Silicon nanotubes: Why not?

A diamond nanowire (CNW), a silicon nanowire (SiNW), a carbon nanotube (CNT), and a silicon nanotube (SiNT) were studied using the semiempirical molecular orbital PM3 method, with confirmations by calculations at the HF/3-21G and HF/3-21G(d) levels. It was shown that the systems with a diamond structure generally possess larger band gaps than their tubular counterparts. Carbon nanotubular structure shows efficient sp2 hybridization and π bonding, thus allowing a high stability of the carbon nanotube structure. In contrast, silicon prefers sp3 hybridization and favors the tetrahedral diamond-like structures, thereby forming the commonly observed nanowires. This distinction can be traced to the differences in the energetics and overlaps of the valence s and p orbitals of C vs Si. Nevertheless, when the dangling bonds are properly terminated, SiNT can in principle be formed. The resulting energy minimized SiNT, however, adopts a severely puckered structure (with a corrugated surface) with SiSi distances ranging from 1.85 to 2.25 Å.