Method for producing multi-layer graphene

Abstract

To provide a method of producing multilayer graphene, capable of producing multilayer graphene having a random layer structure and excellent layer number uniformity.SOLUTION: The method of producing multilayer graphene, includes a step of synthesizing multilayer graphene from a carbon source using an Fe-Cr alloy film as a catalyst. Preferably, the Cr content in the Fe-Cr alloy is 10 to 40 vol.%, and the surface of the crystalline substrate is a (0001) alignment surface.SELECTED DRAWING: Figure 2D

Description

[Technical Field]

[0001] The present invention relates to a method for producing multi-layer graphene. [Background technology]

[0002] Graphite is an electrically conductive material whose crystal structure consists of atomic layers of carbon atoms stacked in layers. In recent years, it has been discovered that single-layer graphene obtained by exfoliation from this crystal exhibits unique electronic properties not found in bulk, such as extremely high mobility. This has led to research into the application of graphene to high-frequency devices, transparent conductive films, flexible devices, and optical sensors.

[0003] On the other hand, when graphene is formed by exfoliation from crystals, the resulting single-layer atomic film is tiny, micron-sized pieces, making mass production and industrial application difficult. In response to this, a method has been developed to synthesize graphene directly on a metal catalyst using chemical vapor deposition (CVD). The number of layers (thickness), stacking structure, and layer uniformity of the graphene formed vary depending on the type of metal used as the catalyst and the crystallinity of the metal film.

[0004] Recently, it has been discovered that multilayer graphene, which has a random (turbostratic) stacking structure in which the stacked graphene layers are randomly rotated in-plane, unlike ordinary graphite, retains the physical properties of single-layer graphene despite being multilayer, and has attracted attention. Single-layer graphene has had several problems in device applications, such as low mechanical strength and low light absorption. However, if the number of layers can be increased while maintaining the excellent electrical properties of single-layer graphene, it is expected that the possibilities for device applications will expand. As a method for synthesizing multilayer graphene having random stacking, a method using iron (Fe) as a catalyst film is known (for example, Non-Patent Document 1). [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] Applied Physics Express 3(2010)025102 Summary of the Invention [Problem to be solved by the invention]

[0006] In one aspect, the present invention aims to provide a method for producing multilayer graphene, which can produce multilayer graphene having a random stacking structure and excellent layer number uniformity. [Means for solving the problem]

[0007] In one embodiment, the disclosed method for producing multi-layer graphene includes synthesizing multi-layer graphene from a carbon source using an Fe—Cr alloy film as a catalyst. [Effects of the Invention]

[0008] According to one aspect, the present invention provides a method for producing multilayer graphene, which can produce multilayer graphene having a random stacking structure and excellent layer number uniformity. [Brief explanation of the drawings]

