A growth method for controlling the number of molybdenum disulfide crystal layers and stacking structure based on temperature gradient and its application

Abstract

The present invention discloses a method for growing molybdenum disulfide crystals based on a temperature gradient to control the number of layers and stacking structure, and its application. This method belongs to the technical field of two-dimensional layered materials and optoelectronic materials. Using molybdenum trioxide powder and sulfur powder as precursors and sodium chloride as an auxiliary agent, chemical vapor deposition is performed to control the growth temperature to 700-900°C, yielding molybdenum disulfide crystals with controllable number of layers and stacking structure. The present invention utilizes a temperature gradient to achieve precise control of the number of layers and stacking structure of molybdenum disulfide crystals. The method of the present invention is highly controllable and repeatable, providing an efficient technical means for the preparation of large-area, high-quality molybdenum disulfide materials, and solving the technical problem of uncontrollable number of layers and stacking structure of multilayer materials. The method of the present invention has precise control and is suitable for preparing high-performance optoelectronic materials with a specific number of layers and stacking structure.

Description

Technical Field

[0001] The present invention belongs to the technical field of two-dimensional layered materials and optoelectronic materials, and in particular relates to a growth method and application thereof for controlling the number of molybdenum disulfide crystal layers and stacking structure based on a temperature gradient. Background Art

[0002] In recent years, with the widespread application of two-dimensional transition metal dichalcogenides (TMDCs) in photodetection and electronic devices, the fine-tuning of their layer number and stacking structure has attracted widespread attention. TMDCs are considered to be ideal materials for high-performance photodetectors due to their unique tunable band gap, excellent nonlinear optical properties, and good material compatibility. However, although a variety of methods have been used to synthesize these materials, especially the preparation of high-quality single-layer and multilayer molybdenum disulfide (MoS2) crystals through chemical vapor deposition (CVD) technology, significant challenges still exist in precisely controlling the number of layers and stacking order.

[0003] Existing CVD methods generally find it difficult to simultaneously achieve large-area uniformity, controllable number of layers, and high crystal quality. Many experiments have shown that by adjusting the growth temperature, reaction atmosphere, and substrate material, the number of layers of TMDCs can be controlled within a certain range, but for structures with more than three layers, it is still very difficult to control the stacking order. These challenges mainly stem from the complexity of TMDCs in anisotropic growth and the difficulty in controlling thermodynamic and kinetic processes. During the CVD growth process, factors such as temperature and precursor concentration directly affect the nucleation and growth behavior of the material, making it difficult to simultaneously achieve large-size crystals, ultra-thin thickness, and controllable stacking structures. More importantly, different stacking structures will significantly affect the optoelectronic properties of TMDCs, so precise control of the number of layers and stacking structure is crucial for the design of next-generation high-performance devices.

[0004] Therefore, there is an urgent need to develop a synthesis method that can precisely control the number of TMDCs layers and the stacking order, which can effectively improve their performance in the fields of photoelectric detection and energy conversion. Summary of the Invention

[0005] To address the above technical issues, the present invention proposes a method for growing molybdenum disulfide crystals using a temperature gradient to control the number of layers and stacking structure, and its application. This method is highly controllable and repeatable, providing an efficient technical means for producing large-scale, high-quality molybdenum disulfide materials, addressing the technical challenge of uncontrollable number of layers and stacking structure in multilayer materials. The method offers precise control and is suitable for preparing high-performance optoelectronic materials with a specific number of layers and stacking structure.

[0006] To achieve the above objectives, the present invention provides the following technical solutions:

[0007] One of the technical solutions of the present invention:

[0008] A method for growing molybdenum disulfide crystals with controlled layer number and stacking structure based on temperature gradient is proposed. Molybdenum trioxide (MoO3) powder and sulfur (S) powder are used as precursors, sodium chloride (NaCl) is used as an auxiliary agent, and chemical vapor deposition is performed. The growth temperature is controlled at 700-900°C to obtain MoS2 crystals with controllable layer number and stacking structure.

[0009] Preferably, the mass ratio of the MoO3 powder, the sulfur powder, and the sodium chloride is 1:100:0.1. For example, when the mass of the MoO3 powder is 1.0 mg, the mass of the sulfur powder is 100 mg, and the mass of the NaCl is 0.1 mg.

[0010] Preferably, the chemical vapor deposition step includes: mixing the MoO3 powder and the NaCl and placing them at the heat source center of a tube furnace, placing the S powder upstream of the tube furnace 15 cm away from the heat source center, placing a Si / SiO2 substrate face to face with the MoO3 powder and the NaCl, and placing the Si / SiO2 substrate directly above the MoO3 powder and the NaCl, heating to the growth temperature for crystal growth, and cooling to room temperature after the growth is completed to obtain MoS2 crystals with controllable number of layers and stacking structure.

[0011] Preferably, the distance between the MoO3 powder and the NaCl and the Si / SiO2 substrate is 4 mm.

