Structure and Properties of Single-Layer MoS2 for Nano-Photoelectric Devices

To meet the need for preparing high-performance nano-optoelectronic devices based on single-layer MoS2, the effects of the heating method (one-step or two-step heating) and the temperature of the MoO3 source on the morphology, size, structure, and layers of an MoS2 crystal grown on a sapphire substrate using chemical vapor deposition are studied in this paper. The results show that MoS2 prepared by two-step heating (the heating of the S source starts when the temperature of the MoO3 source rises to 837 K) is superior over that of one-step heating (MoO3 and S are heated at the same time). One-step heating tends to form a mixture of MoO2 and MoS2. Neither too low nor too high of a heating temperature of MoO3 source is conducive to the formation of MoS2. When the temperature of MoO3 source is in the range of 1073 K to 1098 K, the size of MoS2 increases with the rise in temperature. A uniform large-sized triangle with a side length of 100 μm is obtained when the heating temperature of MoO3 is 1098 K. The triangular MoS2 crystals grown by the two-step heating method have a single-layer structure.


Introduction
As the first two-dimensional material discovered, graphene has shown excellent performance in electrical, optical, thermal, and mechanical aspects [1][2][3][4][5]. However, graphene is a semiconductor without a bandgap, which limits its development in optoelectronic devices. Unlike graphene, bulk, multi-layer, and single-layer MoS 2 materials have bandgaps [6]. Bulk MoS 2 is a semiconductor with an indirect bandgap of 1.2 eV. When the thickness of a MoS 2 film reaches the single-layer level, the bandgap will eventually increase by at least 0.6 eV, due to the two-dimensional confinement effect. The reduction in thickness eventually causes the bandgap of the single-layer MoS 2 film to change from an indirect bandgap to a direct one. This change in bandgap results in photoluminescence of high brightness in single-layer MoS 2 [7]. In addition, single-layer MoS 2 has prominent electrical performance, and the transport calculation of the nonequilibrium Green's function shows that the switching ratio of MoS 2 can reach up to 10 10 [8]. Due to the superior electrical and optical performance of single-layer MoS 2 , it has potential application value in the electronic devices and photoelectronic fields [9][10][11][12][13].
MoS 2 has a hexagonal layered crystal structure. A single-layer MoS 2 is composed of three atomic layers, wherein the upper and lower layers are hexagonal planes composed of sulfur atoms, and the middle metal molybdenum atom layer separates the two layers of sulfur atoms. The common preparation methods for single-layer MoS 2 include micromechanical exfoliated, intercalation of lithium ions, liquid phase ultrasonography, and the chemical vapor deposition method [14][15][16][17].
The micromechanical exfoliated method is simple and quick, and the exfoliated product is mostly single-layer MoS 2 with high carrier mobility. Its disadvantages are that the number of exfoliated layers cannot be controlled, and industrial production is hard to realize. The intercalation of lithium ions method can prepare high-quality single-layer MoS 2 materials through controlling the lithium ion insertion and exfoliated process by an electrochemical lithium battery device, which has the advantages of large size and high quality, but with a complicated operation process, high equipment requirements, and low exfoliated efficiency. The liquid phase ultrasonography method can prepare a single-layer MoS 2 nanosheet. Since this method is not sensitive to water and air, it is suitable for mass production, and the obtained sheet can be easily assembled into a film. However, it is difficult to control its exfoliated degree, leaving a low concentration of the nanosheet solution after cleavage. Its disadvantage is that the ultrasonic power has great influence on the formation of the nanosheet.
The chemical vapor deposition (CVD) method uses sulfur vapor to chemically react with molybdenum oxide vapor to form MoS 2 [18,19]. The single-layer MoS 2 prepared by this method has excellent optical and electrical performance, and can be applied to devices like new-type resonators and transistors based on two-dimensional materials [20,21]. With a simple preparation process, the CVD method can control the structure, morphology, and size of the MoS 2 film by controlling the reaction temperature, time, and gas flow rate. Single-layer MoS 2 films with a size of 20 to 80 µm have been grown by CVD. However, whether larger-sized single-layer MoS 2 films can be prepared by CVD remains unsolved.
In this paper, the effects of heating methods (one-step and two-step heating) and heating temperature of the MoO 3 source on the morphology, size, structure, and layer number of MoS 2 grown on a sapphire substrate by sulfurized MoO 3 via the CVD method are investigated.

