Methane-Mediated Vapor Transport Growth of Monolayer WSe2 Crystals

The electrical and optical properties of semiconducting transition metal dichalcogenides (TMDs) can be tuned by controlling their composition and the number of layers they have. Among various TMDs, the monolayer WSe2 has a direct bandgap of 1.65 eV and exhibits p-type or bipolar behavior, depending on the type of contact metal. Despite these promising properties, a lack of efficient large-area production methods for high-quality, uniform WSe2 hinders its practical device applications. Various methods have been investigated for the synthesis of large-area monolayer WSe2, but the difficulty of precisely controlling solid-state TMD precursors (WO3, MoO3, Se, and S powders) is a major obstacle to the synthesis of uniform TMD layers. In this work, we outline our success in growing large-area, high-quality, monolayered WSe2 by utilizing methane (CH4) gas with precisely controlled pressure as a promoter. When compared to the catalytic growth of monolayered WSe2 without a gas-phase promoter, the catalytic growth of the monolayered WSe2 with a CH4 promoter reduced the nucleation density to 1/1000 and increased the grain size of monolayer WSe2 up to 100 μm. The significant improvement in the optical properties of the resulting WSe2 indicates that CH4 is a suitable candidate as a promoter for the synthesis of TMD materials, because it allows accurate gas control.


Introduction
The discovery of graphene and its unique properties has triggered the development of various types of layered materials [1]. In particular, transition metal dichalcogenides (TMDs), atomically thin semiconductors of the type MX 2 (M = Mo, W; X = S, Se), have attracted considerable attention as their physical and electrical properties are tunable. Depending on their composition and thickness, two-dimensional (2D) TMDs have a variety of electrical properties ranging from metal, to insulator, to semiconductor, which could lead to a new dimension of atomic thickness for future device applications [2,3]. TMD materials have useful device characteristics, such as a high on/off ratio, a wide range of photoluminescence, and a low subthreshold voltage, making them suitable for spintronics and optoelectronics [4]. Among the numerous TMD materials, WSe 2 has been extensively studied because its electrical transport properties can be easily adjusted from p-type to bipolar behavior depending on the type of contact metal [5][6][7]. Bulk WSe 2 crystallizes in the "2H" or trigonal prismatic structure (space group P6 3 /mmc; a = 0.330 nm, c = 1.298 nm), in which each W atom is surrounded by six Se atoms, defining two triangular prisms. It was also reported that the energy band structure of WSe 2 can be altered according to its layer number. WSe 2 shows a direct bandgap of 1.65 eV in the monolayer, compared to an indirect bandgap of 1.2 eV in the multilayered bulk [8,9]. Similar to another 2D layered material, TMD is typically prepared using a mechanical exfoliation method. However, this top-down approach is not suitable for practical high-performance device applications, so bottom-up approaches for large-scale and mass-production have been extensively studied. The chemical vapor deposition (CVD) method is one of the bottom-up approaches that allows the synthesis of large-area TMDs. The CVD growth of TMDs has largely been studied using two different approaches. The first approach is to pre-deposit transition metal sources such as MoO 3 , WO 3 , etc., on the growth substrate and convert them to TMD by sulfidation (or selenization) [10][11][12][13][14][15][16][17][18]. The second is a noncatalytic growth method, in which a transition metal source and sulfur (or selenium) are heat-treated in a growth tube and flowed in a gaseous state to synthesize the TMD layer on a target substrate [19,20]. However, these CVD approaches have not been successful in uniform, high-quality TMD synthesis because it is difficult to control the thickness and nucleation density of TMDs [21]. Recently, to overcome such problems, many researchers have studied various types of promoters and methods applied for CVD-based TMD synthesis to control gas-phase transport of precursors and the reaction of TMD on the growth substrate [22][23][24][25][26]. Ling et al. reported the synthesis of highly-crystalline MoS 2 at a relatively low growth temperature (650 • C) using various aromatic molecules as seeding promoters [13]. In particular, domain size of MoS 2 increased up to 60 µm through vaporized aromatic-molecule catalysts such as perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS) and F 16 CuPc. They also reported that uniform monolayer MoS 2 can be synthesized on the entire area of the SiO 2 /Si substrate; however, the use of such an organic catalyst leaves a residue on the growth substrate that acts as a defect of the synthesized TMD. Another limitation of this method is that it is not applicable to the growth of WS 2 and WSe 2 , which require high growth temperatures. In addition, inorganic materials were also reported in assisted WSe 2 growth methods [15,27]. Liu et al. demonstrated a Cu-assisted self-limited growth (CASLG) method that allowed the synthesis of a high-quality, uniform WSe 2 monolayer while maintaining a balance between high crystallinity and fast growth rates. They explained that Cu atoms, which occupy the hexagonal sites positioned at the center of the six-membered rings of the WSe 2 surface, induce self-limited growth of WSe 2 and prevent unwanted reactions [15]. However, this approach also had disadvantages, for example, the synthesized WSe 2 had small grain sizes with multilayered regions and the vapor pressure of the solid catalyst could not be precisely controlled.
