Preparation method and transfer method of double-layer and few-layer graphene film stack and graphene composite structure

Abstract

The invention discloses a preparation method and a transfer method of double-layer and few-layer graphene film stacks and a graphene composite structure. The method comprises the following steps: 1) preparing a graphene composite structure I which comprises a substrate layer, a graphene layer, a small molecule buffer layer, a polymer flexible layer with low glass transition temperature, a polymer supporting layer and a self-supporting layer; 2) separating the graphene from the substrate layer by using an electrochemical bubbling stripping method to obtain a graphene composite structure II; (3) respectively cleaning the graphene surface in the graphene composite structure II and the graphene surface in the graphene layer / substrate layer structure by using an active oxygen treatment method; laminating the two to obtain a composite structure III; (4) heating the composite structure III to release the self-supporting layer, heating to enable the graphene to be in conformal contact with the substrate, attaching the self-supporting layer again, and removing the substrate layer, and (5) repeating the steps (3) and (4) for n times to obtain a self-supporting layer / polymer supporting layer / polymer flexible layer / micromolecule buffer layer / (n + 2)-layer graphene composite structure IV.

Description

Technical Field

[0001] The present invention belongs to the technical field of graphene stacking and transfer, and particularly relates to a preparation method, a transfer method and a graphene composite structure for stacking double-layer and few-layer graphene films. Background Art

[0002] Graphene is a single-atom-layer hexagonal honeycomb structure composed of covalently connected carbon atoms. The unique crystal structure endows graphene with excellent mechanical, electrical, optical and thermal properties. For example, the carrier mobility of graphene can theoretically reach 1,000,000 cm 2 V -1 s -1 , the Young's modulus can reach 1 TPa, and the fracture strength is as high as 130 GPa, etc. This makes graphene have rich application potential in application fields such as optoelectronic devices, flexible devices, optical communication devices and separation membranes. Double-layer and few-layer graphene have richer and adjustable degrees of freedom, showing novel physical and chemical properties, such as van Hove singularities dependent on the twist angle, enhanced mechanical strength, etc., and have great application potential in fields such as suspended optoelectronic devices, pressure sensors and thermal conductive membranes. However, there are still quite large challenges in realizing large-area graphene films with controllable number of layers on metal substrates by chemical vapor deposition methods at present. Therefore, at present, the wet transfer method assisted by polymethyl methacrylate (PMMA) is widely used to construct double-layer and few-layer graphene films. However, due to the strong binding force between PMMA and graphene, it is difficult to completely remove, and there are more surface and interface pollutants in the constructed double-layer and few-layer graphene films. The pollutants aggregate at the interface to form dense bulges, seriously reducing the excellent properties of the graphene films.

[0003] Yuan et al. (YUAN G, LIU W, HUANG X, et al. Stacking transfer of wafer-scale graphene-based van der Waals superlattices[J]. Nature Communications, 2023, 14(1): 5457.) designed a method based on face-to-face stacking of bilayer and trilayer graphene, and the steps are as follows: Spin-coat a bilayer PMMA film as a protective layer on the graphene grown on Cu(111) / sapphire (first spin-coat 120 k MW, 1 wt% ethyl lactate, 2000 rpm for 1 minute; then spin-coat 996 kMW, 4 wt% ethyl lactate, 2000 rpm for 1 minute). Then, etch the Cu(111) substrate with 1 mol / L aqueous (NH4)2S2O8 solution and etch SiO2 with 1 mol / L aqueous KOH solution. After etching, one piece is used as the lower-layer graphene and the other as the upper-layer graphene. Immerse the lower-layer graphene in the solution, and the optimal wetting angle range of the solution in terms of the IPA / DI water volume ratio is 10 vol% to 25 vol%. By changing the direction of the upper-layer graphene, lower the top graphene to achieve initial alignment, and then completely drain the solution to achieve the stacking of bilayer graphene.

[0004] Currently, the stacking method is mainly constructed by etching the substrate. The time required for etching the substrate is very long, which affects the construction efficiency. In addition, the substrate cannot be reused, resulting in low economic efficiency; there may be residual water molecules that are difficult to eliminate at the interface between the graphene and the substrate, which may cause the graphene to be over-doped; due to the self-cleaning effect of two-dimensional materials, interface bulges will form due to the enrichment of amorphous carbon between graphene layers, affecting the uniformity of the film; this method also cannot precisely control the stacking angle and cannot meet the requirements for angle control of few-layer graphene in specific scenarios.

[0005] Therefore, it is extremely important to develop a fast, clean, and few-layer graphene film transfer method for the application of graphene in multiple fields. Summary of the Invention

