A method for in-situ melting preparation of single-layer MoS2-WS2 lateral heterojunction and application

Abstract

A method for in-situ melting preparation of single-layer MoS2-WS2 lateral heterostructure and application, using oxide MoO3 as molybdenum source, and metal W as tungsten source for reaction, because the metal tungsten is difficult to evaporate to form tungsten vapor, so first in-situ deposition of tungsten on the substrate, then add metal Te as flux, so that the metal tungsten in-situ melting on the substrate, and MoO3 and S powder in-situ reaction on the substrate to form MoS2-WS2 lateral heterojunction, can effectively avoid the formation of thermodynamic more stable alloy. We use three temperature zone tube furnace to control the temperature and heating rate of S powder, MoO3 and substrate respectively, realize the efficient controllable synthesis of heterostructure. Surface photovoltage microscopy shows that this heterojunction can significantly promote the spatial separation of photo-generated electrons and holes, significantly prolong the carrier lifetime, so it has important application potential in solar cells, photo (electric) catalysis and new photoelectric devices.

Description

Technical Field

[0001] This invention belongs to the field of two-dimensional material heterostructure preparation, specifically relating to a method for preparing a single-layer MoS2-WS2 transverse heterostructure. Background Technology

[0002] In recent years, two-dimensional materials have become core candidate materials for next-generation information and energy devices due to their atomic-level thickness, unique band structure, and excellent electrical, optical, and mechanical properties. Typical two-dimensional transition metal dichalcogenides such as MoS2, WS2, MoSe2, and WSe2 have shown broad application prospects in fields such as photoelectric detection, photocatalysis, and flexible electronics because of their direct band gap, strong light-matter interaction, and controllable layer number-dependent properties.

[0003] In two-dimensional materials research, heterostructure construction is considered an important strategy for overcoming the performance limitations of single materials and achieving functional integration. By orderly combining different two-dimensional materials at the atomic scale, band-aligned interface structures can be formed, thereby effectively controlling the generation, separation, and transfer behavior of charges. In particular, the MoS2–WS2 heterojunction with type II band alignment can promote the spatial separation of photogenerated electrons and holes, significantly extending carrier lifetime, and therefore has important application potential in solar cells, photocatalytic water splitting, and novel optoelectronic devices.

[0004] In existing technologies, the preparation methods for two-dimensional heterostructures mainly include the following categories: mechanical transfer, wet chemical synthesis, molecular beam epitaxy (MBE), and chemical vapor deposition (CVD). Mechanical transfer involves artificially stacking two different two-dimensional materials under microscopic manipulation to form a heterostructure. This method is flexible, but it is prone to introducing interface contamination, stacking mismatch, and bubbles during the transfer process, resulting in weak interfacial bonding and affecting charge transfer efficiency. Wet chemical synthesis assembles heterostructures through liquid chemical reactions; the method is simple and low-cost, but the crystallinity and interface quality of the materials often fail to meet the requirements of high-performance devices. Molecular beam epitaxy and CVD directly synthesize two-dimensional heterostructures on a substrate through epitaxial growth. Molecular beam epitaxy allows for atomic-level thickness control, but the equipment is expensive and the yield is limited. CVD can grow two-dimensional materials over a large area and construct heterostructures by controlling the precursor, temperature, and reaction atmosphere, making it the most promising preparation method currently.