[0012] Preferably, the Si / SiO2 substrate also includes a pretreatment step before use; the pretreatment method is: cutting wafer-level Si / SiO2 into independent substrates of 1 cm×1 cm in size, ultrasonically cleaning them with ultrapure water, acetone, isopropanol and ethanol respectively, with an ultrasonic power of 40 W and each ultrasonic cleaning for 10 minutes, and finally blowing them dry with nitrogen to serve as the crystal growth surface.

[0013] Preferably, a single-layer MoS2 crystal is obtained at a growth temperature of 700°C, a double-layer MoS2 crystal is obtained at a growth temperature of 720°C, a triple-layer MoS2 crystal is obtained at a growth temperature of 850°C, and a quadruple-layer MoS2 crystal is obtained at a growth temperature of 900°C.

[0014] Preferably, during the chemical vapor deposition, the crystal growth time is controlled to be 5 minutes.

[0015] The second technical solution of the present invention:

[0016] The present invention also provides a MoS2 crystal with controllable layer number and stacking structure obtained by the above growth method.

[0017] Preferably, the thickness of the MoS2 crystal single layer is 0.71 nm, the average thickness of the double layer is 1.26 nm, the average thickness of the triple layer is 1.85 nm, the average thickness of the quadruple layer is 2.29 nm, and the maximum lateral dimension is 210 μm.

[0018] The third technical solution of the present invention:

[0019] The present invention also provides applications of the MoS2 crystal with controllable layer number and stacking structure in the field of optoelectronics. For example, the MoS2 crystal of the present invention can be prepared into a photodetector with a photosensitivity of 2.3×10 3 , the response rate is 3.3×10 4 A.W. -1 , the detection rate can reach 1.7×10 14 Jones. The MoS2 crystal of the present invention can also be prepared into a polarization-sensitive photodetector.

[0020] Compared with the prior art, the present invention has the following advantages and technical effects:

[0021] 1. This invention utilizes a temperature-driven strategy to precisely control the number of MoS2 crystal layers and their stacking structure, successfully achieving multiple stacking configurations from single to four layers, significantly improving the material's growth controllability. Through precise temperature control, the invention achieves high-quality multilayer MoS2 crystals with excellent photoelectric response characteristics.

[0022] 2. The present invention adopts a dual coupling mechanism of temperature gradient and NaCl auxiliary agent to jointly achieve precise control of the number of layers of MoS2 crystals. With temperature gradient as the main method and the addition of appropriate amount of NaCl as the auxiliary method, the optimal growth conditions of MoS2 crystals are determined, providing a basis for the preparation of high-quality, customized two-dimensional materials with different layers.

[0023] 3. The chemical vapor deposition (CVD) method provided by the present invention is efficient and convenient. During the growth process, no major changes to the existing CVD equipment are required. Only the growth temperature needs to be adjusted to achieve the synthesis of high-quality MoS2 crystals with controllable layer numbers on large-area substrates.

[0024] 4. This invention offers high controllability, effectively addressing the instability of MoS2 multilayer stacks. By precisely adjusting the number of layers and stacking sequence, this invention provides a solid technical foundation for the application of stacking-engineered two-dimensional material devices, particularly the development of high-performance photodetectors. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The accompanying drawings, which constitute part of this application, are intended to provide a further understanding of this application. The exemplary embodiments and descriptions of this application are intended to explain this application and do not constitute an improper limitation on this application. In the accompanying drawings:

[0026] Figure 1 Schematic diagram of the positional relationship between the reaction precursor and the substrate in the growth method of MoS2 layers and stacked structure controlled by temperature gradient provided by the present invention;

[0027] Figure 2 Schematic diagram of the crystal growth process of the controllable growth method of MoS2 layers and stacking structure controlled by two-dimensional temperature gradient provided by the present invention;

[0028] Figure 3 A large-scale optical microscope image of the single-layer MoS2 prepared in Example 1 (a), an optical microscope image of a single crystal of the single-layer MoS2 (b), a Raman imaging image of the single-layer MoS2 (c), a large-scale scanning electron microscope image of the single-layer MoS2 (d), and a scanning electron microscope image of a single MoS2 crystal (e), and size statistics of the single-layer MoS2 crystals in the large-scale field of view (f);

[0029] Figure 4 Schematic diagrams of the atomic structures of the two-dimensional double-layer MoS2 prepared in Example 2 (AA and AB correspond to 3R phase and 2H phase, respectively), high-resolution transmission electron microscopy images of the two stacking configurations of the double-layer MoS2 (c, d), high-angle annular dark-field scanning transmission electron microscopy images of the two stacking configurations of the double-layer MoS2 (e, f), and selected area electron diffraction patterns of the two stacking configurations of the double-layer MoS2 (g, h);

[0030] Figure 5 Schematic diagram of four stacking configurations of the two-dimensional three-layer MoS2 prepared in Example 3 (a), optical microscope images of the four stacking configurations (b), second harmonic generation mapping images of the four stacking configurations (c), Raman spectra of the three-layer MoS2 with four different stacking configurations (d), displacement distances of two characteristic peaks in the Raman spectra of different stacking structures extracted from (d) (e), fluorescence spectra of the three-layer MoS2 with four different stacking configurations (f), and displacement distances of two exciton peaks in the fluorescence spectra of different stacking structures extracted from (f) (g);

