Its salient features include all of those provided by ICSS. 45.2 Features • The center of each surface atom can be “seen” via perfect backscattering of ions at 180°. • Quantitative analysis of composition and structure of solid surfaces can be made easily. • Not only outermost surface layer but also subsurface atomic layers can be analyzed. • Dynamical processes at surfaces can be in situ monitored in a time-resolved manner. 45.3 Instrumentation A CAICISS apparatus is shown schematically in Fig. 45.1 [3–6]. The ion source and the time-of-flight (TOF) energy analyzer form a coaxially configured unit. The basic components of the apparatus consist of the low-energy ion source (far right); the chopper for pulsing the ion beam (just to the left); the specimen (far left) together with the goniometer that can rotate the specimen about the normal and orthogonally intersecting axes; the microchannel plates (center) that detect the scattered ions and neutral atoms, and the TOF measurement electronics (bottom) for their energy analysis. The incident ions pass through a small hole in the center of the microchannel plates on their way to the specimen. 45 Impact Collision Ion Scattering Spectroscopy 277 Fig. 45.1 Schematic of the apparatus of CAICISS In operation, the ion beam leaving the ion source is pulsed by the chopping deflector and the chopping aperture, and then impinges on the specimen. Among the particles (ions and neutral atoms) scattered by the specimen, those that are backscattered at scattering angle of approximately 180° are detected by the microchannel plates. The energy spectra of the scattered particles are obtained based on their TOF measurement. If an ion acceleration tube is installed in the flight path of the scattered particles [5, 7], by applying a synchronized pulsed negative voltage to the tube and thus accelerating only the ions, the ions and neutral atoms can be separated in the measured TOF spectra. A CAICISS apparatus is characterized by (1) detection of particles backscattered at 180°; (2) its coaxial arrangement; and (3) a TOF energy analyzer. These characteristics provide three fundamental advantages in surface analysis described below. 45.3.1 Advantage I: Quantitative Analysis of Surface Composition and Structure This stems from the fact that the scattering angle is 180° and that the analysis is free from ambiguity concerning ion neutralization because both ions and neutral atoms are detected with the same efficiency. Four factors are involved. (a) Superior mass resolution. (b) Analysis with direct visualization due to congruent incident and scattered particle trajectories. 278 M. Aono and M. Katayama Fig. 45.2 Principle of structural analysis by CAICISS (c) Extremely simple analysis of scattered particle trajectory utilizing only the shadowing effect since there is no blocking effect [4]. (d) Ready determination of positional relationships between adatoms and their nearest neighboring atoms, due to ease of double-scattering analysis [8]. Factors (a) and (b) enable quantitative analysis of the outermost surface-layer composition [9–13]. With the ion-beam incidence aligned along the direction in which the shadowing effect occurs, it is as though the crystal model can be taken in hand and observed to determine the composition of the outermost layer. The principle of structural analysis by CAICISS is shown schematically in Fig. 45.2 [4]. In Fig. 45.2a, A and B represent two adjacent atoms near the surface. Varying the angle a between the CAICISS axis and the surface while measuring the intensity of the particles scattered from atom B yields an intensity variation like that shown in Fig. 45.2b. The zero intensity at a = a0 results from the shadowing effect of atom A on atom B. The marked intensity enhancement on both sides of a0, at ac1 and ac2, occurs when the edge of the shadow cone formed by atom A passes through the center of atom B, thus bathing atom B in a focused ion flux (focusing effect). The difference between ac1 and ac2, Dac, corresponds unambiguously to the bond length d between atoms A and B (Dac decreases with increasing d), and a0 corresponds to the bond direction between atoms A and B. The bond direction and bond length between neighboring atoms can therefore be simultaneously determined by measuring changes in intensity, as illustrated in Fig. 45.2b [14–16]. 45.3.2 Advantage II: Observation of Scattering from Subsurface Atomic Layers Contrary to common knowledge concerning conventional ISS, CAICISS enables subsurface observation to depths of several to ten or more atomic layers, for two reasons. The first is the nearly complete absence of a blocking effect by observation of backscattering at 180°. The second is the use of the TOF energy analyzer in CAICISS, which permits detection of noble-gas ions scattered by subsurface atoms 45 Impact Collision Ion Scattering Spectroscopy 279 even though they are neutralized with nearly 100% probability and therefore cannot be detected by an electrostatic energy analyzer. These features provide three key capabilities. (a) Structural analysis of subsurfaces to depths of more than ten atomic layers, and layer-by-layer composition analysis [17–23]. (b) Determination of adatom positions by observation of scattering by substrate atoms [14]. (c) Evaluation of the crystallinity of several to ten or more atomic layers, by observation of the shadowing effect in the aligned direction of the atomic array [24, 25]. 45.3.3 Advantage III: Analysis of Dynamic Surface Processes Several aspects of the CAICISS apparatus make it highly effective for in situ monitoring of various surface processes (e.g., molecular beam epitaxial (MBE) growth) and for time-resolved analysis of dynamical processes at surfaces in real time. Due to its bolt-on structure, the apparatus can be easily attached to other equipment, such as an MBE chamber, through just a single free port (70 mm flange). Since there is no need to install any additional components between the CAICISS attachment port and the specimen, sufficient space can be maintained around the specimen. Moreover, the distance between the CAICISS attachment port and the specimen can be freely chosen. This high degree of freedom is due to the coaxial arrangement of the ion source and the energy detector. These aspects are highly favorable for in situ observation of dynamical processes at surfaces, such as an MBE growth. Furthermore, unlike in electrostatic energy analysis, the TOF-based energy analysis eliminates the need to perform energy scanning in spectral measurements, which enables simultaneous measurement of the entire spectrum, and is thus a further advantage for time-resolved analysis and tracking of dynamic processes. Time-resolved CAICISS spectral measurements provide two types of information. In the first, the spectral peak intensity in single scattering from each of the elements constituting the surface region is proportional to the number of elemental atoms that can be “seen” from the direction of the CAICISS axis, which permits direct monitoring of changes in surface composition and structure [5, 24–29]. The second is related to thin-film growth. When the amount of deposited thin film exceeds a certain thickness (several to ten or more atomic layers), a broad structure appears on the low-energy side of the single scattering peak. This broad structure stems from multiple scattering within the film, and its intensity depends on the film thickness. Monitoring the intensity of the multiple scattering can provide information on the film-growth morphology [25, 30–33]. 280 45.4 M. Aono and M. Katayama Applications 45.4.1 A Monolayer of CaF on Si(111) An analysis of surface structure and composition by CAICISS was performed for a monolayer of CaF on Si(111). The CAICISS spectrum of this surface [14] shown in Fig. 45.3 was obtained with a beam of 2 keV He+ ions at normal ion incidence. Comparison of the Ca and F peak intensities taking the ion scattering cross sections of these elements into account confirms the presence of Ca and F in a ratio of 1:1. Figure 45.4a shows the result [14] of angle-resolved measurement of the intensity of He particles scattered from the Ca atoms in this surface, in the manner shown in Fig. 45.2b. Two remarkable intensity drops at a = 17° and 171°, and the nearby Fig. 45.3 CAICISS spectrum of a monolayer CaF on Si(111) TIME OF FLIGHT (ns) Fig. 45.4 a Normalized intensity of He particles (He+ and He0) scattered from Ca atoms of a monolayer of CaF on Si(111) as a function of the incidence angle, a, in the (110) plane. b Structure of the CaF/Si(111) surface elucidated by CAICISS (a) d=0.064 nm (b) F Ca Si 0.064 0.005 nm (bulk 0.079 nm) 45 Impact Collision Ion Scattering Spectroscopy 281 intensity enhancements are the results of the shadowing (Fig. 45.4b) and focusing effects of F atoms on the Ca atoms. Because of the 1 1 structure of the CaF monolayer, the interlayer distance d between the F and Ca layers represents a structural parameter, and the best fit for this parameter as found by computer simulation is shown by the broken line in Fig. 45.4a, with d = 0.064 nm. This indicates that the Ca–F bond lies approximately 17° relative to the [11 2] direction and has a length of approximately 0.22 nm. References 1. Smith, D.P.: Scattering of low-energy noble gas ions from metal surfaces. J. Appl. Phys. 38, 340–347 (1967) 2. Aono, M., Oshima, C., Zaima, S., Otani, S., Ishizawa, Y.: Quantitative surface atomic geometry and two-dimensional surface electron distribution analysis by a new technique in low-energy ion scattering. Jpn. Appl. Phys. 20, L829–L832 (1981) 3. Katayama, M., Nomura, E., Kanekama, N., Soejima, H., Aono, M.: Coaxial impact-collision ion scattering spectroscopy (CAICISS): a novel method for surface structure analysis. Nucl. Instrum. Meth. Phys. Res. B 33, 857–861 (1988) 4. Aono, M., Katayama, M., Nomura, E., Chasse, T., Choi, D., Kato, M.: Recent developments in low-energy ion scattering spectroscopy (ISS) for surface structural analysis. Nucl. Instrum. Meth. Phys. Res. B 37(38), 264–269 (1989) 5. Aono, M., Katayama, M., Nomura, E.: Exploring surface structures by coaxial impact-collision ion scattering spectroscopy (CAICISS). Nucl. Instrum. Meth. Phys. Res. B 64, 29–37 (1992) 6. Katayama, M.: Exploring surface processes by coaxial impact-collision ion scattering spectroscopy and time-of-flight elastic recoil detection analysis. Current Appl. Phys. 3, 65–69 (2003) 7. Kamiya, I., Katayama, M., Nomura, E., Aono, M.: Separation of scattered ions and neutrals in CAICISS with an acceleration tube. Surf. Sci. 242, 404–409 (1991) 8. Katayama, M., Williams, R.S., Kato, M., Nomura, E., Aono, M.: Structure analysis of the Si (111)√3√3 R30°-Ag surface. Phys. Rev. Lett. 66, 2762–2765 (1991) 9. Katayama, M., Aono, M., Oigawa, H., Nannichi, Y., Sugahara, H., Oshima, M.: Surface structure of InAs(001) treated with (NH4)2Sx solution. Jpn. J. Appl. Phys. 30, L786–L789 (1991) 10. Hashizume, T., Katayama, M., Jeon, D., Aono, M., Sakurai, T.: The Absolute coverage of K on the Si(111)-3x1-K surface. Jpn. J. Appl. Phys. 32, L1263–L1265 (1993) 11. Kawai, M., Liu, Z.-Y., Hanada, T., Katayama, M., Aono, M.: Layer controlled growth of oxide superconductors. Appl. Surf. Sci. 82(83), 487–493 (1994) 12. Kawasaki, M., Takahashi, K., Maeda, T., Tsuchiya, R., Shinohara, M., Ishiyama, O., Yonezawa, T., Yoshimoto, M., Koinuma, H.: Atomic control of the SrTiO3 crystal surface. Science 266, 1540–1542 (1994) 13. Fujino, T., Katayama, M., Inudzuka, K., Okuno, T., Oura, K., Hirao, T.: Surface hydroxyl formation on vacuum-annealed TiO2(110). Appl. Phys. Lett. 79, 2716–2718 (2001) 14. Katayama, M., King, B.V., Nomura, E., Aono, M.: Structure analysis of the CaF2/Si(111) interface in its initial stage of formation by coaxial impact-collision ion scattering spectroscopy (CAICISS). Prog. Theore. Phys. Suppl. 106, 315–320 (1991) 15. Fuse, T., Ryu, J.-T., Fujino, T., Inudzuka, K., Katayama, M., Oura, K.: Adsorption of H on the Ge/Si(001) surface as studied by time-of-flight elastic recoil detection analysis and coaxial impact collision ion scattering spectroscopy. Jpn. J. Appl. Phys. 38, 1359–1362 (1999) 282 M. Aono and M. Katayama 16. Fujino, T., Fuse, T., Ryu, J.-T., Inudzuka, K., Yamazaki, Y., Katayama, M., Oura, K.: Structural analysis of 6H-SiC(0001)√3 √3 reconstructed surface. Jpn. J. Appl. Phys. 39, 6410–6412 (2000) 17. Kato, M., Katayama, M., Chasse, T., Aono, M.: Channeling and backscattering of low energy ions. Nucl. Instrum. Meth. Phys. Res. B 39, 30–34 (1989) 18. Katayama, M., King, B.V., Daley, R.S., Williams, R.S., Nomura, E., Aono, M.: Surface and interface structural analysis by coaxial impact collision ion scattering spectroscopy (CAICISS). In: Yoshimori, A., Watanabe, H. (eds.) Ordering at Surfaces and Interfaces, Vol. 17, pp. 67–72. Springer Series in Material Science (1992) 19. Oura, K., Sumitomo, K., Kobayashi, T., Kinoshita, T., Tanaka, Y., Shoji, F.: Adsorption of H on Si(111)-√3 √3-Ag: evidence for Ag(111) agglomerates formation. Surf. Sci. 254, L460– L464 (1991) 20. Nakanishi, S., Kawamoto, K., Fukuoka, N., Umezawa, K.: Low energy ion scattering analysis of the surface compositional change of Au3Cu(001) induced by oxygen chemisorption. Surf. Sci. 261, 342–348 (1992) 21. Ohnishi, T., Ohtomo, A., Kawasaki, M., Takahashi, K., Yoshimoto, M., Koinuma, H.: Determination of surface polarity of c-Axis oriented ZnO films by coaxial impact-collision ion scattering spectroscopy. Appl. Phys. Lett. 72, 824–826 (1998) 22. Sonoda, S., Shimizu, S., Shen, X.-Q., Hara, S., Okumura, H.: Characterization of polarity of Wurtzite GaN film grown by molecular beam epitaxy using NH3. Jpn. J. Appl. Phys. 39, L202–L204 (2000) 23. Okuno, T., Fujino, T., Shindo, M., Katayama, M., Oura, K., Sonoda, S., Shimizu, S.: Influence of Mn incorporation on molecular beam epitaxial growth of GaMnN film. Jpn. J. Appl. Phys. 41, L415–L417 (2002) 24. Katayama, M., Nomura, E., Soejima, H., Hayashi, S., Aono, M.: Real-time monitoring of molecular-beam epitaxy processes with coaxial impact-collision ion scattering spectroscopy (CAICISS). Nucl. Instrum. Meth. Phys. Res. B 45, 408–411 (1990) 25. Katayama, M., Nakayama, T., McConville, C.F., Aono, M.: Influence of surfactant coverage on epitaxial growth of Ge on Si(001). Phys. Rev. B 54, 8600–8604 (1996) 26. Sumitomo, K., Kobayashi, T., Shoji, F., Oura, K., Katayama, I.: Hydrogen-mediated epitaxy of Ag on Si(111) as studied by low-energy ion scattering. Phys. Rev. Lett. 66, 1193–1196 (1991) 27. Katayama, M., Nakayama, T., McConville, C.F., Aono, M.: Surface and interface structural control using coaxial impact-collision ion scattering spectroscopy (CAICISS). Nucl. Instrum. Meth. Phys. Res. B 99, 598–601 (1995) 28. Fujino, T., Okuno, T., Katayama, M., Oura, K.: Hydrogen segregation and its detrimental effect in epitaxial growth of Ge on hydrogen-terminated Si(001). Jpn. J. Appl. Phys. 40, L1173–L1175 (2001) 29. Fujino, T., Katayama, M., Inoue, S., Tatsumi, A., Horikawa, T., Oura, K.: Quantitative analysis of hydrogen-induced Si segregation on Ge-covered Si(001) surface. Jpn. J. Appl. Phys. 42, L485–L488 (2003) 30. Aono, M., Katayama, M.: A novel method for real-time monitoring of molecular beam epitaxy (MBE) processes. Proc. Jpn. Acad. 65(Ser. B), 137–141 (1989) 31. Katayama, M., Fujino, T., Yamazaki, Y., Inoue, S., Ryu, J.-T., Oura, K.: Coaxial impact-collision ion scattering spectroscopy and time-of-flight elastic recoil detection analysis for in situ monitoring of surface processes in gas phase atmosphere. Jpn. J. Appl. Phys. 40, L576–L579 (2001) 32. Fujino, T., Katayama, M., Yamazaki, Y., Inoue, S., Okuno, T., Oura, K.: Influence of hydrogen-surfactant coverage on Ge/Si(100) hetroepitaxy. Jpn. J. Appl. Phys. 41, L790–L793 (2002) 33. Fujino, T., Katayama, M., Okuno, T., Shindo, M., Tsushima, R., Oura, K.: Thermal stability in the morphology of Ge films on Si(001) grown by hydrogen-surfactant-mediated epitaxy. Jpn. J. Appl. Phys. 42, L63–L66 (2003) Chapter 46 Inelastic Electron Tunneling Spectroscopy Akitoshi Shiotari Keywords Vibrational spectroscopy microscopy Lock-in detection 46.1 Single molecule Scanning tunneling Principle Inelastic electron tunneling spectroscopy (IETS) is a vibrational spectroscopy method. d2I/dV2 signals recorded at tunneling junctions (e.g., metal–insulator–metal interfaces [1], scanning tunneling microscopy (STM) junctions [2–5], and metal– molecule–metal junctions [5, 6]) correspond to electronically excited vibrations as follows. At a tunneling junction, most electrons go through the tunneling barrier without energy loss (elastic tunneling; Fig. 46.1a). However, an additional tunneling channel is opened when the bias voltage V between the electrodes is greater than the vibrational eigenvalue ħX/e of any molecule located in the junction. In this channel, the energy from electrons is used to vibrationally excite the molecule (inelastic tunneling; Fig. 46.1b). Because the total current I is the sum of the elastic and inelastic tunneling currents, the I–V curve has inflection points at V = ± ħX/e (Fig. 46.1c), and thus, the d2I/dV2 curve has a peak and a dip at V = + ħX/e and −ħX/e, respectively (Fig. 46.1e). In contrast to intuitive selection rules of infrared and Raman spectroscopy, definitive selection rules of IETS have not been established; however, recent theoretical studies have proposed valid models for elastic and inelastic tunneling processes and have successfully reproduced experimental IET spectra [7–11]. According to the models, the symmetric property of the molecular orbitals near the Fermi level plays a crucial role in electron-vibration coupling to open the tunneling channels. A. Shiotari (&) Department of Advanced Materials Science, The University of Tokyo, Kashiwa, Japan e-mail: shiotari@k.u-tokyo.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_46 283 284 A. Shiotari Fig. 46.1 Schematic energy diagrams of a elastic and b inelastic tunneling current at an STM junction. c I–V, d dI/dV, and e d2I/dV2 spectra of the total current I (black) together with its elastic component I0 (blue) and inelastic component Iinel (red) 46.2 Features – Chemical characterization at the single-molecule level. – Detection of vibrational/rotational modes of molecules, phonon modes of substrates, and spin flips of magnetic atoms/molecules. – Energy resolution is enhanced with decreasing temperature. – The selection rules are under discussion and becoming clarified theoretically. 46.3 Instrumentation Figure 46.2 shows a schematic diagram of an experimental setup for lock-in detection combined with STM [3]. Because most IETS signals are weak, a lock-in amplifier is useful for detecting the second derivative tunneling current signals during slow ramping of the voltage bias. In general, relative changes in conductance (Dr/r; see Fig. 46.1d) of more than *1% is detectable with this setup. Averaging several d2I/dV2curves recorded over a target molecule is required to obtain a high S/N ratio. IET spectra are sometimes displayed after normalization by the corresponding dI/dV curve, i.e., (d2I/dV2)/(dI/dV). 46 Inelastic Electron Tunneling Spectroscopy 285 Fig. 46.2 Schematic diagram of an experimental setup for STM-IETS measurements. Switch A controls the current feedback circuit. Switch B enables slow ramping and modulation of the sample bias voltage. To perform IETS, Switch A is opened, followed by the closing of Switch B. Reproduced from Ref. [3], Copyright 2005, with permission from Elsevier Peak width (W) in a d2I/dV2 spectrum is expressed by the following equation [1, 12]; W¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi WI2 þ ð1:7Vrms Þ2 þ ð5:4kB T Þ2 ; ð46:1Þ where WI denotes the intrinsic width of the vibration, Vrms is the modulation voltage, kB is the Boltzmann constant, and T is the temperature. As described by Eq. (46.1), the energy resolution of IETS depends on temperature, and thus, samples should typically be kept at temperature below 10 K. 46.4 Applications 46.4.1 Identification of Individual Molecular Isotopes IETS can detect isotope shifts of vibrational modes at the single-molecule level [9, 13–15]. Use of STM-IETS was first reported by Ho et al. [13] who utilized the method to observe acetylene isotopes on surfaces [7, 13, 14]. The individual molecular isotopes were observed by STM as equivalent shapes (Fig. 46.3a). However, the individual IET spectra (Fig. 46.3b, c) displayed peaks corresponding to the antisymmetric C–H(D) stretching modes [7], allowing the isotope composition of each individual molecule to be identified, as labeled in Fig. 46.3a. 286 A. Shiotari Fig. 46.3 a STM images of acetylene molecules on Cu(100). b IET spectra recorded over each molecule in (a). c IET spectra recorded over individual acetylene isotopes on Ni(100). Reproduced with permission from Ref. [14]. Copyright 1998 by American Physical Society 46.4.2 Seeing Through the Skeletal Structure of a Molecule An IETS map (i.e., spatial variation of the d2I/dV2 signal) at a defined voltage bias reflects the excitation process of the corresponding vibrational mode, which can be useful in assigning an STM-IETS signal to a particular vibrational mode [7, 10, 11]. Furthermore, IETS mapping can be used to visualize the chemical bonding structures of single molecules on surfaces. Figure 46.4a shows an IETS map measured over a cobalt phthalocyanine (CoPc) molecule using a carbon monoxide (CO)terminated STM tip [16]. A bias voltage of 1.7 mV was chosen, owing to its proximity to the eigenvalue of the hindered translation mode of CO (*2 meV) (Fig. 46.4c). Because the peak energy is sensitive to interaction with the atoms in CoPc, the “skeleton” of CoPc is imaged in the map (Fig. 46.4a, b). An extremely low temperature (T = 600 mK) and a low modulation voltage (Vmod 1 mV) were required for achieving such a high-energy resolution [see Eq. (46.1)]. 46 Inelastic Electron Tunneling Spectroscopy 287 Fig. 46.4 a IETS map of CoPc on Ag(110), collected using a CO-terminated STM tip. b Schematic of the apparent bonds in (a). c IET spectrum of the bare Ag(110) surface, recorded using the CO-terminated tip. From [16]. 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B 93, 085411/1–085411/7 (2016) Chapter 47 Infrared External-Reflection Spectroscopy Takeshi Hasegawa Keywords Vibrational spectroscopy External reflection Thin film Molecular orientation Polarized IR rays 47.1 Nonmetallic surface Principle Infrared (IR) spectrometry is a representative un-destructive spectroscopic analytical technique, which provides rich molecular information such as molecular conformation, polymorph, packing and orientation. Another significant benefit of using IR spectroscopy is the sensitivity, which is good enough for analyzing monolayer-level thin films. Considering the great quantitative reproducibility, IR spectroscopy is one of the first choices to analyze an ultrathin film particularly of an organic compound. Here, IR reflection spectrometry on a nonmetallic surface, which is called “external reflection,” is described. IR spectroscopy is one of the absorption spectroscopies, and the selection rules are deduced by solving Schrödinger equation with the perturbation theory. We have to note, however, that this is true of a single dipole moment (molecule) in vacuum. To discuss light absorption by a condensed matter, electrodynamics should be employed, since the collection of a huge number of dipole moment can conveniently considered via the electric permittivity. The frequency response of IR absorption to the permittivity is theorized by Kramers–Kronig’s relationship via the convolution theory, which reveals that the permittivity must be a complex [1]. Since the magnetic permittivity can be ignored in IR spectroscopy, the electric relative permittivity, er , is simply related to the refractive index, n, by er ¼ n2 , and therefore, the refractive index is also a complex: n n0 þ in00 . Here, n0 and n00 are the real and imaginary parts of the refractive index, respectively. T. Hasegawa (&) Institute for Chemical Research, Kyoto University, Uji, Kyoto-fu, Japan e-mail: htakeshi@scl.kyoto-u.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_47 289 290 T. Hasegawa By solving Maxwell equations without considering any optical interface, the absorbance spectrum, A, of a bulk matter with a thickness of z is deduced to be [1]: A log10 I 1 4pn00 1 az: z ¼ I0 ln 10 k ln 10 Here, I and I0 are the intensities of the incident and transmitted IR light, respectively, at the wavelength of k. When the optical interface can be ignored, in this manner, the spectrum is simply driven by the “absorption index” of a 4pn00 =k. To discuss absorption spectroscopy on a thin film, the optical interface of the film/substrate boundary must be taken into account, i.e., the continuities of the electric and magnetic fields at boundaries must all be taken into account. If the pand s-polarized IR rays are incident from Phase 1 (mostly air) to the sample (Phase 2) on the substrate (Phase 3), the absorbance spectra of both polarizations are finally obtained as follows [1]. 2 AER;p ¼ ln8pd 10k ðsin2 h1 e3 ÞImðe2;x Þ þ e23 sin2 h1 Imð1=e2;z Þ cos h1 ðe3 1Þðe3 tan2 h1 Þ 8pd2 CpTO TO þ CpLO LO ln 10 k 1 8pd2 n1 cos h1 AER;s ¼ ln 10k Im e2;x e0 1 ð47:1Þ 3 8pd2 Cs TO ln 10 k ð47:2Þ The subscript index, j, of the permittivity, ej , corresponds to the phase number. These equations simply imply that both polarized spectra are theorized by combinations of Im e2;x and Im 1=e2;z , which are often called TO (transverse optic) and LO (longitudinal optic) energy-loss functions, respectively. We have to pay attention that, therefore, we cannot directly compare ER spectra with a bulk spectrum especially for a strong IR absorber, represented by perfluoroalkyl compounds [2]. Since the TO and LO functions are driven by the x- and z-component of the electric permittivity, respectively, an s-polarized ER spectrum reveals only the surface-parallel component of a normal mode in the film, and the absorbance is always negative. On the other hand, the p-polarized ER spectrum depends on the molecular orientation via the x- and z-components, and the angle of incidence, h. Since the denominator involves e3 tan2 h1 , the sign of absorbance changes at Brewster’s angle. To visually understand the complicated coefficients of the TO and LO functions, CpLO and CpTO , respectively, the calculated curves of the coefficients are presented in Fig. 47.1 [1]. As expected, both CpLO and CpTO overturn at the Brewster angle of silicon (tan1 n3 = 73°). These characteristics are interpreted as the surface selection rules of ER spectroscopy. Equations (47.1) and (47.2) are indeed very useful equations, and they Infrared External-Reflection Spectroscopy Fig. 47.1 Coefficients calculated by using n3 = 3.4. CpLO and CpTO are plotted by thin and thick solid lines, respectively; while Cs is given by the dotted line 291 0.4 Coefficient / a.u. 47 C LO 0.2 0 Cs -0.2 C TO -0.4 0 20 40 60 80 Angle of incidence / degree also quantitatively reproduce the ATR and RA (Chap. 48) spectra. In this sense, the equations are the fundamental equations of IR surface spectroscopy. Note that Eqs. (47.1) and (47.2) are deduced from the 3-layer model, which cannot be true of a double-side polished substrate, which requires a 5-layer model [1]. To keep the 3-layer model for using the surface selection rules, we have to employ a single-side polished substrate. A semiconductor wafer is useful for this purpose. 47.2 Features • Vibrational modes in a monolayer-level thin film can be measured for quantitative discussion. • Band intensity (absorbance) is rigorously reproduced by using the equations deduced from Maxwell equations. • Surface selection rule of the p-polarized ER spectrum is a function of the molecular orientation and the angle of incidence. • Both solid and liquid substrates can be employed. 47.3 Instrumentation The measurements are performed on a normal FT-IR bench in laboratory. A reflection attachment (Fig. 47.2) must be used for setting the angle of incidence accurately. 47.4 Applications 47.4.1 Practical Measurements of a Thin Film Figure 47.3 presents IR ER spectra of a 9-monolayer Langmuir–Blodgett (LB) film prepared on a GaAs water [3]. As theoretically expected, the s-polarized spectra 292 T. Hasegawa Fig. 47.2 An overview of a reflector in the sample room of FT-IR To detector polarizer sample angle adjusting stage Fig. 47.3 IR ER spectra of a 9-monolayer LB film of cadmium stearate on a GaAs wafer as a function of the angle of incidence. The left and right panels present results of the s- and p-polarizations, respectively have negative bands only, and the intensity decreases with an increase of the angle of incidence. On the other hand, the p-polarized IR ER spectra consist of positive and negative peaks, and they are overturned when the angle of incidence goes across the Brewster angle. When the angle of incidence is small, the negative bands appear for the antisymmetric and symmetric CH2 stretching vibration bands (maCH2 and msCH2 at 2916 and 2850 cm−1, respectively) and the antisymmetric COO− stretching vibration (maCOO−) band at 1543 cm−1. Judging from the surface selection rule (Fig. 47.1), the negative bands are attributed to the surface-parallel component. In this manner, the CH2 group is revealed to have a parallel orientation to the surface as illustrated in Fig. 47.4. As for the COO− group, the symmetric 47 Infrared External-Reflection Spectroscopy 293 Fig. 47.4 Averaged molecular orientation in the 9-monolayer LB film on Si revealed by the p-polarized IR ER spectrum νa(CH2) νa(COO-) νs(CH2) O O νs(COO-) COO− stretching vibration (msCOO−) band appears at 1433 cm−1 as a positive peak in the p-polarized spectrum, which implies that the COO− group takes a nearly perpendicular orientation to the surface as presented in Fig. 47.4. In this fashion, the p-polarized IR ER spectrum responds to the orientation of each normal mode as well as the angle of incidence. This surface selection rule is understandable by referring toFig. 47.1, which can be calculated by using the permittivity of the substrate, e3 ¼ n2r;3 , only. 47.4.2 Optimal Angle of Incidence for Obtaining High-Quality IR ER Spectra When we simply refer to the coefficient curves in Fig. 47.1, the sensitivity of the p-polarized spectrum would be very high at an angle of incidence near Brewster’s angle, since the absorbance would also become significantly large. Here, we have to recall, however, another important fact that “reflectance” at the substrate surface becomes nil at Brewster’s angle, hB . Figure 47.5 presents reflectance variations for s- and p-polarizations, which are plotted by the dotted and solid curves, respectively. At an angle near the Brewster angle, the reflectance of the p-polarization is very low, which makes the throughput very poor. Therefore, the spectral quality is determined by a balance of the band intensity and the throughput, which can be controlled by changing the angle of 294 1 0.8 Reflectance Fig. 47.5 Reflectance depending on the angle of incidence at the air/Ai interface T. Hasegawa 0.6 s 0.4 p 0.2 0 θ 0 20 40 60 80 Angle of incidence / o incidence. An angle apart from Brewster’s angle by ca. 10° is recommended for the p-polarized ER measurements. For the s-polarization, on the other hand, we do not have to be concerned about the reflectance, and a small angle of incidence should be employed, so that the negative absorbance would become fairly large. References 1. Hasegawa, T.: Quantitative Infrared Spectroscopy for Understanding of a Condensed Matter. Springer, Tokyo (2017) 2. Hasegawa, T., Shimoaka, T., Shioya, N., Morita, K., Sonoyama, M., Takagi, T., Kanamori, T.: Stratified dipole-arrays model accounting for bulk properties specific to perfluoroalkyl compounds. ChemPlusChem. 79, 1421–1425 (2014) 3. Hasegawa, T., Takeda, S., Kawaguchi, A., Umemura, J.: Quantitative analysis of uniaxial molecular orientation in Langmuir–Blodgett films by infrared reflection Spectroscopy. Langmuir. 11, 1236–1243 (1995) Chapter 48 Infrared Reflection–Absorption Spectroscopy Jun Yoshinobu Keywords Vibrational spectroscopy 48.1 Infrared absorption Fourier transform Principle Infrared reflection–absorption spectroscopy (IRAS) is a spectroscopic method for measuring an infrared reflection absorption spectrum due to vibrations of adsorbed atoms and molecules [1–4], where infrared light is incident to an optically flat metal surface at a grazing angle and reflected light is detected. An absorption spectrum is obtained by dividing a “sample” single-reflection spectrum (usually adsorbed surface) by a “reference” single-reflection spectrum (usually clean surface). Figure 48.1 shows the electric vectors of infrared light, which is incident and reflected at a grazing angle on a metal surface. The s-polarized infrared light whose electric vector is perpendicular to the incident plane (i.e., parallel to the surface) changes in phase by 180° due to reflection, and the electric field formed by the incident light and the reflected light is effectively canceled at the surface. On the other hand, the p-polarized light has an electric vector in the plane of incidence; the electric vectors of the incident light and the reflected light are strengthened at the surface. As a result, the vertical component of the standing wave at the surface ðEp? Þ becomes almost doubled as compared with the perpendicular component of the incident p-polarized light. Thus, in IRAS on a metal surface, only p-polarized light becomes effective as probe light. In addition, since the irradiated area of incident light is proportional to 1/cos /, where / is the incident angle, the probed surface area becomes larger as / approaches 90°. Therefore, the absorption 2 intensity of infrared light by the adsorbed molecules is proportional to Ep? = cos /. Since Ep? = cos / has a large maximum at an incident angle close to 90°, the 2 J. Yoshinobu (&) The Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba, Japan e-mail: yoshinobu@issp.u-tokyo.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_48 295 296 J. Yoshinobu Fig. 48.1 Schematic diagram of reflection of infrared light with a grazing angle at a metal surface Fig. 48.2 Schematic diagram of a dynamic dipole moment at a metal surface, normal to the surface (left) and parallel to the surface (right) absorption intensity in IRAS increases several tens of times as compared with the case of transmission infrared absorption spectroscopy. In the case of adsorbed molecules on a metal surface, the dynamic dipole moment normal to the surface becomes almost doubled with its mirror image, but the dynamic dipole moment parallel to the surface is canceled each other (Fig. 48.2). Therefore, a vibrational mode having a dynamic dipole moment normal to the surface is selectively excited by p-polarized light in IRAS; this is the surface normal dipole selection rule. 48.2 Features • Vibrational modes of molecules on a metal surface can be detected. • Surface normal dipole selection rule is applied. • IRAS can be applicable to the surface/interfaces between a metal and vacuum, between a metal and gas, and between a metal and liquid. 48.3 Instrumentation The IRAS measurement system consists of a Fourier transform infrared (FT-IR) spectrometer, several optical mirrors, infrared windows, an optically flat metal sample, and an infrared detector (Fig. 48.3). All the optical paths should be evacuated or purged by dry N2 gas to minimize the absorption by atmospheric water vapor and carbon dioxide. The UHV chamber and mirror boxes are separated by infrared transparent windows, which are made from KBr, BaF2, CaF2, etc. The 48 Infrared Reflection–Absorption Spectroscopy 297 Fig. 48.3 Experimental setup for IRAS. All the optical paths are evacuated by a dry pump, and IR windows (green) are made from KBr. Two infrared detectors are installed in this system [5] infrared light coming from the FT-IR spectrometer is focused on a sample surface. The sample diameter is usually *10 mm. The reflected light is refocused on an infrared detector. In Fig. 48.3, there are two detectors, i.e., a liquid-N2-cooled photoconductive mercury cadmium telluride (MCT) detector and a liquid-Hecooled B-doped Si (Si:B) detector [5]. Figure 48.4 shows IRAS spectra of a “clean” Rh(111) surface using MCT and Si:B detectors. Using the MCT detector, the measurable range is limited above *650 cm−1 (the whole spectrum is shifted upward by 0.002). Using the Si:B detector, the range is effectively limited from 400 to 4000 cm−1. Note that these spectra are almost free from water vapor and CO2 using the IRAS system as shown in Fig. 48.3. Fig. 48.4 IRAS spectra on a “clean” Rh(111) surface. A “sample” single-reflection spectrum was subsequently measured after the measurement of a “reference” single-reflection spectrum on the clean Rh(111) surface to produce IRAS spectra (4 cm−1 resolution; 500 scans for each spectrum) 298 48.4 J. Yoshinobu Applications 48.4.1 Adsorption and Reaction of Cyclohexane on Rh(111) Figure 48.5 shows a series of IRAS spectra (a) C6H12 and (b) C6D12 on Rh(111) as a function of heating temperature [6]. After heating below 181 K, adsorbed cyclohexane molecules remained intact. Normal CH stretching modes are observed between 2800 and 3000 cm−1. Significantly red-shifted CH stretching modes are also observed between 2500 and 2700 cm−1. These peaks originate from the softened CH groups interacting with the Rh surface [6, 7]. The heating at 200 K leads to the decrease in the intensity of CH stretching bands mainly due to desorption of adsorbed C6H12; a new peak appears at 783 cm−1, which is attributed to the out-of-plane CH bending mode of benzene. Thus, a small amount of adsorbed C6H12 was dehydrogenated to benzene at 200 K. Similar experiments were carried out using C6D12 molecules (Fig. 48.5b). Softened CD stretching modes are observed between 1900 and 2050 cm−1 as well as normal CD stretching modes between 2070 and 2200 cm−1. After 210 K heating, most of the adsorbed C6D12 molecules were desorbed. A possible reaction product on the surface is benzene, but the out-of-plane CD bending mode of deuterated benzene (*565 cm−1) is below a detection limit of the MCT detector (*650 cm−1) [6]. Fig. 48.5 IRAS spectra of a C6H12 and b C6D12 as a function of heating temperature. Cyclohexane exposure is 5 shots (h = 0.3 monolayer). After heating to the indicated temperatures, the spectra were recorded at 20 K [6] 48 Infrared Reflection–Absorption Spectroscopy 299 References 1. Hoffmann, F.M.: Surf. Sci. Rep. 3, 107 (1983) 2. Chabal, Y.J.: Surf. Sci. Rep. 8, 21 (1988) 3. Bradshaw, A.M., Schweizer, E.: Adv. Spectrosc. 16, 413 (1988) 4. Madey, T.E., Yates Jr., J.T. (eds.): Vibrational Spectroscopy of Molecules on Surfaces (Methods of Surface Characterization). Springer, New York (1987) 5. Yamamoto, S., Beniya, A., Mukai, K., Yamashita, Y., Yoshinobu, J.: J. Phys. Chem. B 109, 5816 (2005) 6. Koitaya, T., Shimizu, S., Mukai, K., Yoshimoto, S., Yoshinobu, J.: J. Chem. Phys. 136, 214705 (2012) 7. Koitaya, T., Yoshinobu, J.: Chem. Rec. 14, 848 (2014) Chapter 49 Interferometer Displacement Measurement Masaya Toda Abstract The displacement measurement using optical interferometry is based on the observation of an interferogram caused by interfering two reflected lights from a sample surface and a reference mirror surface. Keywords Surface profile 49.1 Topography Deflection Bending Principle The displacement measurement using optical interferometry is based on the observation of an interferogram caused by interfering two reflected lights from a sample surface and a reference mirror surface. The interference fringe is formed by the optical path difference of a half of the light wavelength. Counting simply the number of interference fringe constructs the topography of the sample. The displacement profile along the line interested is obtained. Additionally, analyzing the phase provides sub-nanometer resolution of the topography (Fig. 49.1). White light interferometry with a broadband optical source such as a laser diode is useful for precise surface topographic measurement. The optical path lengths for the sample and the reference mirror must be matched to observe the white light interferogram. Moving the position of the reference mirror, the z positions of the brightest interfering in the height direction are detected. Then, topography of the surface as the contour image is constructed. M. Toda (&) Graduate School of Engineering, Tohoku University, Sendai, Japan e-mail: mtoda@nme.mech.tohoku.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_49 301 302 M. Toda i 0 x Intensity & phase x Interferogram z 0 Topography x Fig. 49.1 Interferometry for topography measurement Camera Beam splitter Laser source z Reference flat (λ/20) Sample substrate Camera Fizeau Interferometer Beam splitter White light source z Reference mirror Sample substrate Michelson Interferometer Fig. 49.2 Example of interferometers 49.2 Instrumentation Fizeau interferometer is often used to measure typically wide area of wafer or glass (Fig. 49.2, left). Since the laser is expanded widely, a long coherence length of the laser is required. The system is stable against vibration because of its long common optical path. Michelson interferometer is also often used (Fig. 49.2, right) to measure the surface topography since the components are simple with one beam splitter and one reference mirror. The interferometer can be installed to a standard microscope by attaching the interferometry objectives. The combined microscope with interferometry objectives provides the magnified surface topographic analysis. The interferometry objectives are commercially available. A Michelson interferometer type is often used for the objectives with magnitude lower than 5 times (Fig. 49.3, left). From 10 to 50 times objectives, a Mirau-type interferometer is used (Fig. 49.3, right). For both types, the light beam from the optical source is split to two beams in the objectives for the sample and the reference mirror. The reflected light from the sample and reference mirror, the interferogram is observed by the camera. 49 Interferometer Displacement Measurement 303 Reference mirror Reference mirror Half mirror Beam splitter (Window) (Glasses) Sample Michelson objective Sample Mirau objective Fig. 49.3 Interferometric objectives Sometimes, the samples are required to be observed through a window such as in a vacuum or pressured chamber. However, when a window is inserted between the sample and the objective, the optical path length to the sample is changed. The white light interferogram cannot be observed through window using a usual-type objective. A few models of Mirau-type objectives are adjustable to change the optical path length, while the adjustment capability of the optical path length of Mirau type is limited to be in a few millimeters. Michelson-type interferometer allows to use a thicker window by inserting the same thick glass between the beam splitter and the reference mirror in the objective. 49.2.1 Features • • • • • • Topography and surface profile, Precise resolution in nanometer range, Non-contact observation optically, Various size of the sample area, Need enough light reflection from sample, and Loss of height by step structure. 49.3 Applications A single fiber can be used to make a Fizeau-type interferometer as shown in Fig. 49.4. The edge of the fiber should be flat to have smooth surface. The edge surface could be coated by thin gold layer to obtain the enough reflection of the light. The graph in Fig. 49.4 shows the example of interferogram signal changing the distance between the optical fiber and the sample surface by piezoelectric actuator. The amplified voltage is recorded and the cycle of voltage gives the 304 M. Toda • Fiber interferometer Piezo actuator Initial position Interferometer sensitivity ~ 3.4 mV/nm Fig. 49.4 Calibration of displacement of fiber interferometer 1585 nm Head Deflection Bimaterial cantilever Differential Reference Intensity Heating stage Fig. 49.5 Cantilever deflection measurement using laser interferometer interferometer sensitivity to convert the distance from the voltage at the initial position (3.4 mV/nm). The deflection of bimaterial cantilever by changing the temperature on a heating stage is monitored by a laser interferometer (Fig. 49.5). The differential curve of interferometric signal gives the distance by counting the number of the zero cross points. When the deflection is changed to one direction (up or down), this method is useful to monitor the dynamics of the deflection. 49 Interferometer Displacement Measurement 305 • Bimaterial cantilever 550 μm2 50 30 10 2D image 50 0 3D image z (μm) z (μm) z (μm) Profile 0 250 500 0.0 -0.5 Profile 0 y (μm) 250 500 x (μm) Fig. 49.6 Topography measurement by white light interferometer The shape of bimaterial cantilever is measured by white light interferometer (Fig. 49.6, left). The residual bending is observed. The 2D image in color is useful to know the bending structure of the cantilever. The topography of the vacuum-packaged membrane is measured (Fig. 49.6, right). The profile of the membrane bended down is observed. The 3D image in color is constructed. The magnified image gives visually understanding of the topography. The profile curve shows the bending structure of the diaphragm. References 1. Toda, M., Seo, Y.J., Kawai, Y., Miyashita, H., Ono, T.: Proceedings of the International Microprocesses & Nanotechnology Conference 25, 1P. (2012) 2. End point detector 3. Sato, M.K., Toda, M., Inomata, N., Maruyama, H., Okamatsu-Ogura, Y., Arai, F., Ono, T., Ishijima, A., Inoue, Y.: Biophys. J. 106, 2458 (2014) 4. Mohd, N., Inomata, N., Toan, N.V., Ono, T., Toda, M.: Proceedings of the International Workshop on Nanomechanical Sensors (2016) Chapter 50 Inverse Photoemission Spectroscopy Kaname Kanai Keywords Unoccupied electronic states Surface states K-resolved inverse photoemission spectroscopy Resonant inverse photoemission spectroscopy Spin-resolved inverse photoemission spectroscopy 50.1 Principle Inverse photoemission spectroscopy (IPES) is a technique that probes unoccupied electronic states between the Fermi level (EF) and the vacuum level of a material [1]. The inverse photoemission process involves an electron entering a sample in an initially unoccupied state, Ek (Fig. 50.1). The electron then decays radiatively to another unoccupied state emitting a photon with energy hm, which is equal to the energy separating the initial and final states. The cross section for inverse photoemission is about 10−4 times that for direct photoemission at an incident or emitted electron energy of about 10 eV. Therefore, the inverse photoemission process has much lower quantum efficiency than direct photoemission; therefore, a highly efficient photon detection system is crucial for experimental measurements. The IPES technique has two measurement modes: Bremsstrahlung isochromat spectroscopy (BIS) mode and tunable photon energy (TPE) mode. In BIS mode, the IPES spectrum is obtained as a function of incident electron energy (Ek), whereas in the TPE mode, the emitted photons are detected by a monochromator with Ek kept constant. BIS mode uses sensitive bandpass filters, which are available for the X-ray and vacuum ultraviolet regions. This has led to the early development of X-ray BIS (XBIS) and ultraviolet BIS (UVBIS) techniques [2, 3]. K. Kanai (&) Department of Physics, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan e-mail: kaname@rs.noda.tus.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_50 307 308 K. Kanai Fig. 50.1 Energy diagram for IPES. In BIS mode, the photon number with constant energy hm emitted from sample is recorded as a function of Ek. Experimentally, V0 is changed to change Ek. In TPE mode, Ek is kept constant and the IPES spectrum is detected by a monochromator 50.2 Features • Unoccupied electronic states can be probed. • The low quantum efficiency of inverse photoemission process requires highly efficient photon detectors. • KRIPES can detect energy band structures above EF. • SRIPES can detect spin-dependent electronic structures above EF. • Low-energy IPES can be applied to soft materials like organic molecules. 50.3 Instrumentation The conventional system for IPES measurements in BIS mode uses a combination of a nearly monoenergetic electron source for exciting the sample and a narrow bandpass photon detector to detect monochromatic photons. Figure 50.2 shows a typical measurement system for BIS mode. The electron gun (e-gun) uses the low-work-function compound BaO as a cathode. The cathode is maintained at a constant bias and heated by a separate rhenium-tungsten filament to obtain a narrow energy distribution for the emitted electrons. The BaO cathode can be operated at about 800 °C. The energy resolution, typically 0.2–0.4 eV, is much less than that for photoemission spectroscopy. The energy distribution of thermal electrons obtained from the e-gun is a major cause of the poor energy resolution of IPES. A bandpass photon detector is composed of a single channel photon multiplier and a SrF2 window. To increase the photon detection efficiency and energy resolution of the detector, the surface of the multiplier is coated with a several-hundred-nanometer-thick layer of KCl. The ionization energy of KCl is about 8.6 eV. SrF2 is transparent to photons with energies of less than about 9.7 eV 50 Inverse Photoemission Spectroscopy 309 Fig. 50.2 Experimental setup for IPES in BIS mode and has a very sharp cutoff above this energy, owing to the absorption edge of the large bandgap insulating halide. This detector can detect photons with hm *9.2 eV. The IPES spectrum is obtained by detecting the number of photons with varying Ek. 50.4 Applications 50.4.1 UPS and IPES UPS and IPES spectra of the C60 film are shown in Fig. 50.3. The molecular orbitals around the energy gap of C60 are well resolved. The electronic structure of C60 is unique for a p-bonded hydrocarbon. C60 is also a strong electron acceptor with a high electron affinity [4]. The IPES spectrum in Fig. 50.3 shows that the threefold t1u state is just above EF, indicating the electron accepting and electron transporting nature of C60. As in this example, IPES is a powerful tool to observe unoccupied electronic states of solids directly. UPS and IPES complement each other for examining the electronic states around EF. In addition, the momentum-dependent k-resolved IPES (KRIPES) spectra obtained by using a collimated electron beam gives a replica of an unoccupied band structure of solids, similar to angle-resolved photoemission spectroscopy. Therefore, the combination of UPS and IPES has been widely used in solid state physics and chemistry. 50.4.2 Resonant Inverse Photoemission Spectroscopy Resonant inverse photoemission spectroscopy (RIPES) is a technique for investigating the unoccupied partial density of states (PDOS) of a particular orbital. Early 310 K. Kanai Fig. 50.3 UPS and IPES spectra of C60 film on an Au substrate. The horizontal axis shows binding energy with respect to EF. Eg denotes energy gap studies demonstrated the use of RIPES to study unoccupied 4f-PDOS of some Ce compounds [5, 6]. Figure 50.4 shows RIPES spectra for Ce compounds measured at the Ce N4,5 absorption edge. The off-resonant and on-resonant spectra were measured at Ek = 80 and 121 eV, respectively. The 4f cross section dramatically increases when Ek is tuned to the Ce N4,5 absorption edges at Ek *120 eV by the interference between the processes with or without 4d to 4f excitation. Although there is a complicated mixture of Ce 4f and 5d contributions to the off-resonant spectra in Fig. 50.4a, the on-resonant spectrum in Fig. 50.4b only gives information about the unoccupied 4f-PDOS. 50.4.3 Spin-Resolved Inverse Photoemission Spectroscopy Spin-resolved IPES (SRIPES) using spin-polarized electrons provides direct access to detailed knowledge of the spin-dependent unoccupied electronic states of ferromagnets, surfaces, interfaces, and thin-film magnetism. Photoemitted electrons from negative-electron-affinity GaAs, which can be obtained by surface treatments with Cs and O2, excited by circularly polarized light is the most efficient way to obtain high currents with considerable spin polarization. SRIPES can be performed by using this emitted spin-polarized electron beam as an excitation source. SRIPES results were reported at the low-Miller-index surfaces of Fe and Ni as model ferromagnetic 3d transition metals by probing the exchange-split majority 50 Inverse Photoemission Spectroscopy 311 Fig. 50.4 a Off- and b on-resonant RIPES spectra of intermetallic Ce compounds [4]. Only 4fPDOS is observed in the on-resonant spectra and minority electronic states just above EF separately in early studies [7–9], and now on spin-polarized surface states originating from the space inversion asymmetry [10]. 50.4.4 Low-Energy Inverse Photoemission Spectroscopy The radiation damage to samples from electron impact during measurements has been a longstanding problem with IPES in the UV, vacuum UV, and soft X-ray range. Recently, low-energy inverse photoemission spectroscopy (LEIPS) in the near-UV range using electrons with kinetic energies of less than 4 eV has been developed [11]. In the LEIPS measurements, radiation damage to the sample from electron impact is greatly reduced. Therefore, this method is especially suitable for soft materials like organic materials. 312 K. Kanai References 1. Woodruff, D.P., Johnson, P.D., Smith, N.V.: Inverse photoemission. J. Vac. Sci. Technol., A 1(2), 1104–1110 (1983) 2. Nijiboer, B.R.A.: Physica (Amsterdam). 12, 461–466 (1946) 3. Dose, V.: VUV isochromat spectroscopy. Appl. Phys. 14, 117–118 (1977) 4. Nakanishi, R., Nogimura, A., Eguchi, R., Kanai, K.: Org. Electr. 15, 2912–2921 (2014) 5. Weibel, P., Grioni, M., Malterre, D., Dardel, B., Baer, Y.: Resonant inverse photoemission: A new probe of correlated systems. Phys. Rev. Lett. 72, 1252–1255 (1994) 6. Kanai, K., Tezuka, Y., Terashima, T., Muro, Y., Ishikawa, M., Uozumi, T., Kotani, A., Schmerber, G., Kappler, J.P., Parlebas, J.C., Shin, S.: Resonance effect on inverse-photoemission spectroscopy of CeRh3, CePd3 and CeSn3. Phys. Rev. B. 60, 5244– 5250 (1999) 7. Scheidt, H., Globl, M., Dose, V., Kirschner, J.: Exchange-split empty energy bands of Fe (110). Phys. Rev. Lett. 51, 1688–1691 (1983) 8. Donath, M.: Spin-dependent electronic structure at magnetic surfaces: the low-Miller-index surfaces of nickel. Surf. Sci. Rep. 20, 251–316 (1994) 9. Berti, G., Calloni, A., Brambilla, A., Bussetti, G., Duò, L., Ciccacci, F.: Direct observation of spin-resolved full and empty electron states in ferromagnetic surfaces. Rev. Sci. Instr. 85, 072901 (2014) 10. Sakamoto, K., et al.: Nat. Commun. 4, 2073 (2013). doi:10.1038/ncomms3073 11. Yoshida, H.: Near-ultraviolet inverse photoemission spectroscopy using ultra-low energy electrons. Chem. Phys. Lett. 539–540, 180–185 (2012) Chapter 51 Kelvin Probe Force Microscope Risa Fuji Keywords Surface potential 51.1 AC bias voltage Surface potential calibration Principle KPFM mode as shown in Fig. 51.1 generates images based on the electric potential of the sample surface. Applying an AC voltage to a conductive cantilever and detecting the resulting electric force makes it possible to observe the surface profile and, at the same time, the surface potential distribution. Here, we consider a cantilever and sample with uniform surfaces. We assume that they are both conductors and that their respective surface potentials are Vt and Vs, respectively. In this case, the electrostatic free energy, G, in a system kept at a constant potential can be expressed by formula (51.1) below, where d is the distance between the cantilever and the sample surface and C is the capacitance between the cantilever and the sample. 1 G ¼ C ðVt Vs Þ2 : 2 ð51:1Þ The electric force, Fez(d), that acts on the cantilever can be expressed by formula (51.2). R. Fuji (&) Global Application Development Center Analytical & Measuring Instruments Division, Shimadzu Corporation, Kanagawa, Japan e-mail: r-fuji@shimadzu.co.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_51 313 314 R. Fuji Fig. 51.1 In KPFM mode, the potentials of sample surface can be measured Fze ðd Þ ¼ @G 1 @C ¼ ðVt Vs Þ2 : @d 2 @d ð51:2Þ Because the capacitance, C, decreases as d increases, the electric force between the cantilever and the sample is always attractive, and the size is proportional to the capacitance gradient. The voltage applied externally to the cantilever can be expressed by formula (51.3). Vt ¼ VDC þ VAC cos xt: ð51:3Þ The electric force at the cantilever can be expressed by formula (51.4). 1 2 1 2 2 ðVDC VS Þ þ VAC þ 2ðVDC VS ÞVAC cos xt þ VAC cos 2xt : 2 @d 2 2 1 @C Fðd; tÞ ¼ Fzm ðd Þ þ ð51:4Þ From this, the amplitude of the vibration electric force corresponding to angular frequency x can be expressed by formula (51.5). Fxe ¼ @C ðVDC VS ÞVAC : @d ð51:5Þ In SPM profile measurement, the distance between the cantilever and the sample is constant and so it can be assumed here that the capacitance gradient and VAC are constant. Therefore, if feedback control is performed for VDC, which is applied to the cantilever, so that Fex is always equal to zero, VDC will be equal to Vs and can be obtained as the potential of the sample surface. 51 Kelvin Probe Force Microscope 51.2 315 Features • Potentials of sample surface can be measured. • In order to correctly observe potential images, the direction and magnitude of the change in the potential deflection signal that occurs when the sample potential is changed must be adjusted. • A conductive sample such as a sample holder is attached before the actual sample is attached. 51.3 Instrumentation In KPFM mode as shown in Fig. 51.2, AC electric signals are applied to a conductive cantilever and images based on the electric potential on the sample surface are generated by detecting the static electricity that occurs between the sample surface and the cantilever. Because topographic images and potential images of the sample surface can be observed at the same time, a potential distribution for the sample’s surface profile can be obtained. With an appropriate AC voltage applied between the cantilever and the sample surface, if the cantilever is moved toward the sample surface, an electric force acts between the cantilever and the sample surface. At this time, although the cantilever vibrates at a constant frequency due to the electric force, if the potential of the sample surface changes, the amplitude of the cantilever vibrations, which are caused by the electric force, also changes. It is known that the amplitude of the cantilever vibrations, which are caused by the electric force, will be equal to 0 if a DC voltage of the same value as the potential of the sample surface is applied to the cantilever. Therefore, performing feedback control of the DC voltage applied to the cantilever so that the amplitude is equal to 0 makes it possible to obtain the potential Fig. 51.2 Experimental setup for KPFM 316 R. Fuji of the sample surface. Also, control of the distance between the cantilever and the sample surface at this time is performed using dynamic mode. 51.4 Applications 51.4.1 Observation of Metals Figure 51.3 shows the topographic image (left) and surface potential image (right) of metals. A copper (Cu) plating was applied to iron (Fe), a cross section of the sample was excised, and the electric potential at their interface was measured. In the topographic image at the left, the interface is not evident. In the electric potential image at the right, however, measurements on the copper side were approximately 0 V, while those on the iron side were a high 90 mV. 51.4.2 Observation of Organic Thin-Film Transistors (FET) Figure 51.4 shows the instrument schematic diagram (left), topographic image (center), and surface potential image (right) of thin-film transistors (FET). This is an example of the shape analysis and potential analysis of organic thin-film transistors. These transistors are attracting attention for applications such as flexible displays. The material is 3-hexylthiophene (P3HT) that offers high mobility. As shown in the instrument schematic diagram below, the actual SPM measurements were performed by grounding the source electrode and independently applying potentials to Fig. 51.3 Topographic image (left) and surface potential image (right) of metals 51 Kelvin Probe Force Microscope 317 Fig. 51.4 Instrument schematic diagram (left), topographic image (center) and surface potential image (right) of thin-film transistors (FET) the gate and drain electrodes to determine how the surface potential changes on the gate. References 1. Nonnenmacher, M., O’Boyle, M.P., Wickramasinghe, H.K.: Kelvin probe force microscopy. Appl. Phys. Lett. 58, 2921 (1991) 2. Morita, S. Atomic/Molecular Nanomechanics. 33 (2011) Chapter 52 Laser Ionization Secondary Neutral Mass Spectrometry Tetsuo Sakamoto Keywords Photo-ionization Polymer analysis 52.1 Mass imaging Laser Trace analysis Principle Laser SNMS is a mass-selective surface imaging technique based on photo-ionization of sputtered neutrals originated in an ion beam irradiation on a solid sample. Atoms, occasionally molecules, are promoted and ionized through photon absorption. Figure 52.1 illustrates two types of photo-ionization schemes. Atoms can be ionized when the photon energy is greater than their ionization potentials. In “non-resonant two photon” scheme, a photon energy greater than a half of the ionization potential is required. In this case, however, higher photon density is needed because the two photons should be absorbed simultaneously. Consequently, the laser used here is usually pulsed lasers to enhance the photon density. Another scheme is called “resonant two photon” ionization. If the laser wavelength is tunable, photon energy can be adjusted to the intermediate excited state. It is known that the excited state has a lifetime in a nano-second order. Therefore, the probability of the absorption of another (second) photon and the atom is ionized. As a result, resonance scheme realizes “element-selective” ionization by adjusting laser wavelength to the energy of the first excited state. In general, tunable laser is expensive and complex. Non-resonant scheme is usually employed using a pulsed laser system at a fixed wavelength. T. Sakamoto (&) Department of Applied Physics, School of Advanced Engineering, Kogakuin University, Tokyo, Japan e-mail: ct13087@ns.kogakuin.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_52 319 320 T. Sakamoto Fig. 52.1 Photo-ionization schemes. Left: non-resonant two photon absorption. Right: resonant two-photon absorption IonizaƟon PotenƟal Excited State Ground State Non-resonant twophoton absorpƟon 52.2 • • • • Resonant two-photon absorpƟon Features High ionization efficiency compared with SIMS, Quantitative analysis is possible, Molecular species other than single atoms can be analyzed, Features of SIMS are included. 52.3 Instrumentation Figure 52.2 depicts the geometry around the specimen in Laser SNMS technique. When a solid surface is bombarded with a pulsed primary ion beam, packets of neutrals are ejected from the surface. A pulsed laser beam passes through the packet and photo-ionization occurs in the region. Then the ionized species are transported into a mass analyzer. Typical instrumental layout is shown in Fig. 52.3. It consists of a pulsed laser system and an ion-beam-induced mass analysis system. Some kinds of laser have been used for SNMS, excimer laser, Nd:YAG laser, and diode-pumped solid state laser. Most of the lasers generate photons at rather long wavelength; therefore, a harmonic generator is used to obtain shorter wavelength in UV range. The pulsed ion beam and the pulsed laser should be synchronized exactly, and usually the synchronization timing is used for also the time-of-flight mass spectrometer as a start event timing. As for the ion beam, if focused ion beam (FIB) is used, high lateral resolution better than 100 nm is realized. 52 Laser Ionization Secondary Neutral Mass Spectrometry 321 Fig. 52.2 Generation of sputtered neutrals and photo-ionization in Laser SNMS SNMS Apparatus TOF MS Pulsed Laser System lens Laser Ionization Volume Pulsed Ion Beam Harmonic generator Sputtered Atoms Sample Fig. 52.3 Typical layout of a Laser SNMS apparatus [1] 52.4 Applications 52.4.1 Highly-Sensitive Analysis of Aromatic Organic Compounds When a laser at wavelength is around deep UV range, aromatic hydrocarbons are effectively ionized through p–p* transition. Figure 52.4 shows a comparison of sensitivity in Laser SNMS at 250 nm and SIMS (without laser ionization) using polyaromatic hydrocarbon (PAH) samples. Laser SNMS showed 30–50 times better sensitivity compared with SIMS. 322 T. Sakamoto Fig. 52.4 Comparison of Laser SNMS and SIMS mass spectra of mixture of PAHs D10-phenanthrene 5,720 counts Intensity [Arb.Units] FIB+Laser(250 nm) Pyrene Anthracene 158 counts FIB only (SIMS) 170 180 190 200 m/z [amu] 120 100 Intensity [counts] m/z=120 Polyhydroxystyrene m/z=104 Polystyrene 80 60 40 20 0 85 90 95 100 105 110 115 120 125 m/z Fig. 52.5 Laser SNMS mass spectrum of polymer blend (polystyrene and polyhydroxystyrene) [2] Another example is polymer. In conventional SIMS, polymer analysis is very difficult because many fragment mass peaks observed instead of characteristic ones attributed to the polymer species. Figure 52.5 shows Laser SNMS of polymer blend (polystyrene and polyhdroxystyrene) at a wavelength of 266 nm. It is clear that “monomer” peaks of each polymer are detected strongly. 52 Laser Ionization Secondary Neutral Mass Spectrometry 323 SIMS analysis Positive ion mode Total ion image Mass image 10 μm 10 μm 4 μm Red; m/z = 91 PS, Green; m/z = 107 PHS PS Laser-SNMS analysis Total ion image n Mass image m/z = 104 PHS n 10 μm 10 μm 4 μm Red; m/z = 104 PS, Green; m/z = 120 PHS OH m/z = 120 Fig. 52.6 Comparison of surface imaging of blend polymer thin film consisting of polystyrene and polyhydroxystyrene, using SIMS (upper) and Laser SNMS (lower) [3] 52.4.2 High-Resolution Imaging Analysis of a Polymer Blend Recent years, solid state laser with a high repetition rate up to 10 kHz is commercially available. In SNMS imaging, the repetition rate is important because higher rate results in shorter analysis time. Figure 52.6 shows imaging analyses of polymer blend thin film surface using SIMS and Laser SNMS. As mentioned before, in Laser SNMS characteristic mass peaks of different polymers can be obtained. “Sea-island structure” of the blend polymer is much clear than that in SIMS mode. The laser SNMS imaging required only 10 min. 324 T. Sakamoto References 1. Sakamoto, T., Koizumi, M.: Resonance enhanced multi-photon ionization of neutral atoms sputtered with Ga-FIB. Appl. Surf. Sci. 255, 901–904 (2008) 2. Sakamoto, T., A new aspect of Laser-SNMS coupled with Ga FIB for polymer analysis, proceedings of SISS-15, 81–82 (2013) 3. Ishikawa, T., Kashiwagi, T., Sakamoto, T., Misawa, K., Fujii, M., Hachiya, M., Noda, H., Endo, K.: Development of a Laser-SNMS instrument for nanoscale analysis and mapping of organic materials. Hyomen-Kagaku. 35, 383–388 (2014). (in Japanese) Chapter 53 Laser Photoelectron Spectroscopy Ryuichi Arafune Keywords Phonon (vibration) energy Electron-phonon coupling 53.1 Electronic structure Principle Photoemission spectroscopy is an extensively used technique to investigate the electronic characteristics of solid surfaces. In this decade, laser photoemission spectroscopy [1], in which a laser is used as the excitation light source instead of a helium discharge lamp and/or synchrotron light source, has been developed. Lasers offer many advantages over these conventional light sources, such as strong intensity, coherency, ultranarrow linewidth (or ultrashort pulse duration), and high tunability of polarization. All these advantages have opened new spectroscopic possibilities. This section introduces an application of laser photoemission spectroscopy [2]: the inelastic process in laser photoemission that allows us to determine phonon energy and the electron-phonon coupling in the photo-excited state. Figure 53.1a shows a schematic diagram of the phonon (vibrational) energy determination through inelastic interaction in the laser photoemission process. Here, we use a metal covered by CO molecules as a model, and photoelectrons excited by a laser with energy hm, which is slightly higher than the work function of the surface. Assuming a flat electron density of states (DOS) and no inelastic scattering in bulk, one can observe that the photoelectron spectrum at the Fermi level is represented by the well-known Fermi-Dirac distribution curve (Fig. 53.1b). However, a fraction of the emitted photoelectrons can lose energy through the excitation of a vibrational mode of the CO molecule having the energy hmvib, where mvib is the molecular vibration frequency. This inelastic process will produce a spectral component with a Fermi-Dirac distribution shape, which shifts to lower R. Arafune (&) International Center for Materials Nanoarchitectonicsm(MANA), National Institute for Materials Science (NIMS), Tsukuba, Japan e-mail: ARAFUNE.Ryuichi@nims.go.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_53 325 326 R. Arafune Fig. 53.1 a Schematic diagram for inelastic interaction in the laser photoemission process. b The photoemission spectrum of the metal surface around the Fermi level. If no interaction occurs between the molecular vibration and the photoelectron emitted from the surface, the spectral line shape is expected to be that of the well-known Fermi-Dirac distribution. c The laser photoemission spectrum obtained while considering the inelastic interaction between the photoelectron and the adsorbed molecule. When the kinetic energy of the photoelectron decreases by hmvib through the excitation of molecular vibration, the photoelectron spectrum will show a step at hmvib below the Fermi level. This spectrum consists of two parts: one (green dotted line) describes the photoelectron emitted without exciting molecular vibrations, while the other (orange dashed line) represents the energy distribution of photoelectrons that has excited the molecular vibration photoelectron kinetic energy by hmvib and has an amplitude determined by the electron-phonon coupling matrix element. When this inelastic component is superposed to the elastic component, a step appears at hmvib below the Fermi edge in the spectrum (Fig. 53.1c). Consequently, the phonon (vibration) energy is determined from the step position. 53.2 Features • Detection of vibrational modes of surface adsorbates and surface phonon. • Especially, powerful for low energy vibrational modes. • In addition to the vibrational (phonon) DOS, the electronic structure around the photoexcited states is important for analysis. • A promising tool for investigating the momentum-resolved Eliashberg function. • Low temperature and high-energy resolution are required for measuring the phonon energy. 53 Laser Photoelectron Spectroscopy 327 Fig. 53.2 Experimental setup for laser photoemission spectroscopy 53.3 Instrumentation The measurement system consists of two components: the laser system and the UHV electron analyzer system. Figure 53.2 shows the experimental setup for laser photoemission spectroscopy. Note that no additional equipment is required for measuring the phonon energy. Since the work function of most materials is within the range of 4–6 eV, a combination of the short pulse-laser and high harmonic generation crystals is usually used. By combining a continuous wave laser and harmonic crystals in actively stabilized cavities, one can produce ultranarrow-linewidth photons with 6 eV energy [3]. The electron energy analyzer is essentially identical to that used in conventional photoemission spectroscopy, provided the energy resolution is sufficiently high (an energy resolution better than 10 meV is strongly required). Recall that the phonon energy is less than several tens of meV. As one analyzes the spectral line shape in the vicinity of the Fermi level cut-off, the refrigeration of the sample is an important technical component for measuring the phonon property using laser photoemission spectroscopy. As the emitted photoelectron has very low energy, a bias voltage is occasionally applied between the sample and the analyzer to minimize the distortion of the spectral line shape to stray magnetic and electric fields and to collect such low-energy photoelectrons effectively. 