[0005] Some progress has been made in the research of preparing two-dimensional heterojunctions using chemical vapor deposition. For example, patent CN202010101665.7 first synthesizes WS2 using WO3 and S powder, and then uses the electrochemical anodic oxidation of Mo foil to prepare multivalent molybdenum oxide foil (MoOx) as a precursor to grow MoS2 on the WS2 surface, successfully synthesizing a MoS2-WS2 vertical heterojunction. Patent CN201911224578.4 proposes a one-step method using S powder, MoO3, and WO3 to synthesize large-size, high-quality MoS2-WS2 vertical heterojunctions on sapphire substrates, effectively avoiding interlayer contamination caused by methods such as mechanical stripping. Lateral heterojunctions, due to their atomically seamless interface, absence of van der Waals gaps, and continuous carrier migration channels, are theoretically more suitable for optoelectronic devices, but their fabrication is more difficult, and a mature process has not yet been established. Patent CN201911084034.2 uses molybdenum oxide foil and WoO3 as precursors to directly grow MoS2-WS2 lateral heterojunctions in one step by controlling the temperature of the two sources. Patent CN202111406267.7 mixes sodium molybdate and sodium tungstate together to form a self-assembled MoS2-WS2 lateral heterojunction. Patent CN202010738875.7 successfully prepares a MoS2-WS2 lateral heterojunction by depositing ammonium molybdate and sodium tungstate directly below the substrate. In the above preparation methods, the tungsten and molybdenum sources are all derived from the same type of oxide or inorganic salt. Their evaporation temperatures are similar during deposition, inevitably leading to the formation of a more thermodynamically stable alloy. This patent proposes a method that uses MoO3 oxide as the molybdenum source and W metal as the tungsten source for the reaction. Since tungsten metal is difficult to evaporate to form tungsten vapor, tungsten is first deposited in situ on the substrate, and then Te metal is added as a flux. In this way, the tungsten metal melts on the substrate and reacts in situ with MoO3 and S powder on the substrate to controllably synthesize a MoS2-WS2 lateral heterojunction, which can effectively avoid the formation of a more thermodynamically stable alloy. Summary of the Invention

[0006] The purpose of this invention is to provide a controllable, high-quality method for synthesizing MoS2-WS2 lateral heterojunctions. To achieve this objective, the technical solution adopted in this invention is as follows: Unlike previous synthesis methods that use the same type of oxide or inorganic salt as the molybdenum and tungsten sources, this invention proposes using MoO3 oxide as the molybdenum source and W metal as the tungsten source for the reaction. Since tungsten metal is difficult to evaporate to form tungsten vapor, tungsten is first deposited in situ on a substrate. Then, Te metal is added as a flux, allowing the tungsten metal to melt in situ on the substrate and react with MoO3 and S powder in situ to form a MoS2-WS2 lateral heterojunction. This effectively avoids the formation of thermodynamically more stable alloys. A three-zone tube furnace is used to synthesize the MoS2-WS2 lateral heterostructure in one step via chemical vapor deposition. The inner layer is MoS2, and the outer layer is epitaxially grown WS2. The heterostructure is triangular with a side length of 5-20 μm. The MoS2-WS2 lateral heterojunction is a single layer with a thickness of approximately 0.8 nm. After the formation of the heterojunction, the surface photovoltage of MoS2 and WS2 increased significantly, indicating that the formation of the heterojunction directly promoted the separation of photogenerated charges.

[0007] The technical solution of the present invention:

[0008] A method for preparing a monolayer MoS2-WS2 lateral heterostructure by in-situ melting includes the following steps:

[0009] Step 1: Mix metals W and Te and grind them thoroughly. Dissolve the mixture in a mixed solvent of ethanol and water (the ratio of ethanol to water is 1:1 to 2:1). Drop the above mixed solution onto a clean SiO2 / Si substrate and then dry it at 50-80℃ for later use.

[0010] Step 2: Place S powder in the first quartz boat, which is placed upstream of the quartz tube. Place MoO3 in the second quartz boat, which is placed midstream of the quartz tube. Place the substrate treated in Step 1 on the third quartz boat, which is placed downstream of the quartz tube.

[0011] Step 3: Evacuate the quartz tube, purge with inert gas, then evacuate again and purge with inert gas again. Repeat this process to ensure an inert environment inside the quartz tube. Then, use a flow meter to maintain a constant inert atmosphere flow rate. Set different heating programs in different areas to create three different temperature zones within the quartz tube: a high-temperature S evaporation zone, a high-temperature MoO3 evaporation zone, and a deposition zone. The inert carrier gas carries S and MoO3 vapors to the vicinity of the substrate surface to react with the molten W on the substrate. The Te on the substrate acts as a flux, helping the W metal melt. After the reaction is complete, allow the quartz tube to cool naturally and remove the substrate. At this point, a MoS2-WS2 lateral heterojunction is obtained on the SiO2-Si substrate.

[0012] Further, in step 1: after cutting the SiO2-Si substrate into small pieces of 1cm×1cm, it is ultrasonically cleaned with acetone, isopropanol and deionized water for 20-40 minutes in sequence, and then the SiO2 / Si substrate is dried with nitrogen for later use.

