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 = CI, 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.