Chemical Vapor Transport Deposition of Molybdenum Disulfide Layers Using H 2 O Vapor as the Transport Agent

Molybdenum disulfide (MoS2) layers show excellent optical and electrical properties and have many potential applications. However, the growth of high-quality MoS2 layers is a major bottleneck in the development of MoS2-based devices. In this paper, we report a chemical vapor transport deposition method to investigate the growth behavior of monolayer/multi-layer MoS2 using water (H2O) as the transport agent. It was shown that the introduction of H2O vapor promoted the growth of MoS2 by increasing the nucleation density and continuous monolayer growth. Moreover, the growth mechanism is discussed.


Introduction
Molybdenum disulfide (MoS 2 ) layers, having unique optical and electrical properties, have attracted extensive interest in the fields of energy generation, electronics, and sensors [1][2][3][4][5][6][7].The growth of large-scale, high-quality MoS 2 layers targeted for silicon integrated device fabrication is still challenging.Vapor deposition has been the predominant method for the growth of large-scale, continuous MoS 2 monolayer or few layers films in recent years [8][9][10].Molybdenum oxides and sulfur are generally used as precursors of MoS 2 .For example, Lee et al. heated MoO 3 powder in sulfur vapor and obtained MoS 2 monolayer and multi-layer films [11].In this method, MoO 3 was first reduced by sulfur vapor to form MoO 3−x , which was then further reacted with sulfur vapor to form MoS 2 [12].MoO 3 acted as a nucleation center promoting crystal growth as well as the introduction of crystal defects.The introduction of defects plays two important roles; one is to promote nucleation for multi-layer growth, and the other is to tailor the electrical properties [13][14][15][16].The MoS 2 domains grown with this method showed different morphologies, e.g.triangle, hexagon, three-point star, as a function of the different atomic ratio of sulfur to molybdenum [17,18].
MoS 2 powder is another commonly used starting material.Wu et al. [19] heated MoS 2 powder at 900 • C in the center of a tube furnace and obtained a MoS 2 monolayer on an insulating substrate downstream of the precursor in a lower temperature zone (~650 • C).The usage of single-precursor MoS 2 powder as the source of Mo and S avoided the introduction of impurities and heterogeneous nucleation during the growth of MoS 2 flakes.Therefore, the MoS 2 monolayer showed a regular triangular shape and high optical quality.This vapor-solid growth method is suitable for the deposition of high-quality monolayer single crystal flake.However, in our recent work, we found that the nucleation is difficult to initiate and the growth temperature window is very narrow, ca.~50 • C [20].These issues could be attributed to the low vapor pressure of MoS 2 powder.The chemical vapor transport method was generally used for the growth of crystals with a solid precursor that has low vapor pressure.For example, Pisoni et al. reported the growth of MoS 2 single crystals using I 2 , Br 2 , and TeCl 4 as transport agents [21].The transport agent converts MoS 2 into high vapor pressure intermediates, which undergo the reverse reaction to deposit MoS 2 onto the substrate.However, the vapor transport agents used in this study are highly toxic and reactive, which could limit their widespread use.
To overcome this limitation, we have investigated ways to improve the nucleation density of MoS 2 using various additives.In this paper, we report the chemical vapor transport growth behavior of MoS 2 monolayer or multi-layer films by using MoS 2 powder as the precursor and water (H 2 O) vapor as the transport agent.In the nucleation stage, H 2 O vapor was introduced into the deposition system and acted as a chemical transport agent.Our mechanistic study suggests that water reacted with MoS 2 to form MoO 2 , which promoted the nucleation of MoS 2 .In the previously mentioned growth methods, the sulfur comes from the sublimation of sulfur or MoS 2 powder and the sulfur flow rate is out of control.Here, the sulfur was formed through the reaction of MoS 2 and water, which provides us a possible way to adjust the sulfur flow rate by controlling the water vapor flow rate.In the second stage, H 2 O vapor was cut off and MoS 2 continuously grew through a simple physical vapor transport process.This novel approach combined the heterogeneous nucleation and homogeneous growth to control the crystal size and thickness of the MoS 2 layer.The thickness of the MoS 2 film obtained ranged from a monolayer to multiple layers.The lateral size of the single-crystal domain is up to 300 µm.

