Preparation and optical properties of colloidal gold monolayers

Colloidal gold adsorbed onto positively charged substrates forms a two-dime nsional layer with a maximum coverage of I colloid/1000 nm(2) as shown by o ptical absorption spectroscopy, small-angle X-ray scattering, and atomic fo rce microscopy. Positively charged substrates are obtained by either chemis orption of end-group-functionalized silanes or by polycation adsorption. Bu lk diffusion controls the gold adsorption; thus lower coverages can also be achieved. The optical spectra not only contain information on single collo ids but also on their mutual interactions, as well as on the polymeric and solvent environments. Absorption spectra based on Mie and Maxwell-Garnett t heory are calculated. The films remain stable on exchange of the external s olvent. The local dielectric environment is shown to not only influence the position of the absorption band but also, more drastically, the intensity (by a factor of 2.5), in agreement with theoretical predictions (+/-15%). P article aggregation induced by branched polycations causes an additional lo w-energy absorption peak.