Ultrathin Bi-Doped Cs₂Ag_xNa_1-xInCl₆ Double Perovskite Nanoplatelets with Stokes-Shifted Photoluminescence

We report the synthesis of ultrathin Bi-doped Cs₂Ag_0.5Na_0.5InCl₆ nanoplatelets (NPLs) through control of the ligand and metal carboxylate concentrations. These ultrathin NPLs, with thickness down to 2.8 nm, exhibit photoluminescence with larger Stokes shift compared to previously reported double perovskite materials of similar kind, which we attribute to greater exciton binding energy and enhanced exciton-phonon coupling in NPLs when their thickness approaches the exciton Bohr diameter. The double perovskite NPLs could self-assemble into ordered structures that exhibited polarized emission with a degree of 0.49. This work establishes morphology control as an effective strategy to modulate the behavior of excitons in double perovskite nanomaterials and expands their optical functionality.