[0013] Further, in step 1, the mass fraction ratio of W to Te is between 1:8 and 1:12. The concentration of the W and Te mixture in a mixed solvent of ethanol and water is between 0.05 mg / ml and 0.1 mg / ml, and the amount of the mixed solution added to the SiO2 / Si substrate is 50 μl to 100 μl / cm³. 2 SiO2 / Si substrate.

[0014] Furthermore, in step 2, the relationship between the mass of S powder, the mass of MoO3, and the substrate obtained in step 1 is as follows: per cm 2 The substrate requires 500mg-1.5g of sulfur powder and 25mg-75mg of moO3.

[0015] Furthermore, in step 2: the distance between the quartz boat containing MoO3 and the quartz boat containing S powder is 18-22cm, and the distance between the quartz boat containing the substrate and the MoO3 is 28-32cm.

[0016] Further, in step 3: the heating program for the S powder zone is to heat from room temperature to 50-80℃ in 10-15 minutes, hold for 35-40 minutes, then heat to 150-200℃ in 15-20 minutes, hold for 10-20 minutes, and then cool naturally to room temperature. The heating program for the MoO3 and substrate zones is to heat from room temperature to 680-720℃ in 60-75 minutes, hold for 10-20 minutes, and then cool naturally.

[0017] Furthermore, in step 3: the inert gas introduced is argon or nitrogen, and the flow rate of the inert gas is maintained at 100-200 sccm.

[0018] The above method produces an in-situ melting method for preparing a monolayer MoS2-WS2 lateral heterojunction for use in photocatalysis and optoelectronic devices.

[0019] The beneficial effects of this invention are:

[0020] 1. This invention proposes a three-temperature-zone controllable synthesis method for monolayer MoS2-WS2 lateral heterostructures. It utilizes three temperature zones to precisely control the heating rates and temperatures of sulfur powder, MoO3, and the SiO2 / Si substrate loaded with W and Te (fluxes). This ensures that the vapors of sulfur powder and MoO3 simultaneously reach the vicinity of the substrate and undergo in-situ reaction deposition, thereby enabling the controllable and repeatable preparation of high-quality monolayer MoS2-WS2 lateral heterostructures. This effectively avoids the formation of thermodynamically more stable alloys.

[0021] 2. After the formation of heterojunction, the surface photovoltage of MoS2 and WS2 increased significantly, indicating that the formation of heterojunction directly promoted the separation of photogenerated charges, extended the carrier lifetime, and improved the photoelectric conversion efficiency. Therefore, it has important application potential in solar cells, photocatalysis, and novel optoelectronic devices. Attached Figure Description

[0022] Figure 1 Optical microscope image of a MoS2-WS2 lateral heterojunction.

[0023] Figure 2 The image shows a Raman spectroscopy diagram of a MoS2-WS2 transverse heterojunction, revealing characteristic peaks of MoS2 in the inner layer and characteristic peaks of WS2 in the outer layer.

[0024] Figure 3 The images show the height of the MoS2-WS2 lateral heterojunction, where (a) is an atomic force microscope (AFM) image and (b) is a height profile along the red dashed line in (a), showing that the heterojunction is a single-layer structure.

[0025] Figure 4 for Mo x W 1-x Optical microscope image of S2.

[0026] Figure 5 for Mo x W 1-x Raman diagram of S2.

[0027] Figure 6 These are surface potential images recorded during alternating switching illumination. Among them, (a) is a single WS2, (b) is a single MoS2, (c) is a single point of WS2 after the formation of the heterojunction, and (d) is a single point of MoS2 after the formation of the heterojunction, respectively, which are surface potential images recorded during alternating switching illumination.

[0028] Figure 7 A comparison of the surface photovoltages of MoS2 and WS2 before and after the formation of the heterojunction. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the embodiments of the present invention will be described in detail and completely below. These embodiments are only used to illustrate a part of the implementation methods of the present invention, and not all of them. All equivalent implementations of the present invention should be included within the scope of the claims of this patent.

[0030] Example 1

[0031] Embodiment 1 of the present invention provides a method for preparing the monolayer MoS2-WS2 transverse heterostructure, which is synthesized in one step by chemical vapor deposition using a three-temperature zone tube furnace.

