A method for constructing multi-layer two-dimensional layered materials based on PVA film

Abstract

A method for building a multilayer two-dimensional layered material based on a PVA film, comprising the steps of preparing a PVA solution; preparing a PVA film; building a twisted homogeneous structure and a heterogeneous structure of a two-dimensional layered material; transferring the two-dimensional heterogeneous structure to a target substrate, etc. The present invention has the following technical effects: (1) The water solubility of the PVA film used ensures that the material interface will not introduce contamination, and the damage to the sample is minimal; (2) The high viscosity of PVA is sufficient to split the same piece of material into two, so the transfer process can accurately control the position of the material and the original orientation of the structural arrangement, meeting the requirements of the preparation of twisted angle homogeneous structures and heterogeneous structures, etc.; (3) A highly flexible, efficient, simple, non-destructive, and low-cost transfer solution is provided for two-dimensional layered materials.

Description

Technical Field

[0001] The present invention belongs to the field of preparation of nanomaterials, and particularly relates to an efficient transfer method for stacking two-dimensional layered materials. Background Art

[0002] Two-dimensional layered materials are crystal plane structures held together by strong in-plane covalent bonds and weak out-of-plane van der Waals forces. Each single layer can be detached from the whole by overcoming the van der Waals forces through mechanical exfoliation, while maintaining a high-quality crystal structure. Heterojunctions constructed from two-dimensional layered materials have the characteristics of a clean interface and are not restricted by lattice matching, which facilitates researchers to design a large number of samples with excellent properties, bringing infinite possibilities for electronic devices.

[0003] In the van der Waals layered structure, the twist angle between two-dimensional layered materials is an additional degree of freedom. Adjusting the angle between two atomic lattices can obtain structures with different properties, and the twist angle allows researchers to design a large number of combinations of van der Waals materials. To stack with a twist angle, it is necessary to know the structural arrangement of the materials to be stacked. The simplest way is to divide the same two-dimensional layered material into different parts, perform translation, apply the twist angle for stacking, and form a van der Waals homojunction with a twist angle.

[0004] Traditional mechanical exfoliation methods use films such as polymethyl methacrylate (PMMA), polypropylene carbonate (PPC), polycarbonate (PC), and polydimethylsiloxane (PDMS) as transfer interlayers, which have problems such as too high temperature requirements and introduction of contaminating elements. The most commonly used dry transfer method is to repeatedly peel the bulk material with tape to obtain a few-layer sample, then stick the few-layer sample on PDMS and tear it off to obtain the required thin-layer sample, and finally transfer it to the target substrate. The problem with this method is that the adhesion between different materials and different substrates is different. During the process of transferring the sample from PDMS to the substrate, the viscosity of PDMS is difficult to control. Especially when preparing homo / heterojunctions, if the adhesion between the material and PDMS is greater than the adhesion between the materials, the transfer process will be extremely difficult, and at the same time, it is easy to damage the sample. In addition, PDMS is likely to leave residual glue on the surface of the sample during the transfer process, contaminating the sample. Moreover, it is very difficult to pick up the sample after being transferred by PDMS, resulting in very low flexibility when constructing multi-layer heterostructures. Polyvinyl alcohol (PVA), as a water-soluble film, has a strong adhesion to two-dimensional layered materials after heating, and its viscosity can be controlled by controlling the temperature, and the water-soluble pollution is small, which has obvious advantages compared with other methods for preparing multi-layer two-dimensional structures. Summary of the Invention

[0005] The object of the present invention is to provide a method for constructing multi-layer two-dimensional layered materials based on PVA films, which has the characteristics of simplicity, pollution-free, low operation difficulty, and flexible transfer, and can achieve non-destructive, rapid and effective construction of multi-layer two-dimensional layered heterostructures and homogeneous structures with twist angles.

[0006] The present invention is achieved through the following technical solutions.

[0007] A method for constructing multi-layer two-dimensional layered materials based on PVA films according to the present invention includes the following steps.

[0008] Step 1: Preparation of PVA solution.

[0009] a. Add PVA particles with a molecular weight of 88,000 to a clean container, and add an appropriate amount of purified water to ensure that the final mass concentration of the solution is controlled at 2%-5%. Too high a concentration will result in insufficient dissolution, and too low a concentration will cause a decrease in the viscosity of the film.

