Probing the Growth Improvement of Large-Size High Quality Monolayer MoS2 by APCVD

Two-dimensional transition metal dichalcogenides (TMDs) have attracted attention from researchers in recent years. Monolayer molybdenum disulfide (MoS2) is the direct band gap two-dimensional crystal with excellent physical and electrical properties. Monolayer MoS2 can effectively compensate for the lack of band gap of graphene in the field of nano-electronic devices, which is widely used in catalysis, transistors, optoelectronic devices, and integrated circuits. Therefore, it is critical to obtain high-quality, large size monolayer MoS2. The large-area uniform high-quality monolayer MoS2 is successfully grown on an SiO2/Si substrate with oxygen plasma treatment and graphene quantum dot solution by atmospheric pressure chemical vapor deposition (APCVD) in this paper. In addition, the effects of substrate processing conditions, such as oxygen plasma treatment time, power, and dosage of graphene quantum dot solution on growth quality and the area of the monolayer of MoS2, are studied systematically, which would contribute to the preparation of large-area high-quality monolayer MoS2. Analysis and characterization of monolayer MoS2 are carried out by Optical Microscopy, AFM, XPS, Raman, and Photoluminescence Spectroscopy. The results show that monolayer MoS2 is a large-area, uniform, and triangular with a side length of 200 μm, and it is very effective to treat the SiO2/Si substrate by oxygen plasma and graphene quantum dot solution, which would help the fabrication of optoelectronic devices.


Introduction
Molybdenum disulfide (MoS 2 ) has attracted attention from researchers in recent years because of its excellent physical properties. MoS 2 has been studied extensively in the field of two-dimensional nano-electronic devices, which has the potential to become an alternative to grapheme [1][2][3][4]. MoS 2 is a two-dimensional transition metal dichalcogenides (TMDs) with an adjustable band gap structure in which atoms in layers are bonded by covalent bond interaction, whereas interlayer atoms are coupled by weak van der Waals forces [5,6]. Monolayer MoS 2 has excellent photoelectric properties due to its high electron mobility and unique band gap structure. The bulk MoS 2 is an indirect bandgap semiconductor with band gap energy about 1.29 eV, band gap energy increases with the decrease of MoS 2 layers number, thus monolayer MoS 2 becomes the direct bandgap semiconductor with 1.90 eV [7][8][9]. Due to the existence of a direct band gap, field effect transistors based on monolayer MoS 2 have higher current switching ratio and electron mobility at room temperature [10,11]. In

