JP6713114B2 - Transparent conductive film containing graphene - Google Patents

Description

The present invention relates to a transparent conductive film including graphene and its protective film.

A conductive planar crystal formed of SP ² -bonded carbon atoms is called a graphene film. Non-Patent Document 1 describes the graphene film in detail. Graphene films are the basic unit of various forms of crystalline carbon films. Examples of crystalline carbon films composed of graphene films are single-layer graphene composed of one graphene film, nanographene that is a laminated body of several to ten nanometer-sized graphene films, and several to several tens of graphene films. There is a carbon nanowall (see Non-Patent Document 2) in which a graphene film laminated body of about layers is oriented at an angle close to perpendicular to the substrate surface. A crystalline carbon film composed of a graphene film is expected to be used as a high frequency device because of its high mobility, and as a transparent conductive film or a transparent electrode because of its high light transmittance.

As a method for producing a graphene film, a method of exfoliating from natural graphite, a method of desorbing silicon by high temperature heat treatment of silicon carbide, a method of forming on various metal surfaces, etc. have been developed so far. Electronic devices using a crystalline carbon film composed of a graphene film have been studied for various industrial applications. Therefore, the advent of a film forming method capable of forming a large-area graphene film with high throughput is desired. A method of forming a graphene film on the surface of a copper foil by a chemical vapor deposition method (CVD) is known (see Non-Patent Document 3 and Non-Patent Document 4). Microwave surface wave plasma CVD (see Non-Patent Document 5), which is one of them, can form a graphene film at a low temperature and in a short time, so that electrons on a substrate such as plastic having low heat resistance can be formed. Fabrication of devices is expected.

International Publication No. 2012/108526

Kumi Yamada, Chemistry and Industry, 61 (2008) pp.1123-1127 Y. Wu, P. Qiao, T. Chong, Z. Shen, Adv. Mater., 14 (2002) pp.64-67 X. Li, W. Cai, J. An, S. Kim, J. Nah, D. Yang, R. Piner, A. Velamakanni, I. Jung, E. Tutuc, SK Banerjee, L. Colombo, RS Ruoff, Science, 324 (2009) pp.1312-1314 X. Li, Y. Zhu, W. Cai, M. Borysiak, B. Han, D. Chen, R. D. Piner, L. Colombo, R. S Ruoff, Nano Letters, 9 (2009) pp.4359-4363 J. Kim, M. Ishihara, Y. Koga, K. Tsugawa, M. Hasegawa, S. Iijima, Applied Physics Letters 98 (2011) pp.091502-1-091502-3

Graphene synthesized by CVD is formed on a catalytic metal such as Cu or Ni, as described in Non-Patent Documents 3 to 5. Therefore, in order to form graphene on an arbitrary substrate, a technique for transferring graphene from the catalytic metal onto the substrate is required. A method of transferring graphene using a polymer such as polymethylmethacrylate (hereinafter sometimes referred to as “PMMA”) is known.

That is, after forming the PMMA layer on the graphene on the catalytic metal, the catalytic metal is removed with an etching solution. Graphene can be formed on an arbitrary substrate by attaching the remaining PMMA layer/graphene on the arbitrary substrate and then removing the PMMA layer with acetone. In the present application, for example, “PMMA layer/graphene” refers to a laminate having a PMMA layer on graphene. The same applies to a laminate having three or more layers. FIG. 5 shows a graphene transfer laminate 20 having a structure of graphene 24/substrate 22 obtained by this method.

It was observed under a microscope that the graphene from which the PMMA layer was removed may crack. It is considered that the cracks lead to deterioration of electric conduction characteristics of graphene. Therefore, it is necessary to establish a transfer technique that does not cause cracks in graphene or to consider a completely different method for forming graphene on an arbitrary substrate. The present invention has been made in view of the above circumstances, and an object thereof is to provide a transparent conductive film having low resistance without deterioration of graphene itself.

As a result of earnest studies to achieve the above-mentioned object, the present inventors have found that the PMMA layer of the protective film is not removed during the transfer of graphene, and the sheet resistance can be reduced with the PMMA layer/graphene as it is. It was The present invention has been completed based on this finding, and the present invention provides the following inventions.

The transparent conductive film of the present invention has a transparent base material, graphene provided on the transparent base material, and a polymethylmethacrylate layer provided on the graphene, and the thickness of the polymethylmethacrylate layer is 20. ~150 nm. In the transparent conductive film of the present invention, graphene may be composed of single-layer graphene or may be composed of two to ten layers of multilayer graphene. In the transparent conductive film of the present invention, it is preferable that the sheet resistance of the laminated portion of the graphene and the polymethylmethacrylate layer is 353Ω or less.

