Plasmon Resonance Shifts of Au-Coated ${\mathrm{Au}}_{2}S$ Nanoshells: Insight into Multicomponent Nanoparticle Growth

In this Letter we present a coherent description of the optical properties of a unique type of nanoparticle, metal-coated dielectric nanoparticles. These structures occur naturally in the form of Au-coated ${\mathrm{Au}}_2\mathrm{S}$ nanoparticles. During the course of nanoparticle growth, the plasmon-related absorption peak undergoes very large shifts in wavelength, from $\sim 650$ to $\sim 900\mathrm{nm}$. We show that this plasmon peak shift is purely classical in origin and is determined solely by the relative thickness of the Au shell and the ${\mathrm{Au}}_2\mathrm{S}$ core diameter. This understanding of the optical properties of these nanoparticles is used to elucidate the nanoparticle growth kinetics.