Method for controllably preparing star-shaped molybdenum disulfide monocrystal and application thereof

Abstract

The present application belongs to the technical field of two-dimensional material preparation and device preparation, and more particularly to a method for preparing star-shaped molybdenum disulfide monocrystals with controllable morphology and application thereof. The present application provides a method for preparing star-shaped monolayer molybdenum disulfide with controllable morphology based on oxygen plasma pretreatment and chemical vapor deposition. By introducing a novel oxygen plasma assisted technology, the growth substrate is pretreated by oxygen plasma, the growth environment of TMDs is improved, and the chemical vapor deposition is coordinated to realize large-scale preparation of high-quality star-shaped molybdenum disulfide monocrystals. The defects of low pattern regulation precision, poor repeatability and insufficient hetero-integration capability in the prior art are solved, and a reliable material basis is provided for its application in high-speed and high-precision electronic devices.

Description

Technical Field

[0001] This invention belongs to the field of two-dimensional material preparation and device fabrication technology, and more specifically relates to a method for preparing star-shaped molybdenum disulfide single crystals with controllable morphology and its application. Background Technology

[0002] Transition metal dichalcogenides (TMDs) are a class of two-dimensional (2D) materials with van der Waals (vdW) layered structures, attracting widespread attention due to their excellent optical, electronic, and mechanical properties. Represented by MoS2, these materials exhibit unique physical properties in the monolayer limit, such as direct band gaps, high surface area-to-volume ratios, and tunable electronic structures, showing great application potential in electronic devices, catalysis, and energy fields.

[0003] Patterning of TMDs has a significant impact on their physical and chemical properties. For example, due to the self-confined nature of crystal growth, the external morphology of a crystal is closely related to its internal crystal structure; therefore, the crystal phase of a material can be inferred from its pattern characteristics. In recent years, researchers have achieved phase control of TMDs through patterning. For instance, Han et al. used a temperature-mediated method to control the phase of Ta... 1+x The crystal phase of S2 was found to contain the T phase Ta. 1+x S2 exhibits a hexagonal morphology, while H phase Ta 1+x S2 exhibits a triangular shape. Nong et al. synthesized a customized 1T' phase MoS2, which displayed an elongated trapezoidal structure, in stark contrast to the triangular H phase MoS2. Furthermore, Yang et al. proposed a "substrate step template growth method," successfully preparing one-dimensional TMDs nanoribbons by controlling the chalcogenide / metal source ratio and utilizing the inductive effect of the step edges, providing a new approach for the high-yield and precise synthesis of specific edge types.

[0004] Despite some progress in TMD pattern control, existing technologies still have the following problems and limitations:

[0005] 1. Complex growth mechanism: The pattern formation of TMDs involves nucleation and different growth rates of different crystal planes, making it difficult to accurately predict and control the final morphology.

[0006] 2. High environmental sensitivity: The growth process of TMDs is highly sensitive to conditions such as temperature, pressure, precursor concentration and reaction time. Small changes may lead to significant morphological differences.

[0007] 3. The gap between theory and practice: Existing theoretical models are difficult to fully guide actual synthesis, resulting in insufficient precision and repeatability of pattern control.

[0008] 4. Functional application is limited: existing methods still face challenges in realizing the hetero-integration of TMDs (such as customizing the number of layers, crystal phase, symmetry and twist angle), which limits its application in high-performance devices.

[0009] Therefore, developing a simple, controllable and efficient TMDs pattern regulation method to realize the precise control of its morphology, crystal phase and function has become a key problem to be solved in this field. SUMMARY

[0010] The purpose of the present application is to provide a method for preparing star-shaped molybdenum disulfide monocrystals with controllable morphology and its application, and specifically to provide a method for preparing star-shaped monolayer molybdenum disulfide with controllable morphology based on oxygen plasma pretreatment and chemical vapor deposition and its application. By introducing a new type of oxygen plasma assisted technology, the growth substrate is pretreated by oxygen plasma, the growth environment of TMDs is improved, and chemical vapor deposition is used to realize large-scale preparation of high-quality star-shaped molybdenum disulfide monocrystals, so as to solve the defects of low pattern regulation precision, poor repeatability and insufficient hetero-integration capability in the above-mentioned prior art, and provide a reliable material basis for its application in high-speed and high-precision electronic devices.

