Synthesize Ultra‐Small CsPbBr ₃ Quantum Dots With Deep Blue Emission by Using NH ₄ Br as Surface Ligand

Halide perovskite CsPbBr 3 quantum dots (QDs) represent a compelling class of luminescent materials, distinguished by their exceptional optical properties. Upon reduction of the size to its exciton Bohr diameter (∼7 nm), due to the quantum confinement effect, the photoluminescence (PL) emission undergoes a transition from intrinsic band‐to‐band green to the highly sought‐after blue emission. However, the realization of deep blue emission (< 460 nm) remains challenging, primarily due to the inherently rapid growth kinetics of CsPbBr 3 QDs, which typically occur on sub‐second timescales. The acquisition of target small‐size QDs necessitates precise regulation of crystallization kinetics to decelerate growth, compounded by the propensity of small QDs to undergo Ostwald ripening and revert to larger, green‐emitting particles. In this study, NH 4 Br is employed as a surface ligand to synthesize CsPbBr 3 QDs, enabling the formation of small QDs (∼5 nm) at room temperature with stable deep blue emission at 456 nm. NH 4 + /NH 4 Br can form a tight and dense capping on small CsPbBr 3 QDs, and thereby effectively retarding the further growth of QDs. This work highlights NH 4 Br as an effective additive for the synthesis of stable blue‐emitting CsPbBr 3 QDs.