Long-persistent Phosphor Research Articles

Novel non-thermoluminescent CaSO4:Dy dosimeters

The non-thermoluminescence afterglow-based dosimetry performance of self-agglomerating pellet-shaped CaSO4:Dy phosphors synthesized through a low-cost, environmentally friendly method is first reported. Thermoluminescence (TL) and afterglow (AG) were analyzed in samples exposed to beta particle irradiation in the dose range from 0.06 to 8.0 Gy. Characteristic TL glow curves consist of an intense TL maximum at 134 °C, a shoulder at 200 °C, and a maximum at 447 °C. CaSO4:Dy exhibits a highly sensitive AG response for 24 hours with linear behavior from 0.06 Gy to 8.0 Gy. A remarkable reproducibility of both the TL and the AG response was observed in repeated irradiation–readout cycles with no need for pre-irradiation annealing. The synthesized CaSO4:Dy exhibits promising properties to be used as an AG-based dosimeter for real-time radiation detection and dosimetry. Moreover, this phosphor might be applied as a long persistent phosphor (LPP), being a cost-effective alternative to other available LPPs.

Broadband near-infrared (NIR) emitting Cr3+ -doped La3 Ga5 SnO14 phosphor with long persistent and photostimulated persistent luminescence.

Recently, long persistent phosphors (LPPs) have attracted significant attention as promising candidates for biomedical applications. However, the serious decrease in luminescence intensity in tissue still remains a major challenge. Therefore, exploring more competitive LPPs and achieving reproducible tissue imaging is crucial. In this study, a new series of near-infrared (NIR) phosphors La3 Ga5 Sn1-x O14 :xCr3+ (x = 0.005-0.05) were synthesized using a high-temperature solid-state method. The as-synthesized samples were characterized using X-ray diffraction, diffuse/photoluminescence spectroscopy, fluorescence decay curves, and thermoluminescence spectroscopy. Upon excitation with ultraviolet light, strong emission bands were observed in the range 600-1200 nm with an optimal doping concentration of x = 0.02mol. Moreover, La3 Ga5 SnO14 :Cr3+ exhibits persistent luminescence due to the presence of suitable energy traps, which prompted the phosphor to emit NIR light even after the removal of the excitation source.

Advances in the preparation of uncopyable photoluminescent security inks from long-persistent phosphors for secure documents

Advances in the preparation of uncopyable photoluminescent security inks from long-persistent phosphors for secure documents

Effect of macropores in titanium dioxide layer on the enhancement of photovoltaic conversion efficiency of long-persistence-phosphor enhanced dye-sensitized solar cells

Mesoporous TiO2 (m-TiO2) have been often employed to improve the photovoltaic conversion efficiencies (PCEs) of the dye-sensitized solar cells (DSSCs). While when incorporated with a long-persistence-phosphor (LPP) layer, only a limited PCE enhancement was obtained for the resultant DSSCs. This is mainly due to that little incident sunlight can transport through the solid m-TiO2 photoanode films and then enter and excite the LPP layer. Here in this work, tunable amount of macropores were introduced into hierarchically porous TiO2 (h-TiO2) using polyvinylpyrrolidone (PVP) as pore-forming agent. As a result, the optimized DSSCs with h-TiO2/LPP photoanodes show the highest PCE of 8.05%, which is improved by 29.00% and 14.51%, compared to those with m-TiO2 and m-TiO2/LPP ones, respectively. Series of analysis indicates that the macropores benefit the transmission of incident sunlight through the h-TiO2 layer and reduce the light reflection at the interface of h-TiO2/LPP. Furthermore, the carbon-doping in TiO2 caused by PVP shifts up the Fermi level of TiO2 and the interface of h-TiO2/LPP enhances the separation of photo-generated carriers, both of which increase the open circuit voltage of the DSSCs. In addition, the DSSCs with h-TiO2-8/LPP photoanodes can well work in dark with a real PCE of 57.37%.

Spiderweb-inspired all-weather CoS quantum dots confined in N-doped carbon for boosted sulfate radical evolution.

Inspired by the working principle of natural spiderweb and long-persistence phosphors, we have synthesized a spiderweb-like nanocomposite in which CoS quantum dots are confined in N-doped carbon frameworks/carbon nanotubes (CNTs). The intimate combination of three-dimensional conductive networks of CoS/CNTs with abundant active sites allows effective capture of sulfate radicals via both physical confinement and chemical bonding and accelerates the redox kinetics significantly. Furthermore, in virtue of the light storing and luminescence behaviors of long-persistence phosphors, the all-weather CoS/CNTs produced can realize an optimum degradation efficiency of 64% under dark conditions. Overall, this work reveals a significant step forward for building a desirable all-weather catalyst with abundant active sites for potential use in degradation under dark conditions.