[0011] To achieve the above-mentioned purpose, the present application provides the following scheme:

[0012] One of the technical schemes of the present application: a method for preparing star-shaped monolayer molybdenum disulfide with controllable morphology based on oxygen plasma pretreatment and chemical vapor deposition is provided, and the steps include:

[0013] The molybdenum source powder and the sulfur source powder are used as precursors, sodium chloride (NaCl) or potassium chloride (KCl) is used as an auxiliary agent, the Si / SiO2 substrate treated by oxygen plasma is used as a growth substrate, and an inert gas is used as a carrier gas. After chemical vapor deposition, the star-shaped monolayer molybdenum disulfide is prepared;

[0014] The molybdenum source powder and the auxiliary agent are mixed and placed opposite to the Si / SiO2 substrate in the chemical vapor deposition;

[0015] The molybdenum source powder includes molybdenum dioxide (MoO2) powder and / or molybdenum trioxide (MoO3) powder, and is preferably molybdenum trioxide (MoO3) powder;

[0016] The sulfur source powder is sulfur (S) powder.

[0017] The present application uses a porcelain boat as a container for containing precursors and a growth substrate. The mixture of molybdenum source powder and auxiliary agent is scattered at the center of the porcelain boat, and a Si / SiO2 substrate is placed upside down above it, with the SiO2 surface facing the mixture. The width of the Si / SiO2 substrate is about 1 cm, and the widest part of the porcelain boat is greater than 1 cm, so the Si / SiO2 substrate can be clamped in the porcelain boat.

[0018] Without the oxygen plasma, the attachment barrier of sulfur atoms at the edge of the formed molybdenum sulfide crystal domain is higher than the diffusion barrier of sulfur atoms on the substrate, at this time the growth is dominated by the attachment of sulfur atoms at the edge, and the product formed according to the growth mechanism is triangular; however, after the oxygen plasma is applied, the substrate surface is rich in oxygen plasma, the attachment barrier of sulfur atoms at the edge of the formed molybdenum sulfide crystal domain is greatly reduced, and is less than the diffusion barrier of sulfur atoms on the substrate, so at this time the growth is dominated by the diffusion of sulfur atoms, and the product formed according to this growth mechanism is star-shaped (polygonal).

[0019] The auxiliary agent (preferably sodium chloride) can reduce the melting point of the molybdenum source powder precursor.

[0020] Further, the mass ratio of the molybdenum source powder, the sulfur source powder and the auxiliary agent is 1:100:0.1.

[0021] Further, the power of the oxygen plasma treatment is 80W, and the time is 5-10min.

[0022] Optionally, the power of the oxygen plasma treatment is 80W, and the time is 5min, to obtain a hexagonal star-shaped molybdenum disulfide single crystal.

[0023] Optionally, the power of the oxygen plasma treatment is 80W, and the time is 10min, to obtain a fractal multi-branch star-shaped molybdenum disulfide single crystal.

[0024] Further, the inert gas is argon, and the flow rate is 30-60sccm.

[0025] Further, the growth temperature of the chemical vapor deposition is 750℃, and the time is 10min.

[0026] Further, the distance between the mixed molybdenum source powder and auxiliary agent and the Si / SiO2 substrate opposite to each other is 0.5cm.

[0027] A temperature higher than the growth temperature will cause cracks, and a temperature lower than the growth temperature will cause incomplete or no volatilization of the molybdenum oxide precursor, so that the target product cannot be obtained; too close distance will cause the precursor to be deposited on the growth substrate surface too early and in large quantities, so that there are a large number of unreacted substances; too far distance will cause the precursor to be sulfidized in the sublimation path and leave the reaction chamber under the action of the gas flow, so that the two-dimensional material product cannot be obtained on the growth substrate surface; too long time will cause etching, and too short time will cause insufficient growth time, so that a large-area product cannot be obtained, only a small-area product can be obtained.

[0028] Further, the step of chemical vapor deposition comprises:

[0029] After mixing the molybdenum source powder and the auxiliary agent, the mixture is placed in the center of the heat source of a tube furnace, the sulfur source powder is placed upstream of the heat source in the tube furnace, and the Si / SiO2 substrate is placed directly above the mixture of the molybdenum source powder and the auxiliary agent, and then the temperature is raised to the growth temperature for crystal growth, and after the growth is completed, the temperature is lowered to room temperature.

[0030] Further, the Si / SiO2 substrate further includes a cleaning step before being subjected to oxygen plasma treatment.

[0031] Optionally, the cleaning step includes: sequentially subjecting the Si / SiO2 substrate to ultrasonic cleaning with ultrapure water, acetone and isopropanol, the ultrasonic power being 40 W, and each ultrasonic cleaning being performed for 10 min, and finally blowing dry with nitrogen.

[0032] The second technical scheme of the present application provides a star-shaped monolayer molybdenum disulfide prepared by the above method.

[0033] The third technical scheme of the present application provides an application of the star-shaped monolayer molybdenum disulfide in the field of optoelectronics.

