Persistent Luminescence Materials Research Articles

Two viologen-based complexes as persistent luminescent materials and their applications in inkless print and anticounterfeiting

Viologen-based complexes have garnered significant attention due to their multifunctional stimuli-responsive properties and tunable persistent luminescence. However, achieving ultralong afterglow emission and color change in these complexes remains a challenge, limiting their application in the field of persistent luminescence. Herein, two novel viologen-based complexes were successfully designed and synthesized using N,N′-bis(2-carboxyethyl)-4,4′-bipyridinium dichloride (H2bybpy) ligands and trimesic acid (H3BTC) with the transition metals Zn(II)/Cd(II). Complexes 1 and 2 display reversible photochromic and electrochromic properties. Interestingly, complexes 1 and 2 in their crystal state exhibit dual fluorescence and persistent luminescence, emitting a green and cyan afterglow when the UV lamp is turned off at 77 K. Incorporating phosphorescent molecules and changing the metal significantly extended the duration of persistent luminescence and altered the hue. The complexes were also demonstrated to have potential applications in inkless printing, color-changing films, data encryption, and anti-counterfeiting. This work enhances the application of viologen-based complexes in light-controlled persistent fluorescence and provides ideas for achieving ultra-long afterglow emission.

Red Persistent Luminescence and Trap Properties of Mg2SiO4: Mn2+, M3+ (M3+ = B3+; Al3+; Ga3+; In3+) Material

Abstract Persistent luminescence (PersL), also called long-lasting phosphorescence or simply afterglow, is a luminescence characterised by the emission of radiation from a few seconds to several days after the excitation source has been switched off. Over the past two decades, research on PersL materials, both in fundamental and applied physics, has developed rapidly; however, the explanation for the physical processes that cause afterglow still needs to be clarified. Today, PersL materials are used mainly for luminescent paints, safety signs and decorations. At the same time, research into using such materials in medicine, information storage, anti-counterfeiting technology, etc., is underway. Currently, information on the long persistent luminescence materials with emission in the blue and green spectral range is widely available. In contrast, the number of publications on the afterglow in the red and near-infrared spectral range is considerably lower. Within the framework of this research, Mg2SiO4: Mn2+; M3+ (M3+ = B3+; Al3+; Ga3+; In3+) materials were synthesised using solid state reaction synthesis. When excited with X-rays, the materials exhibited a broad Mn2+ PersL band with two maxima at approximately 625 nm and 730 nm. After cessation of irradiation, an afterglow of at least 6 hours could be observed. The research focuses on the trap properties of the materials. It was concluded that at least three discrete trap levels with activation energies approximately between 0.4–1.6 eV were present in the samples. Additionally, co-doping with Al3+; Ga3+; In3+ ions improved PersL longevity of the Mg2SiO4: Mn2+ material.

A Novel Near‐Infrared Emitting Sr2LuSbO6:Fe3+ Phosphor with Persistent Luminescence Performance

AbstractThe near‐infrared (NIR) persistent luminescence (PersL) materials have attracted widespread attention owing to the unique self‐sustained light with good penetrability. Currently, most of the reported NIR PersL materials are activated by Cr3+. Considering the potential toxicity of chromium ions, there is an urgent requirement to explore the Cr3+‐free NIR phosphors. Herein, a novel NIR phosphor Sr2LuSbO6:Fe3+ (SLSO:Fe3+) is prepared and its luminescence properties are systematically studied. The Fe3+‐activated Sr2LuSbO6 exhibits a long‐wavelength NIR emission band centered at 890 nm with a 110 nm full‐width at half maximum (FWHM), and its PersL can last over 18 h. The phosphor shows excellent PersL stability even after being dispersed in solutions of different environments for 7 days. Furthermore, the PersL emitted by the phosphor can penetrate the 2 cm thickness beef, evidencing its good NIR PersL penetration property. This work provides a novel NIR PersL materials based on Fe3+ luminescence for technological applications and also contributes to accelerate the development of new‐generation Fe3+‐doped NIR PersL phosphor toward versatile applications.

