Method for preparing star-shaped molybdenum disulfide single crystal with controllable morphology and application of star-shaped molybdenum disulfide single crystal

Abstract

The invention belongs to the technical field of two-dimensional material preparation and device preparation, and particularly relates to a method for preparing star-shaped molybdenum disulfide single crystals with controllable morphology and application of the star-shaped molybdenum disulfide single crystals. The method for preparing the star-shaped single-layer molybdenum disulfide with the controllable morphology is provided based on oxygen plasma pretreatment and chemical vapor deposition, a novel oxygen plasma auxiliary technology is introduced, oxygen plasma pretreatment is conducted on a growth substrate, the growth environment of TMDs is improved, chemical vapor deposition is cooperated, and the morphology of the molybdenum disulfide is controlled. Large-scale preparation of the high-quality star-shaped molybdenum disulfide single crystal is achieved, the defects that in the prior art, pattern regulation and control precision is low, repeatability is poor and heterogeneous integration capacity is insufficient are overcome, and a reliable material basis is provided for application of the high-quality star-shaped molybdenum disulfide single crystal to high-speed and high-precision electronic devices.

Description

Technical Field

[0001] The present invention belongs to the technical field of two-dimensional material preparation and device preparation, and more specifically relates to a method for controllably preparing star-shaped molybdenum disulfide single crystals with controlled morphology and its applications. Background Art

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

[0003] The pattern regulation of TMDs has an important impact on their physical and chemical properties. For example, due to the self-limiting characteristics of crystal growth, the external morphology of the crystal is closely related to its internal crystal structure, so the crystal phase of the material can be inferred from the pattern features. In recent years, researchers have achieved phase control of TMDs through pattern regulation. For example, Han et al. regulated the crystal phase of Ta 1+x S2 by a temperature-mediated method and found that the T-phase Ta 1+x S2 presents a hexagonal morphology, while the H-phase Ta 1+x S2 shows a triangular shape. Nong et al. synthesized 1T'-phase MoS2 by custom synthesis, and the 1T'-phase MoS2 shows an elongated trapezoidal structure, in sharp contrast to the H-phase triangular MoS2. In addition, Yang et al. proposed a "substrate step template growth method" and successfully prepared one-dimensional TMDs nanoribbons by controlling the chalcogen / metal source ratio and utilizing the induction effect of step edges, providing a new approach for the high-yield and precise synthesis of specific edge types.

[0004] Although certain progress has been made in the pattern regulation of TMDs in the prior art, there are still 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, and small changes may lead to significant morphological differences.

[0007] 3. Gap between theory and practice: Existing theoretical models are difficult to fully guide actual synthesis, resulting in insufficient accuracy and reproducibility of pattern regulation.

[0008] 4. Limited functional applications: Existing methods still face challenges in achieving heterogeneous integration of TMDs (such as customized number of layers, crystal phase, symmetry, and twist angle), which limits their application in high-performance devices.

[0009] Therefore, developing a simple, controllable and efficient TMDs pattern regulation method to achieve precise control of its morphology, crystal phase and function has become a key issue to be urgently addressed in this field. Summary of the invention

[0010] The purpose of the present invention is to provide a method for preparing star-shaped molybdenum disulfide single crystals with controllable morphology and its application. Specifically, a method for preparing star-shaped monolayer molybdenum disulfide with controllable morphology based on oxygen plasma pretreatment and chemical vapor deposition and its application are provided. By introducing a new oxygen plasma auxiliary technology, oxygen plasma pretreatment is performed on the growth substrate, the growth environment of TMDs is improved, and chemical vapor deposition is coordinated to achieve large-scale preparation of high-quality star-shaped molybdenum disulfide single crystals, so as to solve the defects of low pattern control accuracy, poor repeatability and insufficient heterogeneous 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 object, the present invention provides the following solutions:

[0012] One of the technical solutions of the present invention is to provide a method for preparing a star-shaped monolayer molybdenum disulfide with controllable morphology based on oxygen plasma pretreatment and chemical vapor deposition, the steps comprising:

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

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

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

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

[0017] The present invention adopts a porcelain boat as a container for holding precursors and growth substrates, wherein a mixture of molybdenum source powder and auxiliary agents is sprinkled at the center of the porcelain boat, and an inverted Si / SiO2 substrate is placed directly 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 that the Si / SiO2 substrate can just fit in the porcelain boat.

