Persistent Phosphors Research Articles

Multifunctional near-infrared long persistent luminescence phosphor BaLu2Al2Ga2SiO12:Cr3+, Tb3+ with good thermal stability, promising quantum efficiency, and excellent environmental resistance

Near-infrared (NIR) persistent luminescence (PersL) phosphors have gained significant study attention in bioimaging and biomedical fields due to the advantages of persistent emission and elimination of biological tissue autofluorescence. However, PersL phosphors generally have difficulty in guaranteeing both good thermal stability and external quantum efficiency (EQE), which limits the multi-functional applications of NIR PersL phosphors. Herein, we developed a novel multifunctional NIR PersL phosphor BaLu2Al2Ga2SiO12:Cr3+ (BLAGSO:Cr3+). Interestingly, this NIR phosphor has good thermal stability (99.7 %@150 °C for BLAGSO:0.03Cr3+) and promising EQE (31.08 % for BLAGSO:0.03Cr3+). To further improve its PersL performance, Tb3+ ion which is believed to act as a modulator of matrix defects and enhances the storage capacity of traps for electrons is introduced into BLAGSO:0.03Cr3+ and an optimal PersL duration of more than 48 h is achieved. Meanwhile, BLAGSO:0.03Cr3+, 0.03 Tb3+ also exhibits good photo-stimulated PersL (PSPL) ability, and the quenched PersL can be rejuvenated by external photo-stimulation. The potential uses of BLAGSO:0.03Cr3+, 0.03 Tb3+ in NIR pc-LEDs and bioimaging are explored and demonstrated. This work is anticipated to drive the research of multifunctional NIR PersL phosphors.

Probing the electron trap-depth distribution in Sr4Al14O25:Eu2+,Dy3+

Sr4Al14O25:Eu2+,Dy3+ is a superluminous persistent luminescence phosphor. Owing to its intrinsic and extrinsic point defects, it tends to luminescence incessantly after excitation, in some cases doing so for as long as 20 h. We report analysis for the energy distribution of its electron traps. The study was done using thermoluminescence (TL) measured on a sample irradiated to 2 Gy at 20 °C. A glow curve measured at 1 °C/s shows two glow peaks at 64 °C (labelled as P1) and at 252 °C (P2). The variation of intensity of peak P1 as a function of heating rate is consistent with competing radiative and non-radiative recombination processes. When the heating rate is faster than 1 °C/s, non-radiative recombinations dominate and the luminescence efficiency drops sharply beyond 70 °C. The activation energy for thermal quenching is estimated to be 0.69±0.06 eV. Once corrected for thermal quenching, the glow curves show a secondary peak (P1A) on the high temperature end of P1. The so-called Tm-Tstop method shows that peaks P1 and P2 gradually shift towards high temperature with preheating temperature Tstop. Indeed, the peak position Tm of both peaks increases with irradiation temperature Tirr. Together, these results suggest that peaks P1 and P2 are each composed of closely spaced overlapping peaks. Analysis of the fractional area between a pair of glow curves measured from the sample irradiated at different temperatures (Tirr) shows that the trap distribution associated with peak P2 is Gaussian whereas that associated with peak P1 follows a non-Gaussian distribution. The activation energy of the electron traps responsible for P1 are estimated to be in the range 0.63–0.68 eV. On the other hand, the Gaussian distribution of trap energy levels responsible for peak P2 suggests that most of the traps have an activation energy of ∼1.58 eV.

Scattering Spheres Boost Afterglow: A Mie Glass Approach to Go Beyond the Limits Set by Persistent Phosphor Composition

