Method for preparing ultra-smooth wrinkle-free graphene film

Abstract

This invention provides a method for preparing ultra-smooth, wrinkle-free graphene films, belonging to the field of graphene film preparation technology. The method includes: smoothing the surface of a highly oriented pyrolytic graphite substrate; depositing a copper or nickel atomic layer on the substrate surface by electron beam evaporation; forming an atomically smooth copper or nickel surface by high-temperature treatment of the copper or nickel atomic layer; using a gaseous carbon source, growing a wrinkle-free monolayer graphene film on the atomically smooth copper surface via chemical vapor deposition; or using highly oriented pyrolytic graphite as a solid carbon source, growing a wrinkle-free multilayer graphene film on an atomically smooth nickel surface via a carbon dissolution-precipitation method. This invention solves the technical problem of wrinkle defects that may occur when using existing CVD methods to prepare graphene films.

Description

Technical Field

[0001] This invention relates to the field of graphene film preparation technology, specifically to a method for preparing an ultra-flat, wrinkle-free graphene film. Background Technology

[0002] Chemical vapor deposition (CVD) is currently the mainstream method for large-scale preparation of high-quality graphene. The core function of this method is to achieve the controllable preparation of large-area, high-quality monolayer or few-layer graphene. Compared with other preparation methods such as mechanical exfoliation and redox methods, graphene prepared by CVD has advantages such as good layer uniformity, low defect density, and excellent electrical properties, which can meet the needs of large-scale applications.

[0003] However, in existing technologies, wrinkles may occur during the fabrication of graphene films using CVD. Current solutions for these wrinkles all have limitations. One approach is the stress-buffered layer method, which involves pre-depositing a stress-buffered material (such as silicon oxide, alumina, or polymer film) on the surface of a copper foil. The flexibility and compatibility of this buffer layer with the graphene and copper foil help alleviate the stress caused by the mismatch in thermal expansion coefficients between the two materials, thus reducing wrinkles. However, the introduction of the buffer layer may affect the catalytic activity of the copper foil, leading to a decrease in graphene growth quality (e.g., uneven layer count, increased defects). Furthermore, the interfacial bonding between the buffer layer and the copper foil / graphene is difficult to control, making it prone to peeling during growth or transfer, affecting fabrication stability. Additionally, it adds a substrate pretreatment step, increasing process complexity and production costs. Secondly, dynamic cooling and stress release processes, through precise control of the cooling rate combined with segmented cooling and inert gas flow regulation, gradually release the internal stress between graphene and copper foil, reducing wrinkle formation. For example, rapid cooling is used at high temperatures and slow cooling at low temperatures to balance stress release and preparation efficiency. However, this method only alleviates some wrinkles and cannot fundamentally eliminate the stress caused by the mismatch in thermal expansion coefficients. For large-area graphene preparation, regional wrinkles will still exist, and precise control of process parameters is difficult, which is not conducive to large-scale production. Thirdly, post-transfer planarization treatment. After the graphene is transferred to the target substrate, wrinkles are eliminated by mechanical flattening, hot pressing, solution immersion, etc. For example, the transferred graphene is placed on an elastic substrate and the wrinkles are smoothed by mechanical pressure, or hot pressing is performed at high temperature to promote the planarization of the graphene. However, this method is a post-processing technique that cannot repair lattice defects that have already occurred at the wrinkles. It may also introduce new impurities or damage during the process. Furthermore, it has limited effect on smoothing deep wrinkles, making it difficult to achieve truly wrinkle-free graphene preparation. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention proposes a method and application for preparing ultra-flat, wrinkle-free graphene films, thereby solving the technical problem of wrinkle defects that may occur when using existing CVD methods to prepare graphene films.

[0005] The technical solution adopted in this invention is as follows: In a first aspect, a method for preparing an ultra-flat, wrinkle-free single-layer graphene film is provided, comprising the following steps: The surface of the highly oriented pyrolytic graphite substrate is smoothed. A copper atomic layer is deposited on the substrate surface by electron beam evaporation; Atomically smooth copper surface is formed by high-temperature treatment of the copper atomic layer; Using a gaseous carbon source, a wrinkle-free monolayer graphene film is grown on an atomically flat copper surface via chemical vapor deposition.

[0006] Furthermore, the surface smoothing treatment of the highly oriented pyrolytic graphite substrate includes: firstly, mechanical polishing with diamond micron powder to remove macroscopic impurities and protrusions on the HOPG surface, and then cleaning with inert gas plasma to remove residual contaminants and oxide layers on the surface; after the surface smoothing treatment, the roughness of the highly oriented pyrolytic graphite substrate surface is ≤0.3nm.

