Band Gap Engineering in Cs₂(NaAg_1-)BiCl₆ Double Perovskite Nanocrystals

Lead-free double perovskite materials, A₂M(I)M'(III)X₆, have recently attracted attention as environment-friendly alternatives to lead-based perovskites, APbX₃, because of both rich fundamental science and potential applications. We report band gap tuning via alloying of Cs₂AgBiCl₆ nanocrystals (NCs) with nontoxic, abundant Na. It results in a series of Cs₂NaAg_1-BiCl₆ (x = 0, 0.25, 0.5, 0.75, and 1) double perovskite NCs, leading to increase in optical band gap from 3.39 eV (x = 0) to 3.82 eV (x = 1) and 30-fold increment in weak photoluminescence. The tuning of band gap has been further explored by electronic structure calculation under the framework of density functional theory (DFT). The latter confirms that the increase in band gap is due to reduction of Ag contribution near valence band maxima (VBM) on incorporation of Na ion in place of Ag. These alloyed double perovskites can have useful potential applications in optoelectronic devices.