AbstractPersistent luminescence phosphor nanoparticles (PersLNPs) offer exciting opportunities for anticounterfeiting, data storage, imaging displays, or AC‐driven lighting applications owing to the possibility to process them as shapable thin coatings. However, despite unique delayed and long‐lasting luminescence, the relatively low storage capacity of persistent phosphor nanoparticles combined with the difficulty of harvesting photons from transparent thin layers drastically hinder the perceived afterglow. In order to enhance persistent luminescence (PersL) of thin coatings, herein a novel approach is proposed based on resonant optical nanostructures. In particular, it is demonstrated that the integration of TiO2 scattering spheres in films (with thickness comprised between 1 and 10 µm) made of ZnGa2O4:Cr3+ PersLNPs enables a significant increase in afterglow intensity due to the combination of effective charging and enhanced outcoupling. As a result, a ≈3.5‐fold enhancement of the PersL is observed in 2 µm‐thick films stuffed with scattering centers using low‐light illumination conditions. Furthermore, inclusion of scattering centers leads to an unprecedented acceleration of the PersL charging speed. These results constitute the first example of photonic engineering applied to enhance the properties of PersL materials coatings.

Incorporation of Eu3+ in ZnGa2O4:Ni2+ for improved NIR persistent luminescence located in second transparency window

AbstractIt is well known that near‐infrared (NIR) persistent phosphors have rather low absorption coefficients of biological tissues for NIR light. However, recent research shows that the phosphors emitting NIR lights in second NIR (NIR‐II, 1000–1350 nm) and third (NIR‐III, 1500–1800 nm) biological window have advantages over that in NIR‐I (650–900 nm). Although ZnGa2O4:Ni2+ outputs NIR emission and afterglow locatedin NIR‐II, the weak signal significant limits its application. In this work, persistent luminescent phosphors of ZnGa2O4:xNi2+, yEu3+ (x = 0–0.013, y = 0.01–0.06) (termed as ZEGN) were synthesized via a traditional high‐temperature solid‐state reaction, which feature a broad emission band in the NIR‐II window. The phosphors exhibit a broad NIR emission at about 1300 nm after ultraviolet (UV) or orange–red lights excitation, arising from the 3T2(3F) → 3A2(3F) transition of Ni2+. However, incorporation of Eu3+ ions, the NIR emission intensity significantly increases with the increase of Ni2+ ion concentration, reaching the maximum by 18 times at x = 0.005. Removing the light source, the sample still outputs intense NIR afterglow and red afterglow that can last over 500 s. It is noteworthy that the red afterglow of Eu3+ shows a dramatically decrease but the NIR afterglow increases with increasing the Ni2+ ion concentration, because of energy transfer. Under the excitation of 282‐nm UV light, the ZGEN sample exhibits a good thermal stability. The phosphor offers a promising application in biological imaging due to broadband NIR‐II light and afterglow.

Mn2+-doped MgGeO3 nanophosphors with controlled shape and optimized persistent luminescence

Mn2+-doped MgGeO3 (MgGeO3:Mn2+) is an efficient persistent phosphor that emits red luminescence for long time after stopping excitation with UV light. For optical and biotechnological uses a precise control of particle size and shape is highly desired since these parameters may have a strong influence on the properties and suitability of phosphor materials for the intended applications. To the best of our knowledge, MgGeO3:Mn2+ has been synthesized by conventional solid-state-reaction, which yields particles of heterogeneous size and shape. Here, we report for the first time in the literature a salt-assisted method for the synthesis of MgGeO3:Mn2+ nanoparticles with uniform shape (nanorods) and a mean size of 350 nm × 99 nm. The rigorous study of the luminescence properties of the MgGeO3:Mn2+ nanorods revealed that whereas the optimum doping level for photoluminescence was 2.0 mol% Mn2+, the best persistent luminescence was attained with just 0.5 mol% Mn2+, which is ascribed to the different mechanisms of both luminescence processes. The optimum persistent nanophosphor showed an intense red emission, which persisted at least 17 h after stopping the excitation. Such excellent properties make the developed nanophosphor an attractive candidate for use in optical and biotechnological applications.

Controlling a defect structure of the ZnGa2O4:Cr3+ spinel through synthesis parameters for persistent luminescence optimization

ZnGa2O4:Cr3+ (ZGO:Cr) is one of the most promising red persistent phosphors for in-vivo imaging due to its 650–750 nm emission band in the range of the so-called first biological window. In this paper, we report on how to control the high brightness persistent luminescence of the smallest ZGO:Cr nanoparticles by optimization of hydrothermal synthesis parameters. The influence of pH solution, the temperature of synthesis, and calcination on the degree of inversion, lattice parameters, morphology, and crystallite size were systematically investigated. The results have shown that different synthesis conditions modify the trap depths distribution, by mixing the occupancy of Zn2+ and Ga3+ sites in the spinel structure, which affects the duration and intensity of persistent luminescence. The correlation between structural changes and optical properties is established and it is possible to control them by selecting the appropriate hydrothermal synthesis parameters.

