Two-dimensional oxides and their role in electron transfer mechanisms with adsorbed species

Insulating or semiconducting oxides prepared in the form of ultrathin films grown on a metal may exhibit phenomena and behaviors that make them quite interesting for their potential applications in catalysis and other fields. In this review we consider in particular what is probably the most relevant one of these phenomena, i.e., the possibility to decouple the wave functions of the metal support and the adsorbed species by means of the thin separating dielectric layer, and consequently to induce charge transfer in one or the other direction, producing collections of positively or negatively charged adsorbates. We describe the basic results that opened up this relatively new field and discuss formation of negatively and positively charged atomic adsorbates and the mechanisms at the basis of the electron flow through the thin dielectric layer. We illustrate some direct consequences of the charge transfer on two-dimensional oxide layers, like the possibility to change the shape of supported clusters, or to dissociate chemical bonds with mechanisms that are different from the respective bulk oxides. We also discuss how the properties of ultrathin films, and in particular the work function of the system, can be modified by selective interstitial or substitutional doping.