328 53.4 R. Arafune Applications 53.4.1 Detection of Vibrational Modes of Molecular Adsorbate Figure 53.3a shows the laser photoemission spectra of the clean and CO covered Cu(001) surfaces excited by the 4.826-eV laser photon at 15 K [4]. While the clean Cu spectrum shows the well-known Fermi-Dirac distribution curve, the spectrum of the CO covered surface shows a step structure at 34.5 meV below the Fermi level. The step position is determined from fitting by using the Fermi-Dirac distribution function or from the differential spectrum as shown in Fig. 53.3b [5]. The dip in the differential spectrum corresponds to the step position. The energy of the step position agrees with the vibrational energy of the frustrated rotation mode. Figure 53.3b also shows the differential spectrum of the Cu(001) covered by isotope substituted CO. The dip for the 13C18O spectrum is red-shifted compared with that for the 12C16O spectrum. This result indicates that the laser photoelectron certainly excites the vibrational mode of the surface adsorbates as expected in the section ‘Principle’. The laser photoelectrons also excite the phonons of the solids, while this example demonstrates that the vibrational mode of only the surface adsorbed Fig. 53.3 a Laser photoemission spectra of clean and CO covered Cu(001) surfaces. The photon energy is 4.826 eV and the sample temperature is 15 K. The green spectrum is for the clean Cu (001), and the red spectrum is the CO covered spectrum. A step structure originating from the excitation of the frustrated rotation mode appears at 34.5 meV below the Fermi level only in the CO covered spectrum. b Differential spectrum of the laser photoemission spectrum of the CO covered Cu(001). While these data are obtained by using a lock-in technique [5], numerical differentiation provides essentially identical data. The red (blue) spectrum is for 12C16O (13C18O) on the Cu(001) surface. The inset highlights the data around the dip. The isotope shift is clearly observed 53 Laser Photoelectron Spectroscopy 329 molecules is excited by the photoelectrons. Thus far, the phonon modes of metals and superconducting diamond have been detected using laser photoemission spectroscopy [6, 7]. Practically, this inelastic interaction through the vibrational elementary excitation by photoelectrons occurs in not only laser photoemission, but also conventional photoemission spectroscopy. Indeed, the inelastic component originating from the phonon excitation in the graphene/graphite spectra has been reported [8]. References 1. Kiss, T., Kanetaka, F., Yokoya, T., Shimojima, T., Kanai, K., Shin, S., Onuki, Y., Togashi, T., Zhang, C., Chen, C.T., Watanabe, S.: Phys. Rev. Lett. 94, 057001 (2005) 2. Arafune, R., Hayashi, K., Ueda, S., Uehara, Y., Ushioda, S.: Phys. Rev. Lett. 95, 207601 (2005) 3. Tamai, A., Meevasana, W., King, P.D.C., Nicholson, C.W., de la Torre, A., Rozbicki, E., Baumberger, F.: Phys. Rev. B. 87, 075113 (2013) 4. Arafune, R., Hayashi, K., Ueda, S., Uehara, Y., Ushioda, S.: Surf. Sci. 600, 3356 (2006) 5. Hayashi, K., Arafune, R., Ueda, S., Uehara, Y., Ushioda, S.: J. Phys. Soc. Japan. 75, 104303 (2006) 6. Minamitani, E., Arafune, R., Yamamoto, M.Q., Takagi, N., Kawai, M., Kim, Y.: Phys. Rev. B. 88, 224301 (2013) 7. Ishizaka, K., Eguchi, R., Tsuda, S., Chainani, A., Yokoya, T., Kiss, T., Shimojima, T., Togashi, T., Watanabe, S., Chen, C.-T., Takano, Y., Nagao, M., Sakaguchi, I., Takenouchi, T., Kawarada, H., Shin, S.: Phys. Rev. Lett. 100, 166402 (2008) 8. Liu, Y., Zhang, L., Brinkley, M.K., Bian, G., Miller, T., Chiang, T.-C.: Phys. Rev. Lett. 105, 136804 (2010) Chapter 54 Lateral Force Microscopy Shiho Moriguchi Keywords Lateral force Topography 54.1 Friction force Dynamic friction coefficient Principle In lateral force microscopy, not only the topographic image of the sample surface but also the distribution image of the lateral force in the same area can be gotten simultaneously. The scan is based on the principle of contact mode in which the vertical force acting between the top of the probe and the sample surface is kept constant in order to measure the shape of the sample surface. The scan direction is the perpendicular direction to the cantilever axis. While scanning, the torsion of the cantilever arising from the lateral force is detected as the horizontal deflection as shown in Fig. 54.1. Three kinds of evaluation approach about friction using lateral force microscopy are as follows: 1. The distribution of the frictional force in the sample surface 2. The value of the dynamic friction coefficient 3. The relative evaluation about the magnitude of the frictional force for samples. Detailed descriptions of these approaches are provided in the applications section. S. Moriguchi (&) Shimadzu Techno-Research, INC, 1, Nishinokyo-shimoaicho Nakagyo-ku, Kyoto, Japan e-mail: s_moriguchi00@shimadzu-techno.co.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_54 331 332 S. Moriguchi Fig. 54.1 Ditecting the horizontal deflection arising from the torsion of the cantilever 54.2 Features • The topographic image of the sample surface and the distribution image of the lateral force in the same area can be gotten simultaneously. • Operational environment is various such as in air, liquid, room temperature, high temperature, and low temperature. • Distribution of the frictional force in the sample surface and the value of dynamic friction coefficient can be evaluated. • The exact value of the cantilever spring constant is required in order to evaluate the absolute value of the frictional force. • The lateral force is influenced from not only the frictional force but also roughness of the sample surface. 54.3 Instrumentation Figure 54.2 shows basic structure of lateral force microscopy. The laser beam irradiated from semiconductor reflects on the backside of the cantilever and goes into the center of the photodetector. The cantilever is bended in vertical direction by the vertical force acting between the top of the probe and the sample surface. The bending is detected as the vertical deflection with 4 elements photodetector. The feedback controller controls the vertical movement of the tube piezo scanner to keep the vertical deflection constant. The normal force acting between the top of the probe and the sample surface is kept constant by this feedback while scanning. 54 Lateral Force Microscopy 333 Fig. 54.2 Basic structure of lateral force microscopy The X/Y scan controller controls the horizontal movement of the tube piezo scanner to scan set area. The scan direction is perpendicular direction to the cantilever axis. The cantilever is twisted by the lateral force acting between the top of the probe and the sample surface. The torsion of the cantilever arising from the lateral force is detected as the horizontal deflection with 4 elements photodetector. The lateral force DF ½nN is calculated from the horizontal deflection DV ½V , the coefficient of the vertical displacement sensitivity and the size of the cantilever as follows: [1, 2] DF ¼ w t3 G 109 DV; 3 l L2 P where w: t: G: l: L: P: cantilever width [m] cantilever thickness [m] rigidity [Pa] probe length [m] cantilever length [m] the coefficient of the vertical displacement sensitivity [V/m]. 334 54.4 S. Moriguchi Applications 54.4.1 The Distribution of the Frictional Force in the Sample Surface Figure 54.3 shows topographic image (left) and horizontal deflection image (right) for the substrate coated with two kinds of metal. The unevenness of the coating which is hard to be confirmed by the topographic image can be clearly seen in the horizontal deflection image. 54.4.2 The Value of Dynamic Friction Coefficient The relationship between the frictional force and the normal force is given by F ¼ lN, where F is the frictional force, l is the dynamic friction coefficient, and N is the normal force. By measuring the fluctuation in the frictional force (F) when the normal force (N) acting between the probe and the sample surface is changed, l can be calculated from F = lN. Figure 54.4 shows the relationship between the lateral force and the normal force which were measured in artificial tears using LFM for the hydrous lens. It was calculated to be the dynamic friction coefficient l = 0.12 from this relationship. Fig. 54.3 Topographic image (left) and horizontal deflection image (right) for the substrate coated with two kinds of metal 54 Lateral Force Microscopy 335 Fig. 54.4 Relationship between the lateral force and the normal force 54.4.3 The Relative Evaluation About Magnitude of the Frictional Force for Samples Figure 54.5 shows the profile of the horizontal deflection for glass (left) and stainless (right). Red line is the profile in trace scanning as shown in Fig. 54.1a, blue line is the profile in retrace scanning as shown in Fig. 54.1c. It indicates that the longer the distance between red line and blue line, the stronger the lateral force. These data were measured with same size normal force, so it turns out that the frictional force of stainless is bigger than one of glass. Fig. 54.5 Profile of the horizontal deflection for glass (left) and stainless (right) References 1. Ogletree, D.F., Carpick, R.W., Salmeran, M.: Calibration of frictional forces in atomic force microscopy. Rev. Sci. Instrum. 67, 3298 (1996) 2. Matsumoto, N., Kobayashi, K.: Investigation of Relevance of the Environment in Friction Force Measurement. Kyoto University FY2003 Nanotechnology Support Project Report. 049 (2003) Chapter 55 Liquid SPM/AFM Akinori Kogure Keywords In situ 55.1 Q factor Spurious peaks FM mode Principle The principle of liquid SPM/AFM is the same as atmospheric AFM (Fig. 55.1). When a probe is near or in contact with a sample surface, an image of the probe– sample interaction with atomic resolution can be obtained. The difference between atmospheric observations and liquid SPM/AFM is that the probe and sample are immersed in a solution. The following modes are possible: Contact mode, Dynamic mode (Amplitude modulation) and Frequency modulation mode. 55.2 Features • Scanning Electron Microscopes and Transmission Electron Microscopes cannot observe samples in solutions, but SPM/AFM can. • For example, in situ observation of biological reactions, catalytic reactions and cell reactions are possible. • In-liquid observation can maintain a more stable environment than in the atmosphere. Therefore, high-resolution observation is possible. A. Kogure (&) AMC Department Testing and Analysis Division, Shimadzu Techno-Research, INC, Kanagawa, Japan e-mail: a_kogure00@shimadzu-techno.co.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_55 337 338 A. Kogure Fig. 55.1 Principle of liquid SPM/AFM 55.3 Instrumentation The principle of the equipment is the same as atmosphere-AFM (Fig. 55.2), and it is composed of the following three. One, laser diode and photo detector. Two, cantilever and holder. Three, scanner. The principle of the equipment is the same as atmosphere-AFM, and it is composed of the following three. First, laser diode and photo detector. Second, cantilever and holder. Third, scanner. The difference between atmospheric AFM is the cantilever and cantilever holder. • Cantilever and holder The back of the cantilever is coated with gold or uncoated because aluminium is unstable due to corrosion. The holder has a liquid cantilever holder and AFM-liquid-dish. • Spring constant In general, the spring constant is 0.02–0.5 N/m for contact mode, 0.1–2 N/m for dynamic mode and 26–40 N/m for FM mode. 55.4 Applications 55.4.1 Observation of DNA in Liquid DNA on a mica surface in a solution can be observed using liquid AFM (Fig. 55.3). In-liquid observation of DNA prevents deformation due to drying, thus making it possible to make an in situ observation. However, SPM/AFM cannot observe floating samples. Therefore, it is necessary to attach the sample to a surface. 55 Liquid SPM/AFM 339 Fig. 55.2 Basic configuration of the SPM/AFM [1] Charge of the substrate A mica surface is negatively charged when processed with magnesium chloride, thus positively charged proteins and DNA will attach to themselves to the surface. Hydrophilic ・ Hydrophobic After salinisation, the mica surface will be hydrophobic. Coat the substrate with adhesive materials such as poly-L-lysin or collagen Cool biomaterial such as living cells to slow down its motion, making it possible to be observed. 55.4.2 Contact mode, Dynamic mode In liquid observation, the contact mode and the dynamic mode have the following features (Table 55.1). 340 A. Kogure Fig. 55.3 Lambda DNA Table 55.1 Comparison of contact mode, dynamic mode and FM mode Operability Advantage Contact mode (in liquid) Dynamic mode (in liquid) FM mode (in liquid) Almost same with operation in air Fast scanning speed Wide scanning can be preformed Some experiences is required Fit for macromolecule, biomaterial, etc. Scanning speed is slow 0.1–2 N/m High skills are required Ultrahigh resolution, ultrahigh sensitivity Disadvantage It’s easy to scratch sample Spring Constant 0.02–0.5 N/m Scanning speed is slow 40 N/m 55 Liquid SPM/AFM 341 Fig. 55.4 Resonance frequency 55.4.3 Resonance of dynamic mode in liquid The cantilever resonance frequency decreases by 1/2–1/5 compared to atmospheric observation due to the viscous resistance of the solution. Also, unnecessary spectral peaks (spurious peaks) appear. Finding an optimal resonance frequency is the necessary for obtaining good images (Fig. 55.4). Reference 1. http://www.shimadzu.com/an/surface/spm/faq/index.html Chapter 56 Low-Energy Ion Scattering Spectroscopy Kenji Umezawa Keywords Low-energy ion scattering 56.1 Atomic collision Ultra-high vacuum Principle Low-energy ions scattering (less than 5 keV) is a powerful tool for the analysis of “first monolayer” [1, 2]. The charged incident particles in this energy region penetrate beyond a monolayer which emerges nearly always as neutral atoms. Figure 56.1 shows a schematic of binary collision. The kinematic relations between incident particles and target elements are given in Eqs. (56.1) and (56.2). E1 ¼ E0 " #2 1=2 M22 M12 sin2 h þ M1 cos h M1 þ M2 E2 4M1 M2 ¼ E0 ðM1 þ M2 Þ2 ð56:1Þ ð56:2Þ For the calculation of the classical trajectory between projectile and target atoms, the modified universal potential (the ZBL potential) as reported by Ziegler, Biersack, and Littmark is commonly used in this energy region [1, 2]. The ZBL potential function depends on the distance r between the nuclei. The ZBL potential is shown by the following equations. K. Umezawa (&) Department of Physics, College of Integrated Arts Sciences, Osaka Prefecture University, Sakai, Osaka, Japan e-mail: umezawa@las.osakafu-u.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_56 343 344 K. Umezawa M1, E1 Fig. 56.1 Schematic representation of an elastic collision between a projectile of mass M1, energy E0, and a target mass M2. After collision, the projectile and the target mass have energies E1 and E2, respectively V ðr Þ ¼ θ M1, E0 M2 M2,E2 Z1 Z2 2 r r e ½0:1818 exp 3:2 þ 0:5099 exp 0:9423 a a r r r þ 0:2802 exp 0:4029 þ 0:02817 exp 0:2016 a a 0:23 1 a ¼ 0:8854 Z1 þ Z20:23 a0 ; ð56:3Þ ð56:4Þ where Z1 is the atomic number of the projectile, Z2 is the atomic number of the target atom, and e is the unit electrical charge, a is the ZBL screening length, a0 is the first Bohr radius. In comparison with LEIS data, the ZBL potential is better than other potential such as the unadjusted Molière potential. But it still does not agree with experimental data. Thus, the force F is obtained from Eq. (56.5). F ¼ rV ðr Þ ð56:5Þ Shadowing and blocking are important concepts between ions and topmost atoms, characteristic of scattering by repulsive potentials. The flux of incident ion beams flows toward nuclei, and then scattered flux is formed according to repulsive potential. The region of “forbidden” space behind the target atom takes the form of a paraboloid with radius Rsh as a function of distance from the target atom. The scattered yields decrease or increase as near-surface atoms are hidden or exposed to ions caused by other atoms. The shadow cones for He+ ions scattering from Au atom are calculated in the ZBL potential in Fig. 56.2. The shadow cone width decreases dramatically with increasing kinetic energy. Using a small-angle approximation and an unscreened coulomb potential, the shadow cone radius Rsh is obtained: 1=2 Rsh ¼ 2 Z1 Z2 e2 d=E ; ð56:6Þ where d is the distance behind the target atom, E is the primary energy, e is the charge of an electron, Z1 and Z2 are atomic numbers of the projectile and target atoms, respectively. Another important concept is blocking [1, 2]. More than two atoms are needed for blocking. The simplest case is two atoms for blocking. Primary ion beams with parallel trajectories are directed at the first atom (a target atom), and then scattered particles undergo the second atom near by the first atom. This interaction results in 56 Low-Energy Ion Scattering Spectroscopy 345 Fig. 56.2 Illustration of shadow cones obtained from the interaction between an Au atom (target atoms) and parallel beams of a 2 keV-4He+ or b 2 MeV-4He+ ions using a ZBL potential. X-axis shows the distance behind Au atom in [Å]. Y-axis shows the shadow cone radius and an impact parameters of incident 4He+ ion beams in [Å]. The edges of the cones are areas of increased flux density arising from the focusing of the ion trajectories an another shadowing phenomenon. It is called blocking, and a hyperboloid-like pattern due to trajectories is called blocking cone. The differential collision cross section for low-energy ion scattering is shown in ref. [1, 2]. The radius of shadow cone in the keV range is almost the same as the nearest-neighboring atomic distance. This is the reason why LEIS is experimentally available for the investigation of surface structures in scattering and recoiling experiments [1–12]. The knowledge of the shape of the shadow cone is applied to determine interatomic distances between the topmost atoms in a crystal. This technique is known as impact collision ion scattering spectroscopy (ICISS), and an observation angle close to 180° is used so that the detected scattered particles experience collision with a near-zero impact parameter [4, 5, 7]. 56.2 Features • Metal or semiconductor surface structural analysis from 1st to 3rd layers can be evaluated. • An ultra-high vacuum chamber combined with LEIS is required for measurements. 56.3 Instrumentation Figure 56.3 shows the equipment of low-energy ion scattering spectroscopy system. It consists of an ultra-high vacuum chamber with the primary ion beam line; (1) ion beam source, (2) micro-channel plate detector (MCP), (3) precision sample manipulator with stepping motors, (4) pre-amplifier, (5) time-to-digital converter 346 K. Umezawa (4ch multiple-stop), (7) pulse generator, (8) low-energy electron diffraction (LEED) optics and Auger electron spectroscopy (AES), (9) softwares to collect data and control stepping motor. A 2 keV-He+ or Ne+ ion beam is typically produced by an ion source with an electron impact type and research grade gas. In general, the multiple scattering due to Ne+ ion beams is less than that due to He+ ion beams. A good feature of He+ ion beams is not involved in isotope. The voltage applied to the chopping plates is 50 V with a pulse width less than 10 n sec. The chopping frequency is typically 100 kHz. Ion beams pass through an MCP which has a center hole with a diameter of 5 mm. An MCP detector (assembly outer diameter = 34 mm) is coaxially mounted along the primary drift tube. The scattered particles are detected by an MCP which is located at 180° against the primary incident beams. The time of flight measurements are carried out using a trigger circuit and a time-to-digital converter which has four independent channels. Each channel independently measures the time between a trigger and stop signals at a time resolution of 10 ns. Data acquisition mode (polar scan and azimuth scan) has been automated and collected in 2.0° steps. 56.4 Applications Figure 56.4 shows the side views of two different types of 3ML-Ag(111) epitaxial growth mode on Cu(111) substrates; Ag ½112//Cu ½11 2 (normal: type-n) and Fig. 56.3 Schematic view of low-energy ion scattering spectroscopy combined with an ultra-high vacuum chamber. An ion beam is collimated to a diameter of 1.5 mm by an aperture. The chopping is carried out when ion beam passes the deflector to produce a pulse beam 56 Low-Energy Ion Scattering Spectroscopy 347 Ag ½112//Cu ½112 (reverse: type-r) [7]. The type-n film has parallel orientation with respect to the Cu substrates, but the type-r orientation has an anti-parallel orientation with respect to the Cu substrates. In these figures, arrows show the directions of incident beams at the particular angle. For example, the arrow labeled “b” indicates that the incident beam hits the atoms perpendicular to the sample surfaces. After getting clean Cu(111) surfaces, a coverage of 3 ML-Ag was evaporated at a rate of about 0.1 ML/min. LEIS measurements were performed from −85° to +85° in 2° increments along the polar angle at the Cu(111) ½11 2 azimuth. In Fig. 56.4c, the symbols “la1n, r” show the intensities from first layers of Ag atoms, and the symbols “la2n, r” show the intensities from second-layers Ag atoms, and the symbols “la3n, r” show the intensities from third-layers Ag atoms of type-n and type-r, respectively. For this case, the type-n versus type-r mode abundances are 23 and 77%, respectively. The peak signals coming from first layers as labeled “sp” (surface peaks at polar angles of ±74°) show the Ag–Ag spacing of 5.0 Å along a ½11 2 azimuth. This can be converted to a Ag nearest-neighbor distance of 2.80 Å, which is slightly less than the Ag–Ag distance (2.89 Å) in a bulk Ag crystal. LEIS study shows the average information of solid surfaces. It is difficult to get the local information on surfaces unlike STM study. There are many examples about surface structural analysis using LEIS study, as well. (a) Normal Ag(111)[11 ]//Cu(111)[11 ] Ag intensity (arb. units) (c) exp. sum. f a la3r la2r sp sp la2r la2n c+d la1n,r la2n 2//Cu(111)[11 ] b la3r -a (b) Reverse Ag(111)[ - c+d -50 0 Polar angle (deg.) 50 Fig. 56.4 a and b show side view of Ag(111) planes for hetero-epitaxial growth on Cu(111) substrates. The arrow-labeled symbols from a through d and sp show the directions of incident beams at particular angles. These labeled arrows correspond to the symbols on figure (c). Figure c Shows time of flight type of low-energy ion scattering intensity as a function of polar angle 348 K. Umezawa References 1. Rabalais, J.W.: Low Energy Ion-Surface Interactions. Wiley, NewYork (1994) 2. Rabalais, J.W.: Principles and Applications of Ion Scattering Spectroscopy. Wiley, New York (2003) 3. Buck, T.M., Wheatley, G.H., Verheij, L.K.: Low energy neon ion scattering and neutralization on first and second layers of a Ni(001) surface. Surf. Sci. 90, 635–647 (1979) 4. Katayama, M., Nomura, E., Kanekama, N., Soejima, H., Aono, M.: Coaxial impact-collision ion scattering spectroscopy (CAICISS): a novel method for surface structure analysis. Nucl. Instrum. Meth. Phys. Res. B 33, 857–861 (1988) 5. Sumitomo, K., Oura, K., Katayama, I., Shoji, F., Hanawa, T.: “A TOF-ISS/ERDA apparatus for solid surface analysis”, “Coaxial impact-collision ion scattering spectroscopy (CAICISS): a novel method for surface structure analysis”. Nucl. Instrum. Meth. Phys. Res. B 33, 871–875 (1988) 6. Souda, R., Aono, M.: Interactions of low energy He+, He0 and He* with solid surfaces. Nucl. Instrum. Meth. Phys. Res. B 15, 114–121 (1986) 7. Umezawa, K., Nakanishi, S., Yoshimura, M., Ojima, K., Ueda, K., Gibson, W.M.: Ag/Cu (111) surface structure and metal epitaxy by impact-collision ion-scattering spectroscopy and scanning tunneling microscopy. Phys. Rev. B 63, 035402 (2000) 8. Williams, R., Kato, M., Daley, R.S., Aono, M.: Scattering cross sections for ions colliding sequentially with two target atoms. Surf. Sci. 225, 355–366 (1990) 9. Robinson, M.T., Yorrens, I.M.: Computer simulation of atomic displacement cascades in solids in the binally-collision approximation, Phys. Rev. B9, 5008–5024 (1974) 10. Yuan, B., Yu, F.C., Tang, S.M.: A database method for binary atomic scattering angle calculation. Nucl. Instrum. Meth. B83, 413–418 (1993) 11. Denier van der Gon, A.W., Smith, R.J., Gay, J.M., O’Connor, D.J., van der Veen, J.F.: Melting of Al surfaces. Surf. Sci. 227, 143–149 (1990) 12. Nieus, H.: Ion scattering spectroscopy and scanning tunneling microscopy: A powerful combination for surface structural analysis. Appl. Phys. A 53, 388–402 (1991) Chapter 57 Low-Energy Electron Diffraction Yoshimi Horio Keywords Surface structure 57.1 Electron diffraction Principle Low-energy electron diffraction (LEED) [1] is one of the diffraction techniques utilizing low-energy electrons and a powerful method for surface structural analysis. The typical range of energies used in LEED is E = 20–500 eV, and the low-energy electron beam irradiates sample surface with normal incidence. The de Broglie wavelength k of the low-energy electron is about 0.05–0.3 nm which is pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi calculated by k Å ¼ 150:4=E ½eV . Since the wavelength is of the order of or less than the interatomic distances, the low-energy electron beam satisfies the atomic diffraction condition. The mean free path of the low-energy electron is of the order of a few atomic layers; then, the event of the elastic diffraction occurs in the very top layers of a sample. Therefore, LEED pattern contains information on the surface atomic arrangement. The LEED spot geometry is understood by the Ewald construction. The semispherical fluorescent screen corresponds to the Ewald sphere, and the diffracted beams produce spots where reciprocal lattice rods intersect the Ewald sphere. So the LEED spots correspond to the two-dimensional surface reciprocal lattices which are deduced from the periodicity of the surface atomic arrangement. On the other hand, the spot intensities as a function of the incident electron energy, so-called I–E spectra (or I–V curves), are necessary to evaluate the atomic positions. Y. Horio (&) Department of Electrical and Electronic Engineering, School of Engineering, Daido University, Nagoya, Japan e-mail: horio@daido-it.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_57 349 350 57.2 Y. Horio Features • Low-energy electron used in LEED is sensitive to the surface structure because of the short mean free path. • LEED spot geometry corresponds to the symmetry of the surface atomic arrangement. • LEED is powerful method to analyze a surface structure by using I–E spectra. • LEED optics can be used not only for the observation of diffraction pattern but also for Auger electron spectroscopy. 57.3 Instrumentation LEED apparatus is schematically shown in Fig. 57.1a, which is evacuated to ultra-high vacuum. Thermo-electrons emitted from heated W filament are accelerated by applying negative voltage (–V) from −20 to −500 V to the cathode filament. The electron beam passing through a Wehnelt cylinder followed by an electrostatic lens is collimated and impinges on a sample surface with normal incidence. The backscattered electrons from the surface propagate to the first grid in the field free space, because the first grid is grounded. The second and third grids are used to reject the inelastically scattered electrons. The potential of the second and third grids is somewhat lower in magnitude, −(V − DV). The greater DV, the brighter the LEED pattern is, but the higher its background intensity is. Then, the retarding voltage should be adjusted to get a LEED pattern with the highest contrast. The fourth grid is grounded to shield the field from the fluorescent screen, because the screen is biased to a high voltage of 3–5 kV in order to reaccelerate the electrons passing through the retarding grids. Finally, elastically diffracted electrons from the sample surface cause fluorescence on the screen which is LEED pattern. There are two arrangements for the viewing directions, (A) front view and (B) reverse view. In the former case, a viewport is placed in front of the backside of the sample and LEED pattern is viewed past the sample as shown by arrow A, where the sample holder should be reasonably small. In the latter case, LEED pattern is viewed through a viewport placed behind the transmission fluorescent screen as shown by arrow B, where the electron gun should be miniaturized. For example, the reverse viewed LEED patterns taken from Si(111)7 7 and Si(001) 2 1 surfaces are shown in Fig. 57.1b and c, respectively. Many fractional order spots tell us the size of unit mesh of the superstructure. 57 Low-Energy Electron Diffraction 351 Fig. 57.1 a Schematic diagram of four-grid LEED apparatus and LEED patterns at E = 50 eV from b Si(111)7 7 and c Si(001)2 1 surfaces 57.4 Applications For example, a LEED analysis of silicon oxynitride (SiON) epitaxial layer on a 6H– SiC(0001) surface is presented. Preparing the SiC(0001) surface by hydrogen-gas etching and subsequent annealing at 1350 °C in nitrogen atmosphere leads to the pffiffiffi pffiffiffi formation of a 3 3 superstructure. Its atomic structure was determined by a quantitative LEED analysis [2]. The LEED spot intensity was measured by means of a computer-controlled data acquisition system equipped with a charge-coupled device (CCD) camera. For structure analysis, intensity versus energy I–E spectra of the LEED spots were measured within an incident energy range of 50–500 eV as shown in Fig. 57.2a. 352 Y. Horio Fig. 57.2 a Experimental (solid lines) and theoretical (dashed lines) I–E spectra and b Ball and pffiffiffi pffiffiffi stick model of top and side views of the SiON surface structure indicating 3 3 superstructure A Barbieri–Van Hove symmetrized automated tensor LEED package [3] was used to determine the atomic positions. Remarkably, the determined structure, denoted the SiON layer, has no dangling bonds in the unit cell, which agrees with the fact that the structure survives in exposure to air. Furthermore, the SiON layer is formed on an unreconstructed SiC(0001) surface with an atomically abrupt interface. Figure 57.2b shows the structure model of SiON layer, which is hetero-doublelayer structure: a silicate monolayer on a silicon nitride monolayer via Si–O–Si bridge bonds. Calculated I–E spectra based on this structure model agree with the experimental ones very well as shown in Fig. 57.2a. The Pendry reliability factor [4] is so small as 0.14 which means highly reliable model. 57 Low-Energy Electron Diffraction 353 References 1. Pendry, J.B.: Low Energy Electron Diffraction. Academic Press, London and New York (1974) 2. Shirasawa, T., Hayashi, K., Mizuno, S., Tanaka, S., Nakatsuji, K., Komori, F., Tochihara, H.: Epitaxial Silicon Oxynitride Layer on a 6H-SiC(0001) Surface. Rhys. Rev. Lett. 98, 136105 (2013) 3. Van Hove, M.A., Moritz, W., Over, H., Rous, P.J., Wander, A., Barvieri, A., Materer, N., Starke, U., Somorjai, G.A.: Automated determination of complex surface structures by LEED. Surf. Sci. Rep. 19, 191–229 (1993) 4. Pendry, J.B.: Reliability factors for LEED calculations. J. Phys. C 13, 937–944 (1980) Chapter 58 Low-Energy Electron Microscope H. Hibino Keywords Low-energy electron diffraction Surface dynamical process Thin film 58.1 Surface structure Principle Low-energy electron microscopy (LEEM) is a projection-type microscopy technique collecting low-energy (typically 1–100 eV) electrons backscattered from the samples for imaging [1]. The practical LEEM instrument was first developed by Telieps and Bauer in the mid-1980s [2]. Currently, standard commercial instruments have a spatial resolution of about 5 nm. Aberration-corrected LEEM instruments, in which a mirror corrector has improved the spatial resolution to 2 nm or less, are also commercially available. Besides, thanks to parallel acquisition of image information and high reflectivity of low-energy electron, LEEM has a high temporal resolution and is suitable for imaging dynamical phenomena at surfaces. By changing the lens strength and using a selected-area aperture, low-energy electron diffraction (LEED) patterns in a *1 lm area can also be obtained. LEEM is an analysis tool indispensable to surface science and nanoscience, especially for dynamical observations of surfaces. Various contrast mechanisms are available for the LEEM image formation. The most typical one is diffraction contrast caused by the difference in diffraction intensity. When the surface is divided into domains with different structures, sufficient diffraction contrast usually appears at a specific energy in the bright-field (BF) LEEM image using the (0,0) beam. Furthermore, the dark-field (DF) LEEM mode using diffracted beams other than (0,0) is also effective for observations of domain structures. When different domains produce diffraction spots at different positions in the LEED pattern, one type of the domains can be exclusively visualized in the DF LEEM image using the corresponding diffraction beam. In addition H. Hibino (&) Kwansei Gakuin University, Sanda, Hyogo, Japan e-mail: hibino.hiroki@kwansei.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_58 355 356 H. Hibino to the diffraction contrast, there are two important contrast mechanisms based on the modification of the phase of electron waves. When the electron waves are reflected from two terraces separated by a step, the reflected waves interfere due to the path length difference, resulting in the step phase contrast. Quantum size contrast appears on the thin film sample because electron waves reflected from the surface and the interface interfere depending on the incident electron energy and the film thickness. 58.2 Features • • • • Fast image acquisition suitable for investigating surface dynamical processes Nanometer-scale spatial resolution Availability of selected-area electron diffraction Various contrast mechanisms which can be used to extract both structural and electronic information about surfaces and thin films • Versatility compatible with various emission microscopic techniques 58.3 Instrumentation There are several instrumental designs for LEEM. Figure 58.1 shows a schematic diagram of one of them, but the basic components are common to many instruments. A 20-keV electron beam emitted from the LaB6 electron gun passes through several lenses and then irradiates the sample as a parallel beam. The sample is biased approximately at the same potential as the electron source and is positioned Fig. 58.1 Schematic illustration of LEEM instrument 58 Low-Energy Electron Microscope 357 *2 mm from the electrically grounded objective lens electrode. The electron beam is decelerated by the strong electric field between the sample and the objective lens down to the energy determined by the potential difference between the electron gun and the sample. The backscattered electrons from the sample are reaccelerated to 20 keV by the same electric field. The uniform electric field in front of the sample has a lens effect, and the sample itself constitutes a part of the objective lens. Such a cathode lens has been used for a long time in emission electron microscopes, which provide images using electrons emitted from the surface by some stimulus such as light and heat. For example, the LEEM instrument can also be used for photoelectron emission microscopy (PEEM), which collects photoelectrons emitted from the sample irradiated with light for imaging. In the LEEM instrument, the electron beam irradiates the sample almost normally and the backscattered electron beam is used for imaging. Therefore, the illumination column and the imaging column are separated by a beam separator. The incident electron beam is deflected by 60° by the beam separator and reaches the sample, and the backscattered electron beam is deflected by the beam separator into the imaging column. After passing through several lenses, backscattered electrons are intensified by the microchannel plate (MCP) and form a magnified image on the phosphor screen. In the LEEM instrument, the LEED pattern can also be acquired by adjusting the lens strengths. The selected-area aperture allows to obtain LEED patterns at desired areas of *1 lm diameter. The inelastic mean free path of electrons is at an atomic layer level when the energy of the electron beam is about 50 eV, which means that LEEM is very surface-sensitive, but also that the LEEM image is susceptible to surface contaminations. Therefore, LEEM is suitable for observations of well-defined surfaces and is usually operated under ultrahigh vacuum. 58.4 Applications LEEM is a powerful tool for investigation of growth and structure of graphene and related two-dimensional materials. Here, some examples of LEEM studies on graphene are shown below. 58.4.1 Dynamical Observations of Graphene Segregation LEEM has provided a lot of important information about elementary processes in graphene growth. Figure 58.2 shows results of in situ LEEM observations of graphene segregation on a Ni foil at around 900 °C. Multilayer graphene was ex situ grown on the Ni foil by chemical vapor deposition (CVD). Carbon solubility in Ni is larger at higher temperatures. The sample was annealed to high temperatures in 358 H. Hibino Fig. 58.2 LEEM images obtained during graphene segregation on the polycrystalline Ni foil and a plot of the graphene edge position as a function of time. The electron beam energy was 2.7 eV the LEEM instrument, until the multilayer graphene was completely dissolved into Ni, and was then cooled to form graphene. It is known that there are three carbon states involved in the graphene segregation; carbon atoms inside the Ni bulk, carbon adatoms on the Ni surface, and carbon atoms inside the graphene lattice [3]. The time evolution of the graphene edge position shows that the graphene initially grew at a constant rate, which could indicate that the growth is governed by the attachment/detachment of carbon atoms at the graphene edge rather than the diffusion of carbon adatoms on the surface. Figure 58.2 also shows that the graphene growth was slowed down after the Ni surface adjacent to the graphene edge was isolated within the graphene area. This suggests that the segregation of carbon atoms from the Ni bulk to the surface is a slower process than the carbon adatom diffusion on the surface. 58.4.2 Orientation Map of Polycrystalline Graphene CVD on Cu foils is a scalable, cost-efficient synthesis method of graphene, but normally provides polycrystalline graphene. For high-performance device applications, we need to control the grain structures in graphene. For this purpose, a visualization method of the grain structures is a prerequisite. Figure 58.3 shows an orientation map of graphene grown on a Cu(100) film at 1000 °C by CVD [4]. Two DF LEEM images using different first-order diffraction beams were overlaid to form the map. The graphene mainly consists of two types of grains. The selected-area LEED patterns indicate that these grains are rotated by 90° with each other. Graphene and a Cu(100) surface have sixfold and fourfold symmetry, respectively. These two orientated grains are a natural consequence of the difference in the space symmetry between graphene and Cu(100). Choice of the substrate orientation is crucial for the growth of single-crystal graphene. 58 Low-Energy Electron Microscope 359 Fig. 58.3 a Orientation map of graphene grown on a Cu(100) film by CVD, which was obtained by overlapping two DF LEEM images obtained using different first-order diffraction beams at 44.5 eV. The LEED patterns in (b) and (c) were obtained from the corresponding encircled areas in (a). The electron beam energy was 35 eV Fig. 58.4 a, b LEEM images of graphene layers grown on 6H-SiC(0001). The electron beam energy values were a 2.5 and b 4.0 eV, respectively. c Low-energy electron reflectivity spectra of the areas indicated in (a). Numbers in (b) and (c) correspond to the number of epitaxial graphene layers 360 H. Hibino 58.4.3 Determination of Number of Graphene Layers The quantum size contrast allows to determine the thickness of thin films. Figure 58.4 shows BF LEEM images of graphene layers grown on 6H-SiC(0001) [5]. The electron beam energy values were (a) 2.5 and (b) 4.0 eV. The image intensity levels in different regions change with the energy in different manners. Figure 58.4c shows the energy dependence of the LEEM intensities (reflectivity spectrum) in areas 1–4. High electron reflectivity windows around 10, 25, and 36 eV reflect the band gaps of graphite, meaning that the low-energy electron reflectivity provides information about both atomic and electronic structures. Intuitively, the oscillatory behaviors at 0–7 eV appear because the interference condition between the electron waves reflected from the surface and the interface changes with the electron energy, but detailed calculations have also shown that the reflectivity spectra exhibit minima due to the resonant transmission of the incident electrons through the quantized electronic states in the graphene layers [6]. The number of the quantized states is directly related to the number of graphene layers, and therefore the number of graphene layers can be digitally counted from the number of minima in the reflectivity spectra. References 1. Bauer, E.: Surface Microscopy with Low Energy Electrons. Springer, New York (2014) 2. Telieps, W., Bauer, E.: An analytical reflection and emission UHV surface electron microscope. Ultramicroscopy 17, 57–65 (1985) 3. McCarty, K.F., Feibelman, P.J., Loginova, E., Bartelt, N.C.: Kinetics and thermodynamics of carbon segregation and graphene growth on Ru(0001). Carbon 47, 1806–1813 (2009) 4. Ogawa, Y., Hu, B., Orofeo, C.M., Tsuji, M., Ikeda, K., Mizuno, S., Hibino, H., Ago, H.: Domain structure and boundary in single-layer graphene grown on Cu(111) and Cu(100) films. J. Phys. Chem. Lett. 3, 219–226 (2012) 5. Hibino, H., Kageshima, H., Maeda, F., Nagase, M., Kobayashi, Y., Yamaguchi, H.: Microscopic thickness determination of thin graphite films formed on SiC from quantized oscillation in reflectivity of low-energy electrons. Phys. Rev. B 77, 075413 (2008) 6. Feenstra, R.M., Srivastava, N., Gao, Q., Widom, M., Diaconescu, B., Ohta, T., Kellogg, G.L., Robinson, J.T., Vlassiouk, I.V.: Low-energy electron reflectivity from graphene. Phys. Rev. B 87, 041406(R) (2013) Chapter 59 Magnetic Force Microscopy Masato Hirade Keywords Stray magnetic field distribution High resolution Magnetic recording bit Magnetic domain structure 59.1 Principle Magnetic force microscope (MFM) is one of the SPM families which can visualize the leakage magnetic field by detecting the magnetic force between a probe and a sample [1]. The magnetic interaction strength is detected by the deflection of the cantilever. A simple principle of MFM was shown in Fig. 59.1. As shown in Fig. 59.1, if the polarity of the probe and the sample are the same, a repulsive force acts on the cantilever. On the other hand, when the polarity of the probe and the sample are opposite, attractive force acts on the cantilever. The resolution of the MFM is determined by the distance between a probe and a sample; it is about several tens nm [2]. There are several techniques for measuring the leakage magnetic field on the sample surface. However, MFM is the method that can measure the leakage magnetic field most easily with high resolution. Research on magnetic domain and magnetic wall behavior in these mesoscopic areas has advanced with the development of MFM. 59.2 Features • Since the basic configuration is the same as general AFM, it does not require large modify from AFM. • Like the general AFM, there is no restriction on the observation environment. M. Hirade (&) Surface Analysis Business Unit, Analytical & Measuring Instruments Division, Shimadzu Corporation, Kyoto, Japan e-mail: hirade@shimadzu.co.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_59 361 362 M. Hirade Fig. 59.1 Measurement principle of MFM. MFM obtains magnetic information on a sample surface by measuring a leakage magnetic field between a probe and a sample. When the probe is S pole (N pole) and a sample is N pole (S pole), it is an attractive force (repulsive force) • It is possible to obtain magnetic domain information with high resolution together with topography image. 59.3 Instrumentation Apart from the fact that the probe used is magnetized, the device configuration of the MFM is the same as the usual SPM. There are two ways to detect the force of MFM. One method is to detect the deflection of the cantilever, and the other is to detect the vibration characteristics of the cantilever. In the former, the force acting on the probe is measured, and in the latter, the force gradient is measured. Generally, it is more sensitive to detect the force gradient. Methods for detecting the force gradient include the AM method and the FM method. The AM method detects the vibration amplitude of the cantilever, and the FM method detects the resonance frequency of the cantilever. Figure 59.2 shows an image obtained method of MFM. First, a height image is obtained, and based on the height information, the probe and the sample are scanned with a certain distance apart, and the force or force gradient is measured. Fig. 59.2 Measurement method of MFM. a first of all, in order to obtain the height image, the sample surface is scanned in constant force mode. b next, the sample surface is scanned in a constant height mode. At that time, height is specified based on the height information acquired in (a) 59 Magnetic Force Microscopy 59.4 363 Applications MFM is often used for observation of recording media such as magnetic head, floppy disk, and hard disk. Here, an observation example (topography image and MFM image) of a silicon substrate on which a cobalt alloy is deposited and a floppy disk is shown. First, the topography image and the MFM image of a silicon substrate on which a cobalt alloy is deposited are shown in Fig. 59.3. As a cantilever, a cantilever on which a cobalt alloy is deposited was used like a sample. From the topography image, it can be confirmed that many particles of about several hundred nanometers which are thought to be cobalt (Fig. 59.3a). Next, the MFM image in the same region is shown in Fig. 59.3b. In the MFM image, a structure greatly different from the topography image was observed. This reflects the magnetic domain structure of the sample surface. In this way, MFM makes it possible to visualize the physical properties (magnetic domain structure) of the sample surface. Next, the observation results of the topography image of the floppy disk (magnetic recording medium) and the MFM image which are no longer used recently are shown in Fig. 59.4. Figure 59.4a shows the height image. No feature can be seen from the topography image shown in Fig. 59.4a. On the other hand, in the MFM image, two regions which were not observed at all in the height image were observed (Fig. 59.4b). One is a flat region, and the other is a region with an asperity structure. Since magnetic information is not written in the flat region, the magnetic domain structure can not be seen, and since the magnetic information is written in the region of asperity structure, the magnetic domain structure is Fig. 59.3 Topography image and MFM image of silicon substrate on which a cobalt alloy is deposited. The scanning range for both images is 4 lm 4 lm. As a cantilever, a cantilever with cobalt alloy deposited was used. a topography image. b MFM image 364 M. Hirade Fig. 59.4 Topography image and MFM image of floppy disk. The scanning range for both images is 80 lm 80 lm. As a cantilever, a cantilever with cobalt alloy deposited was used. a topography image. b MFM image observed. Although the floppy disk observed this time has a horizontal magnetic domain structure, recent hard disks are made to be able to write more detailed magnetic information by making vertical magnetic domain structure, etc. References 1. Martin, Y., K, H.: Wickramasinghe, magnetic imaging by force microscopy with 100 Angstrom resolution. Appl. Phys. Lett. 50, 1455 (1987) 2. Hosaka, S., Kikukawa, A., Honda, Y.: Appl. Phys. Lett. 65, 3407 (1994) Chapter 60 Matrix-Assisted Laser Desorption/Ionization Takaya Satoh Keywords Protein 60.1 Peptide Polymer Imaging mass spectrometry Principle Matrix-assisted laser desorption/ionization (MALDI) is a “soft” ionization technique commonly used in mass spectrometry (Fig. 60.1). It uses organic compounds (typically an organic acid) called “matrix” as a means of facilitating desorption and ionization efficiency. The sample solution is mixed with a matrix solution and spotted on a conductive target plate. The pulsed ultraviolet laser beam irradiates the co-crystallized matrix and sample mixture, thus producing ions off the surface. MALDI can ionize various organic compounds such as proteins, peptides, lipids, metabolites, and synthetic polymers by selecting a suitable matrix. The MALDI mass spectra are easy to interpret because the ions generated are mostly singly charged which means the m/z values measured in the mass spectra are equal to the mass of the ions. However, it can be difficult to analyze the lower mass range (m/z < 500) because of interference by the large abundance of ions originating from the matrix itself. To solve this issue, the ionization technique called surface-assisted laser desorption/ionization (SALDI), which uses nanoparticles or the nanostructure of the plate for facilitating ionization, has been developed for analyzing samples in the low mass range. T. Satoh (&) JEOL Ltd, Tokyo, Japan e-mail: taksatoh@jeol.co.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_60 365 366 T. Satoh Fig. 60.1 Principle of MALDI 60.2 Features • The various organic compounds can be ionized by selecting a suitable matrix compound. • The mass spectra are easy to interpret because singly charged ions are the dominant ions generated. • In combination with a time-of-flight (TOF) mass spectrometer, organic compounds with molecular weights of several 100,000 u can be analyzed. • Accurate mass analysis and tandem mass spectrometry are available for estimating the structure of organic compounds. • Imaging mass spectrometry is available to analyze the localization of organic compounds on a sample surface. The typical pixel size is about 10–100 lm. 60.3 Instrumentation The scheme of MALDI-TOF mass spectrometer is shown in Fig. 60.2. Target plate: The conductive plate where a mixture of matrix and sample solution is spotted for analysis. Ultraviolet laser: A nitrogen laser (k 337 nm) or solid-state lasers such as Nd: YAG (third harmonic wave k 355 nm) and Nd:YLF (third harmonic wave k 349 nm) are used for ionization. Recently, solid-state lasers are used instead of nitrogen lasers due to their longer life and higher speed capabilities. Ion source: The plate is placed on the XY stage in the ion source. The laser irradiation point is changed by moving the XY stage. Typically, the laser irradiation point on the sample surface is observed by using a CCD camera. Mass analyzer:The MALDI ion source can be combined with various mass spectrometers but is most popularly used with TOF mass spectrometers. A TOF mass spectrometer, which can analyze a very wide mass range simultaneously, is best for MALDI which can generate singly charged ions of several 100,000 u. A linear-type TOFMS must be used to analyze these high molecular weight compounds. The 60 Matrix-Assisted Laser Desorption/Ionization 367 Fig. 60.2 Scheme of MALDI-TOFMS Table 60.1 Matrix compounds Matrix Compounds Other name Samples a-Cyano-4-hydroxycinnamic acid 2,5-dihydroxy benzoic acid a-CHCA peptides, lipids, nucleotides DHB Sinapinic acid 3-hydroxy picolinic acid Dithranol SA 3-HPA peptides, nucleotides, oligonucleotides, oligosaccharides peptides, proteins oligonucleotides Polymers reflectron and multi-turn type TOF mass spectrometers which use an ion mirror and electrostatic sectors, respectively, can be used for high-resolution and high mass accuracy measurements. A tandem TOF/TOF mass spectrometer, which can perform high-energy CID measurements, is available for structural analysis. Detector: It detects the ions generated in the ion source. Multi-channel plates and secondary electron multipliers are used as the system detector. 60.4 Applications 60.4.1 Selection of Matrix The selection of matrix compound is an important step for ionizing the target organic compounds. A list of popular matrices is shown in Table 60.1. For ionization of nonpolar compounds such as synthetic polymers, cationization agents are also used with the matrix. 368 T. Satoh Fig. 60.3 Mass spectrum of intact BSA 60.4.2 Proteins and Peptides MALDI is widely accepted for analyzing proteins and peptides. The mass spectrum of intact bovine serum albumin is shown in Fig. 60.3. One of the advantages of MALDI is that it can generate singly charged ions of intact proteins with molecular weights up to several 100,000 u. In this case, these compounds must be analyzed by using a linear TOF mass spectrometer. For the identification of protein mixtures, the proteins are cut into several fragments using protease digestive enzymes such as trypsin. The match scores are statistically calculated by comparing mass lists of peptide mixtures observed in mass spectra with that of an in-silico peptide list made by protein identification software. 60.4.3 Polymer Analysis MALDI is also used for analyzing synthetic polymers and polymer additives (Fig. 60.4). Polymer mass spectra show peaks that are separated at equal intervals which correspond to the monomer units. Additionally, the distribution of polymers and end groups can be analyzed in these mass spectra. The polymer additives are also observed in the low mass region and can be identified by accurate mass analysis and MS/MS analysis. 60.4.4 Imaging Mass Spectrometry More recently, imaging mass spectrometry has been developed to analyze the localization of organic compounds on sample surfaces. The laser irradiation point is 60 Matrix-Assisted Laser Desorption/Ionization 369 Fig. 60.4 Mass spectrum of IRGANOX 1010 doped in PMMA Fig. 60.5 MALDI imaging of phospholipids in mouse brain tissue sections moved step-by-step, taking a mass spectrum for each pixel. The typical pixel size is 10–100 lm. This technique has been applied to various organic compounds such as proteins, peptides, lipids, drugs, and metabolites. Figure 60.5 shows the lipid distribution on mouse brain tissue sections. Chapter 61 Medium-Energy Ion Scattering Tomoaki Nishimura Keywords Surface atomic structure Rumpling 61.1 Surface composition Crystal structure Principle Medium-energy ion scattering is a method to analyze energy of large-angle scattered ions from a sample using hydrogen or helium ion beam accelerated to about several hundred keV. Fast ions gradually lose their energy mainly due to interaction with electrons in the solid, and only a small portion of the incident ions are scattered in large angle by close collisions between the nuclei. Since the energy after large-angle scattering is determined by the mass number of atoms in the sample, element identification is possible from the energy spectrum. Also, since the energy loss is proportional to the travel distance of ions, it is possible to identify the depth from which the detected ions are scattered. From these features, it is possible to analyze the elemental composition in the depth direction from sub-nm to several tens of nanometers as well as high-resolution RBS (HRBS). Combination of a toroidal type electrostatic energy analyzer and 2D position detector is often used as a detector of MEIS. The analyzer can detect scattering yield information of wide scattering angle of 20°–30° at once. As shown in Fig. 61.1, incident ions scattered by deep layer atoms are difficult to emit at a specific angle because there are atoms on the surface side (blocking effect). Therefore, by specifying this direction, it is possible to analyze the atomic position and atomic structure of the surface. T. Nishimura (&) Research Center of Ion Beam Technology, Hosei University, Tokyo, Japan e-mail: t-nishi@hosei.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_61 371 372 T. Nishimura Fig. 61.1 a Side view of ð110Þ zone of fcc crystal. b Angular distribution of the surface peak in the ð 110Þ zone of Cu(110) for 100-keV protons incident in the ½112 channeling direction [1] 61.2 Features • Elemental composition can be measured as a function of depth (sub-nm to several tens nm). • Average information of beam irradiation area can be obtained (typically *1 mm2). • Since high-energy ions are used as probes, position information of nuclei can be obtained directly without being influenced by outer-shell electrons. • It is possible to analyze the atomic structure, crystal structure, and crystallinity of the surface. • The atom position can be obtained with accuracy of 1 pm or less in the z direction. 61.3 Instrumentation The equipment consists of an ion accelerator, a beam line, a goniometer, an energy analyzer, a current integrator, and a system controller. 61 Medium-Energy Ion Scattering 373 61.3.1 Ion Accelerator and Beam Line Ions of several hundred keV are generated using an ion source and acceleration tube. The parallel beam is made by placing two slits in the beam line. 61.3.2 Goniometer Goniometer is used to precisely control the relationship between the crystal axis direction of the sample and the incident beam direction. 61.3.3 Energy Analyzer The energy analyzer consists of an electrostatic toroidal analyzer, a microchannel plate, and a position sensitive detector as shown in Fig. 61.2. Simultaneous analysis of energy and scattering angle is possible by using two-dimensional detector. Since the energy width under a constant voltage is about several% of the incident energy, in order to obtain a wide energy spectrum, the measurement is needed to be performed by changing the voltage applied to the toroidal electrode several times. 61.3.4 Current Integrator Current integrator is used to control the fluence of ion beam. Fig. 61.2 Illustration of toroidal analyzer for medium-energy ion scattering 374 T. Nishimura Fig. 61.3 MEIS spectra of 26 Å HfO2/9 Å Si18O2/Si. Inset close-up of the 76– 88 keV region. Full circles indicate spectra of as-deposited sample, and open circles indicate that of same sample annealed at 1000 °C for 10 s 61.3.5 System Controller It controls the goniometer, the current integrator, and the voltage applied to the toroidal electrode. 61.4 Applications 61.4.1 Analysis of Thin Film Sample (Example of Energy Spectrum Analysis) Figure 61.3 shows how the energy spectrum changes before and after annealing of a sample in which a high-k thin film (HfO2 26 Å) is deposited on a Si18O2(9 Å)/Si sample grown in 18O2 atmosphere [2]. It can be seen that the isotope 18O diffuses into the high-k film. The decrease in the peak width of Si in the spectrum of the sample after annealing indicates the decrease of SiO2 layer. Precise analysis of composition change can be performed using computer simulation software of medium-energy ion scattering [3, 4]. References 1. Copel, M., Gustafsson, T.: Structure of Au(110) determined with medium-energy-ion scattering. Phys. Rev. Lett. 57, 723–726 (1986) 2. Sayan, S., Garfunkel, E., Nishimura, T., Schulte, W.H., Gustafsson, T.: Thermal decomposition behavior of the HfO2/SiO2/Si System. J. Appl. Phys. 94, 928–934 (2003) 3. Nishimura, T.: Computer simulation program for medium-energy ion scattering and Rutherford backscattering spectrometry. Nucl. Instrum. Methods B 371, 97 (2016) 4. Nishimura, T.: MEISwin program. http://www.ionbeam.hosei.ac.jp/en/software.html Chapter 62 Micro-Raman Spectroscopy Katsumasa Fujita Keywords Vibration spectroscopy 62.1 Raman spectroscopy Raman microscopy Principle Raman scattering is an inelastic light scattering, with which molecular or lattice vibration is excited by accepting a part of light energy incident to a material. Since the molecule/lattice vibration is determined by the molecular/lattice structure in the material, measuring the energy loss of light (photon) provides the information of the molecule/lattice vibration excited. Since the light energy is related to the wavelength, the spectrum of scattered light shows the energy shift of photon due to the vibrational excitation. Thus, Raman spectroscopy utilizes Raman scattering effect to analyze sample materials through the inherent molecular/lattice vibration. Figure 62.1a, b shows the energy diagram of Raman scattering and a Raman spectrum of polystyrene beads. The Raman shift is the difference of light wave number before and after Raman scattering given as 1/ki − 1/ks, where ki and ks are the wavelengths of incident and scattered light, respectively. Micro-Raman spectroscopy is used to analyze small samples with a size ranging from sub-micrometers to several hundred micrometers. Although the principle of the analysis is same as typical Raman spectroscopy, apparatuses are slightly modified in order to detect Raman scattering light from a small sample volume. 62.2 Features • Vibrational modes of molecules and lattice structures can be detected. • The spatial resolution is a submicron level in three dimensions. • Mapping of Raman spectra shows distribution of sample materials. K. Fujita (&) Department of Applied Physics, Osaka University, Osaka, Japan e-mail: fujita@ap.eng.osaka-u.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_62 375 376 K. Fujita Fig. 62.1 a Energy diagram for Raman scattering. b Raman scattering spectrum of a polystyrene bead (diameter 500 nm) 62.3 Instrumentation Figure 62.2 shows an optical setup for micro-Raman spectroscopy [1]. A sample is illuminated by laser light tightly focused by an objective lens. The same objective lens is also used to collect scattering light from the sample efficiently. The collected light contains both Rayleigh scattering and Raman scattering, and they are separated by the dichroic mirror shown in Fig. 62.2 so that only Raman scattering light is delivered to a spectrophotometer, where Raman spectra are measured by a multichannel detector, such as a CCD sensor (typically cooled CCD sensors with low dark current). A pinhole placed at the entrance of the spectrophotometer realizes the detection of Raman scattering only from the laser focus by eliminating light scattered at out of focus positions and stray light. This configuration, known as confocal Raman microspectroscopy, allows us to select an arbitrary position of a sample for measurement in 3D. Fig. 62.2 Optical setup for micro-Raman spectroscopy 62 Micro-Raman Spectroscopy 377 Raman spectroscopy can be used to measure molecular/lattice vibrations of any samples, but non-fluorescent ones. The efficiency of Raman scattering is orders of magnitude smaller than that of fluorescent materials. Raman scattering light can be easily hidden by the strong fluorescence. In such a case, one can use a longer wavelength for illumination, such as near-infrared light (NIR). The use of NIR light sufficiently reduces the fluorescent excitation efficiency. However, at the same time, the Raman scattering becomes weak since the efficiency of Raman scattering is inversely proportional to the fourth power of wavelength. The high spatial selectivity in micro-Raman spectroscopy is useful to provide microscopic images with a contrast of molecular/lattice vibrations. The imaging technique using Raman scattering is called as ‘Raman microscopy’ and is effectively utilized to obtain distribution and localization of target materials in a sample. Raman microscopy is performed by repeating micro–Raman spectroscopy measurement with scanning the laser spot on a sample, which can provide 2D or 3D maps of the Raman spectra. Due to the low efficiency of Raman scattering, Raman microscopy requires a long measurement time. For the improvement of the imaging speed, parallel detection of Raman spectra from different points in a sample is performed [2]. 62.4 Applications Since Raman spectroscopy measures molecular/lattice vibrations inherent to the sample, the application ranges in various scientific fields, such as structural chemistry, biology, medicine, pharmaceutics, material science including semiconductor and carbon materials, astronomy, and forensic sciences. There are also many applications in the industry which are growing with the technological development of Raman microscopy. Here, a few examples of micro-Raman spectroscopy measurements are introduced below. Figure 62.3a shows Raman spectra measured from a graphene sheet produced by CVD. The Raman peaks represent vibrational modes typically seen in carbon materials, such as vibrational modes of sp2 carbon (G, 2D) and disorders in carbon systems (D) [3]. Figure 62.3b shows the distribution of the intensities of Raman scattering light in the sheet. The distribution of graphene, graphite, and defects is spatially resolved from the relative intensities of the above Raman bands [4]. The typical spatial resolution of micro-Raman spectroscopy was about a few hundreds of nanometers with using a visible light for sample illumination. Figure 62.4a shows a Raman spectrum of the cytosol of a living cancer cell (HeLa cell). The Raman spectrum contains many Raman peaks that can be assigned to the vibrational modes of intracellular molecules, such as protein, lipid, and DNA. Figure 62.4b shows a Raman image of living HeLa cells [5]. The distribution of cytochrome c, protein, and lipids is shown by the intensities of Raman peaks assigned to pyrrole ring breathing (750 cm−1), peptide’s Amide-I (1689 cm−1), and 378 K. Fujita Fig. 62.3 a Raman spectra of a graphene sheet produced by CVD. b Raman image of a graphene sheet reconstructed by G (1589–1598 cm−1, blue), 2D (2682–2694 cm−1, green), and D (1307– 1387 cm−1, red) bands, which highlight the distribution of graphite, graphene, and defect in the image, respectively Fig. 62.4 a Raman spectrum of a living HeLa cell. b Raman image of living HeLa cells reconstructed by Raman modes assigned to cytochrome c (750 cm−1, green), protein (1689 cm−1, blue), and lipid (2855 cm−1, red) CH2 stretching (2855 cm−1) vibrational modes. Thus, micro-Raman spectroscopy allows visualizing biological cells by using the molecular vibrations, which also allows us to analyze cell states and species without labeling [6]. References 1. Opelik, L., Schmid, T., Zenobi, R.: Modern Raman imaging: vibrational spectroscopy on the micrometer and nanometer scales. Annu. Rev. Anal. Chem. 6, 379–398 (2013) 2. Ando, J., Palonpon, A.F., Sodeoka, M., Fujita, K.: High-speed Raman imaging of cellular process. Curr. Opin. Chem. Biol. 33, 16–24 (2016) 62 Micro-Raman Spectroscopy 379 3. Dresselhaus, M., Jorio, A., Hofmann, M., Dresselhaus, G., Saito, R.: Perspectives on carbon nanotubes and graphene Raman spectroscopy. Nano Lett. 10, 751–758 (2010) 4. Watanabe, K., Palonpon, A.F., Smith, N.I., Chiu, L.-D., Kasai, A., Hashimoto, H., Kawata, S., Fujita, K.: Structued line illumination Raman microscopy. Nat. Commun. 6, 10095 (2015) 5. Hamada, K., Fujita, K., Smith, N.I., Kobayashi, M., Inouye, Y., Kawata, S.: Raman microscopy for dynamic molecular imaging of living cells. J. Biomed. Optics 13, 044027 (2008) 6. Ichimura, T., Chiu, L.-D., Fujita, K., Kawata, S., Watanabe, T.M., Yanagida, T., Fujita, H.: Visualizing cell state transition using Raman spectroscopy. PLoS ONE 9, e84478 (2014) Chapter 63 Microprobe Reflection High-Energy Electron Diffraction Masakazu Ichikawa Keywords Surface 63.1 Interface Crystallinity Structure Topography Principle Microprobe reflection high-energy electron diffraction (l-RHEED) is a reflection high-energy electron diffraction (RHEED) that uses focused electron beams with about 10 nm diameter. The focused electron beams with higher than 10 keV kinetic energy are irradiated on sample surfaces at grazing angles less than 5° to get RHEED patterns from the sample surface nanoareas and analyze their crystallinities. The focused electron beams are scanned over the sample surfaces to get scanning electron microscope (SEM) images by using one RHEED spot intensity as image signals for SEM display. This makes it possible to observe atomic-layer surface nanostructures that change the RHEED spot intensity. The interface nanostructures can also be observed when a RHEED spot intensity from the subsurface area is used for the SEM image signals. The l-RHEED is also called scanning reflection electron microscopy (SREM). 63.2 Features • Atomic-layer nanostructures such as atomic steps, dislocations, and crystalline domains on sample surfaces can be observed because the RHEED spot intensities to get SREM images are sensitively changed by the changes of strains, crystalline structures, and orientations on the sample surfaces [1]. M. Ichikawa (&) Department of Applied Physics, Graduate School of Engineering, The University of Tokyo, Tokyo, Japan e-mail: ichikawa@ap.t.u-tokyo.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_63 381 382 M. Ichikawa • Nanostructures at the interface such as the ones between amorphous layer and crystalline substrate can also be observed when the RHEED spot intensity from the interface is used for SREM images [2]. • Other surface microanalysis techniques such as micro-Auger spectroscopy and characteristic X-ray analysis can easily be combined with the l-RHEED because the focused electron beams are shared in these techniques [3, 4]. • Molecular beam epitaxy (MBE), scanning tunneling microscopy (STM), and X-ray photoemission spectroscopy can easily be combined with the l-RHEED because the space around the samples is wide enough to equip them [3, 4]. • Nanostructures can be formed at given areas on the sample surfaces by using the focused electron-beam-stimulated reactions [5]. 63.3 Instrumentation The l-RHEED is composed of an electron beam gun, analysis chamber, RHEED pattern detector, and SREM image display as shown in Fig. 63.1. (1) Electron beam gun Field emission electron beam gun with high brightness is used to get bright RHEED patterns and SREM images with high quality at about 10 nm beam diameter. (2) Analysis chamber Ultrahigh vacuum condition at about 10−8 Pa is needed to keep clean surface condition because this technique is sensitive to surface contaminations. STM can be combined with this technique to conduct atomic-level observation of the same areas. Characteristic X-ray detector and electron energy analyzer can also be equipped to conduct element and chemical analyses. MBE system can be equipped with the chamber to observe surface structure changes during MBE growths. (3) RHEED pattern detector RHEED patterns are observed on the fluorescent screen. One RHEED spot is selected by an optical lens and an aperture, which is used for the SREM image signal. (4) SREM image display SEM images, i.e., SREM images, are shown on the display, which are obtained by the RHEED spot intensity changes at each sample surface nanoareas. 63 Microprobe Reflection High-Energy Electron Diffraction 383 Fig. 63.1 Composition of microprobe reflection high-energy electron diffraction 63.4 Applications 63.4.1 Layer-by-Layer MBE Growth of Si on Si Substrates It is known that the RHEED intensity oscillation phenomena can be used to control the layer-by-layer crystal growth of thin films because the one period of the oscillation corresponds to one atomic- or one molecular-layer growth [6]. The surface structure changes during the growth, however, had not been known at that time. Figure 63.2 shows SREM images of a Si(111) substrate during Si MBE growth at the substrate temperature of 350 °C when the RHEED oscillations were clearly observed [7, 8]. The dark lines and bright areas of the Si(111) substrate in Fig. 63.2a correspond to atomic steps and terrace areas on the surface, respectively. The terrace areas became dark just after the start of the growth as shown in Fig. 63.2b, c. This was caused by the diffuse scatterings of incident electrons by two-dimensional (2D) islands on the terrace surface areas, which reduced the RHEED spot intensity used for the SREM images. The areas near the atomic steps, 384 M. Ichikawa Fig. 63.2 SREM images of Si(111) surface during Si MBE growth at 350 °C however, became bright. This was caused by the fact that adsorbed Si atoms diffused on the surface and were captured by the nearby atomic steps without formation of 2D islands on these areas. When the growth continued, the SREM images drastically changed (Fig. 63.2d) and then almost the same SREM image as that of the substrate (Fig. 63.2a) was obtained at about six monolayer (ML) growth (Fig. 63.2e). The terrace areas became dark again at about seven ML growth by the diffuse scatterings by the 2D islands. These observations have revealed that thin films grow in layer-by-layer manner by 2D island growth and initial surface nanostructures such as atomic steps are preserved when the one unit layer growth is completed during the RHEED intensity oscillations. 