N, S-doped carbon quantum dot for long persistence phosphor assisted all-weather solar cells

Traditional solar cells can only work during the daytime. The solar cells that can harvest energy in all weather conditions are favorable to solving the energy crisis and ecological issues. We present a novel quantum dot-sensitized all-weather solar cell (AW-SC) that can harvest energy in both day and night conditions. The AW-SC consists of a TiO2/long persistence phosphor (LPP) photoanode sensitized by nitrogen and sulfur co-doped carbon quantum dots (N, S-CQDs) and a Pt counter anode. The N and S doping reduces the band gap of CQDs and enhances the charge injection into the TiO2 layer, resulting in better solar cell performance. The LPP enables the AW-SC to store unabsorbed light from visible to infrared wavelengths and emit green fluorescence to excite the N, S-CQDs in the dark. The optimal AW-SC achieves a power conversion efficiency (PCE) of 18.7% in the dark.

Ultraviolet-C persistent luminescence and defect properties in Ca2Al2SiO7:Pr3+

In recent years, significant attention has been focused on the study of materials with ultraviolet (UV) persistent luminescence (PersL) due to their prospective application possibilities. Although novel UV persistent phosphors are being developed, reports on defect properties and PersL mechanisms are limited. In this study, the incorporation of Pr3+ ions in Ca2Al2SiO7 is analyzed using X-ray diffraction (XRD) and X-ray absorption techniques, whereas PersL properties are characterized by photoluminescence (PL), thermostimulated luminescence (TSL) and electron paramagnetic resonance (EPR) methods. Analysis of the Pr L3-edge X-ray absorption spectra suggests that praseodymium atoms substitute for calcium atoms. The UV-C PersL of the host results from thermally assisted recombination of charge carriers with trap depths within the 0.50–0.87 eV range. Moreover, a partial correlation of PersL and TSL with the decay of X-ray-induced paramagnetic centers was observed. This study expands the knowledge base on the structure of Ca2Al2SiO7 and establishes the optimal Pr3+ concentration for efficient UV-C PersL.

Color-tunable persistent luminescence phosphor for multimode dynamic anti-counterfeiting

Using a solid-state approach, the persistent luminescence property in sodium yttrium silicate which was single-doped or co-doped by europium or cerium was studied. The orthorhombic structure with the space group P212121 could be indexed to all samples, according to X-ray powder diffraction. The luminescence characteristics of these phosphors, including photoluminescence, luminescence degradation, and temperature-dependent emission, were systematically investigated. Under UV (∼365 nm) irradiation, blue/green emissions with broad bands peaking at 450/510 nm attributed to the characteristic 5d→4f/4f65d→4f7 transitions of Ce3+/Eu2+ could be observed in Na3LuSi3O9:Ce3+ and Na3LuSi3O9:Eu2+, respectively. Moreover, blue and green persistent luminescence was also observed in Ce3+ or Eu2+ single-doped samples. Thermoluminescence curve of Na3LuSi3O9:Eu2+, Ce3+ reveals that Na3LuSi3O9:Ce3+ and Na3LuSi3O9:Eu2+ have similar trap distribution. Tunable persistent luminescence can be realized via ratio control of Ce3+ and Eu2+. A possible afterglow mechanism was presented and discussed. The prepared materials have the potential of multimode dynamic luminescence, because they are highly sensitive to the composition, the excitation wavelength, and the detecting time. The outstanding abilities of Na3LuSi3O9:Ce3+, Eu2+ demonstrate the practical capability of this material for fingerprint identification and anti-counterfeiting.

