Ultrafast dynamics of fluorescence-activated CdS nanoparticles in aqueous solutions by femtosecond transient bleaching spectroscopy

The ultrafast dynamics of transient photobleaching and recovery of fluorescence-activated cadmium sulphide (CdS) nanoparticles in aqueous solutions upon treatment with excess cadmium ions have been studied upon excitation at 400 nm with a femtosecond laser system. Three different sized nanoparticles were prepared with a diameter of 5.7, 4.4 and 3.7 nm. The colloids showed strong fluorescence bands at 504, 477 and 458 nm, respectively, which were close to the exciton absorption. Transient photobleaching of less than 1 ps with a peak quite similar to the wavelength of the exciton absorption band was observed for the two larger samples. It was found that the bleaching component with a lifetime of several picoseconds became pronounced upon treatment with excess Cd2+. A much faster component with a subpicosecond lifetime was also observed. These results strongly suggest that the direct recombination of charge carriers dominates in fluorescence-activated CdS nanoparticles.