Method for preparing large-size monolayer molybdenum disulfide film

Abstract

The application discloses a single-layer molybdenum disulfide film large-size growth preparation method. The method comprises the following steps: placing a sapphire substrate on an inert high-temperature gasket in a heating device, placing a sulfur source and a molybdenum source in the heating device, vacuumizing, and then passing nitrogen, heating and treating the sulfur source and the molybdenum source to perform a chemical vapor deposition reaction, and obtaining a molybdenum disulfide film; wherein the inert high-temperature gasket is obtained by immersing dry and clean foamed silicon carbide in a container containing NaClO solution, and then performing immersion treatment under vacuum and drying. The inert gasket in the reaction system is treated by the method of vacuum immersion in NaClO solution, so that the controllability of the preparation process is increased, and the preparation of a large-size, high-quality single-layer molybdenum disulfide film is realized.

Description

Technical Field

[0001] This invention belongs to the field of two-dimensional material preparation technology, specifically relating to a method for large-size growth of monolayer molybdenum disulfide thin films. Background Technology

[0002] In recent years, two-dimensional layered materials have been hailed as promising materials for next-generation nanoelectronic devices due to their unique electronic properties and ultrathin thickness. Among them, molybdenum disulfide (MoD) is a two-dimensional semiconductor material with unique visible light absorption characteristics, and its emission bandgap is closely related to the number of layers. Therefore, MoD is widely used in multi-wavelength functionalized photodetectors. Furthermore, the excellent luminescence properties of direct bandgap monolayer MoD thin films also hold significant application value in photoluminescent and electroluminescent devices. However, at present, developing new growth and fabrication techniques to achieve the fabrication of large-size, uniform monolayer MoD thin films remains a challenge for applications requiring high integration.

[0003] In the prior art, patent CN115058700A discloses a method for preparing molybdenum disulfide thin films. This method uses an alumina pad placed above the molybdenum source to prevent the introduction of excess molybdenum source supply and other impurities. The prepared molybdenum disulfide thin film has a size of approximately 200 μm, but obvious grain boundaries and film gaps can be observed under an optical microscope, indicating that regional continuity and uniformity need to be improved. While this improved method uses an inert pad to address the common problem of excessive growth source supply at the microscopic level, it fails to achieve selective nucleation control of multilayer molecules, thus limiting the size of the molybdenum disulfide thin film and leading to the formation of multilayer molybdenum disulfide. In summary, a new technical method is needed to simultaneously address the growth source supply and the induction and control of chemical bond formation at the microscopic level in order to achieve the preparation of large-size, high-quality monolayer molybdenum disulfide thin films. Summary of the Invention

[0004] The purpose of this invention is to provide a method for large-size growth of monolayer molybdenum disulfide thin films. By improving existing preparation methods, the method uses vacuum impregnation with NaClO solution to treat the inert gasket in the reaction system, thereby increasing the controllability of the preparation process and solving the problems of limited film size and poor uniformity in the prior art.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] A method for large-scale growth of a single-layer molybdenum disulfide thin film includes the following steps: placing a sapphire substrate on an inert high-temperature resistant pad in a heating device; placing a sulfur source and a molybdenum source in the heating device, evacuating the device, and then purging with nitrogen gas; heating the sulfur source and the molybdenum source to perform a chemical vapor deposition reaction to obtain a molybdenum disulfide thin film; wherein, the inert high-temperature resistant pad is obtained by immersing dry and clean foamed silicon carbide in a container containing NaClO solution, performing an impregnation treatment under vacuum conditions, and then drying.

[0007] Preferably, the NaClO solution concentration is 0.08-0.12 mol / L and the vacuum degree is 85-90 kPa.

[0008] Preferably, the method specifically includes the following steps:

[0009] Step S1: Raw material preparation, the raw materials include sapphire substrate, molybdenum source, sulfur source, and inert high-temperature resistant gasket;

[0010] Step S2: Cleaning, the sapphire substrate is cleaned with acetone solution and then cleaned again with isopropanol solution;

[0011] Step S3: Pretreatment, the cleaned sapphire substrate is immersed in a mixed solution of concentrated sulfuric acid and hydrogen peroxide;

[0012] Step S4: Place the molybdenum source on a ceramic boat, place an inert high-temperature resistant gasket inside the ceramic boat, and place a sapphire substrate treated in S3 above the inert high-temperature resistant gasket; place the ceramic boat containing the sulfur source in the upper temperature zone of the dual-temperature zone tubular atmosphere furnace, and encapsulate the dual-temperature tube furnace.