[0031] Figure 6 Schematic diagram and optical microscope image of eight stacking configurations of two-dimensional four-layer MoS2 prepared in Example 4 (a), Raman spectra of three-layer MoS2 with eight different stacking configurations (b), and fluorescence spectra of three-layer MoS2 with eight different stacking configurations (c);

[0032] Figure 7Stokes and anti-Stokes ultra-low frequency Raman spectra of double-layer MoS2 in AA and AB stacking structures of Example 2, including spectra under unpolarized (un), parallel polarization ( / / ) and perpendicular polarization (⊥) conditions (a, b), the inset shows the vibration schematics of the shear mode (SM, purple area) and the breathing mode (BM, green area) and the positions of their characteristic peaks, the intensity ratio of BM and SM in double-layer MoS2 and the intensity ratio of parallel and perpendicular polarization (c), Example 3 Stokes and anti-Stokes Raman spectra of three-layer MoS2 in four stacking structures, including spectra under unpolarized (un), parallel polarization ( / / ) and perpendicular polarization (⊥) conditions (d), the intensity ratio of parallel polarization ( / / ) and perpendicular polarization (⊥) in the three-layer system (e), the theoretical calculation of the electronic band structure of single-layer to four-layer MoS2 in Examples 1-4 (f);

[0033] Figure 8 The angle-resolved polarization Raman spectrum (a) of the two-dimensional monolayer MoS2 prepared in parallel polarization configuration and the angle-resolved polarization Raman spectrum (b) of the perpendicular polarization configuration, the angle-resolved polarization Raman spectrum (c) of the parallel polarization configuration and the angle-resolved polarization Raman spectrum (d) of the AA(A) stacking configuration in Examples 2-4, the angle-resolved polarization Raman spectrum (e) of the parallel polarization configuration and the angle-resolved polarization Raman spectrum (f) of the AB(B) stacking configuration in Examples 2-4;

[0034] Figure 9 Optical microscope images of MoS2 crystals prepared by adding different amounts of NaCl in Comparative Examples 1 to 5, where "NaCl 1.0 mg" represents Comparative Example 2, "NaCl 0.8 mg" represents Comparative Example 3, "NaCl 0.5 mg" represents Comparative Example 4, "NaCl 0.3 mg" represents Comparative Example 5, and "Without NaCl" represents Comparative Example 1;

[0035] Figure 10 Optical microscope images of MoS2 crystals with different numbers of layers grown at different growth temperatures (700°C, 720°C, 850°C, and 900°C) in Examples 1 to 4;

[0036] Figure 11 This is an optical microscope image of a MoS2 crystal grown at a growth temperature exceeding 900°C in Comparative Example 6;

[0037] Figure 12 IV curves (a), photosensitivity (b), responsivity (c), and detectivity (d) of the photodetector based on a single-layer MoS2 crystal prepared in Example 1 under different wavelengths of light sources;

[0038] Figure 13 IV curves (a), photosensitivity (b), responsivity (c), and detectivity (d) of the photodetector based on a single-layer MoS2 crystal prepared in Example 1 at different light intensities at 405 nm;

[0039] Figure 14 Schematic diagram of the structure of the polarization-sensitive photodetector prepared in Example 1 (a), IT curves of the polarization-dependent photocurrent response of 1-3 layers of MoS2 crystals prepared in Examples 1-3 (b), polarization-dependent photocurrent polar coordinate diagram (c) and characteristic light response time curves in fast scanning mode (d). DETAILED DESCRIPTION

[0040] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0041] It should be understood that the terms described herein are intended only to describe particular embodiments and are not intended to limit the present invention. In addition, for numerical ranges herein, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Each smaller range between any intermediate value within a stated value or stated range and any other stated value or intermediate value within the stated range is also encompassed by the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded within the scope.

[0042] Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art. Although only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein may also be used in the practice or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of any conflict with any incorporated document, the contents of this specification shall prevail.

[0043] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments described herein without departing from the scope or spirit of the invention. Other embodiments will be apparent to those skilled in the art from the description of the invention. The description and examples are intended to be illustrative only.

[0044] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0045] All raw materials used in the examples of the present invention are commercially available.

[0046] An embodiment of the present invention provides a growth method for controlling the number of layers and stacking structure of molybdenum disulfide crystals based on a temperature gradient. MoO3 powder and S powder are used as precursors, NaCl is used as an auxiliary agent, and chemical vapor deposition is performed to control the growth temperature to 700-900°C to obtain MoS2 crystals with controllable number of layers and stacking structure.

