Method for producing multi-layer graphene

Abstract

To provide a method for producing multilayered graphene with a randomly stacked structure that can suppress the number of layers of graphene and the in-plane variation of the number of layers of graphene.SOLUTION: A catalyst film containing iron and nickel is recrystallized by performing a first heating treatment in which the catalyst film is heated in an atmosphere containing hydrogen. Then, the catalyst film is subjected to a second heating treatment in an atmosphere containing hydrogen and hydrocarbons to form multiple graphenes stacked with a randomly stacked structure on the surface of the catalyst film.SELECTED DRAWING: Figure 7

Description

[Technical field]

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

[0002] Graphene is a one-atom-thick sheet of carbon atoms arranged in a hexagonal honeycomb lattice. The following techniques are known for producing graphene:

[0003] For example, a method for producing graphene is known that includes the steps of forming an alloy catalyst layer on a substrate and supplying a hydrocarbon gas onto the alloy catalyst layer to form a graphene layer. The alloy catalyst layer is made of at least two metals selected from nickel (Ni), copper (Cu), platinum (Pt), iron (Fe), and gold (Au).

[0004] Also described is a method for producing graphene, which comprises heating a single-crystal thin film of a transition metal epitaxially grown on a single-crystal substrate and supplying carbon to the surface of the transition metal to grow graphene. The transition metal is Fe, Co, Ni, Cu, Mo, Ru, Rh, Pd, W, Re, Ir, Pt, or an alloy thereof. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2011-102231 A [Patent Document 2] JP 2012-232860 A Summary of the Invention [Problem to be solved by the invention]

[0006] Graphene has the advantage of having a very high electron mobility compared to bulk, and therefore its application to high-frequency devices, transparent conductive films, flexible devices, and optical sensors has been studied. However, single-layer graphene has undesirable properties for application to various devices, such as low mechanical strength and low optical absorption rate.

[0007] Multilayer graphene having a random stacking structure is a material that can improve the problems of mechanical strength and light absorption rate while retaining the excellent characteristics of single-layer graphene. Here, Fig. 1 and Fig. 2 are a perspective view and a plan view, respectively, showing the structure of multilayer graphene having a random stacking structure. As shown in Fig. 1 and Fig. 2, the random stacking structure is a structure in which a rotation angle of a multilayer graphene 10 in which a plurality of graphenes 11 are stacked is random around the stacking direction of each layer as the rotation axis.

[0008] Multilayer graphene having a random stacking structure can be manufactured by chemical vapor deposition (CVD) using iron (Fe) as a catalyst film. However, when iron is used as the catalyst film, multilayer graphene having a large number of layers (e.g., 100 layers or more) and a large in-plane variation in the number of layers is manufactured. If the number of layers of multilayer graphene is excessively large, patterning becomes difficult, and it becomes difficult to manufacture a device using the multilayer graphene. Furthermore, if the in-plane variation in the number of graphene layers is large, there is a risk of a decrease in the yield of the device.

[0009] The disclosed technology has been made in consideration of the above-mentioned points, and has an object to provide a method for producing multilayer graphene having a random stacking structure that can suppress the number of graphene layers and the in-plane variation in the number of layers. [Means for solving the problem]

[0010] The method for producing multilayer graphene according to the disclosed technology includes a step of performing a first heat treatment on a catalyst film containing iron and nickel in an atmosphere containing hydrogen to recrystallize the catalyst film, and a step of performing a second heat treatment on the catalyst film in an atmosphere containing hydrogen and a hydrocarbon to form a plurality of graphenes stacked in a random stacking structure on a surface of the catalyst film. Effect of the Invention

[0011] According to the disclosed technology, it is possible to suppress the number of graphene layers and the in-plane variation in the number of layers. [Brief description of the drawings]

