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 means of cluster models density functional calculations. The regular, non defective, surface shows very low reactivity towards Cu atoms; the binding is largely due to polarization mechanisms. This implies that impinging Cu atoms will easily diffuse on the surface and re-evaporate unless trapped at a defect site. In fact, strong bonds are formed between Cu atoms and clusters and some typical point defects at the SiO2 surface. We have analyzed two of these defects, the non-bridging oxygen site (NBO), =Si-O-., and the E' center corresponding to a Si singly occupied sp(3) dangling bond, =Si-.. The Cu clusters interacting with these paramagnetic centers are significantly perturbed by the bonding at the interface, as shown by the different geometrical structures of supported compared to gas-phase clusters. Some observable consequences of the cluster deposition, in particular the appearance of states in the gap of the material, are discussed