Kavli Affiliate: Paul Alivisatos
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| Summary:
Quantum dot (QD) solids are promising optoelectronic materials; further
advancing their function in devices is dependent on understanding their energy
transport mechanisms. The commonly invoked near-field F"orster resonance
energy transfer (FRET) theory often underestimates the exciton hopping rate in
QD solids, yet no consensus exists on the underlying cause. In response, we use
time-resolved ultrafast stimulated emission depletion (TRUSTED) microscopy, an
ultrafast transformation of STED microscopy to spatiotemporally resolve exciton
diffusion in tellurium-doped CdSe-core/CdS-shell QD superlattices. We measure
the concomitant time-resolved exciton energy decay due to excitons sampling a
heterogeneous energetic landscape within the superlattice. The heterogeneity is
quantified by single-particle emission spectroscopy. This powerful multimodal
set of observables provides sufficient constraints on a kinetic Monte Carlo
simulation of exciton transport to elucidate a composite transport mechanism
that includes both near-field FRET and previously-neglected far-field
emission/reabsorption contributions. Uncovering this mechanism offers a
much-needed unified framework in which to characterize transport in QD solids
and new principles for device design.
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