Luminescence mechanism of Sm doped MgAl2O4 nanophosphors: Role of nanoparticle size, samarium content and structural defects

We report structural and luminescence properties of the spinel MgAl2O4:Sm3+ nanophosphors (with Sm3+ content of 0.03, 0.1, 0.2, 0.3, 0.5, 1.0, 1.5, and 3.0 wt%.) synthesized by a co-precipitation method. The X-ray phase analysis and transmission electron microscopy show that the size of the MgAl2O4:Sm3+ nanoparticles varies from 5 nm to 6 nm at a synthesis temperature of 700 °C and from 7 nm to 15 nm at 1000 °C. We demonstrate that the samples synthesized at 700 °C with a samarium content of 1.5 wt% are the most promising candidates for use as orange phosphors. We show that near-surface rare-earth ion segregation arises in the nanoparticles with an average size of 10 nm and the Sm concentrations of 1.0 wt% and 1.5 wt%. Simulation of the MgAl2O4:Sm3+ luminescence spectrum by the modified crystal field theory (MCFT) indicates that Sm ions occupy both octahedral and tetrahedral positions. Complexes with broken bonds, non-equivalent bonds, and complexes with vacancies are revealed and confirmed by EPR studies and theoretical calculations. We demonstrate that the most intense transitions arise in tetrahedral and octahedral complexes with vacancies. The enhancement of the emission efficiency caused by the distortions of the Sm3+ coordination complexes competes with the luminescence quenching caused by surface segregation.