  In this study, the synthesis of multi-walled carbon nanotubes (MWCNTs) was
carried out by chemical vapor deposition (CVD) using propane as the carbon
source and Si as the catalyst support. The effect of CVD process variables such
as temperature, choice of catalyst, etc on the growth behavior of nanotubes has
been examined to understand the catalytic growth of CNTs. The transition metal
catalysts, Fe and Ni, were used in both elemental metal form and in a metal
complex form. In the case of elemental metal catalysts, the respective metals
were deposited over the Si substrate using thermal evaporation following which
nanotubes were synthesized by means of CVD. Subsequent studies of the
synthesized carbon nanostructures employing elemental metal catalysts revealed
a significant influence of the temperature and the catalyst material on the
structure of CNTs. The CNTs synthesized using Ni catalyst were bamboo-like
whereas the CNTs developed employing Fe catalyst were straight tubes with
partial metal filling. Consequently, growth models for the different growth
mechanisms have been proposed. Certain limitations of the above process have
been overcome by employing spin-coating of a metal complex catalyst material on
the Si substrate. The CVD synthesis of nanotubes using metal complex catalysts
always resulted in partially catalyst filled CNTs. More importantly, the metal
complex catalyst could be easily patterned on the Si substrate using
spin-coating and photolithography, which resulted in site selective growth of
partially catalyst filled MWCNTs. Since the entire process of site selective
growth is suitable for conventional device fabrication, this method is a
promising and practical pathway for large-scale fabrication of several magnetic
material filled CNT-based devices.
