Irreversibility in Anion Exchange Between Cesium Lead Bromide and Iodide Nanocrystals Imaged by Single-Particle Fluorescence

Anion exchange is a powerful method to tune the emission wavelength of perovskite CsPbX₃ (X = Cl, Br, or I) nanocrystals across the visible spectrum. CsPbX₃ nanocrystals can possess a number of crystal structures that depend on both their composition and size. Understanding the role these structural variations play during anion exchange is important for their future application in optoelectronic devices. In this work, we used fluorescence microscopy to monitor reaction trajectories of individual nanocrystals as they undergo anion exchange between CsPbBr₃ and CsPbI₃. By varying the concentration of substitutional ion used to induce anion exchange, we quantify differences in reaction times for exchange in opposite directions. CsPbI₃ nanocrystals undergo more abrupt shifts in their emission characteristics as they transform to CsPbBr₃, while CsPbBr₃ nanocrystals exhibit a smoother transition during their transformation to CsPbI₃. Simulations of anion exchange using Monte Carlo trajectories are consistent with a larger degree of structural reorganization when CsPbI₃ nanocrystals undergo anion exchange. Together, these results reveal that there are structural differences between CsPbI₃ nanocrystals synthesized by the hot-injection method and those produced by anion exchange. The narrower distribution of reaction times when CsPbI₃ nanocrystals transform to CsPbBr₃ may produce better compositional homogeneity when this reaction is scaled-up to incorporate these materials into optoelectronic devices.