Ultrafast transient bleaching of CdS nano-particles in various media

Ultrafast dynamics of transient photobleaching and recovery of cadmium sulphide (CdS) nanoparticles of different size ranging from less than 3.5 nm to > 5.0 nm, dispersed in Nafion films, in self-assembled films, and in aqueous solutions were studied upon excitation at 400 nm with a femtosecond laser. All samples gave transient bleaching in less than 1 ps and a multi-exponential recovery in the wavelength region above about 410 nm. The peak wavelength of the transient bleaching and its time dependence together with the recovery behaviour were affected by the particle size, surface treatment and the supporting medium. The peak of the transient bleaching spectrum of larger CdS nanoparticles showed a red-shift during the recovery process, while for the smaller particles no such change was observed. CdS nanoparticles in self-assembled films showed an extremely long-lived bleaching recovery process. Fluorescence-activated CdS nanoparticles in aqueous solutions, which showed very strong and sharp fluorescence near the exciton absorption, showed very fast recovery. These results were explained by the decay of excitons and the recombination of surface trapped electron-hole pairs with different energies depending on the particle size.