JP7586485B2 - How Graphene is Made - Google Patents

Description

Application of Article 30, paragraph 2 of the Patent Act On October 7, 2020, a summary of research on metal catalyst layers, methods for producing metal catalyst layers, methods for producing graphene, and graphene was published in the proceedings of the 39th Electronic Materials Symposium. On October 7, 2020, a summary of research on metal catalyst layers, methods for producing metal catalyst layers, methods for producing graphene, and graphene was published at the 39th Electronic Materials Symposium. On February 10, 2021, Nakajima Ryoto, Meijo University, Graduate School of Science and Engineering, Department of Materials Functional Engineering, published a master's thesis on metal catalyst layers, methods for producing metal catalyst layers, methods for producing graphene, and graphene. On February 10, 2021, a summary of research on metal catalyst layers, methods for producing metal catalyst layers, methods for producing graphene, and graphene was published at the Meijo University Master's Thesis Hearing. On February 26, 2021, an abstract was published in the proceedings of the 68th Spring Meeting of the Society of Applied Physics regarding research on metal catalyst layers, methods for producing metal catalyst layers, methods for producing graphene, and graphene.

The present invention relates to a method for producing graphene .

In Patent Document 1, a film pattern of a catalytic metal is formed on an insulating substrate by vapor deposition, graphene is grown on the film pattern, and then a drain electrode and a source electrode are formed on both sides of the graphene, and a gate electrode is formed on the graphene via a gate insulating film. Patent Document 1 describes that although the catalytic metal film pattern is separated by an insulating film, the graphene extends laterally at the ends of the film pattern, resulting in a structure in which the graphene extends from the film patterns on both sides of the insulating separation film and is connected on the insulating separation film.

JP 2009-164432 A

However, when graphene is produced by the method described in Patent Document 1, there are cases where the mobility of electrons decreases at the boundary where the graphene extending from the film patterns on both sides is connected. In order to improve this point and produce a transistor with better characteristics, a technology that can produce graphene with a large area and graphene with good crystallinity without connecting the graphene in the middle is desired. If graphene with a large area can be obtained, it will be possible to use graphene not only in transistors but also in larger integrated circuits. However, in order to produce graphene with good crystallinity over a large area, it is necessary to enlarge the crystal grains of the metal catalyst layer for growing graphene. Therefore, in order to produce graphene with good crystallinity over a large area, a metal catalyst layer with larger crystal grains and a method for easily producing such a metal catalyst layer are desired.

The present invention has been made in consideration of the above-mentioned conventional situation, and aims to provide a metal catalyst layer capable of producing high-quality graphene, a method for producing a metal catalyst layer, a method for producing graphene, and high-quality graphene.

The metal catalyst layer of the first invention is
A metal catalyst layer laminated and crystallized on a c-plane of a substrate having a corundum structure,
It has a face-centered cubic structure,
The [111] direction is the c-plane axial direction of the substrate;
The [01-1] direction is the [11-20] direction of the substrate.

The method for producing a metal catalyst layer according to the second aspect of the present invention comprises the steps of:
a first deposition step for forming a first layer of crystallized catalytic metal on a surface of a substrate;
a second lamination step of forming a second layer of the catalytic metal in an amorphous state on a surface of the first layer;
a first heat treatment step of performing a heat treatment to crystallize the second layer after the first lamination step and the second lamination step are performed;
Equipped with.

The method for producing graphene according to the third aspect of the present invention includes the steps of:
a carbon raw material laminating step of laminating a carbon raw material on a surface of the metal catalyst layer after carrying out the first heat treatment step in the method for producing a metal catalyst layer according to the second aspect of the present invention;
a second heat treatment step of carrying out a heat treatment after carrying out the carbon raw material lamination step to precipitate graphene between the substrate and the metal catalyst layer;
Equipped with.

The graphene of the fourth invention has no wrinkles in an area of 0.01 cm ² to 180 cm ² .

By using the metal catalyst layer of the first invention, it is possible to produce graphene of good quality.

The method for producing a metal catalyst layer according to the second invention can produce a metal catalyst layer with good crystal quality.

The graphene production method of the third invention makes it easy to produce large-area graphene with good crystallinity and no wrinkles at the interface between the substrate and the metal catalyst layer.

The graphene of the fourth invention can be used in devices that require a large area, such as integrated circuits.

