Batch preparation method of ultra-flat graphene electron microscope support membrane

Abstract

The invention discloses a batch preparation method of an ultra-flat graphene electron microscope support membrane, and belongs to the field of materials. The method comprises the following steps: separating graphene from a pre-oxidized copper metal growth substrate by using an electrochemical stripping method, and transferring the graphene and the pre-oxidized copper metal growth substrate to a polymer composite medium; and then conformally laminating the graphene supported by the polymeric membrane and the electron microscope grid by using a solvothermal lamination method, and removing the polymer medium to obtain the graphene grid. The method disclosed by the invention is simple in process, high in repeatability, strong in compatibility and capable of realizing large-scale preparation and production. The integrity of the graphene support membrane prepared by the method is as high as 90%, and the graphene in the suspended holes of the carrier net has the characteristics of ultra-flatness and good cleanliness.

Description

Technical Field

[0001] The present invention relates to a method for batch preparation of a super-flat graphene electron microscope support film, belonging to the field of materials. Background Art

[0002] Graphene has excellent properties. Suspended graphene can exhibit its intrinsic property characteristics and has low background noise. It is an ideal electron support film material, especially in cryo-electron microscopy imaging analysis, where it can alleviate the gas-liquid interface and sample displacement, and improve the imaging resolution. High-quality single-crystal graphene can be prepared on a large scale through chemical vapor deposition technology. For example, by chemical vapor epitaxial growth on copper (111) / sapphire wafers, super-flat graphene single crystals can be obtained (patent number CN201710523050.1). However, there are few reports on how to achieve batch and high-quality preparation of electron microscope support films by detaching from the graphene growth substrate.

[0003] Currently, the methods for graphene transfer are as follows: (1) The non-transfer etching method (publication number CN 109437176 A) prepares a suspended graphene support film by selectively etching the copper foil growth substrate. This method is applicable to copper foil samples, but the etching aperture is large and the controllability is poor, which easily causes graphene damage. (2) The adhesive-free clean transfer method (publication number CN 106435727 B, application number: CN201710523050.1) uses a low surface tension organic solvent for hot embossing, and then etches away the copper growth substrate. This method requires a high level of operation proficiency, is difficult to mass-produce, and is not applicable to the transfer of super-flat graphene grown on wafers, and is prone to damage due to difficult fitting. (3) The adhesive wet etching method (publication numbers CN 18888418 A, CN111847437A) etches the copper foil graphite directly coated with polymer, and uses a bottom-up film fishing method to align and bond with the grid substrate. This method is only applicable to the small-scale transfer of copper foil graphene that can float, is not compatible with graphene grown on wafers, and has the problem of polymer residue.

[0004] Analysis of the above prior art shows that the preparation method of the graphene-coated grid support film mainly uses a method that sacrifices the growth substrate for transfer. For example, using a porous target grid as the direct transfer medium and a volatile solvent-assisted transfer method by laminating requires etching the growth substrate, which has problems such as the growth substrate being unable to be recycled, long etching time, doping of water oxygen and etching solution metal ions, and incompatibility with copper-based electron microscope grids. And after graphene detaches from the substrate, due to its single-atom layer thickness and lack of a stable and continuous support medium, it is prone to wrinkles and breakage under the action of water with high surface tension during the etching solution cleaning process, making it difficult to prepare in large areas and batches. Also, for the ultra-flat graphene prepared on a hard substrate copper(111) / sapphire wafer, it is difficult to achieve close contact between the grid and graphene using the solvent-assisted direct transfer method. After detaching from the substrate, at stress concentration points such as wrinkles and cavities, graphene is prone to breakage. Therefore, the present invention is specifically proposed to achieve the batch preparation of ultra-flat suspended graphene support films and promote the industrial application of graphene. Summary of the Invention

[0005] The object of the present invention is to provide a method for batch preparation of ultra-flat graphene-coated grids, which uses a small molecule / polymer composite transfer medium to assist in the non-etching peeling method to separate graphene from the metal growth substrate, and is applicable to the high-throughput transfer of ultra-flat graphene grown on a single crystal wafer to an electron microscope grid.

