STM Nanoplasmonics
Roberto Otero
Dep. de Física de la Materia Condensada, Universidad Autónoma de Madrid
IMDEA Nanoscience
The plasmonic modes supported by nanoscale metallic elements are being intensively investigated, due to their potential applications in molecular sensing, communication technology and catalysis. In particular, it was recently found that metallic nanoparticles deposited on solid surfaces with quantum emitters in the gap display a Rabi splitting of their optical spectra characteristic of the strong light-matter coupling regime. Such nanoparticle-on-mirror geometry is actually very similar to the optical resonator that forms between the tip and the sample of a Scanning Tunnelling Microscope, but the last one has a number of advantages such as geometric tunability with sub-angstrom precision or the fact that it can launched by electrical currents instead of light beams that can reach the gap only in very grazing incidences. However, the light emitted due to plasmon excitation by inelastic tunneling events not only contains information about the optical properties of the cavity, but also on the electronic structure of tip and sample. In this seminar I will show a completely experimental method that we have recently developed to separate the optical and electronic structure from STM luminescence spectra. Our results dispel the disagreement between the theoretically expected shifts in the plasmonic modes with changing tip-surface distances and the experimentally observed ones, which had been an issue ever since the discovery of STM-induced luminescence back in the 90’s. Moreover, when extended to photon energies close to the applied bias voltages, our analysis also clarifies the general shape of the emission edge, shedding light on the origin of the overbias emission that has motivated an important debate on the literature, and allowing us to determine the electronic temperature of the junction. Finally, we will show that most of the important changes in the luminescence of the junction when organic molecules are adsorbed on the surface have a purely electronic origin: there are optical changes, much are much more subtle than the observed effects before normalization.