[0047] The temperature gradient of the present invention varies from 700 to 900°C. The present invention suppresses the random appearance of multiple layers by precisely controlling a series of temperature gradients, promotes the growth of MoS2 from a single layer to four layers at a specific temperature, and each temperature corresponds to a specific number of layers, with very few other numbers of layers appearing, proving the effectiveness of temperature control over the number of layers, and the stacking structures of these different numbers of layers have also been fully verified. The growth temperature in the present invention affects the nucleation and growth rate of the crystal. Specifically, a higher temperature can provide more energy, promote the diffusion of the material on the substrate, increase the lateral growth rate of the material, and allow multiple single layers to gradually stack to form a multilayer structure. By controlling the growth temperature, controllable preparation of MoS2 with different numbers of layers is achieved. The present invention controls the growth temperature within the above range, which is conducive to obtaining MoS2 crystals with a controllable number of layers and stacking structure; at the same time, at the above temperature, the precursor MoO3 and the auxiliary agent NaCl will both volatilize, and the precursor MoS2 will nucleate and grow epitaxially on the surface of the SiO2 / Si substrate. NaCl, as an auxiliary agent, plays a key role: it generates non-volatile liquid alkali metal molybdate on the substrate, reducing the nucleation density of MoS2, inhibiting the occurrence of multi-layer disordered stacking, and accelerating the lateral growth of MoS2. This method ensures that the number of MoS2 layers can be precisely controlled under different temperature conditions, growing from a single layer to four layers, with very few other layers appearing, verifying the effectiveness of temperature control on the number of layers, and the stacking structures of different layer numbers have also been fully verified.

[0048] In the embodiment of the present invention, MoO3 is in powder form with a purity higher than 99.5%; NaCl is in powder form with a purity higher than 99.9%; S is in powder form with a purity not lower than 99.5%.

[0049] In an embodiment of the present invention, the MoO3 powder and the NaCl are mixed and placed at the heat source center of a tube furnace, the S powder is placed upstream of the tube furnace 15 cm away from the heat source center, the SiO2 surface of the Si / SiO2 substrate is placed face to face with the MoO3 powder and the NaCl, and the Si / SiO2 substrate is placed directly above the MoO3 powder and the NaCl, the temperature is raised to the growth temperature for crystal growth, and after the growth is completed, the temperature is lowered to room temperature. The final crystal is grown on the SiO2 surface, and a MoS2 crystal with controllable number of layers and stacking structure is obtained.

[0050] In an embodiment of the present invention, the distance between the MoO 3 powder and the NaCl and the Si / SiO 2 substrate is 4 mm.

[0051] In an embodiment of the present invention, the Si / SiO2 substrate further includes a pretreatment step before use. The pretreatment method comprises: cutting wafer-scale Si / SiO2 into individual substrates of 1 cm x 1 cm in size, ultrasonically cleaning them in ultrapure water, acetone, isopropyl alcohol, and ethanol at a power of 40W for 10 minutes each, and finally drying them with nitrogen gas to serve as the crystal growth surface. The substrate has excellent thermal stability and can withstand the temperature fluctuations during the high-temperature CVD process, enabling the stable growth of MoS2 layers with varying numbers of layers.

[0052] In an embodiment of the present invention, the Si / SiO2 substrate is in the form of a sheet, and the number is one sheet.

[0053] In an embodiment of the present invention, a single-layer MoS2 crystal is obtained at a growth temperature of 700°C, a double-layer MoS2 crystal is obtained at a growth temperature of 720°C, a triple-layer MoS2 crystal is obtained at a growth temperature of 850°C, and a quadruple-layer MoS2 crystal is obtained at a growth temperature of 900°C.

[0054] In an embodiment of the present invention, during the chemical vapor deposition, the crystal growth time is controlled to be 5 minutes.

[0055] In an embodiment of the present invention, the heating rate to the growth temperature is 20° C. / min.

[0056] In an embodiment of the present invention, the cooling rate to room temperature is 12° C. / min.

[0057] In an embodiment of the present invention, vapor deposition uses Ar gas as a carrier gas for transporting gas; the flow rate of the Ar gas is 50-70 sccm.

[0058] In an embodiment of the present invention, the thinnest thickness of the MoS2 crystal with controllable two-dimensional layer number and stacking structure is 0.71nm for a single layer, 1.26nm for a double layer, 1.85nm for a triple layer, and 2.29nm for a quadruple layer, with a maximum lateral dimension of 210μm. The MoS2 crystal with controllable two-dimensional layer number and stacking structure provided by the present invention has ultra-high quality and large lateral dimensions.

[0059] The technical solution of the present invention is further illustrated by the following examples.

[0060] Example 1

[0061] S1. Cut wafer-scale Si / SiO2 into independent substrates of 1 cm × 1 cm in size. The Si / SiO2 substrates are ultrasonically cleaned in ultrapure water, acetone, isopropanol, and ethanol, respectively, with an ultrasonic power of 40 W and each ultrasonication for 10 minutes. Finally, they are blown dry with a nitrogen gun for later use.

[0062] S2, using the SiO2 / Si with a size of 1 cm×1 cm obtained by the above process as a growth substrate;

[0063] S3, MoO3 powder with a purity higher than 99.95% and S powder with a purity not lower than 99.95% as precursors, and NaCl powder with a purity higher than 99.99% as an auxiliary agent;

[0064] S4. Weigh 1.0 mg of MoO3 powder and 0.1 mg of NaCl powder, and weigh 100 mg of S powder, and place them at the center and upstream of the heat source of the tube furnace, respectively. At the same time, place the SiO2 / Si substrate prepared in step S2 4 mm above the MoO3 powder and NaCl powder.