[0034] The star-shaped molybdenum disulfide monocrystal prepared by the present application exhibits an in-plane anisotropic atomic structure, in which the lines connecting two opposite outer vertices are arranged along the armchair-shaped crystal direction, and the lines connecting two opposite inner vertices are arranged along the zigzag-shaped direction. Based on these unique properties, the present application is applied in the field of optoelectronics, such as an angle-resolved anisotropic transistor based on the star-shaped molybdenum disulfide monocrystal. These devices switch between a high-resistance path and a low-resistance path. The low-resistance path exhibits an electron mobility as high as 77.6 cm 2 V -1 s -1 , while the electron mobility of the high-resistance path is relatively low, being at most 58 cm 2 V -1 s -1 . The anisotropic transistor in the present application exhibits an anisotropy ratio as high as 1.87.

[0035] The present application discloses the following technical effects:

[0036] The present application provides a simple and efficient preparation method, which can synthesize high-quality, pattern-controllable molybdenum disulfide monocrystals on a substrate in high yield. The method does not need to make substantial changes to the existing chemical vapor deposition device, but only needs to optimize the oxygen plasma treatment conditions and growth parameters, so as to realize the controllable preparation of high-quality star-shaped molybdenum disulfide monocrystals, significantly reducing the preparation cost and technical threshold. Not only does the present application overcome the limitations of traditional morphology control preparation methods, but also opens up a new way for exploring the unique properties of the in-plane armchair-shaped and zigzag-shaped structures and their applications in advanced devices.

[0037] The application adopts an oxygen plasma assisted chemical vapor deposition strategy, and successfully prepares star-shaped MoS2 single crystals with high quality and high purity.

[0038] The application significantly improves the performance of anisotropic transistors prepared when applied to the field of optoelectronics, by virtue of the unique star-shaped MoS2 single crystal structure prepared. 2 V -1 s -1 and 58 cm 2 V -1 s -1 ) and a significant anisotropy ratio (1.87), and realizes the construction of a high-performance angle-resolved anisotropic transistor. BRIEF DESCRIPTION OF DRAWINGS

[0039] The accompanying drawings, which form a part of the present application, are intended to provide further understanding of the present application and are incorporated herein for explanation by way of the illustrative embodiments of the present application. The present application is not limited by the accompanying drawings.

[0040] Figure 1 The device for preparing MoS2 single crystals and the product morphology under different conditions of Example 1-2 and Comparative Example 1 are shown in the figure.

[0041] Figure 2 The microscope photos of MoS2 single crystals prepared in Comparative Example 1 and Example 1-2 are shown in the figure, wherein a is the microscope photo of Comparative Example 1, b is the microscope photo of Example 1, and c is the microscope photo of Example 2.

[0042] Figure 3 The size statistics of MoS2 single crystals prepared in Comparative Example 1 and Example 1-2 are shown in the figure, wherein a is the size statistics of Comparative Example 1, b is the size statistics of Example 1, and c is the size statistics of Example 2.

[0043] Figure 4 The quality characterization of hexagonal star-shaped MoS2 single crystals prepared in Example 1 is shown in the figure, wherein a is arranged from top to bottom in Example 1-2 and Comparative Example 1, b is a Raman mapping, c is an atomic force microscope photo, d and e are angle-resolved polarized Raman spectra of hexagonal star-shaped MoS2 single crystals when a polarizer is vertically placed, f and g are angle-resolved polarized Raman spectra of hexagonal star-shaped MoS2 single crystals when a polarizer is horizontally placed, h is a second harmonic response graph of hexagonal star-shaped MoS2 single crystals in a wavelength range of 810 nm to 990 nm, and i is XPS of hexagonal star-shaped MoS2 single crystals.

[0044] Figure 5Figure 1 is a schematic diagram of a hexastar-shaped MoS2 single crystal, wherein (a) is a transmission electron microscopy (TEM) characterization morphology diagram of the hexastar-shaped MoS2 single crystal prepared in Example 1; (b) is a selected area electron diffraction (SAED) diagram obtained from positions I-VI in (a); (c) is a spherical aberration-corrected transmission electron microscopy (STEM) data of the hexastar-shaped MoS2 single crystal prepared in Example 1; (d) is an analysis result of the lattice spacing in the a and b axial directions in (c); (e) is a structure diagram of three edge atoms (①, ② and ③) in (a); (f) is a complete atomic structure of the entire single crystal inferred from the atomic structure inside the hexastar-shaped MoS2 single crystal and the atomic structure of the edge; and (g) is a high-resolution lattice fringe of the hexastar-shaped MoS2 single crystal.

[0045] Figure 6 Figure 2 is a simulation settlement result of the morphology regulation of Example 1, wherein a is the energy of an oxygen atom existing between a silicon substrate and a monolayer molybdenum disulfide, b is the energy of an oxygen atom existing at a position far from a monolayer molybdenum disulfide domain and on a silicon substrate, c is the energy of an oxygen atom existing above a monolayer molybdenum disulfide domain, d is a schematic diagram of the state of a sulfur atom in a growth process, and e is a potential barrier of processes I, II and III in d.

