applied sciences Article Recent Progress on Irradiation-Induced Defect Engineering of Two-Dimensional 2H-MoS2 Few Layers Guang-Yi Zhao 1, * , Hua Deng 2 , Nathaniel Tyree 3 , Michael Guy 3 , Abdellah Lisfi 3 , Qing Peng 4 , Jia-An Yan 5 , Chundong Wang 6 and Yucheng Lan 3, * 1 2 3 4 5 6 * Nuclear Science and Technology Center, School of Physics, Jilin University, Changchun 130012, China Department of Chemistry, Morgan State University, Baltimore, MD 21251, USA; [email protected] Department of Physics and Engineering Physics, Morgan State University, Baltimore, MD 21251, USA; [email protected] (N.T.); [email protected] (M.G.); [email protected] (A.L.) Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, MI 48109, USA; [email protected] Department of Physics, Astronomy, and Geosciences, Towson University, Towson, MD 21252, USA; [email protected] School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China; [email protected] Correspondence: [email protected] (G.Z.); [email protected] (Y.L.); Tel.: +1-443-885-3752 (Y.L.) Received: 14 January 2019; Accepted: 8 February 2019; Published: 16 February 2019 Abstract: Atom-thick two-dimensional materials usually possess unique properties compared to their bulk counterparts. Their properties are significantly affected by defects, which could be uncontrollably introduced by irradiation. The effects of electromagnetic irradiation and particle irradiation on 2H MoS2 two-dimensional nanolayers are reviewed in this paper, covering heavy ions, protons, electrons, gamma rays, X-rays, ultraviolet light, terahertz, and infrared irradiation. Various defects in MoS2 layers were created by the defect engineering. Here we focus on their influence on the structural, electronic, catalytic, and magnetic performance of the 2D materials. Additionally, irradiation-induced doping is discussed and involved. Keywords: MoS2 ; 2D material; irradiation; review 1. Introduction With the discoveries of zero-dimensional buckyballs in the 1980s  and one-dimensional carbon nanotubes in the 1990s [2,3], various nanomaterials have been synthesized and characterized. Their physical and chemical properties are unique compared with counterpart bulks (graphite, diamond, and amorphous carbon) because of size-induced quantum effects, enabling their great potential in various fields. Two-dimensional (2D) graphene was reported in the 2000s  and excellent electrical behaviors were claimed. Since then investigations of 2D layers were initialized and rapidly developed in recent decades. At present, the 2D layer family spans conductors (such as graphene), insulators (such as hexagonal boron nitride), and semiconductors. Transition-metal dichalcogenide (TMDC) materials, including MoS2 , WS2 , MoSe2 , and WSe2 , are layered semiconductors and have been fabricated into 2D semiconducting layers. MoS2 2D layers have novel potential applications in novel nano-optoelectronics, optical sensors, catalysts, energy storages, and environments. Additionally, MoS2 2D layers show thickness-dependent semiconducting behaviors, being the most interesting and important among TMDC 2D layers. Bulk MoS2 is a semiconductor with an indirect energy band gap of 1.2 eV while mono-layer MoS2 is a direct band-gap semiconductor with an energy band gap of 1.9 eV Appl. Sci. 2019, 9, 678; doi:10.3390/app9040678 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 678 2 of 53 due to 2D quantum confinement. The MoS2 layers have also important applications in valleytronics—a new type of electronics where information is encoded by wave quantum number of electrons. MoS2 low-dimensional materials have been investigated over the past 50 years. MoS2 few-layers (4–5 layers) were first prepared by a peeling process from bulk crystals in the 1960s . Tribological properties of MoS2 films have been studied later in a vacuum or dry air as solid lubricants since the 1990s [6,7]. Compared with bulks, the friction coefficient of MoS2 films was reduced, and the wear life of the films was enhanced. Catalytic properties of MoS2 nanoparticles  and films [9,10] have been investigated from the 1980s. It is believed that edge sites of MoS2 nanomaterials play important roles in electrochemical and catalytic behaviors. MoS2 atomically thin materials, especially mono-layer MoS2 , have been widely investigated in recent years because of their sizable direct band gap, high charge-carrier mobility, and excellent mechanical flexibility [11,12]. The materials have potential applications in the next generation flexible electronics [11,13,14], elastic energy storage, field-effect transistors [15,16], electronic switches , electronic devices such as chemical biosensors [16,17], and optoelectronic devices  such as photodetectors and solar cells . It was demonstrated that chemically active defects, such as sulfur vacancies, significantly control or tune their catalytic activities , electronic transports [21–23], and optical properties . Various forms of irradiation and particle fluxes modulate the density of sulfur vacancies of chemical-vapor deposition (CVD) grown and mechanically exfoliated MoS2 sheets, affecting their chemically active defects. Therefore, irradiation effects have been carried out in recent years. Additionally, these MoS2 devices and sensors have been used in harsh environments, and it is necessary to investigate their tolerance under irradiation. Radiation effects on two-dimensional graphene were recently reviewed [25,26]. Defect engineering and defect-induced properties of other atom-thick mono-layers were summarized as well . The impacts of irradiation on 2H MoS2 two-dimensional materials are reviewed here. 1.1. MoS2 2D Materials MoS2 has two main types of natural phases: hexagonal structure (2H type) with the space group of P63 /mmc (D46h , No. 194) and the trigonal structure (3R type) with the space group of R3m (C53v , No. 160). The 2H phase contains two layers per unit cell, a = 3.15 Å and c = 12.30 Å. Similar to graphite, the 2H type crystals can be easily cleaved to form (0001) layers because of weaker van der Waals forces between (0001) layers. The 3R phase contains three layers per unit cell, stacking in rhombohedral symmetry with trigonal prismatic coordination, a = 3.16 Å and c = 18.33 Å. A third metastable phase, 1T-MoS2 with a tetragonal symmetry, was also artificially produced by intercalating 2H-MoS2 with alkali metals [27–29]. The artificial 1T phase and natural 3R phase are metastable at room temperature and transform to the 2H phase on heating  or IR irradiation  or microwave irradiation . This work focuses on the 2H MoS2 phase. 1T and 3R polytypes will not be discussed. Molybdenum disulfide (2H-MoS2 ) is a transition-metal dichalcogenide with a melting point of 2375 ◦ C. The bulk material is chemically stable in dilute acids and oxygen and insoluble in water. Figure 1a shows the crystallographic structure of 2H-MoS2 bulks. There are two S-Mo-S layers per unit cell stacked in hexagonal symmetry. Each S-Mo-S layer consists of two hexagonal planes of sulfur atoms and an intermediate hexagonal plane of molybdenum atoms. The sandwiched molybdenum atom plane is coordinated through ionic-covalent interactions with the sulfur atoms, forming a stable MoS2 mono-layer as shown in Figure 1b. The MoS2 mono-layers are held together by weak van der Waals interactions. The forces between the mono-layers are not very strong , and the interatomic forces within a mono-layer are sufficient for thermodynamical stability . Therefore, 2H-MoS2 has been widely used as a dry lubricant under high temperature due to the microscaled chemical properties. Appl. Sci. 2019, 9, 678 (a) 3 of 53 (b) Figure 1. (a) Crystallographic structure of 2H-MoS2 bulks and (b) Three-dimensional structure of 2H-MoS2 mono-layers. Figure 2 shows the calculated band gap of MoS2 bulks and that of MoS2 few-layers. The bulk 2H-MoS2 crystal is an indirect band-gap semiconductor with a band gap of 1.23–1.29 eV [32,34–36]. MoS2 bulks can be employed as cathode materials in batteries . MoS2 bulks show very poor catalytic activities for hydrogen evolution reactions. The band gap of MoS2 is a function of layer thickness as shown in Figure 2b–d, and unperturbed by substrate interactions. The mono-layer MoS2 has a direct band gap of 1.8 eV . The band structure changes from the indirect-gap in bulks to the direct-gap in MoS2 mono-layers, being confirmed from experiments [38,39]. The thickness-dependent electronic structure change is closely associated with many corresponding changes in chemical and physical properties. For example, the transition for indirect-to-direct band gap is correlated with changes in mechanical strength, spin density, bond energy, electrical conductivity, and the properties of transistor and sensor devices. MoS2 nanoparticles can adsorb various compounds, including tetrahydrothiophene, thiophene, benzothiophene, dibenzothiophene, and their derivatives . The chemical [29,41] and physical properties [42–44] of MoS2 few-layers were recently reviewed. These novel properties of MoS2 2D materials make them suitable 2D candidates in environmental applications [45,46], catalysts (such as electrocatalysts [47–51] and photocatalysts [48–53]), energy storage (such as lithium ion batteries [48,49,54], supercapacitors [49,54], elastic energy , and solar cells ), as well as sensing . MoS2 mono-layers were also fabricated into ultrasensitive photodetectors [56,57], and integrated circuits to perform the NOR logic operation , besides being employed in drug delivery , thermoelectrics [60,61], piezotronics [62,63], osmotic nanopower generators , and valleytronics . Because of the novel properties and wide applications of 2H-MoS2 2D layers, MoS2 2D layers have been prepared by various physical or chemical methods [41,44–46,66–69]. Individual MoS2 layers were usually obtained by the micromechanical cleavage technique  because of weak interactions between sulfur layers and strong intralayer interactions of MoS2 bulks. The mechanically cleaved MoS2 layers are usually highly crystalline with low defect density. Massive MoS2 layers have successfully been produced through the liquid-phase exfoliation [71,72]. MoS2 few-layers can also be fabricated from CVD. Interested readers can refer to the references herein. The MoS2 -based devices will be used in harsh environments from basic science to industry, such as near nuclear plants/reactors and nuclear medicine imaging. Radiations may damage MoS2 layers by breaking atomic bonds and ionizing atoms. To use the MoS2 devices reliably under irradiation, it is desirable to explore the irradiation effects on MoS2 layers. Appl. Sci. 2019, 9, 678 4 of 53 conductor renders krelax ) 0, le of luminescence that is only lim rate kdefect. In MoS2 of all thickn transition at the K point rema energy. The unusual electronic struct the resulting unique optical beha ters of d-electron orbitals that c and valence bands. Our theoret electronic states of different w orbitals with different spatial conduction band states at the K posed of strongly localized d orbi have minimal interlayer coupling in the middle of the S-Mo-S un FIGURE 4. Calculated band structures of (a) bulk MoS2, (b) quadFigure 2. Calculated band structures of (a) bulk MoS2 , (b) quadri-layer MoS2 , (c) bi-layer MoS2 , and rilayer MoS 2, (c) bilayer MoS2, and (d) monolayer MoS2. The solid states near the Γ point and the (d) mono-layer MoS The solid arrows indicate the lowest Bulk MoS2 is arrows indicate the lowest energy transitions. Bulkenergy MoS2 istransitions. charac2 . originate from a linear combin by anband indirect bandgap. The direct excitonic transitions characterized by terized an indirect gap. The direct excitonic transitions occuroccur at high energies at K at high energies at K point. With reduced layer thickness, the atoms and antibonding pz orbita point. With reduced layer thickness, the indirect band gap becomes larger, while the direct excitonic indirect bandgap becomes larger, while the direct excitonic transistrong interlayer coupling and th transition barely changes. For mono-layer MoS2 in (d), it 2becomes a direct band-gap in d, it becomes a direct semiconductor. tion barely changes. For monolayer MoS on layer thickness. bandgap semiconductor. This dramatic change of electronic The horizontal solid lines in each panel indicate the valence band maximumstruc(VBM) and tively the dotted ture in monolayer MoS2 can explain the observed jump in monolayer conclusion, our study revea lines indicate the conduction band minimum (CBM). The solid blue arrows indicate theIn lowest photoluminescence efficiency. of photoluminescence in MoS2 c energy transitions. Reprinted with permission from Reference . Copyright 2010 American consistent with the theoretical pre Chemical Society. likely to change appreciably with respect to bulk value, bandgap transition in going from 1.2. Irradiation Sourcesbecause the direct excitonic transitions remain at the same MoS2. Such behavior, arising fro energy. Therefore, the enhanced photoluminescence in in MoSof 2, may also arise i Figure 3 shows the spectrum of electromagnetic irradiation. Cosmic rays aretions composed monolayer has to be attributed to a dramatically slower metal dichalcogenides. It point particularly high-energy particles and photons. Primary cosmic rays are composed primarily of 99% electronic relaxation krelax. The decrease of interband relaxcontrolling electronic structure nuclei (about 90% protons, 9% alpha particles, and 1% nuclei of heavier elements) and about 1% ation rate strongly suggests a substantial change in MoS2 exploiting rich d-electron physic solitary electrons. The energy of the cosmic rays is high up to 1020 eV for the primary cosmic rays. electronic structure when going from the bulk to monolayer. to engineering novel optical be When entering the Earth atmosphere, the primary cosmic rays collide with atoms and molecules Electronic structure of bulk and monolayer MoS2 has bonded materials and holds prom to produce secondary cosmic particles with lower energy and electromagnetic waves, including beenpreviouslyinvestigatedthroughabinitocalculations.20,21 applications. gamma-ray, X-ray, neutrons, protons, electrons, and improved alpha particles. The energy of the generated Recent theoretical results using algorithm and gamma rays can be 50computation MeV. These primary and secondary cosmic rays shouldbulk damage microelectronic power predict that the indirect bandgap Acknowledgment. We thank devices because of sufficient energy, causing soft errors in electronic integrated especially MoS2 becomes a direct bandgap semiconductor when circuits, helpful discussion and Professor 21 in small-scale devices. Here,toseveral ion irradiation from GeV to thinned a monolayer, whichand canelectromagnetic readily accountirradiation for measurements. The work was meV are reviewed onour 2Dexperimental MoS2 few-layers. Physical properties of MoS films are also 2 bulks and thick observations. For a detailed comparison Science Foundation CAREER aw covered for comparison with those of 2D MoS few-layers. 2 between experiment and theory, we calculated the band (F.W.) and by DOE-BES Grant D structures of bulk MoS2 and ultrathin MoS2 layers with and T.L.). Y.Z. is supported by a acknowledges support from Scu different thicknesses. We employed density functional periori di Padova and the EAP. theory with generalized gradient approximation using the 22 PWscf package. The calculation results are displayed in THz REFERENCES AND NOTES Figure 4. It is shown that the direct excitonic transition (1) Novoselov, K. S.; Jiang, D.; Sche energy at Audio the Brillouin zone K point barely changes with Cosmic V. V.; Morozov, S. V.; Geim, A. K. rays layer thickness, but the indirect bandgap increases mono102, 10451. (2) Novoselov, K. S.; Geim, A. K.; M tonically as the number of layers decreases. Remarkably, son, M. I.; Grigorieva, I. V.; Dub the indirect transition energy becomes so high in mono2005, 438, 197. layer MoS2 that the material changes into a two-dimen(3) Zhang, Y.; Tan, Y.-W.; Stormer, 201. sional direct bandgap semiconductor. The variation of the (4) Yoffe, A. D. Adv. Phys. 2002, 51 electronic structure in few-layer MoS2 is in accord with (5) Ayari, A.; Cobas, E.; Ogundadegb our experimental data. With the increase of the indirect 2007, 101, No. 014507. Figure 3. Spectrum of electromagnetic irradiation. (6) Gourmelon, E.; Lignier, O.; Hado bandgap in thinner MoS2, the intraband relaxation rate J. C.; Tedd, J.; Pouzed, J.; Salard from the excitonic states decreases and the photolumiCells 1997, 46, 115. nescence becomes stronger. In the case of monolayer (7) Ho, W. K.; Yu, J. C.; Lin, J.; Yu, J. 5865. MoS2, a qualitative change into a direct bandgap semi© 2010 American Chemical Society 1274 DOI: 10.1021/nl903868 Appl. Sci. 2019, 9, 678 5 of 53 1.3. Irradiated MoS2 Materials Defects are created in MoS2 few-layers when the layers are fabricated by exfoliation technique and vapor deposition growth when layers are transferred from substrate to substrate, and placed under electromagnetic radiation and energetic particle fluxes. The defects typically deteriorate the device quality of MoS2 layers. On the other hand, defects in MoS2 also have a beneficial impact on material properties. For instance, defects have been shown to be the dominant dopant in MoS2 , with natural defect variation allowing n-type and p-type regions to coexist across distinct regions of the same sample . The sulfur vacancies can lower Schottky barrier heights of MoS2 . Therefore, it is crucial to fully understand defects in MoS2 to develop a reliable MoS2 -based electronics. In this paper, irradiation-related defects of MoS2 few-layers are focused on. Up to now, MoS2 2D layers have been irradiated under different irradiation sources. Table 1 lists some typical irradiation effects of MoS2 materials. The details are summarized in the following sections. The particle irradiation and electromagnetic irradiation will be reviewed first on structural properties and defects, then theoretical explanations of the irradiation mechanism are discussed. The irradiation-induced band structures, electric, catalytic, and magnetic properties will be summarized at the end. To understand the irradiation-induced defects well, the synthesis/preparation details of the as-prepared MoS2 layers will be described in each section in addition to irradiation. Appl. Sci. 2019, 9, 678 6 of 53 Table 1. Radiation Induced Defects in 2H MoS2 . MoS2 Irradiation Particle Energy Dose/Fluence Irradiation Source Irradiation Conditions Results Ref. 200 µm thickness electron 0.7 MeV 150–600 kGy electron accelerator RT created S vacancies  created S vacancies and Mo vacancies, diamagnetic to ferromagnetic phase transition  formed crystalline islands  200 µm thickness electron 2.0 MeV 20 layer electron 5 keV 100–250 kGy electron accelerator RT 50 nA for 15 min SEM beam in vacuum 106 –109 electron 60 keV induced 2H/1T phase transition  ∼10 layer electron 3–15 keV n/a EPMA in vacuum broke the inversion symmetry  mono-layer electron 80 keV n/a TEM beam in vacuum removed top and bottom S atoms  electrons/nm2 /s STEM beam 400–700 ◦C mono-layer 3000 electron /nm2 in vacuum mono-layer electron 200 keV TEM beam in vacuum created S vacancies, increased electric resistance  mono-layer electron 15 keV 280 µC/cm2 EBL in vacuum produced local strain and changed band structure  mono-layer electron 80 keV 40 A/cm2 TEM beam in vacuum produced holes and Mo5 S3 nanoribbons  amorphous 5–7 layer electron 1 keV 1–10 min EBI in vacuum crystallized  mono-layer U238 1.14 GeV 4000 ions/cm2 heavy ion accelerator in vacuum total damaged  micron thickness Ar+ 500 eV 2.26 × 1015 ions/cm2 plasma UHV produced S vacancies  bi-layer Ar+ 500 eV 1014 –1015 ions/cm2 plasma UHV produced S vacancies and MoS6 vacancy clusters  mono-layer Ar+ 500 eV 2.26 × 1015 ions/cm2 plasma UHV damaged  200 µm thickness proton 3.5 MeV 5 × 1018 ions/cm2 Singletron facility RT preserved lattice structure, produced defects, changed magnetic moments  few-layer proton 10 MeV 1012 –1014 ions/cm2 MC-50 cyclotron n/a decreased electrical conductance  particles/cm2 mono-layer proton 100 keV 1012 –1015 LEAF n/a created defects  bi-layer proton 100 keV 6 × 1014 particles/cm2 LEAF n/a created defects  bulk He2+ 1.66 MeV 900 MGy ion accelerator n/a changed Raman scattering slightly  nanosheet He2+ 1.66 MeV 900 MGy ion accelerator n/a invariant  few-layer He2+ 30 keV 1018 ions/cm2 FIB beam in vacuum mono-layer He2+ 3.04 MeV 8 × 1013 particles/cm2 PTA n/a mono-layer He+ 30 keV 1012 –1016 ions/cm2 HIM in vacuum produced S vacancies  mono-layer He+ 30 keV 1013 –1017 ions/cm2 NFM in vacuum generated disorder  bulk Ar+ 500 eV 3 × 10−3 ions/cm2 STM UHV removed S atoms  micron thickness Ar+ 500 eV 1014 –1015 n/a UHV produced S vacancies at low doses and Mo/MoS6 vacancies at high doses  powder Ar+ 3 keV n/a n/a removed S atoms  ions/cm2 0.1 A/cm2 milled or damaged  produced defects  Appl. Sci. 2019, 9, 678 7 of 53 Table 1. Cont. MoS2 Irradiation Particle Energy Dose/Fluence Irradiation Source Irradiation Conditions 200 nm Ar+ 180 keV 1015 –1017 ions/cm2 IBAD 200 ◦ C in vacuum Results Ref. induced amorphization   bi-layer Ar+ 500 eV 1014 –1015 ions/cm2 n/a UHV produced S vacancies at low doses and Mo/MoS6 vacancies at high doses mono-layer Ar+ 500 eV 1014 –1015 ions/cm2 n/a UHV produced S vacancies at low doses and Mo/MoS6 vacancies at high doses  mono-layer Ar+ 500 eV 3–11 µA/cm2 sputter beam vacuum selectively removed S atoms without significantly depleted Mo atoms  mono-layer Ar+ 13.56 MHz n/a RF plasma RT produced 2H/1T phase transition  mono-layer Ga ion n/a n/a PAMBE 450 ◦ C in vacuum doped Ga, reduced binding energy  mono-layer Ga ion 30 keV 1013 ion/cm2 FIB beam in vacuum produced sub-nm pores and vacancies  bulk Xe 91 MeV 10 ions/µm2 GANIL n/a formed nano-hillocks  few-layer Xe 91 MeV 15 ions/µm2 GANIL n/a formed nano-incisions  ions/µm2 GANIL n/a formed nano-incisions  EBIT n/a induced pits and hillocks  HIRFL vacuum, RT formed hillocks  TOF-SIMS UHV formed defects  mono-layer Xe ion 91 MeV mono-layer Xe ion 25–30 keV 1–4 layer Bi ion 0.45–1.23 GeV mono-layer Mn ion 25 keV 15 1010 ion/cm2 1010 –1012 ions/cm2 1012 –1014 ions/cm2 109 –1011 ions/cm2 bulk C60 20–40 MeV micron thickness γ-ray n/a 5 × 109 R TA RT modified structures  RGIF in air no damaging effects  50–132 layer γ-ray 662 keV 5000 photons 137 Cs source 5–8 layer γ-ray ∼1.2 MeV 120 Mrad 60 Co source RT in air unaffected  RT in air converted to MoOx  1–3 layer X-ray 10 keV 6 Mrad XRD RT in air film UV n/a n/a mercury lamp in O2 (ozone) mono-layer and multilayer UV n/a 1010 –1013 photos/cm2 deuterium lamp multilayer laser λ = 514 nm 1–20 mW for 1–100 s laser few-layer laser λ = 532 nm 300 mW diode laser in air or vacuum mono-layer laser λ = 800 nm 20–50 mJ/cm2 Ti:sapphire laser RT no noticeable degradation  formed oxygen-sulfur bonds  vacuum no structural damage, no oxidation  RT in air stable or damage depending on laser power  patterned and thinned  damaged or unaffected depending on irradiation intensity  EBI: electron-beam irradiation; EBIT: electron-beam ion trap; EBL: electron-beam lithography system; EPMA: electron probe micro-analyzer; FIB: focused ion beam; GANIL: The Grand Accélérateur National d’Ions Lourds (Large Heavy Ion National Accelerator); HIM: helium-ion microscope; HIRFL: heavy ion Research facility in Lanzhou; IBAD: ion-beam assisted deposition; LEAF: low-energy accelerator facility; NFM: NanoFab microscope; PAMBE: plasma-assisted molecular beam epitaxy; PTA: Pelletron tandem accelerator; RF: radio frequency; RGIF: Reactor Gamma Irradiation Facility; RT: room temperature; S: sulfur; SEM: scanning electron microscope; STEM: scanning transmission electron microscope; STM: scanning tunneling microscope; TA: Tandem accelerator; TEM: transmission electron microscope; TOF-SIMS: time-of-flight secondary ion mass spectrometer; UHV: ultra-high vacuum; XRD: X-ray diffractometer. Appl. Sci. 2019, 9, 678 012101-4 8 of 53 Mishra et al. structural degradation of MoS2 layers. However, softening of 2.1 1 1 E2g and enhancement of I(A1g)/I(E2g ) indicates towards dia 2.1. Swift-Heavy Ion Irradiation p-type doping of ML-MoS2 after N2 - and Ga-irradiation. is 0 shifted toward lower Fig. 4(a) shows to Movery 3d5/2 ble Swift-heavy ions are usually accelerated in particle accelerators highpeaks energies, typically binding energies for Ga-irradiated samples relative to that of pol in the MeV or GeV range. They have sufficient kinetic energy and mass to penetrate solid materials pristine ML-MoS 4(b) atoms, shows induce Ga 2p heating, for pristine MLwe 2. Fig. along straight lines. Therefore, heavy ions release sufficient energy displace and Ga irradiated sample. The existence of Ga 2p MoS hav 2 1/2 permanently modify crystal structure, and leave tracks of heavily damaged structure. and Ga 2p3/2 core levels at 1144 and 1117 eV, respectively, 2.6 2.1.1. Uranium-238 Ion Irradiation reveals the incorporation of Ga into MoS2 post Galite irradiation. It isuranium important out that thefound short duration Exfoliated MoS2 mono-layers were irradiated by 1.14 GeV (238to U)point ions . It was by (30 s) and the use of low Ga flux beam equivalent that its electrical properties were significantly changed and the MoS2 -based transistor was destroyed pressure 8 FIG. 2. (a) Mo113d, (b) Mo 3p S22p HRXPS spectra of pristine-MoS and plasma treated (for 1 min and 3 min) layere 2 3/2/N 1s, and (c)(BEP) 2 of 6 10 Torr during Ga-irradiation ensured no ret at a fluence of 4 × 10 ions/cm (4000 ions/µm ). band spectrum for pristine-MoS2 and N2 -irradiated ML-MoS2 samples for (e) 1 min and (f) 3 min. deposition of Ga adlayers on layered-MoS2. Moreover, for gat 2.1.2. Gallium Ion Irradiation Ga-irradiated MoS2 the reduced value of binding energy of Mo Ga-irradiated (2-step treated) MoS2 sam physical damage to the layered-MoS remain aisprocess 2 will(0.2 VBM eV) obtained 0.2 as shown in Fig. 4(c), which bet Michra et al.  challenge, deposited which p-type may mono-layered MoS c-sapphire3(a). substrates using uplift CVD in the I(A )/I(E optimization be addressed by2 on adopting addition, ◦ C has samples and 1g int similar trend obtained for(equivalent NIn2 -irradiated techniquethe and irradiated the layers at 450 for 30 s at a low Ga flux beam pressure: and 2.31, is obtained for Ga-irradiate low brightness mode of nitrogen plasma irradiation for a hence confirming theproduced realization ofap-type ML-MoS2.17,34 ele 6 × 108 torr) in ultra-high vacuum (UHV) conditions. Ga ions were from plasma-assisted treated sample, respectively, as compare longer time. Such reduction in binding energy of VBM also confirms ins MBE system. Figure 4 shows the Raman spectra andof X-ray photoelectron spectra of pristine (XPS) ML-MoS Further, we investigated the effect Ga-irradiation onspectroscopy 2. Also, increase in F theespecially absence Aof1g inelastic scattering of electrons at the surface Mo Ga-irradiated MoS2 .mono-layers. Raman peaks, mode, shifted after3(b) the Ga-irradiation. Fig. after Ga-irradiation shows the ML-MoS 2 Fig. 3(a) shows the Raman spectroscopy of 5/2 of ML-MoS . In Ga-irradiated samples sulfur vacancies and Mo-3d pristine-, peaks shifted towards lower Ga-irradiated samples pristinedegradation of M 2 ing to asthat wellofasthepossible Ga-irradiated, andenergies 2-step,fori.e., Ga-irradiation for relative 1/2 and Ga-2p3/2 were occupied by Ga atomsatand hence enabling the p-type afo MoS2 mono-layer. New core levels were observed 1117 eV and 1144 Ga-2p Fig. 3(c) shows eV, the room temperatu 30 s (step 1)/N2 -irradiation for 1 min (step 2) treated MLbehavior. Fig. 4(d) shows the energy band diagrams of prisrespectively, the incorporation reduced the of Ga into the MoS2 layers. The Ga-irradiation MoSrevealing Ga-irradiated samples. PL tine, N2 -, and 2 samples. Similar to N2 -irradiated MoS2, softening in 1 1 the and p-type ML-MoS realized N2 -been and observ tine ML-MoS value of binding energy of 0.2 eV. Room temperature photoluminescence (PL) spectroscopy indicated 1 2 2 E2g phonon mode by 0.93 cm (2.83 cm ) is obtained for with both methodsbyhas that the optical properties of the MoS2 layers were changed. in MBE. The reported electronic bandgap of Ga-irradiation N2 2. Charged Particle Irradiation (b) Figure 4. (a) Raman spectra and (b) Mo-3d XPS of pristine- and Ga-irradiated mono-layered MoS2 samples . Reprinted with permission from Reference , with the permission of AIP Publishing. Thiruraman et al.  synthesized single-layer MoS2 triangular-flakes via a halide-assisted powder vaporization method and transferred over holes. The suspending MoS2 mono-layers were irradiated by Ga+ ions in a focused ion beam (FIB). The Ga+ ion dose varied from 6.25 × 1012 ions/cm2 to 2.50 × 1013 ions/cm2 . Figure 5 shows high angle annular dark-field (HAADF) images of the MoS2 mono-layers before and after Ga+ ion irradiation for different doses: 0 (pristine), 6.25 × 1012 ions/cm2 , 8.16 × 1012 ions/cm2 , 1.11 × 1013 ions/cm2 , 1.60 × 1013 ions/cm2 , and 2.50 × 1013 ions/cm2 . Statistical analysis showed that Ga+ ion irradiation produced pores with average and maximum diameters of 0.5 nm and 1.0 nm. Within the irradiation dose range, the nanopore density increased with increasing doses. With increasing ion doses, nanopores started to merge, resulting in larger and irregularly shaped pores. For the lowest dose (6.25 × 1012 ions/cm2 ), FIG. 3. (a) pristine-, Ga Ga-irradiatio (step 2) treat Intensity rati modes, i.e., of E12g for Ga (c) Photolum Ga-irradiated temperature. such as type, density and edge termination of defects, is required but canno only the techniques above. In this context, Surwade et al. mentioned th Appl. Sci. 2019, 9, 678 9 of 53 optical signatures were observed in differently prepared defective graph most of the atomic pores were single-molybdenum-based vacancies, while the amount of missing sulfur varied. With an increasing Ga+ ion dose, the number of double-molybdenum-based vacancies 9 increased, and some triple-molybdenum-based vacancies were also found. When the Ga+ ion dose reached to 2.50 × 1013 ions/cm2 , the density of larger pores (with diameter size > 0.8 nm) increased. Metal atomic vacancies were formed with sulfur vacancies while few topological defects and amorphous regions were observed. transport properties of the membranes varied. Figure 5. HAADF images of the pristine and the 30 keV Ga+ ion-irradiated MoS2 mono-layers with different ion doses . Reprinted with permission from Reference . Copyright c 2018 American Chemical Society. Figure 2. Aberration corrected scanning transmission electron micr 2.1.3. Xenon Ion Irradiation Madauβ et al.  fabricated ultra-thin mechanically exfoliated MoS2 sheets on SiO2 substrates and irradiated them using 91 MeV Xe ions. Figure 6a shows2 Atomic force microscopy (AFM) images of the track morphology in MoS2 with various thicknesses. In the case of bulk-like MoS2 , chains of nanosized hillocks were generated and protruded from the surface. For few-layer MoS2 sheets (thickness under 10 nm), 2 individual hillocks were created and mixed with incisions. In even thinner MoS2 sheets, i.e., tri-layered, bi-layered, and mono-layered MoS2 sheets, the major part of the surface track consisted of a continuous incision. The length of the incision varied among tri-layered (3L), bi-layered (2L), and single-layered (1L) sheets. Compared with the surface track length of mono-layers, the length of the incisions was reduced by ∼25% for a bi-layer and by ∼50% for a tri-layer. Additionally, the surface tracks in tri-layered and bi-layered MoS2 were accompanied by protrusions before and after the central incision. Mono-layered MoS2 surface tracks generally consisted only of incisions and no protrusions. Figure 6b shows lattice information around an incision (length 100 nm) created by a projectile. The extended incisions were oriented along the direction of the incoming ion beam and extremely narrow (less than 10 nm). The edges of the incision were relatively straight at an atomic scale. The surrounding lattice remained undisturbed while the violent atomic displacements took place inside the ion track core. Several mechanisms were proposed to explain the ion irradiation damages, such as electrostatic repulsion between the atoms in the ionized region, exciton self-trapping 28,29 changes of the interatomic causing a local lattice distortion, non-thermal melting caused by significant potentials, and phase transitions such as melting due to a thermal spike. It was assumed that the hillock chains in bulk-like MoS2 sheets consisted of nonstoichiometric MoS2 which re-solidified after a phase transition caused by the thermal spike. The nanoscaled incisions should have originated from characterization of single-layer MoS . (a) AC-STEM image of the pris irradiated MoS with different ion doses. (b) High magnification AC-ST vacancies with different atomic configuration. These images were used to analysis of defects shown in Figure S7 and described in the text. In 2D systems, the type of vacancy-defects introduced by ion irradiation c the ion characteristics and kinetic energy. For electron irradiation of M ACS Paragon Plus Environment Appl. Sci. 2019, 9, 678 10 of 53 the material 2D evaporation. It was reported that the threshold for grazing incidence of Xe Mater. 