Visualizing Point Defects in Transition-Metal Dichalcogenides Using

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Visualizing Point Defects in Transition Metal Dichalcogenides Using Optical Microscopy Hye Yun Jeong, Si Young Lee, Thuc Hue Ly, Gang Hee Han, Hyun Kim, Honggi Nam, Zhao Jiong, Bong Gyu Shin, Seok Joon Yun, Jaesu Kim, Un Jeong Kim, Sungwoo Hwang, and Young Hee Lee ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b05854 • Publication Date (Web): 08 Dec 2015 Downloaded from http://pubs.acs.org on December 9, 2015

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Visualizing Point Defects in Transition Metal Dichalcogenides Using Optical Microscopy Hye Yun Jeong†,‡, Si Young Lee†, Thuc Hue Ly†, Gang Hee Han†, Hyun Kim†,‡, Honggi Nam†,‡, Zhao Jiong†, Bong Gyu Shin†, Seok Joon Yun†,‡, Jaesu Kim†,‡, Un Jeong Kim§, Sungwoo Hwang§ & Young Hee Lee†,‡,*

†

IBS Center for Integrated Nanostructure Physics, Institute for Basic Science, Sungkyunkwan

University, Suwon 440-746. Korea. ‡

Department of Energy Science, Department of Physics, Sungkyunkwan University, Suwon 440-

746. Korea. §

Device Laboratory, Samsung Advanced Institute of Technology, Suwon 449-712. Korea.

*Corresponding author E-mail: [email protected]

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ABSTRACT While transmission electron microscopy and scanning tunneling microscopy reveal atomic structures of point defect and grain boundary in monolayer transition metal dichalcogenides (TMDs), information on point defect distribution in macroscale is still not available. Herein, we visualize the point defect distribution of monolayer TMDs using dark-field optical microscopy. This was realized by anchoring silver nanoparticles on defect sites of MoS2 under light illumination. The optical images clearly revealed that the point defect distribution varies with light power and exposure time. The number of silver nanoparticles increased initially and reached a plateau in response to light power or exposure time. The size of silver nanoparticles was a few hundred nanometers in the plateau region as observed using optical microscopy. The measured defect density in macroscale was ~2 × 1010 cm-2, slightly lower than the observed value (4 × 1011 cm-2) from scanning tunneling microscopy.

KEYWORDS Molybdenum disulfide, Point defect distribution, Dark-field optical microscopy, Light illumination, Ag nanoparticle


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The ability to convert indirect bandgap of multilayer transition metal dichalcogenides (TMDs) to direct bandgap of monolayer TMDs and existence of various bandgaps in a series of TMDs1-3 are great advantages over bulk Si, and thus open a new possibility to utilize TMDs for transparent, flexible, and stretchable optoelectronic devices.4-8 Nevertheless, the studies are mostly limited to micron-sized flakes that are typically obtained by mechanical exfoliation. Recently, it has been possible to obtain large-area TMDs. For example, monolayer MoS29, 10 and WS211 were grown to a large area by chemical vapor deposition (CVD), demonstrating the possibility to integrate layered structures. However, control of structural defects is still limited. Structural defects including grain boundaries and point defects have a significant influence on electrical transport12-15 and thermal properties.16 Therefore, a systematic approach is required for analyzing such defects. Information on the atomic rearrangement at grain boundaries or vacancies can be obtained using transmission electron microscopy (TEM) or scanning tunneling microscopy (STM).10, 17-19 However, information on the distribution of such defects in macroscale is not easily accessible, although electrical transport and thermal properties are easily affected by such macroscopic defect distribution. For example, the sheet resistance of graphene is inversely proportional to its grain size, following a scaling law.20 Optical approaches including second harmonic generation21, 22

and liquid crystal23 have been introduced to visualize grain boundaries on CVD-grown MoS2.

Although these methods are noninvasive, sophisticated optical systems are required. In addition, modification of sample surface morphologies by selective oxidation20, 24-26 and decoration with metal nanoparticles on defect sites27,

28

makes it feasible to observe line defects (grain

boundaries, wrinkles, and cracks). Furthermore, these approaches are not suitable for observing point defects. Although point defects of TMDs have been observed by decoration with metal nanoparticles on defect sites of MoS2 using atomic force microscopy (AFM), field-emission scanning electron microscopy (FESEM), and TEM,29 which are still limited to atomistic information, visualizing macroscopic point-defect distribution has not been achieved to date. In this article, we visualize point defects as well as grain boundary distributions on CVD-grown TMDs using dark-field optical microscopy. Visualization was achieved by selectively anchoring Ag nanoparticles on defect sites of TMDs, which were generated by annealing the TiO2/Ag film and aggregated on defect sites of TMDs under white light illumination. While Ag nanoparticles 3 ACS Paragon Plus Environment