MoS 2 Layers Synthesis
MoS 2 was prepared by modifying a previously reported vapor deposition method using a silicon wafer with a 300-nm layer of oxide (SiO 2 /Si) as the substrate [19].The schematic of the vapor deposition setup is shown in Figure 1.MoS 2 powder (99.5% purity, Aladdin, Shanghai, China) was used as the precursor.Before use, the precursor (0.5 g) was loaded into a small quartz glass boat (70 mm in length) and put in the center of the tube furnace (1 inch in diameter, Hefei Kejing Materials technology Co. Ltd., Hefei, China).Before growth, the precursor was flushed under Ar/H 2 (70 sccm, H 2 5%, total pressure of 75 Torr.sccm: standard cubic centimeter per minute) for 10 min at room temperature to remove the air and water absorbed on the precursor.The substrate was put downstream close to the furnace wall.
Coatings 2018, 8, 78 2 of 9 low vapor pressure.For example, Pisoni et al. reported the growth of MoS2 single crystals using I2, Br2, and TeCl4 as transport agents [21].The transport agent converts MoS2 into high vapor pressure intermediates, which undergo the reverse reaction to deposit MoS2 onto the substrate.However, the vapor transport agents used in this study are highly toxic and reactive, which could limit their widespread use.
To overcome this limitation, we have investigated ways to improve the nucleation density of MoS2 using various additives.In this paper, we report the chemical vapor transport growth behavior of MoS2 monolayer or multi-layer films by using MoS2 powder as the precursor and water (H2O) vapor as the transport agent.In the nucleation stage, H2O vapor was introduced into the deposition system and acted as a chemical transport agent.Our mechanistic study suggests that water reacted with MoS2 to form MoO2, which promoted the nucleation of MoS2.In the previously mentioned growth methods, the sulfur comes from the sublimation of sulfur or MoS2 powder and the sulfur flow rate is out of control.Here, the sulfur was formed through the reaction of MoS2 and water, which provides us a possible way to adjust the sulfur flow rate by controlling the water vapor flow rate.In the second stage, H2O vapor was cut off and MoS2 continuously grew through a simple physical vapor transport process.This novel approach combined the heterogeneous nucleation and homogeneous growth to control the crystal size and thickness of the MoS2 layer.The thickness of the MoS2 film obtained ranged from a monolayer to multiple layers.The lateral size of the single-crystal domain is up to 300 μm.

MoS2 Layers Synthesis
MoS2 was prepared by modifying a previously reported vapor deposition method using a silicon wafer with a 300-nm layer of oxide (SiO2/Si) as the substrate [19].The schematic of the vapor deposition setup is shown in Figure1.MoS2 powder (99.5% purity, Aladdin, Shanghai, China) was used as the precursor.Before use, the precursor (0.5 g) was loaded into a small quartz glass boat (70 mm in length) and put in the center of the tube furnace (1 inch in diameter, Hefei Kejing Materials technology Co. LTD., Hefei, China).Before growth, the precursor was flushed under Ar/H2 (70 sccm, H2 5%, total pressure of 75 Torr.sccm: standard cubic centimeter per minute) for 10 min at room temperature to remove the air and water absorbed on the precursor.The substrate was put downstream close to the furnace wall.For the MoS2 growth, the precursor was heated to 1000 °C from room temperature in 30 min under Ar/H2(75 Torr, Ar/H2 70 sccm) and kept at 1000 °C for 1 h.The furnace was then turned off and cooled from 1000 °C to room temperature.During the above heating process, the temperature of the substrate ranged from 710 to 850 °C.The water (H2O) vapor was introduced into the furnace by turning on/off the water valve, which connects the water tube and the Ar/H2 inlet.For typical growth, the water valve was kept open during the whole heating stage.For studies on the influence of H2O on the growth of MoS2, we kept the valve open during the heating stage but limited the water exposure during the synthesis.Optical microscope imaging of the sample was conducted with a Jiangnan MV3000 digital microscope (Nanjing Jiangnan Novel Optics Co. Ltd., Nanjing, China).Tapping mode atomic force microscopy (AFM) was performed on an Agilent 5500 (Agilent Technologies, Palo Alto, CA, USA) in air.Raman spectrum and photoluminescence (PL) were acquired on a Renishawin Via micro-Raman spectroscope (Renishaw, London, UK) with a 532 nm solid-state laser at room temperature.X-ray diffraction (XRD) was carried on a Thermo ARLXTRA (Thermo Electron, Waltham, USA) and ultraviolet visible diffuse reflection spectroscopy (UV-Vis DRS, not including specular reflection) was performed on Shimadzu MPC-3100 (Shimadzu, Tokyo, Japan) with an integrating sphere.

