Lattice Distortion in Mixed-Anion Lead Halide Perovskite Nanorods Leads to their High Fluorescence Anisotropy

Semiconductor nanorods (NRs) with strong fluorescence anisotropy are considered promising polarized light sources for liquid crystal displays. Here, we achieved a high fluorescence anisotropy value of 0.4 in the mixed-anion lead halide perovskite NRs, which constitutes a four-fold enhancement as compared to pristine CsPbBr₃ NRs. This was realized by substituting some smaller bromine anions with larger iodine anions in the orthorhombic CsPbBr₃ NRs via anion exchange. The resulting CsPbBr_1.2I_1.8 NRs kept a highly uniform length of ∼36 nm and width of ∼8.6 nm, and possessed a high emission quantum yield of 83%. On the basis of the DFT calculations, we attribute the improved polarized emission of the mixed-anion perovskite NRs to a moderate structural distortion of PbX₆ octahedra units in the orthorhombic perovskite lattice, which results in a preferred direction of the optical transition dipole moment along the Pb-X-Pb axis. Our study provides a guidance for design and fabrication of perovskite NRs with high fluorescence anisotropy.