[0065] S5. Heat the tube furnace at a heating rate of 20°C / min from room temperature to 700°C, using Ar as the carrier gas at a flow rate of 50 sccm. After the temperature reaches 700°C, use a magnet to move the ceramic boat containing the S powder closer to the center of the heating source, ultimately maintaining a distance of 4 cm from the MoO3 powder and NaCl powder.

[0066] S6. Maintain the temperature at 700°C for 5 minutes for crystal growth, and then cool the system temperature to room temperature at a cooling rate of 12°C / min in an Ar atmosphere to obtain a single-layer MoS2 sample.

[0067] Figure 1 A schematic diagram illustrating the positional relationship between precursors and auxiliary agents in the controllable growth method for MoS2 layers and stacked structures using a temperature gradient, as provided by the present invention. Yellow represents S powder, purple represents MoO3 powder, blue represents NaCl powder, and pink represents the Si / SiO2 substrate. It can be seen that the MoO3 and NaCl powders are located at the heat source center of the tube furnace, the S powder is located upstream of the tube furnace, and the Si / SiO2 substrate is located directly above the MoO3 and NaCl powders.

[0068] Figure 2 A schematic diagram of the crystal growth process for the controllable growth method of MoS2 layers and stacking structures using a temperature gradient provided by the present invention. It can be seen that the number of MoS2 layers can be controlled by controlling a series of temperature gradients from low to high. Specifically, higher temperatures provide more energy, promote diffusion of the material on the substrate, and increase the lateral growth rate of the material, allowing multiple single layers to gradually stack to form a multilayer structure.

[0069] Figure 3 The following are a large-scale optical microscope image of the single-layer MoS2 prepared in Example 1 (a), an optical microscope image of a single crystal of the single-layer MoS2 (b), a Raman imaging image of the single-layer MoS2 (c), a large-scale scanning electron microscope image of the single-layer MoS2 (d), and a scanning electron microscope image of a single MoS2 crystal (e). The size statistics of the single-layer MoS2 crystals in the large-scale field of view (f) are performed. It can be seen that the single-layer distribution in the large area is indeed a single layer, and the Raman spectrum can also verify its single layer and characteristics and high quality. Figure 3 In images (a) and (d), uniformly bright small triangles are observed across a large area of ​​the substrate, confirming their uniform thickness. Magnified optical and scanning electron microscopy images show consistent and uniform thickness of the individual triangles. Furthermore, the triangular morphology demonstrates their most stable morphology, confirming the successful fabrication of MoS2 crystals with controllable layer number and stacking structure.

[0070] Example 2

[0071] The difference from Example 1 is that in step S5, the temperature is increased to 720° C. and maintained at 720° C. for 5 minutes to perform double-layer crystal growth.

[0072] Figure 4 Schematic diagrams of the atomic structures of the two stacking configurations of the two-dimensional double-layer MoS2 prepared in Example 2 (AA and AB correspond to 3R phase and 2H phase, respectively) (a, b), high-resolution transmission electron microscopy images of the two stacking configurations of the double-layer MoS2 (c, d), high-angle annular dark-field scanning transmission electron microscopy images of the two stacking configurations of the double-layer MoS2 (e, f), and selected area electron diffraction patterns of the two stacking configurations of the double-layer MoS2 (g, h); It can be seen that the two stacking configurations of the double-layer MoS2 are fully demonstrated under high-resolution and atomic-level characterization, which also verifies the high-quality single crystal properties of the double-layer MoS2.

[0073] Example 3

[0074] The difference from Example 1 is that in step S5, the temperature is increased to 850° C. and maintained at 850° C. for 5 minutes to perform three-layer crystal growth.

[0075] Figure 5Schematic diagram of the four stacking configurations of the two-dimensional three-layer MoS2 prepared in Example 3 (a), optical microscope images of the four stacking configurations (b), second harmonic generation mapping images of the four stacking configurations (c), Raman spectra of the three-layer MoS2 with four different stacking configurations (d), displacement distances of two characteristic peaks in the Raman spectra of different stacking structures extracted from (d) (e), fluorescence spectra of the three-layer MoS2 with four different stacking configurations (f), displacement distances of two exciton peaks in the fluorescence spectra of different stacking structures extracted from (f) (g); It can be seen that the four stacking configurations of the three-layer MoS2 are fully demonstrated under the characterization of Raman spectroscopy, fluorescence spectroscopy and second harmonic generation, and the intrinsic differences of the four stacking configurations are also verified by the displacement difference of the Raman characteristic peak and the difference between the two exciton peaks in the fluorescence spectrum.

[0076] Example 4

[0077] The difference from Example 1 is that in step S5, the temperature is raised to 900° C. and maintained at 900° C. for 5 minutes to perform four-layer crystal growth.