Methods
Before the MoS 2 growth experiment, SiO 2 /Si substrates were respectively placed in absolute ethanol, acetone, and deionized water solution for ultrasonic cleaning in 10 min, 10 min, 10 min, and dried with a nitrogen gun. Compared to the reported literature, the difference is that SiO 2 /Si substrates are cleaned with PCE-6 plasma cleaner under different time and power conditions. Subsequently, different dosages of 1 mg/mL graphene quantum dot solution were applied to SiO 2 /Si substrates uniformly with oxygen plasma by using VTC-100 vacuum rotary coater. In addition, a quartz boat with 100 mg sulfur powder (Alfa Aesar, Shanghai, China, 99.5%) was transferred to the low temperature zone of the tube furnace constructed by the winding heating belt. Moreover, 2 mg MoO 3 powder (Alfa Aesar, Shanghai, China, 99.95%) was weighed into one end of the quartz boat, and SiO 2 /Si substrate was placed faced down 5 cm apart from the MoO 3 powder, and the quartz boat with MoO 3 powder and SiO 2 /Si substrate was placed in the high temperature zone of tube furnace, as shown in Figure 1a. Figure 1b shows the temperature change diagram of MoS 2 grown by APCVD, high-purity argon gas with a flow rate of 300 sccm was introduced into the tube furnace for 10 min to exclude air in the tube furnace before the start of the reaction. The high temperature zone of the tube furnace was heated from room temperature to 550 • C in 30 min, meanwhile the heating belt began to heat, and the temperature was controlled at 200 • C. The temperature of the high temperature zone continued to 720 • C in 10 min with heating, and maintained the growth temperature for 10 min before cooling down to room temperature. The temperature distribution in the furnace was as follows: The closer the distance from the intermediate heating zone of the tube furnace was, the higher the temperature was [26,27]. Finally, argon gas with a flow rate of 50 sccm was introduced into the tube furnace throughout the reaction experiment, and the pressure of the tube furnace was maintained at a normal pressure. Nanomaterials 2019, 9, x FOR PEER REVIEW 3 of 9 temperature zone of the tube furnace was heated from room temperature to 550 °C in 30 min, meanwhile the heating belt began to heat, and the temperature was controlled at 200 °C. The temperature of the high temperature zone continued to 720 °C in 10 min with heating, and maintained the growth temperature for 10 min before cooling down to room temperature. The temperature distribution in the furnace was as follows: The closer the distance from the intermediate heating zone of the tube furnace was, the higher the temperature was [26,27]. Finally, argon gas with a flow rate of 50 sccm was introduced into the tube furnace throughout the reaction experiment, and the pressure of the tube furnace was maintained at a normal pressure. The new generation high-resolution Raman spectroscopy (LabRAM HR Evolution, HORIBA Jobin Yvon, Paris, France) was used to characterize the surface topography, Raman, and Photoluminescence spectrum of MoS2 systematically. The specific test conditions of the Raman spectroscopy were as follows: 532 nm laser with 1 μm spot diameter, spectral resolution ≤0.65 cm −1 , scan time 5 s, and accumulations number 3 s. The layer number of MoS2 could be determined quickly by the difference in the contrast between MoS2 and SiO2/Si substrate under the optical microscope. Raman spectrum was the scattering spectrum that utilized the inelastic scattering effect of the molecules on photons, which can provide an efficient representation of molecular structure. Photoluminescence spectrum is a form of cold luminescence, it would re-emit photons or electromagnetic waves when SiO2/Si substance absorbs photons or electromagnetic waves of a certain frequency. In addition, surface morphology and thickness of MoS2 grown on SiO2/Si substrate were characterized through interatomic interaction forces by using atomic force microscopy with longitudinal high resolution (AFM, Dimension 3100, Veeco Instruments, Shanghai, China). Moreover, X-ray photoelectron spectroscopy-Auger electron spectroscopy (Theta 300 XPS system, Thermo Fisher, Shanghai, China) was also used to measure chemical and electronic states of MoS2.
Optical microscopy (OM) was the most efficient and intuitive method to characterize layered structural materials. MoS2 was grown on silicon substrate with a layer of 285 nm SiO2 by APCVD. The number, size and film formation of MoS2 can be quickly judged by the contrast between MoS2 and SiO2/Si substrate under the optical microscope.

Effect of Different Oxygen Plasma Treatment Time and Power on the Growth of MoS2
SiO2/Si substrates are cleaned by the PCE-6 oxygen plasma cleaning machine in this experiment. The specific cleaning steps are as follows: First put SiO2/Si substrate into the vacuum chamber and vacuum it to below 1 Pa. Then oxygen gas is introduced to maintain oxygen vacuum in the vacuum chamber to 50 Pa. As the substrate treatment effect is greatly affected by gas pressure, The new generation high-resolution Raman spectroscopy (LabRAM HR Evolution, HORIBA Jobin Yvon, Paris, France) was used to characterize the surface topography, Raman, and Photoluminescence spectrum of MoS 2 systematically. The specific test conditions of the Raman spectroscopy were as follows: 532 nm laser with 1 µm spot diameter, spectral resolution ≤0.65 cm −1 , scan time 5 s, and accumulations number 3 s. The layer number of MoS 2 could be determined quickly by the difference in the contrast between MoS 2 and SiO 2 /Si substrate under the optical microscope. Raman spectrum was the scattering spectrum that utilized the inelastic scattering effect of the molecules on photons, which can provide an efficient representation of molecular structure. Photoluminescence spectrum is a form of cold luminescence, it would re-emit photons or electromagnetic waves when SiO 2 /Si substance absorbs photons or electromagnetic waves of a certain frequency. In addition, surface morphology and thickness of MoS 2 grown on SiO 2 /Si substrate were characterized through interatomic interaction forces by using atomic force microscopy with longitudinal high resolution (AFM, Dimension 3100, Veeco Instruments, Shanghai, China). Moreover, X-ray photoelectron spectroscopy-Auger electron spectroscopy (Theta 300 XPS system, Thermo Fisher, Shanghai, China) was also used to measure chemical and electronic states of MoS 2 .
Optical microscopy (OM) was the most efficient and intuitive method to characterize layered structural materials. MoS 2 was grown on silicon substrate with a layer of 285 nm SiO 2 by APCVD. The number, size and film formation of MoS 2 can be quickly judged by the contrast between MoS 2 and SiO 2 /Si substrate under the optical microscope.

