Supplementary material from "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 due to overlapping excited state absorption, and ground state bleaching. In a recent study a novel 3 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 stimulated emission overpowers absorption providing net optical gain. Prompted by literature reports that replacing cesium with organic ammonium cations, and shifting their morphology from cubes to spheres, alters band edge level spacing and ordering, here we measure stimulated emission 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 its generality, and the importance of clarifying the underlying mechanisms.