“Hot” electrons in metallic nanostructures—non-thermal carriers or heating?

Understanding the interplay between illumination and the electron distribution in metallic nanostructures is a crucial step towards developing applications such as plasmonic photocatalysis for green fuels, nanoscale photodetection and more. Elucidating this interplay is challenging, as it requires taking into account all channels of energy flow in the electronic system. Here, we develop such a theory, which is based on a coupled Boltzmann-heat equations and requires only energy conservation and basic thermodynamics, where the electron distribution, and the electron and phonon (lattice) temperatures are determined uniquely. Applying this theory to realistic illuminated nanoparticle systems, we find that the electron and phonon temperatures are similar, thus justifying the (classical) single-temperature models. We show that while the fraction of high-energy “hot” carriers compared to thermalized carriers grows substantially with illumination intensity, it remains extremely small (on the order of 10 −8 ). Importantly, most of the absorbed illumination power goes into heating rather than generating hot carriers, thus rendering plasmonic hot carrier generation extremely inefficient. Our formulation allows for the first time a unique quantitative comparison of theory and measurements of steady-state electron distributions in metallic nanostructures. Plasmonics: The latest ‘hot’ topic in photo-catalysis Plasmonic metal nanoparticles are attracting considerable attention as they were claimed to produce high energy non-thermal (so-called ‘hot’) electrons for use in photo-catalytic reactions, such as hydrogen dissociation, water splitting, and artificial photosynthesis. Estimating the number of hot electrons generated by a given level of illumination and disentangling it from mundane heating, however, has remained challenging. Now, Yonatan Dubi and Yonatan Sivan from Ben-Gurion University, Israel, have developed a model for determining the electron distribution in a metal nanostructure under continuous wave illumination, allowing, for the first time, a comparison of heating and non-thermal effects in the steady-state electron distributions in metallic nanostructures. They find that non-thermal carrier generation is an extremely small effect, as essentially all absorbed photon energy gives rise to heating. This finding revolutionizes our understanding of plasmon-assisted photocatalysis experiments.