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are formed and diffused out through TiO2 during annealing, Ag nanoparticles are photoexcited by localized surface plasmon resonance under light illumination, transferring electrons to TiO2, and dissolving into Ag+ ions, as reported previously.30 The generated Ag+ ions are reduced and redeposited on defect sites of TMDs. As the light power increased or exposure time was prolonged, the number of Ag nanoparticles increased rapidly initially and reached a constant value at a later stage. The population of Ag nanoparticles deposited on TMDs also relied on the incident light wavelength. The defect density was obtained at the plateau region by counting the number of Ag nanoparticles in a given area that was averaged over the entire region of interest. The estimated defect density on CVD-grown monolayer MoS2 was around 2 × 1010 cm-2, which is slightly less than the value obtained from STM. This approach using dark-field optical microscopy is applicable to a wide range of TMDs for observing macroscale point-defect distribution.

RESULTS AND DISCUSSION Figure 1a is the dark-field optical image of CVD-grown monolayer MoS2 transferred onto the TiO2/Ag substrate. Sol-gel prepared TiO2 layer was spin-coated on Ag film deposited by thermal evaporator. Large-area (2 × 2 cm2) monolayer MoS2 grown on SiO2/Si substrate by CVD was coated with polymethyl methacrylate (PMMA), floated in deionized water, and then simply fished on the prepared TiO2/Ag film. The sample was dried for 20 min under 60 mW white light illumination. This preparation process is briefly illustrated in Supporting Information (SI), Figure S1. The light spot area was 4 cm in diameter. The optical image of MoS2 on SiO2/Si substrate (left inset of Figure 1a) shows a portion of large-area monolayer MoS2 focusing on the empty region in pink. Several remarkable features are shown in Figure 1a: i) bright spots like the milky way, indicating the presence of point defects, are clearly visible and uniformly distributed over the entire area, ii) grain boundary lines are clearly visible, iii) multilayer portions are distinct from monolayer portions, and iv) edges are also identified at the empty region of MoS2. The magnified optical images and FESEM morphological images indicate that the bright spots were Ag nanoparticles or their aggregates with sizes of several hundred nanometers (see SI, Figure S2), where the presence of Ag element was confirmed by energy-dispersive X-ray spectroscopy (EDX) mapping (see SI, Figure S3). Ag nanoparticles generated during the sample preparation


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process (will be discussed later) are anchored preferably at the defect sites such as point defects, grain boundary lines, and edges. Such remarkable visualization of point defects was not clearly obtained in the bright-field image (right inset in Figure 1a). Dark-field image of MoS2 on SiO2/Si substrate (see SI, Figure S4) revealed clear edges at the empty region of MoS2, but no obvious point defects were visible, similar to the aforementioned bright-field image. This clear distinction originates from the enhanced light scattering at the defect sites in dark-field optics. This concept of observation can be generalized to other TMD materials. Monolayer WS2 and WSe2 flakes grown using similar CVD methods with different sources (see the methods section) were identified by Raman spectra (see SI, Figure S5). The point defects and grain boundaries were revealed to be similar to those of monolayer MoS2 flakes (Figures 1b-1d). Further study is needed to investigate the presence of lesser point defects in WSe2 flakes than in other TMD flakes. The effect of white light power on the defect visualization was investigated and shown in Figure 2. The exposure time was set to be 20 min to ensure that the defect density reaches plateau at a given power (see SI, Figure S6). At a low power of 0.1 mW, only point defects were observed. No Ag nanoparticles were observed within the yellow region (no MoS2 layer), indicating a preferable adsorption of Ag nanoparticles at defect sites of MoS2. At the low power region, the defect density increased in proportion to the light power and the size of Ag nanoparticles was reduced from ~800 nm at 0.1 mW to ~400 nm at 1 mW; however, the grain boundary lines were still not observed. At powers larger than 5 mW, the grain boundary lines were observed clearly. Moreover, the defect density increased significantly and plateaued at 20 mW. The size of Ag nanoparticles reached 150-200 nm (see SI, Figure S7). Ag nanoparticles did not adsorb in the empty area even at powers higher than 20 mW. Although point defects are visualized clearly with dark-field optics, which is a great advantage over sophisticated STM or TEM, the ability to measure the point defect density quantitatively still remains questionable. Figure 3a shows the defect density as a function of white light power. The number of Ag nanoparticles was counted for an area of 4 × 4 µm2 from the optical images and averaged over several regions. The density value increased rapidly initially and plateaued to 2.47 ×108 cm-2 at 20 mW. Although previous work showed the generation of defects or damages