MoS 2 Flakes Grown in the Presence of H 2 O Vapor
MoS 2 flakes were prepared on the substrate using H 2 O and MoS 2 powder as illustrated in Figure 2a.performed on Shimadzu MPC-3100 (Shimadzu, Tokyo, Japan) with an integrating sphere.

MoS2 Flakes Grown in the Presence of H2O Vapor
MoS2 flakes were prepared on the substrate using H2O and MoS2 powder as illustrated in Figure 2a. Figure 2b shows the separated triangular MoS2 flakes grown on the SiO2/Si substrate with the H2O vapor valve kept open during the heating of the furnace and growth of the MoS2 flakes.The thickness of the flakes ranged from monolayer to multiple layers.The triangles in dim and uniform color indicate the uniform monolayer MoS2.The bright color triangles are attributed to multi-layer MoS2 with a pyramid-shape structure.The flake lateral size ranged from ca. 20 to 40 μm.
The success of the growth and thickness of the MoS2 flakes were confirmed by Raman spectroscopy.Figure 2c displays the typical Raman spectra of monolayer and multi-layer MoS2 flakes corresponded to the images in Figure 2b.The E2g and A1g modes of MoS2 were observed.The frequency difference between the E2g and A1g mode is thickness-dependent.With the increase of the layer number, the frequency difference valve will increase.The E2g and A1g peaks positions are at 385.0 cm −1 and 404.1 cm −1 (383.3 cm −1 and 409.1 cm −1 ) with a frequency difference of 19.9 cm −1 (25.8 cm −1 ), indicating that the thickness of flakes is monolayer (multi-layer) [22].
Besides Raman spectra, PL is generally used for the identification of the thickness of the MoS2.Mak et al. studied the relationship between the PL quantum yield and layer number.They found that the PL quantum yield drops quickly with the increase of the layer number.Bulk MoS2 is an indirectgap semiconductor showing negligible PL.Few-layer MoS2 shows weak PL due to the confinement effects.Monolayer MoS2 is a direct-gap semiconductor giving out bright PL [23].Figure 2d shows the typical photoluminescence spectra (PL) both of the monolayer and multi-layer MoS2 flakes corresponded to the images in Figure 2b.The excitation wavelength was 532 nm.The PL peaks of monolayer MoS2 are located at 674.5 nm and 622 nm, which are attributed to the A1 and B1 direct excitonic transition emission of the MoS2 monolayer, respectively [9,17,24].We observed that the PL intensity of the monolayer is much stronger than that of the multi-layer sample.

