Copper oxide phases probed via plasmonic light emission in the STM

Abstract

Luminescence spectroscopy with a scanning tunnelling microscope (STM) is employed to differentiate bulk-type Cu _2 O from a Cu _3 O _2 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 Cu _3 O _2 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 Cu _2 O(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, Cu _2 O and Cu _3 O _2 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.

Keywords

• STM luminescence
• plasmon
• exciton
• cuprous oxide
• photon mapping