Blueshift of the surface plasmon resonance in silver nanoparticles studied with EELS

(Dated: March 1, 2013)We study the surface plasmon (SP) resonance energy of isolated spherical Ag nanoparticles dis-persed on a silicon nitride substrate in the diameter range 3.5-26 nm with monochromated electronenergy-loss spectroscopy. A significant blueshift of the SP resonance energy of 0.5 eV is measuredwhen the particle size decreases from 26 down to 3.5 nm. We interpret the observed blueshift usingthree models for a metallic sphere embedded in homogeneous background material: a classical Drudemodel with a homogeneous electron density profile in the metal, a semiclassical model corrected foran inhomogeneous electron density associated with quantum confinement, and a semiclassical non-local hydrodynamic description of the electron density. We find that the latter two models providea qualitative explanation for the observed blueshift, but the theoretical predictions show smallerblueshifts than observed experimentally.INTRODUCTION