Graphene film and method for producing the same

Abstract

An object of the present disclosure is to provide a structure of graphene as a SERS substrate that enhances Raman scattering and a method for manufacturing the same.SOLUTION: According to an embodiment of the present disclosure, there is provided a graphene film including one or a plurality of graphene layers, wherein at least one of the graphene layers is provided with a plurality of through holes and has a plurality of continuous zigzag edges on inner peripheries of the through holes.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to surface-enhanced Raman scattering by graphene. [Background technology]

[0002] Graphene has been proposed as one of the substrates that induces surface-enhanced Raman scattering (SERS) in spectroscopic analysis (see, for example, Non-Patent Document 1).

[0003] The following two mechanisms have been proposed for SERS: The first mechanism is the electromagnetic mechanism, hereafter sometimes abbreviated as EM (Electro-magnetic Mechanism). The second mechanism is a chemical mechanism, hereafter abbreviated as CM (Chemical Mechanism). [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Seiya Suzuki, “Synthesis of Graphene-based Materials for Surface- Enhanced Raman Scattering Applications”, eJ. Surf. Sci. Nanotechnol. 17, 71-82 (2019) Summary of the Invention [Problem to be solved by the invention]

[0005] There is an EM-based "SERS substrate" that can be used as a "substrate" on which the molecules to be measured are placed, but it has the following problems. Resonance with LSP can generate large amounts of heat, which can oxidize the SERS substrate or alter the measurement target. However, LSP stands for Localized Surface Plasmon. The substrate is not uniform, making it difficult to reproducibly fabricate uniform SERS substrates.

[0006] One possible solution to these problems is the development of a SERS substrate based on CM, and graphene is one candidate for such a SERS substrate. Previously, it was confirmed that Raman scattering was enhanced using graphene as a substrate in CM. However, the enhancement was smaller than in EM, and the Raman scattering light generated when a substance was irradiated with light was weak, which meant that the sensitivity of spectroscopic analysis could not be sufficiently improved.

[0007] The present disclosure has been made to solve such conventional problems, and aims to provide a graphene structure as a SERS substrate that enhances Raman scattering and a method for producing the same. [Means for solving the problem]

[0008] The graphene film described in claim 1 of the present invention is a graphene film (92) having one or more layers of graphene (21), characterized in that at least one of the graphenes has a plurality of through holes (22) formed therein and has a plurality of continuous zigzag edges on the inner periphery of the plurality of through holes.

[0009] The graphene film according to claim 1 of the present invention employs a graphene film having continuous zigzag edges, and thereby achieves a Raman scattering enhancement effect greater than a SERS substrate employing a graphene film without continuous zigzag edges based on CM. Therefore, the graphene film according to claim 1 of the present invention can provide a SERS substrate with high sensitivity in spectroscopic analysis.

[0010] The graphene film according to claim 2 of the present invention is characterized in that, when n is a positive integer of 2 or more, the one or more layers of graphene are n-layers of graphene, and the plurality of through holes are provided at the same through-points of the n-th layer and the (n-1)-th layer of graphene.

[0011] The graphene film according to claim 3 of the present invention is characterized in that the n-layer graphene is a two-layer graphene. The graphene film according to claim 4 of the present invention is characterized in that the plurality of through holes have a polygonal shape. The graphene film according to claim 5 of the present invention is characterized in that the polygon is a regular hexagon having zigzag edges on each side.

[0012] A method for producing a graphene film (92) according to claim 6 of the present invention is characterized in that one or more layers of graphene (21) are formed, and a plurality of through holes (22) are formed in at least one of the one or more layers of graphene using hydrogen plasma etching, thereby forming a plurality of continuous zigzag edges on the inner periphery of the through holes.

[0013] The method for producing a graphene film according to claim 6 of the present invention can form continuous zigzag edges. Therefore, the method for producing a graphene film according to claim 6 of the present invention can achieve a higher Raman scattering enhancement effect than a SERS substrate that uses a CM-based graphene film that does not have continuous zigzag edges, thereby providing a SERS substrate with high sensitivity in spectroscopic analysis.

[0014] The method for producing a graphene film according to claim 7 of the present invention is characterized in that the one or more layers of graphene are n-layers of graphene, where n is a positive integer equal to or greater than 2. In this case, the method for producing a graphene film according to claim 7 of the present invention is characterized in that the plurality of through-holes are formed at the same penetration points of the n-th layer and the (n-1)-th layer of graphene using hydrogen plasma etching.

