Geometry and electronic stability of tungsten encapsulated silicon nanotubes

Density functional theory involving generalized gradient approximation (both PW91 and BLYP level calculations are considered and compared) correlation functional is used to investigate the stability of W encapsulated W(n)@Si(6n+6) (n=1-6) hexagonal prism (HP) nanotubes and W(n)@Si(8n+4) (n=1-6) AB(2) type nanotubes. We found that the stability of HP type metal encapsulated silicon nanotubes (MESNTs) exhibits odd-even (O-E) oscillation behavior versus the cluster size which has been proven by the O-E oscillation pattern of the formation energy and the embedded energy. The strong interaction between W and W atoms made W(3)@Si(24) HP type nanotube unstable and we explained why Hiura et al. [Phys. Rev Lett. 86, 1733 (2001)] did not observe clusters bigger than W(2)@Si(18) in their experiment. After that, we proposed a new kind of AB(2) type MESNT in which the strong interaction between W and W atoms has been eliminated effectively, and it might be connected to form longer nanowire structures. Big distortions appeared for both type structures after reoptimized at BLYP level (the initial structures were chosen as those optimized at PW91 level theory) calculation. Metallic character of these two types of MESNTs has been identified and AB(2)-MESNTs were found to be more stable than HP type MESNTs by comparing their linearly fitted total binding energy at BLYP level theory.