Ultrafast electronic dynamics of monodisperse PbS and CdS nanoparticles/nanorods: Effects of size on nonlinear relaxation

Effects of size on nonlinear electronic relaxation processes in highly monodisperse PbS and CdS nanoparticles (NPs)/nanorods (NRs) have been studied using femtosecond transient absorption spectroscopy. All the PbS NPs with different sizes show similar dynamic profiles and an excitation intensity dependence, which is in contrast to the excitation intensity independence observed in previous studies of PbS NPs with broader size distribution. The difference could be due to either the difference in size distribution or, more likely, difference in density and distribution of trap states. The dynamic profiles of CdS NRs and NPs with different sizes all show similar time constants for the slow linear component. However, size dependence has been observed for the fast nonlinear component attributed to exciton-exciton annihilation. This size dependence of the nonlinear component can be explained by considering the size dependence of a number of factors including quantum confinement, surface trap states, and number of excitons per particle. The number of excitons per particle appears to be the dominant factor in the nonlinear decay process.