[0010] b. Place the container on a magnetic heating platform, heat and stir until the PVA is completely dissolved, the solution is clear and transparent without bubbles, and store it sealed at room temperature and let it stand.

[0011] Step 2: Preparation of PVA film.

[0012] Cut a piece of PDMS film and stick it on a clean glass slide. Use a pipette to suck the PVA solution, drop it on the PDMS film, and use the tip of the pipette to scrape the PVA droplet to evenly moisten the PDMS. Then place the glass slide on a heating platform and heat at 60°C for at least 5 minutes. After the water is evaporated, a transparent and uniform PVA film with a thickness in the micron level is finally formed on the PDMS film.

[0013] Step 3: Constructing a two-dimensional layered material heterostructure.

[0014] a. Ultrasonically clean the silicon wafer in acetone, isopropyl alcohol, and deionized water in sequence to remove impurities on the surface of the silicon wafer, and dry the surface moisture with a nitrogen gun.

[0015] b. Repeatedly peel the preferred parent material with tape and stick it on the cleaned silicon wafer, and find the target layered material I to be transferred through a microscope.

[0016] c. Fix the silicon wafer processed in step 3b on the transfer stage, invert and fix the glass slide obtained in step 2 on the robotic arm of the transfer stage, with the PVA film side facing the silicon wafer. Operate the microscope of the transfer stage to focus it on the target layered material I to be transferred on the silicon wafer, control the horizontal movement of the robotic arm so that the PVA film completely covers the target layered material I; control the vertical movement of the robotic arm, and stick the glass slide to the silicon wafer so that the PVA film is completely attached to the target layered material I.

[0017] d. Fix the transfer table robot arm, turn on the heating table and heat it to 75℃, maintain it for 20 seconds, and then stop heating. After the silicon wafer cools to below 40℃, the PVA film solidifies, and the robot arm rises. At this time, the target sample I will be stuck to the PVA film and rise together with the slide.

[0018] e. Replace the matrix material, and after repeated dissociation with tape, stick it on the silicon wafer that has been ultrasonically cleaned with acetone, isopropanol, and deionized water. Use a microscope to find the target layered material II that needs to be transferred, and replace the silicon wafer on the transfer platform with the silicon wafer with the target layered material II. Operate the transfer table microscope to focus it on the target layered material II to be transferred, control the movement of the robotic arm to align the two samples to the desired position, lower the robotic arm to press the glass slide and the silicon wafer together to form a two-dimensional heterogeneous structure. Turn on the heating table and heat it to 75°C, maintain it for 20 seconds, and then stop heating. When the silicon wafer cools to below 40°C and the PVA film solidifies, raise the robotic arm, and the two-dimensional heterogeneous structure is lifted up. The overall structure is as shown. Figure 1 shown.

[0019] Step 4: Transfer the 2D heterostructure to the target substrate.

[0020] a. Based on step 3e, replace the target substrate and fix it on the transfer table. Control the transfer table robot to descend. During the descent, align the two-dimensional heterostructure to the target substrate position and press the glass slide against the substrate. Turn on the heating table, raise the temperature to 90°C and maintain for 20 seconds. Slowly raise the robot arm. The PVA film will separate from the PDMS film and remain on the target substrate.

[0021] b. Place the substrate with the two-dimensional heterostructure and PVA film into purified water and heat it to 80°C. After 30 minutes, the PVA film is completely dissolved. Remove the substrate and use a helium gun to blow away the surface moisture. At this point, the process of building the two-dimensional layered material heterostructure with the help of the PVA film is completed.

[0022] Step 3 described in the present invention can be repeated multiple times to stack the same or different materials, or the same sample can be torn using PVA to apply a twist angle for stacking to form a multilayer two-dimensional heterostructure or a homogeneous structure with a twist angle.

[0023] Compared with the traditional method, the advantages of the present invention are as follows.

[0024] (1) The water solubility of the PVA film used in the present invention ensures that no contamination is introduced into the material interface and minimal damage is caused to the sample.

[0025] (2) The high viscosity of PVA in the present invention is sufficient to split the same piece of material into two, so the transfer process can accurately control the position of the material and the original orientation of the structural arrangement, meeting the requirements of the preparation of torsion angle homogeneous structures and heterogeneous structures.

