Biomedical Engineering Reference
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external part covered preferably semiconducting SWCNTs, suggesting its role
in the fabrication of photonic devices as an interface between electronic and
photonic materials.
Among other atoms, sulphur has been identiied as a triggering agent
in the formation of branched nanotube networks with stacked-cone
morphologies made up of heptagons (negative curvature) and pentagons
(positive curvature).
170
The molecular dynamics (MD) simulations conirmed
that sulphur atoms promote the formation of heptagonal rings, and they are
also more abundant in curved regions (either heptagons or pentagons).
Finally, it is worth mentioning the investigation devoted to the evaluation
of “nucleation”, the irst and most important step of nanotube growth; the
mechanism behind such process has never been accessible to observation and
has remained unaddressed. Therefore, Terrones' research group has created
CNTs by the injection of carbon atoms into the catalytic metal nanoparticles
and subsequent electron irradiation of CNTs partly illed with transition
metals such as Co, Fe, FeCo and Ni.
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When MWCNTs containing metal
particles were irradiated with an electron beam, carbon from graphitic shells
surrounding the metal particles was ingested into the body of the particle and
subsequently emerged as SWCNTs or MWCNTs inside the host nanotubes.
This process was monitored for the irst time with spatial resolution in real-
time studies. Images under TEM (Fig. 9.27) showed a CNT growing from
the metal nanowire in the inner core of the tube (growth speeds were of
only 1 nm/min). The growth of SWCNTs was also observed inside MWCNTs
illed with Fe crystals. Here the growth of the tube was accompanied by the
appearance and migration of an oval domain inside the metallic core.
Figure 9.27
SWCNT growth from a metal crystal. Image after SWCNT has reached a
length of 11 nm. Figure modiied from Rodriguez-Manzo
et al
.
171
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