A batch preparation method for ultra-flat graphene electron microscope support film

Abstract

The present invention discloses a method for batch preparation of an ultra-flat graphene electron microscope support film, which belongs to the field of materials. The method of the present invention comprises: utilizing an electrochemical stripping method to separate graphene from a pre-oxidized copper metal growth substrate, and transferring the result to a polymer composite medium; then utilizing a solvent thermal bonding method to conformally bond the graphene supported by the polymer film to an electron microscope grid, and removing the polymer medium to obtain the graphene grid. The method of the present invention has a simple process, high repeatability, strong compatibility, and can be prepared and produced on a large scale. The graphene support film prepared by the method of the present invention has an integrity of up to 90%, and the graphene in the suspended holes of the grid has the characteristics of ultra-flatness and good cleanliness.

Description

Technical Field

[0001] The invention relates to a batch preparation method of an ultra-flat graphene electron microscope support film, and belongs to the field of materials. Background Art

[0002] Graphene has excellent properties. Suspended graphene can demonstrate intrinsic performance characteristics and has low background noise, making it an ideal electron support film material. In particular, it can alleviate the gas-liquid interface, sample displacement, and improve imaging resolution in cryo-electron microscopy imaging analysis. Chemical vapor deposition technology can achieve large-scale preparation of high-quality single-crystalline graphene. For example, ultra-flat graphene single crystals can be obtained by chemical vapor epitaxial growth on copper (111) / sapphire wafers (Patent No. CN201710523050.1). However, there are few reports on how to achieve mass production and high-quality preparation of electron microscopy support films without the graphene growth substrate.

[0003] The current methods for achieving graphene transfer are: (1) Transfer-free etching method (publication number CN 109437176 A) prepares a suspended graphene support membrane by selectively etching the copper foil growth substrate. This method is suitable for copper foil samples, and the etching aperture is large and the controllability is poor, which easily leads to graphene damage. (2) Glue-free clean transfer method (publication number CN 106435727 B, application number: CN201710523050.1) After hot stamping with a low surface tension organic solvent, the copper growth substrate is etched away. This method requires high operator proficiency and is difficult to mass produce. It is also not suitable for the transfer of ultra-flat graphene grown on wafers and is easily damaged due to bonding difficulties. (3) The wet etching method with adhesive (publication number CN 18888418 A, CN111847437A) etches the copper foil graphene directly coated with polymer and then aligns it with the carrier grid substrate by lifting the film from the bottom up. This method is only suitable for small batch transfer of floating copper foil graphene. It is not compatible with graphene grown on wafers and has the problem of polymer residue.

[0004] Analysis of the above-mentioned prior art shows that the preparation method of the graphene grid support film mainly adopts a method of sacrificing the growth substrate for transfer. For example, the transfer method using a porous target grid as a direct transfer medium and using a volatile solvent to assist the bonding requires etching the growth substrate, which has problems such as the growth substrate cannot be recycled, the etching time is long, water oxygen and metal ion doping of the etching solution, and incompatibility with the copper-based electron microscope grid. Moreover, after the graphene is separated from the substrate, due to the thickness of a single atomic layer and the lack of a stable and continuous supporting medium, it is easy to produce wrinkles and damage during the etching solution cleaning process under the action of water with high surface tension, making it difficult to prepare in large quantities. Moreover, the ultra-flat graphene prepared on the hard substrate copper (111) / sapphire wafer adopts a solvent bonding assisted direct transfer method, which makes it difficult to achieve close contact between the grid and the graphene. After separation from the substrate, the graphene is easily damaged at stress concentration points such as wrinkles and holes. The present invention is specially 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 purpose of the present invention is to provide a method for batch preparation of ultra-flat graphene grids, which uses a small molecule / polymer composite transfer medium to assist in an etching-free stripping method to separate graphene and a metal growth substrate. It is suitable for high-throughput transfer of ultra-flat graphene grown on single crystal wafers to electron microscope grids.