Herein, we report a catalytic growth of the large-area monolayer WSe 2 by utilizing CH 4 (methane) with precisely controlled pressure as t promoter. Through a systematic investigation, it is confirmed that grain size and the nucleation density of WSe 2 can be controlled according to the ratio of carrier gases (Ar/CH 4 ). The gas promoter leads to synthesis of about 100 µm size domains of WSe 2 and significantly reduces nucleation density from 1.6 × 10 5 to 1.5 × 10 2 mm −2 . Various analytical tools such as Raman, photoluminescence (PL), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) analysis are used to demonstrate the properties of synthesized monolayer WSe 2 .

Synthesis of WSe 2
The homemade CVD system was designed to flow gas in both directions with a three-zone furnace and a double-quartz tube (outer: 34 mm diameter, inner: 15 mm diameter tube). The WSe 2 powders were placed in an alumina boat located at the center furnace of the homemade CVD. The SiO 2 /Si substrate was cut to 1 cm × 5 cm size and then placed in the left furnace, about 10 cm from the alumina boat. The CVD system was pumped to the base pressure (2 × 10 −3 torr) by a rotary pump for 10 min and then filled with Ar gas to 760 torr. In the process of increasing the temperature to the WSe 2 growth temperature, the flow direction of the carrier gas (Ar 200 sccm) was reversed to prevent unwanted deposition. After the temperature reached 1050 • C, the flow direction of the carrier gas was reversed again to allow the evaporated precursor to reach the growth substrate. In the synthesis process, the experiment was carried out by flowing a different ratio of Ar and CH 4 (1% diluted at Ar) for 60 min at atmospheric pressure. After the reaction, the furnace was quenched down to room temperature while maintaining the gas flow, and the samples were collected for characterization.

Characterzaion of Synthesized WSe 2
The morphology and size of synthesized WSe 2 samples were characterized using optical microscopy (OM, Olympus DX51, Tokyo, Japan) and a SEM (JEOL JSM-7401F, JEOL, LTD, Tokyo, Japan) operating at 5 kV and 10 µA. The nucleation density and grain size of WSe 2 were analyzed using the Image J program tool. The thickness and surface potential of the WSe 2 monolayer were confirmed by atomic force microscopy and Kelvin probe force microscopy (KPFM) using Park NX10 (Park system, Suwon-si, Gyeonggi-do, Korea) with a Si cantilever Pt-coated tip. X-ray photoelectron spectroscopy analysis was carried out by ESCA2000 spectrometry (Termo Fisher Scientific, Walthan, Massachusetts, U.S.) using monochromatic Al-Kα radiation (1468.6 eV). Raman and photoluminescence spectra were collected with micro-Raman spectroscopy (WITEC Raman system, Ulm, Germany) using a 532 nm laser.

