Generation of surface plasmon excitation of supported metal particles by an external electron beam

Abstract

The relative probabilities for excitation of the various surface plasmons have been derived by use of the coherent state approach for two cases of small aluminum particles, coated with a thin oxide layer and half embedded in a supporting film. Previously [Wang and Cowley, Ultramicroscopy 21 (1987) 77] it was shown that if the supporting film is a dielectric, AlF 3, two surface plasmons (ω 1=8 eV and ω Lo =4 eV) are predicted. If the supporting film is metal Al, there are four surface plasmons (ω 1=11.5 eV, ω 2=10 eV, ω 3=8 eV and ω 4=4 eV). Each frequency was shown to be associated mainly with the excitation of a particular interface. In this paper, the calculations show relative probabilities of excitation which confirm these associations with particular interfaces. Results obtained as a function of impact parameter along different radial directions from the center of the particle and for various directions of the incident beam provide a basis for predicting the form of the STEM images expected to be formed with electrons which have lost the corresponding amounts of energy. Observations of energy loss spectra and images obtained with the particular energy losses made with an HB5 STEM instrument are in general agreement with the predictions except that the relative probabilities for excitation are more nearly equal for the various surface plasmons than predicted. It is suggested that these discrepancies arise from differences between the models used for the theoretical calculations and the actual geometry in the experimental situations.