Large area MoS2 thin film growth by direct sulfurization

In this study, we present the growth of monolayer MoS2 (molybdenum disulfide) film. Mo (molybdenum) film was formed on a sapphire substrate through e-beam evaporation, and triangular MoS2 film was grown by direct sulfurization. First, the growth of MoS2 was observed under an optical microscope. The number of MoS2 layers was analyzed by Raman spectrum, atomic force microscope (AFM), and photoluminescence spectroscopy (PL) measurement. Different sapphire substrate regions have different growth conditions of MoS2. The growth of MoS2 is optimized by controlling the amount and location of precursors, adjusting the appropriate growing temperature and time, and establishing proper ventilation. Experimental results show the successful growth of a large-area single-layer MoS2 on a sapphire substrate through direct sulfurization under a suitable environment. The thickness of the MoS2 film determined by AFM measurement is about 0.73 nm. The peak difference between the Raman measurement shift of 386 and 405 cm−1 is 19.1 cm−1, and the peak of PL measurement is about 677 nm, which is converted into energy of 1.83 eV, which is the size of the direct energy gap of the MoS2 thin film. The results verify the distribution of the number of grown layers. Based on the observation of the optical microscope (OM) images, MoS2 continuously grows from a single layer of discretely distributed triangular single-crystal grains into a single-layer large-area MoS2 film. This work provides a reference for growing MoS2 in a large area. We expect to apply this structure to various heterojunctions, sensors, solar cells, and thin-film transistors.

Two-dimensional layered material MoS 2 with atomically thick layers is one of the most common transition metal dichalcogenides (TMDs) [1][2][3][4] ; it has an indirect energy gap of 1.2 eV in the bulk MoS 2 semiconductor and a direct energy gap of 1.8 eV in the monolayer MoS 2 [5][6][7][8][9] . Single-layer TMDs has an excellent current switch ratio (on/off current ratio) in field effect transistors because of its direct energy gap 10,11 . These advantages can only be possessed by materials with atomic thickness 12 . MoS 2 is a layered structure, which has good lubricity, pressure resistance, and wear resistance. It is mostly used in solid lubricants as well as in high-speed, heavy-duty, high-temperature, and chemical corrosion conditions [13][14][15][16][17] . This material has many potential applications, such as in field effect transistors, electronic devices, light-emitting diodes, sensors, and so on, due to its excellent optoelectronic properties 11,[18][19][20][21][22][23][24][25] . In recent years, it has been found that MoS 2 has semiconductor properties and can exist in the form of a single layer or a few layers 26 . Therefore, two-dimensional materials are widely discussed and studied by scientists. Many methods, including mechanical exfoliation 12,[27][28][29][30] , thermally decomposed ammonium thiomolybdate [31][32][33][34] , sulfurization of Mo/MoO 3 35 , and chemical vapor deposition (CVD) [36][37][38][39][40][41][42] , can be used to synthesize continuous MoS 2 films. These methods are capable of producing many good quality MoS 2 layers, however, achieving large area MoS 2 thin films is challenging. The reason is that MoS 2 tends to transform into nanoparticle and nanotube structures, leading to inefficient production in homogeneous synthesis and largearea MoS 2 thin films, making it challenging to deploy production for electronic devices. Therefore, synthesizing large-area MoS 2 thin films has attracted much research attention.
The growing method of this study is the direct sulfidation using molydenum/molybdenum oxide which also proposed by Lin et al. 35  www.nature.com/scientificreports/ on a sapphire substrate and grown through two stages of heating. In the first stage, the heating time is one hour. The sample is placed into a furnace tube at 500 °C and passed through Ar/H 2 mixed gas (Ar:H 2 = 4:1) under controlled pressure of 1 Torr to convert MoO 3 into MoS 2 . The reaction equation is as follows: In the second stage, the heating time is 30 min. Sulfur powder is placed into a tube first, and Ar/H 2 mixed gas is introduced at 1000 °C. The pressure is controlled at 600 Torr. The MoO 2 in the first stage reacts with sulfur vapor inside the tube to form MoS 2 . The reaction equation is as follows: The advantage of the direct sulfurization of molybdenum oxide is that it can grow a large area of MoS 2 film and is similar to two-stage thermal decomposition.
In 2014, Choudhary et al. 43 ref proposed a large-scale synthesis of MoS 2 on a Si/SiO 2 substrate using a twostep sputtering CVD method. The first step involved the deposition of Mo thin films on the substrate through the sputtering of Mo metal. In the second step, low-pressure CVD was employed to synthesize MoS 2 . Argon gas was used to transport sulfur into the reactor, where it reacted with the Mo film. Upon annealing at a high temperature of 600 °C and subsequent cooling to room temperature, the Mo films were successfully converted to MoS 2 films. The results showed that this method effectively enabled the synthesis of large-area MoS 2 from Mo thin films, and the thickness of the synthesized MoS 2 films could be modulated by varying the sputtering times. The aforementioned studies indicate that large-area MoS 2 thin films can be synthesized using a CVD process. This involves exposing a thin film containing Mo to sulfur powder in an Argon gas environment and subjecting the sample to high temperature conditions. The results demonstrate that this approach can yield MoS 2 thin films with a substantial surface area.
In this study, we present the synthesis of MoS 2 thin films through a CVD process. Our findings indicate that a Mo thin film can be converted into a MoS 2 thin film using this method. Raman mapping, AFM, and highresolution transmission electron microscopy (HRTEM) images confirmed the presence of a monolayer on the 2-inch sapphire wafer. Compared with MoS 2 films grown on Si substrates, electrical measurements of sapphire substrates growth revealed p-type semiconducting behavior with remarkably high electron mobility and on-off ratio. This approach is compatible with conventional semiconductor processes and can be extended to other TMDs and arbitrary substrates by transferring MoS 2 .

