All-electron and pseudo-potential studies of structural and electronic properties of Si chains and nanowires*

Recent experiments\footnote{Y. Wu, et.al., Nature 430, 61 (2004); and references therein} invoke Si nanowires as promising materials for nanoscale electronic and optical devices. We carried out electronic structure calculations of silicon chains and nanowires, by using both the full-potential linearized augmented plane wave (FLAPW) method\footnote{E.Wimmer, H.Krakauer, M.Weinert, AJ Freeman, PRB 24, 864 (1981)} and the pseudopotential plane wave method. We studied two sets of H-terminated one nanometer silicon wires, one oriented along (001) and the other along(111); both show direct band gaps, with the (111) oriented wires showing a smaller gap ($\sim$2.1 eV) than (001) ($\sim$2.5 eV). This trend differs from that reported in the literature \footnote{F. Buda, et.al., PRL 69, 1272 (1992); A. M. Saitta, et.al., PRB 53, 1446 (1996)}, but it is the same in both our all-electron and well converged pseudopotential calculations. We also found that structural relaxations induce different effects on the band structure of differently oriented wires; the band gap change is nearly 0.2 eV between the ideal and relaxed models for (001) while it is negligible for (111) wires.