Dry transfer method of corner two-dimensional material

Abstract

The invention belongs to the field of two-dimensional material preparation, and particularly relates to a dry transfer method of a corner two-dimensional material. On one hand, introduction of impurities is avoided by adopting a plurality of organic polymers for assistance through a splitting step mechanism; on the other hand, by means of the mechanism that the two-dimensional material slides on the h-BN interface and does not exceed the plane geometrical shape of the h-BN interface, the two-dimensional material is locked on the h-BN geometrical interface through laser cutting, the probability that the two-dimensional material wrinkles, bubbles and slides in the transfer process is remarkably reduced, meanwhile, the internal stress of a system is reduced, and the mechanical performance of the system is improved. Therefore, the accuracy of the transfer angle of the corner two-dimensional material is improved; and the phenomena of slippage, traction and the like do not occur between the corner two-dimensional materials, lattice distortion is also weakened, and the quality of the corner two-dimensional materials is further improved. According to the method, reliable technical support is provided for constructing a high-quality strong-correlation physical platform, and the method can be widely applied to the fields of novel quantum devices and the like.

Description

Technical Field

[0001] The present invention belongs to the field of two-dimensional material preparation, and specifically relates to a dry transfer method for corner two-dimensional materials. Background Art

[0002] Since the successful exfoliation of graphene in 2004, two-dimensional material systems represented by graphene, transition metal chalcogenides, and hexagonal boron nitride (h-BN) have shown unique band structures and valley electronics. However, the functionality of single-layer two-dimensional materials has always been limited by their physical dimensions, which has restricted the further development of two-dimensional materials. This bottleneck has been broken with the emergence of corner two-dimensional materials.

[0003] As a revolutionary discovery at the intersection of condensed matter physics and nanotechnology, corner two-dimensional materials are reconstructing human cognition of quantum state control. Through the development of the new research field of corner electronics, people have found that when two two-dimensional materials are stacked at a specific angle, the system will form a moiré superlattice, and cause a significant reconstruction of the electronic energy band, thereby changing the properties of the original two-dimensional material. This not only provides an ideal research platform for strongly correlated physics, but also creates a new method to control the quantum state of materials through the transfer of two-dimensional materials. This two-dimensional material transfer method breaks through the limitations of chemical synthesis of traditional materials, making it possible to design topological quantum states by artificially controlling the moiré periodic potential field, showing revolutionary application prospects in new quantum devices, low-power electronics, and quantum information processing.

[0004] Current methods for transferring two-dimensional materials mainly include dry transfer and wet transfer. Wet transfer usually transfers two-dimensional materials, such as graphene, on metal or silicon dioxide substrates by spin coating a polymer support layer, then corroding the underlying substrate, and finally transferring the material to the target substrate. It is difficult to accurately control the angle, it is easy to cause mechanical damage to the sample, and the solution used usually contaminates the two-dimensional material. Therefore, dry transfer is more used in the preparation process of corner two-dimensional materials.

[0005] The dry corner 2D material transfer methods currently used are mostly "top-to-bottom" transfer methods, that is, through the viscosity of organic matter and the van der Waals force between 2D materials, organic matter such as bisphenol A polycarbonate (PC) is used to pick up the 2D materials layer by layer from top to bottom, and only one transfer process is required to assemble a complete heterojunction. However, this transfer method has problems such as difficulty in accurately controlling the corner, a large amount of stress during transfer, easy introduction of impurities such as bubbles during transfer, and easy wrinkling of the 2D material, which seriously restrict the quality and development of corner 2D material samples.

[0006] Therefore, developing a transfer method that can accurately control the rotation angle, reduce stress, and avoid bubbles and wrinkles is of great positive significance for preparing high-quality two-dimensional material samples with rotation angles and building a strong correlation physics platform. Summary of the Invention

[0007] In view of the above problems or deficiencies, to solve various quality problems existing in the existing transfer methods of two-dimensional materials with rotation angles, the present invention provides a dry transfer method for two-dimensional materials with rotation angles.

[0008] To achieve the above object, the present invention adopts the following technical solutions:

[0009] A dry transfer method for two-dimensional materials with rotation angles, comprising the following steps:

[0010] Step 1: Using a two-dimensional material transfer platform, transfer the bottom layer h-BN and the bottom gate of a preset heterojunction to a silicon oxide substrate using polypropylene carbonate (PPC), and clean the organic substances remaining on the surface of the bottom layer h-BN through annealing and atomic force microscopy.