Materials and Methods
Experimental materials and drug specifications of MoS 2 . A double-side polished sapphire substrate with a thickness of 300 nm was utilized for the growth of MoS 2 . The substrate was pre-coated with a 5 Å MoO 3 film through sputter deposition. Subsequently, direct sulfidation was performed using sulfur powder (S) of 99.98% purity. The purpose is to avoid impurity residues that affect chemical vapor deposition or singlecrystal residue impurities. The quartz tube and ceramic crucible in the high-temperature furnace tube (Tubular thermal) are cleaned with aqua regia. The concentrations of nitric acid and hydrochloric acid in aqua regia are 37% and 68-69%, respectively, to avoid impurities from the previous growth remaining in the cleaning process.
Process and steps of growing MoS 2 by direct sulfurization. The process of this study is shown in Fig. 1, a 2-inch sapphire substrate with 300 nm thickness pre-deposited 5 Å MoO 3 was first prepared, cut into four equal parts, and cleaned with ultrasonic vibration in acetone for 10 min to remove surface impurities and oil stains and achieve a clean surface. Deionized water was used to remove organic solvent-acetone in ultrasonic vibration for 10 min. The sample was collected, and a high-pressure nitrogen gun was used to remove the moisture on the surface of the substrate. Direct sulfurization was used to grow MoS 2 . The substrate was placed under specific pressure and temperature conditions. In the furnace tube system, argon was used as the carrier gas to perform chemical reactions on the surface of the substrate by hot steam vulcanization to generate high-quality, large-area films. The application of direct sulfurization method in the preparation of single-layer TMDs started with the growth of MoS 2 . Initially, 500 sccm of argon gas was introduced to clean the interior of the system. The cavity was kept in a clean environment, and excess water vapor and oxygen in the quartz tube were removed for 10 min. The sample was grown with an argon flow rate of 400 sccm at a heating rate of 30 °C per minute. The growth temperature was 750 °C, which was maintained for 5 min. After the growth was completed, the gas volume was slowly decreased to − 40 sccm per minute to reduce the damage of the newly formed MoS 2 by airflow. The cover was opened once the temperature decreased to 400 °C. After opening the cover, the temperature was reduced to room temperature, and MoS 2 structure was obtained at high temperatures. A 1/4 two-inch sapphire substrate with 1.2 g of sulfur powder (S) as a precursor was placed on a crucible in a furnace tube. The vaporization point of Mo is above 650 °C and the vaporization point of S is above 200 °C; as such, Mo in the gaseous phase under a high-temperature environment will undergo a chemical reaction to produce molybdenum oxide (MoO 3−x ) intermediate. The produced molybdenum oxide intermediate will diffuse to the substrate and react with vaporized sulfur to form MoS 2 film. The distance between the substrate and sulfur layer is 24 cm. The single-layer large-area MoS 2 can be effectively prepared by direct sulfurization. This method not only grows high-quality single-crystal materials but also prepares a large-area uniformly distributed film, which is beneficial to subsequent optoelectronic component manufacturing.   Fig. 2a, where the growth of single-layer MoS 2 is not as good as that at position b, but single-layer MoS 2 covers a larger area. Figure 2d shows the OM image at position d in Fig. 2a, in which the monolayer MoS 2 grows irregularly on the sapphire substrate and has a light blue irregular shape distribution, indicating the existence of monolayer MoS 2 . Figure 2e is the OM image at position e in Fig. 2a, where the gray area is the sapphire substrate, and the scattered blue area is the area where MoS 2 grows.

