Chemical Vapor Deposition of Uniform and Large-Domain Molybdenum Disulfide Crystals on Glass/Al2O3 Substrates

Two-dimensional molybdenum disulfide (MoS2) has attracted significant attention for next-generation electronics, flexible devices, and optical applications. Chemical vapor deposition is the most promising route for the production of large-scale, high-quality MoS2 films. Recently, the chemical vapor deposition of MoS2 films on soda-lime glass has attracted great attention due to its low cost, fast growth, and large domain size. Typically, a piece of Mo foil or graphite needs to be used as a buffer layer between the glass substrates and the CVD system to prevent the glass substrates from being fragmented. In this study, a novel method was developed for synthesizing MoS2 on glass substrates. Inert Al2O3 was used as the buffer layer and high-quality, uniform, triangular monolayer MoS2 crystals with domain sizes larger than 400 μm were obtained. To demonstrate the advantages of glass/Al2O3 substrates, a direct comparison of CVD MoS2 on glass/Mo and glass/Al2O3 substrates was performed. When Mo foil was used as the buffer layer, serried small bilayer islands and bright core centers could be observed on the MoS2 domains at the center and edges of glass substrates. As a control, uniform MoS2 crystals were obtained when Al2O3 was used as the buffer layer, both at the center and the edge of glass substrates. Raman and PL spectra were further characterized to show the merit of glass/Al2O3 substrates. In addition, the thickness of MoS2 domains was confirmed by an atomic force microscope and the uniformity of MoS2 domains was verified by Raman mapping. This work provides a novel method for CVD MoS2 growth on soda-lime glass and is helpful in realizing commercial applications of MoS2.


Introduction
Two-dimensional transition metal dichalcogenides materials, specifically molybdenum disulfide (MoS 2 ), have emerged as an extremely important candidate for low-power, highperformance, and flexible electronics [1][2][3][4][5][6][7]. In order to achieve industry applications, batch production of high-quality and large-scale MoS 2 films at low cost has become a major requirement. Chemical vapor deposition (CVD) has demonstrated great potential in the large-scale production of high-quality MoS 2 films [8][9][10][11]. At present, CVD growth of large-area MoS 2 continuous films larger than 4 inches and the deposition of single MoS 2 domains with sizes up to the millimeter-scale have been realized by independent research groups [8][9][10][12][13][14][15], which greatly promote the industrialization process of MoS 2 . Although those encouraging advancements have been made in the CVD growth of MoS 2 , related studies are still at an early stage for the industrialization of MoS 2 films [16][17][18]. For example, the size of CVD-grown, single-crystalline graphene has reached up to the meter-scale with very fast growth rates [19,20]. However, there is an obvious gap between CVD MoS 2 and graphene both in crystal size and quality [14,15]. More efforts should be made on designing a new growth set-up, searching for possible catalysts and promoters, testing appropriate growth substrates and carefully optimizing the CVD growth parameters to lower the cost and improve the quality and uniformity of CVD MoS 2 [18,21].
Recently, low-cost soda-lime glass has been utilized as a growth substrate for CVD MoS 2 growth [12,[22][23][24][25][26][27][28]. In 2017, Chen et al. synthesized large-size MoS 2 crystals on molten glass at 1050 • C [27]. In 2018, Gao et al. synthesized high-quality bilayer MoS 2 with domain sizes up to 200 µm on molten glass at 830 • C [25]. Zhang et al. reported singlecrystal monolayer MoS 2 grown on molten glass at 850 • C with a domain size larger than 500 µm [24]. Yang et al. from Peking University developed a face-to-face metal precursor supply approach and deposited 6-inch uniform monolayer MoS 2 on the glass [12]. In 2020, Zeng et al. demonstrated bandgap tuning of MoS 2 grown on molten glass by a sphere diameter engineering technique [23]. In 2022, Li et al. reported the evolution of crystalline morphology of MoS 2 grown on glass substrates and provided an effective approach to engineering the morphology of MoS 2 crystals [22]. Commonly, due to the growth temperature of MoS 2 being higher than soda-lime glass's melting point, a piece of Mo foil or graphite needed to be used as a buffer layer between the glass substrates and the CVD system in those works to prevent the adhesion of glass substrates and CVD systems [25][26][27]. However, the MoS 2 morphology and domain size may be critically affected by Mo foil because Mo foil also could be used as the Mo-source in the chemical vapor deposition process [26]. In addition, Mo foils and graphite are very easy to react with oxygen at high growth temperatures, which limits the research approaches to MoS 2 growth on glass substrates, although oxygen has been proven to be an effective gas for improving the size and quality of MoS 2 crystals [8,9,[29][30][31][32]. For example, Zhang's group obtained a 4-inch monolayer MoS 2 film on a sapphire substrate by epitaxial growth using an oxygen-assisted method [13]. Hence, for the aim of Mo-source precise control and to extend the capability of glass substrates for high-quality MoS 2 CVD growth at variable critical experimental conditions, a non-active buffer layer needs to be explored.
