CsPbCl3 → CsPbI3 Exchange in Perovskite Nanocrystals Proceeds through a Jump-the-Gap Reaction Mechanism

Halide exchange is a popular strategy to tune the properties of CsPbX3 nanocrystals after synthesis. However, while Cl ? Br and Br ? I exchanges proceed through the formation of stable mixed-halide nanocrystals, the Cl? I exchange is more elusive. Indeed, the large size difference between chloride and iodide ions causes a miscibility gap in the CsPbCl3-CsPbI3 system, preventing the isolation of stable CsPb(ClxI1-x)(3) nanocrystals. Yet, previous works have claimed that a full CsPbCl3 ? CsPbI3 exchange can be achieved. Even more interestingly, interrupting the exchange prematurely yields a mixture of CsPbCl3 and CsPbI3 nanocrystals that coexist without undergoing further transformation. Here, we investigate the reaction mechanism of CsPbCl3 ? CsPbI3 exchange in nanocrystals. We show that the reaction proceeds through the early formation of iodide-doped CsPbCl3 nanocrystals covered by a monolayer shell of CsI. These nanocrystals then leap over the miscibility gap between CsPbCl(3 )and CsPbI3 by briefly transitioning to short-lived and nonrecoverable CsPb(ClxI1-x)(3) nanocrystals, which quickly expel the excess chloride and turn into the chloride-doped CsPbI3 nanocrystals found in the final product.