Research on the Factors Affecting the Growth of Large-Size Monolayer MoS2 by APCVD

The transition-metal chalcogenides (TMDs) are gaining increased attention from many scientists recently. Monolayer MoS2 is an emerging layered TMD material with many excellent physical and electrical properties. It can be widely used in catalysis, transistors, optoelectronics and integrated circuits. Here, the large-sized monolayer MoS2 is grown on the silicon substrate with a 285-nm-thick oxide layer by atmospheric pressure chemical vapor deposition (APCVD) of sulfurized molybdenum trioxide. This method is simple and it does not require vacuum treatment. In addition, the effects of growth conditions, such as sulfur source, molybdenum source, growth temperature, and argon flow rate on the quality and area of MoS2 are further studied systematically. These analysis results help to master the morphology and optical properties of monolayer MoS2. The high quality, excellent performance, and large-size monolayer MoS2 under the optimal growth condition is characterized by optical microscopy, AFM, XPS, photoluminescence, and Raman spectroscopy. The Raman spectrum and PL mapping show that the grown MoS2 is a uniform triangular monolayer with a side length of 100 μm, which can pave the way for the applications of photodetectors and transistors.


Introduction
In recent years, two-dimensional materials have been a hot topic in Nano-electronic research [1][2][3]. Many scientists have conducted extensive research on graphene with ultra-thin, ultra-strong and super-conductive properties. The pristine graphene has no band gap, so it cannot be used in the logic circuits. To compensate for the lack of band gap in graphene, researchers have begun to focus on the monolayer MoS 2 with its graphene-like structure [4]. Monolayer MoS 2 has a sandwich layered structure, the upper and lower layers are hexagonal planes composed of sulfur atoms, and the middle is a metal molybdenum atomic layer [5]. The atoms in monolayer MoS 2 are covalently bonded, whereas multilayer MoS 2 is bonded by the weak van der Waals force, and the space between adjacent layers is about 0.7 nm [6]. The band gap of MoS 2 decreases with an increase of the number of layers due to quantum confinement [7]. Monolayer MoS 2 has a direct band gap of 1.90 eV, which has the best luminescence properties. However, multilayer MoS 2 has an indirect band gap of 1.29 eV [8,9]. As graphene-like material, the direct-bandgap semiconductor has excellent electrical and optical

Experimental Methods
The specific experiment steps are provided here. First, the 4-inch silicon wafer with a thickness of 285 nm SiO 2 was cut into square small pieces with an area of 4-6 cm 2 by a diamond pen. Then, the SiO 2 /Si substrates were placed into acetone, deionized water, absolute ethanol, and deionized water for ultrasonic cleaning in 15 min, 10 min, 15 min, and 10 min, respectively [22]. And these cleaned substrates were blown dry with the nitrogen gas gun. Subsequently, a certain mass of sulfur powder (purity, 99.5%, Alfa Aesar, Shanghai, China) and MoO 3 powder (purity, 99.95%, Alfa Aesar, Shanghai, China) were weighed by the electronic analytical balance, and then put into two quartz boats, respectively. Next, the first quartz boat with S powder was placed in the low temperature zone of the tube furnace, and then the second quartz boat with MoO 3 powder and SiO 2 /Si substrate was placed in the middle position of the tube furnace, wherein the cleaned SiO 2 /Si substrate was placed face down on the downstream from the MoO 3 powder for 5 cm, as shown in Figure 1a. Afterwards, the argon gas (purity, 99.999%) was inserted into the tube furnace in order to purge the air in the tube furnace, a flow rate of 300 sccm (1 sccm = 1 mL/min) argon gas was then introduced in the tube furnace for 5 min before the heating process. Figure 1b shows the growth mechanism of MoS 2 . During the heating process, argon gas with a flow rate of 35 sccm was continuously supplied as the carrier gas. It can be observed from Figure 1c that the temperature of the SiO 2 /Si substrate rises from room temperature to 550 • C in 20 min initially, then rises to 720 • C in 10 min, and maintains the growth temperature 720 • C in 10 min, and then cools to room temperature naturally. Finally, the samples were taken out and some characterizations on the grown MoS 2 samples were executed. To characterize the surface morphology of grown MoS2, the new generation of high-resolution Raman spectrometer LabRam HR Evolution produced by HORIBA Jobin Yvon, was used. First, the MoS2 sample was placed on the stage of the microscope, then the Hg lamp emit laser light to the MoS2 sample. Subsequently, the scattered light from the monolayer MoS2 was collected and removed by the coupled optical path and the Rayleigh filter, respectively. Finally, the light was be converted into electrical signal by the charge coupled device (CCD), which is a special semiconductor device with many photosensitive elements in the LabRam HR Evolution with the single grating. The specific test conditions were: Spectral range of 300-450 cm −1 ; PL of 600-800 nm; spectral resolution ≤ 0.65 cm −1 ; spatial resolution was Lateral ≤ 1 μm, longitudinal ≤ 2 μm; Raman spectrum of 10 mW; PL spectrum of 25 mW; the scan time of 10 s; and the accumulation time of 3 s. The Raman and photoluminescence characterization were measured by using the LabRam HR Evolution with 532 nm laser and 50 × objective [23][24][25]. At the same time, in order to master the growth mechanism of monolayer MoS2, the X-ray photoelectron spectroscopy and Atomic Force Microscope, were also used.