[0046] Figure 7 Figure 3 is a schematic diagram of an angular resolved anisotropic transistor based on a hexastar-shaped MoS2 single crystal, wherein a is a schematic diagram of an angular resolved anisotropic transistor, b is a microscope diagram of the angular resolved transistor, c is a high-resistance path and a low-resistance path in the angular resolved transistor, d is a transfer curve along a chiral armchair path, e is a transfer curve along a chiral zigzag path, f is an output curve along the chiral armchair path, g is an output curve along the chiral zigzag path, h is a mobility statistic under different paths and different source-drain voltages, i is a switching ratio statistic under different paths and different source-drain voltages, and j is an anisotropy ratio.

[0047] Figure 8 Figure 4 is a schematic diagram of a square wave generator, wherein a is a schematic diagram of a square wave generator, b is a working schematic diagram of the square wave generator, c is a high-resistance path state, d is a state of switching between a high-resistance path and a low-resistance path, e is a low-resistance path state, f is different operation modes of the square wave generator under different source-drain voltages, gate voltages and rotation speeds, g is an anisotropy ratio under different source-drain voltages and gate voltages, and h is an energy consumption under different source-drain voltages and gate voltages.

[0048] Figure 9 Figure 5 is a morphology characterization diagram of products prepared in Comparative Examples 2-5, wherein a is Comparative Example 2, b is Comparative Example 3, c is Comparative Example 4, and d is Comparative Example 5.

[0049] Figure 10 Figure 6 is a microscope photograph of a product prepared in Comparative Example 6.

[0050] Figure 11The morphology and size characterization chart of the product prepared for Comparative Example 7-10 is shown in the figure, wherein a is Comparative Example 7, b is Comparative Example 8, c is Comparative Example 9, and d is Comparative Example 10. DETAILED DESCRIPTION

[0051] The detailed description set forth below of certain exemplary implementations of the application describes and discloses only the specific ways in which the application can be practiced. This description is not intended to limit the scope or application of the application in any way, but to provide specific implementations thereof.

[0052] It should be understood that the terms used herein are for the purpose of describing particular embodiments and are not intended to limit the present application. In addition, for numerical ranges recited in the present application, it is contemplated that each intervening value, to the upper and lower limits of the ranges is also specifically disclosed. Each smaller range that falls within the integer ranges is also specifically disclosed. The upper and lower limits of these smaller ranges can independently be included or excluded in the range.

[0053] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. Although preferred methods and materials are described, any methods and materials similar or equivalent to those described herein can be used in the practice of the present application. All documents mentioned herein are incorporated by reference to disclose and describe in detail the methods and / or materials that are related to the present application. In the case of conflict between the content of the specification and that of any document incorporated herein by reference, the content of the specification controls.

[0054] Many modifications and variations of this application can be made in the light of the above teachings without departing from the spirit and scope thereof, and it is to be understood that all such modifications and variations that fall within the scope of the claims are to be included herein. Other implementations of the application will be apparent to those skilled in the art from consideration of the specification and practice of the application disclosed herein. The specification and examples given are exemplary only.

[0055] As used herein, the terms "comprises", "comprising", "includes", "including", "has", "having", "contains", "containing", or variations thereof, are intended to be open-ended terms that mean including, but not limited to.

[0056] It should be noted that the present application does not describe in detail the conventional operation means in the art, and is not the focus of the present application.

[0057] Example 1

[0058] Preparation of hexagonal star-shaped MoS2 single crystal:

[0059] S1, substrate treatment: N-type doped wafer level Si / SiO2 was cut into 1 cm x 1 cm size independent substrate, respectively through ultrasonic cleaning of ultrapure water, acetone, isopropanol and ethanol, ultrasonic power is 40 W, ultrasonic 10 min each time, dry with nitrogen, then 80 w, 5 min of oxygen plasma pretreatment, as growth substrate;

[0060] S2, CVD growth: Molybdenum trioxide powder (1.5 mg) and sodium chloride (0.15 mg) were mixed and placed in the center of the furnace tube heat source opposite to the growth substrate, with a distance of 0.5 cm, sulfur powder (150 mg) was placed 15 cm upstream of the center of the furnace tube heat source, Ar as carrier gas, flow rate 30 sccm, heating rate 30 ℃ / min to 750 ℃, holding for 10 minutes, hexagonal star-shaped MoS2 single crystal was grown on the growth substrate.

