Optical properties of cluster-assembled nanoporous gold films

We report a spectroscopic ellipsometry investigation of the electronic and morphological properties of cluster-assembled nanoporous metallic gold films deposited from a low-energy supersonic cluster beam. We modeled the optical response of the medium by explicitly introducing a variable degree of porosity in the film and finite-size effects due to the cluster nanometeric size. We deduced the characteristic size of the individual clusters within the nanoporous structure and their depth-resolved arrangement. Comparison with independent data available in the literature yielded extremely good quantitative agreement with the results extracted from our model, in terms of the degree of porosity of the films and of their surface roughness. The cluster-size distribution in the films obtained by the optical model matched extremely well the one obtained by atomic-force microscopy on isolated clusters. This demonstrates that, though the clusters are in contact with one another in the porous film, they retain their crystallographic individuality when assembled in three-dimensional structures.