[0009] [Figure 1A] FIG. 1A is a transmission electron microscope (TEM) photograph showing a cross section of multilayer graphene formed on an iron catalyst film according to the prior art. [Figure 1B] FIG. 1B is an optical microscope photograph showing the top surface of multi-layer graphene formed on an iron catalyst film according to the prior art. [Figure 2A] FIG. 2A is a schematic diagram (part 1) illustrating the procedure of the method for producing multi-layer graphene in embodiment 1. [Figure 2B] FIG. 2B is a schematic diagram (part 2) illustrating the procedure of the method for producing multi-layer graphene in embodiment 1. [Figure 2C] FIG. 2C is a schematic diagram (part 3) illustrating the procedure of the method for producing multi-layer graphene according to the first embodiment. [Figure 2D]FIG. 2D is a schematic diagram (part 4) illustrating the procedure of the method for producing multi-layer graphene according to the first embodiment. [Figure 3] FIG. 3 is a schematic diagram showing the procedure of the method for producing multi-layer graphene in the second embodiment. [Figure 4A] FIG. 4A is a photograph showing a bright-field image of the catalyst film (Fe) of Comparative Example 1 taken by an optical microscope. [Figure 4B] FIG. 4B is a photograph showing a dark-field image of the catalyst film (Fe) of Comparative Example 1 taken by an optical microscope. [Figure 4C] FIG. 4C is a photograph showing a bright-field image of the Fe—Cr alloy film (20% by volume of Cr) of Example 1 taken by an optical microscope. [Figure 4D] FIG. 4D is a photograph showing a dark-field image of the Fe—Cr alloy film (20% by volume of Cr) of Example 1 taken by an optical microscope. [Figure 4E] FIG. 4E is a photograph showing a bright-field image of the Fe—Cr alloy film (40% by volume of Cr) of Example 2 taken by an optical microscope. [Figure 4F] FIG. 4F is a photograph showing a dark-field image of the Fe—Cr alloy film (40% by volume of Cr) of Example 2 taken by an optical microscope. [Figure 4G] FIG. 4G is a photograph showing a bright-field image of the catalyst film (Cr) of Comparative Example 2 taken with an optical microscope. [Figure 4H] FIG. 4H is a photograph showing a dark-field image of the catalyst film (Cr) of Comparative Example 2 taken with an optical microscope. [Figure 5A] FIG. 5A is a photograph showing a dark-field image of the Fe—Cr alloy film (Cr 5% by volume) of Example 3 taken by an optical microscope. [Figure 5B] FIG. 5B is a photograph showing a bright-field image of the Fe—Cr alloy film (10% by volume of Cr) of Example 4 taken by an optical microscope. [Figure 6A] FIG. 6A is a photograph showing a bright-field image of multilayer graphene in Comparative Example 1 (catalyst film: Fe) taken with an optical microscope. [Figure 6B] FIG. 6B is a photograph showing a dark-field image of multilayer graphene in Example 1 (catalyst film: Fe—Cr (20% by volume)) taken by an optical microscope. [Figure 7] FIG. 7 is a diagram showing a Raman spectrum of multi-layer graphene in Example 1 (catalyst film: Fe—Cr (20% by volume)). [Figure 8] FIG. 8 shows a Raman spectrum of multi-layer graphene having a regular AB stacking structure, shown for comparison. DETAILED DESCRIPTION OF THE INVENTION

[0010] (Method of manufacturing multi-layer graphene) The disclosed method for producing multi-layer graphene includes a step of synthesizing multi-layer graphene from a carbon source using an Fe—Cr alloy film as a catalyst.

[0011] The technology disclosed in this application has been completed based on the following problems and findings of the prior art. Specifically, the CVD synthesis method using an iron catalyst film described in Non-Patent Document 1 (Applied Physics Express 3 (2010) 025102) of the prior art forms a thick multilayer graphene film with more than 100 layers, as shown in Figure 1A (in the figure, symbol a indicates multilayer graphene). However, in the microscope image shown in Figure 1B, the variation in the number of layers across different areas can be confirmed as a difference in contrast. Symbol b indicates an area with a large (thick) number of layers of multilayer graphene, and symbol c indicates an area with a small (thin) number of layers of multilayer graphene. Thus, the inventors have discovered that the prior art technology has a problem in that it is difficult to obtain multilayer graphene with a large area and a randomly stacked structure with a uniform number of layers due to poor in-plane uniformity. To further elucidate novel physical properties, and because variation in the number of layers significantly affects yield in device applications, a technology to control the uniformity of the number of layers is desired.

[0012] The disclosed method for producing multilayer graphene includes at least a multilayer graphene synthesis step, and preferably further includes a metal film formation step and an alloying and recrystallization step, and may further include other steps as necessary. By the method for producing multilayer graphene, it is possible to produce multilayer graphene having a random stacking structure and excellent uniformity in the number of layers.

[0013] <Multilayer graphene synthesis process> The multi-layer graphene synthesis step is a step of synthesizing multi-layer graphene from a carbon source using an Fe—Cr alloy film as a catalyst. First, the Fe—Cr alloy film and the method for forming the same will be described, and then the multilayer graphene synthesis process will be described.

[0014] -Fe-Cr alloy film- The Fe—Cr alloy film is an alloy film made of an Fe—Cr alloy containing iron (Fe), chromium (Cr), and unavoidable impurities such as carbon. The Fe-Cr alloy film can be suitably produced by the alloy layer formation process, but is not limited to an Fe-Cr alloy film on a crystalline substrate, and may also be in the form of a foil or plate made of the Fe-Cr alloy.

[0015] The Cr content in the Fe-Cr alloy film is not particularly limited and can be selected appropriately depending on the purpose, but is preferably 5% to 40% by volume, more preferably 10% to 40% by volume, even more preferably 10% to 30% by volume, and particularly preferably 15% to 25% by volume.

