Growth Mechanism of Transition Metal Dichalcogenide Monolayers

May 3, 2016 - Due to their unique optoelectronic properties and potential for next generation devices, monolayer transition metal dichalcogenides (TMD...
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Growth Mechanism of Transition Metal Dichalcogenide Monolayers: The Role of SelfSeeding Fullerene Nuclei Jeffrey D. Cain,†,‡ Fengyuan Shi,†,§ Jinsong Wu,†,§ and Vinayak P. Dravid*,†,‡,§ †

Department of Materials Science and Engineering, ‡International Institute for Nanotechnology, and §Northwestern University Atomic and Nanoscale Characterization Experimental (NUANCE) Center, Northwestern University, Evanston, Illinois 60208, United States S Supporting Information *

ABSTRACT: Due to their unique optoelectronic properties and potential for next generation devices, monolayer transition metal dichalcogenides (TMDs) have attracted a great deal of interest since the first observation of monolayer MoS2 a few years ago. While initially isolated in monolayer form by mechanical exfoliation, the field has evolved to more sophisticated methods capable of direct growth of large-area monolayer TMDs. Chemical vapor deposition (CVD) is the technique used most prominently throughout the literature and is based on the sulfurization of transition metal oxide precursors. CVD-grown monolayers exhibit excellent quality, and this process is widely used in studies ranging from the fundamental to the applied. However, little is known about the specifics of the nucleation and growth mechanisms occurring during the CVD process. In this study, we have investigated the nucleation centers or “seeds” from which monolayer TMDs typically grow. This was accomplished using aberration-corrected scanning transmission electron microscopy to analyze the structure and composition of the nuclei present in CVD-grown MoS2− MoSe2 alloys. We find that monolayer growth proceeds from nominally oxi-chalcogenide nanoparticles which act as heterogeneous nucleation sites for monolayer growth. The oxi-chalcogenide nanoparticles are typically encased in a fullerene-like shell made of the TMD. Using this information, we propose a step-by-step nucleation and growth mechanism for monolayer TMDs. Understanding this mechanism may pave the way for precise control over the synthesis of 2D materials, heterostructures, and related complexes. KEYWORDS: two-dimensional materials, chalcogenides, transition metal dichalcogenides, chemical vapor deposition, growth, inorganic fullerene

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become the most widely used method for the direct growth of TMD monolayers. Such CVD-grown monolayers exhibit high optical18 and electronic quality19 and achieve the best balance between material quality, areal coverage, and potential for scalability. The technique is based on the reaction of transition metal oxide powders (MoO3, WO3) and a chalcogen in the vapor phase to form the TMD of interest. This method has shown incredible versatility and has been used to synthesize the W- and Mo-based dichalcogenides as well as their doped,20 alloyed,21−23 and heterostructured24,25 versions. For these reasons, this method has become an integral part of the progress of the field as a whole. However, while much has been hypothesized, little is known about the true nature of the

he discovery of two-dimensional (2D) transition metal dichalcogenides (TMDs) has stimulated a great deal of work in fields ranging from fundamental studies of charge transport and optical processes in solids to next generation optoelectronic device design. In parallel with these advances, the need for high-quality, large-area monolayer materials has incited an incredible amount of work on thinfilm synthesis and growth. While graphene and the TMDs have historically been isolated via top-down methods such as mechanical exfoliation1,2 (the “scotch-tape” method), solution sonication,3,4 and electrochemical exfoliation,5,6 these methods are plagued by issues of low yield, small lateral dimensions, or poor quality. A variety of vapor-phase techniques have since been introduced in order to alleviate these problems; these include, catalyst-free vapor transport,7,8 metal film sulfurization,9,10 evaporative thinning,11,12 organic precursor chemical vapor deposition,13 and oxide precursor chemical vapor deposition14−17 (CVD). It is the latter technique that has © 2016 American Chemical Society