Structural and optical properties of iron ions doped near-infrared persistent spinel-type phosphors

Structural and optical properties of iron ions doped near-infrared persistent spinel-type phosphors

Role of the thermal treatment on the microstructure of YAGG nanopowders prepared by urea glass route

Yttrium aluminium gallium garnet (YAGG, Y3Al2Ga3O12) doped with rare-earth ions has drawn large attention owing to its optical properties with applications ranging from persistent luminescent phosphors to nanothermometers. Herein, three different YAGG materials were synthesized via the urea glass route followed by thermal treatment, relatively undoped; doped with Ce3+, Cr3+, and Nd3+; and doped with Ce3+, Cr3+, and Yb3+. The garnet formation was studied in situ upon thermal treatment from 300 to 1000 °C using synchrotron powder diffraction. Our results show that with this method, the onset of formation of the garnet is about 860 °C, with comparable cell parameters for both undoped and doped YAGG. A possible growth mechanism of YAGG is therefore discussed on the basis of observed microstructural parameters such as occupancy, microstrain, and crystallite size.

Identification and Quantification of Charge Transfer in CaAl2O4:Eu2+,Nd3+ Persistent Phosphor

AbstractReversible charge transfer between lanthanide ions is identified as storage mechanism for the current workhorse persistent phosphors. Present evidence relies on sophisticated X‐ray absorption spectroscopy to detect the reversible charge transfer. Here, simple optical spectroscopy is used to study the charge transfer in one of the benchmark persistent phosphors CaAl2O4:Eu2+,Nd3+. Based on the observation that both the trapping and de‐trapping processes in CaAl2O4:Eu2+,Nd3+ are thermally activated, forward charge transfer from Eu2+ to Nd3+ and backward charge transfer from Nd2+ to Eu3+ are identified. The percentages of Eu2+ and Nd3+ involved in the charge transfer are >10%, which exceeds the previous estimates in other persistent phosphors. Furthermore, this strategy offers additional advantage of site selectivity, which enables the identification of the distinct contributions of different Nd3+ sites to the charge transfer. These findings underline the significance of reversible charge transfer in persistent phosphors and move towards a complete understanding of persistent luminescence mechanism.

Trap‐Dependent Optical/Thermal Stimulated Luminescence of Gallate Phosphors Charged by UV–visible–NIR light for Multiplexed Data Storage

AbstractUV‐charged optical stimulated single‐band phosphors are being used for single‐level optical data storage because of their ability to easily write/read information by light. In contrast, UV–visible–NIR rechargeable optical stimulated double‐band phosphor is urgently necessary for multiplexed high‐throughput optical data storage. Here, the authors design and synthesize Mn2+,Cr3+ co‐doped zinc gallate storage persistent phosphor that exhibits strong red‐green bicolor persistent luminescence (PersL). Besides the traditional ultraviolet charging, the phosphor can be effectively and repeatedly charged by using visible and NIR light by one‐ or two‐photon process. Interestingly, red and green PersL exhibit the opposite temperature dependence, which is extremely important for multilevel information storage. The mechanism of temperature‐dependent PersL intensity based on different trap types and filling approaches for the double luminescence centers is proposed. The phosphor promises a sophisticated application in optical data storage of wavelength multiplexing.

Near‐Infrared Persistent Phosphor‐Mediated Smart Sensing Light‐Emitting Diodes for Advanced Driver Assistance Systems

Compared with conventional phosphor‐converted near‐infrared light‐emitting diodes (NIR pc‐LEDs), near‐infrared persistent phosphor‐converted LEDs (NIR ppc‐LEDs) are applicable not only in conventional systems but also novel devices in the fields of intelligent security, driverless vehicle technology, and virtual reality. However, NIR ppc‐LEDs have not been extensively investigated. Herein, a novel NIR ppc‐LEDs with the prepared SrAl12O19: Fe3+, Mg2+, Ti4+ NIR persistent phosphors is designed. In relation to pc‐LEDs, the new ppc‐LED exhibits full‐spectral responsive sensing ability during the photostimulated mediation of lattice defects. Furthermore, using the ppc‐LED as a smart sensing LED in a digital NIR imaging system, optical feedback could be converted to a digital signal in a self‐regulated visible‐to‐NIR operation mode. These results imply that the designed ppc‐LED, as a smart sensing LED with full‐spectral responsive ability, greatly enhances the sensitivity of digital NIR imaging, particularly low‐light digital NIR imaging, which has recently drawn considerable attention in the fields of advanced driver assistance systems and VR/AR technology.