[0016] The carbon content is preferably 1.2 mass % or less, more preferably 0.5 mass % or less, and even more preferably 0.1 mass % or less, relative to the total amount of the Fe—Cr alloy. The content of impurities other than carbon is preferably 0.1 mass % or less, more preferably 0.01 mass % or less, and even more preferably 0.001 mass % or less, based on the total amount of the Fe—Cr alloy.

[0017] The average thickness of the Fe-Cr alloy film is not particularly limited and can be appropriately selected depending on the purpose, but is preferably 100 nm to 1,000 nm. The average thickness can be an average value obtained by measuring the thickness of the Fe—Cr alloy film at any five or more points.

[0018] [Method of manufacturing Fe-Cr alloy layer] The method for producing the Fe-Cr alloy layer is not particularly limited and can be appropriately selected depending on the purpose, but it is preferable to produce it by a metal film forming step and an alloying and recrystallization step. The method for producing multi-layer graphene of the present disclosure preferably includes a metal film forming step and an alloying and recrystallization step. By the metal film formation step and the alloying and recrystallization step, an Fe—Cr alloy film that serves as a catalyst for graphene synthesis can be suitably formed on the crystalline substrate.

[0019] <Metal film formation process> The metal film forming step is a step of forming a metal film by (1) depositing Fe and Cr or (2) depositing an Fe—Cr alloy on the surface of a crystalline substrate.

[0020] -Crystalline substrate- The crystalline substrate is not particularly limited and can be appropriately selected depending on the purpose, and examples thereof include a sapphire substrate, a MgO substrate, and a spinel (MgAl2O4) substrate. Among these, a sapphire substrate is preferred, and a sapphire substrate having a (0001)-oriented surface of a sapphire single crystal (commonly referred to as a C-plane) is more preferred. Furthermore, it is preferred that the surface of the crystalline substrate is a (0001)-oriented surface of a sapphire substrate.

[0021] The crystalline substrate preferably has an atomically smooth surface that has been subjected to chemical mechanical polishing. Furthermore, it is preferable to heat treat it in an oxygen atmosphere at about 1,000 to 1,400°C. The heat treatment time is preferably 3 to 24 hours. The average thickness of the crystalline substrate is not particularly limited and can be appropriately selected depending on the purpose, but is preferably 100 nm to 1 mm.

[0022] Suitable methods for forming the metal film include, for example, vapor deposition and sputtering. (1) When a metal film is formed by depositing Fe and Cr (Embodiment 1), a source of Fe and a source of Cr can be used to sequentially deposit an Fe film and a Cr film on the same crystalline substrate, or Fe and Cr can be simultaneously deposited to form a metal film. There are no particular restrictions on the order in which Fe and Cr are deposited and it can be selected appropriately depending on the purpose, but it is preferable to deposit Fe and Cr consistently in a vacuum in order to prevent oxidation of the interface between the Fe film and the Cr film and the inclusion of impurities. (2) When a metal film is formed by depositing an Fe—Cr alloy (Embodiment 2), a metal film (alloy film) can be formed directly on a crystalline substrate using a pre-alloyed Fe—Cr alloy source.

[0023] The temperature of the crystalline substrate when forming the metal film (when depositing the metal film) is preferably room temperature or higher, more preferably 400°C to 600°C. The average thickness of the metal film is preferably 100 nm to 1,000 nm as the total thickness of the Fe film and the Cr film. The Cr content in the metal film is preferably adjusted appropriately so that the Cr content in the resulting Fe-Cr alloy film is within a suitable range (for example, 10% to 40% by volume), preferably 5% to 40% by volume, more preferably 10% to 40% by volume, even more preferably 10% to 30% by volume, and particularly preferably 15% to 25% by volume.

[0024] <Alloying recrystallization process> The alloying and recrystallization step is a step following the metal film formation step in which the metal film is alloyed and recrystallized by heating at 800° C. to 1,100° C. to form an Fe—Cr alloy film.

[0025] A method for alloying and recrystallizing the metal film is preferably to heat it at 800°C to 1,100°C. The heating holding time is preferably 10 minutes to 600 minutes. This allows the metal film to be alloyed and recrystallized to suitably form an Fe-Cr alloy film.