4 (2017) 015034 L Madauß et alions was 2.0 keV/nm. 2D Mater. 4 (2017) 015034 L Madauß et al (a) Figure 4. Transition of SHI modiﬁcations obtained in MoS2 (91 MeV Xe, Se = 19 keV nm–1, Q 1) with varying thickness from bulk-like down to single layer. The direction of the ion beam is depicted by the white arrow. (a) Bulk-like MoS2 shows periodic chains (b) Figure STEM–MAADF images irradiatedMoS (91 MeVwith Xe, nominal angle thicknesses of incidence Q from freestanding single layer 0.2) bulk-like Figure 6. 5. (a) AFM images oftaken Xe from irradiated varying down to 2 of MoS2 suspended on a TEM grid with holes of one micron diameter as shown in the inset in (a). The direction of the ion beam is ◦ single layer (grazing angle θ ≤ 1 ) deposited on SiO substrates and (b) STEM images of irradiated depicted by the white arrow. The brighter features are most likely remnants of the chemicals used in the production of suspended 2 black stretch seen in (a)(grazing and (b) is a part of anθincision the MoS2 middle sheet by the(left projectile. The part MoS2, see methods. ◦ ),into freestanding MoS2 The mono-layers  angle ≤ 0.2cut showing panel) and end visible in (a) and (b) is roughly 20nm long. The round white spots are polycrystalline and/or disordered, and have an apparent lattice 2 (rightspacing panel) of an by one Energy Xe ions:between 91 MeV. ions/µm between 2.8incision and 3.2Å, created consistent with them ion. being Mo clustersof (the spacing latticeFluence: planes in bcc10–15 Mo varies between . 2.23 and 3.85 Å, depending on the orientation). (b)–(d) Atomically resolved images of the incision show that the MoS lattice outside 2 The directions of the ion beam in (a) are marked by white arrows. The black stretches in (b) are of the ion track remains intact. parts of an incision cut into the MoS2 mono-layer by the projectile. The round white spots in (b) are molybdenum clusters. Reprinted with permission from Reference . 1L: mono-layer; 2L: bi-layer; c under 3L:however tri-layer. Copyright terms of the Creative Commons Attribution 3.0the license. not show any protrusions. While complete surface individual hillocks were the absent in these stu- dies . The striking periodicity of the hillocks found track length in these three different MoS2 samples is Hopster al.observed  deposited MoS2 mono-layers on KBr(100) substrates and irradiated them here haset been in several crystalline materials comparable, the length of the incisions is reduced by 35 + 40 + with highly chargedexplanation Xe ions (Xe kineticelecenergy of 25.4 and with and a possible in termswith of a varying ≈25% for a keV bilayer andXe by ≈50% for kinetic a trilayer.energy More- of tronic in-between crystal layers has beenThe dis-fluences over, theranged morphology of 5the as well. 2 . 38.5 keV) in density a vacuum at room temperature. from ×incisions 109 –2 ×changes 1010 ions/cm cussed in observed detail in .on This almost While the incisions in a single layer usually appear as a Hillocks were thefeature MoSdisappears 2 mono-layer surfaces. The MoS2 mono-layers were exfoliated completely in thin, few layer MoS2 sheets (thickness clean cut with straight edges (see ﬁgure 1(b)), bilayer from a single crystal under ambient condition while no details were provided, so it is hard to compare under 10 nm)—although individual hillocks are still and trilayer incisions appear more irregular. In parttheir results with created. The other surfacegroups. track is now a mixture of irregu- icular for the trilayer in ﬁgure 4(c), it can be seen that larly formed hillocks/protrusions and incisions, the the incisions appear more like a series of roundish holes with a diameter of about 10nm each. bilayer MoS2, see ﬁgures 1(b) and 3(a). The data presented above immediately leads to an Guo et al.  deposited 1–4-layer-ed MoS2 on silicon substrates capped with SiO2 . The question from a physics as well as from an In even thinner MoS2 crystallites, three layers and interesting mechanically exfoliated were irradiated by 209 Bi ionsofwith of the 0.45–1.23 2 nanosheets point view:energies How does substrateGeV below, the major partMoS of the surface track consists of a application 10 12 2 and fluences of 1 × 10 –3.6 10 of ions/cm invaries a vacuum at room temperature under in normal incidence. inﬂuence the fabrication of incisions SL-MoS continuous incision. The× length the incision 2? The between trilayer bilayer (2L)observed and single on layer (1L) substrate does indeed play a decisive role, as attributed we will The hillock-like latent(3L), tracks were the surface of irradiated MoS and 2 few-layers as shown in ﬁgure 4(c). The surface tracks in tri- and show further below, but it is by no means a prebilayer MoS2 are accompanied by protrusions before requisite. In ﬁgure 5 we show scanning transmission and after the central incision. Single layer MoS2 sur- electron microscopy (STEM) medium-angle-angularface tracks generally consist of incisions only and do dark-ﬁeld (MAADF) images from freestanding layers 2.1.4. Bismuth Ion Irradiation latter of which resemble the ones found in single- and 6 Appl. Sci. 2019, 9, 678 11 of 53 to the ionization and excitation of energy transfer from 209 Bi ions to the electron system. The induced damages shifted the Raman A1g peak to a higher frequency and increased the intensity ratio between the A1g and E12g modes. 2.1.5. Manganese Ion Irradiation Mignuzzi et al.  mechanically exfoliated natural bulk MoS2 and deposited it on Si substrates covered with SiO2 . The exfoliated MoS2 mono-layers were bombarded with Mn+ (kinetic energy of 25 keV) with different ion doses (1012 –1014 ions/cm2 ) in a UHV. The resulting average inter-defect distance ranged from 1 nm to 10 nm. The generated defects activated new Raman modes around 227 cm−1 , broadened Raman peaks, and shifted Raman modes. 2.1.6. Gold Ion Irradiation Zhai et al.  bombarded MoS2 (0001) bulk surfaces using Au ions with 13.4 MeV. Under ion doses of 1.0 × 1013 ions/cm2 , 1.0–3.5 nm craters were generated on the surface. 2.1.7. Silver Ion Irradiation Bhattacharya et al.  irradiated magnetron-sputtered amorphous MoS2 films of thicknesses 50–750 nm with a 5 × 1015 cm−2 dose of 2 MeV Ag ions. The sliding life of Ag ion-irradiated films increased ten-fold to thousand-fold compared to as-sputtered films. The improvement in wear life was correlated with a significant improvement in adhesion of the films with the substrates and a small increase in the density of the ion-irradiated films. 2.1.8. C60 Ion Irradiation Henry et al.  irradiated MoS2 bulks under 20–40 MeV C60 ions at 300 K. Irradiation was performed at normal incidence and at high fluences (3 × 1011 ions/cm2 ), as well at a grazing incidence of 20◦ and a low fluence of 109 ions/cm2 . The structural modifications occurred in the vicinity of the projectile paths. 2.2. Argon Ion Irradiation Argon ion beams (500 eV) were employed as early as 1987  to irradiate MoS2 crystalline films (cleaved from a single crystal) with a thickness of 0.7 µm. The Ar bombardments created sulfur vacancies and reduced molybdenum of MoS2 edge surfaces. Inoue et al.  studied defects generated by Ar+ ion irradiation of MoS2 bulk surface by scanning tunneling microscopy (STM). A clean MoS2 surface was prepared by cleaving the surface layers of a MoS2 bulk crystal with Scotch tape. The cleaved clean MoS2 was then degassed in a UHV chamber by electrical heating at 538 K for half an hour to provide a clean and contamination-free surface. The sample was irradiated with Ar+ ions at an energy of 500 eV and an irradiation density of 75 × 1011 ions/cm2 . The incident angle of Ar+ ion was ±2–3◦ from the normal to the MoS2 surface. The irradiated MoS2 sample was heated at 583 K for an hour by direct current heating through the substrate to remove residual ions on the sample surface after irradiation. Low-density individual defects were observed. Some surface defects were formed by removal of sulfur atoms from the top MoS2 surface. Some defects may originate from the hybridized dangling bond composed of the Mo-4d orbital and removal of MoS2 layer fragments. Bae et al.  MoS2 prepared slabs with a thickness of several microns from MoS2 crystals using the mechanical exfoliation method and then irradiated them with Ar+ ions with an energy of 500 eV in an UHV, with fluences of 5.65 × 1014 ions/cm2 and 2.26 × 1015 ions/cm2 . The incident angle of Ar+ was almost normal to the surface of the samples. Residual argon on sample surfaces was removed by heating at 583 K for one hour in an UHV after the irradiation process. Figure 7 shows the Raman spectra of Ar+ Appl. Sci. 2019, 9, 678 12 of 53 irradiated samples. After Ar+ irradiation, additional broad satellite peaks appeared in the lower-frequency 1 and A peaks, located at 377–381 cm−1 and 404–406 cm−1 respectively. side of both the E2g 1g The Raman satellite peaks shifted to lower wave-numbers while their intensity increased with increasing Ar+ doses. First-principles calculation showed that new satellite modes were related to molybdenum vacancies, sulfur vacancies, or MoS6 vacancy clusters. The lower shift of the satellites 1 mode should have come from the large cluster vacancy. The A and E1 mode of MoS of the E2g 2 1g 2g materials remained unchanged before and after the irradiation. It is believed that the new peaks were induced by lattice defects introduced by Ar+ irradiation. Additionally, the intensity of the satellite 1 and A modes of peaks increased with the increasing irradiation dose and dominated over the E2g 1g pristine MoS2 , indicating the formation of molybdenum vacancies or MoS6 vacancies. Baker et al.  irradiated MoS2 powders under a 3 keV Ar+ ion beam with a current density of 0.1 µA/cm2 . The Ar+ ion-beam bombardment caused the preferential sputtering of sulfur and produced an amorphous MoSx phase of depth of 3.8 nm. The Mo/S ratio increased with increasing bombardment time, as shown in Figure 8. REV. APPLIED 7, 024001 (2017) PHYS. M.A. Baker et al.r Applied Surface Science 150 (1999) 255–262 256 duction of coatings which exhibit low friction and Arq bombardment and then present a simple method Intensity [arb. units] + 404 Ar -ion fluence fect-induced satellites appear good wear resistance and are thus capable of dry of accurate stoichiometric determination, using only 2 412 1 [ions/cm ] machining. Of the solid lubricants tried to date, the XPS Mo 3d 5r2 and S 2p 3r2 peak positions. With and E2g modes of MoS2 . The MoS 2 shows the most promise in providing the regard to the preferential sputtering of S, changes in 378 fect-induced peaks increases lubricating properties necessary to be used for dry the binding energy and lineshape of the XPS Mo 3d 387 x3 monitored as a function of applications. and S 2p peaks have been sed. We report the resultsmachining of Accurate determination of the 15MoS x stoichiomMoS x composition during the formation of the sul406 2.26 × 10 on the defective monolayer etry is of crucial importance in relating the composiphur depleted altered layer. Investigations of this tion both to coating deposition conditions and to 412 nature have been performed on oxides Že.g., Refs. strate Raman activities under 386 w15–17x. and transition metal sulphides w18x, but to mechanical properties. Studies of sputter-deposited x3 381 cess for the defect-induced MoS x films in which the stoichiometry has been the authors’ knowledge, this is the first of such study 14 quantified have used5.65 Rutherford on MoS 2 . Resulting from the data analysis, a simple ht, based on the comparison × 10 Backscattering Spectroscopy ŽRBS. w10–12x or Electron Probe Mimethod of accurate stoichiometric determination usof the Raman spectra with cro theAnalysis ŽEPMA. w2,10x. Both of 386 these tech- 412ing only the Mo 3d 5r2 and S 2p 3r2 peak positions niques however, are limited in certain was developed and will be presented. This method is bring chemical and physical E 12g thin Afilm 1g 14 analysis depth. applications by their relatively large useful to the analyst as it can be employed to deternalysis of defective structures 0.00 × 10 X-ray Photoelectron spectroscopy ŽXPS. and Auger mine the stoichiometry after atmospheric exposure of s. Electron spectroscopy ŽAES. are two of the most the MoS x film and precludes the need to fit the Mo common techniques used for determining the compopeak. 350 but have not400 450 500 sition of thin film materials, been -1 widely used for determining MoS x stoichiometry. Raman shift [cm ] SERVATION OF The main reason for this is that S is preferentially 2. Experimental MoS2 SURFACES from MoS 2 w8,13,14x. + -irradiated MoS micron crystals Figuresputtered 7. Raman spectra of (bottom) MoS crystalline Ar 2of pristine bulk, 2 FIG. Raman spectrastudy bulk MoS and 2 (bottom) This article will 1. undertake a quantitative of Two standard bulk MoS 2 samples were prepared 14 2 15 ions/cm 2 . 2Reprinted with þ 14 at fluences of (middle) 5.65 × 10 and (top) 2.26 × 10 first of MoS2 samples. The preferential sputtering of S from MoSions/cm under 3 keV for analysis. The bulk samples were a MoS 2 powder Ar -irradiated bulk MoS at fluences of 5.65 × 10 ions=cm 2 2 permission Reference . Copyright 15 c 2017 by2 the American Physical Society. chanical exfoliation from the from(middle) and 2.26 × 10 ions=cm (top). using adhesive tape in the are referred to as bulk since rons. These cleaved MoS2 are satellite peaks appear in the lower-frequency side of both e (17.3 × 7.9 mm2 ) with sliver the E12g and A1g peaks. The Raman intensity of these be heated by passing direct satellite peaks increases when the Arþ dose increases. bstrate in an ultrahigh vacuum This indicates that the new peaks are induced by lattice ples are first degassed at 538 K defects introduced by the Arþ irradiation. The maxima re a clean and contaminationof the defect-induced peaks in the lower-frequency side of pe of samples are made from E12g and A1g peaks are at 377–381 and 404–406 cm−1 , diameter is 3–5 μm) chemirespectively. calation in a hexane solution Similar satellite peaks have been previously observed in . [28,29]. Mono- to few-layer defective MoS2 samples [15,16,18], and were rationalized the colloidal solution of the by resonance and finite-crystal size effects. In the resonanceubstrate. Then these samples Fig. 1. The transient in surface composition showing the preferential etching of S from a MoS powder sample2 when exposed to a 3 keV + ion beam . effect model, defect-induced peaks are assigned E1u Ar and FigureAr8.ionSurface to ato 3 keV at the energy of 500 eV in beam. composition of MoS2 powders when exposed 1 The atomic percent was calculated energy-dispersive spectroscopy. B1u modes, which from are Davydov pairs of X-ray E2u and A1g modesReprinted from carried out with fluences of c Reference . Copyright 1999, with permission from Elsevier. observed by Sekine et al.  in resonance Raman specions=cm2 , estimated from the troscopy. However, these resonance peaks can be observed mes. The incident angle of Arþ only when the incident photon energy is tuned to the exciton mal to the surface of the bulk energies of MoS2 , which is not the case in ordinary Raman d is applied perpendicular to scattering spectroscopies (such as ours) using the same light diation process, residual argon source at the off-resonant photon energy. On the other hand, 2 q Fig. 3b shows XRD spectra of as-deposited and ion Fig. 3b shows XRD spectra of as-deposited and ion irradiated coatings. The as-deposited coating had two irradiated coatings. The as-deposited coating had two peaks: a broad peak at 13.528 and a low 2 u peak at 11.028. peaks: a broad peak at 13.528 and a low 2 u peak at 11.028. Both peaks have been observed previously in IBAD MoS 2 Both peaks have been observed previously in IBAD MoS 2 w22x. The LD and HD ion irradiated coatings exhibited w22x. The LD and HD ion irradiated coatings exhibited only a narrower Ž002. peak at 14.258, with reduced intenonly a narrower Ž002. peak at 14.258, with reduced intenAppl.tests. Sci.Arrow 2019, 9, sity 678 dosage diagram was increased. The sliding UHD‘‘stripe’’ coatingtests. spectra sity as dosage was increased. The UHD coating spectra 13 of 53 diagram of reciprocating sliding ‘‘stripe’’ Fig. as 2. Schematic of reciprocating Arrow ndicates test starting location, turnaround points are marked ‘‘start’’ indicates test starting location, turnaround points are showed no crystalline MoS showed no crystalline MoS 2 peaks at all. The peak at 2 peaks at all. The peak at circles, and sliding cycles for the track segments are indicated circles, and sliding for the track . 13.528 inbytheopen as-deposited coatingcycles is identified as segments the Ž002are 13.528 in the as-deposited coating is identified as the Ž002. numbered. peak of MoS 2 , and yields d-spacings similar to those peak of MoS 2 , and yields d-spacings similar to those reported other sputtered MoS 2 2films. Recently, Dunn et reported for other sputtered MoS films. Recently, Dunn et fordeposited layers nts Žinstead of the normal two in a Wahl recipro- et al. turnaround points ŽinsteadMoS of the normal two (roughly in a recipro- 200 nm) on steel and Si 2substrates via ion-beam al. w34x have attributed the low 2 u peak in the as-deposited. al. w34x have attributed the low 2 u peak in the as-deposited ., th the extra turnaround pointassisted in the middle cating track, withproduced the extra turnaround point in the middle , fully dense with basal-oriented microstructures. deposition. The MoS layers were 2 resulting from coating to widely spaced MoS basal planes coating to widely spaced MoS 2 basal planes resulting from le wear tracks run to the same number of and Ž2. multiple wear tracks2 run to the same number of defects in layers the coatingwere crystal structure. These defectswith were defects in the2+ coating structure.ofThese MoS irradiated ionscrystal at doses 1 ×defects 1015were ions/cm2 , are obtained with minimal The numberdeposited of tests sliding 2cycles are obtained then with minimal number of tests 180 keV Ar removed in the ion irradiated coatings, sharpening and removed in the ion irradiated coatings, sharpening and a needed. and 2coating area needed.16 16 2 Ž0025. × shifting, the peak that of crystalline MoS 2 . temperature shifting the Ž002 towards thatFigure of crystalline 1 × 10 ions/cm and 10towards ions/cm at room in .apeak vacuum. 9a MoS shows X-ray 2. ar tracks and transfer films on balls were Coating wear tracks and transfer films on balls were Fig. 4 is a montage of multi-beam images from crossFig. 4 is a montage of multi-beam images from crossally with a Nomarski microscope. Coating (XRD) examined optically of withthe a Nomarski microscope. and Coatingion-irradiated coatings. The intensity of the (002) diffraction patterns as-deposited sectional TEM of as-deposited, LD, and UHD coatings. sectional TEM of as-deposited, LD, and UHD coatings. wear track depths were measured to within thickness and wear track depths were measured to within 4a◦ and 4b are taken with increasing the objective aperture Fig. 4a and 4b The are taken with plane the objective peak at Fig. 13.5 decreased with irradiation doses. basal peakaperture (2θ = 11◦ ) .. ŽMI.. Michelson interferometry ŽMImain 10 nm using Michelson interferometry encompassing the transmitted beam and Ž002. reflections, encompassing the transmitted beam and Ž002. reflections, disappeared after the4c irradiation. 9b shows spectra ofwithout the samples. The main while Fig. is taken without an Figure objective aperture. SAED Raman while Fig. 4c is taken an objective aperture. SAED peaks patterns inset in each image were taken from the same area patterns inset in each image were taken from the same area 3. Results of the Raman asspectra were kept after the irradiation, while their intensity decreased with increasing each image. as each image. Ž . shows basal-oriented, The as-deposited coating Fig. 4a. showsthe basal-oriented, The of as-deposited coatinglayers analyses of coatings 3.1. Structural analyses of Žcoatings doses. To explore the irradiation effects, microstructure the MoS characterized by 2 Fig. 4a was nanocrystalline domains of MoS 2 , with numerous forked nanocrystalline domains of MoS 2 , with numerous forked high-resolution transmission electron microscopy (HRTEM) as shown in Figure 9c. The w x w x sited, LD and HD coating thicknesses, meaas-deposited, LD and23,34 HD coating thicknesses, meaand The terminated plane defects . The horizontal fringes and terminated plane defects 23,34 . The horizontalas-deposited fringes were all roughly 200 nm, while the UHD sured byinMI, all correspond roughly 200 nm, while S–Mo–S the UHD observed the were images to individual observed in the images correspond to individual S–Mo–S MoS consisted of basal-oriented and nanocrystalline MoS with horizontal (002) fringes. After an 2 2 . reflec0 nm thick. We attribute the 20 nm reduccoatingand wasare180 We attribute the Ž20 reduclayers duenmtothick. interference between 002nm layers and are due to interference between Ž002. reflec2Fig. ess to sputtering losses Žroughly that pre- dose tionofin toions/cm sputtering that pre-became tions. The SAED inset inlosses 4aŽroughly consists of two tions. The SAED pattern and inset inthe Fig.surviving 4a consists of crystalline two irradiation 1thickness × 1015pattern , some regions amorphous, Ž002. ile Code calculations. rather than coating dicted by Profile Code diffuse calculations rather thanto coating sets of arcs. The larger, arcs .correspond sets of arcs. The larger, diffuse arcs correspond to Ž002. regions (still with theandbecause basal orientation) were embedded in the amorphous matrix. After a high ecause the as-deposited IBAD MoS 2 coatcompaction, the as-deposited reflections, the smaller, inner arcs areIBAD due toMoS expanded reflections, and the smaller, inner arcs are due to expanded 2 coatw35 x. ense w35x. ings are fullyspacing dense16 2 basal plane arising from fork and termination basal plane spacing arising from fork and termination irradiation dose ofw345x.spectra × 10of three ions/cm , the MoS regions were nearly amorphous, although few tra of three coatings Žas-deposited, LD and Raman coatings Žas-deposited, LD 2and defects defects w34x. ŽFig. 4b . shows wn in Fig. 3a. All spectra exhibited Raman regions UHD are shown Fig.the3a.surviving Allcoating spectra Žexhibited Raman regions The. TEM imagein of LD Fig. 4b. shows The TEM image of randomly the LD coatingafter crystalline existed. The crystalline oriented the high dose w36x,embedded o those for molybdenite w36x, but intensity bandsdomains similar to molybdenite but intensity small of those basal for oriented crystallites in small domains of basal oriented crystallites embedded in The irradiation-reduced microstructural degraded tribological behaviors ecreased in the ion irradiatedirradiation. coatings. of amorphous the bands decreased in the ioncrystalline irradiated coatings. an matrix. Remaining regions show change an amorphous matrix. Remaining crystalline regions showof MoS2 coatings, such as accelerated wear and eliminated lubrication. 4 (a) 4 Wear 237 (2000) 1–11 K.J. Wahl et al.r K.J. Wahl et al.r 4 Wear 237 (2000) 1–11 (b) K.J. Wahl et al.r Wear 237 (2000) 1–11 2q Ža. XRD and . Raman 3. Ža. XRD and Žb. Raman spectra from IBAD MoS 2 coatings, as-deposited and Fig.ion 3. irradiated withŽb180 keV Ar spectra ions. from with/ cm 1802keV Ar 2q ions. 0 ions / cm2 5 xirradiated 1016 ions 1015 ions IBAD / cm2 MoS 2 coatings, as-deposited and ion (c) Figure 9. (a) XRD patterns, (b) Raman spectra, and (c) cross-sectional HRTEM images of MoS2 coatings on Si substrates as-deposited and irradiated with 180 keV Ar2+ -ions . Insets: selected-area electron diffraction (SAED) patterns. Reprinted from Reference . Copyright c 2000, with permission from Elsevier. Murray et al.  irradiated mechanically exfoliated MoS2 few-layers (5–15 layers) under low-energy Ar (200 eV and 1 ×1013 –1015 ions /cm2 ). The electric resistance increased 100 times after a high dose of irradiation because of induced defects. Bae et al.  also prepared MoS2 bi-layers by chemical exfoliation from MoS2 micron powders in a hexane solution of butyllithium. The prepared bi-layers were then irradiated by Ar+ ions with an energy of 500 eV in an UHV, with fluences of 5.65 × 1014 ions/cm2 and 2.26 × 1015 ions/cm2 . Figure 10 1 modes shows Raman spectra of the MoS2 bi-layers before and after Ar+ irradiation. The A1g and E2g were broadened. It was claimed that a new broad peak emerged at approximately 373 cm−1 , which 1 peak. There was a broad feature at around 435 cm−1 . The mode was was 10 cm−1 away from the E2g fewer fork and termination defects. SAED of the LD film forkAtand defects. SAED the LDFig. film4c., the At the highest implantation dose ŽUHD, Fig. 4c., the At the highest implantation doseofŽUHD, 4c., the ŽUHD, fewer fork andirradiation. termination defects. SAED the LDFig. filmfewer thetermination highest implantation doseof relatively after the The new vibration mode was identified as astillunique signature . reflections. In comparison Ž002. reflections. shows only well defined Ž002enhanced coating contained crystalline regions. Basalshows planes only wellstill defined In comparison coating contained crystalline regions. Basal planes Ž002. reflections. shows only wellstill defined In comparison coating contained crystalline regions. Basal planes Ž . Ž . with Ž002. reflections in Fig. 4a, these reflections are were oriented in many directions, and 100 fringes were with 002 reflections in Fig. 4a, these reflections are were were oriented in many directions, and Ž100. fringes were with Ž002. reflections in Fig. 4a, these reflections are were oriented in many directions, and Ž100. fringes ofshifted theoutward sulfur vacancies. sharper and in reciprocal space. This imalso observed at several orientations. Circular sharper and shifted outward in reciprocal space. This imalso observed at several orientations. Circular areas of sharper and shifted outward in reciprocal space. This im-areas of also observed at several orientations. Circular areas of 2 Ža. as-deposited, and after irradiation with 180 keV Ar 2q ions to Žb. 1 = 10 15 ionsrcm2 Fig. 4. Cross-section TEM images from IBAD MoS 2 coatings Ža. as-deposited, and after irradiation with 180 keV Ar 2q ions to Žb. 1Ž = 10 15Fig. ionsrcm 4. Cross-section TEM images from IBAD MoS 2 coatings Fig. 4. Cross-section TEM images from IBAD MoS 2 coatings a. as-deposited, and after irradiation with 180 keV Ar 2q ions to Žb. 1 = 10 15 ionsrcm2 ŽLD. and Žc. 5 = 10 16 ionsrcm2 ŽUHD.. Fringes running left to right correspond to MoS 216basal planes ŽS–Mo–S.. ŽLD. and Žc. 5 = 10 16 ionsrcm2 ŽUHD.. Fringes running left to right correspond to MoS 2 basal planes ŽS–Mo–S.. ŽLD. and Žc. 5 = 10 ionsrcm2 ŽUHD .. Fringes running left to right correspond to MoS 2 basal planes ŽS–Mo–S.. plies that the Ž002. spacing of the crystalline regions has amorphous material can also be seen by their light pliesconthat the Ž002. spacing of the crystalline regions has amorphous material can also be seen by their light conplies that the Ž002. spacing of the crystalline regions has amorphous material can also be seen by their light concontracted and that these regions have far fewer defects trast. Moreover, basal planes appeared at the circumfercontracted and that these regions have far fewer defects trast. Moreover, basal planes appeared at the circumfercontracted and that these regions have far fewer defects trast. Moreover, basal planes appeared at the circumferthan in the as-deposited coating. In addition, the inner ence of several of the circles. The SAED pattern than shows in athe as-deposited coating. In addition, the inner ence of several of the circles. The SAED pattern shows a than in the as-deposited coating. In addition, the inner ence of several of the circles. The SAED pattern shows a reflection seen in the SAED pattern in Fig. 4a has vanstrong, diffuse background with weak, diffuse spots near seen in the SAED pattern in Fig. 4a has vanreflection strong, diffuse background with weak, diffuse spots near reflection seen in the SAED pattern in Fig. 4a has vanstrong, diffuse background with weak, diffuse spots near ished, consistent with XRD spectra in Fig. 3 and previous the transmitted beam. These spots are due to remaining ished, consistent with XRD spectra in Fig. 3 and previous the transmitted beam. These spots are due to remaining ished, consistent with XRD spectra in Fig. 3 and previous the transmitted beam. These spots are due to remaining results w34x. crystalline regions and can be indexed as Ž002. reflections. results w34x. crystalline regions and can be indexed as Ž002. reflections. results w34x. crystalline regions and can be indexed as Ž002. reflections. Appl. Sci. 2019, 9, 678 DEFECT-INDUCED VIBRATION MODES OF Arþ - … 14 of 53 PHYS. RE + monolayer models contai construct a 5 × 5 monol ~375 386 consider three typical vaca ~451 ~399 the S vacancy, and the Mo Fig. 3, panels (c)–(e). Va 15 2.3 × 10 ×2 images of the monolayer 405 spurious interaction. Elec 386 436 properties are calculated ulation package (VASP) co 14 tional theory with the l 0.0 × 10 1 A1g E 2g exchange and correlation wave method. The plane 350 400 450 500 and the first Brillouin zo -1 Raman shift [cm ] ~ Monkhorst-Pack k-points Figure 10. Raman spectra of MoS2 bi-layers before (bottom) and after (top) Ar+ irradiation at the FIG. 2. Raman spectra of chemically exfoliated bilayer MoS2 structural relaxation is 1 fluence of 2.26 × 1015 ions/cm2 . Reprinted withþpermission from Reference . Copyright c flakes before (bottom) and after Ar irradiation at the fluence of relaxed structures, the har 2017 by the American Physical Society. 2.3 × 1015 ions=cm2 (top). Notice that the bilayer intensity is lated within frozen phon + of the multiplied by aMoS factor of 2 (×2) due to the weakening Bae et al.  claimed that 500 eV.modes are obta 2 mono-layers were destroyed by Ar irradiation with vibration + Raman signal. Chen et al.  sputtered MoS2 mono-layers with Ar ions (500 eV and emission current of dynamical 20 mA), the matrix. Ram defect peaks of Raman scattering were observed and its hydrogen evolution performance was enhanced. Raman off-resonant activit 1 However, no details reported literature. 434were cm−1 for theinAthe 1g and E2g modes, respectively. Notice end , and the polarizab Zhu et al.  treated CVD-grown MoS2 mono-layers under radio-frequency (RF) argon plasma that the peaks are not as sharp as for the bulk cases as the polarizability routine of th with 13.56 MHz at room temperature for 40 s. The very weak Ar plasma had enough kinetic energy signal is very weak, and that there is also a broad peak at Raman are then to trench the S-Mo bond −1 while no enough energy to etch molybdenum and sulfur atoms. Thus, intensities the 2 436 cm . Upon irradiation, also the bilayer system shows α~ yy corresponding to our o plasma treatments induced the lateral sliding of the top S layer and the 2H phase transited to the the emergence of new defect-induced vibration modes. 