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 femtosecond laser at 400 nm. All samples, with the exception of the sample containing the smallest particles, showed maximum bleaching within the time resolution of the experiment. The bleaching then recovered with multi-exponential kinetics. Bi-exponential fits to the recovery of absorbance for samples which exhibited transient photobleaching showed that the lifetime of the short component increased as the size of the particles became larger. This behaviour was attributed to greater average distances between separated electrons and holes in the larger particles. Transient-spectral studies showed that there was a change in the wavelength of maximum photobleaching with time in the cases of 4.3 and 4.9 nm CdS particles. In these cases, the peak of transient photobleaching showed a progressive red shift as the time between the pump and probe pulses was increased. There was no change in the wavelength of maximum 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 nanoparticles in films containing larger particles or to films containing larger particles having different types of trapping sites on the particle surface. It is possible that both factors are contributing to the observed transient photobleaching behaviour.