[0013] Step S5: The air inside the dual-temperature zone tubular atmosphere furnace is evacuated by a mechanical pump; the molybdenum source, sulfur source and substrate are heated; nitrogen is introduced when the vacuum degree of the dual-temperature zone tubular atmosphere furnace reaches 3-5 Pa, and the pressure inside the tubular atmosphere furnace is increased to 0.1-1.0 MPa; molybdenum disulfide thin film is grown by chemical vapor deposition.

[0014] Step S6: After the molybdenum disulfide film is grown, heating is continued for heat preservation, and the temperature is gradually reduced until the dual-temperature zone tubular atmosphere furnace reaches room temperature. The sample in the ceramic boat is then removed to obtain a large-size monolayer molybdenum disulfide film.

[0015] Preferably, the molybdenum source in step S1 is molybdenum trioxide and the sulfur source is sulfur powder.

[0016] Preferably, a flat-surfaced c-phase sapphire substrate with a size of two inches is used.

[0017] Preferably, the cleaning method in step S2 is ultrasonic cleaning.

[0018] Preferably, in step S5, the process of introducing nitrogen gas and bringing the pressure inside the tubular atmosphere furnace to atmospheric pressure is repeated 3-5 times.

[0019] The beneficial effects of this invention are:

[0020] 1) This invention uses a sapphire wafer with a smooth C-surface polished as a substrate to promote the lateral growth of molybdenum disulfide to obtain a continuous thin film with no or few grain boundaries, which can ensure that the thin film has good monocrystalline properties.

[0021] 2) By using foamed silicon carbide and sapphire substrate as a common substrate for CVD deposition, the flow rate of gaseous reactants during the growth of molybdenum disulfide crystals can be limited, thereby controlling the growth of molybdenum disulfide.

[0022] 3) This invention employs a vacuum impregnation process to impregnate NaClO solution into foamed silicon carbide using a vacuum impregnation device, thereby preparing a high-temperature resistant inert gasket. NaClO is a strong base-weak acid salt, which ionizes in aqueous solution to generate hypochlorite ions. During heating, the hypochlorite ions undergo a disproportionation reaction, generating chlorate ions (ClO3). - The molybdenum (Mo) atoms adsorb onto the outermost S atoms of molybdenum disulfide (Mo), effectively inhibiting Mo nucleation on the Mo surface. Simultaneously, it promotes the growth of excess Mo atoms along the edges of the Mo surface, significantly increasing the growth size of monolayer Molybdenum disulfide. This overcomes the shortcomings of CVD deposition using only foamed silicon carbide as an inert spacer, which results in smaller Mo size and uncontrollable layer number. It promotes the lateral growth of Molybdenum disulfide crystals, thereby controlling the thickness of the Molybdenum disulfide film and enabling large-size fabrication.

[0023] 4) This invention involves heating the molybdenum source and sulfur source separately in a dual-temperature zone tubular furnace. The sulfur vapor that evaporates first enters the growth zone along with the argon carrier gas, ensuring the entire reaction is in a state of full sulfur over-sulfurization. Therefore, when the molybdenum source begins to evaporate, it can fully combine with the sulfur vapor, which is beneficial for the growth of molybdenum disulfide films and improves reaction efficiency.

[0024] 5) By introducing nitrogen into the tubular atmosphere furnace, the air in the dual-temperature zone tubular atmosphere furnace can be purged using an inert gas, so as to prevent air from affecting the growth and deposition of molybdenum disulfide thin films in subsequent high-temperature environments.

[0025] 6) This invention prepares molybdenum disulfide using chemical vapor deposition, thereby obtaining a batch of large-size monolayer molybdenum disulfide films, and controls the size and uniformity of the films to a certain extent. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, as well as the beneficial effects of the present invention, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other structures can be obtained based on the structures shown in these drawings without creative effort.

[0027] Figure 1 This is a schematic diagram of the apparatus for preparing large-area monolayer molybdenum disulfide thin films according to the present invention.

[0028] Figure 2 This is a schematic diagram of the vacuum impregnation process in this invention.

[0029] Figure 3 This is an optical image of the large-area monolayer molybdenum disulfide thin film of the present invention.

[0030] Figure 4 The image shows the Raman spectrum of the large-area monolayer molybdenum disulfide thin film of the present invention.

[0031] Figure 5 The PL spectrum of the large-area monolayer molybdenum disulfide thin film of the present invention is shown.

