Tailoring Substitutional Sites for Efficient Lanthanide Doping in Lead-Free Perovskite Nanocrystals with Enhanced Near-Infrared Photoluminescence

The incorporation of rare earth lanthanide ions (Ln³⁺) into lead-free halide perovskite nanocrystals (NCs) is an effective and promising strategy to expand their optical, magnetic, and electrochemical properties. Herein, we designed and synthesized various Ln³⁺ (including Yb³⁺, Er³⁺, and Nd³⁺), doped Sb³⁺- or Bi³⁺-based and Sb³⁺/Bi³⁺ alloyed lead-free perovskite NCs, including vacancy-induced perovskite (A₃B(III)₂X₉), double perovskite (A₂B(I)B (III)X₆), and layered-double perovskite (A₄B(II)B(III)₂X₁₂) NCs with different energy transfer pathways to study the Ln³⁺ dopant photoluminescence (PL). While a small size mismatch between dopant ions and host substitutional sites are critical for efficient doping of many first-row transitional metal ion doped metal chalcogenides, surprisingly, the Ln³⁺ ions, including the large Nd³⁺ ions (112 pm), prefer smaller isovalent Sb(III) octahedral (O_h) sites (90 pm) instead of Bi(III) O_h sites (117 pm) in these lead-free perovskite NCs. Significantly, similar substitutional site-dependent Ln³⁺ doping efficiencies were obtained across all three different perovskite host lattices, despite differences in host-to-dopant energy transfer mechanisms, which can provide strong evidence of the preferred Sb³⁺ substitutional sites for lanthanide dopants in these lead-free perovskite lattices. The efficient Ln³⁺ doping in Sb³⁺-rich perovskite NCs leads to enhanced Ln³⁺ ion PL of the doped NCs. The preference of smaller Sb (III) over Bi(III) substitutional sites for Ln³⁺ dopants is attributed to the relatively high polarizabilities of lanthanide ions and the smaller cationic sites inside [SbX₆]^3- compared with [BiX₆]^3- octahedra. This study provides a fundamental understanding of Ln³⁺ doping behavior in lead-free perovskite NCs and opportunities for designing efficient Ln³⁺-doped functional materials by tuning the microenvironment of the host lattice for enhanced properties.