Ultra-bright and thermally stable deep red emitting doped yttrium zirconate nanoparticles for tunable white LEDs and indoor plant growth

Abstract

To counter the problem of the lack of an efficient red phosphor in commercial LEDs, it is important to develop a red-emitting phosphor with a high quantum efficiency, thermal stability, and size of nanodomain. The present work involves the synthesis of deep red-emitting Y₂Zr₂O₇:Eu³⁺ (YZOE) nanocrystals with a very high color purity, excellent thermal stability, and a higher absolute photoluminescence quantum yield (PLQY) value. The photoluminescence (PL) lifetime and positron annihilation lifetime spectroscopy (PALS) suggested that a very large fraction of the Eu³⁺ ions occupy an isovalent Y³⁺ site with a minimal lattice strain. Triggered by this, the absolute PLQYs for the YZOE nanocrystals reach over 88%, which is much higher than that of its bulkier counterpart, and the highest among the known Eu³⁺-doped oxide based red-emitting phosphors. The color purity of the material (97%) is higher than that of the commercial red phosphor Y₂O₃:Eu³⁺ (92.4%). The optimum red phosphor is able to retain a color purity of 89% at 428 K relative to the PL intensity observed at room temperature, and the activation energy is estimated to be 0.2 eV which demonstrates its excellent thermal stability. Interestingly, the designed red phosphor-converted (pc-LED) was able to provide the reference spectral region which is required for indoor plant growth. Finally, the tunable white LEDs (cool and warm white LED) are fabricated by combining the RGB mixture of the optimized red YZOE nanoparticles with commercial green and blue phosphors together with a 280 nm UV LED chip. By tuning the RGB mixing ratio, the resultant emission from the fabricated pc-LED appears as a warm white light with a correlated color temperature (CCT) of 4164 K, color rendering index (CRI) of 78, and CIE of 0.358, 0.309 which is the ultimate requirement for ideal indoor lighting.