1T phase. We first calculate pho , inRaman A broad new2 mono-layers peak emerges at approximately cm−1 Ma et al.  grew MoS on SiO treated them a vacuum 2 substrates and 373 intensities for the 2. + ions is under 500 eV Arwhich at shifted room temperature. The beam density µA/cm by approximately 10 current cm−1 from thewas E12g3.1–11.2 in Fig. 4. There are two R 0 The molybdenum content remained essentially while theand amount of 2 layers peak. Thisofisthe theMoS same defect-induced peak as constant is observed the A1 mode, which a sulfur decreased significantly during irradiation. The sulfur content of the layers be reduced to −1 could for the bulk system. The broad feature at 435 cm is spectroscopy of MoS2 . 50%. The average sputter yield was 0.03 sulfur atom per Ar+ ion. During the irradiation procedure, relatively enhanced. In the monolayer domain of the vibration mode and the MoS2 mono-layers were selectively de-sulfurized while the basic physical structure of the MoS2 chemically exfoliated samples, no clear signals correspondvibration mode, as show remained largely intact. The photoluminescence (PL) 0yield decreased as sulfur atoms were removed 0 or the A modes are found after ing to either the E frequencies, 391.7 and 40 1 by the Ar+ irradiation. þ irradiation, indicating that the Ar irradiation destroyed our + with previous theories an Ar plasma was also employed to thin MoS2 sheets at room temperature . It was reported that + flakes. monolayer MoS that the monolayer vibrati 2 the top MoS2 layers were entirely removed by the Ar plasma while the bottom MoS2 layers remained largely unaffected. Thus, MoS2 multi-layers with controllable thickness, even MoS2 single-layers, to bulk or few-layer mod could be prepared reliably (with almost 100% success rate). OF DEFECTIVE III. PHONON VIBRATION MODES different due to differe Chen et al.  MoS2 mono-layers using aRAMAN liquid exfoliation method, and annealed AND THEIR SPECTRA MoSprepared 2 SURFACES perpendicular to the surfa them at 300–450 ◦ C in a vacuum. The treated layers were then deposited on Au substrates and exposed induced vibration modes the irradiation-induced defects in theinfluencing MoS2 the to 0.5 keV Ar+ ions To for 1identify min. Defects were introduced into the MoS2 layers, electronic within one monolayer even surfaces, we perform a first-principles calculation of MoS 2 structures of MoS2 . Figure 11 shows XPS spectra of irradiated MoS2 mono-layers. Peaks shifted 406 436 Bilayer Intensity [arb. units] Ar -ion fluence 2 [ions/cm ] because of induced defects. (a) Slab model (b) Pristine (c) Mo vacancy (d) S vacancy (e) MoS6 vacancy 20 Å FIG. 3. monola extende The top vacancy vacancy at the c Appl. Sci. 2019, 9, 678 15 of 53 ACS Nano Figure 11. X-ray photoelectron spectra of the Mo-3d and S-2s in the MoS2 mono-layers on Au substrates before and after Ar+ sputtering. XPS of Au-4f electrons were shown as a standard reference. Reprinted with permission from Reference . Copyright c 2018 American Chemical Society. 2.3. Neon Ion Irradiation Maguire et al.  also irradiated CVD-grown MoS2 mono-layers with Ne+ ions with an energy of 30 keV. With increasing Ne+ ion doses, the A1g and E2g modes were quenched and broadened, indicating the growing disorder induced by the Ne+ ions. 2.4. Alpha-Particle/Helium-Ion Irradiation Alpha particles consist of two protons and two neutrons which are tightly bound together bound together, which is identical to a helium ion (He2+ ). They are produced either from particle accelerators in the form of helium-ion beams or from the process of alpha decay of alpha-particle-emitting radionuclides. Alpha particles have been widely studied in radiopharmaceutical therapy that is a promising treatment approach under active pre-clinical and clinical investigation [119,120]. Figure 3. XPS and Raman characterization of samples before and after introducing defects by Ar+ sputtering for Decayed alpha particles havespectra a kinetic energy MeV, which induce defects and raygenerally photoelectron of the Mo 3d,ofS about 2s, and5Au 4f electrons in the MoS 2/Au sample after annealing at even ‘cut’ the MoS2 nanosheets. sputtering, and sputtering with subsequent annealing at 450 °C. ΔE for the upper two and the lower two plots annealing at 300(helium °C and 450 °C in UHV and after(1.66 sputtering. of the MoS2/Au Isherwood et al.  studied thesample effectsafter of alpha-particle nuclei) irradiation MeV) The MoS2 Raman pe spectra are normalized by the corresponding A1′ peaks. The measured data are shown as dots. Fitted individua on both bulk and liquid-phase exfoliated MoS2 nanosheets using Raman spectroscopy and shown as gray and colored curves, respectively. (c) Raman shift and fwhm of E′ and A1′ peaks for the MoS2/Au energy-dispersive X-ray°C spectroscopy. Liquid-phase exfoliated MoS2oflayers more and 450 °C and after sputtering. (d) Illustration energywere band often alignment fordefective the MoS2/Au sample. The left sputtering, and the right one shows the bands after sputtering. than mechanically exfoliated MoS and CVD-grown MoS layers. Besides water, the solvent used 2 2 during liquid exfoliation could be retained between the MoS2 layers during ultrasonication and of photoemission binding energy (Figure 3a). The intensity ratio of the Mo and S photoemission annealing/cleaning post-procedures. Raman spectroscopy showed a small blueshift of the E12g and properties for MoS2 on Au substra peaks did not change after annealing at 450 °C (Figure S3b). A1g modes in the MoS2These bulk under a high absorbed dose (∼900 MGy), accompanied by a small mean that MoStotal the defects introduced by ion irrad 2 remained stable and without detectable broadening of both peaks. The of exfoliated MoSannealing tolerant the thanband the bulk 2 layers were alignment between Mo formation defects upon alone.less Thisradiation is consistent material. Both spectroscopies proved presence of in amorphous carbon3b,c). in the exfoliated MoS2 decreased by 0.8 with Raman resultsthe presented later the paper (Figure binding energy However, thermal annealing changed the coupling between unchanged after sputte membranes, which could be formed by radiolytic amorphization of residual solventremained and retained MoS beam for 1 min. In contrast to this and the substrate by improving the contact between 2 within the exfoliated nanosheets. 38 sputtering, no MoS and the substrate in MoS /Au, MoS /SLG/Au, and 2 freestanding mechanically2 exfoliated2 MoS few-layers in a helium-ionchange in ΔE was d Fox et al.  irradiated at 300 °C and 450 °C. Re-anneali MoS2/BN/Au or by introducing defects into the 2reducible microscope. Figure 12 shows HRTEM images of a MoS2 few-layer. The hexagonal structure of 450 °C does not change the band substrate CeO2 in MoS2/CeO2/Au. Both eﬀects signiﬁcantly the pristine MoS2 waschange destroyed to analignment amorphous state. Energy-dispersive X-ray spectroscopy To provide more quantitative in the band between MoS 2 and the substrate (EDX) indicated that preferential sputtering of sulfur in theperformance, mechanically exfoliated MoSsputtering, in MoS2 after we perfor and can have a signiﬁcant impactoccurred on the device 2 since many devices dependofonsulfur measurements few-layers. Its stoichiometry waselectronic modifiedand by optoelectronic the preferential sputtering at nanometer on the MoS2/Au 38 Figure 3b shows the R band more alignment of MoS 2. scales. It was believedthe thatenergy He2+levels ions and transfer energy to sulfur atoms than to sputtering. molybdenum after 300 °C and 450 °C annealing As stated in the Introduction, ion irradiation is another way atoms because of the lighter mass of sulfur atoms. Electric properties of the MoS2 layers were altered A1′ modes of MoS2 are easily ob to create defects and defect-induced electronic states in 2D to be semiconducting, metallic-like, or insulating depending on doses. When the dose was above normalized by A1′ modes. As can not show considerable diﬀerence annealing, but after sputtering, E shifted, and both are widened (Fig (at around 362 cm−1) on the lef materials. Heavy ion irradiation is shown to introduce extended defects such as incisions and folds in 2D materials including MoS2.50 Here we focus on lattice defects that can be created by low-energy ion irradiation. We introduced lattice defects into SL MoS2 by low-energy Ar+ sputtering and studied the change 2573 Appl. Sci. 2019, 9, 678 16 of 53 Let Nano Letters (2.56 ± 0.05) × 1018 ion/cm2 , that the crystal(milling) structure of thewas MoS2 observed. was controllably Figure the balls,12c the more eﬃcient the exchange of energy is. complete removalresults of material shows amorphized by the He+ irradiation process. helium ion is much lighter than both sulfur and molybden Stoichiometry Tuning. To gain a better understanding of but is still much closer in mass to sulfur than molybden He2+ -ion fabricated freestanding nanoribbons in MoS few-layers. The edges of the milled regions may 2 the He+ irradiation process, chemical composition analysis was After the collision sulfur atoms will have a lot more energy t 2 + energy-dispersive spectroscopy atoms. This preferential sputtering of sulfur be amorphous or crystalline, depending on He performed beamusing sizes. MoS2 X-ray layers were(EDX). milled molybdenum when the dose Doses from (1.00 ± 0.02) × 1016 to (1.60 ± 0.03) × 1020 He+/ been observed for both electron and argon ion beams in 2 17However, 2, useddose to irradiate mechanically exfoliated, free- ) × cm2 were past. was higher than (1.30 ± 0.03) × 1018 ion/cm the wasa below (1.00 ± 0.02 1014,37 ion/cm our technique enables localized stoichi Letter Letter . When standing, few-layer MoS2 sample. The irradiated region of MoS2 etry modiﬁcation and areas with a range of stoichiometries w MoS2 layers were extensively damaged but not iscompletely etched away. displayed in the high-angle annular dark-ﬁeld (HAADF) produced within a region of just a few tens of nanometers doses above (2.56 ± 0.05) × 1018 He+/cm2 (region complete removal of material was observed for this sampl Nanostructure Fabrication. For few-layer samples MoS2, the interaction volume can be approximated to interaction area (see Supporting Information for an in tigation of sample thickness eﬀect). In this case, the He+ pr size plays a crucial role on the lateral damage extension crystal structure of milled nanosystems. An experiment designed in order to better understand the relationship betw Nano Letters Lettersize and lateral damage extension in the mate the probe This enables control of the width of the amorphous e 2 nm 2 nm region. As an example, defocus was used to adjust the pr size to 12 ± 1, 5.9 ± 0.7, and 1.7 ± 0.2 nm (details on probe size measurement can be found in the Suppor + Figure 2. Stoichiometry alteration of an MoS2 ﬂake by He irradiation. Information). In Figure 3, three diﬀerent edges were fabric (a) STEM image of a freestanding MoS2 ﬂake irradiated with 15 + on a mechanically exfoliated, freestanding MoS2 ﬂake. E diﬀerent He doses. Each region was irradiated with twice the dose of edge was fabricated with a diﬀerent probe size. Each pr the previous region, starting with a dose of (1.00 ± 0.02) × 1016 He+/ delivered the same dose of (1.90 ± 0.04) × 1018 He+/cm2. cm2 at region 1. Regions 9 ((2.56 ± 0.05) × 1018 He+/cm2) and above were milled. (b) Concentration of Mo and S for diﬀerent He+ doses as structure of each edge was imaged in high-resolution T extracted from EDX analysis. The error bars were obtained by the Figure 3a−c corresponds to the ∼12, ∼5.9, and ∼1.7 nm pr ﬁtting of EDX data. sizes, respectively. The lattice structure was not aﬀected distance (white line) from the edge which is approxima equal to the probe size (red circle). (labeled from 1 to 15) was irradiated with twice the dose of the 16 We simulated the distribution of He+ ions for the t previous region with a starting dose of (1.00 ± 0.02) × 10 2 nm + 22 nm 18 + diﬀerent probe sizes used in the experiment. During ima He /cm at region 1. At doses of (2.56 ± 0.05) × 10 He / 2 the2 He+ beam is moved from pixel to pixel (region 29)+and above milling was observed. EDX spectra 18 and milling, + /cm Figure 12. HRTEM image of (a) pristine andcm (b) He -irradiated ( ( 1.00 ± 0.02 ) × 10 He ) of Gaussian distributions was simulate raster scan. An array from each of the regions labeled 1−8 were recorded. The + doses to account for beam overlap during scanning (Suppor evolution of of the stoichiometry of the under material with respect to He2order 1 + 1 MoS2 few-layers . (c) Stoichiometry freestanding an MoS flake different as 2 an the Raman E 2g andspectrum A1g modes of pristine MoSof2 supported an MoS grum Heof. (a) of the E 2g and∼6A1glayer modes pristine ∼6onlayer supported on Information Figure S4). The milled MoS2 edge was assume is shown in Figure 2b. It was observed that as the the He+2 dose 2 + EDX analysis. (d) HRTEM 1 + be found where the dose drops below (1.30 ± 0.03) × dose increased to 1017(left He+/cm (whereand the sample is He+dose. from images of crystalline panel) amorphous (middle he E12g (b) and FWHM A1g modes as acalculated irradiation dose. of The bars ctively. of the Efunction modes as a function Heerror irradiation The error bars 2g and Aof 1g He He+/cm2; a dose below this value was found experimentall predominantly amorphous), preferential sputtering of sulfur 2 + 2 + . image (d)right High resolution-TEM MoS EM of pristine MoS (d)image Highofresolution-TEM image of MoS . (c)image High-resolution TEM2and of panels) pristine extensive damage but not complete milling. The p the He sample occurred. Intuitively clear why sulfur MoS2. nanoribbons fabricated from milling. He it isenergy: 30 keV.produce Reprinted 2 after from 2 after 2 at which the crystal structure was assumed to remain intact should be subjected to preferential erosion compared to . Scale bars are 2 nm. c 2015 The with permission from Reference . Copyrightmolybdenum. American where the ion dose drops below (1.00 ± 0.02) × 1017 He+/ mass of sulfurChemical atoms is three Society. times less than as determined by Raman spectroscopy. Our simulation pred that of molybdenum. Upon collision, He+ transfers more an amorphous region that extends 7, 3.5, and 1.0 nm for th energy to sulfur than to molybdenum. As in the collision of ± 1, 5.9 ±substrate. 0.7, and 1.7 ± 0.2 nm probes, respectively. spherical objects in classical mechanics,supported the closer the masses Fox et al.  also irradiated MBE-grown pristine MoS onof an MgO 2 6-layers scanning TEM (STEM) image in Figure 2a. Each region (a) (b) (c) (d) resolution. A milling approach was milling also developed. resolution. A nanoscale approach was also developed. as spintronics andnanoscale Freestanding ribbons of MoS with pristine crystal structure Freestanding ribbons of MoS What is currently with pristine crystal Figure 4. He+ 2fabricated freestanding nanoribbons in 2D materials. (a) TEM image of astructure 9 nm wide crystalline MoS2 nanoribbon. (b) A 5 nm wide 2 + With increasing He doses, the full2width at half maximum (FWHM) of the E1a 7 (the in-plane Mo-S nm wide crystalline amorphous MoS2 nanoribbon. (c) An amorphous nanoribbon with a minimum width of less than 1 nm. (d) STEM image of2g andof widths to 2 nanoribbon. ∼7 nm(e) Awere fabricated. A new density structuraldown MoS and widths down to ∼7 nm were fabricated. A new 9 nm wide crystalline Mn O nanoribbon milled with HIM. (f) TiO edge milled with HIM. (g) An array of 10 nm A wide peaks 2 3 2 vibration) increased, indicating more in-plane defects during irradiation. The FWHM of 1g nanoribbons milled withbeen HIM. achieved Mn controllability ashasevidenced by electrical 2O3achieved nd pattern nanoscalehas been controllability as evidenced by electrical 2 + decreased with increasing doses, inferring material removal with high He doses. metallic-like, or insulating n 10 characterization. nm wide, with Semiconducting, characterization. Semiconducting, metallic-like, or insulating Tongay al.  irradiated exfoliated with a high-energy He2+ 2 mono-layers MoS2 can be obtained by2etirradiation with diﬀerent doses of ed orientation. MoS can be obtained bymechanically irradiation with diﬀerentMoS doses of 13 ions/cm2 . Upon the He2+ particle irradiation at different + MeV) beamthat (3.04 with doses to 8is ×a 10 He+beyond . We illustrate is aillustrate generalized nanomodiﬁcation ve this the Hethis . We thatup this generalized nanomodiﬁcation also patterning samples Mn2O3 and TiOeV . doses, aapproach new PL peak appeared atsamples 1.78 and the many approach chemicalbyand by alsoof patterning of Mn O and TiO2. intensity of this new peak increased 2 2 3integrated Structural Modiﬁcation. The change in crystal structure to produce MoSwith Structural Modiﬁcation. The change in crystal structure the irradiation dose. The new peak became stronger and broader after thermal annealing at 2 was approaches analyzed 500 by ◦Raman spectroscopy and TEM. The two Figure 3. HRTEM images of the edges of three regions in the same MoS ﬂake with three diﬀerent probe sizes. (a−c) M was analyzed by Raman spectroscopy and TEM. The two . These C, which thermally introduced sulfur vacancies. The integrated intensity offreestanding the main PLmilled peak with 12 ± 1, 5.9 ± 0.7, and 1.7 ± 0.2 nm He probes, respectively. The probe size is indicated by the red dashed circles overlaid on each image. characteristic Raman peaks for pristine MoS are shown in characteristic Raman peaks for pristine MoS olycrystalline or too are shown in white dashed lines approximately show the edge of the amorphous region and agree well with the simulated 2 2 at 1.90 eV increased three-fold while the PL peak position shifted to a higher energy by 20 meV after values for the damage extension 1a. This sample was grown on an MgO substrate by 5309 Figure 1a. This sample was grown on an MgO substrate by f MoSFigure nanoribbons 2 the irradiation. Calculations estimated that approximately one defect per 100 unit cells was generated molecular beam epitaxy (MBE). The peak positions were molecular beam epitaxy (MBE). The peak positions were ransmission electron 2 irradiation −1 under 1013cm dose. −1 be 382.3the and8 × 406.6 for theand E12g406.6 (the cm in-plane measured toions/cm be 382.3 and measured phase of to these for the E12g (the in-plane 2+ beams 14 + 30 keV He Klein et al.  employed toelectrical irradiate MoSHe2 + dose mono-layers on SiO2 /Si Figure Electrical characterization He modiﬁed devices. (a) Double log plot of resistivity versus for a substrate supported, A1g5.(the out-of-plane vibration) Mo−S vibration) and ofAMo−S (the out-of-plane Mo−S vibration) sion. Mo−S Also, vibration) TEM is andmechanically 1g The letters S, I,12and M 16 correspond to regions with semiconducting, insulating, and metallic-like behavior, exfoliated bilayer MoS2 ﬂake. 2 under various doses 10 –10 ions/cm invalues asample scanning helium-ion microscope. modes, respectively. These agree well with the accepted values its requirement ofsubstrates a modes, respectively. These well with the accepted respectively. Inset: Resistivity measurement asagree a of function of gate bias. The resistivity of the pristine (dashed red line) was widely tunable. The 2 35sample irradiated with 1017 He+/cm 35high showed littlethe gate response (solid vibrational, green line). (b) A 20luminescent, nm wide, electrically contacted, MoS2 nanoribbon. properties The from the literature, demonstrating the quality of our Helium-ion bombardments affected intrinsic and valleytronic from the literature, demonstrating the high quality of our g, lack of scalability, ribbon is shown with red false coloring indicating the measured region, and the gold electrodes are also colored. The blue dashed line shows the + sample.adopted A plotofofthe theatomically full width atwithhalf-maximum (FWHM) bar is 500 (c)of Current−voltage (FWHM) plots for the indicated isolating cut made the s a widely sample. A thin plot ofHethe2beam. fullScalewidth at nm. half-maximum of doses as well as for the 20 nm wide MoS2 MoS sheets. nanoribbon. All of the results were +measured at 0 V back gate. 1 function + E12gchemical and A1g peaks as a of He dose is shown in ture23theand the E and A peaks as a function of He dose is shown Maguire 2g et al. 1ggrew MoS2 mono-layers on SiO2 substratesinby a CVD technique and irradiated Figure 1b.(FIB) Thethem widthwith of these peaks was observed change forangle simulation data corresponds wellofwith thetoprobe radiiwas (half the remains entirely intact. In Figure 4b, a 5 structure of the MoS 2 + Figure width these peaks to change for used ion beam He 1b. atThe an energy 30 keV and an+observed of incidence of2 0◦ . These irradiation doses ranged 14 + of the2of Further analysis extension of the 14 amorphous 2 nm wide ribbon with an amorphous structure can be seen. The doses nanoscale above (1.00 ± FWHM). 0.02) × 10 He /cm (The dose error doses above (1.00 ± 0.02) × 10 introduce He /cm (The dose error region from the milled 2edges local FFTs2 .from width of the narrowest structure that the can two be reproduced 1× 1013 ions/cm to 1was×done 1017using ions/cm With increasing He2+ ion doses, characteristic 25 estimatedfrom variation of by beam current, seeof the wasTEM estimated the variation beam current, the images. This data can be found in the Supporting consistentlysee is lessthe than 1 nm, while the structure is no longer probe.was However, a by the Raman peaks quenched and broadened, reflecting the growing disorder with increasing irradiation. Information (Figure S5). The strong agreement between the crystalline MoS (see Figure 1c). The narrowest crystalline 2 in the Supporting Information). discussion of beam stability in the Supporting Information). meter discussion probe size ofarebeam stability experimental results at and simulation that irradiation for this nanoribbon was 7 nm wide as shown with in Figure 4d. MoS2 doses. + 1−1indicates + 1 A new peak appeared 227 cm after the and its intensity increased increasing With greater He doses, the(which FWHMs ofdoses, theirradiated E the Aa 1g peaks With greater Heand FWHMs ofdose the E 2gribbons and are A1gtheoretically peaks stable down to widths of ∼1 nm.19 We 2g and sample is thin was with suﬃcient Helium-ion beams were also employed to mill MoS Some of MoS2during few-layers speculate that . a heating eﬀect regions may be occurring for milling) control of the edge damage range can be achieved increasetoand decrease, respectively. 2 layers een shown be an increase and decrease, respectively. 1 the size of the He+ probe. 1 milling,38 which might destabilize ribbons narrower than 7 by adjusting 2+ sub-nanometer increase the E peak FWHM is due to the wereofthinned to mono-layers under 30 keV He ion beams. + The increase of the E peak FWHM is due to the density The of structural 2g 2gHe dose ((3.10 ± With the probe size (1.0 ± 0.5 nm) and nm. Our future work will aim to address this question. + of more0.06) in-plane defects during The /cm optimized, airradiation. few-layerdefects MoS2 ﬂake was × 1018 He The edges of The these crystalline ribbons exhibit well-deﬁned introduction of2) more in-plane during irradiation. ose.27 introduction This is because selected and nanoribbons were fabricated. InisFigure 4a, a 9 nm lattice conﬁguration, indicating a subnanometer precision of peakthan FWHM because the A peak not only A peak FWHM decreases because the A peak is not only muchA1g larger an decreases 1g 1g ribbon is shown. The well-deﬁned crystal lattice fringes 1g structuring. More importantly, this fabrication technique is not wide aﬀectedinbymore the presence ofacross defects but is alsodemonstrating very sensitive tois also very aﬀected by presence of defects butcrystal m, resulting span the the whole ribbon, that graphene. A 9 nm wide Mn2O3 nanoribbon limitedsensitive to MoS2 or to 36 thickness. is changing samples by material thickness.in36our This is changing in our samples by material use oflayer its ∼0.35 nm Thislayer 5310 DOI: 10.1021/acs.nanolett.5b01673 + Nano Lett. 2015, 15, 5307−5313 with of high Heremoval doses. with This high demonstrates theThis ability of He+ doses. demonstrates the ability of the removal fabrication + + He irradiation alter irradiation the crystal tostructure defect structure by defect ne nanoribbons with to He alter theby crystal introduction and material removal.andFurther Raman data Further is introduction material removal. Raman data is fabricated by this presented S1. The crystal structure was crystal directlystructure was directly in Figure S1. The e useful in orderin toFigure presented high-resolution imaging of a freestanding observedTEM by high-resolution TEM imaging of a freestanding ement.observed However,bythe mechanically exfoliated sample. Figure 1c shows the hexagonal mechanically exfoliated sample. Figure 1c shows the hexagonal dge orientations and + + of pristine MoS . In Figure 1d, at a He+ dose of (1.00 structure + 2 DOI: 10.1021/acs.nanolett.5b Nano Lett. 2015, 15, 5307 ions/cm2. The Mo peak is given in (a) and (b) and S peak in (c) and (d). The fitted spectra along with the constituent peaks and experimental points are also shown. Appl. Sci. 2019, 9, 678 17 of 53 2.5. Proton Irradiation Mathew et al.  prepared MoS2 flakes with a thickness of 200 µm and irradiated them at room temperature using a 3.5 MeV proton ion beam. The Raman spectra of the pristine and irradiated samples at a fluence of 5 × 1018 ions/cm2 are shown in Figure 13. A new peak at 483 cm−1 was clearly visible in the irradiated samples. The appearances of the mode at 483 cm−1 along with the broadening of the mode at 452 cm−1 indicated the presence of lattice defects due to proton irradiation of the samples. The FWHM of the E12g and A1g modes did not increase in the irradiated MoS2 , indicating preserved the to near-surface modesstructure becomewas Raman activeindue resonance region effect,ofasthe irradiated samples. chi-square iteration program that usingthe lattice 29 after the proton irradiation, showing the enhanced The intensity ratio of A /E enhanced 16% 2g observed by Sekine et al. These extra phonons are Davyan–Gaussian functions with a Shir1g 1 interaction of electrons X-ray g the Mo 3d doublet, the peak sepaand A1gphotoelectron modes.29 Thespectroscopy broad peak indicated there were dov with pairsAof the E2g 1g phonons. 1 canand be the order ofvalence LA(M)of irradiated samples ea ratio for 5/2 and 3/2 spin-orbit at 452 cm zone-edge phononsobserved in the irradiated samples the second molybdenum 30 1 phonon. ned to be 3.17 eV was and higher 1.5, respecIn the irradiated sample, a peak at 483 cm is states affected their than +4. The irradiation-induced changes of structures and chemical 1 nding constraints for the S 2p 3/2 clearly visible. Frey et al. reported a peak at 495 cm in magnetic moments. 5 eV and 2, respectively.25,26 The .7 eV observed in the pristine spec(a) d as Mo 3d5/2 and 3d3/2, while the V in Fig. 3(a) is the sulphur 2s pectrum in Fig. 3(b), apart from the additional peaks at 229.6 eV and peak observed at 229.6 eV in Fig. the total Mo signal, which could be r than þ4. The binding energy posi(V) have been reported to be 2 eV V).26 We found a peak at 229.6 eV hich is only 1.0 eV above that of the (b) e spectrum of S consists of S 2p3/2 eV and 162.5 eV and another two eV. The peak at 163 eV in the irrag. 3(d) had increased by 6% in inthe pristine sample. nature of the induced defects and gained using Raman spectroscopy, for characterizing ion irradiation ne and graphite in a recent study.27 pristine and irradiated samples at a 1 cm2 are shown in Fig. 4. The E2g 1 A1g mode at 411 cm Figure are clearly 13. RamanFIG. spectra of (a)spectra irradiated (b)MoS pristine MoS2 nanosheets under 4. Raman of (a) and pristine MoS2 at a flu- a proton fluence of 1 2 and (b) irradiated w frequency sides of E2g and A1g 2 . The deconvoluted modes arepermission labeled in the ence of Reprinted 5 1018 ions/cm 5 × 1018 ions/cm from2. Reference , with the ofspecAIP Publishing. observed in the deconvoluted spectrum; the fitted curve with constituent peaks and experimental points are 2 given. and B1u phonons; ive E1u Kim etthese al.  also micromechanically exfoliated MoS2 few-layers from a bulk MoS2 crystal and fabricated them into MoS2 field-effect transistor (FET) devices on highly doped Si substrates coated with SiO2 . Source and drain electrodes were made by depositing Au/Ti electrodes. The FET Sep 2012 to 220.127.116.11. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions devices were irradiated with a 10 MeV proton beam under fluences of 1012 , 1013 , and 1014 ions/cm2 . Sufficiently high irradiation fluences decreased the electrical current and conductance of the devices while low dose did not change them significantly. The threshold voltage was shifted towards the positive gate voltage direction under proton irradiation. However, these changes were recovered over a time scale of days. It was believed that proton irradiation changed the SiO2 /MoS2 interface states, and the interface trap states at the SiO2 /MoS2 interfaces affected the electrical behaviors of the FET devices. Wang et al.  exfoliated MoS2 bi-layers, transferred them to silicon nitride membranes, and irradiated them at 100 keV protons (H+ ) with a fluence of 6 × 1014 particles/cm2 . Figure 14a-b shows the PL spectra of the bi-layers. Both the suspended and substrate-supported bi-layers showed almost spectra of MoS2 ex the direct bandgap emission after irradiation and the emer3.04 MeV a-particle gence of a defect-induced sideband peak at 1.71 eV. As menpower (i.e., deposit a tioned above, defects induced by the radiation trap the pared to the 100 keV excitons and cause them to be redshifted from the main here. direct band emission at 1.85 eV. For the suspended region, In summary, we Appl. Sci. 2019, 9, 678 18 of 53 cence and direct band we understand this decrease in intensity due to defects, which cause non-radiative recombination and shorten the flakes. For all substra lifetime of the photoexcited carriers. Under all conditions, suspended samples, th the PL intensity in the of the and the indirect emis complete suppression of the indirect emission (1.55–1.60 eV) after thesubstrate-supported irradiation and region the emergence MoS2 is much weaker than that in the suspended region, as of a defect-induced sideband peak at around 1.70 eV. The direct band emission at 1.83 eV increased due to decoupling of shown in Fig. S8 of the supplementary material, consistent tion). In all samples 1.6× after irradiation while the substrate-supported direct band emission by we a factor of 2.7.defect-induced sideba see a 2 with the original reports of Makincreased et al.19 Here, in the direct bandgap emission at disappeared 1.85 eV for the for annealed out by heat After being annealed at 300 ◦ C for 1 h in argon,increase the defect-induced sideband peak substrate-supported regionafter after irradiation. Asindicating with the sus-that pended monolayer Mo both kinds of bi-layers. The indirect band emission was suppressed annealing, pended region, we observe an increase in the defect-induced by a factor of 2.8, l the bi-layers underwent an irreversible indirect-to-direct band-gap et al.et peak at around 1.7 eV, astransition. was reportedWang by Tongay al.11also recombination centers examined MoS and four-layers. TheseAfter multi-layers showed as bi-layers. annealing the samplessimilar at 300 behaviors C forPhys. 1 h in argon, the (2017) 2 tri-layers that, after annealing, t 131101-3 Wang et al. Appl. Lett. 111, 131101 suppressed, indicating material from an indir bandgap semiconduct (c) See supplementa data including Rama results. This research w 1402906 (B.W.) and No. DE-FG02-07ER4 to acknowledge sup Institute of Optical N ION2) (S.Y.). The por Corporation was fun development program (d) 1 G. F. Knoll, Radiation Hoboken, NJ, 2010). 