In this work, high-quality CVD monolayer MoS 2 films grown on glass/Al 2 O 3 substrates were explored. Firstly, to demonstrate the advantages of glass/Al 2 O 3 substrates, MoS 2 films were synthesized on glass/Al 2 O 3 and glass/Mo substrates and characterized with optical microscopy in different regions. At the center of the glass/Mo substrates, serried bilayer seeds could be observed on the monolayer MoS 2 domains, while the MoS 2 domains grown on glass/Al 2 O 3 substrates exhibited a uniform contrast. At the edge of the glass/Al 2 O 3 substrates, monolayer MoS 2 domains with low nucleation density and uniform contrast were grown. As a control, monolayer MoS 2 domains with high nucleation density and multi-layer nucleation sites were grown on the edge region of glass/Mo substrates. Subsequently, the grown MoS 2 thin films were successfully transferred onto SiO 2 /Si substrates and characterized with Raman and photoluminescence (PL) spectroscopy, as well as atomic force microscopy (AFM). A comparison of Raman and PL spectra of MoS 2 domains grown by those two methods further illustrates the advantages of the non-active buffer layer, where uniform MoS 2 domains were synthesized as a result of a onefold supply of Mo-source. Finally, the thickness and uniformity of MoS 2 domains on glass/Al 2 O 3 substrates were confirmed by AFM and Raman mapping.

Experiments and Methods
As depicted in Figure 1, the MoS 2 crystals were grown in a CVD system with a 2-inchdiameter quartz tube and two individual furnaces. In this system, the temperatures of substrates and precursors, carrier gas flow rate, total pressure, and the weight of precursors could be controlled independently. Following the general conditions reported in previous literature [24,25], 1.4 g of sulfur powder and 2 mg of MoO 3 powder were weighed out and placed into separate boats. The boats were placed 25 cm apart in separate regions of the quartz tube to achieve the different temperatures for the S and Mo precursors. Both glass substrates and the buffer layers (Mo foils or Al 2 O 3 lamina) were cleaned with acetone (10 min), isopropanol (10 min) and deionized water (10 min). The size of both the substrates and buffer layers was 20 mm × 20 mm. After loading precursors and substrates, the tube was first pumped down to 0.2 mBar and then filled with Ar to 1000 mBar. The pumping and filling processes were repeated three times to eliminate air and other containment gases in the quartz tube. After that, the temperatures of furnaces I and II were set to 200 and 1050 • C, respectively, with a ramping rate of 10 • C/min. During the growth, high-purity argon was loaded with a flow rate of 20 sccm. After a growth period of 10 min, the furnaces were naturally cooled to room temperature. Nanomaterials 2022, 12, x FOR PEER REVIEW 3 of 12 previous literature [24,25], 1.4 g of sulfur powder and 2 mg of MoO3 powder were weighed out and placed into separate boats. The boats were placed 25 cm apart in separate regions of the quartz tube to achieve the different temperatures for the S and Mo precursors. Both glass substrates and the buffer layers (Mo foils or Al2O3 lamina) were cleaned with acetone (10 min), isopropanol (10 min) and deionized water (10 min). The size of both the substrates and buffer layers was 20 mm × 20 mm. After loading precursors and substrates, the tube was first pumped down to 0.2 mBar and then filled with Ar to 1000 mBar. The pumping and filling processes were repeated three times to eliminate air and other containment gases in the quartz tube. After that, the temperatures of furnaces I and II were set to 200 and 1050 °C, respectively, with a ramping rate of 10 °C/min. During the growth, high-purity argon was loaded with a flow rate of 20 sccm. After a growth period of 10 min, the furnaces were naturally cooled to room temperature.