[0026] The apparatus for alloying and recrystallizing the metal film is not particularly limited and can be appropriately selected depending on the purpose, but it is preferably one that can be used in combination with the apparatus used for synthesizing multilayer graphene, and is preferably a furnace that can maintain high airtightness and has an exhaust system that can reduce the pressure to 10 Pa or less. In addition, it is preferable that supply lines for inert gas, hydrogen gas, etc. used in the synthesis are connected to the apparatus, and that the flow rates of each gas can be controlled by flow meters. The inert gas is not particularly limited and can be appropriately selected depending on the purpose, and examples thereof include argon (Ar) and nitrogen.

[0027] The alloying and recrystallization of the metal film is preferably carried out under atmospheric pressure or reduced pressure in a mixed gas atmosphere of hydrogen and an inert gas, and the hydrogen concentration in the mixed gas is preferably 1% to 30% by volume. The flow rates of hydrogen and inert gas cannot be uniquely defined because they depend on the volume of the apparatus, but the flow rate of hydrogen is preferably 1 sccm to 1,000 sccm, and the flow rate of inert gas is preferably 10 sccm to 10,000 sccm. Here, the unit "sccm" (standard cubic centimeters per minute) is the flow rate (cm) per minute at 1 atm (atmospheric pressure 1,013 hPa) and 25°C. 3 ) is a unit of gas flow converted into 1 sccm. When the gas pressure is 1 atm, 1 [sccm] = 1.667 × 10 -5 [L / s]=6×10 -5 [m 3 / h]=1.667×10 -8 [m 3 / s].

[0028] <Multilayer graphene synthesis process> Next, a step of synthesizing multilayer graphene from a carbon source using the Fe—Cr alloy film as a catalyst (multilayer graphene synthesis step) is carried out.

[0029] -Carbon source- The carbon source is not particularly limited and can be appropriately selected depending on the purpose, and examples thereof include hydrocarbon gases and vaporized alcohols. Examples of hydrocarbon gases include methane (CH4), ethylene (C2H4), and acetylene (C2H2). Examples of alcohols include ethanol and propanol. Of these, acetylene is preferred.

[0030] Examples of methods for synthesizing multilayer graphene include chemical vapor deposition (CVD), molecular beam epitaxy (MBE), etc. Among these, CVD synthesis is preferred. The apparatus used for CVD synthesis is not particularly limited and can be appropriately selected depending on the purpose, but it is preferable that it can be used in combination with the apparatus used to form the Fe—Cr alloy layer, and that it is a furnace that can maintain high airtightness and has an exhaust system that can reduce the pressure to 10 Pa or less. In addition, it is preferable that supply lines for the inert gas, hydrogen gas, and carbon source used in the synthesis are connected to the apparatus, and that the flow rates of each can be controlled by flow meters.

[0031] The multilayer graphene is preferably synthesized by introducing a carbon source into an inert gas atmosphere or a mixed gas atmosphere of hydrogen and an inert gas at atmospheric pressure or reduced pressure. The synthesis temperature for multilayer graphene is preferably 500°C to 800°C, and the temperature is raised to this temperature range before the introduction of the carbon source. The flow rate of the carbon source cannot be unequivocally defined because it depends on the volume of the apparatus, but is preferably 0.1 sccm to 100 sccm. The synthesis time for multilayer graphene depends on the flow rate (or partial pressure) of the carbon source, the temperature, etc., and cannot be uniquely defined, but is preferably 1 minute to 60 minutes.

[0032] (multilayer graphene) The disclosed multilayer graphene has a random stacking structure in which stacked graphenes are randomly rotated in a plane.2 The above is true, and it is preferable that the difference between the maximum and minimum number of layers in the multilayer graphene is 20 layers or less. Multilayer graphene can be suitably produced by the disclosed method for producing multilayer graphene.

