A.A. Mewe

Ellipsometric study of percolation in electroless deposited silver films

Using spectroscopic ellipsometry in the visible and near-infrared spectral range we investigate the optical properties of a growing silver film starting from predeposited gold nanoparticles. The effective pseudodielectric functions, obtained by direct inversion of the ellipsometry spectra, reveal a surface plasmon resonance for the nanoparticulate films. Upon prolonged electroless silver deposition, the resonance shifts to lower energies. The redshift is due the longer electron mean free path in larger silver structures and is analyzed by describing the optical response of the developing silver film in terms of a Lorentz line shape. The position of the oscillator, i.e., its resonance energy, is discussed in relation to the transition from isolated nanoparticles to an interconnected, eventually continuous metal film. This transition is also observed in the optical conductivity which exhibits an abrupt, stepwise increase at the same energy where the aforementioned resonance energy becomes zero. For longer deposition times, the optical spectra can be described in terms of a Drude-like free-electron metal. The development of the Drude–Lorentz parameters, i.e., the relaxation time and electron density, are compared to values for bulk silver; the latter were obtained from an optical measurement on a thick bulk silver sample. The saturation values for the relaxation time and thus the conductivity amount to approximately 40% of the bulk value, in agreement with direct current conductivity measurements on these films.

Self-assembly of nanocolloidal gold films

The unique and new optical, electrical, and magnetic properties of colloidal superstructures as opposed to the bulk characteristics of the constituent materials is attracting the attention of an increasing number of both fundamental scientists and technology-oriented industry. The colloid size used in the assembled structures varies over approximately 3 orders of magnitude and is closely related to the specific application. For photonic band gap materials, the particle size is of the same order of magnitude as the wavelength of light, while for magnetic applications, such as ultrahigh density storage devices, the particle radius is in the low-nanometer range. A combination of the aforementioned physical properties of colloidal matter introduces even more exciting fields of research. Electron transport through monolayers of magnetic nanocrystals or tunable photonic band gap materials, both controlled by applying a magnetic field, are only two examples of the many possibilities.