Effect of Different Oxygen Plasma Treatment Time and Power on the Growth of MoS 2
SiO 2 /Si substrates are cleaned by the PCE-6 oxygen plasma cleaning machine in this experiment. The specific cleaning steps are as follows: First put SiO 2 /Si substrate into the vacuum chamber and vacuum it to below 1 Pa. Then oxygen gas is introduced to maintain oxygen vacuum in the vacuum chamber to 50 Pa. As the substrate treatment effect is greatly affected by gas pressure, this affects discharge power and processing time during glow discharge. Subsequently, the photo-discharge is performed at the discharge frequency of 13.56 MHz, and SiO 2 /Si substrates are processed sequentially at 300 W, 500 W, and 1000 W discharge power for 30, 60, 90, and 120 s. The principle of oxygen plasma cleaning is to convert oxygen into its plasma state by the glow discharge. These high-energy ions bombard the surface of the SiO 2 /Si substrate and easily react with the surface of substrate to increase the surface energy, thereby changing the surface properties of the SiO 2 /Si substrate and remove the organic impurities of the SiO 2 /Si substrate surface, which can achieve a hydrophilic effect, and improve the surface activity of the silicon wafer [28]. The characterization of the SiO 2 /Si substrate before and after oxygen plasma cleaning treatment can be observed by the growth of MoS 2 . The specific growth of MoS 2 under different oxygen plasma treatment time and power are as follows.
It can be found by observing Figure 2 that the growth and morphology of MoS 2 would, respectively, become uniform and better, and the size was also improved with an increase of processing time when the oxygen plasma was used in the low-power and medium-power oxygen plasma cleaning of SiO 2 /Si substrate. This is because the surface roughness is improved significantly after oxygen plasma cleaning, and it can also change the surface characteristics of the SiO 2 /Si substrate. The growth of MoS 2 initially becomes better and more uniform with the increase of processing time when the SiO 2 /Si substrate is subjected to high-power oxygen plasma cleaning, but the treatment time has almost no effect on the growth of MoS 2 after 60 s, which indicates that too long processing time does not always increase the surface activity of the MoS 2 material. As the oxygen plasma cleaning power increases, the density and energy of the plasma increase, and the oxygen plasma treatment speed also increases. At this time, the morphology of MoS 2 is the triangle with uniform size, and multilayer or bulk materials of MoS 2 also appear when the time of cleaning SiO 2 /Si substrate is constant. As shown in Figure 2, it is found that the optimal oxygen plasma cleaning power and time of SiO 2 /Si substrate processing are 500 W and 90 s, respectively. At this time, the morphology of MoS 2 is the best, and the maximum triangular side length can be up to 100 µm. Nanomaterials 2019, 9, x FOR PEER REVIEW 4 of 9 processed sequentially at 300 W, 500 W, and 1000 W discharge power for 30, 60, 90, and 120 s. The principle of oxygen plasma cleaning is to convert oxygen into its plasma state by the glow discharge. These high-energy ions bombard the surface of the SiO2/Si substrate and easily react with the surface of substrate to increase the surface energy, thereby changing the surface properties of the SiO2/Si substrate and remove the organic impurities of the SiO2/Si substrate surface, which can achieve a hydrophilic effect, and improve the surface activity of the silicon wafer [28]. The characterization of the SiO2/Si substrate before and after oxygen plasma cleaning treatment can be observed by the growth of MoS2. The specific growth of MoS2 under different oxygen plasma treatment time and power are as follows. It can be found by observing Figure 2 that the growth and morphology of MoS2 would, respectively, become uniform and better, and the size was also improved with an increase of processing time when the oxygen plasma was used in the low-power and medium-power oxygen plasma cleaning of SiO2/Si substrate. This is because the surface roughness is improved significantly after oxygen plasma cleaning, and it can also change the surface characteristics of the SiO2/Si substrate. The growth of MoS2 initially becomes better and more uniform with the increase of processing time when the SiO2/Si substrate is subjected to high-power oxygen plasma cleaning, but the treatment time has almost no effect on the growth of MoS2 after 60 s, which indicates that too long processing time does not always increase the surface activity of the MoS2 material. As the oxygen plasma cleaning power increases, the density and energy of the plasma increase, and the oxygen plasma treatment speed also increases. At this time, the morphology of MoS2 is the triangle with uniform size, and multilayer or bulk materials of MoS2 also appear when the time of cleaning SiO2/Si substrate is constant. As shown in Figure 2, it is found that the optimal oxygen plasma cleaning power and time of SiO2/Si substrate processing are 500 W and 90 s, respectively. At this time, the morphology of MoS2 is the best, and the maximum triangular side length can be up to 100 μm.

