Hybrid density functional study of armchair SiC nanotubes

Abstract

First-principles calculations for the electronic and geometric structures of three different types of armchair silicon carbide nanotubes from (3, 3) to (11, 11) have been performed using hybrid density functional theory and the finite cluster approximation. Full geometry and spin optimizations have been performed without any symmetry constraints. A detailed comparison of the structures and stabilities of the three types of nanotubes is presented. For type 1 nanotube, the cohesive energy appears to saturate at $4.63\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, whereas for type 2 and 3 nanotubes, the cohesive energy saturates at approximately $4.44\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The dependence of the electronic band gaps on the respective tube diameters, energy density of states, and dipole moments as well as Mulliken charge distributions have been investigated. For type 1 nanotubes, Si atoms moved toward the tube axis and C atoms moved in the opposite direction after relaxation, consistent with other SiC nanotubes found in literature. For type 2 and the newly proposed type 3, this displacement direction is reversed. The band gaps for type 1 nanotubes are larger than bulk $3C\text{\ensuremath{-}}\mathrm{Si}\mathrm{C}$ gap, varying between 2.78 and $2.91\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, while type 2 and type 3 nanotubes have significantly lower band gaps. Unlike the other two types, band gap for type 3 nanotubes decreases monotonically with increasing tube diameter from $1.22\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for the smallest tube to $0.79\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for the largest (11, 11) tube studied here. The corresponding numbers for type 2 are 1.49 and $0.91\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ with an oscillatory pattern. None of the tubes appear to be magnetic. It is expected that these tubes will have interesting and important applications in the field of band gap engineering and molecular electronics.