Optical properties of nanostructured metal films

Nanostructured metal films of platinum, gold and silver up to 675 nm thick we prepared by electrochemical deposition through templates of 700 nm diameter polystyrene spheres assembled as hexagonal close packed monolayer on an evaporated gold surface followed by removal of the template by dissolution in tetrahydrofuran. The reflection spectra of the films at normal incidence were recorded as a function of film thickness and the spectra correlated with the local visual appearance of the film and the surface structure from SEM. For thin films, below one quarter sphere height, the spectra show a single reflectivity dip at a wavelength just below the sphere diameter consistent with surface-plasmon grating-like behaviour. For the thicker films several reflectivity dips are observed which move towards longer wavelength with increasing film thickness. This behaviour is shown to be consistent with a model in which light reflected from the top of the structure interferes with light reflected from within the spherical segment cavities in the film.