Alfred Baalmann

Metal clusters in plasma polymer matrices

Plasma polymer thin films with small isolated silver particles and high volume fractions were prepared by simultaneous plasma polymerisation of hexamethyldisiloxane and metal evaporation. The optical properties of the films were determined by UV–vis spectrophotometry and the electrical behaviour by electrical transport measurements and by impedance spectroscopy. Thermal annealing causes a dramatic change in the DC and AC conductivity, which can be explained by the formation of percolation paths and the reduction of cluster–matrix interactions. UV–vis spectra of Ag/HMDSO composites revealed an additional absorption in the long wavelength region during the annealing process. This effect can be explained by increasing electromagnetic coupling between neighbouring clusters. The results can be simulated by a microscopic model of the embedded metal clusters, in which the isolated spherical clusters are coated by a thin dielectric layer, which prevents agglomeration but allows an electron exchange between the clusters due to an electron hopping process.

Metal clusters in plasma polymer matrices. Part III. Optical properties and redox behaviour of Cu clusters

Using an inert gas evaporation technique in combination with a simultaneous plasma polymerisation, copper clusters with mean diameters between 2.9 to 5.4 nm have been prepared within plasma polymer matrices from hexamethyldisiloxane (HMDSO) monomer. UV-visible spectroscopy of Cu clusters embedded in a plasma polymer film revealed a red shift and a damping of the surface plasmon frequency and resonance, respectively, with increasing thickness and changing nature of copper oxide layers surrounding the clusters. Experimental transmission UV-visible spectra were compared with simulations using the Maxwell–Garnett theory, which is extended to coated spheres. Owing to larger differences in the dielectric constant CuO layers lead to a stronger red shift than Cu2O coatings. The formation and the reduction of the two copper oxide species during successive treatments of Cu clusters with oxygen, hydrogen and carbon monoxide was observed by in situ diffuse reflectance measurements at a wavelength of λ=600 nm and a temperature of 250°C. The redox reactions with O2, CO, H2 are reversible.