1 is a schematic diagram showing a method for producing a metal catalyst layer and a method for producing graphene in Example 1. FIG. 13 shows images obtained by observing, using an SEM, the surfaces of samples in which the temperature for depositing Ni was changed to five different values in the first lamination step in the graphene production method of Example 1. 13 shows the results of measuring each sample by an in-plane X-ray diffraction measurement, in which the temperature for depositing Ni in the first lamination step in the graphene production method of Example 1 was changed to five different temperatures. FIG. 2 is a schematic diagram showing the crystal structure of the surface of a sapphire substrate when viewed from the front. 1 is a schematic diagram showing a crystal structure of a surface of a sapphire substrate after a first lamination step, as viewed from the front. FIG. 1 shows images of the surfaces of the samples of Example 2, Comparative Example 1, and Comparative Example 2 observed using a differential interference contrast microscope. The results are obtained by measuring the samples of Example 2, Comparative Example 1, and Comparative Example 2 by the out-plane X-ray diffraction measurement method. The results are obtained by measuring the samples of Example 2, Comparative Example 1, and Comparative Example 2 by an in-plane X-ray diffraction measurement method. 1 shows an image of the graphene surface of the sample of Example 2 observed using a differential interference contrast microscope, and a result of evaluating the graphene surface of the sample of Example 2 by Raman scattering spectroscopy. FIG. 10 is an image of the surface of graphene of the sample of Example 2 observed with an optical microscope at a position different from that in FIG. 9 , and shows the magnitude of the value of D/G within a rectangular frame shown in the image by using light and dark. This shows the results of measuring the sample of Example 2 by an in-plane X-ray diffraction measurement method after the second heat treatment step was performed. FIG. 11 is a schematic diagram showing the orientation relationship of the sample of Example 3. 13 is an image of the graphene surface of the sample of Example 3 observed using a microscope.

A preferred embodiment of the present invention will be described.

The carbon raw material for the graphene production method of the third invention can be nanodiamond. In this case, graphene can be continuously grown during heat treatment at a growth rate determined by the heat treatment temperature, and the number of graphene layers produced at the interface between the substrate and the metal can be easily controlled.

Next, examples 1 to 3 of the present invention will be described with reference to the drawings.

Example 1
First, a method for producing a metal catalyst layer using a catalytic metal having good crystal quality over the entire surface of a desired substrate, and a method for producing graphene that produces graphene that does not have wrinkles at the interface between the produced metal catalyst layer and the substrate will be described.

First, as shown in FIG. 1A, a sapphire substrate 10 is prepared as a substrate. The outer shape of the sapphire substrate 10 is a square of approximately 10 mm x 10 mm. The sapphire substrate 10 is then set in a chamber of a molecular beam epitaxy device (not shown). The molecular beam epitaxy device can perform molecular beam epitaxy (MBE) (hereinafter simply referred to as MBE). The c-plane ((0001) plane) of the sapphire substrate 10 is the surface (the front is the upper side in FIG. 1; the same applies below). The crystal structure of sapphire is a corundum structure. In other words, the sapphire substrate 10 has a corundum structure.

[Method of producing metal catalyst layer]
Next, as shown in FIG. 1B, a first lamination step is performed to form a first layer 11 of a crystallized catalyst metal on the surface of the sapphire substrate 10. Specifically, the first layer 11 is laminated by crystal growth of Ni, which is a catalyst metal, on the surface of the sapphire substrate 10 using the MBE method. The thickness of the first layer 11 is about 100 nm. The thickness of the first layer 11 may be in the range of 20 nm to 300 nm as long as the aggregation of Ni does not progress significantly. Note that if the thickness of the first layer 11 is less than 20 nm, the Ni layer remains in an island shape, making it difficult to use. At this time, the temperature of the sapphire substrate 10 is 600° C. to 700° C. The crystal structure of Ni is a face-centered cubic structure. In other words, Ni has a face-centered cubic structure. By performing the first lamination step, the crystallized first layer 11 can be laminated on the surface of the sapphire substrate 10.

Next, as shown in FIG. 1(C), a second lamination process is performed to form a second layer 12 of a catalytic metal in an amorphous state on the surface of the first layer 11. Specifically, Ni is evaporated onto the surface of the first layer 11 using an electron beam evaporation method to laminate the second layer 12. The thickness of the second layer 12 is approximately 300 nm. Here, the thickness of the second layer 12 may be in the range of 200 nm to 700 nm, as long as it is a thickness that allows crystallization in the next first heat treatment process. If the thickness is thicker than this, crystallization may also start from the surface of the second layer 12 when the first heat treatment process is performed, and crystallization using the crystal information of the first layer 11 cannot be performed on the surface side of the second layer 12, resulting in polycrystals. By performing the second lamination process, a second layer 12 of Ni in an amorphous state can be laminated on the surface of the first layer 11.

