  The nucleation of carbon nanotubes on small nickel clusters is studied using
a tight binding model coupled to grand canonical Monte Carlo simulations. This
technique closely follows the conditions of the synthesis of carbon nanotubes
by chemical vapor deposition. The possible formation of a carbon cap on the
catalyst particle is studied as a function of the carbon chemical potential,
for particles of different size, either crystalline or disordered. We show that
these parameters strongly influence the structure of the cap/particle interface
which in turn will have a strong effect on the control of the structure of the
nanotube. In particular, we discuss the presence of carbon on surface or in
subsurface layers.
