Influence of Doping Concentration on Defects, Nanopores and Photoluminesence Property of Γ-Al2o3:Mn4+,Mg2+ Nanowire-Phosphor Studied by Positron Annihilation Spectroscopy

The present paper reported the role of Mn4+ or Mg2+ doping concentrations on the structural changes, defects, nanopores and optical property of γ-Al2O3 nanowire-phosphor. γ-Al2O3 nanowire-phosphors doped with Mn4+ and Mg2+ were synthesized by hydrothermal method and post-low temperature treatment, in which glucose, Mn4+ and Mg2+ were used as a reducing agent, dopants and charge compensation ions, respectively. X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL) and photoluminescence excitation (PLE) were utilized, respectively, for determining the structural phase, morphology and red-emitting intensity in studied samples. In particular, positron annihilation spectroscopy (PAS), including positron annihilation lifetime (PAL), Doppler broadening (DB) of positron-electron annihilation energy and electron momentum distribution (EMD) measurements, was employed to investigate the formations and the changes of the lattice structures, defects and nanopores induced by doping process in synthesized materials. Results obtained indicated that the doping concentration of 0.06% was optimal for the substitution of Mn4+ and Mg2+ to two Al3+ sites and the formation of VO-rich vacancy clusters (2VAl+3VO) and large-voids (0.7 nm) with less Al atoms. Those characteristics maximized the electron mobility and thus significantly enhanced the PL and PLE intensities. Moreover, this optimal doping concentration also controlled effectively the size of nanopores, hence it is expected to maintain the high thermal conductivity of γ-Al2O3 nanowire-phosphor. The present study, therefore, demonstrated a potential application of γ-Al2O3 nanowire-phosphor in fabricating the high-color rendering index (CRI) micro-white LEDs.