A Novel Carbon-Assisted Chemical Vapor Deposition Growth of Large-Area Uniform Monolayer MoS2 and WS2

Monolayer MoS2 can be used for various applications such as flexible optoelectronics and electronics due to its exceptional optical and electronic properties. For these applications, large-area synthesis of high-quality monolayer MoS2 is highly desirable. However, the conventional chemical vapor deposition (CVD) method using MoO3 and S powder has shown limitations in synthesizing high-quality monolayer MoS2 over a large area on a substrate. In this study, we present a novel carbon cloth-assisted CVD method for large-area uniform synthesis of high-quality monolayer MoS2. While the conventional CVD method produces thick MoS2 films in the center of the substrate and forms MoS2 monolayers at the edge of the thick MoS2 films, our carbon cloth-assisted CVD method uniformly grows high-quality monolayer MoS2 in the center of the substrate. The as-synthesized monolayer MoS2 was characterized in detail by Raman/photoluminescence spectroscopy, atomic force microscopy, and transmission electron microscopy. We reveal the growth process of monolayer MoS2 initiated from MoS2 seeds by synthesizing monolayer MoS2 with varying reaction times. In addition, we show that the CVD method employing carbon powder also produces uniform monolayer MoS2 without forming thick MoS2 films in the center of the substrate. This confirms that the large-area growth of monolayer MoS2 using the carbon cloth-assisted CVD method is mainly due to reducing properties of the carbon material, rather than the effect of covering the carbon cloth. Furthermore, we demonstrate that our carbon cloth-assisted CVD method is generally applicable to large-area uniform synthesis of other monolayer transition metal dichalcogenides, including monolayer WS2.


Introduction
Two-dimensional (2D) materials have attracted much attention due to their novel physical and chemical properties [1][2][3][4][5]. Graphene, the most studied 2D material, is thin, flexible, remarkably strong, and has exceptionally high electron mobility and thermal conductivity, allowing for a wide range of novel applications [1,2,6]. However, graphene has a zero bandgap, which results in very low on-off ratios in its applications of electronic devices such as transistors [4,5,7]. On the other hand, unlike graphene, transition metal dichalcogenides (TMDCs) have been intensively studied as new 2D layered materials because they have a sizable bandgap and interesting electronic and optical properties [3][4][5]7]. MoS 2 , a family of TMDCs, has been used as a building block for 2D field-effect transistors due to its high carrier mobility and excellent on-off ratios [8][9][10]. In addition, due to its exceptional physicochemical properties, 2D MoS 2 has been extensively used for novel 2D electronics, flexible optoelectronics, and efficient catalysis [11][12][13][14][15]. When MoS 2 is thinned down to a monolayer, its electronic structure and physical symmetries are radically altered, resulting in new physical behavior such as indirect to direct bandgap transitions [16][17][18]. In addition, monolayer MoS 2 exhibits strong light-matter interactions due to its planar exciton confinement effect [16,19,20]. To increase the potential use of monolayer MoS 2 in various applications, it is highly desirable to develop methods for preparing monolayer MoS 2 [8,10,11,15,21,22]. The most well-known mechanical exfoliation method is suitable for producing high-quality single crystalline MoS 2 flakes, but it cannot control the number of layers of the flakes and is unscalable for mass production [23][24][25][26]. In contrast, the chemical vapor deposition (CVD) method can control the number of MoS 2 layers and enables wafer-scale synthesis [27][28][29]. However, the conventional CVD method using MoO 3 and S powder has a problem in that thick MoS 2 films are formed in the center of the substrate and only MoS 2 monolayers are generated at the edge of the thick MoS 2 films [30][31][32][33][34].
This problem is related to the growth mechanism of MoS 2 in the conventional CVD method. The growth of MoS 2 is mainly achieved by the reaction of S with suboxide MoO 3-x species produced from MoO 3 powder [35][36][37]. MoO 3-x is highly volatile and improves the reaction kinetics for the formation of monolayer MoS 2 [35,37]. Monolayer MoS 2 can be effectively formed when the degree of MoO 3-x formation is sufficiently high, whereas thick MoS 2 films are generated when the degree of MoO 3-x formation is low. Therefore, keeping the degree of MoO 3-x formation high in the reaction process is a key condition for large-area uniform growth of high-quality monolayer MoS 2 without forming thick MoS 2 films. To achieve the large-area growth of monolayer MoS 2 , various methods have been reported, including confined-space CVD, reverse-flow chemical vapor epitaxy, inorganic vapor CVD, and metal organic CVD, etc. [38][39][40][41][42][43][44].
In this study, we report a novel carbon cloth-assisted CVD method that uniformly produces high-quality monolayer MoS 2 over a large area on a substrate without forming thick MoS 2 films. As-synthesized monolayer MoS 2 was characterized in detail by Raman/photoluminescence (PL) spectroscopy, atomic force microscopy (AFM), and transmission electron microscopy (TEM). We reveal the detailed growth process of monolayer MoS 2 initiated from MoS 2 seeds by conducting a series of experiments with varying reaction times. In addition, we show that the CVD method employing carbon powder instead of carbon cloth also enables large-area growth of monolayer MoS 2 , confirming the largearea growth of monolayer MoS 2 by the carbon cloth-assisted CVD method is mainly due to reducing properties of the carbon material, rather than the effect of covering the carbon cloth. Furthermore, we confirm that the carbon cloth-assisted CVD method can be used for the synthesis of other monolayer TMDCs such as monolayer WS 2 .