[0012] [Figure 1] FIG. 1 is a perspective view showing a structure of multilayer graphene having a random stacking structure. [Diagram 2] FIG. 1 is a plan view showing a structure of multilayer graphene having a random stacking structure. [Diagram 3] 1A to 1C are diagrams illustrating an example of a method for producing multi-layer graphene according to an embodiment of the disclosed technique. [Figure 4] 1A to 1C are diagrams illustrating an example of a method for producing multi-layer graphene according to an embodiment of the disclosed technique. [Diagram 5] 1A to 1C are diagrams illustrating an example of a method for producing multi-layer graphene according to an embodiment of the disclosed technique. [Figure 6] 1A to 1C are diagrams illustrating an example of a method for producing multi-layer graphene according to an embodiment of the disclosed technique. [Figure 7] FIG. 2 is a diagram showing an example of the configuration of a CVD apparatus used in a first heat treatment and a second heat treatment according to an embodiment of the disclosed technique. [Figure 8A] 1 is a TEM image of a cross section of multi-layer graphene produced by a production method according to a comparative example. [Figure 8B] FIG. 8B is an enlarged view of region A in FIG. 8A. [Figure 9] 1 is an optical microscope image in plan view of multi-layer graphene produced by a production method according to a comparative example. [Figure 10] 1 is an optical microscope image in a plan view of multi-layer graphene produced by a production method according to an embodiment of the disclosed technique. [Figure 11] 1 is a Raman spectrum obtained for multi-layer graphene according to an embodiment of the disclosed technique. [Figure 12A] 1 is an optical microscope image of multi-layer graphene in a plan view when the volume ratio of nickel in the catalyst film is 15%. [Figure 12B] 1 is an optical microscope image of a planar view of multi-layer graphene when the volume ratio of nickel in the catalyst film is 30%. [Figure 12C] 1 is an optical microscope image of a planar view of multi-layer graphene when the volume ratio of nickel in the catalyst film is 45%. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, an example of an embodiment of the disclosed technology will be described with reference to the drawings. In each drawing, the same or equivalent components and parts are given the same reference numerals, and duplicated description will be omitted.

[0014] 3 to 6 are diagrams illustrating an example of a method for producing multilayer graphene according to an embodiment of the disclosed technique. The production method according to this embodiment is particularly suitable for producing multilayer graphene with a random stacking structure having a relatively small number of layers (e.g., about 1 to 10 layers).

[0015] First, a substrate 20 to be used for synthesizing multi-layer graphene is prepared (FIG. 3). For example, a sapphire single crystal substrate (Al2O3) having a (0001) oriented plane (C-plane) on its surface can be suitably used as the substrate 20. For example, the substrate 20 is preferably one having an atomically smooth surface that has been subjected to chemical mechanical polishing (CMP). For example, a magnesium oxide single crystal substrate (MgO) or a spinel single crystal substrate (MgAl2O4) can also be used as the substrate 20.

[0016] Next, a catalyst film 30 containing iron (Fe) and nickel (Ni) is formed on the surface of the substrate 20. Specifically, a first metal film 31 containing mainly iron (Fe) and a second metal film 32 containing mainly nickel (Ni) are sequentially formed on the surface of the substrate 20 (FIG. 4). The first metal film 31 and the second metal film 32 each constitute the catalyst film 30. For example, a vapor deposition method and a sputtering method can be used to form the first metal film 31 and the second metal film 32. The order of forming the first metal film 31 and the second metal film 32 is not particularly limited, and either one may be formed first. The first metal film 31 and the second metal film 32 are preferably formed continuously in a vacuum in order to suppress oxidation of the interface between these films and the inclusion of impurities. The volume ratio of nickel (Ni) in the catalyst film 30 is preferably 40% or more and 80% or less. The thickness of the catalyst film 30 is preferably 100 nm or more and 1000 nm or less. The volume ratio of nickel (Ni) and the thickness of the catalyst film 30 can be controlled by adjusting the film thicknesses of the first metal film 31 and the second metal film 32.

[0017] Next, a first heat treatment is performed in which the catalyst film 30 including the first metal film 31 and the second metal film 32 is heated in an atmosphere including hydrogen (H2). This causes the iron (Fe) included in the first metal film 31 and the nickel (Ni) included in the second metal film 32 to be alloyed. The first heat treatment also recrystallizes the catalyst film 30. The recrystallization of the catalyst film 30 turns the catalyst film 30 into an epitaxial film whose crystal orientation matches the orientation of the crystal plane (e.g., C-plane) of the substrate 20. Furthermore, by performing the first heat treatment in hydrogen (H2) having a reducing effect, it is possible to suppress oxidation of the catalyst film 30.