[0018] When no oxygen plasma is applied, the attachment barrier of sulfur atoms at the edges of the formed molybdenum sulfide crystal domains 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 edges, and the product shape formed according to this growth mechanism is triangular. However, when oxygen plasma is applied, since the substrate surface is rich in oxygen plasma, the attachment barrier of sulfur atoms at the edges of the formed molybdenum sulfide crystal domains is greatly reduced and becomes less than the diffusion barrier of sulfur atoms on the substrate. Therefore, 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 adjuvant (preferably sodium chloride) can reduce the melting point of the molybdenum source powder precursor.

[0020] Furthermore, the mass ratio of the molybdenum source powder, sulfur source powder and adjuvant is 1:100:0.1.

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

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

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

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

[0025] Furthermore, the growth temperature of the chemical vapor deposition is 750 °C and the time is 10 min.

[0026] Furthermore, the distance between the mixture of the molybdenum source powder and the adjuvant and the Si / SiO2 substrate placed opposite to each other is 0.5 cm.

[0027] Temperatures higher than the growth temperature will cause cracks, and temperatures lower than the growth temperature will cause incomplete or non-volatile molybdenum oxide precursors, and the target product cannot be obtained; if the distance is too close, the precursors will deposit on the growth substrate surface prematurely and in large quantities, resulting in a large amount of unreacted substances; if the distance is too far, the precursors will undergo a sulfidation reaction during the sublimation path and leave the reaction chamber under the action of the gas flow, and the two-dimensional material product cannot be obtained on the growth substrate surface; if the time is too long, etching will occur, and if the time is too short, the growth time will be insufficient, and a large-area product cannot be obtained, only a small-area product can be obtained.

[0028] Furthermore, the steps of the chemical vapor deposition include:

[0029] Mix the molybdenum source powder and the adjuvant and place them at the center of the heat source of a tube furnace. Place the sulfur source powder upstream of the tube furnace at the center of the heat source. Place the Si / SiO2 substrate directly above the mixed system of the molybdenum source powder and the adjuvant. Raise the temperature to the growth temperature for crystal growth, and after the growth is completed, cool down to room temperature.

[0030] Furthermore, before the oxygen plasma treatment of the Si / SiO2 substrate, there is also a cleaning step.

[0031] Optionally, the cleaning step includes: successively ultrasonically cleaning the Si / SiO2 substrate with ultrapure water, acetone, and isopropyl alcohol. The ultrasonic power is 40 W, and each ultrasonic cleaning lasts for 10 min. Finally, dry it with nitrogen.

[0032] The second technical solution of the present invention: Provide a star-shaped monolayer molybdenum disulfide prepared by the above method.

[0033] The third technical solution of the present invention: Provide an application of the above star-shaped monolayer molybdenum disulfide in the optoelectronic field.

[0034] The star-shaped molybdenum disulfide single crystal prepared by the present invention exhibits an in-plane anisotropic atomic structure. The line connecting two opposite outer vertices is arranged along the armchair crystal orientation, while the line connecting two opposite inner vertices is arranged along the zigzag direction. Based on these unique properties, the present invention applies it to the optoelectronic field, such as angle-resolved anisotropic transistors based on star-shaped molybdenum disulfide single crystals. These devices switch between high-resistance paths and low-resistance paths. 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, with a maximum of 58 cm 2 V -1 s -1 , and the anisotropic transistors in the present invention exhibit an anisotropy ratio as high as 1.87.