Treatment of the Sapphire Substrate
A sapphire plate was chosen as the substrate to prepare an MoS 2 crystal by vapor deposition. The sapphire substrate was cleaned in a clean room before the growth experiment. The cleaning of the sapphire plate includes two steps: the substrate was first sonicated in acetone, ethanol, and deionized water for 10 min, respectively, and then was dried with a nitrogen gas gun.

Growth of MoS 2 by Vapor Deposition
A chemical vapor deposition double temperature tube furnace, as shown in Figure 1, was adopted as the growth device for MoS 2 . In the experiment, the S powder was first placed in the low temperature zone on the left side of the tube furnace tube, the MoO 3 powder was placed in the high temperature zone in the middle of the furnace tube, and the sapphire substrate was placed flat on the right side of the furnace tube. The purities of the S powder and MoO 3 powder were 99.5%. The sulfur source in the low temperature zone was 15 cm away from the molybdenum source in the high temperature zone, and the molybdenum source was 5 cm away from the sapphire substrate.
Materials 2019, 12 FOR PEER REVIEW 2 product is mostly single-layer MoS2 with high carrier mobility. Its disadvantages are that the number of exfoliated layers cannot be controlled, and industrial production is hard to realize. The intercalation of lithium ions method can prepare high-quality single-layer MoS2 materials through controlling the lithium ion insertion and exfoliated process by an electrochemical lithium battery device, which has the advantages of large size and high quality, but with a complicated operation process, high equipment requirements, and low exfoliated efficiency. The liquid phase ultrasonography method can prepare a single-layer MoS2 nanosheet. Since this method is not sensitive to water and air, it is suitable for mass production, and the obtained sheet can be easily assembled into a film. However, it is difficult to control its exfoliated degree, leaving a low concentration of the nanosheet solution after cleavage. Its disadvantage is that the ultrasonic power has great influence on the formation of the nanosheet. The chemical vapor deposition (CVD) method uses sulfur vapor to chemically react with molybdenum oxide vapor to form MoS2 [18,19]. The single-layer MoS2 prepared by this method has excellent optical and electrical performance, and can be applied to devices like new-type resonators and transistors based on two-dimensional materials [20,21]. With a simple preparation process, the CVD method can control the structure, morphology, and size of the MoS2 film by controlling the reaction temperature, time, and gas flow rate. Single-layer MoS2 films with a size of 20 to 80 μm have been grown by CVD. However, whether larger-sized single-layer MoS2 films can be prepared by CVD remains unsolved.
In this paper, the effects of heating methods (one-step and two-step heating) and heating temperature of the MoO3 source on the morphology, size, structure, and layer number of MoS2 grown on a sapphire substrate by sulfurized MoO3 via the CVD method are investigated.

Treatment of the Sapphire Substrate
A sapphire plate was chosen as the substrate to prepare an MoS2 crystal by vapor deposition. The sapphire substrate was cleaned in a clean room before the growth experiment. The cleaning of the sapphire plate includes two steps: the substrate was first sonicated in acetone, ethanol, and deionized water for 10 min, respectively, and then was dried with a nitrogen gas gun.

Growth of MoS2 by Vapor Deposition
A chemical vapor deposition double temperature tube furnace, as shown in Figure 1, was adopted as the growth device for MoS2. In the experiment, the S powder was first placed in the low temperature zone on the left side of the tube furnace tube, the MoO3 powder was placed in the high temperature zone in the middle of the furnace tube, and the sapphire substrate was placed flat on the right side of the furnace tube. The purities of the S powder and MoO3 powder were 99.5%. The sulfur source in the low temperature zone was 15 cm away from the molybdenum source in the high temperature zone, and the molybdenum source was 5 cm away from the sapphire substrate.  After the tube furnace was vacuumed to 10 Torr with a mechanical pump, it was filled with high-purity argon gas at a flow rate of 70 sccm for 30 min. The evacuation-fill processes were repeated three times to vent the air and impurities in the tube furnace. After that, high-purity argon gas was introduced into the tube furnace, and when the pressure reached 1.2 atm, the exhaust valve was opened and adjusted to make the pressure in the furnace greater than 1 atm. Then, the furnace was heated to a setting temperature and held for a certain time. In the heating process, argon gas was continuously introduced into the double temperature tube furnace at a rate of 70 sccm. As a shielding gas, argon gas was also the medium to transmit the sulfur vapor from the low temperature zone to the high temperature zone. At a high temperature, the molybdenum oxide vapor was reduced by the sulfur vapor. The produced gas compound further reacted with sulfur vapor to generate MoS 2 , which was deposited onto the sapphire substrate to form MoS 2 crystal. Since the gas volume in the tube furnace shrank with the temperature decrease during the cooling process, which decreased the pressure inside the tube, argon gas was continuously supplied until the temperature was completely lowered to room temperature to prevent outside air from entering the tube furnace.

