The Dynamics of Plasmon-Induced Hot Carrier Creation in Colloidal Gold

Abstract

There is an increasing interest in nonequilibrium “hot” carrier generation, created by the decay of collective electronic oscillations on metals known as surface plasmons. Despite extensive efforts, direct observation of the mechanism responsible for generating hot carriers due to plasmon decay has proven challenging. Here, the dynamics of hot carrier generation on gold nanoparticles (Au NPs) are followed with unparalleled detail through ultrafast X-ray absorption spectroscopy (XAS) at the X-ray free-electron laser (XFEL). In Au NPs, the plasmon dephases after 25 fs and the hot carrier population peaks within 105 fs, reaching thermal equilibrium within 1.5 ps. The nonequilibrium carriers display an energy dispersion governed by the density of states of the metal, with some carriers possessing energies surpassing that of a single photon, consistent with the involvement of an Auger heating mechanism distinct from the expected impact excitation that dominates the carrier multiplication step. The most energetic carriers exhibit relatively shorter lifespans, a property that may be critical for exploiting them in applications. This study substantiates hot carrier formation through nonradiative decay as the main decay channel of plasmon resonance. The proposed methodology provides a straightforward approach for real-time tracking of plasmon-induced hot carrier dynamics.