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

[0036] The present invention provides a simple and efficient preparation method, which can synthesize high-quality and pattern-controllable molybdenum disulfide single crystals with high yield on a substrate. This method does not require significant modification of the existing chemical vapor deposition device. By only optimizing the oxygen plasma treatment conditions and growth parameters, the controllable preparation of high-quality star-shaped molybdenum disulfide single crystals can be realized, significantly reducing the preparation cost and technical threshold. It not only overcomes the limitations of traditional morphology control preparation methods but also opens up a new way for exploring the unique properties of in-plane armchair and zigzag structures and their applications in advanced devices.

[0037] The present invention adopts an oxygen plasma-assisted chemical vapor deposition strategy to successfully prepare high-quality and high-purity star-shaped MoS2 single crystals, and the in-plane zigzag and armchair crystal orientation structures thereof have high purity and defect-free characteristics.

[0038] Due to the unique star-shaped molybdenum disulfide single crystal structure prepared by the present invention, the performance of anisotropic transistors prepared for use in the optoelectronic field is significantly improved. Devices based on low-resistance state and high-resistance state channels exhibit excellent electron mobilities (77.6 cm 2 V -1 s -1 and 58 cm 2 V -1 s -1 ) and a significant anisotropy ratio (1.87), realizing the construction of high-performance angle-resolved anisotropic transistors. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and the descriptions thereof are used to explain the present invention and do not constitute an improper limitation to the present invention. In the drawings:

[0040] Figure 1 Schematic diagrams of the devices for preparing MoS2 single crystals in Examples 1-2 and Comparative Example 1 and the product morphologies under different conditions.

[0041] Figure 2 Microscope photos of MoS2 single crystals prepared in Comparative Example 1 and Examples 1-2. Among them, 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 Size statistical charts of MoS2 single crystals prepared in Comparative Example 1 and Examples 1-2. Among them, a is the size statistical chart of Comparative Example 1, b is the size statistical chart of Example 1, and c is the size statistical chart of Example 2.

[0043] Figure 4 Quality characterization diagrams of the hexagonal star-shaped MoS2 single crystals prepared in Example 1. Among them, a from top to bottom are Examples 1-2 and Comparative Example 1 in sequence, b is the Raman surface scan diagram, c is the atomic force microscope diagram, d and e are the angle-resolved polarization Raman spectra of the hexagonal star-shaped MoS2 single crystals when the analyzer is placed vertically, f and g are the angle-resolved polarization Raman spectra of the hexagonal star-shaped MoS2 single crystals when the analyzer is placed horizontally, h is the second harmonic response diagram of the hexagonal star-shaped MoS2 single crystals in the wavelength range from 810 nm to 990 nm, and i is the XPS of the hexagonal star-shaped MoS2 single crystals.

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

[0045] Figure 6 It is the simulation calculation result of the morphology regulation in Example 1. Among them, a is the energy when oxygen atoms exist between the silicon substrate and the monolayer molybdenum disulfide; b is the energy when oxygen atoms exist at a position far from the monolayer molybdenum disulfide crystal domain and on the silicon substrate; c is the energy when oxygen atoms exist above the monolayer molybdenum disulfide crystal domain; d is the schematic diagram of the state of sulfur atoms during the growth process; e is the potential barriers of processes I, II, and III in d.

[0046] Figure 7 In it, a is the schematic diagram of the angular resolution anisotropic transistor based on the hexagonal star-shaped MoS2 single crystal; b is the microscope image of the angular resolution transistor; c are the high-resistance path and low-resistance path in the angular resolution transistor; d is the transfer curve along the armchair crystal orientation path; e is the transfer curve along the zigzag crystal orientation path; f is the output curve along the armchair crystal orientation path; g is the output curve along the zigzag crystal orientation 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.

[0047] Figure 8 In it, a is the schematic diagram of the square wave generator; b is the working schematic diagram of the square wave generator; c is the high-resistance path state; d is the state of the high-resistance path and low-resistance path switching; e is the low-resistance path state; f are the different operating modes presented by the square wave generator when different source-drain voltages, gate voltages, and rotation speeds are used; g is the anisotropy ratio under different source-drain voltages and gate voltages; h is the energy consumption under different source-drain voltages and gate voltages.

