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Nucleation and Vertical Growth of Nano-Graphene Sheets 27 the pretreatment step, while only a continuous film was obtained for growth after pretreatment without Ar+ irradiation (Fig. 5(d)). These results indicate that energetic Ar+ irradiation during the pretreatment (initial growth process) is necessary for CNW growth, and the nucleation of CNWs is incubated in the nanoislands by high density Ar+ irradiation. Therefore, nucleation and CNW growth could be clearly distinguished using the two-step growth technique. Fig. 5. Tilted-view STM images of samples after pretreatment for 15 min (a) with and (b) without Ar+ irradiation. Tilted-view SEM images of samples formed by two-step growth, in which pretreatments were performed (c) with and (d) without Ar+ irradiation.[14] 2.3 Effects of H radicals on CNW growth The effects of radicals were investigated in a multi-beam CVD system. The H2 gas flow rate was changed from 0 to 10 sccm in the second step (CNW growth), and C2F6 and Ar gas flow rates were kept constant at 10 and 5 sccm, respectively. Therefore, several different composition ratios of H/C or H/CF3 would be obtained under these conditions. The chamber was evacuated through a gate valve using a turbo molecular pump, and the total gas pressure was controlled at 2.5 Pa by the valve when the H2 gas flow rate was 5 sccm. The valve position was not changed at various H2 gas flow rates in order to maintain the fluxes of Ar+ ions and CFx radicals. Therefore, the total pressures ranged from 2.2 to 2.8 Pa at H2 flow rates from 0 to 10 sccm. The rf ICP powers applied to H radical source, fluorocarbon source, and Ar+ ion source were 200, 200, and 300 W, respectively. The irradiation period for each sample was 35 min. 2.3.1 Morphological dependence of CNWs on H radicals Figures 6(a)–(e) show tilted-view SEM images of CNWs synthesized for 35 min at different H2 gasflowratesof(a)0,(b)3,(c)5,(d)7,and(e)10sccm.WhentheH2 gasflowratewas0PDF Image | GRAPHENE SYNTHESIS CHARACTERIZATION PROPERTIES
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