[0007] Furthermore, the deposition of a copper atomic layer on the substrate surface by electron beam evaporation includes: using a high-vacuum electron beam evaporation device, heating a copper target material with a purity of 99.999% to an evaporation state by bombardment with an electron beam, and depositing a copper atomic layer on the surface of a highly oriented pyrolytic graphite substrate after planarization, wherein the thickness of the copper atomic layer is 50-200 nm.

[0008] Furthermore, the process of forming an atomically smooth copper surface by high-temperature treatment of the copper atomic layer includes: placing highly oriented pyrolytic graphite after depositing the copper atomic layer in a reaction device, introducing a reducing protective gas, heating to 900-1100℃ at a rate of 10-20℃ / min, holding at that temperature for 2-4 hours, and then cooling to room temperature at a rate of 5-10℃ / min to form an atomically smooth copper surface.

[0009] Furthermore, the growth of a wrinkle-free monolayer graphene film on an atomically flat copper surface via chemical vapor deposition includes: placing highly oriented pyrolytic graphite with an atomically flat copper layer in a reaction apparatus, introducing a mixture of argon and methane as a gaseous carbon source; heating to 950-1050℃, holding at that temperature for 1-1.5h, then cooling to 600℃ at a rate of 2-5℃ / min, and finally allowing it to cool naturally to room temperature to complete the growth of an ultra-flat, wrinkle-free monolayer graphene film.

[0010] The first aspect of the method for preparing ultra-smooth, wrinkle-free monolayer graphene films achieves the following technical effects: significantly reducing graphene defect density and improving crystal quality and intrinsic properties; and achieving precise defect control through a dual-core design: firstly, HOPG is selected as the substrate and subjected to atomic-level planarization to eliminate point defects, line defects, and uneven thickness caused by surface unevenness and grain boundary steps of traditional copper foil substrates; secondly, relying on the extremely close thermal expansion coefficients of HOPG and graphene, combined with the stress buffering and lattice guiding effect of the atomically planar metal layer, wrinkle defects caused by thermal expansion differences are completely suppressed, while eliminating the interference of internal grain boundaries of the metal layer on graphene growth. The resulting graphene has extremely low defect density and good crystal structure integrity, maximizing the preservation of graphene's intrinsic electrical, mechanical, and thermal properties, thus solving the core pain points of high defect density and performance degradation in existing CVD methods for graphene preparation.

[0011] Secondly, another method for preparing ultra-flat, wrinkle-free multilayer graphene films is provided, including the following steps: The surface of the highly oriented pyrolytic graphite substrate is smoothed. A layer of nickel atoms is deposited on the substrate surface by electron beam evaporation; Atomically smooth nickel surface is formed by high-temperature treatment of the nickel atomic layer; Using the highly oriented pyrolytic graphite as a solid carbon source, wrinkle-free multilayer graphene films are grown on an atomically flat nickel surface via a carbon dissolution-precipitation method.

[0012] Furthermore, the surface smoothing treatment of the highly oriented pyrolytic graphite substrate includes: firstly, mechanical polishing with diamond micron powder to remove macroscopic impurities and protrusions on the HOPG surface, and then cleaning with inert gas plasma to remove residual contaminants and oxide layers on the surface; after the surface smoothing treatment, the roughness of the highly oriented pyrolytic graphite substrate surface is ≤0.3nm.

[0013] Furthermore, the deposition of a nickel atom layer on the substrate surface by electron beam evaporation includes: using a high-vacuum electron beam evaporation device, heating a nickel target material with a purity of 99.999% to an evaporation state by bombardment with an electron beam, and depositing a nickel atom layer on the surface of a highly oriented pyrolytic graphite substrate after planarization, wherein the thickness of the nickel atom layer is 50-200 nm.

[0014] Furthermore, the process of forming an atomically flat copper surface by high-temperature treatment of the nickel atomic layer includes: placing highly oriented pyrolytic graphite after depositing the nickel atomic layer in a reaction device, introducing a reducing protective gas, heating to 900-1100℃ at a rate of 10-20℃ / min, holding at that temperature for 2-4 hours, and then cooling to room temperature at a rate of 5-10℃ / min to form an atomically flat nickel surface.