Characterization and Test Methods
An SEM (scanning electron microscope; FEI-QUANTA-400, Thermo Fisher, Hillsboro, OR, USA) was used to observe the morphology of the crystal sample grown by chemical vapor deposition. The acceleration voltage of the SEM was 20 kV. For the sample grown by chemical vapor deposition, its morphology can be directly observed by the SEM. The layer number of MoS 2 was roughly judged according to the contrast of color between MoS 2 and the substrate.
The layer number of MoS 2 was determined by Raman spectroscopy (Wissenschaftliche Instrumente und Technologie GmbH, Ulm, Germany) (Renishaw-UK, with a light-passing efficiency of more than 30%, a spectral range of 200 to 1000 nm, a spectral resolution of 1 cm −1 , and a spatial resolution of 0.5 µm in the lateral direction and 2 µm in the longitudinal direction) and a homemade photoluminescence spectrometer. The wavelength of Raman spectroscopy was 532 nm, and its grating groove density was 1800 L/mm. The wavelength of photoluminescence spectrometer was 700 nm, and its groove density was 150 L/mm.

Effect of Heating Method on the Morphology and Layer Number of MoS 2 Crystals
The effects of the heating method (one-step and two-step heating) on the morphology and layer number of MoS 2 crystals were investigated. The temperature-increase curve of one-step heating is shown in Figure 2a. In this experiment, the high temperature zone (MoO 3 ) and the low temperature zone (S) were heated simultaneously at two constant rates, and they reached the set temperatures at the same time.
Materials 2019, 12 FOR PEER REVIEW 3 After the tube furnace was vacuumed to 10 Torr with a mechanical pump, it was filled with high-purity argon gas at a flow rate of 70 sccm for 30 min. The evacuation-fill processes were repeated three times to vent the air and impurities in the tube furnace. After that, high-purity argon gas was introduced into the tube furnace, and when the pressure reached 1.2 atm, the exhaust valve was opened and adjusted to make the pressure in the furnace greater than 1 atm. Then, the furnace was heated to a setting temperature and held for a certain time. In the heating process, argon gas was continuously introduced into the double temperature tube furnace at a rate of 70 sccm. As a shielding gas, argon gas was also the medium to transmit the sulfur vapor from the low temperature zone to the high temperature zone. At a high temperature, the molybdenum oxide vapor was reduced by the sulfur vapor. The produced gas compound further reacted with sulfur vapor to generate MoS2, which was deposited onto the sapphire substrate to form MoS2 crystal. Since the gas volume in the tube furnace shrank with the temperature decrease during the cooling process, which decreased the pressure inside the tube, argon gas was continuously supplied until the temperature was completely lowered to room temperature to prevent outside air from entering the tube furnace.

Characterization and Test Methods
An SEM (scanning electron microscope; FEI-QUANTA-400, Thermo Fisher, Hillsboro, Oregon ,USA) was used to observe the morphology of the crystal sample grown by chemical vapor deposition. The acceleration voltage of the SEM was 20 kV. For the sample grown by chemical vapor deposition, its morphology can be directly observed by the SEM. The layer number of MoS2 was roughly judged according to the contrast of color between MoS2 and the substrate.
The layer number of MoS2 was determined by Raman spectroscopy (Wissenschaftliche Instrumente und Technologie GmbH, Ulm, Germany) (Renishaw-UK, with a light-passing efficiency of more than 30%, a spectral range of 200 to 1000 nm, a spectral resolution of 1 cm −1 , and a spatial resolution of 0.5 μm in the lateral direction and 2 μm in the longitudinal direction) and a homemade photoluminescence spectrometer. The wavelength of Raman spectroscopy was 532 nm, and its grating groove density was 1800 L/mm. The wavelength of photoluminescence spectrometer was 700 nm, and its groove density was 150 L/mm.

