Vol.9 No.3 2017

Research paper : High quality and large-area graphene synthesis with a high growth rate using plasma-enhanced CVD (M. Hasegawa et al.)−129−Synthesiology - English edition Vol.9 No.3 (2017) the quartz window onto the synthesized graphene film is suppressed more effectively by He/H2/CH4 than Ar/H2/CH4.As in the pretreatment of the substrate, mixed gas using helium or argon for plasma CVD results in a significant difference between the synthesized graphene films. Figure 7 shows the comparison between the cross-sectional TEM images of graphene films synthesized using (a) He/H2/CH4 and (b) Ar/H2/CH4 mixed gases. In the case of mixed gas of helium, a multilayer graphene film consisting of 20 layers was synthesized directly on the copper foil substrate by CVD for 20 min. The layer spacing was 0.34 nm, which is slightly larger than 0.335 nm of graphite. In the thermal CVD on the copper foil substrate, the growth of graphene is limited to two or three layers.[6] In contrast, in the case of plasma CVD of graphene, a much thicker film is grown on the copper foil substrate as shown in this example. This is one of the remarkable features of this method. In the case of mixed gas of argon, on the other hand, the diagonal layered structure in the cross-sectional TEM image was confirmed on the copper foil substrate. A layer spacing of 0.27–0.28 nm, which corresponds to that of CuO(110), indicates the formation of the copper oxide layer during Ar/H2/CH4 plasma CVD. Along the surface of the copper oxide layer, a weak contrast of the layered structure was also confirmed. The layer spacing of 0.34–0.37 nm, however, is much larger than that of graphite, which suggests that the synthesized graphene layer was partly oxidized like the copper foil substrate. In this research we developed a plasma pretreatment method for copper foil substrates, and a plasma CVD method for high-quality graphene by suppressing the impurity incorporation onto the graphene surface. It is found that the plasma pretreatment using He/H2 removes copper oxide on the surface more effectively than using Ar/H2, and is also effective for preventing the substrate from being contaminated by silicon impurities attributed to the sputtering of the quartz window. The plasma pretreatment of the copper foil substrate using He/H2 is found to improve the crystalline of synthesized graphene. The incorporation of silicon impurities from the quartz window onto synthesized graphene films is suppressed using He/H2/CH4 more effectively than using Ar/H2/CH4.Fig. 6 EDX spectrum of Monolayer and/or bilayer graphene synthesized using (a) the Ar/H2/CH4 gas mixture and (b) the He/H2/CH4 gas mixture[13] Copyright (2014) The Japan Society of Applied PhysicsFig. 7 Cross-sectional TEM images of synthesized graphene by surface-wave plasma using (a) the He/H2/CH4 gas mixture and (b) the Ar/H2/CH4 gas mixture[13] Copyright (2014) The Japan Society of Applied Physics 050150100200250300350C KαCu KαSi KαCu LαO KαCu Kβ(b) He/H2/CH4(a) Ar/H2/CH4Energy (eV)Intensity (counts)1050050150100200250300350Energy (eV)Intensity (counts)1050C KαCu KαSi KαCu LαO KαCu Kβ5 nm(b)(a)GrapheneCuCu5 nmGraphene

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