2 R. Dhall, Z. Li, E. Kosm 195702 (2016). 3 J. S. Ross, S. F. Wu, H. Y Q. Yan, D. G. Mandrus, D 1474 (2013). 4 R. Dhall, M. R. Neupane Moore, R. K. Lake, and S 5 T. Y. Kim, K. Cho, W. P T. Lee, ACS Nano 8(3), 2 6 C. X. Zhang, A. K. M. N Fleetwood, M. L. Alle Pantelides, IEEE Trans. N 7 FIG. 2. PL spectra of bilayer MoS2 of taken before andMoS after proton irradiation, J. Lee, M. J. Krupcale, FIG. 5. PL spectra monolayer MoS after irradia-irradiation, FIG. 3. of PL spectra of trilayer MoS2before taken and before andproton after proton Figure 14. PL spectra bi-layer over holes and (b)2 taken deposited on substrates, 2 (a) suspended and after annealing. tion, and after 023106 (2016). andannealing. after annealing. and mono-layer MoS2 (c) suspended over holes and (d) deposited on substrates taken before and calculations by Lake andirradiation coworkers with established that of an6 × 1014 particles/cm2 , and after annealing . after 100 keV proton a fluence increase in thefrom interlayer separation of just 1 Å of is sufficient Reprinted , with the permission AIP Publishing. to induce an indirect-to-direct bandgap transition in this 4 which experimentally AFM material, Wang et al. was confirmed also exfoliated MoSwith 2 mono-layers and transferred them to silicon nitride measurements.4 While these DFT calculations provide a membranes with holes. freestanding MoS 2 mono-layers were then irradiated at 100 keV qualitative explanation of theThe mechanism underlying this protons (H+ ) withbandgap fluences rangingwe from 2 × currently 1012 particles/cm2 to 6 × 1014 particles/cm2 . Figure 14c indirect-to-direct transition, do not have PL an atomistic of the ion interaction the latshows spectrapicture of suspending MoS2 with mono-layers after proton irradiation with a fluence of tice 14 of this material. The suppression of the indirect bandgap 2 6 ×emission 10 particles/cm with and without annealing, compared with those taken before the irradiation. is potentially useful for fabricating optoelectronic Thedevices intensity of the direct band-gap emission suspending mono-layers decreased two-fold after the such as LEDs and solar cells. Spectra taken of after an 13 2 protons/cm and a intermediate proton fluence of 6 10 irradiation due to the irradiation-induced defects. The defects caused non-radiative recombination fluence of 6 1015 protons/cm2 are also shown in andheavier shortened the lifetime of the photo-excited carriers. On the contrary, the intensity of the direct the supplementary material. For the heaviest fluence, the PL band-gap emission at 1.85 eV increased the irradiation for the substrate-supported regions, as intensity decreases, indicating that the effectsafter of radiationinduced defects eventually outweigh the indirect-to-direct shown in Figure 14d. After being annealed at 300 ◦ C for 1 h in argon, the defect-induced sideband bandgap transition. peak disappeared in the bi-layers and mono-layers, and the direct band emission reverted to its original Figures 3 and 4 show MoS2 samples with 3 or more pre-irradiated intensity both kinds of MoS 2 layers. layers in thickness (after for irradiation and before annealing), which, again, show slightly enhanced direct bandgap emis2.6.sion, Electron Irradiation complete suppression of the indirect emission at around 1.4 eV, and a broadened, red-shifted defect induced sideband irradiated MoS single multicrystals with a thickness of about 50 µm by using at Han aroundet1.7al. eV. In addition, the spectra of 2suspended high-energy electrons in ambient conditions at room temperature. The electron dose was 300 kGy layer MoS2 look very similar to those of substrate-supported 14 2 (See Fig. S12 in the supplementary multi-layer MoS 2 (6.70 × 10 electrons/cm ) and the acceleration energy of electrons was 0.7 MeV. There was a material), reflecting the decreased sensitivity to substrate interactions as the material approaches the bulk limit. After annealing, the indirect emission remains suppressed, indicating that an indirect-to-direct bandgap transition is stable FIG. 4. PL spectra of MoS2 with four layers taken before and after proton irradiation, and after annealing. against annealing at 300 C. Also, the defect-induced sideband Appl. Sci. 2019, 9, 678 19 of 53 negligible reduction of sulfur XPS intensity compared to the molybdenum XPS intensity of the samples after the irradiation. However, 1T-like defects were generated in the MoS2 surface, inducing a weak ferromagnetic state at room temperature and improving transport properties. from ref. (3)) show fine distinction between them, substantially undulating domains are Han et al.  prepared single-crystalline MoS2 lamellae with a thickness of 100 µm. The lamellae observed atwere the MFM image of the electron-irradiated sample atconditions the 150 kGy with the electron-irradiated in ambient at room temperature. Figure 15 shows HRTEM images accelerationof energy 0.7 MeV (b). And then these domains are morehoneycomb evident at the increased theofirradiated samples, showing lattices of the MoS2 . The vacancies depended on the acceleration energy and dose. Without irradiation, electron dose of 300 kGy (c). But, they are weakened at the further increased electron doses of the lattice of crystalline MoS2 was a honeycomb few(e),defects, shown in Figure After 400 (d) andwith 600 kGy respectively. Consistently, the MFM15a. images of thebeing increasedelectron-irradiated with 150 kilogray(kGy) dose at 0.7 MeV acceleration energy, additional defects were produced. Double-sulfur vacancies (VS2 ) were acceleration energy of 2.0 MeV also exhibits similar change depending on the electron dose more frequently observed than the monosulfur vacancies (VS ) after electron irradiation. Slightly between 100 (f) and 250 kGy (g). This is in good agreement with the change of the displaced molybdenum atoms were occasionally found, as marked by the red arrow in Figure 15b. magnetizations of Fig. 2, even though the MFM results mainly concern on the magnetic Under electron irradiation with a 300 kGy dose at 0.7 MeV acceleration energy, the honeycomb lattice property along theheavily out-of-planedistorted direction. On(Figure the other hand, is notable that, while the was 15c)itand perfect honeycomb lattices were rarely observed. The numerous VSbetween and V vacancies distorted honeycomb and the irradiation considerably decreased the magnetizations theS2two different acceleration energiesthe have an order difference inlattice Fig. lattice parameters. V defects were more frequently observed under the electron irradiation with a S2no significant influences on the MFM images 2, the high acceleration energy of 2.0 MeV has 100 kGy dose at 2.0 MeV. With an increasing dose of 2.0 MeV electrons, the VS and VS2 concentration (Figs. 5(f) and 5(g))and compared to those 5(b) spacings and (c)) of the low acceleration energy of (b) Zoomed image (110) planes with(Figs. the lattice of 2.71 Å and 1.58 Å, respectively. increased significantly. Molybdenum vacancies were produced too. The lattice parameters were 0.7 MeV. This means thethe SQUID MFM results bulkMoS and2 single surface sensitive, in (a)that shows typicaland honeycomb latticeare of the crystal. The estimated lattice or also significantly increased. Under electron irradiation at higher energy of 10 MeV, different types respectively. distance between the two neighboring Mo atoms is of a = 3.12 Å (c) During the TEM of defects were produced. Under all irradiation, the vacancy densities increased with the electron defects were generated. measurements, monosulfur (VS’s) and line irradiation dose and electron energy. VS2 (a) (b) (c) (d) (e) Figure 15. Zoomed HRTEM images of MoS2 lamellae without electron (a)electron and with electron Figure S10. (a) HRTEM image ofirradiation the H(ii) sample: irradiation with 250 kGy dose at Figureimage S7. (a) image of 2the L(i)inset sample: electron irradiation with 150 kGy dose at 0.7 Figure S6. (a) HRTEM ofHRTEM the pristine FFT image displays hexagonally irradiation with (b)MoS 150. kGy dose at 0.7 MeV energy, (c) 300 kGy dose at 0.7 MeV energy, (d) 100 kGy 2.0 MeV accelerating energy. The fringes of HRTEM image indicate that the number of the accelerating energy.the More are obtained. Especially, VS2 (100) vacancies are frequently arranged spots withMeV a 1×1 structure, where innerdefects andand outer spots250 correspond to the dose at 2.0 MeV energy, (e) kGy dose at 2.0 MeV energy . The distance between two MoS2 layers is about mono- or bi-layers. FFT inset image shows weak additional spots in the neighboring molybdenum atoms is 3.12 Å (a top panel), 3.24 Å (b top panel), 2.94 Å (c top panel), observed than the VS vacancies after electron irradiation. The fringes of HRTEM image hexagonal pattern, relating to The the very increased Vin VMo defects. (c) The lattices S, Vthe S2, and 3.23 Å (d top panel), and 3.22 Å (e top panel). Defects are circled. VSmuch vacancies (a) bottom indicate that the number of the MoS2 layers is about mono- or bi-layers. FFT inset image Figure S8. (a) HRTEM image of the L(ii) sample: electron irradiation with 300 kGy dose at panel come from the irradiation of the electron beam observations. Reprinted from , also significantly increased compared to that ofwith the pristine 2. at with Figure S9. (a) HRTEMare image ofduring the H(i) sample: electron irradiation 100 kGyMoS dose displays an additional spot in the hexagonally arranged spots due to the increased defects. (b) 0.7 AIP MeV accelerating energy. The fringes of HRTEM image indicate that the number of the the permission of Publishing. 2.0 MeV accelerating energy. The fringes of HRTEM image indicate that the number of the The honeycomb lattices are increased compared to that of the pristine MoS2. (c) The red arrow MoS2 layers is about bi- or tri-layers. There rarely exist honeycomb lattices. (b-d) The distorted MoS2 layers is about References fiveor six-layers. VS2-like defects are more frequently observed Rotunno et al.  mechanically exfoliated MoS 2 20-layers and transferred them to carbon-coated indicates the displacement of Mo atom, but these defects are occasionally found. patterns are frequently obtained due to the presence of the many VS and VS2 vacancies. In 1. S. Yan, W. Qiao, X. around He, X. the Guo, L. Xi, and W. Zhong,of Appl. Phys. 106, 012408 to the case of L(x). (b-d) The lattices vacancies areimage increased except the Lett. copper grids. Figure 16a showscompared a transmission electron microscopy (TEM) a typic MoS 2 contrast to the case of L(i), the lattices are considerably decreased compared to that of the (2015). of few-layer. The multi-layers werecase then under a the scanning microscopy beam of (c).irradiated The different intensities main peaks electron in the line profiles of (c) and (d)(SEM) are pristine MoS2. with an accelerating voltage ofattributed 5 keVto and an electron-beam currentB.of 50 nA J.for 15 min. Many S. Tongay, S. S. Varnoosfaderani, R. Appleton, Wu, the electron2.irradiation-induced gliding S or Mo atoms. This suggests thatand theA. VS2F.- Hebard, Appl. Phys. crystalline islands were formed throughout theLett.multi-layers after the irradiation, as shown in like defects are quite similar to101, the 123105 electron(2012). beam-induced 1T phase. The details will be Figure 16b. The selected-area electron diffraction (SAED) patterns indicated that the islands were 3. S. W. Han, Y. H. Hwang, S.-H. Kim,W. S. Yun, J. D. Lee, M. G. Park, S. Ryu, J. S. Park, elsewhere. metallic molybdenum. Under reported high-energy (200 keV) electron irradiation, the MoS2 layers were D.-H. Yoo, S.-P. Yoon, S. C. Hong, K. S. Kim, and Y. S. Park, Phys. Rev. Lett. 110, drastically modified, and their sulfur stoichiometry was changed, inducing the formation of the (2013). molybdenum nanoislands, as shown in Figure 16c.247201 In both cases, massive surface sulfur depletion was induced together with the consequent formation of molybdenum nanoislands. 2D Mater. 3 (2016) 025024 2D Mater. 3 (2016) 025024 2D Mater. 3 (2016) 025024 E Rotunno et al E Rotunno et al Appl. Sci. 2019, 9, 678 20 of 53 (a) (b) (c) Figure 16. TEM images of (a) un-irradiated, (b) 5 keV electron-irradiated, and (c) 200 keV electron-irradiated MoS2 20-layers . The strong reflection spots of the inset SAED come from the MoS0.6 layers while the weak ones from metallic Mo. Reproduced with permission under the Creative Commons license. Copyright c 2016 IOP Publishing. Kim et al.  deposited five to seven atomic layers of MoS2 on a SiO2 /Si wafer at room temperature using the RF magnetron sputtering method. The thickness of the deposed amorphous MoS2 layers was about 4 nm. The films were then exposed under a collimated electron beam with an energy of 1 keV for 1–10 min at room temperature without any additional heating processes. HRTEM Figure 4. (a) HR-TEM image of a MoS2 ﬂake after irradiation. Mo nanocrystals circled in red. The FFT is reporte images indicated that the as-deposited MoS2 was amorphous (left panel 17a).proﬁle Under the pattern in (b). diffraction pattern of the irradiatedin area.Figure (c) Radial intensity of the diffraction Figure 5. (a) TEM image of a MoS ﬂake after SEM irradiation. (b) Electron diffraction pattern of the irradiated area. (c) Radial 2 Figure 1. (a) HAADF-STEM image of the typical MoS before andpattern a close view edge (b). (c) High-from amorphous to crystalline 2 ﬂake intensity proﬁle ofirradiation the diffraction in atoms (b).of the ﬂake electron beam irradiation (EBI) of 1 min, the random re-arranged resolution TEM image revealing the perfect MoS2 honeycomb structure (inset). (d) Electron diffraction pattern. structure, forming MoS2 crystal domains with a size about 5 nm (middle panel in Figure 17a). However, reported in the case of electron irradiated GaN microThis is in agreement with the longer irradiation time would crystalline domains panel in Figure 17a) because of the spacingdamage is. Therefore, the formation of a large(right hex-and structures  devices  due to the breaking of tion provided in ﬁgure 2. of μm and thickness of around 20 monolayers. In by Raman scattering, of MoS compensation of the intrinsic n-type 2, (i.e.bond agonal Moiré pattern means that these the crystallites pre- was Mo-S breaking. The amorphous-crystalline transformation confirmed Therefore, we can conclude th Mg–H complexes. doping). ordercloser to assess theMoS thickness of the we carefully sent a lattice parameter to the one modiﬁcations than in ﬂakes,the 2 The 1 and S depletion, ﬂakes undergo at the atomic as shown in Figure 17b. The two prominent Raman peaks of MoS , the in-plane mode E the the remaining metall 2 2g Different works [19, 20] have recently reported the previous case. inspected their edge. The ﬂake termination, in fact, is level have been studied by means of high resolution in small nanoislands on the surfac that capping or encapsulating layers with the irradiated is because out-of-plane modeMoS A1g2 ,single didThe notdiffraction appear in theof(ﬁgure as-deposited film its amorphous nature. notpattern abrupt 1(b)) butarea present clear steps of with in agreement with . The form TEM and electron diffraction. 1 experireported in ﬁgure 5(b) and its rotational averaged graphene during electron beam irradiation thickness that produce different image intenlands demonstrates the impossib The peak intensities of the E2g and A1different modes increased dramatically after the electron irradiation In ﬁgure 4(a) a high magniﬁcation TEM image of g intensity proﬁle insity. ﬁgure 5(c). Comparing the proﬁle ments has beneﬁcial and protective effects, decreasing The study of the steps intensity a rough the irradiated area isgives reported. Nanometric crystals doping at room temperature. Their XPS measurements provided a stronger proof that the amorphous filmsinversion from intrinsic n in ﬁgure 5(c) withestimation the one reported in ﬁgure 4(c) the the generation of defects and possible lattice breaking of the ﬂake thickness accuracy of between the holes increasing the concentration of s (circled in red)within can bean clearly seen in electron irradiation. Figure shows the XPS spectra of MoS2 samples appearance of a new set ofmonolayers. reﬂections can be 17c seen.resolution sulfur vacancy causes a deep acc and/orcrystallized amorphization.under the 1 keV a few Atomic TEM imaging (bright areas) produced in the continuous MoS2 ﬁlm Inray particular, theenergy two strong peaks at 2.61The andMo-3d peak was de-convoluted band-gap Incharacterized this work we test with the structural and composian Al-K with an of 1486 eV. into ). Modifying drastic assessed that the ﬂakes didregion). not undergo structural α (gray Moirè any fringes also appear where these 1.50under Å werehigh not present in the TEM irradiated sample. chiometry induces the spontaneo tional three stabilitychemical of MoS2 multi-layer ﬂakes ﬂakes, testicrystals overlap with the underlying MoS transformation during the exfoliation process and bonding states of Mo-Mo, Mo-S, and Mo-O in from the as-deposited sample. After 1 aforementioned min of 2 The small lattice distance difference measured Mo nanoislands. dose electron irradiation. We experimentally deterfying that 2there is a closereported relationship, both in terms of retain theconsisted perfect 2H-MoS structure in from electron irradiation, the Mo-3d spectrum only of two peaks originating the states of the the diffraction pattern is consistent with the presence mine the modiﬁcations induced by electron-beam ﬁgure 1(c) and its inset. orientation and lattice parameters, between the only found evidences of of the dramatically large Moiré shown in thepeak image.We SEM irradiation Mo-S Mo-O bonds, with increased intensity of the irradiation to and the crystal lattice and stoichiometry of pattern ﬂake. Similarly, the molybdenum nanograins andMo-S the MoSbonds. presence of several ripplocations Besides these the strong contributions, the weak  induced by 2 In this section we report on the effe multi-layer MoS ﬂakes exfoliated from geological The nanocrystals are responsible for the weak dif- only the S-2p peak2 from the as-deposited sampleÅwere de-convoluted into S-S and S-Mo bonding while exfoliation process, which reﬂection at 2.23 the assigned to metallic Mo, isappear still as straight line in dose on the structural properties o molybdenite via the conventional tape method. The ﬁgure 1(a). spots,The indicated by the also white arrows, observed S-Mo bond remained in the 1 min electron-irradiatedfraction sample. authors checked the samples present. electron irradiation in a Figure 6. Illustration of the protective effect of C coating beam on experiments are carried out both in a transmission both the fast Fourier (FFT) of ﬁgure 4(a), ﬂake after the SEM irradiation. thetransform MoS2pattern The of the electron diffraction The two strong peaks atsymmetry 2.61 and 1.50 Å in cannot be with microscope. under TEM at 400 keV and then measured the Raman scattering a 532 nm laser. It was not clear electron microscope (TEM) and in a scanning electron of ﬁgure 1(d) is also as anwith inset,the and2Hin phase, the electron diffraction In this case also we observed Sy consistent compound. attributed to any known Mo or Moxreported if the inamorphous films were changed under the TEM electron-beam irradiation microscope order to study different irradiation pattern of the same region reported inand ﬁgurelaser 4(b). irradiation. tion of many crystalline islands all Further studies arebeing ongoing order a =inb = 2.74toÅidentify and γ =them. 120°. In ﬁgure 4(c)effect the radial intensity proﬁle ofthe thethree dif- through regimes. LastKarmakar but not least we presence of irradiation etﬁnd al. that theinvestigated the few-layers shownare: in ﬁgure 5(a). to 2the labels regions During the experiment we Theelectron experimentirradiation hasincidentally been carried out According in of situ MoS inside fraction pattern is reported. It has been obtained by a nanometric amorphous carbon layer, unwillingly The noticed that a very thin carbon coating is able to prothe TEM; the beam-reducing apertures by havemechanical been micro-Raman scattering. The few-layered MoS2allsamples were prepared exfoliation andMoirè fringes, resulting fr integrating the pattern intensity along azimuthal of the deposited during the TEM experiments, nanocrystals of the ﬂake is with the unde (1) Free standing MoSthe tect minimizes the MoS2 ﬂakes and completely prevent the for2 ﬂake; this portion − 7 removed and the electron beam has been focused as irradiated a vacuum of 10of×the 10 Mo torr. The thickness of the few-layers was about 7 nm, which direction. the damage induced under by the electron beam irradiation. exhibit incarbon this case a pattern with located over one of the holes of the TEM mation grains. This effect is clearly much as possible in order to maximize the electron The two strongest peaks at 2.74 and 1.58before Å are theand was equivalent to about ten layers of MoS shows the Raman scattering after than one observed in the TEM coated copper grid, therefore here nothecarbon is described in ﬁgure 6 showing a MoS18 2 . Figure 2 ﬂake after the dose. As a result, an(100) areaand of few ofofsquare respec(110)hundreds reﬂections the MoS 1 2 ﬂake, It can be useful to recall that th present either below or over the sample. SEM irradiation. irradiation. In addition to the in-plane E A1girradiated. modes, extra Raman modes appeared nanometers has been uniformly 2g and out-of-plane Results and discussion The weak reﬂection at 2.23 Å, marked by the iodicity depends on the difference Its surface is almost entirely coveredtively. by Mo grains. The ﬂake isthe standing overunder the carbon grid. after being irradiated under 5 keV, 7 keV,The and 10 keV, showing the(2)contrast maximum change keV of the two crystals form damaged area shows a grainy black arrow, is consistent withinthe (110) reﬂection of 10 parameters Three regions, having different grain densities can be STEM image reported in ﬁgure 2(a), and itin canthe be breaking of inversion TEM irradiation similar the two parameters, the lar metallic Mo. irradiation. Obviously, the electron-irradiation-induced defects, resulting noticed. They have been labeled and black dashed lines (3) The circle is a contamination spot. This region was easily distinguished from the otherwise continuous Figure 1(a) shows the typical appearance of the MoS 2 have been drawn imagetotoany highlight them. in the spectra previously observedthe in the TEM, before symmetry. The 15 keV irradiation did on notthelead changes because electron beamthe SEM 4 ﬂakes studied in this work. We limited our attention to contrast exhibited by the pristine region surroundpassed the few-layers without creating any damage. large multilayer ﬂakes having lateral dimension of tens ing it. 5 Matsunaga et al.  irradiated triangular-shaped MoS2 mono-layers grown on SiO2 /Si substrates 2 acceleration voltage of 15 kV and an areal dose of 280 µC/cm2 charge. under an electron beam with an Figure 19a shows an optical image of a mono-layer MoS2 crystal. The right half region was covered by polymethyl methacrylate (PMMA) and was not electron irradiated. Raman scattering indicated 1 peaks of MoS . However, that both regions (with and without irradiation) exhibited the A1g and E2g 2 the two peaks were blue-shifted in the exposed region, as shown in Figure 19b. It was suggested that the electron-beam exposure introduced a local compressive strain in the MoS2 crystal. The induced Appl. Sci. 2019, 9, 678 21 of 53 strain would further change the band structure. The PL peak of the exposed left region, ∼1.85 eV, was blue-shifted 40–50 meV compared with that of unexposed right region (1.81 eV). Figure 19c shows the PL mapping of PL intensity. It was stated that the averaged PL spectrum of the unexposed region of the MoS2 mono-layer could be characterized by a broad peak at 1.81 eV while the exposed region of the same MoS2 sample showed a narrower feature at 1.85 eV, corresponding to a blue shift of 40–50 meV. www.nature.com/scientificreports/ www.nature.com/scientificreports/ (a) 3. HR-TEM images of EBI-treated films. (a) Cross-sectional HR-TEM image of 1EBI-treated min FigureFigure 3. HR-TEM of EBI-treated MoS2 MoS films.2 MoS (a) Cross-sectional HR-TEM image of 1 min Figure 3.images HR-TEM images of EBI-treated (a) Cross-sectional HR-TEM image of 1EBI-treated min EBI-treated 2 films. MoS2Plan-view film.2Plan-view HR-TEM images of (b)the as-deposited sample, (c) 1EBI-treated min sample, and (d) MoS2 film. images of the (b)the as-deposited sample, (c) 1sample, min sample, andsample, (d) MoS film.HR-TEM Plan-view HR-TEM images of (b) as-deposited (c) 1EBI-treated min EBI-treated and (d) min10EBI-treated sample. Insets showpatterns FFT patterns of the of areas as dashed squares. 10 min10EBI-treated sample. Insets showInsets FFT of the areas marked as dashed squares. min EBI-treated sample. show FFT patterns the marked areas marked as dashed squares. 1the E1 peak −1 cm−1 with −1an average value of min1EBI-treated sample. The Raman at 381–382 min EBI-treated The of the E12g dispersed peakatdispersed at 381–382 cmaverage with value an average 1 min 1EBI-treated sample. Thesample. Raman shiftRaman ofshift the of Eshift 381–382 cm with an of value of 2g dispersed 2g peak −1 −1 −1 −1 −1 −1 −1 cm−1. ,cm and the Raman the A peak dispersed at 406–408 cm with an average value of 406.77 381.44381.44 cm 381.44 , and the Raman of the A peak dispersed at 406–408 cm with an average value of 406.77 cm−1. , cm and the Raman shift ofshift the of Ashift peak dispersed at 406–408 cm with an average value of 406.77 cm . 1g 1g 1g −1 −1 −1 The average peak difference (∆k)25.33 was 25.33 cm ,cm which corresponding theto value of six of atomic layers or bulk The average peak(∆k) difference (∆k)cm was,25.33 , which corresponding six atomic layers or bulk The average peak difference was which corresponding to theto value ofthe six value atomic layers or bulk Even thetime EBI time was to (c) 510and min, did not change signifi antly as summarised Even when the EBI timeincreased wastoincreased to 510and 10∆k min, ∆k did not change antly as summarised in MoS2.MoS Even2.MoS when the EBI was increased 5 and min, ∆k did not change signifi antlysignifi as summarised in in (b)2. when Figure 5. XPS spectra ofcrystallinity the (a)the surveyof spectrum, (b) Mothe 3d spectrum, and (c)samples S 2p spectrum from as- improved Table 1. Based onRaman this we estimated that the crystallinity the EBI-treated was Table 1. Based oncomparison, this comparison, we estimated that the of EBI-treated samples was improved Table 1. Based onFigure this comparison, we estimated that crystallinity of EBI-treated samples was improved 1. spectra of as-deposited and EBI-treated MoS filmsthe according to irradiation times of 1, 5, and deposited and 1 min EBI-treated samples. 10 min. Figure 17. (a)through Plan-view HRTEM images ofthe the as-deposited sample, 1 min sample, antly through the atomic rearrangement Mo atoms from amorphous to a crystalline-layered signifi antly the atomic rearrangement of the MoS and S atoms from amorphous to a crystalline-layered signifisignifi antly through the atomic rearrangement of the of Mo and S and atoms from amorphous to a EBI-treated crystalline-layered and 10 min EBI-treated sample . Insets show fast Fourier transformation (FFT) patterns of the structure. structure. structure. FWHM (cm ) Average (cm ) High-resolution TEM analysis was performed in order to confirm the crystalline microstructure areasHigh-resolution marked as dashed squares. (b) Raman spectra of as-deposited and EBI-treated MoS samples TEM analysis was performed in order to confirm the crystalline microstructure High-resolution TEM analysis was performed in order to confirm the crystalline microstructure of theof theof the 2 Sample A E A ∆k E 1after min 19.3 11.5 381.44 406.77 in 25.33 MoSfor films as discussed thein results of the Raman analysis. Figure 3(a) shows a cross-sectional TEM TEM MoS films after EBI, asin discussed results of the Raman analysis. Figure 3(a) shows a cross-sectional MoS2 films asEBI, discussed the results ofthe the Raman analysis. Figure 3(a) shows a cross-sectional TEM 2after an2EBI, irradiation time of 1, 5, and 10 min . (c) Mo-3d XPS spectrum (left panel) and S-2p Table 2. Area ratio of bonding states after the deconvolution of Mo 3d and S 2p spectrums and the calculatedXPS 5 min 17.6 11.6 381.25 406.69 25.44 valueobtained of the S/Mo ratio from the room-temperature as-deposited and 1 min EBI-treated samples. of a 10synthesised MoS film on a SiO /Si wafer by sputtering with EBI a as a image of(right a synthesised MoS film on a SiO /Si wafer obtained by room-temperature sputtering with imageimage of aspectrum synthesised MoS film on a SiO /Si wafer obtained by room-temperature sputtering with EBI as a min 19.1 12.1 381.422 406.64 25.22 2 2 2 2 2 panel) from as-deposited and 1 min electron-irradiated samples . Reproduced with asEBI post-process for 1Five min. toFive seven atomic layers of MoS formed parallel to theto substrate awith thickness of post-process for min. to seven layers of2, AMoS formed parallel the substrate aofthickness of post-process for Table 1 min. toFive seven atomic layers of average MoS formed parallel to the substrate with awith thickness 1. FWHM of E 1 and A peaks in Raman spectra (Fig.atomic 1) and values of E peak positions, 2 2 the composition4.0 of MoSInternational . If both S and Mo have thelicense. same sticking coeffici t on the SiO /Si substrate, the film permission under the(Fig.Creative Commons Attribution and ∆k in Raman map data 2)of for 1, 5, andMoS 10 min EBI-treated samples. nm. Even though the the crystal domain was limited tosticking about 5this nm, iswas clear evidence woulddomain belimited MoS . However, S hasabout a limited lower coeffici t than Mo, the film slightly t in ~4though nm. Even though the size the2 MoS was to about 5this nm, this isdeficie clear evidence ~4 nm.~4Even the size ofsize the MoS crystal domain was to 5 nm, isand clear evidence ofsulphur theof the of the 2 crystal 2of under the normal sputtering conditions. For S to have a sticking coeffici t of 1, two S atoms would have to react with a Mo atom as they arrive onby the substrate. Otherwise, theyPlan-view would desorb, and a sulphur-defi ient MoS images rearrangement from from amorphous to a crystalline structure theprocess. EBI Plan-view TEM atomic rearrangement amorphous to a structure crystalline by theprocess. EBI process. Plan-view TEM images atomicatomic rearrangement from amorphous to a crystalline bystructure the EBI TEM images film would be formed. To maintain the stoichiometry of MoS , the S/Mo ratio arriving at the substrate should be etsulphide al.  prepared aand MoS mono-layer micromechanical cleavage, as shown inFigure Figure 20a.3(b) >2.by Toalso maximise the heating sticking coeffici t of3(b)–(d), S Fig. in our in process, werespectively. kept3(b)–(d), the sputter power asrespectively. low as possible, at about Figure theLiu and and 10 min samples are shown in 3(b)–(d), respectively. 