A method for growing two-dimensional tungsten selenide

Abstract

This invention belongs to the field of materials technology, specifically relating to a method for growing two-dimensional tungsten selenide. Using tungsten selenide as a target material, the method involves heating; when heated to a target temperature, a protective gas is introduced to perform chemical vapor deposition, depositing tungsten selenide onto a substrate. After growth, the material is rapidly cooled to prevent etching of the synthesized tungsten selenide, resulting in two-dimensional tungsten selenide. During the heating process, a protective gas is introduced from the substrate to the target material. The two-dimensional tungsten selenide of this application can achieve different crystal morphologies by controlling the temperature, and the size of the bottom layer of tungsten selenide is relatively good. The tungsten selenide crystals of this application exhibit good uniformity, with significant differences in coupling between layers. There is a uniform strain distribution and interlayer coupling between the two tungsten selenide layers, resulting in high overall quality of the two-dimensional tungsten selenide.

Description

Technical Field

[0001] This invention belongs to the field of materials technology, specifically relating to a method for growing two-dimensional tungsten selenide. Background Technology

[0002] Transition metal chalcogenides (TMDCs) are a new class of two-dimensional layered semiconductor materials, typically expressed by the chemical formula MX2 (M for transition metal and X for chalcogenide), possessing many unique electronic and optical properties. Studies have shown that TMDCs exhibit layer-number dependent physical properties that differ significantly from bulk materials. For example, the transition from monolayer MX2 to bulk MX2 is accompanied by a decrease in band gap and a shift in band structure from a direct band gap semiconductor to an indirect band gap semiconductor. The differences in inversion symmetry between monolayers and multilayers, and between odd-numbered and even-numbered layers, can be distinguished using second harmonic spectroscopy. Besides the changes in properties caused by layer number differences, the stacking configuration between atomic layers also significantly influences the band structure and electronic properties of two-dimensional semiconductor materials. For instance, graphene with an ABA stack exhibits zero-bandgap half-metallic properties, while graphene with an ABC stack is a semiconductor with a gate-tunable bandgap. Recent research indicates that bilayer TMDCs with an AA stack exhibit interfacial ferroelectricity. Furthermore, by controlling the degree of freedom of interlayer rotation, people can significantly influence interlayer electron interactions and the moiré periodic potential field, enabling two-dimensional material systems to exhibit more novel physical properties, such as the Hofstadter butterfly energy spectrum in graphene and hexagonal boron nitride moiré superlattices, as well as the superconducting properties of bilayer and trilayer magic-angle graphene.

[0003] Currently, mechanical exfoliation is the main method for obtaining high-quality two-dimensional single-crystal materials. However, controlling the acquisition of large-area single crystals and two-dimensional materials with specific stacking patterns through mechanical exfoliation remains a significant challenge. Chemical vapor deposition (CVD), on the other hand, is a bottom-up controllable preparation method. By controlling growth conditions such as temperature, gas flow, substrate, and precursor, it is possible to effectively regulate the number of layers, rotation angles, and stacking patterns of the prepared two-dimensional materials. Traditional CVD growth methods tend to randomly introduce nucleation sites during the preparation of TMDCs, leading to the formation of polycrystalline domains and hindering the epitaxial growth of the second crystal layer. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a method for growing two-dimensional tungsten selenide, thereby improving the growth quality of tungsten selenide and obtaining the crystal morphology of two-dimensional tungsten selenide in a controllable manner.

[0005] This invention provides a method for growing two-dimensional tungsten selenide, comprising the following steps: using tungsten selenide as a target material and heating it; when the target temperature is reached, a protective gas is introduced to perform chemical vapor deposition to deposit tungsten selenide on a substrate; after growth is completed, the temperature is rapidly reduced to prevent the synthesized tungsten selenide from being etched, thereby obtaining two-dimensional tungsten selenide; during the heating process, a protective gas is introduced from the substrate to the target material.