Results and Discussion
Figure 2a,b are the optical photographs of glass/Mo and glass/Al2O3 substrates after the growth of MoS2, respectively. The Mo foil buffer layer displays a color of gray, and the Al2O3 buffer layer shows different a color of white. Furthermore, the glass substrates shrunk obviously, and the edge of Mo foil and Al2O3 lamina showed up after the hightemperature growth process [23,26]. As we all know, the shape, distribution, and thickness of MoS2 domains depend on the weight of precursors provided to the substrates. Consequently, as illustrated in Figure 2a,b, non-uniform MoS2 film growth could be observed in different regions due to the different precursors' concentration distributions on the glass substrates [12,33,34]. Typically, in our growth setup, thicker continuous MoS2 films could be grown upstream, closer to the Mo-source. Individual monolayer MoS2 domains tend to be synthesized downstream, farther away from the Mo-source, and abundant growth phenomena could be observed in these regions. Therefore, in order to clearly demonstrate the advantages of glass/Al2O3 substrates, the MoS2 morphology on the center and edge regions, as shown in Figure 2, was analyzed.   [23,26]. As we all know, the shape, distribution, and thickness of MoS 2 domains depend on the weight of precursors provided to the substrates. Consequently, as illustrated in Figure 2a,b, non-uniform MoS 2 film growth could be observed in different regions due to the different precursors' concentration distributions on the glass substrates [12,33,34]. Typically, in our growth setup, thicker continuous MoS 2 films could be grown upstream, closer to the Mo-source. Individual monolayer MoS 2 domains tend to be synthesized downstream, farther away from the Mo-source, and abundant growth phenomena could be observed in these regions. Therefore, in order to clearly demonstrate the advantages of glass/Al 2 O 3 substrates, the MoS 2 morphology on the center and edge regions, as shown in Figure 2, was analyzed.

Results and Discussion
The MoS 2 domains grown on different regions of glass/Al 2 O 3 and glass/Mo substrates are illustrated in Figures 3 and 4. Figure 3a,b are the typical optical microscopy images obtained from the MoS 2 domains grown on the center region of glass/Mo substrates. The MoS 2 domains exhibited triangle shapes and demonstrated lateral sizes larger than 400 µm, which is comparable to the MoS 2 domain sizes on glass substrates reported in other works [12,26]. The large crystal size could be attributed to the smooth, molten surface together with the catalytic role of the glass substrates, as reported in previous works [12,26,27,35,36]. As shown in Figure 3b, numerous tiny islands could be observed on the large MoS 2 triangle domains after additional magnification. The small bilayer islands could be attributed to the Mo foil buffer layer, which provides additional Mo-source during the growth of MoS 2 domains. Figure 3c,d are the representative optical microscopy images obtained from the MoS 2 domains grown on the center region of glass/Al 2 O 3 substrates. Triangle MoS 2 domains larger than 400 µm with uniform color contrast could be observed.
In addition, no small bilayer islands could be observed on the amplified image of the MoS 2 domain, which demonstrates the thickness uniformity of MoS 2 crystal domains grown on the glass/Al 2 O 3 substrates. Figure 4 shows the MoS 2 domains on the edge of the glass/Mo and the glass/Al 2 O 3 substrates. As shown in Figure 4a, the MoS 2 domains on the edge region of glass/Mo substrates generally exhibit star shapes with bright core centers. In contrast, the MoS 2 domains on the edge region of glass/Al 2 O 3 substrates are sparse and generally exhibit triangle shapes with uniform thickness, as shown in Figure 4c. In addition, both the MoS 2 domains grown on glass/Mo and glass/Al 2 O 3 were successfully transferred to SiO 2 /Si substrates. Figure 4b shows the transferred MoS 2 domains with bright core centers that were grown on glass/Mo substrates. Figure 4d shows the transferred MoS 2 domains with uniform thickness that were grown on glass/Al 2 O 3 substrates. The details of the transfer method have been reported in our previous works [24,25,37,38].