[0006] Aiming at the technical problems existing in the prior art, such as slow construction speed of bilayer and few-layer graphene films, graphene interface bulging, interface pollution, poor uniformity, and small area of suspended graphene constructed. The purpose of the present invention is to provide a preparation method for rapid, clean, and flat stacking of bilayer and few-layer graphene films. This method uses an organic small molecule as a buffer layer during the transfer process of graphene wafers, and constructs a hierarchical structure of "self-supporting layer / polymer support layer / polymer flexible layer / small molecule buffer layer / graphene layer". Among them, the interaction force between the small molecule and graphene is weak and easy to remove, which improves the cleanliness of graphene. The middle of the composite film uses a polymer flexible layer (such as PPC, PBT or PA12), which can promote the conformal contact between graphene and the substrate and improve the integrity of the film. The outer "self-supporting layer / polymer support layer" is used to support graphene to prevent the graphene film from curling and breaking during the stacking process. In addition, the electrochemical bubbling method is used to separate graphene from the growth substrate, which greatly improves the stacking efficiency and shortens the construction time. The dry transfer strategy is adopted, and the amorphous carbon between layers and the adsorbed impurities in the air are treated with reactive oxygen to reduce the size of the interlayer bulge during the stacking process and improve the flatness of the stacked few-layer graphene film. Through the above design, the rapid and flat stacking of few-layer graphene films is finally realized. This stacking strategy has good compatibility with the commonly used polymer-assisted transfer method, realizing the rapid, clean, and flat stacking preparation of bilayer and few-layer graphene films at the wafer level, and is expected to promote the application of few-layer graphene in the fields of hot electron luminescence and suspended pressure sensors, etc.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions: In the first aspect, the present invention provides a preparation method for rapid, clean, and flat stacking of bilayer and few-layer graphene films.

[0008] The preparation method for stacking bilayer and few-layer graphene films provided by the present invention includes the following steps: 1) Prepare graphene composite structure one, which includes, in sequence: a base layer, a graphene layer, a small molecule buffer layer, a polymer flexible layer with a low glass transition temperature, a polymer support layer, and a self-supporting layer; wherein, the small molecule buffer layer contains borneol. 2) Use the electrochemical bubbling stripping method to separate the graphene layer in the graphene composite structure one from the base layer to obtain graphene composite structure two, which includes, in sequence: a graphene layer, a small molecule buffer layer, a polymer flexible layer with a low glass transition temperature, a polymer support layer, and a self-supporting layer; 3) Use the active oxygen treatment method to clean the surface of graphene in the graphene composite structure two obtained in step 2) and the surface of graphene in another graphene layer / substrate layer structure for stacking respectively; then dry bond the graphene composite structure two after the active oxygen treatment to the surface of the graphene layer / substrate layer structure to obtain a self-supporting layer / polymer support layer / polymer flexible layer with a low glass transition temperature / small molecule buffer layer / bilayer graphene / substrate composite structure three; 4) Heat the composite structure three to release the self-supporting layer, then heat again to make the graphene conformally contact with the substrate layer, roll press and bond the self-supporting layer again, and use the electrochemical bubbling peeling method in step 2) to separate the graphene from the substrate layer to obtain a "self-supporting layer / polymer support layer / polymer flexible layer with a low glass transition temperature / small molecule buffer layer / bilayer graphene" composite structure; 5) Then repeat steps 3) and 4) n times to obtain a "self-supporting layer / polymer support layer / polymer flexible layer with a low glass transition temperature / small molecule buffer layer / n + 2 layer graphene" composite structure four; n is a natural number greater than or equal to 0.

[0009] In the above method step 1), the substrate layer can be the growth substrate of graphene, which includes a metal layer, and the graphene layer is disposed on the metal layer.

[0010] According to an embodiment of the present invention, the substrate layer can be sapphire / metal thin film or metal foil.

[0011] According to an embodiment of the present invention, the sapphire / metal thin film includes sapphire / copper, sapphire / copper-nickel alloy or sapphire / copper-silicon alloy; the metal thin film in the sapphire / metal thin film can be obtained by sputtering on a sapphire substrate, and the thickness of the metal thin film can be 100 - 1000 nm, such as 200 nm, 500 nm, 800 nm, etc.

[0012] According to an embodiment of the present invention, the metal foil includes copper, nickel, copper-nickel alloy or platinum.

[0013] In the above method step 1), the borneol can include synthetic borneol and / or natural borneol.

[0014] According to an embodiment of the present invention, the mass content of borneol in the small molecule buffer layer is more than 95%, further more than 97%, and still further more than 99%.

[0015] According to an embodiment of the present invention, the small molecule buffer layer is composed of small molecules, specifically composed of borneol.

[0016] In the above method step 1), the temperature range of the low glass transition temperature is 30 - 60 °C.

[0017] Further, the polymer for forming the polymer flexible layer with a low glass transition temperature is selected from at least one of polypropylene carbonate (PPC), polybutylene terephthalate (PBT), and polycaprolactam (PA12).

[0018] In the above method step 1), the polymer for forming the polymer support layer is selected from at least one of polymethyl methacrylate (PMMA), polylactic acid (PLA), polyphenylene aldehyde (PPA), and polycarbonate (PC).

[0019] In the above method step 1), the polymer for forming the self-supporting layer is selected from polydimethylsiloxane (PDMS) or thermal release tape (TRT); In the above method step 1), the method for preparing the graphene composite structure I includes the following steps: Form a graphene thin film on a substrate by chemical vapor deposition to obtain a substrate layer / graphene layer structure; Set a small molecule organic compound containing borneol on the graphene layer by spin coating or sublimation method; Set a polymer solution for forming a polymer flexible layer with a low glass transition temperature on the small molecule organic compound by spin coating, bake and cure; then set a polymer solution for forming a polymer support layer on the polymer flexible layer with a low glass transition temperature by spin coating, bake and cure; Attach a self-supporting layer to the surface of the formed polymer support layer to obtain the graphene composite structure I.

[0020] In the above method, the small molecule organic compound may only contain borneol or may be a mixture of borneol and an organic solvent.

[0021] In the above method, the sublimation method includes: placing the substrate layer / graphene layer structure above the borneol, with the graphene layer facing the borneol, heating the borneol, and the borneol sublimes into a gas and condenses on the surface of the graphene layer to form a buffer layer.