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

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

[0050] Figure 11Morphology and size characterization diagrams of the products prepared in Comparative Examples 7-10, where a is Comparative Example 7, b is Comparative Example 8, c is Comparative Example 9, and d is Comparative Example 10. Detailed implementation manners

[0051] The various exemplary implementation manners of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, characteristics, and implementation schemes of the present invention.

[0052] It should be understood that the terms used in the present invention are only for describing specific implementation manners and are not used to limit the present invention. Additionally, for the numerical ranges in the present invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any intermediate value within any stated value or stated range, as well as each smaller range between any other stated value or intermediate value within the stated range, is also included in the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0053] Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention pertains. Although the present invention only describes preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation 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 related to the documents. In case of conflict with any incorporated document, the content of this specification shall prevail.

[0054] Without departing from the scope or spirit of the present invention, various improvements and changes can be made to the specific implementation manners of the present invention specification, which are obvious to those skilled in the art. Other implementation manners obtained from the present invention specification are obvious to those skilled in the art. The present invention specification and examples are merely exemplary.

[0055] Regarding "comprising", "including", "having", "containing", etc. used herein, they are all open-ended terms, meaning including but not limited to.

[0056] It should be noted that those aspects of the present invention not described in detail are all conventional operation means in the art and are not the focus of the present invention.

[0057] Example 1

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

[0059] S1. Substrate treatment: The N-type doped wafer-level Si / SiO2 is cut into independent substrates with a size of 1 cm × 1 cm, and ultrasonically cleaned with ultrapure water, acetone, isopropyl alcohol, and ethanol respectively. The ultrasonic power is 40 W, and each ultrasonic cleaning lasts for 10 min. Then it is dried with nitrogen, and subsequently, an oxygen plasma pretreatment at 80 W for 5 min is carried out as the growth substrate;

[0060] S2. CVD growth: After mixing molybdenum trioxide powder (1.5 mg) and sodium chloride (0.15 mg), they are placed opposite to the growth substrate at the center of the furnace tube heat source, with a distance of 0.5 cm. Sulfur powder (150 mg) is placed 15 cm upstream of the center of the furnace tube heat source. Ar is used as the carrier gas with a flow rate of 30 sccm. The heating rate is 30 °C / min to 750 °C, and it is maintained for 10 minutes. On the growth substrate, hexagonal star-shaped MoS2 single crystals are grown.

[0061] Example 2

[0062] Preparation of fractal multi-branched star-shaped MoS2 single crystals:

[0063] S1. Substrate treatment: The N-type doped wafer-level Si / SiO2 is cut into independent substrates with a size of 1 cm × 1 cm, and ultrasonically cleaned with ultrapure water, acetone, isopropyl alcohol, and ethanol respectively. The ultrasonic power is 40 W, and each ultrasonic cleaning lasts for 10 min. Then it is dried with nitrogen, and subsequently, an oxygen plasma pretreatment at 80 W for 10 min is carried out as the growth substrate;

[0064] S2. CVD growth: After mixing molybdenum trioxide powder (1.5 mg) and sodium chloride (0.15 mg), they are placed opposite to the growth substrate at the center of the furnace tube heat source, with a distance of 0.5 cm. Sulfur powder (150 mg) is placed 15 cm upstream of the center of the furnace tube heat source. Ar is used as the carrier gas with a flow rate of 30 sccm. The heating rate is 30 °C / min to 750 °C, and it is maintained for 10 minutes. On the growth substrate, fractal multi-branched star-shaped MoS2 single crystals are grown.

