Biomedical Engineering Reference
In-Depth Information
For the catalytic growth of CNTs in the CVD technique, two models
have been proposed to explain the experimental observations: the
base-growth
, which were originally developed for
the catalytic growth of the carbon filament [136]. In the case of
PECVD growth, the catalytic particles are usually found at the tip
and explained by the tip growth model. On the contrary, the base
growth model has been used to explain the vertically aligned carbon
nanotube growth by thermal CVD using iron catalyst. However, the
growth of aligned CNTs is possible through both tip growth and base
growth models, depending on the catalyst and substrate used in the
deposition method.
The SWCNTs grow in the methane CVD process predominantly
via the base growth model [98]. The first step of the CVD reaction
involves the absorption and decomposition of methane molecules
on the surface of transition metal catalytic nanoparticles on the
support surface. Subsequently, carbon atoms dissolve and diffuse
into the nanoparticle interior to form a metal-carbon solid state
solution. Nanotube growth occurs when supersaturation leads
to carbon precipitation into a crystalline tubular form. The size of
the metal catalyst nanoparticle generally dictates the diameter of
the synthesized nanotube. In the base growth mode, the nanotube
lengthens with a close end, while the catalyst particle remains on the
support surface. Carbon feedstock is thus supplied from the “base”
where the nanotube interfaces with the anchored metal catalyst. Base
growth model operates when strong metal-support interactions
exist so that the metal species remain pinned on the support surface.
In contrast, in the tip growth mechanism, the nanotube lengthening
involves the catalyst particle lifted off from the support and carried
along at the tube end. The particle carried along is responsible for
supplying carbon feedstock needed for the nanotube growth. This
mode operates when the metal-support interaction is weak.
Finally, CVD synthesis approaches are highly promising for
producing large quantities of high-quality nanotube materials at large
scale. Controlling nanotube growth with the various CVD strategies
has led to grow organized and well-ordered nanostructures that can
be readily integrated into addressable and designed structures useful
for both fundamental characterization and potential applications in
devices.
and
tip-growth
