Predicting the afterglow duration in persistent phosphors: a validated approach to derive trap depth distributions

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Traditional information encryption materials that rely on fluorescent/phosphorescent molecules are facing an increasing risk of counterfeiting or tampering due to their static reading mode and advances in counterfeiting technology. In this study, a series of Mg2-xZnxSnO4 (x = 0.55, 0.6, 0.65, 0.7 0.75, 0.8) that realizes the writing, reading, and erasing of dynamic information is developed. When heated to 90 °C, the materials exhibit a variety of dynamic emission changes with the concentration of Zn2+ ions. As the doping concentration increased, the ratio of the shallow trap to deep trap changed from 7.77 to 20.86. When x = 0.55, the proportion of deep traps is relatively large, resulting in a higher temperature and longer time required to read the information. When x = 0.80, the proportion of shallow traps is larger and the encrypted information is easier to read. Based on the above features, encryption binary codes device was designed, displaying dynamic writing, reading, and erasing of information under daylight and heating conditions. Accordingly, this work provides reliable guidance on advanced dynamic information encryption.

A multi-functional nano-platform based on LiGa4.99O8:Cr0.01/IrO2 with near infrared-persistent luminescence, "afterglow" photodynamic and photo-thermal functions

Multi-functionalised nano-platforms based on persistent-luminescence nanoparticles (PLNPs) have attracted considerable attention for biomedical applications owing to their lack of background noise and suitability for in vivo imaging without the need for in situ excitation. However, nano-platforms based on PLNPs for continuous photodynamic therapy (PDT) are currently lacking. Herein, we report a nano-platform (LiGa4.99O8:Cr0.01/IrO2, LGO:Cr/IrO2) prepared using PLNPs (LiGa4.99O8:Cr0.01, LGO:Cr) covalently bonded with iridium oxide nanoparticles (IrO2 NPs), producing near-infrared (NIR) persistent luminescence, "afterglow" PDT and photo-thermal therapy (PTT) effects. The LGO:Cr/IrO2 not only exhibits NIR-persistent luminescence at 719 nm and a PTT effect under 808 nm irradiation but also a continuous "afterglow" PDT effect without the need for in situ excitation owing to persistent energy transfer from LGO:Cr to the IrO2 NPs, in turn generating reactive oxygen species (ROS). This multi-functional nano-platform is expected to further promote the application of PLNPs in tumour treatment.

Facilitating Near-Infrared Persistent Luminescence in Cr3+ -Doped Gadolinium Gallium Garnets

Near-infrared persistent luminescence (NIR PersL) materials provide great potential in the fields of night vision, biological imaging, and information encryption. However, among various crystal structures, Cr3+ -doped gallium garnets show inferior PersL property, which turns out to be the bottleneck of their versatile applications. The rational design and facile preparation of high-performance NIR PersL materials are crucial for the emerging applications. In this work, a series of Gd3 Mgx Gex Ga5-2x O12 :Cr3+ (x = 0, 0.25, 0.5, 0.75, 1) is investigated by microwave-assisted solid-state (MASS) approach. Furthermore, by employing chemical composition co-substitution, PersL performance is further improved and the optimum working temperature is adjusted to the lower temperature at 10 °C. Trap level distribution of Gd3 Mg0.5 Ge0.5 Ga4 O12 :Cr3+ phosphor is revealed based on the temperature and fading-time dependent PersL and thermoluminescence property. Further study demonstrates the reduction of the bandgap and the trap distribution forwards at shallow-lying trap energy levels. The synergistic effect, from both energy-band manipulation and trap-level optimization, facilitates NIR PersL in Cr3+ -doped gadolinium gallium garnets. These findings confirm the applicability of MASS-based bandgap and defect level engineering for improving the PersL properties in non/inferior-PersL materials. This burgeoning MASS method may facilitate a wide range of PersL materials for various emerging applications.

Persistent luminescent nanophosphors for applications in cancer theranostics, biomedical, imaging and security

The extraordinary and unique properties of persistent luminescent (PerLum) nanostructures like storage of charge carriers, extended afterglow, and some other fascinating characteristics like no need for in-situ excitation, and rechargeable luminescence make such materials a primary candidate in the fields of bio-imaging and therapeutics. Apart from this, due to their extraordinary properties they have also found their place in the fields of anti-counterfeiting, latent fingerprinting (LPF), luminescent markings, photocatalysis, solid-state lighting devices, glow-in-dark toys, etc. Over the past few years, persistent luminescent nanoparticles (PLNPs) have been extensively used for targeted drug delivery, bio-imaging guided photodynamic and photo-thermal therapy, biosensing for cancer detection and subsequent treatment, latent fingerprinting, and anti-counterfeiting owing to their enhanced charge storage ability, in-vitro excitation, increased duration of time between excitation and emission, low tissue absorption, high signal-to-noise ratio, etc. In this review, we have focused on most of the key aspects related to PLNPs, including the different mechanisms leading to such phenomena, key fabrication techniques, properties of hosts and different activators, emission, and excitation characteristics, and important properties of trap states. This review article focuses on recent advances in cancer theranostics with the help of PLNPs. Recent advances in using PLNPs for anti-counterfeiting and latent fingerprinting are also discussed in this review.