Effect of H2O Vapor on the MoS2 Layers Growth
To investigate the effect of H2O vapor on the MoS2 growth, we limited the time the synthesis was exposed to water vapor.After the precursors were heated to 1000 °C , the water valve was closed after a fixed amount of time during the growth stage: Figure 3a-d  Shown in Figure 3a-d, the shape of the MoS2 prepared without the introduction of H2O is a separated island.Meanwhile in Figure 3e,i-k,m-o, continuous, large-area MoS2 films were observed.This may be due to the presence of H2O vapor, which enhanced the diffusibility of molybdenum and sulfur atoms at domain boundaries, resulting in the continuous growth of monolayer MoS2.The large optical contrast in Figure 3e-p indicates the formation of multiple layers and/or clusters, which may The success of the growth and thickness of the MoS 2 flakes were confirmed by Raman spectroscopy.Figure 2c displays the typical Raman spectra of monolayer and multi-layer MoS 2 flakes corresponded to the images in Figure 2b.The E 2g and A 1g modes of MoS 2 were observed.The frequency difference between the E 2g and A 1g mode is thickness-dependent.With the increase of the layer number, the frequency difference valve will increase.The E 2g and A 1g peaks positions are at 385.0 cm −1 and 404.1 cm −1 (383.3 cm −1 and 409.1 cm −1 ) with a frequency difference of 19.9 cm −1 (25.8 cm −1 ), indicating that the thickness of flakes is monolayer (multi-layer) [22].
Besides Raman spectra, PL is generally used for the identification of the thickness of the MoS 2 .Mak et al. studied the relationship between the PL quantum yield and layer number.They found that the PL quantum yield drops quickly with the increase of the layer number.Bulk MoS 2 is an indirect-gap semiconductor showing negligible PL.Few-layer MoS 2 shows weak PL due to the confinement effects.Monolayer MoS 2 is a direct-gap semiconductor giving out bright PL [23].Figure 2d shows the typical photoluminescence spectra (PL) both of the monolayer and multi-layer MoS 2 flakes corresponded to the images in Figure 2b.The excitation wavelength was 532 nm.The PL peaks of monolayer MoS 2 are located at 674.5 nm and 622 nm, which are attributed to the A1 and B1 direct excitonic transition emission of the MoS 2 monolayer, respectively [9,17,24].We observed that the PL intensity of the monolayer is much stronger than that of the multi-layer sample.