Next, after the first and second lamination steps are performed, a first heat treatment step is performed to crystallize the second layer 12 by heat treatment. In detail, the sapphire substrate 10 on which the first layer 11 and the second layer 12 are laminated is taken out of the chamber of the vacuum deposition device and set in a heat treatment device (not shown). The sapphire substrate 10 on which the first layer 11 and the second layer 12 are laminated is heated to 1000°C in a hydrogen atmosphere and maintained in this state for 30 minutes. Then, the temperature of the sapphire substrate 10 is lowered to 650°C at a rate of 3°C/min, and then to room temperature (for example, about 23°C). In this way, the second layer 12 is crystallized. In this way, as shown in FIG. 1(D), a metal catalyst layer 20 having the first layer 11 and the second layer 12 is generated. Here, the temperature lowering rate is not limited to 3°C/min, and may be 10°C/min.

[Method of producing graphene]
Next, as shown in FIG. 1(E), after the first heat treatment process, a carbon raw material lamination process is performed in which nanodiamond 13, which is a carbon raw material, is applied and laminated on the surface of the metal catalyst layer 20. Nanodiamond is a type of artificial diamond, and has a particle shape with a particle size of several nm. Nanodiamond has a metastable sp ³ bond structure, and can produce graphene at a lower temperature than when other carbon raw materials having a stable sp ² bond structure are used. Nanodiamond 13 is dispersed in an aqueous solution and applied to the surface of the metal catalyst layer 20 using a spin coater. The rotation speed and rotation time of the spin coater when applying nanodiamond 13 to the surface of the metal catalyst layer 20 are, for example, 6000 rpm and 30 seconds.

Next, as shown in FIG. 1(F), after the carbon raw material lamination step, a second heat treatment step is performed in which a heat treatment is performed to precipitate graphene 14 between the sapphire substrate 10 and the metal catalyst layer 20. Specifically, the sapphire substrate 10 with the nanodiamond 13 applied to the surface of the metal catalyst layer 20 is set in a heat treatment device (not shown). Then, the pressure inside the heat treatment device is set to about 10 ⁻⁴ Pa with the sapphire substrate 10 set. Then, the temperature of the sapphire substrate 10 is set to about 900° C. Then, the sapphire substrate 10 is subjected to a heat treatment for about 30 minutes. As a result, C (carbon) of the nanodiamond 13 is dissolved in the metal catalyst layer 20, and the dissolved C precipitates as graphene 14 at the interface between the sapphire substrate 10 and the metal catalyst layer 20. Then, the temperature of the sapphire substrate 10 is lowered to 300° C. at a rate of 3° C./min, and then to room temperature (for example, about 23° C.). At this time, C (carbon) of the nanodiamond 13 dissolved in the metal catalyst layer 20 precipitates as graphene 14, 24 at the interface between the sapphire substrate 10 and the metal catalyst layer 20 and at the interface between the nanodiamond 13 and the metal catalyst layer 20.

Next, as shown in FIG. 1(G), an etching process is performed to remove the metal catalyst layer 20 from the sapphire substrate 10. More specifically, the sapphire substrate 10 is removed from the heat treatment device and immersed in a solution of ferric chloride and pure water in a ratio of 1:3 for 72 hours. This removes the metal catalyst layer 20, the graphene 24 formed on the surface of the metal catalyst layer 20, and the nanodiamonds 13 from the sapphire substrate 10. In this way, the surface of the graphene 14 formed over the entire surface of the sapphire substrate 10 is exposed.

[Verification of the effect of temperature in the first lamination process]
Five types of samples were prepared by carrying out the first lamination step by changing the temperature at which Ni crystals were grown to five different temperatures in the first lamination step in the method for producing a metal catalyst layer in Example 1. The samples were produced by setting the temperatures at which Ni crystals were grown by molecular beam epitaxy (MBE) in the first lamination step to 350° C., 450° C., 550° C., 600° C., and 700° C. Hereinafter, the samples are simply referred to as the 350° C. sample, the 450° C. sample, the 550° C. sample, the 600° C. sample, and the 700° C. sample.

The results of observing these five types of samples with a SEM (scanning electron microscope) are shown in Figures 2(A) to 2(E). In the 350°C sample, Ni is layered on the surface of the sapphire substrate 10 with a slightly rough surface. In the 450°C sample, Ni is layered almost uniformly and flatly on the surface of the sapphire substrate 10. In the 550°C sample, Ni is layered almost uniformly and flatly on the surface of the sapphire substrate 10, but there are some unevennesses in the thickness of the Ni. In the 600°C sample, Ni is layered almost uniformly and flatly on the surface of the sapphire substrate 10, but the unevenness of the Ni thickness is more noticeable than in Example 4. In the 700°C sample, the unevenness of the Ni thickness is more noticeable than in Examples 4 and 5, and the Ni surface grows slightly granular.