[0061] Example 2

[0062] Preparation of fractal multi-branch star-shaped MoS2 single crystal:

[0063] S1, substrate treatment: N-type doped wafer level Si / SiO2 was cut into 1 cm x 1 cm size independent substrate, respectively through ultrasonic cleaning of ultrapure water, acetone, isopropanol and ethanol, ultrasonic power is 40 W, ultrasonic 10 min each time, dry with nitrogen, then 80 w, 10 min of oxygen plasma pretreatment, as growth substrate;

[0064] S2, CVD growth: Molybdenum trioxide powder (1.5 mg) and sodium chloride (0.15 mg) were mixed and placed in the center of the furnace tube heat source opposite to the growth substrate, with a distance of 0.5 cm, sulfur powder (150 mg) was placed 15 cm upstream of the center of the furnace tube heat source, Ar as carrier gas, flow rate 30 sccm, heating rate 30 ℃ / min to 750 ℃, holding for 10 minutes, fractal multi-branch star-shaped MoS2 single crystal was grown on the growth substrate.

[0065] Comparative Example 1

[0066] Preparation of triangular MoS2 single crystal:

[0067] S1, substrate treatment: N-type doped wafer level Si / SiO2 was cut into 1 cm x 1 cm size independent substrate, respectively through ultrasonic cleaning of ultrapure water, acetone, isopropanol and ethanol, ultrasonic power is 40 W, ultrasonic 10 min each time, dry with nitrogen, as growth substrate;

[0068] S2, CVD growth: MoO3 powder (1.5 mg) and NaCl (0.15 mg) were mixed and placed in the center of the furnace tube heat source, and the growth substrate was placed 0.5 cm away from the center of the furnace tube heat source. Sulfur powder (150 mg) was placed 15 cm upstream of the center of the furnace tube heat source. Ar was used as the carrier gas at a flow rate of 30 seem. The temperature was raised at a rate of 30°C / min to 750°C and maintained for 10 min. A MoS2 single crystal in the shape of a triangle was grown on the growth substrate.

[0069] Comparative Example 2

[0070] Compared with Example 1, the only difference is that the CVD growth temperature is 1000°C.

[0071] Comparative Example 3

[0072] Compared with Example 1, the only difference is that the CVD growth time is 15 min.

[0073] Comparative Example 4

[0074] Compared with Example 1, the only difference is that the Ar flow rate is 80 seem.

[0075] Comparative Example 5

[0076] Compared with Example 1, the only difference is that the Ar flow rate is 20 seem.

[0077] Comparative Example 6

[0078] Compared with Example 1, the only difference is that the growth substrate is placed 15 cm downstream of the center of the heat source, specifically:

[0079] S1, Substrate treatment: N-type doped wafer-level Si / SiO2 was cut into individual substrates with a size of 1 cm x 1 cm. The individual substrates were subjected to ultrasonic cleaning with ultrapure water, acetone, isopropyl alcohol, and ethanol, respectively, at an ultrasonic power of 40 W for 10 min each time. Nitrogen was used for drying. Subsequently, an oxygen plasma pretreatment was performed at 80 W for 5 min, and the pretreated substrate was used as the growth substrate.

[0080] S2, CVD growth: MoO3 powder (1.5 mg) and NaCl (0.15 mg) were mixed and placed in the center of the furnace tube heat source. Sulfur powder (150 mg) was placed 15 cm upstream of the center of the furnace tube heat source. The growth substrate was placed 15 cm downstream of the center of the furnace tube heat source. Ar was used as the carrier gas at a flow rate of 30 seem. The temperature was raised at a rate of 30°C / min to 750°C and maintained for 10 min. A MoS2 single crystal was grown on the growth substrate.

[0081] Comparative Example 7

[0082] Compared with Example 1, the only difference is that the CVD growth temperature is 550°C.

[0083] Comparative Example 8

[0084] The only difference from Example 1 is that the CVD growth time is 5 minutes.

[0085] Comparative Example 9

[0086] Compared with Example 1, the only difference is that the molybdenum trioxide powder and sodium chloride were mixed and placed at a distance of 1.2 cm from the center of the furnace tube heat source relative to the growth substrate.

[0087] Comparative Example 10

[0088] Compared with Example 1, the only difference is that the molybdenum trioxide powder and sodium chloride were mixed and placed at a distance of 0.2 cm from the center of the furnace tube heat source relative to the growth substrate.

[0089] Test case

[0090] Figure 1 The diagram shows the apparatus used to prepare MoS2 single crystals in Examples 1-2 and Comparative Example 1, and the morphology of the products under different conditions.

[0091] Figure 2 Microscopic images of MoS2 single crystals prepared in Comparative Example 1 and Examples 1-2 are shown, where a is a microscopic image of Comparative Example 1, b is a microscopic image of Example 1, and c is a microscopic image of Example 2.

[0092] Figure 3 The figures show the size statistics of the MoS2 single crystals prepared in Comparative Example 1 and Examples 1-2, where a is the size statistics of Comparative Example 1, b is the size statistics of Example 1, and c is the size statistics of Example 2.

