A method for preparing heterojunctions with controllable twist angle

Abstract

This invention discloses a method for preparing heterojunctions with controllable torsion angles, belonging to the field of moiré superlattice materials technology. The method first utilizes angle-dependent pump-probe time-resolved spectroscopy to measure the optical response of the lattice orientation of a two-dimensional material, determining its lattice reference orientation and marking the surface. Subsequently, a first marking material is fixed to a substrate, and a second marking material is placed on a precision rotating stage. Observation is performed using an optical microscope. The marking directions of the two materials are first made parallel to determine the initial angle, and then the stage is precisely rotated to the target angle according to a preset torsion angle. Finally, dry transfer and van der Waals bonding of the material are completed while maintaining the angle, forming a heterojunction with a preset torsion angle. This invention achieves a fundamental shift from passive selection to active design of the torsion angle, with a control accuracy better than 0.1 degrees. It eliminates the dependence on expensive equipment such as transmission electron microscopes and has the advantages of universal process applicability, clean interface, and high success rate.

Description

Technical Field

[0001] This invention relates to the field of moiré superlattice materials, and more particularly to a method for preparing heterojunctions with controllable twist angle. Background Technology

[0002] In recent years, with the deepening research on two-dimensional materials, moiré superlattices have become a cutting-edge hot topic in condensed matter physics and materials science due to their unique ability to regulate the electronic band structure and physical properties of materials. Crystal theory states that when two atomically thin two-dimensional materials are stacked, if there is a small angle (twist angle) between the lattice directions, the periodicity of the atomic arrangement will lead to long-wavelength interference fringes, thus forming a moiré superlattice. The period of this superlattice is much larger than the lattice constant of a single-layer material, which can significantly alter the movement of electrons within the material, inducing a series of novel physical phenomena such as correlated electronic states, superconductivity, and topological phases. This provides a completely new material platform for the design of next-generation quantum devices and optoelectronic devices.

[0003] Currently, the mainstream method for fabricating such torsion angle heterojunctions relies on chemical vapor deposition (CVD) growth and mechanical exfoliation and transfer techniques. CVD can directly fabricate two-dimensional materials with specific crystal orientations (such as triangular domains) on a substrate by controlling growth kinetic parameters, but its ability to control the interlayer torsion angle is limited, and it is more suitable for fabricating homogeneous structures. A more common method is mechanical exfoliation combined with dry or wet transfer: first, a single-layer or few-layer two-dimensional material film is exfoliated from the bulk material, and then the two layers are randomly stacked together to form a heterojunction through a transfer process. Afterwards, the torsion angle of the formed heterojunction must be measured using precise characterization techniques such as high-resolution transmission electron microscopy (TEM), second harmonic generation (SHG), or Raman spectroscopy.

[0004] However, the aforementioned existing technologies suffer from a series of fundamental drawbacks: First, the twist angle is completely uncontrollable and random. During the transfer and stacking process, the operator cannot predict or actively set the relative lattice orientation of the two layers of material; the twist angle is determined by chance, making it impossible for researchers to systematically study the structure-property relationship between the twist angle and physical properties, and also making it difficult to conduct "customized" fabrication for specific device requirements. Second, the fabrication efficiency is low and the cost is high. Due to the passive "stack first, measure later" mode, if the measurement results do not meet the research requirements, the sample usually has to be scrapped, resulting in a low success rate and significant waste. The entire process is highly dependent on expensive and complex cutting-edge equipment such as TEM and molecular beam epitaxy (MBE), which has a high barrier to entry and is difficult to promote in ordinary laboratories or industrial environments. Finally, the method itself has limitations. CVD growth is particularly difficult to control the twist angle of heterojunctions; while wet transfer may introduce chemical residues that contaminate the interface, impairing the intrinsic properties of the material.

[0005] In summary, existing technologies are essentially a passive "random preparation, post-selection" approach, lacking a universal preparation method that can actively, precisely, and repeatedly pre-set and achieve the target twist angle. This severely restricts the systematic research and practical application of moiré superlattice materials. Therefore, developing a heterojunction preparation method with controllable twist angle, simple process, and strong universality has become a key technological bottleneck that urgently needs to be overcome in this field. Summary of the Invention

[0006] This application provides a method for preparing heterojunctions with controllable torsion angles, which solves the problem of the passive moiré superlattice material preparation mode in the prior art.