63.4.2 Layer-by-Layer Oxidation of Si Substrates The incident electron beam intensities decrease exponentially in the amorphous materials such as SiO2 by the inelastic scatterings. Then, the electron beams are diffracted only by the crystal Si substrates and the diffracted beams are emitted from the SiO2 surfaces after passing through the SiO2 layers on the Si substrates. Therefore, the SREM images using a RHEED spot intensity show the interface nanostructures between the SiO2 layers and crystal Si substrates. This makes it possible to study the in situ Si oxidation processes [2]. There are two kind of terraces called 2 1 and 1 2 ones on a clean Si(001) surface, where their crystallographic axis directions of Si dimers are perpendicular 63 Microprobe Reflection High-Energy Electron Diffraction 385 Fig. 63.3 SREM images during the oxidation of Si(001) substrate with each other. Then, the 2 1 and 1 2 terraces have dark and bright contrasts, respectively, by the difference of the diffracted electron beam intensities as shown in Fig. 63.3a. There are atomic steps between the adjacent two terraces. Figure 63.3b–d shows the SREM images during the Si(001) substrate oxidation after the oxygen gas injection in the analysis chamber. The images show the repeated terrace contrast reversal without the structure changes. The terrace contrast was not caused by the 2 1 and 1 2 terrace surface structures because they were broken by the oxidation. The terrace contrast was caused by the difference of diffracted beam intensities from the two kinds of the SiO2 and Si interface structures as shown in the insets. The repeated terrace contrast reversal without structure changes has revealed that the Si(001) substrate is oxidized in layer-by-layer manner by 2D island oxidation. 386 M. Ichikawa References 1. Ichikawa, M., Doi, T., Hayakawa, K.: Observation of Si(111) and gold-deposited Si(111) surfaces using microprobe reflection high-energy electron diffraction. Surf. Sci. 159, 133–148 (1985) 2. Watanabe, H., Kato, K., Uda, T., Fujita, K., Ichikawa, M., Kawamura, T., Terakura, K.: Kinetics of initial layer-by-layer oxidation of Si(001) surfaces. Phys. Rev. Lett. 80, 345–348 (1998) 3. Watanabe, H., Ichikawa, M.: Development of a multifunctional surface analysis system based on a nanometer scale scanning electron beam: combination of ultrahigh vacuum-scanning electron microscopy, scanning reflection electron microscopy, Auger electron spectroscopy, and x-ray photoelectron spectroscopy. Rev. Sci. Instrum. 67, 4185–4190 (1996) 4. Maruno, S., Fujita, S., Watanabe, H., Kusumi, Y., Ichikawa, M.: A combined apparatus of scanning reflection electron microscope and scanning tunneling microscope. Rev. Sci. Instrum. 68, 116–119 (1997) 5. Shklyaev, A., Shibata, M., Ichikawa, M.: Nanometer-scale germanium islands on Si(111) surface windows formed in an ultrathin silicon dioxide film. Appl. Phys. Lett. 72, 320–322 (1998) 6. Harris, J.J., Joyce, B.A., Dobson, P.J.: Oscillations in the surface structure of Sn-Doped GaAs during growth by MBE. Surf. Sci. 103, L90–L96 (1981) 7. Ichikawa, M., Doi, T.: Observation of Si(111) surface topography changes during Si molecular beam epitaxial growth using microprobe reflection high-energy electron diffraction. Appl. Phys. Lett. 50, 1141–1143 (1987) 8. Ichikawa, M.: Crystallographic analysis and observation of surface micro-areas using microprobe reflection high-energy electron diffraction. Mater. Sci. Rep. 4, 147–192 (1989) Chapter 64 Multiple-Probe Scanning Probe Microscope Tomonobu Nakayama Keywords Scanning probe microscope Electrical measurements 64.1 Nanocharacterization Principle Multiple-probe scanning probe microscope (MP-SPM) was developed to overcome difficulties in characterizing physical properties of nanoscale structures and materials with conventional SPM families, such as a scanning tunneling microscope (STM), an atomic force microscope (AFM), and the related proximal probe microscopes [1–14]. Conventional SPMs use a single probe to observe surface structures and local properties at a high-spatial resolution down to atomic scale. In the case of STM, as shown in Fig. 64.1a, an electrical current flowing to/from a conductive and atomically sharpened probe from/to a conductive sample establishes a concentration of current at a surface of the sample and this is the reason why STM is very sensitive to local density of states (LDOS) of the surface. Here, since the STM probe and the surface are separated by a small (typically 1 nm) vacuum gap, current is obtained via tunneling of electrons across the spatial gap, providing very accurate control of probe-to-sample distance via a feedback control to maintain the tunneling current constant, for example. As readily understood from Fig. 64.1a, conventional STM is measuring electrical current flowing in the direction perpendicular to the surface plane as shown by the red arrow. Therefore, a single-probe STM is essentially not suitable to measure electrical properties in the direction parallel to sample surfaces, while, in many cases, we want to measure electrical properties of surfaces, nanomaterials placed on surfaces, and device structures fabricated on surfaces. Therefore, multiple-probe T. Nakayama (&) International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Tsukuba, Japan e-mail: NAKAYAMA.Tomonobu@nims.go.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_64 387 388 T. Nakayama Fig. 64.1 Configurations of probes and a sample in a a conventional STM, b a double-probe STM, and c a double-probe AFM. In a and b, samples are both having conducting substrate and an object of interest such as surface structures, nanostructures, or nanomaterials on the surfaces. Red arrows in each panel indicate the directions of electrical currents flowing through the samples scanning tunneling microscope was developed to utilize the capability of SPMs in their high-resolution imaging to nanoscale electrical measurements in parallel to sample surface. As seen in Fig. 64.1b, using two STM probes placed on a conductive substrate with a nanostructure of interest, we can measure electrical current flowing through the nanostructure. However, the current also flows through the conductive bulk substrate, which is usually referred to as a leakage current. This happens when the nanostructure of interest is electrically in contact with conductive substrate. Since such leakage current increases when the interprobe distance increases, smaller interprobe distances are essential to make MP-SPM measurements more sensitive to the nanoscale properties on surfaces of conductive substrates. To completely avoid the leakage current through a bulk substrate, conductive nanostructures are often prepared on insulating substrates, i.e., electronic devices are usually fabricated on insulating silicon oxide layers of silicon wafers. MP-STM is, however, not applicable, and MP-AFM is necessary in this case because a tunneling current between probes and the sample substrates cannot be used for positioning probes on the nanostructure. MP-AFM establishes probe contact to the nanostructure by detecting atomic force acting between them and enables electrical 64 Multiple-Probe Scanning Probe Microscope 389 property measurements of the nanostructure if the AFM probes are conductive as shown in Fig. 64.1c. 64.2 Features • Accurate positioning of all the probes equipped with MP-SPM: The accuracy depends on the spatial resolution achieved in imaging operation by each probes. • Interprobe distances of less than 50 nm: The minimum interprobe distance is determined by shapes of the probes used in MP-SPM measurements. • No special pretreatment required: This feature enables nanoscale characterization of materials and structures which can easily be damaged by lithographic processes (including imaging procedures with SEM) such as inorganic nanostructures and ultimately thin films. • In situ nanofabrication and characterization: Using probes of MP-SPM, it is possible to fabricate artificial nanostructures through atom, molecule, and nanoscale manipulations with probes, and properties of the resulting structures can be evaluated right after the fabrication. • Applicable to samples in various environments: MP-SPM operation in air, ultrahigh vacuum, and even in liquid is possible. MP-SPMs are operated at room temperatures and lower temperatures down to 4 K. • Combining different SPM modes: This feature enables variety of nanoscale measurements depending on the purpose of characterization. Using a part of probes in AFM mode while the other probes in Kelvin probe force microscopy (KPFM) mode is possible, for example. 64.3 Instrumentation MP-SPM has 2–4 independently driven probes, and all the probes essentially work in a manner as same as those in conventional SPMs. Therefore, large part of designing and instrumentation guidelines is quite similar to those for conventional SPM. However, MP-SPM is recognized as a very difficult instrument to be constructed and operated because there are additional points to be considered in order to achieve truly unique and useful MP-SPM measurements. For example, how to geometrically install multiple probes is an important factor to achieve sufficiently small distances between probes, and how to improve instrumental stability is essential to achieve high-spatial resolution in MP-SPM imaging and in establishing precise probe-to-sample contacts. Also a control electronics/software for MP-SPM needs to be developed to make various MP-SPM measurements easier. Probe geometry and instrumental stability: For characterizing nanomaterials, interprobe distances must be at least smaller than the sizes of the nanomaterials. 390 T. Nakayama Therefore, MP-SPMs usually use inclined probes to avoid physical contact between the bases of the probes as shown in Fig. 64.2 to achieve the smallest interprobe distance. This is in contrast to a conventional SPM, where a probe is set in the direction normal to the sample surfaces because such a setup improves instrumental stability through a compact design with increased resonance frequency of the SPM. The drawbacks of probe geometry in Fig. 64.2 are as follows. First, mechanical components to support the probes should be placed considerably away from probe ends, resulting in less instrumental stability as compared with conventional SPMs owing to larger size of the instrument itself. Second, the imaging and measurements are done through off-apex regions of the probes as indicated by A1 and A2 in Fig. 64.2. This would affect spatial resolution of MP-SPM images especially on granular or rough surfaces. However, for reasonably flat surfaces, we can achieve atomic resolution even by the inclined probe setup. So far, there are several different design concepts of MP-SPMs found in the literatures. Especially in the case of multiple-probe atomic force microscopes (MP-AFMs), instrument design depends on the type of force sensor used in MP-AFM. Cantilever-type force sensors are widely used in conventional single-probe AFMs. In the case of MP-AFM, however, most of commercially available cantilever sensors are not suitable for executing nanoscale measurements as shown in Fig. 64.1c because probe apexes of such sensors are not protruded from the end part of the cantilever. Therefore, specially made cantilever sensors should be used for double-probe setup and must be developed for quadruple-probe setup. Moreover, it is possible but tricky to design optical system for independently Fig. 64.2 Typical probe geometry adopted in MP-SPM. Inclining probes are necessary to reduce interprobe distance down to nanometer range. STM images of each probe obtained by another probe are also shown. Adapted from Ref. [13] 64 Multiple-Probe Scanning Probe Microscope 391 detecting deflection of multiple sensors which are closely placed with each other. To avoid this difficulty, self-sensing force sensors such as piezoelectric sensors or tuning fork-type sensors are advantageous for MP-AFM instrumentation. Figure 64.3 shows an example of a tuning fork sensor with metal probe and a quadruple-probe AFM implementation using such sensor probes [14–16]. In any case, it is still challenging task to design sufficiently stable MP-SPM which enables high-spatial resolution and small interprobe distances of around/below 10–50 nm. Especially, optimizing instrumental designs for operating MP-SPM at elevated temperatures, and in liquid environments, are demanded for expanding the field of applications. Fig. 64.3 a Tuning fork-type sensor with a tungsten probe for AFM and STM, and b quadruple-probe AFM/STM instrumentation. Adapted from Refs. [14] and [15] Fig. 64.4 Schematic diagram of control electronics for MP-SPM. MP-SPM hardware (i) is operated by a single computer system (ii), enabling control of positions of quadruple probes and a sample through HV amplifiers integrated in a control electronics (iii). Adapted from Ref. [17] 392 T. Nakayama Control electronics/software: Operation of four independently driven SPM probes is quite difficult if we use four independent control electronics/software for each probe. Therefore, an integrated control electronics has been developed as shown in Fig. 64.4 [17] which enables operation of multiple probes and acquisition of images/data obtained by MP-SPM. In MP-SPM measurements, positional relationship between probes must be identified to analyze the measured data. For this purpose, pattern-matching calculations are proposed and shown to be useful for finding out overlapped parts among multiple images obtained by multiple probes of MP-SPM [17]. 64.4 Application Nanomaterials and nanostructures are often difficult to be connected to macroscopic metal electrodes which are fabricated by lithographic processes. However, MP-SPM can establish electrical contacts to nanoscale materials and structures without such macroscopic electrodes. Figure 64.5 summarizes possible measurements by MP-SPM [13]. Generally, it is believed that four-probe measurements are better than two-probe measurements but this belief is not always true as follows. Inaccuracy of two-probe measurement is owing to a difficulty in estimating contact resistances between probes and a measured object. In the case of MP-SPM measurement on nanowires, by plotting resistances of a single nanowire as a function of length using two probes, we can estimate the effective contact resistances and can deduce the resistance of nanowire itself appropriately [18]. Using MP-SPM, it is also possible Fig. 64.5 Measurement concepts with a conventional single-, b double-, c triple-, and d quadruple-probe SPMs. Adapted from Ref. [13] 64 Multiple-Probe Scanning Probe Microscope 393 to reduce the number of probes from four to three in the case of sheet resistance measurements, for example. This can be done by substituting two contact probes (used as potential probes in four-probe measurements) with one KPFM probe for mapping potential variation over the region carrying electrical current as shown in Fig. 64.6 [14]. MP-SPM measurements are, as compared with those by conventional SPMs, relatively time consuming and difficult even with a help of dedicated electronics and software. However, the achievable measurements certainly provide important information about nanostructures and nanomaterials which cannot be obtained by the other methods. This makes MP-SPM powerful, promising, and indispensable instruments for developing functional materials/structures and future devices at the nanoscale. Fig. 64.6 a MP-AFM image of graphene flake placed on SiO2. b A resistance of 367 X (corresponding to a sheet resistance of 599 X = sq.) measured by four AFM placed at the points shown by P1, P2, P3, and P4 in (a). c The MP-SPM measurement at the same area as in (a) by converting Probe 4 from a contact probe to a noncontact KFM probe. d Potential variation is measured along the line intersecting points P4 and P2 indicated in (a), resulting in a resistance of 397 X (corresponding to a sheet resistance of 554 X = sq.). Excerpted from Ref. [14] 394 T. Nakayama References 1. Aono, M., Jiang, C.-S., Nakayama, T., Okuda, T., Qiao, S., Sakurai, M., Thirstrup, C., Wu, Z.-H.: How to measure the nanoscale physical properties of materials? Oyo Buturi 67, 1361 (1998). [in Japanese] 2. Nakayama, T., Jiang, C.-S., Okuda, T., Aono, M.: Microscope for direct measurements of nanoscale properties: multi-tip scanning tunneling microscope, Keisoku to Seigyo. J. Soc. Instrum. Control Eng. 38, 742 (1999) [in Japanese] 3. Okamoto, H., Chen, D.M.: An ultrahigh vacuum dual-tip scanning tunneling microscope operating at 4.2 K. Rev. Sci. Instrum. 72, 4398 (2001) 4. Grube, H., Harrison, B.C., Jia, J., Boland, J.J.: Stability, resolution, and tip–tip imaging by a dual-probe scanning tunneling microscope. Rev. Sci. Instrum. 72, 4388 (2001) 5. Watanabe, H., Manabe, C., Shigematsu, T., Shimizu, M.: Dual-probe scanning tunneling microscope: Measuring a carbon nanotube ring transistor. Appl. Phys. Lett. 78, 2928 (2001) 6. Shiraki, I., Tanabe, F., Hobara, R., Nagao, T., Hasegawa, S.: Independently driven four-tip probes for conductivity measurements in ultrahigh vacuum. Surf. Sci. 493, 633 (2001) 7. Takami, K., Akai-Kasaya, M., Saito, A., Aono, M., Kuwahara, Y.: Construction of independently driven double-tip scanning tunneling microscope. Jpn. J. Appl. Phys. 44, L120 (2005) 8. Ishikawa, M., Yoshimura, M., Ueda, K.: Development of four-probe microscopy for electric conductivity measurement. Jpn. J. Appl. Phys. 44, 1502 (2005) 9. Guise, O., Marbach, H., Yates Jr., J.T., Jung, M.-C., Levy, J., Ahner, J.: Development and performance of the nanoworkbench: a four tip STM for conductivity measurements down to submicrometer scales. Rev. Sci. Instrum. 76, 045107 (2005) 10. Jaschinsky, P., Coenen, P., Pirug, G., Voigtländer, B.: Design and performance of a beetle-type double-tip scanning tunneling microscope. Rev. Sci. Instrum. 77, 093701 (2006) 11. Matsui, A., Shigeta, Y.: Development of probe-to-probe approach method for an independently controlled dual-probe scanning tunneling microscope. Rev. Sci. Instrum. 78, 106107 (2007) 12. Kim, T.-H., Wang, Z., Wendelken, J.F., Weitering, H.H., Li, W., Li, A.-P.: A cryogenic Quadraprobe scanning tunneling microscope system with fabrication capability for nanotransport research. Rev. Sci. Instrum. 78, 123701 (2007) 13. Nakayama, T., Kubo, O., Shingaya, Y., Higuchi, S., Hasegawa, T., Jian, C.-S., Okuda, T., Kuwahara, Y., Takami, K., Aono, M.: Development and application of multiple-probe scanning probe microscopes. Adv. Mater. 24, 1675 (2012) 14. Nakayama, T., Shingaya, Y., Aono, M.: Multiple-probe scanning probe microscopes for nanoarchitectonic materials science. Jpn. J. Appl. Phys. 55, 1102A7 (2016) 15. Higuchi, S., Kubo, O., Kuramochi, H., Aono, M., Nakayama, T.: A quadruple-scanning-probe force microscope for electrical property measurements of microscopic materials. Nanotechnology 22, 285205 (2011) 16. Higuchi, S., Kuramochi, H., Kubo, O., Masuda, S., Shingaya, Y., Aono, M., Nakayama, T.: Angled long tip to tuning fork probes for atomic force microscopy in various environments. Rev. Sci. Instrum. 82, 043701 (2011) 17. Higuchi, S., Kuramochi, H., Laurent, O., Komatsubara, T., Machida, S., Aono, M., Obori, K., Nakayama, T.: Multiple-scanning-probe tunneling microscope with nanoscale positional recognition function. Rev. Sci. Instrum. 81, 073706 (2010) 18. Kubo, O., Shingaya, Y., Nakaya, M., Aono, M., Nakayama, T.: Epitaxially grown WOx nanorod probes for sub-100 nm multiple-scanning-probe measurement. Appl. Phys. Lett. 88, 254101 (2006) Chapter 65 Nanoscale Angle-Resolved Photoelectron Spectroscopy Koji Horiba Keywords Electronic structure Chemical shift Synchrotron radiation Operando analysis 65.1 Depth profile Principle Photoelectron spectroscopy or electron spectroscopy for chemical analysis (ESCA) is a powerful technique for investigating the electronic structure of solid surfaces. From the angular distribution of valence band photoelectron spectra, we can obtain the dispersion of valence bands in the reciprocal space of solids. This technique is called angle-resolved photoemission spectroscopy (ARPES). On the other hand, the angular distribution of core-level photoelectron spectra corresponds to the probing depth dependence of the photoelectron spectra and can be converted into the depth profiling information. Nano-ARPES is the method for obtaining the lateral and angular distribution of the electronic structure of solids by scanning the nanofocused X-ray as an excitation source on the sample surface along the lateral directions and acquiring the angular distribution of photoelectron spectra at each lateral point. 65.2 Features • Lateral distribution of the electronic structure of solids can be obtained. • Band dispersion or depth profile can be additionally obtained from the angular distribution of photoelectron spectra. K. Horiba (&) Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba, Japan e-mail: horiba@post.kek.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_65 395 396 K. Horiba • Probing depth is limited to a few-nm region from the surface of the samples due to the inelastic mean free path of emitted photoelectron. • Spatial resolution along the lateral directions is limited by the spot size of focused X-ray source. • The use of synchrotron radiation is indispensable for the focused X-ray to nanometer scale. 65.3 Instrumentation Figure 65.1 shows a conceptual illustration of the nano-ARPES system [1]. Nano-ARPES system consists of focusing optics for nanofocused X-ray and an angle-resolved photoelectron spectrometer. A Fresnel zone plate (FZP) is generally used as the focusing optics for the X-ray. We can obtain the lateral (x and y directions) distribution of photoelectron spectra by scanning the samples along the 2-axes lateral directions and acquiring the photoelectron spectrum at each point. Fig. 65.1 Conceptual illustration of nano-ARPES 65 Nanoscale Angle-Resolved Photoelectron Spectroscopy 397 This is the typical experimental scheme of scanning photoelectron microscope (SPEM). For the nano-ARPES measurements, we have to obtain the angular distribution of emitted photoelectrons simultaneously. The up-to-date ARPES spectrometer using a two-dimensional imaging detector enables us to detect the angular distribution of photoelectron spectra without any sample rotation. Thus, we can obtain the angular distribution of photoelectron spectra at each point in the lateral directions. 65.4 Applications 65.4.1 Three-Dimensional (3D) Spatial Distribution Measurements From the angular distribution of core-level photoelectron spectra, we can determine the depth profile of atomic concentration and the chemical bonding states without destruction of the sample. Figure 65.2 shows a schematic illustration of the configuration of photoelectrons with different emission angle. In the case of grazing emission with the large emission angle he, the probing depth of photoelectron l is reduced relative to that in the case of normal emission along the perpendicular direction to the sample surface, because the photoelectrons at a given depth travel through more material, and the probability of inelastic photoelectron scattering increases at the grazing emission. As the simplest example, we consider the case of a uniform film with the thickness of d on a substrate. Total photoelectron intensity is represented by integral in the depth direction of the intensity contribution from a given depth z. Based on simple exponential attenuation for the photoelectrons traveling in solids, the photoelectron intensity from the film IF and the substrate IS is given by Eq. 1, Fig. 65.2 Schematic illustration of emitted photoelectrons from a thin film sample with different emission angle 398 K. Horiba Zd IF ¼ 0 AF exp z dz kF cos he Z1 d zd IS ¼ exp AS exp dz kF cos he kS cos he ð1Þ d where A and k is the bare intensity without attenuation and the inelastic mean free path of the materials, respectively. Then, the thickness of the film can be calculated from the intensity ratio IF/IS as AS kS IF d ¼ kF cos he ln 1 þ AF kF IS ð2Þ In the more complicated case, we have to fit angular distribution curve of relative photoelectron intensity using some computational methods such as a maximum entropy method [2] in order to determine the depth profile of the sample. Thus, we can obtain the spatial distribution along the depth direction. In the combination of depth profiling analysis and SPEM measurements, we can obtain the 3D (lateral x and y, and depth z) distribution of the electronic structure and chemical bonding states of the samples on a nanometer scale. This system is called “3D nano-ESCA” [1]. (a) (b) Fig. 65.3 (a) Schematic illustration of operando SPEM measurements of nanodevice structures. (b) Peak shift of C 1s core-level photoelectron spectra on the graphene FET structures against the gate voltage 65 Nanoscale Angle-Resolved Photoelectron Spectroscopy 399 65.4.2 Operando Analysis of Nanodevice Structures These nano-ARPES or 3D nano-ESCA techniques are particularly appropriate for the analysis of local electronic structures in nanodevice structures, such as metal electrode/graphene interfaces in graphene field effect transistors (FET) [3] and metal nanowire structure for resistive random access memory devices [4]. In addition, the SPEM techniques have potential the “operando” analysis, that is, the analysis of nanodevice structures during the device operation. Figure 65.3a shows a schematic illustration of operando SPEM measurements of FET structures [5]. Source and drain electrodes are put on the specimen. The gate voltage is applied from the substrate used as a back-gate electrode. The top electrodes on the surface are grounded, thereby we can measure the photoelectron spectra under application of back-gate voltage. As an example, the gate-voltage dependence of the peak position of C 1s core level photoelectron spectra on the graphene FET is shown in Fig. 65.3b [6]. The peak shift against the gate voltage can be explained well by a simple simulation based on the shift of the Dirac point due to the carrier doping in the graphene induced by the application of the gate voltage. Thus, it is demonstrated that the operando SPEM measurements successfully observe the carrier injection in the graphene channel by the gate voltage. References 1. Horiba, K., Nakamura, Y., Nagamura, N., Toyoda, S., Kumigashira, H., Oshima, M., Amemiya, K., Senba, Y., Ohashi, H.: Scanning photoelectron microscope for nanoscale three-dimensional spatial-resolved electron spectroscopy for chemical analysis. Rev. Sci. Instrum. 82, 113701/1–113701/6 (2011) 2. Toyoda, S., Okabayashi, J., Oshima, M., Liu, G.L., Liu, Z., Ikeda, K., Usuda, K.: Chemical-state-resolved in-depth profiles of gate-stack structures on Si studied by angular-dependent photoemission spectroscopy. Surf. Interface Anal. 40, 1619–1622 (2008) 3. Nagamura, N., Horiba, K., Toyoda, S., Kurosumi, S., Shinohara, T., Oshima, M., Fukidome, H., Suemitsu, M., Nagashio, K., Toriumi, A.: Direct observation of charge transfer region at interfaces in graphene devices. Appl. Phys. Lett. 102, 241604/1–241604/5 (2013) 4. Horiba, K., Fujiwara, K., Nagamura, N., Toyoda, S., Kumigashira, H., Oshima, M., Takagi, H.: Observation of rebirth of metallic paths during resistance switching of metal nanowire. Appl. Phys. Lett. 103, 193114/1–193114/3 (2013) 5. Nagamura, N., Kitada, Y., Tsurumi, J., Matsui, H., Horiba, K., Honma, I., Takeya, J., Oshima, M.: Chemical potential shift in organic field-effect transistors identified by soft X-ray operando nano-spectroscopy. Appl. Phys. Lett. 106, 251604/1–251604/4 (2015) 6. Fukidome, H., Nagashio, K., Nagamura, N., Tashima, K., Funakubo, K., Horiba, K., Suemitsu, M., Toriumi, A., Oshima, M.: Pinpoint operando analysis of the electronic states of a graphene transistor using photoelectron nanospectroscopy. Appl. Phys. Express 7, 065101/1–065101/4 (2014) Chapter 66 Nonlinear Spectroscopy Shoichi Yamaguchi Keywords Molecular vibration 66.1 Coherent Raman Nonlinear optics Principle Various techniques of nonlinear spectroscopy are now applied to surface and interface analyses. They include sum frequency generation (SFG) and second-harmonic generation (SHG) that are detailed in other sections. SFG provides rich information on molecular structure at an interface by virtue of vibrational spectra, but it is not applicable to buried interfaces sandwiched by dense media absorbing IR light strongly. SHG has wider applicability because it does not use IR but just UV or visible light, but it does not allow for recording vibrational spectra. This section introduces fourth-order nonlinear Raman (FR) spectroscopy that has the advantages of SFG and SHG simultaneously. While physical quantity given by SFG and SHG is second-order nonlinear optical susceptibility (χ(2)), that by FR is fourth-order one (χ(4)). The interface selectivity of FR is due to the same principle as SFG and SHG: χ(4) is zero in the isotropic bulk but nonzero at an interface that are intrinsically anisotropic or noncentrosymmetric. FR can be viewed as the combination of SHG and conventional third-order nonlinear Raman such as impulsive stimulated Raman and coherent anti-Stokes Raman. S. Yamaguchi (&) Department of Applied Chemistry, Graduate School of Science and Engineering, Saitama University, Saitama 338-8570, Japan e-mail: shoichi@apc.saitama-u.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_66 401 402 S. Yamaguchi 66.2 • • • • • Features Vibrational spectra of interfacial molecules can be obtained. Submonolayer sensitivity can be achieved. Raman and hyper-Raman simultaneously active modes are selected. Ambient condition is allowed. Extension to femtosecond time-resolved measurements is straightforward. 66.3 Instrumentation An apparatus of FR spectroscopy is not commercially available, so it has to be built from scratch by a user. Figure 66.1 shows a typical setup of FR spectroscopy in the time domain [1]. The energy diagram of FR is shown in Fig. 66.1a. A noncollinear optical parametric amplifier generates a 6-fs visible pulse that is divided by beam splitters into pump, probe, and reference pulses, as shown in Fig. 66.1b. A sample is irradiated by the pump and probe pulses. In Fig. 66.1c, the sample is an air/liquid interface, but it can be any (liquid/liquid, liquid/solid, gas/solid, solid/solid) interfaces. SHG of the probe pulse from the sample interface is introduced into a photomultiplier tube. The pump pulse induces vibrational coherence impulsively, which modulates SHG of the probe pulse. This pump-induced modulation in the SHG intensity is recorded as a function of pump-probe delay, and this time-domain signal is Fourier-transformed into the frequency domain, resulting in an interface-selective vibrational spectrum. The measurable wavenumber range is 50 * 1800 cm−1, which is limited by the pulse width. A shorter pulse allows a wider wavenumber range. The spectral resolution is approximately equal to the inverse of the maximum pump-probe delay, and it is typically 10 * 20 cm−1. Fig. 66.1 (a) Energy diagram of FR spectroscopy. An ultrashort visible pump pulse induces vibrational coherence that modulates the second-harmonic intensity of a probe pulse. This second-harmonic generation takes place only at interfaces, which makes FR interface-selective spectroscopy. (b), (c) Schematic of the optical setup of FR spectroscopy. Reprinted with the permission from ref. 1: Copyright 2015, Japan Society for Molecular Science 66 Nonlinear Spectroscopy Fig. 66.2 (a) Power spectra of input fundamental (yellow) and output second-harmonic (purple) pulses. The dotted line represents the absorption spectrum of CV in water. (b) Temporal behavior of pump-induced change in second-harmonic intensity generated at the air/water interface. The oscillatory component extracted from the temporal behavior data is shown in the top panel. (c) Fourier-transformed power spectrum of CV at the air/water interface. Reprinted with the permission from ref. 1: Copyright 2015, Japan Society for Molecular Science 403 404 S. Yamaguchi Fig. 66.3 (a) Temporal behavior of pump-induced change in second -harmonic intensity generated at the acidic water/TiO2 (110) interface. (b) Oscillatory component extracted from the temporal behavior data. (c) Fourier-transformed complex spectrum of the acidic water/TiO2 (110) interface. Reproduced from Ref. 2 with permission from the PCCP Owner Societies 66.4 Applications Figure 66.2 shows an example of FR spectroscopy applied to the air/water interface [1]. The input fundamental and output second-harmonic pulses are both in resonance with a solute molecule (cresyl violet, CV) in water as shown in Fig. 66.2a. Resonantly-enhanced SHG at the air/water interface is modulated by the pump pulse, yielding vibrational oscillatory feature in the time domain (Fig. 66.2b). Fourier transformation of the time-domain data provides the interface-selective vibrational spectrum of CV absorbed at the air/water interface, which allows us to see the low-frequency (THz) region as well as the whole fingerprint region (Fig. 66.2). FR spectroscopy is also applicable to liquid/solid interface. Figure 66.3c shows the vibrational spectrum of an acidic water (HCl solution)/TiO2 (110) interface [2]. Interface-specific phonon bands of TiO2 are clearly observed in the imaginary and real parts of χ(4). No other technique is available to obtain a vibrational spectrum of liquid/solid interface with the selectivity of a few molecular diameter thicknesses. References 1. Kuramochi, H., Takeuchi, S., Tahara, T.: Interface-selective fourth-order coherent raman spectroscopy in the time-domain using sub-7-fs pulses. Annual Meeting of Japan Society for Molecular Science, Tokyo (2015) 2. Nomoto, T., Onishi, H.: Fourth-order coherent Raman spectroscopy in a time domain: applications to buried interfaces. Phys. Chem. Chem. Phys. 9, 5515–5521 (2007) Chapter 67 Nuclear Reaction Analysis Markus Wilde and Katsuyuki Fukutani Keywords Ion beam analysis Light element depth profiling (Resonant) nuclear reaction Hydrogen quantitation Hydrogen dynamics 67.1 Principle Nuclear reaction analysis is a method to quantitatively determine the concentration versus depth distribution of light elements in the near-surface region of solids. To detect a specific nucleus A, the analyzed material is bombarded with a beam of projectile ions (a) at a high energy (100 keV–20 MeV) that is sufficient to overcome the Coulomb repulsion barrier to fuse the nuclei of a and A. Conserving the total energy, the resulting nuclear reaction A(a,b)B forms a new nucleus B and emits secondary particles (b: protons (p), neutrons (n), 4He ions (‘a particles’) and/or c-photons) with well-defined high (keV–MeV) energies. The presence of nucleus A in the target is then proven by registering such secondary particles (b) or the reaction product (B) with a suitable detector. The reaction yield (Y) is proportional to the efficiency (‘cross section,’ r) of the nuclear reaction, to the number of incident ions (Qa), and to the content of A (cA) in the target: Y / rQacA. An unknown cA in a sample is thus quantified by normalizing the reaction yield to the number of incident beam ions and by relating this to a measurement of a reference target (standard) with a well-known content of A under identical conditions. The depth information on the target nucleus location obtained through NRA is based on the energy loss (or ‘stopping’) that the charged particles (the projectile ions going into and/or the emitted secondary particles or products exiting from the target) suffer inside the analyzed material due to electronic friction. This energy loss (DE) is proportional to the traveled distance (z) of the respective particle inside the target, i.e., DE = zS, where S = dE/dz (keV/nm) is the so-called stopping power of the material. M. Wilde (&) K. Fukutani Institute of Industrial Science, The University of Tokyo, Tokyo, Japan e-mail: wilde@iis.u-tokyo.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_67 405 406 M. Wilde and K. Fukutani Many nuclear reactions between light elements can occur in the MeV regime [1]. They are insensitive to the chemical bonding state of the target nucleus and—due to different neutron numbers—not only element- but also isotope-specific, i.e., reactions of 1H, 2D, 14N, 15N, and 16O, 18O, for example, are distinctly different. The cross section of each nuclear reaction has its own unique dependence on the collision energy (E): The r(E) variation can be strong or smooth in certain ranges, and some nuclear reactions have narrow resonances at certain energies (Eres), where r is strongly enhanced over the neighboring non-resonant regions. Such narrow resonances in r(E) are highly versatile for depth profiling, because the stopping effect makes the depth in the target at which the nuclear reaction occurs selectable through the incident ion beam energy (Ei). For Ei = Eres, the beam reacts with target nuclei on the sample surface, whereas for an initial Ei larger than Eres, the energy loss below the surface reduces the projectile energy so that it meets the narrow energy window of the resonant reaction at a well-defined probing depth d = (Ei−Eres)/S (Fig. 67.1a). By measuring the reaction yield (Y) as Ei is incrementally scanned across and beyond Eres (Y(Ei), an ‘excitation curve’) the depth distribution of the target nucleus in the sample is thus revealed. Notable reactions for narrow-resonance NRA are 18O(p,a)15N (Eres = 150 keV, 50 eV width) for 18O profiling with proton beams [2] and 1H(15N,ac)12C (Eres = 6.385 MeV, 1.8 keV width) for 1H NRA with 15N beams [3]. In the following, we focus exemplarily on 1H NRA with 15N, as it has found a very large number of applications for the analysis of H at surfaces and interfaces [4]. 1H NRA via 1H(15N,ac)12C is particularly elegant as the reaction emits highly penetrating c-rays of 4.43 MeV that can be registered with a detector outside of the analytic vacuum chamber, and the energy loss of the 15N projectile alone defines the probing depth (Fig. 67.1b). In general, the near-surface depth resolution in resonant NRA is roughly defined by the resonance energy width divided by S, but it decreases in larger probing depths due to the ‘straggling’ effect that broadens the energy distribution of the projectiles due to random scattering in the analyzed Reaction yield (Y ) (a) (b) Depth 0 Vacuum chamber γ ∝ [Hsurface] γ ∝ [Hburied] d E i = E Res E i = E Res+ ΔE H γdetector H ΔE E Res E Res+ ΔE Incident ion energy (E i ) 15 N ion beam Energy loss: ΔE = d . (dE /dz) Sample Fig. 67.1 (a) Principle of narrow-resonance NRA. Due to energy loss by stopping in the target the projectile energy reaches Eres in a probing depth d = (Ei-Eres)/(dE/dz), (b) 1H depth profiling with the 1H(15N,ac)12C reaction. Surface hydrogen reacting with 15N incident at Ei = Eres is distinguished from H buried inside the material, which reacts at Ei = Eres + DE 67 Nuclear Reaction Analysis 407 material. For 15N near 6.4 MeV, S = 1–4 keV/nm, and including an additional Doppler broadening effect due to H-vibration, 1H(15N,ac)12C NRA provides for a resolution of a few nm near the surface before straggling becomes the dominating limitation for the depth resolution from depths of 10–20 nm [4]. Also, nuclear reactions with smoothly varying or plateau-like cross sections (and even with rather broad resonances such as the large, *350 keV wide resonance at 630 keV in the 2D(3He,p)4He reaction cross section widely utilized for D analysis with 3He beams [5]) are useful for depth profiling. In such non-resonant NRA, Ei is kept constant while the energy distribution of the outgoing particles is analyzed with a surface barrier detector near the target. Such a detector typically has to be protected from scattered beam ions by a filter foil, which causes additional energy loss and reduced depth resolution due to straggling. Because heavier particles have larger stopping powers, it is advantageous for 2D(3He,p)4He to detect the product 4 He atoms (rather than the protons) to obtain better depth resolution (10 nm near the surface have been demonstrated [6]). 67.2 Features • Quantitative light element depth profiling • Quasi nondestructive (for many conductive target materials) • For 1H-15N NRA via 1H(15N,ac)12C: (near-surface) depth resolution: 2–5 nm (surface-normal beam incidence); <1 nm (surface-grazing incidence) • Sensitivity for surface-H coverages: a few % of a monolayer (*1013 cm−2) • Sensitivity for volume (‘bulk’) H concentrations: *100 ppm (*1018 cm−3) • Direct zero-point vibration energy measurement of H in adsorption layers. 67.3 Instrumentation The principle and practice of NRA measurements via the resonant 1H(15N,ac)12C reaction have been described in detail [4, 7]. The high energy 15N ion beam near 6.4 MeV (up to *10 MeV, 15N2+) is generated with an electrostatic accelerator. Typically, a Faraday cup upstream of the target measures the ion beam current. Removing it from and inserting it into the beam path can be used to define the acquisition time during which the sample is irradiated by the ion beam at a given 15 N energy. The targets are held electrically insulated in (ultra) high vacuum on manipulation stages, which allows for incidence angle and position alignment. To detect the c-rays from the 1H(15N,ac)12C reaction, lead-shielded bismuth germanate (Bi4Ge3O12, BGO) scintillation crystals coupled to photomultiplier tubes are placed closely to the sample outside the vacuum chamber. The 4.43-MeV c-rays are 408 M. Wilde and K. Fukutani identified through pulse height analysis with a multichannel analyzer. The number of incident 15N projectiles can be evaluated from the sample charge acquired through a current integrator. Quantitative (absolute) H concentrations are obtained through background subtraction, normalizing the c-counts to the number of incident 15 N ions, and calibrating the sensitivity of the c-detection system with a standard sample of precisely known H content (such as Kapton® foil). 67.4 Applications Two major purposes of 1H NRA with 15N are to quantify the H coverage on surfaces and to determine the depth location and quantity of hydrogen at buried interfaces; analytical tasks difficult to accomplish with other methods. Figure 67.2a exemplifies for H-terminated Si(111)-(1 1) that a single layer of surface-H atoms appears in the NRA c-yield profile as a peak at Eres with a FWHM of several (here: 12.7 ± 0.4) keV. A Doppler effect in the 15N–1H collisions due to H-Si zero-point vibration broadens the profile substantially beyond the nuclear reaction resonance width, which causes the nearly Gaussian shape. The profile width allows extracting the zero-point vibrational energy (123.4 ± 4.6 meV for H-Si) [8]. The profile integral is proportional to the H coverage, in this case 1.0 monolayer (ML). (d) 15N (a) (b) (e) (c) Fig. 67.2 NRA c-ray yield profiles from (a) bare Si(111)(1 1)–H and after deposition of 4.5 nm Pb at (b) 360 K, and (c) at 110 K. (d) and (e): illustrations of Pb growth on Si(111) (1 1)–H at 360 K and 110 K, respectively. The solid curves are fits to Gaussian forms [a sum of two Gaussians for (b)]. Adapted with permission from Ref. [9]. Copyright (1999) by the American Physical Society 67 Nuclear Reaction Analysis 409 After depositing 4.5 nm Pb at 110 K, the integral c-yield of profile (c) is the same, but it broadens (to 21.3 ± 1.3 keV), and its center shifts (by 10.3 ± 0.4 keV) above Eres. As neither Si nor Pb dissolve H, this indicates conservation of the full initial Si(111)-(1 1)-H coverage underneath a uniform (*4.4 nm thick) Pb overlayer (Fig. 67.2e), which causes energy straggling and stopping of the 15N ions before they react with H at the Pb/Si interface. Depositing 4.5 nm Pb at 360 K splits the c-yield curve (b) into a surface component at Eres and a second (24.0 ± 8.5 keV wide) peak at 23.5 ± 3.5 keV above Eres (holding 0.4 ML H). Here, the interfacial H is shaded by thicker Pb islands of 10.1 nm height, between which 0.6 ML surface-H on the Si substrate remain directly accessible to the incident 15N beam (Fig. 67.2d), revealing a change in the Pb growth mode between 110 and 360 K. Another important application of 1H(15N,ac)12C NRA is the direct observation of the dynamical behavior of hydrogen in the near-surface region of H-absorbing materials (such as Pd) during desorption from and diffusion into the target, which can be achieved by evaluating the thermal stability of surface-adsorbed and material-absorbed H species in a depth-resolved fashion. In the following example, this technique is applied to identify the two peaks in the H2 thermal desorption spectrum (TDS) (Fig. 67.3a) from H2-exposed Pd(100) that are difficult to interpret without additional information. The NRA profile of identically prepared Pd(100) (Fig. 67.3b) has a dominant peak at Eres indicating 1.0 ML of surface-H and additional c-yield due to *2.6 at. % of H in a few nm wide region below the surface. Probing the surface and the Pd-absorbed H selectively with NRA at two fixed Ei values (arrows in Fig. 67.3b) while raising the sample temperature (T) in increments yields T-dependent NRA signals of the respective H species. These signals drop sharply at temperatures that coincide with the peak positions in the H2 TDS (Fig. 67.3a). The comparison clearly assigns the peak at 180 K to desorption of the Pd-absorbed hydrogen and the one at 330 K to desorption of surface-H. (a) (b) d d Fig. 67.3 (a) H2 TDS trace and (b) NRA profile of Pd(100) exposed at 100 K to 300 L H + H2 (1 L = 1.33 10−4 Pa s). Temperature-dependent NRA signals of the surface (d = 0 nm) and near-surface absorbed (d = 6 nm) H (arrows in (b)) are superimposed in (a). Adapted with permission from Ref. [10]. Copyright (2008) by the American Physical Society 410 Fig. 67.4 Grazing incidence NRA c-ray yield profiles from h = 1–2 nm high Pd nanocrystals exposed to (a) 2 10−5 Pa, (b) 6 10−4 Pa, and (c) 2 10−3 Pa H2 at 90 K. Components of surface-H (red shaded) and of H absorbed inside the nanocrystals (blue curves) are indicated. Inset Grazing incidence NRA geometry and schematic morphology of Pd nanocrystals supported on a thin H-free Al2O3 film on a NiAl(110) backing. Adapted with permission from Ref. [11]. Copyright (2008) by the American Physical Society M. Wilde and K. Fukutani apparent depth (nm) (c) (b) (a) Finally, Fig. 67.4 illustrates NRA under a surface-grazing incidence angle (ai), which—by virtue of increasing the depth resolution - accomplishes the observation of hydrogen inside Pd nanocrystals that measure merely 1–2 nm in height. For such small particles, the width of the Doppler-broadened resonance peak (*10 keV, cf. Figure 67.3b) would preclude discriminating Pd-absorbed H from H on the surface under surface-normal 15N incidence. The grazing incidence geometry (inset in Fig. 67.4, ai = 75°), however, elongates the 15N ion path in the Pd by a factor of 1/cos(ai) which expands the ‘apparent depth’ axis of the NRA profiles (*4 times). On this enlarged scale, the c-yield profile can be broken down into a surface peak (red) and a component for the H absorbed in the interior of the Pd nanocrystals (blue). Raising the H2 pressure (Fig. 67.4b, c) causes the amount of Pd-absorbed H to increase, whereas the surface-H remains saturated [11]. This NRA-based insight into the pressure-dependent H-breathing of Pd nanocrystals has been instrumental in assigning the role of the reactive species in the Pd-catalyzed hydrogenation of 2-butene (C4H8, an olefin with an unsaturated C=C double bond) to Pd-absorbed hydrogen [12]. The example highlights that NRA is even useful to study the behavior of near-surface hydrogen in chemical reactions. 67 Nuclear Reaction Analysis 411 References 1. Trocellier, P., Berger, P., Wilde, M.: Nuclear Reaction Analysis. Encycl. Anal. Chem. 1–17 (2016) 2. Battistig, G., Amsel, G., d’Artemare, E., Vickridge, I.: A very narrow resonance in 18O(p, a)15N near 150 keV: Application to isotopic tracing: I. Resonance width measurement. Nucl. Instrum. Methods Phys. Res. B 61, 369–376 (1991) 3. Amsel, G., Maurel, B.: High resolution techniques for nuclear reaction narrow resonance width measurements and for shallow depth profiling. Nucl. Instrum. Methods Phys. Res. 218, 183–196 (1983) 4. Wilde, M., Fukutani, K.: Hydrogen detection near surfaces and shallow interfaces with resonant nuclear reaction analysis. Surf. Sci. Rep. 69, 196–295 (2014) 5. Alimov, VKh, Mayer, M., Roth, J.: Differential cross-section of the D(3He, p)4He nuclear reaction and depth profiling of deuterium up to large depths. Nucl. Instrum. Methods Phys. Res. B 234, 169–175 (2005) 6. Langley, R.A., Picraux, S.T., Vook, F.L.: Depth distribution profiling of deuterium and 3He. J. Nucl. Mater. 53, 257–261 (1974) 7. Wilde, M.; Ohno, S.; Ogura, S.; Fukutani, K.; Matsuzaki, H.: Quantification of hydrogen concentrations in surface and interface layers and bulk materials through depth profiling with nuclear reaction analysis. J. Vis. Exp. 109, e53452/1—e53452/12 (2016) 8. Fukutani, K.; Itoh, A.; Wilde, M.; Matsumoto, M.: Zero-point vibration of hydrogen adsorbed on si and pt surfaces. Phys. Rev. Lett. 88, 116101/1—116101/4 (2002) 9. Fukutani, K., Iwai, H., Murata, Y., Yamashita, H.: Hydrogen at the surface and interface of metals on Si(111). Phys. Rev. B 59, 13020–13025 (1999) 10. Wilde, M., Fukutani, K.: Penetration mechanisms of surface-adsorbed hydrogen atoms into bulk metals: Experiment and model. Phys. Rev. B. 78, 115411/1—115411/10 (2008) 11. Wilde, M., Fukutani, K., Naschitzki, M., Freund, H.-J.: Hydrogen absorption in oxide-supported palladium nanocrystals. Phys. Rev. B. 77, 113412/1—113412/4 (2008) 12. Wilde, M., Fukutani, K., Ludwig, W., Brandt, B., Fischer, J.H., Schauermann, S., Freund, H. J.: Influence of carbon deposition on the hydrogen distribution in Pd nanoparticles and their reactivity in olefin hydrogenation. Angew. Chem. Int. Ed. 47, 9289–9293 (2008) Chapter 68 Optical Microscopy Kazuya Kabayama and Ryugo Tero Keywords Numerical aperture Fluorescence 68.1 Bright field image Dark field image Principle Major types of optical microscopes include bright field microscopes, dark field microscopes, fluorescence microscopes, phase-contrast microscopes, and differential interference microscopes. Such microscopes have characteristics and features that are unique in terms of the light emitted or absorbed by an object to be observed. For example, a fluorescence microscope distinguishes differences in structure by detecting the concentration distribution of a fluorescent substance or in the fluorescence spectrum when the object emits fluorescence. Bright and dark field microscopes distinguish the target of observation from other objects based on the absorption/reflection and scattering of light, respectively. All of these microscopes have to condense the light from the objects with lenses. The point at which light is condensed is led the “focal point.” The diameter of the condensed light at the focal point cannot be smaller than approximately half of a wavelength due to the wave nature of light. When a very small subject is observed one can see the distributed light intensity (blur), which is given by the point spread function. When two bright spots are very close to each other, the light from them overlap. In order for these overlapping bright spots to be identified as two distinct images, the bright spots have to be separated by the distance “d” given by the following equation: K. Kabayama (&) Department of Chemistry, Graduate School of Science, Osaka University, Osaka, Japan e-mail: kaba@chem.sci.osaka-u.ac.jp R. Tero Department of Environmental and Life Sciences, Toyohashi University of Technology, Toyohashi, Japan e-mail: tero@tut.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_68 413 414 K. Kabayama and R. Tero Fig. 68.1 (a) Schematic of converging angle a. (b, c) Optical paths of (b) a bright field microscope and (c) a dark field microscope in the upright microscope configuration d¼ 0:61 k n sin a where k is the wavelength of light, n is the surface refractive index of the object, a represents half of the converging angle of the objective lens (Fig. 68.1a). This distance d is called the spatial resolution, which lessens as the wavelength (k) becomes longer. “nsina” is collectively referred to the numerical aperture (NA), which is one of the indexes of the objective lens performance. 68.2 Features 68.2.1 Bright Field Microscopy • Most common optical microscopy method. • Identifies a sample by use of differences in transmissivity or reflectance of light between the sample and medium (Fig. 68.1b). • Staining may be necessary to observe a transparent sample. 68.2.2 Dark Field Microscopy • Scattered light from a sample is imaged in a dark field of view (Fig. 68.1c). • High capability of detecting fine samples in the order of nanometer. • Contaminations and rough substrates around the sample also scatter the illumination light and may give a bright background. 68.2.3 Fluorescence Microscopy • A sample is illuminated with excitation light, and fluorescence light from the sample is isolated for observation. 68 Optical Microscopy 415 • Staining or labeling a sample with a fluorescence agent (e.g., dye molecules, fluorescence proteins, and quantum dots) is necessary if the sample itself does not emit fluorescence. • Multiple fluorescent substances can be observed simultaneously. 68.3 Instrumentation 68.3.1 Bright Field Microscope A sample is illuminated with a light source such as a halogen lamp. A part of the illumination light not absorbed by the sample is detected to form an observation image of the sample (Fig. 68.1b). The spatial resolution of a bright field microscope largely varies depending on what type of light source is used as well as how the sample is illuminated. It is possible to clearly image fine structures, which provides a large diffraction angle, when it is illuminated at some incident angle (i.e., an angle with respect to the optical axis). The sample is not coherently illuminated (i.e., parallel light from one direction), but it is illuminated uniformly from various directions, which is generally known as “Köhler illumination.” 68.3.2 Dark Field Microscope A dark field microscope has a device called a “dark field condenser” in its optical path that forms illumination light with a larger incident angle than the converging angle of the objective lens (Fig. 68.1c). It means that the illumination light has larger NA than the objective lens. Only the light scattered or diffracted by the sample enters the objective lens. The details of small particles, microorganisms, etc., even in the order of several nanometers can be captured. 68.3.3 Fluorescence Microscope A sample is modified or labeled with a fluorescent substance, unless the fluorescence from the sample itself is used for observation. The fluorescence-labeled sample is excited by the light with a specific wavelength. The window of this specific wavelength is extracted from the light from a light source (most commonly a mercury lamp) through a bandpass filter, which is called an excitation filter. The fluorescence light from the sample is separated from the excitation light with an optical element (i.e., a dichroic mirror), and another bandpass filter selectively passing the fluorescence light from the sample, which is called an absorption filter or an emission filter. 416 68.4 K. Kabayama and R. Tero Applications Figure 68.2 shows an example of bright field, dark field, and fluorescence images obtained from graphene oxide flakes on a thermally oxidized SiO2/Si substrate. Figure 68.2a shows a bright field image. The graphene oxide flakes have wrinkles, and also fold stacking several layers at some parts. Graphene oxide flakes are observed darker than the surrounding bare substrate surface because of the absorption of light, therefore much darker at the wrinkles and folded parts. Note that optical contrast from the atomic sheet is obtained because of the interference effect of a SiO2/Si substrate with a specific thickness of SiO2 [1]. Figure 68.2b shows a dark field image of the same graphene oxide flakes. Only wrinkles are visible as bright strings, because the scattered light is used for imaging. Flat regions are not visible at bare substrates, or at single-layered or multilayered parts of graphene oxide. The dark filed image detects objects in the order of nanometer, therefore detects the wrinkles of the atomic sheets independently of the SiO2 thickness. Flatness of a thermally oxidized SiO2/Si substrate on a scale of subnanometer is an advantage for the dark field microscopy to suppress scattering of light. Small particulates, some of which are not recognized in the bright field image (Fig. 68.2a) are detected as bright spots. Note that sizes of objects in dark field images depend on the intensity of the scattered light. The width of the bright strings Fig. 68.2 Wrinkled and folded graphene oxide flakes on a thermally oxidized Si wafer with a 90-nm-thick SiO2 layer. (a) Bright field, (b) dark field, and (c) fluorescence images obtained with a reflected light microscope 68 Optical Microscopy 417 and the diameter of the bright spots in Fig. 68.2b are larger than the actual sizes of the wrinkles and particulates. Figure 68.2c shows a fluorescence image of the graphene oxide flakes obtained using 530–550-nm bandpass and >575-nm-longpass filters as excitation and emission filters, respectively. Fluorescent graphene oxide flakes are visible, and the bare substrate surface is not. Fluorescence intensity increases depending on the number of stacked layers. On a SiO2/Si substrate, fluorescence intensity is also affected by the thickness of the SiO2 layer because of the interference effect [2]. References 1. Nagashio, K., Nishimura, T., Kita, K., Toriumi, A.: Mobility variations in mono- and multi-layer graphene films. Appl. Phys. Express. 2, 025003/1–025003/3 (2009) 2. Lambacher, A., Fromherz, P.: Luminescence of dye molecules on oxidized silicon and fluorescence interference contrast microscopy of biomembranes. J. Opt. Soc. Am. B 19, 1435– 1453 (2002) Chapter 69 Optical Second-Harmonic Generation Spectroscopy and Microscopy Khuat Thi Thu Hien and Goro Mizutani Keywords Electronic state 69.1 Structure Surface Interface Principle When the incident light irradiates the surface or interface of a material, two photons with the same photon energy ћx interact with the material together and generate one new photon with the doubled photon energy of 2ћx, as shown in Fig. 69.1a. This phenomenon is known as optical second-harmonic generation (SHG). It is an $ð2Þ electromagnetic irradiation of a second-order nonlinear polarization ~ Pð2Þ ¼ e0 v : $ð2Þ EE / E02 ei2xt created in the medium by the incident field E0 eixt . Here, v is known as the second-order nonlinear electric susceptibility and is a third-rank tensor. $ð2Þ v is vanishing for a centrosymmetric medium within the dipole approximation of the interaction of light with matter. The structure of a surface or interface does not $ð2Þ have center of inversion, so it is SHG active. Allowed surface v elements are reviewed in a textbook of nonlinear optics [1]. Especially, the SHG response from a material with a centrosymmetric bulk structure is very sensitive to its surface because the signal from the bulk is silent. The sensitivity is said to be at least 0.5 monolayer but depends on the sample system. It is noted that in some detailed analysis, the electric quadrupole and other higher order electromagnetic effects have a non-negligible contribution [1]. The SHG spectrum as a function of the incident light photon energy will give information of a resonant electronic level of materials as one can see it in Fig. 69.1b. When ћx or 2ћx is equal to the energy of the real state, oneK.T.T. Hien G. Mizutani (&) School of Materials Science, Japan Advanced Institute of Science and Technology, 923-1292 Ishikawa, Japan e-mail: mizutani@jaist.ac.jp K.T.T. Hien e-mail: ktt-hien@jaist.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_69 419 420 K.T.T. Hien and G. Mizutani Fig. 69.1 SHG principle: (a) SHG from a TiO2 (110) surface, (b) scheme of energy diagram. |g > is the ground state, |v1 > and |v2 > are virtual states, and |r > is a real state. The TiO2 (110) surface is shown as an example for later use or two-photon resonance occurs, respectively. Since light probe has no charge-up problem unlike the spectroscopies using charged particles, the electronic state can be obtained even for insulators. This technique can also be used to study the orientation of asymmetric molecules such as liquid crystal molecules on the surface [2]. Finally, the SHG combined with microscopy can give information of resonant electronic level distribution in two dimensions on the surface and interfaces. Nowadays, femtosecond light sources become very convenient and the SHG microscopy will become easily facilitated in such application. 69.2 • • • • • Features The change of surface and interface electronic levels can be monitored. The orientation and the order of the surface bonds can be detected. The electronic states of insulator can be probed without charge-up effect. Time-resolved spectroscopy as fast as femtosecond level can be performed. Two-dimensional mapping of electronic states or molecular orientation can be performed. 69.3 Instrumentation The optical setup for SHG spectroscopy is shown in Fig. 69.2. The laser light comes from an optical parametric oscillator or generator with a tunable wavelength driven by a Q-switched or mode-locked Nd:YAG laser with light pulse duration of 69 Optical Second-Harmonic Generation Spectroscopy and Microscopy 421 Fig. 69.2 The diagram of SHG spectroscopy. ND Neutral density, PMT Photomultiplier, AMP Amplifier, PC Personal computer nanosecond, picosecond scale with repetition of 10 Hz. The light pulse energy is chosen in order not to damage the sample and is controlled by color glass filters. SHG light pulses are emitted in the same direction as the reflected incident beam due to the Maxwell boundary condition. The generated SHG photons are selected by a monochromator, converted into electric charge by a photomultiplier (PMT), and then the photocurrent is electrically amplified and finally observed on the oscilloscope and display (PC). The polarization of the incident light can be adjusted by polarizers before being focused onto the sample by a lens. The 2x cut filter is added after polarizers in order to cut the SHG generated from the input optics. The x cut filter is put after the sample in order to cut the fundamental light and ensure that only SHG light is guided into detectors. Another polarizer is put before the monochromator to specify polarization of the observed photons. The sample is put either in air or in a vacuum chamber. In order to detect the anisotropy on the sample surface, the sample is put on a rotation stage and is rotated around its surface normal. Normally, the sample surface should be mirror-like flat. The setup for the SHG microscopy is similar to that in Fig. 69.2 with the monochromator with a PMT replaced with a microscope optics equipped with an image-intensified CCD camera. 69.4 Applications As mentioned in the principle section, the SHG technique is excellent in the analysis of the surface electronic states of insulators. Here, we summarize one interesting result by Omote et al. [3]. In this research, the optical SHG response of the rutile TiO2 (110) face has been observed in air as a function of the SHG photon energy 2ћx for polarization combinations of Pin/Pout (open circles) and Sin/Pout (black circles) as shown in Fig. 69.3. The solid and dashed curves are the calculated SHG intensities by an ab initio method. The plane of incidence is parallel to the [001] direction (Fig. 69.1a). The SHG photon energy was scanned from 2.9 to 422 K.T.T. Hien and G. Mizutani Fig. 69.3 The reflected SHG intensities from the relaxed TiO2 (110) surface as a function of the SHG photon energy 2ћx in air with different polarization combinations of Pin/Pout (empty circles) and Sin/Pout (black circles). The solid curve and dashed curve are the calculated SHG intensities. The plane of incidence is parallel to the [001] direction [3] Fig. 69.4 Measured SHG intensity patterns from the TiO2(110) surface as a function of the sample rotation angle around its surface normal 4.8 eV. The onset energy of the SHG resonance at the interface has been found at 2ћx 3.2 eV with the nonlinear polarization at 2ћx perpendicular to the surface. It is different from the bulk value of 3.05 eV. For both configurations, the SHG intensities increase sharply around 2ћx 3.5 eV. From Fig. 69.3, four energies in the SHG resonance at 3.34, 3.65, 4.00, and 4.66 eV were chosen to take the SHG patterns as a function of the sample rotation angle around its surface normal as shown in Fig. 69.4. The input and output polarizations are parallel to the incident plane (Pin/Pout). The SHG intensity is plotted in the radial direction and shows symmetric patterns consisting of two lobes. The intensity is in an arbitrary scale. The zero degree corresponds to the configuration when the plane of incidence includes the [001] direction on the sample (Fig. 69.4a). From these patterns, the second-order susceptibilities of the surface can be calculated. The calculated ð2Þð110Þ vS113 element reproduces much stronger signal for Pin/Pout than for Sin/Pout configuration. Here, suffixes 1 and 3 are defined in Fig. 69.1. This explains why the SHG intensity measured for Pin/Pout is much larger than that for the Sin/Pout polarization combination (Fig. 69.3). On the other hand, in all the patterns, the SHG intensity is higher when the incident plane is parallel to the [001] direction. In this 69 Optical Second-Harmonic Generation Spectroscopy and Microscopy 423 configuration, the incident electric field and the Ti–O–Ti–O– chains including the bridging oxygen atoms (Fig. 69.1) at the interface are in the same plane. According to the theoretical calculation, the nonlinear polarization is dominantly generated by the surface Ti–O–Ti–O– chain. References 1. Brevet, P.-F.: Surface second harmonic generation. Presses polytechniques et universitaires romandes, Lausanne (1997) 2. Chen, W., Feller, M.B., Shen, Y.R.: Investigation of anisotropic molecular orientational distributions of liquid-crystal monolayers by optical second harmonic generation. Phys. Rev. Lett. 63, 2665–2668 (1989) 3. Omote, M., Kitaoka, H., Kobayashi, E., Suzuki, O., Aratake, K., Sano, H., Mizutani, G., Wolf, W., Podloucky, R.: Spectral, tensor, and ab initio theoretical analysis of optical second harmonic from the rutile TiO2(110) and (001) faces. J. Phys.: Condens. Matter 17, S175–S200 (2005) Chapter 70 Particle-Induced X-Ray Emission Koichiro Sera Keywords PIXE Quantitative analysis Non-destructive analysis 70.1 Multipurpose Small quantity Principle PIXE was first proposed by Sven Johansson in 1970 [1]. This method of analysis is based on the emission of characteristic X-rays from a sample after inner-shell ionization with ions from accelerators. In comparison to other analytical techniques based on X-ray spectrometry, such as EPMA and XRF, the ion beams generate fewer continuous X-rays, which dwarf the trace element peaks. Thus, it has an excellent sensitivity down to the ppm level for solids and the ppb level for liquids. Figure 70.1 illustrates the basic principle of PIXE. X-ray production cross-sections can be theoretically estimated [2]. Other essential parameters, such as detection efficiencies, the value of X-ray transmission through an X-ray absorber, the solid angle of the detector, and the number of incident particles, can be experimentally obtained. The elemental concentration can be derived using these quantities. However, the absolute evaluation of these values sometimes results in large errors and certain standard methods are usually adopted. In the internal-standard method, the concentration of a certain element (Ca), denoted by “a,” can be obtained by the following equation: Ca ¼ Cs Ya rXs As Eff s Ys rXa Aa Eff a where subscript “s” indicates the standard element, rX indicates the X-ray production cross-section, A indicates the value of X-ray transmission through an X-ray K. Sera (&) Cyclotron Research Center, Iwate Medical University, Takizawa, Iwate, Japan e-mail: ksera@iwate-med.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_70 425 426 K. Sera (a) (b) Incident Proton Coulomb Interaction M-shell L-shell K-shell Characteristic X-ray Si(Li) detector Electron transition Ejected electron Fig. 70.1 Basic principle of PIXE. (a) Inner-shell ionization (b) X-ray emission absorber, and Eff indicates the detection efficiency. The internal-standard method has been widely applied to bio-medical samples. 70.2 Features • The ability to simultaneously measure all elements heavier than sodium within a short time (3−5 min). • Applicability to the analysis of specimens both in vacuum and air. • A non-destructive analysis with the ability to quantitatively analyze living organisms. • The potential to be a highly sensitive microprobe with a spatial resolution of <0.1 lm. • The ability to quantitatively analyze extremely small samples (*1 lg). 70.3 Instrumentation 70.3.1 Measuring Systems The PIXE system consists of an accelerator, as well as beam transportation, measurement, data collecting, and data analysis systems. Figure 70.2 shows the layout of our PIXE system. We use a three-detector measurement system for in-vacuum PIXE and a two-detector measurement system for in-air PIXE [3, 4] to detect X-rays in the wide energy region and prompt c-rays at the same time. 70 Particle-Induced X-Ray Emission In-air system 427 In-vacuum system Vacuum Si(Li)-1 In-Air Si(Li)-2 Beam from Cyclotron Intrinsic Ge Q-Magnets In-Air Si(Li)-1 Samples Turbo pump Vacuum Si(Li)-2 Bending magnet Pb-Lα Zn-Kα Cu-Kβ Ga-Kα Zn-Kβ Fe-Kβ 180 Ti-Kα 140 Ti-Kβ 100 Cu-Kα 2 Aerosol Ep = 2.9 MeV with a 50 μm Mylar Ni-Kα 10 Mn-Kα 3 Cr-Kα 10 10 Si-K 4 Ca-Kα K-Kβ 10 5 K-Kα Counts/Channel 10 S-Kα Cl-Kα 6 Al-K 10 Fe-Kα Fig. 70.2 Layout of our measurement system 1 10 0 20 60 220 260 300 340 380 420 460 500 Channel Number Fig. 70.3 Typical results of peak fitting and background subtraction 70.3.2 Data Analysis In order to perform the quantification, the peak yields from each spectrum need to be obtained. We developed a computer program “SAPIX” [5, 6]. Figure 70.3 shows a typical result of the peak fitting and background subtraction, where the sample is a dust collected on a filter. In addition to SAPIX, we developed another computer program, “KEI,” for quantifying the elemental concentration [5]. Three computer programs were developed to derive the physical quantities. These were: (1) “ICPER,” [5] which was used to calculate theoretical X-ray production cross-sections, (2) “ABS,” [7] which was used to obtain the transmission of X-rays through various X-ray absorbers and (3) “EFF,” [8] which was used to determine the detection efficiency. 428 K. Sera 70.3.3 Original Quantification Methods Samples such as aerosol and insoluble matter are generally collected on a filter, and an external-standard method [9] is applied to the untreated samples, where the elemental concentrations are obtained as ng/cm2. The standard-free method [10] makes use of the yield of continuous X-rays in a certain energy region. The method allows us to quantitatively analyze samples in extremely small quantities (*1 lg) [11, 12] and untreated samples [13, 14]. The method has been applied to more than 50,000 bio-samples, including various body fluids, organs, nails, and hair. 70.3.4 The Powdered-Internal-Standard Method Although there are many requests for the analysis of powdered samples composed of heavy elements, such as soil, crushed rock, and ash, such samples have been difficult to quantitatively analyze. A powdered-internal-standard method [15] made it possible to quantitatively analyze these types of samples. 