Plasmonic-Induced Luminescence Of Mose2 Monolayers In A Scanning Tunneling Microscope

We report on a scanning tunneling microscopy-induced luminescence in MoSe2 monolayers supported by uniform and nanopatterned gold substrates. Luminescence intensity mappings, recorded with a nanometric spatial resolution, and spectroscopy measurements were performed and analyzed in terms of photon emission processes taking place within the tip-surface gap region, which supports strongly localized optical and electronic states. We found that, excited by tunneling electrons, the light emission is due to the radiative recombinations of free excitons confined within the MoSe2 monolayer. Light emission is observed at positive and negative bias voltages with very different emission rates. The results are interpreted in terms of charge carrier injection in the MoSe2 layer. Additionally, electrodynamic simulations also stress that resonance between the emitted radiation and the surface plasmons formed in the tip-surface gap region plays a critical role in the emission process. When the MoSe2 layer lays on a nanopatterned gold substrate, its luminescence intensity, induced by the tunneling electrons, is enhanced by nearly an order of magnitude. Such an effect is observed here for the first time in scanning tunneling microscopy experiments. The luminescence enhancement is attributed to the surface plasmon properties of the nanopatterned gold substrate, which spectrally match the excitonic transition of the MoSe2 layer. We show that this surface-enhanced scanning tunneling microscope-induced light emission (SESTM-LE) effect is very useful for investigating the photon emission from localized emitters (e.g., quantum wells, quantum dots, molecules) with a subnanometer spatial resolution.