[0011] Step 2: Pre-cut the two-dimensional material to be used into two pieces. Use PPC to pick up the top layer h-BN of the preset heterojunction and place it on the first piece of two-dimensional material, avoiding PPC from contaminating the other piece of two-dimensional material. Among them, the area of the top layer h-BN is smaller than that of the two pieces of two-dimensional materials, so that the two pieces of two-dimensional materials can be cut along the boundary of h-BN subsequently.

[0012] Then raise the temperature to 60-80 °C to make the PPC lose its viscosity and separate from the top layer h-BN. The present invention utilizes the property that PPC has viscosity at 50 °C and loses its viscosity at 60-80 °C. Therefore, PPC is selected to pick up the top layer h-BN in Step 2, and then it is separated from h-BN by raising the temperature, and it will not contaminate the first piece of two-dimensional material.

[0013] Then use a laser to cut the first piece of two-dimensional material along the boundary of the top layer h-BN, so that the cut boundary coincides with the boundary of the top layer h-BN.

[0014] Step 3: Use bisphenol A polycarbonate (PC) to pick up the heterojunction obtained in Step 2 (the top layer h-BN and the first layer of two-dimensional material after cutting), where PC contacts the upper surface of h-BN, and place the lower surface of the first piece of two-dimensional material on the second piece of two-dimensional material after rotating a specific angle; the specific angle is calculated according to the material type of the two-dimensional material with rotation angles.

[0015] Then use chloroform solution to remove PC, and then use a laser to cut the second piece of two-dimensional material along the boundary of the top layer h-BN, so that its boundary also coincides with the boundary of the top layer h-BN.

[0016] The reason for choosing PC instead of PPC in step 3 of the present invention is that PC has better viscosity and softness, which makes the angle more accurate during transfer and bubbles less likely to be generated. When using chloroform solution to remove PC, due to the protection of the top-layer h-BN, the two-dimensional materials at the corners below h-BN are not contaminated by the solution.

[0017] Step 4: Use PC to pick up the heterojunction obtained in step 3, where PC contacts the upper surface of h-BN, place the lower surface of the second two-dimensional material of the heterojunction on the upper surface of the bottom-layer h-BN obtained in step 1, and remove PC to obtain a complete heterojunction of the corner two-dimensional material.

[0018] The complete heterojunction of the corner two-dimensional material, from top to bottom, is the top-layer h-BN, the first two-dimensional material, the second two-dimensional material after cornering, the bottom-layer h-BN, and the bottom gate, where the boundary of the corner two-dimensional material coincides with the boundary of the top-layer h-BN.

[0019] Furthermore, in the above steps, PC or PPC picks up h-BN relying on its own viscosity, while h-BN picks up two-dimensional materials relying on van der Waals force; by using h-BN to pick up two-dimensional materials in the present invention, the contamination of the internal corner two-dimensional materials by organic substances is avoided.

[0020] Furthermore, the annealing process in step 1 is as follows: use an Ar / H2 mixed gas with a H2 volume fraction of 5% - 10%, a flow rate of 80 - 120 sccm, and keep it at a high temperature of 300 - 400 °C for 3 - 5 h.

[0021] Furthermore, when using atomic force microscope (AFM) to clean the bottom-layer h-BN in step 1, the contact mode is used, and the force applied to the tip is 50 - 300 nN.

[0022] Furthermore, the femtosecond laser with a central wavelength of 517 nm and a maximum power of 150 mW is used for laser cutting in steps 2 and 3; its pulsed laser can cut common two-dimensional materials such as graphene within ten layers, MoS2, WS2, WSe2, MoTe2 within three layers, but cannot cut h-BN.

[0023] Furthermore, the two-dimensional materials to be used are single-layer graphene, bilayer graphene, trilayer graphene, single-layer WSe2, bilayer WSe2, single-layer MoTe2, bilayer MoTe2, single-layer MoS2, or bilayer MoS2.

[0024] The principle of the dry transfer method for the two-dimensional material with a twist angle in the present invention is as follows: Since the h-BN interface is superlubricious, other two-dimensional materials are prone to relative sliding on this interface, and this sliding will not exceed the planar geometry of h-BN. Therefore, when the planar shape of the two-dimensional material with a twist angle is made consistent with that of h-BN through laser cutting, it will be "locked" on the geometric interface of h-BN. On the one hand, through the split-step mechanism (separate use of PPC and PC, protection of h-BN) and with the assistance of various organic polymers, the introduction of impurities is avoided. On the other hand, through laser cutting, the two-dimensional material with a twist angle itself is "locked", so the probability of wrinkles and bubbles in the two-dimensional material with a twist angle will be significantly reduced, and there will be no phenomena such as slip and traction between the two-dimensional materials with a twist angle, the internal stress in the system will also be reduced, and the lattice distortion will also be weakened, which further improves the quality of the twist angle of the two-dimensional material with a twist angle.