Results and Discussion
As shown in Figure 1a, monolayer WSe 2 was synthesized on the SiO 2 /Si substrate by a homemade three-zone furnace CVD using WSe 2 powder as a precursor. Briefly, the CVD system can control the temperature at each zone and adjust the direction of the carrier gas as required. During the ramping process for increasing the temperature of the furnace, the carrier gas flowed from the right to left direction to prevent the evaporated precursor from reaching the growth substrate, and the flow direction of the carrier gas was reversed during the growth process to synthesize the WSe 2 monolayer. A 1 × 5 cm 2 SiO 2 /Si growth substrate was placed 10 cm away from the alumina boat containing the precursor. The growth behavior of WSe 2 was investigated by observing the product at the same location as the growth substrate, because the morphology and density of the WSe 2 crystals depended upon the distance between the precursor and the growth substrate [12,16]. Figure 1b illustrates the catalytic growth of WSe 2 crystals via vapor-solid transport mechanism, when CH 4 gas diluted in Ar (1% CH 4 , 99% Ar) was used as both a carrier gas and a promoter. Like other catalysts for the growth of 2D materials, such as the Cu substrate commonly used for graphene growth, CH 4 induces the lateral epitaxy growth of WSe 2 , increasing its grain size while suppressing its vertical growth or deposition. During the synthesis of WSe 2 , methyl radicals and hydrogen are produced by thermal decomposition of CH 4 at the precursor hot zone (1050 • C) [28]. Methyl radicals can react with oxygen atoms on the SiO 2 surface to form O-CH 3 , reducing the nucleation site of WSe 2 . In addition, carbon-related radicals can react with the unstable W vapor to form metastable metallo-organic compounds, which may induce growth of low-defect WSe 2 crystals. Hydrogen is also known to induce the growth of low-defect WSe 2 crystals while suppressing vertical growth into bilayers and multilayers by etching defective WSe 2 [29][30][31]. Figure 1c,d show that while randomly distributed triangular WSe 2 crystals were grown, the size, density, and thickness uniformity of the grown crystal domains varied significantly with or without CH 4 promoters. When WSe 2 was grown without CH 4 gas, grain size of the obtained domains was less than 1 µm and there were many multilayer regions ( Figure 1c). However, when CH 4 gas was used as a promoter, WSe 2 existed mostly as a monolayer with a grain size of more than 10 µm (Figure 1d). These results clearly show that CH 4 gas acts as a promoter for the growth of WSe 2 crystals. triangular WSe2 crystals were grown, the size, density, and thickness uniformity of the grown crystal domains varied significantly with or without CH4 promoters. When WSe2 was grown without CH4 gas, grain size of the obtained domains was less than 1 µ m and there were many multilayer regions ( Figure 1c). However, when CH4 gas was used as a promoter, WSe2 existed mostly as a monolayer with a grain size of more than 10 µ m (Figure 1d). These results clearly show that CH4 gas acts as a promoter for the growth of WSe2 crystals. As various parameters affect the CVD growth of TMDs, substrate size, carrier gas velocity, weights of precursor powders, growth time, and characterization regions were set as constant [8,10,13,18,19,22,26]. Based on this, Figure 2a-d show SEM images of WSe2 according to the CH4 gas ratio. Figure 2a and Figure S1 show that the WSe2 grain size is less than 1 µ m when only Ar gas is used as the carrier gas. Figure 2a and Figure S1 also show some parts of the multilayer WSe2 regions (dark-colored) with a nucleation density of 1.6 × 10 5 mm −2 . By increasing the CH4 gas to 50 sccm, the average grain size of WSe2 was increased to ~6 µ m with a triangular shape and a nucleation density of 5.5 × 10 3 mm −2 (Figures 2b and S2). As the flow of CH4 gas was increased to 100 sccm, the synthesized monolayer WSe2 showed an average grain size of 9 µ m with a nucleation density of 6.8 × 10 2 mm −2 (Figures 2c and S3). Figure 2d and Figure S3 show that the domain size of a single crystal monolayer of WSe2 increased up to 80 μm when flowing 150 sccm of diluted CH4 gas. In this case, the average grain size was 52 μm with a wide distribution due to a lower nucleation density of 156 mm −2 . From a statistical analysis of domain images in Figures S1 to S4, grain size and nucleation density of WSe2 were