  Understanding and controlling the interaction between the excitonic states of
a quantum emitter and the plasmonic modes of a nanocavity is one of the most
relevant current scientific challenges, key for the development of many
applications, from quantum information processing devices to polaritonic
catalysts. In this paper we demonstrate that the tunnel electroluminescence of
C60 nanocrystals enclosed in the plasmonic nanocavity between a metallic
surface and the tip of a Scanning Tunnelling Microscope, and isolated from the
metal surface by a thin NaCl film, can be switched from a broad emission
spectrum, revealing the plasmonic modes of the cavity, to a narrow band
emission, displaying only the excitonic states of the C60 molecules by changing
the bias voltage applied to the junction. Plasmonic emission is found in the
same voltage region in which the rate of inelastic tunnel transitions is large
and, thus, vanishes for large voltages. Excitonic emission, on the other hand,
dominates the spectra in the high-voltage region in which the inelastic rate is
low, demonstrating that the excitons cannot be created by an inelastic tunnel
process. These results point towards new possible mechanisms to explain the
tunnel electroluminescence of quantum emitters and offer new avenues to develop
electrically tuneable nanoscale light sources.
