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 286

Interestingly, pristine, N-doped and P-doped SWCNTs all exhibit a lower
frequency G D Raman peak with similar frequencies, thus indicating that the
defect site is caused by the presence of one extra electron. On the whole, the
behaviour of the G D peak is an effective probe to accurately measure the local
effect of a single defect (including the one determined by dopants) on the
electronic and vibrational properties of sp 2 -hybridised carbon material.
In addition, taking into account the effects determined by localised
defects, the authors have suggested introducing such defects “artiicially” at
speciic sites, where they behave as bouncing centres, thus modulating the
electric current along precise paths. 156 This type of defect can be incorporated
while preserving the connectivity of each carbon atom embedded within the
graphitic lattice. At the same time, it stimulates the design of defect-based
nanostructures for nanoelectronic devices.
Although not mentioned previously, doping of CNTs with different moieties
could occur internally or to the external wall of the tubes. In the second case,
we refer to exohedral doping. In an interesting paper, 157 the authors combined
density functional theory (DFT) and resonance Raman spectroscopy to
disclose the effects of exohedral doping on M@S and S@M DWCNTs), where
a metallic (M) tube is either inside or outside a semiconducting (S) one.
More precisely, there exist four possible conigurations (M@M, M@S, S@S
and S@M), which should present different electronic properties. The authors
have recently reported a detailed Raman spectroscopy analysis carried out
on individual DWCNTs, where both concentric tubes were measured under
resonance conditions, to understand the dependence of their electronic and
optical properties according to their coniguration. 158 Interestingly, they
found that metallic nanotubes were extremely sensitive to doping even when
they were inner tubes, in sharp contrast to semiconducting nanotubes, which
were not affected by doping when the outer shell was a metallic nanotube
(screening effects).
The researchers have also elaborated a strategy based on DWCNTs
luorination, which enabled the speciic suppression of the radial breathing
mode (RBM) and absorption peaks from the outer tubes of DWCNTs. 159
Conversely, Raman signals from the inner tube showed no difference from
the pristine DWCNTs, thus suggesting luorination as a useful method to
preserve the inner tube from having unwanted contact with other substances
that may distract from the inner tube's own characteristics.
In a parallel experiment, DWCNTs were proposed as the preferable
reinforcing iller in polymers. 160 In fact chemical moieties, which were
selectively introduced on the outer tubes (i.e., exohedral incorporation),
imparted effective anchoring sites for strong bonding with polymers, while the
optical properties of the inner tubes of the DWCNTs remained unchanged. On
the whole, these indings promote further exohedral investigations towards
novel nanoscale electronics.