[0015] Furthermore, the growth of wrinkle-free multilayer graphene films on atomically flat nickel surfaces via a carbon dissolution-precipitation method includes: placing highly oriented pyrolytic graphite with an atomically flat nickel layer in a reaction apparatus, using an argon-hydrogen mixed gas as a protective gas, and using highly oriented pyrolytic graphite as a solid carbon source; slowly heating to 1000-1100℃ over 6-7 hours and holding at that temperature for 6-7 hours; cooling to 750℃ at a rate of 0.5-1℃ / min, and then naturally cooling to room temperature to grow an ultra-flat, wrinkle-free multilayer graphene film.

[0016] The second aspect of the method for preparing ultra-flat, wrinkle-free multilayer graphene films includes the technical effects of the first aspect's method for preparing ultra-flat, wrinkle-free single-layer graphene films, plus: employing a carbon dissolution-precipitation mechanism combined with HOPG substrate control, it overcomes the technical contradiction between "low defects" and "large area" while retaining the advantages of large-area preparation. The prepared graphene can achieve centimeter-scale film growth with excellent performance uniformity, providing a material basis for the large-scale high-end applications of graphene and avoiding interface defects and performance fluctuations caused by splicing small-sized graphene.

[0017] Thirdly, an ultra-flat, wrinkle-free graphene film is provided, which is prepared by the ultra-flat, wrinkle-free graphene film preparation method described in the first or second aspect, and can be used to prepare graphene field-effect transistors.

[0018] The technical benefits are as follows: It adapts to the fabrication of high-quality graphene FET devices, enhancing core device performance. The core performance characteristics of graphene field-effect transistors (FETs), such as on / off ratio, carrier mobility, and response speed, place extremely high demands on the defect density, flatness, and crystal integrity of graphene. The graphene prepared in this invention exhibits low defects, no wrinkles, and high flatness, effectively reducing carrier scattering sites and improving carrier mobility, while avoiding the problem of local electric field distortion caused by wrinkles. FET devices fabricated based on this graphene possess the low-defect advantages of HOPG-exfoliated graphene devices, ensuring low leakage current and high stability. Furthermore, the large-area fabrication capability enables array-based and large-scale production of devices, solving the dilemma of existing FET devices where "high performance leads to difficulty in mass production, while mass production limits performance," thus promoting the practical application of graphene in high-frequency, low-power electronic devices. Attached Figure Description

[0019] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the accompanying drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn to scale.

[0020] Figure 1 This is a schematic diagram of the preparation method of ultra-flat and wrinkle-free graphene film in an embodiment of the present invention. Figure 2 This is a Raman spectroscopy result of the graphene film prepared according to Scheme 1 on a SiO2 / Si substrate in this invention. Figure 3 This is an optical micrograph of the graphene thin film prepared according to embodiment two of the present invention. Figure 4 This is a microscopic cross-sectional view of the graphene film prepared using the comparative method in this embodiment of the invention. Detailed Implementation

[0021] The embodiments of the technical solution of the present invention will now be described in detail with reference to the accompanying drawings. These embodiments are merely illustrative of the technical solution of the present invention and are therefore intended to limit the scope of protection of the present invention.

[0022] It should be noted that, unless otherwise stated, the technical or scientific terms used in this application should have the ordinary meaning as understood by those skilled in the art to which this invention pertains.

[0023] The inventors of this application have discovered through research that the formation of wrinkle defects is mainly due to the following factors: In the CVD preparation of graphene, the inconsistency in the thermal expansion coefficients of graphene and the catalytic substrate is one of the core reasons for the wrinkling defects in graphene. There is a significant difference in the thermal expansion coefficients between graphene and the catalytic substrate: graphene has an extremely low thermal expansion coefficient, approaching zero or even being negative, while the thermal expansion coefficient of typical catalytic metal substrates is relatively high, such as the commonly used copper substrate, which has a coefficient of approximately 16.5 × 10⁻⁶. -6 / ℃. This mismatch in thermal expansion coefficients causes stress accumulation throughout the heating, holding, and cooling processes of the CVD process, ultimately leading to wrinkles in the graphene. Specifically, during the heating stage, the copper foil expands significantly in the in-plane direction under high temperature, while the graphene is not yet fully grown, or the small amount of graphene that has grown is stretched by the expansion force of the copper foil due to adhesion between them. During the holding stage, the graphene grows completely on the surface of the copper foil, forming a continuous film. At this point, the graphene and copper foil are in a relatively stable bonded state, but the internal stress caused by the difference in thermal expansion coefficients has already begun to accumulate. During the cooling stage, the copper foil shrinks as the temperature decreases, while the graphene, due to its extremely low thermal expansion coefficient, shrinks much less than the copper foil. The shrinkage of the copper foil generates strong in-plane compressive stress on the graphene attached to its surface. Since graphene is a two-dimensional flexible material, it cannot withstand excessive compressive stress. To release the stress, the graphene will buckle and deform, forming an irregular wrinkled structure. These wrinkle defects are mostly through-hole or regional, ranging in width from a few nanometers to tens of micrometers, and in depth from several nanometers to tens of nanometers. These defects severely disrupt the integrity of the two-dimensional planar structure of graphene, leading to a decline in its electrical properties, such as reduced carrier mobility and increased electrical resistance, thus affecting its application in high-frequency electronic devices. Furthermore, wrinkles easily become stress concentration points, reducing the mechanical stability of graphene and making it prone to breakage during subsequent transfer and device packaging. They also affect the interfacial bonding performance between graphene and other materials, limiting its application in composite materials, coatings, and other fields. While existing technologies can alleviate some wrinkles by optimizing the cooling rate (e.g., slow cooling), they cannot fundamentally eliminate the stress caused by the mismatch in thermal expansion coefficients, making wrinkle defects difficult to avoid. Moreover, slow cooling reduces fabrication efficiency and increases production costs.