Effects of the Growth Temperature on MoS2
Figure 2a is an optical micrograph of MoS2 grown at 670 °C . A small amount of triangular monolayer MoS2 on the substrate, 8-10 μm in size, was observed. The temperature was lower, the MoO3 powder evaporated slowly, and most of the MoO3 was carried away by the argon gas, which resulted in a small amount of MoS2 deposit on the substrate. Figure 2b shows a very regular triangular MoS2, and that the single crystal size can reach up to 40 μm, which indicates that the gas concentration of MoS2 is sufficient and the nucleation density of MoS2 increases when the growth temperature is 720 °C . In Figure 2c, the substrate exists as both triangular MoS2 and some body materials, which is due to the temperature being too high and the MoO3 powder evaporating quickly, thereby forming a mixture of MoO3 and MoS2. The above analysis indicates that 720 °C is the ideal temperature for growth of the monolayer MoS2. To characterize the surface morphology of grown MoS 2 , the new generation of high-resolution Raman spectrometer LabRam HR Evolution produced by HORIBA Jobin Yvon, was used. First, the MoS 2 sample was placed on the stage of the microscope, then the Hg lamp emit laser light to the MoS 2 sample. Subsequently, the scattered light from the monolayer MoS 2 was collected and removed by the coupled optical path and the Rayleigh filter, respectively. Finally, the light was be converted into electrical signal by the charge coupled device (CCD), which is a special semiconductor device with many photosensitive elements in the LabRam HR Evolution with the single grating. The specific test conditions were: Spectral range of 300-450 cm −1 ; PL of 600-800 nm; spectral resolution ≤ 0.65 cm −1 ; spatial resolution was Lateral ≤ 1 µm, longitudinal ≤ 2 µm; Raman spectrum of 10 mW; PL spectrum of 25 mW; the scan time of 10 s; and the accumulation time of 3 s. The Raman and photoluminescence characterization were measured by using the LabRam HR Evolution with 532 nm laser and 50 × objective [23][24][25]. At the same time, in order to master the growth mechanism of monolayer MoS 2 , the X-ray photoelectron spectroscopy and Atomic Force Microscope, were also used.

Effects of the Growth Temperature on MoS 2
Figure 2a is an optical micrograph of MoS 2 grown at 670 • C. A small amount of triangular monolayer MoS 2 on the substrate, 8-10 µm in size, was observed. The temperature was lower, the MoO 3 powder evaporated slowly, and most of the MoO 3 was carried away by the argon gas, which resulted in a small amount of MoS 2 deposit on the substrate. Figure 2b shows a very regular triangular MoS 2 , and that the single crystal size can reach up to 40 µm, which indicates that the gas concentration of MoS 2 is sufficient and the nucleation density of MoS 2 increases when the growth temperature is 720 • C. In Figure 2c, the substrate exists as both triangular MoS 2 and some body materials, which is due to the temperature being too high and the MoO 3 powder evaporating quickly, thereby forming a mixture of MoO 3 and MoS 2 . The above analysis indicates that 720 • C is the ideal temperature for growth of the monolayer MoS 2 .

