Effect of Ho3+ concentration on the structure, morphology and optical properties of Ba0.5Mg0.5Al2O4 nanophosphor

Abstract

Barium magnesium aluminate nanopowders doped with holmium (Ba0.5Mg0.5Al2O4:x% Ho3+) were successfully prepared by the sol-gel method. The Ho3+ concentration was varied up to 2 mol%. X-ray powder diffraction results showed that the Ba0.5Mg0.5Al2O4 nanopowders crystallized in the hexagonal phase of BaAl2O4. Scanning electron microscopy images suggested that the doping influences the particle morphology. Photoluminescence results showed visible emissions observed at around 422 and 733 nm which originated from intrinsic defects within the host material as well as emissions at around 491, 550, 662 and 758 nm which were attributed to the 5F3 → 5I8, 5F4/5S2 → 5I8, 5F5 → 5I8 and 5S2 → 5I7 transitions of Ho3+ respectively. Infrared emissions around 1157 and 1200 nm were attributed to the 5S2,5F4 → 5I6 and 5I6 → 5I8 transitions of Ho3+. Increasing the Ho3+ concentration up to x = 0.8% led to luminescence enhancement, while further increase led to luminescence quenching. Lifetime measurements revealed that the x = 0.8% sample with maximum emission intensity at 550 nm has a strong lifetime component of 7.79 μs and a small component of long lifetime 79 μs, while the x = 2% (maximum doped) sample exhibited some concentration quenching and the contribution of the medium (7.79 μs) component was lower. The Commission Internationale de l’Eclairage (CIE) co-ordinates showed that the emission colour could be tuned towards bluish-purple by mixing of the MgAl2O4 and BaAl2O4 components. The Ho3+ concentration influenced the proportion of green emission colour.