3.1. Geometries and electronic properties
Fig. 1 shows the side-view model of the optimized structures
for zigzag single-walled (6,0) BC3NTs. Geometry optimizations
reveal that the nanotubes can still retain their circular cross
section after N-doping, and the relaxed C–N and B–N bond lengths
are in the range of 1.38–1.43 and 1.51–1.52 Å, respectively. In
general, the C–C bond lengths are not significantly changed upon
N-doping in the considered models. We note that, after geometry
optimization, the N atom is pushed just slightly inward upon
structural relaxation, and the tube diameter decreases from 4.9 Å
in the pristine to 4.8 Å at the site of nitrogen of NB7 model. The
nanotubes do not suffer any significant structural distortions, but
there is a small decrease in the nanotubes diameters at the dopant
site. These data indicate that nitrogen atom can be comfortably
incorporated into a BC3 network at a high N/C atomic ratio. It is
likely associated with the triple-coordination characteristic of
nitrogen atom, which can well match with the sp2
-hybridized
BC3 bonding structure; that is, the geometrical structure of sp2
-
carbon networks will not change greatly when nitrogen atoms are
incorporated. In fact, nitrogen and carbon atoms can be well
bonded as a stable carbon nitride structure such as C3N4 [47]. This
conclusion is consistent with the previous results of the DFT
calculations of N-doped short carbon nanobells [63] and open
(10,0) CNT [64]. It also agrees with the previous experimental
results of Matter et al. [65].