Ultra-thin CoO films grown on different oxide substrates: Size and support effects and chemical stability

We have studied the growth, interface and chemical stability of ultra-thin CoO films supported on different single crystal oxides as substrates, namely SiO2, Al2O3 and MgO. The films have been grown by reactive evaporation of Co in an oxygen atmosphere at room temperature and analyzed in situ by photoemission spectroscopy. Analysis of the Co 2p 3/2 photoemission spectra, supported by theoretical cluster model calculations, reveals size effects for coverages below 5 equivalent monolayers for all substrates. In the case of the Al2O3 substrate, the predicted charge transfer from the substrate to the CoO ultra-thin-film has been experimentally observed. The way of growth of the ultra-thin films is dictated by the crystalline structure of each substrate in terms of the lattice mismatch. It has been found that the quality and stability of the films depend more on the structural parameters of the oxide substrates rather than on their covalent/ionic character. The chemical stability of the films upon air exposure and aggressive oxidizing thermal annealing has also been studied. The most suitable substrate to grow ordered and stable CoO ultra-thin-films is MgO whereas nanoclusters of CoO formed at the early stages of growth on SiO2 appear also to be very stable.