Conventional CVD Method for MoS 2 and WS 2 Synthesis
MoS 2 was synthesized by a CVD method using a two-zone horizontal hot-wall tube furnace equipped with a mass flow controller and a vacuum pump (Edwards Vacuum, west Sussex, United Kingdom). The synthetic scheme is illustrated in Figure 1a. In a 1-inch diameter quartz tube, S powder (0.1 g, Sigma-Aldrich, St. Louis, MO, USA, 99.999%) in an alumina boat was placed upstream, and MoO 3 powder (0.03 g, Sigma-Aldrich, 99.5%) in an alumina boat was put downstream. The growth promoter solution that was prepared by supersaturating NaCl in ethanol was dropped on a clean 300-nm SiO 2 /Si substrate and dried. NaCl serves as a promoter for the growth of MoS 2 [45]. Na+ in NaCl can react with MoO 3-x to form eutectic intermediates possessing a low melting point, promoting the growth of monolayer MoS 2 . The 300-nm SiO 2 /Si substrate was placed face down on the alumina boat containing MoO 3 powder. We used a vacuum pump to lower the pressure of the quartz tube to 5-mTorr or less to remove air in the quartz tube before the reaction. After turning off the vacuum pump, Ar gas (ultra-high purity, 99.999%, Dong-A Gases, Seoul, Korea) flowed at a rate of 100 sccm until reaching atmospheric pressure. After the pressure reached the atmospheric pressure, Ar gas flowed at a rate of 10 sccm. The temperatures of S and MoO 3 powder were independently controlled in two separate heating zones. The MoO 3 powder was heated to 740 • C for 15 min at a rate of ≈47.6 • C min -1 and maintained at 740 • C for 20 min. The S powder was heated to 210 • C for 19 min at a rate of ≈9.7 • C min -1 and maintained at 210 • C for 16 min. After the end of the reactions, the furnace lid was opened to cool the furnace rapidly to room temperature. For the synthesis of monolayer WS 2 , S powder (0.3 g, Sigma-Aldrich, 99.999%) and WO 3 powder (0.05 g, Sigma-Aldrich, 99.9%) were used as precursors. The c-cut sapphire substrate was placed face down on an alumina boat containing WO 3 powder. After the quartz tube was evacuated to 5-mTorr or less, Ar and H 2 gases flowed at a rate of 140 sccm and 20 sccm, respectively, and the chamber pressure was maintained at ≈1.6 Torr. The WO 3 powder was heated to 950 • C for 30 min at a rate of ≈30.8 • C/min and kept at 950 • C for 20 min. The S powder was heated to 210 • C for 32 min at a rate of ≈5.8 • C min -1 and maintained at 210 • C for 18 min.

Carbon Cloth-Assisted CVD Method for Monolayer MoS 2 and WS 2 Synthesis
The synthesis conditions of the carbon cloth-assisted CVD method are the same as those of the conventional CVD method described above, except that carbon cloth is placed on top of MoO 3 and WO 3 powder in an alumina boat for the synthesis of monolayer MoS 2 and WS 2 , respectively.