Garnet-structured persistent luminescence phosphor Ca3Al2Ge3O12:Cr3+ for dynamic anticounterfeiting applications

Fluorescent materials have gradually become a hot spot in the field of anti-counterfeiting. Multifunctional phosphors used in anti-counterfeiting designs still have the problems of disordered reading sequences, difficulty in detection, and easy forging. To resolve these problems, we propose a new flexible dual-mode anti-counterfeiting design using a series of phosphors Ca₃Al₂Ge₃O₁₂:Cr³⁺ (CAG) with deep red persistent luminescence peaking at 722 nm. By adjusting the doping concentration of Cr³⁺ from 2% to 6%, deep red persistent luminescence with different afterglow intensities and durations can be achieved. By performing a series of thermoluminescence (TL) experiments under different conditions, the defects in materials were comprehensively and systematically analyzed. The defects contributed to the deep and shallow traps; this led to an obvious improvement in its long persistent luminescence (LPL). Such a dual-mode system with flexibility and simplicity properties is a good choice not only for anti-counterfeiting, but also for multi-layer information encryption, and a series of demo experiments based on the digital tube, Moss code, QR code, bar-code, school celebration pattern, and love letter information encryption design were implemented. Their dynamic anti-counterfeiting applications have been demonstrated, which provides a new way to rationally design multi-functional luminescent materials.

Tailored Luminescence Output of Bi3+-Doped BaGa2O4 Phosphors with the Assistance of the Introduction of Sr2+ Ions as Secondary Cations

In this work, a tunable luminescence color from yellow to orange of photoluminescence (PL), long persistent luminescence (LPL), and photostimulated luminescence (PSL) is successfully achieved in BaGa₂O₄:Bi³⁺ phosphors with the introduction of Sr²⁺ ions as secondary cations. It is confirmed that broad-band emissions located at 500 and 600 nm originate from the occupation of Bi³⁺ ions at different lattice sites in the BaGa₂O₄ host matrix. The replacement of Sr²⁺ for Ba²⁺ ions makes the emission red-shift from 600 to 650 nm; moreover, two additional emissions appeare at 743 and 810 nm due to the occupational preference of Bi³⁺ ions at Ga³⁺ sites. Furthermore, the doped Sr²⁺ ions promote the reconstruction of the trapping centers, which conduces to the fundamental improvement of the optical storage capacity behavior of Bi³⁺-doped phosphors. Our results clarify the dependence of the luminescence performance on the crystal sites of Bi³⁺ ions with fascinating broad-band emissions in the BaGa₂O₄:0.01Bi³⁺ host matrix and will benefit the design and exploration of Bi³⁺-doped solid solutions for optical storage applications.

Near-infrared mechanoluminescence crystals: a review

Due to the in situ, real-time, and non-destructive properties, mechanoluminescence (ML) crystals have been considered as intelligent stress sensors, which demonstrate potential applications such as in inner crack visualization, light source, and ultrasonic powder recording. Thereinto, it is highly expected that near-infrared (NIR) MLs can realize the visualization of inner biological stress because mechanically induced signals from them can penetrate biological tissues. However, such an energy conversion technique fails to work in biomechanical monitoring due to the limited advances of NIR ML materials. Based on those, some research groups have begun to focus on this field and initially realized this idea in vitro while related advances are still at the early stage. To advance this field, it is highly desirable to review recent advances in NIR ML crystals. In this review, to our knowledge, all the NIR ML crystals have been included in two main groups: oxysulfides and oxides. Besides, the present and emerging trends in investigation of such crystals were discussed. In all, the aim is to advance NIR ML crystals to more practical applications, especially for that of biomechanical visualization in vivo.

Underestimated Color Centers: Defects as Useful Reducing Agents in Lanthanide-Activated Luminescent Materials

Inorganic hosts, such as SrB4 O7 or certain nitrides, intrinsically stabilize Eu2+ even when the dopant is an Eu3+ -based precursor and reducing conditions are not employed in the synthesis. Although this concept is well known in the synthesis of phosphorescent materials, the mechanistic details are scarcely understood. Herein, we demonstrate that trapped charge carriers, such as color centers, can also act as redox partners to stabilize certain oxidation states of activators. Eu-activated CsMgCl3 and CsMgBr3 are used as examples. Upon doping with EuCl3 and in the absence of reducing conditions during the synthesis, dominant cyan or green luminescence from Eu2+ ions was observed. Photoluminescence spectroscopy at 10 K revealed that the reduction is correlated to color centers localized at defects. Although defects are typically undesired in phosphors, we have shown that their role may be underestimated and they could be used on purpose in the preparation of selected inorganic phosphors.

Temperature dependent persistent luminescence: Evaluating the optimum working temperature

Development of persistent luminescent materials has drawn continuous attention in recent years in view of their potential applications in the fields of security night-vision signage, in vivo bio-imaging and optical data storage. Currently, the normative evaluation of a new persistent luminescent material is focused on the light emission spectrum, the afterglow decay curve and the total duration time of the persistent luminescence. In this paper, we investigate the temperature dependent persistent luminescence in some well-known persistent phosphors and relate this to their thermoluminescence properties. The concept of the optimum working temperature is proposed as a new means for evaluating persistent phosphors. It is shown that there is a clear relation between the efficient temperature range of the afterglow output and the thermoluminescence glow curve. The experimental work is supported by simulations of thermoluminescence and afterglow characteristics. The concept of the optimum working temperature for persistent phosphors can be used as an evaluative criterion for applications in various working environments.