Received: March 10, 2016 Accepted: May 3, 2016 Published: May 3, 2016 5440

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commonly employed substrate), ensuring that the nucleation and growth process is the same. Also, because the membranes are electron transparent, this tactic avoids the need for any transfer processes for TEM sample preparation. Transmission electron microscopy specimens of 2D materials are commonly made using a polymer-assisted method in which a thin polymer film, usually poly(methyl methacrylate), is applied to the growth substrate.28 The substrate is then back-etched to free the polymer/monolayer film so that it can be applied to a TEM-friendly substrate. The polymer must then be dissolved in an organic solvent. Directly growing the monolayers onto the ultrathin membranes assures clean samples with no organic residue or sample degradation that may occur during transfer. Furthermore, this avoids the need for any superfluous cleaning processes, such as high-temperature annealing24 or Ar/O2 plasma treatment, which may cause damage to the nucleation centers or obscure the results. The atomic scale structure of the nucleation centers was studied using bright-field (BF) imaging, high-angle annular dark-field (HAADF) imaging, and nanoscale EDS mapping. The few/monolayer samples were imaged at 200 kV in an aberration-corrected dedicated STEM with no beam damage to the flakes. Low-magnification images of the TMD flakes are shown in Figure 1. It was observed that the nucleation centers,

nucleation and growth processes and the specific reaction steps that occur during monolayer formation. Two possible reaction pathways have been previously proposed: (1) suboxide (e.g., MoO3−x) species condense onto the growth substrate, reacting with the chalcogen(s) and forming the TMD; (2) the oxide and chalcogens react only in the vapor phase, and the TMD deposits directly onto the substrate.26,27 However, neither of these mechanisms has been validated nor confirmed. Understanding these mechanisms is integral for the progress of the field and has great implications for suitability of the technique for large-scale growth. Additionally, these concepts underlie the application of the technique for the creation of heterostructured and alloyed 2D semiconductors with precisely controlled structure, position, compositions, and properties. Here, we report a step-by-step nucleation and growth mechanism of CVD monolayers. This has been elucidated through structural and compositional analysis of the nuclei, or “seeds”, at the center of CVD-grown MoS2−MoSe2 monolayers, though we believe the following observations and analysis are readily applicable to the entire TMD family. These seeds are present at the center of the CVD-grown monolayers and are ubiquitous both in this study and in studies throughout the literature. As far back as the first report of CVD-grown MoS2 by Lee et al.,14 their presence has been known and commented on. High-resolution scanning transmission electron microscopy (STEM), nanoscale X-ray energy-dispersive spectroscopy (EDS) mapping, and nanodiffraction reveal that the seeds are actually oxi-chalcogenide/TMD core−shell nanoparticles from which monolayer growth proceeds. The nature of these nuclei sheds light on the dynamics of nucleation and growth and illuminates the true nature of the CVD growth process.

RESULTS AND DISCUSSION The MoS2−MoSe2 monolayers examined in this study were grown using the oxide-based CVD technique outlined above and with parameters standard throughout the literature.14,15,24 In brief, MoO3 powders were placed in an alumina boat at the hot center of a 12 in. tube furnace and the temperature increased to 650 °C; upstream, a mixture of sulfur and selenium powders was placed in a lower temperature zone within the furnace and heated to ∼300 °C. The deposition substrates were suspended, face-down, above the oxide-containing alumina boat, where the MoS2−MoSe2 monolayers were deposited. During the entire process, ultra-high-purity argon was flowed into the chamber to act as a carrier gas and protective inert atmosphere. The parameters used above are representative of those found throughout the literature for Mo-based TMDs, and it can be assumed in all cases that the nucleation and growth of monolayers occur in a similar manner. The exact nature of the mechanism present within this growth process is still unknown, and direct in situ observation of the nucleation and growth phenomena is currently inaccessible. It is then necessary to explore other means by which we can gain insight into these processes; here, that is done by complete characterization of the TMD nucleation centers, which are representative of the first stages of growth of TMD monolayers. In order to effectively study these nucleation centers via STEM techniques, single- and few-layer MoS2−MoSe2 samples were directly grown onto 20 nm thick amorphous SiO2 membranes using the method outlined above. This process was chosen for multiple reasons; first, this strategy mimics the growth of the TMD on SiO2/Si substrates (the most

Figure 1. Scanning transmission electron microscopy images of the MoS2−MoSe2 alloy grown directly onto SiO2 membranes. (a) Lowmagnification HAADF image, multilayer sample. Inset: nanobeam diffraction pattern. (b) Atomic force microscopy image of monolayer MoS2, with the height profile showing the thickness of the nucleation center. (c) Low-magnification BF image of monolayer MoS2−MoSe2 sheets. Inset: monolayer lattice image (scale bar 1 nm). (d) BF image nucleation center.