Effects of the Mass of MoO 3 Powder on MoS 2
The mass of MoO 3 has a direct impact on the growth of MoS 2 crystals. Under the premise that other growth conditions are consistent, 1 mg, 3 mg, and 5 mg of the MoO 3 powder were weighed during the experiment, respectively. Figure 3a is an optical microscope image of MoS 2 when the mass of MoO 3 is 1 mg. It can be found that the size of the triangular single crystal MoS 2 is about 10 µm, the surface of the sample is clean, and the contrast between the sample and the substrate is obvious. This is because the amount of MoO 3 was small, which lead to low nucleation density. Figure 3b shows that the size of single crystal MoS 2 can be as large as 30 µm, and the morphology of the sample was better, which reveals that the Mo:S ratio is >1:2 and the gas concentration of MoS 2 is sufficient in this area. Meanwhile, the nucleation density is high. In Figure 3c, there is no obvious triangular single crystal MoS 2 , and the surface of SiO 2 /Si substrate is covered with fine black particles when the amount of MoO 3 is increased to 5 mg. The reason is that the excessive amount of molybdenum source, which resulted in a large nucleation density and MoS 2, grew toward the bulk material. From the above comparison, it can be found that the optimum mass of MoO 3 is 3 mg. (c) 770 °C . Scale bar: 10 μm.

Effects of the Mass of MoO3 Powder on MoS2
The mass of MoO3 has a direct impact on the growth of MoS2 crystals. Under the premise that other growth conditions are consistent, 1 mg, 3 mg, and 5 mg of the MoO3 powder were weighed during the experiment, respectively. Figure 3a is an optical microscope image of MoS2 when the mass of MoO3 is 1 mg. It can be found that the size of the triangular single crystal MoS2 is about 10 μm, the surface of the sample is clean, and the contrast between the sample and the substrate is obvious. This is because the amount of MoO3 was small, which lead to low nucleation density. Figure 3b shows that the size of single crystal MoS2 can be as large as 30 μm, and the morphology of the sample was better, which reveals that the Mo:S ratio is >1:2 and the gas concentration of MoS2 is sufficient in this area. Meanwhile, the nucleation density is high. In Figure 3c, there is no obvious triangular single crystal MoS2, and the surface of SiO2/Si substrate is covered with fine black particles when the amount of MoO3 is increased to 5 mg. The reason is that the excessive amount of molybdenum source, which resulted in a large nucleation density and MoS2, grew toward the bulk material. From the above comparison, it can be found that the optimum mass of MoO3 is 3 mg.  (c) 770 °C . Scale bar: 10 μm.

Effects of the Mass of MoO3 Powder on MoS2
The mass of MoO3 has a direct impact on the growth of MoS2 crystals. Under the premise that other growth conditions are consistent, 1 mg, 3 mg, and 5 mg of the MoO3 powder were weighed during the experiment, respectively. Figure 3a is an optical microscope image of MoS2 when the mass of MoO3 is 1 mg. It can be found that the size of the triangular single crystal MoS2 is about 10 μm, the surface of the sample is clean, and the contrast between the sample and the substrate is obvious. This is because the amount of MoO3 was small, which lead to low nucleation density. Figure 3b shows that the size of single crystal MoS2 can be as large as 30 μm, and the morphology of the sample was better, which reveals that the Mo:S ratio is >1:2 and the gas concentration of MoS2 is sufficient in this area. Meanwhile, the nucleation density is high. In Figure 3c, there is no obvious triangular single crystal MoS2, and the surface of SiO2/Si substrate is covered with fine black particles when the amount of MoO3 is increased to 5 mg. The reason is that the excessive amount of molybdenum source, which resulted in a large nucleation density and MoS2, grew toward the bulk material. From the above comparison, it can be found that the optimum mass of MoO3 is 3 mg.

