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Abstract

The influence of metal–oxide interactions on the workfunction and band alignment in thin
oxide films is investigated for silica mono- and bilayers grown on Mo(112) and Ru(0001)
supports. By analyzing the position of field-emission resonances and the Kelvin-probe signal
deduced from conductance and force spectroscopy, we have identified a substantial lowering
of the workfunction in the monolayer films, with the oxide bands shifting accordingly. We
explain this observation with a stronger coupling and a shorter binding length of the silica
monolayer to the metal substrate, which removes the effect of electron spill-out, produces a
positive interface dipole and reduces the workfunction of the system. In contrast, the van der
Waals bound bilayer film interacts only weakly with the Ru support, conserving the effect of
electron spill-out and keeping the workfunction high. Direct evidence for the relevance of
interface interactions comes from experiments on buckled silica films, for which regular
workfunction modulations are revealed that follow the topographic height of the film above
the metal surface.