Coupled plasmon resonances in monolayers of metal nanoparticles and nanoshells

We report on the coupled plasmon resonances in a monolayer consisting of metal or metallodielectric nanoparticles with the dipole and quadrupole single-particle resonances. The theoretical models included spherical gold and silver particles and also gold and silver nanoshells on silica and polystyrene cores forming two dimensional random clusters or square-lattice arrays on a dielectric substrate (glass in water). The parameters of the individual particles were chosen so that a quadrupole plasmon resonance could be observed along with the dipole-scattering band. By using an exact multipole cluster-on-a-substrate solution, we showed that particle-substrate coupling can be neglected in the calculation of the monolayer-extinction spectra, at least for the glass-in-water configuration. When the surface particle density in the monolayer was increased, the dipole resonance became suppressed and the spectrum for the cooperative system was determined only by the quadrupole plasmon. The dependence of this effect on the single-particle parameters and on the cluster structure was examined in detail. In particular, the selective suppression of the long-wavelength extinction band was shown to arise from the cooperative suppression of the dipole-scattering mode, whereas the short-wavelength absorption spectrum for the monolayer was shown to be little different from the single-particle spectrum. For experimental studies, the silica/gold-nanoshell monolayers were fabricated by the deposition of nanoshells on a glass substrate functionalized by silane-thiol cross-linkers. The measured single-particle and monolayer-extinction spectra are in reasonable agreement with simulations based on the nanoshell geometrical parameters (scanning electron microscopy data). Finally, we evaluated the sensitivity of the coupled quadrupole resonance to the dielectric environment to show a universal linear relation between the relative shift in the coupled-quadrupole-resonance wavelength and the relative increment in the environment refractive index.