Electron-energy-loss spectroscopy was used to study the quantum size effect in surface-plasmon excitations of small metallic particles, with diameters ranging from 4 to 20 nm. Scanning transmission electron microscopy was used, which provided the capability of observing electron-energy-loss spectra from individual, well-characterized silver particles. The measured relation between the surface-plasmon energy and the particle size shows a definite deviation from that predicted by macroscopic theories, and cannot be well explained by existing microscopic theories.

• Received 4 June 1992

DOI:https://doi.org/10.1103/PhysRevB.46.15421