3(b) (H S)1gas and 1 vaporised ambient . However, these processes require toFig. a high ofas-deposited the as-deposited and 1sulphur 102EBI-treated min EBI-treated samples are shown Fig. of the of as-deposited and 10 min EBI-treated samples are shown in Figure 3(b) the working W, and maintained temperature of up to 700–750 °C. Several researchers have reported a direct 20 sputtering process with a MoSdistance tar- as high as possible, at up to 10–15 cm. As shown in Figs 1 andas-deposited 3, the a Raman spectra and HR-TEM results confi med that keV the as-deposited MoS film MoS mono-layer was then exposed in vacuum under a 80 electron beam shows that the amorphous nature was observed over the entire sample. However, crystallites with shows that the amorphous nature was observed over the entire as-deposited sample. However, crystallites with showsThe thatwell-crystalline the amorphous nature was observed over the entire as-deposited sample. However, crystallites with 2 get at a relatively low substrate temperature of 300–350 °C, but as-depositedremained MoS films show poor crystallinity amorphous. However, we obtained a 2D layered structure for the MoS film after only electron irracompared to other methods . Recent2studies on the post-processing of as-deposited MoS films thermal diation with an energy of with 1 kV for 1 min, as illustrated in Fig. EBI-treated 6(a). Th s is very surprising that such low-energy a size ~5 nm were distributed in an area of 20 nm × 20 nm for the 1 and 10 min samples, as shown aofsize of ~5 nm were distributed in an area of 20 nm × 20 nm for the 1 and 10 min EBI-treated samples, as shown a sizewith of ~5an nm were distributed in an area of 20 nm × 20 nm for the 1 and 10 min EBI-treated samples, as shown intensity of 40 A/cm . Initial defects and holes were induced in the MoS sheet once exposed annealing and laser treatment showed improved crystallinity, but the problems of with high1temperature and highrearrangement for MoS crystallisation.2No additional thermal heating electrons kV encouraged atomic cost remain. process was applied to the substrate, and the temperature resulting from energetic electron bombardment of the in3(c) Fig.in 3(c) and (d). Thsuggest s indicates a startling transformation of theunder MoS film 2amorphous from to a crystalline Fig. 3(c) and Th indicates a startling transformation of the MoS filmofamorphous from amorphous to crystalline in Fig. and (d). Th s indicates as startling transformation the MoS film to a ofcrystalline 2from the experimental as a small substrateof didexposure not exceed 100 °C2irradicondition EBI withirradiation, an energy 1 kV for 1 min, In this paper, we(d). a simple method ofAfter sputtering and81 post-processing with electron beam to the electron beam (Figure 20b). a second under the electron the shown Fig.amorphous 6(b). For thestructure experimental substrate temperature was measured by direct contact with ation (EBI) to obtain crystallinewithin MoS films at amin. low temperature below 100 °C. in The of results, thehowever, structure the EBI 1In the 10EBI-treated min EBI-treated sample, theofarea the of amorphous by theprocess EBI process within min. In the 10 min sample, theof area the amorphous structure bystructure theby EBI process within 1 min. the1In 10 min sample, however, thehowever, area the amorphous a thermocouple. The EBI-treated substrate temperature as-deposited MoS films can be transformed into a crystalline nature after only electron beam irradiation (EBI) exceeded 300 °C only after the EBI process for 10 min. These results holes spread rapidly (Figure 20c) and extended into big holes withsample. diameters of 3–6 nm (Figure 20d) an energy ofincreased 1 kV for 1increased min. There have been several reports onto exploring the evolution of EBI-treated materegion waswith slightly compared to of the 1EBI-treated min Th sbe may from the Mo–S was slightly compared ofstructural the 1EBI-treated Th sbe may from the Mo–S min region was region slightly increased compared to that ofthat the 1that sample. Th sample. s may from thebe Mo–S min rials from amorphous to crystalline as they are being transformed at the local| DOI:10.1038/s41598-017-04222-6 region through the application of a 6 Scientific Reports | 7: 3874 abonds 103 sbreaking irradiation. The MoS converted to Motime. after bonds breaking owing to the excessive energy transfer with the longer irradiation time.insets The insets of3(b)–(d) Fig.long-term 3(b)–(d) high-energy electron beam of several hundred kilovolts tomono-layer several megavolts in the transmission electron microscopy 2transfer 5 S4 nanoribbons owing to the excessive energy transfer with the longer irradiation time. The insets of Fig. 3(b)–(d) bondsafter breaking owing to the excessive energy with longer irradiation The of Fig. (TEM) . Interestingly, our EBI process makes it possible to stimulate the atomic rearrangement of amorphous show theFourier fast Fourier transform (FFT) patterns from the areas marked as dashed squares. They reveal that the MoS underFourier the conditions of a relatively low electron energy of 1the kV andareas short timemarked of 1the min. We think show thefilm fast transform (FFT) patterns from areas marked as dashed squares. They reveal that the showelectron the fast transform (FFT) patterns from as that dashed squares. They reveal that the irradiation. this EBI technique can be easily applied to scale up the synthetic process of MoS films considering its compaticrystal domain of the MoS film transformed from an amorphous state into a hexagonal lattice structure. The crystal domain the MoS2such film transformed from an amorphous state into alattice hexagonal latticeThe structure. The crystal domain ofbility the MoS film from amorphous state MoS into a hexagonal structure. FFT FFT 2processes with conventional PVDtransformed as sputtering and an evaporation. 2 of Komsa et al.  prepared freestanding single-layer samples by mechanical exfoliation of FFT 2 patterns of theof as-deposited showed typical amorphous characteristics widearing. halo ring. In contrast, patterns theDiscussion as-deposited sample showed typical amorphous characteristics wideIn halo ring. In contrast, patterns of the as-deposited samplesample showed typical amorphous characteristics with awith wideawith halo contrast, Results and naturaldiffraction MoS2 1bulk crystals, observed a MoS sheet under an 80 keV electron beam in aofvacuum on 2 and shows the Raman spectra ofand MoS synthesised using sputtering the EBI process withsample differspots in thepatterns FFT patterns of the 1EBI-treated indicated that the c-axis the of crystal min sharpFigure diffraction spots in thefilms FFT patterns of the 1EBI-treated min EBI-treated sample indicated that the crystal sharp sharp diffraction spots in the FFT of the 1bymin sample indicated that the c-axis ofthe thec-axis crystal ent irradiation times. In the as-deposited film grown at room temperature with a working pressure of 5 mTorr, an aberration-corrected HRTEM. It was experimentally observed that the top and bottom sulfur atoms two prominent Raman peaks of MoS , i.e. the in-plane mode (E ) and out-of-plane mode (A ), did not appear structure (space group P6 /mmc) was perpendicular to the substrate. The FFT pattern in Fig. 3(d) shows that the structure (space group P63perpendicular /mmc) was perpendicular to theThe substrate. The FFT pattern in Fig. that 3(d)the shows that the structure (space group P63/mmc) to the substrate. FFT pattern in Fig. 3(d) shows 3 was because of its amorphous nature. However, after the EBI process with as-deposited sample, the peak intensities of crystalline ordering hexagonal symmetry of 10EBI-treated min sample was somewhat inferior toofthattoofthat of the E for (~381 cm for ) and A for (~407 cm ) modes increased dramatically without an additional thermal heating crystalline ordering hexagonal of the 10EBI-treated min EBI-treated sample was somewhat inferior crystalline ordering hexagonal symmetry ofsymmetry the 10the sample was somewhat inferior to that min were removed under the 80 keV electron beam and vacancies were generated in the MoS mono-layers 2 process. As the EBI process time was increased from 1 min to 10 min, the peak intensities of the E and A bands the 1EBI-treated sample. min the min EBI-treated sample. the 1 under min sample. decreased without peak shifts. The highest intensities of the E and A peaks for the 1 min EBI-treated sample the1EBI-treated electron irradiation. The sulfur vacancies agglomerated into line defects due to migration of indicated that the amount of Mo–S bonding may decrease with a longer irradiation time. The difference between 4(a) shows the atomic force microscopy (AFM) profiles at the at edges ofas-deposited theof as-deposited 4(a) shows the atomic force microscopy profiles the as-deposited FigureFigure 4(a) Figure shows the atomic force microscopy (AFM) height at edges ofthe theedges and and and the Raman shifts of the E and A peaks (∆k) was ~25 cm , which is typically shown(AFM) forheight sixprofiles atomicheight layers or the the defects . MoS . MoS films. The average thickness of the MoS film slightly decreased after the EBI process min1EBI-treated minbulkEBI-treated films. average offilm the2(FWHM) MoS2 film slightlyafter decreased after the EBI process 1 min 1EBI-treated MoS The the MoS slightly decreased the EBI process 2 MoS For the 1,25, films. and 10 min EBI-treated samples, The asthickness shown in Table 1, of thethickness full width at half 2 maximum 2average values of4.0 E were 19.3,Figure 17.6, and 19.1 cm , respectively, and those forAFM A were 11.5, and 12.1 cm from toThe nm. 4(b) and (c) show images of(FWHM the, respecas-deposited min samples. from 4.5 nm to 4.0 nm. Figure 4(b) and (c) images show AFM images of the as-deposited and 1EBI-treated min EBI-treated samples. from 4.5 nm4.5 to nm 4.0 nm. Figure and show AFM of11.6the and 1 and min 1EBI-treated samples. tively. FWHM of the4(b) EBI sample was(c) higher than that of the exfoliated monolayer MoS as-deposited of E : 2.8 cmroughness , the FWHM of A : 4.7 cm )at.measured However, thethe crystallinity of our 1of minthe EBI-treated sample atthe room temper- changed However, the measured at centre region of the MoS films signifi antly with the EBI time. However, roughness at the centre region of MoS films changed signifi antly with the EBI time. However, the roughness measured the centre region MoS films changed signifi antly with the EBI time. 2 2 : ~14 cm ) with a 2 ature was superior to that of sputtered MoS films (FWHM of E : ~50 cm , FWHM of A The roughness AFM roughness (Ra) values of the SiO and as-deposited were almost the at same 0.524 The AFM roughness (Rthe of the SiOand and as-deposited were almost the at same 0.524 and The AFM (R of SiO as-deposited samplesample weresample almost the same 0.524 andat and 2 substrate a) values 2 substrate a) values 2 substrate 2 samples Scientific Reports | 7: 3874 |respectively. DOI:10.1038/s41598-017-04222-6 0.523respectively. nm, RThe of 1, the 5,10and min10EBI-treated were 0.696, 0.572 and 0.523 nm, respectively. of 1, 5,10and min EBI-treated samples were 0.5720.541 and nm, 0.541 nm, 0.523 nm, The RaThe values of Rthe 1, the 5, and min EBI-treated samples were 0.696, 0.5720.696, and 0.541 nm, a values a values respectively. Because of theof atomic rearrangement from amorphous to crystalline MoS , the roughness inevitably respectively. Because the atomic rearrangement from amorphous toMoS crystalline MoS , the roughness inevitably respectively. Because of the atomic rearrangement from amorphous to crystalline , the roughness inevitably 2 2 2 2 Mo 3d −1 1 2g −1 1 1g 1g 2g 1 2g S 2p Sample Mo–O Mo–S Mo–Mo S–S S–Mo S/Mo ratio as-deposited 11.54 44.53 43.93 57.23 42.68 2.24 1 min-EBI 8.63 91.37 — — 100 2.00 1 2g 1g 1g 2 2 2 2−x 18 2 2 19 2 23 2 2 20–22 2 2 24 2 2 2 25–28 2 2 2 1 2g 2 1 −1 2g 1g 1 1 29 2 2g 1 2g 5 −1 2g 1g 1g 1g 2g −1 1g 1 −1 1g −1 −1 1g 1 2 −1 30 2 c Reports | 7:| 3874 DOI:10.1038/s41598-017-04222-6 ntific | 7:|3874 | DOI:10.1038/s41598-017-04222-6 orts | Reports 7: 3874 DOI:10.1038/s41598-017-04222-6 1 2g −1 1g 2g −1 4 4 4 Article images denote regions where the channel current is enhanced by ce hift the electric ﬁeld generated by the tip. Most importantly, this he response is not observed at the edge of the (Cr/Au) electrodes ed but is ratherAppl. located in the channel interior. The response Sci. 2019, 9, 678 22 of 53 ACS Nano Article Article ful becomes increasingly prominent when Vtip‑dc is increased, J. Appl. Phys. 117, 135701 (2015) miwith proﬁle. the electrostatic n-type nature of theimages MoSdenote (notregions shown 4−10 2 the whereforce the channelimages currentdenote is enhanced regionsbywhere the channel current is enhanced by EFM, in consistent contrast, detects the EFM, force in contrast, detects electrostatic reIn sample Figurebetween 1c, theasthe right electrode the FET serves asathe e AFM tiphere). and the surface a phase AFM shift tip and of thethe sample surface as phase by shift electric ﬁeld generated the tip. the Most electric importantly, ﬁeld generated this by the tip. Most importantly, this ns grounded source theonVboth applied response to the information left ilever oscillations. Spatial information of and the cantilever oscillations. the Spatial on both is notelectrode. observed at thethe edge of response the (Cr/Au) is not electrodes observed at the edge of the (Cr/Au) electrodes d is nd chargeIndistribution the potential surfaceand is captured charge distribution isincaptured but but isatrather locatedthat the channel but interior. is rather The located responsein the channel interior. The response Figure 1d,at this conﬁguration is reversed, ittheis surface clear mplementary inproviding suchremains complementary a powerful providing a prominent powerful when becomes increasingly becomes Vtip‑dc increasingly is increased, prominent when Vtip‑dc is increased, the measurements, SGM response pinned measurements, near the right contact. r understanding the electronic properties for understanding of semithe electronicwith properties of semiconsistent the n-type nature of consistent the MoS2 with n-type nature of the MoS2 (not shown (not the shown (See section 1 ofmethod the Supporting Information for awe discussion ofmeasurenanostructures. In this work, we conductor apply these nanostructures. measure- In this work, apply here). In Figure 1c,these the right electrode of here). the FET In Figure serves1c, as the the right electrode of the FET serves as the the dependence of the SGM response on drain bias.) This pinning iques potential techniques distributions to probe the current andsource potential in to probe the current andment grounded anddistributions the Vd is appliedgrounded source and the Vd is applied to the left electrode. to the left electrode. eﬀect suggests the formation domain structure ETs. in MoS 2 FETs. of some ﬁxed ed In Figure 1d, this conﬁguration is reversed, In Figure but it1d, is clear this conﬁguration that is reversed, but it is clear that within the channel, in spite of the fact that the the SGMtransistor response was remains pinned near the SGM the right response contact. remains pinned near the right contact. ces 11 AND DISCUSSION AND DISCUSSION fabricated from aRESULTS single-domain MoS2 crystal. (See section 1 of the Supporting Information (See section for a discussion 1 of the Supporting of Information for a discussion of onthe dependence of the drain dependence bias.) Thisof pinning the SGM response on drain bias.) This pinning In parts andThe b of Figure 2, of we EFM images obtained menon here isa demonstrated phenomenon in Figure 1,interest inshow here is demonstrated inSGM Figureresponse 1, in on the ile of interest suggests theour formation of some eﬀect ﬁxed suggests domain structure the formation of some ﬁxed domain structure plot the SGM response of with which one the ofwe our plot back-gated the SGM response of one of back-gated simultaneously SGM mapping foreﬀect opposite polarities of hin within thedetails channel, in spite of the factwithin that the thetransistor channel, was in spite of the fact that the transistor was (see for bias. furtherThese transistors detailsimages on these (seeallow the devices Methods for further on these devices the drain us to construct the evolution 11 as, the Methods fabricated from a single-domain MoS2 crystal. fabricated from a single-domain MoS211 crystal. fabrication). This response and was their obtained fabrication). by conThis response was obtained by conof the potential variation along the channel length, asa we indicate ed In parts and b of Figure 2, we show EFM In parts images a and obtained b of Figure 2, we show EFM images obtained measuring the transistor drain tinuously currentmeasuring (Id) while the transistor drain current (Id) while the plots shown Theofplots reveal eseAFM tipin over simultaneously with the SGMthin mapping for simultaneously opposite polarities with the of SGM mapping for opposite polarities of he the surface rastering of underneath the atomically the AFMthe thin tipEFM over scans. the surface the atomically two consistent features, the ﬁrst ofwere which a workfunctionthe is drain bias. These images construct drain bias. the evolution These images allow us to construct the evolution his this way, we were able to construct channel. (for Inﬁxed this way, tip bias, we able to construct (for ﬁxed tip allow bias, us to the of potential variation along the channel of the length, potential as we variation indicate along the channel length, as we indicate induced potential diﬀerence ∼0.4 eVofbetween thechange MoS of the current change ) the(Ispatial ) caused variation thethe current Vtip‑dc espatial 1 variation SGMof SGM) caused 2(Iand inelectrical the plotssetup shown underneath the EFMinscans. the plots The shown plots reveal underneath the EFM scans. The plots reveal ence tip. gold The electrical by setup the used presence during ofelectrode. these the tip. The used during at the surface of each The second feature is a these or of thethe two consistent features, two consistent is a workfunctionfeatures, the ﬁrst of which is a workfunctionnts is shown in Figure 1b,inand measurements further details is on shown this in Figure 1b, and occurring further details on ﬁrst this of which sudden drop potential within the channel interior, at the ese inducedParts potential of ∼0.4 betweenpotential the MoSdiﬀerence of ∼0.4 eV between the MoS2 and be found in the Methods. Parts setup c and can dbeoffound Figurein1the Methods. c anddiﬀerence d of Figure 1 eVinduced 2 and the gold thechannel surface of each electrode. the Thegold second at thefeature surface is of a each electrode. The second feature is a to SGM scans taken over correspond the channeltoregion SGMfor scans taken overatthe region for sudden drop in potential the channel sudden interior, dropoccurring in potential at within the channel interior, occurring at ). Brightdirections areas in these areaswithin in these rections of the drain bias (Vddiﬀerent of the drain bias (V d). Bright Figure 18. Micro-Raman spectra of 10-layered MoS2 sheets (black curves) and after (red curves) FIG. 7. Micro-Raman spectra measured within before the 25 lm channel (see text). irradiation . Raman-shift versus frequency is plotted, both before (black line) and after (red line) irradiation. (a) (b) (c) energy irradiation. Therefore, we infer that optimal energetic electron irradiation can be utilized for increasing the carrier density of MoS2 devices. We have also investigated the stability of enhanced conductivity with time, which was found to reduce gradually and stabilize above the pre-irradiation value after a few days. It is well-known that the lifetime of MoS2 devices is limited. ance. Above 10 KeV, the If the devices are kept at ambient atmosphere, they get deacHowever, as discussed eartivated veryof soon. It electron-beam is observed that such deactivated devi- was performed the effect irradiation Figure 19. Optical image a2mono-layer MoS crystal. The electron-beam exposure performed ) Optical image of a of monolayer Figure MoS 3. (a) is Optical taken(a) just image after ofEBL a monolayer and development. MoS crystal, Thetaken just after2EBL exposure and development. was performed The onelectron-beam exposurewas on 2 crystal, side of the crystal so that the PMMA only the resist left side remains of the on crystal the right so that side. the (b) PMMA EFM image resist (V remains = 4 on V, the lift right height side. = 60 (b) nm) EFM of image the MoS (V = 4 V, lift height = 60 nm) of the MoS2 tip‑dc 2 tip‑dc ces can also be activated after irradiation. rom 30 KeV plot. In princion only the left side of the crystal while the right side is covered to avoid electron exposure. (b) Raman n in panel (a), obtained after removal crystal shown of all remaining in panel (a), resist. obtained Inset:after magniﬁed removal view of of allthe remaining area identiﬁed resist. Inset: by themagniﬁed green square viewinofthe the area identiﬁed by the green square in the 1 Raman st(G) after EBL andline development. The exposure wasand performed The blue dashed corresponds main toimage. theelectron-beam boundary The of bluethe between dashed line the2 exposed corresponds unexposed to the boundary regions. between (c) Raman the exposed spectra and of the unexposed exposed the exposed for low-energy electron mapping MoS crystal shown inon(a), indicating the Raman shift regions. of the (c) E2g peakspectra as a of function of 1 crystal, indicating the Raman shift of the E crystal, peak as indicating a the Raman shift of the E12g peak as a (blue)(b) areas of the (red) crystal. and the Raman unexposed scattering (blue) map areas of the of the MoS crystal. (d) Raman scattering map of the MoS ne unexposed the right side. EFM image (V(d) = 4 V, lift height = 60 nm) of the MoS 2 2 2g tip‑dc 2 position. (c) PL mapping of the MoS crystal at a wavelength of 670 nm. Reprinted with permission rease density and/ (e) carrier PLmagniﬁed spectra obtained function at the diﬀerent of area position. positions (e) PL within spectra thegreen obtained unexposed at (labeled diﬀerent positions as (i))within and exposed the unexposed (ii−iv) (labeled regions here as (i)) and exposed (ii−iv) regions gposition. resist. in Inset: view of identiﬁed by the square in2thehere 4. PL Effects on vibrational properties after irradiation crystal at a wavelength ofmap 670 at a American wavelength ofChemical 670 nm. panel (f). PL map and of theunexposed MoS identiﬁed in panel (f). ofnm. the MoS 2from 2 crystal cexposed Reference . Copyright 2016 Society. etween the(f)exposed regions. (c)(f)Raman spectra of the the present situation, since 1 shift of the E 2g peak as a ttering map of the MoS2 crystal, indicating the Raman 9732To 9732of irradiation-induced DOI: 10.1021/acsnano.6b05952 DOI: 10.1021/acsnano.6b05952 investigate the impact vacancy we may donsnot across SiO2, (labeled ACS Nano 2016, 10, 9730−9737 ACS Nano 2016, 10, 9730−9737 within the unexposed here as (i)) and exposed (ii−iv) regions 2 creation on vibrational properties, we have also carried out ength 670 nm. bulkofMoS 2, viz., 200 cm /V micro-Raman (MR) measurements before and after irradiaer 9732 density (n) at a particular DOI: 10.1021/acsnano.6b05952 ACS Nano 2016, 10, 9730−9737 tion. Due to restricted electron beam diameter, the damage on ng the relation n ¼ Gl/leA, flake is localized and changes in the Raman spectra can, nd A is the vertical crosstherefore, be seen only around the irradiated area. The passage. Table IV presents Raman-shifts before and after irradiation are plotted in Fig. 7. ults on flakes, where the In addition to the in-plane E12g and out-of-plane A1g modes, , differential conductance extra modes have appeared for 5, 7, and 10 KeV. From 15 ensity is evident for low- ented across the 25 lm channels on V) are plotted for flakes of similar ed) irradiation, with electron beam he figure. Current value above 2 V ensitivity range of the instrument. n irradiation on transport properties of few-layered flakes of MoS2. Suffixes 1 and 2 stand for before and after irradiaenergy of electron irradiation, conductance, and differential conductance both before and after irradiation and carrier S | DOI: NS | DOI:10.1038/ncomms2803 10.1038/ncomms2803 ARTICLE Appl. Sci. 2019, 9, 678 a b bb (a) Monolayer Monolayer Monolayer g h k Bilayer ARTICLE ARTICLE 23 of 53 (b) Bilayer Bilayer Bilayer 2 nm (c) 2 nm 2 nm (d) g 0s h 23 2 nm Figure HRTEM images of a MoS2 mono-layer under electron irradiation . (a)35 MoS 23 35sss2 mono-layer 00s20. 23 hh 23 ssirradiation. 35sss iji (FFT) image. (b) The initial59 i k with without Inset: fast Fourier transformation MoS2 sheet small irradiation-induced vacancies as highlighted by the arrows. (c) Large holes extended from the small vacancies upon electron irradiation of 108 s. (d) Larger holes extended from the small vacancies upon electron irradiation of 261 s. Reprinted with permission under a Creative Commons license. 89 Parkin et al.  prepared MoS2 single-crystalline mono-layers by CVD method and irradiated them with 200 keV electrons in an aberration-corrected TEM. Most generated defects were single sulfur vacancies. The irradiation-induced defects were quantitatively related to the electron dose with the actual defect concentration. Figure 21 shows the defect-induced stoichiometry as a function of electron dose. The elemental composition within the illuminated region was measured by electron X-ray dispersive spectroscopy (EDS). The EDS measurements indicated that the S/Mo atomic ratio was very close the stoichiometric 2:1 ratio before irradiation. The M/Mo 59 89 s lelectron 113ss ratio decreased 59 sss to 89 113 kk 113sremoval s l of 1 |ofInthe situ fabrication suspended sub-nanometre ribbons. l doseFigure with the 89 irradiation because sulfur under electronmolybdenum-sulﬁde irradiation. It should be noted locates at the region labelled by arrow. Scale 100 mm.supporting (b) TEM images of the thin that the EDS intensity ofregion the molybdenum signal remains constant during thebar, irradiation, the fact that the main effect of irradiation to create vacancies while molybdenum atoms by fringe countingwas at the edge. sulfur The straight edge veriﬁes that the membrane is single laye were much more difficult be sputtered. bytocontrast. Scale bar, 100 nm. (c) High-resolution TEM data obtained on the unfolded regio Electron-irradiation-induced transformation of MoS was also reported at high 2 layers is shown inphase the inset. (d) The initial MoS 2 membrane with small irradiation-induced vacan ◦ C in a vacuum . Electron irradiation initiated the phase transition temperatures, such as 400–700 from the small vacancies upon 80 kV electron irradiation of another 108 and 261 s. (g–l) T of 2H-MoS2 at high temperatures, and the transformation increased with increasing electron sub-nanometre ribbon under 80 kVratio electron irradiation. In (c–l) scale bar, 2 nm. doses above 40 × 106 –100 × 106 electrons /nm2 , depending on the temperature. It was also found that MoS2 layers were damaged when the total electron dose exceeded 5.0 × 108 –1.1 × 109 electrons/nm2 with the accelerating voltage of 60 kV. breaking (Supplementary Movie 1), indicating an excellent periodic ﬂu mechanical ribbons. strength. Our ﬁrst-principles simulations alsoThe predict nofofsuspended suspendedmolybdenum-sulﬁde molybdenum-sulﬁdesub-nanometre sub-nanometre ribbons.(a) (a) few-layer MoS22ﬂake ﬂake onTEM TEMgrid. grid. The thinnest vertical dev AAfew-layer MoS on thinnest ﬁde sub-nanometre ribbons. (a) A few-layer MoS ﬂake onhas TEM grid. The thinnest 2 S rectangle that the ribbon a respectable tensile strength up to 30 GPa labelledbybyarrow. arrow.Scale Scalebar, bar,100 100mm. mm.(b) (b)TEM TEMimages imagesofofthe thethinnest thinnestregion. region.The Thethickness thicknessofofthe themembrane membranecan canbe bedetermined determined m.belled (b) TEM images of the thinnest region. The thickness of the membrane can be determined Peierls dist and canisisbe stretched for 9% and before breaking without plastic dge. Thestraight straight edgeveriﬁes veriﬁes thatthe themembrane membrane single layer.The The folded and unfolded regioncan can be be further further determined e.theThe single layer. folded unfolded region determined membrane edge is single layer.that The folded and unfolded region can be further determined deformation (Fig. 4a). Its Young’s modulus is calculated by ﬁtting m.(c) (c)High-resolution High-resolutionTEM TEMdata dataobtained obtainedon onthe theunfolded unfoldedregions. regions.The Thecorresponding correspondingfast fastFourier Fouriertransformation transformation(FFT) (FFT)image image m. btained on the unfolded regions. The corresponding fast Fourier transformation (FFT) curve image the linear elastic region of stress-strain to be about 300 GPa, membranewith withsmall smallirradiation-induced irradiation-inducedvacancies vacanciesas ashighlighted highlightedby bythe thearrows. arrows.(e,f) (e,f)Larger Larger holes holes extended extended initialMoS MoS eheinitial 2 2membrane 16.extended mall irradiation-induced vacancies as highlighted bythat the arrows. (e,f) Larger holes Discussion close to of pure MoS sheets Further calculations on the 2 pon80 80 kVelectron electronirradiation irradiationofofanother another108 108and and261 261s.s.(g–l) (g–l)Time Timeseries seriesofofthe theformation formationand andgrowth growth for for aa suspended suspended on notherkV 108 and 261 s. (g–l) Time series Mo of the formation and growth for a suspended As the tra S ribbon with hybrid functional predict an indirect band gap 5 42 nm. r 80kV kVelectron electronirradiation. irradiation.InIn(c–l) (c–l)scale scalebar, bar, 2 nm. l)80scale bar, 2 nm. with a desirable value of 0.77 eV (Fig. 4b). This gap value not only similar stru promises the optical adsorption into the infrared region but also may be fab Article Appl. Sci. 2019, 9, 678 24 of 53 Figure 21. EDS measured stoichiometry, showing an initial value of S/Mo = 2.01 ± 0.07, as expected for CVD-grown MoS2 where a small amount of sulfur vacancy (∼0.5%) can be present after growth . Reprinted with permission from Reference . Copyright c 2016 American Chemical Society. 2.7. Plasma Irradiation Plasma has been employed to remove impurities and contaminants from surfaces by the collision energy of gas molecules and the chemical action on impurities and contaminants. Various gases (such as argon, oxygen, hydrogen, and nitrogen, as well as their mixtures) have been employed to carry out the cleaning procedures. The plasma is generated by using high-frequency voltages (typically kHz-MHz) to ionize the working gases. The activated species in plasma, including atoms, molecules, ions, electrons, free radicals, metastables, react with contaminants on surfaces. Short-wave ultraviolet (vacuum UV), whose energy is very effective to break most organic bonds of surface contaminants, is produced in plasma too. The technique has been utilized to modify surfaces of MoS2 few-layers in recent years. 2.7.1. Active Nitrogen N2∗ Michra et al.  investigated oxygen-plasma-irradiated MoS2 mono-layers deposited on ◦ C. Figure S spectrum ofsapphires monolayer a holey carbon grid. (b) Measured at 450MoS shows Raman scattering of irradiated MoS2 EDS mono-layers. Raman 2 on 22 − 1 1 −1 towards a lower A1g mode shifted 1.79 the cm Mo towards a higherremains wavenumber and E2g (c) 1.11 cm e. The S intensity drops while intensity constant. EDSmin of N2 plasma irradiation. The peak intensity ratio I A /I and FWHM of 01 ± 0.07, aswavenumber expected after for 3CVD-grown MoS 2 where a small amount of ES 1 peak increased with plasma irradiation time. XPS investigations indicated that N-Mo bonds the E2g pattern of pristine, monolayer MoS2 on a 100 nm thick Si3N4 substrate. (e) formed during the irradiation energies (calculated from Mo-3d5/2 , Mo-3p3/2 , and gives an S sputtering cross-section ofand 75 the ± binding 10 barn. (e) Sum of the intensity ∗ 3/2 S-2p peaks) shifted towards lower binding energy after N2 irradiation. The valence band maximum cross-section(VBM) of 120 ± 20 barn. Note that we ﬁt the SAED data only at low reduced to 0.9 eV after 1 min of irradiation and 0.5 eV after 3 min of irradiation from 1.0 eV of ds 07 np or vs n 2, n 1g 1 2g pristine MoS2 mono-layer. Azcatl et al.  doped MoS2 with nitrogen through a remote N2 plasma surface treatment. Nitrogen covalently bonded to MoS2 upon nitrogen plasma exposures and substituted chalcogen sulfur of MoS2 . The nitrogen doping converted MoS2 to p-type and changed electrical properties. The nitrogen concentration in the doped MoS2 was controlled through adjusting N2 plasma exposure time. XPS measurements indicated that binging energies of Mo-3d5/2 and S-2p3/2 decreased with increasing exposure time. 2 Mono-layer MoS2 heterojunctions, such as intrinsic GaN/p-type MoS2 heterojunction, were also irradiated by N2 plasma under UHV conditions . The values of VBM were reduced to 0.5 eV for −1 MoS2 layers. −1 N2∗ irradiated We have carried out ﬁrst-principles DFT calculations to establish the microscopic origin of the experimentally observed Raman peak shifts as a function of the electron irradiation dose. Monolayer, pristine MoS belongs to space group P6m2 (No. 187), and its two characteristic Raman active modes are E′ (∼384 cm ) and A′1 (∼403 cm ), as discussed previously. 