The touch panel of the present invention has the transparent conductive film of the present invention and a protective film for protecting the polymethylmethacrylate layer of the transparent conductive film. In the touch panel of the present invention, the protective film may be provided on the transparent conductive film via an adhesive layer. The flexible heater of the present invention includes the transparent conductive film of the present invention, a pair of contact electrodes in contact with graphene, and a power source connected to the pair of contact electrodes.

According to the present invention, a low conductive transparent conductive film can be obtained.

FIG. 3 is a schematic sectional view of the transparent conductive film according to the embodiment of the present invention. FIG. 5 is a schematic cross-sectional view for explaining the manufacturing process of the transparent conductive film of the embodiment of the present invention. The Raman spectroscopy spectrum (wide range) of the various transparent conductive films obtained in Example 2 and Example 3. The Raman spectroscopy spectrum (narrow range) of the various transparent conductive films obtained in Example 2 and Example 3. The cross-sectional schematic diagram of the conventional graphene transfer laminated body.

Hereinafter, the transparent conductive film, the touch panel device, and the flexible heater of the present invention will be described based on embodiments and examples. Overlapping description will be omitted as appropriate. In addition, when "-" is described between two numerical values to represent the numerical range, these two numerical values are also included in the numerical range.

FIG. 1 shows a transparent conductive film 10 according to an embodiment of the present invention. The transparent conductive film 10 includes a transparent base material 12, graphene 14, and a polymethylmethacrylate layer 16. The transparent substrate 12 is a thin plate member and is made of, for example, quartz. Examples of the material of the transparent substrate 12 include, in addition to quartz, a flexible transparent resin such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN). In addition, in the present application, “transparent” of the transparent substrate means a material that transmits 90% or more of white light.

The graphene 14 is provided on the transparent substrate 12. The graphene 14 may be composed of single-layer graphene. This is because single layer graphene can be easily synthesized by using the thermal CVD method. Further, the graphene 14 may be composed of two to ten layers of multilayer graphene. This is because double layer graphene having 2 to 10 layers can be easily synthesized by using the plasma CVD method.

When the graphene 14 is composed of two to ten layers of multilayer graphene, the performance such as the gas barrier property of the transparent conductive film 10 is improved as compared with the case of being composed of single layer graphene. The PMMA layer 16 is provided on the graphene 14. The PMMA layer 16 has a thickness of 20 to 150 nm. This is because when the transparent base material 12 having flexibility is used, the flexibility of the transparent conductive film 10 can be secured by setting the thickness of the PMMA layer 16 to 20 to 150 nm.

FIG. 2 shows a manufacturing process of the transparent conductive film 10. First, as shown in FIG. 2A, the graphene 14 is formed on the thin plate-shaped catalyst metal 18. The graphene 14 is obtained by the following synthesis method using plasma treatment (see Patent Document 1). That is, while heating a metal base material containing a carbon source such as an organic compound and the catalytic metal 18, the metal base material is irradiated with the energy of charged particles and electrons in plasma, thereby heating the metal base material. Activates the carbon source to generate graphene. Examples of the catalyst metal 18 include copper, iridium, platinum, and alloys of any of these metals and carbon, in which carbon is difficult to dissolve.

The graphene 14 is generated on the catalytic metal 18 by the carbon source in the metal base material, the trace amount of carbon component attached to the plasma reaction vessel, and the trace amount of carbon component contained in the plasma processing gas. According to this method, graphene can be generated in a shorter time than the conventional thermal CVD method or resin carbonization method. The carbon content in the metal substrate is preferably 4 to 10,000 ppm. Further, the surface roughness Ra of the metal substrate is preferably 200 to 0.095 nm. Furthermore, it is desirable that the temperature of the metal base material be set to 900° C. or lower to generate the graphene 14. As shown in Examples, the transparent conductive film of the present invention is not particularly limited in the method for producing graphene.

Next, as shown in FIG. 2B, a PMMA solution is applied onto the graphene 14 and dried to form a PMMA layer 16 with a thickness of 20 to 150 nm. Then, as shown in FIG. 2C, the catalytic metal 18 is removed by wet etching or the like. Finally, as shown in FIG. 2D, the PMMA layer 16/graphene 14 is placed on the transparent substrate 12 so that the transparent substrate 12 and the graphene 14 are in contact with each other. In this way, the transparent conductive film 10 which is the PMMA layer 16/graphene 14/transparent substrate 12 is obtained.

