From Nonluminescent Cs₄PbX₆ (X = Cl, Br, I) Nanocrystals to Highly Luminescent CsPbX₃ Nanocrystals: Water-Triggered Transformation through a CsX-Stripping Mechanism

We report a novel CsX-stripping mechanism that enables the efficient chemical transformation of nonluminescent Cs₄PbX₆ (X = Cl, Br, I) nanocrystals (NCs) to highly luminescent CsPbX₃ NCs. During the transformation, Cs₄PbX₆ NCs dispersed in a nonpolar solvent are converted into CsPbX₃ NCs by stripping CsX through an interfacial reaction with water in a different phase. This process takes advantage of the high solubility of CsX in water as well as the ionic nature and high ion diffusion property of Cs₄PbX₆ NCs, and produces monodisperse and air-stable CsPbX₃ NCs with controllable halide composition, tunable emission wavelength covering the full visible range, narrow emission width, and high photoluminescent quantum yield (up to 75%). An additional advantage is that this is a clean synthesis as Cs₄PbX₆ NCs are converted into CsPbX₃ NCs in the nonpolar phase while the byproduct of CsX is formed in water that could be easily separated from the organic phase. The as-prepared CsPbX₃ NCs show enhanced stability against moisture because of the passivated surface. Our finding not only provides a new pathway for the preparation of highly luminescent CsPbX₃ NCs but also adds insights into the chemical transformation behavior and stabilization mechanism of these emerging perovskite nanocrystals.