Optical properties of annealed nearly percolated Au thin films

The optical properties evolution of thermally annealed gold (Au) discontinuous films is investigated and correlated with morphological changes. Nearly percolated Au films with different mass thicknesses are deposited by electron beam evaporation and annealed at temperatures ranging from 200 °C to 525 °C at normal atmosphere. Optical characterization is carried out using transmittance and spectroscopic ellipsometry measurements and film morphology is analyzed with scanning electron microscopy (SEM) and atomic force microscopy (AFM). Film depercolation takes place at temperatures between 250 °C and 300 °C depending on the deposited mass thickness. Annealing at lower temperatures improves film connectivity and quenches the localized plasmon resonance of isolated nanostructures. High-temperature annealing results in well-separated particles with significantly different size distributions depending on the deposited mass thickness, indicating that annealing induces dewetting with different pathways depending on the initial film morphology. The optical properties reflect the interplay among these morphological modifications, displaying a large variation for films annealed around the temperature where depercolation takes place. These results evidence that the optical response of nearly percolated Au films can be broadly tuned in a simple manner, making these samples attractive candidates for lithography-free photonic applications.