A method of producing transition metal dichalcogenide heterostructures based on precursor densification CVD combined with stepwise precursor

Abstract

This invention discloses a method for preparing transition metal dichalcogenide heterojunctions based on precursor densification CVD combined with stepwise precursors. The method includes uniformly mixing a first metal oxide precursor with a first flux and then melting it to obtain a first densified precursor; uniformly mixing a second metal oxide precursor with a second flux and then melting it to obtain a second densified precursor; placing the first and second densified precursors at different positions in the high-temperature zone of the CVD reaction chamber; placing a sulfur source or selenium source in the low-temperature zone of the reaction chamber; placing the growth substrate in the high-temperature zone of the reaction chamber; and introducing a carrier gas atmosphere. Under the carrier gas atmosphere, the high-temperature zone of the reaction chamber is heated to a preset growth temperature and held at that temperature to obtain the transition metal dichalcogenide heterojunction on the growth substrate. This method aims to solve the technical problem that releasing different precursors within the same temperature zone can easily lead to uncontrollable mixing, alloying, or random nucleation.

Description

Technical Field

[0001] This invention relates to the field of two-dimensional material preparation, specifically to a method for preparing transition metal dichalcogenide heterostructures based on precursor densification CVD combined with stepwise precursors. Background Technology Two-dimensional transition metal dichalcogenides (TMDCs) possess excellent properties such as atomically thinness, high carrier mobility, strong photoelectric interaction, and tunable bandgap, showing great application potential in many fields such as field-effect transistors, photodetectors, logic memory devices, sensors, flexible electronics, and biomedicine. Two-dimensional heterostructures constructed from various TMDC materials can further expand the material's functionality and application scenarios.

[0002] Based on the bonding mechanism at the interface, TMDC heterostructures can be divided into lateral (in-plane) heterojunctions and vertical (out-of-plane) heterojunctions. Vertical heterojunctions rely on van der Waals forces to achieve vertical stacking of different TMDC layers, possessing atomically clean van der Waals interfaces, and allowing for control of optocoupler characteristics through interlayer rotations. Lateral heterojunctions, on the other hand, rely on covalent bonds between different TMDC domains within the plane to achieve seamless atomic-level connections, forming a one-dimensional interface that enables efficient charge and energy transfer. Due to their unique interface properties, two-dimensional TMDC heterojunctions are ideal materials for next-generation two-dimensional optoelectronic devices such as ultrathin PN junctions, tunneling field-effect transistors, bipolar transistors, and nonlinear optical devices.

[0003] Currently, the mainstream fabrication processes for TMDC heterojunctions are divided into two categories: the manual stacking method combining mechanical exfoliation and dry / wet transfer, and the in-situ synthesis method using chemical vapor deposition (CVD). The manual transfer stacking process is flexible in operation, but it is prone to interfacial impurities, uncontrollable interlayer rotation angles, limited domain size, and is difficult to mass-produce. CVD in-situ growth can directly prepare high-purity heterofilms on the substrate, eliminating interfacial contamination.

[0004] However, TMDC heterojunction systems co-grown with multiple metal sources such as molybdenum and tungsten have the following drawbacks: ① For TMDC heterojunction systems involving multiple metal sources (such as Mo and W), the simultaneous and rapid release and diffusion of different precursors within the same temperature range can easily lead to uncontrollable mixing, alloying, or random nucleation, thereby affecting the interface sharpness, compositional uniformity, and structural repeatability of the heterojunction.

[0005] ② It cannot take advantage of the inherent differences in the molten volatilization temperature of different precursors to selectively achieve the growth of heterojunctions of different transition metal dichalcogenides (i.e., lateral heterojunctions and vertical heterojunctions) in the same CVD process, resulting in a narrow process window and poor operability.