[0022] The temperature for the heating treatment in the sublimation method is 50 - 300 °C, further preferably 100 - 300 °C, and even more preferably 150 - 250 °C; for example, the heating treatment temperature of the sublimation method can be 120 °C, 150 °C, -180 °C, 200 °C, 220 °C, 250 °C, 280 °C, etc.; the heating treatment time is 1 - 30 min, further preferably 5 - 15 min, for example 5 min, 10 min, 15 min, 20 min, 25 min, etc.

[0023] In the above method, the process of setting the small molecule organic compound by the spin coating method includes: dissolving borneol in an organic solvent to form a borneol solution, and then laying or covering the borneol solution on the graphene layer by the spin coating method (such as spin coating with a spin coater); wherein, the organic solvent may include isopropanol and / or ethyl acetate and / or ethyl lactate, preferably isopropanol; the mass concentration of the borneol solution may be 1-50 wt%, preferably 10-30 wt%, such as 10 wt%, 20 wt%, 25 wt%, 30 wt%, etc.; the rotation speed of spin coating may be 500-5000 rpm, further may be 500-2000 rpm, such as 500 rpm, 1000 rpm, 1500 rpm, 2000 rpm, 2500 rpm, 4000 rpm, 5000 rpm, etc.; according to the film formation situation, spin coating of the small molecule organic compound containing borneol can be carried out 1-3 times, and the spin coating time for each time may be 30 s-5 min, such as 1 min, 2 min, 3 min, 4 min, etc.

[0024] In the above method, the polymer solution is uniformly set on the small molecule organic compound by the spin coating method and baked and cured.

[0025] When the polymer flexible layer with a low glass transition temperature is a PPC layer, before spin coating, the PPC (poly(propylene carbonate), with a relative molecular mass of 200,000 or 300,000) is dissolved in anisole (>98%, aladdin) to prepare a PPC polymer solution with a mass fraction of 0.1 g / ml. The rotation speed of spin coating the PPC polymer solution (such as spin coating with a spin coater) may be 500-8000 rpm, further may be 500-4000 rpm, such as 500 rpm, 1000 rpm, 1500 rpm, 2000 rpm, 3000 rpm, 4000 rpm, 5000 rpm, 6000 rpm, etc.; the spin coating time may be 30 s-5 min, such as 1 min, 2 min, 3 min, 4 min, etc.

[0026] After spin coating is completed, the baking temperature for baking and curing PPC can be 80-150 °C, such as 80 °C, 130 °C, 150 °C, etc.; the baking time can be 1-5 min, such as 2 min, 3 min.

[0027] When the polymer support layer is a PMMA layer, the rotation speed of spin-coating the PMMA solution (950 K A4, Microchem Inc.) (e.g., spin-coated by a spin coater) can be 500 - 8000 rpm, further can be 500 - 4000 rpm, such as 500 rpm, 1000 rpm, 1500 rpm, 2000 rpm, 3000 rpm, 4000 rpm, 5000 rpm, 6000 rpm, etc.; the spin-coating time can be 30 s - 5 min, such as 1 min, 2 min, 3 min, 4 min, etc.

[0028] After spin-coating is completed, the baking temperature for baking and curing the PMMA can be 100 - 200 °C, such as 130 °C, 150 °C, 180 °C, etc.; the baking time can be 1 - 5 min, such as 2 min, 3 min.

[0029] In the above method step 2), the electrochemical bubbling method is a process of connecting the sample to be peeled to the negative electrode of the power supply, connecting the platinum electrode to the positive electrode of the power supply, both being placed in the electrolyte, forming an electrolytic cell and applying a voltage to generate hydrogen on the surface of the metal substrate, so that the sample to be peeled is peeled from the metal matrix surface.

[0030] In the above method step 2), the electrolyte used in the electrochemical bubbling method is one or two of sodium hydroxide solution and potassium hydroxide solution, and the concentration of the electrolyte can be 0.1 - 2 mol / L, such as 0.5 mol / L, 1 mol / L or 2 mol / L. The voltage used in the electrochemical bubbling method is 2.5 V - 10 V, such as 10 V; the electrolysis time of the electrochemical bubbling method is 30 - 300 s, such as 60 s.

[0031] In the above method step 2), the electrochemical bubbling method generates hydrogen on the surface of the metal growth substrate by applying a voltage, thereby separating graphene from the growth substrate, and can quickly peel graphene within 30 s - 5 min, improving the construction efficiency.

[0032] In the above method step 2), after the electrochemical bubbling peeling is completed, the following steps are further included: washing the surface of graphene in the graphene composite structure two with water (e.g., deionized water), and then drying naturally.

[0033] According to the embodiments of the present invention, the washing method can be to slowly replace the etching solution with deionized water using a peristaltic pump to fully wash the surface of the graphene layer and improve the automation degree of transfer; the number of washing times can be 1 - 5 times, further can be 3 - 5 times; the washing time for each time can be 5 - 15 min.

[0034] In step 3) of the above method, the interface of the stacked graphene is cleaned by an active oxygen treatment method. The active oxygen treatment method includes oxygen plasma treatment, ultraviolet ozone treatment, photocatalytic oxidation, microwave-assisted oxidation, etc. The treatment time determines the size of the bulge after the graphene is stacked, which is the key of the present invention.

[0035] The active oxygen may be: the active oxygen generated after the oxygen in the air is activated by a Plasma cleaner, or the active oxygen generated after the oxygen is activated by a Plasma cleaner.