17−19 Creating monosulfur vacancies reduces the symmetry of the system, and the symmetry assignments of Raman modes change as well. For simplicity and consistency, modes in the opposite direction has been predicted to be caused by p-type doping in ML-MoS2.26 As pristine MLMoS2 prone to have S vacancies, hence the incorporation of N atoms is favorable during N2 -irradiation.26,27 Therefore, enhancement of compressive strain with increasing25 ofN53 Appl. Sci. 2019, 9, 678 vacancies a lenging esp Using our reduction i carriers. Fu Figure 22. Raman spectraof pristine and N2∗ -irradiated MoS2 mono-layers . Reprinted from Reference , with the permission of AIP Publishing. 2.7.2. Active Oxygen O2∗ ARTICLE Oxygen plasma species include ionized oxygen atoms O+ , excited oxygen atoms O∗ , ionized oxygen molecules O2+ , metastable excited oxygen molecules O2∗ , ozone O3 , ionized ozone O3+ , excited ozone O3∗ , and free electrons. Therefore, oxygen plasma can effectively clean and etch MoS2 2D materials, introducing defects and doping oxygen to MoS2 2D layers. Nan et al.  treated MoS2 layers under oxygen plasma irradiation with 13.56 MHz and 5 W under 5 Pa. As shown in Figure 23 and the inset, the PL intensity increased with increasing plasma irradiation time. The PL enhancement could be increased as high as 100 times over. Considered the unchanged Raman scattering and XPS information, it was concluded that oxygen plasma introduced defects and oxygen bonding in MoS2 , enhancing PL intensity. PL Intensity 10K 15K 10K 5K 0 ation and charge density diﬀerysisorbed on perfect monolayer Figure 23. PL spectra of mono-layer MoS2 after oxygen plasma irradiation with different Figure 5. PL spectra of monolayer MoS2 after oxygen d on defective monolayer MoS2. The durations change of PL intensities with the plasma irradiation time is of shown in the inset. plasma irradiation with diﬀerent durations. The change vacancy (b,d). The positive and c 2014isAmerican Reprinted with permission from Reference .irradiation Copyright times Society. PL intensities with plasma shown inChemical the wn in red and blue, respectively. 4 2 3 inset. (c) et and 0 and 1 10 e/Å for Chen al.  treated MoS multi-layers under oxygen plasma in a reactive ion etcher. Initial 2 n-type MoS2 layers were selected-area doped to p-type, forming p-n junctions in MoS2 . The fabricated monosulfur vacancies are the ructures,21 and therefore our a monosulfur vacancy on the ures of the O2 molecule adsupercell of monolayer MoS2 be a promising method for manipulating the optical properties of MoS2. Here we adopt mild oxygen plasma (13.56 MHz, 5 W, 5 Pa) irradiation to controllably introduce defects in MoS2. It has been reported that plasma irradiation can easily introduce S vacancies in MoS2.23,24 At the same time, the oxygen ions are Nano Letters Appl. Sci. 2019, 9, 678 Nano Letters 26 of 53 highly rectifying diodes exhibited high forward/reverse current ratios and a superior long-term stability at ambient conditions. Kang et al.  treated MoS2 mono-layers with oxygen plasma. The photoluminescence evolved Nano Letters Nano Letters from a higher intense to completely quenched with increasing plasma exposure time because of a direct-to-indirect band-gap transition. The MoS2 lattice was distorted after oxygen bombardment and MoO3 disordered regions were generated in the MoS2 flakes. Islam et al.  deposited MoS2 mono-layers (mechanically exfoliated from crystals) on Si/SiO2 wafers and exposed them under oxygen plasma for several seconds. The plasma treatments were carried out at a power of 100 W operating at 50 kHz, using a gas mixture of oxygen (20%) and argon (80%) with a pressure of 250–350 mTorr. Raman spectroscopy and XPS spectroscopy indicated that MoS2 mono-layers were oxidized and MoO3 was produced in the reaction, 2MoS2 + 7O2 → 2MoO3 + 4SO2 . The resulting MoO3 -rich domains significantly decreased the mobility and conductivity of the fabricated MoS2 mono-layer devices. Ye et al.  exposed CVD-grown MoS2 mono-layers under oxygen plasma at a pressure of 10 Torr with oxygen gas. Figure 24 shows the morphologies of MoS2 exposed to oxygen plasma for 10 s, 20 s, and 30 s. After 10 s of oxygen plasma treatments, short and isolated cracks were observed on the continuous basal plane. Angles of those connected cracks were around 120◦ . After 20 s of oxygen plasma treatments, cracks became longer and were connected to each other, forming a continuous Figure 1. treated Morphology monolayer Figureof1. MoS Morphology synthesized monolayer by CVD method MoSmost on angles Si/SiObetween by CVD method and transferre on Si 2cracks 2 synthesized 2 substrate network. When for 30 of s, the widths the wereoffurther enlarged, ◦ monolayers. (B) Optical microscopy monolayers. image (B) of Optical monolayer microscopy triangles. i image of CVD grown triangular image MoS of CVD grown triangular MoS 2 2 was decomposed into even2 smaller fragments with the interconnected cracks were 120 , and the MoS a thickness ofintensity ∼0.7 nm with thicknessasofme ∼ cannumber be observed. (C) edges. AFM image canstructural beofobserved. CVDchange grown (C)significantly MoS AFM2 with image of CVDthe grown MoS 2 (inset a greater of exposed The decreased of A1g a proﬁle), 1 shows its perfect hexagonal shows lattice its with perfect 2H structure. hexagonal (E) lattice Pho w HAADF image of monolayer HAADF MoS image of monolayer MoS 2 2 shorter wave-numbers and E2g Raman modes of the MoS2 mono-layers, and shifted the A1g mode to were test. transferred to thetheremovable were transferred glassy carbon to the electrode removable for fur g electrochemical test.wave-numbers. MoS2 monolayers electrochemical MoS 2 monolayers and the E12g mode to longer The exposure also decreased PL intensity. The changes thePL glassy carbon electrode, showing the glassy thecarbon high electrode, showing well maintained the high coverage morphology and well aftermtr MoSthat 2 on 2 onoxygen of the MoS Raman and spectra indicated plasma can coverage lower theand MoS 2 crystal symmetry −1 −1 and (H) PL intensity map at and 680 (H) nm PL of intensity the MoS map triangle at 680 in inset nm of of the (G) MoS show triangle high quality in inset of ot cm cm and increase the lattice distortion, which can be attributed to the 2defects that may benefit MoS22 as electrode.catalyst. electrode. electrochemical Figure 1. Morphology of monolayer Figure 1. MoS Morphology of monolayer by CVD method MoS2 synthesized on Si/SiO2 substrate by CVD method and transferre on Si 2 synthesized monolayers. (B) Optical microscopy monolayers. image (B) of Optical monolayer microscopy triangles. i image of CVD grown triangular image MoS of2 CVD grown triangular MoS 2 of ∼0.7 nm2 (inset with a proﬁle), thicknessasofme ∼ can be observed. (C) AFM image can beofobserved. CVD grown (C)MoS AFM2 with imagea thickness of CVD grown MoS shows of its monolayer perfect hexagonal lattice itswith perfect 2H structure. hexagonal (E) lattice Phow HAADF image of monolayer HAADF MoS2 image MoS2 shows were test. transferred to the removable were transferred glassy carbon to the electrode removable for fur g electrochemical test. MoS2 monolayers electrochemical MoS2 monolayers the glassythecarbon high coverage electrode,and showing well maintained the high coverage morphology and well aftermtr MoS2 on the glassy carbon electrode, MoS2 on showing 680(H) nmPL of intensity the MoS2map triangle at 680 in inset nm ofofthe (G)MoS show triangle high quality in inset of ot cm−1 and (H) PL intensity map cm−1atand 2 electrode. electrode. Figure 24. SEM images of CVD-grown MoS2 mono-layers with oxygen plasma treatments for 0 s, 10 s, 20 s, and 30 s oxygen plasma exposure . Both the density and width of the cracks increased Figure 2. Morphology and structure Figure 2. characterization Morphology and of CVD structure grown characterization MoS2 with oxygen of CVD plasma grown treatment. MoS2 witS with exposure time. Reprinted with permission from Reference . Copyright c 2016 American with 0, 10, 20, and 30 s oxygen with plasma 0, 10, exposure, 20, and 30 where s oxygen the cracks plasma appear exposure, in the where basalthp monolayer MoS monolayer MoS 2 Chemical Society. 2 cracks increase with longer exposure cracks increase time. STEM with longer imagesexposure (E,F) show time. theSTEM fresh edges imagesinside (E,F)the show monolayer the fres these exposed edgesedges are found in MoS at2these , with exposed 120° angle. edges (G) in Raman MoS2, with and (H) 120 and S2 terminated edges are found and S2atterminated and shifted peaks after oxygen andplasma shiftedexposure. peaks after oxygen plasma exposure. monolayer. shows higher improvement shows thanhigher that treated improvement by O2 than plasma that treated by O2 Aberration-co plasma tronedges. microscopy (STEM) because of the higher density because of the of exposed the higher edges. density of the exposed characteristic exciton peaks of MoS2 monolayer whose energy separaFor the confocal PL and Raman spectroscopy measurements, a labtion is due to spin-orbit splitting at the top of the valence band at the made laser confocal microscope with a spectrometer was used. With a K point of the 1st Brillouin zone . The integrated PL intensity is uni0.9 NA objective, the focused spot diameter of the laser light was form across the entire area of the pristine MoS2 monolayer. Fig. 1(e) and approximately 300 nm. Scattered light was collected using the same (f) show the integrated absorption intensity mapping image and averobjective and guided to a 50 cm long monochromator equipped with aged absorption spectrum of the pristine MoS2 monolayer, respectively. a cooled CCD through an optical ﬁber with a 200 μm core diameter, The two peaks in the absorption spectrum, at 619 and 669 nm, correwhich acted as a confocal detection pinhole. The excitation lasers Appl. Sci. 2019, 9, 678 27 of 53 spond to B and A excitons, respectively, and are similar to the peak were the 514 nm laser line of an Ar gas laser, with a typical laser positions observed in the PL spectrum (Fig. 1(d)). The large peak at power applied to the sample of 100 μW and an acquisition time of ~460 nm may be understood as a broadening of the high-energy exciton 500 ms per pixel. Confocal absorption spectral imaging was conducted peaks due to phonon–electron coupling . using the same system with a tungsten-halogen lamp as the light Dhall et al.  treated MoS few-layers (15 layers with approximately 12 nm thickness) in 2 Fig. 2(a) shows optical microscope images of the pristine MoS2 source. The experimental details of the confocal absorption spectral immonolayer of MoS2 monolayers with oxygen plasma for aging are explained in the information previous oxygen plasma forsupplementary 3 min. The plasmaof awas generated by and flowing air past treated an electrode supplied times ranging from 10 to 120 s. All of the MoS2 monolayers showed simpaper . with 20 W of RF power at 200 mTorr. The PL efficiency of the few-layers was increased after the ilar morphologies. Fig. 2(b) shows integrated PL intensity mapping images obtained from each sample. The integrated PL intensity was found 3. Results and discussion plasma treatments. to be decreased with oxygen plasma treatment. Fig. 2(c) shows the peak Kim et al.  grew triangular MoS mono-layers on SiO /Si substrates by CVD technique. The position of the 2PL corresponding to A excitons, which indicate that the Fig. 1(a) shows an optical microscope image of a pristine2triangular peak position of the PL corresponding A excitons was gradually on a SiO2/Siwere substrate. Highly crystalline MoS monoMoS2 monolayer MoS then subjected to 2plasma-oxygen treatment for timesto ranging from 10 sredto 2 mono-layers layers typically grow with a triangular shape due to the hexagonal latshifted with increasing oxygen plasma treatment duration. 120 s. Ultra-high pure oxygen gas (99.9999%) was activated by a RF plasma cell, and the working Fig. 3(a) shows the PL spectra of the MoS2 monolayers subjected to tice structure of such crystals . Confocal Raman spectroscopy was 3 Pa [11,12,19,24] (Fig. 1(b)). pure oxygen plasmagas treatment durations. The PL spectra were used to identify the1.3 number of− MoS 2 layers pressure was × 10 under an 1ultra-high oxygen flowforofdifferent 2 sccm. Optical properties averaged from the inside region of each sample. The intensities of the Consistent peak positions and a shift in the in-plane (E 2g) and out-ofof the such as in PL and scattering, changed to with time.decreased Figure 25a ) Raman modes of the vibrations each MoS2Raman were observed plane (A1gmono-layers, peaks corresponding A andtreatment B excitons gradually with increasingand oxygen plasmaMoS treatment duration, possiblytreated due to increased A1g peaks in [12,24]. The wavenumber difference the E12g and shows optical images of between the pristine MoS mono-layer these mono-layers with 2 2 the Raman spectrum has been widely used to identify the number of lattice distortion and defect density as a result of oxygen plasma bomoxygen plasma 10 s,the30value s, 60 s and 120 s. Figure shows integrated mapping images bardment . PL emissionsPL are intensity caused by radiative recombination layersfor because of this difference varies as a 25b MoS2 crystalline radiative recombination function of the number of layers. For the ﬁlm in Fig. 1(a), the peak posiupon photoexcitation. In MoS2 monolayer, obtained from each of the mono-layers. The integrated PL intensity decreased with the oxygen plasma dominates as it only requires one recombination step for the electron tion of A1g is 404.8 cm−1 and the difference from the E12g peak is aptreatment time. Figure 25c showswith thea MoS peak position oftothe mainback PLtopeak. The main position of the , which is consistent proximately 18.4 cm−1 transition its equilibrium state. peak Therefore, the observed PL 2 monolayer. quenching upon oxygen plasma treatment suggests that the recombinaFig. 1(c) and (d) show the integrated PL intensity mapping image and PL spectra gradually red-shifted from 674 nm to 692 nm with an increasing oxygen plasma treatment averaged PL spectrum of the pristine MoS2 monolayer, respectively. tion process in the pristine MoS2 was changed by the bombardment of duration. It was suggested shallow defect states were generated by distortion the oxygen plasma treatments. oxygen, generating lattice and defects. Previous studies of The PL spectrum exhibited peaks at 624that and 674 nm, corresponding to Fig. 2. (a) Optical microscope images of a pristine MoS2 monolayer and monolayers subjected to oxygen plasma treatment for different durations. (b) Confocal PL spectral mapping images, indicating integrated PL intensity, of pristine and treated MoS2 monolayers. (c) Confocal PL spectral mapping images, which show the position of the peak corresponding to A excitons, of pristine and treated MoS2 monolayers. The numbers represent the wavelength. 2 Figure 25. (a) Optical images, (b) confocal PL spectral intensity mapping images, and (c) spectral peak mapping images of pristine and treated MoS mono-layers . The color represents PL intensity in (b) and wavelength of PL peak in c. Reprinted from Reference . Copyright c 2015, with Please cite this article as: M.S. Kim, et al., Photoluminescence wavelength variation of monolayer MoS2 by oxygen plasma treatment, Thin Solid from Elsevier. Filmspermission (2015), http://dx.doi.org/10.1016/j.tsf.2015.06.024 2.7.3. Active Hydrogen H2∗ Ye et al.  exposed CVD-grown MoS2 mono-layers in hydrogen plasma at different temperatures (400–700 ◦ C), as shown in Figure 26. The MoS2 mono-layer were treated to expose more active sites in the basal plane of MoS2 and to improve catalytic activity. No significant changes were observed on the MoS2 when the annealing temperature was 400 ◦ C. The MoS2 was then H2 -treated at 500 ◦ C and small triangular holes with sizes around 1–4 µm appeared. High-density holes with sizes around 10–20 nm were omnipresent in the basal plane of MoS2 . High-density triangular-shaped holes became the prominent part in the original mono-layer MoS2 at 600 ◦ C. 700 ◦ C H2 -treatments led to severe MoS2 decomposition. The hydrogen plasma treatments decreased the peak intensities of Raman scattering and PL spectra, indicating that hydrogen treatments could increase the defects and edges in mono-layer MoS2 . Appl. Sci. 2019, 9, 678 Nano Letters 28 of 53 Nano Letters Nano Letters Nano Letters Letter Letter Figure 26. SEM images of mono-layer MoS2 with 400 ◦ C, 500 ◦ C, 600 ◦ C, and 700 ◦ C H2 annealing, respectively, showing the appearance of small triangle holes . Reprinted with permission from Reference . Copyright c 2016 American Chemical Society. 2.7.4. Other Molecules Fluoride Chen et al.  treated MoS2 multi-layers under fluoride plasma (SF6 , CHF3 , and CF4 ) for Figure 4. p-doping Structure characterization Figure ofp-n CVD 4. Structure grown2MoS characterization Figure of CVD 4.treatment Structure grown atMoS characterization diﬀerent hydrogen Figure of CVD 4.treatment Structure (A−D) grown atMoS SEM characterization diﬀerent with temperatures. hydrogen of monolayer of CVD treatment (A−D) grown atMoS SEM diﬀerent with temperatures. hydrogen of monolay trea 2 with hydrogen 2 with temperatures. 2 images 2 images selected-area to form MoS junctions and diodes. °C2 H with 400, 500,respectively, 600, and 700 showing °C2 H with annealing, 400, appearance 500,respectively, 600,ofand small 700 showing triangle °C2 H with the annealing, 400, appearance STEM 500,respectively, 600, images ofand small (E,F) 700 showing triangle °C show H2holes. highthe annealing, appearance STEMrespectively, images of small (E,F) showing triangle show hol hig the MoS2 with 400, 500, 600, and 700 MoS MoS MoS 2 annealing, 2 the 2holes. with abundant exposed edges and with step-edges. abundant (G) exposed Raman edges and (H) and PL with step-edges. spectra abundant show (G)exposed Raman the edges and (H) andPL with step-edges. spectra abundant show (G)exp R t density nanometer-scale holes formed density inside nanometer-scale the MoS2 layers holes formed density inside nanometer-scale the MoS holes formed density inside nanometer-scale the MoS holes formed inside the MoS 2 layers 2 layers 2 layers decreased intensity after hydrogendecreased annealingintensity caused after by thehydrogen defectsdecreased as annealing revealed intensity caused by SEM after by and thehydrogen STEM. defectsdecreased as annealing revealed intensity caused by SEM after by and thehydrogen STEM. defects as annealing revealedcaused by SEM by and the STEM. defects as revealed Chlorine Murray et al.  irradiated exfoliated MoS2 few-layers (5–15 layers) under low-energy Cl (200 eV and 1 × 1013 –1015 ions/cm2 ). The natural n-type MoS2 layers were doped to p-type and the electric conductivity was reduced after irradiation. Phosphorus Nipane et al.  reported a compatible, controllable, and area selective phosphorus plasma immersion ion implantation process for p-type doping of mechanically exfoliated MoS2 layers using PH3 –He plasma. Homogeneous p-n MoS2 junction diodes were fabricated. Figure 5. HER property characterization Figure 5.ofHER CVDproperty grown MoS characterization Figure hydrogen 5.ofHER CVD annealing property grownatMoS diﬀerent characterization Figure temperatures. hydrogen 5.ofHER CVD annealing (A) property grown LSVatMoS ﬁgure diﬀerent characterization shows temperatures. hydrogen that of MoS CVD annealing grown LSVatMoS ﬁgure diﬀerent shows temperature hydrogen that Mo 2 with 2 with 2 with 2 (A) 2 with has better 500 °C catalytic hydrogen activity annealing than under 400 has and better 500600 °C catalytic °C. hydrogen (B)activity Tafel annealing plots than under show 400 has and better hydrogen-annealed 500600 °C catalytic °C. hydrogen (B)activity Tafel annealing MoS plots than show 400 has lower and better hydrogen-annealed Tafel 600catalytic °C. (B)activity Tafel MoS plots than show 400 lower and hydro Ta 60 2 has 2 has Galliumunder 500 °C hydrogen annealingunder revealing improvedanyelectrochemical treatment, revealing activity improved of MoSany astreatment, a slope HER catalyst revealing activity after improved of hydrogen MoSany asannealing. treatment, a HER catalyst revealing activity afterimproved of hydrogen MoS2 electrochemical asannealing. a HER catalya slope than MoS2 without any treatment, slope than MoS2 without slope than MoS than MoS 2 without 2 electrochemical 2 without 2 electrochemical Mono-layer MoS such as intrinsic GaN/p-type were also 2 heterojunctions, 24 heterojunction, density density became the prominent part density inbecame triangular-shaped the prominent partd about 4 orders of magnitude about less than 4 orders the amount of magnitude used about in less than 4 orders the amount oftriangular-shaped magnitude usedMoS about in less holes than orders the amount oftriangular-shaped magnitude used in less holes than the amount used in holes 24−26 24−26 24−26 24−26 the original monolayer MoS the original monolayer MoS original monolayer literatures. literatures. literatures. literatures. ﬁlm, while 700 °C led to more ﬁlm, while 700 °C led MoS to mo ﬁ irradiated by Ga plasma under UHV conditions . The values of VBM were reduced to 0.2 eV for 2 2 the 2 th 40 In addition to oxygen plasma In exposure, additionhydrogen to oxygenannealing plasma In exposure, addition hydrogen to sample oxygenannealing plasma In exposure, addition hydrogen tomass oxygen annealing plasma exposure, hydrogen annealing severe decomposition severe and sample loss decomposition (Figure 4D).severe and mass sample loss decomposition (Figure 4D).s Ga-irradiated 2 layers. is also anMoS eﬀective method isforalsodefect an eﬀective engineering method of is 2D foralsodefect an eﬀective engineering method of is 2D for defect an eﬀective engineering method 2D for defect 2D This can be explained byalso the decomposition This can be explained of of MoS when the decomposition the Thisengineering can be explained of of MoS when the de th T 2by 2by 43 materials. 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Here, similar usage. strategy was Here, similar strategy was Here, similar strategy was Here, the strategy was the microscale images observed the by microscale SEM, STEM images was observed further the similar by microscale SEM, STEM images was observe furthth Jadwiszczak et al.  exposed mechanically exfoliated MoS few-layers toimage ansites O –Ar plasma used to etch MoS2 in order toused to etch used to etch used to etch MoS expose moreMoS active orderintothe expose moreMoS order tothe expose more order tothe exposelevel. more sites the used toactive image the2inmorphology used change toactive the22in morphology used change toactive image of MoS the2 in morphology u at atomic at atomic leve 2 in sites 2 in sites 2ofinMoS 43 43 43 43 (O2 :Ar basal = 1:3) 2–28 The plane frequency oftothe was 13.52 oxide phase was planefor of MoS basal of MoS2 activity. basal plane of MoS basal plane ofAn MoS its catalytic and improve A plasma its catalytic and toSTEM-HAADF improve AMHz. its catalytic and toSTEM-HAADF improve A planeits A planeF Figure 4E,F2 activity. shows Figure images 4E,F shows of the basal Figure ofcatalytic images 4E,Factivity. shows of the STEM-HAA basal 2 ands.to improve 2 activity. series of SEM images of H2series of CVD SEM grown images MoS of H2 2series SEM2 grown images MoS of H2 2series of CVD SEM images MoS of H are etchedof CVD are etched grown etched CVD grown MoS MoS sample annealed at 500 °C. 2High-density sample annealed holes atwith 500size °C. 2High-density sample MoS annealed holes with 500siM ° 2 2are 2 areat generated under the plasmaetched exposure, changing thein MoS electrical conductivity and carrier mobility shown in Figure 4A−D. Whenshown the annealing in Figuretemperature 4A−D. When was shown the annealing Figuretemperature 4A−D. shown theomnipresent annealing in around Figuretemperature 4A−D. When theomnipresent annealing around 10−20 When nm was are 10−20 in the basal nm was are plane. This around temperature 10−20 in the basal nm was are plane. omnip Th a of the 2D 400materials. °C, no signiﬁcant changes 400were °C, observed no signiﬁcant on the changes MoS 400 °C, observed no signiﬁcant on the changes MoS 400 °C, observed no signiﬁcant oneﬀective the changes MoS observed oneﬀective the MoS indicates that hydrogen annealing indicates is anthat hydrogen way annealing to form indicates is anthat hydrogen way to forin 2 were 2 were 2 were 2 anneal triangles (Figure 4A). As the temperature triangles (Figure increased 4A). As to 500 the temperature °C, triangles (Figure increased 4A). As to 500 the temperature °C, triangles (Figure increased 4A). As toits500 the temperature °C, increased toits500 °C, edge enriched MoS edge enriched MoS edge enriched MoS e beneﬁt electrochemical beneﬁt electrochemic ∗ plasma 2 that would 2 that would 2 that wo Mishra et al.  irradiated MoS under N for 1–3 min and then under Ga flux. The VBM 2 with performance. Similar totriangles the performance. oxygen Similar treatment, to the the performance. oxygen plasma Similar treatment, p the triangles start to be etchedtheand triangles small 2triangular start to beholes etched theand triangles small triangular start to beholes etched with theand small triangular start toplasma beholes etched withand small triangular holes withto the th decrease ofarethe peakThese intensity decrease in 1−4 Raman ofare and the PL peak spectra intensity isphenomena also decrease in Ramanofare and the PL peak spectra intensity is aldi size around 1−4 μm appear. These size around phenomena 1−4 μm areappear. analogous These size around phenomena 1−4 μm appear. analogous size around phenomena μm appear. analogous These analogous was changed. observed here, which observed support here, thewhich conclusion further that observed support here,thewhich conclusion th o to the morphology change ofto graphene the morphology when graphene change of was to graphene the morphology when graphene change of was tocan graphene thefurther morphology when graphene change of wascan graphene when graphene wascan furth Bhimanapati et al.  treated MoSAr/H layers in UV–ozone. The ozone treatment increased 2 vertical annealing could the annealing defects could edges increase hydrogen in the annealing defects could edges inch treated by hydrogen annealingtreated with diﬀerent by hydrogen annealing treated with diﬀerent byhydrogen hydrogen Ar/H annealing treated with increase diﬀerent byhydrogen hydrogen Ar/H annealing with diﬀerent Ar/H rate rate 2 ﬂow rate 2 ﬂow rate 2 ﬂow and 2 ﬂow and monolayer MoS monolayer MoS monolayer MoS m ration. When temperature further ration.increased When temperature to 600 °C, further highration.increased When temperature to 600 °C, highration. increased When temperature to by 600 °C, high- increased to 600 °C, super wettability and enhanced hydrogen evolution reaction of2. further the material changing edge 2. further 2. high- chemistry and surface defects. 1100 1100 DOI: 10.1021/acs.nanolett.5b04331 1100 Nano Lett. 2016, 16, 1097−1103 Nguyen et al.  treated MoS2 layers in UV-ozone and then fabricated the layers into organic photovoltaic cells. The open-circuit voltage, fill factor, and power conversion efficiency increased significantly compared with un-irradiated MoS2 -based solar cells. 3. Electromagnetic Irradiation Electromagnetic irradiation refers to the waves of the electromagnetic field, including gamma rays, X-rays, ultraviolet, (visible) light, infrared, microwaves, and radio waves. The frequency and wavelength of the electromagnetic irradiation is shown in Figure 3. DOI: 10.1021/acs.nanolett.5b043 1100 Nano Lett. 2016, 16, 1097−11 Appl. Sci. 2019, 9, 678 29 of 53 3.1. Gamma-ray Irradiation It is generally accepted that MoS2 macroscopic materials are stiff under gamma-ray irradiation. There is little literature on γ-irradiated bulks. It was reported that MoS2 powders were resistant to a γ-ray irradiation with a dose of 5 × 109 R (1.29 × 106 C/kg) . Lee et al.  mechanically exfoliated MoS2 2D layers (50–132 layers) and exposed them to 5000 γ-ray photons with 662 keV. It was found that the resonance frequency of the layers upshifted immediately after γ-ray exposure and returned to their initial frequency after 60 h. The procedure was repeatable. It was assumed that γ-ray photons generated charges on MoS2 and caused electrostatic forces between MoS2 and substrates, resulting in electrostatic tension and deflection of MoS2 layers. The MoS2 multi-layers were stiff under the γ-ray and showed no irradiation damage. Ozden et al.  prepared MoS2 multi-layers (5–8 layers) through the vapor phase sulfurization and exposed the layers to a γ-ray irradiation of 60 27 Co source (γ-ray energy: 1.1732 MeV and 1.3324 MeV) with a dose of 120 Mrad. The irradiation was carried out at room temperature in an ambient atmosphere. The X-ray photoelectron spectra indicated that MoS2 layers were converted to molybdenum oxide (MoOx ) after the irradiation. It is plausible that the γ-ray displaced or knocked out S atoms while leaving molybdenum atoms unaffected. Maybe the as-produced sulfur vacancies were filled with oxygen atoms to form MoOx . 3.2. X-ray Irradiation There are few reports on X-ray irradiation of MoS2 . Only one paper  reported that MoS2 mono-layers were irradiated with 10 keV X-rays with varying total ionizing doses. It was reported that the MoS2 mono-layers were robust to X-ray radiation, withstanding doses of up to 6 Mrad doses without any noticeable degradation of optical properties. It is generally accepted that X-rays do not affect MoS2 few-layers. Zhang et al.  fabricated single-layer MoS2 FETs and irradiated them under 10 keV X-ray exposure. At room temperature in air, the drain current of the devices decreased significantly under X-ray irradiation up to 10 Mrad. Effective threshold voltage and mobility were degraded with X-ray irradiation dose. It stated that the degradation was consistent with the generation of negatively charged surface states during the X-ray exposure. 3.3. Ultraviolet Light Irradiation Azcatl et al.  treated MoS2 crystals under ultraviolet–ozone exposures at room temperature. Oxygen-sulfur bonds were formed at the top sulfur layer of the MoS2 surface without breaking sulfur-molybdenum bonds. Li et al.  investigated optical behaviors of azobenzene-functionalized MoS2 mono-layers and exfoliated multi-layers on Au substrates. It was reported that UV light could tune doping and the Fermi level of the hybrid structures. Lu et al.  irradiated liquid-exfoliated MoS2 nanosheets in aqueous solutions and found that the MoS2 layers were oxidatively etched because of photon-induced powerful OH∗ radicals. Singh et al.  illustrated multilayer MoS2 FETs under ultraviolet light in N2 atmospheres. The multi-layers were micromechanically exfoliated from natural MoS2 crystals and deposited on SiO2 coated silicon. The wavelength of the UV light was 220 nm and the average intensity was 10 mW/cm2 . Nitrogen gas alone did not affect the electrical properties of MoS2 nanosheets, but nitrogen gas in the presence of UV light remarkably affected the electrical properties of MoS2 nanosheets. Charge-carrier mobility, carrier density, and drain current were enhanced after exposure to nitrogen gas under UV light irradiation because of a possible doping effect. Detailed investigations  showed that the charge-carrier density of single-layer, bi-layer, and few-layer MoS2 nanosheets were reversibly tuned with nitrogen and oxygen gas in the presence of ultraviolet light. The device performance was adjusted by exposure to gases in the presence of UV light. Appl. Sci. 2019, 9, 678 30 of 53 McMorrow et al.  fabricated single-layer and multilayer MoS2 (mechanically exfoliated) on Si substrates into FETs. It was found that the electron mobility increased with UV exposure up to 3.4 ×1010 –2.2 ×1013 photos/cm2 in a vacuum. Raman spectroscopy showed no significant crystalline radiation damage or oxidation degradation under UV exposure. Irradiation-hard MoS2 FET devices are expected. 