Spin Blockades to Relaxation of Hot Multiexcitons in Nanocrystals

Abstract

The rates of elementary photophysical processes in nanocrystals, such as carrier cooling, multiexciton generation, Auger recombination etc., are determined by monitoring the transient occupation of the lowest exciton band. The underlying assumption is that hot carriers relax rapidly to their lowest quantum level. Using femtosecond transient absorption spectroscopy in CdSe/CdS nanodots we challenge this assumption. Results show, that in nanodots containing a preexisting cold "spectator exciton", only half of the photoexcited electrons relax directly to the band-edge and the rest are blocked in an excited state due to Pauli exclusion. Full relaxation occurs only after˜15 ps, as the blocked electrons flip spin. This requires review of numerous studies unaware of this ubiquitous and novel effect, which may facilitate hot carrier energy utilization as well.arXiv:1809.08581v2 [cond-mat.mes-hall]

Unusually Strong Biexciton Repulsion Detected in Quantum Confined CsPbBr3 Nanocrystals with Two and Three Pulse Femtosecond Spectroscopy

“…As shown in a previous report, 32 at such high N0, spin blockades to hot exciton relaxation in the presence of additional BE excitons are relieved and a large majority of the bi-excitons recorded have relaxed to the lowest available levels. 32 The fact that this leads not to a negative signal signifying strong stimulated emission matches the second scenario. In the first scenario discussed by Son and coworkers 20 the second exciton would double the effect of the first on the BE transition regardless of perturbative allowing of transitions to a higher lying spin forbidden mixed exciton state.…”

“…If so, the 2:1 ratio in the bleach signal anticipated for AR of the bi-exciton would be restored for an integral of the TA signal over a To test this premise further, three pulse spectator exciton experiments were conducted on the 6 nm sample. 8,32 Briefly, weak pump TA in a NC sample without any prior excitation, is compared with an identical pump-probe cycle in the same sample after it has been saturated in relaxed mono-excitons. The first, as usual, measures changes in absorbance due to additional Two different scenarios will be considered here to interpret the data.…”

“…Bi-exciton contributions to TA spectra were isolated and compared to that induced by a single excitation using the "spectator exciton" approach. 8,32 Results show that besides induction of A2, a single exciton totally bleaches the 1Se1Sh absorption band. That a second exciton does not change the bleach at the BE, but instead initially bleaches A2 proving it to be an extensively blue-shifted remaining transition to 1Se1Sh and not a partially allowed dark band.…”

“…To answer these questions, we have studied state filling effects on the BE bleach for the first two deposited excitons in strongly QC CsPbBr 3 NDs using two and three pulse broad-band femtosecond TA. Biexciton contributions to TA spectra were isolated and compared to that induced by a single excitation using the “spectator exciton” approach. , Results show that besides induction of A2, a single exciton totally bleaches the initial 1S e 1S h absorption band and that a second exciton does not change the bleach at the BE, but instead initially bleaches A2, proving it to be an extensively blue-shifted remaining transition to 1S e 1S h and not a partially allowed dark band. Later, a picosecond range relaxation process in the nascent biexciton induces a back shift in the position of its induced bleach, partially reviving A2.…”

Relations between absorption, emission, and excited state chemical potentials from nanocrystal 2D spectra

“…Bright-dark exciton equilibration requires separate electron and hole angular momentum/spin relaxation. In CdSe quantum dots, electron-spin flips are reported to be over an order of magnitude slower than exciton-spin flips ( 43 ). It is not yet clear whether separate electron or hole total angular momentum relaxation in PbS quantum dots is slow enough to explain the few hundred nanosecond relaxation from the bright exciton to a more slowly emitting mixture of bright and dark excitons in thermal quasi-equilibrium with each other [in principle, defect ( 17 ) or trap states could also play a role].…”