Conventionally, as shown in FIG. 5, the PMMA layer was removed from the PMMA layer/graphene/substrate and the graphene 24/substrate 22 was used. In this embodiment, the sheet resistance of the PMMA layer 16/graphene 14 can be made lower than the sheet resistance of the graphene 14 when the PMMA layer 16 is removed by not removing the PMMA layer 16. It is considered that this is because cracks hardly occur in the graphene in the manufacturing process of the transparent conductive film 10 and the stress on the graphene 14 in the PMMA layer 16/graphene 14 structure. Specifically, the sheet resistance of the PMMA layer 16/graphene 14 can be 353Ω or less.

The transparent conductive film 10 can be applied to various uses such as a touch panel and a flexible heater. That is, the touch panel according to the embodiment of the present invention includes at least one transparent conductive film 10 and a protective film that protects the PMMA layer 16 of the transparent conductive film 10. The protective film is provided on the transparent conductive film via the adhesive layer. By using the two transparent conductive films 10, a resistive film type or projected capacitive type touch panel can be obtained. Further, by applying a voltage to the four corners of the transparent conductive film 10, a surface-type capacitive touch panel can be obtained.

The flexible heater according to the embodiment of the present invention includes a transparent conductive film 10, a pair of contact electrodes that are in contact with graphene, and a power supply connected to the pair of contact electrodes. A temperature controller may be provided in the flexible heater of the present embodiment to control the amount of electric power supplied from the power source to the contact electrode according to the temperature of the transparent conductive film 10.

Hereinafter, the present invention will be described based on examples, but the present invention is not limited to these examples.

Example 1: Transparent conductive film using graphene by plasma CVD method (1) Preparation of transparent conductive film In a surface wave plasma CVD chamber, a copper foil having a thickness of about 10 μm which is a catalytic metal was set. First, while the temperature of the copper foil is raised by electric heating to maintain it at about 900° C., hydrogen (200 sccm) gas plasma is irradiated on the copper foil for 45 seconds to generate graphene on the copper foil to form graphene/copper foil. Obtained (see FIG. 2(a)). Next, an anisole solution of PMMA (2%) was applied onto the graphene/copper foil, and spin coated at 3000 rpm for 30 seconds.

Then, it was naturally dried at room temperature to obtain a laminated body of PMMA layer/graphene/copper foil (see FIG. 2(b)). Next, the copper foil was removed by etching using 0.5 mol/L ammonium persulfate to obtain a PMMA layer/graphene (see FIG. 2(c)). Then, the PMMA layer/graphene was placed on the quartz substrate such that the quartz substrate of about 40 mm square, which is a transparent substrate, was in contact with the graphene to obtain a transparent conductive film of the PMMA layer/graphene/quartz substrate (FIG. 2). (See (d)).

(2) Sheet resistance measurement of PMMA layer/graphene portion of transparent conductive film PMMA layer/graphene/PMMA layer of quartz substrate/sheet of graphene portion using non-contact sheet resistance measuring instrument (EC-80, manufactured by Napson Corporation) The resistance was measured. The non-contact sheet resistance measuring method is a method of estimating the sheet resistance by reading the change in capacitance due to the generation of eddy current, and is characterized in that the sample is not damaged. The sheet resistance of the PMMA layer/graphene part was 126Ω.

(3) Sheet resistance measurement of graphene after removing PMMA layer from transparent conductive film The effect of the PMMA layer was verified by measuring the sheet resistance of only graphene from which the PMMA layer was removed from the transparent conductive film. First, the transparent conductive film whose sheet resistance was measured above was immersed in acetone to remove the PMMA layer, and a graphene/quartz substrate was obtained. Next, the sheet resistance of the graphene portion of the graphene/quartz substrate was measured and found to be 461Ω. By removing the PMMA layer, the sheet resistance of the film portion on the quartz substrate increased by about four times.

Example 2: Transparent conductive film using graphene by thermal CVD method In order to show the superiority of low resistance of PMMA layer/graphene, preparation of transparent conductive film and various evaluations were performed using graphene manufactured by thermal CVD method. went. A transparent conductive film was prepared and various evaluations were performed in the same manner as in Example 1 except that graphene/copper foil (purchased from Graphene Platform Co., Ltd.) prepared by the thermal CVD method was used. The sheet resistance of the PMMA layer/graphene/graphene/transparent conductive film PMMA layer/graphene portion of the quartz substrate was 353Ω. The sheet resistance of the graphene portion after removing the PMMA layer was 981Ω. From these results, it was found that the PMMA layer/graphene has a lower resistance than the graphene after PMMA removal, regardless of the method for forming the graphene film.

