Figure 2. Theoretical calculations using Density Functional Theory (DFT) within the GGA/PBE approximation defining stable dopant sites.

Relative substitutional energies for B, N, and S dopant are shown for various positions along a (5,5)/(9,0) nanotube (boomerang-type structure). The short dashed lines correspond to substitutional energy in a straight (9,0) and (5,5) tube, respectively. The table shows the energy average over the substitutional sites located at the pentagonal and heptagonal knee position (the energy is relative to that in a periodic (5,5) nanotube). Boron does not promote any type of closure but rather strongly favors structure with a large number of regions exhibiting negative curvature.