SEM analysis.
We analyzed the growth of single-layer MoS 2 thin film. Figure 3 shows the SEM image of junction between large area MoS 2 and scattered MoS 2 grown on the sapphire substrate. The size of MoS 2 in the scattered area is about 20-30 μm. In the OM image, we can see uniform triangular or star-shaped two-dimensional structured MoS 2 (Fig. 4a). Figure 4b shows a MoS 2 SEM image that is single-layer and complete triangular. Taking a triangle as an example, Fig. 4c shows the SEM image of a large-area MoS 2 thin film.

Raman spectroscopy analysis.
We performed Raman measurement on the sapphire substrate sample grown with MoS 2 . A laser with a wavelength of 532 nm was used as the excitation light source. The X-axis in the spectrum represents the Raman shift, and the Y-axis represents the Raman intensity. Figure 5a shows the Raman measurement map of the scattered MoS 2 in the region e of the OM image of MoS 2 in Fig. 2a. We take the black, red, and blue points inside to measure the large-area growth as shown in Fig. 5b. Figure 5c shows the Raman measurement map of large-area MoS 2 corresponding to the three black, red, and blue points in Fig. 5b. The Raman shift is between 386.2 and 405.3 cm −1 , and the value of peak difference (Δk) is about 19.1 cm −1 . The results show that the large-area growth regions are all monolayers. Figure 5d shows the selected OM image of the Raman mapping range. Figure 5e presents the out-of-plane (A 1g ) Raman mapping image of Fig. 5d. The sapphire substrate is identified as the green portion. The distribution of single-layer MoS 2 exhibits a relatively flat profile ranging from light green to yellow, and progressing towards orange-red on the graph. The mapping size of 50 μm × 50 μm shows a limited sampling area, yet the distribution remains consistently flat. This is further verified through Raman measurement, which demonstrates a uniform distribution of single-layer MoS 2 . These findings highlight the potential of MoS 2 as a suitable material for the production of semiconductor devices.  Fig. 6a,b, respectively. In Fig. 6a, the surface of the sapphire substrate and MoS 2 can be clearly seen. Figure 6b shows that the thickness of the grown MoS 2 on the sapphire substrate is 0.72 nm from the height profile of the cross-sectional line in Fig. 6a. This result indicates the successful synthesis of a single-layer MoS 2 structure. In Fig. 6b, there is no substantial difference in profile height between the MoS 2 layers or between the MoS 2 layer and the sapphire surface. The average roughness of MoS 2 films, as measured by AFM, ranges from 0.55 to 2.35 nm. It is important to note that AFM images depict the average distances of atoms on the sample surface, rather than the individual atoms themselves. In Fig. 6c, the 30 μm × 30 μm surface topography map of MoS 2 displays a relatively uniform distribution of  www.nature.com/scientificreports/ molecules, as indicated by the color bar. However, a slight asymmetry in distribution is also observed, suggesting that the MoS 2 surface has a relatively smooth state. Figure 7 shows the PL emission spectrum of MoS 2 . In the measurement, the 532 nm band was used as the laser excitation light source. The spectrum consists of two peaks, corresponding to A1 and B1 excitons at 677 and 627 nm, respectively. Both excitons have a direct excitonic transition from the valence band spin-orbital coupling energy separation 44 . The direct excitonic transition of the two resonances is the K point in the Brillouin zone. In general, the peak of MoS 2 luminescence is about 67 nm, and the converted energy is 1.86 eV, which is the size of the direct energy gap of the MoS 2 film. Based on the above data, the MoS 2 grown has a single-layer structure.

