Method for identifying the number of layers of CVD graphene on copper foil

Abstract

The application discloses a method for identifying the number of layers of CVD graphene on a copper foil, and belongs to the field of materials. The method for identifying the number of layers of CVD graphene on a copper foil comprises the following steps: (1) performing heating and oxidation treatment on the copper foil on which graphene is grown; (2) performing high-resolution imaging on the copper foil after the heating and oxidation treatment in the step (1) by using a scattering near-field optical microscope, recording the optical amplitude signal intensity at different positions, and judging the number of layers of the graphene by comparing the strength change of the optical amplitude signal between the copper foil substrate after oxidation and the graphene with different numbers of layers. The application can quickly identify the number of layers of the graphene by only performing appropriate pre-oxidation treatment, without transferring, and can avoid the pollution and damage of the sample in the transferring process.

Description

Technical Field

[0001] This invention relates to the field of materials, and more specifically to a method for identifying the number of CVD graphene layers on copper foil. Background Technology

[0002] Graphene is a nanomaterial with excellent physical and chemical properties, such as high conductivity, high strength, and high transparency, and has wide applications in various fields. Chemical vapor deposition (CVD) is currently the most promising method for industrializing graphene production. Copper is a common industrial metal, and its cost is much lower than some precious metals (such as platinum and iridium) or rare metals (such as ruthenium and rhodium). This makes using copper as a substrate for CVD-grown graphene highly economical, helping to reduce production costs and promote the commercial application of graphene-related technologies. However, during graphene preparation, changes in carbon source concentration and growth conditions can lead to different numbers of layers in the prepared graphene. Therefore, determining the number of layers is a key indicator for evaluating product quality and consistency. Different applications have strict requirements on the number of graphene layers. For example, graphene used in high-speed electronic devices typically requires a single layer or a few layers to ensure excellent carrier transport performance; while in some composite materials, a specific number of graphene layers may be needed to achieve the best reinforcement effect. Accurately determining the number of graphene layers helps optimize the growth process, improve device performance, and promote the application of graphene in high-speed electronics, optoelectronic devices, sensors, and other fields.

[0003] In existing technologies, optical microscopy can be used to quickly and easily characterize the number of graphene layers. On a silicon substrate with a silicon oxide layer of a certain thickness, when the oxide layer thickness meets certain conditions, color changes are caused by optical diffraction and interference effects, and graphene will exhibit unique color and contrast differences, thus allowing the number of graphene layers to be identified. However, this requires transferring the graphene grown on a copper substrate to a substrate with a significantly different contrast, increasing the complexity and cost of the process, and easily leading to structural damage and contamination during the transfer. The thickness of a single layer of graphene is typically 0.4-0.7 nm, and the number of graphene layers can be directly estimated using the height curve of atomic force microscopy (AFM). However, due to the influence of surface adsorbates, impurities, or defects and wrinkles in the graphene itself, the measured thickness of a single layer of graphene is easily inaccurate. Therefore, judging the number of graphene layers solely by height difference is inaccurate, and a combination of height curves and other measurement methods is usually required, increasing the complexity of characterization. Raman spectroscopy is one of the most commonly used, rapid, non-destructive, and high-resolution techniques for characterizing carbon materials. Graphene has two characteristic peaks at 1580 cm⁻¹. -1 Nearby G peak and 2680 cm -1The nearby 2D peak (overtone peak). As the number of graphene layers (n) increases, the position of the G peak shifts to lower wavenumbers, its shift being related to 1 / n, while the shape of the G peak does not change significantly. Compared to the G peak, the shape and position of the 2D peak change with the increase of the number of graphene layers. With the increase of the number of graphene layers, the 2D peak broadens, its intensity decreases, and a redshift trend appears. In addition, the full width at half maximum (FWHM) of the D peak and the intensity ratio of the G peak to the 2D peak can also be used to measure the number of graphene layers. Overall, the small changes in the shape, intensity, and position of the peaks in the Raman spectrum are closely related to the number of graphene layers, and therefore can be used to characterize the number of graphene layers. However, Raman spectroscopy is only applicable to graphene with AB stacking; it cannot accurately distinguish the number of layers for other, more disordered stacking methods or AA-type graphene. Furthermore, the defects in graphene and the different surface adsorbed substances will also affect the results of Raman spectroscopy characterization. Graphene grown on copper substrates generally needs to be transferred to Si / SiO2 substrates, otherwise the Raman spectrum will have strong fluorescence interference. Summary of the Invention

[0004] To address the aforementioned deficiencies in the prior art, this invention provides a method for identifying the number of CVD graphene layers on copper foil. By performing appropriate pretreatment on the copper substrate with graphene growth, this invention can quickly identify the number of graphene layers without transferring them.