[0015] The method for producing a graphene film according to claim 8 of the present invention is characterized in that the n-layer graphene is two-layer graphene. The method for producing a graphene film according to claim 9 of the present invention is characterized in that the plurality of through holes have a polygonal shape. The method for producing a graphene film according to claim 10 of the present invention is characterized in that the polygon is a regular hexagon having zigzag edges on each side.

[0016] The above disclosures can be combined as much as possible. [Effects of the Invention]

[0017] According to the present disclosure, a Raman scattering enhancement effect can be obtained compared to a SERS substrate employing a graphene film based on CM that does not have continuous zigzag edges, thereby providing a SERS substrate with high sensitivity in spectroscopic analysis. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is an embodiment of a graphene film of the present disclosure. [Figure 2] An example of a graphene film is shown. [Figure 3] An example of graphene is shown below. [Figure 4] 1 shows an example of a through hole. [Figure 5] 1 illustrates an example of the use of the graphene film of the present disclosure. [Figure 6] 1 shows an example of a measurement result using the graphene film of the first embodiment. [Figure 7] 10 is a comparative example of the first embodiment. [Figure 8] FIG. 1 is an AFM (Atomic Force Microscope) image of the sample used in the measurement. [Figure 9] An example of the R6G concentration dependence of Raman peak intensity is shown. [Figure 10] 10 shows a comparative example of Raman peak intensity when multiple graphenes are provided. [Figure 11]10 shows a comparative example of the edge factor E when multiple graphenes are provided. [Figure 12] An example of a graphene film is shown. [Figure 13] An example of the contribution of the zigzag edge to the R6G concentration dependence is shown. [Figure 14] FIG. 10 is an explanatory diagram of parameters. [Figure 15] This is an example (minimum value) of the arrangement of through holes when d is changed, where two hexagons are in contact with each other. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. Note that the present disclosure is not limited to the embodiments shown below. These implementation examples are merely illustrative, and the present disclosure can be implemented in various forms with various modifications and improvements based on the knowledge of those skilled in the art. Note that components with the same reference numerals in this specification and drawings indicate the same components.

[0020] (First embodiment) An embodiment of the graphene film of the present disclosure is shown in Figure 1. A graphene film 92 of this embodiment is laminated on a support substrate 91. The support substrate 91 may be, for example, a SiO2 substrate or a Si substrate.

[0021] The graphene film 92 includes one or more layers of graphene 21. For example, as shown in FIG. 1A, one layer of graphene 21 may be stacked on a support substrate 91. As shown in FIG. 1B, n layers of graphene 21 may be stacked on the support substrate 91, where n is a positive integer greater than or equal to 2. The number of layers of graphene 21 is arbitrary. For example, as shown in FIG. 1B, a first layer of graphene 21#1, a second layer of graphene 21#2, ..., an nth layer of graphene 21#n may be stacked in this order from the support substrate 91 side. For example, a third layer of graphene and a fourth layer of graphene (not shown) may be stacked. Hereinafter, when there is no need to distinguish between one layer and multiple layers, the term "graphene 21" will be used.

[0022] FIG. 2 shows an example of the n-th layer of graphene 21#n shown in FIG. 1(B). The graphene 21#n has a mesh structure formed by a plurality of through-holes 22 formed in the graphene 21. As shown in FIG. 3, the graphene 21 has a carbon atom C and three neighboring carbon atoms in a sp 2 The hexagonal honeycomb structure is formed by bonding the layers together. In this embodiment, an example is shown in which through-holes 22 having a diagonal length φ are arranged so as to be evenly dispersed at intervals d. The single layer of graphene 21 shown in FIG. 1(A) also has a similar structure to the n-th layer of graphene 21#n.

[0023] A plurality of through holes 22 having zigzag ends may be provided at the same penetration location of the n-th graphene layer 21#n and the (n-1)th graphene layer 21#n-1. For example, when n=2, a through hole 22 may be provided at the same penetration location of the second graphene layer 21#2 and the first graphene layer 21#1. When n=3, a through hole 22 may be provided at the same penetration location of the first graphene layer 21#1, the second graphene layer 21#2, and the third graphene layer 21#3. Note that n may be any positive integer equal to or greater than 2.

[0024] In the method for producing a graphene film according to the present embodiment, for example, one or n-th layers of graphene 21 are formed on a support substrate 91, and a plurality of through-holes 22 are formed by passing through the one or n-th layers of graphene 21 using hydrogen plasma etching. Alternatively, a plurality of through-holes 22 are formed by passing through successive layers of the n-th layer of graphene 21, such as the n-th layer to the n-1th layer and the n-2th layer. This allows the production of a graphene film according to the present disclosure. Here, the through-holes 22 can be formed in the one or n-th layers of graphene 21 after the one or n-th layers of graphene 21 are formed, with the through-holes 22 having the same diameter and at the same location. However, it is also possible to form a through-hole 22 each time a layer of graphene 21 is formed.