[0006] In one embodiment of this application, the tungsten selenide target material generally has a high purity of 99.999%. During preparation, it is typically placed in a quartz boat, positioned at the center of a quartz tube. The substrate is preferably a SiO2 / Si substrate, which exhibits strong interaction with WSe2 and has a flat surface and high thermal stability. The substrate is preferably placed 19-25 cm away from the target material to ensure a suitable substrate temperature, thereby obtaining bilayer tungsten selenide with different stacking configurations. The protective gas is preferably argon.

[0007] In the embodiments of this application, the preparation of two-dimensional tungsten selenide is generally divided into four stages: rinsing, heating, growth, and cooling.

[0008] Before heating begins, a rinsing stage is performed by introducing a protective gas to clean the gas path. The protective gas flows from the substrate towards the target. The flow rate of the protective gas is 500 sccm, and the time can be controlled as needed, such as 10 minutes, to thoroughly remove any impurities that may be present in the gas path. During this stage, the gas flow is from the substrate to the target, thereby eliminating the influence of WSe2 vapor nucleation on the substrate during the heating stage and improving the quality of the prepared single crystal. This gas flow direction is a counter-current flow.

[0009] After rinsing, the heating stage begins. The heating program is activated, maintaining the airflow direction (i.e., reverse airflow) and adjusting its magnitude, for example, to 80-120 sccm, preferably 100 sccm. After a certain time, such as 60 minutes, the temperature inside the tube furnace rises uniformly to 1150℃.

[0010] After heating to the target temperature, preferably 1150℃, the direction of the protective gas is adjusted, that is, the protective gas is swept from the target material to the substrate for chemical vapor deposition. The gas pressure is standard atmospheric pressure, and the gas flow rate of the protective gas is 80-120 sccm, preferably 100 sccm. The forward gas flow of the protective gas carries the gaseous molecules formed by WSe2 powder at high temperature to the substrate, where nucleation begins and film is gradually formed. The ultra-high growth temperature and large gas flow rate make the entire growth process last for a short time, such as 3 minutes, to obtain a large WSe2 single crystal.

[0011] After the final growth is complete, the flow of the protective gas is reversed to initiate the cooling stage. To achieve rapid cooling, it is preferable to partially remove the substrate from the device, typically the quartz tube, to expose it to the outside environment, usually air, for rapid cooling. In this invention, the reverse airflow and rapid cooling during the cooling stage prevent thermal etching and improve single-crystal quality.

[0012] The beneficial effect of this invention is that the inventors discovered that suppressing the nucleation sites and density during the temperature-changing stage is key to the controllable preparation of high-quality bilayer and specific stacking configuration TMDCs materials. The inventors have controllably prepared bilayer WSe2 single crystals with both AA and AB stacking configurations (2L-AA, 2L-AB) using a reverse-flow chemical vapor deposition method. During growth, high-purity WSe2 powder is used as a precursor to ensure the stoichiometry and crystal quality of the synthesized WSe2 single crystals. Before reaching the WSe2 growth temperature, a reverse gas flow from the substrate towards the WSe2 source is introduced into a quartz tube to suppress random nucleation during the heating stage, promoting uniform growth of the second layer of WSe2 on the first single crystal. By adjusting the substrate temperature during the growth stage, bilayer WSe2 single crystals with AA and AB stacking configurations can be selectively synthesized. AA-stacked WSe2 tends to be synthesized at higher temperatures (around 950°C), while AB-stacked WSe2 tends to be synthesized at lower temperatures (around 840°C).

[0013] Through a series of experimental characterization and testing analyses, including optical microscopy, atomic force microscopy (AFM), Raman spectroscopy, photoluminescence spectroscopy (PL), and second harmonic generation (SHG), the differences in crystal structure and physical properties between 2L-AA and 2L-AB WSe2 were investigated. This enabled the atomic-level property control of large-size, uniform, and high-quality bilayer WSe2 single crystals. This not only provides a stable and effective route for the controllable chemical vapor phase synthesis of large-size, high-quality transition metal chalcogenide single crystals, but also contributes to the advancement of atomic-level property control of two-dimensional semiconductor materials and the construction of van der Waals heterostructures.

