STM fluorescence of porphyrin enhanced by a strong plasmonic field and its nanoscale confinement in an STM cavity

We have investigated scanning tunneling microscope-induced luminescence (STML) from porphyrin molecules by varying the tip (PtIr, Ag, and Au)/substrate (Pt, Ag, Au, and indium tin oxide) combinations. Strong molecular fluorescence by highest-occupied molecular orbital and lowest-unoccupied molecular orbital transition comparable to plasmon-mediated light is emitted only when both the substrate and the tip are metals but not in other cases. Along with calculations of relative electromagnetic-field powers in the tip-substrate gaps, the enhancement of STML from molecules can be interpreted in terms of the strong plasmon field and its confinement in an STM cavity. We also find rather strong energy-forbidden fluorescence of porphyrin in an Au-tip/porphyrin/Au cavity that occurs under the extremely strong field in the plasmonic nanocavity.