[0032] Specifically, the preparation method of a monolayer MoS2-WS2 transverse heterostructure in this embodiment includes the following steps:

[0033] Step 1: Mix metals W and Te and grind them thoroughly. Dissolve the mixture in a mixed solvent of ethanol and water (ethanol to water ratio of 2:1), and sonicate thoroughly. Drop the mixture onto a clean SiO2 / Si substrate and then dry it.

[0034] The SiO2 / Si was cut into 1cm×1cm pieces and then washed with acetone, isopropanol and deionized water for 20 minutes each. It was then dried with nitrogen gas for later use.

[0035] The mass fraction ratio of W to Te is 1:10. A solution of W and Te is prepared (1 mg of the mixture of W and Te is dissolved in 10 ml of a mixed solution of ethanol and water). 80 μl of the solution is added dropwise to the SiO2 / Si substrate each time and dried at 80 °C.

[0036] Step 2: Select a 2-inch quartz tube, put 1g of S powder in a quartz boat and place it upstream of the quartz tube, put 50mg of MoO3 in another quartz boat and place it in the middle of the quartz tube, and place the substrate treated in Step 1 on a third quartz boat and place it downstream of the quartz tube.

[0037] In this configuration, MoO3 is located between the sulfur powder and the treated SiO2-Si substrate, with a distance of 20 cm between the MoO3 and the sulfur powder, and a distance of 28 cm between the SiO2 / Si substrate and the MoO3.

[0038] Step 3: Evacuate the tubular furnace, then purge with argon gas for cleaning. Repeat this process three times. Control the argon flow rate at 170 sccm using a flow meter to create three distinct temperature zones within the quartz tube: a high-temperature S powder evaporation zone, a high-temperature MoO3 evaporation zone, and a deposition zone. The carrier gas carries S and MoO3 vapors to the vicinity of the substrate surface to react with the molten W on the substrate (Te acts as a flux to help melt the W). After the reaction is complete, allow the quartz tube to cool naturally before removing the substrate. At this point, a MoS2-WS2 lateral heterojunction is obtained on the SiO2 / Si substrate.

[0039] The heating program for the S powder zone was as follows: heating from room temperature to 50°C in 10 minutes, holding for 40 minutes, then heating to 200°C in 15 minutes, holding for 20 minutes, and then naturally cooling. The heating program for the MoO3 and SiO2 / Si substrate zones was as follows: heating from room temperature to 680°C in 65 minutes, holding for 20 minutes, and then naturally cooling.

[0040] In this embodiment, a MoS2-WS2 transverse heterostructure was obtained. Figure 1 This is an optical microscope image of a MoS2-WS2 lateral heterostructure. The formation of the MoS2-WS2 heterostructure was determined by Raman spectroscopy. Figure 2 Furthermore, the heterojunction of MoS2-WS2 is a single-layer structure with a thickness of approximately 0.8 nm. Figure 3 ).

[0041] Comparative Example 1

[0042] When WO3 is used instead of metallic W in the middle region of the tube furnace, and the SiO2-Si substrate is placed in the downstream region, only a very small amount of MoS2-WS2 lateral heterojunction is formed, with the majority being the more thermodynamically stable Mo. x W 1-X S2 alloy ( Figure 4 ), determined by Raman spectroscopy Figure 3 The homogeneous material in it is Mo x W 1-X S2 alloy ( Figure 5 ).

[0043] Example 2

[0044] Embodiment 2 of the present invention provides a method for preparing a single-layer MoS2-WS2 lateral heterostructure. The preparation process is basically the same as in Embodiment 1, except that in this embodiment, the distance between the SiO2-Si substrate and the MoO3 is 32 cm.

[0045] Experiments have shown that a lateral heterojunction of MoS2-WS2 can also be generated when the distance between the SiO2-Si substrate and the MoO3 is 32cm. However, the yield of the MoS2-WS2 lateral heterojunction is lower because the distance between the substrate and the S source and the MoO3 source is greater.

[0046] Comparative Example 2

[0047] All other conditions were the same as in Example 1. The difference was that the distance between the SiO2 / Si substrate and the MoO3 was 40 cm. At this time, it was found that there were almost no MoS2-WS2 lateral heterojunction samples on the substrate. This indicates that the S source and MoO3 source were too far away from the substrate, making it difficult to deposit MoS2 and difficult to sulfide W on the substrate.

