Chemistry Reference
In-Depth Information
Fig. 33 Different atomic
displacements on nanotubes
producing the corresponding
Raman signals
respectively to the atomic displacement along the tube axis and along the circum-
ferential direction. When broadened, the G
peak identifies metallic tubes, while
it presents typical Lorentzian-like shape in the case of semi-conducting ones [
138
].
The D band, located in the 1,200-1,400 cm
1
region, is indicative of the
amount of defects in a sample, and therefore it may represent the abundance
of sp
3
-hybridized atoms present in the sample, on both nanotubes and other
carbonaceous residues. Being associated with the sp
3
carbon, introduced after
chemical modification of the nanotubes, the D band is an important tool for
estimating the degree of functionalization of a sample. According to the increased
level of disorder, the signal is very pronounced in MWCNTs, so that its intensity
cannot be taken into account for the evaluation of the organic functionalization.
The G' band, which can be observed between 2,500 and 2,900 cm
1
, is an overtone
of the D-band. This band is not actually connected to any increment of disorder,
indeed it is still present in Raman spectra of defect-free CNTs. Whereas the G' peak
is of fundamental importance for graphene analysis, since it is indicative of the
level of exfoliation of graphite into individual flakes, its contribution to CNTs
characterization is weak.
In contrast, the RBM is a peculiarity of CNTs describing their isotropic radial
expansion (or contraction) through prominent and sharp peaks localized from 160
and 300 cm
1
. Its frequency depends on the tube diameter, to which is inversely
proportional [
139
].
The RBM peaks also give important information concerning the environment of
the tubes since shape and position are modified according to the presence of bundles
or layers of graphite (MWCNTs).
5.1.2 UV-Vis-NIR Absorption Spectroscopy
Carbon nanotubes can be considered as mono-dimensional objects since the ratio
between their length and diameter is high (aspect ratio). In carbon nanotubes the
density of states (DOS) are non-continuous functions characterized by the presence
of spikes, called van Hove singularities, that are peculiar for each (
n
,
m
) carbon
nanotube. In more detail, CNTs are characterized by different optical absorptions,
according to the transitions between different sets of van Hove singularities,
expressed in Fig.
34
as S
11
or S
22
for semiconducting tubes and as M
11
for metallic
