Simultaneous Determination of Spectral Signatures and Decay Kinetics of Excited State Species in Semiconductor Nanocrystals Probed by Transient Absorption Spectroscopy

Abstract

Characterization of excited-state relaxation of semiconductor nanocrystals by ultrafast spectroscopy is complicated by the fact that an excitation pulse produces a distribution of excited-state populations (e.g., single excitons, biexcitons, triexcitons, etc.). The decay kinetics of these species are usually characterized at specific spectral positions, requiring assumptions about the contribution of each species to the signal at that position. We introduce a method to analyze transient absorption (TA) spectra of nanocrystals by simultaneously isolating the kinetics of different species and their corresponding spectral signals. We use established kinetic models for an ensemble of nanocrystals with multiple excited-state species to describe the measured TA spectra as population-weighted sums of TA spectra of the individual species. The expression we derive describes the decay of the TA spectrum of each (n + 1)-exciton species and the simultaneous rise of the TA spectrum of the n-exciton species. This analysis allows for rigorous determination of the decay kinetics of the various excited-state populations based on their spectral signatures. We apply this analysis to TA spectra of 3.0 nm diameter CdSe quantum dots obtained over a range of pump pulse intensities using global fitting routines to extract spectral signals and kinetics associated with hot excitons, single cooled excitons, and biexcitons. This analysis allows us to extract the TA spectra of biexcitons and single excitons (as well as hot excitons) and subsequently measure the triexciton binding energy. The ratio of the two spectra at the bleach maximum for this sample is 1.6, not 2, as commonly assumed in calculations of efficiencies of multiexciton generation. We show that the strong wavelength dependence of the spectral signatures associated with the kinetics of different species leads to a number of implications on the interpretation of TA spectroscopy data at single wavelengths. Finally, we find a wavelength in the TA spectrum that reports only on multiexciton kinetics. We expect that the approach described here is broadly applicable to analysis of TA data from a variety of nanocrystalline materials.

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