Effect of Graphene Quantum Dot Solution Amount on the Growth of MoS 2
Impurities and defects on the SiO 2 /Si substrate are easily selected when MoS 2 sample is deposited by CVD. Therefore, the processing of SiO 2 /Si substrate would have a greater impact on the nucleation process of MoS 2 . VTC-100 vacuum rotary coating machine is used to spin the graphene quantum dot solution with a particle size of 2 nm on cleaned SiO 2 /Si substrate uniformly in the experiment, and research the effect of different dosages of 1mg/ml graphene quantum dot solution on the growth of MoS 2 by non-uniform nucleation principle. The size, quality, and continuity of monolayer MoS 2 are improved by dropping the graphene quantum dot solution on SiO 2 /Si substrate.

Effect of Graphene Quantum Dot Solution Amount on the Growth of MoS2
Impurities and defects on the SiO2/Si substrate are easily selected when MoS2 sample is deposited by CVD. Therefore, the processing of SiO2/Si substrate would have a greater impact on the nucleation process of MoS2. VTC-100 vacuum rotary coating machine is used to spin the graphene quantum dot solution with a particle size of 2 nm on cleaned SiO2/Si substrate uniformly in the experiment, and research the effect of different dosages of 1mg/ml graphene quantum dot solution on the growth of MoS2 by non-uniform nucleation principle. The size, quality, and continuity of monolayer MoS2 are improved by dropping the graphene quantum dot solution on SiO2/Si substrate.
In order to obtain the optimum dosage of graphene quantum dot solution, SiO2/Si substrates with 5 μL, 10 μL, 15 μL, 20 μL, 25 μL, and 30 μL of 1 mg/mL graphene quantum dot solution are carried out on the growth experiments of MoS2. It can be seen from Figure 3 that different dosages of graphene quantum dot solution on SiO2/Si substrates have an effect on growth of MoS2. It can be observed from Figure 3a-d that many small triangles with a size of 20 μm would appear on SiO2/Si substrate under the same growth conditions, the size of the triangular MoS2 increases gradually with graphene quantum dot solution dosage increase, large area continuous film with 300 μm lateral dimension is formed, and the middle of the MoS2 film is obviously thicker. This is because a certain dosage graphene quantum dot solution can provide a suitable nucleation point for MoS2 on SiO2/Si substrate, which would facilitate the growth of a large-area MoS2 film. In Figure 3e,f, MoS2 sample deposited on SiO2/Si substrate is the triangle with a size of 40 μm, and no uniform continuous film is formed. The reason is that the graphene quantum dots solution dosage is too high and the nucleation sites are excessive. In conclusion, the optimal dosage of 1 mg/mL graphene quantum dot solution is 20 μL by analyzing the above results.

The Optimal Growth Conditions of Monolayer MoS2
In Figure 4, the size of large-area high-quality monolayer MoS2 grown on SiO2/Si substrate with oxygen plasma (500 w, 90 s) and graphene quantum dot solution (20 μL, 1 mg/mL) can be up to 200 μm, and different monolayer MoS2 spliced together to form large area continuous film with