These five types of samples were measured using the in-plane X-ray diffraction measurement method, and the results are shown in Figure 3. In the in-plane method used here, X-rays were incident almost parallel to the surface (c-plane) of the sapphire substrate 10 on which the first lamination process had been carried out, a light receiving unit for receiving the diffracted X-rays was fixed, and the diffracted X-ray intensity was measured by rotating the sapphire substrate 10 around the c-axis.

As shown in FIG. 3, the sapphire substrate 10 in each sample has steep peaks at every 60°, i.e., 0°, 60°, 120°, and 180°. When the surface of the sapphire substrate 10 is viewed from the front, O (oxygen) and Al (aluminum) are arranged as shown in the schematic diagram of FIG. 4. The position of the peak of the sapphire substrate 10 in FIG. 3 corresponds to the diffracted X-rays from the a-plane (11-20) in the crystal of the sapphire substrate 10. The a-plane (11-20) is a plane in the crystal of the sapphire substrate 10 that is perpendicular to the a-axis [11-20] shown in FIG. 4.

In contrast, the samples at 350°C, 450°C, and 550°C show peaks at 30°, 90°, and 150° originating from the (01-1) plane of the first layer 11. In other words, as shown in FIG. 5(A), the [01-1] direction of the first layer 11 in the samples at 350°C, 450°C, and 550°C is shifted by 30° with respect to the a-axis [11-20] of the sapphire substrate 10, and is parallel to the m-axis [1-100] of the sapphire substrate 10.

The 600°C and 700°C samples, like the sapphire substrate 10, show steep peaks at every 60° angle (0°, 60°, 120°, and 180°) originating from the (01-1) plane of the first layer 11. In other words, it can be seen that the [01-1] direction of the first layer 11 in the 600°C and 700°C samples is parallel to the a-axis [11-20] in the crystal of the sapphire substrate 10, as shown in FIG. 5(B).

From these results, it was found that when the temperature for depositing Ni in the first lamination process is increased from 350°C to 550°C, the [01-1] direction of the first layer 11 becomes parallel to the m-axis [1-100] of the sapphire substrate 10, and when the temperature for depositing Ni in the first lamination process is increased from 600°C to 700°C, the [01-1] direction of the first layer 11 becomes parallel to the a-axis [11-20] direction of the crystal of the sapphire substrate 10.

<Example 2, Comparative Example 1, Comparative Example 2>
Next, samples of Example 2 and Comparative Example 1 were prepared by carrying out up to the first heat treatment step in the method for producing a metal catalyst layer of Example 1 on the 550°C sample and the 600°C sample. The sample of Example 2 was obtained by carrying out the second lamination step on the 600°C sample. The sample of Comparative Example 1 was obtained by carrying out the second lamination step on the 550°C sample. The sample of Comparative Example 2 was obtained by carrying out the second lamination step and the first heat treatment step without carrying out the first lamination step.

The surfaces of the second layer 12 (i.e., the surfaces of the metal catalyst layer 20) of the samples of Example 2, Comparative Example 1, and Comparative Example 2 were observed using a differential interference microscope after the first heat treatment process, and the results are shown in Figures 6 (A) to (C). The dotted lines in the figures indicate the boundaries of the crystal grains. As shown in Figure 6 (A), it was found that the metal catalyst layer 20 of the sample of Comparative Example 1 was divided into multiple crystal grains.

In contrast, as shown in FIG. 6B, the sample of Example 2 was found to be composed of a single crystal having the same size as the surface of the sapphire substrate 10, without forming crystal grain boundaries in the metal catalyst layer 20. In other words, the metal catalyst layer 20 of Example 2 is monocrystallized as a single crystal over the entire surface of the sapphire substrate 10, and is laminated on the c-plane of the sapphire substrate 10. Therefore, it was found that when the temperature for depositing Ni in the first lamination process is set to 600°C, a metal catalyst layer 20 of a single crystal having approximately the same size as the surface of the sapphire substrate 10 is generated. Although not shown, the same results were obtained when the sample in which the temperature for depositing Ni in the first lamination process was 700°C was subjected to the first heat treatment process. Therefore, it was found that the temperature for depositing Ni in the first lamination process is preferably 600°C to 700°C.

As shown in FIG. 6(C), it was found that the second layer 12 of the sample of Comparative Example 2 is divided into multiple crystal grains, similar to the sample of Comparative Example 1. It was found that the size of the crystal grains of the second layer 12 of the sample of Comparative Example 2 is rather larger than the crystal grains of the metal catalyst layer 20 of the sample of Comparative Example 1.

Figure 7 shows the results of measuring the samples of Example 2, Comparative Example 1, and Comparative Example 2 using the out-plane X-ray diffraction measurement method. In the out-plane method performed here, X-rays were incident on the surfaces of the samples of Example 2, Comparative Example 1, and Comparative Example 2 at a predetermined angle, while the light receiving section that receives the diffracted X-rays was oriented in a direction that intersects with the surface of the sample, and the diffracted X-rays were measured.

