A method for preparing bilayer MoS2 nanosheets based on a concentric tube furnace with a dual-tube structure

Abstract

The application relates to a double-layer MoS2 nanosheet preparation method based on a double-tube similar concentric architecture type tube furnace. A quartz glass inner tube is erected by using a quartz glass plate in a high-temperature zone of a double-temperature-zone tube furnace, so that the quartz glass inner tube forms a concentric architecture with a quartz glass outer tube. A chalcogen precursor is placed in a low-temperature zone of the quartz glass outer tube, a mixture of a metal molybdenum precursor and a halogen salt is placed at a tube opening of the quartz glass inner tube in the high-temperature zone, and a substrate is placed downstream; in the reaction cavity of the double-tube similar concentric architecture, after the low-temperature zone reaction precursor is sublimated, carrier gas is continuously blown to the high-temperature zone at an argon flow rate of 100 SCCM, participates in the reaction process of the high-temperature zone precursor after sublimation, is deposited on the substrate in the form of nucleation points, and gradually forms large-area and high-density double-layer MoS2 nanosheets by means of continuous reaction at high temperature; and the problems of low layer number control precision, uneven nucleation distribution, easy early consumption of the precursor and low product yield of the existing CVD technology for preparing double-layer MoS2 nanosheets are solved.

Description

Technical Field

[0001] This invention belongs to the field of two-dimensional semiconductor material preparation, and relates to a method for preparing bilayer MoS2 nanosheets based on a concentric tube furnace with a dual-tube structure, and more particularly to a method for preparing bilayer MoS2 nanosheets using a chemical vapor deposition method with a concentric tube structure. Background Technology

[0002] With the rapid development of microelectronic devices toward miniaturization and high integration, traditional bulk semiconductor materials have gradually revealed inherent defects such as insufficient carrier mobility and limited size reduction, making it difficult to meet the performance requirements of next-generation semiconductor devices. Developing new semiconductor materials and preparation technologies has become an urgent task in the fields of materials science and microelectronics.

[0003] The emergence of two-dimensional materials has provided a new path to solving this dilemma. Their unique layered structure endows them with excellent physicochemical and electrical properties, making them a research frontier in the semiconductor field. Among them, transition metal dichalcogenides (TMDCs), as typical two-dimensional semiconductor materials, combine tunable band gaps, high carrier mobility, and good stability, making up for the shortcomings of traditional semiconductor materials. Molybdenum disulfide (MoS2), as the core representative material of TMDCs, has become a key functional structure for next-generation two-dimensional devices due to its interlayer coupling effect, controllable interlayer twist angle, and superior optoelectronic properties. Its high-quality fabrication directly determines the upper limit of device performance, showing irreplaceable application advantages in many fields such as optoelectronic devices and nanoelectronics, and has broad development prospects.

[0004] Chemical vapor deposition (CVD) is currently the mainstream method for preparing two-dimensional TMDCs thin films, offering significant advantages such as large growth area, good film crystallinity, and compatibility with existing semiconductor processes. However, current CVD preparation processes still have significant shortcomings, especially in the preparation of bilayer MoS2 nanosheets. Common technical bottlenecks include low precision in layer number control, uneven nucleation distribution, poor film thickness consistency, and difficulty in controlling the bilayer twist angle. Furthermore, the preparation efficiency is relatively low. These problems severely limit the large-scale application of MoS2 and TMDCs materials. Therefore, developing efficient and stable CVD preparation equipment and supporting technologies suitable for bilayer MoS2 growth is of paramount importance for promoting the industrialization of two-dimensional TMDCs thin films.

[0005] Existing patents related to the preparation of two-dimensional TMDCs materials mostly focus on two-step growth of few-layer MoS2, molybdenum source modification to improve the uniformity of monolayer films, and improvements to high-vacuum sealed environments. These technologies primarily improve the growth quality of monolayer or few-layer MoS2 films by optimizing precursor ratios, temperature programs, substrate treatment methods, or sealed reaction conditions. However, they do not specifically address dedicated processes and architecture designs for bilayer MoS2 nanosheets or other two-dimensional TMDCs nanosheets, nor do they achieve precise control over the number of layers and high-yield preparation of bilayer MoS2 and similar two-dimensional nanosheets. This invention innovatively proposes a concentric dual-tube architecture design. Utilizing a tube-within-a-tube spatial confinement structure, it achieves partitioned placement of the precursor, effectively delaying premature participation of the precursor in the reaction and extending the residence time of the gas-phase precursor. This adapts to the growth requirements of bilayer MoS2 nanosheets, enabling stable, high-precision, and high-yield controllable preparation of bilayer MoS2 nanosheets. Compared to existing publicly available preparation technologies, this invention possesses significant novelty and practical value. Summary of the Invention