[0007] This application provides a method for preparing a heterojunction with controllable torsion angle, comprising the following steps: Step 1: Measure the lattice orientation response of the two-dimensional material using angle-dependent pump-probe time-resolved spectroscopy to obtain the relationship between the peak value of the optical signal and the rotation angle of the sample; determine the lattice reference direction of the material based on this relationship, and make a visual mark on the material surface along the reference direction; Step 2: Transfer and fix the marked first type of two-dimensional material onto the substrate; Step 3: Place the marked second type of two-dimensional material on a precisely rotatable platform, rotate the platform so that the marking direction on its surface is parallel to the marking direction of the first type of two-dimensional material fixed on the substrate, and record the angle reading of the rotating platform at this time as the initial reference angle; Step 4: Calculate and control the rotating platform to rotate from the initial reference angle to the target angle according to the preset target torsion angle, so that a precise relative torsion angle is formed between the two two-dimensional materials; Step 5: While maintaining the target angle, transfer the second two-dimensional material and bring it into contact with the first two-dimensional material. Bond them together through van der Waals forces to form a heterojunction with a preset twist angle.

[0008] Furthermore, in step one, the pump light wavelength for spectral measurement is 400 nm, the probe light wavelength is 800 nm, and the sample rotation step accuracy is 0.1 degrees.

[0009] Furthermore, the crystal structures of the first and second two-dimensional materials in step one include all crystal systems, and are single-layer or few-layer materials of the crystal system.

[0010] Furthermore, the first type of two-dimensional material and the second type of two-dimensional material may be the same type of material or different types of material.

[0011] Furthermore, the determination of the lattice reference direction in step one is based on the following: for hexagonal crystal materials, the peak value of the optical signal appears once every 60 degrees. The direction at which the sample is rotated to the peak maximum value is the lattice reference direction.

[0012] Furthermore, the angular resolution of the precision rotatable platform used in steps three and four is 0.1 degrees, and the repeatability is ±0.05 degrees.

[0013] Furthermore, the material transfer in steps two and five employs a dry transfer process, without the use of chemical solvents.

[0014] Furthermore, the preparation process is carried out in an environment with a cleanliness level better than Class 100, with the ambient temperature controlled at 24-26 degrees Celsius and the relative humidity below 30%.

[0015] One or more technical solutions provided in the embodiments of this application have at least the following technical effects or advantages: In existing technologies, the fabrication of heterostructures, especially two-dimensional material heterostructures, generally relies on methods such as "mechanical exfoliation-random stacking" or "wet transfer." In these methods, the interlayer torsion angle is entirely determined by accidental factors during the operation, making pre-setting and precise control impossible. This prevents researchers and manufacturers from systematically studying the structure-property relationship between the torsion angle and material properties, and further hinders the ability to "design" the torsion angle according to device requirements. During mechanical exfoliation followed by transfer stacking, operators cannot predict or control the lattice orientation of the two layers, making the angle completely uncontrollable. The angle of the stacked sample must first be measured using TEM, SHG, Raman, etc.; if the angle does not meet the requirements, the sample is discarded, resulting in extremely low efficiency. This relies on expensive equipment such as high-resolution TEM, MOCVD, and MBE, and is complex to operate, making it difficult to popularize in general laboratories or industrial applications. Furthermore, CVD growth is typically only applicable to homogeneous structures, and angle control is limited. This makes angle control in heterostructures even more challenging.

[0016] This invention provides a novel, programmable fabrication paradigm. Atom alignment is marked using pump-probe time-resolved spectroscopy under a high-power microscope, aligning with reference directions of the upper and lower layers. Then, using a self-made mechanical rotation transfer platform, the lower layer is rotated before being transferred to the lower layer for bonding, creating a relative twist angle between the two layers. This angle is controlled using a helical micrometer, allowing the twist angle to be adjusted to any preset value (e.g., 0°, 1.1°, 30°, etc.) with an accuracy better than 0.1°, and the final stacking and bonding are completed at this angle. This 0.1° accuracy is tens of times higher than traditional twist angle heterojunction fabrication methods. This shift from "passive acceptance" to "active creation" represents a fundamental methodological innovation in the field of heterojunction fabrication.

[0017] Another significant innovation of this invention lies in its ingenious resolution of the contradiction between high-precision control and process feasibility. While some laboratory-level precision methods can achieve angle control, they are typically cumbersome, have low success rates, and are not scalable. This invention, based on a profound understanding of physics, aligns the reference directions of the upper and lower layers of materials and then uses a precision transfer device with alignment marks and identification criteria to rotate the lower layer to form a preset relative twist angle. This achieves the following advantages: high cleanliness and high-quality interface: This method does not rely on any chemical reagents or extreme conditions, ensuring atomic-level cleanliness and stability of the interface, thereby maximizing the performance advantages brought by twist angle control; strong universality: This method is not only applicable to two-dimensional materials such as WS2, but can also be extended to other two-dimensional materials with hexagonal and orthorhombic lattices, and has broad application prospects. Attached Figure Description

[0018] Figure 1 This is the pump-probe spectrum in this invention; Figure 2 This describes the preparation process of the heterojunction of the present invention.