70.3.5 The Specially-Designed Absorber Some samples contain a certain amount of a specific heavy element, such as iron in soil and rock, and certain metals in ore. These overwhelming peaks in the X-ray spectrum negatively affect the sensitivity of other elements because of their accompanying artifacts, specifically pileup and escape peaks, and tail functions (off-Gaussian components). By selectively decreasing these peaks by using the absorption edges of certain elements, the sensitivity of other elements is considerably improved [16]. Figure 70.4 illustrates an example of a special absorber for use with samples that contain excessive amounts of iron and manganese. The absorption edge of titanium foil drastically decreases the peaks of iron and manganese, while the pinhole and hole take the balance of the X-ray yields over the wide energy region. Thus, sensitivity of heavy elements was improved by nearly two orders, and many peaks of heavy elements, which were not observed with ordinary absorbers, could be detected—as demonstrated in Fig. 70.4. A transmission curve of these complicated absorbers could be precisely obtained using the ABS program [7]. Particle-Induced X-Ray Emission 429 Pinhole: ~200 μm Mn-Kα Fe-Kα Nb-Kα Zr-Kα+Sr-Kβ Rb-Kβ Sr-Kα Rb-Kα Br-Kα Pb-Lβ As-Kα+Pb-Lα Ga-Kα Zn-Kβ Ni-Kβ Zn-Kα 102 100 μm Mylar 500 μm Mylar Hole: ~2 mmφ 2 5520 μg/cm Ti Foil × 2 Cu-Kα Ni-Kα Fe-Kβ Ti-Kα Ti-Kβ CrKα 10 3 K-Kα Mg-K Counts/Channel 104 Ca-Kα Al-K Si-K 105 S-K Cl-K 70 101 100 0 80 160 240 320 400 480 Channel Number Fig. 70.4 Design and effect of an absorber specialized for Mn- and Fe-rich samples 70.3.6 The Development of in-Air PIXE The method of quantitative analysis used in in-air PIXE was established in 2007 [17]. We recalculated the X-ray production cross-sections, since the proton energy was reduced in comparison to the energy in a vacuum. The detection efficiencies of the Si(Li) detector in atmospheric conditions were also determined. The standard-free methods in in-air PIXE were then established [18] and it became possible to quantitatively analyze a single drop of untreated oil. A method for quantitatively analyzing living plants was also developed [19]. The author observed the changes in the elemental concentrations in living plants over time, where iron and calcium showed quite interesting behavior that could be recognized by a reaction to the “apoptosis” induced by proton irradiation. It became possible to quantitatively analyze multiple elements from Al to U with a two-detector measuring system in the in-air PIXE system [4, 20]. As this method is expected to be applicable to experimental animals, it will become a powerful tool for studies in basic medicine. 430 70.4 K. Sera Applications 70.4.1 The Common Utilization of PIXE at the NMCC Our laboratory, the Nishina Memorial Cyclotron Center (NMCC), PIXE and Positron Emission Tomography (PET) has been opened to all researchers in Japan since 1993. More than 120,000 samples have been analyzed using our PIXE system. Table 70.1 shows the rough numbers of classified samples that have been analyzed at the NMCC to date. Several examples of studies using our PIXE system are described below. 70.4.2 Application in the Life and Environmental Sciences Figure 70.5 shows the progress of the number of samples analyzed in the life and environmental sciences. Nearly 40,000 untreated hair samples were analyzed, when more than 25,000 hairs were collected from Asian people to evaluate human exposure to toxic elements. In addition to hair samples, we have established standard-free methods for various types of samples including blood, urine, nail, spinal fluid, saliva, and organs. We have co-operated with many international organizations and used these techniques to solve various environmental problems. The Coordinating Committee of Geo-science Programs in East and Southeast Asia (CCOP) adopted the “Environmental Analysis Support Programme for CCOP and Other Regions”. This program offers free PIXE analyzes at the NMCC to developing countries. Table 70.1 Number of principal samples analyzed at the NMCC Hair (including hair from humans, domestic, experimental animals, and wild animals) 40,000 Aerosol (with particle sizes ranging from nm to a few hundred lm) Body fluids (including blood, urine, spinal fluid, saliva, and sweat) Water (including drinking water, spring water, river water, well water, sea water, and lake water) Geo-science samples (including ore, rock, soil, and fluid inclusion) Organs and tissues (including human, experimental, domestic, and wild animals) Botanical and agricultural samples Veterinary samples (including body hair, body fluids, feather, hoof, and organs) Seaweed, plankton and algae Samples related to the March 11 Tsunami (including sludge, plant, and small ocean life) Biological samples other than plants (including mosquitoes, other insects, and lichens) Other samples (including archeological samples, gemstone, oil, food, drug, and cosmic dust) 16,000 12,000 8000 8000 6000 5000 4000 1400 2000 1000 1000 70 Particle-Induced X-Ray Emission Fiscal Years 2014 2013 2012 2011 2010 2009 2008 2007 2006 2005 2004 2003 2002 2001 2000 1999 1998 1997 1996 1995 1994 1993 1992 1991 1990 431 Clinical Med. Fundamental Med. Dentistry Veterinary Med. Pharmacology Biology Sitology Environment Public Health & Hygiene Agriculture Dendrology 0 1000 2000 3000 4000 5000 6000 7000 Number of Sample Fig. 70.5 Number of samples analyzed in the life and environmental sciences 70.5 Application to Studies in Other Fields In addition to studies in life sciences, our PIXE system has been applied to studies in various research fields, including earth sciences, analytical chemistry, archeology, aquaculture, agriculture, dendrology, wood chemistry, material technology, and astrophysics (Table 70.1). We have two mottos: (1) “We perform quantitative analyzes of all kinds of samples,” and (2) “If you take samples in the morning, we offer you the results in the evening”. 70.6 Application to Problem-Based Studies 70.6.1 Studies on Infectious Disease The prevalence of infectious diseases transmitted by mosquitoes, such as Japanese encephalitis, malaria, dengue, and yellow fever, have become a serious global problem due in part to global warming. In 2012, we developed a method of quantitatively analyzing untreated mosquitoes to clarify their ecology in collaboration with the National Institute of Infectious Diseases, Japan [21]. We have analyzed many species of mosquitoes including Aedes aegypt, Ae. albopictus, and Culex tritaeniorhynchus which are known vectors for numerous infectious diseases. Our method allows us to quantitatively analyze a single mosquito leg, wing, head, or body. 432 K. Sera 70.6.2 The Non-destructive Surface Analysis of Gemstones and Pearls The techniques developed for in-air PIXE have been applied to the non-destructive surface analysis of pearls and gemstones to contribute to the “responsible jewelry” project, which requires materials to be traced to ensure fair business practice in relation to the sales of gemstones throughout the world. In this study, a slight beam was used to irradiate the surface of the sample, as shown in Fig. 70.6. It has become possible to maintain a low sample temperature by introducing He gas and to prevent damaging the sample with irradiation. 70.6.3 Studies on the Problems Caused by the 2011 Tsunami The huge tsunami that struck the east coast of the Sanriku district of Japan on March 11, 2011 caused significant damage and drew a large amount of sludge from the bottom of the sea. This may have exerted negative effects not only on human health but also on the recovery of the marine ecosystem, since the sludge in Japan has been confirmed to be highly contaminated with heavy elements. We have tackled the problem from the following three viewpoints. First, it was necessary for us to grasp the actual condition of heavy-element contamination on land and tideland. From 2011 to 2012, we collected more than 100 sludge samples and nearly 50 plant samples from the Iwate, Miyagi, and Fukushima prefectures. The concentrations of many elements, including Na, S, Cl, K, Ca, Ti, Mn, Cu, As, and Pb, were found to be considerably increased shortly after the tsunami, while the concentrations were found to have decreased remarkably in the samples collected in August 2012. We concluded that the negative effects of heavy elements on human health were no longer serious after one year [22]. Secondly, we investigated the negative effects of heavy elements on the recovery of the marine ecosystem. The marine ecosystem of the Sanriku coast was almost destroyed by the damage from the tsunami. Most of the animals and plants that disappeared have not yet fully recovered. It is essential to observe the processes of heavy-element accumulation in relation to the growth of the small crustaceans that play an important role in the marine food chain. As they grow from less than a few lg to several hundred mg in a short period, they are difficult to analyze with the same preparation and measurement techniques throughout their growing process. Fig. 70.6 Non-destructive surface analysis of a pearl Proton beam He jets for cooling Pearl or Gemstone Si(Li) X-rays 70 Particle-Induced X-Ray Emission 433 We have developed a method for quantitatively analyzing organisms of 1 lg to a few g in weight [12]. It was found that arsenic is mainly incorporated into the detritus food chain, while lead mainly enters the grazing food chain [12]. We must continue to observe the trends in relation to heavy elements in the marine food chain over the long term. Lastly, we examined the negative health effects of heavy elements. The inhalation of dried sludge is a matter to concern for people in the affected area. To evaluate the changes in the elemental concentrations in the people’s bodies before and after the disaster, we collected long hairs from the victims. The hairs were cut into pieces every 1 cm from the root side and then analyzed. As hair is known to grow at a rate of approximately 1 cm per month, the point corresponding to March 11, 2011 can be estimated backwards from the time that the sample was taken. It was found that not only the concentrations of essential elements such as copper, magnesium, and calcium, but also those of toxic elements, such as arsenic and lead, clearly decreased after the tsunami [22]. These findings may be due to drastic changes in the living environments of the victims. In particular, the eating habits of the affected people changed considerably after the tsunami. Before the tsunami, their principal food was mineral-rich marine products. After the tsunami, the victims were forced to eat emergency rations with a lower mineral content, for a period of a few months. Regarding toxic elements, most of the victims took refuge in the hills where there was no sludge and the negative effects of exposure to the contaminated sludge did not seriously affect their health. References 1. Johansson, S.A.E., Campbell, J.L.: PIXE: A novel technique for elemental analysis, pp. 1–347. John Wiley & Sons, Chichester, U. K. (1988) 2. Brandt, W., Lapicki, G.: Energy-loss effect in inner-shell Coulomb ionization by heavy charged particles. Phys. Rev. A 23, 1717–1729 (1981) 3. Sera, K., Yanagisawa, T., Tsunoda, H., Futatsugawa, S., Hatakeyama, S., Saitoh, Y., Suzuki, S., Orihara, H.: Bio-PIXE at Takizawa Facility (Bio-PIXE with a baby cyclotron). Int. J. PIXE 2, 325–330 (1992) 4. Sera, K., Goto, S., Takahashi, C., Saitoh, Y.: Quantitative analysis with a two-detector measuring system in in-air PIXE—design to improve detection sensitivity at low energies. Int. J. PIXE. 23, 55–67 (2013) 5. Sera, K., Futatsugawa, S.: Personal computer aided data handling and analysis for PIXE. Nucl. Instr. Meth. B109(110), 99–104 (1996) 6. Sera, K., Futatsugawa, S.: Spectrum analysis taking account of the tail, escape functions and sub-lines (SAPIX version 4). Int. J. PIXE 10, 101–114 (2000) 7. Sera, K., Futatsugawa, S., Hatakeyama, S., Saitou, Y.: Determination of physical quantities for PIXE by means of PIXE 1-absorption curve. Int. J. PIXE 4, 165–179 (1994) 8. Sera, K., Futatsugawa, S., Matsuda, K.: Determination of physical quantities for PIXE by means of PIXE 2 - efficiency curve. Int. J. PIXE 4, 181–191 (1994) 9. Sera, K., Futatsugawa, S., Saitoh, K.: Method of quantitative analysis making use of bromine in a nuclepore filter. Int. J. PIXE 7–1(2), 71–85 (1997) 434 K. Sera 10. Sera, K., Futatsugawa, S., Matsuda, K., Miura, Y.: Standard-free method of quantitative analysis for bio-samples. Int. J. PIXE 6, 467–481 (1996) 11. Sera, K., Futatsugawa, S., Hatakeyama, S., Saitoh, Y., Matsuda, K.: Quantitative analysis of bio-medical samples of very small quantities by the standard-free method. Int. J. PIXE 7–3(4), 157–169 (1997) 12. Sera, K., Goto, S., Takahashi, C., Saitoh, Y., Kinoshita, K., Matsumasa, M.: Quantitative analysis of small bio-samples of nearly 1 lg. Int. J. PIXE 24–3(4), 161–175 (2014) 13. Sera, K., Futatsugawa, S., Matsuda, K.: Quantitative analysis of untreated bio-samples. Nucl. Instr. Meth. B 150, 226–233 (1999) 14. Sera, K., Futatsugawa, S., Murao, S.: Quantitative analysis of untreated hair samples for monitoring human exposure to heavy metals. Nucl. Instr. Meth. B 189, 174–179 (2002) 15. Sera, K., Futatsugawa, S., Ishiyama, D.: Application of a powdered-internal-standard method combined with correction for self-absorption of X-rays to geological, environmental and biological samples. Int. J. PIXE 9, 63–81 (1999) 16. Sera, K., Futatsugawa, S.: Effects of X-ray absorbers designed for some samples in PIXE analyses. Int. J. PIXE 5, 181–193 (1995) 17. Sera, K., Terasaki, K., Itoh, J., Saitoh, Y., Futatsugawa, S.: Physical quantitative analysis in in-air PIXE. Int. J. PIXE 17, 1–10 (2007) 18. Sera, K., Goto, S., Takahashi, C., Saitoh, Y.: Quantitative analysis of untreated oil samples in in-air PIXE. Int. J. PIXE 20, 77–84 (2010) 19. Sera, K., Goto, S., Takahashi, C., Saitoh, Y.: Standard-free method for living plants in in-air PIXE. Int. J. PIXE 21–1(2), 13–23 (2011) 20. Sera, K., Goto, S., Takahashi, C., Saitoh, Y.: Movement of light elements in living plants measured by means of a standard-free method in in-air PIXE. Int. J. PIXE 23–3(4), 77–91 (2013) 21. Sera, K., Suzuki, H., Sawabe, K., Komagata, O., Goto, S., Takahashi, C., Saitoh, Y.: Standard-free method for quantitative elemental analysis of mosquitoes and small flies. Int. J. PIXE 23–3(4), 93–109 (2013) 22. Sera, K., Goto, S., Takahashi, C., Saitoh, Y., Yamauchi, K.: Effects of heavy elements in the sludge conveyed by the 2011 tsunami on human health and the recovery of the marine ecosystem. Nucl. Instr. Meth. B318, 76–82 (2014) Chapter 71 Penning Ionization Electron Spectroscopy Takuya Hosokai Keywords Electronic structure Molecular orbital distribution Molecular orientation Particle–particle interaction 71.1 Principle Penning ionization electron spectroscopy (PIES) is based on the Penning ionization (PI) phenomenon caused by the interaction between excited metastable atoms and semiconducting or insulating surfaces (Fig. 71.1) [1]. When the excited metastable atoms, typically excited helium atoms in the triplet state [He*(23S; 19.82 eV)], approach the materials, the valence band electron at the surface of the materials (UA, UB, …) transfers to the 1s level of the metastable atom (va) via tunneling. According to the energy conservation law, the energy loss of the transferred valence electron is subsequently used for the electron emission from the 2s level (vb) in the He* atoms. The ejected electron has a kinetic energy (Ek) relative to the vacuum level (Evac); the electron moves to a continuum state in the final state (We) upon the ionization process. As in ultraviolet photoelectron spectroscopy (UPS), the analysis of the kinetic energy using an electron spectrometer provides the valence band spectrum of the material’s surface. A distinct difference between PIES and UPS is the surface sensitivity. While UPS detects photoelectrons emitted from typically *1 nm in depth, PIES is only sensitive to the electrons present at the outermost surface of the solid samples. Therefore, PIES illustrates the spatial distribution of the atomic orbitals (AOs) or molecular orbitals (MOs) of samples (Fig. 71.1b). This means that if molecules are ordered on a solid surface, the PIES intensity provides information on the molecular orientation or substituents protruded at the outermost surface region (Fig. 71.1c). T. Hosokai (&) Research Institute for Measurement and Analytical Instrumentation (RIMA), National Institute of Advanced Industrial Science and Technology (AIST), Ibaraki, Japan e-mail: t.hosokai@aist.go.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_71 435 436 T. Hosokai Fig. 71.1 (a) De-excitation mechanism of a metastable atom through Penning ionization. (b) Interaction between a metastable He atom and diatomic molecules in the gas, and (c) solid phases with different molecular orientations. The corresponding PIES spectrum is shown below in each case PIES is also known as metastable atom electron spectroscopy (MAES), metastable-atom-induced electron spectroscopy (MAIS), metastable quenching spectroscopy (MQS), or metastable atom de-excitation spectroscopy (MDS). All the terms are the same, as the electron spectroscopy uses particles for electron ejection. 71.2 • • • • • Features Electron emission via Penning ionization: particle–particle interaction. Collision energy of metastable atoms is thermal: non-destructive. Ultrahigh surface sensitivity, only the outermost surface layer. Valence band electronic structure similar to UPS. Determination of molecular orientation at the outermost surface layer. 71.3 Instrumentation The measurement system consists of a metastable source chamber and an analysis chamber. The solid-state samples are inserted into the analysis chamber and are set to the measurement position using a manipulator. The metastable source chamber further consists of a source and a quench lamp chamber. The He gas metastable beams are usually produced by cold-discharge-type lamps. In the metastable helium operation, the cold cathode discharges yield mainly He*(23S) species. The discharge is maintained between a tantalum hollow cathode and a stainless-steel skimmer across a pressure gradient created by differential pumping (Fig. 71.2). 71 Penning Ionization Electron Spectroscopy 437 Fig. 71.2 Metastable atom source: (1) copper electrode, (2) Pyrex tube, (3) tantalum cathode, (4) boron nitride nozzle, (5) skimmer, (6) repeller grid, and (7) quench lamp [1] TMP: turbo molecular pump. The figure on the right is a picture of discharging of the metastable source Electrons originating due to the discharge are removed using a repeller grid. For the measurement of the He*(23S) spectra, He*(21S) atoms are quenched via the transition of 21S ! n1P ! 11S (n = 2, 3, 4, etc.) with the light from a helium discharge lamp (quench lamp). More than 99% of the He*(21S) atoms are quenched using this type of lamp. The He*(21S) spectrum can be extracted by taking the difference of the two spectra obtained with the quench lamp on and off. 71.4 Applications 71.4.1 Silicon Dioxide: The Case of an Inorganic Insulator A native silicon dioxide (SiO2) layer with a mean thickness of 1–2 nm is usually formed at the surface of Si wafers by a natural oxidization in the air. An excited He atom irradiated on such Si wafers interacts preferentially with the AOs of oxygen of SiO2. Figure 71.3 shows the PIES spectrum of the SiO2/Si wafers, where the He I UPS spectrum is compared. In the UPS spectrum, there are two peaks (B, C) and one shoulder (A), which is located at the lower binding energy side of B, whereas in the PIES spectrum the A is hardly observed. The features observed in UPS and PIES are assigned as follows: A and B are the non-bonding states of the O 2p orbitals distributed parallel (n║) and normal (n┴) to the SiO2 surface, and C is related to the r (Si-O-Si) bonding state [2]. The difference between the UPS and PIES spectra demonstrates the high-surface sensitivity of the PIES. That is, the He* atoms used as a probe in PIES interact preferentially with the n┴ orbital (B) compared to the n║ orbital (A), because the n┴ orbital spreads more toward the vacuum side of the SiO2 surface. 438 T. Hosokai Fig. 71.3 PIES and HeI UPS spectra of native SiO2/Si wafers [2]. The figure on the right is a schematic of the spatial distribution of the non-bonding orbitals of O atoms Fig. 71.4 (a) PIES spectra of monolayer and bilayer films of ClGaPc (inset) deposited on HOPG. (b) Schematic of molecular orientation and important MO distributions in each film structure in (a) [3] 71.4.2 Polar Organic Molecules on Graphite: An Example of the Determination of Molecular Orientation at the Outermost Surface Layer Chlorogallium phthalocyanine (ClGaPc, Fig. 71.4a inset) is a polar organic metal complex that has the ClGa single bond centered in the Pc ring. On a highly oriented pyrolytic graphite (HOPG), the first monolayer of the ClGaPc molecules shows the Cl atom in the polar ClGa bond directed outward to the vacuum, while the bond is hidden in the second monolayer because of the formation of a stacked bilayer structure (Fig. 71.4b) [3]. PIES enables us to determine such a change in the 71 Penning Ionization Electron Spectroscopy 439 molecular orientation. In Fig. 71.4a, the monolayer PIES spectrum shows very intense C and D bands, which are ascribed to MOs related mainly to the n║ and n┴ orbitals of the Cl atom distributed parallel and perpendicular to the Pc plane, respectively. Spatial distributions of these MOs are shown in Fig. 71.4b. As mentioned, the first monolayer consists of well-ordered molecules with Cl-up orientation, while the second layer in the bilayer films is composed of the molecules with Cl-down orientation. Therefore, the first monolayer spectrum shows strong C and D bands because the He atoms can interact preferentially with the n║(Cl) and n┴(Cl) orbitals protruding to the vacuum side for the Cl-up orientation. On the other hand, these bands appear very weak in the bilayer spectrum, where the n║(Cl) and n┴(Cl) orbitals are hidden in between the paired molecules and are not exposed to the vacuum side (Fig. 71.4b). 71.4.3 Alkanethiols on Metal Surface: Detection of Wave function Spread of Metal-Organic Hybrid State The ability of PIES to detect the wave function spread of AOs and MOs is also effectively demonstrated for metal-organic hybrid systems. Figure 71.5a shows the MAES (PIES) spectra of alkanethiol (CnH2n+1SH, n = 1–3) monolayer Fig. 71.5 (a) MAES (PIES) spectra of an alkanethiolate (CH3S, C2H5S, and C3H7S) monolayer on Pt(111). The vertical arrow indicates the energy position of the 3p-derived band observed in the UPS spectra (not shown). (b) A schematic view of PI on the C3H7S monolayer on Pt(111). Reprinted from ref. [4], Copyright 2015 with permission from Elsevier 440 T. Hosokai chemisorbed on a Pt(111) surface reported by Aoki et al. [4]. In Fig. 71.5a, the S3p-derived band is not prominent for the C2H5S and C3H7S systems despite the clear appearance of alkyl-derived bands. This is because the thiolate species are anchored to the substrate via the S-Pt bond, and He* atoms cannot easily access to the AOs of the inner S atoms (see Fig. 71.5b). Nevertheless, the metal-organic hybrid state can be observed for all the thiolate species as a weak band extending up to the Fermi level (EF). Aoki et al. assigned the hybrid state to a chemisorption-induced gap state localized near the S-Pt bond on the basis of first-principles calculations. The He*(23S) atoms de-excite on the thiolate overlayer via resonance ionization following Auger neutralization, which is a process yielding a self-convoluted spectrum by two final-state photoholes and competes with the PI, giving a broad and featureless PIES spectrum [1]. This is quite remarkable for the CH3S monolayer because the He*(23S) atoms can access closer to the metal substrate upon such a short alkyl chain, yielding an effective overlap between the 2s orbital of He*(23S) and the metal wave functions exposed outside the substrate. References 1. Harada, Y., Masuda, S., Ozaki, H:, Electron spectroscopy using metastable atoms as probes for solid surfaces. Chem. Rev. 97, 1897–1952 (1997) 2. Hosokai, T., Mitsuo, N., Noro, S., Nakamura, T., Kera, S., Sakamoto, K., Ueno, N.: Thickness-dependent electronic properties and molecular orientation of diradical metal complex thin films grown on SiO2. Chem. Phys. Lett. 487, 67–70 (2010) 3. Hosokai, T., Machida, H., Gerlach, A., Kera, S., Schreiber, F., Ueno, N.: Impact of structural imperfections on the energy-level alignment in organic films. Phys. Rev. B. 83, 198310(1–7) (2011) 4. Aoki, M., Masuda, S.: Local electronic structure at organic-metal interface studied by UPS, MAES, and first-principles calculation. J. Electron Spectrosc. Relat. Phenom. 204, 68–74 (2015) Chapter 72 Phase Mode SPM/AFM Hideo Nakajima Keywords AM-AFM 72.1 Dynamic mode Phase Amplitude Viscoelasticity Principle If the cantilever is vibrated at a constant amplitude and frequency (near the resonance frequency of the cantilever) and lightly pressed against the sample, the amplitude and phase will change with a correlation with the shape and physical properties of the sample surface [1]. The method of detecting and imaging the amplitude and phase change of the cantilever at this time is called Phase Mode. Especially in the phase image, images that are not mere surface shapes can be obtained. The main part related to observation such as cantilever motion and feedback control is the same as normal amplitude modulation atomic force microscope (AM-AFM, or known as Dynamic mode, Tapping mode, etc.). The phase image that can be acquired in Phase Mode contains various information. Especially for specimens that are relatively soft materials like polymeric materials and have a smooth surface shape, images with viscoelastic-information dominant can be acquired. However, depending on the type of the cantilever, the strength of the phase signal may be reversed, so care must be taken in interpreting the data. For relatively hard specimens, such as metals and ceramics or specimens with large surface roughness, shape-dominant images can be acquired. In this case, the influence of the waviness of the surface shape becomes small, which helps observe the fine structure. Another amplitude image is an error image with the same feedback as the deviation image. H. Nakajima (&) Global Application Development Center Analytical & Measuring Instruments Division, Shimadzu Corporation, Kyoto, Japan e-mail: hidero@shimadzu.co.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_72 441 442 72.2 H. Nakajima Features – Image data can be acquired with the same operation principle as Dynamic Mode – Visualization of physical property information of polymer material is possible – Visualization of fine structure of surface shape is possible 72.3 Instrumentation In the dynamic mode, the cantilever is vibrated at a constant frequency (near the resonance frequency of the cantilever) by a vibrator installed near the cantilever. When actually scanning the sample surface, feedback control is performed in the height direction so that the amplitude of the cantilever becomes constant. At this time, the excitation frequency component of the cantilever is taken out from the cantilever position signal (vertical deviation signal of the detector) by a lock-in detector to obtain signals of Asind, Acosd, amplitude (A), and phase (d) (Fig. 72.1). Here, the amplitude (A) means the amplitude of the vibrated cantilever, and the phase (d) means the phase delay with respect to the excitation signal. The phase is displayed as an image that shows topographical information and differences in properties across the sample surface, such as viscoelasticity. Because the phase combines a variety of information, this mode provides a useful way to easily observe the differences in properties and minute topography on the sample surface (Fig. 72.2). Asind and Acosd are intermediate signals for detecting amplitudes and phases, and it is difficult to make sense. However, it is a useful signal for actually operating the device because it serves as a measure to stably acquire amplitude and phase signals. 72.4 Applications 72.4.1 Imaging of Viscoelasticity of Polymer Film An example of observing a special polymer film in Phase Mode is shown in Fig. 72.3. In the topographic image (a), the gentle shape of the polymer film is observed. In the phase image (b) of the same field of view as in (a), the contrast which was not seen in the shape image is obtained. This represents the difference in viscoelasticity of the material present on the surface, and it can be observed in three phases separately. 72 Phase Mode SPM/AFM 443 Fig. 72.1 Diagram of the phase detection system Phase signal Fig. 72.2 shows schematic view of Phase Mode. Since a phase delay occurs due to the effect of physical properties and/or fine shape of the sample surface depending on the position, contrast is obtained as an image Fig. 72.3 (a) is a topographic image of a polymer film. Field of view is 10 lm x 10 lm. (b) is a phase image of the same field of view as (a). Differences in viscoelasticity which are not understood by the topographic image are observed 444 Reference 1. García, R., Pérez, R.: Surf. Sci. Rep. 47, 197–301 (2002) H. Nakajima Chapter 73 Photoelectron Diffraction Fumihiko Matsui and Tomohiro Matsushita Keywords Atomic structure Local electronic structure 73.1 Subsurface Adsorbate Element selective Principle The discontinuity of bulk properties at material surfaces and interfaces can give rise to various useful functionalities. The visualization of the three-dimensional atomic arrangement of such structures is essential in materials science and engineering. Photoelectron diffraction (PED) is an element-selective method for local surface structure analysis [1]. The technique uses photoelectrons with kinetic energies in the range of 100 eV–1 keV to analyze the surface; these have an inelastic mean free path on the order of nanometers, corresponding to an escape depth of several atomic layers [2]. The quantum mechanical interference between the photoelectron direct wave emitted from an excited atom, W0 , and the waves scattered by surrounding n 2 P atoms, WS ¼ n uS , are detected as a PED pattern, I ¼ W0 þ WS . The initial i¼1 i core-level atomic orbital and the wave and electric vectors of the incident X-ray determine the quantum numbers of W0 , while the amplitude and phase shift of uSi depend on the i-th atom’s potential and its position. PED patterns from atoms with identical core levels are detected simultaneously; thus, this method is most effective when applied to a system with a preferential orientation of atomic arrangement surrounding every atom that emits photoelectrons. Long-range-ordered structures, such as periodic crystal surfaces, are not a mandatory condition for the technique; F. Matsui (&) Graduate School of Materials Science, Nara Institute of Science and Technology, Nara, Japan e-mail: matui@ms.naist.jp T. Matsushita Japan Synchrotron Radiation Research Institute, Hyogo, Japan e-mail: matusita@spring8.or.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_73 445 446 F. Matsui and T. Matsushita Fig. 73.1 (a) A schematic diagram of forward-focusing peaks and diffraction rings in the photoelectron diffraction pattern. (b) Cu fcc photoelectron diffraction pattern hence, atomic arrangements can be analyzed not only for crystal surfaces, but also for impurity atoms occupying specific sites in the crystal lattice, as well as adsorbate molecules and epitaxial films on the crystal surfaces. PED is a versatile technique, and by changing the energies of the incident photoelectrons and using different scan modes, various types of data can be acquired. When the photoelectron kinetic energy is larger than several hundred eV, forward-focusing peaks (FFPs) and the diffraction rings around them appear in the direction going from the excited atom to the neighboring atoms, as shown in Fig. 73.1; therefore, this allows the investigation of subsurface local structures by PED, using an angular scan mode. In the lower kinetic energy regime, backscattered interference patterns become dominant; using lower energies, the surface adsorption sites and the height of adsorbate from the substrate can be studied using an energy scan mode of PED [3, 4]. Detailed information on local atomic structures can be deduced by comparing measured PED patterns with simulated ones based on possible structural models derived using multiple scattering theory [5, 6]; furthermore, an alternative approach allowing direct visualization of the local atomic arrangement from PED patterns has been developed, known as photoelectron holography; this is described in more detail in the photoelectron holography section. 73.2 Features • Element selective: local- and site-selective three-dimensional atomic arrangement. • Surface sensitive: subsurface dopant structure, adsorbate structure, thin films. • Soft X-ray synchrotron radiation and Mg/Al X-rays are used as an excitation source. 73 Photoelectron Diffraction 73.3 447 Instrumentation Two main measurement methods have been developed for PED: angular scan mode with fixed photon energy, and energy scan mode with fixed detection geometry. Angular scans require a multi-axis, PC-controlled sample manipulator for azimuthal and polar orientation scans, a high-angular-resolution energy analyzer (1 ° or better), and a Mg or Al Ka X-ray source. On the other hand, for energy scan mode measurements, synchrotron radiation is essential. Typical energy ranges used are 50–300 eV for the structural analysis of surface adsorbates, and 300–1200 eV for subsurface regions and thin films, while higher energies are required for analysis of bulk impurities. Display-type spherical mirror analyzers [7], an example of which is shown in Fig. 73.2, enable direct observation around 1p-steradian (60 ) by PED without changing the angles of incident light or the sample orientation. Thus, this apparatus is suitable for studying transition matrix element effects. The analyzer consists of a hemispherical main grid (MG), an outer sphere with obstacle rings (OR), and guard rings (G). Synchrotron light radiation (SR) used for sample excitation is introduced through a hole. The electrostatic field in the space surrounded by the MG, OR, and G is made spherically symmetric with respect to the center of analyzer. The orbits of the electrons emitted from the sample inside the main grid become straight lines while they are subjected to this concentric field after passing through the main grid, and their loci are ellipsoidal, obeying the Kepler’s law. Only electrons with specific kinetic energy converge exactly at the exit aperture and are projected onto a phosphorous screen. The PED patterns are captured by a CCD camera located outside the vacuum chamber. Fig. 73.2 (a) Photograph and (b) schematic diagram of display-type analyzer installed at BL25SU, SPring-8. Smp: sample, RG: retarding grid, MCP: microchannel plates 448 73.4 F. Matsui and T. Matsushita Applications Surface structure analysis: Through high-energy-resolution PED measurements, the local structures of sites with different chemical environments can be investigated. Westphal et al. have clarified the local structure of oxidized silicon surfaces by distinguishing between each chemically shifted Sin+ component [8]. Stereo photograph of atomic arrangements: When a core level is excited by circularly polarized light, the angular momentum of light (helicity) is transferred to an emitted photoelectron. As a result, a shift in the FFP direction inversely proportional to the interatomic distance occurs. This angular circular dichroism of FFP, which is equivalent to the “parallax shift” of photoelectron scattering atoms as seen in the perspective from the photoelectron emitter atom (depicted in Figs. 73.3a, b), can also be used for distance determination [9]. Figures 73.3c, d show examples acquired from an InSb crystal lattice. Diffraction Spectroscopy: Since diffraction patterns differ according to the atomic arrangements surrounding emitter atom sites, these can be “fingerprinted” by their characteristic diffraction patterns. This method enables access to atomic sites located in the subsurface or at buried interfaces, which cannot be accomplished using scanning probe microscopy. Photoelectron and X-ray absorption spectroscopies are also powerful tools for the analysis of surface composition and electronic structure, as all the information contained within the region corresponding to the electron escape depth is mixed in with the spectra. However, by utilizing diffraction information, these data can be disentangled into atomic-site-specific spectra. By combining this diffraction technique with core-level spectroscopy, one can obtain access to the structural details of each atomic site and derive their Fig. 73.3 Schematic diagrams of (a) parallax and (b) the forward-focusing peak shifts on excitation by circularly polarized light. (c) In 3d and (d) Sb 3d PED patterns toward the [111] direction excited by light with left and right helicity. By visual comparison of the two panels (left and right), one can directly identify the different distances between the central atoms 73 Photoelectron Diffraction 449 Fig. 73.4 A series of Ni LMM Auger electron diffraction patterns measured from a Ni thin film grown on a Cu (001) surface at different thicknesses. Atomic layer resolved X-ray absorption spectra for different Ni film thickness were separated by using FFPs. The L3 absorption edge shift corresponding to the surface core-level shift was derived for every atomic site in the Ni thin film [10] individual electronic properties. Auger electrons emitted upon core-level excitation can also be used as an element-specific probe for local structure, as FFPs and diffraction rings appear not only in PED patterns, but also in Auger electron diffraction patterns. By combining Auger electron diffraction and X-ray absorption spectroscopy, surface and subsurface X-ray absorption and magnetic circular dichroism (XAS/XMCD) spectra for each site can be separated. Figure 73.4 shows an example of an application for these techniques for studying the electronic and magnetic structures of a Ni thin film at an atomic level [10], in which surface and interior core-level shifts and magnetic moments are determined individually for each atomic layer around the spin-reorientation transition. References 1. Hüfner, S.: Photoelectron Spectroscopy 3 ed. Springer (2003). Fadley, C. S.: X-ray photoelectron spectroscopy: Progress and perspectives. J. Electron Spectrosc. Relat. Phenom. 2(32), 178–179 (2010) 450 F. Matsui and T. Matsushita 2. Tanuma, S.: Electron attenuation lengths in surface analysis by auger and x − ray photoelectron spectroscopy, In: Briggs, D., Grant, J.T., IM Publications and Surface Spectra Limited, pp. 259–294 (2003) 3. Greber, T., Wider, J., Osterwalder, J.: X-ray photoelectron diffraction in the backscattering geometry: a key to adsorption sites and bond lengths at surfaces. Phys. Rev. Lett. 81, 1654 (1998) 4. Woodruff, D.P.: Adsorbate structure determination using photoelectron diffraction. Surf. Sci. Rep. 62, 1–38 (2007) 5. García de Abajo, F.J., Van Hove, M.A., Fadley, C.S.: Multiple scattering of electrons in solids and molecules: A cluster-model approach. Phys. Rev. B. 63, 75404 (2001) 6. Matsushita, T., et al.: Photoelectron holography with improved image reconstruction. J. Electron Spectrosc. Relat. Phenom. 178–179, 195–220 (2010) 7. Daimon, H.: New display-type analyzer for the energy and the angular distribution of charged particles. Rev. Sci. Instum. 59, 545 (1988) 8. Dreiner, S. et al.,: Local atomic environment of si suboxides at the sio2/si(111) interface determined by angle-scanned photoelectron diffraction. Phys. Rev. Lett. 86, 4068 (2001); Structural Analysis of the SiO2/Si(100) Interface by Means of Photoelectron Diffraction ibid. 93, 126101 (2004) 9. Daimon, H.: “Stereoscopic microscopy of atomic arrangement by circularly polarized-light photoelectron diffraction”. Phys. Rev. Lett. 86, 2034 (2001) 10. Matsui, F., Matsushita, T., Kato, Y., Hashimoto, M., Inaji, K., Daimon, H.: “Atomic-layer resolved magnetic and electronic structure analysis of ni thin film on a cu(001) surface by diffraction spectroscopy” phys. Rev. Lett. 100, 207201 (2008) Chapter 74 Photoelectron Holography Tomohiro Matsushita and Fumihiko Matsui Keywords Atomic structure 74.1 Element-selective Dopant Adsorbate Principle Optical holograms are widely used in our daily life. Three-dimensional structural information is recorded in an optical hologram based on the wave nature of light, and we can see the 3D image on the hologram. Similarly, 3D atomic arrangements can be recorded using the electron wave. When an atom is excited with an X-ray, a photoelectron is emitted. The photoelectron from a localized core level is an excellent element-specific probe for the analysis of atomic structure. Information on the photoelectron-emitting atom and the surrounding atomic configuration is recorded as a photoelectron hologram in the photoelectron intensity angular distribution [1]. Photoelectron holography is a technique for deriving real-space atomic structures from photoelectron diffraction. A three-dimensional atomic image around the photoelectron emitter atom can be obtained directly from the hologram by the following reconstruction calculation method, without assuming any structural models. This technique can be applied to systems with preferential orientations of atomic arrangements around the photoelectron emitter atom. Long-range ordered structure, such as a periodic crystal surface, is not a mandatory condition. Therefore, the atomic arrangements of not only crystal lattices, but also dopant structures in crystals, thin films, and adsorbates on crystalline surfaces can be visualized. T. Matsushita (&) Japan Synchrotron Radiation Research Institute, Hyogo, Japan e-mail: matusita@spring8.or.jp F. Matsui Graduate School of Materials Science, Nara Institute of Science and Technology, Nara, Japan e-mail: matui@ms.naist.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_74 451 452 T. Matsushita and F. Matsui Fig. 74.1 Schematic of photoelectron holography. The measured hologram can be expressed as the sum of the scattering functions of each atom The measured hologram vðkÞ can be approximated by the sum of holograms formed by each atom, as shown in Fig. 74.1, Z vðkÞ ¼ tðk; rÞgðrÞdr; ð74:1Þ where gðrÞ represents the atomic distribution function and tðk; rÞ is a scattering pattern function formed by one scatterer atom located at the position P given by the vector r. When the atomic distribution function is set to gðrÞ ¼ i dðr ai Þ, Eq. (74.1) gives the hologram, where ai is a position vector. Since this approximation ignores multiple scattering, it holds in the case of photoelectrons with kinetic energy of at least a few hundred eV. When the hologram is given, a three-dimensional atomic image gðrÞ can be obtained by solving Eq. (74.1). For solving this equation, a scattering pattern extraction algorithm using maximum entropy (SPEA-MEM [2]) or L1 regularization (SPEA-L1 [3]) is effective. In order to obtain a reliable atomic image, it is necessary to measure the photoelectron emission distribution over 2p steradians. 74.2 • • • • Features Three-dimensional atomic structure. Element-selective. Dopants and adsorbate structures. Surface sensitive. 74 Photoelectron Holography 74.3 453 Instrumentation The instruments required for photoelectron holography are the same as required for photoelectron diffraction. Refer to “Photoelectron Diffraction” section. 74.4 Applications 74.4.1 Bimetal-intercalated graphite superconductor surface [4] Figure 74.2 shows an example of PEH for bimetal-intercalated graphite (Ca, K)C8. The atomic graphene images were acquired at 3.3 and 5.7 Å above the photoelectron emitter (C), and the stacking structures were determined to be AB type and AA type, respectively. The atomic image formed by K 2p photoelectron holography revealed 2 2 periodicity. Fig. 74.2 Atomic structure of bimetal-intercalated graphite 454 T. Matsushita and F. Matsui Fig. 74.3 Element structure reconstructed from an InP photoelectron hologram 74.4.2 Element structure around the emitter atom [5] The scattering pattern function depends slightly on the atomic number Z. By using this feature, it is possible to identify the elements of the scattering atoms (Figure 74.3). References 1. Szöke, A.: In Short Wavelength Coherent Radiation: Generation and Applications, AIP. Conf. Proc. 147, 361–367 (1986) 2. Matsushita, T., Yoshigoe, A., Agui, A.: Electron holography: a maximum entropy reconstruction scheme. Europhys. Lett. 71, 597–603 (2005). Matsushita, T., Guo, F.Z., Matsui, F., Kato, Y., Daimon, H.: Three-dimensional atomic-arrangement reconstruction from an Auger-electron hologram. Phys. Rev. B. 75, 085419 (2007). Matsushita, T., Guo, F.Z., Suzuki, M., Matsui, F., Daimon, H., Hayashi, K.: Reconstruction algorithm for atomic-resolution holography using translational symmetry. Phys. Rev. B. 78, 144111 (2008). Matsushita, T., Matsui, F., Daimon, H., Hayashi, K.: “Photoelectron holography with improved image reconstruction.” J. Electron. Spectrosc. Relat. Phenom. 178–179, 195–220 (2010) 74 Photoelectron Holography 455 3. Matsushita, T.: Atomic Image Reconstruction from Atomic Resolution Holography Using L1Regularized Linear Regression. e-J. Surf. Sci. Nanotech. 14, 158–160 (2016) 4. Matsui, F., Eguchi, R., Nishiyama, S., Izumi, M., Uesugi, E., Goto, H., Matsushita, T., Sugita, K., Daimon, H., Hamamoto, Y., Hamada, I., Morikawa, Y., Kubozono, Y.: Photoelectron holographic atomic arrangement imaging of cleaved bi metal-intercalated graphite superconductor surface. Sci. Rep. 6, 36258 (2016) 5. Matsushita, T., Matsui, F., Goto, K., Matsumoto, T., Daimon, H.: Element assignment for three-dimensional atomic imaging by photoelectron holography. J. Phys. Soc. Jpn. 82, 114005 (2013) Chapter 75 Photoelectron Yield Spectroscopy Hisao Ishii Keywords Ionization energy Work function Environmental measurement Gap states 75.1 Photoelectron Principle PYS is a method to measure the ionization energy of materials (work function in the case of metals) by using photoemission process. A sample surface is irradiated by tunable UV light, and the number of emitted photoelectrons is measured. The quantum yield of photoelectron (Y), which is the number of emitted photoelectrons per photon absorbed, is detected as a function of incident photon energy (hm). The principle is shown in Fig. 75.1 for the case of a material with an energy gap. When hm becomes greater than the threshold ionization energy (Ith) during incremental hm scan, the value of Y starts to increase. Thus, by determining the threshold of the spectrum, the value of Ith can be evaluated. In the case of metal sample, the work function of the sample can be deduced in the similar way. The threshold region of the yield spectrum can be approximated as Y / ðhm Ith Þn , here n depends on the class of the sample materials: n = 2 for metals, n = 1, 3/2, 2, 5/2 for semiconductors. For organic materials, n = 3 is proposed. Practically, the threshold can be determined by linear extrapolation of Y1/n plot. By selecting proper detection system, this method can be performed not only in vacuum, but also in atmospheric condition in contrast to conventional photoelectron spectroscopy. PYS can be also applied to extremely insulating sample by applying sample bias voltage. For the details, see references in ref [1]. H. Ishii (&) Center for Frontier Science, Chiba University, Chiba, Japan e-mail: ishii130@faculty.chiba-u.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_75 457 458 H. Ishii Fig. 75.1 Principle of PYS. The total yield of photoelectron is measured as a function of incidence photon energy. The threshold energy corresponds to ionization energy of sample surface 75.2 Features • Ionization energy (work function in the case of metal) can be determined in both vacuum and atmospheric environments. • The measurement is possible even for extremely insulating materials. • High-sensitivity detection of the photoemission from very weak density of states by using electron multiplier is possible. • By using atmospheric detection, the ionization energy of liquid samples can be measured. 75.3 Instrumentation There are several types of method to detect photoelectrons. In high (or ultrahigh) vacuum, electron multiplier is often used to get high sensitivity. With careful arrangement of signal lines to reduce noise, fA-level ammeter can be also used to measure the current due to photoemission (current-mode measurement). To realize atmospheric measurement, a variation of current-mode measurements has been proposed with maneuver to reduce the space charge near the sample surface in atmosphere such as the application of electric field and gas flow. A special pulse counter of oxygen anions, which are formed by the attachment of photoelectrons to oxygen molecule in air, is also applied to PYS. An example of PYS apparatus is shown in Fig. 75.2. The apparatus consists of a light source, main chamber, preparation chamber, and atmospheric chamber. Monochromatized photon from Xe (500 W) or D2 (150 W) lamps is used as an excitation light. To suppress the absorption of the light by oxygen, the inside of the 75 Photoelectron Yield Spectroscopy 459 Fig. 75.2 An apparatus of PYS and high-sensitivity photoemission spectroscopy monochromator is purged by dry nitrogen gas, and photon energy can be scanned from 1.5 to 8 eV. Aiming at eliminating stray light, a zero-dispersion type of double monochromator was adopted (the exclusion of stray light is essential to detect very weak states like gap states). Excitation light is introduced to the main chamber. By using electrical feedthrough with guard line, the sample is connected to sub-pico ammeter (Keithley 6430) to measure the photocurrent. An electron multiplier is also equipped to perform high-sensitivity measurement of PYS. By switching mirror, the excitation light can be introduced into the atmospheric chamber, where PYS is performed in atmospheric condition. The light is incident vertically onto the sample surface. A ring-shaped electrode to extract electrons is set in front of the sample. By negatively biasing the sample, the photoemission can be kept even in air. For the details, see references in ref [1]. 75.4 Applications 75.4.1 Determination of Work Function In the case of metal sample, the yield curve shows the so-called Fowler function curve. Fowler described the yield function of photoemission from metal sample with work function as the following equations. 460 H. Ishii hm U Y hm U Y ¼ bT F ; ln 2 lnb ¼ ln F kT T kT 2l 3l e e FðlÞ ¼ el 2 þ 2 for l 0 2 3 p2 1 2 e2l e3l þ l el 2 þ 2 for l 0: ¼ 2 6 2 3 2 Fig. 75.3 PYS spectra of a gold film in Fowler plot (a) and square root plot (b) (a) (b) 75 Photoelectron Yield Spectroscopy 461 Fig. 75.4 PYS spectra of a film and single crystal of rubrene [2] film single crystal 0.4 0.2 Yield 1/3 /arb. units 0.6 0.0 -0.2 4.8 5.2 5.6 6.0 Photon Energy/ eV Figure 75.3 shows an example of PYS spectrum for a practical (contaminated) gold film. Figure 75.3(a) is in a Fowler plot, where the work function was determined as 4.26 eV by the Fowler function curve (solid line). If the hm is not close to the threshold, the equation can be approximated as quadratic function Y / ðhm UÞ2 . Thus, the value of U can be easily determined as the threshold in hm-Y1/2 plot as shown in Fig. 75.3(b). 75.4.2 Measurement of Organic Film and Crystal PYS spectra of the evaporated amorphous film and single crystal of rubrene are shown in Fig. 75.4[2]. By assuming Y / ðhm Ith Þ3 relation, after background subtraction, the cubic root of yield is plotted as a function of photon energy hm. From linear extrapolation, Ith of the evaporated film was determined as 5.3 eV, while that of single crystal was 4.9 eV. This result indicates that the ionization energy of evaporated film is not good measure of that of the single crystal: the direct observation of organic single crystals is necessary although conventional photoemission spectroscopy is not easy to perform due to charge-up problem for thick insulating crystal. 462 H. Ishii Fig. 75.5 PYS spectra of low-density polyethylene. Inset is the obtained energy diagram 75.4.3 Observation of Gap States in Energy Gap As an example of gap state observation, PYS spectra of low-density polyethylene film (20 lm thick) are shown in Fig. 75.5. Yield variation in 7 orders of magnitude was clearly observed, indicating the existence of gap states in HOMO-LUMO gap. The threshold is about 3.8 eV, while the ionization energy is 8.5 eV. The observed states are essential to discuss the properties of polyethylene such as contact charging nature. This kind of high sensitivity for gap states has been also applied to various inorganic semiconductors. 75 Photoelectron Yield Spectroscopy 463 References 1. Regarding PYS, see the following review articles, and references therein: Ishii, H., Kinjo, H., Sato, T., Machida, S., Nakayama, Y.: Photoelectron yield spectroscopy for organic materials and interfaces, Chap. 8 (pp. 131–155). In: Ishii, H., Kudo, K., Nakayama, T., Ueno, N. (eds.) Electronic processes in organic electronics: bridging nanostructure, electronic states and device properties. Springer (2015) 2. Nakayama, Y., Machida, S., Minari, T., Tsukagoshi K., Noguchi, Y., Ishii, H.: Direct observation of the electronic states of single crystalline rubrene under ambient condition by photoelectron yield spectroscopy. Appl. Phys. Lett. 93(17) (2008) 173305-1*3 Chapter 76 Photoemission Electron Microscope Toyohiko Kinoshita Keywords Microscope 76.1 Imaging Photoemission Principle PEEM is one of the imaging type photoelectron microscopy [1]. The apparatus is equipped with some electrostatic lens systems, a microchannel plate (MCP) and a fluorescent screen. When excitation photons are injected onto a sample, photoelectrons including secondary electrons are emitted. The lens systems magnify and focus the images of spatial distributions of these electrons from the sample onto the MCP. Then the screen is illuminated by these amplified electrons. By using a charge coupled device (CCD) camera, a magnified image of the emitted electron distributions from the sample surface can be obtained. When a mercury lamp or a deuterium lamp is used as an excitation source, the distribution of the local work function of the surface becomes visible, since the photon energy is about 4 eV. When the excitation light is synchrotron radiation light (SR) and its energy is tuned around the absorption edge of the materials, element-specific imaging is possible. This is because the number of emitted electrons is proportional to the absorption coefficient, which depends on the material. When magnetic circular or linear dichroism effect [2–4] in X-ray absorption edge (XMCD or XMLD) is used, magnetic domain imaging becomes possible. The schematic view in Fig. 76.1 shows the principle of PEEM, especially when magnetic domain imaging is performed. An apparatus equipped with an electron energy analyzer is also developed [5]. If an electron gun is equipped in this setup, low-energy electron microscopy (LEEM) [5, 6] is also available. By using such an apparatus, the photoemission spectra from a small area of sample surfaces can be obtained. T. Kinoshita (&) Japan Synchrotron Radiation Research Institute (JASRI), SPring-8, Hyogo, Japan e-mail: toyohiko@spring8.or.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_76 465 466 T. Kinoshita 76.1.1 Features • The spatial resolution is relatively good, in the order of several tens of nanometers. • Element-specific imaging is possible when tenability of photon energy is used. • The real-time observation can be performed if the incident light is very brilliant. The pump–probe technique is also applicable for dynamical observation. • By using linearly or circularly polarized light, magnetic domain imaging is possible. The method does not apply any magnetic field to the sample, in principle. Therefore, imaging of soft magnetic materials is possible. Imaging of antiferromagnetic domains is also possible. 76.1.2 Instrumentation The apparatus is very simple. As shown in Fig. 76.1, PEEM is equipped with some electrostatic lenses. Usually, it consists of an objective lens, an intermediate lens, and projective lens. In some case, deflector plates, stigmator, and a high-pass filter mesh are equipped in order to obtain images with high quality. 30 * 100 nm of spatial resolution has been reported for this type of simple apparatus. An apparatus equipped with an electron energy analyzer is also available. Figure 76.2 shows a schematic diagram of such an apparatus. In this setup, an electron gun for low-energy electron microscopy (LEEM) is also installed. Not only the electrostatic Fig. 76.1 Schematic view to explain the principle of PEEM, especially when magnetic domain imaging is performed. Depending on the magnetization of each domain and the polarization vector of incident light, numbers of the emitted photons are different. Obtained image reflects the magnified spatial contribution of emitted electrons from the sample 76 Photoemission Electron Microscope 467 lenses but also the magnetic ones are used. In addition, a transfer lens system is present. The analyzer is of the hemispherical type. By using such an apparatus, the photoemission spectra from a small area of sample surfaces can be obtained. This apparatus has two operational modes: an imaging mode and a diffraction mode. In the imaging mode, a real-space image is obtained on the screen. In contrast, in the diffraction mode, information on the reciprocal lattice can be obtained. In this mode, low-energy electron diffraction (LEED), electronic band dispersion, and photoelectron diffraction patterns from local areas can be observed. The analyzer is also useful for the reduction in the extent of chromatic aberration. The * 20 nm of spatial resolution has been reported for this type of setup. Recently, a mirror corrector for the reduction in spherical aberration is realized, which can be installed between (8) and (9) in Fig. 76.2. In this setup, * 3 nm of spatial resolution is available when the CW type excitation light is used in order to prevent the space charge effect [7]. 76.2 Applications One of the specified and largest advantages of SR-assisted PEEM is that one can obtain element-specific magnetic domain images for both ferromagnetic and antiferromagnetic samples. In these studies, XMCD and XMLD-PEEMs have played important roles, because spin orientations in outermost surfaces, interfaces, and Fig. 76.2 Schematic diagram of a PEEM apparatus with hemispherical electron energy analyzer and an electron gun for LEEM measurements [1].The solid and dashed curves represent the electron trajectories for the imaging mode and diffraction mode, respectively. (1) Electron gun. (2) Condenser lens No. 1. (3) Condenser lens No. 2. (4) Condenser lens No. 3. (5) Beam separator. (6) Transfer lens. (7) Field lens. (8) Intermediate lens. (9) Projective lens No. 1. (10) Retarding lens. (11) Imaging lens No. 1. (12) Hemispherical analyzer. (13) Imaging lens No. 2. (14) Acceleration lens. (15) Projective lens No. 2. (16) Projective lens No. 3. (17) MCP and fluorescent screen. (18) Objective lens. (19) Sample. (20) Electron beam injection aperture. (21) Aperture for limiting field of views. (22) Contrast aperture. (23) Slit for analyzer 468 T. Kinoshita substrates can be observed experimentally. This is the advantage of PEEM with X-ray excitation over other microscopic methods. In modern society, we receive great benefit from magnetic recording devices such as memories and hard disks. These magnetic recording devices make use of the phenomenon called exchange bias, and its detailed clarification is necessary to use the phenomenon in a wider range of applications. For example, the observation of the exchange coupling of spins between an antiferromagnetic substrate and a ferromagnetic thin film formed on it is required. The element-specific observation of magnetic domains of various magnetic materials is realized by combining photoemission electron microscopy (PEEM), X-ray magnetic circular and linear dichroisms (XMCD & XMLD) [8]. In Fig. 76.3, an iron (Fe) thin film (thickness, *0.9 nm) was formed on a nickel oxide (NiO) substrate, which is known as a typical antiferromagnetic material, to analyze the magnetic structure. Using various states of magnetic dichroism at the absorption edges of elements constituting the system, information on (1) the magnetic domain originating from the antiferromagnetic distortion of the NiO substrate (yellow frame), (2) three magnetic domains (S1–S3) originating from spin ordering, (3) the ferromagnetic domain of the top layer of the Fe thin film, and (4) the ferromagnetic domain of the interface (in which Fe, Ni, and O are mixed) is obtained. The sample was rotated with respect to the incident direction of the light. From the change in the contrast of the obtained images, the spin directions in each magnetic domain were determined. The exchange coupling of spins among the substrate, top layer, and interface was clarified. This observation technique is considered to be applicable to the development of new materials for inductor circuits, in addition to magnetic recording. The PEEM technique is combined with time-resolved measurement of the movements of the magnetic domain and wall with respect to an external field. Some examples and details are introduced in refs [1–10]. The time-resolved measurements using laser are introduced in ref [11]. Fig. 76.3 Results of observation of magnetic domain in antiferromagnetic substrate and in Fe surface and interface of Fe/NiO(100). The arrows indicate the directions of magnetization (spins) in magnetic domains 76 Photoemission Electron Microscope 469 References 1. Kinoshita, T., Arai, K., Fukumoto, K., Ohkochi, T., Kotsugi, M., Guo, F.Z., Muro, T., Nakamura, T., Osawa, H., Matsushita, T., Okuda, T.: Observation of micro-magnetic structures by synchrotron radiation photoemission electron microscope. J. Phys. Soc. Jpn. 82, 021005/1-021005/24 (2013) 2. Nakamura, T., Suzuki, M.: Recent progress of the X-ray magnetic circular dichroism technique for element-specific magnetic analysis. J. Phys. Soc. Jpn. 82, 021006/1-021006/20 (2013) 3. See, for example, Thole, B.T., Van der Laan, G., Sawatzky, G.A.: Strong magnetic dichroism predicted in the M4, 5 X-ray absorption spectra of magnetic rare-earth materials. Phys. Rev. Lett. 55, 2086–2088 (1985) 4. Amemiya, K.: In this compendium 5. See, for example, Bauer, E.: LEEM and UHV-PEEM: a retrospective. Ultramicroscopy 119, 18–23 (2012) 6. Hibino, H.: In this compendium 7. Taniuchi, Y., Kotani, Y., Shin, S.: Ultrahigh-spatial-resolution chemical and magnetic imaging by laser-based photoemission electron microscopy. Rev. Sci. Instrum. 86, 023701/1-023701/5 (2015) 8. Arai, K., Okuda, T., Tanaka, A., Fukumoto, K., Hasegawa, T., Nakamura, T., Matsushita, T., Muro, T., Kakizaki, A., Kinoshita, T.: Direct observation of spin configuration in an exchange coupled Fe/NiO(100) system by X-ray magnetic circular- and linear- dichroism photoemission electron microscope. J. Appl. Phys. 110 (2011) 084306/1-0843-6/6 (2011) 9. Ohkochi, T., Yamaguchi, A., Hata, H., Goto, M., Nozaki, Y., Kotsugi, M., Nakamura, T., Osawa, H., Kinoshita, T.: Progress in time-resolved photoemission electron microscopy (PEEM) at BL25SU, SPring-8: Rf field excitation of magnetic vortex core gyration. Jpn. J. Appl. Phys. 51, 128001/1 * 128001/2 (2011) 10. Kinoshita, T., Ohkouchi, T., Osawa, H., Arai, K., Fukumoto, K., Okuda, T., Kotsugi, M., Muro, T., Nakamura, T., Matsushita, T.: Status of pump- and probe- time-resolved photoemission electron microscopy at the SPring-8. J. Electron Spectrosco. Relat. Phenom. 185, 389–394 (2012) 11. Kubo, A.: In this compendium Chapter 77 Photoluminescence Yuhei Miyauchi Keywords Semiconductor 77.1 Band gap Exciton Imaging Local conditions Principle Spontaneous emission (radiation) of light from an electronically excited material (except for thermal radiation) is called “luminescence.” In particular, photoluminescence (PL) is light emission occurring after absorption of shorter-wavelength light (higher-energy photons) [1]. In a typical direct-gap semiconductor1, electrons (solid circles) and holes (open circles) that are optically excited into conduction and valence bands, respectively, first relax to lower available energy levels by emitting multiple low-energy phonons and then relax to the ground state by recombination and emission of a photon (Fig. 77.1, ELaser and EPL are the excitation and emission photon energies, respectively). Since the photoexcited electron and hole are oppositely charged, they can attract each other through mutual Coulomb interaction and form a hydrogen-like electron–hole pair called an “exciton.” In a bulk semiconductor, the Coulomb binding energy of excitons is typically smaller than the thermal energy at room temperature (*26 meV), which allows rapid thermal dissociation of excitons, so clear exciton lines can be observed only under low-temperature conditions. In contrast, the PL spectra of various low-dimensional materials, such as atomically thin semiconductors and carbon nanotubes, are dominated by distinct PL features (shown in Applications) arising from excitons even at room temperature, because of large exciton binding energies due to reduced screening and strong quantum confinement effects [2, 3]. PL typically occurs from the lowest energy 1 For radiative electron–hole recombination in an indirect-gap semiconductor, additional phonon emission is involved to compensate the crystal momentum; this process causes slow radiative recombination of electrons and holes, and results in low PL efficiency. Y. Miyauchi (&) Institute of Advanced Energy, Kyoto University, Uji, Kyoto, Japan e-mail: miyauchi@iae.kyoto-u.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_77 471 472 Y. Miyauchi Fig. 77.1 Processes in PL of a direct-gap semiconductor conduction band ELaser EPL E k valence band levels for electrons and holes (or excitons). Thus, PL is quite sensitive to the existence of extrinsic states in a solid such as localized states induced by defects and impurities, which typically have lower energies than intrinsic states; this feature makes PL spectroscopy quite useful for inspection of semiconductors. PL spectroscopy is also frequently used in biology because of its high sensitivity to fluorophores in living organisms, even at the single-molecule level. 77.2 Features • Non- or less invasive measurements. • Applicable to various samples with wide ranges in size, optical density, and surface morphology. • High sensitivity to defects, vacancies, and impurities in semiconductors, and to fluorophores in living organisms. • PL excitation (PLE) spectroscopy is useful for probing optical transitions in various samples for which standard optical absorption spectroscopy is not applicable. • Interpretation of PL (and PLE) spectra is not as straightforward as that of absorption spectra because complicated relaxation processes are involved in PL phenomena. 77.3 Instrumentation A PL spectrometer mainly comprises an excitation light source, excitation and detection optics, a monochromator or spectrograph, photodetectors, and computers for data processing. Figure 77.2 shows a schematic of typical instrumentation for 77 Photoluminescence 473 Fig. 77.2 Typical instrumentation for micro-PL spectroscopy using a laser as the excitation source micro-PL spectroscopy using a laser as the excitation source (mirrors for transferring collimated light are omitted for simplicity). For time-domain PL measurements with time resolution of more than a few tens of picoseconds, the time-correlated single-photon counting method using a pulsed laser and a fast single-channel photodetector is widely used. 77.3.1 Excitation Light Source Typical excitation light sources for PL spectroscopy are lamps or lasers. When broadband light sources such as Xenon arc lamps or supercontinuum lasers are used, high-performance optical band-pass filters or monochromators are necessary to generate monochromatic excitation light with perfect elimination of unwanted wavelengths of light. For PLE spectroscopy, the excitation light wavelength is tuned using a monochromator (or tunable band-pass filters) placed after the broadband light source, and PL signals at a fixed wavelength (using a monochromator and a single-channel detector) or whole PL spectra (using a spectrograph and a multi-channel detector) are recorded as a function of the excitation wavelength. 474 Y. Miyauchi The excitation light power is kept constant for each excitation wavelength, or PL signals are normalized by the excitation light power at each wavelength. For time-domain PL measurements, picosecond or femtosecond lasers with high repetition rates are widely used as excitation sources. 77.3.2 Excitation and Detection Optics In a typical macro-PL spectrometer, mirrors are preferably used for focusing and collimating optics to minimize chromatic aberration. For measurements of liquid samples kept in a standard optical cell, detection is usually from the direction perpendicular to the excitation beam. For macro-PL measurements of solids under low-temperature conditions, the sample is placed in a cryostat, and the detection is from the direction in which direct reflection of the excitation beam by the sample surface is not detected. For the micro-PL measurement of small samples (Fig. 77.2), an objective lens is necessary to focus the excitation light and detect PL emission. The collected PL is sent to the monochromator or spectrograph using flat mirrors (omitted in Fig. 77.2) and focusing optics (mirrors or lenses). Optical filters (typically long-wavelength pass filters) are inserted in the PL collection optics to eliminate unwanted light originating from direct reflection or scattering of the excitation light by the sample. 77.3.3 Monochromator/Spectrograph The most widely used Czerny–Turner-type monochromator comprises an entrance slit, collimating mirror, grating, focusing mirror, and exit slit. Focused light enters through the entrance slit, is collimated by the collimating mirror, and then directed to the grating. The grating disperses the light in different directions depending on its wavelength, and only light of a selected wavelength is directed to the exit slit by the focusing mirror. In order for fast data acquisition, modern PL spectrometers are often equipped with a spectrograph in which an aberration-corrected focusing mirror is used to focus the light dispersed by the grating to spatially different positions on the focal plane. The dispersed light is detected using a multi-channel photodetector such as a charge-coupled device (CCD) or a photodiode array. Each channel of the multi-channel photodetector functions to detect the light intensity as a function of wavelength. Photodetectors: Single-channel photodiodes or photomultipliers (PMTs) are frequently used in PL spectrometers equipped with monochromators. In PL spectrometers equipped with spectrographs, CCD or photodiode arrays are used. A combination of a monochromator or band-pass filters with a fast PMT or an avalanche photodiode, together with a pulsed laser and a time-correlated 77 Photoluminescence 475 single-photon counting system, provides the simplest capability for time-resolved PL measurements at a selected wavelength. Computers: Spectral data acquired by photodetectors are sent to computers to construct and display a PL spectrum, which is a plot of PL intensities as a function of light wavelength or energy of the emitted photons. 