[0006] In view of this, in order to solve the problems of low layer number control precision, uneven nucleation distribution, and premature consumption of precursors in the preparation of bilayer MoS2 nanosheets by existing CVD technology, resulting in low yield, this invention provides a method for preparing bilayer MoS2 nanosheets based on a concentric tube furnace with a dual-tube-like structure. By using a concentric tube-like structure and spatial confinement strategy, the method achieves controllable preparation of large-area, high-density bilayer MoS2 nanosheets. The method is simple to operate and has high product reproducibility.

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] A method for preparing bilayer MoS2 nanosheets based on a concentric tube furnace with a dual-tube structure includes the following steps:

[0009] S1. Place the quartz boat carrying the chalcogenide precursor in the low-temperature zone of the outer quartz glass tube, corresponding to the low-temperature heating coupler of the dual-temperature zone tube furnace. Place the mixture carrying the molybdenum precursor and the halogen salt, along with the substrate used to place the mixture, in the inner quartz glass tube. The inner quartz glass tube and the outer quartz glass tube are concentric and placed in the high-temperature zone of the outer quartz glass tube, corresponding to the high-temperature heating coupler of the dual-temperature zone tube furnace. Close the entire reaction chamber to form a closed space.

[0010] S2. Use a vacuum pump to evacuate the pressure inside the reaction chamber to 1~10 Pa to create a vacuum reaction condition. Set the temperature control of the low temperature zone and high temperature zone of the dual-temperature zone tube furnace to 240~280 ℃ and 780~820 ℃ respectively from room temperature. During the entire reaction, the argon flow rate as the carrier gas is always maintained at 80~120 SCCM. After the dual-temperature zone tube furnace finishes working, the sample can be taken out after natural cooling. During the cooling process, the argon flow rate is kept constant.

[0011] The basic reaction principle of this scheme is as follows: Within a concentrically structured dual-tube reaction chamber, a chalcogenide precursor is sublimated at low temperature in the low-temperature zone. Argon gas, acting as a carrier gas, is continuously blown to the high-temperature zone at a flow rate of 100 SCCM. Argon participates in the reaction process following the sublimation of the molybdenum precursor in the high-temperature zone, depositing as nucleation sites on the substrate. Through continuous reaction at high temperature, large-area, high-density bilayer MoS2 nanosheets are gradually formed on the corresponding substrate. The entire preparation method is carried out using chemical vapor deposition within a concentrically structured dual-tube device, evaporating the precursor and controlling the reaction environment to achieve the gradual growth of MoS2 nanosheet nucleation sites on the substrate.

[0012] Furthermore, in step S1, the reaction chamber has a double-tube structure, including an outer quartz glass tube with a high-temperature zone and a low-temperature zone, and an inner quartz glass tube mounted in the high-temperature zone. The outer and inner quartz glass tubes are concentric. A quartz boat is provided in the low-temperature zone inside the outer quartz glass tube to support the chalcogenide precursor. A mixture of molybdenum precursor and halogen salt, as well as multiple substrates, are provided in the inner quartz glass tube to support the nucleation sites of MoS2 nanosheets.

[0013] Furthermore, in step S1, a rectangular quartz glass plate frame is provided at the bottom of the quartz glass inner tube. The quartz glass inner tube and the rectangular quartz glass plate are welded together as one piece, with one end aligned. A circular hole with a diameter of 10 mm is provided at the center of the other end of the rectangular quartz glass plate, 10 mm from the top, to facilitate the hooking of the quartz glass inner tube.

[0014] Furthermore, in step S1, the outer tube of the quartz glass in the dual-temperature zone tube furnace has a diameter of 50 mm, a wall thickness of 2 mm, and a length of 1400 mm. The inner tube of the quartz glass has a diameter of 25 mm, a wall thickness of 2 mm, and a length of 150 mm. The rectangular quartz glass plate that serves as the bottom support for the inner tube of the quartz glass has a width of 35 mm and a length of 180 mm.

[0015] Furthermore, the substrate preparation method in step S1 is as follows: the wafer-level silicon wafer is cut into small pieces of 1 cm*1 cm, and ultrasonically cleaned with acetone, isopropanol and deionized water for 10 min respectively, and then taken out and dried with nitrogen gas.