[0006] The method for batch preparation of ultra-flat graphene electron microscope support film provided by the present invention comprises the following steps:

[0007] S1, pre-oxidizing the graphene / metal substrate to obtain a graphene / oxidized metal substrate;

[0008] S2. sequentially coating a small molecule buffer layer and a polymer composite media layer on the surface of the graphene / oxidized metal substrate to form a composite media layer / graphene / oxidized metal substrate composite structure;

[0009] S3, laminating a self-supporting layer on the surface of the composite medium layer to obtain a composite structure of self-supporting layer / composite medium layer / graphene / oxidized metal substrate;

[0010] S4, separating the oxidized metal substrate from the composite structure of the self-supporting layer / composite medium layer / graphene by electrochemical stripping;

[0011] S5, laminating the self-supporting layer / composite medium layer / graphene to an electron microscope grid, removing the self-supporting layer and the composite medium layer, and obtaining a graphene / electron microscope grid composite structure;

[0012] The steps S1 and S2 can also be replaced by steps S1* and S2*:

[0013] S1*, sequentially coating a small molecule buffer layer and a polymer composite medium layer on the surface of a graphene / metal substrate to form a composite medium layer / graphene / metal substrate composite structure;

[0014] S2*, pre-oxidizing the composite medium layer / graphene / metal substrate to obtain a composite structure of composite medium layer / graphene / oxidized metal substrate;

[0015] In the batch preparation method of the present invention, the metal substrate can be single crystal copper sputtered on a sapphire wafer;

[0016] The graphene / metal substrate can be prepared by a chemical vapor method.

[0017] In step S1 or S2* of the present invention, the pre-oxidation step is:

[0018] Immersing the graphene / metal substrate in an alcohol-water mixture for 0.5-100 h;

[0019] In the alcohol-water mixture, the volume fraction of water is 20%-80%, and the alcohol can be ethanol, isopropanol or ethylene glycol;

[0020] The function of the pre-oxidation treatment is to weaken the coupling force between the graphene and the metal substrate, thereby avoiding damage to the graphene during the subsequent electrochemical stripping process and ensuring the transfer integrity and success rate.

[0021] In step S2 or S1* of the present invention, the small molecule buffer layer comprises one or more of menthol, borneol, and cyclododecane;

[0022] The polymer composite medium layer comprises a mixture of polypropylene carbonate and polymethyl methacrylate;

[0023] In the mixture of the polypropylene carbonate and the polymethyl methacrylate, the mass fraction of the polypropylene carbonate is 80-99%.

[0024] The present invention combines low surface energy molecules with high molecular weight polymers to reduce interfacial stress and minimize transfer damage.

[0025] In the batch preparation method of the present invention, the thickness of the small molecule buffer layer is 50 nm-10 um, and the thickness of the polymer composite medium layer is 100 nm-10 um.

[0026] In the batch preparation method of the present invention, in step S3, the self-supporting layer is made of polydimethylsiloxane;

[0027] The self-supporting layer is laminated to the graphene surface by roller pressing or vacuum lamination.

[0028] The present invention adopts electrochemical stripping method to achieve large-area rapid transfer, is compatible with the transfer of graphene wafers of different sizes, avoids the problems of traditional etching methods, and realizes high-quality batch preparation.

[0029] In the batch preparation method of the present invention, in step S5, the lamination process is as follows:

[0030] 1) Aligning the porous membrane surface of the electron microscope grid with the graphene surface of the self-supporting layer / composite medium layer / graphene, and using an organic solvent to facilitate close contact between the porous membrane and the graphene interface;

[0031] 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;

[0032] By utilizing the thermal conformal effect of low glass transition temperature polymers, we can further achieve conformal contact between graphene and the electron microscope grid, improve the transfer integrity and uniformity, and significantly enhance the transfer efficiency and quality.

[0033] 3) Soaking and dissolving the composite medium layer by an organic solvent soaking method (preferably acetone), thereby removing the self-supporting layer.

[0034] 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 included is a porous metal membrane or a porous carbon membrane.

[0035] 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%.

[0036] The present invention adopts an etching-free 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.

[0037] 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.

[0038] Compared with the prior art, the present invention has the following beneficial technical effects:

[0039] 1. The present invention weakens the coupling force between graphene and the metal substrate through pre-oxidation, avoiding graphene damage caused by stress concentration during electrochemical stripping, thereby ensuring the integrity and success rate of graphene transfer. The electrochemical stripping method achieves rapid transfer of large-area graphene and is compatible with the transfer of graphene on 2-inch, 4-inch, 6-inch, and 8-inch wafers. It avoids the problems of traditional etching methods such as low efficiency, non-recyclable substrates, high cost, and ion-doped etching solutions, achieving high-quality and mass-produced graphene grids.

