Vol.9 No.3 2017
17/74

Research paper : High quality and large-area graphene synthesis with a high growth rate using plasma-enhanced CVD (M. Hasegawa et al.)−130−Synthesiology - English edition Vol.9 No.3 (2017) 3000250020001500100050025-300 ℃400 ℃600 ℃800 ℃1000 ℃TemperatureLowHighRaman shift (cm-1)Intensity (a. u.)Fig. 8 Raman spectrum of the copper foil heated from room temperature up to 1000 °C at hydrogen atmosphere[14] Copyright (2014), with permission from Elsevier 3 A development of plasma CVD using ultralow carbon sourceFor the industrial application of graphene transparent conductive films, establishment of a synthesis method of high-quality and high-throughput is required. As described previously, the synthesis of graphene by CVD on transitional metal substrates (in particular on copper) is the most promising at the moment.[6] Currently, transmittance of 90 % (four-layers stacking) in the region of visible wavelength and sheet resistance of 30 Ω are indicators of high-performance graphene synthesized by thermal CVD.[7] A demonstration of an organic light-emitting diode (OLED) with graphene anodes which has higher luminous efciency than by using ITO has been reported.[42] Since visible light transmittance of 90–93 % is required for the transparent electrode application of graphene, three or four layers of graphene are necessary. Hence, it is important to improve the controllability of the graphene synthesis for multilayers as well as a single layer. For the realization of the mass production of graphene by a roll-to-roll method, it is required to reduce the thermal load on the apparatus and to attain a signicant reduction of synthesis time. An attempt was made to reduce the thermal load on the apparatus by direct joule heating of the copper foil substrate and to demonstrate roll-to-roll thermal CVD synthesis of graphene at 950 °C by the Sony group.[43] In this example, winding speed of the copper foil substrate was 1.5 mm/sec, and further increase of speed is desired for high-throughput production. Also in order to suppress the microcracks due to thermal expansion and thermal contraction of the copper foil to improve the quality of the graphene, further reduction of the temperature is required. We have developed plasma CVD of graphene to reduce the process temperature and the process time at the same time. By combining low temperature surface-wave plasma CVD with roll-to-roll transfer of copper foil substrate, high-throughput synthesis of graphene with winding speed of 5–10 mm/s was demonstrated by the AIST group.[9][10] The problem of plasma CVD of graphene is the crystal size (domain size) of 10 nm or smaller, which inhibits electrical conductivity. By the large growth rate and high nucleation density of plasma CVD, graphene growth in the two-dimensional direction is prevented, which causes stacking of small akes in multiple layers and deterioration of the controllability of the number of layers. In this study, we attempted to expand the size of graphene crystals and to improve the controllability of the number of layers by reducing the concentration of the carbon source used for graphene synthesis which is expected to suppress the nucleation density. Without supplying carbon-containing gas such as methane, as an ultralow concentration of carbon source, we utilized trace amount of carbon contained in the copper foil and/or supplied from the environment in the reaction chamber. We attempted to expand the crystal size of graphene and improve the electrical conductivity by developing this method. Moreover, we attempted to synthesize AB-stacked bilayer graphene with good controllability in a high yield. This method combines joule heating and hydrogen plasma treatment for the copper foil substrate and it is aimed at the establishment of an industrially advantageous method at lower temperature and requiring shorter time as compared to the conventional thermal CVD method.First of all, we performed only heat treatment at each temperature of 300, 400, 600, 800, and 1000 °C of the copper foil by using direct joule heating in 20 Pa hydrogen for 15 min in the reaction chamber and the foil was cooled down to room temperature. The size of the heat treated sample was 6 × 6 mm2. A copper foil heated at each temperature was examined by Raman spectroscopy (Model: HORIBA XploRa, beam spot of 1 μm in diameter, excitation laser of 632 nm wavelength). It was tested whether graphene was synthesized by only the joule heat treatment in a hydrogen atmosphere as shown in Fig.8. Hydrogen plasma treatment was performed in 30 sccm ow and 5 Pa for 30 s. The surface-wave microwave plasma with low electron temperature which was expected to reduce the ion bombardment was utilized for plasma treatment. The synthesized graphene was transferred to a transparent polymer substrate to measure the electrical conductivity and the optical transmittance. The slightly-adhesive resin film was used as the transparent polymer substrate. The thickness of the resin lm was 41–42 μm. After bonding the resin lm and copper foil with graphene onto the surface of the copper foil substrate, the copper foil was removed by etching using an aqueous solution of ammonium persulfate (0.50 mol/ℓ). The electrical characteristic of the synthesized graphene

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