Anion exchange provides a facile method to tune the emission wavelength of cesium lead halide perovskite (CsPbX₃, X= Cl, Br or I) nanocrystals, making these materials attractive for applications in light emission. However, the complex reaction kinetics of anion exchange limits the compositional uniformity and color purity of the resulting CsPbX₃ nanocrystals. We are using fluorescence microscopy to monitor anion exchange between individual CsPbX₃ nanocrystals, which enables us to quantify heterogeneity in reactivity based on changes in fluorescence intensity at the single-particle level. We observe that switching time for changes in fluorescence intensity during anion exchange are dependent on the concentration of substitutional halide ions in solution. In the case of anion exchange between CsPbCl₃ and CsPbBr₃ nanocrystals, the switching times at a given halide concentration are similar in both reaction directions. Interestingly, 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. We developed different kinetic models to simulate anion exchange in these different systems, in which the activation energies for each exchange event are dependent on the overall reaction progress. Combining the experiments and simulations, we demonstrate that the concentration dependence of fluorescence switching times is controlled by the miscibility of the initial and final structures. The irreversibility in anion exchange between CsPbBr₃ and CsPbI₃ nanocrystals is due to structural differences between CsPbI₃ nanocrystals synthesized by the hot-injection method and those prepared by anion exchange. The distinct transformation kinetics during anion exchange revealed in this talk are important to consider when scaling-up the production of CsPbX₃ nanocrystals with high compositional homogeneity.

Dong Wang

Graduate Student, Washington University in St Louis

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