Optical Properties of Granular Silver and Gold Films

Optical, electrical, and electron-microscopy studies on granular Ag-Si${\mathrm{O}}_2$ and Au-Si${\mathrm{O}}_2$ films, prepared by cosputtering the metal and the insulator, were made over the composition range 10-90% by volume Si${\mathrm{O}}_2$. Electron diffraction patterns indicate crystalline metals with lattice constants equal to those of the bulk metals. For films with less than 50-vol% Si${\mathrm{O}}_2$ the structure consists of amorphous Si${\mathrm{O}}_2$ inclusions in a continuous metal matrix; for films with more than 50-vol% Si${\mathrm{O}}_2$, separate metallic particles are dispersed in an amorphous Si${\mathrm{O}}_2$ continuum. Near the 50-vol% Si${\mathrm{O}}_2$ composition the electrical resistivity increases abruptly and the infrared transmission changes from metal-like to insulatorlike behavior. The absorption and transmission peaks in the visible, observed both in the continuous-metal as well as in the continuous-insulator regimes, are explained by a generalized Maxwell-Garnett theory. It is concluded that metal particles as small as 20 Å have optical constants that do not differ significantly from those of the bulk metals.