"Hot" electrons in metallic nanostructures -- non-thermal carriers or heating?

TL;DR

This paper develops a thermodynamics-based theory to analyze energy flow in illuminated metallic nanostructures, revealing that heating dominates over hot carrier generation, which remains extremely inefficient even at high illumination intensities.

Contribution

It introduces a coupled Boltzmann-heat equation framework that uniquely determines electron and phonon temperatures, clarifying the relative roles of heating and hot carriers in plasmonic systems.

Findings

Electron and phonon temperatures are similar under illumination.

Hot carriers constitute a negligible fraction (~10^{-8}) of total carriers.

Most absorbed energy results in heating rather than hot carrier production.

Abstract

Understanding the interplay between illumination and the electron distribution in metallic nanostructures is a crucial step towards developing applications such as plasmonic photo-catalysis for green fuels, nano-scale photo-detection 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 {\em 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.