with sizes ranging from 10 to 30 nm, are consistently present at the exact center of each triangular flake and nowhere else. This fact precludes the possibility that the observed nanostructures were deposited after the monolayer sheets and confirms that they instead represent the early stages of TMD growth. It was also observed that any secondary or tertiary layer growth initiates at this site. The order of this process is also indirectly confirmed via compositional analysis, as shown later. Direct lattice imaging, coupled with nanobeam diffraction, confirms 5441

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Figure 2. (a) High-magnification HAADF image of the nucleation center and low-magnification image of the underlying sheet. (b) TMD fullerene shell and (c) image intensity plot showing interplanar spacing. (d) Nucleation center from (a), false colored by corresponding fast Fourier transform (see Figure S4, Supporting Information). (e) High-resolution STEM HAADF image of the MoS2−MoSe2 monolayer lattice.

Figure 3. (a) Reference image for (b) EDS maps of the O, Mo, S, and Se distributions within the nucleation center. Scale bars 10 nm. (c) Schematic model of the cross section of the nucleation center with the core−shell fullerene structure.

with the c-axis spacing of multilayer TMDs (MoS2, 6.15 Å; MoSe2, 6.46 Å). Compositional mapping of Mo, S, Se, and O distributions in the nuclei helps to elucidate the details of the structure and further provides the details of the CVD reaction and nucleation process, shown in Figure 3. EDS maps of the nuclei show an inhomogeneous distribution of the elements noted above. Specifically, analysis of the oxygen K series shows that there is a high concentration of oxygen in the core of the fullerene core−shell structure. Conversely, the core is also

that the underlying monolayer sheets exhibit the correct 2H crystal structure (Figure 1a). In Figure 2, we present highmagnification HAADF images showing the morphology of the nucleation centers. It is observed that all of the nuclei exhibit a nanoscale core−shell structure, exactly similar to that of inorganic TMD fullerenes. Specifically, there is multilayer TMD material fully encasing the oxi-chalcogenide core. A line scan of the BF image intensity shows that the interplanar spacing of the multilayer structure is 6.3 ± 0.3 Å, consistent 5442

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not supplied via the suboxide nanoparticle core, which would not have sufficient Mo content for microscale monolayer growth. Later in this stage, the fullerene shell forms as the S/Se concentration in the vapor peaks and a chalcogen-rich atmosphere is reached. The process then stops when the precursor sources are exhausted. This process is shown schematically in Figure 4, and the reaction path is shown in the pseudoternary phase diagram in Figure 4e. It can then be deduced that the monolayer growth process is essentially a “self-seeding” one, in which the suboxide nanoparticles act as the nucleation sites for the growth of monolayers, which grow during stage 2 from fully reduced TMD vapor. In the past, multiple strategies have been introduced to “seed” the growth of TMD monolayers by the use of either large aromatic molecules (perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt, 3,4,9,10-perylene tetracarboxylic acid dianhydride)34 or patterned oxide precursor particles.35 These aim to promote the growth of monolayers by providing nucleation sites. However, in the present method, which again is used most throughout the literature, it can be seen that the seed is provided by the oxi-sulfide/selenide core, eliminating the need for external seeds. This self-seeding process could potentially be controlled via substrate engineering, thereby allowing for the precise positional control of monolayer growth in a single-step growth process.

sulfur- and selenium-deficient with respect to the surrounding area, while the Mo distribution is essentially homogeneous. The above results suggest that the structure is composed of a partially sulfurized/selenized molybdenum trioxide, MoO3−x(S,Se)y, core that is wrapped in a fullerene-like shell of MoS2−MoSe2. This again confirms that the core is deposited before the monolayer due to the presence of oxygen, which would have otherwise been reduced in the later stages of the growth process. Using these two pieces of information, that is, the presence of an oxi-chalcogenide core within a fullerene structure, the exact nature of the nucleation process can be elucidated. The vapor-phase synthesis of TMD fullerenes from oxide precursors is well-studied, and the exact mechanics of formation are known.29−32 This process consists of three stages (Scheme 1): (1) MoO3 evaporates in Mo3O9, Mo4O12, and Scheme 1. Potential Pathways for the Reaction of MoO3 and Chalocogens

