  The nucleation and growth of single wall carbon nanotubes from a
carbon-saturated catalytic particle surrounded by a single sheet of graphene is
described qualitatively by using a very restricted number of elementary
processes, namely Stone-Wales defects and carbon bi-interstitials. Energies of
the different configurations are estimated by using a Tersoff energy
minimization scheme. Such a description is compatible with a broad variety of
size or helicity of the tubes. Several mechanisms of growth of the embryos are
considered: one of them is made more favourable when the tubes embryos are
arranged in an hexagonal network in the graphene plane. All the proposed
mechanisms can be indefinitely repeated for the growth of the nanotubes.
