Density-functional theory study of the electronic structure of thin Si/SiO2 quantum nanodots and nanowires

The atomic and electronic structures of a set of proposed pentagonal thin (1.6 nm in diameter) silicon/silica quantum nanodots (QDs) and nanowires (NWs) with narrow interface, as well as parent metastable silicon structures (1.2 nm in diameter), were studied using cluster B3LYP/6-31G(*) and periodic boundary condition (PBC) plane-wave (PW) pseudopotential (PP) local-density approximation methods. The total density of states (TDOS) of the smallest quasispherical QD (Si-85) corresponds well to the PBC PW PP LDA TDOS of the crystalline silicon. The elongated SiQDs and SiNWs demonstrate the metallic nature of the electronic structure. The surface oxidized layer opens the band gap in the TDOS of the Si/SiO2 species. The top of the valence band and the bottom of conduction band of the particles are formed by the silicon core derived states. The theoretical band gap width is determined by the length of the Si/SiO2 clusters and describes the size confinement effect in the experimental photoluminescence spectra of the silica embedded nanocrystalline silicon with high accuracy.