Trap properties of novel UV-A persistent phosphor Sr3MgSi2O8:Ce3+

For the last few decades, there has been extensive research on the materials that persist to emit optical radiation long after any type of charging has stopped. Most of the established persistent luminescence (PersL) materials emit light in the visible part of the spectrum; however, in recent years, there has been a growing interest in UV-emitting persistent phosphors in recent years. These materials have been tested for various applications, such as photocatalysis, sterilization, and anti-counterfeiting, among others. Here, we report on the X-ray and UV-activated UV-A long PersL of Ce3+-doped Sr3MgSi2O8 material. We prepared samples with varied Ce3+ concentrations using solid–state reaction synthesis in an ambient atmosphere and conducted a thorough investigation using photoluminescence (PL), electron paramagnetic resonance (EPR), and thermally stimulated luminescence (TSL) spectroscopy methods. Our experiments show that the PersL signal of Ce3+ in the 300–450 nm range can be detected for at least 16 h when samples are irradiated with X-rays or UV. The TSL analysis reveals multiple discrete charge traps in the material with activation energies between 0.5 and 1.7 eV. Further EPR measurements confirm the presence of four paramagnetic centers. The thermal stability of these centers was analyzed, and it is established that one of these centers (g1 = 2.0056, g2 = 1.9981, and g3 = 1.9926) gradually decays at room temperature, which is correlated with the PersL processes.

Triggering Long Persistent Luminescence From Functionalizable Carbon Dots‐in‐Carbon Nitrides Composites Via Production of Deep‐Traps

AbstractDispersible and functionalizable long persistent luminescent (LPL) materials are highly desired for flexible use of LPL in various fields. However, synthesizing metal‐free LPL systems with stable long‐lived intermediates and reactive surface groups, which are key requirements for robust LPL and the ability to be functionalized, has proven to be challenging. Herein, the microwave‐assisted preparation of carbon dots (CDs)‐based LPL materials is demonstrated with abundant surface amino groups via in situ embedding of CDs into carbon nitrides (CNs). The incorporation of CDs leads to the formation of deep traps and promotes exciton transmission between CDs and trap states, contributing to the generation of LPL. By varying the embedded CDs, tunable LPL ranging from blue to yellow can be achieved, with a duration exceeding 10 min. Due to the covalent bonding between CDs and CNs, the materials (named CDs@CNs) exhibit robust LPL under ambient conditions and evenly dispersed in water through ultrasonic treatment. Furthermore, the surface amino groups serve as sites for further modification, allowing for on‐demand performance design and control, such as adjustable hydrophilicity–hydrophobicity for various applications. This work provides new insights into designing superior flexible CDs‐based LPL materials and expanding the domain of metal‐free LPL materials.

Synthesis and characterization of TiO2-based supported materials for industrial application and recovery in a pilot photocatalytic plant using chemometric approach

In this contribution, the performance of powdered titanium dioxide (TiO2)–based photocatalysts was evaluated in a pilot photocatalytic plant for the degradation of different dyes, with an investigated volume of 1 L and solar simulated light as irradiation source. Five different samples, synthesized in our laboratories, were tested in the pilot plant, each consisting of TiO2 nanoparticles (NPs) coupled with a different material (persistent luminescent material and semiconductor material) and treated in different thermal conditions. All synthesized samples have been subjected to X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller analysis (BET), and transmission electron microscopy (TEM) characterization, to shed light on the influence of introducing other materials on titania characteristics. To study and evaluate the significance of the parameters affecting the process in the pilot plant, a chemometric approach was applied, by selecting a mathematical model (D-Optimal) to simultaneously monitor a large number of variables (i.e., 7), both qualitative and quantitative, over a wide range of levels. At the same time, the recovery of the synthesized photocatalysts was studied following a novel promising recuperation method, i.e., annulling the surface charge of the suspended samples by reaching the isoelectric point (pHPZC) of each sample, for the quantitative precipitation of TiO2 nanoparticles.Graphical

