Effect of Oxygen Vacancies on the Persistent Luminescence of Y3Al2Ga3O12:Ce3+,Yb3+ Phosphors

Broadband Near-Infrared Light Excitation Generates Long-Lived Near-Infrared Luminescence in Gallates

Persistent luminescence describes the phenomenon whereby luminescence remains after the stoppage of excitation. Recently, upconversion persistent luminescence (UCPL) phosphors that can be directly charged by near-infrared (NIR) light have gained considerable attention due to their promising applications ranging from photonics to biomedicine. However, current lanthanide-based UCPL phosphors show small absorption cross sections and low upconversion charging efficiency. The development of UCPL phosphors faces challenges due to the lack of flexible upconversion charging pathways and poor design flexibility. Herein, we discovered a lattice defect-mediated broadband photon upconversion process and the accompanying NIR-to-NIR UCPL in Cr-doped zinc gallate nanoparticles. The zinc gallate nanoparticles can be directly activated by broadband NIR light in the 700-1000 nm range to produce persistent luminescence at about 700 nm, which is also readily enhanced by rationally tailoring the lattice defects in the phosphors. This proposed UCPL phosphor achieved a signal-to-background ratio of over 200 in bioimaging by efficiently avoiding interference from autofluorescence and light scattering. Our work reported a lattice defect-mediated photon upconversion phenomenon, which significantly expands the horizons for the flexible design of UCPL phosphors toward broad applications ranging from bioimaging to photocatalysis.

Current Developments in Emerging Lanthanide-Doped Persistent Luminescent Scintillators and Their Applications

Lanthanide-doped scintillators have the ability to convert the absorbed X-ray irradiation into ultraviolet (UV), visible (Vis), or near-infrared (NIR) light. Lanthanide-doped scintillators with excellent persistent luminescence (PersL) are emerging as a new class of PersL materials recently. They have attracted great attention due to their unique "self-luminescence" characteristic and potential applications. In this review, we comb through and focus on current developments of lanthanide-doped persistent luminescent scintillators (PersLSs), including their PersL mechanism, synthetic methods, tuning of PersL properties (e. g. emission wavelength, intensity, and duration time), as well as their promising applications (e. g. information storage, encryption, anti-counterfeiting, bio-imaging, and photodynamic therapy). We hope this review will provide valuable guidance for the future development of PersLSs.

Tuning the persistent luminescence property of a Y3Al2Ga3O12:Ce3+,Yb3+ phosphor by controlling the intrinsic oxygen vacancy concentration

Oxygen vacancies (V_O), acting as electron traps, have a significant impact on the persistent luminescence (PersL) property of persistent phosphors. However, the effect of V_O on PersL remains still unclear enough to limit the development of PersL materials. In this study, the V_O concentration of the Y_2.978Ce_0.018Yb_0.004Al₂Ga₃O₁₂ phosphor is accurately controlled by annealing in air and 10%H₂/90%Ar atmospheres at various temperatures. The results show as the annealing temperature increases during the air annealing the V_O concentration, the PersL durations, and the thermoluminescence (TL) intensity constantly decreases, and the three data coincide well with each other, indicating the PersL property of the Y_2.978Ce_0.018Yb_0.004Al₂Ga₃O₁₂ is successfully tuned. Besides, the trap structure of the Y_2.978Ce_0.018Yb_0.004Al₂Ga₃O₁₂ and the charge compensation effect of Yb ions on V_O defects are also discussed. By deconvoluting the TL curves, the Yb trap with a depth of 0.58 eV has been distinctly separated from the V_O traps with a quasi-continuous and broad distribution of depths ranging from 0.58 to 1.21 eV. Our work demonstrates a better understanding of the relationship between V_O and PersL is of great significance to design a high-performance phosphor.

Urea Glass Route as a Way to Optimize YAGG:Ce3+,Cr3+,Pr3+ Nanocrystals for Persistent Luminescence Applications

A new approach for the synthesis of Y₃Al₂Ga₃O₁₂ (YAGG) nanophosphors allowing the preparation of crystallites with sizes starting from 45 nm is presented. The controllability of the energy and trap density of the resulting material samples by annealing temperature was confirmed by thermoluminescence (TL) measurements. It has been shown that the annealing of samples at temperatures up to 1300 °C does not cause any substantial growth of crystallites, still remaining below 100 nm, but leads to changes in the activation energy of the persistent luminescence (PersL) process. On the other hand, annealing above 1400 °C results in grain growth on the submicron scale, which was confirmed by X-ray powder diffraction (XRPD) and electron transmission microscopy (TEM) measurements. In addition, with an increase in the molar ratio of urea to the total amount of metals used (R), qualitative changes are observed in the PersL process occurring from the excited states of Cr³⁺ and Pr³⁺ ions. This proves the influence of the synthesis process, in particular of the metal complexation at its initial stage, on the final structure ordering in the annealed materials. These observations are linked to previously reported defects in the YAGG structure, leading to PersL.

A non-rare earth ion doped broadband emission phosphor located in NIR-II for ethanol concentration detection

In this work, a new NIR phosphor Ca₂GeO₄:xCr was developed, whose emission range (1000-1700 nm) is well within NIR-II, with a full width at half maximum (FWHM) of 215 nm. Interestingly, by sintering under an air atmosphere, the Cr³⁺ ions in the raw material are oxidized to Cr⁴⁺, and the near-infrared light emitted by the material is not the usual luminescence of Cr³⁺ ions located in NIR-I, which can be attributed to the dominant substitution of Cr⁴⁺ ions doped into the lattice sites. A pc-LED fabricated with this phosphor can detect different concentrations of ethanol, which demonstrates its good potential for non-destructive testing applications.

Size-Dependent Persistent Luminescence of YAGG:Cr3+ Nanophosphors

In the current work, YAGG:Cr3+ nanophosphors were synthesized by the Pechini method and then annealed at different temperatures in the range 800–1300 °C. The structure and morphology of the samples were characterized by X-ray Powder Diffraction (XRPD). The lattice parameters and average crystalline sizes as site occupation by Al3+ and Ga3+ ions were calculated from the Rietveld refinement data. To investigate the effect of crystalline size of the materials on their optical properties: excitation and emission spectra were recorded and analyzed. Finally, the effect of crystalline size on the probability of carrier recombination leading to PersL was determined experimentally with thermoluminescence analyses. The Tmax-Tstop method was applied to determine the trap type and particle size (calcination temperature) effect on their redistribution. A correlation between structural changes and trap redistribution was found. In particular, the extinction of high-temperature TL maximum with increasing annealing temperatures is observed, while low-temperature TL maximum increases and reaches a maximum when the lattice parameter reaches saturation.

Stable Metal-Organic Frameworks for Fluorescent Detection of Tetracycline Antibiotics

The rapid detection of antibiotics in agricultural products is of great significance. In this work, two stable fluorescent metal-organic frameworks (MOFs), BUT-178 and BUT-179, are synthesized and used to detect tetracycline antibiotics. Among them, BUT-179 exhibits better performance in the detection of different tetracycline antibiotics in water and eggs. The limits of detection of BUT-179 toward tetracycline, aureomycin, oxytetracycline, and doxycycline all reach the nanomolar level. Furthermore, the cycling tests confirm that BUT-179 can be easily recovered and repeatedly used without an obvious performance loss. This work demonstrates the excellent application potential of MOFs for food safety, especially the fluorescence detection of antibiotics in foods.