Effects of the Mass of S Powder on MoS 2
In Figure 4a, when the mass of S powder was 50 mg, some small triangles and black dots were formed on the substrate, the ratio of Mo:S was <1:2. The sulfur powder was too little to provide enough S gas to react with MoO 3 , which caused the MoO 3 to be deposited on the substrate in the form of particles. Figure 4b shows that the shape of MoS 2 is very regular, and the size of MoS 2 can reach up to 40 µm, which reveals that the nucleation density is high and the gas concentration of MoS 2 is sufficient in this area. Meanwhile, when the Mo:S ratio was >1:2, it increased the size of MoS 2 . When the mass of sulfur powder was increased to 150 mg, the shape of MoS 2 in Figure 4c was the same as that in Figure 4b. When the Mo:S ratio was >1:2, it produced lager triangles. It can be inferred that the mass of S powder should be excessive during the experiment, which is beneficial to the growth of monolayer MoS 2 . The optimum mass of S powder is 100 mg according to the above results.

Effects of the Mass of S Powder on MoS2
In Figure 4a, when the mass of S powder was 50 mg, some small triangles and black dots were formed on the substrate, the ratio of Mo:S was <1:2. The sulfur powder was too little to provide enough S gas to react with MoO3, which caused the MoO3 to be deposited on the substrate in the form of particles. Figure 4b shows that the shape of MoS2 is very regular, and the size of MoS2 can reach up to 40 μm, which reveals that the nucleation density is high and the gas concentration of MoS2 is sufficient in this area. Meanwhile, when the Mo:S ratio was >1:2, it increased the size of MoS2. When the mass of sulfur powder was increased to 150 mg, the shape of MoS2 in Figure 4c was the same as that in Figure 4b. When the Mo:S ratio was >1:2, it produced lager triangles. It can be inferred that the mass of S powder should be excessive during the experiment, which is beneficial to the growth of monolayer MoS2. The optimum mass of S powder is 100 mg according to the above results.

Effects of the Argon Flow Rate on MoS2
The high purity argon gas had two functions during the experiment. The first was to prevent impure gas from entering the tube furnace so that the reaction of sulfurized molybdenum trioxide would be carried out in the argon gas. The second was to transport S gas to the high temperature zone to react with the MoO3 gas. Finally, the MoS2 was transferred on the surface of substrate. Figure 5a shows a mixture of MoO2 and MoS2 on the substrate. Owing to the sufficient S, gas could not be transported to the top of the MoO3, and the synthesis efficiency of MoS2 gas was relatively low, which limited the growth response of MoS2 when the argon flow rate is 20 sccm. In Figure 5b, it is observed that the shape of the MoS2 is triangular, and the MoS2 single crystal has a size of 60 μm when the argon gas flow rate is 35 sccm. The suitable argon flow rate caused the reaction of sulfurized molybdenum trioxide more thoroughly, which can make the gas concentration of MoS2 higher. In Figure 5c, the size of the MoS2 single crystal is greatly reduced, and the number of monolayer MoS2 is also sparse. It can be inferred that the surface of SiO2/Si substrate is not conducive to the nucleation and growth of monolayer MoS2 when the gas flow is 50 sccm. When the flow rate of argon gas becomes larger, the gas concentration and the nucleation density of MoS2 become smaller, resulting in the size of the triangular MoS2 becomes smaller. In summary, the optimal argon flow rate is 35 sccm.
According to the above analysis, the optimal growth conditions for monolayer MoS2 were obtained by atmospheric pressure chemical vapor deposition. The optimal growth parameters were:

