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Abstract

The vibrational properties of well-defined, two-dimensional silica films grown on Ru(0001) are characterized by high-resolution electron energy loss spectroscopy (HREELS). It is an interesting model system because it can adopt both crystalline and vitreous states. A transformation between these states induced by thermal annealing does hardly change the vibrational spectrum despite the redistribution of ring sizes. This holds good for the two intense phonon modes as well as for a variety of weaker modes observed by HREELS. The HREELS spectra allow the characterization of the structural arrangement of the oxygen atoms on the Ru(0001) surface underneath the silica bilayer. The density of oxygen at the interface can be controlled by the oxygen partial pressure during annealing, resulting in a characteristic change of the corresponding signals, which can be assigned to different oxygen structures based on density functional theory calculations. By comparison with quantum mechanical calculations and spectroscopic results from the literature, we assign most of the remaining weak signals observed here to the dipole-inactive modes of the bilayer film, structural imperfections such as patches of monolayer structure, and additional silica particles on top of the bilayer film.