Method for transferring two-dimensional materials to microgrids using water-soluble organic polymer PVA

Abstract

This invention provides a method for transferring two-dimensional materials to a microgrid using the water-soluble organic polymer PVA, comprising the following steps: S1, preparing a PVA solution; S2, preparing a sample to be observed on a silicon wafer; S3, preparing a PVA thin film on the silicon wafer by spin coating; S4, transferring the sample from the silicon wafer to the microgrid; S5, obtaining the transferred product. This method is less likely to damage the sample and is suitable for transferring fragile or complex samples such as few-layer two-dimensional materials and heterojunctions. The microgrid only contacts the dry thin film, thus minimizing the probability of damaging the microgrid and achieving the transfer of a specified sample to a specified location on the microgrid. This also makes this method applicable to transferring samples to in-situ chips.

Description

Technical Field

[0001] This invention relates to a method for wet transfer of two-dimensional materials to a microgrid. This method allows for targeted transfer of a specified sample, minimizing sample loss during the process, and offers advantages such as low contamination, minimal damage to the microgrid, and simple steps, overcoming many problems inherent in traditional wet transfer methods for microgrid sample preparation. Background Technology

[0002] The study of two-dimensional materials has a history of nearly 20 years. In the fabrication of two-dimensional materials and devices, material transfer is a crucial step. Dry transfer and wet transfer are the two most mainstream methods, from which many other transfer techniques have been derived.

[0003] Dry transfer methods, which do not involve contact with the solution, offer advantages such as simplicity, low contamination, and suitability for readily soluble samples. The most basic method is PDMS transfer, which involves peeling the sample from the original substrate and transferring it to a new substrate using PDMS (often spin-coated with polymers such as PBC to enhance adhesion). Another method is van der Waals interaction transfer [Nat. Commun. 7, 11894 (2016)], and the well-known "tear and stack" method for preparing corner materials [Nano Lett. 16, 1989–1995 (2016)] evolved from this. However, the limitation of this method is that it is only suitable for preparing "sandwich" structure samples coated with other two-dimensional materials.

[0004] Unlike dry transfer, the most traditional wet transfer technique is PMMA substrate etching and the derived electrochemical bubbling method [ACS Nano 5, 9927–9933 (2011)]. However, these methods usually suffer from problems such as sample contamination, low controllability, numerous steps, and difficulty in recycling the substrate. Therefore, various improved transfer methods have been proposed, and the PVA wet transfer technology is the method developed in this patent.

[0005] PVA is a water-soluble organic polymer. Its solution, after spin-coating and drying on a substrate, can be directly peeled off from the substrate using tweezers. Based on the water solubility of PVA, several novel wet transfer methods have been developed [Nat. Commun. 11, 3029 (2020)]. For example, the "spin-coating-then-transfer" method, which is the reverse of the conventional sequence, involves spin-coating PVA and PMMA films sequentially onto the original substrate before preparing the sample. The PMMA adsorbed with the sample can be peeled off from the substrate simply by dissolving the PVA layer in water, allowing the original substrate to be reused [Nat. Nanotech. 5, 722–726 (2010)]. In recent years, inspired by the easy peeling of PVA, methods have been developed to directly peel PVA along with the sample from the original substrate and transfer it to a new substrate [Appl. Surf. Sci. 530, 147276 (2020), Adv. Mater. Technol. 3, 1700282 (2018)]. This method has the advantages of low pollution and high controllability. Summary of the Invention

[0006] The purpose of this invention is to address the problems of sample loss or wrinkling, low controllability, and significant contamination in traditional microgrid wet transfer methods, and to propose a reliable, clean, and easy-to-operate two-dimensional material transfer method.

[0007] A method for transferring two-dimensional materials to microgrids using the water-soluble organic polymer PVA specifically includes the following steps:

[0008] Preparation of S1 and PVA solution

[0009] PVA with a degree of hydrolysis of 87-89% and deionized water are mixed in a beaker at a mass ratio of 1:9. The mixture is heated at 70°C and stirred until the PVA is completely dissolved, yielding a 10% PVA solution. Due to the high viscosity of the PVA solution, a large number of bubbles may still be present after dissolution. Allowing the solution to stand for several hours or applying a vacuum can effectively eliminate the bubbles and make the solution completely clear.