As shown in FIG. 7, each sample has a steep peak at approximately 45°. This is a peak derived from the (111) plane of the metal catalyst layer 20. In addition, no peaks derived from the (200) and (220) planes of the metal catalyst layer 20 were observed. Therefore, it was found that the [111] direction of the metal catalyst layer 20 is oriented in the c-plane axis direction of the sapphire substrate 10 in the samples of Example 2, Comparative Example 1, and Comparative Example 2. In addition, the graph shown at the bottom in FIG. 7 is for the sample of Example 2 before the second lamination step and the first heat treatment step, and even at this stage, a slight peak derived from the (111) plane appears. It can be seen that when the first heat treatment step is performed, a steep Ni (111) peak appears as in the spectrum of Example 2.

The results of measuring the samples of Example 2, Comparative Example 1, and Comparative Example 2 using the in-plane X-ray diffraction measurement method are shown in Figure 8. As shown in Figures 8 (A) to (C), in the sapphire substrate 10 of each sample of Example 2, Comparative Example 1, and Comparative Example 2, steep peaks derived from the a-plane appear approximately every 60° at 30°, 90°, and 150° in the measurement profile of the sapphire substrate 10 on the lower side of the figure.

In contrast, as shown in FIG. 8(A), the sample of Comparative Example 1 has peaks at 60°, 120°, and 180° originating from the (01-1) plane of the metal catalyst layer 20, which are oriented 30° out of alignment with the peak of the sapphire substrate 10. As shown in FIG. 8(C), the sample of Comparative Example 2 has peaks at 0°, 60°, 120°, and 180° originating from the (01-1) plane of the second layer 12, which are oriented 30° out of alignment with the peak of the sapphire substrate 10. The average FWHM (full width at half maximum) of the peaks at 0°, 60°, 120°, and 180° of the sample of Comparative Example 2 was 3.7°. In contrast, the average FWHM of the peaks at 60°, 120°, and 180° of the sample of Comparative Example 1 was 8.3°. Therefore, it was found that the sample of Comparative Example 1 had poorer in-plane crystal orientation than the sample of Comparative Example 2.

On the other hand, as shown in FIG. 8B, the sample of Example 2 has steep peaks derived from the (01-1) plane of the metal catalyst layer 20 at approximately every 60° of 30°, 90°, and 150°, similar to the sapphire substrate 10. These peaks appear at the same positions as the peaks of the sapphire substrate 10, and it is clear that the orientation is parallel to the a-axis [11-20] direction of the peaks of the sapphire substrate 10. The average FWHM of the peaks derived from the (01-1) plane of the metal catalyst layer 20 at 30°, 90°, and 150° of the sample of Example 2 was 0.23°. This shows that the crystal orientation of the sample of Example 2 is extremely good. In addition, the maximum value of the FWHM of the peaks derived from the (01-1) plane of the metal catalyst layer 20 at 30°, 90°, and 150° of the sample of Example 2 was 0.25, and the minimum value was 0.18.

[Evaluation of graphene produced using a sample in which the Ni deposition temperature in the first lamination step was set to 650° C.]
Next, the carbon raw material stacking step and the second heat treatment step of the graphene production method of Example 1 were performed on the sample of Example 2, and the metal catalyst layer 20 was removed. The results of evaluation by Raman scattering spectroscopy are shown in Figs. 9(A) and (B). Raman scattering spectroscopy is commonly used in the evaluation of graphene. Graphene can be evaluated based on three types of Raman peaks, namely, D peak, G peak, and G' peak. The D peak is a peak derived from structural defects of graphene and edges of graphene. The G peak is a peak derived from the vibration of stretching in the plane of graphene and sp ² bonds. The G' peak is a peak related to multi-stage transition including interband transition of graphene. Note that, when the G peak and G' peak appear in the result of evaluating graphene using Raman scattering spectroscopy, it indicates that graphene has been produced. Also, by comparing the magnitudes of the G peak and the G' peak, knowledge regarding the number of stacked layers of the generated graphene can be obtained.

The results of evaluation by Raman scattering spectroscopy shown in FIG. 9(B) are those at points A and B in the image of the surface of the graphene 14 of the sample of Example 2 shown in FIG. 9(A) observed using a differential interference microscope. The graphene 14 at point A is multi-layered. The graphene 14 at point B is also multi-layered, but has fewer layers than the graphene 14 at point A. The results of evaluation by Raman scattering spectroscopy at point A were D/G of 0.01 and G'/G of 0.33. The results of evaluation by Raman scattering spectroscopy at point B were D/G of 0.08 and G'/G of 0.29. From these results, it was found that the graphene 14 formed on the surface of the sapphire substrate 10 had good crystallinity and was high-quality multi-layered graphene 14.