[0032] Figure 6 This is an optical image of molybdenum disulfide prepared in the preliminary experiments of this invention. Detailed Implementation

[0033] See Figure 1 In Embodiment 1 of this application, a method for large-scale growth of a monolayer molybdenum disulfide thin film is provided, which includes the following steps:

[0034] Step S1: Preparation of raw materials and experimental equipment. This includes preparing a C-phase sapphire substrate, molybdenum source, sulfur source, foamed silicon carbide, NaClO solution, nitrogen, acetone solution, isopropanol solution, concentrated sulfuric acid, hydrogen peroxide (30%, AR), vacuum impregnation apparatus, dual-temperature zone tubular atmosphere furnace, ceramic boat, and mechanical pump. See the schematic diagram of the entire reaction apparatus. Figure 1 ;

[0035] Step S2: Cleaning. The substrate is cleaned in the acetone solution for approximately 10 minutes; then cleaned in the isopropanol solution for approximately 15 minutes. A mixed solution of concentrated sulfuric acid and hydrogen peroxide is prepared, and the cleaned substrate is immersed in this mixed solution for 1 hour. Existing substrates often contain oxide layers and other impurities; therefore, they need to be cleaned to remove these impurities before subsequent processing. Treating the substrate with a mixed solution of concentrated sulfuric acid and hydrogen peroxide can remove residual organic matter from the surface, giving the substrate better surface hydrophilicity and improving the coverage of the precursor on the substrate surface.

[0036] Step S3: Pretreatment. The cleaned substrate is subjected to high-temperature annealing (1000°C) in air for 2 hours, and then cooled to room temperature (25°C) for later use. An inert high-temperature resistant gasket is prepared. Since chlorate ions can adsorb onto the S atoms of the outermost layer of molybdenum disulfide during the growth of the monolayer, they can effectively inhibit the nucleation and growth of Mo atoms on the already grown monolayer molybdenum disulfide surface. This results in excess Mo atoms on the substrate surface growing epitaxially along the edge of the monolayer molybdenum disulfide, ultimately growing a large-size monolayer molybdenum disulfide film. Furthermore, the presence of the prepared inert gasket prevents a large amount of molybdenum oxide powder from rapidly depositing on the substrate, reducing the growth rate of the molybdenum disulfide film and increasing its uniformity.

[0037] Step S4: Preparation: Place the molybdenum source on a ceramic boat and place the inert high-temperature resistant gasket inside the ceramic boat, and place the cleaned substrate on top of the inert high-temperature resistant gasket; place the ceramic boat containing the sulfur source in the upper temperature zone of the dual-temperature zone tubular atmosphere furnace, and enclose the dual-temperature tube furnace.

[0038] Step S5: Operation: The air inside the dual-temperature zone tubular atmosphere furnace is evacuated using the mechanical pump; the molybdenum source, sulfur source, and substrate are heated separately; nitrogen gas is introduced when the vacuum degree of the dual-temperature zone tubular atmosphere furnace reaches 3-5 Pa, and the pressure inside the tubular atmosphere furnace is increased to 0.1-1.0 MPa; a molybdenum disulfide thin film is grown using chemical vapor deposition; evacuating the dual-temperature zone tubular atmosphere furnace reduces the presence of other substances inside, lowers experimental errors, and meets the requirements of the film formation environment; heating the substrate, molybdenum source, and sulfur source separately reduces heating time, improves work efficiency, and avoids problems caused by material... Sudden temperature increases can damage the internal composition of molybdenum disulfide. Introducing nitrogen into the tubular atmosphere furnace allows for the removal of air from the dual-temperature zone furnace using an inert gas, preventing air from affecting the growth and deposition of molybdenum disulfide in subsequent high-temperature environments. The chemical vapor deposition process involves raising the heating temperatures of the sulfur source and molybdenum source to 180℃ and 700-800℃ respectively within 30 minutes, controlling the reaction time within 10 minutes, and introducing nitrogen at a flow rate of 30-50 sccm. Using chemical vapor deposition to prepare molybdenum disulfide enables the mass production of large-size monolayer molybdenum disulfide films, while allowing for precise control over the film's size and uniformity.

[0039] Step S6: Heating and Cooling. After the molybdenum disulfide film growth is complete, heating continues for heating and cooling, while the temperature is gradually reduced until the dual-temperature zone tubular atmosphere furnace reaches room temperature. The sample is then removed to obtain a large-size monolayer molybdenum disulfide film. This heating process gradually stabilizes the composition of the film, preventing damage from sudden temperature drops and ensuring the quality of the film formation.

[0040] Step S7: Inspection. The quality of the molybdenum disulfide film is tested by using a testing instrument to detect the nucleation and growth inside the molybdenum disulfide film and record the data.