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by strong laser irradiation,31, 32 the power density used in this study was smaller by ten orders of magnitude, negating the possibility of generating defects. More importantly, the plateau region at high power indicates that no new defects or damages were generated during light illumination. Such a density variance in response to the light power strongly suggests that the light promotes anchoring of Ag nanoparticles on defect sites of MoS2. The number of Ag nanoparticles was counted at the plateau region to ensure the reliability of our approach. The average size of Ag nanoparticles at plateau region was 150-200 nm. To estimate the number of defects, we used the defect density obtained from the STM measurements, i.e., 4 × 1011 cm-2 (see SI, Figure S8). By calculating the number of defects within the Ag nanoparticles and the total area of Ag nanoparticles within a given MoS2 area, the defect density obtained from our dark-field optics was 2 × 1010 cm-2 (see SI, calculation method). This value is lower than the measured STM value by 20 times. A similar defect-density evaluation was done with varying light exposure time at a fixed power of 60 mW, as shown in Figure 3b. The defect density reached a plateau at an exposure time longer than 10 min. The counted defect density was again very similar to the power dependence. This consistency in the density evaluation between light power and exposure time in the plateau region strongly indicates that the measurement of defect density is reliable and independent of the process conditions. To prove if there is any sample dependence, the CVD-grown sample was further treated by oxygen plasma (see SI, Figure S9). Defect density was modified by plasma treatment. The dark-field optical image shows the higher density of Ag nanoparticles from the oxygen plasma-treated sample. During oxygen plasma treatment, more defects are generated and oxygen molecules are adsorbed on the defect sites. Therefore, the density of adsorbed Ag nanoparticles increased by about twice after oxygen plasma treatment. The modified defect density was clearly observed with our approaches. We conclude that our method is facile and qualitatively acceptable in estimating the change of point defect density but limitation still exists for quantitative analysis of point defects. To evaluate the light energy dependence, several band filters were used to select wavelength of the light from white light source. The output power was ~0.13 mW in the optical range of 400800 nm. Figure 4 shows defect visualization at various wavelengths. At 780 nm, defects were well visualized but with a lower density compared to those at higher energies. No specific grain boundary lines were formed. At 640 nm, the grain boundary lines were visible and defect density


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greatly increased. The defect density reached maximum at 540 nm with clear boundary lines. For example, the light at 780 nm (1.59 eV) cannot excite electrons at the band edge because the absorption peaks for A and B peaks are located at 1.85 and 1.97 eV2. The defects have been identified to be ~1.75 eV (measured from photoluminescence (PL)).33, 34 We speculate that the point defects shown in Figure 4a are higher-order defects or complexes with environmental gases having the defect-related gap states below 1.59 eV. At 640 nm (1.94 eV), point defects can be directly excited and induces Ag nanoparticle adsorption. At 540 nm (2.3 eV), the increased density is strongly related to the plasmon energy of Ag nanoparticles. The plasmon energy of our sample (Ag particle size of ~ 200 nm) was nearly 2.4 eV, whose values slightly vary with different sizes of Ag nanoparticles (see SI, Figure S10). The plasmon-coupled incident light enhances adsorption of Ag nanoparticles on defect sites. Nevertheless, the defect density was underestimated, implying insufficient Ag supply. The film preparation process in Figure 5a demonstrates how Ag nanoparticles are formed during the substrate preparation process. Sol-gel prepared porous TiO2 layer (thickness: 50 nm) was spin-casted on a 100 nm thick (Figure 5b) Ag film that was deposited by thermal evaporator and further annealed in air at 100 oC for 15 min. During annealing, Ag nanoparticles were embedded both within the porous TiO2 layer (middle panel in Figure 5b) and on its surface due to extended diffusion of Ag atoms (see SI, Figure S11), which was similar to previous report.35 After MoS2 was transferred in deionized water followed by drying in air and PMMA removal, large Ag nanoparticles with a size of ~ 300 nm below MoS2 (not visible), which are tunable with laser power as mentioned before, were formed on TiO2 surface (bottom panel in Figure 5b). EDX mapping of the cross-sectioned region near the bottom panel of Figure 5b clearly revealed the presence of Ag at the surface and below TiO2 layer (Figure 5c). Randomly distributed Ag nanoparticles were observed from the top-view FESEM image (Figure 5d). The inset shows an amplified image of the yellow region, indicating aggregates of Ag nanoparticles. The size and height profile of Ag particles measured by AFM confirms the FESEM images (Figures 5e and f). It has been known that under illumination, Ag nanoparticles anchored on TiO2 substrate in aqueous solution can be dissolved in a form of Ag+ ions by transferring electrons to TiO2 layer.30 The produced Ag+ ions in solution can be further reduced and re-deposited on defect sites of MoS2 to form Ag nanoparticles, in which electrons are supplied by the light illumination: Ag+ 7 ACS Paragon Plus Environment