[0040] 2. By transferring graphene onto a flexible small molecule / polymer substrate, solvent tension induction is used to achieve a clean, close "face-to-face" bond with the target grid. Compared to direct bonding onto a rigid metal growth substrate, bonding efficiency and transfer integrity are significantly improved. Low-surface-energy small molecules isolate the graphene from direct contact with the polymer, while the good solubility of small molecules in organic solvents ensures the surface cleanliness of the transferred graphene grid.

[0041] 3. Utilizing the thermal conformal effect of low-glass-transition-temperature polymers, the graphene is further conformed to the target grid, significantly improving the integrity and uniformity of the graphene transfer. The technical solution of this invention significantly improves the efficiency and quality of graphene transfer to the suspended grid substrate. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 The present invention provides a flow chart of a transfer process for preparing an ultra-flat graphene electron microscope support film from wafer graphene.

[0043] Figure 2 These are photos of the four-inch graphene single crystal wafer being coated with the composite transfer medium, peeled and transferred to the composite medium layer / PDMS support layer, and attached to the carrier grid in Example 1 of the present invention.

[0044] Figure 3 Scanning electron microscope photograph and integrity statistics of the graphene / Quantifoil porous carbon film electron microscope grid prepared in Example 1 of the present invention.

[0045] Figure 4 Transmission electron microscope photograph of the graphene / Quantifoil porous carbon film grid prepared in Example 1 of the present invention, the corresponding selected area electron diffraction image and single crystal statistics.

[0046] Figure 5 Atomic force microscope photo of the graphene / Quantifoil porous carbon film grid prepared in Example 1 of the present invention

[0047] Figure 6This is a cryo-electron microscopy image of the graphene / quantifoil porous carbon film grid loaded with 20s protein prepared in Example 1 of the present invention.

[0048] Figure 7 The Raman spectroscopy and atomic force microscopy were used to characterize the graphene grown repeatedly on the copper substrate recovered in Example 1.

[0049] Figure 8 Optical microscope photograph of graphene transferred from PPC to PDMS (a) and scanning electron microscope photograph of the corresponding graphene grid (b); optical microscope photograph of graphene transferred from PMMA to PDMS (c) and scanning electron microscope photograph of the corresponding graphene grid (d).

[0050] Figure 9 Optical micrograph of electrochemically exfoliated graphene transferred to PDMS without pre-oxidation treatment, and scanning electron micrograph of the prepared graphene grid.

[0051] Figure 10 Atomic force microscopy image of graphene grid without small molecule buffer layer transfer. DETAILED DESCRIPTION

[0052] Unless otherwise specified, the experimental methods used in the following examples are conventional methods.

[0053] Unless otherwise specified, the materials and reagents used in the following examples can be obtained from commercial sources.

[0054] The present invention provides a method for batch preparation of ultra-flat graphene support films for electron microscopes. The method utilizes an electrochemical stripping method to separate graphene from a pre-oxidized copper metal growth substrate and transfers the graphene to a polymer composite medium. The solvent thermal bonding method is then used to conformally bond the graphene supported by the polymer film to an electron microscope grid. The graphene grid is obtained after removing the polymer medium.

[0055] The present method utilizes electrochemical exfoliation to achieve metal substrate separation. The principle behind this method is that water electrolysis generates hydrogen at the interface between the graphene and metal substrate, and the mechanical force of this gas expansion causes interlayer separation. By controlling the voltage and electrolyte composition, the gas generation rate can be precisely controlled, achieving non-destructive separation of the graphene layer from the metal substrate.

[0056] Example 1: Batch preparation of ultra-flat graphene electron microscope support film using 4-inch single crystal graphene wafer

[0057] according to Figure 1 Prepared according to the process shown.

[0058] (1) Pre-decoupling treatment of 4-inch single-crystal graphene film grown by chemical vapor phase method and copper growth substrate (immersion treatment of single-crystal wafer graphene film in ethanol / deionized water mixture to weaken the coupling force between graphene and metal substrate. The volume ratio of ethanol / deionized water is 1:1, the immersion time is 3 hours, and the immersion temperature is room temperature.

[0059] (2) Coating a composite transfer medium to obtain a composite medium / graphene / oxidized metal substrate composite sample: commercially available menthol was dissolved in isopropanol to prepare a uniform solution with a mass fraction of 25%, and the solution was evenly coated at a speed of 1000 rpm to form a low surface energy small molecule buffer layer.

