Halide Perovskite–Chalcohalide Nanocrystal Heterostructures as a Platform for the Synthesis and Investigation of the CsPbCl ₃ –CsPbI ₃ Epitaxial Interface

Halide exchange in lead-based halide perovskites has been studied extensively. While mixed Cl/Br or Br/I alloy compositions can be formed with no miscibility gaps, this is precluded for mixed Cl/I compositions, due to the large difference in Cl^- and I^- ionic radii. Here, perovskite-chalcohalide CsPbCl₃-Pb₄S₃Cl₂ nanocrystal heterostructures are exploited to study the Cl→I exchange and to isolate new types of intermediate structures. The epitaxial interface between the Pb₄S₃Cl₂ chalcohalide and the CsPbCl₃ perovskite significantly influences the intermediate stages of halide exchange in the perovskite domain, leading to coexisting CsPbCl₃ and CsPbI₃ domains, thereby delivering segmented CsPbI₃-CsPbCl₃-Pb₄S₃Cl₂, energetically favorable heterostructures, with partial I-alloying of the CsPbCl₃ domain and at the perovskite-chalcohalide interface. The I:CsPbCl₃ domain between CsPbI₃ and Pb₄S₃Cl₂ enables a gradual lattice expansion across the heterostructure. This design accommodates interfacial strain, with a 5.6% mismatch at the CsPbCl₃-CsPbI₃ interface and a 3.4% mismatch at the perovskite-chalcohalide interface. Full halide exchange leads to CsPbI₃-Pb₄S₃Cl₂ heterostructures. Both in intermediate and fully exchanged heterostructures, the CsPbI₃ domain is emissive. In the intermediate structures, the band alignment between the two perovskite domains is type-I, with the carriers photogenerated in the CsPbCl₃ domain quickly transferring to the CsPbI₃ domain, where they can recombine radiatively.