CONCLUSIONS We have investigated the formation and growth processes in TMD monolayers utilizing diverse analytical techniques. Fewand monolayer samples of a MoS2−MoSe2 alloy were directly grown onto ultrathin SiO2 membranes, and the structure, chemistry, and conformation of the nucleation centers were investigated using aberration-corrected STEM imaging and elemental EDS mapping. The presence of a core−shell structure in which an oxi-sulfide/selenide core is wrapped in a multilayer TMD fullerene is observed at the center of each monolayer flake. Based on the knowledge of the vapor-phase synthesis of TMD fullerenes, the mechanics of the nucleation process of monolayer TMDs are revealed. This is characterized by the evaporation of Mo−O molecular clusters and the condensation of MoO3−x(S,Se)y nanoparticles onto the growth substrate in a chalcogen-deficient atmosphere. Then, as the temperature increases, the complete transformation to the TMD in the optimal sulfur/selenium atmosphere is achieved, resulting in the nucleation and growth of monolayers at the nanoparticles site. Though the nucleation process described here is for a MoS2−MoSe2 alloy, similar “seed” structures have been observed in other studies of TMDs grown via the oxidebased CVD method. This model naturally extends to the other TMDs synthesized with this method regardless of oxide (e.g., WO3) because the mechanism does not depend on the specific transition metal. Until now, knowledge of the basic nucleation and growth dynamics has been lacking and has relied largely on speculation. This knowledge may provide information for future studies of substrate engineering for controlled nucleation and the synthesis of doped, alloyed, and heterostructured 2D materials. Further, it has implications for the suitability and applicability of this method for large-scale deposition.

Mo5O15 molecular clusters;33 (2) the molecular clusters condense onto the deposition substrate, resulting in MoO3−x suboxide nanoparticles; and (3) the nanoparticles are then fully sulfurized to form the inorganic fullerenes.33 The nanoparticles in (2) range in size from 5 to 300 nm, consistent with our results. It is also known that the concentration of the chalcogen vapor will determine the dynamics of the growth process. In a weakly reducing atmosphere (i.e., sulfur/selenium-poor), the formation of suboxide or oxi-chalcogenide particles with an orthorhombic crystal structure will occur.32 Conversely, a sulfur/selenium-rich atmosphere has been shown to result in the formation of TMD fullerenes.33 A TMD monolayer will thus form in a moderately reducing environment.26 Armed with this information, we propose the following mechanism for the formation of the partially reduced core− shell structure and the growth of monolayer TMDs. In stage 1 of the CVD growth process, during the ramp up to the final temperature (Figure 4a,c), a small amount of both the oxide precursor and the S/Se (Tm = 115 °C/220 °C) are in the vapor phase. This results in a weakly reducing environment and the partial reduction of the MoO3 to MoO3−x(S,Se)y molecular clusters, which then condense into nanoparticles (cores) on the SiO2 substrate; this stage is analogous to steps (1) and (2) in the formation of TMD fullerenes. As the temperature of the CVD chamber continues to increase and the final growth temperature is reached (stage 2), there is a transition to a moderate S/Se atmosphere. This results in a more complete reduction of the oxide precursor and full transformation of the MoO3 to Mo(S,Se)2 in the vapor phase. The oxi-sulfide/ selenide core then acts as a heterogeneous nucleation site for the growth of monolayer TMDs, which is supplied from fully reduced Mo(S,Se)2 molecular clusters in the vapor. The Mo is

METHODS Sample Synthesis. Monolayer samples for STEM imaging were synthesized using the standard oxide-based chemical vapor deposition technique. Molybdenum trioxide (MoO3) powder (10 mg) was loaded 5443