Design, Preparation, and Mechanical Property Study of Polylactic Acid/Ca2BO3Cl: Eu2+, Dy3+ Composite Material for 3D Printing.

To break through the simplification of three-dimensional printing (3DP) materials and realize the application of long-persistent phosphors in more fields, polylactic acid doped with Ca2BO3Cl: Eu2+, Dy3+ was prepared in this study. The structure of the mixtures was analyzed and determined by infrared spectroscopy. The luminescence properties of phosphors and the composites were studied by fluorescence spectra and afterglow decay curve measurements. The yellow light of the mixtures could be attributed to the 5d-4f energy level transition of Eu2+. After the excitation of 254 nm ultraviolet lamp, the luminance and duration of the composites could be clearly observed. The mechanical properties of the composite filaments were tested, including maximal force and elasticity modulus. In particular, the influence of humidity on mechanical properties was analyzed in detail. The prepared composite filaments were printed into hollow dodecahedrons and presented in this study. Therefore, the composites had great properties and might be expected to put into practical applications of 3DP technology.

High stability ultra-narrow band self-activated KGaSiO4 long-persistent phosphors for optical anti-counterfeiting.

Optical anti-counterfeiting has been developed as a promising optical-sensing technique. A self-activated KGaSiO4 phosphor was successfully prepared using the traditional solid-state method. The photoluminescence spectra of the as-synthesized phosphors indicate that the ultra-narrow band emission with green light peak at 503 nm is obtained when phosphors are excited by 254 nm UV light. Additionally, the measured afterglow curve shows that the emission of this phosphor can last more than 1200 s after UV excitation stops, which indicates that KGaSiO4 is a potential candidate for anti-counterfeiting materials. The luminescent and decay mechanism are discussed by theoretical calculation and thermo-luminescent spectra in detail. The theoretical model can provide support for explaining the mechanism of narrow band or persistent phosphor.

Enhancement of photoluminescence in [formula omitted] defects activated Zn2-δSiO4-δ persistent phosphor by zinc deficiency

A green long persistent phosphor (LPP) Zn2-δSiO4-δ with VZn" defects as emission centers was synthesized using a conventional solid-state reaction. The samples were analyzed by powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), thermoluminescence (TL) techniques and electron paramagnetic resonance (EPR). Rietveld refinement of the XRD patterns confirmed the willemite structure of Zn2SiO4 with minor impurity of SiO2. There is one main emission peak of the green phosphor at 521 ​nm attributed to VZn"defects as emission centers. TL curves and the spectra of EPR showed that the defect concentration of phosphors was enhanced by sintering in the reducing atmosphere and Zn deficiency to improve the properties of the phosphor. The long-lasting luminescence of Zn2-δSiO4-δ was observed for more than 4 ​h by naked eyes in darkness. The mechanism of photoluminescence of zinc deficiency persistent phosphor was explored.

Photoluminescence and afterglow tuning in ultra-broadband near-infrared Mg3Y2Ge3O12:Cr3+ phosphor via cation substitution

Broadband near-infrared (NIR) long-persistent phosphors with adjustable optical property have been urgently demanded for practical applications in bio-imaging and plant cultivation. Herein, Cr3+ doped Mg3Y2Ge3O12 garnet phosphors with ultra-broadband NIR emissions (600−1200 nm) peaked at 776 nm were prepared via a high-temperature solid-state reaction method, and the cation sites for Cr3+ were well identified and assigned based on the Rietveld refinement and time-resolved emission spectra (TRES). Under the excitation at 436 nm, wide-range modulations in intensity, wavelength (763−834 nm) and bandwidth (143−227 nm) of 4T2→4A2 (4F) transition were achieved via Zn2+/Ca2+ substitution, which completely covers the maximum absorption ranges (715−1050 nm) of bacteriochlorophyll from anaerobic photosynthetic bacteria (PSB). According to the classic Tanabe-Sugano theory, the crystal field parameters around different Cr3+ centers were calculated to explain the spectral tailoring effect. Meanwhile, the afterglow duration was greatly improved from 2 min to 66 and 240 min after co-doping Zn2+/Ca2+, and the trap distribution and mechanism was elaborately explored via thermos-luminescence (TL) experiments. Above results provide an effective strategy for designing NIR-emitting phosphors with desired optical and afterglow performance, which well meets the light requirements of diverse applications especially in plant growth.

