Highly Photoluminescent Sulfide Clusters Confined in Zeolites

The design, synthesis and photoluminescence characterization of sulfide cluster-based zeolitic materials with the general formula Na₈[SiAlO₄]₆S_xCl_2-2x is presented in this report. The employed experimental conditions, which included the thermal treatment of sodium Linde type A (Na-LTA) zeolites in the presence of Na₂SO₄ and NaCl, yielded highly photoluminescent sulfur-zeolites with remarkably large Stokes shifts. An emission band at 650 nm for samples with low sulfur contents is observed when excited at 380 nm, which is fully consistent with S₂ ^- clusters as light-emitting centers. These orange emitting materials display external quantum efficiencies (EQEs) reaching 53% with peculiar high temperature photoluminescence stability up to 450 °C. For higher S/Cl molar ratios, a gradual conversion to near-infrared (NIR) light-emitting centers is observed with a maximum at 780 nm. This novel NIR luminescent species with EQEs of about 10% are attributed to polysulfide clusters (tentatively S₄ ^2- species). Given the high EQE values, large Stokes shifts, and thermal stability, the materials presented in this report can find applications in light down-conversion systems or as phosphors in lighting devices.