Halide Anion-Exchange-Driven Structural Phase Transition in CsPbX3 Perovskites: Alloys versus Heterostructures

All-inorganic lead halide perovskites CsPbX3 (X = Cl, Br, I) hold great promise for developing integrated optoelectronic devices with on-demand optical and electronic properties. The soft crystal lattice and strong ionic-bonding nature allow us to tailor the chemical composition and phase structure of CsPbX3 via anion-exchange reactions at the vapor-solid or the solid-solid interface. The understanding of anion diffusion and exchange mechanisms in CsPbX3 is crucial for the precise control of their structural transformation and promotion of device performance. Herein, we report a controllable growth of CsPbX3 microstructures from alloys to heterostructures by an improved two-step vapor epitaxial method. We demonstrate that the surface reaction of Cl-based precursors on CsPbBr3 microplates facilitates the diffusion and incorporation of Cl ions into the perovskite lattice, leading to the structural transition into CsPb(BrxCl1-x)(3) alloys with a tunable composition and photoluminescence emission. On the contrary, Br ions are more difficult to incorporate into the CsPbCl3 lattice for the reaction of Br-based precursors on CsPbCl3 microplates, which contributes to the formation of CsPbBr3/CsPb(BrxCl1-x)(3) heterojunctions via a diffusion-limited growth mechanism. Density functional theory calculations reveal that the lower dopant formation energy and smaller diffusion barrier of Cl ions in CsPbBr3 as compared to those of Br doping in CsPbCl3 are responsible for the large growth difference. This work not only offers deep insights into the anion-exchange mechanisms but also opens an efficient route to growing various CsPbX3 microstructures with the desired halide compositions and properties for functional device applications.