Effects of the Argon Flow Rate on MoS 2
The high purity argon gas had two functions during the experiment. The first was to prevent impure gas from entering the tube furnace so that the reaction of sulfurized molybdenum trioxide would be carried out in the argon gas. The second was to transport S gas to the high temperature zone to react with the MoO 3 gas. Finally, the MoS 2 was transferred on the surface of substrate. Figure 5a shows a mixture of MoO 2 and MoS 2 on the substrate. Owing to the sufficient S, gas could not be transported to the top of the MoO 3 , and the synthesis efficiency of MoS 2 gas was relatively low, which limited the growth response of MoS 2 when the argon flow rate is 20 sccm. In Figure 5b, it is observed that the shape of the MoS 2 is triangular, and the MoS 2 single crystal has a size of 60 µm when the argon gas flow rate is 35 sccm. The suitable argon flow rate caused the reaction of sulfurized molybdenum trioxide more thoroughly, which can make the gas concentration of MoS 2 higher. In Figure 5c, the size of the MoS 2 single crystal is greatly reduced, and the number of monolayer MoS 2 is also sparse. It can be inferred that the surface of SiO 2 /Si substrate is not conducive to the nucleation and growth of monolayer MoS 2 when the gas flow is 50 sccm. When the flow rate of argon gas becomes larger, the gas concentration and the nucleation density of MoS 2 become smaller, resulting in the size of the triangular MoS 2 becomes smaller. In summary, the optimal argon flow rate is 35 sccm. 720 °C temperature, 3 mg MoO3 powder, 100 mg S powder, and 35 sccm argon gas. At the same time, the Atomic Force Microscope, Raman spectroscopic, and X-ray photoelectron spectrum were used to analyze the prepared monolayer MoS2 sample under optimal growth conditions. (c) 50 sccm. Scale bar: 10 μm.

The Characterization of Monolayer MoS2 under Optimal Growth Conditions
In Figure 6, it is observed that the grown MoS2 is a triangle with a size of 100 μm, which is much larger than the peeled off sample micromechanically. When the growth of MoS2 is under optimal growth conditions, the gas concentration of MoS2 is the largest and the nucleation density reaches a maximum. Moreover, the surface of the grown MoS2 is uniform, and the contrast between the sample and SiO2/Si substrate is obvious. Therefore, the grown MoS2 by optimal conditions can be judged to be a single-layer initially. Raman spectrum has become an effective method for the detection and identification of films prepared by chemical vapor deposition (CVD). The optical properties of monolayer MoS2, obtained under optimal growth conditions, are characterized by Raman and PL spectroscopies. These spectral measurement conditions were in a super-clean lab with constant temperature and humidity. Figure  7a shows two obvious characteristic peaks in the Raman spectrum of MoS2, E 1 2g: 384.6 cm −1 , A1g: 403.6 According to the above analysis, the optimal growth conditions for monolayer MoS 2 were obtained by atmospheric pressure chemical vapor deposition. The optimal growth parameters were: 720 • C temperature, 3 mg MoO 3 powder, 100 mg S powder, and 35 sccm argon gas. At the same time, the Atomic Force Microscope, Raman spectroscopic, and X-ray photoelectron spectrum were used to analyze the prepared monolayer MoS 2 sample under optimal growth conditions.

The Characterization of Monolayer MoS 2 under Optimal Growth Conditions
In Figure 6, it is observed that the grown MoS 2 is a triangle with a size of 100 µm, which is much larger than the peeled off sample micromechanically. When the growth of MoS 2 is under optimal growth conditions, the gas concentration of MoS 2 is the largest and the nucleation density reaches a maximum. Moreover, the surface of the grown MoS 2 is uniform, and the contrast between the sample and SiO 2 /Si substrate is obvious. Therefore, the grown MoS 2 by optimal conditions can be judged to be a single-layer initially.
Materials 2018, 11, 16 FOR PEER REVIEW 6 of 11 720 °C temperature, 3 mg MoO3 powder, 100 mg S powder, and 35 sccm argon gas. At the same time, the Atomic Force Microscope, Raman spectroscopic, and X-ray photoelectron spectrum were used to analyze the prepared monolayer MoS2 sample under optimal growth conditions. (c) 50 sccm. Scale bar: 10 μm.

