A method of transferring a two-dimensional material suitable for use in an electrical device

Abstract

The application discloses a two-dimensional material transfer method suitable for preparing an electrical device, and comprises the following steps: picking up boron nitride by using a polyvinyl alcohol film, picking up a two-dimensional material layer, obtaining a polyvinyl alcohol film-boron nitride-two-dimensional material structure, turning over and transferring to a blank silicon wafer to obtain a two-dimensional material-boron nitride-polyvinyl alcohol film-silicon wafer sequential arrangement structure; picking up the turned-over two-dimensional material-boron nitride-polyvinyl alcohol film-silicon wafer arrangement structure by using a polycarbonate film to obtain a polycarbonate film-two-dimensional material-boron nitride-polyvinyl alcohol film-silicon wafer arrangement structure, soaking in ultrapure water, removing the polyvinyl alcohol film layer and the silicon wafer to obtain a polycarbonate film-two-dimensional material-boron nitride, and after being transferred to a target substrate, removing the polycarbonate film, so that the two-dimensional material is on the boron nitride and is suitable for electrode preparation. The transfer method can reduce bubbles, stress and pollution, and has the characteristics of clean, uniform and complete surface and less polymer residue.

Description

Technical Field

[0001] This application relates to the field of two-dimensional material transfer technology, and specifically to a two-dimensional material transfer method developed for the fabrication of electrical devices. Background Technology

[0002] Two-dimensional materials (2D materials) refer to materials that have one dimension in three-dimensional space within the nanoscale range (1-100 nm). The concept of 2D materials arose after the Geim group at the University of Manchester successfully isolated a single atomic layer of graphene in 2004. Since the discovery of graphene, 2D materials have sparked a tremendous research boom in the scientific community, with graphene and other 2D materials being extensively studied for their unique electrical, optical, mechanical, thermal, and chemical properties.

[0003] Currently, the main methods for preparing two-dimensional materials include mechanical exfoliation, liquid-phase ultrasonic exfoliation, physical vapor deposition, chemical vapor deposition, and atomic layer deposition. One of the key breakthroughs in graphene research is the development of mechanical exfoliation, commonly known as the tape method. This method is simple to operate and can produce samples with few defects and extremely high quality. The flakes produced by this exfoliation method have different sizes and thicknesses and are randomly distributed on the sample substrate. However, subsequent research requires transferring them to different substrates. Therefore, the non-destructive transfer of two-dimensional materials to specific substrates requires the development of corresponding transfer technologies.

[0004] The development of transfer technology is essential for further exploring the applications of two-dimensional materials in new physical phenomena, such as integrating two-dimensional materials onto chips with different functions, fabricating electrical devices, twisted electronics, and heterojunction devices made of different two-dimensional materials. Wet transfer, including precise transfer methods assisted by organic thin films, has a significant drawback: the two-dimensional material is always in direct contact with the film, requiring cleaning with acetone and other wet processes. To improve the cleanliness of the transferred sample, researchers often use high-temperature annealing, which limits the applicability of this method. Substrates sensitive to acid and alkali solvents, and substrates structurally unstable at high temperatures, cannot be used to fabricate samples using this method, or the resulting samples are not clean enough. The latest dry transfer technology utilizes boron nitride pick-and-place stacking, allowing precise manipulation of two-dimensional monolayer samples. During this process, the monolayer flakes produced by mechanical peeling are not directly exposed to the external environment, greatly improving the stability of the two-dimensional material. However, currently, two-dimensional materials transferred by this method are generally completely covered by boron nitride, making electrode fabrication impossible, thus limiting its application in electrical devices. Summary of the Invention

[0005] To address the aforementioned deficiencies in this field, this application aims to provide a two-dimensional material transfer method suitable for fabricating electrical devices, which is a clean transfer method capable of enabling the flipping of two-dimensional material devices.

[0006] According to one aspect of this application, a two-dimensional material transfer method suitable for fabricating electrical devices is provided, comprising:

[0007] A thin layer of hexagonal boron nitride (approximately 40-50 nm) is picked up using a polyvinyl alcohol (PVA) film, and then a target two-dimensional material layer is picked up to form a PVA film-boron nitride-two-dimensional material structure; the structure is then flipped and transferred onto a clean blank silicon wafer to obtain a sequential arrangement structure of target two-dimensional material-boron nitride-PVA film-silicon wafer.

