Alkyl tails of cesium and formamidinium precursors control the growth of lead bromide perovskite nanocrystals.

Access to strongly quantum-confined lead bromide perovskite quantum dots (QDs) remains limited by the rapid formation kinetics of the perovskite phase. A major advance was achieved by introducing tri-n-octylphosphine oxide (TOPO) to coordinate PbBr2 and slow QD growth, although obtaining sub-5 nm CsPbBr3 QDs still required extreme dilution and yielded only tens of micrograms of isolable QDs per milliliter of reaction solution. Here, we introduce molecular design of the A-site cation precursor as a practical handle for controlling QD growth. Using cesium phosphinates, phosphonates, and phosphates with systematically varied aliphatic substituents, we show that the headgroup has only a minor influence on QD growth, whereas the precursor tail length governs the final QD size in a nonmonotonic manner, with a threshold around C8 attributed to changes in precursor organization. Steric tuning of the cesium precursor enabled size control of CsPbBr3 QDs from 18 to 3 nm with up to 40-fold increase in yield relative to the conventional route. In practical terms, the difference is 0.8 mg mL-1 vs. 18 µg mL-1 of isolable 3 nm QDs, with improved size dispersion (26 nm absorption linewidth). Even smaller CsPbBr3 species, estimated at 3-4 PbBr6 octahedra in width, are resolved by in situ absorption spectroscopy, opening the way to interrogate their nucleation and growth. The same strategy is extended to the more challenging FAPbBr3 system, enabling monodisperse QDs as small as 3.8 nm and allowing construction of a practical sizing curve relating FAPbBr3 QD size to the first excitonic absorption peak over the 3.8-12 nm size range.