OVERVIEW ON THE MAJOR RESEARCH ACTIVITIES ON CARBON NANOTUBES BEING DONE IN AMERICA, EUROPE AND ASIA - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 305

attached to C 60 molecules ( Fig. 9.36a,b) and subsequently incorporated inside
SWCNTs (Fig. 9.36c-e); this second step was monitored by HR-TEM subjected
to sub-second time frame recording, which revealed the dynamic behaviour
of the molecule during its cis-trans isomerisation. The use of SWCNTs
provided the double advantage of a low background noise in the images and
a restricted molecular motion inside their inner space. At HR-TEM, the C 60
molecules appeared as a circular shape, while the retinyl group appeared in
either “strong” or “weak” contrast, which suggested its existence in one of
two states when inserted inside the SWCNTs.
A careful evaluation of the images correlated well with an all- trans
isomer. The cis/trans structural transformation, which normally requires
photoactivation, was shown to occur under these conditions; this unprecedent
result has important implications, since it provides a preliminary insight into
how retinal undergoes different conformational changes when it is placed in
a conined space (i.e., inside SWCNTs).
Analogously, the information deriving from HR-TEM and EELS has
proved advantageous in the characterisation of atomic wires produced by
encapsulation of lanthanum atoms attached to fullerene C 80 inside CNTs. 205
These techniques allowed for the identiication of the internalised complex
(La 2 @C 80 ) in the form of dimers and the preservation of their original valence
state.
An additional interesting study with iodine demonstrated that single,
double and triple helixes of iodine chains inside CNTs are inluenced by
SWCNTs' diameter remarkably, as shown by their crystallisation in the
presence of tubes larger than 1.45 nm. 206 Therefore, a critical size cut-off
of the hollow space inside SWCNTs was identiied, on the basis of which
polymorphic and crystal structures of iodine can be generated in a controlled
manner.

9.4.1.2 Synthesis of CNTs

Part of the ongoing projects at Iijima's laboratory deals with the development
of several synthetic procedures for the fabrication and characterisation of
CNTs.
A commonly used technique for the preparation of CNTs consists of CVD
from hydrocarbons. In this regard, the group has selected nine aromatic
hydrocarbons as carbon sources. 207 These hydrocarbons mainly differed in
terms of hybridisation of the atoms attached to benzene rings. In particular,
hydrocarbons with sp 3 moieties included toluene, p -xylene, ethylbenzene and
n -propylbenzene, while species with sp 2 moieties were styrene, allylbenzene
and 1,4-divinylbenzene. In order to compare their SWCNT production