Effect of H 2 O Vapor on the MoS 2 Layers Growth
To investigate the effect of H 2 O vapor on the MoS 2 growth, we limited the time the synthesis was exposed to water vapor.After the precursors were heated to 1000 • C, the water valve was closed after a fixed amount of time during the growth stage: Figure 3a-d  Shown in Figure 3a-d, the shape of the MoS 2 prepared without the introduction of H 2 O is a separated island.Meanwhile in Figure 3e,i-k,m-o, continuous, large-area MoS 2 films were observed.This may be due to the presence of H 2 O vapor, which enhanced the diffusibility of molybdenum and sulfur atoms at domain boundaries, resulting in the continuous growth of monolayer MoS 2 .The large optical contrast in Figure 3e-p indicates the formation of multiple layers and/or clusters, which may be due to the formation of high heterogeneous nucleation density and the Stranski-Krastanov growth mode.The formation of heterogeneous nucleation will be discussed below.Besides the continuous film obtained as described above, the domain size of MoS 2 prepared with H 2 O (shown in Figure 3f-h,p) was larger than those prepared without H 2 O (shown in Figure 3b,c).Figure 4 shows the magnified optical image of the same sample that is shown in Figure 3e.The lateral size of the triangle-shaped MoS 2 flakes ranges from 24 µm to 372 µm.The average lateral size of the MoS 2 flakes prepared without H 2 O introduction was 13 ± 6 µm, while the average size increased to 159 ± 80 µm based on the statistical calculation of the size of the isolated flakes shown in Figure 3a-h, respectively.
The water introduction also has an effect on the thickness of MoS 2 flakes.Based on the frequency difference (24.7 cm −1 , Figure S1) between the E 2g and A 1g mode of MoS 2 and uniform color contract, we can conclude that the MoS 2 flakes prepared without water exposure in Figure 3a-d is multi-layer.In contrast, in those samples prepared in the presence of water (Figure 3e-p), monolayer MoS 2 was observed (as discussed at the end of Section 3.2).Therefore, the introduction of water can reduce the thickness of the MoS 2 flakes.
It is reported that water molecules and carbon atoms can intercalate between the two-dimensional material and the substrate [25][26][27].Although we do not have enough evidence to show the presence of the water intercalation in our sample at high growth temperature (710 • C to 850 • C), we suspect that the molecular structure of water vapor possibly intercalates into the interlayer of the MoS 2 flakes or the interface between the MoS 2 and SiO 2 /Si substrate, which affects the absorption, desorption, and diffusion of the precursor atoms and even the final monolayer growth.
From Figure 3m-o, we can observe some bright features.The white spots are multi-layer MoS 2 .The area with green and yellow color we suspected to be amorphous MoS 2 , MoO 2 , or even organic contamination.To reduce the organic contamination, the silicon wafer substrate was cleaned with hot piranha solution (7:3 concentrated H 2 SO 4 :35% H 2 O 2 ) for 10 min at room temperature, and the vapor deposition system was flushed under Ar/H 2 to remove air-borne contamination before MoS 2 growth.Figure 5 shows the typical UV-Vis DRS of the MoS2 film corresponding to the images shown in Figure 3e-h.The UV-Vis DRS peaks at 665 and 610 nm match the two PL emission peaks (Figure 2d), and are due to the characteristic A1 and B1 direct excitonic transitions of MoS2, respectively [28].
AFM is a commonly used technique for two-dimensional material thickness measurement.Here, we conducted multiple scans of the thickness of the monolayer MoS2 at an edge of the MoS2 flake by AFM. Figure 6 shows the typical AFM image of the edge of the monolayer MoS2 triangle shown circled in black in Figure 3h.A straight trench with a width of ca. 150 nm was observed on the substrate surface, which divided the substrate into two sections.The bottom of the trench is the SiO2/Si substrate.The left side of the trench is MoS2 particles, and the right side of the trench is monolayer MoS2.The thickness of the MoS2 flake is 0.9 ± 0.1nm (Figure 6, Figure S2, and Table S1), indicating that the flake is monolayer.This thickness value, although it significantly deviates from  Figure 5 shows the typical UV-Vis DRS of the MoS2 film corresponding to the images shown in Figure 3e-h.The UV-Vis DRS peaks at 665 and 610 nm match the two PL emission peaks (Figure 2d), and are due to the characteristic A1 and B1 direct excitonic transitions of MoS2, respectively [28].
AFM is a commonly used technique for two-dimensional material thickness measurement.Here, we conducted multiple scans of the thickness of the monolayer MoS2 at an edge of the MoS2 flake by AFM. Figure 6 shows the typical AFM image of the edge of the monolayer MoS2 triangle shown circled in black in Figure 3h.A straight trench with a width of ca. 150 nm was observed on the substrate surface, which divided the substrate into two sections.The bottom of the trench is the SiO2/Si substrate.The left side of the trench is MoS2 particles, and the right side of the trench is monolayer MoS2.The thickness of the MoS2 flake is 0.9 ± 0.1nm (Figure 6, Figure S2, and Table S1), indicating that the flake is monolayer.This thickness value, although it significantly deviates from Figure 5 shows the typical UV-Vis DRS of the MoS 2 film corresponding to the images shown in Figure 3e-h.The UV-Vis DRS peaks at 665 and 610 nm match the two PL emission peaks (Figure 2d), and are due to the characteristic A1 and B1 direct excitonic transitions of MoS 2 , respectively [28].
AFM is a commonly used technique for two-dimensional material thickness measurement.Here, we conducted multiple scans of the thickness of the monolayer MoS 2 at an edge of the MoS 2 flake by AFM. Figure 6 shows the typical AFM image of the edge of the monolayer MoS 2 triangle shown circled in black in Figure 3h.A straight trench with a width of ca. 150 nm was observed on the substrate surface, which divided the substrate into two sections.The bottom of the trench is the SiO 2 /Si substrate.The left side of the trench is MoS 2 particles, and the right side of the trench is monolayer MoS 2 .The thickness of the MoS 2 flake is 0.9 ± 0.1nm (Figure 6, Figure S2, and Table S1), indicating that the flake is monolayer.This thickness value, although it significantly deviates from the expected thickness of monolayer MoS 2 (0.615 nm), is consistent with other AFM measurements of single-layer MoS 2 deposited on a SiO 2 substrate [29,30].In fact, the discrepancy that the measured value by AFM is larger than the theoretical value is common phenomena in the measurement of the thickness of two-dimensional monolayer materials, such as graphene [31].The discrepancy was attributed to the instrument offset due to tip-substrate interaction as well as adsorbed molecules between the monolayer and the SiO 2 substrate [26,31].From the AFM image in Figure 6, we can see that monolayer MoS 2 is smooth and continuous.We measured the root-mean-square (RMS) surface roughness over a 1 µm × 1 µm area.The RMS was 0.22 nm.The trench is probably formed through the rapid diffusion of MoS 2 nucleation along the direction perpendicular to the domain edge.Detailed study of the trench will be reported in future study.In addition to the trench, there are also many white particles on the surface of the MoS 2 flake.These particles should be MoS 2 formed during the growth of the MoS 2 flake, or even contaminations formed during the transport of the sample.
Coatings 2018, 8, 78 6 of 9 the expected thickness of monolayer MoS2 (0.615 nm), is consistent with other AFM measurements of single-layer MoS2 deposited on a SiO2 substrate [29,30].In fact, the discrepancy that the measured value by AFM is larger than the theoretical value is common phenomena in the measurement of the thickness of two-dimensional monolayer materials, such as graphene [31].The discrepancy was attributed to the instrument offset due to tip-substrate interaction as well as adsorbed molecules between the monolayer and the SiO2 substrate [26,31].From the AFM image in Figure 6, we can see that monolayer MoS2 is smooth and continuous.We measured the root-mean-square (RMS) surface roughness over a 1 μm × 1 μm area.The RMS was 0.22 nm.The trench is probably formed through the rapid diffusion of MoS2 nucleation along the direction perpendicular to the domain edge.Detailed study of the trench will be reported in future study.In addition to the trench, there are also many white particles on the surface of the MoS2 flake.These particles should be MoS2 formed during the growth of the MoS2 flake, or even contaminations formed during the transport of the sample.