[0026] (3) The present invention provides a highly flexible, efficient, simple, non-destructive, and low-cost transfer solution for two-dimensional layered materials. Description of the Drawings

[0027] Figure 1 It is a schematic diagram of the final structure after step 3 of the present invention. 1 is a glass slide, 2 is a PDMS film, 3 is a PVA film, 4 is the target two-dimensional layered material I, and 5 is the target two-dimensional layered material II.

[0028] Figure 2 It is an optical image of the final structure of Example 1 of the present invention.

[0029] Figure 3 It is an optical image of the final structure of Example 2 of the present invention. Detailed Embodiments

[0030] The present invention will be further described below in conjunction with the accompanying drawings and through the following embodiments.

[0031] Example 1

[0032] (1) Preparation of the PVA solution: Add 0.25 g of PVA to a clean container, add 10 ml of deionized water, place the container on a magnetic heating platform, heat and stir at 90 °C until the PVA is completely dissolved. The solution is clear and transparent as a whole without bubbles, and is stored sealed at room temperature and allowed to stand to obtain a PVA solution with a mass concentration of 2.5%.

[0033] (2) Preparation of the PVA film: Cut a 4×4 mm-sized PDMS and stick it on a clean glass slide. Use a pipette to aspirate 2 μL of the PVA solution and drop it on the PDMS to form a droplet with a diameter of about 2 mm. Scratch the PVA droplet with the tip of the pipette to evenly moisten the PDMS, and then place the glass slide on a heating platform and heat at 60 °C for 5 minutes. After the water is evaporated, a PVA film with a thickness in the micron level, a diameter of about 2 mm, and transparent and uniform is finally formed on the PDMS.

[0034] (3)Fabricate a two-dimensional layered material heterostructure: Ultrasonically clean the silicon wafer successively with acetone, isopropyl alcohol, and deionized water to remove impurities on the surface of the silicon wafer, and then dry the surface moisture with a nitrogen gun. Repeatedly peel off a small piece of hexagonal boron nitride (HBN) sample with tape. After dissociation for multiple times, attach the sample area to the cleaned silicon wafer, and find the target HBN thin layer to be transferred through a microscope. Fix the silicon wafer with the target sample on the transfer stage, invert and fix the glass slide with PDMS / PVA on the robotic arm of the transfer stage, with the PVA film side facing the silicon wafer. Operate the microscope on the transfer stage to focus it on the HBN thin layer to be transferred on the silicon wafer, control the horizontal movement of the robotic arm so that the PVA film completely covers the HBN; control the vertical movement of the robotic arm, attach the glass slide to the silicon wafer so that the PVA film is completely adhered to the HBN. Fix the robotic arm of the transfer stage, turn on the heating stage and heat it up to 75 °C, maintain for 20 s, and then stop heating. After the silicon wafer cools to below 40 °C, the PVA film solidifies. Raise the robotic arm, and at this time the HBN thin layer will be adhered by the PVA film and rise together with the glass slide.

[0035] Replace the material with vanadium diselenide. Ultrasonically clean the silicon wafer successively with acetone, propanol, and deionized water to remove impurities on the surface of the silicon wafer, and dry the surface moisture with a nitrogen gun. Repeatedly peel off a small piece of vanadium diselenide sample with tape. After dissociation for multiple times, attach the sample area to the cleaned silicon wafer, and replace the silicon wafer on the transfer platform with the silicon wafer with VSe2. Operate the center of the microscope on the transfer stage to focus it on the horizontal plane of the HBN sample, adjust the screw in the horizontal direction of the robotic arm, and focus the center of the microscope image on the target HBN sample. Operate the microscope on the transfer stage again to focus it on the substrate, adjust the screw in the horizontal direction of the transfer stage, and focus the center of the microscope image on the substrate VSe2 sample. During the process of lowering the robotic arm, continuously adjust the overlapping position of the two samples until the PVA film and the silicon wafer are tightly attached. Turn on the heating stage, heat it up to 90 °C and maintain for 10 s, slowly raise the robotic arm, and the PVA film will detach from the PDMS film and remain on the silicon wafer.

[0036] (4)Put the silicon wafer with the sample and the PVA film into deionized water, heat it to 80 °C, and after 30 minutes, the PVA film is completely dissolved. Take out the substrate and gently blow off the surface moisture with a helium gun. Finally, obtain Figure 2 the HBN-coated VSe2 sample on the metal-coated silicon wafer as shown.