[0014] This invention provides a simple, efficient, and universal chemical vapor deposition method for the controllable preparation of bilayer WSe2 single crystals with AA and AB stacking configurations. Large-size, uniform bilayer WSe2 single crystals with different stacking configurations were successfully synthesized controllably. The introduction of a reverse gas flow suppressed random nucleation behavior during WSe2 growth, and WSe2 with different energies in AA and AB stacking configurations was synthesized separately by strictly controlling the growth temperature. The crystal structure and physical properties of the bilayer WSe2 single crystals were characterized and analyzed using a series of experiments including optical microscopy, atomic force microscopy, Raman spectroscopy, and photoluminescence spectroscopy. Second harmonic distortion (HDC) measurements confirmed that the AA and AB stacking configurations of the bilayer WSe2 single crystals possess different crystal structural symmetries. Experimental results show that the reverse gas flow chemical vapor deposition method is not only an effective method for the controllable preparation of bilayer transition metal chalcogenide (WSe2) compounds with different stacking configurations, but also lays the foundation for the atomic layer property control of two-dimensional semiconductor materials and the construction of van der Waals heterostructures.

[0015] The two-dimensional tungsten selenide of this application can be obtained in different crystal forms by controlling the temperature, and the size of the bottom layer of tungsten selenide is better; the tungsten selenide crystals of this application have good uniformity and crystallinity. The overall quality of the two-dimensional tungsten selenide is high. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the tube furnace structure and airflow for WSe2 single crystal growth according to an embodiment of the present invention.

[0017] Figure 2 This is a flowchart illustrating the temperature and airflow control procedure according to an embodiment of the present invention.

[0018] Figure 3 Optical microscope images at different temperatures.

[0019] Among them, (a) is an optical microscope image of WSe2 of 2L-AA grown at 950℃ with a scale bar of 50μm; (b) is an optical microscope image of WSe2 of 2L-AB grown at 840℃ with a scale bar of 50μm.

[0020] Figure 4 Statistical results for monolayer and bilayer WSe2 grown at different substrate temperatures.

[0021] In the images, (a), (c), and (e) are optical microscope images of WSe2 grown at 780℃, 840℃, and 950℃, respectively; (b), (d), and (f) are statistical proportions of different WSe2 crystals grown at different temperatures, with a scale bar of 50 μm.

[0022] Figure 5 The results show the CVD growth of WSe2 monolayer and bilayer at different substrate temperatures.

[0023] (a) and (c) are optical microscope images; (b) and (d) are atomic force microscope images.

[0024] Figure 6 SHG characterization of WSe2.

[0025] Among them, (a) and (b) are side views of the crystal structures of 2L-AB and 2L-AA WSe2, respectively. The small spheres at the top and bottom represent Se atoms, and the large sphere in the middle represents W atoms; (c) is the SHG diagram of 1L, 2L-AA and 2L-AB WSe2.

[0026] (d) and (e) are top views of the crystal structures of 2L-AB and 2L-AA WSe2, respectively.

[0027] Figure 7 The Raman and PL spectra of WSe2 with ab stacking configuration are shown.

[0028] Among them, (a) is the WSe2 Raman spectrum at a single-layer position; (b) is the Raman spectrum at a double-layer position; (c) is the WSe2PL spectrum at a single-layer position; and (d) is the WSe2PL spectrum at a double-layer position.

[0029] Figure 8 The Raman and PL spectra of WSe2 in a AA stacked configuration are shown.

[0030] Among them, (a) is the WSe2 Raman spectrum at a single-layer position; (b) is the WSe2 Raman spectrum at a double-layer position; (c) is the WSe2PL spectrum at a single-layer position; and (d) is the WSe2PL spectrum at a double-layer position.