[0024] The unevenness of the catalytic substrate surface is another important reason for defects in graphene prepared by CVD. The defects it causes mainly include point defects, line defects, and local thickness inhomogeneities, which seriously affect the purity and performance uniformity of graphene. From the perspective of the surface characteristics of the catalytic metal substrate, even after conventional cleaning and annealing pretreatment, there are still a number of uneven factors on the surface of the catalytic metal substrate: First, the microscopic roughness of the catalytic metal substrate itself. The surface roughness of industrially produced copper foil is usually several nanometers to tens of nanometers, with tiny protrusions, depressions, scratches, and grain boundary steps; Second, impurities or oxidation points remaining during the pretreatment process. Although most impurities and oxide layers can be removed by acid washing and reduction annealing, trace impurity particles or local oxidation areas may still remain, forming surface protrusions or abnormal active site areas; Third, mechanical damage to the catalytic metal substrate during handling and placement, such as micro-scratches and indentations, further exacerbates the surface unevenness. During graphene growth, the surface irregularities of the catalytic metal substrate directly affect the adsorption, diffusion, and reconstruction of carbon atoms. At surface protrusions, the adsorption energy of carbon atoms is higher, leading to preferential deposition and the formation of locally thickened graphene (such as bilayer or multilayer graphene), resulting in uneven thickness. At depressions, scratches, or grain boundary steps, the diffusion path of carbon atoms is obstructed, easily forming point or line defects such as vacancies and dislocations, disrupting the honeycomb lattice structure of graphene. Simultaneously, uneven stress distribution at surface irregularities can induce local wrinkles, which, combined with wrinkles caused by mismatched thermal expansion coefficients, further deteriorate the surface quality of graphene. These defects are random and dispersed, making them difficult to completely repair through subsequent processing. Point and line defects significantly reduce the electrical conductivity of graphene, leading to enhanced carrier scattering, and also affecting its thermal conductivity and mechanical properties. Uneven thickness results in regional differences in graphene performance, failing to meet the uniformity requirements of high-end devices. In existing technologies, although surface smoothness can be improved by using copper foil with higher purity and lower roughness or by optimizing the pretreatment process, high-purity copper foil is expensive, and the optimization space of the pretreatment process is limited, so it is impossible to completely eliminate the micro-unevenness on the surface of the copper foil, and the corresponding graphene defect problem cannot be effectively solved.

[0025] Based on the above research findings, and addressing the wrinkling defects caused by thermal expansion coefficient mismatch and substrate surface unevenness in existing CVD methods for graphene preparation, this embodiment provides a method for preparing ultra-smooth, wrinkle-free graphene films. The core idea is to use atomically flat, highly oriented pyrolytic graphite (HOPG) as a substrate, deposit a metal layer (copper or nickel) on the substrate via electron beam evaporation, then treat it at high temperature to form an atomically flat surface, and finally grow graphene through a carbon dissolution-precipitation mechanism. Specifically, the method includes the following steps: S1. The surface of the highly oriented pyrolytic graphite substrate is smoothed. A combination of mechanochemical polishing and plasma cleaning is employed. First, mechanical polishing with diamond micropowder removes macroscopic impurities and protrusions from the HOPG surface. Then, inert gas plasma cleaning (100-200W power, 5-10 min) removes residual contaminants and the oxide layer. This step achieves atomically smooth HOPG surface (roughness ≤0.3nm), providing a clean and smooth substrate for subsequent metal layer deposition. In specific embodiments, argon or nitrogen can be used as the inert gas, with argon being preferred.