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Figure 4. Schematic showing the proposed nucleation and growth dynamics during (a) stage 1 and (b) stage 2. (c) Time vs temperature profile for growth. Stage 1 shows the nucleation of suboxide nanoparticles onto the substrate, and stage 2 shows the growth of the TMD monolayer and fullerene shell. (d) Schematic of the S/Se vapor pressure as a function of growth time. (e) Schematic of the molybdenum−oxygen− chalcogen ternary phase diagram. into an alumina boat and placed at the hot center of a 1 in. diameter tube furnace. Upstream, a 50−50 mixture of sulfur and selenium powders (∼300 mg) was also loaded into an alumina boat at the outer edge of the furnace. The hot center of the furnace was increased to 650 °C over the course of 30 min and held at that temperature for 5 min. The S/Se mixture was brought to ∼300 °C and held there with the same timing. The growth substrates, in this case, 20 nm thick SiO2 membranes, were suspended above the oxide-containing boat. During the entire process, 15 sccm of ultra-high-purity Ar was flowed into the growth chamber to aid in vapor transport and act as a protective inert atmosphere. The entire process takes place at atmospheric pressure. STEM Imaging. All samples were imaged at 200 kV using an aberration-corrected dedicated scanning transmission electron microscope. Specifically, the following microscopes were used: Hitachi HD 2700 STEM and JEOL ARM 200 CF. Samples were gently cleaned prior to imaging with UV-ozone cleaner.

ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation under Grant No. DMR-1507810. This work made use of the EPIC facility of the NUANCE Center at Northwestern University, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF NNCI-1542205); the MRSEC program (NSF DMR-1121262) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois, through the IIN. J.D.C. is supported by the Department of Defense (DoD) through the National Defense Science and Engineering Fellowship (NDSEG) Program. J.D.C. also gratefully acknowledges support from the Ryan Fellowship and the Northwestern University International Institute for Nanotechnology. The authors would also like to acknowledge Harvard University, the University of IllinoisChicago, and Hitachi for the use of their microscopy facilities.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b01705. Figures S1−S4. Additional EDS maps, TEM images, nanoelectron diffraction, and FFT referenced in the caption of Figure 2 (PDF)

REFERENCES (1) Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F. Atomically Thin MoS2: A New Direct-Gap Semiconductor. Phys. Rev. Lett. 2010, 105, 136805. (2) Li, H.; Wu, J.; Yin, Z.; Zhang, H. Preparation and Applications of Mechanically Exfoliated Single-Layer and Multilayer MoS2 and WSe2 Nanosheets. Acc. Chem. Res. 2014, 47, 1067−1075. (3) Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Two-Dimensional Atomic Crystals. Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 10451−10453.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 5444

