Thin silica films on Ru(0001): Monolayer, bilayer and three-dimensional networks of [SiO ₄] tetrahedra

The atomic structure of thin silica films grown over a Ru(0001) substrate was studied by X-ray photoelectron spectroscopy, infrared reflection absorption spectroscopy, low energy electron diffraction, helium ion scattering spectroscopy, CO temperature programmed desorption, and scanning tunneling microscopy in combination with density functional theory calculations. The films were prepared by Si vapor deposition and subsequent oxidation at high temperatures. The silica film first grows as a monolayer of corner-sharing [SiO ₄] tetrahedra strongly bonded to the Ru(0001) surface through the Si-O-Ru linkages. At increasing amounts of Si, the film forms a bilayer of corner-sharing [SiO ₄] tetrahedra which is weakly bonded to Ru(0001). The bilayer film can be grown in either the crystalline or vitreous state, or both coexisting. Further increasing the film thickness leads to the formation of vitreous silica exhibiting a three-dimensional network of [SiO ₄]. The principal structure of the films can be monitored by infrared spectroscopy, as each structure shows a characteristic vibrational band, i.e., ∼1135 cm ^-1 for a monolayer film, ∼1300 cm ^-1 for the bilayer structures, and ∼1250 cm ^-1 for the bulk-like vitreous silica.