[0060] Next, PPC and PMMA were dissolved in anisole to create a uniform mixture at 0.1 g / mL, with a PPC mass fraction of 95%. The mixture was evenly coated once at 1000 rpm and then baked at 130°C for 3 minutes to form a composite transfer medium.

[0061] (3) Forming a PDMS self-supporting layer on the composite medium layer: Polydimethylsiloxane (PDMS) is further laminated onto the composite transfer medium by roller pressing to obtain a composite structure of "PDMS support layer / composite medium layer / graphene / oxidized metal substrate". To ensure the effect after transfer, it is preferred that there are no bubbles in the lamination gap.

[0062] (4) Separate the PDMS support layer / composite medium layer / graphene from the growth substrate using an electrochemical reduction method: A two-electrode electrolysis system is used, with platinum or graphite connected to the positive electrode, and the PDMS support layer / composite medium layer / graphene / oxidized metal substrate is fixed with a clamp and connected to the negative electrode. In a 1 M sodium hydroxide electrolyte, a DC potential of 3 V is applied between the two electrodes for 5 minutes. The interaction between graphene and the growth substrate is weakened by generating uniform hydrogen bubbles and electrochemically reducing the copper substrate. The PDMS support layer / composite medium layer / graphene is then peeled off from the metal growth substrate using a wafer clamp. The separated metal growth substrate is cleaned with deionized water and dried with nitrogen for re-growth of 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 bonding to the carrier grid.

[0063] (5) PDMS support layer / composite medium layer / graphene quantifoil porous carbon film grid bonding: The porous membrane surface of the porous grid is aligned with the graphene surface, and isopropyl alcohol is sprayed on it. The solvent evaporation tension is used to promote close contact between the two bonding surfaces. After the solvent evaporates and dries, further baking treatment is performed to promote the chain segment movement of the PPC polymer supporting the graphene after heating, achieving close contact with the graphene. The baking temperature is 130°C and the baking time is 2 minutes.

[0064] (6) Preparation of graphene grid by solvent immersion and degumming: Soak 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 the graphene and PDMS are separated. After removing the PDMS, the grid is washed twice with acetone at 50°C for 10 minutes each time, and finally washed 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.

[0065] Figure 2 The following are photos of the four-inch graphene single crystal wafer in this embodiment being evenly coated with the composite transfer medium, peeled off and transferred to the composite medium layer / PDMS support layer, and bonded to the carrier grid. It can be seen that the four-inch graphene can be quickly transferred to the PDMS through the electrochemical stripping method, which is conducive to batch bonding with the carrier grid.

[0066] Figure 3 The scanning electron microscope image and integrity statistics of the graphene / Quantifoil porous carbon film electron microscope grid prepared in Example 1 show that the integrity of the prepared graphene after transfer to the quantifoil porous carbon film is as high as 90%.

[0067] Figure 4 The transmission electron microscope image, corresponding selected area electron diffraction image and single crystal statistics of the graphene / Quantifoil porous carbon film support prepared in this example show that the suspended pores of the single-crystalline graphene are clean after being transferred to the quantifoil porous carbon film.

[0068] Figure 5 This is an atomic force microscope photo of the graphene / Quantifoil porous carbon film grid prepared in this example. It can be seen that the prepared graphene grid support film has high cleanliness and ultra-flatness.

[0069] Figure 6 This is a cryo-electron microscopy image of the graphene / quantifoil porous carbon film grid loaded with 20S proteasome prepared in Example 1. It can be seen that graphene can evenly load the ice layer and 20S proteasome.

[0070] Figure 7 Raman spectroscopy and atomic force microscopy were used to characterize the graphene grown on the copper substrate recovered in this example. The results showed that the prepared graphite had no obvious D defect peak (1350 cm -1 ), and its surface is flat, Ra<1 nm.

[0071] Example 2

[0072] The difference from Example 1 is that the order of step 2 and step 1 is interchanged, the composite transfer medium is applied first, and then the decoupling is performed by immersion in an alcohol-water solution, while other conditions remain unchanged. The results show no effect on the transfer results.

[0073] Comparative Example 1

[0074] The difference from Example 1 is that in step 2, only PPC or PMMA single-component polymer is coated, and other conditions remain unchanged.

[0075] Figure 8 Figure a is an optical microscope photo of graphene transferred from PPC to PDMS, and Figure b is a scanning electron microscope photo of the corresponding graphene grid; Figure 7 Figure c is an optical microscope photograph of graphene transferred from PMMA to PDMS, and Figure d is a scanning electron microscope photograph of the corresponding prepared graphene grid.