[0018] In this embodiment, the first heat treatment and the second heat treatment described later are performed using a CVD apparatus. Fig. 7 is a diagram showing an example of the configuration of a CVD apparatus 50 used in the first heat treatment and the second heat treatment described later. The CVD apparatus 50 has an airtight chamber 51, a susceptor 52 provided in the chamber 51, a gas inlet 53 for introducing various gases into the chamber 51, and a gas outlet 54 for discharging the various gases.

[0019] The substrate 20 on which the catalyst film 30 is formed is placed on the surface of the susceptor 52. The susceptor 52 is heated, for example, by a known induction heating method. The substrate 20 and the catalyst film 30 are heated through the susceptor 52. The heating temperature in the first heat treatment is preferably, for example, 800° C. or more and 1200° C. or less. The treatment time in the first heat treatment is preferably 10 minutes or more and 600 minutes or less. During the first heat treatment, hydrogen gas (H2) and an inert gas are introduced into the chamber 51 through the gas introduction part 53. As the inert gas, for example, argon gas (Ar) or nitrogen gas (N2) can be suitably used. The flow rate of the hydrogen gas in the first heat treatment is preferably 1 sccm or more and 1000 sccm or less, and the flow rate of the inert gas is preferably 10 sccm or more and 10000 sccm or less. Here, sccm stands for standard cubic centimeter per minute.

[0020] After the first heat treatment is completed, a second heat treatment is performed in which the substrate 20 and the catalyst film 30 are heated in an atmosphere containing hydrogen (H2) and a hydrocarbon. As a result, a multilayer graphene 10 including a plurality of graphenes stacked in a random stacking structure is formed on the surface of the catalyst film 30 (FIG. 6). In this embodiment, the second heat treatment is performed using the CVD apparatus 50 used in the first heat treatment. That is, the first heat treatment and the second heat treatment are performed successively in the same chamber 51 in which airtightness is maintained. This makes it possible to suppress oxidation and the inclusion of impurities.

[0021] The heating temperature in the second heat treatment is preferably, for example, 800° C. or more and 1200° C. or less, and may be the same as or different from the heating temperature in the first heat treatment. The processing time in the second heat treatment depends on the heating temperature and the type of hydrocarbon used, but is typically preferably 1 minute or more and 60 minutes or less. During the second heat treatment, a hydrocarbon gas is introduced into the chamber 51 through the gas introduction part 53 in addition to hydrogen gas (H2) and an inert gas. For example, argon gas (Ar) or nitrogen gas (N2) can be preferably used as the inert gas. For example, methane gas (CH4) can be preferably used as the hydrocarbon gas. The flow rates of the hydrogen gas (H2) and the inert gas in the second heat treatment may be maintained at the same rates as those in the first heat treatment. The flow rate of the hydrocarbon gas is preferably 0.1 sccm or more and 100 sccm or less.

[0022] The number of graphene layers can be controlled by the volume ratio of nickel (Ni) in the catalyst film 30. The number of graphene layers tends to decrease as the volume ratio of nickel (Ni) in the catalyst film 30 increases. By setting the volume ratio of nickel (Ni) in the catalyst film 30 to 40% or more and 80% or less, multi-layer graphene having 10 or less graphene layers is formed. The number of graphene layers can also be controlled by the amount (concentration) of hydrocarbon gas. The number of graphene layers tends to increase as the flow rate of the hydrocarbon gas introduced into the chamber 51 increases. That is, multi-layer graphene having a desired number of layers can be obtained by the volume ratio of nickel (Ni) in the catalyst film 30 and the flow rate of the hydrocarbon gas.