Effect of Heating Method on the Morphology and Layer Number of MoS2 Crystals
The effects of the heating method (one-step and two-step heating) on the morphology and layer number of MoS2 crystals were investigated. The temperature-increase curve of one-step heating is shown in Figure 2a. In this experiment, the high temperature zone (MoO3) and the low temperature zone (S) were heated simultaneously at two constant rates, and they reached the set temperatures at the same time.  By using SEM to observe the surface morphology of the one-step heating sample, it was found that a large number of white particles or quadrilateral crystals (the intermediate product MoO 2 ) appeared on the surface of a large number of samples, as shown in Figure 3a.
Materials 2019, 12 FOR PEER REVIEW 4 By using SEM to observe the surface morphology of the one-step heating sample, it was found that a large number of white particles or quadrilateral crystals (the intermediate product MoO2) appeared on the surface of a large number of samples, as shown in Figure 3a. The above experimental results can be explained in terms of the chemical reaction and phase transition of the Mo-S-O system. In the experimental temperature region, the following reactions and phase transition can occur in the Mo-S-O system: The effect of temperature on a chemical reaction can be judged according to the dependence of its standard Gibbs free energy change (∆G0) on the temperature (T). If the dependence curve of ∆G0 on T for a chemical reaction has a positive slope, as shown in Figure 4a, then the driving force of reaction decreases with rising temperature. On the contrary, the driving force of reaction increases with rising temperature when the slope of the curve is a negative, as shown in Figure 4b.   The above experimental results can be explained in terms of the chemical reaction and phase transition of the Mo-S-O system. In the experimental temperature region, the following reactions and phase transition can occur in the Mo-S-O system: MoO 3−x(g) + (7 − x)/S 2(g) MoS 2(g) + (3 − x)/2SO 2 (8) The effect of temperature on a chemical reaction can be judged according to the dependence of its standard Gibbs free energy change (∆G 0 ) on the temperature (T). If the dependence curve of ∆G 0 on T for a chemical reaction has a positive slope, as shown in Figure 4a, then the driving force of reaction decreases with rising temperature. On the contrary, the driving force of reaction increases with rising temperature when the slope of the curve is a negative, as shown in Figure 4b.
The slope of the dependence curve of ∆G 0 on T for a chemical reaction depends on the entropy change of the reaction (∆S 0 ). When ∆S 0 is positive, the slope is negative. In contrast, the slope is positive if ∆S 0 is negative. It can be determined that the slopes of the dependence curve of ∆G 0 on T for the reactions of Equations (1), (3), and (5) are negative, which indicates that the driving force of the reaction increases with rising temperature, whereas the dependence curves of ∆G 0 on T for the reactions of Equations (2) and (7)-(9) are positive, which indicates that the driving force for the reaction decreases with rising temperature. Therefore, increasing temperature is favorable for the reactions of Equations (1), (3), and (5), but adverse for the reactions of Equations (2) and (7)- (9).
The effect of temperature on a chemical reaction can be judged according to the dependence of its standard Gibbs free energy change (∆G0) on the temperature (T). If the dependence curve of ∆G0 on T for a chemical reaction has a positive slope, as shown in Figure 4a, then the driving force of reaction decreases with rising temperature. On the contrary, the driving force of reaction increases with rising temperature when the slope of the curve is a negative, as shown in Figure 4b.  The formation of MoO 2 depends on the reaction in Equation (7). The driving force of reaction for Equation (7) decreases with rising temperature. Therefore, the reaction of Equation (7) is easy to take place at low temperature. In addition, in one-step heating, S and MoO 3 sources are heated at the same time. The reaction of S 2(g) and O 2 can decrease the concentration of O 2 in the furnace, which can promote the reaction of Equation (7) and produce MoO 2(s) . Therefore, if the heating of S and MoO 3(s) starts at the same time, there are a large number of white or quadrilateral MoO 2 crystals, as shown in Figure 2a on the surface of the sample.
To reduce the formation of intermediate products, it is necessary to reduce the chance for MoO 3(g) to contact with S 2(g) at low temperatures. To achieve this purpose, a two-step heating method, as shown in Figure 2b, was employed. First, the MoO 3 zone was rapidly heated to 837 K at a rate of 20 K/min, and then slowly heated to 1098 K at a rate of 5 K/min. The heating of the S zone began at the time when the temperature of the MoO 3 zone was heated to 837 K, which can prevent the formation of MoO 2 , since there is no chance for MoO 3(g) and S 2(g) to contact at a temperature lower than 837 K. Figure 3b shows the morphology of the two-step heating sample. It can be seen that there are no MoO 2 white particles on the substrate, and the surface is very clean. In two-step heating, S has not yet sublimated when the temperature of MoO 3 is lower than 837 K, which avoids the contact of MoO 3(g) and S 2(g) at low temperatures and reduces the possibility of generating the intermediate product MoO 2 .