[0019] Figure 1 In the middle: (a) is the angle pump-probe time-resolved spectrum, and (b) is the relationship between the peak value and angle of the pump-probe signal; Figure 2 (a) shows the transfer process of the first layer of two-dimensional material WS2, and (b) shows the preparation process of the WS2 / MoS2 heterojunction with a twist angle of θ. Detailed Implementation

[0020] Unless otherwise defined, 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 invention pertains; the terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to limit the invention; the term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0021] Example 1: A method for preparing a heterojunction with controllable torsion angle, which is to prepare a heterojunction by bonding two two-dimensional materials on a substrate at a set torsion angle; Materials include: The first two-dimensional material: monolayer WS2 (tungsten disulfide, hexagonal crystal system, lattice constant approximately 3.15 Å); The second type of two-dimensional material: monolayer MoS2 (molybdenum disulfide, hexagonal crystal system, lattice constant of about 3.16 Å); Substrate: p-type silicon wafer with 300 nm SiO2 on the surface (used after cleaning); Auxiliary materials: PDMS film, blue film tape; Two-dimensional material pretreatment before the experiment: WS2 and MoS2 monolayers were obtained by mechanical peeling. WS2 and MoS2 bulk materials were repeatedly pasted and peeled off with blue film tape. The tape with thin layer of material was pressed onto the PDMS film and slowly peeled off to obtain monolayer or few-layer WS2 and MoS2. Substrate cleaning: The SiO2 / Si substrate was ultrasonically cleaned with acetone, isopropanol and deionized water for 10 minutes each, and then dried with nitrogen for later use. Experimental steps: S1. Angle-dependent pump-probe time-resolved spectral measurement and orientation marking: pump wavelength: 400 nm (Ti:sapphire laser, 100 fs pulse width), probe wavelength: 800 nm, a polarizer is inserted into the probe optical path, the polarization direction is set to horizontal, the sample is placed on a precision rotating stage, and the rotation step accuracy is 0.1°. The WS2 sample was rotated 360° in 0.1° increments, and the pump-probe time-resolved spectrum was measured at each angle. The change in the pump-probe signal peak value (ΔR / R) as a function of angle was recorded. The above process was repeated to measure the MoS2 sample, and the ΔR / R peak-angle curves of WS2 and MoS2 were extracted (e.g., Figure 1 Figure a shows a partial angle-pumped-detector time-resolved spectrum. Figure 1 b represents the functional relationship between the two-dimensional material and the pump-probe signal); For hexagonal crystal systems WS2 and MoS2, the ΔR / R peak value has a maximum value every 60°. Rotate the sample to the maximum value of the ΔR / R peak value, at which point the atomic arrangement direction (such as the armchair direction) is parallel to the polarization direction. Under a high-power optical microscope (100× objective lens), draw a short straight line on the sample surface along the horizontal direction (i.e. the direction of the maximum peak value) as a reference mark. S2, such as Figure 2 a) Transfer the WS2 with reference mark (still attached to PDMS) to the SiO2 / Si substrate by dry method. After transfer, the reference mark of WS2 can be arbitrarily oriented. Fix the substrate to a self-made precision rotary displacement stage (integrated with a spiral micrometer structure and vernier reading, with an angular resolution of 0.1°). S3, such as Figure 2b. Adjust the microscope focus to the plane where MoS2 is located (still on PDMS), rotate the stage to make the reference mark direction of MoS2 strictly parallel to the reference mark direction of WS2 on the substrate, and record the stage angle reading θ1 at this time; apply a preset torsion angle θ, and calculate the target angle: θ2 = θ1 ± θ; slowly rotate the stage to θ2, at which point the theoretical torsion angle of MoS2 relative to WS2 is θ; while maintaining a stable angle, control the transfer device to slowly press MoS2 down until it contacts WS2, and naturally bond through van der Waals forces to form a WS2 / MoS2 heterojunction, remove PDMS, and complete the preparation of the heterojunction.

[0022] The precision rotary displacement stage is self-made, with an angle range of 360°, a resolution of 0.1°, and a repeatability of ±0.05°. Micro-transfer platform: Three-axis manual displacement stage, stepping accuracy 1μm; Environmental control: The experiment was conducted in a clean bench (Class 100) at a temperature of (25±1)℃ and a humidity of <30%.