Stabilization of Ti3+ in CaTiO3 by doping Y3+ to generate visible light-activated near-infrared persistent luminescence

Near-infrared (NIR) persistent luminescence (PersL) materials have interesting invisible self-luminescent properties, but most of them can only be effectively activated by UV light. In the present study, Ca0.97TiO3:Y0.02 (CaTiO3:Y) is found with visible-light-excited PersL at ∼770 nm lasting at least for 5 h. The PersL is derived from the broad absorption of Ti3+, which is stabilized by Y3+-doping via a charge compensation strategy. The PersL excitation spectrum has a peak at ∼ 400 nm, extending to 720 nm. Y3+-doping at Ca2+ site stimulates the generation of more oxygen vacancies and the stabilization of luminescent Ti3+. The Cr-free property makes CaTiO3:Y an environmentally friendly anti-counterfeiting luminescent material. The interaction between narrow band gap CaTiO3 and proper energy levels of Ti3+ is of vital importance in the successful development of visible-light-excited PersL materials. This study provides a new member of Cr-free NIR PersL material with intense visible-light-excited luminescent property and has certain guidance on developing other visible-light-excited PersL materials.

Tunable afterglow Ca1 - xSrxAl2O4:Eu2+, Nd3+ phosphors with dependence of an excitation wavelength.

In the emerging field of high-capacity information encryption, multicolor, multitemporal, and multimodal luminescence inorganic materials are of great significance. However, conventional inorganic materials lack the flexibility to dynamically adjust the photon transition path, resulting in unicolor luminescence of the sample and reducing the reading and decoding levels. Herein, we elaborately designed the components for constructing dual-phase crystal fields for Eu2+ in phosphors based on a high temperature solid-state method. Specifically, SrAl2O4:Eu2+ crystal with a bright green afterglow and CaAl2O4:Eu2+ crystal with a blue afterglow were obtained in phosphors at the same time. As a result, a tunable afterglow behavior from blue to white was achieved due to the 4f65d1 → 4f7 transition of Eu2+ at different crystal field sites. Finally, the color tunable afterglow sample was used to explore the encryption and decryption processes of information, and the results showed that the prepared material has a good anti-counterfeiting performance, which is promising for the development of long persistent luminescent materials.

Evaluating the potential of persistent luminescence in counteracting urban overheating

In recent years, the use of numerical simulations to model real atmospheric conditions over cities has become increasingly popular. One of the primary objectives of these models is to assess the efficacy of various strategies for mitigating the Urban Heat Island (UHI) phenomenon. At the same time, researchers have developed and studied new adaptive materials for building applications that aim to reduce buildings’ energy consumption and improve urban microclimate conditions, while performing radiative cooling. Among the new generation of passive cooling solutions, persistent luminescent (PL) materials have emerged as a cutting-edge option for energy-saving purposes, owing to their ability to reject the incident solar radiation through both reflection and light emission. Here, the Princeton Urban Canopy Model (PUCM) is used to evaluate the potential of an advanced PL roof coating to counteract urban overheating. The phenomenon of persistent luminescence is modeled for the first time, taking advantage of experimentally obtained parameters coming from previous studies. Results demonstrate how persistent luminescence can effectively mitigate surface overheating reducing the roof’s surface temperature and net shortwave radiation up to 1.15 °C and 35 W/m2 respectively, with consequent benefits to the overall energy balance of the envelope. Such results may be further increased with the optimization of PL materials for engineering solutions.

