This is a copy of the slides presented at the meeting but not formally written up for the volume.

Conductive metal patterns can be fabricated through the selective electroless growth of silver on isolated nanocolloidal gold particles, deposited onto chemically functionalized substrates using microcontact printing. The transition from isolated nanoparticles to continuous, conducting silver films is characterised by a percolation threshold. Using spectroscopic ellipsometry in the visible and near-infrared spectral range on homogeneous particulate films, we have investigated this percolation threshold. From the ellipsometry spectra obtained as a function of electroless silver deposition times, the effective thickness as well as the complex dielectric function of the growing films are determined. For short deposition times, the optical spectra exhibit features characteristic of an effectively insulating, particulate layer. A maximum is observed in the imaginary part of the dielectric function, which shifts towards lower energy with increasing amounts of deposited silver. For thicker films, the appearance of a Drude-like free electron contribution to the optical spectra is exhibited by a strong increase of both the real and imaginary parts of the dielectric function towards lower energy. From the optical spectra, the percolation threshold is identified. The ellipsometry results are discussed in relation to DC conductivity measurements, which also reveal a percolation threshold. Furthermore, our conclusions and the advantages of our approach are discussed in relation to recently published in situ experiments, in which the growth of magnetron sputtered silver nanoparticles is monitored real-time yielding similar optical responses.