[0036] According to an embodiment of the present invention, the active oxygen treatment is to perform active oxygen cleaning using oxygen plasma in an active oxygen cleaner; the power of the active oxygen cleaner is 5 W to 100 W, and the treatment time can be 6 to 60 s. Preferably, the power of the active oxygen cleaner is 20 W to 50 W, and the treatment time can be 15 to 50 s. Specifically, for example, treatment for 36 s at a power of 30 W, treatment for 40 s at a power of 20 W, or treatment for 18 s at a power of 50 W.

[0037] In step 4) of the above method, the heating temperature for releasing the self-supporting layer can be 120 to 180 °C, such as 150 °C or 180 °C; the time can be 3 to 10 min, such as 3 min.

[0038] In step 4) of the above method, the heating condition for conformal contact between the graphene and the base layer is heating at 120 to 180 °C for 3 to 10 min, such as heating for 10 min at 120 °C or heating for 10 min at 150 °C.

[0039] In step 5) of the above method, the value range of n can further be a natural number of 0 ≤ n ≤ 10.

[0040] The above method of the present invention adopts a dry stacking method, which includes laminating the graphene film supported by the composite transfer medium through a roll pressing or static pressing process, avoiding impurities such as water and solution during the stacking process, ensuring the cleanliness of the graphene interface is improved, and avoiding doping.

[0041] In a second aspect, the present invention provides a method for transferring a bilayer or few-layer graphene film.

[0042] The method for transferring a bilayer or few-layer graphene film provided by the present invention includes the following steps: a) Preparing a "self-supporting layer / polymer supporting layer / polymer flexible layer with a low glass transition temperature / small molecule buffer layer / n + 2 layer graphene" composite structure four according to the method described in the first aspect of the present invention; b) Transferring the composite structure four to a target substrate; c) After removing the self-supporting layer, the graphene layer is conformally contacted with the target substrate, and finally the polymer support layer / polymer flexible layer with a low glass transition temperature / small molecule buffer layer is removed to obtain a clean and undamaged bilayer or few-layer graphene film.

[0043] In step b) of the above method, the target substrate can be a wafer substrate such as a silicon wafer, a silicon wafer with an oxide layer (SiO2 / Si), sapphire, quartz, gallium nitride or glass. Among them, the thickness of the oxide layer on the silicon wafer surface can be 300 nm, and O2 plasma cleaning is used.

[0044] In step c) of the above method, the self-supporting layer is removed by heating, and the heating temperature can be 150 - 180 °C; the time can be 3 min - 10 min, for example, 180 °C for 5 min.

[0045] In step c) of the above method, the method of conformally contacting the graphene layer with the target substrate is baking, and the baking temperature can be 120 - 180 °C; the time can be 3 h - 5 h, for example, baking at 180 °C for 3 h.

[0046] In step c) of the above method, the organic solvents for removing the polymer support layer / polymer flexible layer with a low glass transition temperature / small molecule buffer layer (removing glue) can include one or more of acetone, ethanol, isopropanol, banana oil, N-methylpyrrolidone, dichloromethane, chloroform, formic acid, tetrahydrofuran; the above organic solvents can be of UP grade or HPLC purity.

[0047] The presence of the buffer layer containing borneol not only avoids the direct contamination of graphene by the polymer layer, but also can be removed together with the polymer layer (such as PMMA) by an organic solvent, omitting the step of separately removing the buffer layer and shortening the process flow.

[0048] In the third aspect, the present invention provides a bilayer or few-layer graphene film.

[0049] The bilayer or few-layer graphene film provided by the present invention is prepared according to the graphene film transfer method described in the second aspect of the present invention.

[0050] In the fourth aspect, the present invention provides a graphene composite structure.

[0051] The graphene composite structure is the graphene composite structure described in the first aspect of the present invention.

[0052] The graphene composite structure includes, from bottom to top in sequence: a base layer, a graphene layer, a small molecule buffer layer, a polymer flexible layer with a low glass transition temperature, a polymer support layer, a self-supporting layer; Further, the small molecule forming the small molecule buffer layer includes borneol; Further, the glass transition temperature of the polymer flexible layer with a low glass transition temperature is 30-60 °C; and it is selected from at least one of poly(propylene carbonate), poly(butylene terephthalate), and poly(lauryl lactam); Further, the polymer forming the polymer support layer is selected from at least one of polymethyl methacrylate, polylactic acid, polyphenylene dialdehyde, and poly(bisphenol A carborate); Further, the polymer forming the self-supporting layer is selected from polydimethylsiloxane (PDMS) or thermal release tape (TRT).

[0053] Further, the small molecule buffer layer is disposed on the graphene layer, and as an isolation layer between the graphene and the polymer, it ensures the cleanliness of the upper surface of the graphene; Further, the polymer flexible layer with a low glass transition temperature is disposed on the small molecule buffer layer, and a polymer with a low glass transition temperature is used as an intermediate layer to promote the conformal contact between the graphene and the substrate; Further, the polymer support layer is disposed on the polymer flexible layer with a low glass transition temperature to provide support for the graphene layer; Further, the self-supporting layer is disposed on the polymer support layer for self-supporting of the composite structure, facilitating subsequent cleaning of the graphene interface and stacking.