[0037] From Figure 2 the optical image, it can be seen that we obtain the VSe2 sample coated with a thin layer of HBN on the silicon substrate. After the PVA is dissolved in water, the sample and the surface of the silicon wafer are clean enough, and the sample morphology is excellent.

[0038] It should be noted that Example 1 only includes a simple double-layer structure with HBN as the protective layer on the silicon wafer. In some optional embodiments of the present invention, the entire structural step (3) can be repeated multiple times to stack different materials, and there are many choices for the final target substrate.

[0039] Example 2

[0040] (1) Preparation of PVA solution: Add 0.25 g of PVA into a clean container, add 10 ml of deionized water, place the container on a magnetic heating platform, heat and stir at 90 °C until the PVA is completely dissolved. The solution is clear and transparent as a whole and has no bubbles. Seal and store it at room temperature and let it stand still to obtain a PVA solution with a mass concentration of 2.5%.

[0041] (2) Preparation of PVA film: Cut a 4×4 mm PDMS and stick it on a clean glass slide. Use a pipette to suck 2 μL of the PVA solution and drop it on the PDMS film. Scratch the PVA droplet with the tip of the pipette to evenly moisten the PDMS. Then place the glass slide on a heating platform and heat it at 60 °C for at least 5 minutes. After the water is evaporated, finally, a PVA film with a thickness in the micron level, a diameter of about 2 mm, and being transparent and uniform is formed on the PDMS.

[0042] (3) Build a homogeneous structure with a twist angle: Ultrasonically clean the silicon wafer successively with acetone, isopropyl alcohol, and deionized water to remove impurities on the surface of the silicon wafer, and dry the surface moisture with a nitrogen gun. Repeatedly dissociate a small piece of hexagonal boron nitride sample with tape. After dissociating multiple times, stick the sample area on the cleaned silicon wafer and then tear it off. Find the strip-shaped HBN thin layer to be transferred on the silicon wafer through a microscope. Fix the silicon wafer with the target sample on the transfer stage, invert and fix the glass slide with PDMS / PVA on the robotic arm of the transfer stage, with the PVA film side facing the silicon wafer. Operate the microscope of the transfer stage to focus it on the HBN thin layer to be transferred on the silicon wafer, and control the horizontal movement of the robotic arm so that the PVA film completely covers the HBN.

[0043] Operate the robotic arm to lower it, align half of the HBN with the PVA film and separate the other half. Turn on the heating stage and heat it to 80°C. During the heating process, continuously fine-tune the height of the robotic arm's Z-axis to maintain the contact surface between the PVA film and the HBN without deviation. After maintaining the temperature for 10s, slightly lift the robotic arm, which can cause the HBN sheet to crack along the contact surface. If the sample cannot be torn, increase the temperature and try multiple times. After cracking the sample, adjust the fine-tuning screw of the transfer stage robotic arm angle by 0.1°, and adjust the screw in the horizontal direction of the robotic arm to translate and align the two pieces of HBN to re-bond and construct a twist angle homojunction. Lower the robotic arm to make the two samples fit tightly. Fix the transfer stage robotic arm, turn on the heating stage and raise the temperature to 75°C. After maintaining for 20s, stop heating. After the silicon wafer cools below 40°C, the PVA film solidifies. Raise the robotic arm, and at this time, the HBN homojunction will be stuck by the PVA film and rise together with the glass slide.

[0044] Replace the silicon wafer substrate coated with metal, fix the substrate on the transfer stage, operate the transfer stage microscope to focus it on the sample horizontal plane at the center, adjust the screw in the horizontal direction of the robotic arm, and focus the center of the microscope screen on the target sample. Operate the transfer stage microscope to focus it on the substrate again, adjust the screw in the horizontal direction of the transfer stage, and focus the center of the microscope screen on the target position to be transferred. Adjust the screw of the robotic arm's Z-axis to lower the robotic arm, so that the boron nitride homojunction and the PVA film are completely attached to the metal-coated silicon substrate. Turn on the heating stage, raise the temperature to 90°C and maintain for 10s, and slowly raise the robotic arm. The PVA film will separate from the PDMS film and remain on the silicon wafer.

[0045] (4) Put the silicon wafer with the sample and the PVA film into deionized water, heat it to 80°C. After 30 minutes, the PVA film completely dissolves. Remove the substrate, and gently blow the surface moisture with a helium gun. Finally, obtain the HBN twist angle homojunction on the metal-coated silicon substrate.