[0023] In the above description, the catalyst film 30 is formed by sequentially forming the first metal film 31 containing iron (Fe) and the second metal film 32 containing nickel (Ni) on the substrate 20, but the disclosed technology is not limited to this embodiment. For example, an alloy film containing iron (Fe) and nickel (Ni) may be formed as the catalyst film 30 on the substrate 20. That is, the catalyst film 30 may be formed on the substrate 20 by a sputtering method or a deposition method using an Fe-Ni alloy as a supply source.

[0024] In the above description, the catalyst film 30 formed on the substrate 20 is used, but the catalyst film 30 does not have to be formed on the substrate 20. For example, a foil-shaped or plate-shaped catalyst film can be used.

[0025] [Example] Multilayer graphene was produced using the production method according to the embodiment of the disclosed technique described above. A catalyst film containing iron (Fe) and nickel (Ni) was formed on a C-plane sapphire substrate. The volume ratio of nickel (Ni) in the catalyst film was 45%. The volume ratio of iron (Fe) in the catalyst film was 55%. A first heat treatment was performed using a CVD apparatus. The heating temperature in the first heat treatment was 1100°C, and the treatment time was 60 minutes. Argon gas (Ar) was used as the inert gas. The flow rate of argon gas (Ar) was 500 sccm, and the flow rate of hydrogen gas (H2) was 50 sccm. The pressure in the chamber was atmospheric pressure (100 kPa).

[0026] Following the first heat treatment, a second heat treatment was performed using the same CVD apparatus. The heating temperature in the second heat treatment was 1000°C, and the treatment time was 20 minutes. Argon gas (Ar) was used as the inert gas, and methane gas (CH4) was used as the hydrocarbon gas. The flow rate of argon gas (Ar) was 500 sccm, the flow rate of hydrogen gas (H2) was 50 sccm, and the flow rate of methane gas (CH4) was 0.4 sccm. The pressure in the chamber was 50 kPa.

[0027] As a comparative example, multi-layer graphene was produced using a catalyst film that did not contain nickel (Ni) (i.e., contained only iron (Fe)). The production conditions for the comparative example were the same as those for the above-mentioned embodiment. For the embodiment and the comparative example, the first heat treatment and the second heat treatment were simultaneously performed using the same CVD apparatus.

[0028] FIG. 8A is a TEM (Transmission Electron Microscope) image of a cross section of multi-layer graphene produced by a production method according to a comparative example, and FIG. 8B is an enlarged view of region A in FIG. 8A.

[0029] FIG. 9 is an optical microscope image in plan view of multi-layer graphene manufactured by a manufacturing method according to a comparative example. FIG. 10 is an optical microscope image in plan view of multi-layer graphene manufactured by a manufacturing method according to an embodiment of the disclosed technology. These optical microscope images were taken after multi-layer graphene formed on a C-plane sapphire substrate was transferred onto a silicon substrate having a thermal oxide film formed on its surface. In the optical microscope images shown in FIG. 9 and FIG. 10, bright areas are areas with a relatively large number of graphene layers, and dark areas are areas with a relatively small number of graphene layers. The in-plane variation in the number of graphene layers is reflected in the intertwining of bright and dark areas in the optical microscope images.

[0030] In the multilayer graphene according to the comparative example produced using a catalyst film not containing nickel (Ni), the number of graphene layers varied in the range from 1 to about 100. The size of each grain was about several μm.

[0031] On the other hand, the multi-layer graphene according to the embodiment of the disclosed technology, which was produced using a catalyst film containing nickel (Ni), had a number of graphene layers in the range of about 1 to 5 layers. The size of each grain was about several tens of μm. That is, the multi-layer graphene according to the embodiment of the disclosed technology has a significantly smaller number of graphene layers and a significantly smaller variation in the number of layers compared to the multi-layer graphene according to the comparative example.

[0032] FIG. 11 is a Raman spectrum obtained for the multi-layer graphene according to the embodiment of the disclosed technique. -1 The absence of peaks due to defects around 2700 cm suggests that the multi-layer graphene obtained is of high quality. -1 The peak shape in the vicinity is a single peak, which indicates that the obtained multilayer graphene has a random stacking structure.

