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Nucleation and Vertical Growth of Nano-Graphene Sheets 31 At H2 gas flow rates of 5–7 sccm, where CNWs were definitely formed, a large amount of C was contained mainly in the deposits, while F and Si were rarely detected, which indicates that the Si substrate was fully covered with carbon nanostructures, despite irradiation with fluorocarbon radicals. On the other hand, even when CNWs were not obtained (H2 gas flow rates of 0 and 10 sccm), F and C were detected, which suggests that a fluorocarbon monolayer is present on the Si substrates. There is also a correlation between the heights of the CNWs shown in Fig.7 and the F contents in the deposits; CNWs with increased height contain lower F content. It is well known that H atoms scavenge F atoms, which results in the formation of by-products such as HF. CNWs were rarely formed at a low H2 flow rate of 3 sccm or less, because F atoms on the top of the growing CNWs were not sufficiently scavenged. In contrast, CNWs were not formed at a high density of H atoms, because excess H atoms would remove both F and C atoms from the growth surface. Even when CNWs were grown at H2 gas flow rates of 5–7 sccm, not all F atoms were scavenged, which suggests that other parameters, such as the acceleration voltage, flux of Ar+ ions, and the surface temperature require optimization. 2.4 Effects of ions on CNW growth The effects of ions on CNW growth were investigated in a multi-beam CVD system. A first subject is what type of combination of radicals and ions is effective on the initial growth of CNWs. Various combinations of radicals and ions were employed to the first-step at the two-step growth. Secondly, dependence of CNW growth on energy and flux of ions are discussed. Energy and flux of ions during the second step were varied, and changes in surface morphologies of deposits are studied. 2.4.1 Synergetic effects of radicals and ions on CNW growth The combinations of irradiation species used in the pretreatment step were varied. The pretreatment step consisted of irradiation with Ar+ ions and/or fluorocarbon radicals and/or H radicals. In all samples, in situ ellipsometry revealed that CNWs were not obtained only by the pretreatment step. The CNW growth process was then carried out for 35 min after pretreatment without exposure to the atmosphere between the pretreatment and CNW growth steps. Figure 10 shows tilted-view SEM images of the samples after the pretreatment and the CNW growth processes. In Fig. 10, pretreatments were composed of irradiation with (a) energetic Ar+ ions at 200 eV (Ar+), (b) CF3 radicals (CF3), (c) H radicals (H), (d) Ar+ + CF3, (e) Ar+ + H, (f) CF3 + H, and (g) Ar+ + CF3 + H. The conditions for the CNW growth process (second step) were constant for all samples. As shown in Figs. 10(b), (c), and (f), no CNWs were observed, but a thin film was obtained after the CNW growth process with pretreatments consisting of CF3, H, and CF3 + H. In contrast, CNWs were successfully formed on the Si substrates after CNW growth with pretreatments of Ar+, Ar+ + CF3, Ar+ + H, and Ar+ + CF3 + H, as shown in Figs. 10(a), (d), (e), and (g), respectively. It is noted that CNWs were formed only when irradiation with Ar+ ions was included in the pretreatment. The CNW heights were different for each sample; however, it is not meaningful to estimate the growth rate, due to the different starting time of CNW growth. In contrast, the morphologies were almost the same when CNWs were obtained, as shown in Figs 10(a), (d), (e), and (g). From SEM observations, it is considered that irradiation with Ar+ is crucial for subsequent CNW growth; irradiation with Ar+ is one of the key factors for the formation of Si substrate conditions ideal for CNW growth.PDF Image | GRAPHENE SYNTHESIS CHARACTERIZATION PROPERTIES
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