Transparent conducting films (TCFs) have become highly important core materials used in LCDs, touch panels, solar cells, and other applications. Currently, indium tin oxide (ITO), which contains a rare metal, indium, is used in nearly all TCFs, and the development of alternative materials in order to prevent rising costs and supplement an unstable supply has become a necessity.
Graphene is a one-atom-thick sheet, composed of carbon atoms. Optical transparency over a broad wavelength, ranging from visible to infrared light, and high electrical conductivity are superb properties of graphene from an application point of view, and it is hoped that graphene-based TCFs will become an alternative to ITO-based TCFs.
Graphene was discovered by Dr. Andre Geim and Dr. Konstantin Novoselov of the University of Manchester in 2004, by attaching a piece of adhesive tape to a piece of graphite and peeling it off. The amount of graphene that could be obtained by this method was extremely limited, and it is apparent that a method of graphene synthesis applicable to the continuous large-area production was essential in order for it to be adapted for industrial use. Consequently, a chemical vapor deposition (CVD) method, which produces graphene on the surface of nickel and copper by pyrolyzing carbon-containing methane gas, was developed. This has made large-scale synthesis of graphene possible and increased the potential for industrial applications; however, the fact that this method requires the pyrolysis of methane gas to occur at 1000 °C makes continuous production difficult, and this remains a problem to be solved.
At the Nanotube Research Center, we have been working on technologies for low-temperature, large-area synthesis of nanocrystalline diamond thin films by applying our unique microwave plasma CVD equipment and method, and have been making efforts to modify this method for application to low-temperature, large-area CVD synthesis of graphene. We have succeeded in the synthesis of large-area graphene up to the size of A3 paper at a low temperature of 300 °C. We are now able to produce graphene-based TCFs that have a visible light transmittance of about 80 % and sheet resistance of 1 to 2 kΩ/sq. We have fabricated a test model of an electrostatic capacity-type touch panel by applying the graphene-based TCFs, and confirmed its performance (Figure 1). Figure 2 shows an A1-size graphene transparent sheet that was fabricated by connecting four A3-size sheets. We have been working on the development of roll-to-roll deposition technologies for graphene by improving our current method.