Luminescence from Cuprous Oxide in a Scanning Tunneling Microscope: Competition between Plasmonic and Excitonic Response

Conventional photoluminescence and light emission spectroscopy with a scanning tunneling microscope (STM) are used to compare the optical response of bulk Cu2O(111) and 1–10 nm-thick Cu2O/Au(111) films. According to electron diffraction, X-ray photoelectron spectroscopy, and STM, both systems share identical structural and electronic properties and expose the same thermodynamically preferred (√3 × √3) R30° termination. Their luminescence behavior exhibits, however, distinct differences. While spectra of bulk Cu2O are dominated by the recombination of free and defect-bound excitons, a plasmonic response either from interface or localized plasmons is detected for the supported films. The latter arises from electromagnetic coupling between the STM tip and the Au(111) support, whereby the Cu2O adlayer only controls the inelastic tunneling rates and thus the emitted intensity. Our paper discusses why the unique exciton response of Cu2O is covered by plasmon excitations in oxide films as thick as 10 nm. On this basis, intrinsic limitations of STM luminescence spectroscopy are addressed when exploring spatially confined optical modes in dielectric materials.