In Situ Observation of Emission Behavior during Anion-Exchange Reaction of a Cesium Lead Halide Perovskite Nanocrystal at the Single-Nanocrystal Level.

Postsynthesis anion-exchange reaction of cesium lead halide (CsPbX3; X = Cl, Br, and I) perovskite nanocrystals (NCs) has emerged as a unique strategy to control band gap. Recently, the partially anion-exchanged CsPb(Br/I)3 NC was reported to form an inhomogeneously alloyed heterostructure, which could possibly form some emission sites depending on the halide composition in the single NC. In this work, we observed the in situ emission behavior of single CsPb(Br/I)3 NCs during the anion-exchange reaction. Photon-correlation measurements of the single NCs revealed that the mixed halide CsPb(Br/I)3 NC exhibited single-photon emission. Even when irradiated with an intense excitation laser, the single NC exhibited single-photon emission with a photoluminescence spectrum of a single peak. These results suggested that the heterohalide compositions of the CsPb(Br/I)3 NC do not form any emission sites with different band gap energies; instead, the NC forms emission sites with uniform band gap energy as a whole NC via quantum confinement.