Anti‐Counterfeiting Application of Persistent Luminescence Materials and Its Research Progress

AbstractPersistent luminescent materials can continue to emit light after stopping excitation, and their stored energy is gradually released during the decay time after persistent emitting, which makes the color and luminance changes in slow decay and plays an important role in protecting information and combating forgery. When combined with other anti‐counterfeiting technologies to form a multi‐modal and multi‐process anti‐counterfeiting design, it perfectly hides the real information and has a broad application prospect in anti‐counterfeiting logo design. A brief overview of the design of three different persistent luminescence‐based anti‐counterfeiting applications in unimodality to achieve information encryption in the time dimension. The energy stored in the trap is studied to be released by different physical stimuli and applied to stimulus‐like responsive persistent luminescent materials, and the design of persistent luminescence anti‐counterfeiting applications based on many different processes in dual‐modality, obtaining the dynamic anti‐counterfeiting effect of multiple processes in multiple modes. This work reviews the research background of persistent luminescent materials in the field of anti‐counterfeiting in recent years, compares the important breakthrough progress of persistent luminescent materials applied to anti‐counterfeiting research, and finally puts forward the current problems, future challenges, and development directions.

Achieving tunable long persistent luminescence in metal organic halides based on pyridine solvent

The research of long persistent luminescence (LPL) materials has yield brilliant results in many fields. However, the efforts are still needed for the regulation of the LPL performance. In this work, a series of LPL metal organic halides with rich halogen-bond interactions, Py-CdX2 (X = Cl, Br, I) were synthesized through self-assembly by CdX2 and pyridine solvent. The steady-state emission redshifted and phosphorescence lifetime declined as the halogen atoms are aggravated. Three halides exhibit adjustable emission from blue to green and multiple phosphorescence from green to yellow at room temperature by changing the excitation wavelengths. Surprisingly, Py-CdX2 can emit the visible color-tunable LPL from green to yellow after removing different excitation sources at ambient conditions. Combing the results of theoretical calculation and experimental analysis, it is found that heavy atom effect and the rich intermolecular halogen bond help realize LPL and multiple triplet states originated from the pyridine ring and the halogens.

Chromatically polarized Afterglow: Photo-controlled and anisotropic persistent luminescence from doped polymeric systems

Organic persistent luminescent (OPL) materials showing persistent luminescence at room temperature have caught significant research attention for decades. Integrating chromatic polarization into an OPL system with tunable emissions is attractive and challenging but rarely reported. Herein we propose a strategy to realize linear-polarization-dependent afterglow color with reversible photo-patterning capability. This is accomplished through the use of an acridone-based Förster-resonance energy transfer (FRET) system, which is coupled with aligned Au-nanorods. The composite shows reversible UV-lithographic capability attributed to the generation of radicals by the acridone luminophore. Importantly, this system allows for the variation of chromatic afterglow based on the linear polarized angles relative to the aligned direction of AuNRs. The multiple delayed emission peaks are absorbed in a polarized-dependent manner due to the anisotropic plasmonic resonance of the aligned Au-nanorods. With the photo-patterning capability, polarization- and humidity-dependent afterglow, this approach holds the potential to contribute to various applications of OPL materials, such as sensors, information encryption, and anti-counterfeiting.

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+ -dopedgalliumgarnets 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.

Chargeable persistent luminescence 3D-printed scaffolds: A stepwise tactic for osteosarcoma treatment

Osteosarcoma is the most frequent malignant primary bone tumor with a poor prognosis and remains a significant issue in clinics due to the unrepairable bone defect following surgery and the lingering tumor cells. Photodynamic therapy (PDT) with its non-invasive nature and spatiotemporally controllable feature exhibits specific therapeutic efficiency, but it is limited by phototoxicity caused by prolonged laser exposure. Herein, we design and engineer an irradiation-free bifunctional PDT scaffold (BG@SAO-RB) by integrating persistent luminescence material (SrAl2O4:Eu, Dy) and photosensitizer (rose bengal) into a 3D-printed bioactive glass scaffold for stepwise osteosarcoma elimination and bone defect repair. Owing to the design of a rechargeable internal light source, the constructed scaffolds with reactive oxygen species generation ability attain long-term and efficient PDT for osteosarcoma, leading to tumor cell killing and proliferative inhibition. The findings from tumor growth characteristics and pathological sections also confirm that BG@SAO-RB scaffolds achieve successful osteosarcoma ablation through long-term PDT in tumor-bearing nude mice. Notably, the as-designed scaffolds promote osteogenic differentiation of rat bone marrow mesenchymal stem cells and accelerate bone regeneration, which was evidenced by the improved radiological and histological manifestations. This work broadens the biomedical application of persistent luminescence materials and implicates an efficient treatment paradigm for osteosarcoma elimination and subsequent bone tissue regeneration.

