Broad Dual Emission from Cs₂Zr_1–xMo_xCl₆: Enhancing the NIR Emission in Lead-free Vacancy Ordered Double Perovskites

Near-infrared (NIR) emitting materials find a variety of applications in photodynamic therapy, national security, food inspection, and bioimaging, but achieving efficient NIR luminescence in lead-free perovskite materials is still challenging. Herein, we synthesized a series of lead-free Mo⁴⁺ alloyed Cs₂ZrCl₆ vacancy-ordered double perovskites with different Mo concentrations. These materials display tunable dual luminescence bands due to intrinsic host emission in the blue region and dopant induced d–d transitions emitting in the NIR region. Host- and dopant-induced emission can be excited at different energies, as revealed by spectroscopic studies. Minimum interaction between the host and dopant octahedra accounts for the dual emission in these 0D materials, further confirmed by density functional theory (DFT) calculations. Alloying Mo⁴⁺ reduces the band gap of these materials and DFT calculations confirm that the decrease in band gap is due to the increased contribution of Mo 4d states at the conduction band minima. Moreover, the structural stability of the host material also increases after Mo⁴⁺ alloying. Our study explored the tunable dual band emission in visible and NIR regions and enhanced structural stability in Mo⁴⁺ alloyed Cs₂ZrCl₆, thereby opening new avenues in the development of Pb-free NIR-emitting phosphors.