CsPbCl 3 → CsPbI 3 Exchange in Perovskite Nanocrystals Proceeds through a Jump-the-Gap Reaction Mechanism.

Halide exchange is a popular strategy to tune the properties of CsPbX 3 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 CsPbCl 3 -CsPbI 3 system, preventing the isolation of stable CsPb(Cl x I 1- x ) 3 nanocrystals. Yet, previous works have claimed that a full CsPbCl 3 → CsPbI 3 exchange can be achieved. Even more interestingly, interrupting the exchange prematurely yields a mixture of CsPbCl 3 and CsPbI 3 nanocrystals that coexist without undergoing further transformation. Here, we investigate the reaction mechanism of CsPbCl 3 → CsPbI 3 exchange in nanocrystals. We show that the reaction proceeds through the early formation of iodide-doped CsPbCl 3 nanocrystals covered by a monolayer shell of CsI. These nanocrystals then leap over the miscibility gap between CsPbCl 3 and CsPbI 3 by briefly transitioning to short-lived and nonrecoverable CsPb(Cl x I 1- x ) 3 nanocrystals, which quickly expel the excess chloride and turn into the chloride-doped CsPbI 3 nanocrystals found in the final product.