[0054] The method of the present invention electrochemically rapidly exfoliates graphene by using a pre-designed transfer medium as a support layer, uses a polymer with a low glass transition temperature such as PPC, PBT or PA12 as an intermediate layer to promote the conformal contact between the graphene and the substrate, uses small molecules as an isolation layer between the graphene and the polymer to ensure the cleanliness of the upper surface of the graphene, and treats the graphene interface with reactive oxygen before stacking multiple layers of graphene to clean the amorphous carbon accompanying the growth of graphene and the hydrocarbons adsorbed in the air during the stacking process, reducing the bulges generated during the stacking process.

[0055] Compared with the prior art, the present invention has the following beneficial technical effects: 1. The present invention can realize the clean stacking preparation of few-layer graphene films. Through the "graphene / small molecule buffer layer / polymer flexible layer with a low glass transition temperature (such as PPC) / polymer support layer (such as PMMA)" composite structure, the direct contact between PMMA and graphene is effectively avoided, reducing the residue of surface contaminants and the doping of water and oxygen.

[0056] 2. The present invention uses a polymer with a low glass transition temperature (such as PPC) as the intermediate support layer of the transfer medium. During the stacking process, heating promotes the interaction between graphene and the substrate, ensuring conformal contact during the interlayer stacking process and improving the stacking integrity and uniformity.

[0057] 3. By treating the interlayer with reactive oxygen, the present invention reduces the interlayer bubbles during the stacking process, improves the flatness, and enables the flat stacking preparation of wafer-level bilayer graphene films.

[0058] 4. The present invention achieves clean stacking between layers through van der Waals force dry lamination picking, improving the cleanliness of few-layer graphene films.

[0059] 5. The present invention realizes interlayer stacking through the electrochemical bubbling stripping method, which can achieve rapid stacking of wafer-level graphene films and improve the stacking preparation efficiency.

[0060] 6. The angle of the few-layer graphene stacked by the present invention can be controlled. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] Figure 1 It is a schematic process flow diagram for the rapid, clean, and flat stacking transfer of few-layer graphene films assisted by reactive oxygen; Figure 2 It is a photograph (a) of wafer-level bilayer graphene prepared by stacking in Example 1, its optical microscope image (b), sheet resistance data (c), and Raman mapping data (d); Figure 3 It is a graphene film (a) transferred onto a silicon wafer with a 300 nm thick SiO2 in Example 2, its optical microscope image (b), and a graphene film (c) transferred onto a silicon wafer with a 300 nm thick SiO2 in Comparative Example 1 and its optical microscope image (d); Figure 4 It is the AFM characterization (a) (b) of the few-layer graphene film transferred onto SiO2 / Si prepared in Example 2; the AFM characterization (c) (d) of the few-layer graphene film transferred onto SiO2 / Si prepared in Comparative Example 1; Figure 5 It is the Raman mapping image (a) (b) (c) of the graphene film after transfer in Example 2; Figure 6 It is an image (a) of the graphene film transferred onto a suspended substrate in Example 3 and its SEM images (b) (c), and a comparison image (d) in Comparative Example 2 and its SEM images (e) (f); Figure 7Statistics of the integrity of different layers of graphene transferred to a suspended substrate in Example 3 (a) and SEM image characterization (b) (c) (d) (e) (f). Detailed implementation mode

[0062] The present invention will be further described in detail below in conjunction with the detailed implementation mode. The provided embodiments are only for clarifying the present invention, rather than limiting the scope of the present invention. The following provided embodiments can be used as a guide for those of ordinary skill in the art to make further improvements, and do not constitute any limitation to the present invention in any way.

[0063] The experimental methods in the following embodiments are all conventional methods unless otherwise specified, and are carried out according to the techniques or conditions described in the literature in this field or according to the product specifications. The materials, reagents, etc. used in the following embodiments can be obtained from commercial channels unless otherwise specified.

[0064] Example 1 As Figure 1As shown in the process flow chart, ordinary commercial borneol Borneol (97%) is dissolved in isopropanol with a mass fraction of 25 wt%. The borneol solution is evenly covered on the surface of a 4-inch sapphire / Cu(111) graphene wafer sample using a spreader at a speed of 500 rpm. Then, a PPC (relative molecular weight 200,000) solution (concentration 0.1 g / ml) dissolved in anisole (>98%, Aladdin) is spin-coated onto the graphene / borneol surface at a speed of 500 rpm. Then, it is baked on a heating table for 3 min at a baking temperature of 80 °C. After baking, a PMMA solution (950 K A4, Microchem Inc.) is spin-coated at a speed of 500 rpm and baked on a hot table at a baking temperature of 150 °C for 3 min. Then, PDMS is rolled and bonded to the PMMA surface to obtain a composite structure of "PDMS / PMMA / PPC / borneol / graphene / growth substrate". It was placed in a 1 mol / L sodium hydroxide solution and stripped by bubbling at a voltage of 10 V for 60 s. The positive electrode was connected to the metal platinum electrode in the electrolyte, and the negative electrode was connected to the metal growth substrate of the graphene. The graphene film was then washed with ionized water for 3 times, each time for 15 min, to obtain the composite structure 2 "PDMS / PMMA / PPC / small molecule buffer layer / graphene", and the composite structure 2 was dried in air. After drying, the composite structure 2 was placed together with graphene / copper (111) / sapphire (i.e., a sapphire substrate sputtered with 200 nm copper) in an active oxygen cleaning machine (Diener, pico, Germany) for active oxygen cleaning at 30 W for 36 s. The treated film was attached to the treated graphene / growth substrate surface (so that the graphene surfaces were in contact with the graphene surfaces) to obtain the composite structure 3. The PDMS was released by heating (release temperature 180 °C, heating time 3 min), and then heated at 150 °C for 10 min to promote the chain segment movement of the polymer flexible layer PPC, thereby achieving conformal contact between the graphene film and the growth substrate. PDMS was again laminated on its surface by roller pressing, and then double-layer graphene was obtained by electrochemical bubbling exfoliation. The above steps were repeated three times to obtain a stack of 5 layers of graphene. After natural drying, it was transferred to the target silicon substrate, and the PDMS was released by heating (release temperature 180 °C, time 5 min), and then baked on a heating table at 180 °C for 3 h; finally, acetone was used to remove the glue (PMMA / PPC layer / small molecule buffer layer) to obtain a five-layer graphene film transferred to the silicon wafer, and its optical image is shown in Figure 2. Figure 2 (a). 2 (b) represents the characterization results of stacked double-layer graphene under 5x, 20x, and 100x objective lenses; 2 (c) illustrates the surface resistance characterization of the constructed five-layer graphene, and clear stratification can be seen; 2 (d) is the Raman surface scanning characterization of the five-layer graphene stacked by this method.