[0033] The area of the multi-layer graphene is not particularly limited and can be appropriately selected depending on the purpose. 2 More than 5,000 μm is preferable. 2 More preferably, 10,000 μm or more 2 The above is even more preferable. The maximum number of layers in multilayer graphene (N max ) and minimum (N min ) and the difference (N max -N min ) is preferably 20 layers or less, more preferably 15 layers or less, and even more preferably 10 layers or less. The number of layers in the multilayer graphene is not particularly limited and can be appropriately selected depending on the purpose, and may be 2 to 200 layers, 5 to 100 layers, 50 to 100 layers, or 10 to 50 layers. As for the distribution of the number of layers in the two-dimensional (planar) direction in the multilayer graphene, a specific number of layers is preferably uniformly distributed in order to obtain uniform chemical properties and optical properties, but the number of layers may be distributed with a range.

[0034] The method for identifying multilayer graphene is not particularly limited and can be appropriately selected depending on the purpose. For example, as shown in the Raman spectroscopy spectrum of multilayer graphene in Example 1 (catalyst film: Fe—Cr alloy film (Cr 20% by volume)) in FIG. 7 described later, -1 , and 2700 cm -1 The formation of multilayer graphene can be confirmed by observing a clear peak around 2700 cm. -1 The shape of the peak in the vicinity is a single peak, which indicates that the obtained multilayer graphene has random stacking.

[0035] Multilayer graphene can be identified using an optical microscope based on the difference in contrast with the crystalline substrate or Fe-Cr alloy layer, and its area and number of layers can be determined. Another method for identifying the number of layers is to directly measure the step between the crystalline substrate or Fe-Cr alloy layer and the multilayer graphene using an atomic force microscope, and the number can be calculated from the thickness of single-layer graphene (approximately 0.34 nm).

[0036] Multilayer graphene can be used as a two-dimensional material that exhibits unique physical properties not found in multilayer graphene with an AB stacking structure. For example, compared to multilayer graphene with an AB stacking structure, multilayer graphene can be used in high-frequency devices due to its high electron mobility, in chemical sensors due to its large specific surface area, and in optical sensors due to its high optical absorption coefficient.

[0037] Hereinafter, embodiments of the disclosed method for producing multi-layer graphene will be described with reference to the drawings, but the present invention is not limited to the following embodiments. Note that in the following drawings, for convenience of illustration, the relative sizes and thicknesses of some components are not shown.

[0038] [Embodiment 1] The synthesis of multilayer graphene by CVD will be explained as an example with reference to FIG. 2. 2A to 2D are schematic diagrams showing the steps of a method for producing multi-layer graphene in embodiment 1. Embodiment 1 includes a metal film forming step, an alloying and recrystallization step, and a multi-layer graphene synthesis step, and the metal film forming step is a step of depositing Fe and Cr on the surface of a crystalline substrate to form a metal film.

[0039] First, a sapphire single crystal having a (0001) oriented surface (commonly referred to as the C-plane) is prepared as the crystalline substrate 1 (FIG. 2A). The crystalline substrate 1 preferably has an atomically smooth surface that has been subjected to chemical mechanical polishing, and is preferably further subjected to heat treatment in an oxygen atmosphere at 1,000°C to 1,400°C for 3 to 24 hours.

[0040] Next, a metal film containing Fe and Cr is formed on the crystalline substrate 1 by, for example, vapor deposition or sputtering (FIG. 2B). Using Fe and Cr supply sources, an Fe film 2 and a Cr film 3 are sequentially deposited on the same crystalline substrate 1, or Fe and Cr are simultaneously deposited to form the metal film. To prevent oxidation of the interface between the Fe and Cr films or the inclusion of impurities, it is preferable to deposit Fe and Cr consistently in a vacuum. The temperature of the crystalline substrate 1 during deposition is preferably room temperature or higher, more preferably 400°C to 600°C. The average thickness of the metal film is preferably 100 nm to 1,000 nm. The amounts of Fe and Cr supplied are appropriately adjusted so that the Cr content in the metal film falls within a suitable range (e.g., 10% to 40% by volume).