[0006] The batch preparation method of the ultra-flat graphene electron microscope support film provided by the present invention includes the following steps: S1. Perform pre-oxidation treatment on the graphene / metal substrate to obtain a graphene / oxidized metal substrate; S2. Coating a small molecule buffer layer and a polymer composite medium layer on the surface of the graphene / oxidized metal substrate in sequence to form a composite structure of the composite medium layer / graphene / oxidized metal substrate; S3. Bond a self-supporting layer on the surface of the composite medium layer to obtain a composite structure of the self-supporting layer / composite medium layer / graphene / oxidized metal substrate; S4. Separate the oxidized metal substrate from the composite structure of the self-supporting layer / composite medium layer / graphene by an electrochemical peeling method; S5. Bond the self-supporting layer / composite medium layer / graphene to an electron microscope grid, and remove the self-supporting layer and the composite medium layer to obtain a graphene / electron microscope grid composite structure; The steps S1 and S2 can also be replaced by steps S1* and S2*: S1*. Coating a small molecule buffer layer and a polymer composite medium layer on the surface of the graphene / metal substrate in sequence to form a composite structure of the composite medium layer / graphene / metal substrate; S2*. Pre-oxidize the composite medium layer / graphene / metal substrate to obtain a composite structure of the composite medium layer / graphene / oxidized metal substrate; In the batch preparation method of the present invention, the metal substrate can be single-crystalline copper sputtered on a sapphire wafer; The graphene / metal substrate can be prepared by chemical vapor deposition.

[0007] In step S1 or S2* of the present invention, the steps of the pre-oxidation treatment are as follows: Immerse the graphene / metal substrate in an alcohol-water mixture for 0.5 - 100 h; In the alcohol-water mixture, the volume fraction of water is 20% - 80%, and the alcohol can be ethanol, isopropanol or ethylene glycol; The function of the pre-oxidation treatment is to weaken the coupling force between graphene and the metal substrate, avoid the breakage of graphene during the subsequent electrochemical exfoliation process, and ensure the transfer integrity and success rate.

[0008] In step S2 or S1* of the present invention, the small molecule buffer layer includes one or more of menthol, borneol and cyclododecane; The polymer composite medium layer includes a mixture of poly(propylene carbonate) and poly(methyl methacrylate); In the mixture of poly(propylene carbonate) and poly(methyl methacrylate), the mass fraction of poly(propylene carbonate) is 80% - 99%.

[0009] The present invention combines low surface energy molecules with polymer polymers to reduce interfacial stress and reduce transfer breakage.

[0010] In the batch preparation method of the present invention, the thickness of the small molecule buffer layer is 50 nm - 10 μm, and the thickness of the polymer composite medium layer is 100 nm - 10 μm.

[0011] In the batch preparation method of the present invention, in step S3, the self-supporting layer is made of polydimethylsiloxane; The self-supporting layer is adhered to the graphene surface by roll pressing or vacuum lamination.

[0012] The present invention uses electrochemical exfoliation to achieve large-area and rapid transfer, is compatible with the transfer of graphene on wafers of different sizes, avoids the problems of traditional etching methods, and realizes high-quality batch preparation.

[0013] In the batch preparation method of the present invention, in step S5, the lamination process is as follows: 1) Oppositely buckle the surface of the porous membrane layer of the electron microscope grid with the graphene surface of the self-supporting layer / composite medium layer / graphene, and use an organic solvent to promote the close adhesion of the contact interface between the porous membrane and graphene; 2) baking at a specific temperature (40-150° C.) to soften the polymer composite medium layer and conformally fill the pores of the carrier mesh; By utilizing the thermal conformal effect of low glass transition temperature polymers, the conformal contact between graphene and the electron microscope grid is further achieved, the transfer integrity and uniformity are improved, and the transfer efficiency and quality are significantly improved; 3) Soaking and dissolving the composite medium layer by an organic solvent soaking method (preferably acetone), thereby removing the self-supporting layer.

[0014] In the batch preparation method of the present invention, in step S5, the electron microscope grid can be a microgrid of Au, Cu, Ni, or Ni-Ti mesh, and the porous membrane contained therein can be a porous metal membrane or a porous carbon membrane.

