Ultrathin, ordered oxide films on metal surfaces

Metal oxides and oxide thin films are extensively used as active catalysts and catalytic supports, as well as in many other important technical applications. Unlike TiO2, which is a semiconductor and can be investigated using a variety of surface science techniques, most metal oxides are insulators, which seriously restricts their use as model surfaces with modern surface science techniques. This difficulty can be circumvented by synthesizing ultrathin oxide films a few nanometers in thickness with well-defined structures, that mimic the corresponding bulk oxides yet are thin enough to be sufficiently conducting. In this review, preparations, structures, electronic and chemical properties of four representative oxides, alumina, magnesium oxide, silica, and titania, are addressed. Of these MgO is found to grow in a layer-by-layer fashion, allowing preparation of crystalline thin film structures with varying thicknesses. Crystalline TiO2 and Ti2O3 can also be synthesized, whereas SiO2 and Al2O3, although amenable to synthesis as well-defined monolayer structures, have only been grown to date as amorphous multilayers.