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 (Oh) sites (90 pm) instead of Bi(III) Oh 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.