Stimulated emission to absorption cross-section ratio in quantum-confined lead halide perovskite nanocrystals: cation, shape and size independence

A nanocrystal's cross-section for stimulated emission is regularly hidden in ultrafast transient absorption spectra owing to overlapping excited-state absorption and ground-state bleaching. In a recent study, a novel three-pulse femtosecond spectroscopic method, coined the 'spectator exciton' approach, was used to overcome this in quantum-confined CsPbBr3 nanocrystals. Results proved that stimulated emission takes place already in the single-exciton state, but with a probability roughly an order of magnitude weaker than band-edge absorption. Only in bi-excitons does stimulated emission overpower absorption, providing net optical gain. Prompted by literature reports that replacing caesium with organic ammonium cations and shifting their morphology from cubes to spheres alters band edge level spacing and ordering, we measured stimulated emission in the same way in quantum-confined spheroidal FAPbBr3 nanodots of similar dimensions. Results prove that at room temperature, the unusual ratio between a single-exciton bleach and its stimulated emission cross-section observed in CsPbBr3 nanodots is unchanged by cation exchange and morphological alterations. The robustness of the measured cross-section ratios to these structural modifications stresses their generality and the importance of clarifying the underlying mechanisms. This article is part of the discussion meeting issue 'Excitonic frontiers'.