[0016] Furthermore, in step S1, the chalcogenide precursor is solid sulfur powder, the mixture of molybdenum precursor and halogen salt is a mixture of solid MoO3 powder and NaCl solution, and the substrate is a 300 nm SiO2 / Si wafer.

[0017] Furthermore, in step S2, the dual-temperature zone tubular furnace is connected to a gas filling and evacuation device via an external gas passage. The gas filling and evacuation device includes a gas cylinder for storing carrier gas, a flow meter for controlling the flow rate of carrier gas, and a vacuum pump for evacuating the reaction chamber. The gas passage transmits carrier gas to the reaction chamber and simultaneously collects working waste gas.

[0018] Furthermore, the temperature control process in the low-temperature zone and the high-temperature zone in step S2 is as follows: In the low-temperature zone, the temperature is raised from room temperature to 150 ℃ in 10 min; then raised to 260 ℃ in 16 min; held at 260 ℃ for 14 min; and then lowered to room temperature. In the high-temperature zone, the temperature is raised simultaneously with the low-temperature zone, from room temperature to 800 ℃ in 26 min at a rate of 30 ℃ / min; then held at 800 ℃ for 14 min; and then lowered to room temperature.

[0019] The bilayer MoS2 nanosheets were prepared using the above-described method.

[0020] The beneficial effects of this invention are as follows:

[0021] 1. The present invention discloses a method for preparing bilayer MoS2 nanosheets based on a concentric tube furnace with a dual-tube structure. In the high-temperature zone of the dual-temperature tube furnace, a quartz glass inner tube is mounted using a quartz glass plate, forming a concentric structure with the quartz glass outer tube. A chalcogenide precursor is placed in the low-temperature zone of the outer quartz glass tube, while a mixture of a molybdenum precursor and a halogen salt is placed at the opening of the inner quartz glass tube in the high-temperature zone, with a substrate placed downstream. This method utilizes chemical vapor deposition (CVD). After the chalcogenide precursor in the low-temperature zone is volatilized, a sulfur atmosphere is blown to the inner quartz glass tube in the high-temperature zone using a carrier gas, where it reacts with the volatilized molybdenum precursor to form a low-valence molybdenum oxide. The intermediate product is further sulfided into MoS2 under the sulfur atmosphere and deposited on the substrate to form nucleation sites, achieving bilayer growth and lateral expansion of MoS2 nanosheets under high-temperature conditions. The concentric dual-tube architecture, with its tubular design, regionally distributes the precursors, effectively delaying their participation in the reaction and preventing premature consumption and nucleation. The inner quartz glass tube within the concentric design effectively creates localized precursor aggregation, and the resulting confinement effect extends the precursor residence time, promoting the growth of bilayer MoS2 nanosheets. This method is compatible with chemical vapor deposition, is simple to design, and produces highly reproducible products, enabling the fabrication of large-area, high-density bilayer MoS2 nanosheets.

[0022] 2. The bilayer MoS2 nanosheet preparation method based on a concentric tube furnace with a dual-tube structure disclosed in this invention enables the precursor to be placed in separate regions, effectively delaying the precursor's participation in the reaction and avoiding premature consumption and disordered nucleation. The quartz glass inner tube forms a local confinement effect, promoting precursor aggregation and prolonging the residence time, significantly improving the growth efficiency and uniformity of the bilayer MoS2 nanosheets. This method is compatible with conventional CVD equipment, has a simple structural design, controllable parameters, and high repeatability, and can stably prepare large-area, high-density bilayer MoS2 nanosheets, meeting the needs of large-scale fabrication of two-dimensional semiconductor devices.

[0023] Other advantages, objectives, and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination, or may be learned from practice of the invention. The objectives and other advantages of the invention can be realized and obtained through the following description. Attached Figure Description

[0024] To make the objectives, technical solutions, and advantages of the present invention clearer, the preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, wherein:

[0025] Figure 1 This is a schematic diagram of the overall structure of the dual-tube concentric architecture CVD device of the present invention;

[0026] Figure 2 This is a schematic diagram of the quartz glass inner tube structure in this invention;

[0027] Figure 3 For the present invention Figure 1 The left view;

[0028] Figure 4 The image shows an OM diagram of the high-density bilayer MoS2 nanosheets prepared in the examples;

[0029] Figure 5 The images shown are OM images of the 3R / 2H bilayer MoS2 nanosheets prepared in the examples. From left to right, they are the OM images of the 3R phase and the OM images of the 2H phase.

