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29

2.2.3. A-CNTs Grown via Vapor-Phase CVD

In contrast to what was observed with the pre-deposited catalyst CVD
technique, longer growths of A-CNTs are possible through vapor-phase CVD, and it
has been shown that CNTs can be grown in excess of 1 cm in-length though this
process [43]. Due to the added complexity of the vapor-phase delivery method, the
reaction parameters can become more difficult to optimize, making it harder to
produce higher-quality CNTs. The added degree of freedom with having to actively
control the catalyst content not only makes growth optimization more time-
consuming, but also will inherently result in CNTs with more metal content.
However, if the embedded metal content of the CNTs is not of concern, the act of
constantly adding more catalyst to the system during synthesis via this process can
help stave off catalyst death and allow for longer A-CNTs. Another downfall of this
technique is that the CNTs tend to not be uniform in diameter. As depicted in Figure
2.9, the "top” of the CNTs are considerably thicker than the "middle”, and especially
the "bottom”.

Table 2.2 - Parameters for vapor-phase CVD growth.

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