Prolonged Red Persistent Luminescence in Bi3+ Single-Doped LiGa5O8: Regulating Traps by Site Selective Occupation.

Persistent luminescence (PersL) materials with impressive superiorities in optical performance have shown tremendous potential for information encryption application. Herein, a Bi3+-doped LiGa5O8 PersL material is facilely synthesized by a simple solid-state reaction. Site selective and preferential occupation of Bi3+ ions at distinct Ga3+ and Li+ polyhedra sites in LiGa5O8 endows it with a bimodal emission at 511 and 718 nm upon UV excitation at 254 nm. After removing the light source, an intense red afterglow was observed due to the generation of more and deeper traps classified as the oxygen vacancy defect (VO••) and the impurity defect (BiLi••). It is rarely reported in inorganic phosphors to achieve the Bi3+ single doping-activated red PersL. The LiGa5O8:x% Bi3+ (with x = 0.5-5.0) samples also exhibit tunable PL and time-dependent PersL properties. Combined with the Morse Code, a multiple information encryption model based on the abnormal optical properties of LiGa5O8:x% Bi3+ (x = 0, 0.5, and 5.0) samples was designed for high-level dynamic anticounterfeiting. These achievements further validate the use of the Bi3+ single doping strategy to explore excellent PersL materials and expand their multifunctional optical applications.

Unveiling the Potential of Sunlight‐Driven Multifunctional Blue Long Persistent Luminescent Materials via Cutting‐Edge Trap Modulation Strategies

AbstractNumerous challenges persist in the field of long persistent luminescence (LPL) materials, including the unclear mechanism of persistent luminescence, low efficiency of sunlight excitation, and limited availability of commercial materials. To address these issues, a LPL material, SrAl2Si2O8:x% Eu2+,y% Dy3+, is synthesized for excitation by UV lamps or sunlight. Dy3+ doping remarkably extends the afterglow duration from 2 to 7 h under UV lamp excitation, exhibiting a remarkable enhancement of 350%. Notably, SrAl2Si2O8:4.0% Eu2+, 3.0% Dy3+ shows 2 h afterglow duration when exposed to sunlight, surpassing other LPL materials and expanding potential applications. XPS and XANES measurements confirm Eu2+ as the luminescent and persistent luminescent centers, while Dy3+ increases trap diversity. Through meticulous analysis of the EPR and XPS data, it is elucidated that the material encompasses two distinct types of traps, namely oxygen vacancies and may be traps. This provides a basis for subsequent trap analysis in LPL materials. Microstructural analysis examined ion coordination environment changes due to Eu2+ and Dy3+ doping. The mechanical luminescent properties are characterized, and a plausible afterglow mechanism is proposed based on the experimental results, offering insights for further research into LPL materials.

Quasi‐Continuous Defect Levels in Broadband Gap: A New Strategy for High‐Temperature Long Persistent Luminescence Materials

AbstractLong persistent luminescence (LPL) materials widely employed in the fields of emergency lighting and anti‐counterfeiting are mostly used at room temperature. As temperatures rise, the performance of LPL materials deteriorates dramatically, which hinders their application in in vivo imaging, high‐temperature display, and information storage. Herein, a multifunctional material LiGa5O8:Tb3+ (LGT) with green high‐temperature LPL (HT‐LPL) and blue cathodoluminescence (CL) is reported. Its LPL performance is anomalously enhanced with increasing temperature, and the duration time is more than 8 h at 423 K. With combined temperature‐dependent decay curves and thermoluminescence analyses, the unique quasi‐continuous defect levels are found in the band gap. The high‐concentration carriers in deep traps are frozen at room temperature and activated only at high temperatures, accompanied by changes in energy transfer pathways. The excellent HT‐LPL makes LGT a light‐emitting component of next‐generation smart wearable devices, as well as high‐temperature warning equipment in deep well exploration at a depth of 4500 m. The intense anti‐degradation blue CL makes it suitable for field emission displays, while the manipulable emission property makes it suitable for high‐level anti‐counterfeiting. This study fills a gap in HT‐LPL materials and opens up a new gateway for the efficient design of HT‐LPL and other multifunctional materials.