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(solution) + e- Ag0 (MoS2). This process is similar to chemically activated Ag ions on MoS2 under light illumination.36 The Ag particles with sizes of several hundred nanometers can be easily generated in our process so that they can be observed via dark-field optics. The light illumination plays a key role in promoting the related chemical reactions by an efficient charge transfer process. The density of Ag+ ions and consequently Ag nanoparticles on MoS2 is therefore efficiently modulated by the light power/exposure time. The number of adsorbed Ag nanoparticles on the defect sites increased initially and reached a plateau due to saturated defect sites. In addition, the incident light is coupled to plasmons in Ag nanoparticles, confirmed by the strong reaction at the incident light matching with plasma frequency of Ag nanoparticles. The generation of electrons in MoS2 is another rate-determining step to Ag+ ion reduction, confirmed again the significantly reduced reaction with incident light energy smaller than the MoS2 bandgap. To see clearly how Ag atom can be adsorbed on defect sites in MoS2, we performed density functional calculations. Two model calculations were considered for preferable Ag adsorption (Figure 6): i) direct adsorption of Ag atom on S vacancy site and ii) adsorption of Ag atom on oxygen atom-saturated S vacancy. Total energy calculations were performed with density functional theory (DFT) using an open source, ab initio code Quantum ESPRESSO.37 The exchange correlation energy was described by the relativistic generalized gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) exchange model.38 The lattice parameter of MoS2 unit cell was 3.18 Å, and 4 × 4 supercell was fully relaxed until atomic forces were less than 0.05 eV/Å. 12 × 12 × 1 Monkhost-pack grid K-point was used for k-point sampling. The adsorption energy was calculated by Ead(Ag) = Etotal(Ag+MoS2(v)) – Etotal(Ag) – Etotal(MoS2(v)). We calculate Ead(Ag) = + 2.15 eV, suggesting that simple Ag adsorption on S vacancy site is energetically unfavorable. By the similar calculations, oxygen adsorption energy on S vacancy site, Ead(O) = - 8.69 eV, forming strong chemical bonds between oxygen and Mo with a bond length of 2.07 Å. This implies that defect sites are likely saturated with oxygen atoms or molecules. Ag atom can be adsorbed on oxygen-saturated sulfur vacancy easily under ambient conditions. When Ag atom is adsorbed on oxygen-saturated sulfur vacancy, Ag adsorption energy Ead(Ag-O) = - 0.51 eV with a bond length (Ag-O) of 2.33 Å. Meanwhile, the bond length of (Mo-O) is strengthened to 2.01 Å. These calculations strongly suggest that Ag atoms can be


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aggregated on oxygen atom-saturated defect sites rather than directly on sulfur vacancy sites in MoS2. The theoretical predictions are in good agreements with the increased defect density by oxidation (see SI, Figure S12). We further demonstrate that Ag particles are adsorbed on the defect sites and grain boundaries (a collection of defects in the form of a line). Figure 7a shows a local area of empty space (left) with two MoS2 grains merging in the middle of the right part. Ag nanoparticles were clearly anchored along the boundary line and edge (Figure 7c), although those Ag nanoparticles anchored at the point defects were washed out mostly during TEM grid preparation process. The grain boundary was identified by different colors at different orientations, in which two grain orientations were deviated from each other by 23° (Figure 7b). The modification of optical properties of monolayer MoS2 where Ag nanoparticles are anchored on defect sites was probed using Raman spectroscopy with 532 nm laser excitation (Figure 8). The Raman mapping image for A1g intensity of MoS2 revealed non-uniform intensity distribution. In particular, the intensities of A1g near Ag nanoparticles were stronger than the other areas on bare MoS2. Figure 8b shows the representative peak profiles from three different positions: MoS2/SiO2 substrate (bottom), MoS2/TiO2/Ag substrate (middle), and MoS2/Ag nanoparticle/TiO2/Ag. The upshift of A1g peak positions on Ag nanoparticles/TiO2/Ag and TiO2/Ag compared to that of SiO2 substrate is a signature of charge compensation of n-type MoS2.39 It is of note that E12g peak position on Ag nanoparticles/TiO2/Ag was downshifted compared to those on TiO2/Ag and SiO2 substrate. The deconvolution of E12g peak clearly showed downshift of E’+ (384.6 cm-1) with emerging E’- (380.3 cm-1) at the lower energy side. This could be explained by the tensile strain imposed by Ag nanoparticles. In addition, both Raman intensities were enhanced near Ag nanoparticles because of the high reflectance of Ag film and local electrical field enhancement by the presence of Ag nanoparticles.40 Figure 8c demonstrates that the intensity enhancement of A1g peak is larger than that of E12g peak. The intensity ratio of E12g peak to A1g peak decreased significantly to 0.2 on Ag nanoparticle/TiO2/Ag substrate from ~0.8 on SiO2 substrate (inset). In general, A1g peak intensity becomes higher by p-doping.29 This was done in our case by providing electrons of MoS2 to Ag+ ions to form Ag nanoparticles.


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CONCLUSION In summary, we successfully visualized point defects with dark-field optical microscopy by preparing a simple TiO2/Ag substrate without involving heavy chemistry. Our approach provides a qualitative way of analyzing point defects and can be used to monitor defect distribution that may vary with various growth conditions of large-area TMD materials during CVD. Therefore, this method can be complementary to sophisticated STM or TEM observations and may be useful for preliminary comprehensive analysis of point defect distribution for CVD-grown TMD materials.

