Preparation and spectral characteristics of gold particles in Langmuir-Blodgett films

Langmuir-Blodgett films incorporating complex ions of gold(III) have been fabricated from air-water monolayers of arachidic acid, dimethyldioctadecylammonium bromide and dihexadecyl phosphate. The films, and their reactions with reducing agents to produce colloidal gold particles, have been characterised by surface pressure/area per molecule (π-A) isotherms, UV-vis. and XPS spectroscopy, atomic force microscopy (AFM) and transmission electron microscopy (TEM). Colloidal gold particles, produced in films exposed to carbon monoxide, were examined by AFM and TEM. The gold particles, as determined by TEM, were of a mean size of 10 nm and were randomly distributed in the films. Solvent immersion studies were also conducted in order to investigate the effect of the dielectric medium on the position of the gold colloid plasmon band. The position of the wavelength maximum of the gold plasmon band of the gold-LB film in air was not consistent with a just a layer of surfactant surrounding the particles. From the position of the surface plasmon band peak an effective refractive index of 1.35 is calculated for the LB film, which is considerably lower than that of a dense surfactant film. Also, the band maximum did not shift to any measurable extent when the films were immersed in water. We suggest that the reason for these observations is that air is trapped in the film and this lowers the refractive index of the film and prevents water penetration into the film. Immersion of a gold-LB film in the high refractive index liquid CS2 resulted in a redshift in the gold plasmon band, which was a consequence of the aggregation of the gold particles due to the dissolution of the surfactant film in the surrounding liquid.