Atomic Scale Insights into the Early Stages of Metal Oxidation: A Scanning Tunneling Microscopy and Spectroscopy Study of Cobalt Oxidation

Understanding the mechanisms driving the early stages of oxidation of metallic surfaces is of fundamental importance in fields such as nanocatalysis, electrochemistry, and nanoelectronics. In this work, the early, stages of oxidation of ultrathin Co films deposited on the Fe(001)-p(1 x 1)O surface have been investigated by means of Auger electron spectroscopy, scanning tunneling microscopy, and scanning tunneling spectroscopy. The oxidation is initiated by homogeneous oxide nucleation over the atomically flat Co terraces, inducing the development of a smooth oxide wetting layer when the islands coalesce. Atomically resolved images reveal that the oxide wetting layer is highly defective, possibly due to the presence of oxygen vacancies. Constant current scanning tunneling microscopy images acquired in different tunneling conditions, as well as scanning tunneling spectroscopy, reveal the distinct electronic properties of the oxide nuclei with respect to the chemisorbed phase. The fundamental band gap develops since the early stages of oxide nucleation. Moreover, spectroscopic curves acquired in the near-field-emission regime reveal a significant lowering of the sample work function induced by the oxide development. Our results represent a remarkable case in which metal oxidation can be studied at the atomic-scale level.