METHODS Synthesis of monolayer MoS2, WS2, and WSe2 Ammonium heptamolybdate (AHM, Sigma-Aldrich, 431346) powder as a Mo precursor was dissolved in deionized water. The solution (6 µl) was dropped onto a quartz wafer (2 mm × 20 mm). The quartz wafer was put into a dry oven (~80 oC) next to the target wafer in the reactor that was coated by sodium cholate (SC) solution as described previously. S source (200 mg) was placed inside and sublimated prior to growth process. The Mo heating zone was heated to 780 oC at a ramping rate of 78 oC/ min, and the temperature of S zone was ramped up to 210 oC (42 o

C/min). During the entire process, N2 (500 sccm) was injected as a carrier gas. To synthesize tungsten dichalcogenides (WS2, WSe2), ammonium metatungstate hydrate

(AMT, Sigma Aldrich, 463922) as a W source was dissolved in deionized water (0.1 g/ml). Sulfur (200 mg) and Selenium (500 mg) were introduced to furnace to grow WS2 and WSe2, respectively. Instead of N2 used in the MoS2 synthesis process, hydrogen (5 sccm) was injected to reduce tungsten oxide to grow WS2 and WSe2. The W heating zone was heated to 800 oC at a ramping rate of 80 oC/ min and maintained for 5 minute. When the temperature of the W heating zone reached its maximum, S and Se zone temperature reached 210 oC and 400 oC, respectively. Preparation of substrate Ag film (100 nm) was deposited by thermal evaporation on SiO2/Si substrate. The Titanium (IV) butoxide solution (TiO2, Sigma-Aldrich, 510718, 5wt% in n-butanol) was spin-coated (2000 10 ACS Paragon Plus Environment

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rpm, 60 s) onto the Ag/SiO2/Si substrate and annealed at 100 oC for 15 min in air. During annealing, Ag nanoparticles were formed on the surface of TiO2 layer by diffusion through porous TiO2 layer. Decoration of Ag nanoparticles on defective sites of TMDs The CVD-grown TMDs were transferred to TiO2/Ag/SiO2/Si substrate with PMMA support. The PMMA A4 (MicroChem, 4 wt% in Anisole) was spin-coated onto the as-grown TMDs/SiO2 (at 1500 rpm for 60 s). The TMDs and PMMA support were detached from SiO2 by placing the TMDs/SiO2 into a hot 1M KOH solution for a few minutes. PMMA/TMD layer was then detached from SiO2/Si substrate and then floated onto the KOH solution. Next, the PMMA/TMDs were rinsed using deionized water for 4 times. Finally, the PMMA/TMDs were picked up by the prepared substrate and then illuminated using a white light source (LDLS EQ99FC) for 20 min. The light intensity and wavelength were controlled using a neutral density (ND) filter (0.1~ 200 mW) and a band filter (420 ~ 780 nm). During illumination, Ag ions were diffused to the defect sites of TMDs. PMMA was removed by acetone and the sample was rinsed with isopropyl alcohol (IPA) and deionized water for several times. TEM sample preparation The sample was transferred to a quantifoil TEM grid with a copper-supported thin film (TedPella, 200 mesh, copper, 1.2 µm holes). After Ag nanoparticle decoration under PMMA/MoS2 film during white light irradiation, the PMMA/MoS2 film with substrate was immersed into deionized water, fished using a TEM grid, and dried under ambient conditions. PMMA was then removed by acetone and the sample was rinsed with IPA. Characterization The morphology of TMDs on TiO2/Ag was examined using optical microscopy (20 × and 100 × magnification, WEISS, Axio Imager 2), AFM (Nano Navi) in tapping mode, and FESEM (JEOL JSM7000F). Raman mapping was performed using confocal Raman microscopy (NTEGRA Spectra, NT-MDT) system with an exciting laser wavelength of 532 nm and a 100 × objective (NA = 0.7) lens. The microstructure of MoS2 was obtained using TEM (JEMARM 200 F).


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Focused ion beam (SII SMI3050TB) was performed to visualize elements of nanoparticles and their cross-sectional structures.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS This work was supported by IBS-R011-D1 and by the Human Resources Development program (No. 20124010203270) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy.

ASSOCIATED CONTENT Supporting Information Available: Supporting figures S1-S12 and calculation method. This material is available free of charge via the Internet at http://pubs.acs.org.