obtained as a function of the CH4 gas ratio (Figures 2e and S5). Generally, increasing the CH4 gas ratio yielded a lower nucleation density of monolayer WSe2 with a larger grain size. The catalytic effect of CH4 on the synthesis of large-grain monolayer WSe2 was similar to the catalytic growth of other 2D materials (graphene, h-BN, MoS2, WSe2, etc.) [11,15,[32][33][34][35][36]. As various parameters affect the CVD growth of TMDs, substrate size, carrier gas velocity, weights of precursor powders, growth time, and characterization regions were set as constant [8,10,13,18,19,22,26]. Based on this, Figure 2a-d show SEM images of WSe 2 according to the CH 4 gas ratio. Figure 2a and Figure S1 show that the WSe 2 grain size is less than 1 µm when only Ar gas is used as the carrier gas. Figure 2a and Figure S1 also show some parts of the multilayer WSe 2 regions (dark-colored) with a nucleation density of 1.6 × 10 5 mm −2 . By increasing the CH 4 gas to 50 sccm, the average grain size of WSe 2 was increased to~6 µm with a triangular shape and a nucleation density of 5.5 × 10 3 mm −2 (Figure 2b and Figure S2). As the flow of CH 4 gas was increased to 100 sccm, the synthesized monolayer WSe 2 showed an average grain size of 9 µm with a nucleation density of 6.8 × 10 2 mm −2 (Figure 2c and Figure S3). Figure 2d and Figure S3 show that the domain size of a single crystal monolayer of WSe 2 increased up to 80 µm when flowing 150 sccm of diluted CH 4 gas. In this case, the average grain size was 52 µm with a wide distribution due to a lower nucleation density of 156 mm −2 . From a statistical analysis of domain images in Figures S1-S4, grain size and nucleation density of WSe 2 were obtained as a function of the CH 4 gas ratio (Figure 2e and Figure S5). Generally, increasing the CH 4 gas ratio yielded a lower nucleation density of monolayer WSe 2 with a larger grain size. The catalytic effect of CH 4 on the synthesis of large-grain monolayer WSe 2 was similar to the catalytic growth of other 2D materials (graphene, h-BN, MoS 2 , WSe 2 , etc.) [11,15,[32][33][34][35][36]. We also investigated the effects of the CH4 promoter on the morphological and optical properties of synthesized WSe2 via the nondestructive analysis tools of Raman spectroscopy and PL. Figures  3a,b show the typical Raman mapping (at center wavelength: ~252 cm −1 ) obtained with and without the CH4 promoter, respectively. When CH4 was used as a carrier gas, the grain size was about 80 µ m with a uniform and strong intensity of E 1 2g peak over the synthesized WSe2 crystals (Figure 3a). This result is consistent with the SEM results in Figure 2d. On the other hand, when only Ar was used as the carrier gas, the intensities of the measured E 1 2g peaks were much lower and nonuniform ( Figure  3b). Figure 3c shows the differences in the typical Raman spectra of WSe2 crystals grown with and without a CH4 promoter. In the case of CH4-assisted growth, Raman peaks corresponding to E 1 2g and A1g modes of single-layered WSe2 were observed ( Figure S6). When only Ar gas was used, a relatively low E 1 2g peak and an additional small peak at 307 cm −1 (corresponding to B 1 2g resonance mode of WSe2) were observed. In general, the B 1 2g peak is only active on the bilayer or multilayer of WSe2 [5,37]. We also noted that carbon-related Raman signals such as D peak (~1350 cm −1 ), G peak (~1600 cm −1 ), or 2D peak (~2700 cm −1 ) were not observed. These results indicate that CH4 acted only as a promoter during WSe2 synthesis and did not leave other carbon-related residues. We noted that the WSe2 growth temperature (700~750 °C) was too low to form a carbon layer by the reaction of methane on the surface of the SiO2/Si substrate [38]. The optical properties of the synthesized WSe2 and the effect of the CH4 promoter were further investigated using micro-PL with a 532 nm laser. Figure 3d shows the PL mapping of WSe2 synthesized using a CH4 promoter (CH4:Ar = 150:50). The synthesized WSe2 grain exhibited a uniform PL intensity at the 760 nm wavelength, which is equivalent to the PL value measured with exfoliated and synthesized single-crystal monolayer WSe2 [5,37,39]. On the other hand, when only Ar (200 sccm) was used as a carrier gas, the PL of synthesized WSe2 had a low intensity and showed a wide distribution due to the formation of bilayers and multilayers of WSe2, as shown in Figure 3e. The synthesis effects of CH4 gas were demonstrated