[0006] Therefore, there is an urgent need to provide a method to improve the formation quality of heterojunctions by transforming explosive volatilization into a melt-release, controllable source mode, and to achieve selective growth of transverse or vertical heterojunctions in the same CVD process based on the inherent differences in the volatilization temperature of the precursor melt. Summary of the Invention

[0007] To address the aforementioned shortcomings of existing technologies, this invention provides a method for preparing transition metal dichalcogenide heterojunctions based on precursor densification CVD combined with stepwise precursors, thereby solving the aforementioned technical problems.

[0008] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows: A method for preparing transition metal dichalcogenide heterojunctions based on precursor densification CVD combined with stepwise precursors is provided, comprising the following steps: S1: The first metal oxide precursor and the first flux are uniformly mixed and melt-treated to obtain the first densified precursor; the second metal oxide precursor and the second flux are uniformly mixed and melt-treated to obtain the second densified precursor. S2: The first densification precursor and the second densification precursor are placed at different positions in the high-temperature zone of the chemical vapor deposition reaction chamber, the sulfur source or selenium source is placed in the low-temperature zone of the reaction chamber, the growth substrate is placed in the high-temperature zone of the reaction chamber, and a carrier gas atmosphere is introduced. S3: Under a carrier gas atmosphere, the high-temperature zone of the reaction chamber is heated to the preset growth temperature, and the reaction is maintained at the temperature to obtain a transition metal dichalcogenide heterojunction on the growth substrate. The first and second densification precursors have different volatilization temperatures and are released sequentially during the heating process, forming a stepwise power supply.

[0009] Traditional powder precursors in CVD suffer from large surface areas, rapid and uncontrollable volatilization, leading to simultaneous large-scale releases from multiple sources, uncontrolled mixing of gaseous components, and alloying. In this invention, flux is mixed with precursor powder and pre-melted to form a densified composite precursor. This process produces two key effects: First, the physical morphology changes from loose powder to dense bulk, significantly reducing the effective volatilization surface area and transforming volatilization from rapid bursts to slow, stable, and controllable releases. This fundamentally alters the precursor supply kinetics, optimizing the interface sharpness, compositional uniformity, and structural repeatability of the heterojunction. Second, the introduction of the flux modulates the chemical potential and vapor pressure characteristics of the metal oxides, resulting in different volatilization temperatures for the two different precursors. This inherent difference can be effectively mitigated by sequential supply, preventing component mixing caused by simultaneous large-scale releases of two metal sources.

[0010] Furthermore, since the two densified precursors have different volatilization temperatures, the different volatilization properties of the two precursors can be utilized more effectively, and different temperature control processes can be adopted to achieve the selective generation of transition metal dichalcogenide heterojunctions.

[0011] Furthermore, in step S1, the first metal oxide precursor is MoO3, the first flux is NaCl, and the mass ratio of MoO3 to NaCl is 8-10:1. The second metal oxide precursor is WO3, and the second flux is NaCl, with a mass ratio of WO3 to NaCl of 8-10:1.

[0012] In this invention, MoO3-NaCl and WO3-NaCl have different volatilization temperatures. MoO3 begins to volatilize significantly at about 700°C, while WO3 requires about 780-800°C or higher to volatilize in large quantities. This difference causes the Mo source to be released preferentially and nucleate and grow MoS2 crystal domains on the substrate first. The W source is then released and continues to participate in the reaction.

[0013] Furthermore, the process parameters for the melting treatment in step S1 are as follows: under a closed, air-free state, the temperature is raised to 700°C or above, held for 3-10 minutes, and then cooled.

[0014] Furthermore, in step S2, the amount of sulfur source used is 0.5-3.5 times the total mass of the precursor; the carrier gas is an argon-hydrogen mixture, composed of 1%-5% H2 and 95%-99% Ar.

[0015] In this invention, a mixed carrier gas of 1%-5% H2 and 95%-99% Ar is introduced during the CVD preparation of two-dimensional heterojunctions. First, argon acts as an inert carrier to continuously transport sulfur source precursor vapor to the substrate reaction zone, while the gas flow carries away residual oxygen, water vapor, and reaction byproducts in the furnace, maintaining a clean reaction environment. Second, low-concentration hydrogen provides a weak reducing atmosphere, which can reduce trace oxides on the substrate and quartz boat surfaces, suppress the generation of impurities from high-temperature oxidation of the sulfur source, and prevent oxide defects from damaging the heterojunction interface quality.