[0048] Comparative Example 3

[0049] All other conditions were the same as in Example 1, except that the distance between the SiO2 / Si substrate and the MoO3 was 20 cm. In this case, almost no MoS2-WS2 lateral heterojunction sample was found on the substrate. Because the substrate was too close to the S powder and MoO3, overgrowth was likely to occur on the substrate, making it difficult to epitaxially grow WS2 outside the monolayer MoS2.

[0050] Example 3

[0051] Embodiment 3 of the present invention provides a method for preparing a single-layer MoS2-WS2 transverse heterostructure. The preparation process is basically the same as in Embodiment 1, except that the flow rate of the inert gas argon is changed to 120 sccm.

[0052] Experiments have shown that MoS2-WS2 transverse heterojunctions can also be generated at a flow rate of 120 sccm.

[0053] Comparative Example 4

[0054] All other conditions were the same as in Example 1, except that the flow rate of the inert argon gas was changed to 50 sccm. At this point, very few MoS2-WS2 lateral heterojunctions were found on the substrate, and most samples were in an amorphous state. This is because the low flow rate resulted in a slow lateral growth rate, leading to fewer samples and their amorphous state.

[0055] Example 4

[0056] Example 4 of this invention provides a method for preparing a single-layer MoS2-WS2 lateral heterostructure. The preparation process is basically the same as in Example 1, except that in this example, 50 μl of a mixed solution of W and Te is dropped onto a SiO2 / Si substrate and dried. Experiments have shown that MoS2-WS2 lateral heterojunctions can also be formed under these conditions.

[0057] Comparative Example 5

[0058] All other conditions were the same as in Example 2, except that 20 μl of a mixed solution of W and Te was dropped onto a SiO2-Si substrate and dried. At this point, very few MoS2-WS2 lateral heterojunctions were found on the substrate, and most samples were in an amorphous state. This is because the W content was very low, and it was difficult for MoS2 to nucleate on the substrate for subsequent lateral epitaxial growth.

[0059] Example 5

[0060] Example 5 of this invention provides a method for preparing a single-layer MoS2-WS2 lateral heterostructure. The preparation process is basically the same as in Example 1, except that the temperature of the sulfur powder is maintained at 160°C. Experiments have shown that a MoS2-WS2 lateral heterostructure can also be formed under these conditions, indicating that sufficient sulfur vapor can reach the vicinity of the substrate to participate in the reaction.

[0061] Comparative Example 6

[0062] All other conditions were the same as in Example 1, except that the temperature of the S powder was maintained at 130°C. At this point, it was found that there were almost no MoS2-WS2 lateral heterojunctions on the substrate, and most samples were quadrilateral. This is because insufficient S source makes it difficult for MoO3 and W to be completely sulfurized.

[0063] Example 6

[0064] Embodiment 6 of this invention provides a method for preparing a single-layer MoS2-WS2 lateral heterostructure. The preparation process is basically the same as in Embodiment 1, except that the temperature of both MoO3 and the substrate is increased to 720°C. Experiments have shown that MoS2-WS2 lateral heterojunctions can also be generated under these conditions.

[0065] Example 7

[0066] Example 7 of this invention provides a method for preparing a single-layer MoS2-WS2 lateral heterostructure. The preparation process is basically the same as in Example 1, except that, according to the heating time in Example 1, the temperature of MoO3 is raised to 680°C and the temperature of the substrate is raised to 700°C within the same time period. Experiments have shown that MoS2-WS2 lateral heterojunctions can also be generated under these conditions.

[0067] Comparative Example 7

[0068] All other conditions were the same as in Example 1, except that the temperature of both MoO3 and the substrate was increased to 630°C. At this point, it was found that there were almost no MoS2-WS2 lateral heterojunctions on the substrate. Because the temperature was too low, it was difficult for a reaction to occur near the substrate and for the sample to be deposited.

[0069] Example 8

[0070] This embodiment 8 provides an application of a single-layer MoS2-WS2 lateral heterojunction, specifically, as follows: Figure 6 and Figure 7 As shown, this heterostructure significantly extends carrier lifetime due to its excellent charge separation performance, and therefore has important application potential in solar cells, photocatalysis and novel optoelectronic devices.

