Valence electron excitations and plasmon oscillations in thin films, surfaces, interfaces and small particles

Abstract

Plasmon oscillation is a collective excitation of electrons in a valence band of a solid material. The motion and polarization of valence electrons under the impact of a fast moving charged particle directly reflect the solid state properties of the material. Oscillations of surface charges depend sensitively on dielectric properties of the material and, more importantly, on the geometrical configuration of the media. The advances of electron microscopy techniques have made it possible to study local excitations from each individual particle smaller than a few nanometers in diameter. Dielectric response theory has shown remarkable success in describing the observed valence-loss spectra and resonance modes. This review gives a systematic description on the classical electron energy-loss theory and its applications in characterizing interband transition and plasmon excitations in thin films, surfaces, interfaces, isolated particles and supported particles of different geometrical configurations. These fundamental studies are important for characterizing many advanced nanophase and nanostructured materials of technological importance. This article is focused on quantitative calculation of valence-loss spectra acquired from different geometrical configurations of dielectric objects. The classical energy-loss theory is equivalent to the quantum mechanical theory, provided all the scattered electrons are collected by the spectrometer. The hydrodynamic model is also described to include fluctuation of electron density in metallic particles smaller than 10 nm in diameter. Applications of valence electron excitation spectroscopy are demonstrated using numerous experimental results.