[0016] Furthermore, in step S2, the substrate is one of SiO2 / Si wafer, sapphire wafer, mica wafer or fluorine-phlogopite wafer; the substrate faces downward and is located 5-10 mm upstream of the precursor.

[0017] Furthermore, in step S3, the preset growth temperature is 720-850℃, and the temperature is maintained for 3-8 minutes; the flow rate of the carrier gas is 40-100 sccm.

[0018] In this invention, a flow rate of 40-100 sccm is set. Too low a flow rate will prevent timely removal of impurities and insufficient precursor vapor supply, leading to uneven film growth and heterojunction interface delamination. Too high a flow rate will dilute the gaseous reactants and shorten the contact time between the precursor and the substrate, resulting in a thinner film with more defects. This flow rate range balances gas phase transport efficiency, impurity purging capability, and precursor deposition reaction time, ensuring uniform and dense growth of the heterojunction film.

[0019] Furthermore, in step S3, a one-step heating process is used to directly and slowly heat the high-temperature zone to the preset growth temperature and hold it at that temperature to obtain a transverse heterojunction.

[0020] Furthermore, the temperature of the low-temperature zone is set at 350℃ and the temperature of the high-temperature zone is set at 800℃, and the temperature is maintained for 3-8 minutes.

[0021] In this invention, at a low-temperature zone temperature of 350°C, sulfur powder has completely volatilized inside the tubular furnace, forming sulfur vapor. When the high-temperature zone temperature rises to approximately 700-750°C, MoO3 preferentially sublimates and reacts with the sulfur vapor, initially forming triangular monolayer MoS2 domains on the substrate surface. Due to the large number of unsaturated dangling bonds and defect sites at the edges of the MoS2 domains, they exhibit high chemical activity, constituting a high-energy surface. When the temperature further increases to 780-800°C, WO3 begins to volatilize significantly, and the W source enters the gas phase. At this point, the nucleation barrier of WS2 on the active edges of the formed MoS2 domains is significantly lower than that on the inert basal surface (van der Waals surface) of MoS2. Therefore, WS2 preferentially undergoes in-plane and out-of-plane epitaxial growth along the edges of the MoS2 domains, forming a "core-shell" lateral heterojunction with a clear covalent bond interface.

[0022] Furthermore, under slow heating conditions, different metal sources exhibit distinct sequential release characteristics due to their differences in volatilization temperature. MoO3 preferentially volatilizes and participates in the reaction at lower temperatures, first forming MoS2 domains on the substrate; only when the temperature continues to rise to the volatilization range of WO3 does the W source begin to supply in large quantities. This time-separated precursor supply mode is conducive to realizing the stepwise growth process of "the first type of TMDs grows completely first, and the second type of TMDs grows laterally along its edge," which is a dynamic guarantee for the formation of high-quality lateral heterojunctions.

[0023] Furthermore, in step S3, a two-step heating process is adopted: first, the high-temperature zone is heated to the first preset temperature and held, and then rapidly heated to the second preset temperature and held, to obtain a vertical heterojunction.

[0024] Furthermore, the temperature is first raised to 730℃ and held for 1 minute, then raised to 830℃ and held for 3 minutes within 5 minutes.

[0025] In this invention, at a low-temperature zone temperature of 350℃, sulfur powder has completely volatilized inside the tubular furnace, forming sulfur vapor. A two-step heating strategy is a key technical means to achieve vertical heterojunctions. The first step involves holding at 730℃ for 1 minute, allowing MoO3 to preferentially release and fully nucleate and grow into a complete MoS2 monolayer on the substrate. At this point, the temperature has not yet reached the threshold for large-scale volatilization of WO3, and only a small amount of W source is released, ensuring that the MoS2 monolayer domains can form completely without interference from the W source. The second step involves rapidly heating to 830℃ and holding for 3 minutes, bringing the system into a high-energy state. Under the high-temperature condition of 830℃, on the one hand, WO3 volatilizes in large quantities, significantly increasing the concentration and supersaturation of the W source in the gas phase; on the other hand, the high temperature lowers the nucleation barrier of WS2 on the MoS2 substrate, increasing the probability of basal surface nucleation. When the surface nucleation rate exceeds the edge epitaxial rate, WS2 begins to nucleate directly on the MoS2 monolayer surface and grows in a vertical direction, forming a van der Waals stacked vertical heterojunction.

