Tuning the surface plasmon resonance by preparation of gold-core/silver-shell and alloy nanoparticles

For many applications like surface enhanced Raman scattering or fluorescence microscopy in which the optical field enhancement associated with surface plasmon excitation is exploited, tunability of this collective resonance over a wide energy range is required. For this purpose we have investigated Au/Ag alloyed and Au-core/Ag-shell nanoparticles with different shell thicknesses. The core-shell nanoparticles were prepared by subsequent deposition of Au and Ag atoms on quartz substrates followed by diffusion and nucleation. The surface plasmon frequency could be stabilized at 2.2 eV independent of the Ag-shell thickness. Annealing of the core-shell nanoparticles makes possible tuning of the resonance frequency from 2.2 eV (lambda = 564 nm) to 2.6 eV (lambda = 477 nm). Theoretical modelling allows us to attribute this observation to the formation of alloy nanoparticles.