[0093] Depend on Figures 2-3 It can be seen that the morphology of MoS2 single crystals changed significantly with the addition and continuous increase of oxygen plasma. When the substrate was not modified with oxygen plasma, the obtained MoS2 single crystals had a triangular morphology, while after the addition of oxygen plasma modification, the obtained MoS2 single crystals had a star-shaped morphology. Further increasing the oxygen plasma modification time, the obtained MoS2 single crystals tended to be polygonal, with many more angles growing on the basis of the star shape. In addition, the addition of oxygen also caused a significant change in the size of the product, and the presence of oxygen caused the MoS2 single crystals to gradually increase in size.

[0094] Figure 4The image shows the quality characterization of the hexagonal star-shaped MoS2 single crystal prepared in Example 1. From top to bottom, a represents the Raman spectra of Examples 1-2 and Comparative Example 1; b is a Raman surface scan; c is an atomic force microscope image; d and e are the angle-resolved polarized Raman spectra of the hexagonal star-shaped MoS2 single crystal when the analyzer is placed vertically; f and g are the angle-resolved polarized Raman spectra of the hexagonal star-shaped MoS2 single crystal when the analyzer is placed horizontally; h is the second harmonic response diagram of the hexagonal star-shaped MoS2 single crystal in the wavelength range of 810 nm to 990 nm; and i is the XPS of the hexagonal star-shaped MoS2 single crystal.

[0095] Depend on Figure 4 The atomic force microscopy image shows that the monolayer thickness is approximately 0.7 nm; the Raman peak of the hexagonal MoS2 single crystal is at 383 cm⁻¹. -1 and 407cm -1 This proves that it is a monolayer MoS2 structure; Raman surface scan data shows that the star-shaped MoS2 sample is homogeneous; the hexagonal star-shaped MoS2 single crystal has a second harmonic generation response, and it responds to incident light in the wavelength range of 810nm to 990nm, and a second harmonic signal is obtained at 1 / 2 wavelength of the incident light. The signal is dependent on the incident wavelength, and the response is the highest for incident light at 850nm.

[0096] Figure 5 (a) is a transmission electron microscopy (TEM) morphology image of the hexagonal star-shaped MoS2 single crystal prepared in Example 1; (b) is a selected area electron diffraction pattern obtained from positions I-VI in (a); (c) is spherical aberration TEM data of the hexagonal star-shaped MoS2 single crystal prepared in Example 1; (d) is the lattice spacing analysis results of the a and b axes in (c); (e) is the atomic structure diagram of the three edges (①, ② and ③) in (a); (f) is the complete atomic structure of the entire single crystal inferred from the atomic structure inside and at the edges of the hexagonal star-shaped MoS2 single crystal; (g) is the high-resolution lattice fringes of the hexagonal star-shaped MoS2 single crystal.

[0097] Depend on Figure 5 Selected area electron diffraction, high-resolution transmission electron microscopy, and annular dark-field scanning transmission electron microscopy confirmed that it is a single-crystal structure with clearly defined atoms in the interior and at the edges. Connecting the two outer diagonals forms an armchair-shaped structure, while connecting the two inner diagonals forms a zigzag structure. The star-shaped MoS2 single crystal was transferred to a copper mesh using wet chemical etching. Figure 5 Image a shows a transmission electron microscope (TEM) image of one of the single crystals. To verify its intrinsic single-crystal properties, selected area electron diffraction (SAED) analysis was performed at each vertex region of the star-shaped crystal domains. Figure 5The SAED pattern of b shows that all diffraction spots are arranged along the same crystal direction, confirming the uniform lattice structure of MoS2 domains. The interplanar spacing calculated from the diffraction spots is highly consistent with the theoretical value. In addition, SAED analysis detected (110) and (100) plane reflections, indicating that the MoS2 sample has a preferred orientation along these crystal directions. Figure 5 The c is an aberration-corrected scanning transmission electron microscopy (AC-STEM) image of MoS2 domains. The interplanar spacing measured along the a-axis and b-axis is corresponding to the (100) and (010) planes, respectively. The calculation process is described in detail in Figure 5 d. Figure 5 e shows the atomic structure of different edges of the crystal domain, not only revealing the preferential formation of zigzag edges (ZZ edges), but also explicitly defining the intrinsic atomic configuration of star-shaped domains (schematic diagram see Figure 5 f). This structure analysis shows that MoS2 domains have anisotropic characteristics: the connecting lines between the relatively concave points are arranged along the ZZ direction, and the remaining edges follow the armchair (AC) direction. To further verify the lattice structure of the sample, high-resolution TEM (HRTEM) analysis was performed. Figure 6 g shows clear lattice fringes diffracted by the domain region. To reduce errors, the interplanar spacing of the adjacent 10 fringes was measured, and the calculated interplanar spacing of adjacent fringes is consistent with the (100) plane spacing.