[0041] Next, the metal film including the Fe film 2 and the Cr film 3 is alloyed and recrystallized to form an Fe—Cr alloy film 5 suitable as a catalyst for multilayer graphene synthesis (Figure 2C). A furnace with a highly airtight exhaust system capable of reducing pressure to 10 Pa or less is used as the apparatus for multilayer graphene synthesis, as described below. Supply lines for inert gas, hydrogen gas, carbon source, etc. are connected to the apparatus, and the flow rates of each are controlled by flow meters. After introducing the crystalline substrate 1 on which the metal film including the Fe film 2 and the Cr film 3 has been deposited into the apparatus, the residual oxygen concentration in the furnace is reduced by repeating multiple cycles of evacuation and inert gas refill. The alloying and recrystallization of the metal film are preferably performed in a mixed gas atmosphere of hydrogen and inert gas at atmospheric pressure or reduced pressure. The hydrogen concentration in the mixed gas is preferably 1% to 30% by volume, the hydrogen flow rate is preferably 1 sccm to 1,000 sccm, and the inert gas flow rate is preferably 10 sccm to 10,000 sccm. By heating the metal film at 800°C to 1,100°C and holding it for about 10 minutes to 600 minutes, the metal film including the Fe film 2 and the Cr film 3 is alloyed and recrystallized, and an Fe—Cr alloy film 5 can be formed.

[0042] Next, multilayer graphene 10 is synthesized from a carbon source by CVD using an Fe—Cr alloy film 5 as a catalyst (FIG. 2D). The synthesis temperature for multilayer graphene is preferably 500°C to 800°C, and the temperature is raised to this temperature range prior to the introduction of acetylene as a carbon source. The flow rate of acetylene is preferably 0.1 sccm to 100 sccm. The synthesis time for multilayer graphene is approximately 1 minute to 60 minutes, depending on the flow rate (or partial pressure) of acetylene and the temperature. As a result of the above, multilayer graphene having a random stacking structure is obtained on the surface of the Fe—Cr alloy film 5.

[0043] [Embodiment 2] 2A, 3, 2C, and 2D are schematic diagrams showing the steps of a method for producing multilayer graphene in embodiment 2. Embodiment 2 is the same as embodiment 1, except that the metal film formation step is a step of depositing an Fe—Cr alloy on the surface of a crystalline substrate to form a metal film.

[0044] Next, an Fe—Cr alloy is deposited on the crystalline substrate 1 ( FIG. 2A ) to form a metal film 4 by, for example, evaporation or sputtering ( FIG. 3 ). A pre-alloyed Fe—Cr alloy source is used to form the metal film (Fe—Cr alloy film) 4 directly on the crystalline substrate 1. Deposition is preferably carried out in a vacuum to prevent oxidation of the interface between the Fe and Cr films and the inclusion of impurities. The temperature of the crystalline substrate 1 during deposition is preferably room temperature or higher, more preferably 400°C to 600°C. The average thickness of the metal film is preferably 100 nm to 1,000 nm. The Fe—Cr alloy source is appropriately prepared so that the Cr content in the metal film falls within a suitable range (e.g., 10% to 40% by volume).

[0045] Next, the metal film 4 is alloyed and recrystallized to form an Fe—Cr alloy film 5 suitable as a catalyst for synthesizing multilayer graphene (FIG. 2C). Multilayer graphene 10 is synthesized from a carbon source by a CVD method using the Fe—Cr alloy film 5 as a catalyst (FIG. 2D). As a result of the above, multilayer graphene having a random stacking structure is obtained on the surface of the Fe—Cr alloy film 5. [Example]

[0046] Hereinafter, the disclosed method for producing multi-layer graphene and the multi-layer graphene will be described in more detail based on examples, but the present invention is not limited to the following examples.

[0047] Example 1 According to the method of embodiment 1, multi-layer graphene was produced under the following conditions. A metal film was formed using Fe and Cr sources so that the Cr content in the resulting Fe—Cr alloy film was 20% by volume, and alloying and recrystallization were carried out to form the Fe—Cr alloy film of Example 1. The size of the crystalline substrate 1 was 1 cm x 1 cm, the temperature of the crystalline substrate 1 when the metal film was formed was room temperature, the average thickness of the metal film was 100 nm, the hydrogen concentration in the mixed gas was 1 vol%, the flow rate of hydrogen was 1 sccm, and the flow rate of the inert gas was 100 sccm. The heating temperature for alloying and recrystallization was 1,000° C., and the heating time was 60 minutes.

[0048] The bright-field and dark-field optical microscope images of the obtained Fe—Cr alloy film (20% by volume of Cr) of Example 1 are shown in FIG. 4C and FIG. 4D, respectively.