[0010] S2. Prepare the sample to be observed on a silicon wafer.

[0011] This step uses a mechanical peeling method. First, the material is peeled off with tape and then repeatedly adhered. Next, PDMS is used to peel off the two-dimensional material from the tape and attach it to the pre-cleaned silicon wafer. Finally, an ideal peeled sample is selected under a microscope.

[0012] S3. PVA thin film is prepared on silicon wafer by spin coating.

[0013] Place the silicon wafer in a spin coater, and use a dropper to draw an appropriate amount of PVA solution and drop it onto the wafer. Set the spin coating program to 1000 rpm for 6 seconds, followed by 3000 rpm for 2 minutes. After spin coating, dry the wafer on a hot plate at 70°C for 1-2 minutes. A uniform, dry PVA film is then obtained. The spin coating and drying parameters can be adjusted according to actual conditions.

[0014] S4. Transfer the sample from the silicon wafer to the microgateway.

[0015] Place the target microgrid on clean filter paper, and lay the film flat on the surface of the microgrid. Under a microscope, confirm that the target sample is in the appropriate position on the microgrid. Then, use a dropper to draw an appropriate amount of deionized water and drop it onto the PVA film to dissolve the film, leaving the sample on the microgrid. Under a microscope, confirm whether the sample has been successfully transferred and whether there is any PVA residue. If there is residue, repeat the above dissolution steps.

[0016] This step is crucial for the transfer operation. Because PVA films generally adhere worse to various substrates than PMMA films, and the microgrids are soft and fragile, the microgrids with PVA films attached must never be immersed in water. Instead, they must be dissolved by dripping water, and during dissolution, the film must be firmly pressed against both sides of the microgrid using duckbill tweezers to prevent the film from being deflected by the water droplets. The deionized water used to dissolve the PVA can be preheated to improve the dissolution effect.

[0017] S5. Obtain the transferred finished product.

[0018] Dry the microgrid and store it in a dedicated sample box, or test it directly using a transmission electron microscope (TEM).

[0019] This invention addresses the problems of existing microgrid sample preparation techniques, such as poor controllability of direct sample addition and unsuitability for complex samples like heterojunctions, as well as low controllability, susceptibility to contamination, and potential sample loss associated with PMMA wet transfer. It proposes a sample preparation method with multiple advantages. This method ingeniously applies PVA wet transfer to microgrid sample preparation. During the transfer process, the sample is less prone to wrinkling or loss, there is virtually no contamination, high spatial controllability, and the original substrate can be recycled.

[0020] Compared with the prior art, the technical effects of the present invention are as follows:

[0021] (1) This method is not easy to damage the sample and is suitable for the transfer of fragile or complex samples such as few-layer two-dimensional materials and heterostructures.

[0022] (2) Compared with ordinary dry and wet sample preparation methods, the microgrid in this method only comes into contact with the dry film, thus minimizing the probability of damaging the microgrid.

[0023] (3) This method attaches the thin film to the microgrid, so it can transfer the specified sample to the specified position of the microgrid, just like dry transfer. This makes this method also applicable to transferring samples to the in-situ chip. Attached Figure Description

[0024] Figure 1 This is a flowchart illustrating the PVA transfer process.

[0025] Figure 2 A micrograph of the corner hBN on the silicon wafer before spin coating, as shown in the example;

[0026] Figure 3 The image shown is a micrograph of the corner hBN on the silicon wafer after spin coating with PVA in the example. The color difference before and after spin coating is caused by thin film interference.

[0027] Figure 4a A low-magnification micrograph of hBN at the corner of the copper mesh for the example (PVA has been fully dissolved);

[0028] Figure 4b A high-magnification micrograph of hBN at the corner of the copper mesh used in this example (PVA has been fully dissolved);

[0029] Figure 5a A TEM photograph showing the overall appearance of the sample in the example;

[0030] Figure 5b This is a TEM image of the lower layer hBN in the example sample;

[0031] Figure 5c This is a TEM image of the upper layer hBN of the example sample;

[0032] Figure 5d This is a TEM image of the corner region of the sample in the example;

[0033] Figure 6a The electron diffraction pattern of the lower layer hBN in the example;

[0034] Figure 6b The electron diffraction pattern of the upper layer hBN in the example;

[0035] Figure 6c The electron diffraction pattern of the rotation angle hBN is shown in the example. Detailed Implementation

[0036] The specific technical solutions of the present invention will be described with reference to the embodiments.

