  The molecular dynamics method, based on an empirical potential energy
surface, was used to study the effect of catalyst particle size on the growth
mechanism and structure of single-walled carbon nanotubes (SWNTs). The
temperature for nanotube nucleation (800-1100 K), which occurs on the surface
of the cluster, is similar to that used in catalyst chemical vapor deposition
experiments, and the growth mechanism, which is described within the
vapor-liquid-solid model, is the same for all cluster sizes studied here (iron
clusters containing between 10 and 200 atoms were simulated). Large catalyst
particles, that contain at least 20 iron atoms, nucleate SWNTs and have a far
better tubular structure than SWNTs nucleated from smaller clusters. In
addition, the SWNTs that grow from the larger clusters have diameters that are
similar to the cluster diameter, whereas the smaller clusters, which have
diameters less than 0.5 nm, nucleate nanotubes that are approximately 0.6-0.7
nm in diameter. This is in agreement with the experimental observations that
SWNT diameters are similar to the catalyst particle diameter, and that the
narrowest free-standing SWNT is 0.6-0.7 nm.
