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 267

be, on average, ~44% longer than SWCNTs, thus providing two to four times
higher conductivity in transparent conductors than SWCNTs and unsorted
nanotubes.
With regard to this last aspect, the authors have described that the
performance of transparent conductors, produced using monodisperse
metallic SWCNTs, can be increased by 5.6 and 10 times in the visible and IR
regions, respectively. 99 Moreover, provided monodisperse metallic SWCNTs
with Angstrom-level resolution in diameter are used, semitransparent
conductive coatings with tunable optical transmittance can be produced.
Finally, the use of special catalysts has resulted in remarkable progress
towards selective growth of monodisperse samples. 100 As concerns this aspect,
the authors have described how DGU process could easily separate SWCNTs
produced by carbon monoxide (CO) disproportionation on bimetallic Co-Mo
catalysts (CoMoCAT growth strategy). 101
Interestingly, the accurate comparison between NIR spectra and AFM
images revealed large variations among growth methods and effective sorting
by DGU. 102 To reiterate, the researchers have elaborated a counting-based
method, aimed at quantifying the semiconducting tubes present in several
fractions of as-grown or processed SWCNTs, providing useful information
about sample compositions in view of future techniques that should control
the sample's characteristics precisely.
A few ongoing projects are also related with optical and electronic
properties of CNTs.
More precisely, the optical properties of semiconducting SWCNTs are
governed by excitons. The excitons created in semiconductors with an ultra-
short laser pulse initially possess a deinite phase relationship between
themselves. However, scattering among the tubes and with charged carriers,
phonons, impurities and defects can lead to dephasing and, eventually,
population relaxation. An experimental study of ultrafast exciton dephasing
in semiconducting SWCNTs utilising a femtosecond four-wave mixing (FWM)
technique has been recently reported. 103 Throughout this investigation, the
Hersam group discovered that both exciton-exciton and exciton-phonon
scattering have profound effects on the dephasing process.
Another study on near-ield photoluminescence (PL) of semiconducting
single SWCNTs revealed large DNA-wrapping-induced red shifts of the exciton
energy, which were two times higher than the value indicated by confocal
microscopy. 104 As a consequence, two distinct PL bands (attributable to
DNA-wrapped and unwrapped nanotube segments) were clearly identiied,
with a transition between these two energy levels smaller than the spatial
resolution of about 15 nm. This was conirmed by another experiment on
the exciton energy transfer in pairs of semiconducting nanotubes, using high-
resolution optical microscopy. 105 PL bands were observed, with intensities
correlated with inter-tube distance. As expected, the eficient energy transfer