Mechanism of MoS2 Growth in the Presence of H2O Vapor
The results obtained in Figure 3 suggest that H2O vapor promoted the growth of MoS2 film.We hypothesized that the H2O vapor reacted with MoS2 powder to give molybdenum oxide.Then the molybdenum oxide evaporated and deposited on the substrate, acting as heterogeneous nucleation center, from which the molybdenum oxide reacted with sulfur at a lower temperature and transformed into the MoS2 layer [12].The following reactions should have occurred during the growth of MoS2 [32]: the expected thickness of monolayer MoS2 (0.615 nm), is consistent with other AFM measurements of single-layer MoS2 deposited on a SiO2 substrate [29,30].In fact, the discrepancy that the measured value by AFM is larger than the theoretical value is common phenomena in the measurement of the thickness of two-dimensional monolayer materials, such as graphene [31].The discrepancy was attributed to the instrument offset due to tip-substrate interaction as well as adsorbed molecules between the monolayer and the SiO2 substrate [26,31].From the AFM image in Figure 6, we can see that monolayer MoS2 is smooth and continuous.We measured the root-mean-square (RMS) surface roughness over a 1 μm × 1 μm area.The RMS was 0.22 nm.The trench is probably formed through the rapid diffusion of MoS2 nucleation along the direction perpendicular to the domain edge.Detailed study of the trench will be reported in future study.In addition to the trench, there are also many white particles on the surface of the MoS2 flake.These particles should be MoS2 formed during the growth of the MoS2 flake, or even contaminations formed during the transport of the sample.

Mechanism of MoS2 Growth in the Presence of H2O Vapor
The results obtained in Figure 3 suggest that H2O vapor promoted the growth of MoS2 film.We hypothesized that the H2O vapor reacted with MoS2 powder to give molybdenum oxide.Then the molybdenum oxide evaporated and deposited on the substrate, acting as heterogeneous nucleation center, from which the molybdenum oxide reacted with sulfur at a lower temperature and transformed into the MoS2 layer [12].The following reactions should have occurred during the growth of MoS2 [32]:

Figure 1 .
Figure 1.Schematic illustration of the MoS2 growth setup.