[0031] Figure 9 This is a scan of a two-layer WSe2.

[0032] Among them, (a) is an optical microscope image of WSe2 with AA stacking; (b) is a PL intensity scan of WSe2 with AA stacking; and (c) is a Raman spectral density image of WSe2 with AA stacking. 2g 1 (d) Peak intensity scan image; (e) Optical microscope image of WSe2 with AB stacking; (f) PL intensity scan image of WSe2 with AB stacking; (c) Raman spectroscopy image of WSe2 with AB stacking. 2g 1 Peak intensity scan.

[0033] Figure 10 Optical microscope images of WSe2 growth results under different airflow strategies.

[0034] Among them, (a) is an optical microscope image of the WSe2 growth result without using the reverse airflow strategy; (b) is an optical microscope image of the WSe2 growth result using the reverse airflow strategy.

[0035] Figure 11 Optical microscope images of WSe2 growth results under different cooling strategies.

[0036] Among them, (a) shows the WSe2 growth result without using the substrate rapid cooling strategy, which shows etching behavior; (b) shows the WSe2 growth result using the substrate rapid cooling strategy. Detailed Implementation

[0037] Example 1

[0038] A method for growing two-dimensional tungsten selenide, employing, as follows Figure 1 The process was carried out in a tube furnace structure, using WSe2 powder (99.999% purity) as the reaction source. During preparation, 1g of WSe2 powder was placed in a quartz boat at the center of a quartz tube. Simultaneously, the substrate was placed approximately 19-25cm downstream within a quartz tube on the furnace wall to ensure a suitable substrate temperature. This is crucial for preparing bilayer WSe2 single crystals with different stacking configurations. Here, a pre-cut 1cm x 2cm SiO2 / Si substrate was selected, as it exhibits strong interaction with WSe2, has a flat surface, and high thermal stability.

[0039] Figure 2 This is a schematic diagram of the WSe2 growth process, which is divided into four stages: rinsing, heating, growth, and cooling.

[0040] During the rinsing stage, after placing the WSe2 powder and substrate, valves 1 and 4 are closed, and valves 2 and 3 are opened. A 500 sccm Ar rinsing gas is introduced into the rinsing gas path for 10 minutes to thoroughly remove any impurity gases that may be present in the gas path. At this time, the gas flow is from the substrate to the source, eliminating the influence of WSe2 vapor nucleation on the substrate during the heating stage and improving the quality of the prepared single crystal. This gas flow direction is the reverse gas flow.

[0041] After rinsing, the heating stage begins. The heating program is activated, and the airflow direction is kept constant, with its size adjusted to 100 sccm. After 60 minutes, the temperature inside the tube furnace rises uniformly to 1150℃.

[0042] With valves 2 and 3 closed and valves 1 and 4 opened, the gas flow shifts from the source to the substrate, initiating the growth stage. The Ar gas forward flow carries gaseous molecules formed from the WSe2 powder at high temperature to the substrate, where nuclei begin to form and gradually solidify. The ultra-high growth temperature and high gas flow rate allow for the acquisition of a large WSe2 single crystal within 3 minutes of the entire growth process.

[0043] The airflow is then reversed to enter the cooling stage. At this time, a portion of the quartz tube is quickly extracted to ensure that the quartz tube containing the substrate is exposed to the air, so as to achieve rapid cooling.

[0044] Example 2

[0045] By changing the substrate placement position—from the inner edge of the furnace cavity (approximately 950°C) to the center of the furnace cavity wall (approximately 840°C), and finally to the outer edge of the furnace cavity (approximately 780°C)—and thus altering the substrate temperature, monolayer and bilayer WSe2 single crystals with different stacking arrangements can be selectively fabricated. 2L-AAWSe2 tends to form on substrates at 950°C, while 2L-AB forms on substrates at 840°C. Monolayers (1L) are predominantly found on substrates at even lower temperatures, around 780°C. Figure 3 (a) and Figure 3 (b) Optical microscope images of bilayer WSe2 with typical AA stacking (0° twist angle) and AB stacking (60° twist angle) are shown respectively. The size of the first layer of bilayer WSe2 in both stacking methods exceeds 100 μm. This shows that the present application can control the temperature to change the stacking method and the size.

