Effects of particle size and spacing on the optical properties of gold nanocrystals in alumina

Au-particle/Al₂O₃-matrix nanocomposite thin films with a narrow Au particle size distribution were fabricated by radio-frequency magnetron cosputtering. The films were characterized both chemically and structurally using x-ray photoelectron spectroscopy, x-ray diffraction, and transmission electron microscopy. The optical absorption of samples with various Au particle sizes and concentrations were measured using an ultraviolet/visible-spectrometer. Both the Au nanoparticle size and the Au concentration increase result in a redshift of the Au surface-plasmon resonance energy. The redshift due to the size effect is concluded as mainly originating from the size-dependent dielectric function of Au core electrons and a semiquantitative description of its size dependence is presented. The experimental result describing the concentration effect suggests a deviation from the conventional Maxwell-Garnett effective medium theory, which can be corrected by introducing the multipole effect based on the dipole-dipole interaction model.