In Situ Observation of Emission Sites during the Halide Exchange Reaction in Single Cesium Lead Halide Perovskite Nanocrystals

Control of the bandgap and emission properties of cesium lead halide (CsPbX3; X = Cl, Br, and I) perovskite nanocrystals (PNCs) can be realized through a postsynthesis halide exchange reaction. Previously, a mixed halide PNC (CsPbBrxI(3–x)) with a size comparable to its Bohr diameter was found to exhibit uniform bandgap energy as a whole NC via quantum confinement even with an inhomogeneous halide composition. The quantum confinement effect led to the single-photon emission from the single PNC. In this work, we investigated the emission behavior of a single mixed halide PNC with a size larger than the Bohr diameter during the halide exchange reaction. We revealed that single CsPbBr3 PNCs before the halide exchange reaction exhibited multiphoton emission, which was attributed to the formation of some emission sites in the single PNCs due to polaronic confinement. During the halide exchange reaction, we observed a change from multiphoton emission to single-photon emission, which was not observed in quantum confined PNCs. On the basis of emission lifetime analysis, we revealed that the change was attributed to quenching of the emission sites. Additionally, a change in PL anisotropy was observed during the halide exchange reaction due to lattice rearrangement induced by incoming halide ions. These results provide important insights into the emission behavior of PNCs with sizes larger than the Bohr diameter.