Cu atoms and clusters on regular and defect sites of the SiO2 surface. Electronic structure and properties from first principle calculations

The interaction of isolated Cu atoms and small Cu clusters, from Cu-2 to Cu -5, with the dehydroxylated surface of silica has been investigated by mean s of cluster models density functional calculations. The regular, non defec tive, surface shows very low reactivity towards Cu atoms; the binding is la rgely due to polarization mechanisms. This implies that impinging Cu atoms will easily diffuse on the surface and re-evaporate unless trapped at a def ect site. In fact, strong bonds are formed between Cu atoms and clusters an d some typical point defects at the SiO2 surface. We have analyzed two of t hese defects, the non-bridging oxygen site (NBO), =Si-O-., and the E' cente r corresponding to a Si singly occupied sp(3) dangling bond, =Si-.. The Cu clusters interacting with these paramagnetic centers are significantly pert urbed by the bonding at the interface, as shown by the different geometrica l structures of supported compared to gas-phase clusters. Some observable c onsequences of the cluster deposition, in particular the appearance of stat es in the gap of the material, are discussed.