[0065] Comparative Example 1

[0066] Preparation of triangular MoS2 single crystals:

[0067] S1. Substrate treatment: The N-type doped wafer-level Si / SiO2 is cut into independent substrates with a size of 1 cm × 1 cm, and ultrasonically cleaned with ultrapure water, acetone, isopropyl alcohol, and ethanol respectively. The ultrasonic power is 40 W, and each ultrasonic cleaning lasts for 10 min. Finally, it is dried with nitrogen as the growth substrate;

[0068] S2, CVD Growth: After mixing molybdenum trioxide powder (1.5 mg) and sodium chloride (0.15 mg), they are placed opposite to the growth substrate at the center of the furnace tube heat source, with a distance of 0.5 cm. Sulfur powder (150 mg) is placed 15 cm upstream of the center of the furnace tube heat source. Ar is used as the carrier gas with a flow rate of 30 sccm. The temperature is increased at a rate of 30 °C / min to 750 °C and maintained for 10 minutes. On the growth substrate, triangular MoS2 single crystals are grown.

[0069] Comparative Example 2

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

[0071] Comparative Example 3

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

[0073] Comparative Example 4

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

[0075] Comparative Example 5

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

[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: The N-type doped wafer-level Si / SiO2 is cut into independent substrates with a size of 1 cm × 1 cm, and they are ultrasonically cleaned with ultrapure water, acetone, isopropyl alcohol, and ethanol respectively. The ultrasonic power is 40 W, and each ultrasonic cleaning lasts for 10 min. Then they are dried with nitrogen, and subsequently, an oxygen plasma pretreatment is carried out at 80 W for 5 min to serve as the growth substrate;

[0080] S2, CVD Growth: After mixing molybdenum trioxide powder (1.5 mg) and sodium chloride (0.15 mg), they are placed at the center of the furnace tube heat source. Sulfur powder (150 mg) is placed 15 cm upstream of the center of the furnace tube heat source. The growth substrate is placed 15 cm downstream of the center of the furnace tube heat source. Ar is used as the carrier gas with a flow rate of 30 sccm. The temperature is increased at a rate of 30 °C / min to 750 °C and maintained for 10 minutes. On the growth substrate, MoS2 single crystals are grown.

[0081] Comparative Example 7

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

[0083] Comparative Example 8

[0084] Compared with Example 1, the only difference is that the CVD growth time is 5 min.

[0085] Comparative Example 9

[0086] Compared with Example 1, the only difference is that the distance between the center of the furnace tube heat source and the growth substrate after mixing molybdenum trioxide powder and sodium chloride is 1.2 cm.

[0087] Comparative Example 10

[0088] Compared with Example 1, the only difference is that the distance between the center of the furnace tube heat source and the growth substrate after mixing molybdenum trioxide powder and sodium chloride is 0.2 cm.

[0089] Test Example

[0090] Figure 1 Schematic diagrams of the apparatus for preparing MoS2 single crystals in Examples 1-2 and Comparative Example 1 and the product morphologies under different conditions.

[0091] Figure 2 Microscope photos of the MoS2 single crystals prepared in Comparative Example 1 and Examples 1-2, where 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.

[0092] Figure 3 Size statistical charts of the MoS2 single crystals prepared in Comparative Example 1 and Examples 1-2, where a is the size statistical chart of Comparative Example 1, b is the size statistical chart of Example 1, and c is the size statistical chart of Example 2.

[0093] It can be seen that Figure 2 - Figure 3 with the addition and continuous increase of oxygen plasma, the morphology of the MoS2 single crystal has changed significantly. When the substrate is not modified with oxygen plasma, the obtained MoS2 single crystal has a triangular morphology, while after the addition of oxygen plasma modification, the obtained MoS2 single crystal has a star-like morphology. Further increasing the oxygen plasma modification time, the obtained MoS2 single crystal approaches a polygonal shape, and many corners grow on the basis of the star shape; and the addition of oxygen also makes the size of the product change significantly. The presence of oxygen makes the MoS2 single crystal gradually increase.

