Highly Emissive Divalent-Ion-Doped Colloidal CsPb_1-xM_xBr₃ Perovskite Nanocrystals through Cation Exchange

Colloidal CsPbX₃ (X = Br, Cl, and I) perovskite nanocrystals (NCs) have emerged as promising phosphors and solar cell materials due to their remarkable optoelectronic properties. These properties can be tailored by not only controlling the size and shape of the NCs but also postsynthetic composition tuning through topotactic anion exchange. In contrast, property control by cation exchange is still underdeveloped for colloidal CsPbX₃ NCs. Here, we present a method that allows partial cation exchange in colloidal CsPbBr₃ NCs, whereby Pb²⁺ is exchanged for several isovalent cations, resulting in doped CsPb_1-xM_xBr₃ NCs (M= Sn²⁺, Cd²⁺, and Zn²⁺; 0 < x ≤ 0.1), with preservation of the original NC shape. The size of the parent NCs is also preserved in the product NCs, apart from a small (few %) contraction of the unit cells upon incorporation of the guest cations. The partial Pb²⁺ for M²⁺ exchange leads to a blue-shift of the optical spectra, while maintaining the high photoluminescence quantum yields (>50%), sharp absorption features, and narrow emission of the parent CsPbBr₃ NCs. The blue-shift in the optical spectra is attributed to the lattice contraction that accompanies the Pb²⁺ for M²⁺ cation exchange and is observed to scale linearly with the lattice contraction. This work opens up new possibilities to engineer the properties of halide perovskite NCs, which to date are demonstrated to be the only known system where cation and anion exchange reactions can be sequentially combined while preserving the original NC shape, resulting in compositionally diverse perovskite NCs.