Figure 10(A) is an image of the surface of the graphene 14 of the sample of Example 2 at a position different from that of Figure 9, observed using a microscope. Figure 10(A) shows that the graphene near the top layer of the multi-layer graphene 14 is generated in an island shape at adjacent positions, and these island-like graphene 14 are bonded at the position of the dotted line. Figure 10(B) is a diagram showing the magnitude of the value of D/G in each part within the rectangular frame in Figure 10(A) by light and dark. In Figure 10(B), the brighter the color tone, the larger the value of D/G, and the darker the color tone, the smaller the value of D/G. As shown in Figure 10(B), the position of the dotted line in Figure 10(A) corresponds to the position where adjacent island-like graphene are bonded, but even in this part, the brightness is generally uniform. Therefore, it was found that in the sample of Example 2, in the region where multiple island-shaped graphenes 14 formed in adjacent positions are bonded, no defect regions indicated by a large D/G ratio are generated, and the bond is in a state of good crystal quality. This can be explained by the fact that the graphene layer shown below exhibits high orientation with well-aligned crystal orientation. In other words, when island-shaped graphenes 14 with aligned crystal orientation are bonded, the bonds are smoothly connected and no defects are generated.

The results of measuring the sample of Example 2 by the in-plane X-ray diffraction measurement method after the second heat treatment process are shown in FIG. 11. The results in FIG. 11 were obtained by measuring the diffraction angles of the sapphire substrate 10, the metal catalyst layer 20, and the graphene 14 at 37.8°, 76.5°, and 65.8°. As shown in FIG. 11, the peaks derived from the a-plane of the sapphire substrate 10 appear steeply at 60° intervals of 0°, 60°, and 120°. The peaks derived from the (01-1) plane of the metal catalyst layer 20 appear steeply at 60° intervals of 0°, 60°, and 120°, just like the sapphire substrate 10, and appear in the same positions as the peaks of the sapphire substrate 10.

In contrast, the (10) diffraction of graphene 14 in the sample of Example 2 appears steeply at 60° angles of 30°, 90°, and 150°. The average FWHM of the peaks of graphene 14 in the sample of Example 2 at 30°, 90°, and 150° was 0.18°. This indicates that the crystal orientation of graphene 14 produced in the sample of Example 2 is extremely good. In addition, the maximum FWHM of the peaks of graphene 14 in the sample of Example 2 at 30°, 90°, and 150° was 0.22, and the minimum was 0.14.

In order to explain the above orientation relationship, a schematic diagram showing the relationship of the atomic arrangement of each layer (sapphire substrate 10, metal catalyst layer 20, graphene 14) is shown in FIG. 12. It can be seen that the pointed direction of the six-membered ring of graphene 14 is the same as the [11-20] axial direction of the sapphire substrate 10 and the [01-1] direction of the metal catalyst layer 20. Furthermore, it can be seen that the [10] direction of graphene 14 is shifted by 30° from the a-axis direction of the sapphire substrate 10 and faces the m-axis [1-100] direction. Unlike the case of this embodiment, graphene formed on a Ni catalyst using a conventional method does not show such excellent orientation that a half-width is shown. In view of the above, it is believed that the excellent orientation of this embodiment is due to the fact that graphene 14 is precipitated and grown at the interface between the crystallized metal catalyst layer 20 and the sapphire substrate 10. That is, in an interface where the orientation (molecular arrangement) of the top and bottom (between the metal catalyst layer 20 and the sapphire substrate 10) is perfectly aligned, carbon atoms are forced to precipitate only at certain interface atomic positions, and when each carbon atom forms a six-membered ring of graphene 14, the orientation relationship of the six-membered ring is also uniquely defined. As a result, it is believed that the orientation relationship of graphene 14 has a fixed relationship with the sapphire substrate 10 and the metal catalyst layer 20 as shown above.

Example 3
FIG. 13 shows the result of observing the surface of a sample obtained by carrying out the method for producing a metal catalyst layer and the method for producing graphene in Example 1 with an optical microscope. The sapphire substrate 10 used in the sample in Example 3 has a rectangular shape of 10 mm×15 mm. As shown in FIG. 13, it was found that the sample in Example 3 has high-quality graphene 14 with few defects produced over the entire surface of the sapphire substrate 10 in the image. The graphene 14 is present over the entire surface in FIG. 13. The white island-like regions in contrast correspond to multi-layer graphene 14 having a large number of layers.