[0065] Depend onFigure 2 It can be seen that the five-layer graphene interface stacked by this method is clean and has good uniformity, without obvious charge and stress doping.

[0066] Example 2 As Figure 1 As shown in the process flow chart, commercially available Borneol (97%) is dissolved in isopropanol with a mass fraction of 25 wt%. The Borneol solution is evenly covered on the surface of a 1 cm×1 cm graphene / copper(111) / sapphire (i.e., sapphire substrate sputtered with 200 nm copper) sample using a spin coater at a rotation speed of 2000 rpm. Then, a solution of PPC (relative molecular mass 200,000) dissolved in anisole (>98%, Aladdin) (concentration 0.1 g / ml) is spin-coated onto the graphene / Borneol surface at a rotation speed of 2000 rpm. After that, baking is carried out. It is baked on a heating table for 3 min at a baking temperature of 80 °C. After baking, a PMMA solution (950 KA4, Microchem Inc.) is spin-coated at a rotation speed of 2000 rpm and baked at 150 °C for 5 min. Then, PDMS is pasted on the PMMA surface to obtain a composite structure I "PDMS / PMMA / PPC / Borneol / graphene / growth substrate". It is placed in a 1 mol / L sodium hydroxide solution for bubble separation (stripping is carried out by bubbling for 60 s with a 10 V voltage. The positive electrode is connected to a platinum electrode in the electrolyte, and the negative electrode is connected to the copper growth substrate of graphene); the composite structure I is separated from the copper substrate, and after being placed in deionized water for cleaning and drying, a composite structure II is obtained; the composite structure II and a new piece of graphene / copper(111) / sapphire (i.e., sapphire substrate sputtered with 200 nm copper) are put into an active oxygen cleaning machine (Diener, pico, Germany) and cleaned with air plasma at a power of 35 W for 30 s. The processed film is pasted onto the graphene / growth substrate surface to obtain a composite structure III. PDMS is released by heating at 180 °C, and then conformal contact is carried out by heating at 150 °C for 10 min. PDMS is rolled and pasted again, and electrochemical bubble stripping is used to obtain bilayer graphene with a composite transfer medium. This process is repeated n times to obtain a composite structure IV "PDMS / PMMA / PPC / Borneol / n + 2-layer graphene". The silicon wafer with a 300 nm thick oxide layer is cleaned with active oxygen (Diener, pico, Germany) at a machine power of 300 W, a gas flow rate of 10 sccm, and a time of 10 min. The composite structure III is transferred onto the cleaned silicon wafer. PDMS is released on a heating table at 180 °C for 5 min, and then baking is continued at 180 °C for 3 h to promote the conformal contact between graphene and the substrate. Finally, acetone is used to remove the glue (PMMA / PPC layer / small molecule buffer layer) to obtain a few-layer graphene film transferred onto the silicon wafer, as Figure 3as shown in (a) and (b); its AFM atomic force microscope characterization is as Figure 4 shown in (a) and (b); its Raman surface scanning characterization is as Figure 5 shown in (a), (b) and (c).

[0067] It can be seen from Figure 3 that compared with the traditional stacking method, the graphene interface contaminants disappear under a 100-fold optical microscope.

[0068] It can be seen from Figure 4 that compared with the traditional stacking method, through the treatment of interfacial reactive oxygen species, the contaminants are effectively removed at the micron scale.

[0069] It can be seen from Figure 5 that the few-layer graphene stacked by this method has no obvious stress and charge doping, and has good uniformity.