[0041] In step S1 of this embodiment, the molybdenum source is molybdenum trioxide, the sulfur source is sulfur powder, and the sapphire substrate is a C-plane polished sapphire substrate with a flat surface and a crystal plane index of (0001). The surface bevel angle of the substrate is 0.2 degrees, and the size is two inches. The C-phase sapphire substrate has atomically flat steps, and its orientation consistency can ensure good single crystallinity of the film, promoting the lateral growth of molybdenum disulfide to obtain a continuous film with no or few grain boundaries.

[0042] In this embodiment, the cleaning method in step S2 is ultrasonic cleaning. Ultrasonic cleaning uses powerful ultrasonic waves to generate shock waves in a liquid to impact the dirt in the liquid, thereby removing impurities from the surface and interior of the substrate to ensure the quality of the substrate and thus ensure the quality of the film formation in the later stage.

[0043] In this embodiment, the immersion treatment in step S3 involves immersing the substrate treated in step S2 in a mixed solution of concentrated sulfuric acid and hydrogen peroxide with a volume ratio of 3:1 for 1 hour. This method not only further removes adsorbed impurities and organic matter from the substrate surface, but also gives the substrate better surface hydrophilicity, which helps to improve the size of the molybdenum disulfide thin film prepared subsequently.

[0044] In this embodiment, the sulfur source and the molybdenum source are molybdenum trioxide powder and sulfur powder, respectively, and the mass ratio of the sulfur source to the molybdenum source is 24:5.

[0045] The inert gasket prepared in step S3 of this embodiment is obtained by immersing dry and clean foamed silicon carbide in a container 1 containing 100 ml of 0.08-0.12 mol / L NaClO solution, placing it in a vacuum impregnation apparatus, and performing vacuum impregnation treatment under a vacuum degree of 85-90 kPa, followed by drying. Figure 2 As shown, the vacuum impregnation apparatus includes a tank 2, which is connected to a vacuum pump 3 via piping. A dryer 4 is installed on the connecting pipe to evacuate the tank. A pressure gauge 5 is installed on the top of the tank, and a valve 6 is also provided on the tank to adjust the vacuum level. The process is carried out in two stages: Vacuum treatment stage: The tank valve is closed, and the vacuum pump is started to gradually expel the gas in the vacuum impregnation system (including the sample). Repressurization stage: The vacuum pump is turned off, and the tank valve is opened to restore the system pressure to atmospheric pressure. The pressure difference between the inside and outside of the system is used to force the NaClO solution to quickly enter the pores of the foamed silicon carbide.

[0046] In step S3 of this embodiment, the optimal concentration range of the NaClO solution used is 0.08-0.12 mol / L. When the NaClO solution concentration is below 0.08 mol / L, the number of chlorate ions in the reaction process is small and cannot be completely adsorbed onto the S atoms, resulting in the simultaneous appearance of multilayer and monolayer molybdenum disulfide. When the NaClO solution concentration reaches 0.2 mol / L, some monolayer molybdenum disulfide is etched and destroyed at high temperature, ultimately yielding very small and irregularly shaped monolayer molybdenum disulfide.

[0047] To remove oxygen and water vapor, step S5, which involves introducing nitrogen and bringing the pressure inside the tubular atmosphere furnace to normal, is repeated 3-5 times. Through multiple cycles, the oxygen and water vapor inside the dual-temperature zone tubular atmosphere furnace are reduced, thereby improving the film formation quality.

[0048] The two-inch single-layer molybdenum disulfide film obtained based on the above embodiments is as follows: Figure 3 As shown, the molybdenum disulfide film obtained under an optical microscope exhibits good uniformity and high quality, further demonstrating the effective benefits of this technical solution.

[0049] Figure 4 This is the Raman spectrum of the sample prepared in this embodiment, which contains two peaks E. 2g(S-Mo)

[0050] 383cm -1 A1 g(S-Mo) 403cm -1 These correspond to the transverse and longitudinal vibration modes of molybdenum disulfide, respectively, and the test results are consistent with previous studies on molybdenum disulfide. 1 2g Peak and A 1g The wavenumber difference Δω between peaks can be used to identify the thickness of two-dimensional molybdenum disulfide films. The wavenumber difference Δω between sample peaks is 20 cm⁻¹. -1 This corresponds to a single-layer molybdenum disulfide film.

[0051] Figure 5 The A exciton emission peak of the sample prepared in this embodiment is at 690 nm, with a small fluctuation at 640 nm to the left, corresponding to the B exciton peak of the sample prepared in this invention. PL testing is a non-destructive characterization technique that can quickly and conveniently characterize defects, impurities, and luminescence properties in semiconductor materials. Based on the emission peak position, the approximate band gap of our prepared molybdenum disulfide film can be calculated to be approximately 1.80 eV, corresponding to the band gap of a single layer of molybdenum disulfide.

