The investigation of Cr3+-activated gallate-based near-infrared long-persistent phosphors
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Near infrared (NIR) long-persistent phosphors have gained considerable attention in recent years due to their potential applications in security and defense, solar energy utilization, and in vivo deep-tissue bio-imaging. The present work aims to discover and develop two novel series of chromium ion (Cr3+)-activated, gallate-based NIR persistent phosphors: Cr3+-doped β-Ga2O3 nanowires and Cr3+-doped zinc gallogermanate powders and ceramics. The Cr3+-doped β-Ga2O3 nanowires were synthesized by hydrothermal reaction followed by calcination. After exposed to 250370 nm UV light for several minutes, β-Ga2O3:Cr3+ nanowires exhibit persistent luminescence in the 650850 nm wavelength range with an afterglow time of more than 4 h. No persistent NIR luminescence was observed in nanowires before. The Cr3+-doped zinc gallogermanates were prepared by a solid-state reaction method with general chemical formula of ZnxGayGezO(x+(3y/2)+2z): tCr3+, mR, where R is a co-dopant selected from a group consisting of alkaline earth ions, rare earth ions, and Li+ ions; x, y, and z are integers from 1 to 5; t is 0.01 mol% to 2 mol% based on the total moles of Ga; and m is 0 to 2 mol% based on the total moles of Ga. Any combinations of these variables can always produce materials with remarkable NIR persistent luminescence. In this dissertation, we used Zn3Ga2Ge4O14:0.1%Cr3+ as an example to demonstrate the superb capabilities of Cr3+-doped zinc gallogermanates in excitation energy absorption, storage, NIR light conversion, and NIR light persistent luminescence. The NIR persistent luminescence in Cr3+-doped zinc gallogermanates can be effectively activated by both UV light and solar radiation. Remarkably, the materials can be quickly and repeatedly charged by solar radiation in any weather conditions, at any moment between sunrise and sunset, at any outdoor locations, and in various aqueous solutions. Seconds to minutes of solar radiation activation can result in days to weeks (> 400 h) of NIR persistent luminescence. The 400 h of persistence time is about two orders of magnitude longer than that achieved previously by others. The afterglow mechanisms of both β-Ga2O3:Cr3+ nanowires and Cr3+-doped zinc gallogermanates were studied based on the results of afterglow decay curves and thermoluminescence measurements and related models were proposed.