[0015] The ultra-flat graphene electron microscope support film prepared by the method of the present invention has a completeness of ≥90% in the suspended area, a surface roughness of ≤1 nm, and a single-crystal graphene proportion of ≥99%.

[0016] The present invention adopts a non-etching method to achieve large-area and high-quality transfer of wafer graphene to a grid substrate, thereby realizing the recycling of the growth substrate to reduce costs, avoiding long etching time, water oxygen and metal ion doping of the etching solution, and being compatible with substrates that are not resistant to etching solutions, such as copper-based electron microscope grids, for batch preparation.

[0017] The ultra-flat graphene electron microscope support film prepared by the present invention can be used for preparing electron microscope grids, nanoimprinting, low-temperature cryo-electron microscopes, hydrogen-deuterium separation, etc.

[0018] Compared with the prior art, the present invention has the following beneficial technical effects: 1. The present invention weakens the coupling force between graphene and the metal substrate by pre-oxidation treatment, avoids the graphene damage caused by stress concentration during electrochemical stripping, and ensures the integrity and success rate of graphene transfer. The electrochemical stripping method realizes the rapid transfer of large-area graphene and is compatible with the transfer of 2-inch, 4-inch, 6-inch and 8-inch wafer graphene. It avoids the problems of low efficiency, non-recyclable substrate, high cost, and ion doping of etching solution in traditional etching methods, and realizes high-quality and batch preparation of graphene carrier networks.

[0019] 2. By transferring graphene to a flexible small molecule / polymer substrate, the solvent tension is used to induce a clean and tight "face-to-face" fit with the target grid. Compared with direct bonding on a hard metal growth substrate, the bonding efficiency and transfer integrity are significantly improved. Low surface energy small molecules are used to isolate the direct contact between graphene and polymers, and the good solubility of small molecules in organic solvents ensures the surface cleanliness of the graphene grid after transfer.

[0020] 3. By utilizing the thermal conformal effect of polymers with low glass transition temperature, conformal contact between graphene and the target carrier grid is further achieved, significantly improving the transfer integrity and uniformity of graphene. Through the technical solution of the present invention, the efficiency and quality of the transfer of graphene to a suspended carrier grid substrate have been significantly improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a flowchart of the transfer process for preparing a super-flat graphene electron microscope support film from wafer graphene provided by the present invention.

[0022] Figure 2 It is a photo of uniformly coating a composite transfer medium on a four-inch graphene single crystal wafer, peeling and transferring it to a composite medium layer / PDMS support layer, and attaching a carrier grid in Example 1 of the present invention.

[0023] Figure 3 It is a scanning electron microscope photo and integrity statistics of a graphene / Quantifoil porous carbon film electron microscope carrier grid prepared in Example 1 of the present invention.

[0024] Figure 4 It is a transmission electron microscope photo of a graphene / Quantifoil porous carbon film carrier grid prepared in Example 1 of the present invention, the corresponding selected area electron diffraction pattern, and single crystal property statistics.

[0025] Figure 5 It is an atomic force microscope photo of a graphene / Quantifoil porous carbon film carrier grid prepared in Example 1 of the present invention. Figure 6 It is a cryo-electron microscopy image photo of a graphene / quantifoil porous carbon film carrier grid loaded with 20s protein prepared in Example 1 of the present invention.

[0026] Figure 7 It is the Raman spectroscopy and atomic force microscope characterization of repeating the growth of graphene on the copper substrate recovered in Example 1.

[0027] Figure 8 It is an optical microscope photo of transferring graphene to PDMS by PPC (a) and a scanning electron microscope photo of the corresponding prepared graphene carrier grid (b); an optical microscope photo of transferring graphene to PDMS by PMMA (c) and a scanning electron microscope photo of the corresponding prepared graphene carrier grid (d).

[0028] Figure 9 It is an optical microscope photo of transferring electrochemically exfoliated graphene to PDMS without pre-oxidation treatment, and a scanning electron microscope photo of the prepared graphene carrier grid.

[0029] Figure 10 It is an atomic force microscope photo of a graphene carrier grid without transfer of a small molecule buffer layer. Detailed implementation mode

[0030] Unless otherwise specified, the experimental methods used in the following examples are all conventional methods.