[0030] Figure 6 The images shown are SEM images of the 3R / 2H bilayer MoS2 nanosheets prepared in the examples. From left to right, they are SEM images showing the 3R phase and the 2H phase.

[0031] Figure 7 The image shows the Raman spectrum of the bilayer MoS2 nanosheets prepared in the examples.

[0032] Figure 8 Raman E of the 3R / 2H bilayer MoS2 nanosheets prepared in the examples 1 2g and A 1g Mode intensity mapping, Figure 8 (a), (b), (c), and (d) are Raman E values ​​of 3R phase bilayer MoS2 nanosheets, respectively. 1 2g Mode intensity mapping diagram, showing Raman E of 2H phase bilayer MoS2 nanosheets 1 2g Mode intensity mapping diagram, showing Raman A of 3R phase bilayer MoS2 nanosheets 1g Mode intensity mapping diagram, showing Raman alpha of 2H phase bilayer MoS2 nanosheets1g Mode intensity mapping;

[0033] Figure 9 The figures show the second harmonic intensity polarization polarization diagram and mapping diagram of the 3R / 2H bilayer MoS2 nanosheets prepared in the examples. Figure 9 (a), (b), (c), and (d) respectively present the polarization polarization spectrum of the second harmonic signal intensity of 3R phase bilayer MoS2 nanosheets, the mapping diagram of the second harmonic signal intensity of 3R phase bilayer MoS2 nanosheets, the polarization polarization spectrum of the second harmonic signal intensity of 2H phase bilayer MoS2 nanosheets, and the mapping diagram of the second harmonic signal intensity of 2H phase bilayer MoS2 nanosheets. Detailed Implementation

[0034] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0035] This embodiment provides a method for preparing bilayer MoS2 nanosheets based on a concentric tubular furnace with a dual-tube structure. The reaction chamber uses a concentric dual-tube structure: the outer quartz glass tube has a diameter of 50 mm, a wall thickness of 2 mm, and a length of 1400 mm, and is divided into a high-temperature zone and a low-temperature zone; the inner quartz glass tube has a diameter of 25 mm, a wall thickness of 2 mm, and a length of 150 mm. The inner tube is welded to a rectangular quartz glass plate with a width of 35 mm and a length of 180 mm, with one end aligned. A circular hole with a diameter of 10 mm is drilled at the center of the other end of the glass plate, 10 mm from the top. The inner quartz glass tube is horizontally supported by this glass plate in the high-temperature zone of the outer quartz glass tube, forming a concentric dual-tube layout.

[0036] In this embodiment, the substrate is multiple 300 nm SiO2 / Si wafers, the chalcogenide precursor is 300 mg of solid S powder, and the mixture of molybdenum precursor and halogen salt is a mixture of 12 mg MoO3 powder and 0.5 ml of 5% wt NaCl aqueous solution. The carrier gas is high-purity argon, the working pressure of the reaction chamber is controlled at 1~10 Pa, and the argon flow rate is constant at 100 SCCM. The specific preparation method includes the following steps:

[0037] 1. Substrate pretreatment

[0038] The wafer-level silicon wafer is cut into 1 cm × 1 cm pieces, and then ultrasonically cleaned with acetone, isopropanol and deionized water for 10 min each. After cleaning, the pieces are removed and dried with nitrogen gas for later use.

[0039] 2. Material filling and cavity sealing

[0040] A quartz boat carrying solid S powder is placed in the low-temperature zone of the outer quartz glass tube, corresponding to the low-temperature heating zone of the dual-temperature zone tube furnace; a mixture of MoO3 and NaCl is placed at the opening of the inner quartz glass tube, and the pretreated SiO2 / Si substrate is placed in the downstream region of the inner quartz glass tube; after the materials are placed, the entire reaction chamber is sealed to form a closed reaction space.

[0041] 3. Vacuum and Atmosphere Setup

[0042] Start the vacuum pump to evacuate the pressure inside the reaction chamber to 1~10 Pa to establish a vacuum reaction environment; turn on the argon gas source and use a flow meter to stabilize the argon gas flow rate at 100 SCCM, and keep the carrier gas flowing throughout the process.

