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Nucleation and Vertical Growth of Nano-Graphene Sheets 33 Ar+ ion energy of 200 eV, the heights of grown films increase with increasing the Ar+ ion flux. This means absorption enhancement of CF3 radicals by ion irradiation. It is deduced that the ion flux determines the amount of dangling bonds produced on the growth surface. The rate of dangling bond production should be lower than the rate of nucleation site production in order to induce vertical growth of nanographene sheets and result in CNW growth. On the other hand, at an Ar+ ion flux of 3.8 A/cm2, the heights of grown films decrease with increasing the Ar+ ion energy. This indicates etching effects of the grown films by the ion irradiation. Therefore, the ion energy is also a critical factor for CNW growth, because its strength determines the production of dangling bonds, nucleation, and etching. Therefore, Ar+ irradiation conditions with limited fluxes (3.3–3.8 μA/cm2) and energies (200–250 eV) along with CF3 and H densities in the order of 1011 cm-3 must be critical factors for CNW growth. 250 ygre nenoi +rA 160 )Ve( CNWs formation (c) h=12 nm 200 (b) h=0 nm 1.8 (e) h=25 nm (h) h=25 nm (f) h=25 nm 50 nm 5.4 No deposition (a) h=14 nm (d) h=18 nm (g) h=25 nm Film Formation 3.3 3.8 4.5 Ar+ ion flux (μA/cm2) Fig. 11. Chart of Ar+ ion irradiation for CNW growth for fluxes ranging from 1.8 to 5.4 μA/cm2 and ion energies ranging from 160 to 250 eV. The height (h) of each sample is indicated in each SEM image.[13] The roles of fluorocarbon radicals and hydrogen radicals are discussed. At the top of the growing CNW, the edge would be terminated by CF3. A H radical migrates to the edge and the terminated F atom at the top is removed by abstraction with a H radical. The HF molecule generated is exhausted to the gas phase. At the same time, dangling bonds are generated on C atoms and CF3 radicals from the fluorocarbon radical source or migrating on the CNWs are absorbed at the dangling bonds. As a result, a new C-C bond is produced. Repetition of these processes results in CNW growth. It is considered that graphitization would be affected by high energy ion irradiation as well as thermal assist.PDF Image | GRAPHENE SYNTHESIS CHARACTERIZATION PROPERTIES
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