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5. Pu, J.; Zhang, Y. J.; Wada, Y.; Wang, J. T. W.; Li, L. J.; Iwasa, Y.; Takenobu, T. Fabrication of Stretchable MoS2 Thin-Film Transistors using Elastic Ion-gel Gate Dielectrics. Appl. Phys. Lett. 2013, 103, 023505. 6. Lopez-Sanchez, O.; Lembke, D.; Kayci, M.; Radenovic, A.; Kis, A. Ultrasensitive Photodetectors Based on Monolayer MoS2. Nat. Nanotechnol. 2013, 8, 497-501. 7. Yin, Z. Y.; Li, H.; Li, H.; Jiang, L.; Shi, Y. M.; Sun, Y. H.; Lu, G.; Zhang, Q.; Chen, X. D.; Zhang, H. Single-Layer MoS2 Phototransistors. ACS Nano 2012, 6, 74-80. 8. Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S. Electronics and Optoelectronics of Two-Dimensional Transition Metal Dichalcogenides. Nat. Nanotechnol. 2012, 7, 699-712. 9. Han, G. H.; Kybert, N. J.; Naylor, C. H.; Lee, B. S.; Ping, J. L.; Park, J. H.; Kang, J.; Lee, S. Y.; Lee, Y. H.; Agarwal, R.; et al. Seeded Growth of Highly Crystalline Molybdenum Disulphide Monolayers at Controlled Locations. Nat. Commun. 2015, 6, 6128. 10. van der Zande, A. M.; Huang, P. Y.; Chenet, D. A.; Berkelbach, T. C.; You, Y. M.; Lee, G. H.; Heinz, T. F.; Reichman, D. R.; Muller, D. A.; Hone, J. C. Grains and Grain Boundaries in Highly Crystalline Monolayer Molybdenum Disulphide. Nat. Mater. 2013, 12, 554-561. 11. Yun, S. J.; Chae, S. H.; Kim, H.; Park, J. C.; Park, J. H.; Han, G. H.; Lee, J. S.; Kim, S. M.; Oh, H. M.; Seok, J.; et al. Synthesis of Centimeter-Scale Monolayer Tungsten Disulfide Film on Gold Foils. ACS Nano 2015, 9, 5510-5519. 12. Ghorbani-Asl, M.; Enyashin, A. N.; Kuc, A.; Seifert, G.; Heine, T. Defect-Induced Conductivity Anisotropy in MoS2 Monolayers. Phys. Rev. B 2013, 88, 245440. 13. Zhu, W. J.; Low, T.; Lee, Y. H.; Wang, H.; Farmer, D. B.; Kong, J.; Xia, F. N.; Avouris, P. Electronic Transport and Device Prospects of Monolayer Molybdenum Disulphide Grown by Chemical Vapour Deposition. Nat. Commun. 2014, 5, 3087. 14. Islam, M. R.; Kang, N.; Bhanu, U.; Paudel, H. P.; Erementchouk, M.; Tetard, L.; Leuenberger, M. N.; Khondaker, S. I. Tuning the Electrical Property via Defect Engineering of Single Layer MoS2 by Oxygen Plasma. Nanoscale 2014, 6, 10033-10039. 15. Yu, Q. K.; Jauregui, L. A.; Wu, W.; Colby, R.; Tian, J. F.; Su, Z. H.; Cao, H. L.; Liu, Z. H.; Pandey, D.; Wei, D. G.; et al. Control and Characterization of Individual Grains and Grain Boundaries in Graphene Grown by Chemical Vapour Deposition. Nat. Mater. 2011, 10, 443-449. 16. Ding, Z. W.; Pei, Q. X.; Jiang, J. W.; Zhang, Y. W. Manipulating the Thermal Conductivity of Monolayer MoS2 via Lattice Defect and Strain Engineering. J. Phys. Chem. C 2015, 119, 16358-16365. 17. Najmaei, S.; Liu, Z.; Zhou, W.; Zou, X. L.; Shi, G.; Lei, S. D.; Yakobson, B. I.; Idrobo, J. C.; Ajayan, P. M.; Lou, J. Vapour Phase Growth and Grain Boundary Structure of Molybdenum Disulphide Atomic Layers. Nat. Mater .2013, 12, 754-759. 18. Zhou, W.; Zou, X. L.; Najmaei, S.; Liu, Z.; Shi, Y. M.; Kong, J.; Lou, J.; Ajayan, P. M.; Yakobson, B. I.; Idrobo, J. C. Intrinsic Structural Defects in Monolayer Molybdenum Disulfide. Nano Lett. 2013, 13, 2615-2622. 19. Hong, J. H.; Hu, Z. X.; Probert, M.; Li, K.; Lv, D. H.; Yang, X. N.; Gu, L.; Mao, N. N.; Feng, Q. L.; Xie, L. M.; et al. Exploring Atomic Defects in Molybdenum Disulphide Monolayers. Nat. Commun. 2015, 6, 6293. 20. Duong, D. L.; Han, G. H.; Lee, S. M.; Gunes, F.; Kim, E. S.; Kim, S. T.; Kim, H.; Ta, Q. H.; So, K. P.; Yoon, S. J.; et al. Probing Graphene Grain Boundaries with Optical Microscopy. Nature 2012, 490, 235-239.