[0026] Existing technologies mostly use metal foils such as copper foil and nickel foil as substrates, and the surface flatness depends on pretreatments such as polishing and annealing, making it difficult to achieve atomic-level flatness. This embodiment selects a different substrate and treatment method, using HOPG as the substrate, and combining mechanical and chemical polishing with plasma cleaning to easily achieve an atomically flat surface, solving the graphene defects caused by substrate unevenness from the source. HOPG itself has a naturally atomically flat surface, and after fine processing, surface micro-defects can be eliminated, providing a flat substrate for subsequent metal layer deposition and graphene growth, solving the graphene defect problem caused by the unevenness of traditional copper foil surfaces from the source; more importantly, the thermal expansion coefficients of HOPG and graphene are extremely close (graphene's thermal expansion coefficient is close to zero, and HOPG's thermal expansion coefficient is also extremely low, with highly matched values), and HOPG has extremely strong structural stability at high temperatures, with negligible deformation. This characteristic makes HOPG the optimal material for stress control, ensuring that the deformation of HOPG, graphene, and metal interlayers is synchronized during high-temperature growth and cooling, and that stress differences are not generated due to thermal expansion differences. This effectively inhibits the accumulation and transmission of stress between interfaces, and fundamentally prevents graphene from wrinkling due to stress.

[0027] S2. Depositing a metal atomic layer on the substrate surface by electron beam evaporation. High vacuum electron beam evaporation equipment (vacuum degree ≤1×10) is used. -6 A metal target is heated to an evaporation state by electron beam bombardment, and a metal atomic layer is deposited on the planarized HOPG surface. The deposition rate is controlled at 0.1-0.5 nm / s, and the thickness of the metal atomic layer is 50-200 nm. In some embodiments, the metal target is preferably made of copper or nickel.

[0028] This step uses electron beam evaporation to form a uniform and dense metal atomic layer on the HOPG surface. Under high temperature, the metal atoms gain sufficient activity to diffuse and reconstruct along the HOPG surface. Relying on the lattice guidance effect of HOPG, an atomically flat metal surface is formed. The atomically flat structure can reduce the grain boundaries inside the metal layer, avoid the uneven graphene thickness and defects caused by grain boundary steps, and provide a carrier for subsequent graphene growth.

[0029] S3. Forming an atomically smooth metal surface by high-temperature treatment of the metal atomic layers. The deposited metal layer of HOPG is placed in a tube furnace, and a reducing protective gas is introduced. The temperature is increased to 900-1100°C at a rate of 10-20°C / min and held for 2-4 hours. Then, it is cooled to room temperature at a rate of 5-10°C / min. At high temperature, metal atoms diffuse and reconstruct fully, forming an atomically flat copper / nickel metal surface guided by the HOPG lattice, eliminating grain boundaries and stress within the metal layer. In some embodiments, the reducing protective gas can be an argon-hydrogen mixture with a volume ratio of 9:1, or an argon-hydrogen mixture with a volume ratio of 9.5:0.5.

[0030] For steps S2 and S3, existing technologies typically use metal foil directly, which contains numerous grain boundaries and is prone to stress due to its mismatch with the thermal expansion coefficient of graphene. This embodiment employs a different method for metal layer preparation and structural control: a nanometer-thick metal layer is deposited by electron beam evaporation, followed by high-temperature crystallization to form an atomically flat metal layer. This eliminates grain boundaries and, relying on the stability of HOPG, suppresses stress transmission, thus preventing wrinkles.

[0031] S4. Growing ultra-smooth, wrinkle-free graphene films on atomically flat metal surfaces This step can be implemented in two ways, such as Figure 1 As shown, the first method uses a gaseous carbon source to grow an ultra-smooth, wrinkle-free graphene film via CVD; the second method uses a solid carbon source to grow an ultra-smooth, wrinkle-free graphene film via carbon deposition. The following details the specific implementation processes of steps S1-S4 for both methods, along with steps S1-S3 described above: Implementation Plan 1: Using a gaseous carbon source, an ultra-smooth, wrinkle-free monolayer graphene film is grown on the surface of an atomically flat copper layer. This scheme uses copper as the metal layer carrier and achieves ultra-smooth, wrinkle-free graphene growth through the synergistic effect of the HOPG substrate and the atomically flat copper layer. The specific implementation steps are as follows: HOPG substrate pretreatment: Highly oriented pyrolytic graphite with dimensions of 2cm×2cm and a thickness of 0.5mm was selected and mechanically and chemically polished using diamond micropowder with a particle size of 50nm. The polishing pressure was controlled at 0.1MPa and the polishing time was 30min. Subsequently, the polished HOPG was placed in a plasma cleaner, and argon gas (flow rate 20sccm) was introduced at a power of 100W for a cleaning time of 5min. The surface roughness of the substrate was ensured to be ≤0.3nm by AFM (atomic force microscopy) inspection, which met the atomic level flatness standard.