Carbon Powder-Assisted CVD Method for Monolayer MoS 2 Synthesis
The synthesis conditions of the carbon powder-assisted CVD method are the same as those of the conventional CVD synthesis method described above, except that activated carbon powder is mixed with MoO 3 powder. The mixing ratios of MoO 3 powder to activated carbon powder used in each experiment were 1:1, 1:2, 1:3, 1:4, 1:5, and 1:10, respectively.

Characterization
Raman spectra and maps were obtained using a 532-nm laser with 100 µW focused through a 100× objective at room temperature. Scanning electron microscopy (SEM) images and energy-dispersive X-ray spectroscopy (EDS) data were taken at 5 kV using a JSM-7900F (JEOL) microscope operating from 1 to 15 kV (JEOL Ltd., Tokyo, Japan). AFM measurement was performed in noncontact mode on an Anton-Paar Tosca 400 AFM instrument (Anton Paar, Sumida, Austria). TEM measurements were performed using a JEM-2100F microscope (JEOL Ltd., Tokyo, Japan).  Figure S1. The growth of such thick MoS 2 films and partial formation of monolayer MoS 2 have been commonly observed in MoS 2 growth by the conventional CVD method using MoO 3 and S powder as precursors [30][31][32][33][34]. An AFM height image of monolayer MoS 2 shows small particles grown nonuniformly on the surface of monolayer MoS 2 ( Figure 1g). The line profile obtained along the dotted red line in Figure 1g shows that the thickness of the synthesized monolayer MoS 2 is~0.72 nm, which is consistent with the reported thickness of monolayer MoS 2 ( Figure 1h) [19,46].