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21. Yin, X. B.; Ye, Z. L.; Chenet, D. A.; Ye, Y.; O'Brien, K.; Hone, J. C.; Zhang, X. Edge Nonlinear Optics on a MoS2 Atomic Monolayer. Science 2014, 344, 488-490. 22. Cheng, J. X.; Jiang, T.; Ji, Q. Q.; Zhang, Y.; Li, Z. M.; Shan, Y. W.; Zhang, Y. F.; Gong, X. G.; Liu, W. T.; Wu, S. W. Kinetic Nature of Grain Boundary Formation in As-Grown MoS2 Monolayers. Adv. Mater. 2015, 27, 4069-4074. 23. Kim, D. W.; Ok, J. M.; Jung, W. B.; Kim, J. S.; Kim, S. J.; Choi, H. O.; Kim, Y. H.; Jung, H. T. Direct Observation of Molybdenum Disulfide, MoS2, Domains by Using a Liquid Crystalline Texture Method. Nano Lett. 2015, 15, 229-234. 24. Ly, T. H.; Duong, D. L.; Ta, Q. H.; Yao, F.; Vu, Q. A.; Jeong, H. Y.; Chae, S. H.; Lee, Y. H. Nondestructive Characterization of Graphene Defects. Adv. Funct. Mater. 2013, 23, 5183-5189. 25. Ly, T. H.; Chiu, M. H.; Li, M. Y.; Zhao, J.; Perello, D. J.; Cichocka, M. O.; Oh, H. M.; Chae, S. H.; Jeong, H. Y.; Yao, F.; et al. Observing Grain Boundaries in CVD-Grown Monolayer Transition Metal Dichalcogenides. ACS Nano 2014, 8, 11401-11408. 26. Rong, Y. M.; He, K.; Pacios, M.; Robertson, A. W.; Bhaskaran, H.; Warner, J. H. Controlled Preferential Oxidation of Grain Boundaries in Monolayer Tungsten Disulfide for Direct Optical Imaging. ACS Nano 2015, 9, 3695-3703. 27. Kim, K.; Lee, H. B. R.; Johnson, R. W.; Tanskanen, J. T.; Liu, N.; Kim, M. G.; Pang, C.; Ahn, C.; Bent, S. F.; Bao, Z. N. Selective Metal Deposition at Graphene Line Defects by Atomic Layer Deposition. Nat. Commun. 2014, 5, 4781-4789. 28. Yu, S. U.; Park, B.; Cho, Y.; Hyun, S.; Kim, J. K.; Kim, K. S. Simultaneous Visualization of Graphene Grain Boundaries and Wrinkles with Structural Information by Gold Deposition. ACS Nano 2014, 8, 8662-8668. 29. Shi, Y. M.; Huang, J. K.; Jin, L. M.; Hsu, Y. T.; Yu, S. F.; Li, L. J.; Yang, H. Y. Selective Decoration of Au Nanoparticles on Monolayer MoS2 Single Crystals. Sci. Rep. 2013, 3, 1839. 30. Kazuma, E.; Sakai, N.; Tatsuma, T. Nanoimaging of Localized Plasmon-Induced Charge Separation. Chem. Commun. 2011, 47, 5777-5779. 31. Paradisanos, I.; Kymakis, E.; Fotakis, C.; Kioseoglou, G.; Stratakis, E. Intense Femtosecond Photoexcitation of Bulk and Monolayer MoS2. Appl. Phys. Lett. 2014, 105, 041108. 32. Castellanos-Gomez, A.; Barkelid, M.; Goossens, A. M.; Calado, V. E.; van der Zant, H. S. J.; Steele, G. A. Laser-Thinning of MoS2: On Demand Generation of a Single-Layer Semiconductor. Nano Lett. 2012, 12, 3187-3192. 33. Tongay, S.; Suh, J.; Ataca, C.; Fan, W.; Luce, A.; Kang, J. S.; Liu, J.; Ko, C.; Raghunathanan, R.; Zhou, J.; et al. Defects Activated Photoluminescence in Two-Dimensional Semiconductors: Interplay between Bound, Charged, and Free Excitons. Sci. Rep. 2013, 3, 2657. 34. Chow, P. K.; Jacobs-Gedrim, R. B.; Gao, J.; Lu, T. M.; Yu, B.; Terrones, H.; Koratkar, N. Defect-Induced Photoluminescence in Monolayer Semiconducting Transition Metal Dichalcogenides. ACS Nano 2015, 9, 1520-1527. 35. Kulczyk-Malecka, J.; Kelly, P. J.; West, G.; Clarke, G. C. B.; Ridealgh, J. A.; Almtoft, K. P.; Greer, A. L.; Barber, Z. H. Investigation of Silver Diffusion in TiO2/Ag/TiO2 Coatings. Acta. Mater. 2014, 66, 396-404. 36. Daeneke, T.; Carey, B. J.; Chrimes, A. F.; Ou, J. Z.; Lau, D. W. M.; Gibson, B. C.; Bhaskaran, M.; Kalantar-zadeh, K. Light Driven Growth of Silver Nanoplatelets on 2D MoS2 Nanosheet Templates. J. Mater. Chem. C 2015, 3, 4771-4778. 37. Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I.; et al. QUANTUM ESPRESSO: a Modular and Open-