[0078] Figure 6 Schematic diagram and optical microscope image (a) of the eight stacking configurations of the two-dimensional four-layer MoS2 prepared in Example 4, Raman spectra of the eight different stacking configurations of the four-layer MoS2 (b), and fluorescence spectra of the eight different stacking configurations of the four-layer MoS2 (c). It can be seen that the eight stacking configurations of the four-layer MoS2 are fully confirmed by the characterization of Raman and fluorescence spectra, and the intrinsic differences of the eight stacking configurations are also verified by the difference in the displacement of the Raman characteristic peak and the difference between the two exciton peaks in the fluorescence spectrum.

[0079] Figure 7 The Stokes and Anti-Stokes ultra-low frequency Raman spectra of double-layer MoS2 in AA and AB stacking structures of Example 2, including spectra under unpolarized (un), parallel polarization ( / / ) and perpendicular polarization (⊥) conditions (a, b), the illustrations show the vibration diagrams of shear mode (SM, purple area) and breathing mode (BM, green area) and the positions of their characteristic peaks, the intensity ratio of BM and SM in double-layer MoS2 and the intensity ratio of parallel and perpendicular polarization (c), the Stokes and Anti-Stokes Raman spectra of triple-layer MoS2 in four stacking structures of Example 3, including spectra under unpolarized (un), parallel polarization ( / / ) and perpendicular polarization (⊥) conditions (d), the intensity ratio of parallel polarization ( / / ) and perpendicular polarization (⊥) in the triple-layer system (e), the theoretical calculation of the electronic band structure of single-layer to quadruple MoS2 of Examples 1-4 (f). We used ultra-low frequency Raman spectroscopy analysis to study the double-layer (Figure ab) and triple-layer ( Figure 7d) Characteristic peaks of the breathing mode (BM) and shear mode (SM) of the system. Ultra-low frequency Raman spectroscopy has been widely used to identify the crystalline phase or stacking order of few-layer TMDCs. In addition, through parallel and perpendicular polarization measurements, the unique characteristics of the shear mode and breathing mode were successfully identified, which helps to accurately determine the subtle changes in the stacking order of different layers. Figure 7 The intensity ratio of BM to SM (expressed as I B / I S ) can be effectively used as a parameter to distinguish the bilayer regions in two bilayer samples. B / I S ~0.4 is identified as AB superposition, and I B / I S ~0.85 is identified as AA, ( Figure 7 The pink coordinate axis in c indicates that the interlayer coupling is stronger than that of AB. However, in the three-layer system, we found that SM and BM almost merged into one peak in the ultra-low frequency Raman spectrum, which indicates that their respective intensities are not as obvious as those observed in the two-layer system. Therefore, I B / I S The ratio cannot be used directly to identify the stacking structure of the three layers. In our method to quantitatively evaluate the difference between the two modes, we used polarization technology. This method helps to clearly distinguish between SM and the parallel to the sample plane (I / / ) and vertical (I ⊥ ) features of the BM. In this case, I / / is the sum of the parallel components of SM and BM. On the contrary, I ⊥ It consists only of the vertical component of the SM, which results in the BM being completely ineffective in the vertically polarized configuration. Therefore, for the three-layer system, the I B / I S The ratio is converted to I / / I ⊥ Than, now I in the three layers B / I S Refers to the strength ratio between the bottom two layers of a three-layer structure. / / I ⊥ (AB stacking is about 2.5, AA stacking is about 4, Figure 7 The purple coordinate axis of c) and the I of three-layer MoS2 / / I ⊥ (ABA is about 3, AAB and ABB are about 6, AAA is close to 9, Figure 7 The purple coordinate axis of e) and the interlayer BM and SM intensity ratio ( Figure 7 The comparative analysis of the pink coordinate axis of e in Figure c and e is shown in Figures c and e. From the perspective of the three layers, Figure 7 E in, I in AAA / / I⊥ The intensity ratio is the highest, followed by AAB and ABB, and ABA is the lowest. This clearly shows that the intensity ratio depends on the stacking structure of double-layer and triple-layer MoS2, providing a valuable tool for determining the stacking order of few-layer MoS2. In general, in ultra-low frequency Raman spectroscopy, whether using I B / I S Or I / / I ⊥ The information of its stacking structure and strength changes can be obtained, but I / / I ⊥ The factors taken into consideration are more comprehensive, and the changing trends are relatively consistent.

[0080] Figure 8 The angle-resolved polarization Raman spectrum (a) of the two-dimensional monolayer MoS2 in parallel polarization configuration and the angle-resolved polarization Raman spectrum (b) in perpendicular polarization configuration prepared in Example 1, the angle-resolved polarization Raman spectrum (c) of the parallel polarization configuration and the angle-resolved polarization Raman spectrum (d) in perpendicular polarization configuration of the AA(A) stacking configuration in Examples 2-4, and the angle-resolved polarization Raman spectrum (e) of the parallel polarization configuration and the angle-resolved polarization Raman spectrum (f) in perpendicular polarization configuration of the AB(B) stacking configuration in Examples 2-4.

[0081] Comparative Example 1

[0082] Same as Example 1, except that no NaCl powder is added to S4.