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Tunable Chemical Compositions and Optical Properties. J. Am. Chem. Soc. 2014, 136, 3756−3759. (23) Su, S. H.; Hsu, Y. T.; Chang, Y. H.; Chiu, M. H.; Hsu, C. L.; Hsu, W. T.; Chang, W. H.; He, J. H.; Li, L. J. Band Gap-Tunable Molybdenum Sulfide Selenide Monolayer Alloy. Small 2014, 10, 2589−2594. (24) Gong, Y. J.; Lin, J. H.; Wang, X. L.; Shi, G.; Lei, S. D.; Lin, Z.; Zou, X. L.; Ye, G. L.; Vajtai, R.; Yakobson, B. I.; Terrones, H.; Terrones, M.; Tay, B. K.; Lou, J.; Pantelides, S. T.; Liu, Z.; Zhou, W.; Ajayan, P. M. Vertical and In-Plane Heterostructures from WS2/MoS2 Monolayers. Nat. Mater. 2014, 13, 1135−1142. (25) Zhang, X. Q.; Lin, C. H.; Tseng, Y. W.; Huang, K. H.; Lee, Y. H. Synthesis of Lateral Heterostructures of Semiconducting Atomic Layers. Nano Lett. 2015, 15, 410−415. (26) Ji, Q.; Zhang, Y.; Zhang, Y.; Liu, Z. Chemical Vapour Deposition of Group-VIB Metal Dichalcogenide Monolayers: Engineered Substrates from Amorphous to Single Srystalline. Chem. Soc. Rev. 2015, 44, 2587−2602. (27) Shi, Y.; Li, H.; Li, L. J. Recent Advances in Controlled Synthesis of Two-Dimensional Transition Metal Dichalcogenides via Vapour Deposition techniques. Chem. Soc. Rev. 2015, 44, 2744−2756. (28) Dumcenco, D.; Ovchinnikov, D.; Marinov, K.; Lazić, P.; Gibertini, M.; Marzari, N.; Sanchez, O. L.; Kung, Y.-C.; Krasnozhon, D.; Chen, M.-W.; Bertolazzi, S.; Gillet, P.; Fontcuberta i Morral, A.; Radenovic, A.; Kis, A. Large-Area Epitaxial Monolayer MoS2. ACS Nano 2015, 9, 4611−4620. (29) Feldman, Y.; Frey, G. L.; Homyonfer, M.; Lyakhovitskaya, V.; Margulis, L.; Cohen, H.; Hodes, G.; Hutchison, J. L.; Tenne, R. Bulk Synthesis of Inorganic Fullerene-like MS2 (M = Mo, W) from the Respective Trioxides and the Reaction Mechanism. J. Am. Chem. Soc. 1996, 118, 5362−5367. (30) Tenne, R.; Seifert, G. Recent Progress in the Study of Inorganic Nanotubes and Fullerene-Like Structures. Annu. Rev. Mater. Res. 2009, 39, 387−413. (31) Wiesel, I.; Arbel, H.; Albu-Yaron, A.; Popovitz-Biro, R.; Gordon, J. M.; Feuermann, D.; Tenne, R. Synthesis of WS2 and MoS2 Fullerene-Like Nanoparticles from Solid Precursors. Nano Res. 2009, 2, 416−424. (32) Feldman, Y.; Wasserman, E.; Srolovitz, D. J.; Tenne, R. HighRate, Gas-Phase Growth of MoS2 Nested Inorganic Fullerenes and Nanotubes. Science 1995, 267, 222−225. (33) Zak, A.; Feldman, Y.; Alperovich, V.; Rosentsveig, R.; Tenne, R. Growth Mechanism of MoS2 Fullerene-like Nanoparticles by GasPhase Synthesis. J. Am. Chem. Soc. 2000, 122, 11108−11116. (34) Ling, X.; Lee, Y. H.; Lin, Y.; Fang, W.; Yu, L.; Dresselhaus, M. S.; Kong, J. Role of the Seeding Promoter in MoS2 Growth by Chemical Vapor Deposition. Nano Lett. 2014, 14, 464−472. (35) Han, G. H.; Kybert, N. J.; Naylor, C. H.; Lee, B. S.; Ping, J.; Park, J. H.; Kang, J.; Lee, S. Y.; Lee, Y. H.; Agarwal, R.; Johnson, A. T. Seeded Growth of Highly Crystalline Molybdenum Disulphide Monolayers at Controlled Locations. Nat. Commun. 2015, 6, 6128− 6134.