Effect of Temperature on the Morphology and Layer Number of MoS 2 Crystals
The temperature directly affects the sublimation rate of MoO 3 and S, thus affecting the growth of MoS 2 crystal. To eliminate the influence of temperature on the volatilization rate and bring about a change in the S concentration in the tube furnace, the temperature of the low temperature zone is set at 453 K. The effect of the MoO 3 temperature (1048 K, 1073 K, 1098 K, 1123 K) on the growth of a single-layer MoS 2 crystal is studied with a molybdenum source dosage of 0.02 g, a holding time of 30 min, a gas flow rate of 100 sccm, and a distance between the carrier and the molybdenum source of 4~5.5 cm. Figure 5 shows the effect of MoO 3 temperature on the morphology of a single-layer MoS 2 crystal obtained by SEM. It can be seen that, as shown in Figure 5a, there is almost nothing on the substrate when the temperature of MoO 3 is 1048 K. The reason for this is that at low temperatures, there is not enough MoO 3 sublimated, and the amount of MoO 3 participating in the chemical vapor deposition reaction is insufficient. When the temperature of MoO 3 is 1073 K, as shown in Figure 5b, the temperature in the high temperature zone just exceeds the melting point of MoO 3 (1068 K), which make it is possible for more MoO 3 to be evaporated and reduced by S 2(g) to form MoS 2 . The size of the single-layer MoS 2 depends on the evaporation pressure of MoO 3 . As shown in Figure 6, the evaporation pressure of MoO 3 increases with rising temperature. When the temperature of MoO 3 is 1073 K, the side length of the triangular MoS 2 is approximately 30 µm. Obviously, the size of the triangular single-layer MoS 2 on the substrate is not sufficiently large. When the temperature of MoO 3 is 1098 K, as shown in Figure 5c, an amount of complete and independent triangular MoS 2 crystals with a side length of approximately 100 µm appears. This results from the higher temperature increasing the evaporation rate of MoO 3 and the amount of MoO 3 evaporated, providing sufficient MoO 3(g) to react with S 2(g) ; thus, more MoS 2 is formed and deposited on the substrate for crystal growth. From Figure 5b,c, it can be seen that the grown single crystals are regular triangles, the surface colors of the single crystals are uniform, and the comparisons with the substrates are apparent. It can be preliminarily judged that the triangle crystals are single-layer.
Materials 2019, 12 FOR PEER REVIEW 6 surface colors of the single crystals are uniform, and the comparisons with the substrates are apparent. It can be preliminarily judged that the triangle crystals are single-layer.  When the temperature of MoO3 is 1123 K, as shown in Figure 5d, the size of MoS2 does not become larger but diminishes. However, the MoS2 crystal number increases under this condition: a large number of regular triangular MoS2 crystals are contacted or stacked together to form multi-layered MoS2. Since the ∆S0 for Equation (5) is positive and ∆G0 decreases with rising temperatures, the driving force of the reaction for Equation (5) increases with rising temperature. Therefore, when the temperature of MoO3 rises to 1123 K, the evaporation pressure of MoO3 is much higher, which can increase the concentration of MoS2(g) in the furnace. The critical size of the atomic cluster for MoS2 to form nucleation decreases with an increasing concentration of MoS2(g). The decrease of the critical size of the atomic cluster for MoS2 to form nucleation can lead to the   When the temperature of MoO3 is 1123 K, as shown in Figure 5d, the size of MoS2 does not become larger but diminishes. However, the MoS2 crystal number increases under this condition: a large number of regular triangular MoS2 crystals are contacted or stacked together to form multi-layered MoS2. Since the ∆S0 for Equation (5) is positive and ∆G0 decreases with rising temperatures, the driving force of the reaction for Equation (5) increases with rising temperature. Therefore, when the temperature of MoO3 rises to 1123 K, the evaporation pressure of MoO3 is much higher, which can increase the concentration of MoS2(g) in the furnace. The critical size of the atomic cluster for MoS2 to form nucleation decreases with an increasing concentration of MoS2(g). The decrease of the critical size of the atomic cluster for MoS2 to form nucleation can lead to the continuous formation of the MoS2 crystal nucleus, thus suppressing its growth. Therefore, when the temperature of MoO3 is 1123 K, a large number of multi-layered MoS2 crystals are formed. When the temperature of MoO 3 is 1123 K, as shown in Figure 5d, the size of MoS 2 does not become larger but diminishes. However, the MoS 2 crystal number increases under this condition: a large number of regular triangular MoS 2 crystals are contacted or stacked together to form multi-layered MoS 2 . Since the ∆S 0 for Equation (5) is positive and ∆G 0 decreases with rising temperatures, the driving force of the reaction for Equation (5) increases with rising temperature. Therefore, when the temperature of MoO 3 rises to 1123 K, the evaporation pressure of MoO 3 is much higher, which can increase the concentration of MoS 2(g) in the furnace. The critical size of the atomic cluster for MoS 2 to form nucleation decreases with an increasing concentration of MoS 2(g) . The decrease of the critical size of the atomic cluster for MoS 2 to form nucleation can lead to the continuous formation of the MoS 2 crystal nucleus, thus suppressing its growth. Therefore, when the temperature of MoO 3 is 1123 K, a large number of multi-layered MoS 2 crystals are formed.
In addition, at this higher temperature, the evaporation rate of MoO 3 is much higher, and MoO 3(g) cannot be completely reduced by S 2(g) in time. The decomposition of the excessive MoO 3(g) can form MoO 2(s) . Therefore, when the temperature of MoO 3 is 1123 K, the deposits on the substrate are a mixture of MoS 2 and disulfide.