[0094] Figure 4Mass characterization diagrams of the hexagonal star-shaped MoS2 single crystal prepared in Example 1. Among them, a are the Raman spectra of Example 1-2 and Comparative Example 1 from top to bottom, b is the Raman mapping, c is the atomic force microscopy image, d and e are the angular-resolved polarization Raman spectra of the hexagonal star-shaped MoS2 single crystal when the analyzer is placed vertically, f and g are the angular-resolved polarization Raman spectra of the hexagonal star-shaped MoS2 single crystal when the analyzer is placed horizontally, h is the second harmonic generation 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] It can be seen from Figure 4 that the atomic force microscopy image shows that the monolayer thickness is about 0.7 nm; the Raman peaks of the hexagonal star-shaped MoS2 single crystal are 383 cm -1 and 407 cm -1 , which proves that it is a monolayer MoS2 structure; the Raman mapping 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 the incident light in the wavelength range of 810 nm to 990 nm, and a second harmonic signal is obtained at 1 / 2 of the wavelength of the incident light. The signal is dependent on the incident wavelength and has the highest response to the incident light of 850 nm.

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

[0097] It can be seen from Figure 5 that the selected area electron diffraction, high-resolution transmission electron microscopy, and annular dark field scanning transmission electron microscopy confirm that it is a single crystal structure, with clear atoms inside and at the edge. The two outer corner diagonals are connected to form an armchair structure, and the two inner corner diagonals are connected to form a zigzag structure. The star-shaped MoS2 single crystal was transferred to a copper grid using a wet chemical etching technique. Figure 5 a in Figure 5The SAED pattern shown in b of [ID] demonstrates that all diffraction spots are arranged along the same crystal direction, confirming the uniform lattice structure of the MoS2 domains. The lattice plane spacing calculated from the diffraction spots is in good agreement with the theoretical value. In addition, SAED analysis detected the reflections of the (110) and (100) lattice planes, indicating a preferred orientation of the MoS2 sample along these crystal directions. Figure 5 c of [ID] is an aberration-corrected scanning transmission electron microscope (AC-STEM) image of the MoS2 domains. The lattice plane spacings measured along the a-axis and b-axis are both corresponding to the (100) and (010) lattice planes respectively. The calculation process is detailed in Figure 5 d of [ID]. Figure 5 e of [ID] shows the atomic structures at different edges of the domains, revealing not only the preferential formation of the zigzag edge (ZZ edge), but also the intrinsic atomic configuration of the star-shaped domains (schematic diagram shown in Figure 5 f of [ID]). This structural analysis indicates that the MoS2 domains have anisotropic characteristics: the line connecting the relatively concave points is 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) was used for analysis. Figure 5 g of [ID] shows clear lattice fringes of the diffraction in the domain area. To reduce errors, 10 adjacent fringes were selected to measure the spacing, and the calculated spacing between adjacent fringes is which is consistent with the lattice plane spacing of the (100) plane.

[0098] Figure 6 For the simulation results of the morphology regulation in Example 1, where a is the energy when oxygen atoms exist between the silicon substrate and the monolayer molybdenum disulfide, b is the energy when oxygen atoms exist at a position far from the monolayer molybdenum disulfide domains and on the silicon substrate, c is the energy when oxygen atoms exist above the monolayer molybdenum disulfide domains, d is a schematic diagram of the state of sulfur atoms during the growth process, and e is the potential barriers of processes I, II, and III in d.

[0099] It can be seen from Figure 6 that process I in c is the migration on the silicon substrate, process II is the combination with the existing molybdenum disulfide without the assistance of oxygen atoms, and process III is the 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, the preparation of anisotropic transistors is exemplified as follows: specifically:

[0101] First, spin-coat AZ 1512 photoresist on the substrate (3000 rpm, 30 s), and then form a uniform photoresist film through soft baking (90 °C, 1 min). Define 12 electrode patterns with a width of 4 μm by a laser lithography machine (exposure energy 120 mJ / cm 2 ), develop (MF-319, 50 s), and then hard bake (110 °C, 2 min) to cure the pattern. Deposit a 5-nm chromium adhesion layer and a 60-nm gold conductive layer in sequence by electron beam thermal evaporation (deposition rate , substrate temperature 80 °C). Finally, obtain complete electrodes through ultrasonic-assisted stripping (soak in acetone + 40 kHz ultrasonic for 10 s). After SEM detection, the electrode line width is controlled within 4 ± 0.2 μm, and the sheet resistance ≤ 2.5 μΩ·cm, meeting the device requirements. Then, obtain gold electrodes by thermal evaporation, and the thickness of the gold electrodes is 65 nm.