[0049] Next, multi-layer graphene 10 was synthesized from acetylene, a carbon source, by a CVD method using the Fe—Cr alloy film as a catalyst. The flow rate of acetylene was 0.5 sccm, and the synthesis temperature and synthesis time of the multi-layer graphene were 600° C. and 5 minutes, respectively. FIG. 6B shows a bright-field image of the multilayer graphene obtained in Example 1 (catalyst film: Fe—Cr (20% by volume)) taken by an optical microscope.

[0050] Example 2 An Fe—Cr alloy film and multilayer graphene of Example 2 were produced in the same manner as in Example 1, except that the metal film was formed so that the Cr content in the resulting Fe—Cr alloy film was 40% by volume. The bright-field and dark-field optical microscope images of the obtained Fe—Cr alloy film (Cr 40% by volume) of Example 2 are shown in FIG. 4E and FIG. 4F, respectively.

[0051] Examples 3 to 5 The Fe—Cr alloy films and multilayer graphene of Examples 3 to 5 were produced in the same manner as in Example 1, except that the metal film was formed so that the Cr content in the resulting Fe—Cr alloy film was 5 vol %, 10 vol %, and 30 vol %. FIG. 5A shows a dark-field optical microscope image of the Fe—Cr alloy film (5% by volume of Cr) obtained in Example 3, and FIG. 5B shows a dark-field optical microscope image of the Fe—Cr alloy film (10% by volume of Cr) obtained in Example 4.

[0052] (Comparative Example 1) A catalyst film (Fe) and multilayer graphene of Comparative Example 1 were produced in the same manner as in Example 1, except that the metal film was formed so that the Cr content in the resulting Fe—Cr alloy film was 0 vol % and the Fe content was 100 vol %. The bright-field and dark-field images of the obtained catalyst film (Fe) of Comparative Example 1 taken by an optical microscope are shown in FIG. 4A and FIG. 4B, respectively. FIG. 6A shows a bright-field image of the multilayer graphene obtained in Comparative Example 1 (catalyst film: Fe) taken by an optical microscope.

[0053] (Comparative Example 2) A catalyst film (Cr) and multilayer graphene of Comparative Example 2 were produced in the same manner as in Example 1, except that the metal film was formed so that the Cr content in the resulting Fe—Cr alloy film was 100% by volume. The bright-field and dark-field images of the obtained catalyst film (Cr) of Comparative Example 2 taken by an optical microscope are shown in FIG. 4G and FIG. 4H, respectively.

[0054] A uniform contrast is observed in the bright-field image (Fig. 4A) of the catalyst film (Fe) of Comparative Example 1, but a pattern containing numerous irregular lines is observed in the dark-field image (Fig. 4B). These are due to the surface irregularities and the grain boundaries inside the film, and indicate the non-uniformity of the film. In the dark-field image (FIG. 4D) of the Fe—Cr alloy film (20% by volume of Cr) of Example 1, the pattern seen in the catalyst film (Fe) of Comparative Example 1 is not observed, which indicates that a highly uniform alloy film with no grain boundaries and a flat surface has been formed. Although not shown, fewer grain boundaries were formed in the Fe—Cr alloy film (Cr 30% by volume) of Example 5 compared to Example 1, and a relatively large number of grain boundaries could be confirmed in the dark-field image ( FIG. 4F ) of the Fe—Cr alloy film (Cr 40% by volume) of Example 2. In the dark-field image (FIG. 4H) of the catalyst film (Cr) of Comparative Example 2 shown for reference, very fine crystal grains are formed, resulting in an inhomogeneous film with a high grain boundary density.

[0055] 5A and 5B show dark-field images of the Fe—Cr alloy film (5% by volume Cr) of Example 3 and the Fe—Cr alloy film (10% by volume Cr) of Example 4, which are examples of Fe—Cr alloy films with low Cr content. In both cases, patterns due to grain boundaries and irregularities are present, and it is clear that the films are less uniform than the Fe—Cr alloy film (20% by volume Cr) of Example 1. From these results, the Fe-Cr alloy film (20% by volume of Cr) of Example 1 is most suitable as a catalyst film.

[0056] FIG. 6B shows a dark-field optical microscope image of the multilayer graphene in Example 1 (catalyst film: Fe—Cr (20% by volume)) produced by performing the alloying recrystallization step and the multilayer graphene synthesis step, and FIG. 6A shows a bright-field optical microscope image of the multilayer graphene in Comparative Example 1 (catalyst film: Fe).