[0098] Figure 6 is the simulation settlement result of morphology regulation of example 1, wherein a is the energy of oxygen atoms existing between the silicon substrate and the monolayer molybdenum disulfide, b is the energy of oxygen atoms existing far from the monolayer molybdenum disulfide crystal domain and on the silicon substrate, c is the energy of oxygen atoms existing above the monolayer molybdenum disulfide crystal domain, d is a schematic diagram of the state of sulfur atoms in the growth process, and e is the potential barrier of processes I, II and III in d.

[0099] As can be seen from Figure 7 , process I in c is migration on the silicon substrate, process II is combination with the existing molybdenum disulfide without the assistance of oxygen atoms, and process III is combination with the existing molybdenum disulfide with the assistance of oxygen atoms; e shows that the potential barriers of processes I, II and III are 0.347 eV, 0.955 eV and 0.074 eV, respectively.

[0100] Next, taking the hexagonal star-shaped MoS2 single crystal prepared in example 1 as the material, an anisotropic transistor is prepared to illustrate, specifically:

[0101] First, AZ 1512 photoresist was spin-coated onto the substrate (3000 rpm, 30 seconds), followed by soft baking (90℃, 1 minute) to form a uniform film. Then, a laser lithography machine (exposure energy 120 mJ / cm²) was used. 2 Twelve electrode patterns, each 4 μm wide, were defined, developed (MF-319, 50 seconds), and then hard-baked (110°C, 2 minutes) to cure the patterns. A 5 nm chromium adhesion layer and a 60 nm gold conductive layer were sequentially deposited using electron beam thermal evaporation (rate...). The substrate temperature was 80℃. Finally, a complete electrode was obtained through ultrasonic-assisted exfoliation (acetone immersion + 40kHz ultrasound for 10 seconds). SEM analysis showed that the electrode linewidth was controlled at 4±0.2μm and the sheet resistance ≤2.5μΩ·cm, meeting device requirements. A gold electrode with a thickness of 65nm was then obtained using thermal evaporation.

[0102] Figure 7 In Figure 1, a is a schematic diagram of an angle-resolved anisotropic transistor based on a hexagonal star-shaped MoS2 single crystal; b is a microscope image of the angle-resolved transistor; c shows the high-resistance and low-resistance paths in the angle-resolved transistor; d is the transfer curve along the armchair-shaped crystal path; e is the transfer curve along the sawtooth-shaped crystal path; f is the output curve along the armchair-shaped crystal path; g is the output curve along the sawtooth-shaped crystal path; h is the mobility statistics under different paths and different source-drain voltages; i is the on / off ratio statistics under different paths and different source-drain voltages; j is the anisotropy ratio.

[0103] Depend on Figure 8 As can be seen, a is a schematic diagram of the 12-electrode anisotropic device, b is a microscope photograph of the 12-electrode anisotropic device, with the electrodes labeled sequentially as 1-12; in c, within the angle-resolved transistor, pathways 1-7, 3-9, and 5-11 are high-resistance pathways along the armchair-shaped crystal orientation, while pathways 2-8, 4-10, and 6-12 are low-resistance pathways along the zigzag crystal orientation; the maximum mobility along the armchair-shaped direction is 58 cm⁻¹. 2 V -1 s -1 The average migration rate was 52.6 cm. 2 V -1 s -1 The migration rate along the zigzag direction was the highest at 77.6 cm. 2 V -1 s -1 The average migration rate was 69 cm. 2 V -1 s -1Figures h and i show the mobility and on / off ratio statistics for different directions under different source voltages, respectively. The statistics show that the mobility along the sawtooth shape is higher than that along the armchair shape, and the on / off ratio along the sawtooth shape is also slightly higher than that along the armchair shape. Figure j shows the on / off ratio of high and low resistance paths under different source-drain voltages and different gate voltages, which is the anisotropy ratio.

[0104] The following describes the fabrication of a square wave generator based on a hexagonal star-shaped molybdenum sulfide single-crystal anisotropic transistor. Specifically:

[0105] To prepare a square wave generator: a 12-electrode anisotropic transistor is used as the rotor, and a set of source and drain electrodes is fixed at both ends of the rotor as the stator. During the rotation of the 12-electrode anisotropic transistor, different electrodes contact the stator respectively to generate switchable electronic signals.

[0106] Figure 8 a is a schematic diagram of the square wave generator; b is a schematic diagram of the square wave generator's operation; c is the high-impedance path state; d is the state of switching between the high-impedance path and the low-impedance path; e is the low-impedance path state; f is the different operating modes of the square wave generator when using different source-drain voltages, gate voltages, and rotational speeds; g is the anisotropy ratio at different source-drain voltages and gate voltages; h is the energy consumption at different source-drain voltages and gate voltages.

