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2.2. Vapor-Phase CVD
2.2.1. Introduction to Vapor-Phase CVD Growth
As compared to the pre-deposited catalyst method, the vapor-phase CVD
process allows for the growth of CNTs on much more complex surfaces. In this
technique, the catalyst precursor is dissolved in an organic solvent and introduced
with the carbon feedstock through liquid delivery. As the solution evaporates and
the dissociated precursors pass through the system, the catalyst particles will land
on surfaces in the reaction chamber, where the carbon will interact with them to
initiate the growth of a CNT. This opens up the possibility for CNT growth on
surfaces which are more complex or otherwise are not suitable for the pre-
deposition of a catalyst. It has been shown that CNTs can be readily grown on many
surfaces, ranging from oxidized silicon to quartz and even on the surfaces of carbon
fibers via this method [38]. Similarly to pre-deposited catalyst CVD, it is also
possible to pattern the growth of CNTs through vapor-phase CVD. By patterning an
oxide layer on a silicon substrate, it has been shown that CNTs will only grow on the
oxide, allowing for the selective growth of 2D and even 3D patterns such as pillars
and "daisies" [39].
While it may be considered a downfall for most applications, the CNTs grown
via vapor-phase CVD tend to be of a lower quality, due in part to the fact that the
amount of catalyst is not nearly as well-controlled. This tends to result in CNTs
which have varied diameters and are not nearly as crystalline (at least for the outer
coaxial layers] and may also have an amorphous or pyrolized carbon coating. The