Claims

1. A method for preparing a monolayer MoS2-WS2 transverse heterostructure by in-situ melting, characterized in that, Includes the following steps: Step 1: Mix metals W and Te and grind them thoroughly. Dissolve the mixture in a mixed solvent of ethanol and water. Drop the mixture onto a clean SiO2 / Si substrate and then dry it at 50-80°C for later use. Step 2: Place S powder in the first quartz boat, which is placed upstream of the quartz tube. Place MoO3 in the second quartz boat, which is placed midstream of the quartz tube. Place the substrate treated in Step 1 on the third quartz boat, which is placed downstream of the quartz tube. Step 3: Evacuate the quartz tube, purge with inert gas, evacuate again, and purge with inert gas again. Repeat this process to ensure an inert environment inside the quartz tube. Then, use a flow meter to maintain a constant inert atmosphere flow rate and set different heating programs in different areas to form three different temperature zones in the upper, middle, and lower reaches of the quartz tube: a high-temperature S evaporation zone, a high-temperature MoO3 evaporation zone, and a deposition zone, respectively. The inert carrier gas will carry the vapors of S and MoO3 to the vicinity of the substrate surface to react with the molten W on the substrate. The Te on the substrate acts as a flux to help the metal W melt. After the reaction is complete, the quartz tube is allowed to cool naturally and the substrate is removed. At this point, a MoS2-WS2 lateral heterojunction is obtained on the SiO2-Si substrate.

2. The method for preparing a monolayer MoS2-WS2 transverse heterostructure by in-situ melting according to claim 1, characterized in that, In step 1: After cutting the SiO2-Si substrate into small pieces of 1cm×1cm, it is ultrasonically cleaned with acetone, isopropanol and deionized water for 20-40 minutes in sequence, and then the SiO2 / Si substrate is dried with nitrogen for later use; the ratio of ethanol to water is 1:1~2:

1.

3. The method for preparing a monolayer MoS2-WS2 transverse heterostructure by in-situ melting according to claim 1, characterized in that, In step 1: the mass fraction ratio of W to Te is between 1:8 and 1:12; the concentration of the W and Te mixture in the mixed solvent of ethanol and water is between 0.05 mg / ml and 0.1 mg / ml; and the amount of the mixed solution added to the SiO2 / Si substrate is 50 μl to 100 μl / cm³. 2 SiO2 / Si substrate.

4. The method for preparing a single-layer MoS2-WS2 lateral heterostructure by in-situ melting according to claim 1, characterized in that, In step 2: the relationship between the mass of S powder, the mass of MoO3, and the substrate obtained in step 1 is as follows: per cm 2 The substrate requires 500mg-1.5g of sulfur powder and 25mg-75mg of moO3.

5. The method for preparing a monolayer MoS2-WS2 transverse heterostructure by in-situ melting according to claim 1, characterized in that, In step 2: the distance between the quartz boat containing MoO3 and the quartz boat containing S powder is 18-22cm, and the distance between the quartz boat containing the substrate and the MoO3 is 28-32cm.

6. The method for preparing a monolayer MoS2-WS2 transverse heterostructure by in-situ melting according to claim 1, characterized in that, In step 3: the heating program for the S powder temperature zone is to heat from room temperature to 50-80℃ in 10-15 minutes, hold for 35-40 minutes, then heat to 150-200℃ in 15-20 minutes, hold for 10-20 minutes, and then cool naturally to room temperature; the heating program for the MoO3 and substrate temperature zones is to heat from room temperature to 680-720℃ in 60-75 minutes, hold for 10-20 minutes, and then cool naturally.

7. The method for preparing a monolayer MoS2-WS2 transverse heterostructure by in-situ melting according to claim 1, characterized in that, In step 3: the inert gas introduced is argon or nitrogen, and the flow rate of the inert gas is maintained at 100-200 sccm.

8. A method for preparing a monolayer MoS2-WS2 transverse heterojunction by in-situ melting according to any one of claims 1-7.

9. A monolayer MoS2-WS2 lateral heterojunction prepared by in-situ melting according to any one of claims 1-7 is used in photocatalysis and optoelectronic devices.

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