First Evidence of Electron Trapped Ln2+ Promoting Afterglow on Eu2+, Ln3+ Activated Persistent Phosphor‐Example of BaZrSi3O9:Eu2+, Sm3+

AbstractBaZrSi3O9:Eu2+, Sm3+ (Em:525 nm) is prepared. The role played by the trivalent co‐doping ion Sm3+ in the afterglow and the type of trap are clarified. BaZrSi3O9:Eu2+, Sm3+ is found to produce Sm2+ during the excitation by X‐ray absorption near‐edge structure (XANES), etc., and it is thus proved that Sm3+ exists as an electron trap in the afterglow process. In the field of persistent phosphors activated by Eu2+ and Re3+ such as Sm3+ or Dy3+ having been widely utilized as emergency guide lights, clock faces, etc. for > 25 years, for the first time it is successfully observed that after excitation Re2+ is formed, transferring its electron to 5d band of Eu2+, returning to Re3+ by itself, where the decrease in Sm2+ coincides with the increase in Sm3+, and the two decay time τ1 and τ2 of PL (5D0→7F0) of Sm2+ coincides with the two evolution time of PL (5d→4f) of Eu2+. The behavior of electron transfer from Sm2+ to Eu2+ as a key of afterglow is detected. The detailed afterglow mechanism is proposed by analysis of thermoluminescence and defect reaction, which is very important for the in‐depth investigation of the long afterglow material and the further improvement of the mechanism.

A new insight into the mechanism of persistent luminescence phosphors SrS: Eu2+, Pr3+

The red persistent luminescence phosphors SrS: , (x = 0∼0.001, y = 0∼0.003) were synthesized by the solid-state reaction method in a weak reducing atmosphere of active carbon. The synthetic parameters, photoluminescence characteristics, and dynamic properties of thermoluminescence of the phosphors were studied systematically. According to the results of photoluminescence spectra of SrS: , , the energy level scheme of the phosphors was confirmed quantitatively. Based on the studies of the scanning electron microscope (SEM), UV-Visible absorbance spectra, afterglow decay curves, thermoluminescence(TL) spectra, and X-ray photoelectron spectroscopy (XPS) spectra, we found that the SrS: , (SSEP) phosphor presented the best persistent luminescence property, which mainly resulted from its outstanding absorbance characteristic, deeper depth of () defects, and fewer defect concentration belonging to the higher-order kinetics. Those comprehensive results deduced that a complex defect ()- should present in the Eu2+ doping phosphors, and the Pr3+ doping will produce defects, which can suppress the formation of () defects and increase their energy depth.

Persistent Luminescence Zn2GeO4:Mn2+ Nanoparticles Functionalized with Polyacrylic Acid: One-Pot Synthesis and Biosensing Applications.

Zinc germanate doped with Mn2+ (Zn2GeO4:Mn2+) is known to be a persistent luminescence green phosphor with potential applications in biosensing and bioimaging. Such applications demand nanoparticulated phosphors with a uniform shape and size, good dispersibility in aqueous media, high chemical stability, and surface-functionalization. These characteristics could be major bottlenecks and hence limit their practical applications. This work describes a one-pot, microwave-assisted hydrothermal method to synthesize highly uniform Zn2GeO4:Mn2+ nanoparticles (NPs) using polyacrylic acid (PAA) as an additive. A thorough characterization of the NPs showed that the PAA molecules were essential to realizing uniform NPs as they were responsible for the ordered aggregation of their building blocks. In addition, PAA remained attached to the NPs surface, which conferred high colloidal stability to the NPs through electrostatic and steric interactions, and provided carboxylate groups that can act as anchor sites for the eventual conjugation of biomolecules to the surface. In addition, it was demonstrated that the as-synthesized NPs were chemically stable for, at least, 1 week in phosphate buffer saline (pH range = 6.0-7.4). The luminescence properties of Zn2GeO4 NPs doped with different contents of Mn2+ (0.25-3.00 mol %) were evaluated to find the optimum doping level for the highest photoluminescence (2.50% Mn) and the longest persistent luminescence (0.50% Mn). The NPs with the best persistent luminescence properties were photostable for at least 1 week. Finally, taking advantage of such properties and the presence of surface carboxylate groups, the Zn2GeO4:0.50%Mn2+ sample was successfully used to develop a persistent luminescence-based sandwich immunoassay for the autofluorescence-free detection of interleukin-6 in undiluted human serum and undiluted human plasma samples. This study demonstrates that our persistent Mn-doped Zn2GeO4 nanophosphors are ideal candidates for biosensing applications.