Characterization by Raman Spectroscopy
Based on the Raman scattering effect, Raman spectroscopy can be used to study the structure of molecules by analyzing the scattering spectrum different from the incident light frequency. There are five vibration modes (E 2 2 g , E 1 2 g , E 1g , E 1u , and A 1g ) in MoS 2 . For the Raman spectroscopy test of MoS 2 , only E 1 2 g and A 1g modes are observed [22,23]. The characteristic peak position of the Raman spectrum of MoS 2 is related to the film thickness. The Van der Waals force increases as the layer number decreases. When the layer number of the film decreases gradually, the characteristic peak corresponding to the A 1g vibration mode happens to red shift (shifts to the long wavelength side), and the characteristic peak corresponding to the E 1 2 g vibration mode happens to blue shift (shifts towards the blue end of the spectrum). When MoS 2 is a bulk material, the E 1 2 g peak is located near 382 cm −1 , and the A 1g peak is located near 407 cm −1 . When the layer number of MoS 2 is reduced to a single layer, the E 1 2 g peak shifts towards the blue end, to approximately 385 cm −1 , and the A 1g peak shifts to the long wavelength side, to approximately 403 cm −1 . The A 1g peak corresponding to MoS 2 film of different layers differs from the E 1 2 g peak, and thus the layer number of MoS 2 could be determined based on the difference between the detected A 1g peak and E 1 2 g peak [24]. Figure 7 shows the results of the clean sapphire substrate and the single-layer triangular MoS 2 samples deposited on the substrate obtained by Raman spectroscopy. By comparing Figure 7a,b, it can be inferred that the peak appearing at a wavenumber of 416 cm −1 in Figure 7b is caused by the influence of the sapphire substrate Al 2 O 3 . The temperature in the high temperature zone of sample 1 (the sample in Figure 5b) is 1073 K, and the temperature in the high temperature zone of the sample 2 (the sample in Figure 5c) is 1098 K. It can be seen that the E 1 2 g peak of the single-layer triangular MoS 2 in the two samples is approximately 385 cm −1 , and the A 1g peak is approximately 403 cm −1 . The wavenumber difference is 19.2 cm −1 and 20.7 cm −1 , respectively. This indicates that the deposits on the sapphire substrate of sample 1 and sample 2 are single-layer MoS 2 , which is consistent with the predicted results of SEM.
Materials 2019, 12 FOR PEER REVIEW 7 In addition, at this higher temperature, the evaporation rate of MoO3 is much higher, and MoO3(g) cannot be completely reduced by S2(g) in time. The decomposition of the excessive MoO3(g) can form MoO2(s). Therefore, when the temperature of MoO3 is 1123 K, the deposits on the substrate are a mixture of MoS2 and disulfide.

