A method of marking the edge profile of suspended graphene

Abstract

The application discloses a method for marking a suspended graphene edge profile, and introduces a metal element into an electrolyte for transferring graphene by an electrochemical bubbling method; during electrification, the metal element is electrocrystallized at the graphene edge to form metal nanoparticles, so that the graphene edge profile is marked. Through the metal nanoparticle modification on the graphene edge, it is possible to observe the suspended graphene morphology under an optical microscope and a scanning electron microscope, so that quantitative measurement on key parameters such as the number of graphene layers and layer intercorner is realized.

Description

Technical Field

[0001] This invention relates to two-dimensional material characterization technology and microscopic positioning technology for suspended two-dimensional materials, specifically to a method for marking the edge contours of suspended graphene. Background Technology

[0002] Graphene, composed of a single layer of carbon atoms, is a representative of two-dimensional materials and possesses many unique properties, showing great promise for applications. By controlling the number of graphene layers, the interlayer rotation angle, and different morphologies, more extraordinary physical properties can be discovered. In recent years, twisted graphene has opened up new research avenues for realizing superconductivity. [1 -2].

[0003] The following are some methods for characterizing the number of layers and the twist angle of graphene:

[0004] 1. Optical Microscope: By transferring graphene onto a specific substrate, its morphological contrast can be observed under a microscope. [3] This contrast primarily arises from three aspects: the layer absorption of graphene, the interference effect of graphene itself, and the multiple reflections and interference effects caused by the substrate. [4] It is particularly important to emphasize that the multiple reflections and interference effects caused by the substrate dominate the morphological contrast. Therefore, when graphene is transferred to a microgrid copper grid, it is almost impossible to effectively observe the morphology of the suspended graphene using an optical microscope. [4] .

[0005] 2. Raman spectroscopy: Raman scattering spectroscopy can provide key information about graphene, such as lattice vibrations, electronic structure, and electron-phonon coupling. [5] From this, we can obtain information about the number of graphene layers, mass, stacking method, boundary type, and external environment. [5] However, when using Raman scattering spectroscopy to characterize suspended graphene, it is often impossible to describe large areas. In mapping mode, the suspended sample is also difficult to characterize because it is easy for the position of the suspended sample to shift under incident light.

[0006] 3. Scanning Electron Microscopy: By adjusting the instrument parameters, the morphological differences between different layers of multilayer graphene can be clearly observed on a copper substrate. This difference is due to the varying degrees to which different numbers of graphene layers attenuate secondary electrons on the copper surface. [6] However, when it comes to suspended graphene, the lack of a substrate limits the clarity of the sample's morphology, making it difficult to obtain clear morphological features. [6] .

[0007] 4. Transmission electron microscopy: For suspended graphene located on a microgrid copper mesh, the number of layers can be inferred by utilizing the characteristics of edge wrinkles, or the number of layers and stacking angle can be resolved through dark-field imaging of graphene. [7] However, this method requires operation under relatively harsh experimental conditions and is technically challenging. Common bright-field imaging, due to the difficulty in obtaining adequate topographic contrast, cannot precisely select specific locations for further testing.

[0008] 5. Atomic Force Microscopy (AFM): This technique studies the surface structure and properties of materials by detecting extremely weak interatomic forces between the sample surface and the probe. Typically, graphene needs to be transferred to a flat substrate for measurement. However, due to the relatively low surface flatness of suspended graphene on a microgrid copper grid, it is susceptible to damage and breakage, making effective characterization difficult using this method.

[0009] In summary, methods such as optical microscopy, Raman spectroscopy, scanning electron microscopy, and atomic force microscopy are insufficient for obtaining the morphological contrast of suspended graphene. Although transmission electron microscopy can distinguish the number of layers and stacking angles to some extent, its characterization size is limited, and it is costly and difficult to operate. Currently, there is no simple, low-cost, and easy-to-operate method to effectively obtain the morphological contrast information of suspended graphene. Furthermore, accurately locating graphene with different numbers of layers and different rotation angles is also an urgent problem to be solved. References:

[0010] 1.Cao,Y.,Fatemi,V.,Fang,S.et al.Unconventional superconductivity inmagic-angle graphenesuperlattices.Nature 556,43–50(2018).https: / / doi.org / 10.1038 / nature26160.