[0046] From Figure 3 the optical image, it can be seen that we obtain the sample of the HBN twist angle homojunction on the silicon substrate coated with titanium-nickel metal. The final surface morphologies of the substrate and the sample both show excellent performance, and there is no residual trace of PVA.

[0047] The above-described embodiments only represent several implementation manners of the present invention. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the present invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several deformations and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the present invention patent should be subject to the appended claims.

Claims

1. A method for constructing a multi-layer two-dimensional layered material based on a PVA film, characterized in that It includes the following steps: Step 1: Preparation of PVA solution: a. Add PVA particles with a molecular weight of 88,000 to a clean container, add an appropriate amount of purified water, and control the final solution concentration at 2% - 5%; b. Place the container on a magnetic heating platform, heat and stir until the PVA is completely dissolved, the solution is clear and transparent without bubbles, and store it sealed at room temperature and let it stand; Step 2: Preparation of PVA film: Cut a piece of PDMS film and stick it on a clean glass slide. Use a pipette to suck the PVA solution, drop it on the PDMS film, and use the tip of the pipette to scrape the PVA droplet to evenly moisten the PDMS. Then place the glass slide on the heating platform and heat at 60 °C for at least 5 minutes. After the water is evaporated, finally, a PVA film with a thickness in the micron level and transparent and uniform is formed on the PDMS film; Step 3: Construct a two-dimensional layered material heterostructure: a. Ultrasonically clean the silicon wafer in turn with acetone, isopropyl alcohol, and deionized water to remove impurities on the surface of the silicon wafer, and blow dry the surface moisture with a nitrogen gun; b. Repeatedly peel the preferred parent material with tape and stick it on the cleaned silicon wafer. Find the target layered material I to be transferred through a microscope; c. Fix the silicon wafer processed in step 3b on the transfer platform, invert and fix the glass slide obtained in step 2 on the robotic arm of the transfer platform, with the PVA film side facing the silicon wafer; Operate the microscope of the transfer platform to focus it on the target layered material I to be transferred on the silicon wafer, control the horizontal movement of the robotic arm so that the PVA film completely covers the target layered material I; Control the vertical movement of the robotic arm and stick the glass slide to the silicon wafer so that the PVA film is completely adhered to the target layered material I; d. Fix the robotic arm of the transfer platform, turn on the heating platform and raise the temperature to 75 °C, maintain it for 20 s, and then stop heating; After the silicon wafer cools below 40 °C, the PVA film solidifies, raise the robotic arm, and the target sample I is picked up by the PVA film and rises together with the glass slide; e. Replace the parent material, repeatedly dissociate it with tape, and stick it on the silicon wafer ultrasonically cleaned with acetone, isopropyl alcohol, and deionized water. Find the target layered material II to be transferred through a microscope, and replace the silicon wafer on the transfer platform with the silicon wafer with the target layered material II; Operate the microscope of the transfer platform to focus it on the target layered material II to be transferred, control the movement of the robotic arm so that the two samples are aligned to the designed position, lower the robotic arm to stick the glass slide and the silicon wafer tightly to form a two-dimensional heterostructure; Turn on the heating platform and raise the temperature to 75 °C, maintain it for 20 s, and then stop heating; After the silicon wafer cools below 40 °C, the PVA film solidifies, raise the robotic arm, and the two-dimensional heterostructure is lifted; Step 4: Transfer the two-dimensional heterostructure to the target substrate: a. On the basis of step 3e, replace the target substrate and fix it on the transfer platform. Control the robotic arm of the transfer platform to lower. During the lowering process, align the two-dimensional heterostructure to the position of the target substrate and stick the glass slide and the substrate tightly; Turn on the heating platform, raise the temperature to 90 °C and maintain it for 20 s, slowly raise the robotic arm, and the PVA film will detach from the PDMS film and remain on the target substrate; b. Place the substrate with the two-dimensional heterostructure and the PVA film into purified water, heat it to 80 °C, and after 30 minutes, the PVA film is completely dissolved. Remove the substrate and gently blow off the surface moisture with a helium gun to complete the construction of the two-dimensional layered material heterostructure.

2. The method for constructing a multi-layer two-dimensional layered material based on a PVA film according to claim 1, characterized in that The step 3 is repeated multiple times to stack the same or different materials to form a multi-layer two-dimensional heterostructure.

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