[0033] Figures 12A, 12B, and 12C are optical microscope images in plan view of multi-layer graphene when the volume ratio of nickel (Ni) in the catalyst film is 15%, 30%, and 45%, respectively. These optical microscope images were taken after multi-layer graphene formed on a C-plane sapphire substrate was transferred onto a silicon substrate with a thermally oxidized film formed on its surface. As the volume ratio of nickel (Ni) in the catalyst film increases, the number of graphene layers decreases, and the in-plane variation in the number of layers is suppressed.

[0034] As described above, the method for producing multilayer graphene according to the embodiment of the disclosed technology includes the following steps: (1) A step of recrystallizing a catalyst film containing iron and nickel by performing a first heat treatment in an atmosphere containing hydrogen, and (2) A step of forming a plurality of graphenes stacked in a random stacking structure on the surface of the catalyst film 30 by performing a second heat treatment in an atmosphere containing hydrogen and hydrocarbon. The production method according to the embodiment of the disclosed technology makes it possible to produce multilayer graphene having a random stacking structure that can suppress the number of graphene layers and the in-plane variation in the number of layers.

[0035] The following supplementary notes are further disclosed regarding the above embodiment. (Appendix 1) A step of performing a first heat treatment of heating the catalyst film containing iron and nickel in an atmosphere containing hydrogen to recrystallize the catalyst film; and performing a second heat treatment of heating the catalyst film in an atmosphere containing hydrogen and a hydrocarbon to form a plurality of graphenes stacked on a surface of the catalyst film, the graphenes having a random stacking structure. A method for producing multi-layer graphene.

[0036] (Appendix 2) The method further includes forming the catalyst film on a substrate by sequentially forming a first metal film containing iron and a second metal film containing nickel on the substrate; The first heat treatment causes the iron contained in the first metal film and the nickel contained in the second metal film to be alloyed. The manufacturing method described in Appendix 1.

[0037] (Appendix 3) The method further includes a step of forming an alloy film containing iron and nickel on the substrate as the catalyst film. The manufacturing method described in Appendix 1.

[0038] (Appendix 4) The substrate is a single crystal sapphire substrate. The method for producing the present invention described in Appendix 2 or Appendix 3.

[0039] (Appendix 5) The volume ratio of nickel in the catalyst film is 40% or more and 80% or less. 5. The method of claim 1, wherein the

[0040] (Appendix 6) The first heat treatment and the second heat treatment are carried out consecutively in the same airtight chamber. 6. The method of claim 1,

[0041] (Appendix 7) The number of graphene layers is controlled by the volume ratio of nickel in the catalyst film. 7. The method of any one of claims 1 to 6.

[0042] (Appendix 8) The heating temperature in the first heat treatment and the second heat treatment is 800° C. or more and 1200° C. or less. 8. The method of claim 1, wherein the

[0043] (Appendix 9) The number of layers of the graphene is 10 or less. 9. The method of any one of claims 1 to 8. [Explanation of symbols]

[0044] 10 Multilayer graphene 20 Substrate 30 Catalyst membrane 31 First metal film 32 Second metal film 50 CVD equipment 51 Chamber 52 Susceptor

Claims

1. A step of performing a first heat treatment of heating the catalyst film containing iron and nickel in an atmosphere containing hydrogen to recrystallize the catalyst film; and performing a second heat treatment on the catalyst film in an atmosphere containing hydrogen and a hydrocarbon to form a plurality of graphenes stacked on a surface of the catalyst film, the graphenes having a random stacking structure. A method for producing multi-layer graphene.

2. The method further includes forming the catalyst film on a substrate by sequentially forming a first metal film containing iron and a second metal film containing nickel on the substrate, The first heat treatment causes the iron contained in the first metal film and the nickel contained in the second metal film to be alloyed. The method of claim 1 .

3. The method further includes a step of forming an alloy film containing iron and nickel on the substrate as the catalyst film. The method of claim 1 .

4. The substrate is a single crystal sapphire substrate. The method according to claim 2 or 3.

5. The volume ratio of nickel in the catalyst film is 40% or more and 80% or less. The method according to any one of claims 1 to 4.

6. The first heat treatment and the second heat treatment are carried out successively in the same airtight chamber. The method according to any one of claims 1 to 5.

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