[0005] This invention first provides a method for identifying the number of CVD graphene layers on copper foil, comprising the following steps: (1) The copper foil with graphene grown on it is subjected to heating oxidation treatment; (2) Use a scattering near-field optical microscope to perform high-resolution imaging on the copper foil after heating and oxidation in (1), record the intensity of optical amplitude signals at different positions, and determine the number of graphene layers by comparing the changes in the intensity of optical amplitude signals between the oxidized copper foil substrate and graphene with different numbers of layers.

[0006] In the above method, step (1) involves heating at 100-150°C for 1-2 minutes.

[0007] Specifically, the heating oxidation treatment is heating at 100°C for 1 minute.

[0008] In the above method, in step (2), when the scanning range of the high-resolution imaging does not exceed 50 μm × 50 μm, the scanning mode is selected as PsHet, the resolution is set to 20-30 nm, the scanning speed is set to not exceed 6 μm / min, and the near-field light source is selected as 950-1050 cm. -1 The mid-infrared laser outputs a second- or third-order optical amplitude image.

[0009] In the above method, in step (2), when the scanning range of the high-resolution imaging is greater than 50 μm × 50 μm, the scanning mode is selected as Serpent, and Skip is set to 1 or 2; the resolution is set to 20-30 nm, the scanning speed is set to no more than 6 μm / min, and the near-field light source is selected as 950-1050 cm. -1 The mid-infrared laser outputs a second- or third-order optical amplitude image.

[0010] In the above method, the graphene is grown on a copper foil substrate by chemical vapor deposition.

[0011] A slight heating oxidation process was performed on the copper substrate on which graphene was grown. The purpose of this was twofold: first, to reduce charge transfer between the graphene and the copper substrate to avoid interfering with subsequent measurements; and second, to weaken the near-field signal of the copper substrate so as not to overshadow the near-field signal of the graphene.

[0012] High-resolution imaging of graphene was achieved using s-SNOM (scattering near-field optical microscopy). Due to the difference in carrier concentration between the oxidized copper foil substrate and graphene layers of varying numbers, this difference is reflected in the intensity of the optical amplitude signal after coupling with excitation light, and can therefore be detected by near-field optical microscopy. By comparing the intensity changes of the optical amplitude signal in different regions, the number of graphene layers can be determined.

[0013] The present invention has the following beneficial effects: (1) Compared with optical microscopy and Raman spectroscopy, which require transferring the graphene grown on the copper substrate to other substrates for characterization, the present invention only requires appropriate pre-oxidation treatment and does not require transfer, thus avoiding contamination and damage to the sample during the transfer process. (2) Compared with atomic force microscopy, this invention does not use height difference but optical intensity as the basis for judgment, thus avoiding interference from factors such as surface adsorbates, impurities or defects and wrinkles in graphene itself on the test results; (3) Compared with Raman spectroscopy, the method of the present invention is not only applicable to graphene with AB stacking, but also can accurately distinguish the number of layers of graphene with other stacking methods that are more disordered or AA type; at the same time, two-dimensional images are simpler and more intuitive than spectra, and do not require complex data analysis and comparison. Attached Figure Description

[0014] Figure 1 This is an image of a single crystal region of oxidized copper foil under an optical microscope.

[0015] Figure 2 Images of different crystal regions of oxidized copper foil under an optical microscope.

[0016] Figure 3 The graphene monolayer was CVD grown on oxidized copper foil, with a scale bar of 4 μm.

[0017] Figure 4 The graphene is a multilayer graphene grown by CVD on oxidized copper foil, with a scale bar of 4 μm.

[0018] Figure 5 This involves the analysis and comparison of height signals and optical amplitude signals at the same location. Detailed Implementation

[0019] The present invention will be further described in detail below with reference to specific embodiments. The embodiments given are only for illustrating the present invention and are not intended to limit the scope of the present invention.

[0020] Unless otherwise specified, the experimental methods described in the following examples are conventional methods.

[0021] Unless otherwise specified, the quantitative experiments in the following examples are all repeated three times, and the results are averaged.

[0022] Unless otherwise specified, all materials and reagents used in the following examples are commercially available.

[0023] Example 1 1. Growth of CVD graphene A commercial copper foil (99.9% purity) measuring 4 cm × 8 cm and 10 μm thick was immersed in a 20% hydrochloric acid aqueous solution for 10 min. The copper foil was then removed and immersed in deionized water, ultrasonically cleaned at 60 Hz for 10 min to remove adsorbed acid and impurities, and then dried with high-purity nitrogen. The copper foil was then placed in a high-temperature tube furnace, and 200 sccm of argon and 50 sccm of hydrogen were introduced, followed by heating. Annealing was carried out for 1 h at 1020 °C. The gas conditions were then changed to 250 sccm of argon and 25 sccm of hydrogen, while maintaining the temperature. 4 sccm of methane (1% purity, the remainder being argon) was introduced, and growth was carried out for 10 min. During this process, multilayer graphene single crystals formed on the copper foil surface. The furnace was then opened for cooling. When the temperature dropped to approximately 600 °C, the methane supply was stopped, while maintaining constant argon and hydrogen flow rates. After the temperature returned to room temperature, the gas supply was stopped, and the sample was removed.

