Spectroscopic studies of liquid solutions of R6G laser dye and Ag nanoparticle aggregates

We have found that R6G laser dye in a concentration of 0.1 g l(-1) mixed with a solution of aggregated silver nanoparticles exhibits a new emission band with a maximum at 612 nm. This band does not exist in pure dye of comparable concentration or in a mixture of dye with a solution of single silver nanoparticles. A qualitatively similar red-shifted emission band is observed in pure R6G dye at very high concentration (3.8 or 16.7 g l(-1)). In both cases, no changes occur to the shapes of the absorption spectra of the dye. We explain the observed spectral changes in terms of J-aggregates of R6G molecules whose formation is probable in the presence of Ag aggregates with a complicated surface structure and is much less likely in the case of adsorption of dye molecules on single Ag nanoparticles. Alternatively, many features observed in the experiment can be explained by an enhancement of the rates of spontaneous radiative transitions in the proximity of metallic particles, which is due to a modification of the local density of electromagnetic modes in the vicinity of metal surfaces at energies resonant with surface plasmon resonances.