The scanning tunneling microscope (STM) provides access to atomic-scale properties of a conductive sample. While single-molecule tip functionalization has become a standard procedure, fluorescent molecular probes remained absent from the available tool set. Here, the plasmonic tip of an STM is functionalized with a single fluorescent molecule and is scanned on a plasmonic substrate. The tunneling current flowing through the tip-molecule-substrate junction generates a narrow-line emission of light corresponding to the fluorescence of the negatively charged molecule suspended at the apex of the tip, i.e., the emission of the excited molecular anion. The fluorescence of this molecular probe is recorded for tip-substrate nanocavities featuring different plasmonic resonances, for different tip-substrate distances and applied bias voltages, and on different substrates. We demonstrate that the width of the emission peak can be used as a probe of the exciton-plasmon coupling strength and that the energy of the emitted photons is governed by the molecule interactions with its environment. Additionally, we theoretically elucidate why the direct contact of the suspended molecule with the metallic tip does not totally quench the radiative emission of the molecule.

4 Physical Institute Solids and Nanostructures Georg August Universität Göttingen Göttingen Germany

CIC nanoGUNE BRTA Donostia San Sebastián Spain

Department of Electricity and Electronics FCT ZTF UPV EHU Leioa Spain

Donostia International Physics Center Donostia San Sebastián Spain

IKERBASQUE Basque Foundation for Science Bilbao Spain

Institute of Experimental and Applied Physics University of Regensburg Regensburg Germany

Institute of Physics Czech Academy of Sciences Prague Czech Republic

Materials Physics Center CSIC UPV EHU Donostia San Sebastián Spain

Université de Strasbourg CNRS IPCMS Strasbourg France

Université Paris Saclay CNRS Institut des Sciences Moléculaires d'Orsay Orsay France

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