Spectral sensitization and supersensitization of AgBr nanocrystals studied by ultrafast fluorescence spectroscopy

Spectral sensitization and supersensitization of the silver bromide nanocrystals were studied by the femtosecond fluorescence up-conversion technique. An annihilation-free fluorescence from J-aggregates of the Dye 1 (3,3'-disulfopropyl-5,5'-dichloro-9-ethylthiacarbocyanine) adsorbed on the silver bromide nanocrystals (edge-to-edge distance from 40 to 900 nm) of different shapes (cubic and octahedral) was measured. Very fast nonexponential fluorescence decays were observed with a fast component from 0.4 to 2.5 ps, and an average decay time from 1.1 to 5.5 ps, depending on the type and size of AgBr grains. The average fluorescence decay time is several times longer on the cubic grains than that on the octahedral grains. Upon addition of a supersensitizer (SS) (3,3'-disulfopropyl-9-ethyl-4,5,4',5'-dibenzothiacarbocyanine), which is coadsorbed on the surface of silver bromide grains, the fluorescence decay became several times faster with a mean decay time of 0.60-1.3 ps. Different models of supersensitization were examined. The results were analyzed in the framework of the "hole-trapping" supersensitization model. Octahedral grains with different sizes were examined. The effective hole-trapping (electron transfer from SS to the excited J-aggregate) rate constant was found to be independent of the grain size and equal to similar to0.60 ps(-1). The increase of the electron injection rate (from the excited J-agrgregate to the AgBr conduction band) with the increase of the grain size is observed and a possible mechanism is discussed.