Discussions
In 2017, Zhu et al. reported the presence of two types of seed centers in the growth mechanism of MoS 2 , namely: (1) molybdenum-oxysulfide (MoO x S 2-y ) nanoparticles or nanocrystals for growing multilayer MoS 2 and (2) two main edge-terminated irregular polygons for the growth of monolayer MoS 2 , the S-Mo Klein edge and the Mo zigzag edge, which appear roughly in equal quantities 45 . The morphology then evolves into a nearly triangular shape with predominant Mo meandering edges. Using a CVD system to grow MoS 2 , the impact of gas flow rate on MoS 2 growth can be studied. The Ar flow rate was varied between 200 and 500 sccm. Results show that a faster flow rate mainly produced single layers, while a slower flow rate resulted in more layers. When the deposition rate exceeds the growth rate of MoS 2 , S-dominant growth occurs, which is the mechanism for monolayer MoS 2 growth. Conversely, when the deposition rate is slower than the growth rate of MoS 2 , the dominant growth is molybdenum-oxysulfide, which is the mechanism for minority layer MoS 2 growth. In 2018, Zhou et al. further investigated the seed crystals responsible for the two growth mechanisms and the impact of temperature, in addition to gas flow rate, on the deposition rate 46 . The study's experimental results revealed that temperature with a central nucleation point played a significant role in the formation of the seed crystals.
In the same time, Taheri et al. shown that a three-stage mechanism was proposed for the growth of MoS 2 47 . The first stage involves the evaporation of MoO 3 , which may be facilitated by the presence of sulfur. This can either result in the partial reduction of MoO 3 to form MoS 2 or the creation of molybdenum oxysulfide, both of which are volatile in nature. The evaporated molybdenum oxide then diffuses to regions on the substrate with lower molybdenum concentrations, as a result of the chemical potential difference. The experimental results by Taheri et al. revealed that under the conditions used in their study, diffusion lengths can attain values in the tens of micrometers range. This process plays a role in mitigating local morphological changes at the nanoscale within the precursor MoO 3 films. In other words, the precursor film grows uniformly on the substrate as a local source of molybdenum, but does not dictate the ultimate morphology of the final MoS 2 . The second stage entails sulfidation to produce MoS 2 through processes such as substitution of sulfur, lattice reorganization, and material redistribution. This leads to the uniform formation of single-crystal triangles of MoS 2 across the substrate. The third stage involves the growth and merging of these triangles to form continuous MoS 2 . During this stage, MoS 2 nanoparticles evaporate and are redeposited along the edges of the discrete triangles. The high density of triangles and long diffusion length allow for easy merging of the triangles, resulting in the filling of any gaps and the formation of continuous triangles with minimal overgrowth and voids. Meanwhile, the nanoparticles undergo evaporation and are not consumed. Unlike the growth process of CVD, there are no specific nucleation sites that cause morphological changes in MoS 2 . Instead, the process of vapor diffusion and nucleation under sulfidation occurs uniformly and locally at the microscale across the entire substrate, resulting in continuous MoS 2 coverage. A unique aspect of the synthesis process is the cessation of carrier gas flow during the cooling step. Although MoS 2 has a melting temperature of 1185 °C, its vapor pressure is small but not negligible at temperatures above 700 °C. By stopping the flow of the carrier gas, the trapped MoS 2 vapor remains in equilibrium with the grown  www.nature.com/scientificreports/ the growth of continuous MoS 2 . According to the flow direction of Argon gas, the growth of MoS 2 progresses from a large coverage area to a smaller one, from fragmentation to large-area growth. By avoiding low-pressure growth, the process in a low-pressure environment minimizes unnecessary gas-phase reactions, which can lead to uneven distribution of gasified sulfur atoms. The resultant differences in the concentration of sulfur atoms can result in changes in the morphology of MoS 2 from fragmentation to a more uniform, large-area distribution. The Raman spectroscopy measurement showed that the grown MoS 2 exhibits the characteristics of a monolayer. Raman mapping is the best method for analyzing the number of layers [48][49][50] . The PL spectral analysis results indicated that the distribution of the grown MoS 2 monolayer is uniform, as demonstrated by the mapping diagrams obtained with the excitation light peak at 627 nm or 677 nm. The thickness of the sapphire substrate with periodic growth of MoS 2 was analyzed using AFM. The selected area was confirmed to be a single layer, consistent with the results obtained from the Raman and PL spectroscopy analysis.

Conclusions
The research can be divided into two parts. In the first part, we refer to the relevant literature and successfully grow large-area MoS 2 in some areas of the sapphire substrate by changing the parameters through direct sulfurization. The single-layer thin film successfully overcomes the limitations of growing a large-area single-layer MoS 2 thin film in chemical vapor deposition. In addition to adjusting the parameters, the method can achieve a continuous thin film of about 500 μm × 500 μm. The second part is the analysis of the results of Raman spectroscopy, photoluminescence spectroscopy, atomic force microscopy, and scanning electron microscopy. The large-area MoS 2 film is composed of scattered triangular MoS 2 . The MoS 2 film grown has monolayers. In summary, desirable modulated layers with a large area of MoS 2 can be achieved by growth techniques using sulfided pre-deposited transition metals through sequential transition metal deposition and direct sulfurization.