[0011] 2.Cao,Y.,Fatemi,V.,Demir,A.et al.Correlated insulator behavior athalf-filling in magic-anglegraphene superlattices.Nature 556,80–84(2018).https: / / doi.org / 10.1038 / nature26154.

[0012] 3. R. R. Nair et al. Fine Structure Constant Defines Visual Transparency of Graphene. Science 320, 1308-1308 (2008). DOI: 10.1126 / science.1156965.

[0013] 4. Dan Bing et al. Optical contrast for identifying the thickness of two-dimensional materials, Optics Communications, Volume 406, 2018, Pages 128-138, ISSN 0030-4018,

[0014] https: / / doi.org / 10.1016 / j.optcom.2017.06.012.

[0015] 5. Wu Juanxiaa, Xu Hua, Zhang Jin. Raman Spectroscopy of Graphene[J]. Acta Chimica Sinica,

[0016] 2014, 72(3):301-318. DOI: 10.6023 / A13090936.

[0017] 6. Shihommatsu et al. Formation Mechanism of Secondary Electron Contrast of Graphene Layers on a Metal Substrate. ACS Omega. 2017 Nov 13; 2(11):7831-7836.

[0018] 7. Na Min Young et al. Dark-field Transmission Electron Microscopy Imaging Technique to Visualize the Local Structure of Two-dimensional Material; Graphene. Applied Microscopy.

[0019] 10.9729 / AM.2015.45.1.23. Summary of the Invention

[0020] To address the shortcomings of the existing technologies, this invention provides a method for marking the edge contours of suspended graphene. The method aims to use an electrochemical approach to mark the edges during the graphene transfer process, enabling the visualization of the edges of suspended graphene under optical electron microscopes and scanning electron microscopes. This allows the observation of its morphology (including the number of layers, interlayer rotation angles, and the presence of helical structures), while also enhancing the ease of sample positioning under transmission electron microscopes.

[0021] To achieve its objectives, the present invention employs the following technical solution:

[0022] A method for marking the edge contours of suspended graphene is characterized by introducing a metal element into the electrolyte of an electrochemical bubbling transfer method for graphene. During the energizing process, the metal element undergoes electrocrystallization at the graphene edge to form metal nanoparticles, thereby marking the graphene edge contours. Specifically, the method includes the following steps:

[0023] Step 1: Coat the surface of the graphene film grown directly on the metal substrate with PMMA and bake to form a PMMA layer / graphene film / metal substrate composite.

[0024] Step 2: Adhesive tape is adhered to one side of PMMA to form a composite of tape layer / PMMA layer / graphene film / metal substrate;

[0025] Step 3: Prepare an electrolyte containing metal elements;

[0026] Step 4: Using a platinum wire as the anode and the tape layer / PMMA layer / graphene film / metal substrate composite as the cathode, electrochemical bubbling peeling is performed in the electrolyte prepared in step 3. After the tape layer / PMMA layer / graphene film is peeled off from the metal substrate, the current is continued to be applied, so that the metal elements undergo electrocrystallization at the edge of the graphene to form metal nanoparticles, thereby marking the edge contour of the graphene.

[0027] Step 5: After removing the tape layer / PMMA layer / graphene film from the electrolyte and rinsing it, transfer it to the target substrate and dry it.

[0028] Step 6: Immerse the dried target substrate in acetone to remove the tape layer / PMMA layer on the graphene film, and then remove it.

[0029] This invention utilizes an electrochemical method to electrocrystallize metal elements at the edges of graphene, forming metal nanoparticles and thus marking the graphene edge contours. The electrocrystallization process mainly manifests in the following two forms: (1) After the metal elements are adsorbed within the graphene layer, they generate adsorbed atoms through a discharge reaction. These adsorbed atoms then diffuse to the step defect sites at the graphene edges, thereby forming nanoparticles. (2) The metal elements first diffuse to the step defect sites in the solution, and then preferentially undergo a discharge reaction at these step defects, thereby forming nanoparticles. The grain size and number of metal nanoparticles can be controlled by adjusting parameters such as reaction time, metal element concentration in the solution, voltage, or current.

[0030] Furthermore, the introduction of metal elements into the electrolyte can be achieved through various methods, including at least one of the following: electrochemical methods, methods for directly dissolving metal compounds, methods for reacting metals with other metal compounds, redox reaction methods, and biological reduction methods.

[0031] Furthermore, the metallic elements encompass a variety of types, including at least one of a variety of metallic elements such as iron, copper, nickel, zinc, and calcium.