[0032] Electron beam evaporation of copper atomic layers: The pretreated HOPG is placed in a high-vacuum electron beam evaporation apparatus, the chamber is closed, and a vacuum of 5 × 10⁻⁶ is drawn. -7 Pa; a copper target with a purity of 99.999% was selected, the electron beam power was set to 80W, the deposition rate was 0.1-0.5nm / s, and the thickness of the deposited copper atomic layer was 50-200nm; during the evaporation process, the thickness data was fed back in real time by a thin film thickness monitoring instrument to ensure that the copper layer is uniform and dense.

[0033] High-temperature crystallization to form an atomically flat copper surface: After depositing the copper layer, the HOPG is transferred to a tube furnace, and an argon-hydrogen mixed gas (volume ratio 9:1, total flow rate 50 sccm) is introduced. The temperature is raised to 900℃ at a rate of 10℃ / min and held for 2 hours to allow the copper layer atoms to fully diffuse and reconstruct, forming an atomically flat copper surface guided by the HOPG lattice. Then, the temperature is lowered to room temperature at a rate of 5℃ / min.

[0034] CVD growth of graphene: HOPG with an atomically flat copper layer is placed in a tube furnace, and a mixture of argon and methane (volume ratio 99:1, total flow rate 5 sccm) is introduced as the gaseous carbon source; the temperature is raised to 950-1050℃ and held for 1-1.5h, where methane catalyzes the decomposition of the atomically flat copper layer to grow monolayer graphene. Then, the temperature is lowered to 600℃ at a rate of 2-5℃ / min, and then allowed to cool naturally (without holding) to room temperature to complete the growth of an ultra-flat, wrinkle-free monolayer graphene film.

[0035] Characterization and Verification: Raman spectroscopy revealed that the G peak of wrinkle-free graphene was located at 1580-1590 cm⁻¹ under an excitation wavelength of 532 nm. -1 The 2D peak is located at 2670-2685 cm⁻¹. -1 The graphene was transferred to a SiO2 / Si wafer using a transfer method. The D-peak intensity was extremely low (D / G peak intensity ratio ≤ 0.05), indicating a low defect density. AFM analysis showed that the graphene surface roughness was ≤ 0.5 nm, with no obvious wrinkles or protrusions. Figure 2 As shown.

[0036] Implementation Plan 2: Using a solid carbon source, grow an ultra-smooth, wrinkle-free multilayer graphene film on the surface of an atomically flat nickel layer. HOPG substrate pretreatment: basically the same as in implementation scheme one, except that the power of the plasma cleaner is 200W and the cleaning time is 10min.

[0037] Electron beam evaporation of nickel atomic layers: A nickel target with a purity of 99.999% is selected. The difference from Implementation Scheme 1 is that the electron beam power is set to 90W, the deposition rate is 0.1-0.5nm / s, and the thickness of the deposited nickel atomic layer is 50-200nm. The requirements for evaporation vacuum degree and thickness monitoring are the same as in Implementation Scheme 1 to ensure the uniformity of the nickel layer.

[0038] High-temperature crystallization to form an atomically flat nickel surface: This is basically the same as in Scheme 1, except that the volume ratio of the argon-hydrogen mixture is 9.5:0.5, the temperature is raised to 1100℃ at a rate of 20℃ / min, held for 4 hours, and the cooling rate is 10℃ / min.

[0039] Graphene growth by carbon dissolution-precipitation method: An argon-hydrogen mixed gas (volume ratio 9:1, total flow rate 500 sccm) is used as the protective gas, and HOPG (HOPG with a nickel layer deposited on it) is used as the solid carbon source. The HOPG with an atomically flat nickel layer is placed in a tube furnace and slowly heated to 1000-1100℃ over 6-7 hours, and held at that temperature for 6-7 hours. The carbon atoms in the HOPG dissolve into the nickel layer. The temperature is then lowered to 750℃ at a rate of 0.5-1℃ / min, and the carbon atoms precipitate to form multilayer graphene. The graphene is then allowed to cool naturally (without holding) to room temperature to complete the growth of an ultra-flat and wrinkle-free multilayer graphene film.

