ULTRA-FAST PHOTORESPONSES OF CDS NANOPARTICLES IN NAFION FILMS

The ultra-fast dynamics of transient absorbance bleaching and recovery of cadmium sulphide nanoparticles with sizes ranging from 2.7 to 4.9 nm in Nafion polymer films was studied upon excitation with a femtosec ond laser at 400 nm. All samples, with the exception of the sample con taining the smallest particles, showed maximum bleaching within the ti me resolution of the experiment. The bleaching then recovered with mul ti-exponential kinetics. Bi-exponential fits to the recovery of absorb ance for samples which exhibited transient photobleaching showed that the lifetime of the short component increased as the size of the parti cles became larger. This behaviour was attributed to greater average d istances between separated electrons and holes in the larger particles . Transient-spectral studies showed that there was a change in the wav elength of maximum photobleaching with time in the cases of 4.3 and 4. 9 nm CdS particles. In these cases, the peak of transient photobleachi ng showed a progressive red shift as the time between the pump and pro be pulses was increased. There was no change in the wavelength of maxi mum bleaching with time for Nafion films containing 3.5 Mm particles. The differences in transient spectral behaviour could be due either to their being a more polydisperse size distribution of CdS nanoparticle s in films containing larger particles or to films containing larger p articles having different types of trapping sites on the particle surf ace. It is possible that both factors are contributing to the observed transient photobleaching behaviour.