[0025] Furthermore, the graphene 21 formed on the support substrate 91 may be transferred to another support substrate for use.

[0026] To form the graphene 21, graphene flakes obtained by cleaving graphite, for example, can be used on the support substrate 91. This allows for a high-quality graphene thin film with few defects to be obtained. Furthermore, if hydrogen plasma etching is used to form the through-holes 22, a zigzag edge structure can be formed on all sides of the inner periphery of the through-holes 22 while maintaining the honeycomb structure of the graphene 21. In other words, the entire inner periphery of the through-holes 22 has zigzag edges. For example, the through-holes 22 can be formed as polygons having zigzag edges of a desired length on each side.

[0027] FIG. 4 shows an example of the through-hole 22. The polygon is a regular hexagon with zigzag edges on each side. 13 from carbon C 17 The carbon C has a zigzag structure. 12 from carbon C 18 It can be thought of as a zigzag structure. 11 from carbon C 19 It can be thought of as a zigzag structure, with carbon C 20 from carbon C 24 The carbon C has a zigzag structure. 19 from carbon C 25 It can be thought of as a zigzag structure. 20 from carbon C 26 It can be considered a zigzag structure.

[0028] During spectroscopic analysis, for example, as shown in FIG. 5 , a graphene film 92 transferred to a transparent support substrate 91A is placed in contact with an object 81, and measurement light 83 from a light source 82 is irradiated from the transparent support substrate 91A side. The distance d shown in FIG. 2 is less than the beam diameter of the measurement light 83. The measurement light 83 passes through the transparent support substrate 91A and the graphene film 92 and is irradiated onto the object 81, and Raman scattered light from the object 81 is enhanced via the graphene film 92. This Raman scattered light 84 is observed by a photodetector 85.

[0029] The form of spectroscopic analysis is not limited to that shown in Fig. 5, and the support substrate 91 does not have to be transparent. For example, when the specimen is a liquid, the specimen may be dropped onto a graphene film on a support substrate, irradiated with a light source, and the Raman scattered light may be observed with a photodetector.

[0030] The graphene film 92 of this embodiment has a graphene nanomesh structure with zigzag edges, in which a plurality of through holes 22 with a plurality of continuous zigzag edges are provided, and therefore a strong Raman scattering enhancement effect can be obtained. Hereinafter, the graphene nanomesh structure with zigzag edges provided in the graphene film 92 of this embodiment will be referred to as "zGNM."

[0031] (Second embodiment) Figure 6 shows an example of measurement results using the graphene film 92 of the first embodiment. Figure 7 shows a comparative example without through-holes 22. The graphene used was three-layer graphene flakes obtained by cleaving graphite on a SiO2 / Si substrate. This allows for high-quality graphene with few defects to be obtained.

[0032] 8, an unprocessed region (hereinafter sometimes referred to as the unprocessed region) and a region of the graphene film 92 of the first embodiment (hereinafter sometimes referred to as the zGNM region) were fabricated in one flake of the graphene element, and Raman scattering from these two regions was measured in a single spectroscopic mapping. This makes it possible to ignore uncertainties such as changes in light intensity for each measurement, contamination on the graphene surface, and the degree of adhesion to the support substrate.

[0033] To investigate the effect of the zigzag edges on Raman scattering enhancement, hexagonal through-holes 22 were arranged in a triangular lattice pattern at 200 nm intervals in the zGNM region so that many zigzag edges could be obtained within a measurement beam diameter of approximately 700 nm. The diagonal length φ of the through-holes 22 was set to approximately 100 nm. The hexagonal holes visible in the zGNM region in Figure 8 are the through-holes 22, and the SiO2 of the support substrate 91 is exposed within the through-holes 22. In this case, the ends of the through-holes 22 form the zigzag edges of the graphene.

[0034] A laser Raman microscope with a laser beam of 532 nm wavelength was used for the Raman scattering measurements. The laser Raman microscope used was a Renishaw inVia Raman microscope. In order to compare the spectra obtained from each measurement, all of the Raman spectroscopy spectra shown below are measured at ~950 cm. -1 The spectral intensity is normalized by the intensity of the Raman peak originating from Si, which appears at the surface of the element. This allows the spectral intensity to be considered as the intensity per unit area of ​​the element. R6G (rhodamine 6G) molecules, which are common in this field, were used as the probe molecule to investigate the SERS effect. The R6G molecules were adsorbed onto the graphene element by immersing the element in R6G solutions with concentrations of 0.1, 1, 10, and 100 μM, which correspond to approximately 0.05, 0.5, 5, and 50 ppm.