Brighten strontium aluminate long-persistence materials via optimizing defect energy level distribution

Afterglow brightness of inorganic persistent luminescent (PersL) materials upon ceasing light excitation represents an important parameter for various glow-in-the-dark applications. Although the emission wavelength of PersL materials is developed to cover over the whole spectral range from ultraviolet to near infrared light, and light emission is sufficiently long-lasting with a fractional number of rare-earth doped PersL materials commercially available, PersL materials with high initial afterglow brightness at room-temperature condition remain rarely reported. Employing trap-controlled temperature dependency of persistent luminescence mechanism, defect energy level distribution, and microwave-assisted solid-state reaction, here a highly bright green-emitting strontium aluminate phosphor is reported. Higher initial afterglow brightness (a photometric luminance of 99.8 cd/m2, enhanced 1.7 times than that of the commercial one) is achieved due to the massive aggregation of charge carriers at shallow-trap energy levels. The relation between initial afterglow brightness and trap energy level distribution is investigated. The glowing-in-the-dark composite panel composed of Sr4Al14O25:Eu2+, Dy3+ and Polydimethylsiloxane can light up the safety sign and the map of fire evacuation route. These findings may provide a major step forward to developing high-brightness PersL materials and are expected to further benefit a wide range of PersL material systems for practical glow-in-the-dark applications.

Orange-red persistent luminescence in Mn2+-Doped Sr3Y2Ge3O12 films for dynamic anti-counterfeiting

The research focus in the field of anti-counterfeiting has shifted toward the secure luminescent label technology, which has gained prominence. The exceptional luminescence properties of persistent luminescence materials are important in the achievement of effective anti-counterfeiting measures. It is in this context that Sr3Y2Ge3O12 can serve as an excellent matrix material. In this study, it is found that Sr3Y2Ge3O12: 4.00% Mn2+ exhibits a luminescence intensity, persistence time, and stability that meet the practical requirements for applications. It is shown that the fabricated persistent luminescence film can be used in anti-counterfeiting applications, thereby, demonstrating the possibility to broaden the application scope of persistent luminescence materials. An emission band centered at 634 nm and a high-energy inflection point at 578 nm are observed in the samples excited at 260 nm. Fluorescence emission at 634 nm is also achieved through X-ray excitation. Through thermoluminescence, photoluminescence, and phosphorescence spectroscopy, a comprehensive investigation of the trap characteristics and inherent mechanisms of the long persistent phosphorescence has been conducted. The novel orange-red Sr3Y2Ge3O12: Mn2+ phosphor is extensively examined.

Experimental and theoretical study on the green persistent luminescence phosphor Ca5Ga6O14:Bi3+

In this study, we developed a type of green persistent luminescence (PersL) material Ca5Ga6O14:Bi3+, which could be excited by UV light ranging from 270 nm to 360 nm with peak at 325 nm and exhibited a broad afterglow band peaking at 515 nm with the duration of about 11 min, which can be ascribed to the electron transitions between the 6s orbital and the 6p orbitals of Bi3+ ion. The thermoluminescence investigation and the First-principle calculation results revealed that the 6s orbital of Bi3+ was about 0.2 eV above the valence band maximum, while the 6p orbitals were 0.6–1.0 eV below the conduction band minimum. Moreover, we found that O vacancies and −2 charged Ga vacancies could form defect levels just above valence band and serve as the hole-trap centers, playing the key role in the process of the afterglow of Ca5Ga6O14:Bi3+. The possible afterglow mechanism was discussed in detail. This work offers an efficient way to construct Bi3+ doped PersL phosphors with desired properties.