Highly Stable White Light Emission from III-Nitride Nanowire LEDs Utilizing Nanostructured Alumina-Doped Mn 4+ and Mg 2 .

In this report, red-emitting alumina nanophosphors doped with Mn 4+ and Mg 2+ (Al 2 O 3 :Mn 4+ , Mg 2+ ) are synthesized by a hydrothermal method using a Pluronic surfactant. The prepared samples are ceramic-sintered at various temperatures. X-ray diffraction shows that Al 2 O 3 :Mn 4+ , Mg 2+ annealed at 500 °C exhibits a cubic γ-Al 2 O 3 phase with the space group Fd 3 m -227. The tetragonal δ-Al 2 O 3 and rhombohedral α-Al 2 O 3 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 Mn 4+ ion, under a light excitation of 460 nm. A strong zero-phonon line (ZPL) emission is observed in the luminescence spectra of δ-Al 2 O 3 :Mn 4+ , Mg 2+ at 298 K, whereas a weak one is observed in those of α- and γ-Al 2 O 3 :Mn 4+ , Mg 2+ . 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 x Ga 1- x N nanowire chips with Al 2 O 3 :Mn 4+ , Mg 2+ 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 Al 2 O 3 :Mn 4+ , Mg 2+ nanophosphors which is attributed to the increased heat dissipation in the nanowire LEDs.