Light absorption by the clusters of colloidal gold and silver particles formed during slow and fast aggregation

Spectra of absorption (400-800 nm) by the aggregates of colloidal gold (5, 15, and 30 nm in diameter) and silver (20 nm in diameter) particles were st udied experimentally and theoretically. It was revealed that, during fast a ggregation corresponding to the diffusion-limited cluster aggregation (DLCA ), the pattern of spectra is dependent on the size of primary particles. Sp ectra with the additional absorption maximum in the red region are observed for 15 and 30 nm gold and 20 nm silver particles, while the absorption spe ctrum for 5 nm particles is characterized by only one maximum shifted to th e red region. The slow aggregation resulted in a decrease in plasmon absorp tion peak with no marked shift to the red region and to the broadening of l ong-wave absorption wing. From data on electron microscopy, typical branche d DLCA-clusters were formed during fast aggregation, whereas small compact aggregates and a noticeable number of single particles were observed in a s ystem during slow aggregation. The computer model of the diffusion-limited cluster-cluster aggregation was used to explain these results. Optical prop erties of aggregates were calculated using coupled dipole method (CDM or DD A) and the exact method of a multipole expansion. Corrections for the size effect were introduced into the bulk optical constants of metals for nanosi zed particles. It was shown that a modified version of DDA (Markel et al., Phys. Rev: B, 1996, vol. 53, no. 5, p. 2425) allows us to explain the patte rn of experimental spectra of DLCA-aggregates and their dependence on a mon omer size. The exact method was applied to calculate: the extinction cross sections of metallic aggregates demonstrating strong electrodynamic interac tion between particles. The number of higher multipoles that are required t o adequately describe this interaction is much larger than the number of te rms of an ordinary Mie series and is the main obstacle to the exact calcula tion of the spectra of metallic aggregates with a large number of particles .