Catching hot electrons in a single molecule

Efficiently utilizing the hot carriers – electrons and holes whose energy distribution deviates significantly from the equilibrium distribution, is the key to a broad range of emerging applications, for example, photovoltaics and photocatalysis. Even though the exploitation of hot carriers from nanoscale systems has tremendous potential, optimizing and realizing nanostructures tailored for the above-mentioned applications continues to be challenging. The primary rationale behind this roadblock is the inability to directly investigate hot carriers at their intrinsic spatial length (~ Angstrom), time (~ femtoseconds) and energy (~ electron volts) scales, due to the difficulty in achieving all the three resolutions simultaneously in state-of-the-art experiments. By developing broadband nonlinear spectroscopy at the atomic space-time resolutions, researchers from Max Planck Institute for Solid State Research (MPI-FKF) and Politecnico di Milano have now succeeded in capturing the dynamics of hot carriers in a single molecule.

The pursuit to utilize hot carriers in nanoscale metals to develop highly efficient photovoltaic devices, drive and control single molecule photochemistry and to realize ultrafast all-optical switching from metasurface configurations is long-standing. The hot carriers are present in the atomic scale volumes of a nanoscale system and have a broad spectrum of energies. Although their effective exploitation has tremendous potential, optimizing and realizing nanostructures tailored for the above-mentioned applications continues to be challenging.

A direct visualization of the hot carriers is required in order to understand their intricate behaviour at the atomic scales. In our work, we have introduced atomic scale microscopy and nonlinear optical spectroscopy to directly visualize hot carriers at their intrinsic length (~ Angstrom), time (~ femtoseconds) and energy (~ electron volts) scales, a feat, which has been beyond reach until date.

The spatiotemporal dynamics of the hot carriers probed by a unique design of pump-probe spectroscopy in a picocavity revealed a complex energy dependent behaviour. The atomic scale diffusion and relaxation of the hot carriers is much faster for high-energy carriers compared to the low-energy carriers. The excitation of hot carriers in the picocavity enables ultrafast (~ 10 THz) all-optical control over the broadband (~ eV) anti-Stokes electronic resonance Raman scattering (ERRS) and the four-wave mixing (FWM) signals generated at the atomic length scale.

Unleashing the potential of our technique, we have mapped the distribution of the hot carriers and the nonlinear optical susceptibilities (χ⁽³⁾, related to FWM) in a single graphene nanoribbon (GNR) at the atomic length scales. We show that both ERRS and FWM signals are more efficiently generated along the edges of the GNR — the manifestation of the atomic-scale nonlinear optical microscopy. The atomic-scale nonlinear optical microscopy reveals the sub-molecular scale variation of the optical properties of a molecule, which were previously accessible only in ensemble measurements at the macroscopic length scales.

More information: Luo, Y., Sheng, S., et al.  Visualizing hot carrier dynamics by nonlinear optical spectroscopy at the atomic length scale. Nature Communications (2025). DOI: https://doi.org/10.1038/s41467-025-60384-2