For example, in the sample of Comparative Example 1, the metal catalyst layer 20 is divided into multiple crystal grains. Therefore, when graphene 14 is produced on the surface of the sapphire substrate 10 using the sample of Comparative Example 1, carbon atoms diffuse at low temperatures in the areas where the crystal grain boundaries are located, and poor-quality graphene with many defects is formed during the heating process. In addition, the orientation of the metal catalyst layer 20 in the sample of Comparative Example 1 differs for each grain. Therefore, the orientation of the island-like graphene 14 grown in each grain is not aligned, and defects occur in the island-like graphene 14 where they are bonded.

In contrast, in the sample of Example 3, the metal catalyst layer 20 is single-crystallized over the entire surface of the sapphire substrate 10 by carrying out the method for producing the metal catalyst layer of Example 1, there are no grain boundaries, carbon does not diffuse into grain boundaries at low temperatures, and high-quality graphene grows. Therefore, the graphene 14 in the sample of Example 3 was produced without defects over the entire surface of the sapphire substrate 10. In addition, because the metal catalyst layer 20 is single-crystallized and has no grains, the orientation direction of the graphene islands is uniform, and even when the graphene islands are bonded to each other, no defects occur at the bond between adjacent graphene islands.

In addition to the above-mentioned high quality, when the metal catalyst layer 20 single-crystallized by the method of the present invention is used, wrinkles do not occur in the graphene produced when, for example, after the second heat treatment process for precipitation growth, the graphene is cooled at a temperature drop rate slower than about 30°C/min. During cooling, the sapphire substrate 10, the metal catalyst layer 20, and the graphene 14 each shrink differently and non-uniformly due to differences in their respective thermal expansion coefficients. For this reason, in conventional methods, stress is applied to the graphene 14, causing wrinkles in the graphene 14.

However, in the sample of Example 3, graphene 14 is generated between the sapphire substrate 10 and the metal catalyst layer 20, and since the graphene 14 does not have a bond to the sapphire substrate 10 and the metal catalyst layer 20, the sapphire substrate 10 and the metal catalyst layer 20 on the top and bottom tend to slide against each other. This makes it possible to match the different thermal contraction relationships, and with the slow cooling rate (about 30°C/min) as described above, sufficient sliding occurs between the sapphire substrate 10 and the metal catalyst layer 20 over the entire substrate surface, completely relieving stress, and preventing the graphene 14 from wrinkling.

At this time, graphene 14 is generated between the sapphire substrate 10 and the metal catalyst layer 20, and a structure in which graphene 14 is sandwiched between the sapphire substrate 10 and the metal catalyst layer 20 from above and below in a flat gap (i.e., a sandwich structure) is also effective in preventing wrinkles from being formed. In the conventional method in which the metal catalyst layer 20 has many grains, a larger gap is generated between the metal catalyst layer 20 and the sapphire substrate 10 at the grain boundaries than the surrounding area. In addition, the metal catalyst layer may shift at the grain boundaries, making the graphene 14 more likely to deform at these areas, and wrinkles may be formed in the graphene 14. Therefore, in this embodiment, the metal catalyst layer 20 is single crystallized and does not have grains, which is also considered to be one of the reasons why wrinkles do not occur. This wrinkle-free state of graphene 14 is maintained even after the metal catalyst layer 20 is removed by etching, and wrinkle-free graphene 14 can be obtained over the entire surface of the sapphire substrate 10.

The diameter of a c-plane sapphire substrate currently available is approximately 6 inches. Therefore, according to the method for producing a metal catalyst layer of the present invention, it is possible to produce a single crystal of a metal catalyst layer 20 having a size roughly similar to the surface of a c-plane sapphire substrate having a diameter of approximately 6 inches. Therefore, when the method for producing a metal catalyst layer of the present invention and the method for producing graphene of the present invention are performed using a c-plane sapphire substrate having a diameter of 6 inches, it is possible to produce graphene 14 having no wrinkles with an area of approximately 180 cm ^2. As described above, according to the present invention, as shown in FIG. 13, it is possible to produce graphene 14 having no wrinkles in an area at least larger than 0.01 cm ² , which was difficult to achieve with the conventional method. In other words, the graphene 14 of the present invention has no wrinkles in an area of 0.01 cm ² to 180 cm ² .

Next, we will explain the effects of the above example.

The metal catalyst layer 20 of the present invention is layered and crystallized on the c-plane of a sapphire substrate 10 having a corundum structure. The metal catalyst layer 20 has a face-centered cubic structure. The [111] direction of the metal catalyst layer 20 is the c-plane axial direction of the sapphire substrate 10. The [01-1] direction of the metal catalyst layer 20 is the [11-20] direction of the sapphire substrate 10. By using this metal catalyst layer 20, it is possible to produce graphene 14 of good quality.

