Chemistry Reference
In-Depth Information
Fig. 34 Densities of states diagrams of metallic SWCNT (
left
) and semiconducting SWCNT
(
right
)
ones. Therefore the chirality of the tubes can be studied by their absorption spectra
(Fig.
35
), where transitions between the valence and conductive bands are well
distinguished [
140
].
Beside the position of the peaks, the shape also affords significant information.
When bundled, the vis-NIR spectrum of CNTs is characterized by broad and
low-defined van Hove singularities, which become sharp and well-separated
when nanotubes are individualized by means of surfactants [
141
]. The absorption
features even disappear with a high degree of functionalization, as a consequence
of the
ˀ
-conjugation that is progressively destroyed, changing dramatically the
electronic arrangement of the tubes.
Although this technique provides exhaustive indications about CNTs chirality
and debundling degree, it also presents some drawbacks, which limit its effective
routine employment as a characterization tool. Among these, the most difficult
to achieve is to obtain a stable dispersion of nanotubes, from which impurities have
been totally removed [
142
]. Indeed, at present, a 100% pure reference sample
of nanotubes is not yet available.
5.1.3
Infrared Spectroscopy
CNTs possess weak dipole moments, resulting in Infrared (IR) vibrational modes
that are difficult to detect. For this reason IR spectroscopy is much less used than
Raman spectroscopy for nanotube characterization.
Nevertheless, this technique can produce relevant information on CNTs purity,
electronic structure, and degree of functionalization with good reliability. Earlier
reflectance observations of SWCNTs as nonpurified powder were able to reveal