(4) O’Neill, A.; Khan, U.; Coleman, J. N. Preparation of High Concentration Dispersions of Exfoliated MoS2 with Increased Flake Size. Chem. Mater. 2012, 24, 2414−2421. (5) Liu, N.; Kim, P.; Kim, J. H.; Ye, J. H.; Kim, S.; Lee, C. J. LargeArea Atomically Thin MoS2 Nanosheets Prepared Using Electrochemical Exfoliation. ACS Nano 2014, 8, 6902−6910. (6) Anto Jeffery, A.; Nethravathi, C.; Rajamathi, M. Two-Dimensional Nanosheets and Layered Hybrids of MoS2 and WS2 through Exfoliation of Ammoniated MS2 (M = Mo,W). J. Phys. Chem. C 2014, 118, 1386−1396. (7) Wu, S.; Huang, C.; Aivazian, G.; Ross, J. S.; Cobden, D. H.; Xu, X. Vapor−Solid Growth of High Optical Quality MoS2 Monolayers with Near-Unity Valley Polarization. ACS Nano 2013, 7, 2768−2772. (8) Clark, G.; Wu, S.; Rivera, P.; Finney, J.; Nguyen, P.; Cobden, D. H.; Xu, X. Vapor-Transport Growth of High Optical Quality WSe2 Monolayers. APL Mater. 2014, 2, 101101. (9) Zhan, Y.; Liu, Z.; Najmaei, S.; Ajayan, P. M.; Lou, J. Large-Area Vapor-Phase Growth and Characterization of MoS2 Atomic Layers on a SiO2 Substrate. Small 2012, 8, 966−971. (10) Orofeo, C. M.; Suzuki, S.; Sekine, Y.; Hibino, H. Scalable Synthesis of Layer-Controlled WS2 and MoS2 Sheets By Sulfurization of Thin Metal Films. Appl. Phys. Lett. 2014, 105, 083112. (11) Huang, Y.-K.; Cain, J. D.; Peng, L.; Hao, S.; Chasapis, T.; Kanatzidis, M. G.; Wolverton, C.; Grayson, M.; Dravid, V. P. Evaporative Thinning: A Facile Synthesis Method for High Quality Ultrathin Layers of 2D Crystals. ACS Nano 2014, 8, 10851−10857. (12) Hanson, E. D.; Shi, F.; Chasapis, T. C.; Kanatzidis, M. G.; Dravid, V. P. Two-Dimensional Bismuth-Rich Nanosheets Through the Evaporative Thinning of Se-Doped Bi2Te3. J. Cryst. Growth 2016, 436, 138−144. (13) Kang, K.; Xie, S.; Huang, L.; Han, Y.; Huang, P. Y.; Mak, K. F.; Kim, C.-J.; Muller, D.; Park, J. High-Mobility Three-Atom-Thick Semiconducting Films with Wafer-Scale Homogeneity. Nature 2015, 520, 656−660. (14) Lee, Y.-H.; Zhang, X.-Q.; Zhang, W.; Chang, M.-T.; Lin, C.-T.; Chang, K.-D.; Yu, Y.-C.; Wang, J. T.-W.; Chang, C.-S.; Li, L.-J.; Lin, T.W. Synthesis of Large-Area MoS2 Atomic Layers with Chemical Vapor Deposition. Adv. Mater. 2012, 24, 2320−2325. (15) Najmaei, S.; Liu, Z.; Zhou, W.; Zou, X.; Shi, G.; Lei, S.; 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. (16) Liu, B.; Fathi, M.; Chen, L.; Abbas, A.; Ma, Y.; Zhou, C. Chemical Vapor Deposition Growth of Monolayer WSe2 with Tunable Device Characteristics and Growth Mechanism Study. ACS Nano 2015, 9, 6119−6127. (17) Shaw, J. C.; Zhou, H.; Chen, Y.; Weiss, N. O.; Liu, Y.; Huang, Y.; Duan, X. Chemical Vapor Deposition Growth of Monolayer MoSe2 Nanosheets. Nano Res. 2014, 7, 511−517. (18) Ji, Q.; Zhang, Y.; Gao, T.; Zhang, Y.; Ma, D.; Liu, M.; Chen, Y.; Qiao, X.; Tan, P.-H.; Kan, M.; Feng, J.; Sun, Q.; Liu, Z. Epitaxial Monolayer MoS2 on Mica with Novel Photoluminescence. Nano Lett. 2013, 13, 3870−3877. (19) Schmidt, H.; Wang, S.; Chu, L.; Toh, M.; Kumar, R.; Zhao, W.; Castro Neto, A. H.; Martin, J.; Adam, S.; Ö zyilmaz, B.; Eda, G. Transport Properties of Monolayer MoS2 Grown by Chemical Vapor Deposition. Nano Lett. 2014, 14, 1909−1913. (20) Zhang, K.; Feng, S.; Wang, J.; Azcatl, A.; Lu, N.; Addou, R.; Wang, N.; Zhou, C.; Lerach, J.; Bojan, V.; Kim, M. J.; Chen, L. Q.; Wallace, R. M.; Terrones, M.; Zhu, J.; Robinson, J. A. Manganese Doping of Monolayer MoS2: The Substrate Is Critical. Nano Lett. 2015, 15, 6586−6591. (21) Feng, Q.; Zhu, Y.; Hong, J.; Zhang, M.; Duan, W.; Mao, N.; Wu, J.; Xu, H.; Dong, F.; Lin, F.; Jin, C.; Wang, C.; Zhang, J.; Xie, L. Growth of Large-Area 2D MoS(2)((1)‑x) Se(2)x Semiconductor Alloys. Adv. Mater. 2014, 26, 2648−2653. (22) Li, H.; Duan, X.; Wu, X.; Zhuang, X.; Zhou, H.; Zhang, Q.; Zhu, X.; Hu, W.; Ren, P.; Guo, P.; Ma, L.; Fan, X.; Wang, X.; Xu, J.; Pan, A.; Duan, X. Growth of Alloy MoS(2x)Se2(1‑x) Nanosheets with Fully 5445

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