The method for producing a metal catalyst layer of the present invention includes a first lamination step of forming a first layer 11 of crystallized Ni on the surface of a sapphire substrate 10, a second lamination step of forming a second layer 12 of amorphous Ni on the surface of the first layer 11, and a first heat treatment step of performing a heat treatment to crystallize the second layer 12 after performing the first lamination step and the second lamination step. With this configuration, a metal catalyst layer 20 with good crystal quality can be produced.

The graphene production method of the present invention includes a carbon raw material lamination step of laminating nanodiamonds 13 on the surface of the metal catalyst layer 20 after performing the first heat treatment step in the above-mentioned metal catalyst layer production method, and a second heat treatment step of performing a heat treatment to precipitate graphene 14 between the sapphire substrate 10 and the metal catalyst layer 20 after performing the carbon raw material lamination step. This configuration makes it easy to produce large-area graphene 14 with good crystallinity and no wrinkles at the interface between the sapphire substrate 10 and the metal catalyst layer 20.

The graphene 14 of the present invention has no wrinkles over an area of 0.01 cm ² to 180 cm ^2. This configuration allows it to be used in devices requiring a large area, such as integrated circuits.

The carbon raw material in the graphene production method of the present invention is nanodiamond 13. With this configuration, graphene 14 can be continuously grown during heat treatment at a growth rate determined by the heat treatment temperature, and the number of layers of graphene 14 produced at the interface between the sapphire substrate 10 and the metal catalyst layer 20 can be easily controlled.

The present invention is not limited to the first and second embodiments described above with reference to the drawings, and the following embodiments are also included within the technical scope of the present invention.
(1) In the first embodiment, etching is performed using a solution of ferric chloride and pure water. However, this is not limited to this, and etching may also be performed using nitric acid, sulfuric acid, hydrochloric acid, hydrogen peroxide, or a mixture of these acids.
(2) In the first embodiment, a sapphire substrate having a c-plane ((0001) plane) as its surface is used. However, the present invention is not limited to this, and gallium oxide or the like may also be used.
(3) In the first embodiment, Ni is used for the first and second layers, but this is not limiting, and other metals including transition metals such as Cu, Co, Fe, Mn, W, Re, Os, Ir, Pt, Au, etc. may be used. Of course, alloys such as AuNi and NiCu may also be used.
(4) In the first embodiment, nanodiamond is used as the carbon raw material. However, this is not limiting, and other carbon raw materials such as amorphous carbon may be used.
(5) In the first embodiment, the MBE method is used, but other methods capable of forming a first layer of crystallized catalytic metal may be used. For example, a metal catalyst layer may be formed by a vapor phase growth method. In the first embodiment, the electron beam deposition method is used, but other methods capable of forming a second layer of amorphous catalytic metal on the surface of the first layer, such as a deposition method using resistance heating or a lamination method using plating, may be used.
(6) In the first embodiment, the thickness of the first layer is 100 nm, but the thickness is not limited to this and may be 20 nm to 300 nm.
(7) In the first embodiment, the thickness of the second layer is 300 nm, but the thickness is not limited to this and may be 200 nm to 700 nm.
(8) Within the scope of the present invention, the two-dimensional material to be precipitated is not limited to graphene, but may be other two-dimensional materials such as _MoS2 , _MoTe2 , WS, Mo1 _{- xWxS2} , _{FeTiS2 , TaSe2} , BN, _WTe2, etc. Of course, a multilayer structure in which these two-dimensional materials are layered may also be used.

10...Sapphire substrate (substrate)
11: First layer 12: Second layer 13: Nano diamond (carbon raw material)
14... Graphene 20... Metal catalyst layer

Claims (4)

a first lamination step for forming a first layer of a crystallized catalytic metal on a c-plane of a substrate having a corundum structure;
a second lamination step of forming a second layer of the catalytic metal of the same type as the first layer in an amorphous state on a surface of the first layer;
a first heat treatment step of performing a heat treatment to crystallize the second layer after the first lamination step and the second lamination step to generate a metal catalyst layer having the first layer and the second layer;
Equipped with
The metal catalyst layer has a face-centered cubic structure,
the [111] direction of the metal catalyst layer is the c-plane axial direction of the substrate,
The method for producing graphene , wherein the [01-1] direction of the metal catalyst layer is the [11-20] direction of the substrate. a carbon raw material lamination step of laminating a carbon raw material on a surface of the metal catalyst layer after the first heat treatment step is performed;
a second heat treatment step of carrying out a heat treatment after carrying out the carbon raw material lamination step to precipitate graphene between the substrate and the metal catalyst layer;
2. The method of producing graphene according to claim 1 , comprising : The method for producing graphene according to claim 2 , wherein the carbon raw material is nanodiamond . 2. The method of claim 1 , which produces wrinkle-free graphene over an area of 0.01 cm2 ^to 180 cm2 ^.

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