77.4 Applications Figure 77.3(a) shows a PL spectrum of monolayer tungsten diselenide (WSe2), which is an atomically thin, direct-gap [2] semiconductor, on a Si/SiO2 substrate measured at room temperature. PL intensities are plotted as a function of photon energy. The major peak observed at 1.66 eV corresponds to PL due to radiative recombination of excitons. Figure 77.3(b) shows an optical image of the sample. Figure 77.3(c) shows a PL spectral image of the same sample, constructed by assigning the integrated PL intensities of excitons at each position for emission photon energies in the range of 1.655–1.665 eV to each color according to the color map, and plotted as a function of x- and y-coordinates. The monolayer part is a direct-gap semiconductor and shows high PL intensity (orange, yellow, or white colors), and the other parts with more than two layers are indirect-gap semiconductors and show low or negligible PL intensity (purple or blue colors). Fluctuations in PL intensity are observed in the monolayer part; this indicates Fig. 77.3 a PL spectrum, b Optical image, and c PL spectral image of monolayer WSe2 476 Y. Miyauchi Fig. 77.4 PLE map of SWCNTs dispersed in an organic solvent [4]. PL intensities assigned to each color according to the color map are plotted as a function of excitation and emission photon energies existence of local inhomogeneity. Because of the high sensitivity of PL to the local conditions of the sample, PL spectral imaging provides rich information about even slight differences in local electronic structure. Figure 77.4 shows an example of PLE mapping of single-walled carbon nanotubes (SWCNTs) dispersed in an organic solvent [4]. PLE spectroscopy is a powerful tool for obtaining information, similar to (but not exactly equivalent to) optical absorption, about the optical transitions of a specific species in a mixture. Various kinds of SWCNTs specified by two integers (n, m) exist in the sample, and it is difficult to obtain a pure optical absorption spectrum for only a specific (n, m) type simply by using standard absorption spectroscopy, because absorption contributions from many other (n, m) species in the mixture are inevitable. In contrast, the PLE spectrum of a specific (n, m) type is easily obtained by virtue of (n, m)-dependent emission photon energies. The inset shows a PLE spectrum of the (7, 5) species, extracted as a vertical cut of the PLE map at the emission photon energy of the (7, 5) species of around 1.19 eV. References 1. Fox, M.: Optical properties of solids. Oxford University Press Inc. (2001) 2. Mak, K.F., Shan, J.: Photonics and optoelectronics of 2D semiconductor transition metal dichalcogenides. Nat. Photon 10, 216–226 (2016) 3. Miyauchi, Y.: Photoluminescence studies on exciton photophysics in carbon nanotubes. J. Mater. Chem. C. 1, 6499–6521 (2013) 4. Miyauchi, Y., Hirori, H., Matsuda, K., Kanemitsu, Y.: Radiative lifetimes and coherence lengths of one-dimensional excitons in single-walled carbon nanotubes. Phys. Rev. B. 80, 081410(R)/1-081410(R)/4 (2009) Chapter 78 Photon Emission from the Scanning Tunneling Microscope Makoto Sakurai Keywords Photon Inelastic scattering Radiative transition 78.1 Far-field Tunneling Principle Photon emission from a scanning tunneling microscope (STM) is based on photon creation due to inelastic scattering of electrons tunneling from a STM tip through the tunneling gap under the application of bias voltage (Fig. 78.1). The tip is used for the input of tunneling electrons to a sample surface, then photons created as a function of the tip position and bias voltage give unique information about the inelastic tunneling through the intensity, energy, and polarization of the visible or infrared light. The optical properties are formed by the photon creation process, which is summarized mainly in the following four principles: (1) radiative decay of tip-induced plasmons, (2) radiative transition across the bandgap in semiconductors, (3) radiative dipole transition between electronic states across a tunneling gap, and (4) intramolecular radiative transition. The light is detected at a far-field distance, and then its optical properties are influenced by the propagation process from the gap to far-fields. 78.2 Features • Photon is created as a result of inelastic scattering of tunneling electrons. • Tunneling electrons are injected using a STM tip under the application of bias voltage between a tip and a sample. M. Sakurai (&) WPI-Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Tsukuba, Japan e-mail: SAKURAI.Makoto@nims.go.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_78 477 478 M. Sakurai Fig. 78.1 Schematic of photon emission from the STM (PE-STM) and photograph of system using optical fiber bunch to collect light emitted from tunneling gap [1] • Photon map is obtained as a function of a STM tip position. • Tip-induced light includes information about electronic states related to the tunneling and local optical properties near the tunneling gap. • Quantum efficiency of PE-STM is of the order of 10−4–10−7 photons/electron. 78.3 Instrumentation The system for measuring photon emission from the STM (PE-STM) consists of STM, light collecting and guiding parts, and photo-detectors. Quantum efficiency (QE) of the photon creation is of the order of 10−4–10−7 photons/electron (see Fig. 78.2a–d) under the STM operation with a tunneling current of *110−10–10−8 Fig. 78.2 a–d Four mechanisms of photon creation at tunneling gap [2]. e–h Schematic of collection methods for light emitted from tunneling gap. Methods e–g are used mainly in UHV-chambers. Method h is used mainly in air with high numerical aperture objective lens (oil immersion). The parabolic mirror has large collection angles and is used in conventional cathodoluminescence microscopy 78 Photon Emission from the Scanning Tunneling Microscope 479 A and a sample bias voltage of ±1.2–±5 V, and then it is required to collect the weak light emitted from the tunneling gap effectively. As for silver tips, the light intensity is enhanced by the multiple scattering at the gap. There are mainly four methods of the light collection (Fig. 78.2e–h). In the case of optical fiber bunches (Fig. 78.2e), the end plate of the fiber bunches is placed near the gap at an angle of *30 ° with respect to the surface [1, 2]. The light propagates through the optical fibers to a photo-detector placed on the outside of a UHV-chamber. The increase of individual fiber bunches enables measurement of the angle-resolved light emission simultaneously [3]. In the case of the lens and the parabolic mirror (Fig. 78.2f–h), precise adjustment of their positions is required to collect and guide light effectively. As for optical detectors, a photomultiplier tube with low dark current is widely used to count the number of photons. Energy of the light is measured using a system combining the grating with a low-temperature charge-coupled device camera. 78.4 Applications PE-STM was innovated by Prof. J.K. Gimzewski in 1988 [4] and has been applied to several samples with high spatial resolution. 78.4.1 Light Emission from Atomic Patterns Formed by Atom-Manipulation Hydrogen (H) atoms on a H-terminated Si(001) surface are extracted by a STM tip under the application of appropriate bias voltage. Then atomic scale patterns of exposed Si dangling bonds are made artificially by the scanning of the tip (Fig. 78.3a, b). In Fig. 78.3c, the left and right figures show a STM topographic occupied state image of exposed dangling bonds forming the letter P on a H-terminated Si(001) surface before and after observation of the photon map (middle), which is recorded as a function of the tip position scanning at Vs = −3 V, It = 8 nA, and vs = 9 nm/s. The STM images demonstrate that a PE-STM is non-destructive. The high spatial resolution of the photon map is formed by the difference of the Si surface state, related to the direct dipole transition across the tunneling gap (Fig. 78.2b) [5]. In Fig. 78.3d, a single Si dangling bond is observed at the left bottom corner in the occupied (left) and unoccupied (middle) state STM images. A photon map (right in Fig. 78.3d) is recorded at Vs = −3 V, It = 3 nA, and vs = 9 nm/s. The magnified images (Fig. 78.3e) clearly show the dynamical flip-flop motion of the Si dangling bond during the scanning of the tip, demonstrating an atomic scale light switch (left bottom in Fig. 78.3e) based on a voltage-controlled flip-flop motion of a single atom [6]. 480 M. Sakurai Fig. 78.3 a Schematic of tip-induced extraction of hydrogen (H) atom on Si(001)-(2 1)-H surface. b STM image of Si dangling bond wires on Si(001)-(2 1)-H surface. c STM image of letter P of Si dangling bonds before and after the experiment. Corresponding photon intensity map (photon map) (middle) as position of STM tip [5]. d Occupied (left) and unoccupied (middle) state STM images of isolated single Si dangling bond on Si(001)-(2 1)-H surface. Corresponding photon map (right). e Magnified STM image in d and corresponding light intensity map recorded simultaneously. Note that dynamical flip-flop of single Si dangling bond is observed on both images [6] The atomic patterns of Si dangling bonds on a H-terminated Si(001) surface can be used as atomic lithography, because metal atoms deposited on the surface are preferentially adsorbed onto Si dangling bond sites [7]. STM images recorded before and after Ag deposition demonstrate the atomic precision of the lithographic technique (Fig. 78.4a). The radiative decay of tip-induced plasmons on Ag atomic patterns was measured for the investigation of atomic scale photon manipulation [8]. The top panels in Fig. 78.4 (b) show occupied-state STM images of an isolated Ag nanocluster and dense arrangement of Ag nanoclusters formed on terminated Si(001) surfaces. A tungsten tip was positioned onto the Ag nanoclusters at It = 3 nA (blue arrows in Fig. 78.4b), and their optical spectra were measured with varying Vs. The light intensity increases largely for the arranged Ag nanoclusters. Quantum efficiency of the light emission is plotted against Vs in Fig. 78.4c, indicating the large enhancement of QE by the dense arrangement. For isolated Ag nanoclusters, the tip-induced plasmon is localized on the nanocluster. For arranged Ag nanoclusters, the tip-induced plasmon excited below the tip apex induces other plasmons on neighboring Ag nanoclusters. The large enhancement of the light emission is due to radiative decay of the plasmons extended over the densely arranged Ag nanoclusters with the onset voltage of 3 V. The results obtained by the combination of a PE-STM with the atom-manipulation demonstrate atomic scale photon manipulation. 78 Photon Emission from the Scanning Tunneling Microscope 481 Fig. 78.4 a Top and bottom panels show STM images of tip-induced pre-patterns of Si dangling bonds, recorded before and after Ag deposition [7]. b STM images of isolated and arranged Ag nanoparticles formed through tip-induced pattern formation. Corresponding optical spectra of light emitted from tunneling junction at each tip position under application of various Vs. c Quantum efficiency is plotted against Vs [8] 78.4.2 Surface Plasmon-Mediated Emission Radiative decay of surface plasmons in a PE-STM has been studied for a single Au nanowire [9] and Au thin films [9–11]. Radiation of the surface plasmon excited at the interface between metal and glass (or ITO-coated glass) is measured through a transparent substrate using the detection geometry as shown in Fig. 78.2h. The tip-induced plasmon at the tunneling junction couples to the surface plasmon adjacent to the junction, and the surface plasmon propagating along the interface decays radiatively. Note that radiation from the surface plasmon shows a typical in-plane scattering angle [12], which is different from the angular distribution of dipole radiation on a surface [2, 13]. 482 M. Sakurai 78.4.3 Radiative Bandgap Transitions In the application of a PE-STM to semiconductors with direct bandgap transitions, electrons tunneling from the STM tip under the operation of Vs propagate as minority carriers in the conduction band and achieve radiative recombination with holes with a quantum efficiency of the order of 10−4–10−5 photons / electron (Fig. 78.2c). For a p-type GaAs(110) surface (Fig. 78.5a), STM-induced light emission is observed at positive Vs (Fig. 78.5c). The energy of the light emitted from a GaAs surface at T = 300 K is the same as the energy width of the bandgap of GaAs (Fig. 78.5b), and does not depend on the applied bias voltage (Fig. 78.5d) [2, 14]. Injection efficiency of hot electrons tunneling from a tip through a single Ag nanocluster was estimated by the light emission due to radiative recombination in a GaAs substrate [14]. The efficiency decreases locally on Ag nanoclusters (Fig. 78.5g, h) and does not depend on the height of nanoclusters (0.35–1.1 nm). The energy shift observed in Fig. 78.5f suggests the necessity of additional energy of *0.7 eV for the ballistic transport through a single Ag nanocluster to a GaAs surface. The results show that the highly spatial resolved PE-STM is a useful system for local characterization of optical crystals and devices. Fig. 78.5 a STM image of cleaved GaAs(110) surface. b Schematic band diagram of GaAs. c Intensity of tip-induced light emission from p-type GaAs(110) surface is plotted against Vs. d Optical spectra of tip-induced light emitted from GaAs(110) surface for each Vs [2, 14]. e STM image of Ag nanoclusters on a GaAs(110) surface. f Light intensity of tip-induced light emission for GaAs area and single Ag nanocluster on GaAs(110) surface against Vs. g STM image and h photon map recorded simultaneously at Vs = 2.2 V, It = 0.3 nA, vt = 6 nm/s, and T = 100 K [14] 78 Photon Emission from the Scanning Tunneling Microscope 483 78.4.4 Intramolecular Radiative Transition Tip-induced light emission from molecules (Fig. 78.2d) needs spatial isolation of a molecule from a surface to reduce quenching of the excited state in the molecule. In optical spectra of tip-induced light emission from molecules, radiative transitions between vibronic energy levels, which are formed by electronic energy levels and vibrational energy levels of a molecule, are observed. In the case of a single ZnEtiol molecule on a NiAl(110) surface, thin Al2O3 layers with a thickness of * 0.5 nm are inserted to reduce the quenching [15]. For 5-ML TPP molecular layers on a Au (111) surface, a few molecular layers at the bottom work as isolation from a substrate [16]. In the optical spectra of the tip-induced light emission from TPP molecules, the peaks in Fig. 78.6a are due to vibronic transitions. The transitions from the highly excited state S1(1) to the ground state S0(0) (Fig. 78.6c) are observed in the (1,0) transition in Fig. 78.6a, in addition to the conventional transitions from the lowest excited state S1(0) to the ground state S0(v 0) (Fig. 78.6b). Note that the transition from S1(1) breaks Kasha’s rule. In tip-induced light emission from a molecular chromophore suspended between a surface and a tip (Fig. 78.6d), the oligothiophene chain has a role in the isolation [17]. In their optical spectra (Fig. 78.6e), the vibronic features are observed beside the narrow main peaks reflecting the optical transition between energy levels of the chromophore. Fig. 78.6 a PE-STM spectra emitted from 5-ML TPP molecules for different configurations at T = 80 K. Schematic of various vibronic transitions: b normal luminescence and c hot luminescence. Reprinted with permission from Ref. [16]. Copyright 2009 Nature Publishing Group. d Scheme of experimental configuration: emitting molecule is suspended between tip and sample. e Typical light emission spectra for different suspended molecular devices at T = 4.5 K. Reprinted with permission from Ref. [17]. Copyright 2016 American Chemical Society 484 M. Sakurai 78.4.5 Polarized Light Emission from the STM Polarization of the tip-induced light at the tunneling gap includes information about the photon creation process. But polarization of the light detected at a far-field is usually influenced by the geometrical configuration formed by the tip and the sample surface [2, 18, 19]. In the case of metal, for example, an electric dipole oscillating parallel to a metal surface (s-polarized light emission) does not propagate to far-fields because of destructive interference between the direct light and the reflected light at the surface [2, 12]. Therefore, the measurement of polarized light in PE-STM using the conventional geometry as shown in Fig. 78.2g is limited to semiconductor surfaces under the consideration of the geometrical effect. In spin polarized electrons tunneling from a ferromagnetic tip, GaAs is used as a spin detector (Fig. 78.7a). Radiative transition of the spin-polarized electrons tunneling from a Ni tip in a GaAs crystal creates circularly polarized light according to the optical selection rules in a GaAs crystal [20]. The spin polarization measured for both polarities of the magnetic field demonstrates the tunneling of spin-polarized electrons (Fig. 78.7b). An optical fiber bunch with a linear polarizer at the end face is used to detect linearly polarized light emission in the STM (Fig. 78.7c) [21]. The radiative dipole transition between the surface states across the tunneling gap (Fig. 78.2b) creates linearly polarized light depending on the orbital symmetry of the surface state as shown in Fig. 78.7d. The linearly polarized features related to the dipole transition Fig. 78.7 a Experimental setup for detection of circular polarized light emitted from tunneling gap. b Spin polarization vs. injection energy in GaAs(110) surface from Ni tip. Reprinted with permission from Ref. [20]. Copyright 1992 American Physical Society. c Linearly-polarized PE-STM. d p- and s-polarized light creation by radiative dipole transition between tip and sample. e Total intensity Itotal, f p-polarized intensity I? , g s-polarized intensity Ik , and h ratio Ik =I? of emitted light from gap between tip and Si(001)-(2 1) sample as function of Vs at It = 8 nA and hv = 1.91 eV [21] 78 Photon Emission from the Scanning Tunneling Microscope 485 at the tunneling gap demonstrate that optical selection rules still apply in the light emission (Fig. 78.7e–h). The result suggests that orbital symmetries of surface states are analyzed by a PE-STM in an energy-resolved manner at the spatial resolution of STM. References 1. Thirstrup, C., Sakurai, M., Aono, M.: Photon emission STM using optical fiber bunches. J. Surf. Ana. 4, 152–158 (1988) 2. Sakurai, M., Thirstrup, C., Aono, M.: New aspects of light emission from STM. Appl. Phys. A 80, 1153–1160 (2005) 3. Sakurai, M., Aono, M.: Light creation and propagation in the narrow space between a nanoscale Ag cluster and a tungsten tip. Surf. Sci. 495, L834–L838 (2001) 4. Gimzewski, J.K., Reihl, B., Coombs, J.H., Schlittler, R.R.: Photon emission with the scanning tunneling microscope. Z. Phys. B-Condensed Matter 72, 497–501 (1988) 5. Thirstrup, C., Sakurai, M., Aono, M.: Visible light emission from atomic scale patterns fabricated by the scanning tunneling microscope. Phys. Rev. Lett. 82, 1241–1244 (1999) 6. Sakurai, M., Thirstrup, C., Aono, M.: Light emission from a single atom. Surf. Sci. 526, L123–L126 (2003) 7. Sakurai, M., Thirstrup, C., Aono, M.: Nanoscale growth of silver on prepatterned hydrogen-terminated Si(001) surfaces. Phys. Rev. B 62, 16167–16174 (2000) 8. Sakurai, M., Thirstrup, C., Aono, M.: Scanning tunneling microscope-induced light emission from nanostructures formed by Ag clusters. Phys. Rev. B 64, 045402 (2001) 9. Bharadwaj, P., Bouhelier, A., Novotny, L.: Electrical excitation of surface plasmons. Phys. Rev. Lett. 106, 226802 (2011) 10. Divitt, S., Bharadwaj, P., Novotny, L.: The role of gap plasmons in light emission from tunnel junctions. Opt. Express 21, 27452–27459 (2013) 11. Rogez, B., Cao, S., Dujardin, G., Comtet, G., Moal, E., Mayne, A., Duchemin, E.B.: The mechanism of light emission from a scanning tunneling microscope operating in air. Nanotechnology 27, 465201 (2016) 12. Novotny, L., Hecht, B.: Principles of Nano-Optics, Cambridge (2006) 13. Sommerfeld, A.: Partial differential equations of physics. Academic, New York (1949) 14. Sakurai, M., Wang, Y.G., Aono, M.: Inelastic scattering in electron transport from a metal tip through a nanoscale nanocluster into a GaAs substrate. Surf. Sci. 602, L45–L48 (2008) 15. Qiu, X.H., Nazin, G.V., Ho, W.: Vibrationally resolved fluorescence excited with submolecular precision. Science 299, 542–546 (2003) 16. Dong, Z.C., Zhang, X.L., Gao, H.Y., Luo, Y., Zhang, C., Chen, L.G., Zhang, R., Tao, X., Zhang, Y., Yang, J.L., Hou, J.G.: Generation of molecular hot electroluminescence by resonant nanocavity plasmon. Nat. Photo. 4, 50–54 (2010) 17. Chong, M.C., Vargas, L.S., Bulou, H., Boeglin, A., Scheurer, F., Mathevet, F., Schull, G.: Ordinary and hot electroluminescence from single-molecule devices: controlling the emission color by chemical engineering. Nano Lett. 16, 6480–6484 (2016) 18. Anisimovas, E., Johansson, P.: Tip-geometry effects in circularly polarized light emission from a scanning tunneling microscope. Phys. Rev. B 59, 5126–5133 (1999) 19. Pierce, D.T., Davies, A., Stroscio, J.A., Celotta, R.J.: Polarized light emission from the metal-metal STM junction. Appl. Phys. A 66, S403–S406 (1998) 20. Alvarado, S.F., Renaud, P.: Observation of spin-polarized-electron tunneling from a ferromagnet into GaAs. Phys. Rev. Lett. 68, 1387–1391 (1992) 21. Sakurai, M., Thirstrup, C., Aono, M.: Optical selection rules in light emission from the scanning tunneling microscope. Phys. Rev. Lett. 93, 046102 (2004) Chapter 79 Photo-Stimulated Desorption Akihiko Ikeda and Katsuyuki Fukutani Keywords Electronic excitation Phonon excitation Lifetime Laser-induced thermal desorption 79.1 Potential energy surface Principle Photo-stimulated desorption (PSD) is desorption of adsorbed molecules from surfaces induced by photon irradiation. PSD occurs either in thermal or nonthermal ways. In the former case, the energy of light absorbed by solid surfaces is converted to heat, leading to the thermally activated desorption, where the desorbed molecules are in thermal equilibrium with the surface at a certain temperature. In the latter case, on the other hand, the photo-excitation is classified into two regimes, phonon excitation and electronic excitation of the adsorbate-surface complex depending on the photon energy. The absorbed light energy is transferred to the adsorbate through phonon–phonon or electron–phonon interaction, which eventually leads to breaking of the adsorbate-surface bond. In the case of electronic excitation, the excitation is further classified into either core electron or valence electron excitation. Since the core electron excitation is usually followed by the Auger deexcitation, this is often called the Auger-stimulated process. In these two cases, the adsorbate becomes electrically unstable upon photo-excitation causing nuclear motion of adsorbates, which is schematically illustrated for the valence electron excitation in Fig. 79.1 [1, 2]. Here, the bonding electron between the adsorbate and surface is assumed to be excited to the antibonding state. Due to the repulsive nature of the excited-state potential, this results in the breaking of the chemical bond of the adsorbate-surface A. Ikeda Institute for Solid State Physics, University of Tokyo, Kashiwanoha, Chiba 277-8581, Japan K. Fukutani (&) Institute of Industrial Science, University of Tokyo, Megoru-Ku, Komaba, Tokyo 153-8505, Japan e-mail: fukutani@iis.u-tokyo.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_79 487 488 A. Ikeda and K. Fukutani Fig. 79.1 Schematic illustration of the MGR model. Two curves represent the ground and excited states’ potentials. Electronic excitation between the two states is induced by photo-absorption followed by relaxation to the ground state due to the finite lifetime. Upon excitation, the adsorbate receives a force determined by the gradient of the excited-state potential. In the case of a short residence time (Relax 1), the adsorbate is recaptured because the kinetic energy acquired on the excited potential is not sufficient to surpass the surface barrier. On the other hand, the adsorbate is desorbed with a kinetic energy Ek in the case of a long residence time (Relax 2) complex. This is called the Menzel-Gomer-Redhead (MGR) model. A key for PSD is the lifetime of the excited state. For appreciable desorption to occur, the lifetime needs to be sufficiently long, typically a few 10 fs. As illustrated in Fig. 79.1, a short lifetime leads to recapture of the adsorbate. The potential shape and lifetime depend on the adsorbate system, and are reflected on the kinetic energy of the desorbing molecule both in the perpendicular and parallel directions. When the excitation is accompanied by charge transfer to the adsorbate, the excited-state potential becomes attractive due to the image charge effect, which is called the Antoniewicz model [1]. 79.2 Features • PSD occurs in either thermal or nonthermal ways. • Nonthermal PSD is induced via phonon or electronic excitation of the surface-adsorbate complex. • PSD via electronic excitation reflects the surface electronic structure including the ground and excited states and the excited-state lifetime. • The kinetic energy distribution and angular distribution are helpful to uncover the PSD process. 79 Photo-Stimulated Desorption 79.3 489 Instrumentation A typical setup for the PSD study is shown in Fig. 79.2, which consists of a light source for desorption and a detector of desorbing molecules. As a light source, either of discharge lamps, lasers, and synchrotron radiation (SR) is used depending on the necessary wavelength and pulse duration. SR offers photons with a wide range of wavelength from X-rays to infrared light, whereas the light wavelength of >190 nm is typically available with lasers and discharge lamps. While a quadrupole mass spectrometer detects the desorbed molecules in a mass-selective way, resonance-enhanced multiphoton ionization (REMPI) or laser-induced fluorescence allows for the internal-state selective detection. In either case, with a use of a pulsed light source for desorption, time-of-flight (TOF) measurements can be performed by tuning the time difference between the desorption light and detection of desorbing molecules. With these techniques, the final-state kinetic energy in translational and internal degrees of freedom is measured, which reflects the potential shape and lifetime of the excited-state intermediate. When ultrashort pulsed laser is used, the pump–probe spectroscopy allows for directly probing the excited state: Two-photon photoemission is able to detect the electronically excited-state energy level [3], whereas sum-frequency generation at the surface measures the vibrational dynamics of the adsorbed molecule [4]. In combination with the surface spectroscopy such as infrared spectroscopy and electron energy loss spectroscopy, furthermore, the reduction of particular molecular species can be probed under photon irradiation, which gives information on the initial state of desorbing species. 79.4 Applications 79.4.1 Thermal and Nonthermal PSD PSD via thermal and nonthermal regimes is demonstrated for Xe atoms physisorbed on the Au(001) surface [5]. Figure 79.3a shows the Xe desorption intensity at a photon energy of 6.4 eV as a function of laser fluence. At a low fluence, the desorption intensity increases linearly, while it reveals a nonlinear dependence at a Fig. 79.2 Schematic drawing of the experimental setup for PSD 490 A. Ikeda and K. Fukutani Fig. 79.3 a Desorption intensity of Xe from Au(001) as a function of the desorption laser fluence. b Time-of-flight spectra of desorbing Xe from Au(001) following pulsed laser irradiation. Solid curves are fits of Maxwell–Boltzmann distribution. Although the temperature obtained by fits depends on the laser fluence, the three curves are described by a temperature of about 300 K. c Simulation of the sample temperature rise and laser-induced thermal desorption. The dotted curve indicates the incoming laser intensity profile, and the shaded area denotes the rate of thermal desorption of adsorbates. Adapted from [5] high fluence. As shown in Fig. 79.3b, TOF’s of Xe desorbed at a high laser fluence represents a Maxwell–Boltzmann distribution with a translational temperature of 300 K irrespective of the laser fluence. This value is in agreement with a simulation of the temperature rise and thermal desorption shown in Fig. 79.3c. At a low laser fluence, on the other hand, the desorption strongly depends on the excitation photon energy: While a significant desorption occurs at 6.4 eV, no desorption is observed at 2.3 eV as displayed in Fig. 79.3b. At a low laser fluence, nonthermal PSD is operative as a single-photon process, and PSD is supposed to occur via transient formation of Xe− on Au(001). Figure 79.4a shows a model potential for such an ionic intermediate, which reveals an attractive character due to the image charge effect. Upon excitation, the Xe atom is attracted toward the surface on the excited-state potential, and neutralization of the adsorbate by electron transfer from Xe− to the surface causes transition from the excited state to the ground state, where the neutral adsorbate is repelled from the surface leading to desorption. The translational energy of desorbing Xe and desorption cross section simulated on the basis of the potentials are shown in Fig. 79.4b, c as a function of the intermediate lifetime, where the experimental results are well reproduced by the lifetime of 15 fs. 79 Photo-Stimulated Desorption 491 79.4.2 Internal-State Distribution and Initial-State Selectivity of PSD Small molecules such as NO, CO, and O2 among others are chemisorbed on various substrates, which represent PSD of chemisorbed systems. NO is adsorbed at the on-top and hollow sites of Pt(111) as schematically shown in the inset of Fig. 79.5. Figure 79.5a shows the rotational energy distribution of NO desorbed from Pt(111) upon photon irradiation of 6.4 eV [6]. The distribution is highly non-Boltzmann, and the two spin–orbit states are out of thermal equilibrium reflecting the nonthermal PSD. The peculiar feature of the rotational energy distribution is supposed Fig. 79.4 a Model potential of Xe and Xe− on Au(001) and a schematic of the Antoniewicz model of Xe desorption. b Simulated translational temperature (TDC) and c PSD cross section as a function of the Xe− lifetime. Adapted from [5] Fig. 79.5 a Rotational energy distribution of NO photo-desorbed from Pt(111) at a high NO coverage. b Reflection absorption infrared spectroscopy for NO on Pt(111) at a low coverage. The absorption peak at 1490 cm−1 corresponding to NO adsorbed at a hollow site is reduced in intensity with photon irradiation of 6.4 eV. Adapted from [6, 7] 492 A. Ikeda and K. Fukutani to reflect the tilted NO structure in the adsorption state. When photon irradiation is performed for the hollow site species, the NO coverage depletion is clearly recognized in the infrared spectra as shown in Fig. 79.5b. Nevertheless, no PSD of NO is observed, which indicates breaking of the N-O bond leading to photodissociation of NO in contrast to PSD of NO at the on-top site in Fig. 79.5a [7]. References 1. Avouris, Ph, Walkup, R.E.: Fundamental mechanisms of desorption and fragmentation induced by electronic transitions at surfaces. Ann. Rev. Phys. Chem. 40, 173 (1989) 2. Zhou, X.-L., Zhu, X.-Y., White, J.M.: Photochemistry at adsorbate/metal interfaces. Surf. Sci. Rep. 13, 73 (1991) 3. Petek, H., Ogawa, S.: Femtosecond time-resolved two-photon photoemission studies of electron dynamics in metals. Prog. Surf. Sci. 56, 239 (1997) 4. Arnolds, H., Bonn, M.: Ultrafast surface vibrational dynamics. Surf. Sci. Rep. 65, 45 (2010) 5. Ikeda, A., Matsumoto, M., Ogura, S., Fukutani, K., Okano, T.: Photostimulated desorption of Xe from Au(001) surfaces via transient Xe- formation. Phys. Rev. B. 84, 155412 (2011) 6. Fukutani, K., Murata, Y., Schwarzwald, R., Chuang, T.J.: UV-laser-induced desorption of NO from Pt(111). Surf. Sci. 311, 247 (1994) 7. Song, M.-B., Suguri, M., Fukutani, K., Komori, F., Murata, Y.: Laser-induced desorption and etching at surfaces. Appl. Surf. Sci. 79/80, 25 (1994) Chapter 80 Piezoresponse Force Microscope Masato Hirade Keywords Ferroelectric material High-resolution imaging 80.1 Piezoelectric effect Piezoelectric response Principle Ferroelectric material generates an electric charge when pressure is applied. Also, it will expand and contract when a voltage is applied. In Piezoresponse Force Microscope (PFM), a conductive cantilever is used to apply a voltage to a local region of a ferroelectric material, and distortion caused by the piezoelectric effect of the ferroelectric material is detected as a deflection of the cantilever and imaged [1]. In the measurement, the polarized state of the sample is measured with respect to the AC voltage applied between the sample and the probe by whether the relationship between expansion and contraction of the sample strain is in-phase or reversed (Fig. 80.1). Piezoelectric response of ferroelectric material of nm order can be detected. PFM is a useful technique for local polarization/phase transition of ferroelectric material, observation of domain boundary, and so on. 80.2 Features • Adding a lock-in amplifier and a function generator to the basic AFM configuration makes it possible to measure piezoelectric response. • Like the general AFM, there is no restriction on the observation environment. • Information on the piezoelectric response can be obtained together with the topography image. M. Hirade (&) Surface Analysis Business Unit, Analytical & Measuring Instruments Division, Shimadzu Corporation, Kyoto, Japan e-mail: hirade@shimadzu.co.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_80 493 494 M. Hirade Fig. 80.1 Measurement principle of PFM. In PFM, AC voltage was applied between the probe and the sample. At that time, by detecting the deflection of the cantilever corresponding to the AC voltage applied between the probe and the sample, the piezoelectric response characteristic of the ferroelectric material can be imaged 80.3 Instrumentation PFM is a microscope to visualize the polarization direction of ferroelectric materials and so on. In PFM measurement, apply an AC voltage between the probe and the sample with the conductive probe in contact with the sample. As a result, local distortion is induced along the polarization direction of the ferroelectric material. By detecting this distortion as deflection of the cantilever, the polarization direction at each part of the ferroelectric material is visualized. Figure 80.2 shows the general PFM configuration. In addition to the usual SPM, the PFM consists of a function generator that applies an AC voltage between the probe and the sample and a lock-in amplifier that detects a signal corresponding to the applied AC voltage. Fig. 80.2 A schematic diagram of PFM. In addition to general SPM components, PFM consists of a function generator for applying an AC voltage between the probe and the sample and a lock-in amplifier for acquiring the signal from the cantilever corresponding to the applied AC voltage. In addition, a voltage is applied between the probe and the sample, and it is necessary to use a conductive probe 80 Piezoresponse Force Microscope 80.4 495 Applications Ferroelectric materials are used in various fields such as actuators, sensors, and memories. Here, examples of observation of the ceramic ferroelectric material and the polymer ferroelectric material are shown. First, an observation example of the ceramic ferroelectric material is shown in Fig. 80.3. As shown in Fig. 80.3a, in the topography image, only the domain structure is observed, and no other characteristic surface structure is observed. On the other hand, in the PFM image shown in Fig. 80.3b, many structures which were not observed in the topography image were observed. These indicate that polarization exists even in a flat region in the topography image. Since ceramic ferroelectric materials contain lead which is a harmful substance, researches on polymer ferroelectric materials which do not contain them are actively conducted. Next, an observation example of the polymer ferroelectric material is shown in Fig. 80.4 [2]. Like the ceramic ferroelectric, the domain structure is clearly observed in the topography image (Fig. 80.4a). On the other hand, in the PFM image of the polymer ferroelectric material, no polarization was observed in the domain, and polarization was strongly observed in the domain boundary. By simultaneously obtaining the topography image and the PFM image as described above, it is possible to relate the surface structure and the polarization characteristic. Fig. 80.3 Topographic image and PFM image of the ceramic ferroelectric material. The scanning range for both images is 8 8 lm. a Topography image. b PFM image 496 M. Hirade Fig. 80.4 Topography image and PFM image of ferroelectric polymer. The scanning range for both images is 20 20 lm. This image was provided by the Okamura laboratory at the Tokyo University of Science. a Topography image. b PFM image References 1. Gruverman, A., Auciello, O., Tokumoto, H.: Nanoscale investigation of fatigue effects in Pb (Zr, Ti)O3 films. Appl. Phys. Lett. 69, 3191–3193 (1996) 2. It was provided by Okamura laboratory of Tokyo University of Science Chapter 81 Positron-Annihilation-Induced Desorption Spectroscopy Takayuki Tachibana and Yasuyuki Nagashima Keywords Positron Electron–positron pair annihilation Desorption induced by electronic transitions 81.1 Principle PAID refers to ion desorption from solid surfaces initiated by ionization of surface atoms via electron–positron pair annihilation. It occurs even if the kinetic energy of incident positrons is lower than the threshold energies for electron-stimulated desorption (ESD) of ions initiated by electron-impact excitation of surface atoms. Figure 81.1 shows a schematic of the PAID process. It is known that positrons annihilate with electrons and two 511-keV c-rays are generated. However, the cross section of the pair annihilation is smaller than that of positron–electron collisions. Thus, when positrons impinge on solid targets, they rapidly slow down to the thermal energy and then diffuse in the bulk before annihilation. If the incident energy is lower than a few keV, most of the positrons diffuse back to the surface because they are implanted into the target at a depth small compared to their diffusion length. After the diffusing positrons reach the surface, they may be trapped at the surface and eventually annihilate there [1]. The annihilation with electrons at the surface results in ion desorption induced by electronic transitions (DIET). The process of pair annihilation does not require kinetic energy of the positrons. Therefore, PAID can occur even if the incident energy is below the threshold for ESD. In addition, because of the positron trapping at the surface, the pair T. Tachibana (&) Department of Physics, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima-ku, Tokyo 171-8501, Japan e-mail: tachibana@rikkyo.ac.jp Y. Nagashima Department of Physics, Tokyo University of Science, Tokyo, Japan e-mail: ynaga@rs.kagu.tus.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_81 497 498 T. Tachibana and Y. Nagashima Fig. 81.1 Schematic of the PAID process. a An incident low-energy positron rapidly slows down to the thermal energy level in the bulk material and then diffuses back to the surface, where b pair annihilation results in the ionization of a surface atom and where c desorption may be induced by electronic transitions annihilation may cause the desorption more efficiently than the electron and the positron impact. 81.2 Features • The analysis of desorbed species provides information about the elements on the surface. • Enhanced surface selectivity, elimination of indirect desorption by secondary electrons, and extremely low beam damage. • Information about desorption dynamics is obtained following just a single ionization process. 81.3 Instrumentation Figure 81.2 shows the time-of-flight (TOF) measurement system for PAID ions [2]. Slow positrons are transported to the target using an axial magnetic field efficiently. A NaI (Tl) scintillator coupled with a photomultiplier tube is used to detect 511-keV c-rays emitted from the target by the pair annihilations. The desorbed positive ions from the target are accelerated by the electric field between the target and the grounded disk. They are deflected by another electric field between the parallel plates. Although the ions are forced to spiral around the axial magnetic field, their cyclotron radius is relatively large, because the ions are much more massive than the positron. Thus, the ions entering the electric field between the parallel plates are strongly deflected and directed to a microchannel 81 Positron-Annihilation-Induced Desorption 499 Fig. 81.2 Schematic of a TOF measurement system for the PAID ions [2] plate (MCP). A TOF spectrum is obtained through an analysis of the time interval between the signals from the detectors. 81.4 Applications 81.4.1 Ion Desorption Induced by Positron Annihilation Figure 81.3 shows an example of the PAID spectra from TiO2(110) surface for several incident positron energy values between 13 and 44 eV [2]. The peak near time zero was due to the detection of annihilation c-rays. Another peak was observed at approximately 2.4 µs and was attributed to the desorption of O+ ions from the target. In ESD from the TiO2 surface, the initial threshold of O+ ion desorption has been reported to be 34 eV, which corresponds to the ionization energy of Ti 3p core electrons [3]. Creation of other deep core holes in Ti and O atoms also led to O+ ion desorption. Thus, the O+ ions detected below 34 eV were desorbed because of positron annihilation with core electrons on the TiO2 surface, but not because of impact ionizations. The PAID yield of the O+ ions is 10−4 ions/e+ for 24-eV positrons, which is greater than the ESD yield at 180 eV estimated from the desorption cross section [4, 5]. 500 T. Tachibana and Y. Nagashima Fig. 81.3 TOF spectra triggered by the emission of the annihilation c-rays for positron impact energies (Ei) ranging from 13 to 44 eV [2] References 1. Charlton, M., Humberston, J.: Positron Physics. Cambridge University Press, Cambridge, UK (2000) 2. Tachibana, T., Hirayama, T., Nagashima, Y.: Positron-annihilation-induced ion desorption from TiO2(110). Phys. Rev. B. 89, 201409(R)/1-201409(R)/4 (2014) 3. Knotek, M., Feibelman, P.: Ion desorption by core-hole auger decay. Phys. Rev. Lett. 40, 964– 967 (1978) 4. Lee, J., Zhang, Z., Yates, Jr, J.: Electron-stimulated positive-ion desorption caused by charge transfer from adsorbate to substrate: oxygen adsorbed on TiO2(110). Phys. Rev. B. 79, 081408 (R)/1-081408(R)/4 (2009) 5. Lee, J., Zhang, Z., Yates, Jr, J.: Electron-stimulated positive-ion desorption caused by charge transfer from adsorbate to substrate: oxygen adsorbed on TiO2(110). Phys. Rev. B. 79, 209904 (E)/1 (2009) Chapter 82 p-Polarized Multiple-angle Incidence Resolution Spectrometry Takeshi Hasegawa Keywords Vibrational spectroscopy pMAIRS IR transparent surface Thin film Quantitative molecular orientation analysis 82.1 Principle Multiple-angle incidence resolution spectrometry (MAIRS) is a unique technique built on a chemometric idea to simultaneously reveal the TO and LO energy-loss function spectra (see Chapter 47) of an identical thin film sample deposited on an infrared (IR) transparent substrate [1]. To prevent the polarization dependence of FT-IR, an improved technique of pMAIRS is developed, which employs only p-polarization. pMAIRS is now becoming a promising technique for quantitative molecular orientation analysis in a thin film [2]. Here, only the pMAIRS technique is described. Figure 82.1 presents schematics of the pMAIRS measurements of the in-plane (IP) and the out-of-plane (OP) spectra of an identical thin film deposited on an IR transparent substrate. Both measurements are imagined with the surface-normal incidence. Since the OP measurement using the electric field oscillation parallel to the wavenumber vector is a virtual measurement, direct measurements cannot be performed. Instead, multiple-angle incidence measurements are performed, and the collection of the single-beam spectra, S, is decomposed to have the sIP and sOP single-beam spectrum by using Eq. (82.1). 1 0 rIP;1 sobs;1 B sobs;2 C B rIP;2 S@ A¼@ .. .. . . 0 1 rOP;1 sIP s rOP;2 C þ U Rp IP þ U: A sOP sOP .. . ð82:1Þ T. Hasegawa (&) Institute for Chemical Research, Kyoto University, Uji, Kyoto-Fu, Japan e-mail: htakeshi@scl.kyoto-u.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_82 501 502 T. Hasegawa U receives nonlinear responses to Rp that is a theoretically obtained matrix: Rp ¼ cos2 hj þ sin2 hj tan2 hj .. . tan2 hj .. . ! ; being a function of the angle of incidence of the jth measurement, hj . Since the polarization has only two degrees of freedom, i.e., parallel and perpendicular to the surface, the number of constituents of the classical least squares (CLS) regression [3] is robustly fixed, which makes the CLS analysis work ideally to decompose S into sIP and sOP [1–4]. After this measurement is repeated for both the sample and background, they are used for obtaining explicit absorbance spectra. AIP ¼ log sSIP sBIP and AOP ¼ log sSOP sBOP : The AIP and AOP spectra correspond to the conventional transmission and reflection–absorption (RA) spectra, respectively [1], as presented in Fig. 82.2, although the surface enhancement is not available for the OP spectrum. pMAIRS has a great benefit that the orientation angle of each chemical group can be very easily determined without considering optical parameters. If the experimental condition of the angles of incidence is optimized [5], the orientation angle, /, can readily be calculated by a simple band intensity ratio of the IP and OP spectra (Eq. (82.2)). / ¼ tan 1 rffiffiffiffiffiffiffiffiffi 2AIP : AOP ð82:2Þ Since both AIP and AOP spectra are of an identical spot on an identical sample, the experimental reproducibility is quite high, which enables us to analyze the (b) Out-of-plane (OP) (a) In-plane (IP) E sIP E sOP Fig. 82.1 Schematic concept of pMAIRS-IP and OP measurements. A thin film is deposited on the surface of a substrate 82 p-Polarized Multiple-angle Incidence Resolution Spectrometry 503 molecular orientation in a film having a surface roughness. Spin-coating, drop-casting, and dip-coating films can be a good analytical target for pMAIRS. 82.2 Features • Quantitative molecular orientation analysis in a thin film for each chemical group. • Orientation angle is determined without using optical parameters. • The IP and OP spectra are simultaneously obtained from an identical sample. • Impervious to the surface roughness of the film. • The average orientation angle is obtained irrespective of the degree of crystallinity of the film. 82.3 Instrumentation 2850 2873 2918 2961 OP Absorbance 2850 0.01 IP 2955 Fig. 82.2 IR pMAIRS spectra of a 5-monolayer Langmuir–Blodgett film of cadmium stearate deposited on both sides of a germanium (n = 4.0) plate 2931 2917 The pMAIRS equipment is made of an angle-controllable sample stage, which is installed in the sample room of FT-IR. The angle of incidence can be accurately and precisely controlled by using a step motor via software of FT-IR. For a better measurement of a very thin film, an MCT (mercury-cadmium-telluride) detector is always employed. Since pMAIRS spectra are obtained from single-beam (light intensity) spectra, the stability of the MCT detector is crucial. To make the detector fully stable, at least two hours are spent after filling the Dewar flask of the detector with liquid nitrogen before starting pMAIRS measurements. 3100 3050 3000 2950 2900 2850 2800 2750 2700 Wavenumber / cm -1 504 T. Hasegawa The lowest wavenumber limit of pMAIRS for a silicon substrate is about 700 cm−1. Silicon substrates made by the FZ method yield much better results than those made by the CZ method, i.e., no disturbing peak at about 1100 cm−1 appears in the results. For obtaining satisfying results, careful air-purging inside the spectrometer is strongly recommended. 82.4 Applications 82.4.1 Molecular Orientation Analysis in a Thin Film Having a Surface Roughness Molecular schemes of zinc tetraphenylporphirin (ZnTPP) that is a promising semiconductor compound are presented in Fig. 82.3 [6]. ZnTPP consists of a porphyrin skeleton with four phenyl rings. Thanks to the phenyl rings, this compound is soluble in an organic solvent, which can be spread on a substrate by a wet process to have a thin film. The molecular arrangement is influenced by the solvent as well as the film-preparation technique such as the spin-coating and drop-casting techniques. IR pMAIRS spectra of ZnTPP films are presented in Fig. 82.4, in which the IP and OP spectra are plotted by the red and blue curves, respectively [6]. The top pMAIRS spectrum is for a film made by the spin-coating technique from a chloroform (Chl) solution. The IP and OP spectra agree with each other not only for the spectral shape, but also for the intensity in the entire wavenumber range. This perfect agreement straightforwardly implies that the molecules are randomly oriented. On the other hand, the bottom spectrum exhibits a significantly apparent difference between the IP and OP spectra, which implies that the molecules take a highly oriented arrangement. This film is prepared by using a slowly evaporating solvent of 1,2,4-trichlorobenzene (TCB) spread by the drop-casting technique followed by thermal annealing. In this manner, the molecular orientation can be controlled by changing the evaporation time of the solvent [6], which is vividly visualized by the band intensity ratios. The orientation angles of /por and /ph defined in Fig. 82.3 for the TCB-DC film are obtained as 11° and 78°, respectively, by using Eq. (82.2), since the bands at ca. 800 and 750 cm−1 are attributed to the C–H out-of-plane deformation vibration mode on the porphyrin and phenyl rings, respectively [6]. The summation of these angles, 89°, implies that the porphyrin and phenyl rings are mutually orthogonal to each other quantitatively as illustrated in Fig. 82.3. In fact, the molecular configuration is confirmed by X-ray analysis [6]. In this manner, pMAIRS is powerful for revealing the molecular orientation in a thin film even with a surface roughness, and the reproducibility is also quite high. Of course, a similar analysis can be performed for an amorphous film of a polymer [7]. 82 p-Polarized Multiple-angle Incidence Resolution Spectrometry 505 Fig. 82.3 Schematics of the vibration modes and the angles between the substrate surface-normal and the direction of the transition moment: the angles of /por and /ph correspond to the c(C–H)por (a) and c(C–H)ph (b) modes, respectively Fig. 82.4 IR pMAIRS spectra of thin ZnTPP films prepared different combinations of solvent and film-preparation techniques 506 T. Hasegawa Fig. 82.5 Correlation of IR pMAIRS spectra with GIXD patterns 82.4.2 Correlation of IR pMAIRS Spectra with Polymorph Another great potential of using IR pMAIRS is that the polymorph can be discussed. The right panel of Fig. 82.5 presents a part of four IR pMAIRS spectra selected from Fig. 82.4 near 800 cm−1. For both IP and OP spectra, the four spectra are all consisted of two bands at 803 and 798 cm−1. Since this band region is known to respond to crystallinity [7], the identical films are subjected to the grazing-angle incidence X-ray diffraction (GIXD) measurements, which are presented in the left panel. Since GIXD also provides information for both IP and OP directions, the combination analysis with pMAIRS is quite convenient and useful. The four XRD patterns are found to be combinations of the top and bottom patterns as found in the pMAIRS spectral changes. As a matter of fact, the top and bottom results are assigned to the monoclinic crystals having different “refcodes” of CCDC [6]. In this manner, therefore, once the correlation between pMAIRS spectra and GIXD patterns is revealed, even polymorph can be discussed by measuring the IR pMAIRS spectra only. References 1. Hasegawa, T.: A novel measurement technique of pure out-of-plane vibrational modes in thin films on a nonmetallic material with no polarizer. J. Phys. Chem. B 16, 4112–4115 (2002) 2. Hasegawa, T.: Advanced multiple-angle incidence resolution spectrometry for thin-layer analysis on a low-refractive-index substrate. Anal. Chem. 79, 4385–4389 (2007) 3. Hasegawa, T.: Quantitative Infrared Spectroscopy for Understanding of a Condensed Matter. Springer, Tokyo (2017) 4. Hasegawa, T.: A new approach to analysis of molecular structure in thin films: infrared multiple-angle incidence resolution spectrometry. Appl. Spectrosc. Rev. 43, 181–201 (2008) 5. Shioya, N.. Norimoto, S.. Izumi, N.. Hada, M.. Shimoaka, T.. Hasegawa, T.: Optimal experimental condition of IR pMAIRS calibrated by using an optically isotropic thin film 82 p-Polarized Multiple-angle Incidence Resolution Spectrometry 507 exhibiting the Berreman effect. Appl. Spectrosc. 70, 901–910 (2016). DOI:10.1177/ 0003702816658673 6. Hada, M., Shioya, N., Shimoaka, T., Eda, K., Hada, M., Hasegawa, T.: Chem. Eur. J. 70, 16539–16546 (2016) 7. Shioya, N., Shimoaka, T., Eda, K., Hasegawa, T.: Phys. Chem. Chem. Phys. 17, 13472–13479 (2015) Chapter 83 Quartz Crystal Microbalance Yuji Teramura and Madoka Takai Keywords Quartz crystal Mass change detection Real-time measurement Label-free measurement 83.1 Dissipation factor Principle A quartz crystal microbalance (QCM) is an acoustic transducer that converts mass changes on the sensor surface of an oscillating quartz-crystal resonator into an electronic signal. QCM is possible to capture an extremely small amount of adhering material (ng/cm2 sensitivity) quantitatively from the change in resonance frequency (Df) under vacuum, in the gas phase and also in a liquid environment [1– 3]. QCM has been used for monitoring of thin-film deposition, gas sensors, biosensors and numerous applications. In addition, QCM-based sensor is a potential tool for physical property of adhering material by analyzing dissipation factor (D) together with the detection of Df (QCM-D) [4]. When molecules adsorb onto the QCM-D sensor surface, Df (= fi−f0) and DD (= Di−D0) are calculated in situ in respond to the temporal changes from f0 and D0 at t = 0 to fi and Di at t = ti (Fig. 83.1). Mass changes due to the adsorption of molecules are detected from Df (Fig. 83.1a). The dissipation parameter D provides structural properties like viscoelastic character (Fig. 83.1b). When the DD value is high, the adsorption actually induces high viscosity thus the oscillation energy is lost more rapidly by friction or strong interaction with surrounding molecules. On the other hand, when the DD value is low, the elasticity effectively maintains the Y. Teramura (&) M. Takai (&) Department of Bioengineering, The University of Tokyo, Tokyo, Japan e-mail: teramura@bioeng.t.u-tokyo.ac.jp M. Takai e-mail: takai@bis.t.u-tokyo.ac.jp Y. Teramura Department of Immunology, Genetics and Pathology (IGP), Rudbeck Laboratory, Uppsala University, Uppsala, Sweden © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_83 509 510 Y. Teramura and M. Takai (a) f0, D0 fi<f0, Di> D0 ~ 1/f0 ~ 1/fi Δ f = fi-f0 Δ D = Di-D0 Time (b) Low viscoelas city High viscoelas city ΔDhigh ΔDlow ~ ΔDhigh > ΔDlow ~ decay me, τ Low viscoelas city High viscoelas city Time Fig. 83.1 Schematic diagrams of QCM-D measurement. a Changes of the frequency of the oscillating sensor when the mass change occurs. b Difference in dissipations signal between different viscous layers oscillation energy and then the oscillations slowly decay. Simultaneous measurement of Df and DD makes QCM-D allow real-time and in situ kinetic analysis of biological reactions such as interactions of proteins, DNAs, cellular membranes, and cells. 83.2 • • • • Features Mass change at nano gram level is detected. Viscoelasticity change is also measurable quantitatively. Available for vacuum, gas and solution conditions. High insight in real time monitoring by non-labeling with high sensitivity. 83 Quartz Crystal Microbalance 83.3 511 Instrumentation The experimental set up of flow system is shown in Fig. 83.2, and that of vacuum system is shown Fig. 83.3. AT-cut quartz crystals were used for the QCM-D measurement (See Appendix for details). The resonant frequency of the crystal, which is connected with the oscillator circuit, is measured with a frequency counter (Fig. 83.2). The quartz crystal is mounted to the chamber so that one side faces the liquid with samples and the other side facing air is used for connecting the electrodes to the electric circuit. In a vacuum setup, the quartz crystal is introduced in a vacuum chamber at the vicinity of the sample (Fig. 83.3). 83.4 Applications QCM-D is applied to various fields relating to chemistry, biology, biomedical science, etc. (Figure 83.4a) (representative references are shown in Appendix). Recently QCM devices are widely used for the analytical tool for surface waste Sample Sensor chip Oscillator Circuit Temperature controlled flow cell Power Frequency counter Fig. 83.2 Schematic illustration of the experimental QCM-D set up under flow (gas and liquid) condition Fig. 83.3 Set up of QCM sensor for thickness monitoring in vacuum evaporation system 512 Y. Teramura and M. Takai Fig. 83.4 a Applications of QCM(D) devices and b illustrations of typical examples to analyze surface interactions and surface reactions interactions and surface reactions in wet conditions (Fig. 83.4b). Two experimental examples of QCM-D measurement in wet conditions are shown below. 83.4.1 Surface Reaction on Poly(Ethylene Glycol)Conjugated Phospholipid Derivatives Here we show some results of surface reactions using QCM-D measurement where the QCM-D analyses could reveal the new finding in cell surface engineering with poly(ethylene glycol)-conjugated phospholipid (PEG-lipid) derivatives [6]. The whole cell surface can be homogeneously coated through hydrophobic interactions, this coating is useful to regulate immune reactions post cell transplantation [7, 8]. QCM-D is a powerful analysis tool to study the interactions between PEG-lipid derivatives and mimic membrane to understand how the immobilization was taking place and the polymer states changed on the surface by the immobilization. We examined how the molecular weight and the density of PEG would influence on the protein interaction. We used three biotin-PEG-lipids (PEG molecular weights: 1, 5, 40 k, Fig. 83.5a) and monitored the interaction between streptavidin and a 83 Quartz Crystal Microbalance 513 (a) (b) (c) Fig. 83.5 Experimental design. a Chemical structures of biotin-PEG(1, 5, 40 k)-DPPEs. b Schematic representation of the CH3-SAM surface modified with biotin-PEG-DPPE and streptavidin binding. c QCM-D measurements of Df and DD over time using biotin-PEG(1, 5, 40 k)-DPPE with various concentrations. (Reprinted from Ref. [5] with partial modification with permission from Elsevier) 514 Y. Teramura and M. Takai self-assembled monolayer (SAM) surface (Fig. 83.5b, c). It turns out that although the area occupied by 40 k-PEG chains is much larger than that of 1 k- and 5 k-PEG chains, the 40 k-PEG does not prevent protein adsorption completely. Both the molecular weight of the PEG chain and the graft density must be taken into account for the design of molecular structure in terms of inhibiting from protein adsorption. Interestingly, the DD is negative when streptavidin binds to the biotin-PEG(1 k, 5 k)-DPPE modified surfaces. On the other hand, the DD is positive when streptavidin binds to the biotin-PEG(40 k)-DPPE-modified surface. Multiple binding of streptavidin to biotins on neighboring PEG chains reduces the viscoelasticity of surfaces modified with biotin-PEG(1, 5 k)-DPPEs. This explains the negative DD, because DD reflects the sum of the changes in membrane thickness and viscoelasticity. In contrast, streptavidin binds at a one-to-one stoichiometry with the 40 k biotin-PEG-lipids and increases the viscoelasticity of PEG chains. This information is helpful to understand what happens to immobilized functional molecules on a living cell surface by PEG-lipid derivatives. Other methods like confocal microscopy and surface plasmon resonance (SPR) cannot detect the difference in the streptavidin binding among PEG chains with different molecular weight. 83.4.2 Surface Interaction Between Materials and Cells Understanding material-cell interactions is important for tissue and cell engineering. Figure 83.6 shows an example of cell-analytical QCM-D system. Direct observation using QCM-D and optical-microscopy achieves the investigation of several cellular behaviors dynamically after growing cells one by one to eliminate cell–cell contact. The initial cellular behaviors of several cells (adsorption, attachment and spreading of L929 mouse fibroblasts) on the gold electrode of QCM-D are classified into three regions according to the slope of the Df plots (Fig. 83.7a): region I, cell adsorption and desorption; region II, attachment and spreading; and region III, secretion of microexudates. When the number of cells increases thus the cell–cell distance decreases, region III disappears and the slope of the Df plots in region II becomes steeper (Fig. 83.7b). The adhesion strength between the cells and the gold electrode shown in Fig. 83.7a, b indicates the cellular behavior depending on the Fig. 83.6 Schematic illustration of cell-analytical QCM-D system 83 Quartz Crystal Microbalance (a) III cell 515 II cell (b) cell III: remodeling Rinse II (13 cells/mm2) II: cell attachment, spreading I: cell adsorption I: cell adsorption II: cell attachment, spreading Fig. 83.7 a Df plots of L929 cellular behavior on a gold electrode surface (the injected cell number was 13 cells/mm2), and the photograph of the L929 cells on gold in region II and region III. b Df plots of L929 cellular behavior on a gold electrode in high cell density (500 adherent cells/mm2), and the photograph of L929 cell on gold in region II. Scale bars stand for 100 lm density of attached cells. The initial cell attachment behavior, especially the strength of cell adhesion to the material surface, is evaluated quantitatively by determining the slope of the Df plots using the QCM-D system [8]. Appendix Quartz Crystal for QCM Sensor Piezoelectric effect is the phenomenon to generate an electric change in response to strain when external mechanical stress is applied to some crystalline materials. In particular, silicone dioxide, SiO2, is the most common as a crystalline material in piezoelectric resonators, and a-quartz, a specific crystalline form of SiO2 has been used for QCM–based device. The SiO2-based device stably generates a frequency by piezoelectric effect and inverse piezoelectric effect. Oscillation modes and temperature property depend on how to cut out the crystalline. In other words, the direction and magnitude of the piezoelectric straining is directly controlled by the angle of cutting out of the crystalline. So-called AT-cut quartz crystals are used for QCM applications. AT-cut type is general oscillators which were cut at the angle of 35° 15′ to z-axis of artificial crystalline, and can generate a frequency from 1 MHz to several hundreds MHz. For AT-cut type oscillator, the thickness of quartz plate is an important parameter to determine its oscillating frequency. Resonance occurs when the thickness of the quartz plate is an odd integer of half wavelengths of the induced wave. 516 Y. Teramura and M. Takai The resonant frequency f, is given by: f ¼n vq ¼ nf0 2tq ð83:1Þ vq 2tq ð83:2Þ f0 ¼ The resonant frequency for n = 1 is called fundamental resonant frequency. Where vq is the wave velocity (speed of sound) in the quartz plate and tq is the thickness of the quartz plate. The unit is MHz. Here, n (odd number) is the overtone, n = 3, 5, 7 is the third, fifth, seventh overtone, and so on. Equation 83.1 indicates that oscillating frequency is inversely proportional to the thickness of the quartz plate. For example, the thickness is approximately 33 µm when the oscillating frequency is 50 MHz. Df ¼ f f 2f 2 1 Dm ¼ Dm ¼ n 0 Dm ¼ n Dm mq tq qq C vq q q ð83:3Þ Here, the mass per area of the crystal, mq , is related to its thickness, tq , by mq ¼ tq qq ðkg=m2 Þ, where qq is the density of the quartz. Here, vq ¼ 3400 ðm=sÞ and qq ¼ 2650 ðkg=m3 Þ are given for an AT-cut quartz crystal. When the resonant frequency of the quartz plate is 5 MHz at its fundamental mode (n = 1), C gives 17:7 ðng=cm2 =HzÞ. The sensitivity increases with a factor n by operating at different overtones where it increases with the square of the fundamental frequency, f0 . This relationship between Df and Dm is also known as Sauerbrey equation [Sauerbrey, G. Zeitschrift Fur Physik, 155, 206–222 (1959)]. Quartz Crystal for QCM Sensor Dissipation of the quartz crystal can be also measured at the same time when the frequency change is measured during molecules adsorption onto the surface. The energy of the oscillating crystal dissipates or loses from the system when the driving voltage is turned off. Therefore, in situ monitoring of the viscoelastic changes associated with adsorption of molecules can be useful information. The energy dissipation (D factor) is a dimensionless quantity, which is defined as follows; D¼ Edissipated 2pEstored ð83:4Þ Where Edissipated is the dissipated or lost energy during one period of oscillation, and Estored is the total stored energy in the oscillation system. D value is the ratio of 83 Quartz Crystal Microbalance 517 Edissipated and Estored, indicating the energy loss in the whole system in response to Df change. According to the equivalent electric circuit, Eq. (83.4) is represented as D¼ R1 2pfL1 ð83:5Þ where R1 and L1 are an inductance and resistance, respectively and f is the frequency. The equivalent electric circuit corresponding to a mechanical model, gives s ¼ 2L1 =R1 where s is the decay time constant. Therefore, D¼ 1 pf s ð83:6Þ References for Further Information about QCM Application Particles 1. Grabar, K.C., et al.: Two-dimensional arrays of colloidal gold particles: a flexible approach to macroscopic metal surfaces. Langmuir 12, 2353–2361 (1996) 2. Serizawa, T., et al.: Electrostatic adsorption of polystyrene particles with different surface charges onto the surface of an ultrathin polymer film. Colloids Surf. A, 164, 237–245 (2000). 3. Chen, K.L., Elimelech, M.: Aggregation and deposition kinetics of fullerene (C60) nanoparticles. Langmuir 22, 10994–11001 (2006). 4. Johannsmann, D., et al.: Dissipation in films of adsorbed nanospheres studied by Quartz Crystal Microbalance (QCM). Anal. Chem. 81, 8167–8176 (2009). Lipids 5. Keller, C.A., Kasemo, B.: Surface specific kinetics of lipid vesicle adsorption measured with a quartz crystal microbalance. Biophys. J. 75, 1397–1402 (1998). 6. Richter, R.P., Brisson, A.R.: Following the formation of supported lipid bilayers on mica: a study combining AFM, QCM-D, and Ellipsometry. Biophys. J. 88, 3422–3433 (2005). 7. Briand, E., et al.: Combined QCM-D and EIS study of supported lipid bilayer formation and interaction with poreforming peptides. Analyst, 135, 343–350 (2010). Biological Materials 8. Rodahl, M., et al.: Simultaneous frequency and dissipation factor QCM measurements of biomolecular adsorption and cell adhesion. Faraday Discuss. 107, 229–246 (1997). 518 Y. Teramura and M. Takai 9. Hovgaard, B.H., et al.: Quartz crystal microbalance studies of multilayer glucagon fibrillation at the solid-liquid interface. Biophys. J. 93, 2162–2169 (2007). 10. Olsson, A.L. J. et al.: Novel analysis of bacterium-substratum bond maturation measured using a quartz crystal microbalance. Langmuir 26, 11113–11117 (2010). 11. Baranova, N.S., et al.: The inflammation-associated protein TSG-6 cross-links hyaluronan via hyaluronan-induced TSG-6 oligomers. J. Biol. Chem. 286, 25675–25686 (2011). 12. Tymchenko N. et al.: Reversible changes in cell morphology due to cytoskeletal rearrangements measured in real-time by QCM-D. Biointerphases 7, 1–9 (2012). Polymer/Ceramics/Metals 13. Rao, A., et al.: Effect of deprotection extent on swelling and dissolution regimes of thin polymer films. Langmuir 22, 10009–10015 (2006). 14. Alves, N.M. et al.: Assembling and crosslinking of polyelectrolyte multilayer films of chitosan and alginate studied by QCM and IR Spectroscopy. Macromol. Biosci. 9, 776–785 (2009). 15. Zhang, G., Wu, C.: Quartz crystal microbalance studies on conformational change of polymer chains at interface. Macromol. Rapid Commun. 30, 328– 335 (2009). 16. Nagasawa, D., et al.: Role of interfacial water in protein adsorption onto polymer brushes as studied by SFG spectroscopy and QCM. J. Phys. Chem. C 119, 17193–17201 (2015). 17. Lee, H.-S., et al.: Reversible swelling of chitosan and quaternary ammonium modified chitosan brush layers: effects of pH and counter anion size and functionality. J. Mater. Chem. 22, 19605–19616 (2012). 18. Deakin, M.R.; and Melroy, O. R.: Monitoring the Growth of an Oxide Film on Aluminum In Situ with the Quartz Crystal Microbalance. J. Electrochem. Soc. 136, 349–352 (1989). 19. Itoh, T. et al.: In situ simultaneous measurement with IR-RAS and QCM for investigation of corrosion of copper in a gaseous environment. Corros. Sci. 39, 193–197 (1997). 20. Liu, B., et al.: Enantiopure metal–organic framework thin films: oriented surmof growth and enantioselective adsorption. Angew. Chem. Int. Ed. 51:807– 810 (2012). Biomaterials 21. Boulmedais, F., et al.: Polyelectrolyte multilayer films with pegylated polypeptides as a new type of anti-microbial protection for biomaterials. Biomaterials 25, 2003–2011 (2004). 83 Quartz Crystal Microbalance 519 22. Marx, K.A., et al.: Quartz crystal microbalance biosensor study of endothelial cells and their extracellular matrix following cell removal: Evidence for transient cellular stress and viscoelastic changes during detachment and the elastic behavior of the pure matrix. Anal. Biochem., 343, 23–34 (2005). 23. Modin, C., et al.: QCM-D studies of attachmand differential spreading of pre-osteoblastic cells on Ta and Cr surfaces. Biomaterials 27, 1346–1354 (2006). 24. Feiler, A.A., et al.: Adsorption and viscoelastic properties of fractionated mucin (BSM) and bovine serum albumin (BSA) studied with quartz crystal microbalance (QCM-D). J. Colloid Interface Sci. 315, 475–481 (2007). 25. Ahola, S, Turon, X, Osterberg, M, Laine, J and Rojas, O.J.: Enzymatic hydrolysis of native cellulose nanofibrils and other cellulose model films: effect of surface structure. Langmuir 24, 11592–11599 (2008). 26. Wie, Q. et al.: Protein Interactions with Polymer Coatings and Biomaterials. Angew. Chem. Int. Ed. 53, 8004–8031 (2014). 27. Kushiro, K. et al.: Simultaneous characterization of protein–material and cell– protein interactions using dynamic QCM-D analysis on SAM surfaces. Biomater. Sci. 4, 989–997 (2016). 28. Chen, R. et al.: Biomaterials Act as Enhancers of Growth Factors in Bone Regeneration. Adv. Func. Mater. 26, 8810–8823 (2016). Sensors (Gas/bio/humidity) 29. Wang, J., et al.: Dendritic nucleic acid probes for dna biosensors. J. Am. Chem. Soc. 120, 8281–8282 (1998). 30. Tombelli, S., et al.: Aptamer-based biosensors for the detection of HIV-1 Tat protein. Bioelectrochemistry 67, 135–141 (2005). 31. Peh, W.Y.X., et al.: Understanding ligand binding effects on conformation of Estrogen Receptor DNA complex: A combinational QCM-D and SPR study. Biophys. J. 92, 4415–4423 (2007). 32. Boyadjiev, S.I., et al.: Preparation and characterization of ALD deposited ZnO thin films studied for gas sensors. Appl. Surf. Sci. 387, 1230–1235 (2016). 33. Yao, Y., et al.: Novel QCM humidity sensors using stacked black phosphorus nanosheets as sensing film. Sens. Actuat. B, 244, 259–264 (2017). Drug 34. Ai, H., et al.: Nano-encapsulation of furosemide microcrystals for controlled drug release. J. Control. Release 86, 59–68 (2003). 35. Deligöz, H., Tieke, B.: QCM-D study of layer-by-layer assembly of polyelectrolyte blend films and their drug loading-release behavior. Colloids Surf. A 441, 725–736 (2014). 36. Ayad, M.M., et al.: pH-Responsive sulphonated mesoporous silica: a comparative drug release study. RSC Advances 6, 57929–57940 (2016). 520 Y. Teramura and M. Takai Energy/Battery 37. Katz, E., et al.: A non-compartmentalized glucose∣O2 biofuel cell by bioengineered electrode surfaces. J. Electroanal. Chem. 479, 64–68 (1999). 38. Caia, M., et al.: Investigation of thermal and electrochemical degradation of fuel cell catalysts. J. Power Sources, 160, 977–986 (2006). 39. Tavassola, H., et al.: Solvent Oligomerization during SEI Formation on Model Systems for Li-Ion Battery Anodes. J. Electrochem. Soc. 159, A730–A738 (2012). 