Effect of electromagnetic interactions on plasmon excitations in silver particle ensembles

The optical properties of silver nano-particle ensembles are studied as a function of particle density by analyzing the light emission excited via electron injection from an STM tip. The particles are prepared with distinct dome and disk-like shapes on a Al2O3/NiAl(1 1 0) support. The particle density is varied over one order of magnitude by changing the Ag deposition temperature and the defect concentration in the oxide surface. With increasing density, a pronounced blue shift of the Ag plasmon mode is observed for ensembles of dome-like particles, whereas disk-like particles show relatively constant resonance energies. The observed evolution of plasmon energy as a function of particle density reflects the influence of electromagnetic interactions in the ensemble, as verified by model calculations.