Figure 1 .
Figure 1.Schematic illustration of the MoS 2 growth setup.
Figure 2b shows the separated triangular MoS 2 flakes grown on the SiO 2 /Si substrate with the H 2 O vapor valve kept open during the heating of the furnace and growth of the MoS 2 flakes.The thickness of the flakes ranged from monolayer to multiple layers.The triangles in dim and uniform color indicate the uniform monolayer MoS 2 .The bright color triangles are attributed to multi-layer MoS 2 with a pyramid-shape structure.The flake lateral size ranged from ca. 20 to 40 µm.

Figure 2 .
Figure 2. (a) Schematic illustration of the MoS 2 growth using H 2 O and MoS 2 powder; (b) Optical images of MoS 2 grown on a SiO 2 /Si substrate with H 2 O vapor for 10 min; (c) Typical Raman spectra and (d) Photoluminescence (PL) spectra of the monolayer (1L-MoS 2 ) and multi-layer MoS 2 (ML-MoS 2 ) flakes according images shown in Figure 2b.

Figure 3 .
Figure 3. Optical images of the MoS2 grown on a SiO2/Si substrate with varying the amount of H2O vapor released into the furnace.The images in each row are of the same sample but measured at different areas.From the left to right, the deposition temperature decreases as a result of the differences in the location.The amount of H2O vapor into the furnace is controlled by adjusting the length of time that the H2O valve is open: (a-d) 0 min, (e-h) 10 min, (i-l) 20 min, and (m-p) 60 min.For (f,j,k), we intentionally scratched the sample to show the contrast between the MoS2 film and the SiO2/Si substrate (bright orange color).The scale bars represent 100 μm.

Figure 4 .
Figure 4. Magnified optical images of the same sample that is shown in Figure 3e.The images were measured at different locations.From left to right, the growth temperature was gradually decreasing.

Figure 3 . 9 Figure 3 .
Figure 3. Optical images of the MoS 2 grown on a SiO 2 /Si substrate with varying the amount of H 2 O vapor released into the furnace.The images in each row are of the same sample but measured at different areas.From the left to right, the deposition temperature decreases as a result of the differences in the location.The amount of H 2 O vapor into the furnace is controlled by adjusting the length of time that the H 2 O valve is open: (a-d) 0 min; (e-h) 10 min; (i-l) 20 min; and (m-p) 60 min.For (f,j,k), we intentionally scratched the sample to show the contrast between the MoS 2 film and the SiO 2 /Si substrate (bright orange color).The scale bars represent 100 µm.

Figure 4 .
Figure 4. Magnified optical images of the same sample that is shown in Figure 3e.The images were measured at different locations.From left to right, the growth temperature was gradually decreasing.

Figure 4 .
Figure 4. Magnified optical images of the same sample that is shown in Figure 3e.The images were measured at different locations.From left to right, the growth temperature was gradually decreasing.

Figure 5 .
Figure 5.Typical ultraviolet visible diffuse reflection spectroscopy (UV-Vis DRS) of MoS2 corresponding to the images in Figure 3b.

Figure 6 .
Figure 6.Atomic force microscopy (AFM) image (a) and cross-section (b) (along the blue line marked in (a)) of monolayer MoS2 grown on the SiO2/Si substrate corresponding to the images in Figure 3h.The scale bars represent 1 μm in AFM image.

Figure 5 .
Figure 5.Typical ultraviolet visible diffuse reflection spectroscopy (UV-Vis DRS) of MoS 2 corresponding to the images in Figure 3b.

Figure 5 .
Figure 5.Typical ultraviolet visible diffuse reflection spectroscopy (UV-Vis DRS) of MoS2 corresponding to the images in Figure 3b.

Figure 6 .
Figure 6.Atomic force microscopy (AFM) image (a) and cross-section (b) (along the blue line marked in (a)) of monolayer MoS2 grown on the SiO2/Si substrate corresponding to the images in Figure 3h.The scale bars represent 1 μm in AFM image.

Figure 6 .
Figure 6.Atomic force microscopy (AFM) image (a) and cross-section (b) (along the blue line marked in (a)) of monolayer MoS 2 grown on the SiO 2 /Si substrate corresponding to the images in Figure 3h.The scale represent 1 µm in AFM image.