Enhancing near infrared persistent luminescence from Cr^3+-activated zinc gallogermanate powders through Ca^2+ doping

Near infrared (NIR) long-persistent luminescence phosphors are promising for applications ranging from night-vision surveillance to in vivo bioimaging. Yet, the luminescence brightness and afterglow period remain insufficient for the reported persistent phosphors with both the activator content optimized and the host material defined. Here, we show that the emission profile of the emerging NIR persistent phosphors of Cr3+-activated spinel zinc gallogermanate (emission at 650–850 nm from the 2E→4A2 transition of Cr3+) can be improved through the incorporation of non-luminescent, divalent calcium (Ca2+) into the host lattice. We found that a slight introduction of 3% Ca2+ ions into the formulated afterglow material was able to enhance its persistent luminescence intensity (recorded after 300s stoppage of the excitation light) by about 15 fold. This was possibly ascribed to the engineering of tetrahedral trapping defects (spinel inversion) surrounding the emitting Cr3+ ions at the octahedral sites and the reduction of luminescence quenching centers in the crystal, enacted by the calcium doping. The simple performance-enhancing route described here has an immediate implication for other visible and NIR persistent phosphors engaged in a plethora of photonic and biomedical applications.