Part II • XAFS: Principles • XANES/NEXAFS • Applications 1 X-ray Absorption Spectroscopy (XAS) X-ray Absorption spectroscopy is often referred to as - NEXAFS for low Z elements (C, N, O, F, etc. K-edge, Si, P, S, L-edges) or - XAFS (XANES and EXAFS) for intermediate Z and high Z elements. 2 NEXAFS, XANES and EXAFS • NEXAFS (Near Edge X-ray Absorption Fine Structures) describes the absorption features in the vicinity of an absorption edge up to ~ 50 eV above the edge (for low Z elements for historical reasons). • It is exactly the same as XANES ( X-ray Absorption Near Edge Structures), which is often used together with EXAFS (Extended X-ray Absorption Fine Structures) to describe the modulation of the absorption coefficient of an element in a chemical environment from below the edge to ~ 50 eV above (XANES), then to as much as 1000 eV above the threshold (EXAFS) • NEXAFS and XANES are often used interchangeably • XAFS and XAS are also often used interchangeably 3 XAFS of free atom In rare gases, the pre-edge region exhibits a series of sharp peaks arising from bound to bound transitions (dipole: 1s to np etc.) called Rydberg transitions 4 XAFS of small molecules Small molecules exhibit transitions to LUMO, LUMO + and virtual orbital, MO in the continuum trapped by a potential barrier (centrifugal potential barrier set up by high angular momentum states and the presence of neighboring atoms), or sometimes known as multiple scattering states 5 XAFS: the physical process Dipole transition between quantum states • core to bound states (Rydberg, MO below vacuum level, -ve energy, the excited electron remains in the vicinity of the atom)- long life time-sharp peaks • core to quasi-bound state (+ve energy, virtual MO, multiple scattering states, shape resonance, etc.); these are the states trapped in a potential barrier, and the electron will eventually tunnel out of the barrier into the continuum-short lifetime, broad peaks • core to continuum (electron with sufficient kinetic energy to escape into the continuum) - photoelectric effect. XPS &EXAFS 6 XAFS: the physical process cont’ Scattering of photoelectron by the molecular potential – how the electron is scattered depends on its kinetic energy • Low kinetic energy electrons - Multiple scattering (typically up to ~ 50 eV above the threshold, the region where bound to quasi-bound transitions take place); e is scattered primarily by valence and shallow inner shell electrons of the neighboring atoms - XANES region •High kinetic energy electrons (50 -1000 eV) are scattered primarily by the core electrons of the neighboring atoms, single scattering pathway dominates - EXAFS region 7 Electron scattering • Free electron (plane wave) scattered by an atom (spherical potential) travels away as a spherical wave • Electrons with kinetic energy > 0 in a molecular environment is scattered be the surrounding atoms • Low KE e- is scattered by valence electrons, undergoes multiple scattering in a molecular environment • High KE e- is scattered by core electrons, favors single scattering Multiple Scattering Single Backscattering 8 9 10 NEXAFS of free atom & unsaturated diatomic molecules States trapped in the potential barrier are virtual MO’s or multiple scattering states (quasi bound states) Free atom free e with KE >0 diatomic with unsaturated bonding N2, NO, CO etc. bound states * Asymptotic wing of the coulomb potential supports the Rydberg states 11 NEXAFS of Free Atom: Ar L3,2-edge hv 3d 0.40 2p3/2 - 4s 123meV 0.35 Relative Intensity 0.30 4d 0.25 244.00 244.25 244.50 244.75 Photon Energy Ar (gas) 2p6 2p54s1 [Ne]3s23p6 14s1 2p Core hole j=l±s 3d 0.20 2p1/2 - 4s 5d 4d 0.15 6d 2p1/2 2p3/2 5d 6d 0.10 0.05 Hund’s rule (lower binding) 244 245 246 247 248 Photon Energy (eV) 249 250 251 12 NEXAFS of atom and small molecules HBr Kr M5,4 (3d5/2,3/2) M5,4 (3d5/2,3/2) MO Rydberg transitions in HBr remain strong and a broad transition to molecular orbital emerges 13 NEXAFS of Nitrogen π* IP σ* Intensity (arb. units) N2 400.0 400.5 401.0 401.5 Photon Energy (eV) 402.0 1s - * Vibronic structures in the 1 s to * transition 14 Molecular orbital illustration CO N2 * MO axis C2H2 * * C 1s C 1s 15 16 E = (E*-Eo) 1/r Why would this correlation exist? i) Multiple scattering theory ii) Simple picture- particle in a boxthe shorter the bond the farther the energy separation iii) Scattering amplitude Z 17 18 NEXAFS of molecules oriented on a surface (angular dependence of resonance intensity) When molecules adsorb on a surface, their molecular axis is defined by the axis of the substrate. Therefore, angle dependent experiments can be made by rotating the substrate with respect to the polarization of the photon beam. Using selection rules, the orientation of the molecule on a surface can be determined. H H C=C H H C-C 19 Carbon Allotropes Hexagonal Diamond (Lonsdaleite) Diamond Graphene Graphite Nanotube 20 XANES of Benzene LUMO + 1 LUMO core 21 *1 C-H* Increasing no. of benzene rings XANES systematic *2 Tom Regier unpublished 22 Chemical systematic C60 CN T 23 24 d - electron in transition metals The dipole selection rule allows for the probing of the unoccupied densities of states (DOS) of d character from the 2p and 3p levels – M3,2 and L3,2-edge XANES analysis (relevance: catalysis) 25 Filling of the d band across the period 3d 4d 5d 26 Decreasing whiteline intensity across the period as the d band gets filled Transition metal systematic L3,2/M3,2 Whiteline and unoccupied densities of d states • 3d metal: the L-edge WL for early 3d metals is most complex due to the proximity of the L3,L2 edges as well as crystal field effect; the 3d spin-orbit is negligible • 4d metals: the L3,L2 edges are further apart, WL intensity is a good measure of 4d hole population • 5 d metals: The L3 and L2 are well separated but the spin orbit splitting of the 5d orbital becomes important, j is a better quantum number than l, WL intensity is a good measure of d5/3 and d3/2 hole populations 27 3d metals Hsieh et al Phys. Rev. B 57, 15204 (1998) 28 d- band filling across 5d row EFermi Ta W Pt Au 5d metal d band Intensity (area under the curve) of the sharp peak at threshold (white line) probes the DOS T.K. Sham et al. J. Appl. Phys. 79, 7134(1996) 3p3/2-5d 2p3/2-5d 29 d - charge redistribution in 4d metals Ag Pd 4d charge transfer upon alloying I. Coulthard and T.K. Sham, Phys. Rev. Lett., 77, 4824(1996) 30 Analysis of d hole in Au and Pt metal In 5 d metals with a nearly filled/full d band such as Pt and Au, respectively, spin orbit coupling is large so d5/2 and d3/2 holes E population is not the same F DOS 5d5/2 Selection rule: dipole, Pt l = ± 1, j = ± 1, 0 L3, 2p3/2 5d5/2.3/2 L2, 2p1/2 5d3/2 T.K. Sham et al. J. Appl. Phys. 79, 7134(1996) 5d3/2 31 Au L3 2p3/2 - 5d 5/2,3/2 Au L2 2p1/2 - 5d3/2 M. Kuhn and T.K. Sham Phys. Rev. 49, 1647 (1994) 32 * * Phys. Rev. B22, 1663 (1980) 33 Probing depth profile with light: applications photon e ion (photon) surface 0.1-1 nm near surface nm-10 nm Interface bulk-like X-ray probes: Light-matter Interaction: Absorption & Scattering Morphology Structure Electronic properties soft x-ray 10-103 nm hard x-ray 34 Soft X-ray vs. hard X-ray Soft x-ray usually cannot penetrate the entire sample measurements cannot be made in the transmission mode. Yield spectroscopy is normally used ! One absorption length (t1 = 1 or t1 = 1/ ) is a good measure of the penetration depth of the photon Example of one absorption lengths Element density(g/cm3) hv(eV) mass abs (cm2/g) Si 