[0032] Furthermore, the electrolyte is a NaOH solution or KOH solution with a concentration of 0.1-2M.

[0033] Furthermore, the applied current and energizing time during the electrochemical bubbling peeling process are controlled to ensure that the metal elements undergo electrocrystallization at the edge of the graphene to form metal nanoparticles.

[0034] Furthermore, the electrochemical bubbling stripping process can employ a constant current source or a constant voltage source.

[0035] Furthermore, in step 4, after the tape layer / PMMA layer / graphene film / metal substrate composite is fixed, it is added to the electrolyte as a cathode to ensure that the tape layer / PMMA layer / graphene film remains in contact with the metal substrate after being peeled off from the metal substrate.

[0036] Furthermore, the graphene can be suspended multilayer twisted graphene, suspended multilayer spiral graphene, suspended multilayer twisted spiral graphene, or suspended element-doped graphene, or it can be other two-dimensional materials with good conductivity, such as suspended two-dimensional titanium disulfide, suspended two-dimensional vanadium sulfide, suspended two-dimensional titanium carbide, and other two-dimensional materials with good conductivity.

[0037] Furthermore, the method of the present invention is applicable to graphene films grown on various metal substrates.

[0038] Furthermore, any type of target substrate can be selected as needed. For example, the target substrate can be a copper mesh, nickel mesh, molybdenum mesh, gold mesh, or nylon mesh, or it can be silicon, germanium, diamond, quartz, mica, sapphire, gallium arsenide, indium phosphide, gallium nitride, silicon carbide, PET, boron nitride, iron foil, or gallium oxide.

[0039] Furthermore, the suspended graphene with edge contour marking according to the method of the present invention is beneficial for characterization methods of various sample positioning, such as optical microscopy, scanning electron microscopy, transmission electron microscopy, scanning probe microscopy, Raman spectroscopy, etc.

[0040] Compared with existing technologies, the beneficial effects of this invention are reflected in:

[0041] 1. This invention modifies the edges of suspended graphene with metal nanoparticles, making the edges of the suspended graphene clearly distinguishable under different characterization methods. By analyzing the deposition location of the nanoparticles, the morphological characteristics of the graphene edges can be obtained, leading to information such as the number of graphene layers, rotation relationships, and helical structure. This modification also facilitates precise sample positioning during further transmission electron microscopy characterization, effectively saving time previously wasted on sample positioning during experiments. This invention successfully solves the problem of obtaining morphological contrast information from suspended graphene, providing a new means and method for the study of two-dimensional materials.

[0042] 2. The method of this invention can transfer graphene with marked edge contours onto various types of target substrates, such as semiconductors, metals, ceramics, and polymers. This diversity of substrate selection significantly improves the morphological contrast of graphene and makes it more widely applicable in different application fields.

[0043] 3. The method of the present invention can not only achieve high-precision positioning of samples during transmission electron microscopy characterization, but is also widely applicable to various characterization methods on various substrates, including but not limited to precise positioning in the fields of optics, electricity, mechanics, thermal, and magnetism. Attached Figure Description

[0044] Figure 1 This is a process flow diagram of the electrochemical bubbling method in Embodiment 1 of the present invention.

[0045] Figure 2 This is a schematic diagram of the electrochemical bubbling stripping device in Embodiment 1 of the present invention.

[0046] Figure 3 This is an optical image of the suspended multilayer spiral graphene on the microgrid copper grid in Embodiment 1 of the present invention without edge modification.

[0047] Figure 4This is an optical image of the suspended multilayer helical graphene on the microgrid copper grid with edge modification in Embodiment 1 of the present invention.

[0048] Figure 5 This is a scanning electron microscope image of suspended multilayer spiral graphene on a microgrid copper grid without edge modification in Embodiment 1 of the present invention.

[0049] Figure 6 This is a scanning electron microscope image of the suspended multilayer spiral graphene with edge modification on the microgrid copper grid in Embodiment 1 of the present invention.

[0050] Figure 7 This is a transmission electron microscope (TEM) image of the suspended multilayer helical graphene with edge modification on the microgrid copper grid in Embodiment 1 of the present invention.

[0051] Figure 8 This is a compositional distribution diagram of the metal nanoparticles modified with the edge of the suspended multilayer helical graphene in Example 1 of the present invention, where (a) is the morphology image of the graphene edge nanoparticles, and (b) and (c) are the distribution of iron and oxygen elements corresponding to (a), respectively.

