Molecular radicals are efficient electroluminescent emitters due to the spin multiplicity of their electronic states. The excited states often exhibit a complex composition with multiple significant electronic configurations, which are essential for their optoelectronic properties but difficult to probe directly. Here we use light-scanning tunneling microscopy to investigate such an excited state by visualizing the response of a single radical molecule to a laser excitation. We observe characteristic atomic-scale spatial photocurrent patterns that can be tuned by applied bias voltage. We interpret these patterns as resulting from decay of an excited doublet state through sequential electron transfers with the tip and the substrate. The relative contributions of two dominating electronic configurations involved in this excited state are tuned by the applied voltage. This approach thus allows for disentangling the components of multiconfigurational excited states in single molecules.

• Publikační typ
• časopisecké články MeSH

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.

• Publikační typ
• časopisecké články MeSH

Spontaneous fluorescence rates of single-molecule emitters are typically on the order of nanoseconds. However, coupling them with plasmonic nanostructures can substantially increase their fluorescence yields. The confinement between a tip and sample in a scanning tunneling microscope creates a tunable nanocavity, an ideal platform for exploring the yields and excitation decay rates of single-molecule emitters, depending on their coupling strength to the nanocavity. With such a setup, we determine the excitation lifetimes from the direct time-resolved measurements of phthalocyanine fluorescence decays, decoupled from the metal substrates by ultrathin NaCl layers. We find that when the tip is approached to single molecules, their lifetimes are reduced to the picosecond range due to the effect of coupling with the tip-sample nanocavity. On the other hand, ensembles of the adsorbed molecules measured without the nanocavity manifest nanosecond-range lifetimes. This approach overcomes the drawbacks associated with the estimation of lifetimes for single molecules from their respective emission line widths.

• Klíčová slova
• STM, TCSPC, TEPL, nanocavity, photoluminescence, phthalocyanine,
• Publikační typ
• časopisecké články MeSH