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

Gaurav Singh,† Michael A. Guericke,† Qing Song,‡ and Marcus Jones*,† †Department of Chemistry, University of North Carolina at Charlotte, 9201 University City Boulevard, Charlotte, North Carolina 28223, United States ‡

IBM Research Almaden, 650 Harry Road, San Jose, California 95120, United States

*S Supporting Information

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.

Introduction

Colloidal semiconductor quantum dots (QDs) have garnered tremendous scientifi c and technological interest due to their unique physical and chemical properties.1,2  Their size-tunable optoelectronic properties and processability make them an exciting class of materials for a wide range of device applications.3−10  The majority of studies on QD electronic properties has so far focused mostly on the photophysics of single exciton states, including their relaxation dynamics,11−14  their coupling with phonon modes,15−17  and their interaction with surface or ligand states.18−21  However, experimental realization of multiexciton generation (MEG)22−26  in QDs and the potential of MEG-based solar cells27−30  to make a signifi cant leap in photoconversion effi ciency has garnered signifi cant interest in the study of multiexciton states.31−34  Multiexciton states are equally important for the practical realization of optical gain in QD-based lasers35,36  or for using QDs as entangled photon sources.37,38  On the other hand, nonradiative recombination of multiply excited states39  is thought to play a key role in QD blinking and QD photocharging,40,41  and its understanding has important implications for using QDs as reliable biomarkers,42,43  bright light-emitting diodes (LEDs),44  or single photon sources.7,45  Ultrafast transient absorption (TA) and time-resolved photoluminescence (TRPL) spectroscopy have been the mainstay techniques used to probe multiple exciton dynamics in QDs.46−49  TA is a powerful technique that can have a very high temporal resolution, can be extended to very long lifetimes, and can probe any optically active transition, including, e.g., intraband relaxation dynamics in CdSe QDs.50  It is typically a low repetition rate measurement (kHz) requiring relatively high excitation intensities, but much higher repetition rate experiments are also possible.51  Recent work has extended TA studies to the study of single-particle ultrafast dynamics,52  which is very challenging because it requires the determination of very small changes in probe-beam intensity. Ultrafast TRPL has been used to study multiexciton dynamics of a few tens of picoseconds or less,47  but the upconversion technique suff ers from a restricted time window that makes it impractical to study most QD dynamics. Timecorrelated single photon counting (TCSPC) is an alternative TRPL technique, which requires low light intensities, utilizes comparatively simple instrumentation, and is able to probe dynamics from tens of picoseconds to microseconds. It is a powerful tool for understanding QD carrier recombination dynamics;53  however, the time resolution is not as short as TA, and in ensemble experiments, it is challenging to separate exciton luminescence from multi- or charged-exciton emission because their spectral shifts are often similar to the inhomogeneously broadened exciton line widths, making the contributions of weakly emitting species hard to identify on top of an already complex multiexponential fluorescence decay.54,55 For these reasons, ultrafast TA has most often been used to study multiexciton dynamics in colloidal QDs.

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