[0046] Example 3

[0047] To quantitatively analyze the effect of growth temperature Figure 4 The synthesis ratios of 1L, 2L-AA, and 2L-AB WSe2 crystals at different substrate temperatures were summarized. Maintaining a growth time of 3 min, a WSe2 source temperature of 1150 °C, and a gas flow rate of 100 sccm, the synthesis ratios at a substrate temperature of 780 °C were summarized. Figure 4 a and Figure 4 As shown in b, 95% of the generated crystals are monolayer WSe2, and the amount of WSe2 in other layers is negligible. When the substrate temperature rises to 840℃, as... Figure 4 c and Figure 4 As shown in diagram d, the proportion of monolayer WSe2 decreases significantly, accounting for only 6% of the single crystal. Bilayer WSe2 dominates, with 2L-AA WSe2 accounting for 48% and 2L-AB WSe2 accounting for 43%. A very small amount of multilayer WSe2 also appears. When the substrate temperature is further increased to 950℃, as... Figure 4 e and Figure 4 As shown in f, the proportion of 2L-AA WSe2 single crystals increases to 93%, and the double-layer WSe2 with AB stacking is rare, with almost no single-layer WSe2 single crystals visible.

[0048] Example 4

[0049] The size of bilayer WSe2 does not increase indefinitely with prolonged growth time. Increasing the growth time cannot increase the size of the first or second layer of WSe2 single crystals indefinitely. The nucleation density will continuously increase with time, eventually forming WSe2 single crystals with twin interfaces and uneven thickness. Experiments have verified that under these conditions, 3 minutes is the optimal growth time, producing bilayer WSe2 with the first layer typically ranging from 80-120 μm in size, and the second layer size controllable between 10-20 μm. Figure 5 a and Figure 5 As shown in Figure c, an optical microscope clearly reveals the distinct boundary between the first and second layers of 2L-AA and 2L-AB WSe2. Both WSe2 single crystals with different stacking methods exhibit very smooth surfaces, with no obvious surface defects observed, confirming the uniformity of the prepared WSe2 single crystals. Figure 5 b and Figure 5 Figure d shows an AFM scan with a triple height comparison of the substrate, the first WSe2 single crystal layer, and the second WSe2 single crystal layer. By measuring the height at the step, the step thickness between the second WSe2 layer and the first WSe2 layer, as well as between the first layer and the substrate, can be clearly seen, showing a uniform and flat surface morphology.

[0050] Example 5

[0051] To further explore the differences in crystal structure between 2L-AA and 2L-AB WSe2, we used second harmonic generation to characterize monolayer and bilayer single crystals with two different stacking schemes. Because SHG is highly sensitive to the inversion symmetry of the crystal, the inversion symmetry is broken from the bulk WSe2 to the center of the monolayer WSe2, thus exhibiting a very strong SHG signal, such as... Figure 6 As shown in c. For 2L-ABWSe2 single crystal, the in-plane W-Se bond directions are opposite in the top and bottom layers, as shown in the cross-sectional diagram. Figure 6 As shown in Figure a, its top view is as follows: Figure 6 As shown in diagram d, this structure is centrosymmetric, thus the SHG signal is suppressed. In contrast, the W-Se bonds in the 2L-AA WSe2 single crystal have the same orientation within the plane, as shown in the cross-sectional diagram. Figure 6 As shown in b, the top view is as follows: Figure 6 As shown in e, it does not possess central inversion symmetry and exhibits a very strong SHG signal.