40. Harms, H.A., et al.: In situ investigation of dye adsorption on TiO2 films using quartz crystal microbalance with a dissipation technique. Phys. Chem. Chem. Phys. 14, 9037–9040 (2012). References 1. Bense, E.: Improved quartz crystal microbalance technique. J. Appl. Phys. 56, 608–626 (1984) 2. Auge, J.: Hauptmann, P; Eichelbaum, F; and Rösler, S; Quartz crystal microbalance sensor in liquids. Sens. Actuat. B 19, 518–522 (1994) 3. Marx, K.A.: Quartz Crystal Microbalance: A useful tool for studying thin polymer films and complex biomolecular systems at the solution − surface interface. Biomacromol 4, 1099–1120 (2003) 4. Voinova, M.V., Rodahl, M., Jonson, M., Kasemo, B.: Viscoelastic acoustic response of layered polymer films at fluid-solid interfaces: continuum mechanics approach. Phys. Scr. 59, 391 (1999) 5. Teramura, Y., Kuroyama, K., Takai, M.: Influence of molecular weight of PEG chain on interaction between streptavidin and biotin-PEG-conjugated phospholipids studied with QCM-D. Acta Biomater. 30, 135–143 (2016) 6. Teramura, Y.; and Iwata, H, Cell surface modification with polymers for biomedical studies. Soft Matter 6, 1081–1091 (2010) 7. Teramura, Y., Iwata, H.: Bioartificial pancreas microencapsulation and conformal coating of islet of Langerhans. Adv. Drug Deliv. Rev. 62, 827–840 (2010) 8. Watarai, E.; Matsuno, R.; Konno, T.; Ishihara, K.; and Takai M, QCM-D analysis of material-cell interactions targeting a single cell during initial cell attachment, Sens. Actuat. B171–172, 1297–1302 (2012) Chapter 84 Reflectance Difference Spectroscopy Ken-ichi Shudo and Shin-ya Ohno Keywords Optical measurement phenomena 84.1 Spectroscopy Anisotropy Ultrafast Principle Reflectance difference spectroscopy (RDS) is a linear optical method capable of performing highly sensitive measurements to the reflectance anisotropy (RA) of solid surfaces, providing information on the surface structure and electronic states near the surface [1, 2]. This method can be applied to various solid surfaces under ultrahigh vacuum, ambient gas conditions at high pressure, and liquid conditions. In the case of anisotropic structures with a- and b-axes at the sample surface (Fig. 84.1), the reflectance difference (RD) amplitude, D~r =~r , is defined as: D~r ð~r a ~r b Þ ¼ ~r ð~r a þ ~r b Þ=2 where ~r a and ~r b are complex reflectance values associated with the a- and bpolarized components, respectively, of the incident light. Due to chemical or physical distortion in the surface structure (e.g., by molecular (atomic) adsorption, deposition asymmetry, or photostimulation), spectroscopic features of the RD amplitude are discussed in terms of the deformation at the surface [1]. A proportionality is assumed between the change in the spectral profile and the distortion. The complex RD amplitude is decomposed into real and imaginary parts as follows: K. Shudo (&) S. Ohno Faculty of Engineering/Science, Yokohama National University, Yokohama, Japan e-mail: ken1@ynu.ac.jp S. Ohno e-mail: sohno@ynu.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_84 521 522 K. Shudo and S. Ohno Concave Mirrors (Polarizer) p-pol. Sample Spectrometer b s-pol. a Light Source(s) Rot. Mirror & Lens Fig. 84.1 Schematic illustrating the measurement of the reflectance difference (RD) amplitude. Traditionally the sample was rotated at a minimum frequency of *50 Hz to eliminate drift [3]. In most cases, the light is incident close to the normal direction to the sample surface D~r Dr ¼ þ iDh ~r r In actual RD measurements, the real part can be more accurately measured than the imaginary part, which is sometimes obtained from the real part by using the Kramers–Kronig relation. For precise determination of the amplitude, a photoelastic modulator (PEM) is used to induce oscillatory retardation in the optical phase of the component along the a-axis (or b-axis), for some arbitrary angle of the analyzer (Fig. 84.2). The intensity of the light, after passing through the analyzer, has a DC component (I0) with small oscillatory components of the harmonic frequency series (Ix, I2x, I3x, …). With a phase modulation of d = d0 sin xt, the relative intensity is given as follows: Ix 2DhJ 1 ðd0 Þ sin xt; I 0 1 þ Dr r J 0 ðd0 Þ 2 Dr I 2x r J 2 ðd0 Þ cos 2xt I0 1 þ Drr J 0 ðd0 Þ where Jn are Bessel functions of order n. The real and imaginary parts of the RD amplitude can be simultaneously derived from these relations. 84.2 Features • It has a high sensitivity to the surface in ultrahigh vacuum, environmental conditions, and intermediate conditions. • In situ nondestructive measurements can be conducted. 84 Reflectance Difference Spectroscopy 523 Fig. 84.2 Modern optical setup of reflectance difference spectroscopy (RDS). An anisotropic structure, such as the illustrated single-domain Si(001)-(2 1) surface with dimer rows, is assumed for the sample. In this setup, a pair of photoelastic modulators (PEMs) and a second polarizer comprise the analyzer, and the reflected light passes through a monochromator and then into the photodetector • Real-time observation, from millisecond (with conventional lamp and lock-in amplifier) to sub-picosecond (with an ultrashort pulsed laser during pump-and-probe measurements), is enabled by an adequate light source. • Modulation spectroscopy technique is required for precise and stable measurements with lock-in detection; the pump-and-probe method should be applied for the measurement of ultrafast phenomena via time-domain spectroscopy. • Reflectance signal is roughly interpreted in terms of dielectric constants. • In addition to simple crystallography, the surface structure and electronic states can be quantitatively analyzed at the atomic scale. 84.3 Instrumentation In conventional spectroscopy, a xenon lamp is often used as a light source that covers the range from the near infrared, visible to the near ultraviolet (1–6 eV). A prism polarizer with an extinction ratio of >105 is highly desirable for the atomic-scale sensitivity, since the corresponding RD amplitude from the surface RA is < *10−3 times stronger than the dominant (symmetric) component from the bulk. 524 K. Shudo and S. Ohno When the sample is rotated, an oscilloscope can trace the oscillation of the RD amplitude, and nowadays, a PEM (typical frequency: x 50 kHz) is used to increase the precision of measurements. At this high frequency, a lock-in amplifier is required for extraction of the Ix and I2x components. Most studies have considered only the real part (Dr/r), because the accuracy of the analysis is generally higher than that of the imaginary part (Dh). In time-domain spectroscopy, reflected light is passed through the monochromator, and the light intensity is usually measured with a photomultiplier. Pulse lasers are used to observe very fast phenomena. In ultrafast measurements of, for example, phonons or molecular vibrations, a mode-locked titanium:sapphire laser is used (pulse width:*10 femtoseconds), and optical RA is detected by a pair of photodiodes (see Fig. 84.4). 84.4 Applications 84.4.1 Lattice Distortion Induced by Surface Oxidation Figure 84.3 shows an example of RD spectra obtained from a clean single-domain Si(001)-(2 1) and the oxidized surface in the monolayer regime of the same surface. Typical features on the oxidized surface in the monolayer regime give rise to positive peaks in the spectra. For example, the peak at 3.4 eV is associated with E1 and E0’ transitions at the L and C points, respectively. The peak at 4.4 eV corresponds to the E2 transition at the X point. These spectral features are attributed to the strain at the SiO2/Si interface [4]. A value of 2% is estimated for distortion of the Si lattice along the ½110-direction, indicating that the RD features are sensitive to the lattice deformation. Furthermore, an activation energy of 0.16 ± 0.03 eV, associated with temperatures varied in 583–823 K range, was determined from the 0.4 E2 E 1, E'0 0.2 (b) Oxidized 10 3 x Δ R/R Fig. 84.3 Example of RD spectra of a a clean single-domain Si(001)(2 1) surface and b the oxidized surface in the monolayer regime of the same surface [5] (color is added from a figure in Ref. [5], under permission of Copyright 2008, American Physical Society) 0.0 -0.2 (a) Clean -0.4 -0.6 -0.8 2.0 2.5 3.0 3.5 4.0 Photon energy (eV) 4.5 5.0 84 Reflectance Difference Spectroscopy 525 Arrhenius plot of the oxidation time corresponding to oxidation in the monolayer regime [5]. 84.4.2 Measurement of Ultrafast Dynamics Associated with Lattice Vibration Using an ultrafast pulse laser, a similar method can be applied for measuring the electron dynamics, excitation/relaxation kinetics, and lattice dynamics in real time. To analyze these types of transient phenomena, a pump-and-probe measurement (Fig. 84.4) is taken using a femtosecond pulse equipped with an optical delay line [1]. RA is measured via the electro-optical (EO) sampling method, where each polarization component of the reflected probe beam is detected by a pair of photodiodes (electrically connected in opposite polarities) placed behind a polarization beam splitter. These measurements yield data on propagation of the intramolecular vibration associated with adsorbed molecules as well as the coherent lattice vibration of crystals. In the case of graphite, the pristine surface emits a phonon at (a) (b) (c) Fig. 84.4 a Typical optical setup for measuring real-time lattice motion on a surface using, among others, a polarization beam splitter (PBS) and a photodiode (PD). b Resultant real-time RD intensity of a graphite surface. The intensity signal is plotted as a function of the delay time of the probe after the pump pulse. c Fourier transformation of the time-domain RD in (b), corresponding to the coherent motion of the lattice 526 K. Shudo and S. Ohno 47 THz (G mode), whereas the defective surface gives rise to another at *40 THz (D mode), which is manifested as a slight decrease in Fig. 84.4c. Numerous dynamic and transient features are elucidated from the perspective of ultrafast phenomena [1, 6]. References 1. Shudo, K., Katayama, I., Ohno, S. (eds): Frontiers in optical methods: nano-characterization and coherent control, springer series in optical sciences, Vol. 180, ISBN 978–3-642-40593-8 (2013, Springer-Verlag GmbH, Berlin/Heidelberg) 2. Weightman, P., Martin, D.S., Cole, R.J., Farrell, T.: Reflectance anisotropy spectroscopy. Rep. Prog. Phys. 68, 1251 (2005) 3. Aspnes, D.E.: Above-bandgap optical anisotropies in the reflectance spectra of some cubic semiconductors. J. Vac. Sci. Technol., B 3, 1138 (1985) 4. Fuchs, F., Schmidt, W.G., Bechstedt, F.: Understanding the optical anisotropy of oxidized Si (001) surfaces. Phys. Rev. B 72, 075353 (2005) 5. Ohno, S., Kobayashi, H., Mitobe, F., Suzuki, T., Shudo, K., Tanaka, M.: Monolayer oxidation on Si(001)-(2 1) studied by means of reflectance difference spectroscopy. Phys. Rev. B 77, 085319 (2008) 6. Katayama, I., Koga, S., Shudo, K., Takeda, J., Shimada, T., Kubo, A., Hishita, S., Fujita, D., Kitajima, M.: Ultrafast dynamics of surface-enhanced raman scattering due to Au nanostructures. Nano Lett. 11, 2648 (2011) Chapter 85 Reflection High-Energy Electron Diffraction Yoshimi Horio Keywords Surface structure 85.1 Thin film growth Electron diffraction Principle Reflection High-Energy Electron Diffraction (RHEED) [1] is one of the powerful methods for surface structural analysis. The incident electron beam that is accelerated by high voltage over 10 kV impinges a sample surface in grazing incidence. The high-energy electron accompanied with de Broglie wavelength less than about 0.01 nm is diffracted at surface atomic array. Many reflected electrons from the surface go to the front and form the RHEED pattern on the fluorescent screen. Though high-energy electrons have relatively long mean free path, RHEED is surface sensitive as the detection depth of about 1 nm. The reason is that the incident and reflected electrons are very grazing. Strictly, the detection depth depends on both the energy and the glancing angle of incident electron. The geometry of the diffraction spots in RHEED pattern tells us the periodicity of the atomic arrangement at surface. The spot intensity is strongly influenced by the position of the surface atoms. Then, the dependence of spot intensity on the glancing angle of the incident electron beam, rocking curve, is utilized for the surface structural analysis. Specular spot intensity, especially, changes depending on the surface roughness or surface step density. For the layer-by-layer growth mode, the intensity oscillates every growing atomic layer. The RHEED intensity oscillation [2] is used as the monitor of the number of growing atomic layer. Y. Horio (&) Department of Electrical and Electronic Engineering, School of Engineering, Daido University, Nagoya, Japan e-mail: horio@daido-it.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_85 527 528 85.2 Y. Horio Features • RHEED has surface sensitivity so that the detection depth is nearly less than 1 nm. • One can see the periodicity of surface atomic arrangement from the geometry of diffraction spots. • One can obtain the lattice distance from the location of diffraction spots. • 2D or 3D islands on substrate surface can be distinguished whether the RHEED pattern is reflection or transmission type. • Several evaporation sources can be installed for in situ observation of the film growth. • RHEED intensity oscillation can be used for the monitoring of the growing film in atomic level. 85.3 Instrumentation RHEED apparatus is simply constructed with an electron gun, a fluorescent screen and a sample manipulator as shown in Fig. 85.1a. Incident electron beam, which is produced and accelerated over 10 kV in an electron gun, is focused on a fluorescent screen by a magnetic lens. The divergence angle of the beam is desirable to be less Fig. 85.1 a Schematic diagram of RHEED apparatus, and RHEED patterns from b Si(111) 7 7 pffiffiffi pffiffiffi and c Si(111) 3 3 Al surfaces, which are shown upside down 85 Reflection High-Energy Electron Diffraction 529 than 10−4 rad for high coherency, and the diameter of focused spot on the screen should be less than 0.1 mm. The incident beam is controlled by deflection coils #1 and #2 to impinge on sample surface in grazing angle. Usually, the incident glancing angle is as small as 0.5–7 deg. Therefore, the irradiated area on the sample surface is about 1–10 mm long along the beam, and the information from RHEED is averaged one in such a macroscopic area. Images of RHEED patterns are incorporated in a computer by CCD camera system. Locations and intensities of diffraction spots on the screen are monitored and continuously recorded in process of time. For the measurement of rocking curves, the incident electron beam should be continuously changed by the following two methods. One is to control the current passing in the pair of deflection coils #1 and #2 as shown in Fig. 85.1a. Other is to mechanically tilt the electron gun or the sample holder. Since the space in front of the sample surface is free in RHEED, it is easy to combine with other instruments for simultaneous crystal growth and other analysis methods. Because, the screen is far away from the sample and does not face the sample surface, the light from the heated sample does not disturb the RHEED observation in contrast to LEED. RHEED is more suitable to analyze dynamical changes of surface structure during sample heating and deposition of species on the surface, which is opposite to LEED for static analysis of surface structure. pffiffiffi For example, RHEED patterns taken from Si(111) 7 7 and Si(111) 3 pffiffiffi 3 Al surfaces are shown in Fig. 85.1b, c, respectively. These patterns were especially obtained by energy-filtered RHEED [3]. Since inelastic scattering electrons are excluded by retarding meshes, which is set in front of the screen, the patterns are high contrast with diminished background intensity. 85.4 Applications As shown in Fig. 85.2, deposition of 1/3 monolayer of Al on clean Si(111) 7 7 surface at room temperature followed by annealing up to about 750°C makes the pffiffiffi pffiffiffi surface 3 3 Al superstructure. RHEED rocking curves of several diffraction spots of 0 0; 13 13 ; 23 23 ; 1 1 for ½112 incidence and 0 0; 1 0; 1 0; 2 0; 2 0 for ½101 incidence were measured as indicated by solid curves in Fig. 85.2a. These spot intensities were calculated by dynamical diffraction theory based on several structure models [4]. The best-fitted model to the experimental result is T4 site model as shown in Fig. 85.2b. In this model, Al atoms adsorbed on the T4 sites whose locations are just on the second layer Si atoms. Due to the Al deposition, the substrate Si atoms up to the fourth layer depth are relaxed. The calculated rocking curves based on the T4 site model are indicated by dashed lines in Fig. 85.2a. The calculated rocking curves reproduce the experimental ones very well. 530 Y. Horio pffiffiffi pffiffiffi 12 and ½101 Fig. 85.2 a RHEED rocking curves from Si(111) 3 3 Al surface for ½1 incidences. For the comparison, experimental (solid lines) rocking curves are shown with the theoretical (dashed lines) ones which are calculated based on the T4 site structure model of Si(111) pffiffiffi pffiffiffi 3 3 Al surface shown in (b) References 1. Ichimiya, A., Cohen, P.I.: Reflection high-energy electron diffraction. Cambridge University Press, Cambridge, UK (2004) 2. Joyce, B.A., Dobson, P.J., Neave, J.H., Woodbridge, K., Zhang, J., Larsen, P.K., Bolger, B.: RHEED studies of heterojunction and quantum well formation during MBE growth—from multiple scattering to band offsets. Surf. Sci. 168, 423–438 (1986) 3. Horio, Y.: Zero-loss reflection high-energy electron diffraction patterns and rocking curves of the Si(111) 7 7 surface obtained by energy filtering. Jpn. J. Appl. Phys. 35, 3559–3564 (1996) 4. Horio, Y.: Structure analysis of Si(111) √3 √3—A1 surface by energy-filtered RHEED. Surf. Rev. Lett. 4, 977–983 (1997) Chapter 86 Resonant Inelastic X-Ray Scattering Yoshihisa Harada Keywords Elementary excitation Tunneling Low temperature 86.1 Element specificity Radiative decay Principle In the RIXS process, a core electron of a particular element is resonantly excited to an unoccupied state by a monochromatized incident X-ray, which is a process called X-ray absorption (XAS). Subsequent decay of a valence electronic state to the created core hole will occur within the lifetime of the core hole (*femtoseconds). RIXS is a radiative part of this decay process, and the emitted X-ray energy spreads from the incident energy (elastic scattering) to lower energies (inelastic scattering). Figure 86.1 shows a schematic drawing of the RIXS process. Since RIXS involves XAS as an intermediate state, it naturally has element and chemical state selectivity, which is quite useful for physical chemistry. In the final state, RIXS leaves a variety of element-specific excitations. The loss energies of the inelastic scattering have information about the energy of various elementary excitations such as phonon, magnon, spinon, crystal field (ex. dd) excitation, orbiton, charge transfer excitation, plasmon, all related to the lattice, spin, orbital and charge degrees of freedom, respectively. The scattering process is governed by the following Kramers–Heisenberg formula, 2 X hf jHint jnihnjHint jii d2 r /hf jHint jii þ E dXdx E þ h x þ iC i n i jni Y. Harada (&) Institute for Solid State Physics, The University of Tokyo, Chiba, Japan e-mail: harada@issp.u-tokyo.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_86 531 532 Y. Harada Fig. 86.1 a Schematic drawing of RIXS process for a charge excitation as an example. b Detecting elementary excitations with the related degrees of freedom and energy range where r is a cross section of the scattering, X and x are the incident and emitted X-ray energies, Hint describes interaction between photon and electrons in a material, i, n and f in the bracket are initial, intermediate and final states, respectively, Ei and En are the energy of the initial and intermediate eigenstates, hxi is the excitation energy of the incident X-ray, and C is the lifetime broadening of the intermediate state. RIXS corresponds to the second term, which can be approximately rewritten in the form of the product of P(x) that comprises properties of X-ray and core electrons, and dynamical structure factor S(q, x) that exhibits properties of elementary excitations, d2r dXdx jPðxÞj2 S ð~ q; xÞ res S(q, x) is directly related to the imaginary part of the density response function v′′(q, x) of those excitations, Sð~ q; xÞ ¼ 1 v00 ð~ q; xÞ p 1 expðx=kB TÞ RIXS can experimentally provide v′′(q, x) and thus widely applied to various bulk systems, surfaces and interfaces. 86 Resonant Inelastic X-Ray Scattering 86.2 533 Features • Element-specific information can be obtained by core excitation of a particular element. • RIXS can directly probe response functions that explain many exotic properties of strongly correlated systems. • Orbital-specific information can be obtained by applying the dipole selection rule and polarization correlation to RIXS. • RIXS provides bulk-sensitive information through photon-in / photon-out process and is applicable to insulator, solution, gas and so on. • RIXS can also provide information around the solid surface by tuning the excitation energy to the surface state. • Element specificity also contributes to the surface selectivity when a particular element is present only on the surface. • Electronic and magnetic fields can be applied to samples in the RIXS process, and thus, RIXS is suitable for operando spectroscopy of energy materials. 86.3 Instrumentation RIXS measurement requires a focused emission spot and a spectrometer that consists of an optical grating and an X-ray detector. The energy resolution of RIXS is in principle determined by the size of the emission spot, arm length of the spectrometer, line density of the grating and the spatial resolution of the detector. The incident synchrotron X-ray beam can be focused down to 1 lm using a postfocusing mirror system (often KB mirror configuration is applied), or Fresnel zone plate (FZP) when an extreme focusing smaller than 1 lm is required. Energy resolution is E/DE * 1000–2000 (typical) and now reaches E/DE > 10,000 at the best performance. Probing depth of RIXS is on the order of 100 nm for solids and 1000 nm for liquids. For a surface-selective measurement, several methods can be applied: (i) grazing emission configuration to enhance the surface signal, (ii) tuning the excitation energy to a particular absorption edge of a particular element characteristic for the surface states and adsorbates, (iii) using the element specificity when a particular element resides only on the surface. Figure 86.2 shows typical setup for RIXS experiments of liquid/gas/solid interfaces. 534 Y. Harada Fig. 86.2 Experimental setup for RIXS. Typical sample setup for liquid/gas/solid interface experiments using an X-ray transparent thin membrane is also shown 86.4 Applications 86.4.1 Intermediate Oxidation State at the SiO2/Si Interface Figure 86.3 shows O K-edge RIXS of SiO2/Si(111) interface, which is a good example to show how RIXS can extract element specifically inequivalent chemical states at the interface [1]. At the interface, Si–O bond formation is gradually modulated and mostly two types of Si–O configurations are expected as visualized by ab initio calculation and denoted as P1 and P3. The electronic states at the interface (Fig. 86.3e, f) are noticeably different from those of bulk SiO2 (Fig. 86.3d), and the interfacial electronic states strongly depend on the intermediate oxidation states at the interface. Experimental O K-edge RIXS spectra obtained by tuning the excitation energy to select each Si-O configuration (Fig. 86.3b, c) are quite well reproduced by the ab initio calculation. In this experiment, resonant excitation plays a key role to select oxygens only present at the interface. 86.4.2 Surface Chemical Bond of SO2 Monolayer on Ni(100) Figure 86.4 shows another example of RIXS application to study the chemical bond formation at the solid surface [2]. In this case, oxygen atoms in the adsorbed SO2 are present on the surface; thus, element selectivity of RIXS can only be used to extract the surface electronic states. In addition, total reflection geometry of the 86 Resonant Inelastic X-Ray Scattering 535 Fig. 86.3 O 1s RIXS spectra for the 1.8-nm-thick SiO2/Si (111) structure by resonant excitation to pre-edge peaks in O 1s XAS (b, c) and ionization excitation (c), compare with the corresponding calculated O 2p DOS (red lines) for (d) SiO2, (e) P3 atoms of SiO2/Si(111) interface with P3 and (f) P1 atoms of SiO2/Si(111) interface with P1. The calculated DOS spectrum was the average of each oxygen atom spectrum for amorphous SiO2 and that of each interface oxygen atom spectrum in the case of P1 and P3. Gaussian broadening was applied for the O 2p DOS using experimental and lifetime broadening (total 1.1 eV). Red and yellow balls correspond to oxygen and silicon atoms, respectively incident beam to the Ni surface (Fig. 86.4a) dramatically enhances signals from the surface. Figure 86.4b shows O K-edge RIXS spectra of randomly oriented multilayer SO2 obtained by postedge excitation and monolayer SO2 on Ni‖100) obtained by p* resonance, respectively. Black and red spectra of the monolayer SO2 correspond to in-plane and out-of-plane orbitals, respectively, which are extracted from the RIXS results in two independent detection geometries (Fig. 86.4). Appearance of the broad feature denoted N is unambiguously originating from strong surface chemical bond between SO2 and Ni. The excitation to the p* resonance enhances the out-of-plane component which is otherwise very weak to detect. Using DFT-based model cluster calculations contribution of each orbital to the surface chemical bond can be identified as Fig. 86.4c. 536 Y. Harada Fig. 86.4 O K-edge RIXS of SO2 on Ni(100) surface. a Schematic drawing of the optical configuration of incident and emitted X-rays [2] to the Ni(100) surface. b O K-edge RIXS spectra of multilayer and monolayer SO2 on Ni(100) with DFT-based calculations. c MOs corresponding to each RIXS peak 86 Resonant Inelastic X-Ray Scattering 537 References 1. Tokushima, T., Harada, Y., Takata, Y., Sodeyama, K., Tsuneyuki, S., Nagasono, M., Kitajima, Y., Tamenori, Y., Ohashi, H., Hiraya, A., Shin, S.: r-bonding contribution of a strong p-acceptor molecule: Surface chemical bond of SO2 on Ni(100). Phys. Rev. B 78, 085405/1– 085405/5 (2008) 2. Yamashita, Y., Yamamoto, S., Mukai, K., Yoshinobu, J., Harada, Y., Tokushima, T., Takeuchi, T., Takata, Y., Shin, S., Akagi, K., Tsuneyuki, S.: Direct observation of site-specific valence electronic structure at the SiO2/Si interface. Phys. Rev. B 73, 045336/1–045336-4 (2006) Chapter 87 Rutherford Backscattering Spectrometry Daiichiro Sekiba Keywords Rutherford scattering 87.1 Ion beam analysis Electrostatic accelerator Principle RBS is a method to determine the absolute elemental composition, usually of thin films (t * several tens or hundreds nm) deposited on substrates. Ion beams, such as 4 He+, accelerated by electrostatic accelerators up to *MeV are used as proves. The swift ion can be close to a nucleus in a target and induces the elastic scattering due to the coulomb potential. This phenomenon was found by Johannes (Hans) Wilhelm Geiger and Ernest Marsden under the direction of Ernest Rutherford, and interpreted by Rutherford on 1911 [1]. The mass M2 of elements in target can be determined by the kinetic energy E1 of the scattered prove particle with the mass of M1, which had initially the kinetic energy E0. Readers can find the derivation of the relationship among these parameters in text books of classical dynamics, as follows. The “k” in front of E0 is often called k-factor. 0 E1 ¼ @ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi12 M22 M12 sin2 h A M1 þ M2 M1 cos h þ E0 kE0 Here h signifies the scattering angle with respect to the beam incident direction. The elemental composition can determined analytically from the differential cross section described as follows. D. Sekiba (&) Institute of Applied Physics, University of Tsukuba Tandem Accelerator Complex, Tsukuba, Japan e-mail: sekiba@tac.tsukuba.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_87 539 540 D. Sekiba qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2 cos h þ 1 ðM1 =M2 Þ2 sin2 h 2 Z1 Z2 e2 1 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi rðE; hÞ ¼ 2E sin4 h 1 ðM1 =M2 Þ2 sin2 h The depth profiles of each element are determined by using stopping power of target against the incident beam. The stopping powers of compounds are usually well approximated by Bragg’s rule [2]. In general, the procedure of depth profile determination is not straightforward, so that simulation software is often employed [3]. 87.2 • • • • Features Absolute quantification of heavy element in light matrix Depth profile with nm-scale depth resolution Nondestructive Sensitive to lattice relaxation by means of ion beam channeling 87.3 Instrumentation Figure 87.1 shows the typical setup of RBS system. Usually tandem-type (or sometimes single-end-type) electrostatic accelerator is used to create the incident beam. In the case of tandem-type accelerator, initially negative ions are made and transferred to the terminal shell through the bending magnet (analyzer magnet). The excessive electrons in the negative ion are removed in terminal shell by stripper gasses or stripper foil. Then, the produced positive ion is again accelerated toward the scattering chamber. The kinetic energy and charged state of incident beam are monochromatized by the final analyzer magnet and slit system. Several MeV is useful for the light incident beam, such as 4He+ or 4He2+, and several tens MeV is often employed for the heavy incident beams. The divergence of the incident beam is suppressed down to several mrad by means of the double slit system, which consists of two x–y slits (1 1 * 2 2 mm2) separated more than 1 m. The shaped incident beam is dosed onto the sample surface, and the scattered particles (ion and/or neutral atom) are detected by solid-state detector (SSD). The SSDs should be able to analyze the energy of scattered particles. For the detection and energy analyzing of the scatted particles, boron-doped Si detector (a kind of p–n junction) or Si surface barrier detector (a kind of Schottky barrier) (SSBD) is used. The signal current is analyzed the set of preamplifier, linear amplifier and multi-channel analyzer (a kind of analog-digital converter). The typical energy resolution and depth resolution are *30 keV and several tens nm, respectively. The normalization of the incident beam can be done by using, for example, the 87 Rutherford Backscattering Spectrometry 541 Fig. 87.1 Schematics of setup of RBS including electrostatic accelerator combination of beam chopper and Faraday cup. The beam normalization by the sample current is not recommended, because the amount of emitted secondary electrons due to the beam irradiation strongly depends on the work function of the sample, which can change with the sample conditions. 542 87.4 D. Sekiba Applications 87.4.1 Observation of Depth Profile Change on Hydrogen Storage Film Some metals and metal compounds show the reversible metal–insulator transition through the hydrogen absorption and desorption. When the thin films of these materials are deposited on a glass surface, they often show the property of switchable mirror [4]. Mg2Ni is one of such metal compounds [5]. Figure 87.2a shows a typical structure of the Mg2Ni-based switchable mirror. In this study, the substrate was a Si wafer instead of a glass to avoid the charge up due to the ion beam irradiation. Figure 87.2b shows the setup of the RBS measurement. The scattering angle h in this case is 150°. Figure 87.2c shows the RBS spectra taken on the new (as-deposited) and old (degraded due to the repetition of hydrogen absorption and desorption) samples. Figure 87.2d displays the enlarged view of the spectral feature of O, Mg and Ni. Readers can see that O appears only in the old sample, and the profile of Mg is drastically changed due to the repetition of hydrogen absorption and desorption. From the detailed fitting with a simulation software [3], we can know that the origins of the degeneration are creation of MgO and MgH2 layer between the Pd catalyst layer and Mg–Ni layer. Fig. 87.2 a The structure of sample. b The geometry of the experimental setup. c RBS spectra taken on the sample shown in Fig. 87.2 (a). The spectra on as-deposited and after the repetition of hydrogen adsorption and desorption are shown. d The enlarged view of the spectra around the signals of O, Mg and Ni 87 Rutherford Backscattering Spectrometry 543 References 1. Rutherford, E.: LXXIX. The scattering of a and b particles by matter and the structure of the atom, Philosophical Magazine Series 6, 21: 669–688, (1911) 2. Bragg, W.-H., Kleeman, R.: XXXIX. On the a particle of Radium, and their loss of range in passing through various atoms and molecules, Phil. Mag. Dec. 318–340, (1904) 3. For example, SIMNRA, (http://home.mpcdf.mpg.de/*mam/) 4. Huiberts, J.-N., Griessen, R., Rector, J.-H., Wijngaarden, R.-J., Dekker, J.-P., de Groot, D.-G., Koeman, N.-J.: Yttrium and lanthanum hydride films with switchable optical properties. Nature 380, 231–234 (1996) 5. Sekiba, D., Horikoshi, M., Abe, S., Ishii, S.: Mg segregation in Mg-rich Mg-Ni switchable mirror studied by Rutherford backscattering, elastic recoil detection analysis, and nuclear reaction analysis, J. Appl. Phys. 106, 114912/1–114912/5 (2009) Chapter 88 Scanning Capacitance Microscopy Nobuyuki Nakagiri Keywords SPM density 88.1 MOS Capacitance Semiconductor Dopant and carrier Principle As shown in Fig. 88.1, a very small MOS (metal-oxide-silicon) structure is formed by contacting the metallic tip of the AFM (Atomic Force Microscopy) to the oxide surface of the silicon sample. Various physical properties can be obtained from capacitive measurements using the MOS capacitor; for example, the thickness and dielectric constant for the oxide layer, and the dopant profile and conductivity type for the silicon substrate. Typical C-V curves for p-type and n-type silicon and their dC/dV-V curves are shown in Fig. 88.1. Since the dC/dV-V curves have a positive peak for p-type silicon and a negative peak for n-type, it is possible to determine the dopant type from the SCM measurement. The peak amplitude depends on dopant density of silicon, namely, low dopant density corresponds to a relatively higher amplitude. 88.2 Features • SCM can be constructed by adding some electronics to a typical AFM. • Developing probes suitable for SCM probe is effective for improving contrast and repeatability in SCM imaging. • Simultaneous SCM/AFM imaging is useful to determine dopant types and densities of a silicon sample. N. Nakagiri (&) Tsukuba Research Laboratory, Nikon Corporation, Tsukuba, Japan e-mail: nobuyuki.nakagiri@gmail.com © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_88 545 546 N. Nakagiri Fig. 88.1 Configuration of SCM and C-V, dC/dV-V curves. A MOS structure is formed by the cantilever tip and semiconductor sample. The capacitance changes with increasing bias voltage V • SCM can be applied to study working semiconductor devices such as MOS during operation. • SCM can be used for characterization of the silicon oxide of a silicon sample. 88.3 Instrumentation A schematic diagram of SCM is shown in Fig. 88.2. The SCM is constructed by adding a capacitance sensor and other electronic circuits to a standard AFM [1, 2]. An ac modulation voltage and a variable dc bias voltage are applied to the sample. The metal tip of the cantilever is connected electrically to the capacitance sensor. The sensor can measure tip-sample capacitance change with a sensitivity of 10−19 F. The ac component of the output signal (dC/dV) is detected using a lock-in amplifier, that is, SCM measures not the capacitance itself but derivatives of it, mostly to eliminate the effect of stray capacitance. Experiments can be conducted under ambient atmospheric conditions. 88.4 Applications 88.4.1 Observation of Silicon Oxide/Silicon Sample This sample has three different regions as shown in Fig. 88.3 [3, 4]. Those regions are n-type regions, p-type regions, and high-density n-type regions indicated by I, 88 Scanning Capacitance Microscopy 547 Fig. 88.2 Schematic diagram of SCM/AFM is composed of the standard AFM and added electronics for SCM A p-type A' n-type silicon dioxide silicon electrode AFM image SCM image Fig. 88.3 Schematic diagram of the sample and simultaneously obtained AFM and SCM images II, and III, respectively. The n-type region I is an n-type silicon wafer of 4–6 Xcm without any doping. The p-type region II was made by implanting BF2+ at a dose of 7 1012 atoms/cm2 at 100 keV and annealed at 950 °C for 30 min. The high-density n-type metallic region III was made by implanting As at a dose of 3 1015 atoms/cm2 at 120 keV and annealed 1000 °C for 30 min. Since the surface of the sample was entirely covered with a thermal oxide layer, it is stable under ambient conditions. The other side of the sample was an electrode. 548 N. Nakagiri As shown in Fig. 88.3, the three different regions are clearly seen in the SCM image. On the other hand, the regions I and II could not be distinguished by AFM (or optical microscopy, not shown in the figure), because there is no height difference between them. It took about 5 min to take the simultaneous SCM and AFM images. The SCM image was taken with an 80 kHz, 4 Vp-p modulation voltage, and 0 V dc bias. The scan area was 20 lm 20 lm. The time constant of the lock-in amplifier was 3 ms in these cases. The longer the time constant is, the better the signal-to-noise (S/N) ratio becomes. Since the SCM images can be taken at various dc bias voltages, dC/dV-V curves can be derived for the different region. The curves can be also obtained by sweeping the bias voltage, while the probe is located at one position. One sweep of bias voltage from −10 to 10 V or from 10 to −10 V was conducted over 10 s. 88.4.2 Observation of MOS-FET Figure 88.4 shows SCM images of a MOS-FET (metal–oxide–semiconductor field-effect transistor). The cross-section was prepared by cutting a commercially available DRAM. The upper SCM image was taken at the gate voltage of 0 V. Any p-channel was not seen in this image as expected. On the other hand, a p-channel was induced under the gate by applying −4 V as clearly seen in the lower SCM image. This demonstration indicates that the SCM is a useful tool for characterization of working semiconductor devices [5, 6]. Fig. 88.4 Schematic diagram of the sample and SCM images taken at the gate voltage of 0 and −4 V 88 Scanning Capacitance Microscopy 549 References 1. Barrett, R.C., Quate, C.F.: Charge storage in a nitride-oxide-silicon medium by scanning capacitance microscopy. J. Appl. Phys. 70, 2725 (1991) 2. Yamamoto, T., Suzuki, Y., Miyashita, M., Sugimura, H., Nakagiri, N.: Development of a metal patterned cantilever for scanning capacitance microscopy and its application to the observation of semiconductor devices. J. Vac. Sci. Technol. B15, 1547 (1997) 3. Nakagiri, N., Yamamoto, T., Sugimura, H., Suzuki, Y.: Application of scanning capacitance microscopy to semiconductor devices. J. Vac. Sci. Technol. B14, 887 (1996) 4. Yamamoto, T., Suzuki, Y., Sugimura, H., Nakagiri, N.: SiO2/Si system studied by scanning capacitance microscopy. Jpn. J. Appl. Phys. 35, 3793 (1996) 5. Takasaki, Y., Yamamoto, T.: Cross-section analysis of electric devices by scanning capacitance microscope. Microelectronics Reliability. 39(6–7), 987 (1999) 6. Nakakura, C.Y., Hetherington, D.L., Shaneyfelt, M.R., Shea, P.J., Erickson, A.N.: Observation of metal–oxide–semiconductor transistor operation using scanning capacitance microscopy. Appl. Phys. Lett. 75, 2319 (1999) Chapter 89 Scanning Electrochemical Microscopy Yasufumi Takahashi Keywords Electrochemical imaging 89.1 Chemical sensing Microelectrode Principle Scanning electrochemical microscopy (SECM) is a probe microscopy technique, in which an ultramicroelectrode (UME) is used as a probe. SECM has been used to characterize and image the local electrochemical properties of various materials by scanning the sample surfaces with an UME. The probe current reflects the electrochemical processes occurring in the small space surrounded by the probe and the substrate. Figure 89.1 shows SECM two basic measurement modes. Electrode miniaturization improves spatial resolution and offers several advantages, such as low double-layer charging currents, low ohmic drops, fast mass transport, and diffusion rate-limiting steady-state current. The steady-state current of the voltammogram of the UME is described as follows: i ¼ 4nFDCa ð1Þ where F is Faraday’s constant; D and C are the diffusion coefficient and bulk concentration of the redox species, respectively; and a is the radius of the UME. SECM has been used to monitor short-life chemicals, local catalytic reactivities, corrosion, and cell consume/release chemicals. A particularly useful application of SECM is the observation of the respiration activity of single embryo because of its non-invasive nature. Y. Takahashi (&) Division of Electrical Engineering and Computer Science, Kanazawa University, Kanazawa 920-1192, Japan e-mail: yasufumi@se.kanazawa-u.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_89 551 552 Y. Takahashi (a) FB mode PosiƟve FB (b) GC mode NegaƟve FB substrate MRed MOx MRed MRed MRed Enzyme MOx ConducƟve substrate InsulaƟng substrate Fig. 89.1 Schematic view of SECM a feedback (FB) mode, b generation collection (GC) mode. FB mode is used to detect the redox activity of a sample surface. A mediator is oxidized or reduced at the UME tip and diffuses to the substrate electrode, where it is regenerated; this establishes a redox cycle and enhances the current through the UME. This situation is called positive FB. If the sample surface is an inert electrical insulator, the redox cycle does not occur; thus, the UME current decreases. This situation is called negative FB. In GC mode, the UME tip collects redox species generated from a substrate electrode. For biological applications, electrochemically active species can be generated from the samples (enzymes and cells) located on an insulator without applying a potential. This is also referred to as GC mode [1] 89.2 • • • • Features Simplified electrochemical measurements under steady-state conditions. Detection of chemical species in localized volumes. Electrochemical measurements in highly resistive media. Monitoring of electrochemical processes over short time domains. 89.3 Instrumentation The basic SECM system consists of four major components: an electrochemical cell (including UME, counter electrode, and reference electrode), a current detector (current amplifier or potentiostat), a micro-/nano-positioner, and a data acquisition system (Fig. 89.2). A vibration-free table is necessary to provide adequate vibration isolation for high-resolution measurements. In a typical SECM measurement, a two-electrode setup is employed because the current flow is extremely small 89 Scanning Electrochemical Microscopy 553 DC motor controller DC motor Piezo (z) Current Amp Probe Reference PC FPGA Piezo Controller Piezo (XY) Culture dish Optical microscopy vibration-free table Fig. 89.2 Experimental setup for SECM [2] (pA–nA), and therefore, the IR drop is negligible. If the sample needs external bias, a bipotentiostat is required. The UME is held on a micro-/nano-positioner, which allows movement and positioning relative to the interface under investigation. Various positioners have been employed in SECM instruments, where the best choice depends on the type of measurement and spatial resolution required. To acquire the data, conventional AD/DA or field-programmable gate array boards are available. 89.4 Applications 89.4.1 Respiratory Activity Measurement of Single Embryos Figure 89.3 shows single embryo’s respiration activity measurement by using SECM. The decrease of the oxygen reduction current signal reflects respiration activity and indicates the status of the embryos. The oxygen concentration profiles are analyzed by theoretical spherical diffusion, which is effective to quantitative evaluation of oxygen consumption rate (F). When an embryo reached the stage of a morula with a 74-µm radius on day six after in vitro fertilization, the oxygen concentration difference between the bulk solution and the morula surface was 7 µM. The oxygen consumption rate of a single morula was estimated to be 554 Y. Takahashi Fig. 89.3 Plots of the oxygen concentration profile versus the rs/(r + rs) value (B) at 37 °C in a water-saturated atmosphere of 5% CO2 and 95% air. rs and r are the radius of the sample and the distance from the sample surface, respectively. The photographs and the profile correlate to a blastocyst on day 8. Bar, 200 lm. Tip radius, 0.92 lm [3] (1.40 ± 0.27) 10−14 mol/s. After the SECM analysis, the embryo was continuously cultured for another two days and grew to the stage of a blastocyst with a 100µm radius. For the blastocyst, the oxygen consumption was found to be in the range of (2.50 ± 0.46) 10−14 mol/s < F < (4.49 ± 0.50) 10−14 mol/s. The oxygen consumption of the morulae on day six after in vitro fertilization was strongly related to the morphological quality of the embryo. The morulae showing a larger oxygen consumption rate developed into blastocysts of a larger size. 89.4.2 Nanoscale Topography Imaging and Neurotransmitter Detection Figure 89.4 shows the topography images of PC12 cells and neurotransmitter detection by using SECM–scanning ion conductance microscopy (SECM-SICM). SECM is a powerful tool for investigating the spatial distribution of neurotransmitter release and for quantitatively analyzing single vesicles. SICM is an effective tool for live cell topography imaging and local chemical ejection. An advantage of SECM–SICM is that the SICM probe filled with electrolyte can be used to apply different reagents for the local stimulation of the cell. Therefore, voltage-driven application of K+ ions was realized by SECM–SICM itself to achieve both the local 89 Scanning Electrochemical Microscopy 555 Fig. 89.4 Topography images of differentiated PC12 cells using SECM-SICM. The arrows show the dendritic structures. A series of current spikes corresponding to neurotransmitter release detected after (b) whole cell stimulation with 105 mMK+ using another micropipette and c voltage-driven delivery of K+ ions using a SICM barrel [4] depolarization of the cell membrane and simultaneous detection of the neurotransmitter. With local stimulation, a low frequency of current spikes compared to the entire cell stimulation was observed (Fig. 89.4c). 556 Y. Takahashi References 1. Takahashi, Y., Kumatani, A., Shiku, H., Matsue, T.: Scanning probe microscopy for nanoscale electrochemical imaging, doi:10.1021/acs.analchem.6b04355 2. Takahashi, Y.: Development of high-resolution scanning electrochemical microscopy for nanoscale topography and electrochemical simultaneous imaging. Electrochemistry 84, 662– 666 (2016) 3. Shiku, H., Shiraishi, T., Ohya, H., Matsue, T., Abe, H., Hoshi, H., Kobayashi, M.: Oxygen consumption of single bovine embryos probed by scanning electrochemical microscopy. Anal. Chem. 73, 3751–3758 (2001) 4. Takahashi, Y., Shevchuk, A.I., Novak, P., Zhang, Y.J., Ebejer, N., Macpherson, J.V., Unwin, P.R., Pollard, A.J., Roy, D., Clifford, C.A., Shiku, H., Matsue, T., Klenerman, D., Korchev, Y. E.: Angew. Chem. Int. Ed. 50, 9638–9642 (2011) Chapter 90 Scanning Electron Microscope Energy Dispersive X-Ray Spectrometry Masaki Morita Keywords SEM-EDS X-ray analysis Elemental analysis Characteristic X-ray X-ray spectroscopy 90.1 Principle SEM-EDS performs an elemental analysis on a material’s surface. The high-energy electron beam of the scanning electron microscope (SEM) interacts with the sample material and a characteristic X-ray is generated. Energy-dispersive X-ray spectrometer (EDS, EDXS) then detects the characteristic X-ray. When a high-energy electron beam contacts the sample, the incident electron ejects an inner shell electron from one of the atoms in the sample (Fig. 90.1-1). The atom is left with a vacancy in its inner shell (Fig. 90.1-2). This is an unstable state and an outer shell electron drops into fill the vacancy (Fig. 90.1-3). However, this electron has the energy of the outer shell, which is much higher than that allowed for the inner shell. The excess energy is emitted in the form of an X-ray, which have an energy equal to the difference in energy between the inner and the outer shells. As each element has a unique energy difference between the inner and outer shells, the emitted X-ray is referred to as the characteristic X-ray. SEM-EDS analysis obtains what elements are present in the surface of a material from the characteristic X-ray peak energies of the acquired X-ray spectrum (qualitative analysis), and its elemental compositions from the characteristic X-ray intensities (quantitative analysis). Furthermore, SEM-EDS obtains the elemental distribution of a selected area by synchronizing the electron beam probe position with EDS acquisition. M. Morita (&) JEOL Ltd, Tokyo, Japan e-mail: mamorita@jeol.co.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_90 557 558 M. Morita (1) (2) Electron beam Secondary electron Ejects Vacancy K shell L shell M shell (3) Characteristic X-ray Fig. 90.1 Flow of characteristic X-ray generation. 1 Electron beam ejects the electron from the K shell. 2 The vacancy is generated in the K shell as the ejected electron goes out from the atom as the secondary electron. 3 An electron from the L shell drops to the K shell, and the characteristic X-ray is emitted Detector Detector elem element FET SEM Digital pulse processor X-ray Electron probe MCA Elemental map Sample Electron probe position Fig. 90.2 SEM-EDS system. The graphs show the output signals of each component 90.2 Features • SEM-EDS simultaneously performs multi-element (normally B to U) analysis in a selected area of a material’s surface. • Elemental compositions and elemental distribution related to an electron microscope images can be evaluated. Scanning Electron Microscope Energy Dispersive … Fig. 90.3 X-ray spectrum of olivine. The spectrum was acquired by a silicon drift detector (SDD), which is one type of solid-state detector. Accelerating voltage of SEM was 15 kV. The sample was polished and then coated by carbon 559 Mg X-ray Counts 90 O Si Fe 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 X-ray energy [keV] • It is easily possible to attach an EDS detector to almost any SEM. The detector is compact and lower cost than other X-ray spectrometers. 90.3 Instrumentation SEM-EDS is an attached EDS to SEM. EDS consists mainly of three components: a detector, a digital pulse processor, and a multi-channel analyzer (MCA). Figure 90.2 shows a typical SEM-EDS system and the output signal of each component. The detector has a detector element, which converts the detected X-ray to a measurable signal such as charge, calorie, or light intensity. The detector element of a typical EDS detector uses a solid-state detector, which converts an incoming X-ray to a charge pulse [1]. The incoming X-ray in the solid-state detector generates electron–hole pairs, where the quantity is proportional to the energy of the incoming X-ray. The solid-state detector system obtains the energy of each individual incoming X-ray by measuring the number of the generated electrons as the charge pulse. The solid-state detector outputs a voltage signal, which is an integration of the charge pulse, via an FET preamplifier. The solid-state detector requires cooling, either by Peltier cooling or liquid nitrogen, in order to operate with a good energy resolution. The digital pulse processor converts the outputted voltage signal to an X-ray pulse, where its amplitude is proportional to the energy of the incoming X-ray [2]. In this conversion, a time constant for this conversion decides an energy resolution of X-ray detection and a throughput of the EDS. This throughput is given as an output count rate of X-rays. Choosing a longer time constant gives better energy resolution; however, the output count rate is lower. The MCA makes an X-ray spectrum by counting the X-ray pulses output from the digital pulse processor. The MCA assigns the X-ray pulse to the appropriate energy channel and then counts the X-ray pulses for each energy channel. The 560 M. Morita X-ray spectrum is a histogram of the counts versus the X-ray energy by the accumulation of X-ray pulses entering each X-ray energy channel. The SEM-EDS obtains the elemental distribution by synchronizing the electron probe position of the SEM with the X-ray spectrum acquisition. The elemental distribution, often referred to as elemental mapping, displays intensity distribution of each individual characteristic X-ray at each image pixel. 90.4 Applications 90.4.1 Qualitative and Quantitative Analysis Figure 90.3 shows an X-ray spectrum of olivine acquired by SEM-EDS. The X-ray spectrum has typically several peaks. These peaks are of characteristic X-rays from the containing elements of the sample. In the SEM-EDS analysis, it is possible to identify the elements from the peak positions and calculate the abundance of the identified elements from the peak intensities by a matrix correction [3–5]. The characteristic X-ray intensity for an element varies not only with its abundance but also with the type and abundance of other elements in the sample, physically. Below are the three main matrix effects that are typically corrected: • Atomic number correction(Z) • Absorption correction(A) • Fluorescence correction(F) SEI BEI Si Na Ca P 100 m Fig. 90.4 SE image, BSE image, and elemental maps of granite. Accelerating voltage of SEM was 15 kV. The sample was polished and then coated by carbon 90 Scanning Electron Microscope Energy Dispersive … 561 The matrix correction of SEM-EDS analysis corrects the characteristic X-ray intensities and then calculates the abundances. The typical detection limit of common SEM-EDS is about 0.1 ms%, although this detection limit depends on the elements detected, the samples, and the acquisition conditions. 90.4.2 Elemental Map Figure 90.4 shows a secondary electron (SE) image, a backscattered electron (BSE) image, and elemental maps of granite. The intensity of SE and BSE is related to the surface topography and the average atomic number, respectively. The BSE image can also show the composition distribution; however, it cannot show trace elements or the specific element present. The elemental maps acquired by EDS can obtain precise elemental distribution. The spatial resolution of an elemental map is lower than SE and BSE images [6]. The spatial resolution of an elemental map is typically a few microns, which is larger than the size of the electron probe, due to the electron beam spread inside the material and the mean free path of X-ray being longer than that of the electron. SEM-EDS, when coupled with SE/BSE imaging, is a powerful technique that can allow a greater understanding of a sample. References 1. Knoll, G.F.: Radiation Detection and Measurement, Wiley (2000) p. 353 2. G.F. Knoll, Radiation Detection and Measurement. Wiley (2000) p. 577 3. Philibert, J., Tixier, R.H.: Quantitative Electron Probe Microanalysis. In: Heinrich, K.F.J. NBS Spec. Publ., vol. 298, p. 23 (1991) 4. Packwood, P.H., Brown, J.D.: A Gaussian expression to describe /(qz) curves for quantitative electron probe microanalysis. X-Ray Spectrom. 10, 138–146 (1981) 5. Brown, J.D., Packwood, R.H.: Quantitative electron probe microanalysis using Gaussian /(qz) Curves. X-Ray Spectrom. 11, 187–193 (1982) 6. Castaing, R., Descamps, J.: Sur les bases physiques de l’analyse ponctuelle par spectrographie X. J. Phys. Radium 16, 304–317 (1955) Chapter 91 Scanning Electron Microscopy Yasuyuki Okano Keywords Secondary electron Composition information 91.1 Backscattered electron Shape information Principle Scanning electron microscope (SEM) is an instrument that can image and analyze specimens using a focused electron beam. When the focused electron beam irradiates a specimen, various signals are generated in consequence of the interaction of the incident electron with atoms in the specimen (Fig. 91.1) [1]. Among the signals generated from the surface of the specimen, secondary electrons (SEs) and backscattered electrons (BSEs) can be detected for observation of shape and composition (material contrast). The only SEs generated at shallow depth on the specimen surface are detected because they have low energy (usually less than 50 eV). The yield of SE largely depends on the incident angle of the electron beam to the specimen surface. It is therefore suitable for shape observation of the specimen. The BSE has high energy compared to the SE (Fig. 91.2) [1]. The yield of BSE depends on the atomic number of the specimen. It is suitable for composition observation. In addition to such shape observation and composition observation, it is also possible to perform elemental analysis by detecting characteristic X-rays with energy-dispersive X-ray spectrometry (EDS) which is an attachment device of SEM. Y. Okano (&) Advanced Technology Development Team, SM Research and Development Department, SM Business Unit, JEOL Ltd, Tokyo, Japan e-mail: yaokano@jeol.co.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_91 563 564 Y. Okano Fig. 91.1 Origin and information depth of secondary electrons (SEs), backscattered electrons (BSEs), Auger electrons (AEs), and X-ray in the diffusion cloud of electron range for normal incidence of the primary electrons (PE) Fig. 91.2 Schematic energy spectrum of emitted electrons consisting of secondary electrons (SEs) with ESE 50 eV, low-loss electrons (LLEs) with energy losses of a few hundreds of eV, backscattered electrons (BSEs) with EBSE >50 eV and peaks of Auger electrons (AEs), where U is the primary electron energy 91.2 Features • A nano-sized structure can be observed when we use a SEM with the Schottky emitter which has high image resolution of a few nm. • Shape, composition, voltage contrast, and crystallographic information can be obtained. 91 Scanning Electron Microscopy 565 • A low vacuum SEM permits to observe an insulating sample and a specimen with water content without any pretreatment (including metal evaporation or dehydration). • Elemental and crystallographic analysis can be done by using analysis instrument such as an EDS or an electron backscatter diffraction (EBSD) system. • SEMs have a variety of products which include tabletop and floor-top with high resolution. We can select an instrument for any purpose. 91.3 Instrumentation The SEM is composed of several parts (Fig. 91.3): (a) an electron source for generating electrons; (b) an electron optical system which includes condenser lens, objective lens, and deflector for focusing electrons generated from an electron source to form a small probe and for scanning; (c) a chamber and a stage for moving a specimen in vacuum. (d) signal detectors for detecting electrons generated from a specimen; (e) an evacuation system for keeping the stage and the electron optical column in vacuum; (f) an image control system for displaying the detected signal. Tungsten hairpin or LaB6 chip is generally used as electron sources. For high-resolution SEM, a field-emission electron source with a tungsten single crystal chip or Schottky emitter with ZrO2 reservoir is used [1, 2]. A small probe with high luminance is formed from the electron beam generated from the electron source by Fig. 91.3 Schematic diagram of the scanning electron microscope 566 Y. Okano using a condenser lens and an objective lens. The electron probe scans the sample surface by a deflector. The specimen is put on a specimen stage with X/Y axis movement, rotation, and tilt mechanism. The SE and BSE generated from the specimen surface are detected by the detectors. An Everhart–Thornley detector (E–T detector) for detecting SE and a semiconductor (P–N junction type) detector for detecting BSE (BSE detector) are generally used in SEM [1]. In the E–T detector, SEs are accelerated and collided with a scintillator to which a high-voltage is applied. Electrons are converted into light in the scintillator. The converted light is amplified by the photomultiplier tube and sent to the image control system. The BSEs are directly detected by the BSE detector. BSEs produce electron-hole pairs in the p–n junction. These charged carriers are separated and are collected as an image signal. The chamber and electron optical column are evacuated to about 10−3 Pa. The electron gun chamber of the field-emission SEM (FE-SEM) with Schottky emitter is kept at ultra-high vacuum of about 10−8 Pa using ion pump. The detected signal is displayed as a SEM image on the image monitor by synchronizing with the scanning of the electron beam. 91.4 Applications 91.4.1 Difference Between SE and BSE Figure 91.4 shows two images of ceramic capacitors on an electrical board taken by SE and BSE at landing voltage of 5 kV. The SE image (Fig. 4a) has shape information of the specimen. Since the amount of SE generated from the specimen increases at the edge, the signal becomes brighter. The BSE (Fig. 4b) image has composition information because the yield of BSE depends on atomic number. The ceramic capacitors consist of Ni and BaTiO3. A contrast difference between the two materials can be clearly seen. Fig. 91.4 SEM images of ceramic capacitors at a landing energy of 5 keV. a SE image which has shape information, b BSE image which has composition information 91 Scanning Electron Microscopy 567 91.4.2 Deceleration Method The deceleration method enables to observe the fine surface roughness of 10 nm or less. The deceleration method is a mode in which a bias voltage is applied to the specimen. Aberration of the primary electron beam can be reduced by applying a bias voltage to the specimen [3, 5]. This enables high-resolution observation, especially, at low landing voltage, for example, 1 kV or less. Because of low landing energy, samples can be prevented from damage. The signal-to-noise ratio (S/N) is also improved because many electrons emitted from the specimen are detected by the lens detector (TTL detector) using specimen bias voltage (Fig. 91.5a). Fig. 91. 5b) shows mesoporous silica with bias voltage -5 kV set at a landing voltage of 300 V. The fine structure of the surface can be clearly observed by using deceleration method. 91.4.3 Observation with Low Vacuum SEM For observation of non-conductive specimens, specimen charging artifacts need to be eliminated. One of common methods is sputter coating of the specimen with a conductive layer. The other method is low vacuum mode. In low vacuum mode, it is possible to set the specimen chamber to a low vacuum of several tens to several hundreds of Pa. Non-conductive specimens can be observed in low vacuum conditions without sputter coating. Figure 91.6 is a schematic diagram of charge reduction by low vacuum observation. In the low vacuum, the gas molecules (for example N2) introduced into the specimen chamber is ionized by incident electrons, which neutralize the charge on the specimen. The image below shows lily pollen at a landing voltage of 7 kV. The non-conductive specimen without charging can be observed by using low vacuum SEM. Fig. 91.5 Schematic diagram and typical trajectories of electrons with specimen bias (a), and SEM image of mesoporous silica (b) at the landing energy 300 eV and the bias voltage −5 kV 568 Y. Okano Fig. 91.6 Schematic diagram of charge reduction by low vacuum (a). SEM image of lily pollen (b) at the landing energy 7 keV Fig. 91.7 BSE image and EDS mapping image of Au@TiO2 yolk-shell materials [4]. a BSE image. b EDS mapping of Au. The probe current was 440 pA, and the acquisition time for the EDS mapping was 50 min. The landing energy was 4 keV. The specimen bias voltage was −5 kV 91.4.4 Elemental Analysis Using EDS When an electron beam is irradiated to a specimen, X-ray photons are generated in the beam-specimen interaction volume. The X-ray has specific energies (characteristic X-ray) which depend on atom number. EDS identifies the element by detecting this characteristic X-ray. Figure 91.7 shows a SEM image and an EDS mapping of Au@TiO2 taken at a landing voltage of 4 kV. It is possible to visualize the distribution of Au elements by detecting the characteristic X-rays (Au Ma). The particles of the order of 10 nm can also be recognized by combining with the deceleration method. 91 Scanning Electron Microscopy 569 References 1. Reimer, L.: Scanning Electron Microscopy: Physics of Image Formation and Microanalysis. Springer Verlag (1998) 2. Fransen, M., Vanrooy, L., Tiemeijer, P., Overwijk, M., Faber, J., Kruit, P.: On the electron-optical properties of the ZrO/W schottky electron emitter. Adv Imaging Electron Phy. 111, 91–127 (1999) 3. Pease, R.: Low-voltage scanning electron microscopy. 176–187 (1967) 4. Terasaki, O., Cho, H., Cho, M., Jeong, H., Asahina, S., Sakuda, Y., Suga, M., Kazumori, H., Kudo, M., Nokuo, T., Liu, Z., Stevens, S., Anderson, M., GaleanoNunez, D., Schuth, F., Kjellman, T., Alfredsson, V., Han, L., Che, S., Deng, H., Yaghi, O., Cho, K., Ryoo, R.: Novel structural characterisations of insulating and electron beam sensitive materials employing low voltage high resolution scanning electron microscopy. JEOL News 48, 21–31 (2013) 5. Suga, M., Asahina, S., Sakuda, Y., Kazumori, H., Nishiyama, H., Nokuo, T., Alfredsson, V., Kjellman, T., Stevens, S., Cho, H., Cho, M., Han, L., Che, S., Anderson, M., Schuth, F., Deng, H., Yaghi, O., Liu, Z., Jeong, H., Stein, A., Sakamoto, K., Ryoo, R., Terasaki, O.: Recent progress in scanning electron microscopy for the characterization of fine structural details of nano materials. Prog. Solid State Chem. 42, 1–21 (2014) Chapter 92 Scanning Helium Ion Microscope Keiko Onishi and Daisuke Fujita Keywords Microscope Nanoscale fabrication 92.1 Ion beam Secondary electron Principle Scanning helium ion microscope (SHIM) is based on the similar principle with field emission scanning electron microscope (FE-SEM) [1]. The difference between them is that scanning beam of SHIM is a positively charged helium ion (He+) beam from a gas field ion source (GFIS), but not a negatively charged electron beam. An enlarged image of the sample surface is obtained like FE-SEM. Helium gas is field-ionized almost only from the top-most atoms by applying a high voltage to a sharp tip made of monocrystalline refractory metals in a diluted helium gas. Only the He+ beam emitted from a single atom is focused by the ion optical system and is scanned over the sample surfaces. If a backscattered ion detector is equipped, the secondary electrons (SE) and backscattered ions (BSI) can be acquired simultaneously. SHIM can observe the sample image with less current than FE-SEM. If the neutralizing flood gun is equipped, it is easier to observe insulating materials than FE-SEM. SHIM can also be used for direct nanofabrication like focused ion beam (FIB) systems. Since He+ beam does not have such a sputtering capability as a gallium ion beam, it cannot process on the micron-scale, but nanoscale ultrafine modification utilizing the nanoscale-focused He+ beam is possible. If a gas introduction system is installed, deposition of gas-decomposition microstructures by a precisely controlled He+ beam is possible. K. Onishi (&) D. Fujita Research Center for Advanced Measurement and Characterization, National Institute for Materials Science, Ibaraki, Japan e-mail: ONISHI.Keiko@nims.go.jp D. Fujita e-mail: FUJITA.Daisuke@nims.go.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_92 571 572 K. Onishi and D. Fujita 92.2 • • • • • Features very high spatial resolution of sub-nanometer scale (SE image) materials contrast imaging using BSI large depth of focus insulating surface imaging without conductive coating high-resolution nanoscale lithography and ion-beam-induced deposition 92.3 Instrumentation Like conventional FE-SEMs, SHIM device is composed of an optical column (ion optics), a sample chamber, a vacuum system, and an imaging control part (Fig. 92.1). The optical column consists of an GFIS, focusing lens, scanning deflectors, objective lens, and creates a scanning helium ion beam probe with high brightness. A monocrystalline-sharpened tip made of high-melting-point metals such as tungsten is used as an ion source. The specimen is mounted on a specimen stage with XYZ-axis movement, rotation, and tilting mechanism. Compared to ultra-high resolution FE-SEM, relatively large samples can be observed with the same or higher resolution. The specimen chamber has an Everhart–Thornley (ET) detector for the secondary electron detection for the SE imaging. A microchannel plate (MCP) is used for the backscattered ion detection and imaging. The MCP is movable and can be inserted above the sample when the BSI imaging is required. The electron flood gun is used for the neutralization of positively charged samples. A gas injection system (GIS) can be equipped as one of the nanofabrication options for the Fig. 92.1 Schematic structure of a SHIM system 92 Scanning Helium Ion Microscope 573 ion-beam-induced deposition. While the vacuum of the optical column (*10−6 Pa) and the specimen chamber (*10−5 Pa) is kept at high vacuum (HV) level, and the base pressure of the ion source (*10−8 Pa) is kept at ultra-high vacuum (UHV) level. The image control system includes the power supplies for ion optics control, the operation control, the signal detection, and the monitor for observation. 92.4 Applications As an example of the insulator observation, Fig. 92.2 shows the SE image of the surfaces of a gecko’s foot [2]. Although the sample is insulating, it is not prepared with conductive coating. The He+ ion beam and electron beam are alternately irradiated for each line of the scan. Since the dried gecko is an insulator, without Fig. 92.2 SE images of the surfaces of a gecko’s foot [2] 574 K. Onishi and D. Fujita Fig. 92.3 (a) SE image of nanoscale NIMS logo on Al surface, fabricated by SHIM. (b) SE image of Pt pillars deposited on Si surface by SHIM with GIS [2] electron beam irradiation, the specimen is immediately charged up. The charged-up image becomes darkened, and it is impossible to observe the true image. However, by irradiating an appropriate amount of electron beam, it is possible to observe with the wide range of magnification, from a sub-millimeter scale to a nanometer scale. As an example of nanoscale processing, Fig. 92.3 (a) and (b) shows nanoscale lithography pattern created on an Al surface using a bitmap image input, and the nanorod deposition on a Si substrate using He+ beam decomposition of a Pt organic compound [2]. By irradiating the surface with a larger He+ dose compared with that of normal observation conditions, the SHIM can be used as a direct nanolithography tool. By spot irradiation of He+ beam to the Pt organic compound gas injected near the surface by GIS, pillars of decomposition products, considered to be made of Pt, can be produced. The diameter of the Pt pillars is considerably larger than that of the He+ ion beam, but it is still on the nanometer scale. As for other application tools, including operando or in situ observations, transmission SHIM imaging mode, variable temperature sample stages [3], variable voltage application tools [4], have been developed. References 1. Guo, H.X., Fujita, D.: Scanning Helium Ion Microscopy, Characterization of Materials, pp. 1– 9. Wiley (2012) 2. Onishi, K.: Development and Shared Use of Advanced Nanomaterial Evaluation Techniques Using Scanning Helium Ion Microscope. Kenbikyo 48, 154–158 (2013) 92 Scanning Helium Ion Microscope 575 3. Guo, H.X., Wang, C.X., Miyazawa, K., Masuda, H., Fujita, D.: Thermal decomposition of fullerene nanowhiskers protected by amorphous carbon mask. Scientific Reports 6, 38760 (2016) 4. Sakai, C., Ishida, N., Masuda, H., Nagano, S., Kitahara, M., Fujita, D.: Active voltage contrast imaging of cross-sectional surface of multilayer ceramic capacitor using helium ion microscopy. Appl. Phys. Lett. 109, 051603 (2016) Chapter 93 Scanning Near-Field Optical Microscopy/Near-Field Scanning Optical Microscopy Tetsuya Narushima Keywords Optical near-field microscopy 93.1 Nanoscopy Imaging Scanning probe Principle Scanning near-field optical microscopy (SNOM)/near-field scanning optical microscopy (NSOM) is one of the scanning probe microscopies, especially for investigation of optical properties and phenomena in nanometer scale. SNOM/NSOM observation provides high spatial resolution of 10–100 nm that conventional optical microscopes do not achieve, in principle. Spatial resolution of the conventional microscope is provided with the size of focal spot, which cannot be shrunk far below wavelength k of visible light (400–800 nm). Under light illumination, small structures, such as an aperture or a sphere, absorb or scatter the light. The scattered light repropagates through free space. At the same time, confined optical fields are also induced around the small structures. If the structures are sufficiently small compared with k, the non-propagating optical field, i.e., “optical near-field” is localized in a spatial region of 10–100 nm of their vicinity. In SNOM/NSOM, distance between a SNOM/NSOM probe and sample surface are regulated to directly illuminate samples with confined optical near-field around a tip part of the probe, as shown in Fig. 93.1. Because of this tiny near-field spot, high spatial resolution of *(k/10)−(k/100) is achievable in near-field optical images. In addition to the near-field images, the distance control also provides topographic information on the sample surface, i.e., a topographic image. Instead of illuminating the sample, the near-field probe is alternately used to pick up local optical response of the sample [1]. T. Narushima (&) Institute for Molecular Science, Okazaki, Japan e-mail: naru@ims.