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 311

from 130 to 210 cm -1 . After annealing, sharp lines appeared in the range of
300-370 cm -1 , owing to the formation of inner tubes and conversion into
DWCNTs. HR-TEM conirmed such results by showing a typical CeCp 3 @
SWCNT on the top-right panel, while the middle and bottom panels showed
the sample transformed into Ce@DWCNT.
The production of DWCNTs can also be induced by peapods 221 or by
encapsulation or ferrocene molecules as precursors. 222 This catalytic process
offers new opportunities for controlled materials design. Moreover, from the
electronic point of view, this structural change from SWCNTs to DWCNTs is
accompanied by the conversion of semiconducting tubes into metallic ones,
as a consequence of the doping-induced transition into a metallic phase. This
is conirmed by a massive enhancement in conductivity of doped SWCNTs,
thus providing new insights on 1D electronic systems.
By changing the nanotube diameter it is possible to modulate the
tubes' properties. Along that direction, the research group has attempted
to clarify whether dye molecules encapsulated in SWCNTs can exhibit a
photosensitising function. 223 For photosensitisation to occur, it is necessary
that the absorption band of the encapsulated dye is located in the spectral
range where the semiconducting SWCNTs have a low extinction coeficient. In
the experiments, squarylium (SQ) dye was chosen as favourite dye because of
its small dimensions (thus suitable for encapsulation inside SWCNTs of 1.4 nm)
and its absorption bands not overlapping with SWCNTs. Photoluminescence
measurements clearly revealed that the energy of the excited state of the
encapsulated SQ dye (SQ@SWCNTs) was transferred to the SWCNTs, thus
exhibiting the photosensitising function inside the SWCNTs.
X-ray diffraction, instead, indicated that the dye inside the tubes was
placed off-centre in comparison with chloroform molecules of pristine tubes.
The difference could be attributed to the π -conjugated structure, which could
determine π - π interactions with the tube wall, 224 thus shifting SQ molecules
off the SWCNT centre.
A small part of Kataura's research is devoted to the separation of CNTs
with different techniques. One example is the recent gel-based separation of
metallic and semiconducting SWCNTs on the basis of the selective adsorption
of semiconducting SWCNTs on agarose gel. 225 A gel containing a mixture of
SWCNTs and sodium dodecyl sulphate was frozen, thawed and squeezed,
affording a solution containing 70% of pure metallic SWCNTs and a gel with
95% of pure semiconducting SWCNTs. Interestingly, ield-effect transistors,
produced on the so-separated semiconducting SWCNTs, demonstrated that
their electronic properties were preserved.