[0008] The above-mentioned flipped two-dimensional material-boron nitride-PVA film-silicon wafer arrangement structure is picked up using polycarbonate film (PC) to obtain PC film-two-dimensional material-boron nitride-PVA film-silicon wafer arrangement structure;

[0009] The above PC film-two-dimensional material-boron nitride-PVA film-silicon wafer arrangement structure was immersed in ultrapure water. Since PVA film is water-soluble, the PVA film layer and silicon wafer can be removed to obtain PC film-two-dimensional material-boron nitride.

[0010] After the above PC film-two-dimensional material-boron nitride arrangement structure is fixedly moved to the target substrate, the PC film is removed with chloroform (trichloromethane) to obtain the two-dimensional material-boron nitride-target substrate arrangement structure, thereby realizing that the two-dimensional material is located on top of the boron nitride, which is suitable for electrode preparation.

[0011] According to some embodiments of this application, the thickness of the PVA film is 0.5 to 1 μm.

[0012] According to some embodiments of this application, the preparation of a PVA film includes: preparing a PVA solution with a mass fraction of 2% to 3%, uniformly spreading it on a bare silicon wafer, and baking it until a film is formed.

[0013] According to some embodiments of this application, the baking temperature is 60-65°C and the baking time is 4-5 hours.

[0014] According to some embodiments of this application, the thickness of the PC film is 0.5 to 1 μm.

[0015] According to some embodiments of this application, the preparation of PC film includes: preparing a PC solution with a mass fraction of 6-7%, dropping it onto a clean glass slide surface, and baking it until a film is formed.

[0016] According to some embodiments of this application, the baking temperature is 80-85°C and the baking time is 5-6 minutes.

[0017] According to some embodiments of this application, removing the PVA film and silicon wafer includes: immersing the PC film-two-dimensional material-boron nitride-PVA film-silicon wafer in ultrapure water and heating it at 60-65°C for 30-60 minutes.

[0018] According to some embodiments of this application, removing the PC film includes: heating the two-dimensional material on the PC film to 180-200°C to melt it after it comes into contact with the substrate, and then immersing it in chloroform for 10-60 seconds.

[0019] Compared with the prior art, this application has at least the following beneficial effects:

[0020] This application provides a two-dimensional material transfer method developed for the fabrication of electrical devices. The two-dimensional material is picked up twice using two polymer films with different solubilities, and the two polymers are used alternately for transfer, thereby realizing the flipping of the two-dimensional material device.

[0021] This application utilizes boron nitride (h-BN) stacked two-dimensional material devices to reduce bubbles, stress, and contamination. The transferred two-dimensional material has the characteristics of clean, uniform, and complete surface with little polymer residue.

[0022] The device transferred by the transfer method of this application can achieve device flipping, with the surface of the two-dimensional material exposed without h-BN coating, which facilitates the subsequent integration of the two-dimensional material onto chips with different functions for the fabrication of electrical devices and the realization of torsional electronics research. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of a two-dimensional material transfer process in an example embodiment of this application.

[0024] Figure 2 The boron nitride to be picked up is shown in the example embodiment of this application.

[0025] Figure 3 Optical images of h-BN and graphene captured by PVA in an example embodiment of this application.

[0026] Figure 4 An optical image of the sample transferred to the target substrate after the PC is melted (left) in an example embodiment of this application. Detailed Implementation

[0027] The technical solution of this application will be clearly and completely described below with reference to the embodiments of this application. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0028] It should be particularly noted that similar substitutions and modifications made to this application are obvious to those skilled in the art, and they are all considered to be included in this application. Those skilled in the art can obviously make modifications or appropriate alterations and combinations to the methods and applications described herein without departing from the content, spirit, and scope of this application to implement and apply the technology of this application. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments.

[0029] Unless otherwise specified, this application is conducted under standard conditions or conditions recommended by the manufacturer. The raw materials or excipients used, as well as the reagents or instruments used, whose manufacturers are not specified, are all conventional products that can be obtained commercially.

[0030] The following is a detailed description of this application.

