NIR-Emitting Er ³⁺ and Yb ³⁺ Alloyed Cs ₂ (Na x Ag 1– x )BiCl ₆ Lead-Free Double Perovskite Nanocrystals

We report the synthesis of all-inorganic erbium (Er 3+ ) and ytterbium (Yb 3+ ) alloyed Cs 2 Na x Ag 1– x BiCl 6 ( x = 0, 0.25, 0.5, 0.75, 1) double perovskite (DP) nanocrystals (NCs). These NCs exhibit sharp near-infrared (NIR) photoluminescence (PL) at 1540 and 1000 nm, corresponding to the characteristic f – f transitions of Er 3+ and Yb 3+, respectively. Er 3+ and Yb 3+ alloyed Cs 2 (Na/Ag)BiCl 6 DPs show significantly enhanced NIR emission compared with that of the Er 3+ and Yb 3+ alloyed pristine Cs 2 AgBiCl 6 . Specifically, Cs 2 Na 0.75 Ag 0.25 BiCl 6 exhibits a 27- and 12-fold increase in NIR emission from Er 3+ and Yb 3+, respectively, when compared with the Na-free alloyed Cs 2 AgBiCl 6 DP. The multifold enhancement in the PL is attributed to the efficient energy transfer from the host lattice to the lanthanide ions (Ln 3+ ) via self-trapped excitons (STEs). Furthermore, Er 3+ and Yb 3+ coalloyed Cs 2 Na 0.75 Ag 0.25 BiCl 6 NCs were synthesized, revealing that Yb 3+ also acts as an effective energy transfer channel, further sensitizing the NIR emission of Er 3+ . These findings provide fundamental insights into the design and synthesis of efficient NIR-emitting nanomaterials.