[0025] In summary, by splitting the existing transfer process into several steps and combining and applying the step-by-step mechanism, the present invention makes the transfer process highly controllable, greatly improves the angular accuracy of the two-dimensional material sample with a twist angle, significantly reduces the probability of wrinkles, bubbles and slip during the transfer process of the material, and at the same time reduces the internal stress in the system. The present invention provides a reliable technical support for constructing a high-quality strongly correlated physical platform, which is conducive to the two-dimensional material with a twist angle showing a wider range of applications in the fields of new quantum devices and the like. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 is a flowchart of the present invention.

[0027] Figure 2 is a schematic diagram of the complete heterojunction structure of the two-dimensional material with a twist angle after transfer of the present invention.

[0028] Figure 3 is a photomicrograph of the twist bilayer graphene sample prepared in the embodiment under an optical microscope.

[0029] Figure 4 is a photomicrograph of the device obtained after micro-nano processing of the twist bilayer graphene sample prepared in the embodiment under an optical microscope; where the letters A to G respectively represent different electrodes.

[0030] Figure 5 is a graph of the change of the longitudinal resistance with the filling number of the device obtained after micro-nano processing of the twist bilayer graphene sample prepared in the embodiment through transport measurement.

[0031] Figure 6 is for the twist bilayer graphene sample prepared in the embodiment in Figure 4 the measured twist angle map near the indicated electrodes.

[0032] Figure numerals: 1-top layer h-BN, 2-first two-dimensional material, 3-second two-dimensional material after turning, 4-bottom layer h-BN, 5-bottom grid. DETAILED DESCRIPTION

[0033] The technical solution of the present invention is described in detail below in conjunction with the embodiments and drawings.

[0034] Example

[0035] A dry transfer method for corner two-dimensional materials, the specific steps are as follows:

[0036] Step 1: Use a two-dimensional material transfer platform and an organic carrier composed of polydimethylsiloxane (PDMS) and polypropylene carbonate (PPC), in which PDMS plays a supporting role; raise the temperature of the transfer platform to 50°C, use the above-mentioned carrier to first pick up the bottom layer h-BN, and then use h-BN to pick up the graphite bottom grid and place it on the marked silicon wafer.

[0037] Annealing was performed at 300°C in an Ar / H2 mixture (H2 volume fraction was 5%) for 3 hours. After annealing, an atomic force microscope (AFM) was used in contact mode to apply a force of 100 nN to the needle tip to clean the underlying h-BN surface. Cleaning was completed when the AFM height map showed no fluctuations.

[0038] Step 2: First, use a femtosecond laser with a central wavelength of 517nm and a maximum power of 150mW to cut a single-layer graphene sample into two pieces; raise the temperature of the transfer platform to 50°C, use an organic carrier composed of PDMS and PPC, pick up the top layer of h-BN, and place it on the first single-layer graphene, avoiding contamination of the other graphene to be used during the process.

[0039] The temperature is then raised to 80°C, causing the PPC to lose its viscosity and separate from the top h-BN layer, leaving the top h-BN and single-layer graphene heterojunction on the silicon wafer.

[0040] Then use a femtosecond laser to cut along the boundary of the top layer of h-BN so that the boundary of the graphene underneath it coincides with it.

[0041] Step 3: At 100°C, pick up the top layer of h-BN and the cut graphene in step 2 using a carrier made of PDMS and bisphenol A polycarbonate (PC), rotate it 1.1° (specific angle), and place it on the second graphene.

[0042] Then, chloroform solution was used to remove PC, and the graphene under the top h-BN layer was protected by the top h-BN layer and was not contaminated by the chloroform solution.

[0043] Use a laser again to cut the rotated graphene along the boundary of the top-layer h-BN, so that it also coincides with the boundary of the top-layer h-BN.

[0044] Step 4: At 120 °C, use PDMS and a PC carrier again to pick up the heterojunction in Step 3 and slowly place it on the bottom-layer h-BN that has been cleaned in Step 1 at 140 °C. Thus, the entire transfer process is completed; finally, use chloroform solution to remove the PC.

[0045] Use an optical microscope to observe the sample obtained in this embodiment, as Figure 3 shown, Figure 3 The white part in the middle is the part where the top-layer h-BN and the rotated bilayer graphene below it are located. It can be observed that there are no wrinkles in the rotated two-dimensional material sample area, and there are no other bubbles except in the upper right corner.