[0107] Depend on Figure 9 As can be seen from the schematic diagram of the square wave generator, the anisotropic transistor is the rotor, and there are two fixed source and drain electrodes as the stator. When the armchair-shaped electrode is in contact with the stator, the high-resistance path is open. When the armchair-shaped electrode has just left the stator and the sawtooth-shaped electrode has just contacted the stator, the high-resistance path and the low-resistance path are switched. When the sawtooth-shaped electrode is in contact with the stator, the low-resistance path is open. Applying different source and drain voltages, gate voltages, or rotational speeds to the square wave generator will change the square wave generation mode and obtain different on / off ratios and power consumption. The electronic square wave generator based on the star-shaped molybdenum sulfide single-crystal anisotropic transistor has an on / off ratio of 1.3-1.9 and a maximum power consumption of 4000 microwatts.

[0108] Figure 9 The images show the morphological characteristics of the products prepared in Comparative Examples 2-5, where a represents Comparative Example 2, b represents Comparative Example 3, c represents Comparative Example 4, and d represents Comparative Example 5.

[0109] Depend on Figure 10It can be seen that when the temperature of CVD growth is too high (1000℃), cracks will appear in the material; when the CVD growth time is too long (15 min), etching will occur; when the flow of carrier gas is too large (80 sccm), the reaction will be insufficient due to the excessive gas flow speed, resulting in low material quality; when the flow of carrier gas is too low (20 sccm), there will be insufficient sulfur source due to the slow gas flow speed, making it difficult to generate the product.

[0110] Figure 11 The microscope photos of the products prepared for Comparative Example 6 are shown in the figure, from which it can be seen that, compared with Example 1 and Comparative Example 1, when the oxygen plasma pretreatment is reduced or the relative position of the mixture of molybdenum trioxide powder and sodium chloride and the growth substrate is changed, the product morphology will be significantly affected.

[0111] ​ The morphology and size characterization figures of the products prepared for Comparative Examples 7-10 are shown in the figure, in which a is Comparative Example 7, b is Comparative Example 8, c is Comparative Example 9, and d is Comparative Example 10. It can be seen from the figure that when the growth temperature is insufficient, it is difficult to form the product (almost no product can be seen on the substrate), when the growth time is insufficient, the size of the obtained material is small, when the growth distance is too long, the product is difficult to deposit on the surface of the growth substrate, only a very small amount of sample can be observed, and almost no star-shaped crystals can be observed, and when the growth distance is too close, almost all the obtained products are precursors that have not yet reacted, therefore, the growth temperature should not be too low, the growth time should not be too short, and the growth distance should be moderate.

[0112] The various embodiments in the specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0113] The above description of the disclosed embodiments enables a person skilled in the art to implement or use the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to these embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for morphology-controllable preparation of star-shaped monolayer molybdenum disulfide single crystals based on oxygen plasma pretreatment and chemical vapor deposition, characterized in that the steps include... include: Using molybdenum source powder and sulfur source powder as precursors, sodium chloride or potassium chloride as auxiliary agent, an oxygen plasma-treated Si / SiO2 substrate as growth substrate, and an inert gas as carrier gas, the star-shaped monolayer molybdenum disulfide single crystal was prepared by chemical vapor deposition. In the chemical vapor deposition process, the molybdenum source powder and the auxiliary agent are mixed and placed opposite to the Si / SiO2 substrate; The molybdenum source powder includes molybdenum dioxide powder and / or molybdenum trioxide powder; The sulfur source powder is sulfur powder; The mass ratio of the molybdenum source powder, sulfur source powder, and auxiliary agent is 1:100:0.1; The oxygen plasma treatment has a power of 80 W and a duration of 5-10 min; The inert gas is argon, with a flow rate of 30-60 sccm; The growth temperature for the chemical vapor deposition was 750 °C, and the time was 10 min. The molybdenum source powder and the auxiliary agent are placed at a distance of 0.5 cm from each other relative to the Si / SiO2 substrate.

2. The method as described in claim 1, characterized in that, The chemical vapor deposition step includes: The molybdenum source powder and the auxiliary agent are mixed and placed in the center of the heat source of a tube furnace. The sulfur source powder is placed upstream of the tube furnace in the center of the heat source. The Si / SiO2 substrate is placed directly above the mixture of the molybdenum source powder and the auxiliary agent. The temperature is raised to the growth temperature for crystal growth. After the growth is completed, the temperature is lowered to room temperature.

3. The method as described in claim 1, characterized in that, The process of treating Si / SiO2 substrates with oxygen plasma also includes a cleaning step.

4. A star-shaped monolayer molybdenum disulfide single crystal prepared by the method described in any one of claims 1-3.

5. The application of the star-shaped monolayer molybdenum disulfide single crystal as described in claim 4 in the field of optoelectronics.

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