Vol.9 No.3 2017
15/74

Research paper : High quality and large-area graphene synthesis with a high growth rate using plasma-enhanced CVD (M. Hasegawa et al.)−128−Synthesiology - English edition Vol.9 No.3 (2017) Furthermore, the molecular dynamics simulation of energetic ion bombardment of He+, Ne+, Ar+, Kr+ and Xe+ for SiO2 have also been reported.[21] The sputtering yield for SiO2 substrates increased along with the atomic number of impact ions. The lightest He+ among these ions could not efficiently transfer its energy to surface atoms on the SiO2 substrate even at ion energy of 100 eV, and the sputtering yield by He+ was almost zero. On the other hand, Ar+ effectively showed sputtering of SiO2.[21] Therefore, it was shown that it was possible to suppress the deposition of silicon and oxygen from the quartz window onto the copper foil substrate by the He/H2 plasma pretreatment, and the cleaning of the copper foil surface could be performed effectively. The syntheses of graphene by Ar/H2/CH4 and He/H2/CH4 plasma CVD on a copper foil substrate pretreated with Ar/H2 and He/H2 plasma was examined and compared with that without such pretreatment. The Raman spectra of the synthesized graphene by plasma CVD are shown in Fig. 4. The G-band (1520 cm-1) overlapping with the D’-band and the D-band (1320 cm-1) were observed for the as-received copper foil substrate prepared by He/H2/CH4 plasma CVD. However, the 2D-band was not observed on the as-received substrate prepared without plasma pretreatment. In the case of Ar/H2/CH4 plasma CVD subsequent to the Ar/H2 plasma pretreatment, the 2D-band (2650 cm-1) was observed with half the intensity of the G-band. In the case of He/H2/CH4 plasma CVD subsequent to the He/H2 plasma pretreatment, the 2D-band (2650 cm-1) was observed with the same intensity as the G-band. This result, combined with the result of the cross-sectional TEM image, indicates that the crystalline of graphene synthesized using He/H2/CH4 plasma CVD at low-temperature (350–400 °C) on a He/H2 plasma pretreated copper foil substrate is better than that synthesized on an Ar/H2 plasma pretreated substrate, as will be described later. That is, it suggests that the He/H2 plasma pretreatment successfully removes copper oxide and impurities on the substrate surface and recovers the catalytic effect of the copper surface for graphene synthesis. In plasma CVD of graphene, the synthesis is completed within several tens of seconds, and has the potential to synthesize graphene in a short time. In order to realize the continuous synthesis of high-quality graphene of high-throughput, it is necessary to sufciently remove the oxide and contaminations prior to the synthesis. Figure 5 shows a comparison of the details of the spectra at around Si 2p binding energy for graphene lms synthesized using He/H2/CH4 and Ar/H2/CH4 plasma CVD. The Si 2p (103.0 eV) was observed clearly in the spectrum of the graphene film synthesized by Ar/H2/CH4 plasma CVD, but not in the spectrum of that synthesized by He/H2/CH4 plasma CVD. In order to obtain more detailed information with respect to the impurities contained in the graphene film, an elemental analysis of very thin (one or two layers) films was conducted by EDS (Fig. 6). In the case of the synthesized graphene film using Ar/H2/CH4 plasma CVD, about 2 % silicon was detected, whereas less than 0.8 % silicon including the background signals was detected in the graphene film synthesized using He/H2/CH4 plasma CVD. Therefore, from the results of impurity analysis of XPS and EDS, it is concluded that the incorporation of silicon from Raman shift (cm-1)━ He/H2 plasma treatment━ Ar/H2 plasma treatment━ As-receivedIntensity (a. u.)30002500200015001000500250200150(b) He/H2/CH4Binding energy (eV)(a) Ar/H2/CH4Binding energy (eV)Intensity (cps)Intensity (cps)9510010511011525020015095100105110115Fig. 4 Raman spectrum of graphene synthesized on the copper foil substrate[13]Copyright (2014) The Japan Society of Applied Physics Fig. 5 XPS spectrum of Si 2p binding energy for synthesized graphene using (a) the Ar/H2/CH4 gas mixture and (b) the He/H2/CH4 gas mixture[13] Copyright (2014) The Japan Society of Applied Physics

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