ATOMIC AND ELECTRONIC-STRUCTURE OF NEUTRAL AND CHARGED SINOM CLUSTERS

Using molecular orbital approach and the generalized gradient approxim ation in the density functional theory, we have calculated the equilib rium geometries, binding energies, ionization potentials, and vertical and adiabatic electron affinities of SinOm clusters (n less than or e qual to 6,m less than or equal to 12). The calculations were carried o ut using both Gaussian and numerical form for the atomic basis functio ns. Both procedures yield very similar results. The bonding in SinOm c lusters is characterized by a significant charge transfer between the Si and O atoms and is stronger than in conventional semiconductor clus ters. The bond distances are much less sensitive to cluster size than seen for metallic clusters. Similarly, calculated energy gaps between the highest occupied and lowest unoccupied molecular orbital (HOMO-LUM O) of (SiO2)(n) clusters increase with size while the reverse is the n orm in most clusters. The HOMO-LUMO gap decreases as the oxygen conten t of a SinOm cluster is lowered eventually approaching the visible ran ge. The photoluminescence and strong size dependence of optical proper ties of small silica clusters could thus be attributed to oxygen defec ts. (C) 1998 American Institute of Physics.