[0046] Subsequently, use micro-nano processing technology to process the obtained sample into a circular device, as Figure 4 shown, where the letters A to G respectively represent different electrodes. Through transport measurement, at a temperature of 6 K, use a source meter to change the bottom gate voltage. Under an AC current bias of 10 nA, the change in the longitudinal resistance at the FG port is measured by the four-terminal resistance measurement method, as Figure 5 shown. At a temperature of 6 K, we observed a series of resistance peaks at integer fillings, which indicates the existence of a correlated insulating state, that is, electrons enter a strongly correlated state. In addition, by calculating the transfer angle through the carrier concentration, the transfer angle at this port is obtained as 1.09°, which is only 0.01° different from our actual transfer angle of 1.1°. The transfer angles at other ports are as Figure 6 shown, which indicates the uniformity of the sample and further proves the characteristics of the sample in this embodiment, such as no wrinkles, few bubbles, and little stress.

[0047] As can be seen from the above embodiments, based on the dry transfer method of the rotated two-dimensional material provided by the present invention, we obtained a rotated two-dimensional material sample without wrinkles, almost no bubbles, highly uniform in angle, and with little stress. Its transport data indicates the successful construction of a strongly correlated physical platform. Therefore, the present invention is of great significance for both the research on strongly correlated physics and the practical applications in new quantum devices and low-power electronics.

Claims

1. A dry transfer method for corner two-dimensional materials, characterized in that: The following steps are involved: Step 1: Using a two-dimensional material transfer platform, polypropylene carbonate (PPC) is used to transfer the bottom layer h-BN and bottom gate of the preset heterojunction to a silicon oxide substrate, and the organic matter remaining on the surface of the bottom layer h-BN is cleaned by annealing and atomic force microscopy; Step 2: Pre-cut the two-dimensional material to be used into two pieces, use PPC to pick up the top layer h-BN of the preset heterojunction, and place it on the first two-dimensional material; wherein the area of ​​the top layer h-BN is smaller than the two two-dimensional materials; Then the temperature is raised to 60-80°C to make PPC lose its viscosity and separate from the top layer of h-BN; Then, the laser is used to cut the first two-dimensional material along the boundary of the top h-BN layer, so that the boundary after cutting coincides with the boundary of the top h-BN layer. Step 3: Pick up the heterojunction obtained in step 2 using bisphenol A polycarbonate PC, wherein PC contacts the upper surface of h-BN, and place the lower surface of the first two-dimensional material on the second two-dimensional material after rotating at a specific angle; the specific angle is calculated based on the material type of the corner two-dimensional material; Then, after removing the PC using a chloroform solution, a laser is used to cut the second 2D material along the boundary of the top h-BN layer so that its boundary also coincides with the boundary of the top h-BN layer. Step 4: Use PC to pick up the heterojunction obtained in step 3, wherein the PC contacts the upper surface of h-BN, place the lower surface of the second layer of two-dimensional material of the heterojunction on the upper surface of the bottom layer h-BN obtained in step 1, and remove the PC to obtain a complete heterojunction of the corner two-dimensional material; The complete heterojunction of the corner two-dimensional material comprises, from top to bottom, a top layer of h-BN, a first piece of two-dimensional material, a second piece of two-dimensional material after the corner, a bottom layer of h-BN and a bottom gate, wherein the boundary of the corner two-dimensional material coincides with the boundary of the top layer of h-BN.

2. The dry transfer method of corner two-dimensional material according to claim 1, characterized in that: In the above step, PC or PPC picks up h-BN by relying on its own viscosity, while h-BN picks up two-dimensional materials by relying on van der Waals force.

3. The dry transfer method of corner two-dimensional material according to claim 1, characterized in that: The annealing process in step 1 is: Use Ar / H2 mixed gas with a H2 volume fraction of 5% to 10% and a flow rate of 80 to 120 sccm at a high temperature of 300 to 400°C for 3 to 5 hours.

4. The dry transfer method of corner two-dimensional material according to claim 1, characterized in that: In step 1, the atomic force microscope is used to clean the bottom h-BN in contact mode, and the force applied to the needle tip is 50 to 300 nN.

5. The dry transfer method of corner two-dimensional material according to claim 1, characterized in that: The laser cutting in steps 2 and 3 uses a femtosecond laser with a central wavelength of 517 nm and a maximum power of 150 mW.

6. The dry transfer method of corner two-dimensional material according to claim 1, characterized in that: The two-dimensional material to be used is single-layer graphene, double-layer graphene, triple-layer graphene, single-layer WSe2, double-layer WSe2, single-layer MoTe2, double-layer MoTe2, single-layer MoS2 or double-layer MoS2.

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