The Optimal Growth Conditions of Monolayer MoS 2
In Figure 4

Characterization of Monolayer MoS2
It can be found by observing Figure 5a that there are two characteristic peaks in Raman spectrum of monolayer MoS2. The E 1 2g peak position of in-plane vibration mode is located at 383.3 cm −1 , and the A1g peak position of out-of-plane vibration mode is located at 402.7 cm −1 , the peak distance between two peaks is 19.4 cm −1 , and A1g/E 1 2g ≈ 1.05, which indicates the triangular MoS2 is monolayer [29]. In Figure 5b, monolayer MoS2 film has the strongest luminescence peak at 683.6 nm due to the direct excitation of excitons. It can be known from the conversion relationship between wavelength and electron volts that the corresponding transition energy level at the luminescence peak is about 1.82 eV, and there is also a B peak at 1.98 eV due to the interaction of electrons in the 3d orbital of molybdenum atoms, which can further prove that the sample is a large-area monolayer MoS2 with good film quality [30,31]. In addition, Raman mapping is carried out to observe film-forming quality and uniformity of triangular monolayer MoS2, and the color is very uniform, which also indicates that the sample is monolayer MoS2 with highly uniform quality. Atomic force microscopy (AFM) is used to characterize the surface topography of layered structural materials. The thickness of MoS2 on SiO2/Si substrate is characterized by AFM, as shown in Figure 6. The film and triangular MoS2 are very uniform in color, and the thickness of MoS2 is about 0.83 nm, which indicates that MoS2 is monolayer.
X-ray photoelectron spectroscopy (XPS) is the advanced analytical technique in microscopic analysis of electronic materials and components. It uses photoelectron binding energy as abscissa

Characterization of Monolayer MoS 2
It can be found by observing Figure 5a that there are two characteristic peaks in Raman spectrum of monolayer MoS 2 . The E 1 2g peak position of in-plane vibration mode is located at 383.3 cm −1 , and the A 1g peak position of out-of-plane vibration mode is located at 402.7 cm −1 , the peak distance between two peaks is 19.4 cm −1 , and A 1g /E 1 2g ≈ 1.05, which indicates the triangular MoS 2 is monolayer [29]. In Figure 5b, monolayer MoS 2 film has the strongest luminescence peak at 683.6 nm due to the direct excitation of excitons. It can be known from the conversion relationship between wavelength and electron volts that the corresponding transition energy level at the luminescence peak is about 1.82 eV, and there is also a B peak at 1.98 eV due to the interaction of electrons in the 3d orbital of molybdenum atoms, which can further prove that the sample is a large-area monolayer MoS 2 with good film quality [30,31]. In addition, Raman mapping is carried out to observe film-forming quality and uniformity of triangular monolayer MoS 2 , and the color is very uniform, which also indicates that the sample is monolayer MoS 2 with highly uniform quality.

Characterization of Monolayer MoS2
It can be found by observing Figure 5a that there are two characteristic peaks in Raman spectrum of monolayer MoS2. The E 1 2g peak position of in-plane vibration mode is located at 383.3 cm −1 , and the A1g peak position of out-of-plane vibration mode is located at 402.7 cm −1 , the peak distance between two peaks is 19.4 cm −1 , and A1g/E 1 2g ≈ 1.05, which indicates the triangular MoS2 is monolayer [29]. In Figure 5b, monolayer MoS2 film has the strongest luminescence peak at 683.6 nm due to the direct excitation of excitons. It can be known from the conversion relationship between wavelength and electron volts that the corresponding transition energy level at the luminescence peak is about 1.82 eV, and there is also a B peak at 1.98 eV due to the interaction of electrons in the 3d orbital of molybdenum atoms, which can further prove that the sample is a large-area monolayer MoS2 with good film quality [30,31]. In addition, Raman mapping is carried out to observe film-forming quality and uniformity of triangular monolayer MoS2, and the color is very uniform, which also indicates that the sample is monolayer MoS2 with highly uniform quality.  X-ray photoelectron spectroscopy (XPS) is the advanced analytical technique in microscopic analysis of electronic materials and components. It uses photoelectron binding energy as abscissa and relative intensity as ordinate to make photoelectron spectroscopy. The relevant information of MoS2 is obtained by analyzing the energy spectrum. It can be seen from Figure 7 that Mo3d5/2 and 3d3/2 binding energy in Mo4+ is about 228.5 eV and 232.5 eV, respectively. At the same time, 2P3/2 and 2P1/2 binding energy in S2p is about 162.2 eV and 163.3 eV, respectively. The percentages of the S2p element and Mo3d element are 15.79% and 7.58%, respectively, which also indicates MoS2 is monolayer.

