Growth Mechanism
To begin with, I shortly explain the general growth mechanism of carbon nanotubes
that is basically common for different growth methods.
First of all, the exact way in which nanotubes are formed is not completely
understood. One distinguishes
between extrusion or tip-growth,
and base-growth, which are both a
three-step process. As
schematically sketched in Fig.1,
first, a round or pear-shaped
precursor for the formation of
nanotubes is formed on the surface
of the metal catalyst. Second, the
carbon diffuses on the sides of the
precursor leaving, however, the top
of it free – that is the reason for
the hollow core of the
nanotube. Out of this a rod-like carbon structure is formed. By the base growth
(extrusion) the nanotube grows upwards from the metal particle that remains
attached to the substrate. By the tip growth the particle detaches and stays on the
top of the growing nanotube. Depending on the size of the catalyst particles, singlewalled
or multi-walled nanotubes are grown.