[0031] Unless otherwise specified, the materials, reagents, etc. used in the following examples can all be obtained from commercial channels.

[0032] The batch preparation method of the super-flat graphene electron microscope support film provided by the present invention separates graphene from a pre-oxidized copper metal growth substrate by an electrochemical stripping method and transfers it to a polymer composite medium; then, by a solvothermal lamination method, the graphene supported by the polymer film is conformally laminated with an electron microscope grid, and the graphene grid is obtained after removing the polymer medium.

[0033] The principle of using the electrochemical stripping method to achieve metal substrate stripping in the method of the present invention is as follows: hydrogen gas is generated at the interface between graphene and the metal substrate by the electrolysis of water reaction, and the interlayer separation is achieved through the mechanical force of gas expansion. By controlling the voltage and the composition of the electrolyte, the gas generation rate can be precisely regulated to achieve non-destructive separation of the graphene layer and the metal substrate.

[0034] Example 1. Batch preparation of super-flat graphene electron microscope support film using 4-inch single-crystal wafer graphene According to Figure 1 The shown process is carried out for preparation.

[0035] (1) Pre-decoupling treatment of the 4-inch single-crystal graphene film grown by chemical vapor deposition and the copper growth substrate (the single-crystal wafer graphene film is soaked in a mixed solution of ethanol / deionized water to weaken the coupling force between graphene and the metal substrate. The volume ratio of ethanol / deionized water is 1:1, the soaking time is 3 hours, and the soaking temperature is room temperature).

[0036] (2) Coating the composite transfer medium to obtain a composite sample of composite medium / graphene / oxidized metal substrate: commercially available menthol is dissolved in isopropanol to prepare a homogeneous solution with a mass fraction of 25%, and it is spin-coated once at a speed of 1000 rpm to form a low-surface-energy small molecule buffer layer.

[0037] Then, PPC and PMMA are dissolved in anisole to prepare a homogeneous mixed solution with a concentration of 0.1 g / mL, wherein the mass fraction of PPC in PPC / PMMA is 95%. It is spin-coated once at a speed of 1000 rpm and heated and baked at 130 °C for 3 minutes to form a composite transfer medium.

[0038] (3) Form a PDMS self-supporting layer on the composite medium layer: Continuously laminate polydimethylsiloxane (PDMS) on the composite transfer medium by roll pressing to obtain a composite structure of "PDMS support layer / composite medium layer / graphene / oxidized metal substrate". To ensure the transfer effect, it is preferred that there are no bubbles in the lamination gap.

[0039] (4) Separate the PDMS support layer / composite medium layer / graphene from the growth substrate using electrochemical reduction: Adopt a two-electrode electrolysis system, connect platinum or graphite to the positive electrode, and fix the PDMS support layer / composite medium layer / graphene / oxidized metal substrate with a fixture and connect it to the negative electrode. In a 1 M sodium hydroxide electrolyte, apply a DC potential of 3 V between the two electrodes for a duration of 5 minutes. Utilize the generation of uniform hydrogen bubbles and the electrochemical reduction of the copper substrate to weaken the interaction between graphene and the growth substrate, and then peel the PDMS support layer / composite medium layer / graphene from the metal growth substrate using a wafer fixture. After the separated metal growth substrate is cleaned with deionized water and dried with nitrogen, it is used for regrowing graphene. The peeled PDMS support layer / composite medium layer / graphene is cleaned in an ionic solution, dried at room temperature, and then used for further lamination with the grid.

[0040] (5) Lamination of the PDMS support layer / composite medium layer / graphene on the quantifoil porous carbon film grid: Oppose the porous film layer surface of the porous grid to the graphene surface, spray isopropanol, and utilize the solvent evaporation tension to promote the close contact of the two lamination surfaces. After the solvent has evaporated completely, further baking treatment is carried out to promote the chain segment movement of the PPC polymer supporting graphene after heating, achieving close contact with graphene. The baking temperature is 130 °C and the baking time is 2 minutes.