from the representative PL spectrum of each PL mapping shown in Figure 3f. Based on these optical property data, it was confirmed that when using CH4 as a promoter in the WSe2 growth process, large WSe2 grains with uniform monolayers can be synthesized. We also investigated the effects of the CH 4 promoter on the morphological and optical properties of synthesized WSe 2 via the nondestructive analysis tools of Raman spectroscopy and PL. Figure 3a,b show the typical Raman mapping (at center wavelength:~252 cm −1 ) obtained with and without the CH 4 promoter, respectively. When CH 4 was used as a carrier gas, the grain size was about 80 µm with a uniform and strong intensity of E 1 2g peak over the synthesized WSe 2 crystals (Figure 3a). This result is consistent with the SEM results in Figure 2d. On the other hand, when only Ar was used as the carrier gas, the intensities of the measured E 1 2g peaks were much lower and nonuniform (Figure 3b). Figure 3c shows the differences in the typical Raman spectra of WSe 2 crystals grown with and without a CH 4 promoter. In the case of CH 4 -assisted growth, Raman peaks corresponding to E 1 2g and A 1g modes of single-layered WSe 2 were observed ( Figure S6). When only Ar gas was used, a relatively low E 1 2g peak and an additional small peak at 307 cm −1 (corresponding to B 1 2g resonance mode of WSe 2 ) were observed. In general, the B 1 2g peak is only active on the bilayer or multilayer of WSe 2 [5,37]. We also noted that carbon-related Raman signals such as D peak (~1350 cm −1 ), G peak (~1600 cm −1 ), or 2D peak (~2700 cm −1 ) were not observed. These results indicate that CH 4 acted only as a promoter during WSe 2 synthesis and did not leave other carbon-related residues. We noted that the WSe 2 growth temperature (700~750 • C) was too low to form a carbon layer by the reaction of methane on the surface of the SiO 2 /Si substrate [38]. The optical properties of the synthesized WSe 2 and the effect of the CH 4 promoter were further investigated using micro-PL with a 532 nm laser. As shown in the topology images obtained through tapping mode AFM, the thickness of the synthesized WSe2 is uniform to ~0.7 nm, corresponding to the thickness of the monolayer (Figure 4a) [40,41]. A KPFM image of the monolayer WSe2 showed a reduction in surface potential of ~300 meV in WSe2 due to the electrostatic screening effect and charge distribution of WSe2 (Figure 4b) [42]. The work function of the Pt-coated tip was ~4.3 eV, which was obtained by measuring the surface potential of highly oriented pyrolytic graphite (HOPG) ( Figure S7). Since the work function of the SiO2/Si substrate was 4.6 eV, the work function of the synthesized WSe2 was estimated to be ~4.3 eV. This value is equivalent to the work function value of the exfoliated monolayer WSe2 [43]. Figures 4c,d show the XPS results of the synthesized monolayer WSe2 with four W-4f peaks (W 4+ 4f7/2: 32.8 eV, W 4+ 4f5/2: 34.8 eV, W 6+ 4f7/2: 36 eV, and W 6+ 4f5/2: 38.2 eV) and two Se-3D peaks (Se 3d5/2: 55.1 eV and 3d3/2: 55.9 eV). The two W 4+ 4f peaks correspond to the binding energy of W bonded to Se atoms, while the two Se-3d peaks point to the binding energy of Se bonded to W atoms. The two W 6+ 4f peaks correspond to the binding energy of the W atoms bonded to the O atoms, resulting from the exposure of the synthesized WSe2 to air during the XPS analysis. Additionally, there was no W-4f peak at 32 eV and 34 eV, which represent the 1T phase; therefore, it can be confirmed at the WSe2 of the 2H phase. These results are consistent with previous reports on WSe2 [12,44].  Figure 3d shows the PL mapping of WSe 2 synthesized using a CH 4 promoter (CH 4 :Ar = 150:50). The synthesized WSe 2 grain exhibited a uniform PL intensity at the 760 nm wavelength, which is equivalent to the PL value measured with exfoliated and synthesized single-crystal monolayer WSe 2 [5,37,39]. On the other hand, when only Ar (200 sccm) was used as a carrier gas, the PL of synthesized WSe 2 had a low intensity and showed a wide distribution due to the formation of bilayers and multilayers of WSe 2 , as shown in Figure 3e. The synthesis effects of CH 4 gas were demonstrated from the representative PL spectrum of each PL mapping shown in Figure 3f. Based on these optical property data, it was confirmed that when using CH 4 as a promoter in the WSe 2 growth process, large WSe 2 grains with uniform monolayers can be synthesized.