[0026] Furthermore, under rapid heating conditions, the system traverses a large temperature range in a short period, causing the Mo and W sources to reach high volatilization flux states almost simultaneously. The concentrations of both metal precursors in the gas phase increase simultaneously, leading to a significant increase in local supersaturation. Under high supersaturation conditions, the second type of TMDs can not only grow at the edges of existing domains but also more easily nucleate on the surface of existing monolayers. This is because high supersaturation lowers the critical nucleus size for surface nucleation, increasing the probability of surface nucleation and thus promoting the formation of vertical heterojunctions.

[0027] In summary, there is a synergistic effect between the heating rate and the growth temperature of this invention. That is, the combination of slow heating and lower temperature is conducive to the formation of lateral heterojunctions, while the combination of rapid heating and higher temperature is conducive to the formation of vertical heterojunctions.

[0028] The present invention also provides a transition metal dichalcogenide heterojunction prepared by the above method, wherein the heterojunction is a MoS2 / WS2 lateral heterojunction or a MoS2 / WS2 vertical heterojunction; the lateral heterojunction has a core-shell spatial distribution structure, with MoS2 located in the central region and WS2 located in the edge region.

[0029] The beneficial effects of this invention are as follows: (1) Traditional powder precursors in CVD suffer from large specific surface areas, rapid and uncontrollable volatilization, leading to simultaneous large-scale release from multiple sources, uncontrollable mixing of gas phase components, and alloying. By mixing the flux with the precursor powder and pre-melting it, a uniform and dense composite precursor is formed. This process produces two key effects: firstly, the physical morphology changes from loose powder to dense bulk, significantly reducing the effective volatilization surface area and transforming volatilization from rapid bursts to slow, stable, and controllable release. This fundamentally alters the precursor supply kinetics, optimizing the interface sharpness, compositional uniformity, and structural repeatability of the heterojunction. Secondly, the introduction of alkali metal halide flux modulates the chemical potential and vapor pressure characteristics of metal oxides, resulting in different volatilization temperatures for the two different precursors (MoO3 begins to volatilize significantly at about 700℃, while WO3 requires about 780–800℃ or higher to volatilize in large quantities). This inherent difference causes the Mo source to be released preferentially and nucleate and grow MoS2 domains on the substrate first, while the W source is released subsequently and continues to participate in the reaction. Thus, the timing of the source supply effectively avoids the mixing of components caused by the simultaneous large-scale release of the two metal sources.

[0030] (2) The final configuration of the heterojunction depends on the competition between two growth modes: edge epitaxial growth (leading to lateral heterojunctions) and surface nucleation stacking (leading to vertical heterojunctions). At lower temperatures (800℃), the energy barrier for edge epitaxy is lower than that for surface nucleation, and lateral growth is dominant. At higher temperatures (830℃), the energy barrier for surface nucleation decreases due to thermal activation, while the gas phase supersaturation increases, and the surface nucleation stacking mode becomes dominant. The densification treatment and stepwise supply of the precursor provide the necessary kinetic basis for this temperature regulation.

