Concentric nanoshells and plasmon hybridization

Abstract

The optical properties of metal nanostructures are related to their plasmon response, which is sensitively dependent on nanostructure geometry and environment. The metallodielectric, core-shell structure of nanoshells represents a unique geometry allowing for the systematic tunability of the plasmon resonance of the nanostructure. This is accomplished by varying the relative dimensions of the core and shell layers. Fabrication of a nanoshell particle with a strong plasmon resonance is dependent on shell quality, which is strongly dependent on the careful preparation of the metal shell. The resonant response of metal nanostructures can also be modified through plasmon-plasmon interactions. This work focuses on the fabrication of nanoparticles with a multilayer, concentric-shell structure consisting of a silica core, inner gold shell layer, silica spacer layer, and an outer gold shell layer. This concentric nanoshell particle is fabricated through the controlled growth of a nanometer-scale silica layer around a preformed nanoshell. The silica layer was found to increase the thermal and chemical stability of the nanoshell particles. A second gold shell could be grown on this layer to generate the concentric nanoshell particle. This layered nanoparticle geometry has a plasmon resonance dependent on the interaction between the inner and outer shell plasmons. This interaction can be explained in terms of a sphere-cavity model of plasmon hybridization derived from a semi-classical model of the plasmon resonance. Varying the dimensions of the concentric shell layers can independently and systematically control the plasmon resonance of the inner and outer shell, which effects the interaction between the two plasmons. The coupling between the inner and outer shell plasmons was investigated experimentally by varying the concentric nanoshell dimensions, specifically examining how the spectral detuning of the inner and outer shell resonances and spatial interaction between inner and outer shell plasmons determine the nanoparticle's optical properties. Calculations using Mie scattering theory to model the nanoshell plasmon response agree quantitatively with experimental measurements of the nanoshell plasmon resonance in both the single-layer and multi-layer regime.