Simultaneously excited downshifting/upconversion luminescence from lanthanide-doped core-shell lead-free perovskite nanocrystals for encryption and data storage

Metal ion doping is the most viable method to modulate the optical properties of lead-free Cs₂AgInCl₆ double perovskite (DP) nanocrystals (NCs). However, despite the fact that doping multiple cations can allow for emission color tuning, their random distribution into the perovskite matrix poses severe limitations caused by uncontrollable and adverse energy interactions between the dopants, resulting in poor emission color purity and photoluminescence (PL) quenching. To address this issue, we designed Cs₂AgInCl₆-based core-shell NCs doped with different metal ions in the two layers via an epitaxial growth strategy, with the aim to spatially separate the diverse optically active centers. We selected Cs₂AgInCl₆:Bi as the core and Cs₂AgInCl₆:Yb, Er as the shell and successfully constructed Cs₂AgInCl₆:Bi@Cs₂AgInCl₆:Yb, Er core-shell NCs. The so-designed core-shell NCs allow for non-interacting dual-modal luminescence (upconversion/downshifting) yielding not only broadband self-trapping exciton orange emission under ultraviolet (370 nm) excitation but also upconversion green light emission under near-infrared (980 nm) excitation, so far unprecedented in DP NCs. Taking advantage of the high photostability and stability against moisture bestowed by the shell coating, we demonstrate that the resulting core-shell DP NCs can be successfully employed as luminescent ink for dual-mode anti-counterfeiting labels. These results not only provide a novel design strategy for multi-emissive lead-free NCs, but can also represent a milestone to realize controllable luminescent materials for anti-counterfeiting and information storage.