Example 3: Transparent conductive film in which the PMMA layer was re-formed In order to investigate the cause of the low resistance of the PMMA layer/graphene, the following verification experiment was conducted. First, a PMMA layer was formed again on the graphene/quartz substrate obtained by removing the PMMA layer obtained in Example 2 to obtain a transparent conductive film of the PMMA layer/graphene/quartz substrate. The method for reforming the PMMA layer was the same as the first method for forming the PMMA layer in Example 2. The sheet resistance of the PMMA layer/graphene portion of this transparent conductive film was 921Ω. The sheet resistance of the graphene portion before reforming the PMMA layer was slightly decreased from 981Ω. Considering that the sheet resistance of the PMMA layer/graphene before the removal of the PMMA layer in Example 2 was 353Ω, it is presumed that PMMA has almost no doping effect on graphene.

Example 4: Raman spectroscopic analysis of transparent conductive film Raman spectroscopic analysis of a transparent conductive film was performed from the viewpoint of evaluating the crystal quality of graphene. By observing the peak shift of the Raman spectrum and observing the stress on the graphene, and observing the intensity of the D band indicating a defect, the damage to the graphene can be verified. FIG. 3 is Raman spectroscopy spectra (wide range) of various transparent conductive films obtained in Examples 2 and 3. The wavelength of the laser light used was 532 nm.

The Raman spectroscopic spectrum of the PMMA layer/graphene/quartz substrate obtained in Example 2 is in the lower stage, the Raman spectroscopic spectrum of the graphene/quartz substrate after removing the PMMA layer obtained in Example 2 is in the middle stage, and in Example 3 The Raman spectroscopy spectrum of the obtained PMMA layer/graphene/quartz substrate after reforming the PMMA layer is shown in the upper row. In addition, graphene was described as "Gr" and the quartz substrate was described as "glass". Comparing the spectra, the intensity of the D band (in the vicinity of 1350 cm ⁻¹ ) showing the presence or absence of defects was almost unchanged. This suggests that there is almost no damage to graphene before and after removing the PMMA layer. However, since the laser diameter used in the Raman spectroscope is about 1 μm, the evaluation is performed at this size.

FIG. 4 is a partially enlarged view of FIG. In other words, as the shift is likely to observe the G band (1600 cm ^-1 vicinity) and 2D band (2700 cm ^-1 vicinity), the 1560～1640Cm ^-1 and 2650～2730Cm ^-1 of Raman spectrum of FIG. 3 (broad) The Raman spectrum (narrow band) in FIG. 4 was selected. As shown in FIG. 4, the peak positions of the G band and the 2D band of the spectrum of the transparent conductive film after removing the PMMA layer (middle stage) are compared with the respective peak positions of the spectrum of the transparent conductive film before removing PMMA (lower stage). And was shifting to the negative side.

Further, the peak positions of the G band and 2D band of the spectrum (upper part) of the transparent conductive film after reforming the PMMA layer are the same as the peak positions of the G band and 2D band of the spectrum (middle part) of the transparent conductive film after removal of the PMMA layer. Almost unchanged. From this result, it is presumed that the stress applied to the graphene before the removal of the PMMA layer was released by the removal of the PMMA layer, or the force extending by the removal of the PMMA layer was applied to the graphene. It is speculated that this releasing or stretching force caused micro-sized cracks in the graphene and increased the sheet resistance of the graphene after removing the PMMA layer.

DESCRIPTION OF SYMBOLS 10... Transparent conductive film 12... Transparent base material 14... Graphene 16... Polymethylmethacrylate layer 18... Catalyst metal 20... Graphene transfer laminated body 22... Substrate 24... Graphene

Claims (7)

A transparent substrate, having graphene provided on the transparent substrate, and a polymethylmethacrylate layer provided on the graphene,
Raman spectroscopic spectrum peaks using a laser beam having a wavelength of 532 nm were observed at 1600 to 1606 cm ⁻¹ and 2690 to 2699 cm ⁻¹ ,
A transparent conductive film having a thickness of the polymethylmethacrylate layer of 20 to 150 nm. In claim 1,
A transparent conductive film in which the graphene is composed of single-layer graphene. In claim 1,
A transparent conductive film in which the graphene is composed of two to ten layers of multi-layer graphene. In any one of Claim 1 to 3,
A transparent conductive film having a sheet resistance of 353Ω or less at a laminated portion of the graphene and the polymethylmethacrylate layer. A touch panel comprising the transparent conductive film according to claim 1 and a protective film for protecting the polymethylmethacrylate layer of the transparent conductive film. In claim 5,
A touch panel in which the protective film is provided on the transparent conductive film via an adhesive layer. A flexible heater comprising the transparent conductive film according to claim 1, a pair of contact electrodes in contact with the graphene, and a power source connected to the pair of contact electrodes.

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