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source Software Project for Quantum Simulations of Materials. J. Phys.Condens. Mat. 2009, 21, 395502. 38. Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett.1996, 77, 3865-3868. 39. Mouri, S.; Miyauchi, Y.; Matsuda, K. Tunable Photoluminescence of Monolayer MoS2 via Chemical Doping. Nano Lett. 2013, 13, 5944-5948. 40. Gong, C.; Huang, C. M.; Miller, J.; Cheng, L. X.; Hao, Y. F.; Cobden, D.; Kim, J.; Ruoff, R. S.; Wallace, R. M.; Cho, K.; et al. Metal Contacts on Physical Vapor Deposited Monolayer MoS2. ACS Nano 2013, 7, 11350-11357.


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Figures:

Figure 1. Visualization of defects for various CVD-grown TMDs. (a) Dark-field optical image of CVD-grown MoS2, and bright-field optical images from monolayer MoS2 on SiO2/Si substrate (left inset) and TiO2/Ag/SiO2/Si substrate (right inset). The structural defects including grain boundaries, point defects, and edges on monolayer MoS2 were visualized using dark-field optical microscopy. (b), (c), and (d) Dark-field optical images of MoS2, WS2, and WSe2. Scale bar is 20 µm.


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Figure 2. Modulation of defect distribution with white light power. Dark-field optical images of MoS2 on TiO2/Ag substrate irradiated for 20 min with white light at (a) 0.1, (b) 1, (c) 5, (d) 20, (e) 60, and (f) 200 mW. Yellow-dotted lines indicate empty regions of monolayer MoS2. The spot diameter of the illuminating white light is around 4 cm.


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Figure 3. Defect density determination of CVD-grown MoS2. Defect density as a function of (a) white light power and (b) exposure time. Red line indicates the number of Ag nanoparticles on MoS2 with an area of 1cm2. Blue line indicates the extracted defect density by taking into account the number of defects obtained from STM observations. The green line is the defect density obtained from STM measurement.


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Figure 4. Modulation of defect distribution with light wavelength. The light energy was selected with a band filter from white light source; (a) 780, (b) 640, (c) 540 and (d) 420 nm. Light energy is (a) lower and (b-d) higher than absorption peak position of MoS2. Red-dotted lines in (c) and (d) indicate bilayer or multilayer. Yellow-dotted lines indicate empty region of monolayer MoS2. The monolayer MoS2 was illuminated under 0.13 mW light power.


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Figure 5. Substrate preparation process. (a) Schematic illustration of the formation and seeding of Ag nanoparticles on defect sites of monolayer MoS2, (b) FESEM cross-sectional view followed by deposition processes (Ag, TiO2, and MoS2), (c) Cross-sectional TEM image and the corresponding EDX mapping image to identify Ag nanoparticles, (d) FESEM image of point defect distribution and enlarged point defect (inset), (e) AFM surface morphology using a tapping mode, and (f) the height profile of the Ag nanoparticle indicated by the yellow circle in (e).


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Figure 6. Calculation of adsorption energy of Ag atom on defect sites. DFT-GGA calculations were performed to obtain the adsorption energy calculations. i) Case 1: direct adsorption of Ag atom on S vacancy and ii) case 2: adsorption of Ag atom on oxygen atom-saturated S vacancy. The adsorption energy: Ead(Ag)= + 2.15 eV (case 1), unstable and Ead (Ag-O) = - 0.51 eV (case 2).


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Figure 7. TEM images of Ag nanoparticles deposited along the grain boundary of CVD-grown MoS2. (a) TEM image of monolayer MoS2 after decorating with Ag nanoparticle followed by transferring onto a TEM grid. Ag nanoparticles were nucleated along grain boundary and edge, (b) Selective-area electron diffraction (SAED) patterns obtained on grain 1 and 2, and (c) Darkfield TEM images showing Ag nanoparticles located on the grain boundary (top) and on the edge (bottom). Many Ag nanoparticles were detached during transfer onto the TEM grid.


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Figure 8. Raman characterization of CVD-grown MoS2 on Ag nanoparticles using a 532 nm laser (300 µW). (a) Raman mapping image with the A1g peak intensity. The intensity of MoS2 on Ag nanoparticles is enhanced compared to that without Ag nanoparticles and highlighted around them. (b) Raman spectra of MoS2 at various positions. The substrate affects Raman peak shift and intensity. The insert graph shows downshift of E12g peak and peak-split into two peaks because of the strain applied to MoS2 by Ag nanoparticles. (c) Enhancement of Raman intensity and the intensity ratio (inset).


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Table of Contents Graphic and Synopsis


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Visualizing Point Defects in Transition-Metal Dichalcogenides Using

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