[0083] Comparative Example 2

[0084] Same as Example 1, except that the amount of NaCl powder added in S4 is 1.0 mg.

[0085] Comparative Example 3

[0086] Same as Example 1, except that the amount of NaCl powder added in S4 is 0.8 mg.

[0087] Comparative Example 4

[0088] Same as Example 1, except that the amount of NaCl powder added in S4 is 0.5 mg.

[0089] Comparative Example 5

[0090] Same as Example 1, except that the amount of NaCl powder added in S4 is 0.3 mg.

[0091] Figure 9The optical microscope images of MoS2 crystals prepared by adding different amounts of NaCl in Comparative Examples 1 to 5 are shown. When no NaCl is added (ie, Comparative Example 1), MoS2 with regular morphology cannot be grown, or even cannot be grown at all; when 1.0 mg of NaCl is added (ie, Comparative Example 2), it will fuse and deteriorate the growth environment at high temperature, and cause a violent reaction and no MoS2 crystals can be obtained; when 0.8 mg of NaCl is added (ie, Comparative Example 3), the auxiliary agent can quickly lower the melting point of MoO3, but the growth environment is still deteriorated, resulting in only the generation of nucleation points of MoS2 crystals, which limits their lateral growth; when 0.5 mg of NaCl is added, the MoS2 crystals can be grown at high temperatures, and the growth environment can be deteriorated. When NaCl is added (i.e., Comparative Example 4), MoS2 crystals can be grown rapidly, but the growth rate cannot be controlled, resulting in irregular morphology; when 0.3 mg NaCl is added (i.e., Comparative Example 5), MoS2 crystals can be grown rapidly and the morphology is regular triangles, but the layer distribution cannot be accurately controlled, and it can be seen that the thickness is uneven, which is suitable for multi-layer growth; these data show that in order to ensure that the content of NaCl auxiliary agent is neither too little nor too much, only by selecting an accurate amount (0.1 mg) can high-quality MoS2 crystals with regular morphology and controllable number of layers be grown (Note: In order to illustrate the accuracy of this phenomenon, at least three batches were used to prove the repeatability and reliability of the experiment).

[0092] Figure 10 These are optical microscope images of MoS2 crystals with different numbers of layers grown at different growth temperatures (700°C, 720°C, 850°C, and 900°C) in Examples 1-4. It can be seen that adjusting the growth temperature under appropriate NaCl auxiliary agent content can more accurately achieve precise control of the number of layers from a single layer to four layers. This once again verifies the dual coupling effect of the temperature gradient and the auxiliary agent (Note: In order to illustrate the accuracy of this phenomenon, at least three batches were used to demonstrate the repeatability and reliability of the experiment. A single layer could be reproduced in the experiment, but the size of the single-layer triangles was not fixed. Some triangles were large, and some were small. Therefore, different magnifications were selected to more clearly see the single-layer triangles).

[0093] Comparative Example 6

[0094] The difference from Example 1 is that in step S5, the temperature is increased to 930° C. and maintained at 930° C. for 5 minutes to allow crystal growth.

[0095] Figure 11 This optical microscope image of a sample grown at a growth temperature exceeding 900°C in Comparative Example 6 shows that the number of layers exceeds five and even exceeds 900°C, demonstrating the limitations of using a temperature gradient to grow crystals with higher numbers of layers, or the need for more precise control of the temperature gradient range. Significant control of the number of layers is only achieved within the 700-900°C range, further demonstrating the crucial role of the temperature gradient.

[0096] Application Example 1

[0097] The single-layer MoS2 crystal prepared in Example 1 was made into a photodetector. The specific preparation process was as follows: the device was prepared based on the single-layer MoS2 crystal synthesized in Example 1, and the electrode formed by thermally evaporating Au with a thickness of 150 nm using a mask plate was transferred to the MoS2 crystal as the source electrode and the drain electrode by means of a transfer electrode. Four different wavelengths of light (including 365 nm, 405 nm, 450 nm and 515 nm) were used as light sources, as well as a light source with a fixed wavelength and different illumination power to obtain a photodetector.

[0098] The electrical and photoelectric tests of the single-layer MoS2 photodetector prepared in Example 1 were carried out using a probe station. The results are shown in Figure 12 and Figure 13 . Figure 12 IV curves (a), photosensitivity (b), responsivity (c) and detection rate (d) of the photodetector based on single-layer MoS2 crystal prepared in Example 1 under light sources of different wavelengths; comparing the dark current and the photocurrent response when applying different wavelengths, the light response after adding light shows a trend of first increasing and then decreasing as the wavelength increases from 365nm to 515nm, and the maximum corresponding wavelength is at 405nm. It can be clearly seen that the single-layer MoS2 crystal prepared by this method has the highest responsivity to 405nm.