[0102] Figure 7 In (a), it 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 path and low-resistance path in the angle-resolved transistor; (d) is the transfer curve along the armchair crystal orientation path; (e) is the transfer curve along the zigzag crystal orientation path; (f) is the output curve along the armchair crystal orientation path; (g) is the output curve along the zigzag crystal orientation 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] As can be seen from Figure 7 , (a) is a schematic diagram of a 12-electrode anisotropic device, and (b) is a microscope photograph of the 12-electrode anisotropic device. The electrodes are sequentially labeled 1-12 in order; in (c), in the angle-resolved transistor, the 1-7, 3-9, and 5-11 paths are the paths along the armchair crystal orientation for transmission, which are high-resistance paths, and the 2-8, 4-10, and 6-12 paths are the paths along the zigzag crystal orientation for transmission, which are low-resistance paths; the maximum mobility along the armchair direction is 58 cm 2 V -1 s -1 , and the average mobility is 52.6 cm 2 V -1 s -1 , the maximum mobility along the zigzag direction is 77.6 cm 2 V -1 s -1 , and the average mobility is 69 cm 2 V -1 s -1, Figures h and i are the statistics of mobility and on / off ratio in different directions under different source voltages. It can be seen from the statistics that the mobility along the zigzag direction is higher than that along the armchair direction, and the on / off ratio along the zigzag direction is also slightly higher than that along the armchair direction; in j, the on / off ratios of high- and low-resistance paths under different source-drain voltages and different gate voltages are the anisotropy ratios.

[0104] Next, a square-wave generator is prepared based on the anisotropic transistor of hexagonal star-shaped molybdenum disulfide single crystal. Specifically:

[0105] Preparation of the square-wave generator: Use a 12-electrode anisotropic transistor as the rotor, and fix a set of source-drain electrodes at both ends as the stator. During the rotation of the 12-electrode anisotropic transistor, different electrodes are in contact with the stator respectively, generating a switchable electronic signal.

[0106] Figure 8 In, a is a schematic diagram of the square-wave generator; b is a working schematic diagram of the square-wave generator; c is the high-resistance path state; d is the state of switching between the high-resistance path and the low-resistance path; e is the low-resistance path state; f is different operating modes presented by the square-wave generator when different source-drain voltages, gate voltages and rotation speeds are used; g is the anisotropy ratio under different source-drain voltages and gate voltages; h is the energy consumption under different source-drain voltages and gate voltages.

[0107] As Figure 8 can be seen, in the schematic diagram of the square-wave generator, the anisotropic transistor is the rotor, and there are also two fixed source-drain electrodes as the stator; when the electrode in the armchair direction is in contact with the stator, the high-resistance path is conducting at this time. When the electrode in the armchair direction just leaves the stator and the electrode in the zigzag direction just comes into contact with the stator, this is the state of switching between the high-resistance path and the low-resistance path. When the electrode in the zigzag direction is in contact with the stator, the low-resistance path is conducting at this time; applying different source-drain voltages, gate voltages or rotation speeds to this square-wave generator will change the generation mode of the square wave and obtain different on / off ratios and power consumptions. The electronic square-wave generator prepared based on the anisotropic transistor of star-shaped molybdenum disulfide single crystal has an on / off ratio ranging from 1.3 to 1.9 and a maximum power consumption of 4000 microwatts.

[0108] Figure 9 Figures a-d are the morphological characterization diagrams of the products prepared in Comparative Examples 2-5. Among them, a is Comparative Example 2, b is Comparative Example 3, c is Comparative Example 4, and d is Comparative Example 5.