Ultraviolet-A Persistent Luminescence of a Bi3+-Activated LiScGeO4 Material.

Long persistent phosphors (LPPs) with ultraviolet (UV) luminescence have great potential for application in the fields of biomedicine, environmental, and catalysis. However, it is currently limited by the design and development of remarkable UV LPPs with a suitable spectral region and an ultralong afterglow decay time. Herein, we develop a new type of Bi3+-activated LiScGeO4 LPP, which exhibits bright ultraviolet-A (UVA) persistent luminescence (PersL). Because of the existence of numerous stabilized effective traps, the as-synthesized phosphors can undergo an ultralong PersL decay time far longer than 12 h. The PersL properties, effective trap depths, distributions, and types, as well as the possible mechanism for the PersL behavior of LiScGeO4:Bi3+, are comprehensively surveyed utilizing PersL excitation spectra, PersL decay analyses, thermoluminescence experiments, and X-ray photoelectron spectroscopy. This work can cover the shortage of LPPs in the UV region and also can lay the foundation for the development of more excellent UV LPPs toward versatile novel applications.

Enhancement of Long-Persistent Phosphorescence by Solid-State Reaction and Mixing of Spectrally Different Phosphors.

Rare-earth-doped oxide-based phosphors have attracted great interest as light-emitting materials for technical applications and fundamental research because of their high brightness, tunable emission wavelength, and low toxicity, as well as chemical and thermal stability. The recent development of rare-earth-doped nanostructured materials showed improved phosphorescence characteristics, including afterglow and lifetime. However, the development of highly efficient phosphors remains challenging in terms of brightness and long persistence. Herein, novel protocols were developed for improving phosphorescence characteristics based on the energy transfer effect by chemical mixing of spectrally different phosphors. This protocol is based on the simple mixing method of different phosphors, which is totally different from the conventional methods but provides much brighter persistent phosphorescence. Simple chemical mixing methods significantly improved the afterglow intensity and lifetime of green and blue phosphors regardless of mixed time when subjected to a high-temperature solid-state reaction. In particular, chemical mixing after a high-temperature solid-state reaction enhanced the phosphorescence intensity more effectively than did chemical mixing before the reaction. We achieved increased luminescence of the phosphor, which is 10 times greater than that of the control sample, from all of the chemical mixing methods, which resulted in more efficient energy transfer than previously reported studies. Chemical mixing of three spectrally different phosphors was also performed to achieve multistep energy transfer for the first time, exhibiting a much higher afterglow intensity (∼2 times) than that of single-step energy transfer. This study provides a novel and simple method for the production of bright and long-persistent phosphors and thus expands their application range.

Two Are Better Than One: A Design Principle for Ultralong-Persistent Luminescence of Pure Organics.

Because of their innate ability to store and then release energy, long-persistent luminescence (LPL) materials have garnered strong research interest in a wide range of multidisciplinary fields, such as biomedical sciences, theranostics, and photonic devices. Although many inorganic LPL systems with afterglow durations of up to hours and days have been reported, organic systems have had difficulties reaching similar timescales. In this work, a design principle based on the successes of inorganic systems to produce an organic LPL (OLPL) system through the use of a strong organic electron trap is proposed. The resulting system generates detectable afterglow for up to 7 h, significantly longer than any other reported OLPL system. The design strategy demonstrates an easy methodology to develop organic long-persistent phosphors, opening the door to new OLPL materials.

UV–Vis-NIR broadband-photostimulated luminescence of LiTaO3:Bi3+ long-persistent phosphor and the optical storage properties

Abstract Trap depth is an important factor in the design of long-persistent (LPL) or photostimulated luminescence (PSL) materials, lithium tantalate is a promising matrix with abundant defect traps owing to the volatile lithium. The new long-persistent luminescence material LiTaO3:Bi3+ exhibits intense indigo blue photoluminescence (PL) and long-persistent luminescence with the maximum peak located at 430 nm, which is ascribed to the 3P1 → 1S0 transition of Bi3+, while the decay rate of afterglow increases when the concentration of Bi3+ dopant improves. Under near-infrared (NIR) stimulation, the synthetic phosphor shows bright indigo blue photostimulated luminescence, but the UV–Vis stimulation light can quench afterglow rapidly due to its efficient photostimulation effect related to the distribution and depth of defect traps in matrix crystal. By means of photostimulated luminescence spectra, thermoluminescence (TL) curves and X-ray photoelectron spectroscopy (XPS), the impact of antisite-tantalum defect traps on luminescence property is characterized. The findings give insight into the mechanism of photostimulated luminescence and expand the application field of long-persistent luminescence materials in optical information storage.