[0070] Example 3 Dissolve common commercially available borneol in ethyl lactate with a mass fraction of 30 wt%. Use a spin coater to evenly cover the borneol solution on the surface of 1 cm × 1 cm graphene / copper (111) / sapphire, with a rotation speed of 2000 rpm. Dissolve PPC in anisole with a mass fraction of 15 wt%. Then spin coat the PPC solution onto the graphene / borneol surface at a rotation speed of 3000 rpm. Then bake it at 80 °C for 3 min. Then spin coat the PMMA solution (950 K A4, Microchem Inc.) at 2000 rpm and bake it at 150 °C for 5 min. Finally, laminate PDMS on the PMMA surface by rolling to obtain Composite Structure I; place it in a 1 mol / L sodium hydroxide solution, use a 10 V voltage to bubble for 60 s to peel off, separate Composite Structure I from the sapphire substrate, and then rinse and dry it on the deionized water surface to obtain Composite Structure II. Put Composite Structure II and another piece of graphene / growth substrate into an active oxygen cleaning machine (Diener, pico, Germany) and clean it with active oxygen. Treat it at a power of 30 W for 36 s. Stick the cleaned Composite Structure II on the surface of the graphene / growth substrate to obtain Composite Structure III. Heat and release PDMS at 180 °C for 5 min, and then heat it at 120 °C for 10 min to achieve conformal contact. Stick a new piece of PDMS, use electrochemical bubbling to peel off and clean and dry to obtain bilayer graphene with a composite transfer medium. Repeat this process n times to obtain Composite Structure IV, and transfer the few-layer graphene film to a silicon substrate with suspended holes, heat and release PDMS at a release temperature of 180 °C for 5 min, and then bake it on a heating table at 180 °C for 3 h; finally, use acetone and isopropyl alcohol to remove the PMMA / PPC layer / small molecule buffer layer to obtain the few-layer graphene film transferred to the suspended substrate, as Figure 6As shown in (a), Figure 6 Figures (b) and (c) are SEM result characterizations of the stacking of this method. The integrity statistics results of 1 - 5 layers of graphene stacked using this method are as follows Figure 7 .

[0071] It can be seen from Figure 6 that the composite transfer medium designed based on the conformal principle can improve the integrity of the suspended substrate and reduce cracks and wrinkles.

[0072] It can be seen from Figure 7 that as the number of graphene stacking layers increases, the transfer integrity of the suspended substrate continuously increases.

[0073] Comparative Example 1 Use a spin coater to evenly coat the PMMA solution (950 K A4, Microchem Inc.) on the surface of the graphene / copper(111) / sapphire growth substrate sample at a rotation speed of 2000 rpm, and bake it at 150 °C for 5 min to obtain a "PMMA / graphene / growth substrate" composite structure; place it in a 1 mol / L ammonium persulfate solution and etch for 5 h. Then, use a peristaltic pump to change the etching solution to deionized water. After that, separate the PMMA / graphene composite film from the sapphire substrate and rinse it on the deionized water surface; use the graphene / growth substrate to fish out the PMMA / graphene floating on the liquid surface. After the interface is dried, etch it again with the etching solution. Rinse the obtained PMMA / bilayer graphene on the deionized water surface 3 times, 15 min each time; repeat this step n times to obtain a PMMA / n + 2 layer graphene composite film. Perform reactive oxygen cleaning on a silicon wafer with a 300 nm thick oxide layer at a power of 300 W, an oxygen flow rate of 10 sccm, and a time of 10 min. Transfer the composite film to the cleaned silicon wafer, let it dry naturally, and then bake it on a heating stage for 3 h at a temperature of 180 °C; finally, remove the glue with acetone to obtain the few - layer graphene film transferred to the silicon wafer as shown in Figure 3 (c), and its optical microscope characterization is as shown in Figure 3 (d). Its AFM atomic force microscope characterization is as shown in Figure 4 (c) and (d).

[0074] Comparative Example 2 Use a spin coater to evenly coat the PMMA solution (950 K A4, Microchem Inc.) on the surface of the graphene / growth substrate at a rotational speed of 5000 rpm, bake it on a heating stage for 5 min at a baking temperature of 150 °C. Then, attach PDMS to the PMMA surface to obtain a PDMS / PMMA / graphene / growth substrate composite structure. Immerse it in a 1 mol / L sodium hydroxide solution, and separate the PDMS / PMMA / graphene from the copper (111) / sapphire growth substrate by the electrochemical bubbling method. Rinse it 3 times on the deionized water surface, 15 min each time. Then, dry the composite film in a fume hood, transfer the composite film to a new graphene / growth substrate, release the PDMS at 180 °C on a hot stage, then roll and attach a new piece of PDMS and immerse it in the sodium hydroxide solution for bubbling separation to obtain PDMS / PMMA / bilayer graphene. Repeat this process n times. Finally, obtain a PDMS / PMMA / n + 2 layer graphene film, dry it in a fume hood and then release it onto the target suspended substrate. Finally, remove the glue with acetone vapor. The transfer result is as shown in Figure 6 shown in (d), and the SEM characterization is as shown in Figure 6 (e) and (f).

[0075] The present invention has been described in detail above. For those skilled in the art, without departing from the spirit and scope of the present invention and without unnecessary experiments, the present invention can be implemented within a relatively wide range under equivalent parameters, concentrations, and conditions. Although specific embodiments of the present invention are given, it should be understood that the present invention can be further improved. In general, according to the principle of the present invention, this application intends to include any changes, uses, or improvements to the present invention, including those that depart from the scope disclosed in this application but are made with conventional techniques known in the art. Some basic features can be applied according to the scope of the following appended claims.

