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 284

Figure 9.24 Schematic representation of doped carbon nanotubes.
Despite the successful incorporation of P in MWCNTs, doping SWCNTs
with large contents of P is dificult because P atoms are larger than C ones,
thus increasing the disorder within the hexagonal carbon structure. Moreover,
P by itself decreases the catalytic eficacy of iron during nanotube growth. In
addition, smaller amounts of doping elements have to be inserted into the
growth environment of SWCNTs in order to obtain samples of acceptable
quality, but they are harder to detect. Anyhow, the authors have elaborated a
successful methodology for a uniform doping of SWCNTs with P. 151
It emerged that in comparison with N-doped nanotubes, both P and
N doping created new states due to the presence of the doping species,
which appeared close to the Fermi energy. However, while for N this state
lied beneath the conduction bands, causing all N-doped nanotubes to be
metallic, P created a highly localised state that did not affect the conduction
bands, and therefore they did not inluence the semiconductive or metallic
character of the nanotubes. This was in agreement with another study, in
which density functional theory (DFT) calculations on phosphorus (P) and
phosphorus-nitrogen (PN)-doped SWCNTs were reported. 152 The results of
this study demonstrated that substitutional P and PN doping created localised
electronic states that acted as scattering centres. It was also predicted that the
energy for P-doped tube formation is lower as the curvature increases, thus
making narrow-diameter nanotubes preferable when compared with large-
diameter nanotubes. Nevertheless, for low doping concentrations (1 doping
site per ~200 atoms), the PN- and P-doped nanotubes displayed promising
properties (e.g., 50% reduction in the elongation upon fracture, constant
Young's modulus, stable conductance) for components in sensors operating
at the molecular level.
Although less discussed, doping of CNTs could be responsible for
changes in the magnetic properties of CNTs. In order to address this point,
a theoretical study was carried out for >80 sp 2 -like species (in the range
of 600-2,300 atoms) to estimate the magnetic response when additional