[0031] Currently, existing two-dimensional material transfer methods generally suffer from several problems, including the inability to transfer samples prepared by mechanical peeling, the inability to accurately align with the target substrate for directional transfer, the inability to transfer multiple two-dimensional materials stacked together, and the inability to transfer only a single sample to the substrate. Furthermore, the inability to flip the transferred device makes it difficult to fabricate electrodes for electrical devices, which is not conducive to on-chip integration. Alternatively, the transfer success rate may be low, making it difficult to apply to the stacking of multilayer structures.

[0032] To address the aforementioned problems in this field, this application first utilizes PVA to pick up a thick layer of h-BN, leveraging the stronger van der Waals forces between h-BN and other monolayer two-dimensional materials to sequentially pick up mechanically peeled two-dimensional materials; then, the PVA film with stacked two-dimensional materials is peeled off, flipped, and transferred to a clean silicon wafer; next, PC is used to pick up the stacked two-dimensional materials on the PVA film, thereby achieving device flipping; subsequently, the transferred two-dimensional materials are immersed in ultrapure water. Since the PVA film is soluble in water while the PC film is insoluble, the two-dimensional materials can be transferred onto the PC film after the PVA film dissolves, and finally transferred to the target substrate.

[0033] The technical solution of this application is further described below with reference to specific embodiments, and the preparation process is as follows: Figure 1 .

[0034] Example

[0035] PC solution preparation and PC transfer template (stamp) creation:

[0036] PC solution preparation: The solvent is chloroform, and the PC concentration is prepared to be 6-7% by mass. The solution is dissolved by sonication for 3 hours. The adhesive is then applied to two glass slides (1-2 drops are applied with a dropper, and the glass slides are kept clean. Before applying, the slides are blew with nitrogen or cleaned with isopropanol). The solution is baked at 80°C for 5 minutes until a film is formed.

[0037] PC stamp making: Use a punch to make a 2mm diameter polydimethylsiloxane (PDMS) cylinder with a height of about 2mm. Place it on a glass slide that has been cleaned with isopropyl alcohol. Use a punch to make holes in the bottom double-sided adhesive tape and 3M tape respectively. Adhere the PC film with 3M tape to cover the PDMS cylinder, artificially creating a curved surface to facilitate the subsequent transfer process.

[0038] PVA solution preparation and PVA transfer template (stamp) creation:

[0039] PVA solution preparation: The solvent is high-purity water, and the PVA concentration is 2-3% by mass. Take 0.5 mL of PVA solution and spread it evenly over a two-inch bare silicon wafer. Place it on a heating table at 60-65℃ and bake for 4-5 hours until a film is formed.

[0040] PVAstamp fabrication: Use a punch to make a PDMS cylinder with a diameter of 2mm and a height of about 2mm. Place it on a glass slide that has been cleaned with isopropyl alcohol. Use a punch to make holes in the bottom double-sided adhesive tape and 3M tape respectively. Adhere the PC film with 3M tape to cover the PDMS cylinder.

[0041] Flipping and Transfer of Two-Dimensional Materials:

[0042] Using a PVAstamp, a layer of h-BN with a thickness of about 50 nm was picked up at 60-65 °C, cooled to 50-55 °C and lifted to pick up the h-BN; graphene and other two-dimensional materials were picked up in sequence in this way to form a heterojunction.

[0043] Remove the PVA stamp, flip the glass slide containing the PVA stamp, and slowly cut the PVA film along the edge of the stamp using sharp tweezers or a sharp knife; gently peel the PVA film off the PDMS cylinder with tweezers and slowly place it on a clean silicon wafer;

[0044] Note: The operation should be slow and the force applied evenly. Otherwise, uneven force will cause stress that can wrinkle the PVA film and lead to device breakage. Do not pull the film easily, otherwise the device may be easily torn.

[0045] Use a new PC stamp to pick up the PVA film with the stacked heterojunction devices. Preheat the heating stage to 60-80°C. The curled PVA film will unfold. Lower the PC stamp to pick up the PVA film. Once the PC film and PVA film are in full contact, turn off the heating stage and allow it to cool naturally to 40-22°C. The PC film will then pick up the silicon wafer at the same time.