To provide a clear explanation of the different morphologies of the MoS 2 domains grown on glass/Mo and glass/Al 2 O 3 substrates, a schematic illustration of the underlying growth mechanism was plotted. Typically, as shown in Figure 5a,b, the flat glass plane will be condensed into an oblate, sphere-like shape at a temperature higher than its molten point [23]. Subsequently, the Mo foil or Al 2 O 3 buffer layer under glass substrates would be exposed to the CVD system. The Mo foil could be used as the Mo-source in the hightemperature CVD process, which has been demonstrated in previous work [26]. Therefore, excess Mo-source would be supplied and diffused on the glass/Mo substrates' surface. Consequently, the MoS 2 domains with small bilayer islands and bright core centers were grown at the center and edge regions, respectively. On the contrary, the strong chemical bonds of Al 2 O 3 make it stable at high temperatures. Therefore, as shown in Figure 5c,d, the Al 2 O 3 buffer layer would not affect the Mo-source supply during the process of MoS 2 growth and MoS 2 crystals with uniform thickness were grown. Furthermore, as discussed in previous works [12,26,27,35,36], the large domain size of MoS 2 at the center region of glass substrates could be ascribed to the smooth, molten glass surface under high temperature, together with the catalytic role of Na in the glass substrates, which reduce the energy barrier in the CVD process.  Figure 3a,b are the typical optical microscopy images obtained from the MoS2 domains grown on the center region of glass/Mo substrates. The MoS2 domains exhibited triangle shapes and demonstrated lateral sizes larger than 400 μm, which is comparable to the MoS2 domain sizes on glass substrates reported in other works [12,26]. The large crystal size could be attributed to the smooth, molten surface together with the catalytic role of the glass substrates, as reported in previous works    Figure 4 shows the MoS2 domains on the edge of the glass/Mo and the glass/Al2O3 substrates. As shown in Figure 4a, the MoS2 domains on the edge region of glass/Mo substrates generally exhibit star shapes with bright core centers. In contrast, the MoS2 domains on the edge region of glass/Al2O3 substrates are sparse and generally exhibit triangle shapes with uniform thickness, as shown in Figure 4c. In addition, both the MoS2 domains grown on glass/Mo and glass/Al2O3 were successfully transferred to SiO2/Si substrates. Figure 4b shows the transferred MoS2 domains with bright core centers that were grown on glass/Mo substrates. Figure 4d shows the transferred MoS2 domains with uniform thickness that were grown on glass/Al2O3 substrates. The details of the transfer method have been reported in our previous works [24,25,37,38]. Raman spectroscopy is one commonly used spectroscopic technique to investigate phonons as well as the vibrational, rotational and other low-frequency modes in twodimensional materials, which could be used to provide information about both crystal quality as well as estimate the number of layers of MoS 2 domains [39][40][41][42]. Therefore, Raman spectra were collected to further characterize the properties of MoS 2 grown on glass/Mo and glass/Al 2 O 3 . Figure 6a,c display the Raman spectra of the transferred MoS 2 domains on SiO 2 /Si substrates that were grown on the center regions of glass/Mo and glass/Al 2 O 3 substrates, respectively. As shown in Figure 6a, two characteristic Raman peaks were found at 385.4 cm −1 and 405.2 cm −1 in the spectral range, which can be assigned to in-plane vibration modes of Mo and S in the opposite direction (E 1 2g ) and an out-of-plane vibration mode of S atoms (A 1g ). The frequency difference for the MoS 2 domains grown on glass/Mo substrates was 19.7 cm −1 . For the MoS 2 domains grown on glass/Al 2 O 3 substrates, the E 1 2g and A 1g peaks are located at 387.3 cm −1 and 405.2 cm −1 . Compared with the MoS 2 domains grown on