Extending the Weakly Confined Regime of Perovskite Nanocrystals for Fast Emission at Low Temperature

Weakly confined semiconductor nanocrystals have distinct photophysical properties that arise from coherently delocalized excitons. Experimental observation of these properties has been untested for sizes well beyond those of the Bohr exciton. We produce a size series of CsPbBr₃ weakly confined perovskite nanocrystals (WC-PNCs) with volumes up to ∼1000 times that of the Bohr exciton using a continuous injection procedure that controls growth at elevated temperatures. Through this reaction scheme, we are able to prepare WC-PNCs with high quantum yields and homogeneity, allowing for the observation of size-dependent optical properties. Single WC-PNCs at cryogenic temperatures exhibit excitonic emission at a radiative rate proportional to the nanocrystal volume. This dependence is consistent with coherent delocalization of the weakly confined exciton over the entire PNC volume and results in exceptionally fast radiative lifetimes (∼35 ps on average for the largest 80 nm PNCs). The properties of excitons in the weakly confined regime are well suited to the grand challenge of discovering materials for next-generation, light-based quantum technologies.