Highly Stable White Light Emission from III-Nitride Nanowire LEDs Utilizing Nanostructured Alumina-Doped Mn⁴⁺ and Mg²

In this report, red-emitting alumina nanophosphors doped with Mn⁴⁺ and Mg²⁺ (Al₂O₃:Mn⁴⁺, Mg²⁺) are synthesized by a hydrothermal method using a Pluronic surfactant. The prepared samples are ceramic-sintered at various temperatures. X-ray diffraction shows that Al₂O₃:Mn⁴⁺, Mg²⁺ annealed at 500 °C exhibits a cubic γ-Al₂O₃ phase with the space group Fd3m-227. The tetragonal δ-Al₂O₃ and rhombohedral α-Al₂O₃ phase is obtained at 1000 and 1300 °C, respectively. Cube-like nanoparticles in a size of ∼40 nm are observed for the alumina heated at 500-1000 °C. The size and red-emitting intensity of the phosphors remarkably increased with annealed temperature ∼1300 °C. Emission spectra of the phosphors show strong peaks at 678 and 692 nm due to ² E _g → ⁴ A ₂ transitions of the Mn⁴⁺ ion, under a light excitation of 460 nm. A strong zero-phonon line (ZPL) emission is observed in the luminescence spectra of δ-Al₂O₃:Mn⁴⁺, Mg²⁺ at 298 K, whereas a weak one is observed in those of α- and γ-Al₂O₃:Mn⁴⁺, Mg²⁺. The alumina phosphors exhibited an excellent waterproof ability during 60 days in water and good thermal stability in the range of 77-573 K. A warm-white light-emitting diode (WLED) fabricated using In Ga_1- N nanowire chips with Al₂O₃:Mn⁴⁺, Mg²⁺ red-emitting nanophosphors presents a high color rendering index of ∼95.1 and a low correlated color temperature of ∼4998 K. Moreover, the current-voltage characteristic of the nanowire LEDs could be improved using Al₂O₃:Mn⁴⁺, Mg²⁺ nanophosphors which is attributed to the increased heat dissipation in the nanowire LEDs.