[0035] Figures 6 and 7 show peak decomposition at the measurement points using the Bayesian method. In both cases, the spectrum is resolved into the G and D bands derived from graphene, as well as peaks a through m derived from R6G molecules. The G band originates from the honeycomb structure of graphene, and its Raman peak intensity is proportional to the area of ​​the graphene. The G band is smaller in the zGNM region than in the unprocessed region. On the other hand, the D band originates from intracrystalline scattering called intervalley scattering, and is proportional to the amount of defects in the honeycomb structure. Therefore, the D band is very small in the unprocessed region, but appears in the microfabricated zGNM region.

[0036] Among the multiple peaks originating from the R6G molecules, we focused on the peaks named i=a, k, and q, and the dependence of the Raman peak intensity on the R6G concentration is shown in Figure 9. The Raman peak intensity P per unit element area in the unprocessed region i The Raman peak intensity M per unit element area on the support substrate 91 is indicated by a white circle. i are shown as black circles. For ease of viewing, each point is slightly shifted along the horizontal axis. P i and M i becomes large, and at any concentration, M i P iIt can be seen that the Raman scattering is enhanced more strongly on the zGNM region than on the unprocessed region.

[0037] On the SiO2 surface, there is almost no enhancement of Raman scattering, and no peaks due to R6G molecules are observed. Therefore, the observation of peaks due to R6G molecules on both substrates indicates that both the unprocessed and zGNM regions have the effect of enhancing Raman scattering from R6G molecules.

[0038] Considering that no peaks due to R6G molecules are obtained on the SiO2 surface, it is noteworthy that the Raman peak intensity per element area is higher in the zGNM region than in the all-graphene substrate, even though the substrate area contributing to the enhancement of Raman scattering from R6G molecules is small. This suggests that there is a strong Raman scattering enhancement effect at the zigzag edges around the through-holes 22, which is present in the zGNM region but not in the unprocessed region.

[0039] (Third embodiment) In this embodiment, when the number of graphene layers is one, the through holes of graphene 21 do not have zigzag edges but are circular holes with a random edge structure. Graphene 21 when the number of graphene layers is two, three, and four has hexagonal through holes 22 with zigzag edges. The arrangement of through holes 22 is the same from the first to second layers when the number of graphene layers is two, from the first to third layers when the number of graphene layers is three, and from the first to fourth layers when the number of graphene layers is four.

[0040] 10 and 11 show comparative examples of the effect of providing multiple layers of graphene 21. FIG. 10 shows the Raman scattering intensity (arb. unit) per unit element area of ​​peak i, and FIG. 11 shows the edge factor E(i) (arb. unit) of peak i. The edge factor E is an index representing the effect of the edge. In this disclosure, the Raman peak intensity per unit zigzag edge length [1 / μm] when the zigzag edge is straight is used. As shown in FIGS. 10(a) and 11(a), 0.1 μM R6G molecules were used to compare the first to third layers. As shown in FIGS. 10(b) and 11(b), 1 μM R6G molecules were used to compare the second to fourth layers. The comparison results showed that the Raman peak intensity and edge factor E were maximum for two layers and decreased as the number of layers increased.

[0041] Therefore, it can be seen that by using two layers of graphene 21, a higher Raman scattering enhancement effect can be obtained compared to the cases where the number of graphene layers is other than two.

[0042] (Fourth embodiment) In the graphene film 92 of this embodiment, no through-holes 22 are formed in the first graphene layer 21#1. In this embodiment, as shown in Fig. 12 , the measurement light 83 that passes through the through-holes 22 in the graphene 21#2 is irradiated onto the first graphene layer 21#1. Therefore, enhanced Raman scattering by the graphene 21#1 can be obtained also from inside the through-holes 22.

[0043] For example, the first layer of graphene 21#1 may have the through-holes 22, the second layer of graphene 21#2 may not have the through-holes 22, and the third layer of graphene 21#3 may have the through-holes 22.

[0044] Furthermore, for example, the first layer of graphene 21#1 may have the through-holes 22, the second layer of graphene 21#2 may have the through-holes 22, and the third layer of graphene 21#3 may not have the through-holes 22.

[0045] It should be noted that the combinations of the graphene 21#n and the presence or absence of the through-hole 22 are not limited to these.

[0046] (Fifth embodiment) Fig. 13 shows an example of the contribution of the zigzag edge to the R6G concentration dependence. From the graph showing the edge factor E, it can be seen that R6G is selectively adsorbed to the zigzag edge. Therefore, in this embodiment, it is assumed that the Raman peak intensity obtained at the zigzag edge is the sum of the contribution from graphene 21 and the contribution from the zigzag edge, as shown in Fig. 14.