Characterization by Raman Spectroscopy
Based on the Raman scattering effect, Raman spectroscopy can be used to study the structure of molecules by analyzing the scattering spectrum different from the incident light frequency. There are five vibration modes (E 2 2 g, E 1 2 g, E1g, E1u, and A1g) in MoS2. For the Raman spectroscopy test of MoS2, only E 1 2 g and A1g modes are observed [22,23]. The characteristic peak position of the Raman spectrum of MoS2 is related to the film thickness. The Van der Waals force increases as the layer number decreases. When the layer number of the film decreases gradually, the characteristic peak corresponding to the A1g vibration mode happens to red shift (shifts to the long wavelength side), and the characteristic peak corresponding to the E 1 2 g vibration mode happens to blue shift (shifts towards the blue end of the spectrum). When MoS2 is a bulk material, the E 1 2 g peak is located near 382 cm −1 , and the A1g peak is located near 407 cm −1 . When the layer number of MoS2 is reduced to a single layer, the E 1 2 g peak shifts towards the blue end, to approximately 385 cm −1 , and the A1g peak shifts to the long wavelength side, to approximately 403 cm −1 . The A1g peak corresponding to MoS2 film of different layers differs from the E 1 2 g peak, and thus the layer number of MoS2 could be determined based on the difference between the detected A1g peak and E 1 2 g peak [24]. Figure 7 shows the results of the clean sapphire substrate and the single-layer triangular MoS2 samples deposited on the substrate obtained by Raman spectroscopy. By comparing Figures 7a and 7b, it can be inferred that the peak appearing at a wavenumber of 416 cm −1 in Figure 7b is caused by the influence of the sapphire substrate Al2O3. The temperature in the high temperature zone of sample 1 (the sample in Figure 5b) is 1073 K, and the temperature in the high temperature zone of the sample 2 (the sample in Figure 5c) is 1098 K. It can be seen that the E 1 2 g peak of the single-layer triangular MoS2 in the two samples is approximately 385 cm −1 , and the A1g peak is approximately 403 cm −1 . The wavenumber difference is 19.2 cm −1 and 20.7 cm −1 , respectively. This indicates that the deposits on the sapphire substrate of sample 1 and sample 2 are single-layer MoS2, which is consistent with the predicted results of SEM.

Characterization of Photoluminescence Spectra
In bulk MoS 2 , no photoluminescence is observed. When it is gradually thinned to a few layers, or even a single layer, the band gap of MoS 2 changes to a direct band gap, and the fluorescence effect is obviously enhanced, with two characteristic peaks appearing near 1.8 eV and 2.0 eV. The characteristic peak corresponding to 1.8 eV is the direct band gap of MoS 2 , and the characteristic peak corresponding to 2.0 eV is the energy band splitting caused by the spin orbit coupling of MoS 2 [7,25]. The photoluminescence spectrum can be used as an effective means of characterizing the layer number of MoS 2 . Figure 8 shows the results of the clean sapphire substrate and the single-layer triangular MoS 2 samples deposited on the substrate obtained by photoluminescence spectra. By comparing Figure 8a,b, it can be believed that the peak appearing at a wavelength of 692.7 nm in Figure 8b is caused by the influence of the sapphire substrate Al 2 O 3 . Sample 1 (the sample in Figure 5b) shows a strong characteristic peak at the wavelength of λ 1 = 665.8 nm, and sample 2 (the sample in Figure 5c) shows a strong characteristic peak at λ 2 = 671.4 nm. According to the Planck-Einstein relation, it can be determined that the corresponding transition energy level E = 1.86 eV at the luminescence peak λ 1 = 665.8 nm, and the corresponding transition energy level E = 1.85 eV at the luminescence peak λ 2 = 671.4 nm, which is in accordance with the band gap width of single layer MoS 2 . It can be seen that both the characteristic peaks around 670 nm of two samples are intrinsic transition peaks, further verifying that the MoS 2 crystals deposited on the substrate in Figure 5b,c are of single-layer structures. By comparing the fluorescence efficiency of two samples at the characteristic peaks around 670 nm, it can be seen that the fluorescence efficiency of sample 1 is higher than that of sample 2.