Controlling persistent luminescence in nanocrystalline phosphors.

Persistent luminescent phosphors can store light energy in advance and release it with a long-lasting afterglow emission. With their ability to eliminate in situ excitation and store energy for long periods of time, they are promising for broad applications, including background-free bioimaging, high-resolution radiography, conformal electronics imaging and multilevel encryption. This Review provides an overview of various strategies for trap manipulation in persistent luminescent nanomaterials. We highlight key examples in the design and preparation of nanomaterials with tunable persistent luminescence, particularly in the near-infrared range. In subsequent sections, we cover the most current developments and trends concerning the use of these nanomaterials in biological applications. Moreover, we assess their advantages and disadvantages compared with conventional luminescent materials for biological applications. We also discuss future research directions and challenges, such as insufficient brightness at the single-particle level, and possible solutions to these challenges.

Shortwave Ultraviolet Persistent Luminescence of Sr2MgSi2O7: Pr3+

Currently, extensive research activities are devoted to developing persistent phosphors which extend beyond the visible range. In some emerging applications, long-lasting emission of high-energy photons is required; however, suitable materials for the shortwave ultraviolet (UV-C) band are extremely limited. This study reports a novel Sr2MgSi2O7 phosphor doped with Pr3+ ions, which exhibits UV-C persistent luminescence with maximum intensity at 243 nm. The solubility of Pr3+ in the matrix is analysed by X-ray diffraction (XRD) and optimal activator concentration is determined. Optical and structural properties are characterised by photoluminescence (PL), thermally stimulated luminescence (TSL) and electron paramagnetic resonance (EPR) spectroscopy techniques. The obtained results expand the class of UV-C persistent phosphors and provide novel insights into the mechanisms of persistent luminescence.

An attempt to enhance the afterglow luminescence of NIR light emitting long persistent phosphor Zn3Ga2Ge2O10:Cr3+ by Pr3+ co-doping

In this study we have focused on the improvement of luminescence intensity and the long afterglow luminescence of Cr3+ doped Zn3Ga2Ge2O10 long persistent phosphor by adding an appropriate amount of Pr3+ ions in proportion with Ga concentration. We have successfully developed a series of long persistent Zn3Ga(2-x-y)CrxPryGe2O10 (where x = 0, 0.04 and y = 0, 0.01, 0.02, 0.03, 0.04) phosphors by high temperature solid state reaction method. The structural properties of the developed phosphors were characterized by X-ray diffraction method. The diffuse reflectance spectra were recorded at room temperature and it shows three strong absorbance bands at the wavelength of 256, 412 and 570 nm due to the transitions of 4A2g→4T1g (4P), 4A2g→4T1g (4F) and 4A2g→4T2g (4F) of Cr+3 ions which are also confirmed from the photoluminescence (PL) excitation spectra. PL emission analysis revealed that developed phosphors have a strong narrow luminescence around 698 nm and significantly enhanced with the Pr3+ content upto its optimum value (0.02). Afterglow NIR light emission from the developed phosphors was captured by night vision monocular and it is found that the phosphors have long persistent luminescence for more than 24 h. Thermoluminescence (TL) measurement was also carried out for better understanding the kinetic mechanism and luminescence process in these materials. This will enable to calculate the trapping parameters associated with prominent glow peaks.