Structure, energetic stability and tunable electronic properties of [B.sub.x][C.sub.y][N.sub.z] armchair nanotubes: a theoretical study on the influence of diameter and local carbon concentration

In this work we use density functional theory to perform a systematic theoretical study on the energetic stability, structure and electronic properties of armchair [B.sub.x][C.sub.y][N.sub.z] nanotubes consisting of C and BN phases segregated in opposite diametrically ribbons focusing the calculations on the influence of the tube diameter and local carbon concentration on such properties. Interestingly, we have found that these nanotubes stabilize in a form presenting an ellipsoidal profile for the cross section, in contrast to the usual circular shape. For each investigated diameter, we closely follow how the band gap evolves during the transformation of BN into a C tube as the atomic stripes are being replaced. In fact, we have found that nanotubes with the same minimum width of C region present the same band gap despite their different diameters. In particular we have found a variety of systems that have tunable band gaps ranging from 0.27 eV up to 1.72 eV which could become of interest for specific applications.