Persistent Luminescence of ZnGa2O4:Cr3+ Transparent Glass Ceramics: Effects of Excitation Wavelength and Excitation Power.

Highly transparent glass ceramics, synthesized from a glass precursor with a nominal composition of SiO₂/ZnO/Ga₂O₃/Na₂O/Cr₂O₃ (65:17:23:5:0.125), were prepared by glass crystallization. This method enables the synthesis of up to 50 wt.-% of size-controllable Cr³⁺-doped ZnGa₂O₄ nanocrystals embedded in an SiO₂-rich glass matrix after crystallization at 900-1000 °C for 10 min. Rietveld refinements and transmission electron microscopy imaging showed that the nanoparticle sizes are ca. 16 and 33 nm for the glass ceramics annealed at 900 and 1000 °C, respectively. Photoluminescence measurements of these glass ceramics revealed intense deep red ( λ = 695 nm) persistent emission that is detectable for up to 10 h, similar to that previously observed for ZnGa₂O₄ powder. As the glass ceramics exhibit good transparency, the persistent glow arises from the entire sample volume, which is of interest for outdoor applications and for more detailed studies of the mechanism for the persistent luminescence. Thermoluminescence measurements showed a wide distribution of trap depths centered at ca. 325 K. The traps can be charged not only under UV light but also under visible-light excitation from λ = 365 to 625 nm. Electron paramagnetic resonance spectroscopy measurements of the Cr³⁺ ions showed clearly that the ZnGa₂O₄ nanocrystals in the glass ceramics exhibit the same types of defects and disorder as those in microcrystalline ZnGa₂O₄ powder, and this suggests that antisite defects are the dominant traps responsible for the persistent luminescence. It is proposed that the ability to charge ZnGa₂O₄ particles with visible light can be attributed to the combination of a two-photon absorption and a local electric field owing to antisite defects in the vicinities of some of the Cr³⁺ ions. [ABSTRACT FROM AUTHOR]