[0037] The process of this invention is as follows Figure 1 As shown.

[0038] In this embodiment, the corner hexagonal boron nitride is transferred to a microgate to measure the corner value. The original substrate used is a silicon wafer (with a 260nm oxide layer), and the microgate is a 230-mesh copper mesh with a carbon film.

[0039] Hexagonal boron nitride (hBN) is a widely used material. Currently, some research has been conducted on the luminescence [Nano Lett. 21, 2832-2839 (2021); Opt. Express 30, 10596-10604 (2022); Nat. Mater. 21, 896–902 (2022)], ferroelectricity [Adv. Mater. 34, 2203990 (2022)], and reflectivity [Chinese Phys. B 31, 064207 (2022)]. There are roughly two methods for determining the angle of twisted hBN: one is to introduce angle control during preparation, and the other is to determine the angle after random preparation. In the second type of method, selected area electron diffraction (SAED) is the most commonly used technique.

[0040] The basic operating steps are as described above. To prepare the corner hBN, after the hBN is first peeled off to the silicon wafer, PDMS is reattached to the tape. A suitable sample is selected on the PDMS and applied over the existing hBN on the silicon wafer. Micrographs of the samples before and after spin coating are shown below. Figure 2 and Figure 3 Photographs of the hBN at the corner of the copper mesh after wet transfer are shown below. Figure 4a and Figure 4b As shown. A copper mesh was placed under a double tilt bar and observed using a TEM. TEM micrographs of different parts of the sample are shown below. Figures 5a to 5d Electron diffraction patterns are visible Figures 6a to 6c The diffraction pattern indicates that the rotation angle of the sample is approximately 29.29°.

Claims

1. A method for transferring two-dimensional materials to a microgrid using the water-soluble organic polymer PVA, characterized in that, Includes the following steps: Preparation of S1 and PVA solution PVA and deionized water are mixed and heated at 70°C with stirring until PVA is completely dissolved to obtain a PVA solution. S2. Prepare the sample to be observed on a silicon wafer. The peeled sample was obtained by mechanical peeling. S3. PVA thin film is prepared on silicon wafer by spin coating. Place the silicon wafer in a spin coater, use a dropper to draw up PVA solution and drop it onto the silicon wafer; set the spin coating program, and dry it on a hot plate after spin coating to finally obtain a uniform and dry PVA film; S4. Transfer the sample from the silicon wafer to the microgateway. Place the target microgrid on clean filter paper, lay the film flat on the surface of the microgrid, and confirm the target sample is in the appropriate position of the microgrid under a microscope. Then, use a dropper to draw up deionized water and drop it onto the PVA film to dissolve the film, leaving the sample on the microgrid; confirm under a microscope whether the sample has been successfully transferred and whether there is any PVA residue; if there is residue, repeat the above dissolution steps. S5. Obtain the transferred finished product. Dry the microgrid and store it in a dedicated sample box, or test it directly using a transmission electron microscope.

2. The method for transferring two-dimensional materials to a microgrid using the water-soluble organic polymer PVA according to claim 1, characterized in that, In S1, the degree of alcoholysis of PVA used is 87-89%; After dissolving PVA, allow it to stand or apply a vacuum to eliminate air bubbles.

3. The method for transferring two-dimensional materials to a microgrid using the water-soluble organic polymer PVA according to claim 1, characterized in that, The specific method of mechanical peeling described in S2 is as follows: first, use tape to peel off the sample material to be observed, and after multiple adhesions, use PDMS to peel off the two-dimensional material from the tape and attach it to the pre-cleaned silicon wafer. Finally, select the ideal peeled sample under a microscope.

4. The method for transferring two-dimensional materials to a microgrid using the water-soluble organic polymer PVA according to claim 1, characterized in that, In S3, the spin coating conditions are: spin coating at 1000 r / min for 6 s, followed by spin coating at 3000 r / min for 2 min; the drying conditions are: drying at 70℃ for 1-2 min; finally, a uniform and dry PVA film is obtained.

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