Lithium incorporation into a silica thin film: Scanning tunneling microscopy and density functional theory

The interaction of Li with a thin silica film grown on Mo(112) has been studied with low-temperature scanning tunneling microscopy (STM) and density functional theory (DFT). Thanks to the porous silica structure, single Li atoms are able to penetrate the oxide layer and bind at the metal-oxide interface. From the distinct topographic signature of the embedded atoms, the interfacial adsorption sites are determined to be along the furrows of the Mo(112) surface. With increasing Li coverage, the formation of an ordered superstructure is observed, where stripes of high and low Li density alternate in the direction perpendicular to the Mo furrows. The ordering effect is explained by the anisotropic screening response of the Mo(112) surface. The Li becomes cationic upon adsorption at the interface and substantially lowers the work function of the silica/Mo system. This work-function decrease is directly deduced from the downshift of the silica conduction band, as determined with STM conductance spectroscopy and DFT calculations. The derived interrelation between work function and Li coverage strongly deviates from the universal behavior found on metal surfaces, reflecting the superior screening of the Li+ ions in their binding sites at the metal-oxide interface.