The temperature dependence of the growth rate of C nanofibers by plasma-enhanced chemical vapor deposition with Ni, Co and Fe catalysts was studied. A common low activation energy of 0.23 to 0.4eV, much lower than that for thermal deposition, was extrapolated. The C diffusion on the catalyst surface and the stability of the precursor molecules, C2H2 or CH4, were investigated by means of ab initio plane-wave density functional calculations. A low activation energy of 0.4eV was found for C surface diffusion on Ni and Co(111) planes; much lower than that for bulk diffusion. The energy barriers for C2H2 and CH4 dissociation were at least 1.3 and 0.9eV, respectively, on Ni(111) planes or step edges. Hence, the rate-limiting step for plasma-enhanced growth was C diffusion on the catalyst surface, while an extra barrier was present for thermal growth due to gas decomposition.

Surface Diffusion - the Low Activation Energy Path for Nanotube Growth. S.Hofmann, G.Csányi, A.C.Ferrari, M.C.Payne, J.Robertson: Physical Review Letters, 2005, 95[3], 036101 (3pp)