[0047] Raman peak intensity M[1 / μm 2 ] is expressed by the following formula: (Number 1) S0M=S1·P+L·E (1) where the parameters are as follows: S0 is the area of ​​the unit cell. S1 is the area of ​​graphene in a unit cell. M is the Raman peak intensity per unit element area on graphene 21 of zGNM (with holes) [1 / μm 2 ]. P is the Raman peak intensity per unit element area on pristine (hole-free) graphene 21 [1 / μm 2 ]. L is the length of the zigzag edge in the unit cell [μm]. E is the Raman peak intensity per unit zigzag edge length [1 / μm].

[0048] S0 is expressed by the following equation using the interval d between the through holes 22.

number

[0049] S1 is expressed by the following equation using the interval d between the through holes 22 and the length a of one side of the hexagonal through holes 22.

number

[0050] When the length of one side of the hexagonal through-hole 22 is a, the length L of the zigzag end is the sum of the six sides, so it is 6a. Furthermore, in this embodiment, the following equation holds between the interval d of the through-holes 22 and the length a.

number

[0051] Therefore, in this embodiment, the following equation holds true:

number

[0052] Transforming equation (5) gives the following equation:

number

number

[0053] When regular hexagons are arranged in a triangular lattice, the unit cell shown by the dotted line is a rhombus made up of equilateral triangles with a side length of d. In this case, the lattice spacing d is shortest when the regular hexagons are closest together, as shown in Figure 15, and in this case d = √3a.

[0054] In this embodiment, an example in which the through-holes 22 are hexagonal is shown, but the same applies to other shapes of the through-holes 22.

[0055] Furthermore, the smaller the spacing d between the through-holes 22, the stronger the Raman peak intensity, and further, the Raman peak intensity is maximized when the through-holes 22 are closely packed together. In other words, the thinner and denser the mesh structure, the greater the enhancement.

[0056] (Other embodiments) In the above-described embodiment, graphene 21 may be transferred from a SiO2 substrate or a Si substrate to a flexible substrate. This allows the support substrate 91 to be a flexible substrate, making it possible to easily perform spectroscopic analysis on curved surfaces such as fruit and human skin. As shown in FIGS. 6 and 7, the present disclosure can enhance Raman scattering in each wavelength band. Therefore, the present disclosure allows the graphene film 92 of this embodiment to be attached to the surface of a measurement target, thereby enabling spectroscopic analysis of trace substances present on the surface of the measurement target.

[0057] Therefore, the following measurements are possible: - Testing for coronavirus and other viruses Criminal investigation (detection of drugs, poisons, bloodstains, etc.) Food testing (detection of pesticides and food additives, etc.) Diagnosis of illness and health checks (e.g., examining physical condition and illness from components of sweat, tears, exhaled breath, and saliva) · Guessing the origin of wine or coffee and classifying its flavor [Explanation of symbols]

[0058] 21: Graphene 22:Through hole 91: Support substrate 91A: Transparent support substrate 92: Graphene film

Claims

1. A graphene film having one or more layers of graphene, At least one of the graphenes has a plurality of through holes, The through holes have a plurality of continuous zigzag edges on the inner periphery thereof. Graphene membrane.

2. When n is a positive integer of 2 or more, the one-layer or multiple-layer graphene is n-layer graphene, the plurality of through holes are provided at the same penetration points of the n-th layer and the (n-1)th layer of graphene; The graphene film of claim 1 .

3. The n-layer graphene is a two-layer graphene. The graphene film of claim 2.

4. The shape of the plurality of through holes is polygonal. The graphene film of claim 1 .

5. The polygon is a regular hexagon with zigzag edges on each side. The graphene film of claim 4.

6. forming one or more layers of graphene; forming a plurality of through holes in at least one of the one or more layers of graphene using hydrogen plasma etching, thereby forming a plurality of continuous zigzag edges around the inner periphery of the through holes; A method for producing graphene films.

7. When n is a positive integer of 2 or more, the one-layer or multiple-layer graphene is n-layer graphene, The plurality of through holes are formed at the same penetration points of the n-th and (n-1)-th graphene layers using hydrogen plasma etching. The method for producing a graphene film according to claim 6 .

8. The n-layer graphene is a two-layer graphene. The method for producing a graphene film according to claim 7 .

9. The shape of the plurality of through holes is polygonal. The method for producing a graphene film according to claim 6 .

10. The polygon is a regular hexagon with zigzag edges on each side. The method for producing a graphene film according to claim 9 .

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