Adsorption of Cu, Pd, and Cs atoms on regular and defect sites of the SiO2 surface

The interaction of isolated Cu, Pd, and Cs atoms on regular and defect sites of the SiO2 surface has been studied with cluster models. Hartree-Fock, density functional theory, and multireference configuration interaction methods have been used to characterize the bonding of the metal atoms at the following sites: bridging oxygens at the regular surface, =Si-O-Si=, Si singly occupied sp(3) dangling bonds (E' centers), =Si-., nonbridging oxygen centers, =Si-O-., and neutral oxygen vacancies, =Si-Si=. The bonding with the nondefective sites of the surface is very weak, <0.2 eV. Sticking of the atoms occurs only at the defect sites with adsorption energies ranging from 1 to 3 eV. Several spectral signatures of the bond at the silica defects have been considered: core level shifts, impurity levels in the band gap, and optical transitions.