Characterization of Photoluminescence Spectra
In bulk MoS2, no photoluminescence is observed. When it is gradually thinned to a few layers, or even a single layer, the band gap of MoS2 changes to a direct band gap, and the fluorescence effect is obviously enhanced, with two characteristic peaks appearing near 1.8 eV and 2.0 eV. The characteristic peak corresponding to 1.8 eV is the direct band gap of MoS2, and the characteristic peak corresponding to 2.0 eV is the energy band splitting caused by the spin orbit coupling of MoS2 [25,26]. The photoluminescence spectrum can be used as an effective means of characterizing the layer number of MoS2. Figure 8 shows the results of the clean sapphire substrate and the single-layer triangular MoS2 samples deposited on the substrate obtained by photoluminescence spectra. By comparing Figure 8a and b, it can be believed that the peak appearing at a wavelength of 692.7 nm in Figure 8b is caused by the influence of the sapphire substrate Al2O3. Sample 1 (the sample in Figure 5b) shows a strong characteristic peak at the wavelength of λ1 = 665.8 nm, and sample 2 (the sample in Figure 5c) shows a strong characteristic peak at λ2 = 671.4 nm. According to the Planck-Einstein relation, it can be determined that the corresponding transition energy level E = 1.86 eV at the luminescence peak λ1 = 665.8 nm, and the corresponding transition energy level E = 1.85 eV at the luminescence peak λ2 = 671.4 nm, which is in accordance with the band gap width of single layer MoS2. It can be seen that both the characteristic peaks around 670 nm of two samples are intrinsic transition peaks, further verifying that the MoS2 crystals deposited on the substrate in Figure 5b and c are of single-layer structures. By comparing the fluorescence efficiency of two samples at the characteristic peaks around 670 nm, it can be seen that the fluorescence efficiency of sample 1 is higher than that of sample 2.

Conclusions
(1) The two-step heating method to prepare MoS2 is superior to the one-step heating method. Mixed compounds of MoO2 and MoS2 are easily formed by the one-step heating method. The two-step heating method can avoid the contact of S2(g) with MoO3(g) and the formation of MoO2 at low temperatures.
(2) The growth of MoS2 crystal depends on the temperature. Neither too low nor too high of a heating temperature of MoO3 is conducive to the formation of MoS2. In the range of 1073 K to 1098 K, the size of MoS2 increases with the rise in temperature. A uniform large-sized triangle with a side length of 100 μm is obtained when the heating temperature of MoO3 is 1098 K.
(3) The test results of SEM, Raman spectroscopy, and photoluminescence spectroscopy show that the triangular MoS2 crystals grown by the two-step heating method are single-layer structures.

Conclusions
(1) The two-step heating method to prepare MoS 2 is superior to the one-step heating method. Mixed compounds of MoO 2 and MoS 2 are easily formed by the one-step heating method. The two-step heating method can avoid the contact of S 2(g) with MoO 3(g) and the formation of MoO 2 at low temperatures.
(2) The growth of MoS 2 crystal depends on the temperature. Neither too low nor too high of a heating temperature of MoO 3 is conducive to the formation of MoS 2 . In the range of 1073 K to 1098 K, the size of MoS 2 increases with the rise in temperature. A uniform large-sized triangle with a side length of 100 µm is obtained when the heating temperature of MoO 3 is 1098 K.
(3) The test results of SEM, Raman spectroscopy, and photoluminescence spectroscopy show that the triangular MoS 2 crystals grown by the two-step heating method are single-layer structures.