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30 Graphene – Synthesis, Characterization, Properties and Applications However, the H radical density appeared to decrease with further increase of the H2 gas flow rate above 7 sccm, which was rather due to the device limitation; it was difficult to efficiently maintain ICP at high flow rates and high pressures. The results shown in Figs. 6–8 suggest that the H radical density has a strong influence on the nucleation and morphology of the resulting CNWs, and there would be an optimum ratio of H radical density to fluorocarbon flux or other species. Moreover, it is noted that CNWs were not formed at a H2 gas flow rate of 10 sccm, although the H radical density was almost the same as that at 5 sccm. It is presumed that in the case of high flow rate of H2 gas, the dissociation rate from H2 molecules to H atoms was low and/or by-products containing H, such as HF and CHx, were generated. Under such conditions, H2 molecules would disturb the transport of other important species and chemical reactions. In addition, by-products related to H would influence the amount of CFx radicals present, which has an important role for CNW growth when using a fluorocarbon/hydrogen system. Therefore, further investigation regarding the role of H2 molecules is required. 2.3.2 Compositional dependence of CNWs on H radicals Figure 9 shows atomic composition ratios measured by XPS for CNWs grown at different H2 flow rates during a simultaneous irradiation process. The composition ratios of F/C and Si/C were estimated from the intensities of the C 1s, F 1s, and Si 2p peaks using ionization cross-section value of each peak. It should be noted that difference in electron inelastic mean free path of each photoelectron and that in surface roughness are not considered at this estimation. Therefore, although absolute values of estimated composition ratios are not precise, qualitative tendencies can be discussed based on them. 1.0 0.8 0.6 0.4 0.2 00 2 4 6 8 10 H2 gas flow rate (sccm) Fig. 9. Atomic composition ratios of CNWs grown at different H2 flow rates. The flow rates of C2F6 and Ar were 10 and 5 sccm, respectively. F/C Si/C Atomic Composition ratioPDF Image | GRAPHENE SYNTHESIS CHARACTERIZATION PROPERTIES
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