Vol.9 No.3 2017
21/74

Research paper : High quality and large-area graphene synthesis with a high growth rate using plasma-enhanced CVD (M. Hasegawa et al.)−134−Synthesiology - English edition Vol.9 No.3 (2017) 21.510.50101001000H2 plasma treatment without CH4 gasPlasma CVD with CH4/H2 gasesD-band/G-band peak intensity ratioHall mobility (cm2/Vs)Fig. 14 A4 size large area graphene transparent conductive lm Transmittance : 92 % and sheet resistance : less than 500 ΩFig. 15 The relationship between the Hall mobility and Raman intensity D-band/G-band ratio[15] Plasma CVD using CH4/H2 gas mixture, Hydrogen plasma treatment without CH4 gasCopyright (2015), with permission from Elsevier source, the crystalline quality of synthesized graphene was signicantly improved, and the controllability of the number of layers was also improved. As carbon sources for graphene synthesis, we utilized trace amount of carbon contained in the copper foil and carbon supplied from the environment in the reaction chamber. Graphene of 60 % AB-stacked bilayer graphene and 40 % of disoriented bilayer graphene was synthesized. The sheet resistance of bilayer graphene was 951 Ω on average, and the carrier mobility was 1000 cm2/Vs at room temperature. The sheet resistance of 130 Ω was attained after doping with a gold chloride solution.4 The development of large area graphene synthesis technologyWe attempted to synthesize A4 size large area graphene by combining plasma treatment and joule heating using ultralow carbon sources developed in this study. Figure 14 shows an A4 size large area graphene transparent conductive lm which was synthesized on a copper foil substrate using the above method and then transferred onto a PET film. The optical transmittance of only graphene was 92 % (3.6 layers) and the sheet resistance was less than 500 Ω without doping. In this way, we succeeded in fabricating a A4 size large area graphene transparent conductive lm using plasma treatment of ultralow carbon sources developed in this study.5 The relationship between Hall mobility and crystalline qualityWe examined the correspondence between Hall mobility and crystalline quality by Raman spectroscopy measurements of van der Pauw devices which measured the Hall mobility. Here, the Raman intensity ratio of the D-band and the G-band was an index for graphene crystalline quality. The relationship between the Hall mobility and the intensity ratio of the D-band/G-band is shown in Fig. 15. In this study, we prepared two graphene films. One was synthesized by plasma CVD using methane and hydrogen gas, the other was synthesized by combining plasma treatment and the joule heating method without using methane gas carried out in this study. In the case of plasma CVD of the previous method, the intensity ratio of the D-band/G-band was higher and mobility was 10–100 cm2/Vs. On the other hand, in the case of the novel method, the intensity of the D-band/G-band was lower and mobility was 100–1000 cm2/Vs, and we succeeded in achieving 10 times larger electric conductivity than the previous method. From the intensity ratio of Raman signals of the D-band and the G-band, it was possible to estimate the graphene domain size.[51] In the case of plasma CVD of the previous method, the estimated graphene domain size was 17 nm. On the other hand, in the case of the novel method which combined hydrogen plasma treatment and joule heating, it was 170 nm. Also from this result, we conrmed that the domain size had expanded to 10 times larger than the previous method. 6 Estimation of domain size by dark-field TEMTo directly estimate the domain size of the synthesized graphene lm by plasma treatment of this study, we measured the graphene film by dark-field TEM. Graphene samples transferred to a TEM grid using polymethyl methacrylate (PMMA) were prepared. Figure 16(a) shows a bright-field TEM image, and Fig. 16(b) shows the selected area electron diffraction pattern (SAED). The graphene lm seemed to be relatively uniform in the bright-eld TEM image, but many family spots were observed in the SAED. The results suggest that several domains with different orientations exist. The

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