A Multipulse Time-Resolved Fluorescence Method for Probing Second-Order Recombination Dynamics in Colloidal Quantum Dots

Abstract

The ability to generate and utilize multiexcited electronic states in colloidal quantum dots (QDs) is key to a growing range of QD technologies, but the factors that control their radiative and nonradiative recombination dynamics are not yet fully resolved. A significant barrier toward a greater understanding of these species is the fact that their spectroscopic signatures are energetically close to dominant exciton transitions, making it difficult to separate their decay contributions in inhomogeneously broadened ensembles. Here we describe a multipulse technique wherein a controllable number of 80 MHz laser pulses are used to generate different excited state populations, which are then monitored using time-resolved fluorescence. By changing the number of pulses and using a general data analysis method we are able to separate second-order emission generated by absorption of two or more laser pulses from first-order contributions generated by just one pulse. Furthermore, we show that it is possible to determine the nature of the multiexcited state by comparing the second-order emission intensity to models of QD decay dynamics. We find that in our sample of CdSe/CdS core/shell QDs the second-order emission is dominated by emissive trion states rather than biexcitons. Our spectroscopic technique offers a powerful new way to study multiexcited QDs, and the insights that will be gained from this and future studies could be an important step toward harnessing multiexcitons and other multiexcited states in new QD technologies