Light scattering spectra of colloidal gold aggregates: experimental measurements and theoretical simulations

ddWe report on the experimental spectra of light scattered at 90 degrees by colloidal gold particles (diameter 30 nm) and aggregates built from these particles during the salt aggregation process. The time dependence of spectra in the range 350-850 nm was measured by using an luminescence attachment to Specord M-40 spectrophotometer. The aggregation of sol was accompanied by essential increasing the single particle spectrum maximum (near 580 nm), its displacement to the red part in the spectrum, and by a broadening of the long wavelength wing of extinction. To explain these findings, we used a computer diffusion-limited cluster-cluster aggregation model. The optical properties of aggregates including light scattering spectra were computed by the coupled dipole method (CDM or DDA). The bulk optical constants of metals were modified by the size-limiting effect of nanoparticles. Our calculations showed a direct correlation between cluster particle number (N similar or equal to 10-50) and intensity of scattered light. However, this correlation failed for 100-particle clusters. It can be supposed that DDA model is not adequate for accurate prediction of light scattering properties of large aggregates.