[0109] As Figure 9It can be seen that when the temperature of CVD growth is too high (1000 °C), cracks will appear in the material; when the CVD growth time is too long (15 min), etching will occur; when the flow rate of the carrier gas is too large (80 sccm), due to the too fast gas flow rate, the reaction will be insufficient and the material quality will be low; when the flow rate of the carrier gas is too low (20 sccm), due to the too slow gas flow rate, there will be insufficient sulfur source, making it difficult to generate the product.

[0110] Figure 10 It is a microscopic photograph of the product prepared in Comparative Example 6. As can be seen from the figure, compared with Example 1 and Comparative Example 1, when reducing the oxygen plasma pretreatment or changing the relative position of the mixture of molybdenum trioxide powder and sodium chloride and the growth substrate, it will have a significant impact on the morphology of the product.

[0111] Figure 11 It is a graph showing the morphology and size characterization of the products prepared in Comparative Examples 7 - 10. Among them, a is Comparative Example 7, b is Comparative Example 8, c is Comparative Example 9, and d is Comparative Example 10. As can be seen from the figure, 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 obtained material has a smaller size; when the growth distance is too long, the product is difficult to deposit on the surface of the growth substrate, and only a very small amount of samples can be observed, and there is almost no star shape of the crystal; when the growth distance is too close, almost all are precursors that have not had time to react. Therefore, the growth temperature should not be too low, the growth time should not be too short, and the growth distance needs to be appropriate.

[0112] In this specification, each embodiment is described in a progressive manner. The key point of each embodiment is to illustrate the differences from other embodiments. For the same or similar parts among the embodiments, reference can be made to each other.

[0113] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be obvious to those skilled in the art. The general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention 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 controllably preparing star-shaped monolayer molybdenum disulfide with a controllable morphology based on oxygen plasma pretreatment and chemical vapor deposition, characterized in that the steps Including: Using molybdenum source powder and sulfur source powder as precursors, sodium chloride or potassium chloride as an adjuvant, an oxygen plasma-treated Si / SiO2 substrate as a growth substrate, and an inert gas as a carrier gas, the star-shaped monolayer molybdenum disulfide is prepared by chemical vapor deposition. In the chemical vapor deposition, the molybdenum source powder and the adjuvant are placed opposite to the Si / SiO2 substrate after being mixed. The molybdenum source powder includes molybdenum dioxide powder and / or molybdenum trioxide powder. The sulfur source powder is sulfur powder.

2. The method according to claim 1, characterized in that, The mass ratio of the molybdenum source powder, the sulfur source powder and the adjuvant is 1:100:0.

1.

3. The method according to claim 1, wherein The power of the oxygen plasma treatment is 80 W and the time is 5 - 10 min.

4. The method according to claim 1, characterized in that, The inert gas is argon and the flow rate is 30 - 60 sccm.

5. The method according to claim 1, characterized in that, The growth temperature of the chemical vapor deposition is 750 °C and the time is 10 min.

6. The method according to claim 1, wherein The distance between the molybdenum source powder and the adjuvant after being mixed and the Si / SiO2 substrate placed opposite is 0.5 cm.

7. The method according to claim 1, characterized in that, The steps of the chemical vapor deposition include: After mixing the molybdenum source powder and the adjuvant, place them at the heat source center of the tube furnace, place the sulfur source powder upstream of the tube furnace at the heat source center, place the Si / SiO2 substrate directly above the mixed system of the molybdenum source powder and the adjuvant, heat up to the growth temperature for crystal growth, and cool down to room temperature after the growth is completed.

8. The method according to claim 1, wherein Before the oxygen plasma treatment of the Si / SiO2 substrate, there is also a cleaning step.

9. A star-shaped monolayer molybdenum disulfide prepared by the method according to any one of claims 1 - 8.

10. An application of the star-shaped monolayer molybdenum disulfide according to claim 9 in the optoelectronic field.

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