[0052] Figure 9 Optical images are shown of transferring multilayer graphene samples onto a silicon dioxide layer / silicon substrate in Embodiment 1 of the present invention, with energizing times of 5 minutes (a), 30 minutes (b), 45 minutes (c), and 60 minutes (d), respectively. Detailed Implementation

[0053] To more clearly and understandably illustrate the purpose and content of this invention, the specific embodiments of this invention will be described in detail below with reference to examples. The following content is merely illustrative and explanatory of the invention, and those skilled in the art can modify the described specific embodiments or adopt similar methods, as long as they do not depart from the scope defined by the claims, they shall fall within the protection scope of this invention.

[0054] Example 1

[0055] This embodiment provides a method for marking the edge contours of suspended graphene, and the process flow diagram is shown below. Figure 1 The specific steps are as follows:

[0056] Step 1: Using a spin coater, PMMA is spin-coated onto the surface of multilayer graphene directly grown on a silicon-doped platinum foil substrate (3000 rpm, 1 min). The sample is then placed on a 160°C heating stage and baked for 5 min to evaporate the solvent, forming a PMMA layer / graphene film / Pt composite.

[0057] Step 2: Adhesive tape is adhered to one side of the PMMA layer, and the tape is cut along the edge of the substrate shape to form a tape layer / PMMA layer / graphene film / Pt composite. The tape is used as a support because the graphene edge nanoparticle modification process requires a relatively long electrochemical reaction time, preventing graphene agglomeration during the bubbling transfer process.

[0058] Step 3: Using 1M NaOH solution, with iron foil as the anode and cathode respectively, apply a constant current of 0.05A and pass the current for 30 minutes to introduce iron into the electrolyte.

[0059] Step 4: Using a platinum wire as the anode and the tape / PMMA / graphene film / Pt composite as the cathode, the tape / PMMA / graphene film / Pt composite is held in place by metal tweezers to ensure that even if the tape / PMMA / graphene film detaches from the Pt substrate during the entire energizing process, it remains in contact with the Pt substrate electrode. Figure 2 As shown, electrochemical bubbling peeling is performed in the Fe-containing electrolyte prepared in step 3, maintaining a constant current of 0.05A. After the tape layer / PMMA layer / graphene film is peeled off from the metal substrate, the current is continued for a total of 30 minutes, so that the metal elements undergo electrocrystallization at the edge of the graphene to form metal nanoparticles, thereby marking the edge contour of the graphene.

[0060] Step 5: Remove the tape layer / PMMA layer / graphene film from the electrolyte, rinse it twice in clean water, and then use the target substrate microgrid copper mesh to retrieve the sample and allow it to dry under natural conditions.

[0061] Step 6: Immerse the dried target substrate in acetone for 10 hours to remove PMMA. During this process, the tape will gradually detach from the sample as the PMMA dissolves.

[0062] In this embodiment, the same operation was performed using a 1M NaOH solution without metal elements to obtain suspended multilayer spiral graphene without edge modification.

[0063] In this embodiment, the same operation was performed with different bubbling times of 5 min, 30 min, 45 min, and 60 min, and the graphene was transferred to a silicon dioxide substrate to obtain multilayer graphene with different degrees of modification.

[0064] Figure 3 An optical image of suspended multilayer helical graphene on a microgrid copper mesh, without edge finishing, is presented. In the image, the whitish areas indicated by the black dashed circles represent thicker multilayer helical graphene layers, making it difficult to distinguish the individual layers and resulting in low overall contrast and clarity. Areas with fewer layers are almost transparent and difficult to discern. Figure 4 This image shows an optical image of suspended multilayer helical graphene with edge modification on a microgrid copper mesh. The white lines in the image are formed by nanoparticles generated by electrochemical reactions, which are distributed at the edge of the graphene, vividly demonstrating the edge structure of the helical graphene.

[0065] Figure 5 The image shows a scanning electron microscope image of suspended multilayer helical graphene on a microgrid copper grid without edge modification. The number of layers cannot be distinguished in the image, and the overall contrast is low.

[0066] Figure 6 Scanning electron microscope images of suspended multilayer helical graphene with edge modification on a microgrid copper grid are shown. The black lines represent nanoparticles generated by electrocrystallization during the electrochemical reaction. These nanoparticles are distributed along the edge of the graphene, which clearly shows the edge structure of the helical graphene.

