Electron Transfer at Oxide Surfaces. The MgO Paradigm: from Defects to Ultrathin Films

Electron transfer (ET) is a fundamental process in physics, chemistry, and biology. Charge transfer determines phenomena like oxidation and reduction, bond activation and bond breaking in chemical reactions, formation of radical species, and charge transport and charge trapping in nanoelectronic devices, just to mention a few examples. ET processes are the basis of technologically relevant fields. One of the best characterized systems is MgO, a simple stoichiometric binary oxide with rock-salt structure, which exhibits well-defined surfaces and is stable under operating conditions. Despite this apparent simplicity, a lot of work spanning several decades has been necessary to unravel the atomistic details of reactivity of this material. When going from a (001) single crystal surface, where the number of such sites is negligible, to a powder sample, where these sites represent a significant fraction of the total exposed surface, one observes a complete change of reactivity, from totally inert to highly reactive.