[0041] (6) Remove the glue by solvent immersion to prepare the graphene grid: Immerse the PDMS support layer / composite medium layer / graphene / grid in acetone at room temperature for 2 hours. During this period, the menthol / PMMA-PPC composite transfer medium is dissolved, and graphene is separated from PDMS. After removing PDMS, soak and clean it twice with acetone at 50 °C for 10 minutes each time, and finally soak and clean it once with isopropanol at 50 °C. Isopropanol, as a low surface tension solvent, further removes residues and reduces wrinkles on the graphene surface. After drying at room temperature, the graphene / quantifoil porous carbon film grid is obtained.

[0042] Figure 2 This is a photo of the four-inch graphene single crystal wafer in this example uniformly coated with the composite transfer medium, peeled and transferred to the composite medium layer / PDMS support layer, and laminated with the grid. It can be seen that the four-inch graphene can be quickly transferred to PDMS by electrochemical peeling, which is beneficial for batch lamination with the grid.

[0043] Figure 3Scanning electron microscope photographs and integrity statistics of the graphene / Quantifoil porous carbon film electron microscopy grids prepared in Example 1. It can be seen that after the prepared graphene is transferred to the Quantifoil porous carbon film, the integrity is as high as 90%.

[0044] Figure 4 Transmission electron microscope photographs, corresponding selected area electron diffraction pictures and single crystallinity statistics of the graphene / Quantifoil porous carbon film grids prepared in this example. It can be seen that the inside of the suspended holes is clean after the single crystal graphene is transferred to the Quantifoil porous carbon film.

[0045] Figure 5 Atomic force microscope photographs of the graphene / Quantifoil porous carbon film grids prepared in this example. It can be seen that the support film of the prepared graphene grid has the characteristics of high cleanliness and ultra-flatness.

[0046] Figure 6 Cryo-electron microscopy imaging photographs of the graphene / quantifoil porous carbon film grids loaded with 20S proteasome prepared in Example 1. It can be seen that graphene can uniformly load the ice layer and 20S proteasome.

[0047] Figure 7 Raman spectroscopy and atomic force microscopy characterization of the recycled copper substrate for repeated growth of graphene in this example. The results show that the prepared graphite has no obvious D defect peak (1350 cm -1 ), and its surface is flat, with Ra < 1 nm.

[0048] Example 2

[0049] The difference from Example 1 is that the order of steps 2 and 1 is interchanged. The composite transfer medium is coated first, and then decoupled by soaking in an aqueous alcohol solution, with other conditions remaining unchanged. It shows that there is no influence on the transfer result.

[0050] Comparative Example 1 The difference from Example 1 is that only a single-component polymer of PPC or PMMA is coated in step 2, with other conditions remaining unchanged.

[0051] Figure 8 In Figure a, it is an optical microscope photograph of transferring graphene to PDMS by PPC. In Figure b, it is a scanning electron microscope photograph of the corresponding prepared graphene grid. Figure 7 In Figure c, it is an optical microscope photograph of transferring graphene to PDMS by PMMA. In Figure d, it is a scanning electron microscope photograph of the corresponding prepared graphene grid.

[0052] From Figure 8It can be seen that, with other transfer conditions remaining the same, when graphene is transferred separately using PPC and PMMA, there is almost no graphene coverage on the carrier grid. PPC has poor mechanical properties and is prone to breakage when transferred to PDMS, resulting in a low integrity of the graphene coverage on the prepared carrier grid. PMMA can transfer graphene to PDMS completely, but due to its high glass transition temperature, conformal bonding between graphene and the carrier grid cannot be achieved, and there is almost no graphene coverage on the carrier grid.

[0053] Comparative Example 2 The difference from Example 1 is that the pre-oxidation treatment in Step 1 is omitted, and other conditions remain unchanged.

[0054] Figure 9 Shown in the figure are an optical microscope photograph (left) of graphene electrochemically exfoliated and transferred to PDMS without pre-oxidation treatment, and a scanning electron microscope photograph (right) of the prepared graphene carrier grid. It can be seen from the figure that without substrate pretreatment, when graphene is peeled from the copper substrate to PDMS, parallel cracks and breakage are likely to occur due to stress concentration, resulting in a low integrity of the graphene support film transferred to the carrier grid.