[0024] 2. Pretreatment of the copper substrate with graphene growth. Cut the copper foil with graphene grown in step 1 to an appropriate size (2 × 2 cm). 2 Then, it is placed on a heating platform and heated at 100 ℃ for 1 min for oxidation treatment. Because graphene has a protective effect on the copper substrate, the color contrast between the oxidized and unoxidized parts can be clearly seen.

[0025] 3. High-resolution imaging of graphene was performed using a near-field scattering optical microscope. The oxidized copper foil from step 2 was placed on the sample stage of the near-field scattering optical microscope. The sample was introduced, and the location of the layer to be identified was determined. A scanning range of 15 μm × 15 μm was selected based on the sample area size. The scanning mode was PsHet, the resolution was set to 20 nm, the scanning speed was set to 6 μm / min, and a 980 cm⁻¹ near-field light source was selected. -1 The mid-infrared laser outputs a third-order optical amplitude image (O3A).

[0026] Figures 1-4 The graphene prepared by the above method is shown in different images, representing different regions.

[0027] Figure 1 The copper foil is oxidized, and the single crystal region shows a single color under an optical microscope; the white area is graphene grown on the copper foil; the center is the cantilever of the s-SNOM probe.

[0028] Figure 2 The image shows an oxidized copper foil, with different crystal regions displaying different colors under an optical microscope; the white area represents graphene grown on the copper foil; and the center represents the cantilever of the s-SNOM probe.

[0029] Figure 3 This image shows a single-layer graphene grown by CVD on an oxidized copper foil. The left image is an AFM image, with the hexagonal region in the middle representing graphene; the right image is a third-order optical image of s-SNOM. The optical images show a very uniform contrast on the graphene surface, indicating that it is a single layer.

[0030] Figure 4 This image shows multilayer graphene grown by CVD on oxidized copper foil. The left image is an AFM image, with the darker areas representing graphene. The right image is a third-order optical image of s-SNOM; the contrast between the multilayer graphene and copper oxide is more pronounced, and there are also differences in contrast between layers, allowing for rapid identification of the number of graphene layers. All graphene layers in the image are bilayer or trilayer structures.

[0031] Figure 5 At the same scribed position, due to the unevenness of the substrate surface, AFM images cannot determine the number of graphene layers by height difference; however, near-field optical microscopy images can clearly distinguish the intensity changes of optical amplitude of graphene with different numbers of layers. The intensity at the scribed position is 1 for a single graphene layer, 2 for three layers, and 3 for two layers.

[0032] The method of this invention pre-oxidizes the copper foil substrate of CVD graphene and then scans the sample using suitable parameters, enabling rapid identification of the number of layers in the prepared graphene. This method is unaffected by transfer, impurities, defects, or stacking patterns. It can be used for rapid identification before CVD graphene processing and offers advantages such as simple operation, sample recyclability, high accuracy, and intuitive test results.

Claims

1. A method for identifying the number of CVD graphene layers on copper foil, comprising the following steps: (1) The copper foil with graphene grown on it is subjected to heating oxidation treatment; (2) Use a scattering near-field optical microscope to perform high-resolution imaging on the copper foil after heating and oxidation in (1), record the intensity of optical amplitude signals at different positions, and determine the number of graphene layers by comparing the changes in the intensity of optical amplitude signals between the oxidized copper foil substrate and graphene with different numbers of layers.

2. The method according to claim 1, characterized in that: In step (1), the heating oxidation treatment is heating at 100-150℃ for 1-2 min.

3. The method according to claim 1 or 2, characterized in that: In step (2), when the scanning range of the high-resolution imaging does not exceed 50 μm × 50 μm, the scanning mode is selected as PsHet, the resolution is set to 20-30 nm, the scanning speed is set to no more than 6 μm / min, and the near-field light source is selected as 950-1050 cm. -1 The mid-infrared laser outputs a second- or third-order optical amplitude image.

4. The method according to claim 1 or 2, characterized in that: In step (2), when the scanning range of the high-resolution imaging is greater than 50 μm × 50 μm, the scanning mode is selected as Serpent, and Skip is set to 1 or 2; the resolution is set to 20-30 nm, the scanning speed is set to no more than 6 μm / min, and the near-field light source is selected as 950-1050 cm. -1 The mid-infrared laser outputs a second- or third-order optical amplitude image.

5. The method according to any one of claims 1-4, characterized in that: The graphene was grown on a copper foil substrate by chemical vapor deposition.

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