Chapter 99 - Femtosecond time-resolved studies on spectral sensitization of AgBr nanocrystals

Spectral sensitization and supersensitization of silver bromide nanocrystals are studied by the femtosecond fluorescence up-conversion technique. Fluorescence from J-aggregates of cyanine dyes adsorbed on AgBr nanocrystals (40-900 nm) with different shapes (cubic or octahedral) is measured. Fast non-exponential fluorescence decays are observed with a fast component ranging from 400 fs to 2.5 ps depending on the type and size of the crystals. The rates of electron injection from J-aggregate dye to the conduction band of AgBr at various conditions are determined. The increase of the electron injection rate with the increase of the grain size is observed and explained by the space charge theory. Upon addition of a super-sensitizer (SS), which is different cyanine dye co-adsorbed on the surface of silver bromide grains, the fluorescence decay becomes several times faster. The results are analyzed in the framework of the “hole-trapping” supersensitization model. The effective hole trapping (electron transfer from SS to the excited J-aggregate) rate constant is found to be independent of the grain size and equal to ∼0.60 ps -1 . By fluorescence anisotropy measurement, the ultrafast energy transfer of the J-aggregates on the octahedral crystal (200 nm) is studied. The anisotropy decays are biphasic with two time constants of ∼100 fs and a few picoseconds, the short time constant being assigned to energy transfer between epitaxially arranged J-aggregates on the {111} surface.