[0057] In Comparative Example 1 (catalyst film: Fe), many regions with different color shading (contrast) were observed. This difference in contrast is due to differences in the thickness of the multilayer graphene film, indicating a large variation in the number of layers. In general, the non-uniformity in the number of graphene layers can be confirmed by contrast in microscopic observation, and the size can be estimated from this. The maximum length of each region with equivalent contrast was approximately tens of μm to hundreds of μm. On the other hand, in the multilayer graphene of Example 1 (catalyst film: Fe—Cr (20% by volume)), the contrast was constant across the entire substrate, and the difference between the maximum and minimum number of layers in the multilayer graphene was calculated to be 20 or less, indicating that multilayer graphene with a uniform number of layers was obtained. Note that the region with a uniform number of layers obtained in this example was equivalent to the size of the substrate used.

[0058] FIG. 7 shows the Raman spectrum obtained from the multilayer graphene shown in FIG. 6B of Example 1 (catalyst film: Fe—Cr (20% by volume)). 1600cm -1 , and 2700 cm -1 A clear peak was observed around 2700 cm, confirming the formation of graphene. -1 The peak shape around this region was a single peak, indicating that the multilayer graphene obtained had random stacking. For comparison, Figure 8 shows the 2700 cm peak obtained from highly oriented pyrolytic graphite (HOPG). -1 The Raman spectrum around 2695 cm is shown. Generally, graphene in HOPG is stacked in a regular Bernal (AB) pattern, and in graphite crystals with such a stacking structure, the peak observed in the same range is on the low wavenumber side (2695 cm). -1 It is known that the structure has a shoulder near the center.

[0059] Furthermore, the following notes are disclosed: (Appendix 1) A method for producing multi-layer graphene, comprising the step of synthesizing multi-layer graphene from a carbon source using an Fe-Cr alloy film as a catalyst (reference numeral 10, Figures 2C-D). (Appendix 2) The method for producing multi-layer graphene according to Appendix 1, wherein the Cr content in the Fe—Cr alloy film is 10% by volume to 40% by volume (reference numeral 5, FIG. 2C). (Appendix 3) A step of forming a metal film by (1) depositing Fe and Cr or (2) depositing an Fe-Cr alloy on a surface of a crystalline substrate; and then, heating the metal film at 800°C to 1,100°C to alloy and recrystallize the metal film to form the Fe—Cr alloy film (FIGS. 2A to 2C and 3). (Appendix 4) 4. The method for producing multilayer graphene according to claim 3, wherein the crystalline substrate is a sapphire substrate having a (0001) oriented surface, and the surface is the (0001) oriented surface. (Appendix 5) 5. The method for producing multilayer graphene according to any one of claims 1 to 4, wherein the synthesis temperature of the multilayer graphene is 500°C to 800°C. (Appendix 6) 6. The method for producing multi-layer graphene according to any one of claims 1 to 5, wherein the carbon source is acetylene. (Appendix 7) A multilayer graphene having a random stacking structure, Its area is 1,000 μm 2 That's all, The multilayer graphene is characterized in that the difference between the maximum and minimum number of layers in the multilayer graphene is 20 or less (reference numeral 5, FIG. 6B). [Explanation of symbols]

[0060] 1. Crystalline substrate 2 Iron film (Fe film) 3 Chromium film (Cr film) 4 Metal film (Fe-Cr alloy film) 5. (Alloyed and recrystallized) Fe-Cr alloy film 10 Multilayer graphene

Claims

1. A method for manufacturing a sapphire substrate, comprising the steps of: (1) depositing Fe and Cr, or (2) depositing an Fe—Cr alloy on a (0001) oriented surface of the sapphire substrate to form a metal film; Next, the metal film is alloyed and recrystallized by heating at 800°C to 1,100°C to form an Fe—Cr alloy film having a Cr content of 10% by volume to 40% by volume; synthesizing multilayer graphene from a carbon source using the Fe—Cr alloy film as a catalyst.

2. 2. The method for producing multilayer graphene according to claim 1, wherein the synthesis temperature of the multilayer graphene is 500°C to 800°C.

3. 3. The method for producing multi-layer graphene according to claim 1, wherein the carbon source is acetylene.

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