2.33 1840 3.32 x103 100 8.6 x104 Graphite 1.58 300 4.02x104 t1(m) 1.3 0.05 0.16 35 Soft x-ray spectroscopy: unique features • XAFS measurement in the soft x-ray (short absorption length) region is often made using yield spectroscopy Electron yield (total, partial, Auger) X-ray fluorescence yield (total, selected wavelength) Photoluminescence yield (visible, light emitting materials) XEOL (X-ray excited optical luminescence) and TRXEOL (Time-resolved X-ray Excited Optical Luminescence) • Since soft X - ray associates with the excitation of shallow core levels, the near-edge region (XANES/ NEXAFS) is the focus of interest for low z elements and shallow cores 36 E.G.: the Interplay of TEY and FLY hv Diamond-like Graphite-like Amorphous TEY c-BN h-BN a-BN FLY 80 nm Si(100) Cubic BN film grown on Si(100) wafer ? Preferred orientation of the h-BN film? 37 Boron K-edge b 3 TEY FLY c-BN h-BN a-BN TEY 2 Intensity (a. u.) hv normal * 4 1 20 0 40 0 60 0 90 I (1s - *) minimum 0 190 200 210 220 Photon Energy (eV) 1 * a Si(100) FLY Inthesity (a. u.) 3 glancing 2 4 I (1s - *) maximum 0 20 0 40 0 60 0 90 190 X.T. Zhou et.al. JMR 2, 147 (2006) 200 210 220 Photon Energy (eV) 38 XAFS and XEOL (Optical XAFS) E XEOL XAFS PLY Mono CB hvop Edge Abs. hvex EF 200 850 Wavelength (nm) VB 39 XEOL - Energy Domain • X-ray photons in, optical photons out Illustrations BN nanowires Si nanowires ZnS nanoribbons 40 40 Boron nitride nanotube (BNNT) B-O bond Oxygen BNNT W.-Q. Han et al. Nanoletter. 8, 491-494 (2008) h-BN Liu et. al. (UWO) unpublished 41 Si K-edge XEOL of silicon nanowires 4000 12000 3 Si K-edge Si 10000 Intensity (arb. units) 9000 SiO2 1840 1850 1860 Photon Energy TEY 2 (a) 11000 Intensity 3000 1847.5 eV 1842 eV (b) 2000 1 1000 0 0 300 1870 difference curve 400 500 600 700 Wavelength (nm) hvex(eV) 1890 8000 7000 1867 1851 1847.5 6000 5000 1845 4000 1842 1840 3000 1838.5 1830 2000 530 nm 460 nm 1000 200 300 400 500 630 nm 600 700 800 Wavelength (nm) T.K. Sham et al. Phys. Rev. B 70, 045313 (2004) 42 42 Photoluminescence: Si K-edge Si nanowire TEY 40 Intensity (arb. units) FLY PLY zero order 30 460 nm 20 530 nm 630 nm 10 Si SiO2 0 1830 1840 1850 1860 1870 1880 Photon Energy (eV) 43 43 ZnS hetero-crystalline nano-ribbon 520 nm X.-T. Zhou et al., J. Appl. Phys. 98, 024312(2005) 520 nm 280 K 10 K Total 332 nm ZnSOLnw XAS (zincZB blend) 1020 300 400 500 600 1030 1040 1050 Energy (eV) 700 Wavelength (nm) 332 nm Total 0-14 ns OLnw ZnS W XAS (wurtzite) 300 400 500 600 Wavelength (nm) R.A. Rosenberg et al., Appl. Phys. Lett. 87, 253105(2005) 700 800 1020 1030 1040 1050 Energy (eV) 44 XEOL - Time Domain • Timing - resolved XEOL (TRXEOL) • Illustrations ZnO: Nanodeedle vs Nanowire CdSe-Si: Hetero nanostructures 45 45 TRXEOL at APS T.K. Sham & R.A. Rosenberg, ChemPhysChem 8, 2557-2567 (2007) 46 46 Time-gated spectroscopy • Select time window(s) • Obtain spectra /yields of photons arriving within that window 153 ns 50 Fast time window 100 Time (ns) 150 Slow time window 47 47 CdSe -Si hetero nanoribbons hv = 1100 eV CdSe 0-20 ns Si X.H. Sun et al. J. Phys. Chem., C, 111, 8475(2007) total optical yield 20-150 ns 48 48 R.A. Rosenberg et. al. Appl. Phys. Lett., 89, 243102(2006) CdSe-Si Heterostructure: Se L3,2 - edge Intensity (arb. units) 0.2 TEY Se L3,2-edge Un-gated 0 - 20 ns 0.1 20 - 150 ns 1400 1420 1440 1460 1480 1500 1520 Photon Energy (eV) 49 STXM: Scanning Transmission X-ray Microscopy STXM @ the SM beamline @CLS Courtesy of A.P. Hitchcock, McMaster Fresnel Zone Plates 30nm J.G. Zhou, J. Wang, H.T. Fang, C.X. Wu, J.N. Cutler, T.K. Sham, Chem. Comm. 46, 2778 (2010) 50 50 Mico/nano spectroscopy of N-CNT TEM STXM S7 S3 S1 S2 S8 S9 ______ 500 nm (S2) J. Zhou, J. Wang et al. J. Phys. Chem. Lett. (2010) DOI: 10.1021/jz100376v 51