Thick MoS 2 Films and Monolayer MoS 2 Synthesized Using the Conventional CVD Method
Raman and photoluminescence (PL) analyses were performed on the as-synthesized monolayer MoS 2 . The Raman spectrum of monolayer MoS 2 shows that the frequency difference between the E 1 2g mode located at 382 cm −1 and the A 1g mode located at 403 cm −1 was approximately 21 cm −1 (Figure 1i), which is consistent with that of the reported monolayer MoS 2 [31]. Due to the direct bandgap of monolayer MoS 2 , the PL spectrum of MoS 2 shows a strong A exciton peak at 1.84 eV (Figure 1j) [19,47]. Raman and PL maps show that monolayer MoS 2 exhibited nonuniform Raman and PL peak intensities, confirming that monolayer MoS 2 had nonuniform optical and electronic properties (Figure 1k-m). Even when H 2 was used as a carrier gas with Ar for the MoS 2 synthesis, thick MoS 2 films were formed in the center of the substrate and some flakes of monolayer MoS 2 were partially formed at the edge of the thick films ( Figure S2). Figure 2a shows a schematic illustration of the carbon cloth-assisted CVD growth of monolayer MoS 2 . The experimental conditions were the same as those of the conventional CVD synthesis growth, except that the MoO 3 powder contained in the alumina boat was covered with carbon cloth. Unlike the conventional CVD method, this carbon cloth-assisted CVD method enables the growth of monolayer MoS 2 without forming thick films in the center of the substrate. Figure 2b is an optical image showing MoS 2 grown on a 300-nm SiO 2 /Si substrate by the carbon cloth-assisted CVD method, confirming that there were no thick films in the center of the substrate. Figure 2c-e show the low-magnification and high-magnification optical images for regions A, B, and C in Figure 2b, respectively, confirming that monolayer MoS 2 grew relatively uniformly throughout the substrate without forming thick MoS 2 films in the center of the substrate. Additional data on the optical characterization of the MoS 2 grown using the carbon-assisted CVD method are shown in Figures S3 and S4. In addition, we observed that the size of the monolayer MoS 2 decreased when we moved from region A to region C in Figure 2b. The change in the size of the monolayer MoS 2 can be explained as follows; on regions A and B located upstream, a sufficient amount of S vapor reacts with MoO 3-x to form large monolayer MoS 2 , whereas on region C located downstream, the amount of S vapor reaching region C is relatively small, resulting in relatively limited reactions with S vapor and MoO 3-x . To investigate the mechanisms of the carbon cloth-assisted CVD growth of monolayer MoS 2 , materials formed on carbon cloth during the growth were analyzed. Figure 3a,b show low-magnification and high-magnification SEM images of carbon cloth obtained after the carbon cloth-assisted CVD growth, respectively, confirming that the surface of the carbon cloth was entirely covered with nanoplates with a size of two to three microns. Raman analysis confirms that these nanoplates consisted of MoS 2 and MoO 2 (Figure 3c) [48]. In addition, EDS analysis shows that the nanoplates were composed of Mo, S, and O, and the proportion of O was very large compared to the proportion of S (Figure 3d), which confirms that the nanoplates were mostly composed of MoO 2 and were partially composed of MoS 2 . MoO 2 is a byproduct that is frequently formed in the conventional CVD growth of MoS 2 using MoO 3 and S powder as precursors. MoO 2 is nonvolatile and has a high melting point, so it remains once it is formed on the substrate. One of the important roles of carbon cloth in carbon cloth-assisted CVD growth is to prevent MoO 2 from forming on the SiO 2 /Si substrate by allowing MoO 2 to form on the carbon cloth ( Figure S5). Another role of carbon cloth is to improve the reaction kinetics for MoS 2 growth by facilitating the formation of suboxide MoO 3-x species formed from MoO 3, as carbon acts as a reducing agent.  Figure 4a is an AFM image of the monolayer MoS 2 synthesized using the carbon clothassisted CVD method, which shows that the surface of monolayer MoS 2 was clean without any particles, unlike monolayer MoS 2 synthesized using the conventional CVD method. The line profile obtained along the dotted red line in Figure 4a shows that the thickness of the synthesized monolayer MoS 2 was~0.988 nm, which is consistent with the reported thickness of monolayer MoS 2 (Figure 4b) [19,46]. Additional AFM data of monolayer MoS 2 are shown in Figure S6. Figure 4c shows an optical image of monolayer MoS 2 grown on an SiO 2 /Si substrate. The Raman spectrum (Curve 1) of monolayer MoS 2 taken at point 1 shows the Raman peaks of the E 1 2g mode located at 380 cm −1 and the A 1g mode located at 401 cm −1 (Figure 4d) [31]. Curve 2 in Figure 4d shows the Raman spectrum obtained from the substrate at point 2. The PL spectrum (Curve 1) of monolayer MoS 2 shows a strong peak at 1.84 eV (Figure 4e), which is consistent with the A exciton peak due to the direct bandgap of monolayer MoS 2 [19,47]. Curve 2 in Figure 4e is the PL spectrum obtained from the substrate at point 2. Raman and PL maps of MoS 2 show that monolayer MoS 2 exhibited uniform Raman and PL peak intensities, confirming that monolayer MoS 2 had a uniform chemical composition and electronic structure (Figure 4f-h). Figure 4i Figure 5e shows the Raman spectra of MoS 2 synthesized at each reaction time, confirming that the flakes and films synthesized at all reaction times were composed of MoS 2 . Figure 5f shows the PL spectra of MoS 2 formed at each reaction time, confirming that all MoS 2, except for the MoS 2 seeds formed at the reaction time of five minutes, exhibited a strong A exciton peak at 1.84 eV, indicating that the as-synthesized MoS 2 flakes and layers were monolayers. We believe that the variation of the PL peak position originated from the variation of strain or defects of the as-synthesized monolayer MoS 2 [49].

