Spin blockades to hot multi-exciton relaxation in nanocrystals: Case of CdTe

In a recent three pulse “spectator exciton” experiment on quantum confined CdSe nanocrystals, spin orientation conflicts between hot and cold electrons in the same doubly excited nanocrystal were shown to block the former from relaxing to the band edge. This blockage was ultimately overcome by phonon-assisted flipping of the hot carrier’s spin, allowing it to pair with its band edge counterpart within tens of picoseconds. To test the generality of this novel phenomenon, the same approach is used here on nanodots of another II-VI semiconductor, CdTe. Despite difficulties arising from slower electron cooling rates and faster bi-exciton recombination times relative to CdSe, the results demonstrate similar spin blockades to hot bi-exciton cooling in CdTe nanocrystals. This study confirms the general occurrence of spin blockade phenomena in II-VI quantum dots, underscoring the potential of this mechanism for assisting hot electron harvesting. It similarly highlights the inherent limitations of using band edge bleaching signals as ensemble exciton counters when multi-excitons are involved.