Ultrafast electronic relaxation in colloidal gold (I) sulfide nanoparticles

Abstract

Ultrafast electronic relaxation dynamics in Au₂S colloidal nanoparticles have been studied using fs transient absorption spectroscopy. The electronic absorption spectrum of the nanoparticles exhibits a broad featureless absorption with increasing intensity from the near-IR into the visible and UV, indicating that Au₂S is an indirect bandgap semiconductor. The electronic relaxation dynamics have been measured with 390 nm excitation and probing at 790 and 850 nm. The transient absorption decay profiles can be fit to a double exponential with time constants of 600 fs and 23 ps. The fast decay can be assigned to trapping of electrons from the conduction band to shallow trap states or from shallow traps to deep traps, while the long decay is assigned to recombination from shallow or deep trap states. The overall fast relaxation can be attributed to a high density of intrinsic or surface trap states. This fast decay is non-radiative and consistent with no observable luminescence at room temperature. EXAFS data show a 20% decrease in the first coordination shell for nanoparticles relative to bulk, which suggests a large number of surface dangling bounds that can contribute to a high density of surface trap states.