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_93 577 578 T. Narushima Fig. 93.1 Schematic view of SNOM/NSOM system, along with obtainable topographic and near-field images (transmission) for a S-shaped gold nanostructure. The near-field images show spatial resolution is higher than that obtained by conventional optical microscopes. Spatial distributions of transmitted optical near-field are dependent on excitation wavelengths 93.2 Features • SNOM/NSOM measurement realizes spatial resolution of 10–100 nm in a horizontal direction with high reproducibility. • Various kinds of optical spectroscopic techniques based on polarization, Raman scattering, nonlinear, ultrafast optical processes, and so forth can be adopted with high spatial resolution. • Topographic image is also obtained simultaneously with near-field optical images. • Direct analysis of (local) optical properties/processes is possible based on associated structural information of the sample. 93.3 Instrumentation An SNOM/NSOM system equips with a probe as a source of optical near-field and a scanning stage. The near-field probe is a key component to determine spectroscopic function as well as spatial resolution [2]. There are two major types of the near-field probe: apertured type and apertureless one. In the former apertured probe, leakage of optical field from a small aperture at a metallic film under light 93 Scanning Near-Field Optical Microscopy/Near-Field … 579 illumination is utilized as a confined optical near-field (Fig. 93.2). The apertured structure is usually fabricated at a tip part of a sharpened optical fiber coated with metallic film. Optical near-field is observed at any small structures as well as the aperture under light illumination. Because of this, a nanoparticle placed at a sharp tip [3, 4] or the sharp tip itself [5] is also analogously used as apertureless near-field sources. In conventional optical microscopes, spatial resolution is determined by the size of a focal spot which is focused with a lens. In principle, the spot size cannot be shrunk far below wavelength of light. In contrast, the size of near-field spot brought by either of the above near-field probes can be made sufficiently small compared with wavelength of light. The small near-field spot consequently provides high spatial resolution of near-field images. Detailed features of each probe are described. Apertured near-field probe maintains most of the optical properties of incident light such as polarization, because the near-field is generated through optical fiber. In contrast, the near-field of the apertureless probe is considerably dominated by material property and shape of the tip part. Spatial resolution of the apertured probe is solely determined by aperture diameter, which remains at 50 nm at best on a routine basis because of its poor throughput and signal-to-noise ratio. Since region of the tip part of the apertureless probe can be confined more for near-field generation, it has been reported the sharp tip realizes super resolution of single nanometer level [6]. As described above, we need to properly consider suitable experimental configuration as well as probe type for each measurement. Fig. 93.2 Apertured near-field probe of a tapered optical fiber coated with metallic film 580 93.4 T. Narushima Applications 93.4.1 Polarization-Dependent SNOM/NSOM Measurements for Nanostructured Materials Metallic nanomaterials interact strongly with optical field through plasmon resonance and yield enhancement of the localized optical fields in their proximity. The intensity of the localized optical field of the nanostructures is dependent on structural geometries such as shape, configuration, and spacing [7]. Figure 93.3(b, c) shows near-field (nonlinear) two-photon excitation optical images for dimers of gold spheres [8]. Strong optical fields were visualized at the interstitial sites between the spheres, only where the incident polarization was aligned along the dimeric axes. The interaction of optical field with nanostructures involving chiral shapes as another structural geometry is actively discussed [9–11]. The chiral nanostructures behave like chiral molecules and show optical activity. To reveal the nanoscopic origin of the optical activity, circular dichroism (CD) spectroscopic technique was adopted for the SNOM/NSOM system [12]. This technique enabled us to investigate the relationship between the macroscopic optical activity for the entire nanostructure and the local optical activity in the individual nanostructure [13–15]. Macroscopic CD spectra for arrayed samples of chiral nanostructures: S-shaped and its mirrored structures are shown in Fig. 93.4a. The CD spectra were symmetric with respect to zero level of CD signal (DA = 0), which clearly reproduces mirror symmetry of the chiral pair. Figure 93.4b shows near-field CD distributions for the chiral nanostructures. Both positive and negative local CD signals coexisted in the individual nanostructures. Also, the local CD distribution properly showed Fig. 93.3 Near-field two-photon excitation images excited at 780 nm for nanodimers of gold spherical particles (diameter 100 nm). Panel (a) shows a topographic image, which was simultaneously obtained with the two-photon excitation images of panels (b) and (c). Arrows in the panels indicate direction of linear polarizations [8]. Reproduced with permission from ref. 8. Copyright 2008, The Japan Society of Applied Physics 93 Scanning Near-Field Optical Microscopy/Near-Field … 581 Fig. 93.4 Macroscopic and local CD for a pair of chiral “S”-shaped nanostructures. a Macroscopic CD spectra were measured with the use of arrayed samples. b Distributions of local CD in the individual nanostructures observed with a near-field CD microscopy. The CD value was defined as DA = ALCP − ARCP, where ALCP and ARCP represent the absorbances for left and right circularly polarized lights, respectively. Reproduced from Ref. 12 with permission from the PCCP Owner Societies. Reprinted with permission from Ref. 14. Copyright 2014 American Chemical Society antisymmetric relation brought by structural symmetry of the chiral pair of nanostructures. References 1. Saiki, T., Matsuda, K.: Near-field optical fiber probe optimized for illuminationcollection hybrid mode operation. Appl. Phys. Lett. 74, 2773–2775 (1999) 2. Novotny, L., Hecht, B.: Principles of Nano-Optics, 1st edn, pp. 173–224. Cambridge University Press, Cambridge (2006) 3. Anger, P., Bharadwaj, P., Novotny, L.: Enhancement and quenching of single molecule fluorescence. Phys. Rev. Lett. 96, 113002 (2006) 4. Hoeppener, C., Novotny, L.: Antenna-based optical imaging of single Ca2 transmembrane proteins in liquids. Nano Lett. 8, 642–646 (2008) 5. Keilmann, F., Hillenbrand, R.: Near-field microscopy by elastic light scattering from a tip. Phil. Trans. Roy. Soc. Lond. A 362, 787–805 (2004) 6. Chen, C., Hayazawa, N., Kawata, S.: A 1.7 nm resolution chemical analysis of carbon nanotubes by tip-enhanced Raman imaging in the ambient. Nat. Commun. 5, 3312 (2014) 582 T. Narushima 7. Okamoto, H., Narushima, T., Nishiyama, Y., Imura, K.: Local optical responses of plasmon resonances visualised by near-field optical imaging. Phys. Chem. Chem. Phys. (Perspective) 17, 6192–6206 (2015) 8. Okamoto, H., Imura, K.: Near-Field Optical Imaging of Nanoscale Optical Fields and Plasmon Waves. Jpn. J. Appl. Phys. 47, 6055–6062 (2008) 9. Vallius, T., Jefimovs, K., Turunen, J., Vahimaa, P., Svirko, Y.: Optical activity in subwavelength-period arrays of chiral metallic particles. Appl. Phys. Lett. 83, 234–236 (2003) 10. Kuwata-Gonokami, M. et al.: Giant optical activity in quasi-two-dimensional planar nanostructures. Phys. Rev. Lett. 95, 227401-1-4 (2005) 11. Schäferling, M., Dregely, D., Hentschel, M. Giessen, H.: Tailoring enhanced optical chirality: Design principles for chiral plasmonic nanostructures. Phys. Rev. X 2, 031010-1-9 (2012) 12. Narushima, T., Okamoto, H.: Circular dichroism nano-imaging of two-dimensional metal nanostructures. Phys. Chem. Chem. Phys. 15, 13805–13809 (2013) 13. Narushima, T., Okamoto, H.: Strong nanoscale optical activity localized in two-dimensional chiral metal nanostructures. J. Phys. Chem. C. 117, 23964–23969 (2013) 14. Narushima, T., Hashiyada, S., Okamoto, H.: Nanoscopic study on developing optical activity with increasing chirality for two-dimensional metal nanostructures. ACS photonics 1, 732– 738 (2014) 15. Hashiyada, S., Narushima, T., Okamoto, H.: Local optical activity in achiral two-dimensional gold nanostructures. J. Phys. Chem. C. 118, 22229–22233 (2014) Chapter 94 Scanning Probe Microscopy Ken Nakajima Keywords Atomic resolution Topography Interaction Thermal conductivity Infrared absorption and reflection 94.1 Principle Scanning probe microscopy (SPM) is a kind of microscopy that generates images of surface features by mechanically scanning a physical probe over the specimen under study, in which the concomitant response of a detector is measured. This generic term encompasses STM, SFM, SNOM, SCM, SKPM, SICM, etc., where “X” of SXM denotes interactions between the probe and the specimen. For instance, “T” of STM expresses “tunneling current” and “F” of SFM “force.” Depending on the detail of interaction force, SFM has more specific commonly used names such as AFM (atomic force), MFM (magnetic force), and FFM (friction force). The resolution of each SPM varies somewhat with a kind of interaction, but some reach an atomic resolution. The nature of an SPM probe depends on the type of SPM being used. However, certain characteristics are common to all SPMs: the probe must have a very sharp apex to realize high-resolution feature. Many SPMs can image several interactions simultaneously. In case of AFM-based SPMs, AFM is used to trace a surface topography, and other quantities are being measured at the same time to visualize their two-dimensional image associated with the topographic image. K. Nakajima (&) School of Materials and Chemical Technology, Tokyo Institute of Technology, Tokyo, Japan e-mail: nakajima.k.aa@m.titeach.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_94 583 584 94.2 K. Nakajima Features • Many different types of surface images are obtained with use of different SPMs. • Spectroscopic measurement can be performed by regulating a control physical quantity. • Multiple combinations of SPMs are possible such as STM/AFM, AFM/Infrared. • SPM can be operated in ambient condition, UHV, gas atmosphere, and liquid. • Temperature control is easily associated with many SPMs. 94.3 Instrumentation To form images, SPMs scan the probe over the surface laterally in raster manner using piezoelectric scanner (see Fig. 94.1). At each discrete point in the raster scan (scanning plane is the XY-plane), a value is recorded. These recorded values are displayed as a SPM image. There are two different modes of operation. In constant interaction mode, a feedback loop is used to vertically move the probe up and down in the Z-axis to maintain a constant interaction. This interaction depends on the type of SPM. In case of static SFM, the interaction is the force, that is, a cantilever deflection. The Z position of the probe (voltage applied to Z piezoelectric scanner) is recorded and displayed as a topographic image. In constant height mode, the probe is not moved in the Z-axis during the scan. Instead, the value of the interaction under study is recorded. This recorded information is displayed as interaction map. Constant height imaging is more difficult if the scanning is performed on uneven surfaces. To avoid this problem, usually any SPMs are associated with AFM which traces the topographic feature keeping a gap distance constant during detecting the interaction quantities. Fig. 94.1 Principle of SPM operation 94 Scanning Probe Microscopy 94.4 585 Applications 94.4.1 Scanning Thermal Microscopy Here describes a rather new SPM, scanning thermal microscopy (SThM), an SPM method in which a thermal sensor is integrated into the probe to measure both topography and thermal properties [1]. The thermal probes are microfabricated and use thermocouple as thermal sensing elements. Since the thermocouple is located at the apex of the probe, a local absolute temperature map can be generated if the thermal probe is calibrated against a standard thermocouple. The thermal conductivity contrast of the specimen under study can be also recorded. In this mode of operation, the thermocouple is heated slightly above the ambient temperature using an optical laser of AFM. When the thermal probe is scanned over a sample, the amount of heat flow from the thermocouple to the specimen is governed by its thermal conductivity. In Fig. 94.2, bismuth telluride nanocrystalline film co-sputtered with carbon shows the thermal conductivity variations at the crystallite and grain boundaries. This is the first time the thermoelectric behavior of this material was demonstrated with a SThM technology. Fig. 94.2 Thermal conductivity map of bismuth telluride co-sputtered with carbon nanocyrstalline films. The scan size is 1.0 µm. Relative thermal conductivity variations signify the difference in the thermal conduction in the crystallites and at the grain boundaries. (Image courtesy: Prof. B. R. Mehta, IIT-Delhi) 586 K. Nakajima Fig. 94.3 IR reflection signal (1900 cm−1) overlaid on height image of pentacene molecule. The scan size is 3.3 µm. (Image courtesy: Bruker Nano Surfaces Division) 94.4.2 AFM/Infrared AFM/Infrared (IR) is one of the future-promising instrumentation that extends AFM into the chemical regime by providing IR absorption and reflection imaging based on scattering-type scanning near-field optical microscopy (sSNOM). IR light from a laser source is split in asymmetric Michelson interferometer, and one part is focused onto the end of a metallized AFM tip, which acts as a “nanoscale antenna.” In contact with a sample, evanescent fields at the tip apex result in a local sample polarization, which in turn acts back on the tip polarization, eventually converting the polarization as radiation into the far-field [2]. The phase-sensitive detection of this backscattered light from the tip is realized by combining the interferometer and a MCT detector. This technique offers a straightforward way to directly obtain the near-field IR absorption and reflection, the nanoscale analogue to conventional, and far-field FTIR absorption and reflection. Figure 94.3 shows IR reflection signal (1900 cm−1) overlaid on height image of pentacene molecule, showing monolayer sensitivity of this technique. References 1. Goeckeritz, J., Aden, G., Chand, A.: Nanometer Thermal Conductivity Mapping Using Laser-based Scanning Thermal Microscopy. MRS Online Proceeding Library Archive 1754, 81–86 (2015) 2. Xu, X.G., Tanur, A.E., Walker, G.C.: Phase controlled homodyne infrared near-field microscopy and spectroscopy reveal inhomogeneity within and among individual boron nitride nanotubes. J. Phys. Chem. A 117, 3348–3354 (2013) Chapter 95 Scanning Transmission Electron Microscopy Koji Kimoto Keywords Annular dark-field (ADF) Annular bright-field (ABF) 95.1 Bright-field (BF) Principle Scanning transmission electron microscopy (STEM) [1, 2] is a method of observing a small area using an incident electron probe, which is scanned on a thin specimen (Fig. 95.1). Various electron signals from the specimen, including transmitted electrons, diffracted electrons, thermal diffuse scattered electrons, and secondary electrons, are simultaneously measured as a function of the position of the incident electron probe, resulting in two-dimensional STEM images. Bright-field (BF), annular BF, and annular dark-field (ADF) imaging are normally applied. The spatial resolution of STEM images basically depends on the size of the incident probe, and atomic resolution has already been realized. STEM combined with analytical techniques, such as energy-dispersive X-ray spectroscopy (EDX) and electron energy-loss spectroscopy (EELS), allows us to perform chemical analyses with a high spatial resolution. 95.2 Features • High-resolution imaging can be performed using a fine incident electron probe. • Various STEM images, such as BF, ABF, and ADF images, are simultaneously observed. K. Kimoto (&) Electron Microscopy Group, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan e-mail: kimoto.koji@nims.go.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_95 587 588 K. Kimoto Fig. 95.1 Schematic diagram of STEM • The spatial resolution and imaging property of each STEM image depend on the incident probe size and the detected signal. • Combination with analytical methods enables chemical analysis with high spatial resolution. 95.3 Instrumentation STEM can be performed using a dedicated STEM instrument and a TEM instrument under a probe-forming condition. A schematic diagram of STEM is shown in Fig. 95.1. An incident electron, whose typical acceleration voltage is 60–300 kV, is focused on a specimen using condenser lenses and an objective lens. The objective lens in STEM is a probe-forming lens, whereas the objective lens in TEM is an image-forming lens. A condenser aperture, which is also called an objective aperture in STEM, defines the convergence angle (e.g., 20 mrad) of the incident 95 Scanning Transmission Electron Microscopy 589 probe. The incident probe is scanned using deflection coils. An aberration corrector for STEM is installed between the objective lens and the condenser lenses. High-brightness electron sources are required to realize a small intense probe. A few (BF, ABF, and ADF) STEM detectors are installed in a viewing chamber of the microscope, in which a diffraction pattern is projected. Figure 95.2 shows an example of diffraction patterns in STEM; a BF disk (000) and diffracted disks (e.g., 110) are overlapped. Each STEM detector measures the electrons of each detection angle range. By changing the camera length (i.e., the magnification of diffraction patterns), the detection angle range can be optimized. The BF detector covers a small scattering angle range (e.g., 0–5 mrad). The ADF detector, which has a donut shape, covers a relatively large scattering angle range (e.g., 50–200 mrad) to detect thermal diffuse scattering, although typical Bragg angles are less than a few tens of mrad. The ABF detector covers the outer part of the BF disk (e.g., 10–20 mrad). The incident probe size depends on the brightness of the electron source, the wavelength k of the electrons, the convergence semiangle a, and the aberrations of the objective lens (e.g., third-order spherical aberration C3). Although a wide convergence angle is necessary to reduce the diffraction limit (1.22k/a), the effect of aberration becomes evident (e.g., C3a3). Recent aberration correctors can eliminate the following aberrations: defocus, twofold astigmatism, axial coma, threefold astigmatism, axial star, fourfold astigmatism, and third-order spherical aberrations. The alignment of these aberrations is critical to realize high spatial resolution. The spatial resolution of STEM-based techniques also depends on the propagation of the incident probe in the specimen. The incident electrons focused on the crystalline specimen propagate along atomic columns (so-called channeling), which is a major feature of the incident electron propagation in STEM. Fig. 95.2 Diffraction pattern of STEM and schematic of detection angle ranges of BF, ABF, and ADF detectors 590 95.4 K. Kimoto Applications BF, ADF, and ABF imaging have different properties and mechanisms, and their combination is useful for material characterization. Figure 95.3 shows examples of ABF and ADF images of SrTiO3 (001). Each imaging property is described as follows. 95.4.1 BF Imaging The BF image contrast in STEM is equal to that of a BF image in conventional transmission electron microscopy (CTEM) according to the reciprocity [3]. The convergence angle and BF detector angle in STEM correspond to the objective aperture angle and illumination angle in CTEM, respectively. If the probe size is smaller than the lattice fringe distance, we can observe coherent lattice fringes in BF-STEM images. A large STEM convergence angle, in which a BF disk and diffraction disks are overlapping, is required to observe lattice fringes, and it shows contrast reversal with varying specimen thickness and defocus of the objective lens, as well as lattice fringes of CTEM. Fig. 95.3 ABF (left) and ADF (right) images of SrTiO3 obtained at an acceleration voltage of 300 kV 95 Scanning Transmission Electron Microscopy 591 95.4.2 (HA)ADF Imaging ADF imaging is a powerful imaging technique because of its intuitive interpretability, atomic resolution, and compositional sensitivity [1]. High-angle (HA) ADF imaging is considered to be incoherent imaging, in which the image contrast is the convolution between atomic arrangements and the probe intensity distribution [4]. The contrast is considered to be the integrated intensity scattered by each atom in an atomic column, and atomic columns are usually observed as bright dots. Therefore, HAADF imaging has the potential to identify even the type and number of atoms in each atomic column. It has been reported that the ADF image contrast is proportional to Zn, where Z is the atomic number and n = 1.5–2.0. The parameter n depends on the ADF detection angle range. The Z2 dependence is similar to Rutherford scattering, although the high-angle scattering is mainly due to thermal diffuse scattering, as shown in Fig. 95.2. ADF imaging is suitable for visualizing heavy atoms. 95.4.3 ABF Imaging ABF imaging is utilized for visualizing relatively light elements. Atomic columns are observed as dark spots (Fig. 95.3). There are a few reports on interpreting the imaging mechanism, and the contrast is roughly proportional to Zn, where n * 1/3 [5]. (a) (b) ADF contrast (%) 0.04 Fig. 95.4 ADF image of single-layer graphene obtained at 80 kV 0.07 592 K. Kimoto 95.4.4 Quantitative ADF Imaging ADF imaging has become a standard imaging technique, and we can quantitatively compare experimental results with simulated results [6, 7]. The quantitative ADF contrast can be calculated using the following optical parameters: ADF detection angle range, aberrations of the objective lens, defocus spread, and effective source distribution. Figure 95.4 shows experimentally obtained and simulated ADF images of single-layer graphene ADF image obtained at an acceleration voltage of 80 kV. The quantitative ADF contrast is the ratio of the ADF detector current to the incident probe current. The difference between the results of the experiment and simulation is mainly due to the shot noise in the ADF signal. References 1. Pennycook, S.J., Nellist, P.D.: Scanning Transmission Electron Microscopy. Imaging and Analysis, Springer, New York (2011) 2. Tanaka, N.: Scanning Transmission Electron Microscopy of Nanomaterials. Imperial College Press, London (2015) 3. Cowley, J.M.: Image contrast in a transmission scanning electron microscope. Appl. Phys. Lett. 15, 58–59 (1969) 4. Pennycook, S.J., Jesson, D.E.: High-resolution Z-contrast imaging of crystals. Ultramicroscopy 37, 14–38 (1991) 5. Findlay, S.D., Shibata, N., Sawada, H., Okunishi, E., Kondo, Y., Ikuhara, Y.: Dynamics of annular bright field imaging in scanning transmission electron microscopy. Ultramicroscopy 110, 903–923 (2010) 6. LeBeau, J.M., Stemmer, S.: Experimental quantification of annular dark-field images in scanning transmission electron microscopy. Ultramicroscopy 108, 1653–1658 (2008) 7. Yamashita, S., Koshiya, S., Nagai, T., Kikkawa, J., Ishizuka, K., Kimoto, K.: Quantitative annular dark-field imaging of single-layer graphene-II: atomic-resolution image contrast. Microscopy 64, 409–418 (2015) Chapter 96 Scanning Transmission X-Ray Microscopy Yasuo Takeichi Keywords X-ray imaging NEXAFS Chemical imaging Magnetic imaging 96.1 Spectromicroscopy Principle Scanning transmission X-ray microscopy (STXM) is a method to obtain a microscopic image of the raster-scanned sample by detecting the transmission intensity of the focused X-rays. As drawn in Fig. 96.1, a Fresnel zone plate (FZP) is often used to focus the soft X-rays from the synchrotron radiation sources [1]. An order-sorting aperture (OSA) is used to omit the zeroth and higher order diffractions. The photon energy can be tuned around the absorption edge of a specific element. The spatial resolution, i.e. the focusing size of the X-rays in the soft X-ray STXM is typically 20–100 nm. It is in principle determined by the diffraction limit of the lithographically fabricated FZPs [1]. The most important measurement mode in the STXM is an “image stack,” that is, a number of images at different photon energy points to obtain a dataset with space (XY) plus energy (E) dimensions. From the dataset, one can obtain a local spectrum to analyze the near-edge X-ray absorption fine structure (NEXAFS). Y. Takeichi (&) Institute of Materials Structure Science, High Energy Accelerator Research Organization, Tsukuba, Ibaraki, Japan e-mail: yasuo.takeichi@kek.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_96 593 594 Y. Takeichi Sample Y Transmission High Low FZP High Low Absorption Sample X OSA Sample X-ray detector Fig. 96.1 Principle of the soft X-ray scanning transmission X-ray microscopy 96.2 Features • Element distribution contrast images can be obtained. • NEXAFS spectrum of a specific area down to < 100 nm can be obtained. • X-ray magnetic circular dichroism (XMCD) and linear dichroism (LD) experiments can be performed. • In situ environment such as chemical cells, wet cells, static and radio-frequency fields can be applied. 96.3 Instrumentation As summarized in Ref. [2], soft X-ray STXMs are widely introduced to second- and third-generation synchrotron radiation facilities. Many of them are based on a design developed at the Advanced Light Source [3]. On the other hand, efforts are made to introduce STXM without building a dedicated beamline [4]. In both cases, the microscopes are designed with features as follows; Synchrotron radiation X-rays from an undulator or bending magnet source is monochromatized, and then focused using the FZPs. The focal distance from the FZP to the sample is typically several mms. The sample should be placed at the focal point within the depth of focus of * lm. The sample position is raster scanned with piezoelectric scanner, and then monitored with laser interferometic sensors. The photomultiplier tube is often used to detect the transmitted X-rays in the pulse-counting manner. Fully digitized control electronics are used to realize a fast scan with the dwell time at each pixel of* milliseconds. 96 Scanning Transmission X-Ray Microscopy 595 In principle, all the optics should be in vacuum, however, the ultra-high vacuum is not necessary. The STXM chamber containing the components from the FZP to the detector are often separated from the synchrotron beamline by a thin Si3N4 membrane window. This allows a quick replacement of the sample and feasibility for in situ environments. The samples should not be too thick (this results in no transmitted signals) or too thin (no significant absorption). Ideally, the sample should have the optical density OD ¼ InðI0 =I Þ 1 at the target photon energy. For this purpose, the specimens for STXM are often prepared by dispersing the particles onto Si3N4 membranes or grid meshes with support films. Larger samples are usually sectioned by using microtomes or focused ion beam (FIB). 96.4 Applications 96.4.1 Chemical imaging by image stack measurement The image-stack dataset realizes a better illustration of the chemical compositions than an image obtained at a fixed photon energy. This can be done by statistical analysis methods such as singular value decomposition (SVD) [5], cluster analysis, and principal component analysis [6]. Figure 96.2 shows an example of chemical composition mapping on a core-shell polymeric microsphere [7]. The sample was prepared by microtome sectioning after being embedded in epoxy resin. The reference C 1s spectra of divinylbenzene (DVB), ethylene glycol dimethylacrylate (EGDMA), and an aliphatic epoxy were measured separately as Fig. 96.2a. The image-stack dataset were decomposed using the SVD method. Physically, this procedure corresponds to obtaining fitting coefficients of the three chemical species to the NEXAFS spectrum at each pixel. The images at specific photon energies are shown in Fig. 96.2(b–e), and the color composite map displayed in Fig. 96.2f clearly represents the distribution of chemical compositions. 96.4.2 Magnetic domain observation by XMCD measurement Polarization-dependent experiments such as XMCD and LD are possible as with the bulk spectroscopy. Figure 96.3 shows an example of the XMCD experiments performed on an Nd–Fe–B permanent magnet [4]. The specimen was a thin section prepared by using FIB. The photon energy was set to 1003 eV (Nd M4-edge) and the polarization dependent measurement was done using an APPLE-II type 596 Y. Takeichi Fig. 96.2 a C 1 s NEXAFS spectra of divinylbenzene (DVB), ethylene glycol dimethylacrylate (EGDMA), and an aliphatic epoxy. b–e Optical density images at 282.0, 285.1, 288.4, and 300.0 eV. f Color composite map representing epoxy (red), DVB (green), and EGDMA (blue). Reprinted from Ref. [7], Copyright 2007, with permission from Elsevier undulator radiation. The magnetic domain image shown in Fig. 96.3b was obtained by taking the asymmetry A ¼ ðIL IR Þ=ðIL þ IR Þ of the successively obtained images with left- and right-handed polarization. The maze and stripe domains characteristic of the thin film with strong perpendicular anisotropy was successfully observed. 96 Scanning Transmission X-Ray Microscopy Fig. 96.3 a Optical density image and b X-ray magnetic circular dichroism asymmetry image of an Nd–Fe–B specimen. Reprinted with permission from ref. [4]. Copyright 2016, American Institute of Physics (a) 597 (b) 1um Optical density 0.0 2.2 Asymmetry -0.14 +0.14 References 1. Attwood, D.: Soft X-rays and Extreme Ultraviolet Radiation: Principles and Applications, Chap. 9. Cambridge University Press (1999) 2. Hitchcock, A.P.: Soft X-ray spectromicroscopy and ptychography. J. Elec. Spectrosc. Relat. Phenom. 200, 49–63 (2015) 3. Kilcoyne, A.L.D., Tyliszczak, T., Steele, W.F., Fakra, S., Hitchcock, P., Franck, K., Anderson, E., Harteneck, B., Rightor, E.G., Mitchell, G.E., Hitchcock, A.P., Yang, L., Warwick, T., Ade, H.: Interferometer-controlled scanning transmission X-ray microscopes at the Advanced Light Source. J. Synchrotron Rad. 10, 125–136 (2003) 4. Takeichi, Y., Inami, N., Suga, H., Miyamoto, C., Ueno, T., Mase, K., Takahashi, Y., Ono, K.: Design and performance of a compact scanning transmission X-ray microscope at the Photon Factory. Rev. Sci. Instrum. 87, 013704 (2016) 5. Koprinarov, I., Hitchcock, A.P., Li, W.H., Heng, Y.M., Stöver, H.D.H.: Quantitative Compositional Mapping of Core-Shell PolymerMicrospheres by Soft X-ray Spectromicroscopy. Macromol. 34, 4424–4429 (2001) 6. Lerotic, M., Mak, R., Wirick, S., Meirer, F., Jacobsen, C.: MANTiS: a program for the analysis of X-ray spectromicroscopy data. J. Synchrotron Rad. 21, 1206–1212 (2014) 7. Hitchcock, A.P., Morin, C., Zhang, X., Araki, T., Dynes, J., Stöver, H., Brash, J., Lawrence, J. R., Leppard, G.G.: Soft X-ray spectromicroscopy of biological and synthetic polymer systems. J. Elec. Spectrosc. Relat. Phenom. 144, 259–269 (2005) Chapter 97 Scanning Tunneling Microscopy Yukio Hasegawa Keywords Surface atomic structure Surface morphology Surface electronic states Atom manipulation 97.1 Tunneling current Principle STM is a surface microscope with extremely high spatial resolution, which enables us to see atoms on surfaces. When a sharp metal needle is located at a very proximate distance (*1 nm) from the sample surface (left panel in Fig. 97.1), tiny amount of electrical flow, called a tunneling current, is induced between them. Since the current is so sensitive to the variation in the tip-sample gap distance, atomic-scale surface corrugation can be detected by monitoring the current during the lateral scanning of the tip over the surface. The tunneling current, which originates from quantum mechanics, is due to the overlap of the wave functions of the probe tip and sample surface. Since both wave functions decay rapidly in the gap region, the amount of the overlap and thus the tunneling current change drastically with the gap distance. A simple calculation tells us that the tunneling current is proportional to exp(−10z√U), where z (nm) is the gap distance between the tip and sample and U (eV) is a potential barrier height or (averaged) work function. Using a typical number of U (5 eV), the current is found to change by an order of magnitude with the gap-distance variation of 0.1 nm, which is less than typical atomic size. Since the tunneling current is closely related to the wave functions, or electronic states, STM images also reflect the spatial distribution of the electronic states of the sample surfaces. According to the Tersoff–Hamann theory, STM image is a spatial mapping of the amplitude of surface wave functions whose energy is around the Fermi level. Usually, the wave function has a large amount of amplitude around surface atoms, and therefore, the atomic structure can be observed in the STM Y. Hasegawa (&) The Institute for Solid State Physics, The University of Tokyo, Kashiwa, Japan e-mail: hasegawa@issp.u-tokyo.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_97 599 600 Y. Hasegawa Fig. 97.1 (left) Schematic of the tip and sample in STM. (right) Potential diagram for electron tunneling between the tip and sample surface image. But some adsorbates are observed dark in the image when they reduce the wave function amplitude there. With the negative bias voltage applied to the sample, STM probes the wave functions of the sample whose energy level is below the Fermi energy (filled states, right panel in Fig. 97.1), whereas the states above the Fermi energy (empty states) can be probed by the positive sample bias voltage. Because of the polarity dependence, STM images taken at both polarities look different, in particular, on semiconductor surfaces (e.g., Figure 97.3). In more quantitative description, electronic states whose energy level is between the Fermi energy EF and EF + eV, where V is the bias voltage applied to the sample, contribute to the tunneling current and STM images (right panel in Fig. 97.1). In other words, the tunneling current is proportional to the integral of the local density of states in the energy range. When the bias voltage is raised so that the integral range covers the energy level of surface states, the contrast of the STM image may change drastically reflecting the newly involved states (e.g., Figure 97.4). The electronic sensitivity of the tunneling current also makes STM a local probe of surface electronic states with atomic-scale spatial resolution, whose function is specifically called scanning tunneling spectroscopy (STS). Since STM utilizes the tunneling current as a probe, the sample has to be conductive electrically. STM cannot be used for the characterization of insulating materials. 97.2 Features • Visualization of surface atomic structure. • Observation of surface morphology with atomic-scale spatial resolution. • Local probing and spatial mapping of surface electronic states around the Fermi level. • Manipulation of atoms and molecules on surfaces for nanofabrication. 97 Scanning Tunneling Microscopy 601 • Investigation of local excitation and response on surfaces by electrical field and/or current. 97.3 Instrumentation In order to precisely control the gap distance between the probe tip and sample surface and scan the tip over the surface, the tip is mounted on a piezo-actuator with which one can precisely control the tip position in the xyz directions through an application of voltages on it. In usual STM imaging mode, which is often called constant current mode, the gap distance is feedback-controlled with the applied voltage on the piezo so that the tunneling current is kept constant. In this mode, the height information for the image is obtained by monitoring the piezo-voltage. The maximum span of the piezo is *1 lm, which indicates that the tip has to be brought within the range by other methods. Several coarse approaching mechanisms, such as Pan-type, Besoke-type, which are named after the inventors’ names, and various types of inertial sliders, have been developed. The design of the STM unit basically depends on which type of coarse approaching is employed. In order to achieve high performance of STM, mechanical and electrical noises have to be minimized. Designing the compact STM unit to achieve high resonance frequency between the tip and sample is crucial to suppress low-frequency vibrations from outside. A whole system is situated on a vibration-isolation table. A spring suspension system, an elastomer (e.g., Viton) stack, and an eddy-current damper are often used inside the chamber for the vibrational suppression (Fig. 97.2). Since the amount of tunneling current is typically < 100 pA, significant care should be taken to its noise level, and appropriate shielding has to be installed on the current line if necessary. The electrical noise in high voltage amplifiers that drive the piezo-actuator sometimes limits the performance and should also be cared. 97.4 Applications 97.4.1 Atomically Resolved Imaging of the Si(111)7 x 7 Surface Since the observation of 1/7 ordered spots in an electron diffraction pattern, atomic structure of the Si(111)7 x 7 reconstructed surface had been a long-standing unsolved issue in surface science community until Binnig, Rohrer et al. [1] observed the atomically resolved real-space image of the surface with STM. The first STM image showed 12 protrusions in the rhombus unit cell and revealed inhomogeneous contrasts between the two triangular halves, indicating the 602 Y. Hasegawa Fig. 97.2 Schematic of STM instrumentation Fig. 97.3 STM images of the Si(111)7 x 7 surface taken with the sample bias voltage of negative (a) and positive (b) polarities existence of 12 adatoms and stacking fault within the unit cell, respectively. Later, detailed analysis confirmed that the reconstructed structure can be described well by a dimer–adatom–stacking fault (DAS) model proposed by Takayamagi et al. [2]. Figure 97.3 shows typical STM images taken on the Si(111)7 x 7 surface by negative (a) and positive (b) sample bias polarities, implying (a) and (b) correspond to a spatial mapping of filled and empty states of the surface, respectively. In both images, 12 silicon adatoms, which are separated by 0.77 nm, are clearly resolved in the rhombus unit cell. Whereas all adatoms look similar in the empty-state image (b), in the filled-state image (a), the adatoms in the left-pointing triangles look brighter than those in the right-pointing triangles. The contrast difference can be explained by the difference in electronic states, and it is indeed due to the presence 97 Scanning Tunneling Microscopy 603 of stacking fault in the left-pointing halves (faulted half). A careful observer may notice that within the triangular halves the adatoms located at the corner (corner adatoms) look brighter than those at the center (center adatoms). These are also due to electronic origin and can be explained by the amount of electron donation from the adatom to neighboring rest atoms. 97.4.2 Imaging of Molecular Orbitals STM provides us not only the atomic structure of the sample surface but also the spatial mapping of its electronic states. Here, imaging of molecular orbitals is presented as an example of the electronic state observation [3]. As mentioned in principle, an STM image corresponds to a spatial mapping of electronic states whose energy levels are in a range of the Fermi level EF to EF + eV, where V is the applied sample bias voltage. In the case of a single molecule, the energy levels of the electronic states are discrete, and the filled and empty states nearest to the Fermi level are called highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), respectively. As shown below, one can obtain the spatial mapping of HOMO and LUMO states of a molecule by setting the bias voltage so that the energy level of the state is situated within the probing range. Figure 97.4 shows STM images taken on a pentacene molecule adsorbed on a substrate. Since direct molecular adsorption modifies the molecular orbitals through Fig. 97.4 (top) STM images of a pentacene molecule taken with the sample bias voltage of 2.5 V (left), +0.5 V (center), and +1.8 V (right). (bottom) Calculated HOMO (left) and LUMO (right) states of a free pentacene molecule. Reprinted with permission from ref. [3]. Copyright 2005 by American Physical Society 604 Y. Hasegawa the hybridization with the electronic states of the substrate, the observed molecule is situated on an atomically thin insulating layer (NaCl) formed on a metal substrate (Cu(111)). The top-left STM image was taken with the sample bias voltage of −2.5 V. Since the energy level of the HOMO state is −2.4 eV from EF, which is within the probing range of EF to EF −2.5 eV, the spatial mapping of the HOMO state is imaged. A characteristic pattern showing a node along the long axis and 4 nodes along the short axis of the molecule, which are expected from a theoretical calculation (left bottom), is clearly observed. References 1. Binnig, G., Rohrer, H., Gerber, Ch., Weibel, E.: 7 7 reconstruction on Si(111) resolved in real space. Phys. Rev. Lett. 50, 120 (1983) 2. Takayanagi, K., Tanishiro, Y., Takahashi, M., Takahashi, S.: Structural analysis of Si(111)7 7 by UHV transmission electron diffraction and microscopy. J. Vac. Sci. Technol. A3, 1502 (1985) 3. Repp, J., Meyer, G., Stojković, S.M., Gourdon, A., Joachim, C.: Molecules on Insulating Films: Scanning-Tunneling Microscopy Imaging of Individual Molecular Orbitals. Phys. Rev. Lett. 94, 026803 (2005) Chapter 98 Scanning Tunneling Spectroscopy Keisuke Sagisaka Keywords Tunneling spectroscopy Local density of states (LDOS) Differential conductance LDOS map 98.1 Principle Spectroscopic measurement performed with STM, referred to as scanning tunneling spectroscopy (STS), provides information proportional to the local density of states (LDOS), the number of states per unit energy, of the sample surface. The STS measurement is usually made by positioning the STM tip over a target feature of a surface, deactivating the feedback loop to fix the STM tip height, applying a sample bias ramp, and recording tunneling current (I) or differential conductance (dI/dV). Assuming that the tip LDOS and the tunneling matrix element are independent of the energy e, an approximate form of the tunneling current with a bias voltage V using the Tersoff–Hamann model is [1] I/ eV Z qs ðEF þ e; r0 Þd ð1Þ 0 where qs is the sample surface LDOS, and r0 is the center of curvature of the tip. The differential conductance with respect to V is found to be [1] dI / qs ðEF þ eV; r0 Þ: dV ð2Þ K. Sagisaka (&) Research Center for Advanced Measurement and Characterization, National Institute for Materials Science, Tsukuba, Japan e-mail: SAGISAKA.Keisuke@nims.go.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_98 605 606 K. Sagisaka Equation (2) indicates that dI/dV is proportional to the surface LDOS. V = 0 corresponds to the Fermi energy of the sample surface so that STS can detect both occupied and unoccupied states, depending on the polarity of the bias voltage. Since the precision of tip positioning across the surface relies upon that of the STM, STS can perform a measurement with the same spatial resolution as the STM. This enables detection of very local features, such as single atoms, single molecules, and even the internal structure of a single molecule. With appropriate instrument configuration, STS is routinely performed under various environments such as magnetic field, light illumination, and applied stress, to study the responses of surface electronic states to these external perturbations. When STS is performed at every pixel during STM imaging, it is referred to as current-imaging-tunneling spectroscopy (CITS) [2] or spectroscopic imaging (SI) [3]. This mode is time-consuming for a large number of measurements but provides a very informative data set with the surface LDOS, the energy and the position across the surface. With this mode of data collection, one can generate spectra at selected positions or surface LDOS maps at selected energies. Another mode to acquire a map of surface LDOS is known as dI/dV imaging: recording the dI/dV signal through a lock-in amplifier simultaneously during topographic imaging at a fixed sample bias. Interpretation of spectroscopic data or dI/dV maps obtained by the approaches as discussed above requires some care. The tip height prior to STS is usually regulated by a set point (combination of I and V, that is, the integrated LDOS at the tip position from the Fermi level to eV). If the variation of LDOS is large across the surface, the tip height used to measure spectra may not be constant between different areas. This causes difficulty for relative analysis of position-dependent spectroscopic data. To reduce this effect, normalization has been proposed by Feenstra et al. [4]. It is also proposed that a ratio of dI/dV maps with two bias voltages can be effective to eliminate the set point effect [5]. 98.2 Features • Local density of states of the surface in both occupied and unoccupied states can be measured with atomic resolution. • A dI/dV map provides spatial distribution of local density of states at selected energies. • It is possible to perform STS under various environments such as magnetic fields, light illumination, and applied stress, to study the response of surface electronic states to these perturbations. • Energy resolution is determined by sample and tip temperatures and sample bias modulation when lock-in detection is used. 98 Scanning Tunneling Spectroscopy 98.3 607 Instrumentation The measurement can be performed in conventional STM apparatus but it requires functions to disable the feedback electronics and to sweep the bias voltage during the STM observation. Nowadays commercially available controllers usually include these functions with their software and electronics. To acquire a dI/dV spectrum, numerical differentiation of an I–V curve or direct detection of dI/dV signals by a lock-in technique is used. The former usually does not require any extra setting and its advantage is rapid measurement so that it is useful to check the conditions of a sample surface and tip apex easily. However, since the signals always include a finite quantity of noise, numerical differentiation may exaggerate this noise or may cause serious broadening of a spectrum, potentially misleading the interpretation. The latter method requires a lock-in amplifier and auxiliary equipment such as an adder for the bias voltage to superimpose an AC modulation voltage (sine waves with a small amplitude). This technique can directly extract the dI/dV signal with high signal-to-noise ratio due to the noise filtering capability of the lock-in amplifier. When the lock-in technique is employed, the energy resolution of STS is limited by modulation voltage Vmod, as well as by thermal broadening governed by the Fermi–Dirac distribution function. One example to evaluate the energy broadening at temperature T is [6] dE ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð3:3kB T Þ2 þ ð2:5Vmod Þ2 ; ð3Þ where kB is the Boltzmann constant. Figure 98.1 shows a test result of the modulation voltage effect on a single point spectroscopy performed on the Pb(111) surface. Broadening in spectra is more prominent with a larger Vmod. Fig. 98.1 Effect of modulation voltage on STS using a lock-in amplifier. Set point: +5 mV and 0.5 nA. Sample temperature: 0.4 K. Modulation frequency: 410 Hz 608 98.4 K. Sagisaka Applications Figure 98.2 shows an example of STS performed on the Si(001) surface, where the top silicon atoms are dimerized. Spectra of I, dI/dV, normalized dI/dV, and for comparison, projected DOS (PDOS) of silicon dimer calculated by density functional theory (DFT) are displayed. Here, normalization of dI/dV (divided by I/V) as proposed by Feenstra et al. [2] was employed. Both dI/dV and normalized spectra reveal electronic states of the dangling bond of the silicon dimer (p1, p2, p*1, and p*2), but the normalized spectrum compares better with the theoretically obtained PDOS. Figure 98.3 shows an example of dI/dV imaging applied to a quantum well structure fabricated on the Si(001) surface [7]. The dI/dV images were recorded through the lock-in detection. In contrast to the topographic features, the dI/ Fig. 98.2 STS performed on the Si(001) surface. The top three panels show spectra of I, dI/dV, and dI/dV normalized by (I/V). Set point: +1.0 V and 0.5 nA. Sample temperature: 78 K. Inset displays an STM image of Si dimers. The bottom panel shows projected density of states of a Si dimer calculated by DFT. A band gap of 0.5 eV is inserted between occupied and unoccupied states to compensate for the well-known underestimate of the band gap in DFT 98 Scanning Tunneling Spectroscopy 609 Fig. 98.3 Topographic and dI/dV images of a quantum well structure fabricated on the Si(001) surface [7]. The silicon dimer row sectioned with dots behaves as a quantum well, which shows that the number of maxima varies with energy. Set point: 0.5 nA, Modulation voltage: 20 mV, Modulation frequency: 6.5 kHz. Sample temperature: 78 K dV images uncover the spatial distribution of electronic structure in the quantum well. With sample bias voltage (energy), the higher indices of quantum states are visualized. References 1. A rigorous assessment of the tunneling current and dI/dV can be found in Chen, C. J.: Introduction to Scanning Tunneling Microscopy: 2nd edn. Oxford University Press (2008) 2. Hamers, R.J., Tromp, R.M., Demuth, J.E.: Surface electronic structure of Si(111)-(7 7) resolved in real space. Phys. Rev. Lett. 56, 1972 (1986) 610 K. Sagisaka 3. Lee, J., Slezak, J.A., Davis, J.C.: Spectroscopic imaging STM studies of high-Tc superconductivity. J. Phys. Chem. Solids 66, 1370 (2005) 4. Feenstra; R. M., Stroscio, J. A.; Fein; A.P., Tunneling spectroscopy of the Si(111)21 surface. Surf. Sci. 181, 295 (1987) 5. Kohsaka, Y., Taylor, C., Fujita, K., Schmidt, A., Lupien, C., Hanaguri, T., Azuma, M., Takano, M., Eisaki, E., Takagi, H., Uchida, S., Davis, J.C.: An intrinsic bond-centered electronic glass with unidirectional domains in underdoped cuprates. Science 315, 1380 (2008) 6. Morgenstern, M., Haude, D., Meyer, C., Wiesendanger, R.: Experimental evidence for edge-like states in three-dimensional electrons systems. Phys. Rev. B 64, 205104 (2001). Also, derivation of energy broadening is discussed in ref [1] 7. Sagisaka, K., Fujita, D.: A parabolic quantum well on a single dimer row of the Si(001) surface studied by scanning tunneling microscopy. J. Phys: Conf. Ser. 100, 052002 (2008) Chapter 99 Soft X-Ray Absorption Fine Structure Kenta Amemiya Keywords Electronic structure Chemical species Light elements Anisotropic structure 99.1 Element selectivity Principle Soft X-ray absorption fine structure (SXAFS) is based on the excitation of core-level electrons to the unoccupied states around the vacuum level, which is accompanied with soft X-ray absorption. Since each element has its own core-level energy, the SXAFS provides with element-specific information. The intensity of the X-ray absorption I, corresponding to the electron excitation, is given by 2 I ¼ wf jbe rjwi d Ef Ei hx ; where wi and wf are the wave functions of the initial and final states, Ei and Ef are the energy levels of them, and ê and ħx are the polarization vector and the photon energy of the incident X-ray [1]. Since the initial state, corresponding to the core level, is well defined, one can obtain information on the unoccupied final state. 99.2 Features • Element-selective information is obtained by using the core-level excitation. • Electronic states of the unoccupied orbitals are investigated, leading to the identification of chemical species. K. Amemiya (&) Institute of Materials Structure Science, High Energy Accelerator Research Organization, Ibaraki, Japan e-mail: kenta.amemiya@kek.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_99 611 612 K. Amemiya • Anisotropy in the electronic structure is observed by using linearly polarized soft X-rays, which enables us the determination of the molecular orientation. • Typical probing depths are a few and a few hundred nm in the electron and fluorescence-yield modes, respectively, in the soft X-ray region. 99.3 Instrumentation SXAFS measures X-ray absorption intensity as a function of incident photon energy, but it is often difficult to directly measure absorbance in the transmission mode, in which the intensity of the X-rays is monitored upstream and downstream of the sample. This is because the attenuation length of the soft X-ray is in the nanometer or micrometer region in solid materials, depending on the photon energy. If the transmission mode is not available, the electron- or fluorescence-yield modes are adopted, in which the Auger electrons or fluorescence X-rays emitted in the core-hole relaxation process after the X-ray absorption are detected as illustrated in Fig. 99.1. Since the number of Auger electrons or fluorescence X-rays is proportional to the number of absorbed X-rays, one can obtain the X-ray absorption intensity at each photon energy. In the electron-yield mode, a simpler experimental setup, total-electron-yield mode, is also available, in which the drain current from the sample, which compensates the emitted Auger electrons and secondary electrons, is measured with an ammeter. SXAFS requires soft X-rays with variable photon energies, which are available at synchrotron radiation facilities. The photon energy is chosen so as to excite appropriate core level of the element, of which one wants to observe the chemical and electronic states. For instance, the 1s electron is excited for investigating light elements such as C, N, and O, whose energy is several hundred eV. In the case of 3d transition metals such as Mn, Fe, and Co, the 2p level, whose energy is 400– 1000 eV, is suitable to be excited in order to investigate the electronic state of the Sample A X-rays Retarding voltage e Electron detector Fig. 99.1 Typical experimental setup for the SXAFS measurement Fluorescence X-ray detector 99 Soft X-Ray Absorption Fine Structure 613 3d orbitals. The linearly polarized X-rays are used to observe anisotropy in geometrical or electronic structures. The sample rotation system is effective to investigate the orientation of adsorbed molecules or anisotropic electronic structures. 99.4 Applications 99.4.1 Characterization of Unoccupied Molecular Orbitals Figure 99.2 shows O K-edge SXAFS data for carbon monoxide. The molecular orbitals are occupied up to the 5r level, which has a r-bonding character, while the anti-bonding 2p and higher-energy orbitals are unoccupied. Therefore, the strong X-ray absorption peak observed at *535 eV corresponds to the electron excitation from the O 1s core level to the 2p orbitals and the absorption peak at *550 eV to the O 1s ! 6r excitation. Thus, one can obtain information on the energy diagram for the unoccupied states, which lead to the identification of the chemical species. 99.4.2 Orientation of p Conjugated Molecules Figure 99.3 shows the angle-dependent C K-edge SXAFS spectra for a ring-type molecule, thiophene. Sharp peaks appearing between 285 and 290 eV originate Energy (eV) 6σ 2π 5σ 1π 4σ 3σ C 1s Absorption Intensity (arb. units) 5 4 3 2 1 0 O 1s 520 530 540 550 560 570 Photon Energy (eV) Fig. 99.2 Energy diagram of the molecular orbital carbon monoxide (left) and X-ray absorption spectrum taken at the oxygen K-edge (right) 614 12 Partial Electron Yield (arb. units) Fig. 99.3 Incidence-angle dependence of the C K-edge SXAFS spectrum for thiophene (C4H4S) molecule adsorbed on a Au(111) surface. The polarization vector, E, represents the direction of the electric field of X-ray K. Amemiya C K edge 10 E π∗1 hν 8 θ 6 4 π∗2+σ∗ C-S θ = 15 o θ = 55 o θ = 90 o σ∗C-C 2 0 280 290 300 310 Photon Energy (eV) from the electron excitation from C 1s to the p-type anti-bonding orbitals, p*1 and p*2, consisting of the C 2p orbitals, whose direction is normal to the molecular plane. In the case of the X-ray absorption at the s-type core level, the highest absorption intensity is obtained when the polarization vector is parallel to the direction of the p-type orbital, to which the electron is excited. Therefore, the peak intensity corresponding to the p*1 and p*2 orbitals becomes highest when the polarization vector is perpendicular to the molecular plane of thiophene. Thus, it is concluded from Fig. 99.3 that the thiophene molecule adsorbs on the surface with a flat-lying configuration. Reference 1. Stöhr, J.: NEXAFS Spectroscopy. Springer, Berlin (1992) Chapter 100 Spectroscopic Ellipsometry Takumi Moriyama Keywords Polarization Optical constants Dielectric constant Film thickness 100.1 Refractive index Principle When the linearly polarized light is reflected from a clean surface or a surface covered by a thin film, its polarization changes and the light becomes elliptically polarized. Ellipsometry measures this change in the polarization state of light upon reflection from a surface [1]. As a result of the measurement, ellipsometric angles W & D are obtained. 100.2 Instrumentation Optical setup of spectroscopic ellipsometer is shown in Fig. 100.1. It contains five main blocks, and the measurement has five steps: 1. Incident light beam is generated from a white light source, 2. linear polarization state is established by a polarizer, 3. when the incident light is reflected upon surface of a sample, the polarization state is changed, 4. polarization state is measured by analyzer, 5. data is detected by spectrometer combined with photomultiplier tube (PMT)/CCD/photo diode array (PDA) [2]. T. Moriyama (&) Analytical Technology Center, HORIBA TECHNO SERVICE Co., Ltd, Kyoto, Japan e-mail: takumi.moriyama@horiba.com © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_100 615 616 T. Moriyama Fig. 100.1 Optical setup of spectroscopic ellipsometer 100.3 Feature • Film thickness (d) and optical constants (n&k) could be obtained simultaneously even for unknown films. • The d and n&k for each layer could be obtained for multilayer structure. • It has superior sensitivity for ultrathin films evaluation. • In addition to d and n&k, it is possible to obtain information about material properties like: band gap, crystallinity, composition, and electrical properties. • The d and n&k could be monitored during film deposition. • It is possible to investigate air–liquid and liquid–solid interfaces. • Spectroscopic ellipsometry is very useful technic for development of new materials. 100.3.1 Basics of Optical Modeling In case of ellipsometrical measurements, the intensities or polarization state ratios (complex relative amplitude attenuation) are measured and the ellipsometrical angles W & D are calculated. No direct access exists to the parameters in which we are usually interested, such as the dielectric functions ðeÞ, refractive indices ðe n Þ, compositions, and film thicknesses (d). 100 Spectroscopic Ellipsometry 617 For semi-infinite case (bulk with no overlayer), e n can be calculated by Eq. (1) below. This is the only case that the refractive index could be uniquely determined directly from the measurement. " #12 ~n1 1q 2 2 ¼ sin / 1 þ tan / : ~n0 1þq ð1Þ e n1; e n 0 : Refractive index of semi-infinite substrate and medium, respectively / : Angle of incidence q : Polarization state ratios In general, for any planar structure on the substrate, W & D could be calculated if thicknesses and refractive indices are known. On the other hand, for the inverse case, even if W & D are known, d and e n could not be directly calculated. In order to obtain d and e n for each layer, the modeling is required. Modeling approach is based on the assumption that the measured W(k) and D(k) are changing at each wavelength according to dispersion law, discussed below. Determination of material properties could be done by describing the fundamental response of a material to an applied electromagnetic field. Each material has unique energy dependance of dielectric function e. In visible-near UV range, dielectric response is determined almost entirely by the electronic properties of a material. For example, glass and inorganic insulators are transparent in the near-infrared/visible ranges, due to the high binding energy of the bound electrons, and start to absorb only in the UV or deep-UV range. Relationship between dielectric function and refractive index given in (2) below is very important for the creation of materials optical model. e¼e n2 ð2Þ e ¼ e1 þ ie2 : ð3Þ where e1, e2 : Real and imaginary parts of the complex dielectric function and e n ¼ n ik: ð4Þ n, k : Real (refractive index) and imaginary (extinction coefficient) parts of the complex refractive index, usually called optical constants. 618 T. Moriyama In the case of materials with complicated electronic systems, one just has to summarize the individual dielectric responses of the crystal lattice, bound (few binding energies may exist) and free electrons. If the dielectric response of the material is determined successfully, the optical response can be calculated from Eq. (2). A mathematical description of the dielectric properties of a material, as well as its optical properties, as a function of energy (wavelength) is provided by dispersion law (formulae). 100.3.2 Flow of Analysis Procedure Flow of spectroscopic ellipsometric data analysis procedure is shown in Fig. 100.2: 1. Measurement data points W & D are obtained, 2. an optical model is constructed (assuming each layer’s thickness d and dispersion law for e n ) and calculated data points are obtained, 3. two sets of data points are numerically compared and the fitting of parameters is performed to minimize the “mean square error (v2)” and 4. layer thicknesses and optical constants spectra are obtained. Fig. 100.2 Flow for analysis procedure of spectroscopic ellipsometric data 100 Spectroscopic Ellipsometry Mes Th r N P 619 : Pair of measured data : Pair of calculated data : Standard deviation : Number of data points : Number of fitted parameters 100.3.3 Effective Medium Approximation All materials are inhomogeneous on atomic scale. Part of macroscopically homogeneous materials is inhomogeneous on microscopic scale [3]. If each separate region is large enough to possess their own dielectric identities, but small compared to wavelength of light, effective medium theory (EMT) can be used. Several EMT models exist, but most famous are Maxwell-Garnett (MG) and Bruggeman effective medium approximation (EMA). MG expression is used to calculate effective dielectric function in case of small inclusions inside the host material, while EMA is used for the case of random topologically symmetric microstructure. Examples of EMA models are shown in Fig. 100.3. We can describe polycrystalline material (crystalline + amorphous), microscopic roughness (material + void), and interlayer (mixture of neighboring materials) by using EMA models. Fig. 100.3 Examples of EMA models: a poly Si, b surface roughness, c interlayer 620 100.4 T. Moriyama Application Spectroscopic ellipsometry (SE) is very powerful technique for the characterization of existing materials, structures, deposition tools, and development of the new ones. In the semiconductor industry, SE started to replace laser ellipsometry about 30 years ago. In addition to determination of layer’s thickness, optical constants, presence of the interface and surface layers, determination of materials band gap, crystallinity, composition (for compound semiconductors), and electrical properties (resistivity, carrier density, and mobility) became possible using SE. SE is widely used in academy and industry for the following application fields: semiconductors, displays, solar cells, solid state lighting, optical devices, optical and functional coatings, fabricated on various substrates like Si, glass, compound semiconductors, and plastic films. Recently, the application field of SE is increasingly spreading to chemistry, metallurgy, biology, and other fields. 100.4.1 Example of Data Analysis Data analysis of SOI (silicon on insulator) sample made by SIMOX (separation by implantation of oxygen) method is shown in Fig. 100.4 [4]. The structure is SiO2/ Si/SiO2 on Si substrate. As one can see from W & D spectra, for the energies below 3 eV, this sample is transparent so the interference fringes contain the information from all the layers Fig. 100.4 Example of multilayer structure analysis (SIMOX) 100 Spectroscopic Ellipsometry 621 and interfaces in this structure. On the other hand, spectrum above 3 eV contains the information only from the upper SiO2 layer and its interface with Si, due to the strong absorption of UV light at Si interface. Three-layer model has big misfit at energies below 3 eV. Introduction of the interface layer, which is a mixture of 26% of amorphous Si (a-Si) with SiO2, improves the fit drastically. This fact suggests the existence of not fully oxidized SiO2 layer (interlayer) just above the Si substrate. References 1. Azzam, R.M.A., Bashara, N.M.: Ellipsometry and Polarized Light, North Holland, Amsterdam (1987) 2. Tompkins, H.G., Irene, E.A.: Handbook of Ellipsometry, Willian Andrew, New York (2005) 3. Aspnes D.E.: Handbook of Optical Constants of Solids, chapter 5, 104 (1985) 4. Hirakawa, S., Nabatova-Gabain, N., Wasai, Y., Iida, H.: HORIBA Technical Reports English edition, 19, vol. 5 (2003) Chapter 101 Spin- and Angle-Resolved Photoelectron Spectroscopy Taichi Okuda Keywords Very-low-energy electron diffraction (VLEED) Mott scattering Spin–orbit interaction Spin-exchange interaction Spin texture 101.1 Principle SARPES is a method in which the feature of spin resolution is added to normal ARPES measurement (see Sec. Angle-Resolved UPS). By SARPES measurement, one can obtain all the information of quantum states of electrons, i.e., energy, momentum, and spin, and thus, the measurement is sometimes called as “complete experiment.” SARPES is, therefore, one of the most powerful tools to investigate the complete electronic structure (band structure with spin resolution) of solids. The key function, “spin resolution,” of SARPES can be realized by using spin detectors which are illustrated in Fig. 101.1. So far, many types of spin detectors such as Mott detector [1], spin-LEED detector [2], diffuse scattering detector [3], VLEED detector [4] have been developed. Among them, the Mott detector (see Fig. 101.1a, c), in which an asymmetric scattering probability of highly accelerated electrons at a target is used to detect the spin polarization of electrons, is the most widely used spin detector. Since the asymmetry is caused by spin–orbit interaction (SOI), heavy elements such as Au or Th are used as the material for the target to enhance the effect of SOI. However, the scattering probability of electrons is very low (*10−2 orders) because most of the injected electrons penetrate into the target. This leads to an extremely low efficiency for spin detection (*10−4 orders). Efficiency of the other spin detectors (spin-LEED, defuse scattering) that use the asymmetric scattering by SOI for spin detection is almost the same as Mott detector. Therefore, SARPES measurement is usually time consuming, and due to the low efficiency, the energy and angular (momentum) resolutions of electron analyzer T. Okuda (&) Hiroshima Synchrotron Radiation Center (HSRC), Hiroshima University, Hiroshima, Japan e-mail: okudat@hiroshima-u.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_101 623 624 T. Okuda (a) (c) (b) (d) Fig. 101.1 Schematic diagrams of a Mott detector and b VLEED detector. The principle of spin detection of Mott detector is spin–orbit interaction of electron with nuclei of target atom (c). In the VLEED spin detector, spin dependent electron reflection caused by the energetically shifted bands of spin-up and spin-down states by exchange interaction of ferromagnetic target is utilized for spin detection (d) have to be lowered. Very-low-energy electron diffraction (VLEED) spin detector is the one that can overcome the drawback of Mott detector. As shown in Fig. 101.1b, electron spin polarization is detected by measuring the intensity asymmetry of reflected electrons by positively and negatively magnetized ferromagnetic target. Since the energy of spin-up and spin-down bands of ferromagnetic material is different from each other, the scattering probability of the incoming low-energy electron (*10 eV) by the magnetized target depends on the spin polarization of electron (Fig. 101.1d). Since the electron reflectivity of low-energy electron is one order higher and the spin-resolving power is also 2–4 times higher than Mott detector, the efficiency of VLEED spin detector can be 100 times higher than the Mott detector. Thanks to the higher efficiency, about 10 times higher energy and angular resolutions have been achieved recently [5]. 101 Spin- and Angle-Resolved Photoelectron Spectroscopy 101.2 625 Features • Spin-resolved band dispersion can be directly observed. • Spin-resolved Fermi surface mapping or constant energy contour mapping is available. • Not only ferromagnetic materials but also spin split states of nonmagnetic materials caused by spin–orbit interaction can be observed. • Three-dimensional vector of electron spin can be determined by using several sets of spin detectors. 101.3 Instrumentation As presented in Fig. 101.2, SARPES system consists of an electron energy analyzer and a spin detector. Usually, hemispherical analyzer is used for the detection of energy and emitted angle of photoelectron. After analyzing energy and angle of photoelectron, the spin polarization is measured with a spin detector. In case of Mott detector, electron is accelerated to high energy (more than 20 kV) and injected to heavy materials target (Au or Th). Scattered electrons are detected by electron detectors (Channeltron or micro-channel plate) which are installed at the left and right (up and down) sides of the target (see Fig. 101.1a). The intensity asymmetry of left and right (or up and down) detectors, A = (Il − Ir)/(Il + Ir), is proportional to the spin polarization of electron, which is connected with the so-called effective Sherman function (Seff) that represents the spin-resolving power of spin detector with the equation, P = A/Seff. In the VLEED detector, low-energy electron (*10 eV) is reflected by magnetized ferromagnetic target (Fe, Co, Fe-O, and so on.). The intensity asymmetry of reflected electrons by positively and negatively magnetized targets, A = (I+ − I−)/ (I++I−), is proportional to the spin polarization. In case of VLEED detector, only one electron detector (e.g., channeltron) is used. Thus, the unwanted instrumental asymmetry, which is sometimes caused by the different trajectory of electron to two different electron detectors, in case of Mott detector, can be avoided. Since the direction of spin vector does not change by electric field, one can measure both in-plane and out-of-plane spin components by using electron deflector as in the figure. Thus, using pair of spin detectors which are orthogonally installed, one can measure all the spin components of electron and investigate the spin vector of electrons [6, 7]. 