[0067] Figure 7 Transmission electron microscopy (TEM) images of suspended multilayer helical graphene with edge-modified microgrids are presented. The helical structure of the graphene is clearly visible, making the number of layers easily identifiable. This provides a precise basis for subsequent electron diffraction measurements and high-resolution image acquisition.

[0068] Figure 8 This demonstrates that the nanoparticles contain elements such as iron (Fe) and oxygen (O), with the oxygen originating from the oxidation of the nanoparticles in the air.

[0069] Figure 9 Optical images are provided showing the electrochemical transfer of multilayer graphene samples onto a silicon substrate with a 300 nm silicon dioxide layer at 5, 30, 45, and 60 minutes, respectively. See also... Figure 9 In (a) of the diagram, when the energizing time is 5 minutes, the graphene is peeled off from the platinum substrate using a bubbling method, with no metal nanoparticles attached to the edges. See also Figure 9 In (b) of the diagram, after 30 minutes of energization, metal nanoparticles can be observed adhering to the edges of the graphene. (See also...) Figure 9 In (c), after 45 minutes of energization, the number and size of nanoparticles attached to the graphene edge increased, and the edge markings became clearer. See also Figure 9 In (d), after 60 minutes of power-on, in addition to the nanoparticles attached to the edges of the graphene, nanoparticles also began to be deposited on the surface of the graphene.

[0070] By comparison Figure 3 and Figure 4 ,as well as Figure 5 and Figure 6It can be clearly observed that the method of the present invention effectively solves the problem of obtaining morphological contrast information of suspended graphene. Furthermore, from... Figure 7 and Figure 8 It can also be seen that this edge modification not only improves the characterization of suspended graphene, but also enables precise sample positioning during further transmission electron microscopy characterization, thereby effectively reducing the ineffective time spent on sample positioning during the experiment. According to Figure 9 The observation results show that the size, quantity, and distribution of nanoparticles can be precisely controlled by adjusting the experimental conditions, thereby meeting different needs and applications.

[0071] The above are merely exemplary embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for marking the edge contour of suspended graphene, characterized in that: A metal element is introduced into the electrolyte for electrochemical bubbling transfer of graphene. During the energizing process, the metal element undergoes electrocrystallization at the edge of the graphene to form metal nanoparticles, thereby marking the edge contour of the graphene. The process includes the following steps: Step 1: Coat the surface of the graphene film grown directly on the metal substrate with PMMA and bake to form a PMMA layer-graphene film-metal substrate composite. Step 2: Adhesive tape is adhered to one side of PMMA to form a composite material consisting of tape layer-PMMA layer-graphene film-metal substrate; Step 3: Prepare an electrolyte containing metal elements; Step 4: Using a platinum wire as the anode and the tape layer-PMMA layer-graphene film-metal substrate composite as the cathode, electrochemical bubbling peeling is performed in the electrolyte prepared in step 3. After the tape layer-PMMA layer-graphene film is peeled off from the metal substrate, the current is continued to be applied, so that the metal elements undergo electrocrystallization at the edge of the graphene to form metal nanoparticles, thereby marking the edge contour of the graphene. Step 5: After removing the tape layer-PMMA layer-graphene film from the electrolyte and rinsing it, transfer it to the target substrate and dry it. Step 6: Immerse the dried target substrate in acetone to remove the tape layer - PMMA layer on the graphene film, and then remove it.

2. The method for marking the edge contour of suspended graphene according to claim 1, characterized in that: Methods for introducing metal elements into electrolytes include at least one of electrochemical methods, methods for directly dissolving metal compounds, and biological reduction methods.

3. The method for marking the edge contour of suspended graphene according to claim 1, characterized in that: The metallic element is at least one of iron, copper, nickel, and zinc.

4. The method for marking the edge contour of suspended graphene according to claim 1, characterized in that: The electrolyte is a NaOH solution or a KOH solution.

5. The method for marking the edge contour of suspended graphene according to claim 1, characterized in that: In step 4, after the tape layer-PMMA layer-graphene film-metal substrate composite is fixed, it is added to the electrolyte as a cathode to ensure that the tape layer-PMMA layer-graphene film remains in contact with the metal substrate after being peeled off from the metal substrate.

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