The decay kinetics of optical excitations in semiconductor nanoparticles are complicated by both the diversity of possible intermediate states and by heterogeneity among the particles. Multiple population-period transient spectroscopy (MUPPETS) uses a sequence of six ultrafast laser pulses and two periods of time evolution (2D) to reveal three important, new features in these kinetics. (1) In bare CdSe particles, the exciton decay must be described by 3 or more exponentials, possibly implying a cascade through multiple states. MUPPETS measurements show that the decay on individual particles is exponential.  The complex kinetics are solely due to heterogeneity in the barrier for electron trapping at the surface. A model based on the dipole moment of the particles can explain the broad, observed distribution. (2) In CdSe/ZnS particles at high fluence, a biexciton signal is measureable. Its nonexponential shape has been attributed to additional contributions from higher excitons and/or photoproducts. MUPPETS is able to extract a biexciton signal free from such interferences. The biexciton decay is shown to be intrinsically nonexponential, a result that is not predicted by standard theories of Auger relaxation. (3) Despite the passivating effects of a ZnS shell, nonradiative decay both persists and is spread over times ranging from 1 ps to many nanoseconds. This result is easily attributed to subsets of particles with a variety of passivation defects. Surprisingly, MUPPETS indicates none of the heterogeneity implied by this model. We suggest that mutual polarization of the exciton and surface charges causes a time-dependent decrease in the exciton cross-section.Based upon work supported by the National Science Foundation under CHE-1111530.

Exciton and biexciton dynamics in CdSe nanoparticles by 2D-MUPPETS

247th National Meeting (2014)

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