[0040] Characterization and Verification: AFM analysis showed that the surface roughness of the thick graphene was ≤0.5 nm. Optical microscopy revealed no obvious wrinkles or protrusions on the surface. Cross-sectional transmission electron microscopy (TEM) characterization showed that the thick graphene had a multilayered parallel structure with straight interlayers and a smooth surface, free from stress-induced wrinkles. Figure 3 As shown.

[0041] Existing CVD methods rely on a carbon source pyrolysis-adsorption-reconstruction mechanism, which is susceptible to substrate defects and stress. Implementation scheme two employs a different graphene growth mechanism, using a carbon dissolution-precipitation method. At high temperatures, carbon atoms from the carbon source (either solid or gaseous) dissolve in an atomically flat nickel layer. Once the carbon atoms reach saturation in the metal layer, they preferentially precipitate on the metal layer surface as the temperature slowly decreases. Leveraging the lattice matching of the atomically flat metal layer and the flat substrate constraint of HOPG, combined with the synergistic effect of the atomically flat metal layer and the HOPG substrate, the graphene growth orientation can be precisely controlled, producing a graphene film with a lattice orientation consistent with the metal layer and a smooth, wrinkle-free surface. This simultaneously achieves ultra-flatness and wrinkle-free characteristics, eliminating the need for subsequent planarization treatment.

[0042] The ultra-flat, wrinkle-free graphene film can be prepared through the above steps S1-S4. It can be seen that the process of the present invention is simple, without the need for a buffer layer or complex post-processing. Relying on the synergistic effect of HOPG and atomically flat metal layer, the target performance can be achieved in a single growth process without the need for post-processing after the graphene film growth is completed, thus reducing the complexity and cost of the process.

[0043] Comparative approach: Growing graphene films on sapphire substrates with copper deposits. To compare the preparation effects using different substrates, following the process flow and parameters of Implementation Scheme 1, a 1-micron-thick copper layer was deposited on a sapphire substrate, and graphene was grown on the copper layer using chemical vapor deposition. The resulting graphene could not achieve atomic-level flatness, exhibiting fluctuations of approximately 50 nm. This phenomenon can be demonstrated by cross-sectional sampling, such as... Figure 4 As shown, by depositing a protective layer on the graphene surface and then sampling the cross-section using a focused ion beam, it can be found that the surface of the sapphire substrate is flat, but after copper plating, it cannot be guaranteed that the grown graphene layer will be flat.

[0044] This embodiment also provides an ultra-flat, wrinkle-free graphene film, which is prepared using the ultra-flat, wrinkle-free graphene film preparation method described above.

[0045] This embodiment also provides an application of an ultra-flat, wrinkle-free graphene film, based on which graphene field-effect transistors (FETs) are fabricated. FET devices fabricated based on this graphene film possess the low-defect advantages of HOPG-exfoliated graphene devices, ensuring low leakage current and high stability. Furthermore, the large-area fabrication capability enables array-based and large-scale production of these devices, solving the dilemma of existing FET devices where "high performance leads to difficulty in mass production, while mass production results in limited performance," thus promoting the practical application of graphene in high-frequency, low-power electronic devices.

[0046] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention, and they should all be covered within the scope of the claims and specification of the present invention.

Claims

1. A method for preparing an ultra-flat, wrinkle-free single-layer graphene film, characterized in that, Includes the following steps: The surface of the highly oriented pyrolytic graphite substrate is smoothed. A copper atomic layer is deposited on the substrate surface by electron beam evaporation; Atomically smooth copper surface is formed by high-temperature treatment of the copper atomic layer; Using a gaseous carbon source, a wrinkle-free monolayer graphene film is grown on an atomically flat copper surface via chemical vapor deposition.

2. The method for preparing an ultra-smooth, wrinkle-free single-layer graphene film according to claim 1, characterized in that, The surface smoothing treatment of the highly oriented pyrolytic graphite substrate includes: firstly, mechanical polishing with diamond micron powder to remove macroscopic impurities and protrusions on the HOPG surface, and then cleaning with inert gas plasma to remove residual contaminants and oxide layers on the surface; after the surface smoothing treatment, the roughness of the highly oriented pyrolytic graphite substrate surface is ≤0.3nm.

