First-principles optical properties of silicon and germanium nanowires

In this work we study the optical properties of hydrogen-passivated, free-standing silicon and germanium nanowires, oriented along the [I 00] [I I 0] [I I I I directions with diameters up to about 1.5 nm, using ab-initio techniques. In particular, we show how the electronic gap depends on wire's size and orientation; such behaviour has been described in terms of quantum confinement and anisotropy effects, related to the quasi one-dimensionality of nanowires. The optical properties are analyzed taking into account different approximations: in particular, we show how the many-body effects, namely self-energy, local field and excitonic effects, strongly modify the single particle spectra. Further, we describe the differences in the optical spectra of silicon and germanium nanowires along the [100] direction, as due to the different band structures of the corresponding bulk compounds. (C) 2007 Elsevier B.V. All rights reserved.