Modern engineering fields put forward requirements for optical temperature sensors, which need natural-light excitation/storage and near-infrared (NIR) afterglow emission for some special conditions. Here, NIR persistent luminescent phosphors of ZnGa 2−x (Mg/Ge) x O 4 :Cr 3+ (x = 0−1.25) have been synthesized. The incorporation of Mg 2+ /Ge 4+ ions in ZnGa 2 O 4 :Cr 3+ resulted in more defect clusters of "Mg Ga ′ −Ge Ga• " and "Zn Ga ′ −Ge Ga • " and interstitial oxygens (O Int ). Increasing the calcination temperature and Mg 2+ /Ge 4+ doping both contributed to the generation of O Int . Higher efficiency of visible light excitation was observed, mainly due to the defect clusters and O Int. The samples exhibited a bright NIR emission at 695 nm by exposure to UV or visible light, and the NIR signal can last longer than 1 h after the stoppage of excitation. Incorporation of Mg 2+ /Ge 4+ and increasing the calcination temperature both resulted in a deeper trap depth. However, the density of trapped charge carriers takes the dominant role in the persistent luminescence. Therefore, the x = 0.25 sample, having the most trapped charge carriers, exhibits the best afterglow performance. The prepared phosphor exhibited a temperature-dependent persistent luminescence behavior, which can charge natural light and release NIR light repeatedly many times, indicating that they are the potential natural-light rechargeable materials for temperature sensing.

Traditional fluorescent anti-counterfeiting materials usually display monochromatic luminescence at a fixed excitation mode, which greatly reduces the efficiency of anti-counterfeiting applications. Recently, developing a multilevel anti-counterfeiting material with tunable photoluminescence is a hot topic in the arena of advanced anti-counterfeit research. Here, spinel-structured solid solutions of ZnGa 2−x (Mg/ Ge) x O 4 :0.001Mn (x = 0−1.2) have been successfully synthesized by a high-temperature solid-state reaction. In this solid solution, Mn 2+ and Mn 4+ ions were substituted for the tetrahedral site (Zn site) and the octahedral site (Ga site), respectively, which emitted a green light at ∼505 nm with afterglow and a red light at ∼668 nm with the absence of afterglow. The Mn 2+ and Mn 4+ luminescent centers in ZnGa 2 O 4 were effectively regulated by the incorporation of Mg 2+ /Ge 4+ , which resulted in more Mg 2+ ions occupying the Zn site and thus leading to more Mn 4+ ions in the Ga site and less Mn 2+ ions in the Zn site. Therefore, doping Mg 2+ /Ge 4+ contributed to a greatly enhanced red emission for Mn 4+ ions at ∼668 nm and a weakened green emission for Mn 2+ ions at ∼505 nm. The luminescent materials prepared in this study show dynamic and multicolor changes and a higher anti-counterfeiting security, indicating that they have potential for use in advanced luminescent anti-counterfeit materials. The research results in this work provide guidance for the development of multimode luminescent materials in anti-counterfeiting applications.

Near-infrared (NIR) emitting persistent phosphors of Cr3+-doped zinc gallogermanate have emerged for in vivo bioimaging with the advantage of no need for in situ excitation. However, it is challenging to synthesize well-dispersed and uniform spherical particles with high brightness, high resolution, and distinguished NIR long afterglow. In this work, Zn3Ga2Ge2O10:Cr3+ (ZGGC) monospheres were directly synthesized by a facile hydrothermal method with the assistance of citric anions (Cit3–), which emit an NIR emission at ∼696 nm and exhibit excellent NIR-persistent luminescence with rechargeability. Controlled experiments indicated that the shape evolution of the ZGGC product is significantly affected by Cit3–, solution pH, and the duration and temperature of hydrothermal reaction. Furthermore, compositional influence on the crystal structure, bandgap, trap depth, and luminescence characteristics of Zn y Ga2Ge2O10−δ:Cr3+ (y = 2.8, 3.0, 3.2) were investigated in detail, which allows us to construct an energy level diagram of the ZGGC host, Cr3+ ions, and electron traps. It was found that the bandgap and conduction-band minimum (CBM) are significantly affected by the Zn content, while the valence-band maximum (VBM) is not. The y = 3.0 sample exhibited the best persistent luminescence, owing to its deepest defects. The ZGGC-NH2 prepared through surface functionalization of ZGGC spheres showed distinguished NIR long afterglow, low toxicity, and great potential for in vitro cell imaging and in vivo bioimaging in the absence of excitation. Moreover, the persistent luminescence signal from the ZGGC-NH2 can be repeated in vivo through in situ recharge with external excitation of a red LED lamp, indicating that the ZGGC-NH2 is suitable for applications in long-term in vivo imaging.

To develop reliable optical anti-counterfeiting materials, higher requirements are put forward for innovative anti-counterfeiting phosphors, which are hardly observed under natural light but recognizable under specified conditions. Herein, novel multimode...

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