Colloidal Synthesis of Lead-Free Silver–Indium Double-Perovskite Cs₂AgInCl₆ Nanocrystals and Their Doping with Lanthanide Ions

Abstract

Recently, lead halide perovskite (LHP) materials have shown potential for many optoelectronic applications. However, the structural instability and toxicity concerns necessitated robust and heavy-metal-free alternatives. We report a synthetic route to Cs2AgInCl6 double-perovskite nanocrystals (DP NCs) that have the crystal structure replacing two lead ions in LHP by a pair of silver and indium ions. Cubic-shaped Cs2AgInCl6 DP NCs were obtained with a narrow size distribution, which showed superior stability against moisture that rivaled core–shell-structured CdS/ZnS NCs. Cs2AgInCl6 DP NCs showed discrete optical properties: (i) multiple absorbing states that resolved the parity-forbidden band-edge transition at 3.33 eV and next—following higher order parity—allowed transition at 4.88 eV and (ii) multiple emission states that consisted of the band edge emission at 350 nm and emissions from defective states that peaked at 395 nm. The synthetic route was further exploited for isovalent doping with lanthanide ions, which yielded Yb-doped, Er-doped, and Yb/Er-codoped Cs2AgInCl6 DP NCs. The characteristic f–f transition emissions were observed in infrared at 996 nm for Yb3+ and at 1537 nm for Er3+ dopants.