3. The method for preparing an ultra-smooth, wrinkle-free single-layer graphene film according to claim 1, characterized in that, The method of depositing a copper atomic layer on the substrate surface by electron beam evaporation includes: using a high-vacuum electron beam evaporation equipment, heating a copper target material with a purity of 99.999% to an evaporation state by bombardment with an electron beam, and depositing a copper atomic layer on the surface of a highly oriented pyrolytic graphite substrate after it has been flattened, wherein the thickness of the copper atomic layer is 50-200 nm.

4. The method for preparing an ultra-smooth, wrinkle-free single-layer graphene film according to claim 1, characterized in that, The process of forming an atomically smooth copper surface by high-temperature treatment of the copper atomic layer includes: placing highly oriented pyrolytic graphite after depositing the copper atomic layer in a reaction device, introducing a reducing protective gas, heating to 900-1100℃ at a rate of 10-20℃ / min, holding at that temperature for 2-4 hours, and then cooling to room temperature at a rate of 5-10℃ / min to form an atomically smooth copper surface.

5. The method for preparing an ultra-smooth, wrinkle-free single-layer graphene film according to claim 1, characterized in that, The method of using a gaseous carbon source to grow a wrinkle-free monolayer graphene film on an atomically flat copper surface via chemical vapor deposition includes: placing highly oriented pyrolytic graphite with an atomically flat copper layer in a reaction apparatus, introducing a mixture of argon and methane as the gaseous carbon source; heating to 950-1050℃, holding at that temperature for 1-1.5 hours, then cooling to 600℃ at a rate of 2-5℃ / min, and finally allowing it to cool naturally to room temperature to complete the growth of an ultra-flat, wrinkle-free monolayer graphene film.

6. A method for preparing an ultra-flat, wrinkle-free multilayer graphene film, characterized in that, Includes the following steps: The surface of the highly oriented pyrolytic graphite substrate is smoothed. A layer of nickel atoms is deposited on the substrate surface by electron beam evaporation; Atomically smooth nickel surface is formed by high-temperature treatment of the nickel atomic layer; Using the highly oriented pyrolytic graphite as a solid carbon source, wrinkle-free multilayer graphene films are grown on an atomically flat nickel surface via a carbon dissolution-precipitation method.

7. The method for preparing an ultra-flat, wrinkle-free multilayer graphene film according to claim 6, characterized in that, The surface smoothing treatment of the highly oriented pyrolytic graphite substrate includes: firstly, mechanical polishing with diamond micron powder to remove macroscopic impurities and protrusions on the HOPG surface, and then cleaning with inert gas plasma to remove residual contaminants and oxide layers on the surface; after the surface smoothing treatment, the roughness of the highly oriented pyrolytic graphite substrate surface is ≤0.3nm.

8. The method for preparing an ultra-flat, wrinkle-free multilayer graphene film according to claim 6, characterized in that, The method of depositing a nickel atom layer on the substrate surface by electron beam evaporation includes: using a high-vacuum electron beam evaporation equipment, heating a nickel target material with a purity of 99.999% to an evaporation state by bombardment with an electron beam, and depositing a nickel atom layer on the surface of a highly oriented pyrolytic graphite substrate after it has been flattened, wherein the thickness of the nickel atom layer is 50-200 nm.

9. The method for preparing an ultra-flat, wrinkle-free multilayer graphene film according to claim 6, characterized in that, The process of forming an atomically flat copper surface by high-temperature treatment of the nickel atomic layer includes: placing highly oriented pyrolytic graphite after depositing the nickel atomic layer in a reaction device, introducing a reducing protective gas, heating to 900-1100℃ at a rate of 10-20℃ / min, holding at that temperature for 2-4 hours, and then cooling to room temperature at a rate of 5-10℃ / min to form an atomically flat nickel surface.

10. The method for preparing an ultra-flat, wrinkle-free multilayer graphene film according to claim 6, characterized in that, Using the highly oriented pyrolytic graphite as a solid carbon source, a wrinkle-free multilayer graphene film is grown on an atomically flat nickel surface via a carbon dissolution-precipitation method. The process includes: placing highly oriented pyrolytic graphite with an atomically flat nickel layer in a reaction apparatus, using an argon-hydrogen mixture as a protective gas, and using the highly oriented pyrolytic graphite as the solid carbon source; slowly heating to 1000-1100℃ over 6-7 hours and holding at that temperature for 6-7 hours; cooling to 750℃ at a rate of 0.5-1℃ / min, and then naturally cooling to room temperature to grow an ultra-flat, wrinkle-free multilayer graphene film.

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