Copper oxide phases probed via plasmonic light emission in the STM

Luminescence spectroscopy with a scanning tunnelling microscope (STM) is employed to differentiate bulk-type Cu₂O from a Cu₃O₂ surface oxide at the nanometre scale. Although plasmonic modes in the tunnelling gap are responsible for the emission response in both cases, the luminescence cross-section shows a characteristic dependence on bias voltage for the two oxide phases. On the Cu₃O₂ surface oxide, the emission yield peaks at the 1st field emission resonance of the STM junction, when particularly high inelastic tunnelling rates are realized between tip and sample. On Cu₂O(111) patches, on the other hand, direct and defect-mediated electron recombination from the oxide conduction to the valence band is responsible for plasmon excitations in the STM cavity. Given the different excitation mechanisms, Cu₂O and Cu₃O₂ oxide regions become distinguishable in bias-dependent STM luminescence maps with nanometre precision. We expect this optical contrast scheme to be applicable whenever blends of dielectric materials with different band gaps coexist on the surface. (paper)