3.4. Visible Light Irradiation Liu et al.  tested photocatalytic performance of N-doped MoS2 nanoflowers under visible light. It was reported that the N-doped MoS2 nanoflowers showed excellent photocatalytic activities and durability on the elimination of the organic pollutants under visible light irradiation. Laser has been widely employed in Raman scattering of MoS2 few-layers. Laser has also been used to thin multilayered MoS2 down to a single-layer two-dimensional crystal [141,142] (power density up to 80–140 mW/µm2 ) and generate ripples of MoS2 [142,143]. It is reported that the upper atoms of MoS2 layers can be removed by high-power lasers because of laser ablation [112,141]. Early investigation indicated that natural MoS2 crystals do not undergo any kind of significant laser-assisted oxidation when exposed to high laser power (up to 32,000 W/cm2 ) (wavelength: 632.8 nm)  while their Raman mode intensity changes slightly with laser power. For microcrystalline MoS2 powders, Raman scattering indicated that MoS2 oxidized to MoO2 under a high-power laser. Raman mode intensity and position were also significantly affected by the laser power. Paradisanos et al.  mechanically exfoliated MoS2 few-layers from a bulk natural crystal and subsequently deposited them on Si/SiO2 wafers, irradiated the few-layers under a pulsed laser with 800 nm wavelength and 1 kHz repetition rate. No modification of MoS2 mono-layers was observed up to a certain single-pulse fluence of 50 mJ/cm2 (2.5 mW) while damage occurred beyond the fluence via the material ablation. Further work indicated that MoS2 mono-layers were practically unaffected by a low-power (600 µW) pulsed laser irradiation when exposed to 103 pulses at a fluence of 20 mJ/cm2 (lower than the damage threshold). However, the MoS2 mono-layers were damaged upon 105 pulses at 20 mJ/cm2 . The A1g and E2g intensities were almost constant with pulse times, then rapidly decreased at a critical exposure time. The abrupt decrease possibly came from ablation and eventual sublimation of the MoS2 atoms. Gu et al.  in situ studied Raman scattering of MoS2 layers during the laser thinning of MoS2 . Due to the high surface-area-to-volume ratio, thinner MoS2 layers are less stable and easier to decompose under high-power laser irradiation because of laser-induced thermal effects and sublimation. Tran Khac et al.  irradiated MoS2 layers under laser power of 1 mW, 5 mW, and 10 mW for an exposure time of 60 s. No significant changes were observed in topographic images of the irradiated regions under the 1 mW laser. However, significant amounts of particles or adsorbates were formed on the MoS2 surface after irradiated under 5 mW and 10 mW lasers. To minimize thermal effects on atomically thin MoS2 during Raman spectral measurements, low laser powers (0.14 mW to 2 mW) are usually employed  to avoid potential laser-induced local surface temperatures. Lu et al.  employed a focused laser beam to directly pattern MoS2 mono-layers and few-layers. Focused laser beam irradiation modified and thinned MoS2 layers to create well-defined structures and controllable thickness. Alrasheed et al.  used a 532 nm laser to irradiate mechanically exfoliated MoS2 few-layers (less than 5 layers) on SiO2 /Si substrates. The power ranged between 0.93 and 8.3 mW and the treatment time was 0.01–180 s. In ambient conditions, MoS2 nanosheets were etched and amorphous MoS2 redeposited on the nanosheets at low laser powers while the few-layers were oxidized and MoS2 nanoparticles formed at high laser powers. The nanoparticle formation and oxidation were dependent Appl. Sci. 2019, 9, 678 31 of 53 on the number of layers and laser exposure time. The Raman intensity and Raman peak width changed with laser treatments, as shown in Figure 27. ACS Applied Materials & Interfaces Research Article Figure 27. Raman intensity and linewidth (FWHM) of (a) mono-layer-ed (1L), (b) tri-layer-ed (3L), and (c) 5-layer-ed (5L) MoS2 nanosheets laser-treated in ambient conditions. Reprinted with permission from Reference . Copyright c 2018 American Chemical Society. 3.5. Infrared Light Irradiation Fan et al.  irradiated lithium intercalated 1T MoS2 layers with thickness of a few microns under Figure 5.a Raman intensity, linewidth shift of (a) 1L, (b) 3L, and (c)MoS 5L MoS in converted ambient conditions. 2 nanosheets near-IR laser (780 nm)(FWHM), in air atand ambient temperature. The were laser-treated completely to 2 layers 2H counterpart under the irradiation. 2H MoS2 multi-layers were grown through pulsed mid-infrared laser deposition  (wavelength of 7.0–8.2 µm). No any IR damage was reported. 3.6. Terahertz Wave Irradiation MoS2 multi-layers were fabricated into terahertz modulators  applied in high-speed communications. Terahertz conductivities of MoS2 few-layers were experimentally measured [149,150] from THz spectroscopy. However, there was no report on terahertz damage on MoS2 . Theoretical simulations indicated that the THz absorption of mono-layer MoS2 was very low and the maximum THz absorption of mono-layer MoS2 was approximately 5% . The sum of reflection and absorption losses of mono-layer MoS2 was lower than that of graphene by one to three orders of magnitude. Figure 6. AFM images and Raman measurements on 1L MoS2 nanosheets laser-treated in vacuum. (a) Optical image and (b) AFM image of laser3.7. Microwave Irradiation treated MoS 2 nanosheet. Higher-magniﬁcation AFM images of each treatment locations for (c) 1 min and (d) 30 s laser treatments. (e) Raman spectra before and after the 30 s laser treatment. Few-layer MoS2 were fabricated into two-dimensional nanoelectromechanical systems (NEMS) as ultralow-power, high-frequency tunable oscillators and ultrasensitive resonant transducers . These devices can operate in the very high frequency band (up to ∼120 MHz). Ultra-thin MoS2 was also fabricated to MoS2 transistors operating at gigahertz frequencies [153,154]. All these devices worked at microwave range. Therefore, it is necessary to characterize the microwave damages to MoS2 few-layers. It was reported that microwave can accelerate catalytic reactions  because of the formation of hot-spots within catalysts. However, no significant differences were detected between microwave irradiated MoS2 catalysts (size in microns) and untreated MoS2 during the sulfating reaction . Zhao et al.  irradiated MoS2 powders under a microwave (1 kW power, 2450 MHz) at 200 ◦ C. It was reported that the (001) basal planes of the MoS2 crystal structure were cracked into (100) edge planes under microwave irradiation, creating additional active edge sites. The Raman intensity of microwave irradiated materials increased significantly after microwave treatments. The irradiated 0 efficiently because of the microwave-induced cracks. couldand capture Figure 7.MoS AFM2 images Raman Hg measurements on 4L MoS nanosheets laser-treated in vacuum. (a) Optical image and (b) AFM image of laser2 treated MoS2 nanosheet. Higher-magniﬁcation AFM images of each treatment locations for (c) 30 s and (d) 2 min laser treatments. (e) Raman spectra before and after the 2 min laser treatment. instant thinning for laser powers of 4.7 and 8.3 mW regardless of the treatment time. However, for 0.93 mW, we see the formation of anomalous particles around the treatment sites. In fact, we observe nanoparticles forming around all the treatment 18108 DOI: 10.1021/acsami.8b04717 ACS Appl. Mater. Interfaces 2018, 10, 18104−18112 Appl. Sci. 2019, 9, 678 32 of 53 Xu et al.  treated chemically exfoliated MoS2 nanosheets in an inert nonpolar solvent of 1,2-dichlorobenzene under 2.45 GHz microwave irradiation at 130 ◦ C in an inert atmosphere, with an output power of 500 W. 2H MoS2 nanosheets converted to the 1T phase in minutes. 4. Other Irradiation 4.1. Ultrasonic Wave Irradiation MoS2 nanosheets have been prepared by ultrasonication through liquid exfoliation technique [19,41,46,71]. The physical properties were affected by ultrasonication conditions. For example, Gao et al.  sonicated MoS2 powders in N,N-dimethylformamide for 2–10 h to chemically exfoliate MoS2 nanosheets. Crystalline MoS2 few-layers were produced after 2 h sonication and the size of the nanosheets decreased gradually with increasing sonication time. The exfoliated MoS2 nanosheets showed ferromagnetic properties at room temperature, in contrast to pristine MoS2 bulks which showed diamagnetism only. The saturation magnetizations of the ultrasonicated nanosheets increased with the ultrasonication time and the decreasing crystalline size. MoS2 nanomaterials were also prepared by ultrasonicating slurrys of molybdenum hexacarbonyl and sulfur in 1,2,3,5-tetramethylbenzene (isodurene) with a high-intensity ultrasound (20 kHz) under Ar atmosphere . The nanostructured MoS2 had a higher surface area and showed higher catalytic activities for thiophene hydrodesulfurization. 4.2. Thermal Irradiation MoS2 bulks sublimate at 450 ◦ C and melt at 2375 ◦ C. MoS2 crystals are not stable in the presence of oxygen, resulting in MoO3 . It was reported that the material oxidized at 315–375 ◦ C in air and converts to MoO3 at 400 ◦ C . MoS2 mono-layers are not very stable at ambient conditions either. Peto et al.  examined atomic-resolution STM images of the basal plane of mechanically exfoliated MoS2 mono-layers after one month and one year of ambient exposure (in air, at room temperature and ambient light). STM measurements clearly revealed modifications in the atomic structure of the MoS2 basal plane during the ambient exposure. New point-defects were formed after one month of the ambient exposure and the defect concentration increased 20–30 times. The point-defects increased 100–500 times after one year of exposure. The oxidation of MoS2 basal planes yielded 2D MoS2− x Ox layers. Chen et al.  annealed the liquid-exfoliated MoS2 mono-layers on various substrates (SiO2 , Au, graphene, BN, and CeO2 ) in a vacuum. Thermal annealing did not introduce any noticeable defects into MoS2 layers up to 450 ◦ C and MoS2 remained stable, as shown in Figure 11. Other group reported that thermal annealing in a vacuum caused S vacancies in MoS2 at 500 ◦ C . Donarelli et al.  annealed MoS2 crystals in UHV. Single sulfur vacancies were generated in bulks while the S/Mo ratio in the bulk did not change with the annealing temperature up to 400 ◦ C. They also examined the liquid-exfoliated MoS2 few-layers. The S/Mo ratio of MoS2 few-layers gradually decreased once the annealing temperature increased up to 400 ◦ C. Two kinds of sulfur vacancies (single and double) were introduced in the molybdenite layers. The threshold for the formation of double vacancies typically occurred upon thermal annealing at 200 ◦ C. Nan et al.  mechanically exfoliated MoS2 mono-layers from bulk crystals and transferred them to Si wafers with SiO2 capping layer. The mono-layers were then annealed at temperature of 350 ◦ C or 500 ◦ C for 1 h in a vacuum (0.1 Pa). Figure 28a shows PL intensity image of an as-prepared MoS2 mono-layer, which was uniform across the whole sample. After being annealed for 1 h at 350 ◦ C in a vacuum, the PL intensity was enhanced by 6-fold (Figure 28b). At the same time, the PL peak blue-shifted after the annealing. Figures 28c-d show another MoS2 mono-layer before and after annealing at 500 ◦ C for 1 h in a vacuum. The PL image becomes highly inhomogeneous after the annealing. More detailed investigation indicated that the PL enhancement and inhomogeneity Appl. Sci. 2019, 9, 678 exposed to ambient air.physical At the same time, the and explained by the adsorption of PL O2peak and blue to ∼1.81 eV, as shown by the green curve H2O shifts molecules on MoS (p-type doping, with O 2 2 as 18 inthe Figure 1g. These phenomena been reported dominant contributor). Thehave as-prepared MoS2 is and explained by the physical adsorption of O2 and H2O molecules on MoS2 (p-type doping, with O2 as the dominant contributor).18 The as-prepared MoS2 is to 30 and 89after times, respectively) therespectively pumping original values. Furthermore, the and they are very stable after those locations drop slightly ∼30 min with a only power of ∼0.5 m respectively) after the pumping and they 33are very stable after of 53 ∼30 min with a power of ∼0.5 m should come from the reaction of oxygen (due to not very high vacuum condition) with MoS2 at high temperature and followed chemical bonding of oxygen molecules to MoS2 . (a) (b) (c) (d) Figure 28. PL intensity images of as-prepared (a,c) and annealed MoS2 mono-layers in a vacuum for ◦ CPL intensity images ofpermission monolayerfrom MoS2Reference : (a) as-prepared, (b) annealed c in vacuum for 1 h at 3 1 h at (b) 350 ◦ C and Figure (d) 5001. . Reprinted with . Copyright down to 0.1 Pa. The images have the same color bar; PL intensity images of another monolayer Mo 2014 American Chemical Society. annealed in vacuum for 1 h at 500 C, (f) after pumped down to 0.1 Pa. The images have the same color b from locations AD in the images; (h) change of normalized PL intensities (as compared to the origi Yamamoto et al.  mechanically exfoliated MoS2 single-layers and few-layers from MoS2 bulk BD throughout the images annealing and pumping process. Figure intensity of monolayer MoS 2: (a) as-prepared, (b) annealed in vacuum for 1 h at 3 crystals, deposited them onto 1. 300PLnm thick SiO 2 /Si substrates. The MoS2 layers were then exposed to down to 0.1 Pa. The images have the same color bar; PL intensity images of another monolayer Mo ◦ ◦ an Ar/O2 mixture at temperatures ranging The flow rates of Ar O2 were annealed in vacuum forfrom 1 h at27 500CC,to(f)400 afterC. pumped down to 0.1 Pa. and The images the ’ NAN ET AL. VOL. 8have NO.same 6 ’ color 5738b 1.0 L/min and 0.7 L/min, Raman measurements indicated that the oxygen fromrespectively. locations AD in thespectroscopy images; (h) change of normalized PL intensities (as compared to the origi BD throughout pumping process. treatments led to triangular etch pits onthe theannealing surfaces and of the atomically thin MoS2 . MoS2 was etched preferentially along the crystallographic directions of the zigzag edges with a preferential termination, resulting in uniform orientations with NAN ET AL. of the pits, as shown in Figure 29a–e. The pit size increased VOL. 8 ’ NO. 6 ’ 5738 increasing oxidation exposure (Figure 29f), and the growth rate was larger at higher temperature. However, the numbers of etch pits per unit area was uncorrelated with oxidation time, oxidation temperature, and MoS2 thickness but varied significantly from sample to sample. It was assumed that the oxidative etching in MoS2 layers on SiO2 was initiated at intrinsic defect sites. Additionally, oxygen exposure above 200 ◦ C significantly diminished the electron density in MoS2 single-layers and oxygen treatments at 400 ◦ C resulted in conversion of MoS2 layers to MoO3 platelets. No molybdenum oxide (MoO3 ) was detected after oxygen treatments below 340 ◦ C. Tongay et al.  annealed MoS2 single-layers in a vacuum. Figure 30 shows room temperature PL spectra of a MoS2 mono-layer annealed at 450 ◦ C in a vacuum for different annealing time. A 40 min annealing at 450 ◦ C enhanced the PL intensity by over 50 times. The FWHM of the PL peak decreased and the PL peak position shifted slightly with annealing time. The thermal annealing did not degrade the crystalline quality of the material. Appl. Sci. 2019, 9, 678 34 of 53 The Journal of Physical Chemistry C Article f 2. (a−d) AFM images of and MoS at 320 °C Figure 1. (a) An of single-layer trilayer (3L), and(1L), four-layer (4L)Figure MoS(2L), at 1L 320 °C 2L for 3 h. 2 oxidized(4L) 2 on SiO 2 after oxidation Figure 29.AFM (a)image AFM images(1L), of bilayer MoS(2L), bi-layer tri-layer (3L), and four-layer 2 single-layer for surrounded (a) 1, (b) 3,by(c) 4, andlines (d)on 6 h. bars are 2 μm. (e) The The inset shows an optical image of this ﬂake before oxidation. (b−e) Close-up images of the areas dashed (b)The 1L,scale (c) 2L, ◦ average distance r from the center to the apex of triangular pits as a with triangular (d) on 3L, and (e)2 4L in the oxidation panel (a). The scale bars areC 500for nm. 3 (f)hSchematic of hexagonal lattice an of theoptical MoS2 structure SiO after at 320 .drawing The inset shows image of thispits. flake before function of oxidation time. The layers red line is ﬁt. Two dashed lines indicate the (1̅010) S and (101̅0) Mo edges. The little blue circles correspond to 2 S atoms in outer constituent above andThe inset is an AFM oxidation. (b–e)interior Close-up of the areas surrounded bythat dashed lines onispit(b) 1L, (c) 2L, (d)MoS 3L,2 after image of the a typical triangular formed on single-layer islands suggests Mo (101̅ 0) edge more stable, and below the Mo (red circles) constituentimages layer. Evidence for MoS 2 nanocrystal for 4 h. as The scale bar 300(g)nm. that it is decorated by S atoms in one of two possible conﬁgurations that cannot be distinguished oxidation in our experiment described in is text. Proﬁles and (e) 4L in the panel (a) . The scale bars are 500 nm. (f) The average distance r from the center of pits along the dashed lines in (b−e). to the apex of triangular pits as a function of oxidation time . The inset is an AFM image of a no obvious simple corr The observed oxidati SiO2 are in sharp contra on the same SiO2 surf SiO2 results in circular unlike atomically thin supported graphene i single-layer being the pits in single-layer grap the surface, and the n time and temperature. T graphene on SiO2 is du charged impurities in impurities is signiﬁcan thickness. Thus, for thic is predominantly activa the etch pits have nearl layer depth.36 The oxid similar in character to t rather than graphene oxidative etching of atom defect sites on the surfa of the density of pits after oxidation at variou from 106 to 109 cm−2, reported density of intr atoms such as tungste crystal,37,38 indicating th initiating etching. Previous scanning p emission measurement oxidation leads to the basal plane surface of b microcrystalline MoS2 results in a peak at 820 terminal oxygen atom normalized intensity o oxidation temperature 820 cm−1 in pristine sin (black line). However, t the Si peak at ∼520 cm− even at 340 °C for 2 h conclude that the peak stretching mode in M mode of MoS2.42 This i MoO3-related peaks su spectrum of oxidized M of MoS2 does not ch suggesting that no M spectroscopy clearly remains after oxidation of MoO3 formation, th and 06),h.18,19,23,26 After oxidation for exact an hour, etch pits with an average points to Mo-edge4,(101̅ (3M Water-Soluble Wave Solder Tape 5414). The thicknesses though the structure 3 2 nm on the size edge of 6.3(and × for 10locations triangular formed onatomic single-layer MoS after oxidation 4 h.areThe scale barsurfaces is 300(Figure nm. 2a). identiﬁed bypit optical contrast, force of typical MoS2 were of the 2reconstructed offormed additional sulfur Nano Additional oxygen likely treatment leads on to lateral growth of the microscopy (AFM), and RamanLetters spectroscopy.30,31 To remove atoms terminating the Mo-edge) depends the c 2013 American Reprinted with permission from Reference . Copyrighttriangular Society. as shown inChemical Figure 2b−d. The distance r from 19,23,26 adhesive residue, all samples were annealed in an H2/Ar Further work chemical environment andpits, substrate. the center to the apex of themicroscopy triangular or pits increases almost mixture for 2 h at 350 °C. The ﬂow rates of Ar and H2 are 1.7 using high-resolution transmission electron linearly with a growth rate of approximately 70 nm/h, as shown and 1.8 L/min, respectively. This hydrogen treatment leads to scanning tunneling microscopy could resolve the issue and in Figure 2e, but the density of pits is nearly constant during no chemical modiﬁcation of the MoS2 basal plane (see also elucidate the electronic and magnetic properties of these the oxygen treatment, indicating that the oxidative etching is Supporting Information for an AFM image and Raman spectra edges. not initiated homogeneously but at speciﬁc sites on the surface of MoS2). After preannealing MoS2 samples in H2, they were Figure 1g shows the proﬁles of the pits along the dashed lines of atomically thin MoS2. exposed to an Ar/O2 mixture at temperatures ranging from 27 in Figure 1b−e. The pits are mostly single-layer-deep (∼0.7 Figure 3a−d shows AFM images of MoS samples of various to 400 °C. The ﬂow rates of Ar and O2 are 1.0 and 0.7 L/min, nm) on single- and few-layer MoS2, indicating a very high2 thicknesses after oxidation at 320 °C for 1 h. In Figure 3a, the respectively. The nanoscale structure of oxidized MoS2 was degree of anisotropy in etching along the basal plane versus the density of etch pits formed on the single-layer MoS2 ﬁlm is 7.5 characterized by AFM in tapping mode using silicon cantilevers double-layer-deep c-axis, though we×do cm−2, while observe the pit density on single-layer MoS2 in Figure 106occasionally with a nominal tip radius of >10 nm (NCH, Nanoworld), and Figure 3e,f). pits on few-layer MoS 3b is22samples orders of(see magnitude larger(The than larger that in Figure 3a. Figure the composition and oxidation state were determined using in Figure 1g is artifactthickness with etch depth of the pits on single-layer MoS 2 Raman spectroscopy (Horiba Jobin Yvon Raman microscope) of single- toan4-layer 3c shows a MoS2 ﬂake caused by the limitation of the tapping mode AFM with 2400 gratings per mm and a solid state laser with a ﬁxed pits on the surfaces. The density of pits onto4-layer MoS2 is 3.5 determine the thickness of −2an, which atomically thin than membrane on excitation wavelength of 532 nm. is larger the densities on surfaces of × 108 cm 7 rough SiO2.32) The AFM images any cleartrilayer sign of (2.7 × 108 cm−2) cm−2 ) and single-layer (9.0do×not 10show the reaction products (presumably MoO3); we discuss this in RESULTS AND DISCUSSION 25645 dx.doi.org/10 more detail below. Figure 1a shows a typical AFM topographic image of atomically Our MoS2 crystals are expected to have a 2H structure,1,2 thin MoS2 supported on SiO2 after oxygen annealing at 320 °C where the triangular lattices of adjacent layers are 180°-inverted for 3 h (see the inset for an optical image of this ﬂake). The relative to each other. Therefore, the triangular pits formed on oxygen treatment results in etch pits on the surfaces of singlethe surfaces are also expected to have 180°-inverted and few-layer MoS2. We observe the formation of etch pits even orientations among even and odd numbers of layers. Such after air annealing (see Supporting Information for an AFM trends can be seen in Figure 1a. However, we also observe the image), suggesting that the etching process is independent Figure 1. of Drastic enhancement photoluminescence of monolayer TMDs by thermal annealin Figure 30. PL spectra mono-layer MoSof2 in forroom-temperature different annealing time . Theintensity anneal temperature triangular pits with same orientations on even and odd layerthe partial pressure of oxygen gas. As shown in Figure 1b−e, as a function of annealing time. The anneal is at 450 °C in vacuum. laser excitation intensity is ﬁxed at MoS number-thickness regions (see Supporting Information for anThe ◦ C in amonolayer 2 c is 450 vacuum. Reprinted with permission from Reference . Copyright 2013 American the shape of the pits is triangular and their orientations are AFMspectrum image), suggesting that itand is the top surface that is monolayer MoS2. (Inset) Raman spe PL measurements. (b)observations Normalized PL for pristine optimally annealed ﬂat terrace. These identical over each atomically Chemical Society. continuous across the layer-number-thickness indicate that the triangular shapesmonolayer of the pits reﬂect latticePL intensity enhancement of monolayer MoSeboundary. thermal anneal at 250 °C for 15 min. T annealed MoSthe 2. (c) 2 upon Because of this ambiguity, we cannot be certain of the of the MoS2 basal plane and that thePL edges of the pitsenhancement thermal anneal at 300 °C for 10 min. The peak shift by surface ∼17 meV. (d) intensity of monolayer WSeorder 2 upon correlation between the stacking of MoS layers and the 2 along theIrradiation: zigzag directions with only a single chemical 4.3.areHybrid UV–Ozone Treatments meV. All the PL measurements are taken at room temperature in ambient condition. orientations of the triangular pits; however, the observations of termination, that is, terminated on either the Mo-edge (101̅0) only a single etch-pit orientation within a single terrace, and the or S-edge (1̅010) (see Figure 1f). The observation of only three Ultraviolet–ozone technique dry-cleaning method to clean surfaces by the of opposite orientations for material diﬀerent layer preferred edge orientations rulescleaning out armchair-oriented edges is a observation thicknesses within a single strongly that theof oxidation by for which there of are contaminants six possible identical edges. Our decomposition through ultraviolet irradiation andcrystal, the suggests chemical action termination is globally determined to be along only one of the experiments are unable to resolve whether the preferred edge ozone. During cleaning, organic compounds decompose to volatile substances by ultraviolet Mo or S terminated zigzag edges. is the Mo-edge or S-edge; however,surface evidence from other studies ■ lights and by strong oxidation of ozone. The has been employed to25643−25649 irradiate 2D MoS2 25644 technique dx.doi.org/10.1021/jp410893e | J. Phys. Chem. C 2013, 117, mono-layers [163,164] and few-layers [109,165–167]. Wang et al.  rapidly treated MoS2 1–3 layers that were mechanically exfoliated from crystals. These was no any change before and after the UV–ozone treatments. It was assumed that the irradiation effect was below detection because of the rapid treatment (30 s) and nitrogen protection that reduced high oxidation and mobility change. Le et al.  prepared MoS2 nanosheets using a sonication exfoliation method and UV–ozone treated the MoS2 multi-layers for 15–30 min. The MoS2 layers were converted to MoOx . Burman et al.  treated liquid-exfoliated MoS2 few-layers (15–16 layers) in UV–ozone atmosphere for 1.5 h. It was found that the Mo/S atomic ratio decreased, and atomic percentage of oxygen increased with the expose time. Azcatl et al.  exposed the mechanically exfoliated MoS2 layers under ultraviolet-ozone at room temperature. They found that oxygen-sulfur bonds were formed at the top sulfur layer while the sulfur-molybdenum bonds were kept besides removal of adsorbed carbon contamination from the MoS2 surface. Figure 2. Eﬀects of exposure to diﬀerent gas species on the PL intensity of annealed monolayer TMDs. (a) Change in annealed but measured in vacuum value) upon exposure to H2O alone, O2 alone, and ambient air. The pressure of these ga Torr, respectively. Trion X− and exciton X0 peak positions are indicated. (b) Modulation of the PL intensity of monolayer Mo purging (50 and 100 Torr, respectively) and pumping. (c) Modulation of the PL intensity of monolayer WSe2 as a function cross section (barn) cross section (barn) evolution of a MoS2 shee for practically all TMDs is within the energies commonly First, freestanding singl used in TEM studies. The displacement thresholds for chalcogen atoms in the pared by mechanical exfo (top) layer facing the beam proved to be considerably higher tals, followed by characte Appl. Sci. 2019, 9, 678 35 of 53 than for the bottom chalcogen layer, as the displaced atom is a Si þ 90 nm SiO2 subs ‘‘stopped’’ by the other layers. However, after a vacancy is TEM support film (Quan The ultroviolet–ozone technique was also used to mill MoS few-layers to mono-layers 2 created in the bottom layer, the threshold energy for the top . . The TEM grid wa Compared to mechanically exfoliated MoS single-layers, the PL intensity of the UV–ozone milled 2 S atom in MoS2 to be displaced and fill the vacancy is about evaporating isopropanol MoS2 mono-layers was enhanced by 20–30 times. 8.1 eV. This is similar in magnitude to the threshold for was etched with KOH. Ab displacingand S atom from Simulations the bottom layer (6.9 eV), and thus imaging was carried out 5. Irradiation Mechanism Theoretical formation of vacancy columns should be possible even at TITAN microscope at a Besides the experimental work reviewed in the previous sections, there are lots of theoretical 80 kV when lattice vibrations are accounted for. T for The contrast difference b approaches to investigate the irradiation mechanism of MoS2 2D nanosheets. d transition metals even higher, sinceenergy they are to initial clearly detectable in the Komsa et al.  calculated the is displacement threshold and bonded the minimum kinetic six neighbors similarly by the Susing layer.theFor single-layer nature of th energy of the recoil atom to kickand an atom from stopped MoS2 mono-layers, density-functional about 20 eVexchange-correlation is required to displace contrast would be ident theory with theinstance, Perdew-Burke-Ernzerhof function.Mo Theatom sulfur displacement threshold energy of MoS to incident-electron energies from its2 was site 6.9 in eV, thecorresponding MoS2 lattice, which corresponds toof about This90iskeV. also confirmed in About 20 eV was required to displace molybdenum atom from site in MoS2 mono-layer electron energy of 560one keV. Naturally, under suchits conditions cessive diffraction spots f lattices, corresponding to an electron energy of 560 keV. Therefore, the sulfur sub-layers were easier the S sublattice is quickly destroyed. Formation of transition ent intensity, whereas for destroyed whilemetal it wasvacancies unlikely to generate molybdenum vacancies in molybdenum sub-layers is thus considered highly unlikely. . The analyzed inten under TEM observations (where 200 keV electron beams were employed). Figure 31 shows the With regard to possible vacancy agglomeration under spots was found to be 1:0 calculated cross-section of sulfur atoms. Lattice vibrations were taken into account, assuming continuous irradiation, we found that creation of a vacancy During continuous ima a Maxwell-Boltzmann velocity distribution. The cross denotes the experimentally determined alter theshows formation energy in athe neighboring sites energies. ber of The vacancy sites (excl cross-section fordoes MoS2not . The inset the same data for larger range of electron in the semiconducting TMDs. Thus, we do not expect by crack forma atom displacement cross-section was estimated from the electron threshold energy bypanied using the accelerated formation of large vacancy clusters. In the shrinkage of the membra McKinley-Feshbach formalism. sputtered atoms as in Ref 20 ing was found to be 1.8 b, 150 with the calculated cross s that the theoretical estim 100 15 cies in the parameters of t 50 distribution) at energies b 0 In Figs. 4(d) and 4(e) w 100 150 200 50 10 electron energy (keV)  for the single and based on atomic structu from the DFT calculation 5 the experimental TEM im can clearly be distinguis 0 contrast relative to the 60 65 70 75 80 85 90 95 100 pristine area. We find t electron energy (keV) ratios are 0.9 (0.9) for a Figure 31. Cross-section for sputtering a sulfur atom from MoS2 mono-layers as calculated through FIG. 3 (color online). Cross section for sputtering a sulfur single the McKinley-Feshbach formalism and the dynamical values of the displacement thresholds . and 0.2 (0.2) for t atom from MoS2 , WS2 , and TiS2 sheets as calculated through Having shown that va Reprinted with permission from Reference . Copyright c 2012 by the American Physical Society. the McKinley-Feshbach formalism and the dynamical values of under electron irradiatio the displacement displacementthreshold thresholds. Dotted are thewith data for the observations. The calculated energy was inlines agreements experimental they could be consecutiv static lattice, and solids linesobserved are the results of calculations wheremicroscopes with Single sulfur vacancies (VS’s) were frequently under transmission electron cies deliberately introdu lattice voltage vibrations taken into 200 account, a the Maxwellan 200 kV acceleration (the are electron energy, keV isassuming higher than displacement energy of calculate the formation e velocity The cross denotes the experisulfur atoms (90Boltzmann keV) while lower thandistribution. that of molybdenum atoms, 560 keV). and consider dono mentally determined cross of section for MoS inset shows for the MoS2MoS 2 . The Table 2 compares formation thresholds five kinds of defect configurations bulks2 and As, and Sb; double accep same data forfor a larger of electron energies. mono-layers. The thresholds single range S vacancies were very low, both in bulk and mono-layers. Single-line S-vacancy and double-line S-vacancy configurations are generated with increasing single S-vacancy concentration by prolonged electron irradiation. In addition, the formation thresholds for 035503-3 Mo-vacancy and DIV are not very high (18–29 eV). Appl. Sci. 2019, 9, 678 36 of 53 Table 2. Comparison of formation thresholds in eV for various vacancy configurations of MoS2 bulks and mono-layers . Comfiguration Bulk (eV) Mono-layer (eV) single S-vacancy single Mo-vacancy single-line S-vacancy double-line S-vacancy DIV 14.4 24.8 39.2 66.2 28.8 5.7 18.8 30.5 62.3 23.3 DIV: di-vacancy comprised of nearest neighbor single sulfur vacancies and single molybdenum vacancies. Kretschmer et al.  combined analytical potential molecular dynamics with Monte Carlo simulations to simulate helium and neon ion irradiation of MoS2 nanosheets deposited on SiO2 substrates. Their simulation indicated that substrates governed the defect production of irradiated MoS2 layers. The irradiation-induced defect production was dominated by backscattered ions and sputtered substrate atoms, rather than by the direct helium and neon ion impacts. Molecular dynamics simulations were also employed to study the production of defects in MoS2 mono-layers under noble gas ion irradiation, such as He, Ne, Ar, Kr and Xe . Sulfur atoms were sputtered away predominantly from the top or bottom layers by the ion irradiation, depending on the incident angle, ion type, and ion energy. 