Yixi Zhuang

Four Cr ⁴⁺ ‐activated transparent glass‐ceramics containing different species of silicate nano‐crystals ( Zn ₂ SiO ₄ , Mg ₂ SiO ₄ , Li ₂ ZnSiO ₄ , and Li ₂ MgSiO ₄ ) were successfully prepared. Absorption spectra, photoluminescence spectra, lifetime decay curves, and quantum yield of these transparent glass‐ceramics were measured. According to the crystal field strength of Cr ⁴⁺ ‐incorporated tetrahedral sites, the broadband near‐infrared (NIR) luminescence of Cr ⁴⁺ can be tailored from 1130 to 1350 nm and the lifetime of Cr ⁴⁺ luminescence can be prolonged from 6 to 100 μs. Quantum yield in the transparent glass‐ceramics containing Li ₂ ZnSiO ₄ nano‐crystals reached at 17%, which is the highest value of NIR luminescence in transition‐metal‐activated glass materials.

We made a systematic investigation on the optical properties of non-doped and Cr-doped Zn_xGa₂O_3+x (0.98 ≤ x ≤ 1.02) spinel crystals. Absorption, photoluminescence excitation, and persistent luminescence excitation spectra indicated that the bottom of the conduction band (CB) was affected (i) by compositional deviation from stoichiometry or (ii) by atmosphere control in a similar way. By constructing an energy level diagram, a broad distribution of trap depth in a Cr³⁺-doped Zn_xGa₂O_3+x sample with a composition of excess Zn or in one prepared in an O₂ atmosphere was attributed to lowering of the CB bottom, which resulted in faster decay of persistent luminescence in these two samples. Defect chemical reactions revealed that the anti-site species of Zn^′ _Ga may be the reason for lowering of the CB bottom. A self-redox effect of Cr ions induced by the compositional deviation was found and explained by the defect reactions. The investigation indicated that the composition with slight Zn deficiency is important to obtain a narrow distribution of trap depths and longer persistent luminescence, however the self-reduction effect of Cr ions in the Zn deficient composition also needs to be considered.

Thermoluminescence peaks for Cr³⁺ emissions shift to higher temperature due to the partial substitution of Al for Ga sites in ZnGa₂O₄:Cr,Bi phosphors, which indicates that the electron traps responsible for the red persistent luminescence are tuned to deeper levels by Al-doping. Al-containing phosphors with deeper traps showed a lower persistent luminescence intensity than Al-free samples at 20 °C because the ambient thermal energy is too low to release trapped electrons in deeper levels; however, the Al-containing persistent phosphors perform better at 80 °C with greater thermal energy. At 20 °C, the trapped electrons in the Al-containing phosphors can be efficiently released with the assistance of additional NIR photostimulation. NIR photostimulated red persistent luminescence (NIR-to-red mode) as presented in the Al-containing ZnGa₂O₄:Cr,Bi phosphors could be a promising technique for new in vivo imaging systems.

Transparent glass‐ceramics containing Li ₂ ZnSiO ₄ :Fe nanocrystals were prepared by melt‐quenching method followed by post‐heat‐treatment process. The as‐prepared glasses and glass‐ceramics showed red to near‐infrared photoluminescence centered at 730 nm, ascribed to Fe ³⁺ ions in tetrahedral coordination. The intensity of the photoluminescence was enhanced by two technologically simple techniques—the valence state of irons was controlled from Fe ²⁺ to Fe ³⁺ ions using oxidizing agents, whereas the coordination state was compulsorily converted from octahedral to tetrahedral symmetry by performing a ceramization process. The presence of Fe ²⁺ ions was considered a major origin for Fe ³⁺ photoluminescence quenching. Oxidation of Fe ²⁺ and conversion of Fe ²⁺ ions into tetrahedral symmetry contribute to suppression of energy transfer from Fe ³⁺ emitters to Fe ²⁺ quenching centers.

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