[0076] Depend on Figure 8 As can be seen, when graphene is transferred onto PPC and PMMA alone, keeping all other transfer conditions the same, the grid is virtually free of graphene coverage. PPC has poor mechanical properties and is prone to breakage when transferred onto PDMS, resulting in low graphene grid coverage. While PMMA can transfer graphene completely onto PDMS, its high glass transition temperature prevents conformal bonding between the graphene and the grid, resulting in virtually no graphene coverage.

[0077] Comparative Example 2

[0078] The difference from Example 1 is that the pre-oxidation treatment in step 1 is omitted, and other conditions remain unchanged.

[0079] Figure 9 Optical microscope photograph of graphene electrochemically exfoliated and transferred to PDMS without pre-oxidation treatment (left), and scanning electron microscope photograph of the prepared graphene grid (right). As can be seen from the figure, without substrate pretreatment, graphene is easily peeled from the copper substrate to PDMS, which is prone to parallel cracks and breakage due to stress concentration, resulting in low integrity of the graphene support film transferred to the grid.

[0080] Comparative Example 3

[0081] The difference from Example 1 is that in step 2, the PPC / PMMA layer is directly coated on the metal substrate, and other conditions remain unchanged.

[0082] Figure 10 This is an atomic force microscope photo of a graphene grid without a small molecule buffer layer transfer. As can be seen from the figure, the surface of the graphene without a small molecule layer is wrinkled and highly contaminated.

Claims

1. A method for batch preparation of an ultra-flat graphene electron microscope support film, comprising the following steps S1, S2, S3, S4, and S5, or steps S1*, S2*, S3, S4, and S5: S1, pre-oxidizing the graphene / metal substrate to obtain a graphene / oxidized metal substrate; S2. sequentially coating a small molecule buffer layer and a polymer composite media layer on the surface of the graphene / oxidized metal substrate to form a composite media layer / graphene / oxidized metal substrate composite structure; S1*, sequentially coating a small molecule buffer layer and a polymer composite medium layer on the surface of a graphene / metal substrate to form a composite medium layer / graphene / metal substrate composite structure; S2*, pre-oxidizing the composite medium layer / graphene / metal substrate to obtain a composite structure of composite medium layer / graphene / oxidized metal substrate; S3, laminating a self-supporting layer on the surface of the composite medium layer to obtain a composite structure of self-supporting layer / composite medium layer / graphene / oxidized metal substrate; S4, separating the oxidized metal substrate from the composite structure of the self-supporting layer / composite medium layer / graphene by electrochemical stripping; S5, laminating the self-supporting layer / composite medium layer / graphene to an electron microscope grid, removing the self-supporting layer and the composite medium layer, and obtaining a graphene / electron microscope grid composite structure; The metal substrate is a copper single crystal thin film prepared on a sapphire wafer; In step S1 or S2*, the pre-oxidation step is: soaking 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; In step S2 or S1*, the small molecule buffer layer comprises one or more of menthol, borneol, and cyclododecane; The polymer composite medium layer comprises a mixture of polypropylene carbonate and polymethyl methacrylate; In the mixture of the polypropylene carbonate and the polymethyl methacrylate, the mass fraction of the polypropylene carbonate is 80-99%.

2. The batch preparation method according to claim 1, characterized in that: The thickness of the small molecule buffer layer is 50 nm-10 um, and the thickness of the polymer composite medium layer is 100 nm-10 um.

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

4. The batch preparation method according to claim 1 or 2, characterized in that: In step S3, the self-supporting layer is laminated to the graphene surface by a roller pressing method or a vacuum lamination method.

5. The batch preparation method according to claim 1 or 2, characterized in that: In step S5, the lamination process is as follows: 1) Aligning the porous membrane surface of the electron microscope grid with the graphene surface of the self-supporting layer / composite medium layer / graphene, and using an organic solvent to facilitate close contact between the porous membrane layer and the graphene interface; 2) at a specific temperature, softening the polymer composite medium layer and conformally filling the pores of the carrier mesh; 3) Soaking and dissolving the composite medium layer by an organic solvent soaking method, thereby removing the self-supporting layer.

6. The batch preparation method according to claim 1 or 2, characterized in that: In step S5, the electron microscope grid is a microgrid of Au, Cu, Ni, or Ni-Ti mesh, and the porous membrane contained therein is a porous metal membrane or a porous carbon membrane.

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