[0046] Place the stamp with the silicon wafer in ultrapure water at 60-65℃ and heat for 30-60 minutes to remove the PVA film. Allow the silicon wafer to detach naturally, leaving the sample on the PC. This method is highly successful for selecting h-BN samples with 50-60nm and neat edges.

[0047] Release the PC stamp containing the sample onto a specific electrode or substrate. Before releasing the sample, preheat the heating stage to 60–80°C to prevent thermal stress from causing the sample to curl. Then lower the stamp and heat it to 180–200°C to melt the sample after it contacts the substrate. After that, immerse it in chloroform for 10–60 seconds to remove the PC film.

[0048] Comparative Example 1

[0049] Two-dimensional materials are grown using PVA film transfer chemical vapor deposition (CVD).

[0050] Preparation of the sample to be transferred: Gently blow the surface of the two-dimensional material to be transferred with a nitrogen gun to ensure the cleanliness of the sample surface; prepare a PVA solution with a solubility of 2% to 3% by mass, take a small amount of PVA solution and spread it evenly on the CVD grown substrate, place it on a heating stage at 60 to 65°C and bake for 4 to 5 hours until film is formed.

[0051] Separation of two-dimensional material from growth substrate: The position of the two-dimensional material on the growth substrate is determined by optical microscope. First, a cut is made on the edge of the PVA film without the two-dimensional material growing with pointed tweezers as the starting point for separation. Then, the PVA film is slowly torn up along the cut corner with tweezers until the entire PVA film is completely torn apart. At this time, the two-dimensional material to be transferred is stuck on the PVA film and separated from the growth substrate.

[0052] Transferring a PVA film with two-dimensional material attached to a target substrate: Use tweezers to place the side of the PVA film with the two-dimensional material attached onto the target substrate, cover it with a clean silicon wafer, and press lightly to promote the adhesion between the target substrate and the PVA film. Then place the target substrate on a glass slide and heat it on a heating stage at 60-80°C for 5-10 minutes, while pressing gently to further promote the adhesion between the PVA film and the target substrate.

[0053] Remove the PVA film and take off the silicon wafer pressed on the target substrate. Immerse the target substrate in ultrapure water for 10-30 minutes to remove the PVA film on the surface of the two-dimensional material. Then, gently blow the surface of the target substrate with a hot air blower or nitrogen gun to remove water droplets from the surface of the target substrate, thus obtaining the target substrate with the transferred two-dimensional material.

[0054] The method described in Comparative Example 1 simply transfers CVD-grown two-dimensional materials onto a specific substrate. Because CVD-grown two-dimensional materials have many defects, they are unsuitable for use in electrical devices.

[0055] Comparative Example 2

[0056] The PVA solution was prepared and the stamp was made according to the method described in the embodiment.

[0057] Transfer using PVA film: A PVA stamp is used to pick up the target 2D material at 60–65°C, then cooled to 50–55°C and lifted. The PVA film can then pick up the 2D material. This method is repeated to pick up other 2D materials sequentially to form a heterojunction. The PVA film containing the sample is then released onto a specific substrate. Before releasing the sample, the heating stage is preheated to 80°C, and the stamp is lowered. Once the sample on the PVA film contacts the substrate, it is cooled to room temperature to 25°C, and then the substrate is lifted together. The stamp with the silicon wafer is then immersed in water at 60–65°C for 30–60 minutes to remove the PVA film. The silicon wafer is allowed to detach naturally, leaving the sample on the wafer.

[0058] Comparative Example 3

[0059] The PC solution was prepared and the stamp was made according to the method described in the embodiment;

[0060] PC film transfer: Pick up the target two-dimensional material with a PC stamp at 80-85℃, and then pick up other two-dimensional materials in sequence to form a heterojunction. Select the substrate to be transferred, lower the PC stamp, and heat it to 180-200℃ to melt the sample after it contacts the substrate. Then immerse it in chloroform for 60 seconds and remove the PC film.

[0061] Comparative Example 4

[0062] The PVA and PC solutions were prepared and the stamp was fabricated according to the method described in the embodiment.

[0063] A thick layer of h-BN is first picked up using a PVA or PC film. Taking advantage of the stronger van der Waals forces between h-BN and other monolayer two-dimensional materials, the mechanically exfoliated two-dimensional materials are sequentially picked up. Other steps are the same as in Comparative Example 2 or 3, transferring the material to a specific substrate using the PVA or PC film transfer method.