6. Irradiation-Induced Properties Pure 2H-MoS2 crystalline bulks are indirect band-gap semiconductors and show diamagnetic from 10 K to room temperature. Their physical properties are sensitive to structures and defects. Irradiation produces defects in MoS2 few-layers and affects their properties significantly. Below, the optical properties, electronic properties, catalytic properties, and magnetic properties are summarized. 6.1. Band Structures Bulk MoS2 is a semiconductor with an indirect band gap of 1.23 eV [36,170]. The band gap of the material depends on thickness , as shown in Figure 2. Figure 32a shows the band structure of MoS2 mono-layers without vacancies. MoS2 mono-layers have a direct band gap of 1.8 eV, resulting in strong absorption, PL bands [38,39], and electroluminescence  near 680 nm. Defects affect the band gap of MoS2 layers significantly. Figure 32b–c shows calculated band structures of MoS2 mono-layers with defects. Negative (S vacancies) and positive (Mo vacancies) charges transferred to the orbitals of nearest neighbor Mo or S atoms around the vacancies, resulting in additional states at the bonding level and at mid-gap of the band structures. The K-point direct band gap remained intact for MoS2 mono-layers with S vacancies, while the gap value was reduced. The direct band gap at K-point was reduced too for MoS2 mono-layers with Mo vacancies. 6.2. Electric Properties The density-functional method and the Green’s function approach indicated that structural defects and grain boundaries were principal contributors to electric transport properties of MoS2 mono-layers . Atomic vacancies could significantly reduce the conductance of the 2D layers. In experiments, MoS2 2D materials were deposited on various substrates and connected to metal electrodes to fabricate FETs to detect electric properties under irradiation. Durand et al.  studied electrical transport properties of CVD-grown MoS2 mono-layers on SiO2 /Si substrates. The carrier density was significantly increased, and the mobility was reduced under a low-energy electron irradiation (5.0 keV) in an UHV environment. It was believed that the electron-irradiation generated defects in MoS2 layers and caused Coulomb potentials at the MoS2 /SiO2 interfaces. 135701-3 Karmakar et 135701-3 al. Karmakar 135701-3 Karmakar et al. et al. J. Appl. Phy Appl. Sci. 2019, 9, 678 37 of 53 FIG. 1. Total b ent vacancy c monolayers, viz gle SV, SL S-v single Mo-vaca cancy compris vacancies (DIV C high-symmet onal Brillouin are plotted w Appearance of states for differ tions is evident (a) (b) (c) intact for a assumptions may approximately only for S-vacancies, intact for remains SV, remains albeit with assumptions may band approximately hold only for S-vacancies, intact forreduce SV, may approximately hold hold only forremains S-vacancies, structure for different vacancy configurations in MoS Figure 32. Total assumptions 2 mono-layers (a) without SL DL and DL gh thesulfur other vacancy configurations need accounting ofboth both SL along and have both a C-point direct the other vacancy configurations needconfigurations a proper of a proper SL and have the other vacancy need a proper accounting of DL vacancy, (b) with single vacancies, and (c)accounting single molybdenum vacancies Γ-M-K-Γ DIV cases, direct b isolated atom reference energies. The isolated reference enerDIV cases, direct band-gap at K-point is re isolated atom reference energies. The isolated reference enerhigh-symmetry directions of hexagonal Brillouin zone . DIV cases, direct band-g isolated atom reference energies. The isolated reference enerIn some studie gies for individual Mo Sand S atoms are 1.89 and 1.08 eV, like some Refs. 14 and 35 gies for individualgies Mofor andindividual S atoms are 1.89 1.08 eV,1.89 In some studies, likw Mo and atoms are and In 1.08 eV,studies, ◦ C through Lu et al.  grew MoS single-crystalline few-layers on SiO /Si substrates at 700 2 2 gated MoS2single (0001 respectively, arenegligible. not negligible. gated MoS2 (0001) surface respectively, whichrespectively, are not negligible. gated MoSwith surS whichwhich are not 2 (0001) CVD method, and then transferred them to pre-patterned SiO2 /Si substrates with Au electrodes For DL vacancies, it was indicated that the created vacancy induced gap For DL vacancies,For it was indicated that the created vacancy induced gap states are situated de DL vacancies, it was indicated that the created vacancy induced gap stat by the poly(methyl methacrylate)-assisted method. The fabricated MoS2 devices were then cancy get arranged a staggered configuration bytheseindicates This indicates that cancy may get arranged in amay staggered configuration by configuration This indicates levels may as may get arranged in a in staggered by that thatact these electron-irradiated cancy in a scanning electron microscope with a 30 keV electron beam. The This electron removing S atoms alternatively from two nearest neighbor conduction. With s removing S atomsµC/cm alternatively from that two the nearest neighbor conduction. With single SV, we too single observ atoms from two nearest neighbor With 2 . It S dose was 100–1800 removing was foundalternatively threshold voltage shifted to the negative conduction. side and lines. We have observed thatformation thestagformation energies of stagsuch in lines. We have observed that the formation energies of such deep states insuch addition to thestates additio lines. We observed that the energies of stagdeep deep states in addi the mobility increased upon thehave increasing electron doses. gered and columnar DLsame defects areonto ofHowever, thex membranes, same order. However, incr geredParkin and etcolumnar DL are of order. with increasing S-vacancy conce gered anddefects columnar DLthe defects are of the same order. However, with with increasin al.  deposited single-crystalline MoS SiN contacted 2 mono-layers Therefore, in the present study, the DL configuration consists DL cases and also Therefore, the present configuration consists DL cases and also inwith presence Moin(M Therefore, in the DL present study, the DL configuration consists DL cases and ofalso p them to Auin electrodes. Thestudy, fabricated device was then irradiated under a TEM electron beam of columnar S vacancies. In addition, in Ref. 17, calculated (DIV) vacancies, an of S found vacancies. In addition, in Ref. 17, calculated (DIV) vacancies, antibonding gap states are 200columnar keV. It was that the electrical resistance increased with the electron irradiation dose. of columnar S vacancies. In addition, in Ref. 17, calculated (DIV) vacancies, antibon prediction ofactual the actual electron required to getcontribute contribute to doping prediction electron beam energy required tobeam get energy contribute to In addition, these addit Kim etofal.the actual prepared few-layer MoS FETs, andbeam investigated the effect of under prediction of the energy required to irradiation getdoping. to doping. In a 2 electron MeV proton exposures. The electrical properties of the devices were nearly unchanged in response a sustained production of these defects is very high (more spin-polarized and S a10sustained production of these defects is very high (more spin-polarized SO-coupled and con a sustained production of these defects is very high (more and spin-polarized andthus SO-co 12 2 to low protons/cm ). The current conductance the devices thanKeV). 80 (10 KeV). the successive experimental sections, wenetic netic order forvacanc syste than 80fluence KeV).proton In than theirradiation successive experimental sections, we andnetic order forofsystems with different 80 In theInsuccessive experimental sections, we order for systems w 13 –1014 protons/cm2 ). The electrical changes significantly decreased after high fluence irradiation (10 Onestudy recent have observed that in practical situation, electron beam of One recent onfirs se have observed that in practical situation, electron beam of electron One recent first-prin have observed that in practical situation, beam of first-principles contributed to proton-irradiation-induced traps in the SiO substrate layers and atin the interfaces 2introducing face defects in Mo far lower energy is capable of modification of also includes the eff face defects MoS far lower energy is capable of introducing modification of face far lower energy is capable of introducing modification of 2 defects in MoS2 al between the MoS2 layers electronic SiOproperties 2 layers. structural which indicates th in different structural forms. of MoS which indicates that adsorption ofthat S ato in different forms. electronic properties of and MoSthe 2 which indicates ad in different structural forms. electronic properties of MoS 2 2 Islam et al.  measured electrical properties of MoS2 mono-layers exposed under oxygen Figure 1 exhibits a comparison of the impact of defects localized shallow Figure 1 exhibits aFigure comparison of the impact of defects localized shallow localized state at the valence ba 1 exhibits a comparison of the impact of defects shallow state plasma. The plasma was generated from a gas mixture of oxygen (20%) and argon (80%) under a , revealing on the total band-structure of monolayer MoS understandably in , revealing on the total band-structure of monolayer MoS understandably in contrast with the presen 2 revealing theattotal band-structure of2 monolayer MoS2of, the understandably in contrac pressure of 250–350on mTorr 50 kHz. Figure 33a shows the ID -VDS curves device at a back-gate appearance ofatadditional states both at bonding level atleading leading toresults deep ga appearance of additional statesofboth bonding level and atcurves leading to deep pr appearance additional both atDS bonding levellinear and atand gap deep gap sta voltage VG = 40 V for various plasma exposure states time. The ID -V were around thestates. zero toThe mid-gap. These states signify the contribution from addirent investigation mid-gap. These states signify the states contribution addirent investigation also support the modific mid-gap. These signify from the the contribution from addi- ofrent investigation also sa bias, showing Ohmic behaviors. Figure 33b demonstrates exposure-time dependence electric negative (S-vacancies) or positive (Mo-vacancies) levels described tional negative (S-vacancies) orexponentially positive (Mo-vacancies) levels as described indrain Ref. 36,aswhere tionaltional negative (S-vacancies) positive (Mo-vacancies) levels as described inforR resistance. The resistance increased withor increasing plasma exposure time. The charges transferred toorbitals the orbitals of nearest neighbor (nn)than layer, in addition charges transferred to the orbitals of to nearest neighbor (nn) layer, in (nn) addition to four the state charges transferred the of nearest neighbor layer, inshallow addition to thn current and mobility decreased exponentially with plasma exposure time, dropping more Mo or S-atoms around the vacancies. Charge transport in edge, defect states Mo or of S-atoms around vacancies. transport inIt wasedge, defect states are also observed to a orders magnitude only a totalaround of 6 Charge s plasma exposure. Charge claimed thatinthe significant Moafter or the S-atoms the vacancies. transport edge, defect states are for low carrier-density regime can be explained by MoS conduction band degradation of electronic properties was caused by the creation of insulating MoO -rich disordered for low carrier-density regime can be explained by MoS conduction band edge. 2 MoS2 for low carrier-density regime can be explained 3by conduction band edge.ed 2 hopping through such vacancy-induced localized gapFor each vaca domains inthrough the MoShopping sheet upon oxygen plasma exposure. hopping vacancy-induced localized gap- localized For each configuration, inc 2 such through such vacancy-induced gap- vacancyFor each vacancy 16 16 16 Figure 1 implies that the K-point direct band-gap states. coupling reduces implies Figure that the1 K-point band-gap states. Figure 1 states. coupling reduces coupling the total reduces energy of implies direct that the K-point direct band-gap thethe to TABLE II. SO-coupled non-collinear magnetic moment (m) zcomponents along x, y,z and z directions TABLE II. SO-coupledTABLE non-collinear magnetic moment (m)magnetic components along x, y,components and directions moments for res nea II. SO-coupled non-collinear moment (m) along and x, y,resultant and directions and atoms and for the entire system. atoms and for the entireatoms system. and for the entire system. Configuration Configuration mx, my, mz (nn Mo) Configuration (nn Mo) x, m y, mz Total m (nn Mo) mx, mym ,m z (nn Mo) Total (nn Mo) x, my, mz (entire system) Total m (nnmm Mo) mx, m ource–drain bias plasma exposure time leads to the gradual increase of the tunnel barrier raised by the eﬀective medium semiconductor 2 device. The ID h the increase of 4 Sci. 2019, 9, 678 Appl. ound to be 10 . 2 lated to be 6 cm D/dVG), where L is d CG ¼ 303rA/d is h 3r 3.930 is the ¼250 nm) is the 38 of 53 f the same device nied view of the (pristine MoS2) stingly, the drain Figure 33. (a) ID -VDS curves of the single-layer MoS2 device after different plasma exposure time . Fig. 3 (a) ID vs. VDS characteristics curve for the single-layer MoS2 ncreasing oxygen (b) Resistance of the device as a function of plasma exposure time . Republished with permission device after diﬀerent plasma exposure times. (b) Resistance of the of RSC Publisher, Reference . c 2014. Permission conveyed through Copyright Clearance seen in Fig. 2(b) device as afrom function of plasma exposure time. The green line is the Center, Inc. is displayed in a linear ﬁt of the logarithmic resistance as a function of exposure Properties duration. 5 nA for the 6.3. as-Catalytic Molybdenum is relatively abundant and cheaper than noble metals. Thus, molybdenum compounds have been used as electrocatalysts and photoelectrocatalysts to supersede noble metal catalysts. It was found that MoS2 edges were catalytically active for hydrogen evolution reaction Nanoscale, 2014, 6, 10033–10039 | 10035 (HER) while the basal surfaces were catalytically inert. So MoS2 nanomaterials have been employed as catalysts to generate hydrogen [69,176–179] because of more catalytic edges. MoS2 nanoparticles were first used as photocatalysts  and electrocatalysts  to generate hydrogen in the 2000s. The catalytic properties were controlled by edge sites of the MoS2 materials. MoS2 2D films were more active and employed as photocatalysts  late in 2016 for water purification, Irradiation can introduce defects in MoS2 layers, which create more HER active sites. Sulfur vacancies can activate and optimize hydrogen evolution . Additionally, electrical conductivity can be tuned by the introduced defects. Therefore, catalytic activities of MoS2 HER can be significantly enhanced by irradiation defect engineering. Tao et al.  treated the CVD-grown MoS2 thin films by Ar plasma. The active site density of MoS2 thin films increased five times to 7.74 × 1016 cm2 after the Ar plasma treatments. HER performance was enhanced significantly too. However, long-time Ar treatments etched MoS2 , not benefited the HER behaviors. The MoS2 sheets were also treated by O2 plasma . The O2 plasma treatments led to the formation of both Mo-O and S-O bonds, generating more active sites with the increasing plasma time, enhancing the HER activity significantly. 6.4. Magnetic Properties 2H-MoS2 bulks are temperature-dependent diamagnetic . It was reported that irradiation improved magnetic ordering in single crystals of MoS2 . Karmakar et al.  investigated electron-irradiation-induced magnetic behaviors of MoS2 crystals. The irradiation was carried out under 30 keV electrons with exposure time of 30 min. The effective magnetic moment of single crystals increased from 0.42 µB per Mo-ion to 1.11 µB per Mo-ion on account of irradiation. Han et al.  investigated magnetism of electron-irradiated MoS2 single crystals. The diamagnetic MoS2 single crystals transformed into ferromagnetic state after irradiation up to room temperature, as shown in Figure 34. The irradiation-induced magnetic phase transition was largely attributed to the strain around the irradiation-induced vacancies. Appl. Sci. 2019, 9, 678 39FIG. of 532. Comparison of magne curves after subtracting the diam or paramagnetic background fr data of Figs. 1 and S1, supplem material. The electron dos changed at low (a), (c) and high acceleration energies (see Table Figure 34. Magnetization curves of crystalline MoS2 lamellae with a thickness of 100 microns after various electron irradiation . Left panel: under low-energy electron irradiation. L(i): 150 kGy and 0.7 MeV; L(ii): 300 kGy and 0.7 MeV; L(iii): 600 kGy and 0.7 MeV. Right panel: under high-energy electron irradiation. H(i): 100 kGy and 2.0 MeV; H(ii): 250 kGy and 2.0 MeV. Magnetic fields were parallel to ab-plane or c-plane of the lamellae. Reprinted from Reference , with the permission of AIP Publishing. Mathew et al.  irradiated MoS2 sheets with a thickness of 200 µm at room temperature using a 3.5 MeV proton ion beam. The pristine sample was diamagnetic in nature. After irradiated at a fluence of 1 × 1018 ions/cm2 , ferromagnetic ordering was induced in the MoS2 , as shown in Figure 35a. More detailed work indicated that the induced magnetization depended on the irradiation dose. Figure 35b shows the magnetization of irradiated MoS2 crystalline flakes that were measured at 300 K and 5 kOe as a function of proton ion fluence from 1 × 1017 ions/cm2 to 5 × 1018 ions/cm2 . The magnetization of the pristine flakes was negative as well as in irradiated flakes at a fluence of 1 × 1017 ions/cm2 . The magnetization became positive after the sample was irradiated at a fluence of 2 × 1018 ions/cm2 and increased with the irradiation doses. However, the magnetization decreased when the flakes were irradiated at a fluence of 5 × 1018 ions/cm2 . Mathew et al.  also exposed MoS2 crystals (200 µm thickness) under 0.5 MeV proton irradiation at an ion fluence of 1 × 1018 ions/cm2 (same fluence at which magnetic ordering was observed using 2 MeV protons) while ferromagnetism was not observed in the low-energy proton irradiation. After annealing at 350 ◦ C for 1 h in Ar flow, a weak magnetic hysteresis loop was observed. The weak magnetic signal was also observed in the irradiated samples at a lower fluence of 2 × 1017 ions/cm2 and a lower energy of 0.5 MeV proton irradiation. Based on these phenomena, the appearance of observed magnetism in proton-irradiated MoS2 flakes were due to a combination of defect moments arising from vacancies, interstitials, deformation and partial destruction of the lattice structure, such as the formation of edge states and reconstructions of the lattice. Zhang et al.  reported weak ferromagnetism phenomenon in MoS2 nanosheets and attributed FIG. 3. HRTEM images of (a) the pristine MoS2 and (b)–(e) electron beam-irradiated samples (see Figs. S4–S8, supplementary material). (f)–(j) Hist the magnetic properties tosize thedistribution presenceofof atoms.irradiation Later, first-principles computations summarize that the theunsaturated countable atomsedge after electron complies with a Gaussian profile (red curves) with the peak p centers at around 1.4 Å for thecalculations pristine MoS2. were carried out to predict ferromagnetism in zigzag and density-functional theory nanoribbons of MoS2 [186–188], magnetism in MoS2 clusters , and magnetic edge state of hydrodesulfurizated MoS2 particles . It was generally accepted that  zigzag MoS2 edges were ferromagnetic and metallic whereas armchair MoS2 edges were nonmagnetic and semiconducting. The predication was experimentally approved in MoS2 few-layers . It stated that grain boundaries or defects in the nanosheets were responsible for the ferromagnetism of MoS2 nanosheets. Appl. Sci. 2019, 9, 678 102103-2 Mathew et al. Appl. Phys. Lett. 101, 102103 (2012) (b) (blue), and 10 K (red) irradiated MoS2 at a fl ions/cm2. An enlarged isotherms near the orig inset. The variation of 40 of 53 fluence is shown in ( and field cooled (ZFC ization vs temperature an applied field of 500 ated MoS2 sample 5 1018 ions/cm2. Th estimated magnetic temperature plot near shown in the inset. 2 magnetization at 300 K as a function of io 1 1017 ions/cm2 to 5 of coercivity is found to increase with ion fluence at all temperatures used in this study, although at 10 K, it is almost constant. A large variation of coercivity with temperature indicates the presence of long-range magnetic ordering in the irradiated samples. The zero field cooled (ZFC) and field cooled (FC) magnetizations at an applied field of 500 Oe are given in Fig. 2(c). An insight into the nature of magnetic ordering (ferro- or ferri-magnetic) can be gained by analyzing the variation of susceptibility with temperature. The inverse of susceptibility plot is shown as an inset of Fig. 2(c). The value of Curie temperature (Tc) is estimated to be 895 K. The nature of the curve near Tc, a concave curvature with respect to the temperature axis, is characteristic of ferrimagnetic ordering whereas for a ferromagnetic material, this curvature near Tc would be convex.23 The variation of magnetization at 300 K with an applied field of 5 kOe is plotted in Fig. 2(d) for different fluences. The magnetization of the pristine sample and sample irradiated at a fluence of 1 1017 ions/cm2 are negative, at a fluence of 1 1018 ions/ cm2, the sample magnetization becomes positive and at 2 1018 ions/cm2, the magnetism in the sample increases further, and at a fluence of 5 1018 ions/cm2, it has FIG. 1. M vs H curve (a) at 300 K and (b) at 10 K for a pristine and irradiated decreased. The dependence of magnetization on the irradiaFigure 35. (a) M vs H curve for a pristine without irradiation and an irradiated MoS micron-thick MoS 2 after subtracting the substrate Si contribution. tion fluence observed in Fig. 2(d) (the bell shaped curve) 2 18 2 indicates thatmagnetization the role of the implanted and thus of the proton fluence flake atordering a fluence ofto5the×presence 10 protons/cm and (b) as protons a function magnetic can be due of defects such effect of end-of-range for the observed magnetism is as atomic vacancies, displacements, and saturation of 18 a va17 protons/cm 2 to 2 . defects from 1 × 10 5 × 10 protons/cm Reprinted from Reference minimal, as it had been shown in the case of proton- , with the cancy by the implanted same sample was subseDownloaded 08 Sepprotons. 2012 The to 18.104.22.168. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions irradiated HOPG.19,20,24 It was demonstrated that 80% of the quently irradiatedofatAIP cumulative fluences of 2 1018 and permission Publishing. measured magnetic signal in the 2 MeV Hþ irradiated 5 1018 ions/cm2 to probe the evolution of induced magneHOPG originates from the top 10 nm of the surface.24 To tism with ion fluence. radiation-induced modification in our samples near the Magnetizations asstated a functionthat of field isothermsedges at a flu- of probe Recent work zigzag MoS 2 few-layers can induce ferromagnetism. Therefore, surface region, we used XPS and Raman spectroscopy. ence of 5 1018 ions/cm2 are shown in Fig. 2(a). An defects, such sulfur vacancies, canis given convert diamagnetic 2H-MoS nanosheets ferromagnetism . The modifications in atomic 2bonding and core-level enlarged viewas of the M-H curves near the origin in electronic structure can be probed using XPS. The XPS specthe inset and the decrease of coercivity with increasing temIrradiation could create numerous defects in MoS 2D materials as discussed in the previous sections. tra of a 2 pristine sample and the sample irradiated at a fluence perature is clear from the plot. A plot of coercivity vs ion flu18 2 ions/cmfew-layers are shown in Figs. 3(a)–3(d). FittingMore of of 5 of 10MoS ence at variousshould temperatures is givenmagnetic in Fig. 2(b). The value So irradiation affect properties significantly. fruitful outputs 2 are expected in irradiated MoS2 few-layers. 7. Conclusions and Outlook FIG. 2. (a) M vs H curve at various temfrom 400 K (black), 95 K Semiconducting MoS2 2D layers have unique transportperatures properties have been applied in (blue), and 10 K (red) forand a pristine and irradiated MoS at a fluence of 5 10 various devices. Their band-gap, doping, photonic, electric, electronic, chemical, and ions/cm . An enlarged viewmagnetic, of the M-H isotherms near the origin is shown in the bio-properties can be tuned effectively and simply by defect engineering. exposures, inset. The variation Various of coercivity vsirradiation ion fluence is shown in (b). (c) Zero field and field cooled (ZFC and FC) magnetincluding swift-heavy ions, argon ions, alpha particles, protons, electrons, electromagnetic waves, can ization vs temperature measurements in applied field ofproperties 500 Oe for an irradi-essentially. Various significantly induce defects in MoS2 few-layers to manipulate an various ated MoS sample at a fluence of 10 ions/cm . The inverse of the defects, mainly sulfur vacancies and molybdenum vacancies, 5estimated have been generated/created in MoS2 magnetic susceptibility vs temperature plot near T (900–950 K) is layers. MoS2 layers can be significantly activated, functionalized,shown and modified. The irradiation-induced in the inset. 2(d) The value of at 300 K with H ¼ 5 kOe defects are beneficial for several applications including: solar magnetization cells [19,194–196], batteries [54,197,198], as a function of ion fluence from 1 10 ions/cm to 5 10 ions/cm . supercapacitors , thin film transistors [15,16,80,86,110,138,200–205], sensors [17,55,56,106,206–210], hydrogen generators [8,47,50,69,134,176,177,183,211,212], and applications in thermoelectrics [213–216], piezotronics , valleytronics , and environments [45,49]. MoS2 2D layers were damaged simultaneously under the irradiation and lost their unique properties. Therefore, it is critical to control the irradiation exposure to tune MoS2 properties by the irradiation-induced defect engineering while avoid potential damages. Besides MoS2 2D layers, the irradiation techniques can be employed to tune physical properties of other disulfide 2D materials, such as NiS2 urea-electrocatalysts , WS2 hybrid catalysts , as well as selenide water-electrocatalysts , perovskite electrocatalytic nanoparticles , C3 N4 catalytic materials , hydroxide nanosheet battery-electrodes , oxide photocatalysts , low-dimensional thermoelectric materials [224,225], graphene and carbon nanotubes [226–228], and even porous electrodes [229,230]. 18 2 2 18 2 2 c 17 2 18 2 Downloaded 08 Sep 2012 to 22.214.171.124. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions Author Contributions: G.Z. and Y.L. conceived the idea, designed and went through with the manuscript. H.D. wrote the section of alpha-particle irradiation. N.T. and Y.L. wrote the section of UV irradiation. M.G. and Y.L. wrote the section of proton irradiation. A.L. was in charge of the section of magnetic properties. Q.P. took charge of the theoretical analysis on irradiation. C.W. summarized background of MoS2 materials. J.-A.Y. was in charge of Raman scattering. G..Z. and Y.L. wrote other sections. All authors provided constructive comments on the manuscript. Appl. Sci. 2019, 9, 678 41 of 53 Funding: Y.L. is partially supported by the Army Research Laboratory under Cooperative Agreement Number W911NF-12-2-0022. A.L. is thankful to the financial support from the National Science Foundation through research grant NSF DMR 1206380. C.W. thanks the financial support from the National Natural Science Foundation of China (grants 51502099), the National Key Research and Development Program of China (Grant No. 2017YFE0120500), and the Hubei “Chu-Tian Young Scholar” program. Acknowledgments: The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Army Research Laboratory or the U. S. Government. The U. S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation herein. Conflicts of Interest: The authors declare no conflict of interest. Abbreviations The following abbreviations are used in this document: 2D AFM CBM CVD EBI EBIT EBL EDS EDX EPMA FET FFT FIB FWHM GANIL HAADF HER HIM HIRFL HRTEM IBAD LEAF MBE NEMS PAMBE PL RF RT SAED SEM STEM STM TMDC TEM TOF-SIMS UV UHV VBM VUV XPS XRD two-dimensional atomic force microscopy conduction band minimum chemical vapor deposition electron-beam irradiation electron-beam ion trap electron-beam lithography electron X-ray dispersive spectroscopy energy-dispersive X-ray spectroscopy electron probe micro-analyzer field effect transistor fast Fourier transformation focused ion beam full width at half maximum The Grand Accélérateur National d’Ions Lourds (Large Heavy Ion National Accelerator) high angle annular dark-field hydrogen evolution reaction helium-ion microscopy heavy ion Research facility in Lanzhou high-resolution transmission electron microscopy ion-beam assisted deposition low-energy accelerator facility molecular beam epitaxy nanoelectromechanical systems plasma-assisted molecular beam epitaxy photoluminescence radio-frequency room temperature selected-area electron diffraction scanning electron microscopy scanning transmission electron microscopy scanning tunneling microscope Transition-metal dichalcogenides transmission electron microscopy time-of-flight secondary ion mass spectrometry ultraviolet ultra-high vacuum valence band maximum vacuum ultraviolet X-ray photoelectron spectroscopy X-ray diffraction Appl. 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