Ab initio Molecular Dynamics Study on Small Carbon Nanotubes

Ab initio molecular dynamics simulations are performed on small single wall nanotubes. By structural relaxation,the equilibrium C-C bond lengths and bond angles are determined. Our result shows that for both zigzag and armchair nanotubes there are two nonequivalent bond lengths. One bond stretches from that of the graphene sheet, while the other shrinks. Small variations on bond angles are also shown. Energy bands are calculated for the optimized structures. It is found that the intrinsic curvature of the very small nanotube greatly modifies the energy band which can no longer be well described in the tight-binding zone-folding picture. In our calculation very small nanotubes are metallic. The energy per atom fits quite well with the relation of E(R) = E0 + f/R2 even for the extreme small radius. The implications of the results on the properties of small nanotubes are discussed.

Density-functional theory investigation of energy gaps and optical properties of Hg_(1-x)Cd_x Te and In_(1-x)Ga_x As

We use a modified Becke-Johnson exchange plus a local density approximation correlation potential within the density functional theory to investigate the electronic structures of Hg1-xCdxTe and In1-xGaxAs with x being 0, 0.25, 0.5, 0.75, and 1. For both of the two series, our calculated energy gaps and dielectric functions （real part 61 and imaginary part 62） are in agreement with the corresponding experimental results with x being between 0 and 1. The calculated zero-frequency refractive index varies greatly with x for Hg1-xCdxTe, but changes little with for In1-xGaxAs, which is consistent with the real parts of their dielectric functions. Therefore, this new approach is satisfactory to describe the electronic structures and the optical properties of the semiconductors.