Surface plasmon dispersion of a cluster-assembled silver nanoparticle film

We report an investigation with high-resolution electron-energy-loss spectroscopy of a thin Ag film grown by deposition and aggregation of ${\mathrm{Ag}}_3^{\mathrm{-}}$ clusters on graphite. We find the plasmon mode of the nanostructured film (particle size ∼10 nm) at about 3.85 eV near the Brillouin-zone center, representing a blueshift of 0.15 eV with respect to the surface of a bulk Ag crystal. With increasing momentum parallel to the surface, the plasmon shows a very shallow positive dispersion and a distinct broadening. The results suggest a decrease in the $s-d$ coupling in our nanoparticle film compared with the bulk.