The Characterization of Monolayer MoS2 under Optimal Growth Conditions
In Figure 6, it is observed that the grown MoS2 is a triangle with a size of 100 μm, which is much larger than the peeled off sample micromechanically. When the growth of MoS2 is under optimal growth conditions, the gas concentration of MoS2 is the largest and the nucleation density reaches a maximum. Moreover, the surface of the grown MoS2 is uniform, and the contrast between the sample and SiO2/Si substrate is obvious. Therefore, the grown MoS2 by optimal conditions can be judged to be a single-layer initially. Raman spectrum has become an effective method for the detection and identification of films prepared by chemical vapor deposition (CVD). The optical properties of monolayer MoS2, obtained under optimal growth conditions, are characterized by Raman and PL spectroscopies. These spectral measurement conditions were in a super-clean lab with constant temperature and humidity. Figure  7a shows two obvious characteristic peaks in the Raman spectrum of MoS2, E 1 2g: 384.6 cm −1 , A1g: 403.6 Raman spectrum has become an effective method for the detection and identification of films prepared by chemical vapor deposition (CVD). The optical properties of monolayer MoS 2, obtained under optimal growth conditions, are characterized by Raman and PL spectroscopies. These spectral measurement conditions were in a super-clean lab with constant temperature and humidity. Figure 7a shows two obvious characteristic peaks in the Raman spectrum of MoS 2 , E 1 2g : 384.6 cm −1 , A 1g : 403.6 cm −1 , which is fitted by the Gaussian-Lorenze curve. The E 1 2g peak corresponds to the vibration of sulfur atoms in the horizontal plane, and A 1g peak represents the vibration of sulfur atoms in the vertical horizontal plane direction. Due to the gradual increase of the van der Waals force, the E 1 2g characteristic peak was blue-shifted, and the A 1g characteristic peak was red-shifted when the number of layers was reduced [26]. Therefore, the layer number of MoS 2 sample can be determined based on the wavenumber difference of two characteristic peaks A 1g and E 1 2g modes. It can be seen from the above two peak positions in Figure 7a that the wavenumber difference is about 19 cm −1 , and the intensity ratio of E 1 2g /A 1g ≈ 0.95, which indicates that the grown MoS 2 sample is a monolayer. Figure 7b is a triangular MoS 2 single crystal with Raman mapping image of the A 1g peak intensity (4000 points total), which is randomly selected on the grown MoS 2 sample. It can be seen from Figure 7b that the thickness of triangular MoS 2 is uniformly high. To the best of our knowledge, when MoS 2 transforms from bulk material into monolayer material, the band gap changes from indirect band gap to direct band gap, and the fluorescence efficiency is enhanced significantly. Figure 7c shows the PL spectrum fitted by the Gaussian-Lorenze curve, the two main exciton peaks of the monolayer MoS 2 luminescence are located at 642 nm (B excitons) and 691 nm (A excitons), respectively. The neutral excitons of A and B arise from valence band to conduction band transitions within the direct band gap of monolayer MoS 2, directly. The relative contribution of excitons and triangles determines the peak position of PL spectrum. The shape of PL spectrum is consistent with the previous reported result of monolayer MoS 2 obtained by mechanically exfoliated, CVD, physical vapor deposition (PVD), and so on [27][28][29]. Due to strain during thermal growth and different test environments, the peak position of A and B exciton results in a small fluctuation shift, which is within the allowable range [30]. According to the conversion relationship between wavelength and electron volt, the photon energy is inversely proportional to the wavelength: In the formula (1), the symbol E, h, C, λ and k refer to the energy, Planck constant, light speed, wavelength and constant, respectively. Their units respectively are eV, J·s, nm/s, nm and J/eV.