[0023] Because of the anisotropy of optical response determined by crystal symmetry, regardless of whether the material is hexagonal, orthorhombic or other crystal systems, its optical response has an angular dependence consistent with the lattice symmetry, so it can have good applications. Moreover, as long as the material has an angle-dependent optical response (which is satisfied by almost all two-dimensional materials), its lattice orientation can be determined by measuring its angle-dependent pump-probe spectrum (or similar spectroscopic methods). By using pump-probe time-resolved spectroscopy to optically mark the lattice orientation of a material, and since the optical response of a two-dimensional material (such as carrier relaxation and exciton dynamics) is lattice symmetry dependent, its lattice orientation (such as armchair or zigzag orientation) can be determined by rotating the sample and measuring its optical response peak. Then, mechanical rotation alignment is performed based on the marked orientation. A high-precision rotary displacement stage is used to precisely control the relative angle between the two layers of material with the marked orientation as a reference.

[0024] This technology achieves integrated control of the torsion angle from design to fabrication, moving from passively accepting random angles and then selecting them to actively presetting target angles. It eliminates the dependence on expensive equipment such as high-resolution electron microscopes, allowing orientation calibration to be completed using only optical microscopes and spectral systems, thus improving the repeatability, success rate, and efficiency of fabrication. It is suitable for systematically studying the relationship between torsion angle and performance. It solves the problem of "not knowing the lattice orientation" through optical orientation marking, solves the problem of "not being able to precisely control the angle" through mechanical rotation alignment, and solves the problem of "interface contamination and damage" through dry transfer stacking. Ultimately, the angular resolution reaches 0.1°, which is 10–50 times higher than traditional methods. No post-screening is required, the success rate is significantly improved, and only an optical microscope, a spectroscopic system, and a self-made rotating stage are needed. TEM / MBE is not required. It is applicable to all two-dimensional materials that can be mechanically exfoliated (hexagonal, orthorhombic, etc. crystal systems), dry transfer, no chemical residue, clean interface, and preservation of intrinsic physical properties. The method can be extended to the preparation of multilayer stacks and complex heterostructures.

[0025] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. For those skilled in the art, the present invention can have various modifications and variations. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a heterojunction with controllable twist angle, characterized in that, The method comprises the following steps: Step 1: measuring the lattice direction response of the two-dimensional material by using angle-dependent pump-probe time-resolved spectroscopy, obtaining the relationship between the peak value of the optical signal and the sample rotation angle, determining the lattice reference direction of the material, and marking the reference direction on the surface of the material; Step 2: transferring and fixing the first two-dimensional material on a substrate; Step 3: placing the second two-dimensional material on a platform that can be precisely rotated, rotating the platform, and making the surface mark direction of the second two-dimensional material parallel to the mark direction of the first two-dimensional material fixed on the substrate, recording the angle reading of the rotating platform at this time as the initial reference angle; Step 4: calculating and controlling the rotating platform to rotate from the initial reference angle to the target angle according to the preset target twist angle, so as to form an accurate relative twist angle between the two two-dimensional materials; Step 5: transferring the second two-dimensional material to contact with the first two-dimensional material under the condition of maintaining the target angle, and bonding by Van der Waals force to form a heterojunction with a preset twist angle.

2. The production method according to claim 1, characterized by, In step 1, the wavelength of the pump light used for spectroscopy is 400 nm, the wavelength of the probe light is 800 nm, and the sample rotation step precision is 0.1 degrees.

3. The preparation method according to claim 1, characterized in that, The crystal structure of the first and second two-dimensional materials in step 1 includes all crystal systems, and is a single-layer or few-layer material of the hexagonal crystal system.

4. The method of claim 1, wherein, The first two-dimensional material and the second two-dimensional material are the same material or different materials.

5. The preparation method according to claim 1, characterized in that, In step 1, the determination of the lattice reference direction is based on the fact that for a hexagonal crystal system material, the peak value of the optical signal appears a maximum value every 60 degrees, and the sample is rotated to the peak maximum value, and the direction is the lattice reference direction.

6. The method of claim 1, wherein, The angle resolution of the precise rotating platform used in steps 3 and 4 is 0.1 degrees, and the repeat accuracy is plus or minus 0.05 degrees.

7. The preparation method according to claim 1, characterized in that, The material transfer in steps 2 and 5 uses a dry transfer process without using chemical solvents.

8. The method of claim 1, wherein, The preparation process is carried out in an environment with cleanliness better than 100 levels, the environmental temperature is controlled at 24-26 degrees Celsius, and the relative humidity is less than 30 percent.

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