Illuminating Photocatalytic and Charge Transfer Mechanisms in Plasmonic Nanoparticle Systems.

Abstract

Plasmonic metal nanoparticles can be tuned to very efficiently convert incoming visible (solar spectrum) photons into hot charge carriers within the nanoparticles. When a material, either a molecule or semiconductor, is chemically attached to the nanoparticle, the energetic carriers can transfer into the material. Once in the attached material, the energetic charge can provide current for a device, or induce a photochemical reaction. Classical models of photo-induced charge transfer in plasmonic metals suggest that the efficiency of this process is extremely low. The vast majority of the energetic charge carriers rapidly decay within the metal and are never transferred into the neighboring molecule or semiconductor. The studies in this dissertation demonstrate a system that effectively bypasses this inefficient conventional mechanism of charge transfer. They show that a system made up of silver nanocubes and an adsorbed dye molecule (methylene blue) experiences high rates of direct metal-to-molecule charge transfer, bypassing the decay and thermalization process normally taking place in the nanoparticle. In this direct charge transfer mechanism, the yield of extracted hot carriers from plasmonic nanoparticles can be significantly higher than in conventional systems. Analysis of the results within the framework of this direct mechanism points toward a method of engineering numerous systems for efficient charge generation and extraction from plasmonic nanoparticles, with many potential applications.