glass/Mo substrates, the frequency difference was reduced to 17.9 cm −1 , indicating a monolayer thickness for the measured MoS 2 domains. Moreover, the full width at half maximum (FWHM) of the E 1 2g peak also reduced from 5.6 cm −1 to 3.8 cm −1 , demonstrating the high quality of the MoS 2 domains grown on glass/Al 2 O 3 substrates. The reduced frequency difference and FWHM results from the different morphologies of MoS 2 domains grown on glass/Mo and glass/Al 2 O 3 substrates, as shown in Figure 3. In addition, PL spectroscopy was performed for the transferred MoS 2 domains grown on glass/Mo and glass/Al 2 O 3 substrates, and the obtained spectra are shown in Figure 6b,d. Both those two types of MoS 2 have a characteristic peak (A exciton peak) at 1.84 eV, which is in agreement with previous studies [24,25]. The FWHMs of the A exciton peaks for MoS 2 domains grown on glass/Mo and glass/Al 2 O 3 substrates are 1.02 and 0.96 eV, respectively. The reduced PL FWHMs of MoS 2 domains grown on glass/Al 2 O 3 could also be attributed to its uniform thickness and high crystal quality. To provide a clear explanation of the different morphologies of the MoS2 domains grown on glass/Mo and glass/Al2O3 substrates, a schematic illustration of the underlying growth mechanism was plotted. Typically, as shown in Figure 5a,b, the flat glass plane will be condensed into an oblate, sphere-like shape at a temperature higher than its molten point [23]. Subsequently, the Mo foil or Al2O3 buffer layer under glass substrates would be exposed to the CVD system. The Mo foil could be used as the Mo-source in the hightemperature CVD process, which has been demonstrated in previous work [26]. Therefore, excess Mo-source would be supplied and diffused on the glass/Mo substrates' surface. Consequently, the MoS2 domains with small bilayer islands and bright core centers were grown at the center and edge regions, respectively. On the contrary, the strong chemical bonds of Al2O3 make it stable at high temperatures. Therefore, as shown in Figure  5c,d, the Al2O3 buffer layer would not affect the Mo-source supply during the process of MoS2 growth and MoS2 crystals with uniform thickness were grown. Furthermore, as discussed in previous works [12,26,27,35,36], the large domain size of MoS2 at the center region of glass substrates could be ascribed to the smooth, molten glass surface under high temperature, together with the catalytic role of Na in the glass substrates, which reduce the energy barrier in the CVD process. Raman spectroscopy is one commonly used spectroscopic technique to investigate phonons as well as the vibrational, rotational and other low-frequency modes in two-dimensional materials, which could be used to provide information about both crystal quality as well as estimate the number of layers of MoS2 domains [39][40][41][42]. Therefore, Raman spectra were collected to further characterize the properties of MoS2 grown on glass/Mo and glass/Al2O3. Figure 6a,c display the Raman spectra of the transferred MoS2 domains on SiO2/Si substrates that were grown on the center regions of glass/Mo and glass/Al2O3 substrates, respectively. As shown in Figure 6a, two characteristic Raman peaks were found at 385.4 cm −1 and 405.2 cm −1 in the spectral range, which can be assigned to in-plane vibration modes of Mo and S in the opposite direction (E 1 2g) and an out-of-plane vibration mode of S atoms (A1g). The frequency difference for the MoS2 domains grown on glass/Mo substrates was 19.7 cm −1 . For the MoS2 domains grown on glass/Al2O3 substrates, the E 1 2g and A1g peaks are located at 387.3 cm −1 and 405.2 cm −1 . Compared with the MoS2 domains grown on glass/Mo substrates, the frequency difference was reduced to 17.9 cm −1 , indicating a monolayer thickness for the measured MoS2 domains. Moreover, the full width at half maximum (FWHM) of the E 1 2g peak also reduced from 5.6 cm −1 to 3.8 cm −1 , demonstrating the high