Green Afterglow of Undoped SrAl₂O₄

Undoped SrAl₂O₄ nanocrystals were obtained via solution combustion using urea as fuel. The afterglow properties of undoped SrAl₂O₄ were investigated. Green afterglow from undoped SrAl₂O₄ is visible to the human eye when the 325 nm irradiation of a helium-cadmium laser (13 mW) is ceased. The afterglow spectrum of undoped SrAl₂O₄ is peaked at about 520 nm. From the peak temperature (321 K) of the broad thermoluminescence glow curve, the trap depth of trap levels in undoped SrAl₂O₄ is estimated to be 0.642 eV using Urbach's formula. Based on first-principles density functional calculations, the bandstructures and densities of states are derived for oxygen-deficient SrAl₂O₄ and strontium-deficient SrAl₂O₄, respectively. Our results demonstrate that the green afterglow of undoped SrAl₂O₄ originates from the midgap states introduced by oxygen and strontium vacancies. The observation of green afterglow from undoped SrAl₂O₄ helps in gaining new insight in exploring the afterglow mechanisms of SrAl₂O₄-based afterglow materials.