As shown in the topology images obtained through tapping mode AFM, the thickness of the synthesized WSe 2 is uniform to~0.7 nm, corresponding to the thickness of the monolayer (Figure 4a) [40,41]. A KPFM image of the monolayer WSe 2 showed a reduction in surface potential of~300 meV in WSe 2 due to the electrostatic screening effect and charge distribution of WSe 2 (Figure 4b) [42]. The work function of the Pt-coated tip was~4.3 eV, which was obtained by measuring the surface potential of highly oriented pyrolytic graphite (HOPG) ( Figure S7). Since the work function of the SiO 2 /Si substrate was 4.6 eV, the work function of the synthesized WSe 2 was estimated to bẽ 4.3 eV. This value is equivalent to the work function value of the exfoliated monolayer WSe 2 [43]. Figure 4c,d show the XPS results of the synthesized monolayer WSe 2 with four W-4f peaks (W 4+ 4f 7/2 : 32.8 eV, W 4+ 4f 5/2 : 34.8 eV, W 6+ 4f 7/2 : 36 eV, and W 6+ 4f 5/2 : 38.2 eV) and two Se-3D peaks (Se 3d 5/2 : 55.1 eV and 3d 3/2 : 55.9 eV). The two W 4+ 4f peaks correspond to the binding energy of W bonded to Se atoms, while the two Se-3d peaks point to the binding energy of Se bonded to W atoms. The two W 6+ 4f peaks correspond to the binding energy of the W atoms bonded to the O atoms, resulting from the exposure of the synthesized WSe 2 to air during the XPS analysis. Additionally, there was no W-4f peak at 32 eV and 34 eV, which represent the 1T phase; therefore, it can be confirmed at the WSe 2 of the 2H phase. These results are consistent with previous reports on WSe 2 [12,44].

Conclusions
In summary, we developed a CH4-assisted vapor transport growth method to obtain highquality monolayer WSe2 crystals with large domain sizes. Unlike other promoter s or growth promoters previously reported (polymer, halide, and metal), CH4 only acts as a promoter for WSe2 growth without producing any residue. Moreover, the nucleation density of WSe2 was tuned (from 1.6 × 10 5 to 1.5 × 10 2 mm −2 ) by using the gas-phase CH4 promoter with precise flow control. The characterization of the synthesized monolayer WSe2 by Raman, PL, and KPFM confirmed that CH4 is a suitable candidate as a promoter for the growth of high-quality monolayer WSe2. Finally, our CH4assisted growth approach may be applicable for the controlled growth of high-quality single crystals of other TMDs.

Supplementary Materials:
The following are available online at www.mdpi.com/xxx/s1: Figures S1-S4-The SEM results at each synthesis condition for statistical analysis, Figure S5-Distribution of grain sizes in WSe2 according to carrier gas ratios, Figure S6-Raman spectrum of synthesized monolayer WSe2, Figure S7-The work function of Pt-coated AFM tip and reference HOPG, and Figure S8-XPS wide-range spectra of synthesized WSe2.

Conclusions
In summary, we developed a CH 4 -assisted vapor transport growth method to obtain high-quality monolayer WSe 2 crystals with large domain sizes. Unlike other promoter s or growth promoters previously reported (polymer, halide, and metal), CH 4 only acts as a promoter for WSe 2 growth without producing any residue. Moreover, the nucleation density of WSe 2 was tuned (from 1.6 × 10 5 to 1.5 × 10 2 mm −2 ) by using the gas-phase CH 4 promoter with precise flow control. The characterization of the synthesized monolayer WSe 2 by Raman, PL, and KPFM confirmed that CH 4 is a suitable candidate as a promoter for the growth of high-quality monolayer WSe 2 . Finally, our CH 4 -assisted growth approach may be applicable for the controlled growth of high-quality single crystals of other TMDs.

Supplementary Materials:
The following are available online at http://www.mdpi.com/2079-4991/9/11/1642/s1: Figures S1-S4-The SEM results at each synthesis condition for statistical analysis, Figure S5-Distribution of grain sizes in WSe 2 according to carrier gas ratios, Figure S6-Raman spectrum of synthesized monolayer WSe 2 , Figure S7-The work function of Pt-coated AFM tip and reference HOPG, and Figure S8-XPS wide-range spectra of synthesized WSe 2 .

Conflicts of Interest:
The authors declare no conflict of interest.