Growth of cobalt on a VO(1Â 1Â 1) surface: Template, surfactant or encapsulant role of the oxide nanolayer?

Cobalt overlayers, from submonolayer coverages to several monolayers thick, have been grown on to an oxygen-terminated VO(1 1 1) bilayer supported on a Rh(1 1 1) surface. Scanning tunnelling microscopy (STM) images show that the Co deposits exhibit a rough morphology at room temperature, but that annealing in ultrahigh vacuum (UHV) up to 670 K results in a remarkable ordering of the VO-Co layer system, producing flat structures which display the same lattice parameters as the bare VO(1 1 1) surface. X-ray photoelectron spectra (XPS) and scanning tunnelling spectroscopy (STS) differential conductance spectra reveal that the VO bilayer segregates on top of the Co layer already during the Co deposition at room temperature, encapsulating the Co film. It is proposed that the segregation of the VO layer is driven by the low surface free energy of the oxygen-terminated VO(1 1 1) phase.