[0052] In the preliminary experiments of this invention, the growth of molybdenum disulfide on a sapphire substrate using unimpregnated NaClO foamed silicon carbide as an inert spacer was also studied. The process parameters were as follows: the mass ratio of sulfur source to molybdenum source was 24:5; the temperature of the sulfur source region was 180℃; the temperature of the molybdenum source region was 700-800℃; the chemical vapor deposition reaction time was 10 minutes; only foamed silicon carbide was used as the inert spacer; the sapphire substrate was a smooth, C-face-polished sapphire substrate with a crystal plane index of (0001); the surface bevel angle of the substrate was 0.2 degrees; and the size was two inches. All other process flows remained unchanged. The research results are as follows: Figure 6 As shown, molybdenum disulfide single crystals with a size of approximately 64 nm were observed under a 200x microscope. It can be seen that the molybdenum disulfide grown by CVD using only foamed silicon carbide as an inert spacer is relatively small, with thick nucleation sites appearing on some triangular single crystals, affecting the uniformity of the single crystals and failing to show a clear film-forming trend, making it difficult to prepare large-size molybdenum disulfide films. Therefore, impregnating foamed silicon carbide with NaClO solution under vacuum as an inert spacer is also a necessary condition for successfully preparing uniform, large-size monolayer molybdenum disulfide films.

[0053] The above are merely preferred embodiments of the present invention and do not limit the scope of the patent. Any equivalent structural or procedural transformations made based on the description and drawings of the present invention, or direct or indirect applications in other related technical fields, are similarly included within the scope of patent protection of the present invention.

Claims

1. A method for large-size growth of a single-layer molybdenum disulfide thin film, characterized in that, Includes the following steps: A sapphire substrate is placed on an inert high-temperature resistant pad in a heating device. A sulfur source and a molybdenum source are placed in the heating device, and after evacuation, nitrogen gas is introduced. The sulfur source and molybdenum source are heated to perform a chemical vapor deposition reaction to obtain a molybdenum disulfide film. The inert high-temperature resistant pad is obtained by immersing dry and clean foamed silicon carbide in a container containing NaClO solution, impregnating it under vacuum, and then drying it. The concentration of NaClO solution is 0.08-0.12 mol / L, and the vacuum degree of vacuum impregnation is 85-90 kPa. The preparation method includes the following specific steps: S1: Raw material preparation, the raw materials include sapphire substrate, molybdenum source, sulfur source, and inert high-temperature resistant gasket; S2: The sapphire substrate is cleaned with acetone solution and then cleaned again with isopropanol solution; S3: The cleaned sapphire substrate is immersed in a mixed solution of concentrated sulfuric acid and hydrogen peroxide; S4: Place the substrate that has been soaked in S3 above the inert high-temperature resistant pad, place the molybdenum source below the inert high-temperature resistant pad in the lower temperature zone of the dual-temperature zone tubular atmosphere furnace, and place the sulfur source in the upper temperature zone of the dual-temperature zone tubular atmosphere furnace; the ports at both ends of the dual-temperature tubular furnace need to be sealed. S5: Evacuate the dual-temperature zone tubular furnace; heat the molybdenum source, sulfur source, and substrate; when the vacuum degree of the dual-temperature zone tubular atmosphere furnace reaches 3-5 Pa, introduce nitrogen gas and reduce the pressure inside the tubular furnace to the range of 0.1-1.0 MPa; then grow a molybdenum disulfide thin film using chemical vapor deposition. S6: After the molybdenum disulfide film is grown, heating is continued for heat preservation, and the temperature is gradually reduced until the dual-temperature zone tubular atmosphere furnace is cooled to room temperature. The sample in the ceramic boat is then removed to obtain a large-size monolayer molybdenum disulfide film.

2. The method for large-size growth of monolayer molybdenum disulfide thin films according to claim 1, characterized in that, The molybdenum source is molybdenum trioxide, and the sulfur source is sulfur powder.

3. The method for large-size growth of a single-layer molybdenum disulfide thin film according to claim 1, characterized in that, The sapphire substrate is made of sapphire wafers with a smooth C-surface polishing.

4. The method for large-size growth of a single-layer molybdenum disulfide thin film according to claim 1, characterized in that, The cleaning method in S2 is ultrasonic cleaning.

5. The method for large-size growth of a single-layer molybdenum disulfide thin film according to claim 1, characterized in that, The volume ratio of concentrated sulfuric acid to hydrogen peroxide in the S3 mixed solution is 3:1.

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