Analysis of structural defects and their influence on red-emitting γ-Al2O3:Mn4+,Mg2+ nanowires using positron annihilation spectroscopy

The present paper reported on the analysis of structural defects and their influence on the red-emitting γ-Al₂O₃:Mn⁴⁺,Mg²⁺ nanowires using positron annihilation spectroscopy (PAS). The nanowires were synthesized by hydrothermal method and low-temperature post-treatment using glucose as a reducing agent. 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. Three PAS experiments, namely, positron annihilation lifetime (PAL), Doppler broadening (DB), and electron momentum distribution (EMD), were simultaneously performed to investigate the formations of structural defects in synthesized materials. Obtained results 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 oxygen vacancy (V_O)-rich vacancy clusters (2V_Al + 3V_O) and large voids (~0.7 nm) with less Al atoms. Those characteristics reduced the energy transfer between Mn⁴⁺ ions, thus consequently enhanced the PL and PLE intensities. Moreover, this optimal doping concentration also effectively controlled the size of nanopores (~2.18 nm); 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-performance optoelectronic devices.