Dye-embedded YAG:Ce@SiO composite phosphors toward warm wLEDs through radiative energy transfer: preparation, characterization and luminescence properties.

The most common yellow phosphor for wLEDs, YAlO:Ce (YAG:Ce), suffers from a deficiency of red in its spectral content of light. In this paper, a new strategy is provided to tailor the Ce spectral profile through surface-located dye molecules of ATTO-Rho101, which feature intense, broad absorption in the green-yellow spectral region of Ce emission as well as bright red emission. Sphere-shaped and highly dispersed micrometer and nanometer-sized YAG:Ce (micro/nano-YAG:Ce) was synthesized through a modified solvothermal method. Surface SiO coating and simultaneous dye embedding were performed on the solvothermally derived YAG:Ce, heat-treated micro-YAG:Ce and commercial phosphors. Efficient radiative transfer/reabsorption from Ce in the inner core of YAG to the dye molecules in the SiO outer shell, irrespective of the size of the phosphors, was demonstrated in the accumulated YAG:Ce@SiO + 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:Ce@SiO + dye phosphors on blue LED chips yielded a warm wLED (R∼ 93), but an R of only ∼79 was obtained for the wLED with commercial YAG:Ce@(SiO + dye) 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 Ce→ Ce oxidation upon activation by heating or photoirradiation and then quench the photoluminescence (PL) of micro-YAG:Ce even after surface modification by SiO, YAG or being embedded in an epoxy resin matrix. High calcination temperatures greatly improved the PL stability of micro-YAG:Ce 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:Ce@(SiO + dye) due to the incomplete and/or loose SiO layer grown during multiple surface modifications.