Conclusions
In this paper, large-area high-quality uniform triangle monolayer MoS2 is successfully grown on an SiO2/Si substrate with oxygen plasma treatment and graphene quantum dot solution by APCVD. Effects of substrate processing conditions, such as oxygen plasma treatment time, power, and the amount of graphene quantum dot solution on growth quality and the area of monolayer MoS2 are studied in detail. The optimal dosage of 1mg/ml graphene quantum dot solution is 20 μL, and the optimal power and time of SiO2/Si substrate processing are 500 W and 90 s, respectively. Analysis and characterization of monolayer MoS2 are carried out by Optical Microscopy, AFM, XPS Raman, and Photoluminescence Spectroscopy. The results show that the grown sample on the SiO2/Si substrate under optimal cleaning conditions is a large-area high-quality, uniform, triangular monolayer MoS2 with a side length of 200 μm, and it is very effective to treat SiO2/Si substrate by X-ray photoelectron spectroscopy (XPS) is the advanced analytical technique in microscopic analysis of electronic materials and components. It uses photoelectron binding energy as abscissa and relative intensity as ordinate to make photoelectron spectroscopy. The relevant information of MoS 2 is obtained by analyzing the energy spectrum. It can be seen from Figure 7 that Mo3d 5/2 and 3d 3/2 binding energy in Mo 4+ is about 228.5 eV and 232.5 eV, respectively. At the same time, 2P 3/2 and 2P 1/2 binding energy in S 2p is about 162.2 eV and 163.3 eV, respectively. The percentages of the S 2p element and Mo 3d element are 15.79% and 7.58%, respectively, which also indicates MoS 2 is monolayer. Nanomaterials 2019, 9, x FOR PEER REVIEW 7 of 9 and relative intensity as ordinate to make photoelectron spectroscopy. The relevant information of MoS2 is obtained by analyzing the energy spectrum. It can be seen from Figure 7 that Mo3d5/2 and 3d3/2 binding energy in Mo4+ is about 228.5 eV and 232.5 eV, respectively. At the same time, 2P3/2 and 2P1/2 binding energy in S2p is about 162.2 eV and 163.3 eV, respectively. The percentages of the S2p element and Mo3d element are 15.79% and 7.58%, respectively, which also indicates MoS2 is monolayer.

Conclusions
In this paper, large-area high-quality uniform triangle monolayer MoS2 is successfully grown on an SiO2/Si substrate with oxygen plasma treatment and graphene quantum dot solution by APCVD. Effects of substrate processing conditions, such as oxygen plasma treatment time, power, and the amount of graphene quantum dot solution on growth quality and the area of monolayer MoS2 are studied in detail. The optimal dosage of 1mg/ml graphene quantum dot solution is 20 μL, and the optimal power and time of SiO2/Si substrate processing are 500 W and 90 s, respectively. Analysis and characterization of monolayer MoS2 are carried out by Optical Microscopy, AFM, XPS Raman, and Photoluminescence Spectroscopy. The results show that the grown sample on the SiO2/Si substrate under optimal cleaning conditions is a large-area high-quality, uniform, triangular monolayer MoS2 with a side length of 200 μm, and it is very effective to treat SiO2/Si substrate by oxygen plasma and graphene quantum dot solution, which can pave the way for the fabrication of optoelectronic devices.

Conclusions
In this paper, large-area high-quality uniform triangle monolayer MoS 2 is successfully grown on an SiO 2 /Si substrate with oxygen plasma treatment and graphene quantum dot solution by APCVD. Effects of substrate processing conditions, such as oxygen plasma treatment time, power, and the amount of graphene quantum dot solution on growth quality and the area of monolayer MoS 2 are studied in detail. The optimal dosage of 1mg/ml graphene quantum dot solution is 20 µL, and the optimal power and time of SiO 2 /Si substrate processing are 500 W and 90 s, respectively. Analysis and characterization of monolayer MoS 2 are carried out by Optical Microscopy, AFM, XPS Raman, and Photoluminescence Spectroscopy. The results show that the grown sample on the SiO 2 /Si substrate under optimal cleaning conditions is a large-area high-quality, uniform, triangular monolayer MoS 2 with a side length of 200 µm, and it is very effective to treat SiO 2 /Si substrate by oxygen plasma and graphene quantum dot solution, which can pave the way for the fabrication of optoelectronic devices.