[0031] Furthermore, there is a synergistic effect between the heating rate and the growth temperature. The combination of slow heating and lower temperature is conducive to the formation of lateral heterojunctions; the combination of rapid heating and higher temperature is conducive to the formation of vertical heterojunctions. Ultimately, by utilizing the inherent differences in the evaporation temperatures of different precursors, the growth of lateral or vertical heterojunctions can be selectively achieved in the same CVD process, resulting in a strong process window and high operability. Attached Figure Description

[0032] Figure 1 Schematic diagram of a chemical vapor deposition apparatus for preparing WS2 / MoS2 heterojunctions; Figure 2 (a) is the Raman spectrum of the WS2 / MoS2 lateral heterojunction; Figure 2 (b) is the single-point PL spectrum of the WS2 / MoS2 lateral heterojunction; Figure 3 (a) is an optical microscope image of the WS2 / MoS2 heterojunction; Figure 3(bc) Raman mapping image of the WS2 / MoS2 heterojunction; Figure 4 Optical microscope images of the products in Examples 1-3; Figure 5 An optical microscope image of the product in Example 4; Figure 6 Optical microscope image of the product in Comparative Example 1; Figure 7 An optical microscope image of the product in Example 5; Figure 8 The images shown are optical microscope images of the WS2 / MoS2 vertical heterojunction in Example 6 and the Raman spectrum of the heterojunction interface region. Figure 9 The images shown are optical microscope images of the WS2 / MoS2 vertical heterojunction in Example 7 and the Raman spectrum of the heterojunction interface region. Detailed Implementation

[0033] The specific embodiments of the present invention are described below to enable those skilled in the art to understand the present invention. However, it should be understood that the present invention is not limited to the scope of the specific embodiments. For those skilled in the art, various changes are obvious as long as they are within the spirit and scope of the present invention as defined and determined by the appended claims. All inventions utilizing the concept of the present invention are protected.

[0034] Unless otherwise specified, all raw materials used in the embodiments of the present invention are commercially available analytical grade materials.

[0035] Example 1 A method for preparing transition metal dichalcogenide heterojunctions based on precursor densification CVD combined with stepwise precursors includes the following steps: (1) Weigh 10mg of MoO3 powder and 1mg of NaCl powder respectively, mix them evenly and place them at the front end of the first quartz boat; weigh 20mg of WO3 powder and 2mg of NaCl powder, mix them evenly and place them at the rear end of the first quartz boat.

[0036] (2) Figure 1 As shown, the first quartz boat is placed in the heating center zone, i.e. the high-temperature zone, of the CVD tube furnace. Under normal pressure and closed conditions, a mixed carrier gas consisting of 95% Ar and 5% H2 is introduced to clean the tube furnace system for 5 minutes.

[0037] In this embodiment, a mixed gas of 95% Ar and 5% H2 is used to clean the furnace pipeline. The core purpose is to quickly replace the air in the pipeline, thoroughly remove oxygen and water vapor, effectively prevent sulfur from being oxidized to form sulfur dioxide impurities when the sulfur powder is heated and melted, effectively prevent oxygen from causing the sulfur powder to spontaneously combust at high temperatures, and at the same time create a clean atmosphere in advance.

[0038] (3) After cleaning, start the tube furnace heating program, set the temperature of the heating center area to 800℃, and the holding time to 3min. After melting and pre-treating the precursor, cool to obtain two densified precursors.

[0039] (4) Weigh 40 mg of sublimed sulfur powder and place it in the second quartz boat in the upstream low temperature zone, with the mica sheet substrate placed above the first quartz boat.

[0040] (5) Set the temperature of the low-temperature zone to 350℃ and the temperature of the high-temperature zone to 800℃, introduce the mixed carrier gas, and carry out the sulfidation reaction for 3 minutes under normal pressure. The mixed carrier gas is an argon-hydrogen mixture, which consists of 5% H2 and 95% Ar, and the flow rate of the mixed carrier gas is 40 sccm.

[0041] (6) After the reaction is complete, turn off the heating, keep the carrier gas flowing, and wait for the furnace temperature to cool naturally to below 100°C before opening the furnace and taking out the substrate to obtain the MoS2 / WS2 lateral heterojunction product.

[0042] The product obtained in Example 1 is denoted as Figure 4 1-1.