The strongest luminescence peak is at 691.6 nm, the transition energy level is about 1.80 eV in Figure 7c. It is known that the direct band-gap of monolayer MoS 2 is 1.8-1.9 eV, which can further prove that the sample is a monolayer MoS 2 . Figure 7d is the power-dependent PL spectrum of monolayer MoS 2 under ambient condition, it can be seen that the PL intensity increases as laser power increases, and the relative positions of the A and B neutral excitons in the PL spectrum take place red-shifting, when the laser power increases due to the n-type doping of SiO 2 /Si substrate.
In order to obtain the binding energy of grown MoS 2 , the X-ray photoelectron spectroscopy (XPS) of Theta 300 XPS system, produced by Thermo Fisher, was adopted. The specific test conditions were: Source Gun Type: Al K Alpha; Spot Size: 650 µm; Lens Mode: Standard; Analyser Mode: Pass Energy 30.0 eV of CAE; Energy Step Size: 0.100 eV; Number of Energy Steps: 181, the number of scans in S 2p and Mo 3d is 8 and 7, respectively. The X-ray photoelectron spectrum image of the MoS 2 sample is shown in Figure 8, it can be observed that the MoS 2 sample have Mo 4+ and S 2− signals, which is consistent with the results reported in the literature [31]. The 3d orbital peaks of Mo are at 229.17 eV and 232.17 eV, corresponding to 3d 5/2 and 3d 3/2 orbitals respectively. The 2p orbital peaks of S are at 161.97 eV and 163.27 eV, corresponding to 2p 3/2 and 2p 1/2 orbitals respectively. At the same time, the atomic percentages of Mo 3d and S 2p elements are 5.42% and 11.36%, respectively, which indicates that the ratio of the atomic numbers of Mo and S is approximately 1:2. It indicates the presence of monolayer MoS 2 . photon energy is inversely proportional to the wavelength: In the formula (1), the symbol E, h, C, λ and k refer to the energy, Planck constant, light speed, wavelength and constant, respectively. Their units respectively are eV, J·s, nm/s, nm and J/eV.  photoluminescence spectrum corresponding to Figure 6. The black line corresponds to Figure 6a, and the red line corresponds to Figure 6b; (d) is the power-dependant PL spectrum under ambient condition.
The strongest luminescence peak is at 691.6 nm, the transition energy level is about 1.80 eV in Figure 7c. It is known that the direct band-gap of monolayer MoS2 is 1.8-1.9 eV, which can further prove that the sample is a monolayer MoS2. Figure 7d is the power-dependent PL spectrum of monolayer MoS2 under ambient condition, it can be seen that the PL intensity increases as laser power increases, and the relative positions of the A and B neutral excitons in the PL spectrum take place red-shifting, when the laser power increases due to the n-type doping of SiO2/Si substrate.
In order to obtain the binding energy of grown MoS2, the X-ray photoelectron spectroscopy (XPS) of Theta 300 XPS system, produced by Thermo Fisher, was adopted. The specific test conditions were: Source Gun Type: Al K Alpha; Spot Size: 650 μm; Lens Mode: Standard; Analyser Mode: Pass Energy 30.0 eV of CAE; Energy Step Size: 0.100 eV; Number of Energy Steps: 181, the number of scans in S2p and Mo3d is 8 and 7, respectively. The X-ray photoelectron spectrum image of the MoS2 sample is shown in Figure 8, it can be observed that the MoS2 sample have Mo 4+ and S 2− signals, which is consistent with the results reported in the literature [31]. The 3d orbital peaks of Mo are at 229.17 eV and 232.17 eV, corresponding to 3d5/2 and 3d3/2 orbitals respectively. The 2p orbital peaks of S are at 161.97 eV and 163.27 eV, corresponding to 2p3/2 and 2p1/2 orbitals respectively. At the same time, the atomic percentages of Mo3d and S2p elements are 5.42% and 11.36%, respectively, which indicates that the ratio of the atomic numbers of Mo and S is approximately 1:2. It indicates the presence of monolayer MoS2.  Figure 9a shows that the surface roughness of MoS2 sample is extremely low, the contrast and height of the sample surface are not changed obviously, which indicates the formation of monolayer MoS2 [32]. The height of the MoS2 sample in Figure 9b is around 0.72 nm. Therefore, it further evidences that the grown MoS2 is a single layer.  Figure 9a shows that the surface roughness of MoS 2 sample is extremely low, the contrast and height of the sample surface are not changed obviously, which indicates the formation of monolayer MoS 2 [32]. The height of the MoS 2 sample in Figure 9b is around 0.72 nm. Therefore, it further evidences that the grown MoS 2 is a single layer.