[0055] Comparative Example 3 The difference from Example 1 is that in Step 2, a PPC / PMMA layer is directly coated on the metal substrate, and other conditions remain unchanged.

[0056] Figure 10 Shown in the figure is an atomic force microscope photograph of a graphene carrier grid without small molecule buffer layer transfer. It can be seen from the figure that without the small molecule layer, the graphene surface has wrinkles and a lot of contamination.

Claims

1. A batch preparation method of a super-flat graphene electron microscope support film, comprising the following steps: S1. Pre-oxidize the graphene / metal substrate to obtain a graphene / oxidized metal substrate; S2. Sequentially coat a small molecule buffer layer and a polymer composite medium layer on the surface of the graphene / oxidized metal substrate to form a composite structure of the composite medium layer / graphene / oxidized metal substrate; S3. Bond a self-supporting layer to the surface of the composite medium layer to obtain a composite structure of the self-supporting layer / composite medium layer / graphene / oxidized metal substrate; S4. Separate the oxidized metal substrate from the composite structure of the self-supporting layer / composite medium layer / graphene by an electrochemical stripping method; S5. Bond the self-supporting layer / composite medium layer / graphene to an electron microscope grid, and remove the self-supporting layer and the composite medium layer to obtain a graphene / electron microscope grid composite structure; The steps S1 and S2 can also be replaced by steps S1* and S2*: S1*. Sequentially coat a small molecule buffer layer and a polymer composite medium layer on the surface of the graphene / metal substrate to form a composite structure of the composite medium layer / graphene / metal substrate; S2*. Pre-oxidize the composite medium layer / graphene / metal substrate to obtain a composite structure of the composite medium layer / graphene / oxidized metal substrate.

2. The mass production method according to claim 1, characterized in that: The metal substrate is a copper single crystal thin film prepared on a sapphire wafer; In step S1 or S2*, the steps of the pre-oxidation treatment are: Immerse the graphene / metal substrate in an alcohol-water mixture; In the alcohol-water mixture, the alcohol is one or more of ethanol, isopropanol or ethylene glycol.

3. The mass production method according to claim 1 or 2, characterized in that: In step S2 or S1*, the small molecule buffer layer contains one or more of menthol, borneol and cyclododecane; The polymer composite medium layer contains a mixture of poly(propylene carbonate) and poly(methyl methacrylate); In the mixture of poly(propylene carbonate) and poly(methyl methacrylate), the mass fraction of poly(propylene carbonate) is 80-99%.

4. The mass production method according to claim 1 or 2, characterized in that: The thickness of the small molecule buffer layer is 50 nm - 10 μm, and the thickness of the polymer composite medium layer is 100 nm - 10 μm.

5. The mass production method according to claim 1 or 2, characterized in that: In step S3, the self-supporting layer is polydimethylsiloxane.

6. The mass production method according to claim 1 or 2, characterized in that: In step S3, the roller pressing method or the vacuum bonding method is used to bond the self-supporting layer to the graphene surface.

7. The mass production method according to claim 1 or 2, characterized in that: In step S5, the bonding process is as follows: 1) Oppositely buckle the surface of the porous membrane layer of the electron microscope grid with the graphene surface of the self-supporting layer / composite medium layer / graphene, and use an organic solvent to promote the tight bonding of the contact interface between the porous membrane layer and the graphene; 2) At a specific temperature, soften the polymer composite medium layer and conformally fill the grid pores; 3) Soak and dissolve the composite medium layer by an organic solvent immersion method, and then remove the flexible support layer.

8. The mass production method according to claim 1 or 2, characterized in that: In step S5, the electron microscope grid is a microgrid of Au, Cu, Ni, Ni-Ti grid, and the porous membrane included is a porous metal membrane or a porous carbon membrane.

9. A super-flat graphene electron microscope support film prepared by the method according to any one of claims 1-8; The integrity of the support film in the suspended area is ≥90%, and the proportion of single-crystal graphene is ≥99%.

10. Application of the ultra-flat graphene electron microscope support film according to claim 9 in the preparation of electron microscope grids, nanoimprinting, cryogenic electron microscopy, and hydrogen-deuterium separation.

Images