Effect of adsorbed molecules on hot electron relaxation in gold nanoshells

Abstract

Summary form only given. The optical response of metal nanoparticles is dominated by the electrons and thus is extremely rapid. For bulk metals, the relaxation of hot electrons is understood as an electron-phonon interaction! The relaxation of hot electrons in nanoparticles is more complex and affected by size and embedding medium. Nanoshells are nanoparticles with a thin metal shell coating a dielectric core. For nanoshells, the excitation wavelength of the collective electron oscillation (plasmon resonance) depends on the core size and shell thickness. Nanoshells with gold sulfide core and a gold shell have a plasmon resonance tunable from 600-950 nm. When p-aminobenzoic acid aniline, n-propylamine, or p-mercaptobenzoic acid were added to an aqueous nanoshell solution, the molecules were bound to the gold surface by the amine or thiol groups. Enhanced Raman signals were detected from the bound molecules due to the large local electric fields at the metal nanoshell surface, confirming the binding of the molecules. The transient bleaching of the nanoshell solutions was measured in a degenerate pump-probe experiment. The induced change in transmission occurred because of the change in the dielectric function of the gold shell for a hot electron distribution. The mechanism by which the adsorbed molecules affect the electron relaxation could be energy transfer from the hot electrons to adsorbed molecules or perturbation of the electronic potential of the metal by the bound molecules. This perturbation could induce a decrease in Coulomb screening in the nanoshells.