[0043] 4. Dual-zone temperature control program

[0044] Simultaneously start the heating programs for the low-temperature and high-temperature zones of the dual-zone tubular furnace:

[0045] Low temperature zone: The temperature is raised from room temperature to 150 ℃ in 10 min, then raised to 260 ℃ in 16 min, and held at 260 ℃ for 14 min;

[0046] High temperature zone: The temperature is increased from room temperature to 800 ℃ at a rate of 30 ℃ / min over 26 min, and then held at 800 ℃ for 14 min.

[0047] During the reaction, sulfur powder in the low-temperature zone sublimates to form sulfur vapor, which is transported to the high-temperature zone by argon carrier gas; the mixture of MoO3 and NaCl in the high-temperature zone volatilizes and reacts with the sulfur atmosphere to generate low-valence molybdenum oxide, which is further sulfided into MoS2 and deposited on the substrate surface to form nuclei, and continues to grow into bilayer MoS2 nanosheets under high temperature and confined environment.

[0048] 5. Cooling and Sampling

[0049] After the temperature control program is completed, the dual-temperature zone tube furnace heating system is turned off, and the reaction chamber is allowed to cool naturally to room temperature. The argon flow rate is kept constant at 100 SCCM throughout the cooling process. After the chamber cools to room temperature, the reaction chamber is opened and the substrate is removed, thus obtaining a large-area, high-density, and uniformly thick bilayer MoS2 nanosheet.

[0050] Appendix Figure 2 As the core component of this invention, the quartz glass inner tube and outer tube form a near-concentric structure. Figure 3In a dual-temperature zone tube furnace, a chalcogenide precursor is placed in the low-temperature zone of the outer quartz glass tube, while a mixture of a molybdenum precursor and a halogen salt is placed at the opening of the inner quartz glass tube in the high-temperature zone, with the substrate positioned downstream. This method utilizes chemical vapor deposition (CVD). After the chalcogenide precursor in the low-temperature zone is volatilized, a sulfur atmosphere is blown to the inner quartz glass tube in the high-temperature zone via a carrier gas, where it reacts with the volatilized molybdenum precursor to form a low-valence molybdenum oxide. The intermediate product is further sulfidized to MoS2 under the sulfur atmosphere and deposited on the substrate to form nucleation sites. Under high-temperature conditions, bilayer growth and lateral expansion of MoS2 nanosheets are achieved, effectively synthesizing large-area, high-density bilayer MoS2 nanosheets. The concentric architecture of the dual-tube type includes a tube-tube design that places the precursor in different regions, effectively delaying the process of the precursor participating in the reaction and avoiding premature consumption and nucleation of the precursor. The quartz glass inner tube in the concentric design effectively causes the local aggregation of precursors, and the resulting confinement effect prolongs the residence time of the precursors, promoting the growth of bilayer MoS2 nanosheets.

[0051] The prepared high-density bilayer MoS2 nanosheets were observed using an optical microscope (OM), such as... Figure 4 As shown, due to differences in stacking structure, the bilayer MoS2 nanosheets exhibit two stable 3R and 2H phases ( Figure 5 ), and its corresponding SEM image ( Figure 6 Its uniform bilayer structure can be verified. Raman spectroscopy characterization (…) Figure 7 and Figure 8 ), resulting in an E with high intensity and narrow bandwidth 1 2g and A 1g The characteristic peaks of the mode further verified the bilayer structure and crystallinity of the MoS2 nanosheets. Second harmonic generation (SHG) is often used to characterize and verify the nonlinear optical properties of two-dimensional materials. By using crystal symmetry and signal intensity characteristics, the successful preparation of bilayer MoS2 nanosheets and the identification of their interlayer stacking structure were effectively verified. Figure 9 The polarization spectra and signal intensity mapping of the 3R and 2H phases in bilayer MoS2 nanosheets are shown, clearly characterizing their phase distribution and polarization response.

[0052] As can be seen from the above embodiments, the present invention adopts a concentric structure of dual tubes to prepare high-quality bilayer MoS2 nanosheets with large area and high density. The changes in parameters such as precursor ratio, dual-temperature zone holding time, and carrier gas conditions will directly affect the uniformity of the number of nanosheets and the crystal quality. In actual preparation, the process parameters can be optimized according to the application requirements to obtain bilayer MoS2 nanosheets suitable for different scenarios.

