Facilitating Low-Energy Activation in the Near-Infrared Persistent Luminescent Phosphor Zn_{1+ x}Ga_{2-2 x}Sn_xO₄:Cr³⁺ via Crystal Field Strength Modulations

Cr³⁺-activated persistent luminescent phosphors with a spinel structure are emerging materials in bio-imaging applications for their distinctive features of deep biotissue penetration and rechargeable near-infrared persistent emission. To realize the long-term and multicycle imaging purpose, reactivation using in situ external light with deep biotissue penetration is an alternative strategy, apart from the efforts on trap modulations for the host lattice. However, recharging with high-energy ultraviolet /visible photons will result in low activation efficiency because of the absorption by biological tissues. Here, we report a low-energy photon-rechargeable near-infrared persistent material, with rechargeable efficiency 400 times higher than that of the ZnGa₂O₄:Cr³⁺ reference material. The crystal field strength and band gap energies can be tailored by control of cation occupancy, contributing the red shift of both the persistent luminescence excitation and emission spectra of the optimized complex spinel samples. The persistent emission red shift is of interest for improved deep tissue penetration for bioimaging, whereas the persistent excitation red shift facilitates the activation of persistent luminescence by low-energy radiation. Furthermore, it demonstrates that increasing the rate of cation site inversion in the spinel can lead to higher storage capacity of charge trapping.