Monolayer MoS 2 Synthesized Using the Carbon Powder-Assisted CVD Method
The MoS 2 synthesis was conducted using the carbon powder-assisted CVD method to determine whether the large-area growth of monolayer MoS 2 without forming thick MoS 2 films is because carbon acts as a reducing agent or because carbon cloth physically covers the MoO 3 precursor. For carbon powder-assisted CVD synthesis, experiments were conducted by mixing MoO 3 powder and carbon powder in ratios of 1:1, 1:2, 1:3, 1:4, 1:5, and 1:10, respectively.
Figure 6a-f shows low-magnification and high-magnification optical images of monolayer MoS 2 synthesized with various mixing ratios of carbon powder to MoO 3 powder, confirming that monolayer MoS 2 was grown on the substrate over the large area without forming thick films in the center of the substrate. We demonstrated that the carbon material, acting as a reducing agent, plays an important role in the large-area uniform synthesis of monolayer MoS 2 . When the mixing ratio of MoO 3 powder to carbon powder was 1:1, the MoS 2 had a nonequilateral triangle shape, which means that MoS 2 has low crystallinity ( Figure 6a). This is because when the ratio of carbon powder is low, the degree of the formation of suboxide MoO 3-x species formed during the reaction process is low, so the reaction kinetics deteriorate. When the mixing ratio of the MoO 3 powder to the carbon powder was from 1:2 to 1:10, MoS 2 with an equilateral triangle shape and high crystallinity was formed. Among them, the largest monolayer MoS 2 was obtained when the mixing ratio of MoO 3 powder to carbon powder was 1:5 ( Figure 6e). When the mixing ratio of the MoO 3 powder to the carbon powder was further changed to 1:10, the size of the monolayer MoS 2 became small (Figure 6f). The Raman spectra confirm that all synthesized flakes exhibited Raman peaks at the E 1 2g mode and the A 1g mode of MoS 2 (Figure 6g). Figure 6h shows the PL spectra of the MoS 2 synthesized with various mixing ratios of carbon powder to MoO 3 powder. As the ratio of carbon powder increased, monolayer MoS 2 with higher crystallinity was produced, which showed higher PL intensity. The PL spectra of the MoS 2 show strong A exciton peaks at 1.84 eV when the mixing ratio of MoO 3 powder to carbon powder was 1:4 and 1:5, confirming that the as-synthesized MoS 2 flakes were high-quality MoS 2 monolayers. However, when the ratio of carbon powder to MoO 3 powder is too high, MoO 3 is reduced to suboxide MoO 3-x species and further reduced to form MoO 2 or Mo, which rather hinders the growth of monolayer MoS 2 . Thus, under this condition, the size of the monolayer MoS 2 became smaller again and the PL intensity decreased.
We performed the synthesis of monolayer MoS 2 using graphite powder mixed with MoO 3 powder ( Figure S8). Like the activated carbon powder-assisted CVD method, the graphite powder-assisted CVD method led to the synthesis of monolayer MoS 2 over a large area on the substrate. These results confirm that the reducing property of carbon is the main factor inducing the large-area growth of monolayer MoS 2 .

Growth Mechanism of Monolayer MoS 2 in the Carbon-Assisted CVD Growth
During the carbon-assisted CVD growth of monolayer MoS 2 , MoO 3 is reduced by carbon to form volatile suboxide MoO 3−x species, which are further sulfurized to form MoS 2 on an SiO 2 /Si substrate. The proposed reaction mechanism is as follows [50,51].
In this paper, we showed that the reaction kinetics for the growth of monolayer MoS 2 can be improved by using carbon materials. When no carbon materials were used, thick MoS 2 films were formed in most areas on the substrate and Figure S1), whereas when carbon materials were used, monolayer MoS 2 was formed in most areas on the substrate (Figure 2, Figure 6 and Figure S3). Thus, we believe that the carbon materials improve the reaction kinetics for the growth of monolayer MoS 2 and suppress the formation of thick MoS 2 films.
The generally accepted mechanism for the growth of monolayer MoS 2 involves the nucleation of tiny suboxide MoO 3-x seeds on the substrate surface followed by subsequent sulfurization of these seeds and subsequent growth of monolayer MoS 2 [50]. Thus, suboxide MoO 3-x species play a key role in the growth of monolayer MoS 2 . By using carbon cloth or carbon powder, we effectively increased the degree of the formation of suboxide MoO 3-x species, leading to the growth of monolayer MoS 2 in most areas on the substrate. On the other hand, the formation of thick MoS 2 films can be achieved by either the direct nucleation of nonvolatile MoO 3 or MoO 2 clusters on the substrate followed by subsequent sulfurization.