[0053] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing bilayer MoS2 nanosheets based on a concentric tube furnace, characterized in that, Includes the following steps, S1. Place the quartz boat carrying the chalcogenide precursor in the low-temperature zone of the outer quartz glass tube, corresponding to the low-temperature heating coupler of the dual-temperature zone tube furnace. Place the mixture carrying the molybdenum precursor and the halogen salt, along with the substrate used to place the mixture, in the inner quartz glass tube. The inner quartz glass tube and the outer quartz glass tube are concentric and placed in the high-temperature zone of the outer quartz glass tube, corresponding to the high-temperature heating coupler of the dual-temperature zone tube furnace. Close the entire reaction chamber to form a closed space. S2. Use a vacuum pump to evacuate the pressure inside the reaction chamber to 1~10 Pa to create a vacuum reaction condition. Set the temperature control of the low temperature zone and high temperature zone of the dual-temperature zone tube furnace to 240~280 ℃ and 780~820 ℃ respectively from room temperature. During the entire reaction, the argon flow rate as the carrier gas is always maintained at 80~120 SCCM. After the dual-temperature zone tube furnace finishes working, the sample can be taken out after natural cooling. During the cooling process, the argon flow rate is kept constant.

2. The method for preparing bilayer MoS2 nanosheets as described in claim 1, characterized in that, In step S1, the reaction chamber has a double-tube structure, including an outer quartz glass tube with a high-temperature zone and a low-temperature zone, and an inner quartz glass tube mounted in the high-temperature zone. The outer and inner quartz glass tubes are concentric. A quartz boat is provided in the low-temperature zone inside the outer quartz glass tube to support the chalcogenide precursor. A mixture of molybdenum precursor and halogen salt, as well as multiple substrates, are provided in the inner quartz glass tube to support the nucleation sites of MoS2 nanosheets.

3. The method for preparing bilayer MoS2 nanosheets as described in claim 1, characterized in that, In step S1, a rectangular quartz glass plate frame is provided at the bottom of the quartz glass inner tube. The quartz glass inner tube and the rectangular quartz glass plate are welded together as one piece, with one end aligned. A circular hole with a diameter of 10 mm is provided at the center of the other end of the rectangular quartz glass plate, 10 mm from the top.

4. The method for preparing bilayer MoS2 nanosheets as described in claim 1, characterized in that, In step S1, the outer tube of the dual-temperature zone tube furnace has a diameter of 50 mm, a wall thickness of 2 mm, and a length of 1400 mm. The inner tube of the quartz glass has a diameter of 25 mm, a wall thickness of 2 mm, and a length of 150 mm. The rectangular quartz glass plate that serves as the bottom support for the inner tube has a width of 35 mm and a length of 180 mm.

5. The method for preparing bilayer MoS2 nanosheets as described in claim 1, characterized in that, The substrate preparation method in step S1 is as follows: the wafer-level silicon wafer is cut into small pieces of 1 cm*1 cm, and ultrasonically cleaned with acetone, isopropanol and deionized water for 10 min respectively, and then taken out and dried with nitrogen gas.

6. The method for preparing bilayer MoS2 nanosheets as described in claim 1, characterized in that, In step S1, the chalcogenide precursor is solid sulfur powder, the mixture of molybdenum precursor and halogen salt is a mixture of solid MoO3 powder and NaCl solution, and the substrate is a 300 nm SiO2 / Si wafer.

7. The method for preparing bilayer MoS2 nanosheets according to any one of claims 1 to 6, characterized in that, In step S2, the dual-temperature zone tubular furnace is connected to a gas filling and evacuation device via an external gas passage. The gas filling and evacuation device includes a gas cylinder for storing carrier gas, a flow meter for controlling the flow rate of carrier gas, and a vacuum pump for evacuating the reaction chamber. The gas passage transmits carrier gas to the reaction chamber and simultaneously collects working waste gas.

8. The method for preparing bilayer MoS2 nanosheets as described in claim 7, characterized in that, The temperature control process in step S2 for the low-temperature zone and the high-temperature zone is as follows: In the low-temperature zone, the temperature is raised from room temperature to 150 ℃ in 10 min; then to 260 ℃ in 16 min; and held at 260 ℃ for 14 min; after which the temperature is lowered to room temperature. In the high-temperature zone, the temperature is raised from room temperature to 800 ℃ in 26 min, with a heating rate of 30 ℃ / min, starting simultaneously with the low-temperature zone; then held at 800 ℃ for 14 min; and after which the temperature is lowered to room temperature.

9. The bilayer MoS2 nanosheets prepared by the method of claim 8.

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