[0052] Example 6

[0053] We characterized the optical properties of the WSe2 bilayer film using Raman and PL spectroscopy. Figure 7 c. Figure 8 c and Figure 7 d、 Figure 8The PL spectra of WSe2 shown in d exhibit dominant peaks at 755 nm for monolayer, 775 nm for AB-stacking bilayer, and 775 nm for AA-stacking bilayer. Figure 7 a, Figure 8 a and Figure 7 b、 Figure 8 b shows the Raman spectra of WSe2 at monolayer and bilayer sites of 2L-AB and 2L-AA WSe2. 2g 1 The peak is at 250cm -1 B represents the out-of-plane vibration of WSe2. 2g The characteristic peak is at 309 cm⁻¹ -1 .

[0054] Figure 9 b and Figure 9 The PL intensity scans of different WSe2 bilayers in e show a clear boundary between the bright monolayer and the darker bilayer. Figure 9 c and Figure 9 f of E 2g 1 The Raman intensity scans are very uniform, indicating that 2L-AB and 2L-AA WSe2 have high crystallinity and crystal quality. Their contrast matches that of the optical images.

[0055] Example 7

[0056] The growth results of WSe2 were obtained by employing different airflow strategies, such as Figure 10 As shown, (a) represents the WSe2 growth result without using the reverse airflow strategy. The entire WSe2 growth process, namely the four stages of rinsing, heating, growth, and cooling, uses forward airflow, and is otherwise the same as in Example 1; (b) represents the WSe2 growth result in Example 1 using the reverse airflow strategy. It can be seen that the WSe2 growth result of this application is significantly better than the WSe2 growth result without using the reverse airflow strategy.

[0057] The growth results of WSe2 were obtained by employing different cooling strategies, such as Figure 11 As shown, (a) represents the WSe2 growth result without using a rapid substrate cooling strategy, exhibiting etching behavior; that is, during the cooling stage of WSe2 growth, the heat source is turned off, but the quartz tube is not removed, otherwise it is the same as in Example 1; (b) represents the WSe2 growth result of Example 1 using a rapid cooling strategy. It can be seen that the WSe2 growth result of this application is significantly better than the WSe2 growth result without using a rapid cooling strategy.

[0058] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of protection of this application is limited to these examples; within the framework of this application, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of different aspects of one or more embodiments of this application as described above, which are not provided in detail for the sake of brevity.

[0059] One or more embodiments in this application are intended to cover all such substitutions, modifications, and variations that fall within the broad scope of this application. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of one or more embodiments in this application should be included within the protection scope of this application.

Claims

1. A method for growing two-dimensional tungsten selenide, characterized in that, Using tungsten selenide as the target material, heating is performed. When the target temperature is reached, a protective gas is introduced to perform chemical vapor deposition, depositing tungsten selenide onto the substrate. The growth time is 3 minutes. After growth, the temperature is rapidly reduced to prevent the synthesized tungsten selenide from being etched, resulting in two-dimensional tungsten selenide. The method of rapidly reducing the temperature to prevent tungsten selenide deposition is to expose the device containing the substrate to the outside environment for rapid cooling. During the heating process, a protective gas is introduced from the substrate to the target material at a flow rate of 80-120 sccm. The substrate is a SiO2 / Si substrate; The target temperature is 1150℃, and the gas pressure for chemical vapor deposition is standard atmospheric pressure. During chemical vapor deposition, a protective gas is blown from the target to the substrate at a flow rate of 80-120 sccm. Both tungsten selenide and the substrate are placed in a tube furnace for chemical vapor deposition. The tungsten selenide is placed in the middle of the tube furnace, and the substrate is 19-25 cm away from the tungsten selenide.

2. The method for growing two-dimensional tungsten selenide as described in claim 1, characterized in that, The protective gas is argon.

3. The method for growing two-dimensional tungsten selenide as described in claim 1, characterized in that, Before heating begins, a protective gas is introduced to purge the gas path. The protective gas flows from the substrate towards the target.

4. The method for growing two-dimensional tungsten selenide as described in claim 3, characterized in that, Before heating begins, a protective gas is introduced to purge the gas path at a flow rate of 500 sccm.

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