Application to Other TMDCs
In addition, to confirm that the carbon cloth-assisted CVD method applies to the synthesis of other monolayer TMDCs, we performed the synthesis of monolayer WS 2 using the conventional CVD method and the carbon cloth-assisted CVD method, respectively. Figure 7a is an optical image of the monolayer WS 2 synthesized using the conventional CVD method. The size of the monolayer WS 2 was as small as four microns, and its shape was not an equilateral triangle. Raman and PL mappings at the 2LA mode and A 1g mode of WS 2 show that the monolayer WS 2 exhibited nonuniform Raman and PL peak intensities, confirming that the monolayer WS 2 had nonuniform optical and electronic properties (Figure 7b-d). Figure 7e is an optical image of monolayer WS 2 synthesized using the carbon cloth-assisted CVD method. The size of the monolayer WS 2 was approximately 13.5 microns, and its shape was an equilateral triangle. Raman and PL mappings at the 2LA mode and A 1g mode of WS 2 show that the monolayer WS 2 exhibited uniform Raman and PL peak intensities, confirming that monolayer WS 2 had a uniform chemical composition and electronic structure (Figure 7f-h). Figure 7i shows an AFM image of the monolayer WS 2 synthesized using the carbon cloth-assisted CVD method, confirming that the surface of the monolayer WS 2 was clean without any particles. The line profile shows that the thickness of the monolayer WS 2 was~0.69 nm, which is consistent with the reported thickness of the monolayer WS 2 (Figure 7j) [52,53].
The growth of high-quality monolayer WS 2 by the carbon cloth-assisted CVD method can be explained as follows. For the synthesis of monolayer WS 2 , WO 3 powder was used as a precursor. The WO 3 has a significantly high melting point (1473 • C) and its vapor pressure is very low at the reaction temperature (950 • C). Thus, the conventional CVD method produces small monolayer flakes of WS 2 with very low coverage on the substrate ( Figure S9). When carbon cloth is placed on top of WO 3 powder, carbon acts as a reducing agent and increases the degree of the formation of suboxide WO 3-x species to improve the reaction kinetics for the formation of monolayer WS 2 . Thus, under this condition, triangular monolayer WS 2 with increased size forms uniformly on the substrate ( Figure S9). Consequently, we confirmed that the carbon cloth-assisted CVD method is generally applicable to the synthesis of high-quality monolayer WS 2 .

Figure 7.
Conventional CVD growth and carbon cloth-assisted CVD growth of monolayer WS 2 . (a) Optical image of monolayer WS 2 synthesized using the conventional CVD method. (b,c) Raman maps of monolayer WS 2 synthesized using the conventional CVD method, taken at the 2LA mode and the A 1g mode of WS 2 , respectively. (d) PL map of monolayer WS 2 synthesized using the conventional CVD method. (e) Optical image of monolayer WS 2 synthesized using the carbon cloth-assisted CVD method. (f,g) Raman maps of monolayer WS 2 synthesized using the carbon cloth-assisted CVD method, taken at the 2LA mode and A 1g mode of WS 2 , respectively. (h) PL map of monolayer WS 2 synthesized using the carbon cloth-assisted method. (i,j) AFM image and height line profiles of monolayer WS 2 synthesized using carbon cloth-assisted CVD method.

Conclusions
We developed a novel carbon-assisted CVD method for large-area uniform growth of high-quality monolayer MoS 2 . Using the carbon cloth-assisted CVD method, we synthesized high-quality monolayer MoS 2 uniformly over a large area on the substrate without forming thick MoS 2 films. Through detailed analyses of the carbon cloth that was used in the reaction and experiments with varying reaction times, we revealed the mechanisms for the large-area growth of high-quality monolayer MoS 2 . In addition, we showed that the carbon powder-assisted CVD method also produces high-quality monolayer MoS 2 over a large area on the substrate. This confirms that the uniform large-area growth of MoS 2 using the carbon cloth-assisted CVD method is mainly due to the reducing properties of the carbon material. Furthermore, we demonstrated that the carbon cloth-assisted CVD method can be generally used to synthesize monolayer WS 2 .
Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/nano11092423/s1, Figure S1: Additional optical characterization of the MoS 2 grown using the conventional CVD method, Figure S2: MoS 2 synthesized by the CVD method using H 2 as a carrier gas with Ar, Figure S3: Additional optical characterization of the MoS 2 grown using the carbon cloth-assisted CVD method, Figure S4: MoS 2 flakes grown using the carbon cloth-assisted CVD growth, Figure S5: MoO 2 -MoS 2 nanoplates grown after the carbon cloth-assisted CVD growth and after the conventional CVD growth, Figure S6: Additional AFM data of monolayer MoS 2 synthesized using the carbon cloth-assisted CVD method, Figure S7: Size distribution of monolayer MoS 2 synthesized at reaction times of 5 min, 15 min, and 20 min using the carbon cloth-assisted CVD method, respectively, Figure S8: Graphite powder-assisted CVD growth of monolayer MoS 2 , Figure S9: Monolayer WS 2 synthesized on a c-cut sapphire substrate using the conventional CVD method and the carbon cloth-assisted CVD method.

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