[0064] Comparative Examples 2-4 all used mechanical exfoliation methods to obtain two-dimensional materials. In Comparative Examples 2 and 3, regardless of whether the transfer method involved PVA or PC film, the malleability of the polymer film easily led to breakage of the single-layer two-dimensional material during the transfer process, which is highly detrimental to the fabrication of electrical devices.

[0065] To prevent the problem of two-dimensional material breakage during the transfer process, Comparative Example 4 uses boron nitride to pick up the two-dimensional material and then transfer it to a specific substrate. However, the surface of the two-dimensional material transferred by this method is coated with boron nitride, which cannot be directly used to prepare electrodes or other electrical devices.

[0066] The transfer method described in this application utilizes two polymer films with different solubilities to pick up the two-dimensional material twice, alternating between the two polymers for transfer, thereby achieving the flipping of the two-dimensional material device. The surface of the two-dimensional material is exposed, while h-BN is underneath, facilitating the subsequent integration of the two-dimensional material onto chips with different functions for the fabrication of electrical devices and the realization of torsional electronics research.

[0067] The above description of the embodiments is only for the purpose of helping to understand the method and core ideas of this application. It should be noted that, for those skilled in the art, several improvements and modifications can be made to this application without departing from the principles of this application, and these improvements and modifications also fall within the protection scope of the claims of this application.

Claims

1. A two-dimensional material transfer method suitable for fabricating electrical devices, characterized in that, include: A boron nitride layer is picked up using a polyvinyl alcohol film, and then a two-dimensional material layer is picked up to obtain a polyvinyl alcohol film-boron nitride-two-dimensional material. The polyvinyl alcohol film-boron nitride-two-dimensional material is flipped and transferred onto a silicon wafer to obtain a two-dimensional material-boron nitride-polyvinyl alcohol film-silicon wafer; The two-dimensional material-boron nitride-polyvinyl alcohol film-silicon wafer is picked up using a polycarbonate film to obtain a polycarbonate film-two-dimensional material-boron nitride-polyvinyl alcohol film-silicon wafer; After removing the polyvinyl alcohol film and silicon wafer, the polycarbonate film-two-dimensional material-boron nitride is transferred to the target substrate, and then the polycarbonate film is removed. In this process, after the polycarbonate film is removed, the surface of the two-dimensional material is exposed without boron nitride coating.

2. The transfer method according to claim 1, characterized in that, The boron nitride layer is a thin hexagonal boron nitride layer; the thickness of the boron nitride layer is 40-50 nm.

3. The transfer method according to claim 1, characterized in that, The thickness of the polyvinyl alcohol film is 0.5 to 1 μm.

4. The transfer method according to claim 3, characterized in that, The preparation of the polyvinyl alcohol film includes: preparing a polyvinyl alcohol solution with a mass fraction of 2% to 3%, spreading it evenly on a bare silicon wafer, and baking it until a film is formed.

5. The transfer method according to claim 4, characterized in that, The baking temperature is 60-65℃, and the baking time is 4-5 hours.

6. The transfer method according to claim 1, characterized in that, The polycarbonate film has a thickness of 0.5–1 μm.

7. The transfer method according to claim 6, characterized in that, The preparation of the polycarbonate film includes: preparing a polycarbonate solution with a mass fraction of 6-7%, dropping it onto the surface of a clean glass slide, and baking it until a film is formed.

8. The transfer method according to claim 7, characterized in that, The baking temperature is 80-85℃, and the baking time is 5-6 minutes.

9. The transfer method according to any one of claims 1-8, characterized in that, The removal of the polyvinyl alcohol film and silicon wafer includes: immersing the polycarbonate film-two-dimensional material-boron nitride-polyvinyl alcohol film-silicon wafer in water and heating it at 60-65°C for 30-60 minutes.

10. The transfer method according to claim 9, characterized in that, The removal of the polycarbonate film includes: heating the polycarbonate film to 180-200°C to melt it after the two-dimensional material on the polycarbonate film comes into contact with the substrate, and then immersing it in chloroform for 10-60 seconds.

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