J-aggregates on metal nanoparticles characterized through ultrafast spectroscopy and near-field optics.

Illumination of metal nanoparticles at the plasmon resonance produces enhanced evanescent fields on the nanoparticles' surfaces. The unusual strength of the field makes it a target for exploring photoinduced phenomena at the nanoscale, if efficient functionalization or coating of the nanoparticle surface with appropriate chromophores is possible. One direction is to use cyanine dyes that form monolayers of J-aggregates on the surface of noble metal nanoparticle colloids. The unique, collective electronic properties of J-aggregates produce excitons with enormous extinction coefficients that are of interest for their efficient energy transfer, electron transfer, and nonlinear optical properties. In that vein, we report our results on time-resolved spectroscopy and near-field scanning optical microscopy (NSOM) of J-aggregate exciton dynamics on Ag and Au nanoparticle colloids. Ultrafast transient absorption studies show that J-aggregate exciton lifetimes on Ag nanoparticles are much longer than on Au nanoparticles, with a 300 ps lifetime that is two orders of magnitude longer than the electronic processes in the nanoparticles themselves. Complementary NSOM studies of the colloids show that fluorescence from the J-aggregates on the Ag nanoparticles is induced by the scanning probe. These results may be significant for improving the nanophotonic performance of hybrid materials for nanoscale applications.