Dye-embedded YAG:Ce3+@SiO2 composite phosphors toward warm wLEDs through radiative energy transfer: preparation, characterization and luminescence properties

Abstract

The most common yellow phosphor for wLEDs, Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce³⁺), suffers from a deficiency of red in its spectral content of light. In this paper, a new strategy is provided to tailor the Ce³⁺ spectral profile through surface-located dye molecules of ATTO-Rho101, which feature intense, broad absorption in the green-yellow spectral region of Ce³⁺ emission as well as bright red emission. Sphere-shaped and highly dispersed micrometer and nanometer-sized YAG:Ce³⁺ (micro/nano-YAG:Ce³⁺) was synthesized through a modified solvothermal method. Surface SiO₂ coating and simultaneous dye embedding were performed on the solvothermally derived YAG:Ce³⁺, heat-treated micro-YAG:Ce³⁺ and commercial phosphors. Efficient radiative transfer/reabsorption from Ce³⁺ in the inner core of YAG to the dye molecules in the SiO₂ outer shell, irrespective of the size of the phosphors, was demonstrated in the accumulated YAG:Ce³⁺@SiO₂ + dye powder upon blue light excitation; this enhanced its red emission. Fluorescence microscopy was demonstrated to be a powerful tool to identify the reabsorption phenomenon of the powdered materials. Packaging the heat-treated micro-YAG:Ce³⁺@SiO₂ + dye phosphors on blue LED chips yielded a warm wLED (R_a ∼ 93), but an R_a of only ∼79 was obtained for the wLED with commercial YAG:Ce³⁺@(SiO₂ + dye)₅ due to the low concentration of phosphors dispersed in the epoxy resin and the resulting decreased reabsorption by dye molecules. Surface-protonated amine species were found to induce Ce³⁺ → Ce⁴⁺ oxidation upon activation by heating or photoirradiation and then quench the photoluminescence (PL) of micro-YAG:Ce³⁺ even after surface modification by SiO₂, YAG or being embedded in an epoxy resin matrix. High calcination temperatures greatly improved the PL stability of micro-YAG:Ce³⁺ through the removal of surface-capped species. The dye in the silica matrix showed high stability against heating and irradiation due to the so-called “caging effects”; however, decreased photo-stability was found in commercial YAG:Ce³⁺@(SiO₂ + dye)₅ due to the incomplete and/or loose SiO₂ layer grown during multiple surface modifications.