Origin of Shifts in the Surface Plasmon Resonance Frequencies for Au and Ag Nanoparticles

Abstract

Optical properties of noble metal (Au, Ag) nanoclusters are of remarkable interest owing to the understanding fundamental issues of electronic properties in the small metallic clusters and their applications in nonlinear optics. The surface plasmon resonance (SPR), defined as resonance frequency of coherently oscillating free electrons with the exciting light, occurring in the near-UV-Visible region leads to the major applications. We discuss results, both experimental and theoretical, reporting the red or blue shift of the SPR frequency with decreasing noble metal cluster size as an effect of embedding matrix and surrounding porosity as well as in free noble metal nanoclusters without matrix. Reduction of the electron density (spillout effect) in the small nanoclusters and the interband screening of electrons in noble metals for larger nanoclusters shift the frequency of light absorbed either to the red or blue region, respectively, with decreasing cluster size. A strong dependence of the cluster size with the SPR frequency is discussed using time dependent local density approximation (TDLDA) with the consideration of porosity of surrounding medium. As the most recent report, quantum effect is also considered to understand blue shift of the SPR frequency of individual Ag nanoclusters with reducing size.