Abstract Submitted to the    NANOTUBE'04 Conference:

Lowering the growth temperature of carbon nanotubes and nanofibres is an essential step for their direct integration in devices. Plasma enhanced synthesis has mainly been pursued to achieve vertical alignment and little is known to date about possible growth activation and low temperature growth. We present a systematic study of the temperature dependence of the growth rate and the structure of as-grown nanofibers using a plasma enhanced chemical vapor deposition system with a C2H2/NH3 gas mixture and sputtered or evaporated, patterned thin transition metal films as catalysts [1]. An onset of growth is observed at temperatures as low as 120 C. We derive growth activation energies of 0.23-0.4 eV. This is much lower than what is reported for thermal deposition (1.2-1.5 eV) [2] and similar to the activation energies of carbon surface diffusion on the corresponding transition metal [3]. The carbon diffusion on the catalyst surface and the stability of the precursor gas (C2H2) are investigated by first principles plane wave density functional calculations using the CASTEP code. We find a low activation energy (~0.5 eV) for surface diffusion of carbon atoms on the Ni and Co (111) planes. A high barrier exits for the (100) planes, which, however, do not feature dominantly in the Wulff construction for equilibrium shapes of fcc catalyst clusters [4]. Even in this case a relatively low barrier subsurface C diffusion pathway is found. The barrier for catalytic C2H2 dissociation appears to be higher than 0.5 eV on Ni(111). Thus we suggest that the limiting step for plasma-enhanced growth is carbon diffusion on the catalyst surface, whilst an extra barrier can be present in pure thermal growth for the gas decomposition on the catalyst surface. Bulk diffusion can efficiently contribute only at high temperatures, but surface diffusion is always active. This could also be a possible low temperature growth path for single walled carbon nanotubes.

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3. J. F. Mojica et al Surf. Sci. 59, 447 (1976)
4. E. M. McCash, Surface Chemistry, Oxford University Press (2001)