Size dependence of structural, electronic, elastic, and optical properties of selenium nanowires: A first-principles study

We have studied the structural, elastic, and optical properties of selenium nanowires, as well as bulk selenium, by performing first-principles density functional theory calculations. The nanowires are structurally similar to bulk trigonal Se, in that they consist of hexagonal arrays of helices, though there is a slight structural rearrangement in response to the finite size of the nanowires. These small structural changes result in Young's modulus decreasing slightly for progressively thinner nanowires. However, there is a significant effect on electronic structure and optical properties. The thinner the nanowire, the greater the band gap, and the greater the anisotropy in optical conductivity. The latter is due to the effects of finite size being much more marked for the case where the electric field is polarized perpendicular to the helical axis, than in the case where the polarization is parallel to c. For the case of bulk Se, we obtain good agreement with experimental data on the structure, elastic constants, and dielectric function.