626 T. Okuda Spin detector 1 X Coils Z Spin filter target Channeltron Y Deflector Coils Z Channeltron hν MCP Deflector Spin detector 2 Y X Z Hemispherical Analyzer Fig. 101.2 Illustration of spin-ARPES system with VLEED spin detectors. After analyzing energy and emitted angle of photoelectron by the hemispherical analyzer, electrons are delivered into spin detectors through 90° electron deflectors. By using the deflector, one can measure spin polarization along both the in-plane (X or Y) and out-of-plane (Z) directions of the crystals, and the use of two spin detectors allows one to measure all the spin components 101.4 101.4.1 Applications 3D Spin Vector Analysis of Exchange Split Ni (111) Surface States SARPES has been originally utilized to measure spin-polarized electron of ferromagnetic materials that is caused by spin-exchange interaction. As discussed above by using pair of spin detectors, not only spin polarization of the electronic states but also the spin orientation can be obtained. Here, we see such an example on Ni (111) surface [8]. Figure 101.3a shows the valence band spectrum of Ni (111) and its spin polarization as the function of binding energy along the x-, y-, and z-axes of Ni (111) obtained by two sets of Mott detectors. The axes are defined in (b). In addition to the bulk Ni d bands (K1 and K3), three surface states (S1, S2, and S3) are observed. From the data set of spin polarizations, one can obtain the easy axis of magnetization as in (b). Not only large exchange splitting of bulk band, but small exchange splitting of surface bands are also resolved in (c). 101 Spin- and Angle-Resolved Photoelectron Spectroscopy 627 Fig. 101.3 a ARPES results of the Ni (111) valence band at the C point as well as the spin polarization along the x-, y-, and z-axes of the Ni crystal. b Polarization map obtained from the spin-polarization components in (a). Bright color matches the direction of magnetization. c Spin-resolved energy distribution curves of Ni (111). In addition to the bulk band (K3), difference between majority spin state (red) and minority spin state (blue) is clearly seen also in surface states (S1 and S2) 101.4.2 Evidence of Rashba Effect on Au Atomic Chain on Si (557) Originating from the crystal inversion asymmetry and the strong spin–orbit interaction, the so-called Rashba spin split can emerge at materials surface or interface even for nonmagnetic materials. The most famous example of such Rashba spin split surface states is the Shockley states of Au (111), which shows clear split surface bands in ARPES. The evidence of spin polarization in these split bands has 628 T. Okuda been reported by SARPES measurement using Mott detector [9]. Similar splitting surface states were observed on a Si (557) surface decorated with one-dimensional atomic Au chains in ARPES as shown in Fig. 101.4a. However, the origin of splitting band was the subject of a big debate. Among many possibilities, Rashba spin split was also one of the candidates of its origin. However, at the same time it was also challenging to measure the spin polarization directly by SARPES since the band splitting is much smaller and the band dispersion is much steeper than the surface states of Au (111). Higher efficiency and energy and angular resolutions of SARPES measurement by using VLEED spin detector helped to solve the issue. Figure 101.4b, c shows the SARPES results of the electronic band of Au atomic chain on Si (557) taken by a VLEED detector. Red and blue curves in (b) are photoemission intensity distribution curves of spin-up and spin-down states taken at different photoelectron emission angles (related with electron momentum). The two (a) (b) (c) Fig. 101.4 a ARPES results of Au/Si (557), b its SARPES spectra, and c the polarization map. Red and blue in (b) and (c) represent spin-up and spin-down states, respectively 101 Spin- and Angle-Resolved Photoelectron Spectroscopy 629 splitting bands are clearly observed as spin-up and spin-down bands. In spin-polarization map, spin-polarization reversal with respect to the time reversal symmetry point at M which is the strong evidence of Rashba effect is also revealed. References 1. Kisker, E., Clauberg, R., Gudat, W.: Electron spectrometer for spin-polarized angle- and energy-resolved photoemission fom ferromagnets. Rev. Sci. Instrum. 53, 1137 (1982) 2. Wang, G.-C.: Polarized low-energy-electron diffraction from W(100). Phys. Rev. B 23, 1761 (1981) 3. Unguris, J., Pierce, D.T., Celotta, R.J.: Low-energy diffuse scattering electron-spin polarization analyzer. Rev. Sci. Instrum. 57, 1314 (1986) 4. Tillmann, D., Thiel, R., Kisker, E.: Very-low-energy spin-polarized electron diffraction from Fe(001). Z. Phys. B: Condens. Matter 77, 1 (1989) 5. Okuda, T., Miyamoto, K., Miyahara, H., Kuroda, K., Kimura, A., Namatame, H., Taniguchi, M.: Efficient spin resolved spectroscopy observation machine at Hiroshima Synchrotron Radiation Center. Rev. Sci. Instrum. 82, 103302 (2011) 6. Hoesch, M., Greber, T., Petrov, V.N., Muntwiler, M., Hengsberger, M., Auwärter, W., Osterwalder, J.: Spin-polarized Fermi surface mapping. J. Electron Spectrosc. Relat. Phenom. 124, 263 (2002) 7. Okuda, T., Myamoto, K., Kimura, A., Namatame, H., Taniguchi, M.: A double VLEED spin detector for high-resolution three dimensional spin vectorial analysis of anisotropic Rashba spin splitting. J. Electron Spectros. Relat. Phenomena 201, 23 (2015) 8. Okuda, T., Lobo-Checa, J., Auwärter, W., Morscher, M., Hoesch, M., Petrov, V., Hengsberger, M., Tamai, A., Dolocan, A., Cirelli, C., Corso, M., Muntwiler, M., Klöckner, M., Roos, M., Osterwalder, J., Greber, T.: Exchange splitting of the three Gamma surface states of Ni(111) from three-dimensional spin- and angle-resolved photoemission spectroscopy. Phys. Rev. B 80, 180404(R) (2009) 9. Okuda, T., Myamoto, K., Takeichi, Y., Miyahara, H., Ogawa, M., Harasawa, A., Kimura, A., Matsuda, I., Kakizaki, A., Shishidou, T., T, Oguchi: Large out-of-plane spin polarization in a spin-splitting one-dimensional metallic surface states on Si(557)-Au. Phys. Rev. B 82, 161410 (R) (2010) Chapter 102 Spin-Polarized Scanning Electron Microscopy Teruo Kohashi Keywords Secondary electrons 102.1 Spin polarization Surface magnetization Principle Spin-polarized scanning electron microscopy (spin SEM) is a method to visualize magnetization distribution at the surface of a ferromagnetic sample [1–4], whose principle is summarized in Fig. 102.1. Polarization of the electron spin within ferromagnetic materials is the origin of the materials’ magnetization, and this spin polarization is maintained while the electrons are emitted from the sample surface as secondary electrons. In spin SEM, the secondary electrons are transferred to a spin detector, where the spin polarization can be detected by taking advantage of spin–orbit interaction, such as in Mott detectors [5]. The magnetization of the emission point of the secondary electrons on the sample surface can thus be obtained. Consequently, a magnetic domain image can be obtained by scanning the sample with respect to the primary electron beam in a spin SEM system. 102.2 Features • Magnetization at the sample surface is visualized through the spin polarization of secondary electrons. • Probing depth is very short (typically 1 nm), which requires a clean sample surface. • The lateral spatial resolution is better than 10 nm (3 nm at best [6]). T. Kohashi (&) Nano-Process Research Department, Center for Technology Innovation, Research & Development Group, Hitachi, Ltd., Tokyo, Japan e-mail: teruo.kohashi.fc@hitachi.com © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_102 631 632 T. Kohashi Fig. 102.1 Principle of spin SEM • Magnetization direction can be resolved into three-dimensional components. • Magnetization information is detected independent of the topography of sample surface. 102.3 Instrumentation A typical structure of the spin SEM is shown in Fig. 102.2, where three chambers are mounted on a vibration-free stage. The sample is put into the load-lock chamber, where the vacuum condition is better than 1 10−5 Pa. The sample is then transported to the preparation chamber, where the pressure is about 1 10−7 Pa. Here, the sample surface is cleaned by argon ion sputtering/milling, and Auger analysis assures that any contamination on the surface has been removed and the outermost layer is a magnetic material. The sample is then transferred to the observation chamber, where the vacuum condition is better than 1 10−7 Pa to keep the sample surface clean. The main equipment is installed here, such as a sample stage, an electron gun to produce the primary electron beam, an electron optical system for collecting and transporting the secondary electrons, and a spin detector. Signals from the spin detector are transferred to the signal analyzer, and magnetic domain structures are imaged. 102.4 Applications: Visualization of HDD Recorded Bits One of the characteristics of spin SEM is its high spatial resolution, which is a very powerful function to study high-density magnetic recording devices. The recording bit length of a hard disk drive (HDD) becomes shorter as the recording density becomes higher. Therefore, it is necessary to study the bit shape in detail to achieve higher recording density. An example of a high-resolution image obtained when examining the recording bit shape of a perpendicular recording medium by using 102 Spin-Polarized Scanning Electron Microscopy 633 Fig. 102.2 Schematic of the spin SEM chamber configuration the spin SEM is introduced in Fig. 102.3 [7]. The sample is an HDD perpendicular recording medium with a CoCrPt recording layer. Main-signal bits with lengths of 254, 127, 64, and 42 nm were recorded over the background bits with a bit length of 25 nm. Figure 102.3a, b shows recorded bit images obtained using the spin SEM. The black and white contrasts show the opposite magnetization components perpendicular to the sample surface. Tracks run vertically in the images, and each black and white area seen in the main tracks denotes one recorded bit. It can be seen that the main signal is clearly recorded. The tiny (25 nm long) bits that were recorded as background can also be identified between the main tracks. Figure 102.4 shows the high-magnification topography (a) and magnetic domain (b) images of the 127-nm bits in the main tracks, which were obtained at the same time in the same field of view. In Fig. 102.4a, fine structures of 10 nm or less can be seen, which are later confirmed to be the grain structures of the medium by a TEM measurement. On the other hand, the shapes of the 127-nm recorded bit are clearly visible in Fig. 102.4b. We can see that the contrast in the magnetic domain image is not affected by the grain structure shown in the topography image. This demonstrates one advantage of the spin SEM as it gives us magnetic information independent of the sample surface topography. Moreover, very fine structures were observed around the bit boundaries and track edges, and the sizes of the small structures are similar to those of the grains. This enables us to study the magnetization of each small grain in this medium by comparing the topography and magnetic domain images. These results show that spin SEM has a high spatial resolution better than 10 nm, as fine as each grain size, and can image nanoscale 634 T. Kohashi Fig. 102.3 HDD recorded bit images on a perpendicular recording medium [7] Fig. 102.4 High-magnification image of a recorded medium [7]: topography image (a) and magnetic domain image (b) of the perpendicular magnetization component magnetic structures, such as small irregularities of the bit shapes around the bit boundaries or track edges in the recording medium. References 1. Koike, K., Hayakawa, K.: Scanning electron microscope observation of magnetic domains using spin-polarized secondary electrons. Jpn. J. Appl. Phys. 23, L187–L188 (1984) 102 Spin-Polarized Scanning Electron Microscopy 635 2. Unguris, J., Hembree, G.G., Celotta, R.J., Pierce, D.T.: High resolution magnetic microstructure imaging using secondary electron spin polarization analysis in a scanning electron microscope. J. Microsc. 139, RP1–RP2 (1985) 3. Oepen, H.P., Kirschner, J.: Imaging of magnetic microstructures at surfaces: the scanning electron microscope with spin polarization analysis. Scan. Microsc. 5, 1–16 (1991) 4. Allenspach, R.: Ultrathin films: magnetism on the microscopic scale. J. Magn. Magn. Mater. 129, 160–185 (1994) 5. Mott, N.F.: The scattering of fast electrons by atomic nuclei. Proc. R. Soc. London Ser. A 124, 425–442 (1929) 6. Koike, K.: Spin-polarized scanning electron microscopy. Microscopy 62, 177–191 (2013) 7. Kohashi, T., Konoto, M., Koike, K.: High-resolution spin-polarized scanning electron microscopy (spin SEM). J. Electron Microsc. 59, 43–52 (2010) Chapter 103 Spin-Polarized Scanning Tunneling Microscopy Toyo Kazu Yamada Keywords Spin-polarized tunneling current 103.1 Spin polarization Magnetism Principle Spin-polarized scanning tunneling microscopy (SP-STM) is a powerful tool to visualize spin-polarization vectors of sample surface atoms. Variety of unique new magnetic properties have been discovered and studied, such as non-collinear magnetic exchange coupling between monolayer films, magnetoresistance of single atoms, single molecules, and nanomagnets, as well as skyrmion or magnon in magnetic films [1–5]. SP-STM measurements can be performed with an ultrahigh-vacuum (UHV)STM setup combined with a scanning tunneling spectroscopy (STS) technique, while SP-STM requires a spin-polarized tip. Figure 103.1 shows the principle, in which local density of states (LDOS) of a tip and a sample is spin-polarized near the Fermi energy (EF). Now only 3 spin-up electrons exist for the tip [spin polarization (P) = (3 − 0)/(3 + 0) = 100%], while 3 spin-up and 1 spin-down electrons for the sample [P = (3 − 1)/(3 + 1) = 50%]. In (a), tip and sample spin-polarization vectors are parallel. Then, tunneling probability can be described as 3 3 + 0 1 = 9, where no spin-flip during tunneling is assumed. Next, the sample spin-polarization vector is reversed (antiparallel configuration, see Fig. 103.1b). In this case, tunneling probability is 3 1+ 0 3 = 3. SP-STM detects this difference in current due to different spin-polarization vector configurations between the tip and the sample. This mechanism is comparable to tunnel magnetoresistance. T.K. Yamada (&) Graduate School of Advanced Integration Science, Chiba University, Chiba, Japan e-mail: toyoyamada@faculty.chiba-u.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_103 637 638 T.K. Yamada Fig. 103.1 Spin-polarized electron tunneling process between the spin-polarized tip and the sample in the case of a parallel and b antiparallel spin-polarization (magnetization) configurations. [1–5] 103.2 Features • • • • Spin-polarization vectors of sample surfaces can be detected. Magnetoresistance can be detected at the atomic scale. Spin-polarized STM tips are required. Combined with IETS or magnetic coils, magnon or hysteresis loop can be measured. • SP-STM has been performed in UHV at *10 mK–300 K. 103.3 Instrumentation SP-STM system is based on a UHV-STM setup. All experiments have been performed in UHV to prevent contamination adsorption on the tip and the sample surfaces. Single impurity atoms break local symmetry, and therefore, spin polarization can be quenched. To detect spin-polarized current, it is necessary to decrease current noise as much as possible (typical current noise: Inoise < 1 pA). Figure 103.2 shows an example of SP-STM measurement system, where a layerwise antiferromagnetically coupled Mn(001) film is scanned by a spin-polarized tip. The most important (and difficult) point to make success SP-STM measurements is how to obtain a stable spin-polarization vector at the tip apex. Recent trends are uses of Fe-coated W tips, Mn-coated W tips, Cr-coated W tips, and bulk-Cr tips [1–6]. The Fe-coated tips have been used since they have the highest spin polarization of *40%, while recently most of the SP-STM researchers use the antiferromagnetic tips because of no stray fields (P * 10%) [2]. 103 Spin-Polarized Scanning Tunneling Microscopy 639 STM spin-polarized tip bct-Mn(001) film Spinpolarized tunneling current 10ML 9ML z 8ML x y Fig. 103.2 Experimental setup of spin-polarized STM measurements. Spheres and black and white arrows denote atoms and spin-polarization vectors, respectively. Blue and red arrows denote spin-up and spin-down electrons, respectively. SP-STM measurements can be performed in UHV at 10 mK–300 K. [2–5] Table 103.1 Spin-polarization vectors of spin-polarized tips SP tips Thickness Direction Spin polarization Refs. Fe-coated W Mn-coated W Cr-coated W Cr-coated W Bulk-Cr 2–10 nm 3–6 nm >10 nm 2–10 nm – In-plane In-plane In-plane Out-of-plane Out-of-plane *40% *9% *10% *10% – [1–5] [2] [2] [2] [6] These tips were prepared in a UHV preparation chamber before setting in STM. First, a W tip chemically etched in air was introduced into the preparation chamber through a load lock chamber and cleaned (Ar+ sputter with a subsequent anneal to 2200 K). Next, magnetic films were deposited at 300 K on the cleaned W tip. Proper annealing of 500–600 K could increase crystalline quality of the film [2]. On the other hand, the bulk tip requires a different manner. A chemically etched bulk-Cr tip was cleaned inside the UHV-STM setup by applying voltage pulses ( 10 V) between the tip and the sample to remove oxide layers at the apex [6]. Depending on materials and thickness, the tip spin-polarization directions can be controlled by different magnetic anisotropies. Table 103.1 shows experimentally obtained tip spin-polarization vectors of SP-STM tips. Using these clean spin-polarized tips combined with STS, the spectroscopy dI/dV map shows spin contrast. 103.4 103.4.1 Applications Magnetic Imaging Experimentally obtained SP-STM results of Mn(001) films grown on Fe(001) are shown in Fig. 103.3 [2–5]. Seventh to tenth monolayers (ML) are exposed on the 640 T.K. Yamada Topographic image Spin-polarization map 8ML 9ML y z x 10ML Fig. 103.3 Spin-polarized STM results on bct-Mn(001) films with an Fe-coated W tip in UHV at 300 K. Topographic image (100 100 nm2) and spin-polarization dI/dV map at +0.2 V were obtained simultaneously. [2–5] surface as shown in the STM topographic image (100 100 nm2). Since Mn(001) films higher than fourth layer have the same LDOS, non-spin-polarized W tips show no contrast; however, spin-polarized Fe-coated W tips with an in-plane sensitivity (see Table 103.1) show spin contrast in the dI/dV map at +0.2 eV (Fig. 103.3). Alternating spin contrast between the monolayers means that Mn atoms in the monolayer couple ferromagnetically, while they couple antiferromagnetically between the layers. Thus, we could obtain spin structures of nanomaterial by means of SP-STM. 103.4.2 Single-Molecule Junction Using the STM tip, we can manipulate single atoms or single molecules. Left panels as shown in Fig. 103.4 show examples, where letters “N” and “S” were written with 17 Fe atoms, and “smile” was drawn with 11 CO single molecules. Using this STM manipulation technique, we can measure conductance through the target (see Fig. 103.4). Here, we measure I–z curve instead of dI/dV. By measuring the tunneling current (I), the tip is approached to the target molecule. When the tip contacts with the molecule, the measuring current becomes constant, forming a single-molecule junction. Thus, we can measure conductance through the molecule. Using the spin-polarized tips, we can measure magnetoresistance (MR) through a single molecule (see right panels as shown in Fig. 103.4). Single phthalocyanine molecules were deposited on the Mn(001) films as shown in Fig. 103.3. First, the Fe-coated W tip was gently contacted to the molecule adsorbed on the ninth monolayer, in which spin polarization is parallel to the tip spin polarization and measured conductance “I”. Next, the same tip was contacted to the molecule adsorbed on the eighth monolayer, in which spin polarization is antiparallel to the tip spin polarization and measured conductance “II”. Thus, MR through a single 103 Spin-Polarized Scanning Tunneling Microscopy 641 Fig. 103.4 STM manipulations of single atoms and molecules in UHV at 5 K. Magnetoresistance measurements through a single molecule by means of SP-STM [3] phthalocyanine molecule was obtained by MR = (I–II)/II = −50% [3], which suggests that such a 1-nm-size p-conjugated organic molecule can be useful as a new nanomaterial for near-future spintronic devices. References 1. Wiesendanger, R.: Spin mapping at the nanoscale and atomic scale. Rev. Mod. Phys. 81, 1495 (2009) 2. Yamada, T.K., Vazquez de Parga, A.L.: Room temperature spin-polarizations of Mn-based antiferromagnetic nanoelectrodes. Appl. Phys. Lett. 105, 183109 (2014) 3. Bagrets, A., Schmaus, S., Jaafar, A., Kramczynski, D., Yamada, T.K., Alouani, M., Wulfhekel, W., Evers, F.: Single molecule magnetoresistance with combined antiferromagnetic and ferromagnetic electrodes. Nano Lett. 12, 5131–5136 (2012) 4. Yamada, T.K., Martinez, E., Vega, A., Robles, R., Stoeffler, D., Vazqeuz de Parga, A.L., Mizoguchi, T., van Kempen, H.: Spin configuration in a frustrated ferromagnetic/ antiferromagnetic thin-film system. Nanotechnology 18, 235702 (2007) 5. Yamada, T.K., Bischoff, M.M.J., Heijnen, G.M.M., Mizoguchi, T., van Kempen, H.: Observation of spin-polarized surface states on ultrathin bct Mn(001) films by spin-polarized scanning tunneling spectroscopy. Phys. Rev. Lett. 90, 056803 (2003) 6. Rodary, G., Girard, J.-C., Largeau, L., David, C., Mauguin, O., Wang, Z.-Z.: Atomic structure of tip apex for spin-polarized scanning tunneling microscopy. Appl. Phys. Lett. 98, 082505 (2011) Chapter 104 Spin-Resolved Photoemission Electron Microscopy Keiki Fukumoto Keywords Magnetic domains 104.1 Imaging Synchrotron X-ray Principle When the order of magnetic spins is locally different, it is the so-called magnetic domains. Domain structures can be randomly formed or it can be also formed by the competition of magnetic energies such as magnetic dipole, exchange, anisotropy, and Zeeman energies. Photoemission electron microscopy (PEEM) with polarized sources, such as spin-polarized electron beam [1], polarized UV light [2], linearly [3] or circularly [4] polarized X-rays, is one of the techniques to image magnetic domain structures (spin-resolved PEEM: SR-PEEM). In particular, a combination of PEEM with X-ray magnetic circular dichroism (XMCD-PEEM) for ferromagnetic domains is one of the most developed methods, which will be introduced in this chapter. The details of XMCD will be described elsewhere in this book, therefore, briefly here. A micron-sized ferromagnetic material with relatively weak magnetic anisotropy and exchange energies (i.e., FeNi alloy) creates a magnetic closure domain to reduce the dipole energy, as schematically shown in Fig. 104.1a, in which the magnetization directions are indicated by black arrows. A blue thick arrow indicates the incidence of circularly polarized X-ray (CPX), and its spin direction is given by a gray arrow. Figure 104.1b shows conceptual drawing of X-ray absorption spectra (XAS) in regions A and B obtained by CPX around the Fe 2p absorption edge. The 2p edge splits into two (2p3/2 and 2p1/2) by the strong spin–orbit interaction. The red curve in Fig. 104.1b is XAS in region A, where the magnetization direction is more parallel compared with other areas. On the other hand, the blue curve in Fig. 104.1b is from region B with more antiparallel configuration. The difference of photoabK. Fukumoto (&) Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Ibaraki, Japan e-mail: keiki@post.kek.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_104 643 644 K. Fukumoto Fig. 104.1 Brief introduction of XMCD sorption, the XMCD contrast, can be explained by two-step model using the 2p3/2 edge in Fig. 104.1c. The first step is the difference of excitation probabilities depending on the angle between spin direction of CPX and magnetization directions. The second step is the ratio of detection of the excited spin up and down electrons defined by 3d empty states. By taking into account these two steps, the transition probabilities from 2p3/2 core level into 3d valence level are defined. Using PEEM, locally different XMCD signal can be imaged. 104.2 Features • Spatial resolution is a few tens of nm. • PEEM should be equipped in a high vacuum or in an ultrahigh vacuum chamber. • Microspectroscopic XMCD-PEEM allows to pixel by pixel analysis of spin and orbital moments using sum rule. • By the virtue of using synchrotron X-ray, magnetic domains can be imaged with element selectively. • Using the pulse feature of synchrotron X-ray and synchronizing it to external (magnetic) field pulses, dynamics of magnetic domain can be imaged. 104.3 Instrumentation The contrast of PEEM images reflects locally different photoemitted electron intensities, which depend on surface morphology, workfunction, absorption coefficient, and also electron density of states. The photoemitted electrons are accelerated toward an objective lens by the potential difference to the sample. The 104 Spin-Resolved Photoemission Electron Microscopy 645 Fig. 104.2 Procedure to create a magnetic domain structure image extracting voltage is approximately 20 kV in 2 mm for the high-resolution mode. After passing through electrostatic or magnetic lenses, the number of electrons is amplified by channel plates and converted to photons by a fluorescence screen to image by a CCD camera. The lateral resolution of PEEM is on the tens of nm defined by chromatic and spherical aberrations. Figure 104.2 shows one procedure to create a magnetic domain image by XMCD-PEEM. Here assuming again that the sample is a square-shaped micronsize and the photon energy of CPX is tuned at the Fe 2p3/2 edge. Figure 104.2a and b illustrates the imaging of magnetic domain structures using PEEM with reversed grayscale contrasts by left and right CPXs. Taking difference of two PEEM images obtained by left and right CPX, the magnetic contrast will be enhanced (Fig. 104.2c). 104.4 104.4.1 Applications Estimation of Spin and Orbital Moments with Spatial Resolution One pioneer work estimating the spin and orbital moments with element selectively in sub-micron spatial resolution using XMCD-PEEM has been reported in Ref. [4], for which the sample was an epitaxially grown Co/Ni double layer on a Cu(001) surface. Co and Ni layers were prepared as a crossed wedge with slope rotated by 90° with respect to each other, as schematically drawn in Fig. 104.3a. In the region of interest in 646 K. Fukumoto Fig. 104.3 a is a schematic drawing of a crossed wedge Co/Ni layer on a Cu (001) surface. b and c are maps of spin and orbital moments, respectively [4]. Figure 104.3 adapted with permission from Ref. [4] Copyrighted by the American Physical Society this study, the thickness range for the Co and Ni layers were 1.4 to 2.6 monoatomic layer (ML) and 11 to 14 ML, respectively. The spin direction of the Co/Ni layer was determined by the competition of thickness dependent magneticrystalline anisotropy energies of these two layers. They were in-plane and out-of-plane on the upper and bottom parts of the white dotted line in the figure. PEEM images were taken at around the Ni 2p absorption edge with arbitrary energy steps with both left and right circularly polarized X-rays, and an XMCD spectrum (difference of the two spectra) was obtained from each pixel. By analyzing the data set using sum rule, the maps of spin and orbital moments have been successfully obtained and shown in Fig. 104.3b and c, respectively. The thicknesses of Ni and Co layers were given at the top and right axes, respectively. Figure 104.3b gives also the magnetization directions, in which different features of magnetic domain structures were recognized in the upper and lower parts separated by the white dotted line. The bottom part of the image shows two grayscale contrasts with out-of-plane magnetizations, and the magnetizations were up or down to the paper plane. The spin direction switches from out-of-plane to in-plane direction with increasing the Co thickness and decreasing the Ni thickness. The upper part shows four contrasts with in-plane <110> easy axis. The spin moment was uniform in whole area; however, the orbital moment drastically changed when spin direction switched from in-plane to out-of-plane across the white dotted line. By pixel by pixel analysis of microspectroscopic XMCD-PEEM 104 Spin-Resolved Photoemission Electron Microscopy 647 images on the crossed wedge sample, the spin and orbital moments relating to the magnetocrystalline anisotropy have been successfully estimated. The measurements have been performed at the twin helical undulator beamline for BL25SU of SPring-8. 104.4.2 Dynamics of Magnetic Domain Structures Pulsed feature of synchrotron X-ray is suitable for time-resolved experiments with a pump and probe scheme. Here, an example of imaging the dynamics of magnetic domain structures by synchronizing CPX pulses with magnetic field pulses will be introduced [5]. The experiments were performed at the soft X-ray beamline BL25SU of SPring-8. Diagram of experimental setup of Ref. [5] is shown in Fig. 104.4. One filling pattern of electron bunches of the storage ring was a combination of one train bunch and 2.92 MHz isolated single bunches. Magnetic field pulses created by shining femtosecond laser pulses to a fast photodiode were synchronized to the isolated bunches. Photoemittted electrons by the bunch train were blocked by reducing the channel plate voltage. A sample was a disk-shaped FeNi with the dimension of the diameter of 6 um and the thickness of 100 nm, which had a magnetic vortex structure and an XMCD-PEEM image of which is shown in Fig. 104.5d. The grayscale continuously changes around the center of the disk. Appling magnetic field pulse pushes the vortex core close to the edge of the disk, and subsequently, the core shows damped oscillation back to the center of the disk. Temporal motion of the core is plotted in Fig. 104.5g. XMCD-PEEM images were stored at each delay between CPX pulses and magnetic field pulses from −2 to Fig. 104.4 Diagram of time-resolved XMCD-PEEM experiments [5] 648 K. Fukumoto Fig. 104.5 Time evolution of magnetic domain structure in a disk-shaped FeNi [5] 78 ns with 0.5 ns step. To create one image at each delay, the photoemission signal with approximately 300,000 pulses was accumulated. Therefore, the dynamics has to be repeatable on each magnetic field pulse. References 1. Duden, T., Bauer, E.: Magnetization wrinkle in thin ferromagnetic films. Phys. Rev. Lett. 77, 2308–2311 (1996) 2. Marx, G.K.L., Elmers, H.J., Schonhens, G.: Magneto-optical linear dichroism in threshold photoemission electron microscopy of polycrystalline Fe films. Phys. Rev. Lett. 84, 5888–5891 (2000) 3. Nolting, F., Scholl, A., Stohr, J., Seo, J.W., Fompeyrine, J., Seigwart, H., Locquet, J.-P., Anders, S., Luning, J., Fullerton, E.E., Toney, M.F., Scheinfein, M.R., Padmore, H.A.: Direct observation of the alignment of ferromagnetic spins by antiferromagnetic spins. Nature 405, 767–769 (2000) 104 Spin-Resolved Photoemission Electron Microscopy 649 4. Kuch, W., Gilles, J., Kang, S.S., Imada, S., Suga, S., Kirschner, J.: Magnetic-circulardichroism microspectroscopy at the spin reorientation transition in Ni (001) films. Phys. Rev. B 62, 3824–3833 (2000) 5. Fukumoto, K., Matsushita, T., Osawa, H., Nakamura, T., Muro, T., Arai, K., Kimura, T., Otani, Y., Kinoshita, T.: Construction and development of a time-resolved x-ray magnetic circular dichroism photoelectron emission microscopy system using femtosecond laser pulses at BL25SU SPring-8. Rev. Sci. Instrum. 79, 063903–063907 (2008) Chapter 105 Super-Resolution Microscopy Kazuya Kabayama and Ryugo Tero Keywords PALM STORM STED Stimulated emission Moiré pattern 105.1 SIM Localization accuracy Principle The term “super-resolution microscopy” (SRM) includes all the optical methodologies featuring spatial resolutions exceeding the diffraction limits of optical microscopes. Until now, major microscopic methods such as the photo-activated localization microscopy (PALM)/stochastic optical reconstruction microscopy (STORM), stimulated emission depletion (STED) microscopy, and structured illumination microscopy (SIM) have been developed (Fig. 105.1). The two-photon excitation microscopy is also considered as a super-resolution microscopy in certain cases [1]. As described in the chapter of optical microscopy, the spatial resolution (d) is approximately a half of the wavelength of the light used for observation. Two objects closer than d are not identified with a conventional optical microscope (Fig. 105.1a). The SRM techniques make it possible to image and dissect structures finer than 200 nm even using visible light (approximately 400–750 nm in wavelength). Their capacities are mainly utilized in fluorometric imaging and observation of biologic samples. PALM/STORM: The location of each fluorescent molecule is determined from the peak of the Gaussian distribution of fluorescence, not forming an image from all fluorescent molecules (Fig. 105.1b). The location data is used to construct a dot K. Kabayama (&) Department of Chemistry, Graduate School of Science, Osaka University, Osaka, Japan e-mail: kaba@chem.sci.osaka-u.ac.jp R. Tero Department of Environmental and Life Sciences, Toyohashi University of Technology, Toyohashi, Japan e-mail: tero@tut.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_105 651 652 K. Kabayama and R. Tero Fig. 105.1 Principles of representative SRMs. a Conventional optical (wide-field) microscopy (OM) light from a fluorochrome spreads following the point spread function (PSF) (top). The PSF of two (or more) fluorochromes closer than the spatial resolution (d) overlaps each other, if they are excited simultaneously. The size of a focused excitation beam is also determined by PSF. b PALM/STORM fluorescence from each fluorochrome is separately obtained, and each location is determined as the center of PSF (top and middle). The location of all the fluorochromes are dotted and overlaid (bottom). c STED A focused excitation beam illuminates all the fluorochromes included in PSF (top). The toroidal-shape STED beam suppresses the fluorescence at the peripheral of PSF (middle). Only the fluorochromes at the center emit fluorescence as if they are illuminated by excitation light with a smaller spot size than the PSF of excitation beam (bottom). d SIM: A square sample has stripe shapes, which pitches are narrower than the spatial resolution of the microscope d (the pitch of the right half is 10% narrower than that of the left half). The sample is illuminated with light with a periodic intensity. The Moiré patterns have larger pitches (orange and purple arrows in the top image) than the pitch of the sample; thus, they are resolved using the microscope with d. The Moiré pattern depends on the rotation angle of the structured illumination (bottom) image of the distribution of the fluorescent molecules. Note that the “resolution” is determined by the localization accuracy of each molecule, not by the spatial resolution when two or more molecules are observed simultaneously. The localization accuracy of several nanometers is possible if a sufficiently high signal-to-noise ratio is achieved [2]. This approach is called “fluorescence imaging with one-nanometer accuracy” (FIONA). STED microscopy: The apparent point spread function (PSF) of the laser scanning microscopy is reduced by the superimposed “STED beam” on the excitation laser beam (Fig. 105.1c). The STED beam is torus-shaped and can forcibly transfer the excited fluorescent molecules to the ground state. SIM: A sample is illuminated with light having a patterned structure such as a grid shape to generate the Moiré effect (Fig. 105.1d). Higher spatial resolution is 105 Super-Resolution Microscopy 653 obtained mathematically from the diffraction generated by the periodically patterned illumination. 105.2 105.2.1 Features PALM/STORM • Significantly higher spatial resolution than other SRMs. • Lower temporal resolution. 105.2.2 STED Microscopy • Live imaging is possible with the same temporal resolution as a conventional confocal laser scanning microscope (CLSM). • The resolution along the Z-axis is the same as that in a conventional CLSM. 105.2.3 SIM • Has an advantage over other SRMs in imaging thicker sections and 3D imaging. • Live imaging is possible. The general features of these SRMs are summarized in Table 105.1. Note that since all SRMs are being continuously improved, the principles and structures of the microscopies are not limited to those described here. Table 105.1 Comparison of SRMs Principle Lateral spatial resolution (nm) Vertical spatial resolution (nm) Number of frames necessary for structuring one image Temporal resolution Requirement on fluorochrome PALM/STORM STED SIM Localization accuracy of single molecules 15–50 Stimulated emission Moiré pattern 30–100 90–130 20–200 500–700 <300 Several hundreds to tens of thousands images One image sec–min msec Six to nine images (2D-SIM) Approx. 15 images (3D-SIM) msec–sec Blinking fluorescent molecules are generally used [3,4] Somewhat limited None (fluorochromes having slow discoloring are preferable) (continued) 654 K. Kabayama and R. Tero Table 105.1 (continued) Irradiation energy required for constructing one image PALM/STORM STED SIM Very large (it can be mitigated using a suitable fluorescent probe) [3, 4] Very large (it can be mitigated using a suitable fluorescent probe) [5] Medium 105.3 Instrumentation 105.3.1 PALM/STORM Its structure is based on the conventional wide-field microscope or the total internal reflection fluorescence microscope (TIRFM). A sample is labeled with a blinking fluorochrome, which is controlled with an optical switch. The sample images are obtained repeatedly up to approximately several tens of thousands of times using a highly sensitive camera. The center of the luminescent spot of every molecule in each image is decided by computer processing, and a super-resolution image is finally obtained by superimposing the images. 105.3.2 STED Microscopy Its structure is based on the conventional galvanometer-mirror CLSM. A sample is illuminated with superimposed two types of pulsed laser beams, for excitation and for stimulated emission of fluorochromes. The STED beam is formed to a toroidal shape with a phase modulator inserted in the optical path. 105.3.3 SIM Various types of microscopes such as the conventional wide-field microscope and CLSM are available. The sample is illuminated by light having a stripe-shaped pattern which is formed with diffraction grating. Several images are taken at different rotation angles of the illumination pattern. After the images are superimposed, a super-resolution image is obtained by mathematical processing based on Fourier analysis. 105 Super-Resolution Microscopy 105.4 655 Applications Figure 105.2 shows examples of SIM images in comparison with wide-field images obtained for actin filaments (Fig. 105.2a, b) and fluorescence beads (Fig. 105.2c, d). A conventional wide-field fluorescence image shows that actin filaments with various width and brightness exist (Fig. 105.2a), but the SIM image obtained at the same position clearly visualizes that many of them consist of bundles of fine filaments (Fig. 105.2b). The images of fluorescence beads obtained by the wide-field microscopy (Fig. 105.2c) and SIM (Fig. 105.2d) demonstrate the reduction of the apparent PSF by the SIM method. Fig. 105.2 Fluorescence images of (a, b) actin filaments and (c, d) fluorescent beads in cells observed with conventional wide-field microscopy (a and c) or with SIM (b and d). Scale bars correspond to 10 µm in (a, b) and 2 µm in (c, d). (Provided by Dr. Yasuhiro Hirano, Osaka University, Japan.) 656 K. Kabayama and R. Tero References 1. Schermelleh, L., Heintzmann, R., Leonhardt, H.: A guide to super-resolution fluorescence microscopy. J. Cell Biol. 190, 165–175 (2010) 2. Yildiz, A., Forkey, J.N., McKinney, S.A., Ha, T., Goldman, Y.E., Selvin, P.R.: Myosin V walks hand-over-hand: single fluorophore imaging with 1.5-nm localization. Science 300, 2061–2065 (2003) 3. Betzig, E., Patterson, G.H., Sougrat, R., Lindwasser, O.W., Olenych, S., Bonifacino, J.S., Davidson, M.W., Lippincott-Schwartz, J., Hess, H.F.: Imaging intracellular fluorescent proteins at nanometer resolution. Science 313, 1642–1645 (2006) 4. Uno, S.N., Kamiya, M., Yoshihara, T., Sugawara, K., Okabe, K., Tarhan, M.C., Fujita, H., Funatsu, T., Okada, Y., Tobita, S., Urano, Y.: A spontaneously blinking fluorophore based on intramolecular spirocyclization for live-cell super-resolution imaging. Nat. Chem. 6, 681–689 (2014) 5. Hofmann, M., Eggeling, C., Jakobs, S., Hell, S.W.: Breaking the diffraction barrier in fluorescence microscopy at low light intensities by using reversibly photoswitchable proteins. Proc. Natl. Acad. Sci. U.S.A. 102, 17565–17569 (2005) Chapter 106 Surface Acoustic Wave Shinya Sasaki 106.1 Principle The amplitude of a surface acoustic wave has the maximum value A at the surface, and it attenuates exponentially as it goes inside. The penetration depth d for the attenuation of the amplitude to A/e depends on wavelength k. Therefore, the higher the frequency of the wave, the shallower the region it propagates through. For a homogeneous and isotropic material, the phase velocity C of the surface acoustic wave propagating through the material is expressed approximately by the following formula in terms of Young’s modulus E, Poisson’s ratio m, and density q. 0:87 þ 1:12t C¼ 1þt sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi E 2qð1 þ tÞ ð106:1Þ If the medium is homogeneous, the phase velocity of the surface acoustic wave is independent of frequency f. However, when the density or Young’s modulus has a distribution in the direction of the depth, the phase velocity changes in the propagation region, reflecting the material properties. In other words, in the case of a surface where the film is present on a substrate, the phase velocity of the surface acoustic wave reflects the properties of the thin film surface at higher frequencies. However, at lower frequencies, it reflects the properties of the substrate, which is in the deeper region. Because of this, when a thin film is present on the surface, the phase velocity C of the surface acoustic wave becomes complicated to use for the consideration of the effect of the thin film. In addition, it can be considered as a function of the seven respective physical properties of the thin film as well as the S. Sasaki (&) Department of Mechanical Engineering, Tokyo University of Science, Tokyo, Japan e-mail: s.sasaki@rs.tus.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_106 657 658 S. Sasaki substrate, namely Young’s modulus E and E′, Poisson’s ratio m and m′, density q and q′, film thickness d, and frequency f. C ¼ CðE; E 0 ; t; t0 ; q; q0 ; d; f Þ ð106:2Þ In order to determine the physical properties, it is necessary to fit the theoretical curve by the following nonlinear least squares (Marquardt) method to minimize the difference between the scattering curve C(f) obtained from the experiment and the theoretical value. X ½Cðfk Þ CðE; E 0 ; t; t0 ; q; q0 ; d; fk Þ ! mim 2 ð106:3Þ k In addition to Young’s modulus, the physical properties that determine the scattering curve C(f) consist of film thickness, density, and Poisson’s ratio, but it is possible to estimate the value even by considering at least two of the properties as the variables by using the Marquardt method. However, in order to obtain a highly accurate and reliable value, it is recommended that the values of film thickness and density are measured in advance by analytical procedures such as the use of a spectroscopic ellipsometer and substituting them by known values in the theoretical formula. 106.2 Features • Nondestructive measurement for elastic modulus, thickness and density of films. • High precision analysis of ultra-thin and multilayer films. • Rapid and easy measurement. 106.3 Instrumentation As shown in Fig. 106.1, the measuring system is mainly comprised of short pulse lasers and its irradiation optical system, a sample movement stage, a voltage converter probe for detecting the surface acoustic wave, an oscilloscope for recording the signal, and a personal computer for controlling the entire system and analyzing the data [1, 2]. A nitrogen gas laser of wavelength 337.4 nm is used as the laser beam source. A short pulse laser of pulse length 0.5 ns and a maximum energy 0.4 mJ is passed through a cylindrical lens to form a line beam approximately 10 mm wide and irradiated onto the surface of the sample with a frequency of 10 Hz. To generate a broadband surface acoustic wave, the energy density of the irradiation light is crucial and optimization by taking measures such as reducing the irradiation energy by introducing an optical filter is essential in case a material gets damaged 106 Surface Acoustic Wave 659 Fig. 106.1 Schematic representation of the laser-induced surface acoustic wave system on receiving the laser radiation. The surface acoustic wave, which propagates the sample surface, is detected by a piezoelectric converter probe situated at a position several millimeters away from the laser irradiation location. By placing polyvinylidene fluoride (PVDF), a piezoelectric film between the wedge-shaped metallic tip portion and the sample, the pressure-voltage-converted electric signal up to a high-frequency region (250 MHz) is recorded in the oscilloscope as a waveform signal. To measure the phase velocity, the accuracy of the distance from the laser irradiation position to the detecting sensor is required and, therefore, a high positioning accuracy on the order of 1 lm on the sample movement stage is essential. Now, in order to eliminate the response lag effect of the waveform detector, it is necessary to conduct the measurements at two or more points that have different distances between the laser irradiation position and the detector. 106.4 Applications Figure 106.2 shows the result of a specific measurement example of Young’s modulus by the nanoindentation method and SAW measurement for various thin films on the silicon (100) substrate. For both silicon oxide and silicon nitride hard films, Young’s modulus is practically equal, but the value by the nanoindentation method has a large standard deviation. For the nanoindentation method, the local physical property was measured, and therefore, whenever there is any unevenness in the film, it is reflected as such in the measured value. In the case of the relatively thick polyimide film with a thickness of 5.5 lm, although the average value is somewhat equal, the value obtained by the SAW measurement shows some 660 SAW Nanoindentation Elastic modulus GPa Fig. 106.2 Comparison of nanoindentation and SAW method in measuring elastic modulus of thin films on Si (100): CVD silicon oxide (1 lm), CVD silicon nitride (0.2 lm with intermediate layer: of SiO2: 0.1 lm), polyimide (5.5 lm) and nickel (0.3 lm) S. Sasaki SiO2 SiN Polyimide Nickel variation. This might be due to the inability to detect the waveform signal of sufficient intensity because of the large vibration-damping of the polyimide film and the effect of measurement noise. For a nickel film of 0.3 lm thickness, the nanoindentation method varied greatly and there was a high possibility that the measurement was not accurate. This may be attributed to the surface roughness of the nickel film. References 1. DIN EN 15042-1:2006-06: Thickness measurement of coatings and characterization of surfaces with surface waves—part 1 Guide to the determination of elastic constants, density and thickness of films by laser induced surface acoustic waves 2. Schneider, D., Schltrich, B., Sceibe, H.J., Ziegele, H., Griepentrog, M.: A laser-acoustic method for testing and classifying hard surface layers. Thin Solid Films 332, 157–163 (1998) Chapter 107 Surface Enhanced Raman Scattering Katsuyoshi Ikeda Keywords Vibrational spectroscopy Surface plasmon resonance Single molecule detection Electronic resonance 107.1 Principle SERS is based on plasmonic enhancement of Raman scattered signals near a metal surface. The excitation of surface plasmon polaritons is accompanied by local field enhancement in the vicinity of the metal surface. In the case of a dimer of Ag nanospheres, for example, such field enhancement occurs in the gap region as shown in the left panel of Fig. 107.1. This “electromagnetic” effect can enhance the signal intensity by a factor of more than 106 [1]. This remarkably large effect is related to the fact that both excitation and scattering processes can gain intensity from plasmon resonances (see the right panel of Fig. 107.1). In addition, SERS may gain extra intensity from non-electromagnetic mechanisms such as electronic resonance of target molecules or charge transfer (CT) resonances at metal–molecule interfaces, which can provide information on interfacial electronic structures. On the other hand, optical excitation of surface plasmons must be mediated by metal nanostructures in SERS measurement. Hence, the enhancement factor is quite sensitive to the size and shape of the nanostructures, resulting in relatively low reproducibility of SERS spectra. Moreover, measurable SERS is normally obtained only at surfaces of coinage metals; surface plasmon polaritons are strongly damped at other metal surfaces such as transition metals. This sometimes limits practical application of SERS. K. Ikeda (&) Department of Physical Science and Engineering, Nagoya Institute of Technology, Nagoya 466-8555, Japan e-mail: kikeda@nitech.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_107 661 662 K. Ikeda Fig. 107.1 Local field distribution near a dimer of Ag spheres with diameter of 20 nm under illumination of 524 nm, which was calculated by the discrete dipole approximation method. The bright spot in the gap region indicates that SERS effect is expected in this small area selectively. An energy diagram of Raman scattering is also illustrated in the right panel. a0M and a0k denote a polarizability tensor of metal nanoparticles and a Raman tensor of molecules, respectively 107.2 Features • A small number of molecules can be detected in the vicinity of metal surfaces. • Both vibrational and electronic information can be obtained at metal–molecule interfaces. • Surface selection rules operate for SERS. • In situ, spectroscopy is possible in various media with strong absorption in the frequency range of molecular vibrations. • Plasmon-active metal nanostructures are required to measure SERS. 107.3 Instrumentation The SERS measurement can be conducted at a SERS-active metal substrate using a conventional Raman spectrometer consisting of an excitation laser, optical filters, and a CCD spectrometer. An Ar+ ion laser with 514.5-nm radiation or a He–Ne laser with 632.8-nm radiation is frequently utilized as a light source. The excitation beam is applied to the sample surface through the laser line filter. Scattered signals from the sample are then introduced to the CCD spectrometer after removing the Rayleigh line using the optical notch filters. To benefit from the surface enhancement effect, the excitation wavelength must be overlapped by a surface plasmon resonance band of the metal substrate. The activity of surface plasmon resonances is typically introduced by nanostructuring of the metal substrate. Such a roughened 107 Surface Enhanced Raman Scattering 663 surface with broad resonances is easily obtained by application of electrochemical oxidation and reduction cycles. Nanolithographic fabrication of well-shaped nanopatterns with relatively narrow resonances is also now frequently utilized to improve the sensitivity and reproducibility of the signal enhancement. 107.4 107.4.1 Applications Single Molecule Detection The normal Raman scattering cross section is essentially small. Nevertheless, single molecule detection has been extensively discussed in SERS spectroscopy because of the extremely large signal enhancement of SERS. Since the electromagnetic enhancement alone may not be enough for achieving single molecule detection, electronic resonances of target molecules or CT resonances at metal–molecule interfaces are frequently utilized to obtain further signal enhancement. Figure 107.2 shows an example of single molecule SERS measurement reported by Kneipp et al. [2]. The upper panel shows temporal variation of SERS intensity at a distinct Raman band of crystal violet molecules attached to small silver clusters in water at an average of 0.6 molecules in the probed volume. The bottom panel shows the statistical analysis of the intensity fluctuation, indicating Brownian motion of the clusters. It is believed that this behavior corresponds to counting the number of the molecules in the measurement spot. Fig. 107.2 Time sequence of SERS signals of the 1174 cm−1 Raman line of crystal violet molecules attached to silver nanoclusters in water and their statistical analysis of the Raman line at an average of 0.6 molecules in the probed volume. Reprinted with the permission from Ref. [2]. Copyright 2006 by American Physical Society 664 K. Ikeda Fig. 107.3 Electrochemical potential and excitation energy dependence of SERS spectra of p-aminothiophenol on Ag electrode in 0.1 M NaClO4 aqueous solution, measured with 488.0 and 632.8 nm excitation. Reprinted with the permission from Ref. [3]. Copyright 1994 American Chemical Society 107.4.2 Electrochemical SERS SERS can detect vibration motions with energy of approximately 0.01–0.1 eV using incident photons with energy of 1–5 eV. This is a significant advantage for in situ vibrational spectroscopy such as spectro-electrochemistry. Moreover, the relatively large excitation energy in SERS can provide an opportunity to obtain electronic information at metal–adsorbate interfaces. Figure 107.3 shows an example of electrochemical SERS of p-aminothiophenol adsorbed on a silver electrode [3]. For 488.0 nm (2.54 eV) excitation, three peaks at 1142, 1391, and 1440 cm−1 disappeared at around −0.45 V versus SCE during the negative going scan of electrochemical potential. For 632.8 nm (1.96 eV) excitation, the potential dependence of these peaks was shifted negatively. This can be explained by considering electronic structures at the silver–adsorbate interface; these “potential-dependent” peaks gain extra intensity from CT resonance at the interface through Herzberg–Teller type vibronic coupling. 107.4.3 Field-Gradient SERS SERS is based on field localization at the nanoscale. This also leads to an increase in the field gradient. Therefore, one can expect that the selection rules under the dipole approximation may be broken in SERS. Figure 107.4 shows SERS and Raman spectra of graphene sheets with and without SERS-active silver nanodimer arrays [4]. It is known that the zone-boundary D and D’ phonons can be observed only when intravalley or intervalley scattering of excited phonons is resonantly 107 Surface Enhanced Raman Scattering 665 Fig. 107.4 SERS and Raman spectra of defect-free graphene sheets with and without field gradient. Reprinted with the permission from Ref. [4]. Copyright 2013 American Chemical Society induced by graphitic defects. Since the zone-center G phonon is allowed ordinarily, only the G band is found in the Raman spectrum of the defect-free graphene. On the other hand, SERS spectra showed very strong D and D’ bands especially when the incident light is polarized along the dimer long axis. In this polarization condition (r plasmon excitation), the electromagnetic field is strongly confined within the gap region of the dimer. When the polarization direction is perpendicular to the dimer axis (p* plasmon excitation), the field localization is rather small. The polarization dependence of the D and D’ band intensities in SERS strongly suggests that the dipole forbidden transition was induced by the gradient fields of the strongly confined plasmon polaritons. Indeed, the relative intensities of D and D’ bands were controlled by the gap separation, which is related to the degree of the field gradient. In vibrational spectroscopy, control of selection rules may open up a novel way for surface characterization. References 1. Pettinger, B.: Light scattering by adsorbates at Ag particles: quantum-mechanical approach for energy transfer induced interfacial optical processes involving surface plasmons, multipoles, and electron-hole pairs. J. Chem. Phys. 85, 7442–7451 (1986) 2. Kneipp, K., Kneipp, H., Kneipp, J.: Surface-enhanced raman scattering in local optical fields of silver and gold nanoaggregates-from single-molecule raman spectroscopy to ultrasensitive probing in live cells. Acc. Chem. Res. 39, 443–450 (2006) 3. Osawa, M., Matsuda, N., Yoshii, K., Uchida, I.: Charge transfer resonance raman process in surface-enhanced raman scattering from p-aminothiophenol adsorbed on silver: herzberg-teller contribution. J. Phys. Chem. 98, 12702–12707 (1994) 4. Ikeda, K., Takase, M., Hayazawa, N., Kawata, S., Murakoshi, K., Uosaki, K.: Plasmonically nanocinfined light probing invisible phonon modes in defect-free graphene. J. Am. Chem. Soc. 135, 11489–11492 (2013) Chapter 108 Surface Magneto-Optic Kerr Effect Takeshi Nakagawa Keywords Magnetism 108.1 Ultrathin films Optical transition Principle Surface magneto-optic Kerr effect (SMOKE) is based on magnetic circular dichroism (MCD), where the absorption for right and left circular polarized light is different because electronic states are split due to exchange interaction and spin– orbit coupling [1]. Figure 108.1 shows a schematic energy diagram for optical transition from doubly degenerated dxz, dyz (l = 2, ml = ±1) to pz (l = 1, ml = 0) states in a ferromagnetic material magnetized along z direction [2]. The degenerated d states are split into up and down spin states by exchange interaction (Dex) and further split into d(x + iy)z (ml = +1) and d(x − iy)z (ml = −1) by spin–orbit coupling (Dso). The dipolar selection rule allows the transition with Dl = ±1 and Dml = ±1. Dml = +1(−1) corresponds to the transition by left (right) circularly polarized light. Thus, the transition for left and right circular light is different, resulting in MCD. Upon the reflection at the magnetic materials, linearly polarized light, which can be coherent sum of right and left circular light, is transformed into elliptically polarized light with its major polarization axis rotated from the initial polarization upon the reflection (Fig. 108.2), which is Kerr rotation and ellipticity. Note that the Kerr rotation and ellipticity in principle depends on the electronic structure, photon energy, and geometry, and is not directly proportional to the magnetic moment. Evaluation of the spin and orbital magnetic moments in ultrathin films requires other measurements such as x-ray magnetic circular dichroism (XMCD). T. Nakagawa (&) Department of Molecular and Material Sciences, Kyushu University, Fukuoka, Japan e-mail: naka@kyudai.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_108 667 668 T. Nakagawa Fig. 108.1 Schematic energy diagram for the optical transition from d to p states in a ferromagnetic material magnetized along z direction. The d states are split by exchange (Dex) and spin–orbit (Dso) interactions Fig. 108.2 SMOKE setup for longitudinal configuration. Linearly polarized light is transformed into elliptically polarized light upon the reflection at the surface of the magnetic material. The sample is placed in a vacuum chamber, while optics are placed in the air. Polar and transverse configurations are also shown 108.2 • • • • Features Magnetization hysteresis curves can be measured. Magnetization easy axis can be determined. Sensitivity down to monolayer is achieved. Magnetic moment cannot be evaluated. 108 Surface Magneto-Optic Kerr Effect 108.3 669 Instrumentation Figure 108.2 shows a simple SMOKE system, consisting of a laser (a few mW, continuous wave), polarizers, and photo detector. Linearly polarized light is converted to elliptically polarized light upon the reflection at the magnetic material. Kerr rotation is measured with the second polarizer (analyzer) and the detector. On the other hand, with a quarter-wave plate in addition, Kerr ellipticity is measured. Stable light sources and polarizers with high extension ration (>106) would be greatly preferred. Even with the simple experimental setup, the detection sensitivity can be achieved as high as 1 10−3 degree, which is the same order of magnitude of the typical Kerr rotation for in-plane magnetized 1 ML Fe thin films. Using a laser with variable wavelength light source offers additional information such as band structures [3]. Sensitivity of the SMOKE measurement may be improved by adopting modulation technique. The simultaneous determination of the Kerr rotation and ellipticity is a more practical benefit associated with the use of the modulation technique [3]. For a SMOKE measurement, a sample is placed in a vacuum chamber, and optics such as laser, polarizers are set up outside the chamber. The light passes through vacuum windows. Inevitably, the SMOKE in the vacuum chamber is affected by the vacuum windows. The birefringence of the windows deteriorates the system sensitivity because of the higher background signal caused by the unwanted polarization. The deterioration can be partially overcome by a quarter-wave plate [1] or window compensation technique [4]. The SMOKE measurement utilizes three configurations, polar, longitudinal, and transverse as shown in Fig. 108.2. The polar configuration is employed for perpendicular magnetization, while the longitudinal and transverse configurations are employed for in-plane magnetization. In the transverse configuration, polarization does not change, but the reflectivity changes when the magnetization is turned over, which is measured without the analyzer. Normally, the polar Kerr rotation and ellipticity are much larger, one order of magnitude, than the longitudinal ones since the reflected light enhances the magneto-optical interaction in the polar configuration, while it cancels in the longitudinal configuration [5, 6]. 108.4 108.4.1 Applications Thickness-Dependent Magnetization Measurement SMOKE is not sensitive to slight modification of the electronic structure. It is applicable to the studies of thickness-dependent magnetization in ultrathin films. Figure 108.3 shows the magnetization curves and the Kerr rotation of in-plane magnetized Fe ultrathin films prepared on clean Si(111) and N-terminated Si(111) as a function of Fe thickness [7]. The excitation source used is a diode laser of 670 T. Nakagawa Fig. 108.3 Magnetization curves taken by longitudinal SMOKE for Fe ultrathin films on (a) Si (111) and (b) N/Si(111) surfaces. (c) Thickness dependence for Kerr rotation is shown for Fe/Si (111) (black squares) and Fe/N/Si(111) (red circles) 1.95 eV. Since Si has high reactivity with Fe, forming iron silicide at the interface, passivate layers such as N–Si interface are necessary to prevent silicide formation. Thickness-dependent measurement shows the onset of the ferromagnetism of Fe/Si (111) is 7 ML, and Fe/N/Si(111) surfaces 9 ML at T = 298 K. This thickness dependence indicates that the extrapolation of the Kerr rotation gives an intercept almost at 0 ML for Fe/N/Si(111), suggesting no magnetically dead layer. On the other hand, the intercept for Fe/Si(111) is at 3 ML, suggesting that the 3 ML Fe at the interface forms silicide. The larger Kerr rotation of Fe/N/Si(111) for 15 ML or thicker, compared with that of Fe/Si(111), also indicates that the nitride interface effectively precludes the silicide formation. It is noted that missing of ferromagnetism at T = 298 K for Fe/N/Si(111) below 9 ML is due to both the low Curie temperature and the superparamagnetic effect, which is experimentally verified by XMCD at low temperatures. 108.4.2 Spin Reorientation Transition and Determination of Easy Axis In ultrathin films, a perpendicular magnetization is frequently observed, which is not stable in thick films due to the dominant contribution of the magnetic dipolar interaction. The perpendicular magnetization in the ultrathin films is stabilized by magnetocrystalline anisotropy energy, which comes from anisotropic electronic structures in thin films and at surfaces/interfaces. Figure 108.4 shows a temperature-dependent spin reorientation transition between the perpendicular and the in-plane magnetization in 6 ML Fe/Ag(100) [8]. At 310 K, the polar SMOKE shows a hysteresis loop. Although the longitudinal SMOKE also shows a hysteresis 108 Surface Magneto-Optic Kerr Effect 671 Fig. 108.4 Temperature dependence of magnetization curves by SMOKE on Fe(6 ML)/Ag(100) for the polar (right) and the longitudinal (left) configuration. Reprinted with permission from Ref. [8]. Copyright 1993 by American Physical Society loop at 310 K, the longitudinal hysteresis is an artifact due to misalignment of the magnetic field. Above 370 K, the polar SMOKE shows no hysteresis, while the longitudinal SMOKE shows hysteresis loops with much lower Kerr intensity. With increasing temperature, the magnetization easy axis in Fe (6 ML)/Ag(100) changes the direction from the out-of-plane to in-plane. References 1. Qui, Z.Q., Bader, S.D.: Surface magneto-optic Kerr effect. Rev. Sci. Instrum. 71, 1243 (2000) 2. Bruno, P., Suzuki, Y., Chappert, C.: Magneto-optical Kerr effect in a paramagnetic overlayer on a ferromagnetic substrate. Phys. Rev. B 53, 9214 (1996) 3. Sato, K.: Measurement of magneto-optical Kerr effect using piezo-birefringent modulator. Jpn. J. Appl. Phys. 20, 2403 (1981) 4. Arnold, C.S., Venus, D.: Simple windows-compensation method for improving the signal-to-noise ratio in measurements of the magneto-optic Kerr effect in ultrathin films. Rev. Sci. Instrum. 66, 3280 (1995) 5. Zak, J., Moog, E.R., Liu, C., Bader, S.D.: Magneto-optics of multilayers with arbitrary magnetization directions. Phys. Rev. B 43, 6423 (1991) 6. Yokoyama, T., Nakagawa, T., Takagi, Y.: Magnetic circular dichroism for surface and thin film magnetism. Int. Rev. Phys. Chem. 27, 449 (2008) 7. Eguchi, K., Takagi, Y., Nakagawa, T., Yokoyama, T.: Growth process and magnetic properties of iron nanoparticles deposited on Si3N4/Si(111)-(8 8). Phys. Rev. B 85, 174415 (2012) 8. Qui, Z.Q., Pearson, J., Bader, S.D.: Asymmetry of the spin reorientation transition in ultrathin Fe films and wedge grown on Ag(100). Phys. Rev. Lett. 70, 1006 (1993) Chapter 109 Surface Plasmon Resonance Kaoru Tamada Keywords Surface plasmon resonance Biosensor Fluorescence enhancement 109.1 Kretschmann Refractive index Principle “Surface plasmon polariton (SPP)” is collective oscillation of electrons at a metal– dielectric interface, which holds a specific longitudinal wave vector, ksp, lying along with interface as a function of frequency. x ksp ðxÞ ¼ c sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi em ðxÞed ðxÞ ; em ðxÞ þ ed ðxÞ ð109:1Þ where em (=e′m + ie″m) and ed (=e′d + ie″d) are complex dielectric functions of metal and dielectic materials. x and C are the angular frequency and the light velocity in vacuum. This equation is called as SPP dispersion equation. When a laser light is irradiated to a metal–dielectric interface with an incident angle h, the photon wave vector of the incident light projected to the metal–dielectric interface, kxph, is described as: x kph ðx Þ ¼ x pffiffiffiffiffiffiffiffiffiffiffiffi ed ðxÞ sinh; c ð109:2Þ when kxph = ksp, a resonance coupling between photons and SPP can be maintained. This phenomenon is called as “surface plasmon resonance (SPR).” In general, Kretschmann-based prism couplers are utilized to achieve the above condition (Fig. 109.1), where evanescent waves at the interface under the total internal reflection couple with SPP on metal thin films (typically 50 nm-thick gold or K. Tamada (&) Institute for Materials Chemistry and Engineering, Kyushu University, Fukuoka, Japan e-mail: tamada@ms.ifoc.kyushu-u.ac.jp © Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_109 673 674 K. Tamada (a) (b) Fig. 109.1 a Kretschmann-based prism coupler for SPR measurement (top) and the typical angular scan data to determine SPR resonance angle (h0) (bottom). The resonance angle shifts to larger angles when molecules adsorb on gold surface. b Schematics of wave vector matching for resonance coupling of photon and SPP at a metal–dielectric interface. At h0, the photon wave vector projected to the interface (x–y plane) matches the SPP wave vector, i.e., kxph = ksp silver). The resonance condition can be tuned by the incident angle h. The SPR coupling can be detected experimentally by monitoring the reflection light intensity [1, 2]. At the resonance angle (h0), the reflected light intensity becomes minimum as shown in Fig. 109.1a. Surface-sensitive sensing measurement with SPR is performed by use of the metal surface at the opposite side of prism (open surface). The SPR wave excited at a metal/dielectric interface propagates in the x–y plane, while electric field penetrates into the dielectric medium and decays exponentially. The penetration depth in the dielectric medium is typically a few hundred nanometers, and the refractive index (dielectric functions) at this depth region determines the excitation condition of SPP (see Eq. 109.1). Thus, adsorption of small molecules having different refractive index from original medium (e.g., protein adsorption in water) can be detected sensitively by the resonance angle shift. This type of SPR, excited on metal thin film, is called as “propagation” mode of SPR, in contrast to metal nanoparticle-based SPR (“localized” mode) [3]. Both SPR are widely used for analytical tools, because of their surface sensitive property, based on light confinement and enhancement effects. Those are used not only for refractive index detections but also for fluorescence or vibration signal enhancement (e.g., plasmon-enhanced fluorescence [4], surface enhanced Raman spectroscopy (SERS) [5]). The fields related to SPR are really wide, but this chapter focuses on a guidance of basic technique based on “propagation” mode of SPR with a prism coupler. For a grating-based SPR or a long-range SPR with Otto configurations, please refer the following references [6, 7]. 109 Surface Plasmon Resonance 109.2 675 Features • Surface sensitive detection method with metal thin film. • Typically used for detection of refractive index change, which leads to label-free, real-time biosensor application. • SPR is used to enhance fluorescence or molecular vibration signals as well. 109.3 Instrumentation One example of experimental setup for SPR measurement is schematically shown in Fig. 109.2. In a Kretschmann geometry system, a triangular prism (LaSFN9 glass) is index-matched to LaSFN9 substrates with the gold (or silver) films via immersion oil. A p-polarized He–Ne laser beam (k = 632.8 nm, 5 mW) is mechanically chopped in conjugation with a lock-in amplifier before entering the prism. The intensity of the laser beam reflected at the prism–gold interface is monitored by a photodiode detector. The change of reflectivity as a function of the incident angle is recorded as “angular scan” data on a h–2h rotation stage, while kinetics scan data are recorded as a function of time at a fixed incident angle. The resolution of measured resonance angle is 0.01°–0.001°. Before the measurement of samples, the angular scan of the bare gold film was taken in order to determine the exact gold film thickness and the complex dielectric constant, em (=e′m + ie″m), by curve fitting with Fresnel’s equations. Next, probe molecules are immobilized on gold (in many cases, by injection of thiolated probe molecules into the fluid cell), and then target molecules are injected. At each step, adsorption reaction is real-time monitored by the change in reflect

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