The conjecture that, as in bulk semiconductors, hot multiexcitons in nanocrystals cool rapidly to the lowest available energy levels is tested here by recording the effects of a single cold "spectator" exciton on the relaxation dynamics of a subsequently deposited hot counterpart. Results in CdSe/CdS nanodots show that a preexisting cold "spectator exciton" allows only half of the photoexcited electrons to relax directly to the band-edge. The rest are blocked in an excited quantum state due to conflicts in spin orientation. The latter fully relax in this sample only after ∼25 ps as the blocked electrons spins flip, prolonging the temporal window of opportunity for harvesting the retained energy more than 100 fold! Common to all quantum-confined nanocrystals, this process will delay cooling and impact the spectroscopic signatures of hot multiexcitons in all envisioned generation scenarios. How the spin-flipping rate scales with particle size and temperature remains to be determined.
Bibliographical noteFunding Information:
S.R. holds the Lester Aronberg Chair in Chemistry. S.R. acknowledges financial support from the Israel Science Foundation (Grant # 163/16). R.B. gratefully acknowledges support of the Israel Science Foundation (ISF Grant No. 189-14) and the binational US−Israel Science Foundation (BSF Grant No. 2015687). T.G. thanks the Lady Davis Fellowship Trust and the Raymond and Janine Bollag Post-Doctoral Fellowship Fund for a fellowship. S.R. and T.G. thank Dr. O. Liubashevski and Dr. J. Dana for technical help. E.L. acknowledges financial support from the Israel Science Foundation Projects 914/15 and 1508/14. J.D. acknowledges the Marie-Sklodowska Curie action H2020-MSCA-ITN-642656 (PHONSI) for fellowship support.
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