We construct quasi-spherical Si nanocrystallites consisting of 17, 29, 47, 71, and 99 atoms with diameters from 0.8 to 1.6 nm embedded in an SiO2 network. All atoms have saturated bonds: Si is four-fold connected and O is two-fold connected. The models comprise 400-600 atoms and have lattice parameters of about 2 nm. The networks are subjected to a bond switching algorithm yielding models of nanocrystalline Si embedded in amorphous silica.
Subsequently, we employ density functional methods. As a result of the DFT-optimization we find that the Si nanocrystals are free of defects at the interface to the host matrix. Si-Si distances within the Si nanocrystallites are strained, the strain itself tailors off to the suboxide interface. The excess energy of the optimized models with respect to crystalline silicon and vitreous silica scales linearly with the surface of the Si nanoparticles. The interfacial energy of the nc-Si/SiO2 interface is calculated to 1.5 J/m2. We observe an increase of the band gap with decreasing cluster size due to the quantum-confinement effect. The highest occupied states of the valence band are located at Si-Si bonds close to the interface; the corresponding charge density forms a shell-like structure around the central core of the nanocrystal. The lowest unoccupied states are centered within the nanocrystal.