[0043] Example 2 The preparation steps of Implementation 2 are the same as those of Example 1, except that the mass ratio of WO3 to MoO3 is changed, that is, 15 mg of MoO3 and 15 mg of WO3 are weighed.

[0044] The product obtained in Example 2 is denoted as Figure 4 1-2.

[0045] The preparation steps of Implementation 3 are the same as those of Example 1, except that the mass ratio of WO3 to MoO3 is changed, that is, 20 mg of MoO3 and 10 mg of WO3 are weighed.

[0046] The product obtained in Example 3 is denoted as Figure 4 1-3.

[0047] In Examples 1-3, although the ratio of WO3 to MoO3 was varied within the range, the precursors were more fully volatilized, and both MoS2 and WS2 could nucleate and grow, forming a "core-shell" lateral heterojunction with a clear covalent bond interface, high crystallinity, and excellent product quality.

[0048] Example 4 The preparation steps of Example 4 are the same as those of Example 1, except that the vulcanization reaction time (i.e., the holding time) is 4 minutes. The product obtained in Example 4 is as follows: Figure 5 As shown.

[0049] When the sulfidation reaction time is 4 min, both MoS2 and WS2 can nucleate and grow, forming a transverse heterojunction with a "core-shell" structure and a clear covalent bond interface.

[0050] Example 5 Example 5 was prepared using the same method as Example 1, except for the vulcanization reaction time, which was 5 minutes. The resulting product was as follows: Figure 7 As shown.

[0051] When the sulfurization reaction time is 5 minutes, the precursor is continuously supplied, the TMDs domain regions continue to grow, and the film area increases (e.g., Figure 7 As shown in the figure, the gaps between the domain regions are gradually filled, eventually forming a "core-shell" lateral heterojunction with a covalent bond interface.

[0052] Comparative Example 1 The preparation method of this comparative example is the same as that of Example 1, except that the vulcanization reaction time is 2 minutes. The product obtained in Comparative Example 1 is as follows: Figure 6 As shown.

[0053] When the sulfidation reaction time is too short (i.e., 2 min in Comparative Example 1), the precursor volatilizes insufficiently, the TMDs domains grow incompletely, the domain size is small, the heterojunction interface is not fully formed, and the product quality is poor.

[0054] Comparative Example 2 The preparation method of this comparative example is the same as that of Example 1, except that the vulcanization reaction time is 12 min. Excessive heat preservation time may cause the already formed heterostructure to undergo interface reconstruction due to thermodynamic driving force or secondary nucleation at undesirable locations.

[0055] Example 6 A method for preparing transition metal dichalcogenide heterojunctions based on precursor densification CVD combined with stepwise precursors includes the following steps: (1) Weigh 10mg of MoO3 powder and 1mg of NaCl powder respectively, mix them evenly and place them at the front end of the first quartz boat; weigh 20mg of WO3 powder and 2mg of NaCl powder, mix them evenly and place them at the rear end of the first quartz boat.

[0056] (2) Place the first quartz boat in the heating center area of ​​the CVD tube furnace, i.e. the high temperature zone, and under normal pressure and closed conditions, introduce a mixed carrier gas consisting of 95% Ar and 5% H2 to clean the tube furnace system for 5 minutes.

[0057] (3) After cleaning, start the tube furnace heating program, set the temperature of the heating center area to 800℃, and the holding time to 3min. After melting and pre-treating the precursor, cool to obtain two densified precursors.

[0058] (4) Weigh 40 mg of sublimed sulfur powder and place it in the second quartz boat in the upstream low-temperature zone. The mica sheet substrate is placed above the first quartz boat. (5) The low-temperature zone temperature is set to 350°C, and the high-temperature zone adopts step-by-step temperature control. First, the temperature is raised to 730°C and held for 1 minute within 48 minutes, and then rapidly raised to 830°C within 5 minutes. An argon-hydrogen mixed carrier gas is introduced, and a sulfidation reaction is carried out at atmospheric pressure for 3-5 minutes to generate a vertical heterojunction on the substrate. The mixed carrier gas is an argon-hydrogen mixed gas, which consists of 5% H2 and 95% Ar, and the flow rate of the mixed carrier gas is 40 sccm.