Claims

1. A preparation method for stacking double-layer and few-layer graphene films, comprising the following steps: 1) Prepare Graphene Composite Structure I, where Graphene Composite Structure I includes, arranged in sequence: a base layer, a graphene layer, a small molecule buffer layer, a polymer flexible layer with a low glass transition temperature, a polymer support layer, and a self-supporting layer; among them, The small molecule buffer layer contains borneol; 2) Use the electrochemical bubbling exfoliation method to separate the graphene layer in the graphene composite structure one from the substrate layer to obtain a graphene composite structure two, which includes, in sequence: a graphene layer, a small molecule buffer layer, a polymer flexible layer with a low glass transition temperature, a polymer support layer, and a self-supporting layer; 3) Use the active oxygen treatment method to clean the surface of the graphene in the graphene composite structure two obtained in step 2) and the surface of the graphene in another graphene layer / substrate layer structure for stacking; then dry-bond the graphene composite structure two after the active oxygen treatment to the surface of the graphene layer / substrate layer structure to obtain a composite structure three of self-supporting layer / polymer support layer / polymer flexible layer with a low glass transition temperature / small molecule buffer layer / double-layer graphene / substrate layer; 4) Heat the composite structure three to release the self-supporting layer, then heat again to make the graphene conformally contact with the substrate layer, roll and bond the self-supporting layer again, and use the electrochemical bubbling exfoliation method in step 2) to separate the graphene from the substrate layer to obtain a self-supporting layer / polymer support layer / polymer flexible layer with a low glass transition temperature / small molecule buffer layer / double-layer graphene composite structure; 5) Then repeat steps 3) and 4) n times to obtain a self-supporting layer / polymer support layer / polymer flexible layer with a low glass transition temperature / small molecule buffer layer / n + 2 layer graphene composite structure four; The n is a natural number greater than or equal to 0.

2. The method according to claim 1, wherein: In step 1), the glass transition temperature of the polymer flexible layer with a low glass transition temperature is 30 - 60 °C; And / or, in step 1), the polymer with a low glass transition temperature is selected from at least one of poly(propylene carbonate), poly(butylene terephthalate), and poly(lauryl lactam); And / or, in step 1), the polymer forming the polymer support layer is selected from at least one of polymethyl methacrylate, polylactic acid, polyphthalaldehyde, and polycarbonate; And / or, in step 1), the polymer forming the self-supporting layer is selected from polydimethylsiloxane or a thermal release tape; And / or, in step 2), the electrolyte used in the electrochemical bubbling method is one or both of sodium hydroxide solution and potassium hydroxide solution; And / or, in step 2), the concentration of the electrolyte is 0.1 - 2 mol / L; And / or, in step 2), the voltage used in the electrochemical bubbling method is 2.5 V - 10 V; And / or, in step 2), the electrolysis time of the electrochemical bubbling method is 30 - 300 s; And / or, in step 4), the heating condition for making the graphene conformally contact with the substrate layer is heating at 120 - 180 °C for 3 - 10 min.

3. The method according to claim 1 or 2, characterized in that: In step 3), the method for treating reactive oxygen is selected from at least one of the following: oxygen plasma treatment, ultraviolet ozone treatment, photocatalytic oxidation, and microwave-assisted oxidation.

4. The method according to claim 3, wherein: In step 3), the treatment with reactive oxygen is carried out by using oxygen plasma for reactive oxygen cleaning in a reactive oxygen cleaning machine; the power of the reactive oxygen cleaning machine is 5 W to 100 W, and the treatment time is 1 s to 100 s.

5. The method according to claim 1, wherein: In step 1), the method for preparing the first graphene composite structure includes the following steps: Forming a graphene film on a substrate by chemical vapor deposition to obtain a substrate layer / graphene layer structure; Setting a small molecule organic compound containing borneol on the graphene layer by spin coating or sublimation; Setting a polymer solution for forming a polymer flexible layer with a low glass transition temperature on the small molecule organic compound by spin coating and curing; Then setting a polymer solution for forming a polymer support layer on the polymer flexible layer with a low glass transition temperature by spin coating and curing; Laminating a self-supporting layer on the surface of the formed polymer support layer to obtain the first graphene composite structure.

6. The method according to claim 5, characterized in that: The small molecule organic compound includes the borneol or the borneol and an organic solvent, and the organic solvent includes isopropanol and / or ethyl acetate and / or ethyl lactate.

7. A method for transferring a bilayer or few-layer graphene film, characterized in that: The method includes the following steps: a) Preparing a self-supporting layer / polymer support layer / polymer flexible layer with a low glass transition temperature / small molecule buffer layer / n+2 layer graphene composite structure four according to the method described in any one of claims 1-6; b) Transferring the composite structure four to a target substrate; c) After removing the self-supporting layer, making the graphene layer conformally contact with the target substrate, and finally removing the polymer support layer / polymer flexible layer with a low glass transition temperature / small molecule buffer layer to obtain a clean and undamaged bilayer or few-layer graphene film.

8. The method according to claim 7, characterized in that: In step c), the self-supporting layer is removed by heating; and / or, in step c), the organic solvents for removing the polymer support layer / polymer flexible layer with a low glass transition temperature / small molecule buffer layer include one or more of acetone, ethanol, isopropanol, banana oil, N-methylpyrrolidone, dichloromethane, chloroform, formic acid, and tetrahydrofuran.

9. Graphene composite structure, characterized in that: The graphene composite structure sequentially includes from bottom to top: a substrate layer, a graphene layer, a small molecule buffer layer, a polymer flexible layer with a low glass transition temperature, a polymer support layer, and a self-supporting layer; wherein, the small molecule buffer layer contains borneol.

10. The graphene composite structure according to claim 9, wherein: The glass transition temperature of the formed polymer flexible layer with a low glass transition temperature is 30 to 60 °C; and / or, the polymer with a low glass transition temperature is selected from at least one of poly(propylene carbonate), poly(butylene terephthalate), and poly(lauryl lactam); and / or, the polymer for forming the polymer support layer is selected from at least one of polymethyl methacrylate, polylactic acid, polyphenyl dialdehyde, and polycarbonate; and / or, the polymer for forming the self-supporting layer is selected from polydimethylsiloxane or a thermal release tape.

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