quality of the MoS2 domains grown on glass/Al2O3 substrates. The reduced frequency difference and FWHM results from the different morphologies of MoS2 domains grown on glass/Mo and glass/Al2O3 substrates, as shown in Figure 3. In addition, PL spectroscopy was performed for the transferred MoS2 domains grown on glass/Mo and glass/Al2O3 substrates, and the obtained spectra are shown in Figure 6b,d. Both those two types of MoS2 have a characteristic peak (A exciton peak) at 1.84 eV, which is in agreement with previous studies [24,25]. The FWHMs of the A exciton peaks for MoS2 domains In order to further characterize the MoS 2 domains grown on glass/Al 2 O 3 substrates, atomic force microscopy (AFM) and Raman mapping were performed after they were transferred onto SiO 2 /Si substrates. Figure 7a displays the AFM images obtained from a typical triangle-shaped MoS 2 domain. A thickness of 0.7 nm was demonstrated through the AFM characterization, as shown in Figure 7b. This is consistent with the thickness of the monolayer MoS 2 . Figure S1 displays the optical microscope and AFM images of the CVD-grown MoS 2 on the center region of glass/Al 2 O 3 substrates. As shown in Figure S1b, numerous tiny islands could be observed on the large MoS 2 triangle domains. Figure 7c,d show the Raman intensity mappings recorded at 387.3 cm −1 and 405.2 cm −1 , respectively. The Raman mappings on the intensity of E 1 2g mode and A 1g mode reveal a uniform color contrast, further demonstrating the thickness uniformity and good crystallinity of MoS 2 grown on glass/Al 2 O 3 substrates. grown on glass/Mo and glass/Al2O3 substrates are 1.02 and 0.96 eV, respectively. The reduced PL FWHMs of MoS2 domains grown on glass/Al2O3 could also be attributed to its uniform thickness and high crystal quality. In order to further characterize the MoS2 domains grown on glass/Al2O3 substrates, atomic force microscopy (AFM) and Raman mapping were performed after they were transferred onto SiO2/Si substrates. Figure 7a displays the AFM images obtained from a typical triangle-shaped MoS2 domain. A thickness of 0.7 nm was demonstrated through the AFM characterization, as shown in Figure 7b. This is consistent with the thickness of the monolayer MoS2. Figure S1 displays the optical microscope and AFM images of the CVD-grown MoS2 on the center region of glass/Al2O3 substrates. As shown in Figure S1b, numerous tiny islands could be observed on the large MoS2 triangle domains. Figure 7c,

Conclusions
In this work, the two-dimensional semiconductor MoS2 was successfully synthesized on glass/Al2O3 substrates for the first time. The advantages of glass/Al2O3 substrates for CVD MoS2 growth were demonstrated by a direct comparison of glass/Mo substrates. Op-

Conclusions
In this work, the two-dimensional semiconductor MoS 2 was successfully synthesized on glass/Al 2 O 3 substrates for the first time. The advantages of glass/Al 2 O 3 substrates for CVD MoS 2 growth were demonstrated by a direct comparison of glass/Mo substrates. Optical microscopy revealed that MoS 2 crystals grown on both glass/Mo and glass/Al 2 O 3 substrates were conventionally triangle-shaped and had a lateral size larger than 400 µm. Numerous bilayer islands and bright core centers could be observed on the surface of MoS 2 domains at the center region and edge region of glass/Mo substrates. As a control, MoS 2 domains grown on glass/Al 2 O 3 substrates exhibited uniform optical contrast, demonstrating the thickness uniformity of those MoS 2 domains. The Raman and PL comparison of MoS 2 domains further confirmed this. In addition, the thickness of MoS 2 domains was characterized by atomic force microscopy and the homogeneity of MoS 2 domains grown on glass/Al 2 O 3 substrates was further demonstrated by Raman mapping. These results demonstrate a novel method to produce MoS 2 films on glass substrates and are of great value for future commercial applications of MoS 2 films.