[0059] (6) After the reaction is complete, turn off the heating, keep the carrier gas flowing, and wait for the furnace temperature to cool naturally to below 100°C before opening the furnace and removing the substrate to obtain the MoS2 / WS2 vertical heterojunction product (e.g. Figure 8 (As shown).

[0060] Example 7 Example 7 is prepared using the same method as Example 5, except for the vulcanization reaction time (i.e., holding time), which is 5 minutes. The product obtained in Example 7 is as follows: Figure 9 As shown. Extending the vulcanization time to 5 minutes still yielded a vertical heterojunction, but the quality was slightly inferior to that of Example 6.

Claims

1. A method for preparing transition metal dichalcogenide heterojunctions based on precursor densification CVD combined with stepwise precursors, characterized in that, Includes the following steps: S1: The first metal oxide precursor and the first flux are uniformly mixed and melt-treated to obtain the first densified precursor; the second metal oxide precursor and the second flux are uniformly mixed and melt-treated to obtain the second densified precursor. S2: The first densification precursor and the second densification precursor are placed at different positions in the high-temperature zone of the chemical vapor deposition reaction chamber, the sulfur source or selenium source is placed in the low-temperature zone of the reaction chamber, the growth substrate is placed in the high-temperature zone of the reaction chamber, and a carrier gas atmosphere is introduced. S3: Under a carrier gas atmosphere, the high-temperature zone of the reaction chamber is heated to the preset growth temperature, and the reaction is maintained at the temperature to obtain a transition metal dichalcogenide heterojunction on the growth substrate. The first and second densification precursors have different volatilization temperatures and are released sequentially during the heating process, forming a stepwise power supply.

2. The method according to claim 1, characterized in that, In step S1, the first metal oxide precursor is MoO3, the first flux is NaCl, and the mass ratio of MoO3 to NaCl is 8-10:

1. The second metal oxide precursor is WO3, and the second flux is NaCl, with a mass ratio of WO3 to NaCl of 8-10:

1.

3. The method according to claim 1, characterized in that, In step S1, the process parameters for the melting treatment are as follows: under a closed, air-free state, the temperature is raised to 700°C or above, held for 3-10 minutes, and then cooled.

4. The method according to claim 1, characterized in that, In step S2, the amount of sulfur source used is 0.5-3.5 times the total mass of the precursor; the carrier gas is an argon-hydrogen mixture, consisting of 1%-5% H2 and 95%-99% Ar.

5. The method according to claim 1, characterized in that, In step S2, the substrate is one of SiO2 / Si wafer, sapphire wafer, mica wafer or fluorine-phlogopite wafer; the substrate is facing down and located 5-10 mm upstream of the precursor.

6. The method according to claim 1, characterized in that, In step S3, the preset growth temperature is 720-850℃, and the temperature is maintained for 3-8 minutes; the flow rate of the carrier gas is 40-100 sccm.

7. The method according to claim 1, characterized in that, In step S3, a one-step heating process is used to directly and slowly heat the high-temperature zone to the preset growth temperature and hold it at that temperature to obtain a transverse heterojunction.

8. The method according to claim 1, characterized in that, In step S3, a two-step heating process is adopted. First, the high-temperature zone is heated to the first preset temperature and held, and then rapidly heated to the second preset temperature and held to obtain a vertical heterojunction.

9. The method according to claim 8, characterized in that, The two-step heating process is as follows: first, the temperature is raised to 730°C and held for 1 minute, and then the temperature is raised to 830°C and held for 3 minutes within 5 minutes.

10. The transition metal dichalcogenide heterojunction prepared by the method according to any one of claims 1-9, characterized in that, The heterojunction is a MoS2 / WS2 lateral heterojunction or a MoS2 / WS2 vertical heterojunction; the lateral heterojunction has a core-shell spatial distribution structure, with MoS2 located in the central region and WS2 located in the edge region.

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