Biomedical Engineering Reference
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eficiencies, their thermal decomposition pathway was considered, which
indicated that C
2
H
3
/C
2
H
4
were the major products formed, and thus the major
precursors for SWCNT growth. The authors also showed that the diameter of
SWCNTs could be tuned by changing the C
2
H
4
concentration.
Interestingly, the researchers are responsible for the irst identiication
of the smallest CNTs and their characterisation through aberration-corrected
HR-TEM.
208
This tube species showed a chiral index of (3, 3), (4, 3) or (5, 1).
The tubes were grown inside SWCNTs with the diameter of 1.0-1.2 nm (Fig.
9.37), although the (3, 3) nanotube was rather unstable under the electron
beam and, therefore, could not survive alone without the protection of outer
nanotube. The closing cap of such tube was demonstrated to correspond to a
half-dome of C
20
fullerene, which consists of six pentagons only.
Figure 9.37
(a) HR-TEM image of the cap region of (3, 3)
@
(10, 6) DWCNT,
(b) simulated images, and (c) schematic model. (d) The inner cap structure can be
modelled as half a C
20
fullerene consists of six adjacent pentagons (shown in yellow).
Reproduced from Guan
et al
.
208
with permission.
As just mentioned, the authors have reported eficient methods for the
growth of SWCNTs, but they have also disclosed the mechanism behind
this process through
ex situ
microscopic and spectroscopic analyses. More
precisely, the research group has elucidated the effect of water on the catalysts
employed in CVD.
209
They proved that catalyst deactivation is a consequence
of carbon coating. Water molecules are said to assist this process by removing
such coating and reviving catalysts' activity during CVD.
Another in-depth investigation has been recently performed to disclose
the mechanism associated with the growth of CNTs in general and of their
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