[0099] Figure 13 IV curves (a), photosensitivity (b), responsivity (c), and detectivity (d) of the photodetector based on a single-layer MoS2 crystal prepared in Example 1 at 405 nm under different light intensities; Figure 13 Under 405nm light, the optical power is 4.54μW·cm -2 to 129.8 μW·cm -2 The power dependence of the photodetector was studied in the range of . The results showed that the photocurrent increased significantly with the increase of light intensity. In addition, the photosensitivity of the photodetector prepared in Example 1 was 2.3×10 3 , the response rate is 3.3×10 4 A.W. -1 , the detection rate can reach 1.7×10 14 Jones. In summary, the above device indicators all show that this photodetector outperforms most photodetectors based on two-dimensional materials. In particular, it has ultra-high photoresponse characteristics among molybdenum disulfide-based phototransistors, which is the highest value reported in this field.

[0100] Application Example 2

[0101] The two-dimensional 1-3 layer MoS2 crystals prepared in Examples 1-3 are made into polarization-sensitive photodetectors. The structure of the photodetector is shown in FIG. Figure 14 In a, the specific preparation process is: based on the two-dimensional 1-3 layer MoS2 crystal synthesized in Examples 1-3, the electrode formed by thermally evaporating Au with a thickness of 150nm using a mask plate is transferred to the MoS2 crystal as the source electrode and the drain electrode by means of a transfer electrode, light with a wavelength of 405nm is used as the light source, and a half-wave plate is placed above the light source to achieve a change in the direction of the light source, thereby obtaining a polarization-sensitive photodetector.

[0102] The polarization-sensitive photodetectors prepared in Examples 1-3 were tested electrically and photoelectrically using a probe station. The direction of the laser beam was changed by rotating the half-wave plate. The results are shown in Table 1. Figure 14 . Figure 14 Figure a is a schematic diagram of the structure of the polarization-sensitive photodetector prepared in Example 1, b is the IT curve of the polarization-dependent photocurrent response of 1-3 layers of MoS2 crystal prepared in Examples 1-3, c is the polarization-dependent photocurrent polar coordinate diagram, and d is the characteristic light response time curve in fast scanning mode. Figure 14 As can be seen from part b, when the direction of the laser beam changes periodically over time, a periodically changing photocurrent will be generated. Figure 14 Extract the data from part b of the figure and draw a polar coordinate diagram of the polarization-dependent photocurrent, as shown in Figure 14 As shown in part c. Figure 14 Part c shows that the photocurrent changes periodically with the rotation angle, reaching maximum values ​​at 90° and 270° and minimum values ​​at 0° and 180°. The anisotropy ratios of the photocurrents obtained from 1-3 layers of MoS2 crystals were calculated to be 1.36, 1.44, and 1.52, respectively. The above results show that the two MoS2 crystals prepared in Example 1 have strong in-plane photoelectric anisotropy, providing an ideal platform for multifunctional optoelectronic devices.

[0103] The above are merely preferred embodiments of the present application, but the scope of protection of the present application is not limited thereto. Any changes or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in this application should be included in the scope of protection of the present application. Therefore, the scope of protection of the present application should be based on the scope of protection of the claims.

Claims

1. A method for growing molybdenum disulfide crystals by controlling the number of layers and stacking structure based on a temperature gradient, characterized in that: The following steps are involved: S1. Cut wafer-scale Si / SiO2 into independent substrates of 1 cm × 1 cm in size. The Si / SiO2 substrates are ultrasonically cleaned in ultrapure water, acetone, isopropanol, and ethanol, respectively, with an ultrasonic power of 40 W and 10 min each time. Finally, they are blown dry with a nitrogen gun for later use. S2, using the Si / SiO2 with a size of 1 cm × 1 cm obtained by the above process as a growth substrate; S3, MoO3 powder with a purity higher than 99.95% and S powder with a purity not lower than 99.95% as precursors, and NaCl powder with a purity higher than 99.99% as an auxiliary agent; S4. Weigh 1.0 mg of MoO3 powder and 0.1 mg of NaCl powder, and also weigh 100 mg of S powder, and place them at the center and upstream of the heat source of the tube furnace, respectively. At the same time, place the SiO2 / Si substrate prepared in step S2 4 mm above the MoO3 powder and NaCl powder. S5. Heat the tube furnace at a heating rate of 20°C / min from room temperature to 700°C, using Ar as the carrier gas at a flow rate of 50 sccm. After the temperature reaches 700°C, use a magnet to move the ceramic boat containing the S powder closer to the center of the heating source, ultimately maintaining a distance of 4 cm from the MoO3 powder and NaCl powder. S6, maintaining the temperature at 700-900° C. for 5 min for crystal growth, and then cooling the system temperature to room temperature at a cooling rate of 12° C. / min in an Ar atmosphere to obtain a molybdenum disulfide crystal; A single-layer MoS2 crystal was obtained at a growth temperature of 700°C, a double-layer MoS2 crystal was obtained at a growth temperature of 720°C, a triple-layer MoS2 crystal was obtained at a growth temperature of 850°C, and a quadruple-layer MoS2 crystal was obtained at a growth temperature of 900°C. The average thickness of a single molybdenum disulfide crystal is 0.71 nm, the average thickness of a double layer is 1.26 nm, the average thickness of a triple layer is 1.85 nm, and the average thickness of a quadruple layer is 2.29 nm. The maximum lateral size is 210 μm.

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