Optical properties of quasi one-dimensional chains of gold nanoparticles

Nanopores in alumina membranes can serve as reaction vessels for the generation of nanosized gold particles. In addition, they enable a quasi one-dimensional arrangement of nanoparticles, the optical properties of which can easily be investigated due to their transparency in the visible and near ultraviolet (UV) regions. Gold colloids inside the pores were produced either by thermal decomposition of [Au-55(PPh3)(12)Cl-6] Clusters or by loading the pores with preformed colloids. The clusters as well as the colloids were transferred into the pores by simple immersion, and if necessary supported by applying a vacuum. The [Au-55(PPh3)(12)Cl-6] clusters were decomposed over the temperature range of 100 to 800 degreesC, using pores of different diameters. Transmission electron microscopy (TEM) was used to investigate the resulting nanoparticles. At decomposition temperatures up to ca. 500 degreesC, no specific influence of pore size or temperature was observed: 4-5 nm colloids were formed. However, temperatures > 500 degreesC resulted in colloids of up to 10-11 nm being formed. The optical properties of these and of preformed gold colloids in the membranes were studied. The extinction spectra of the colloidal assemblies generated from clusters exhibited two absorption peaks, caused by excitation of the plasmon resonance along the long and the short axes of the wire-like arranged particles. The optical extinctions were measured with unpolarized and polarized light (0 and 90 degrees). Depending on the angle of polarization, the polarized light caused either a blue- or a red-shift in the absorption maximum. Theoretical calculations, using the so-called generalized Mic theory and Maxwell Garnet theory, confirmed the experimentally observed behavior of these gold/alumina nanocomposites.