Structure and vibrational spectra of crystalline SiO2 ultra-thin films on Mo(1Â 1Â 2)

The structure and the vibrational properties of ultra-thin SiO2 films (one to three layers, 1-3 L) epitaxially grown on a Mo(1 1 2) substrate have been determined by first principle DFT calculations. For 2 L and 3 L films the structures considered are derived from those of β-cristobalite, β-tridymite, α- or β-quartz; for 1 L films we considered regular arrays of isolated SiO4 tetrahedra, one-dimensional (SiO3)n chains, or an hexagonal two-dimensional (SiO2.5) structure. The 1 L films have the c(2 × 2) pattern observed experimentally. On the basis of the comparison of the computed frequencies with IR and HREEL spectra, of the existing data on film thickness, periodicity, and surface reactivity, we conclude that the film consists of a single silica layer with hexagonal c(2 × 2) structure.