Fluorescent single-molecule STM probe

TL;DR

This paper presents a novel fluorescent single-molecule STM probe that uses tip-induced plasmonic interactions to study molecular fluorescence and trion-plasmon coupling at the nanoscale.

Contribution

It introduces a new method combining STM and fluorescence spectroscopy to analyze single-molecule emissions and plasmonic interactions with high spatial resolution.

Findings

Emission peak width correlates with trion-plasmon coupling strength.

Photon energy depends on molecule-environment electrostatic interactions.

Direct contact with metal does not fully quench molecular fluorescence.

Abstract

The plasmonic tip of a scanning tunnelling microscope (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 (trion). 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 trion-plasmon coupling strength and that the energy of the emitted photons is governed by the molecule interactions with its electrostatic 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.