New Journal of Physics | 2021
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 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.