Long-persistent phosphorescence in Eu2+-doped calcium borate chloride for optical data storage

Yellow-emitting optical data storage phenomena were observed in a series of Ca2BO3Cl: Eu2+, Ln3+ (Ln = La, Ce, Pr, Sm, Gd, Tb, Ho, Tm, Yb, Lu) long-persistent phosphors. Eu2+ in Ca2BO3Cl could emit yellow light located at 580 nm ascribed to the 5d-4f transitions when optical data was being read out. As co-activators, Ln3+ had various effects in causing afterglow and changing trap concentration. Optical data storage phenomena occurred for each combination of Eu2+ and Ln3+ in calcium borate chloride host. Thereinto, prepared Ca2BO3Cl: Eu2+, Ce3+ phosphor presented a decent afterglow and optical storage property. Its afterglow decay time was more than 1 h at room temperature and had a bright yellow optical signal at 473 K Ca2BO3Cl: Eu2+, Ho3+ showed an optimum optical storage property, which remained bright yellow light even after 7 days of data storage. This outstanding optical data storage performance was mainly derived from Ho3+ increased the trap concentration of oxygen vacancies. The crystal structure, excitation and emission spectra, afterglow decay curves and thermoluminescence curves were measured and analyzed in all Eu2+ and Ln3+ doped Ca2BO3Cl samples. A simple optical data storage method was set up and effective optical signals were observed. Based on these results, detailed optical data storage processes and possible mechanism were studied and discussed.

Enhanced performance of CdS/CdSe quantum dot-sensitized solar cells by long-persistence phosphors structural layer

Light absorption plays an important role in improving the power conversion efficiency (PCE) of quantum dot-sensitized solar cells (QDSSCs). In this study, a multifunctional long-persistence phosphor (LPP) layer was introduced into the CdS/CdSe QDSSCs via a simple doctor blade method. The LPP layer can simultaneously improve the light harvesting and photo charge transfer in CdS/CdSe QDSSCs. As a result, their short-circuit current and corresponding PCE are effectively enhanced. The PCE can reach up to 5.07%, which is about 24% larger than that of the conventional CdS/CdSe QDSSCs without LPP layer. The solar cells can work in dark for a while due to the long-lasting fluorescence of the LPP layer. This research provides an effective way to improve the PCE of QDSSCs, and finds the possibility for all-weather QDSSCs.

Synthesis, luminescent and structural characteristics of Sr4Al14O25:Eu2+ and Sr4Al14O25:Eu2+, RE3+ (RE = Nd, Dy) long persistent nanophosphors for solid state lighting

Effective green long persistent phosphors (LPP) Sr4Al14O25:Eu2+ and Sr4Al14O25:Eu2+, RE3+ (RE = Nd, Dy) were prepared using solution combustion procedure at 600 °C and reheated for two hours at 1150 and six hours at 1350 °C in tube furnace in the reducing environment (in presence of 95 % N2 and 5 % H2). The effect of codopants (Nd3+ and Dy3+) on the luminescent and structural characteristics of materials were studied using photoluminescence (PL) spectroscopy, powder X-ray diffraction (PXRD), Fourier transformation infrared (FTIR) and transmission electron microscopy (TEM). The influence of concentration of dopant ions on the PL intensity was examined by varying Eu2+ concentration (0.01-0.05 mol) in host matrix. The incorporation of 0.03 mol Eu2+ ion in Sr4Al14O25 lattice produces maximum PL intensity. On 363 nm excitation, strong broad emission band is observed at 480−520 nm corresponding to 4f65d1→4f7 transition of divalent europium ions. The persistency and luminescence intensity is found to improve by doping Nd3+ and Dy3+ ions as codopants and also with increasing the temperature. PXRD study confirms the formation of single phase orthorhombic crystal symmetry with Pmma space group. The crystallinity of the materials increases with temperature. FTIR spectra determine the stretching and bending modes of vibrations to confirm the formation of metal-oxygen bonds. TEM images demonstrate the synthesis of nearly spherical shaped nanoparticles.

Luminescence and color properties of Ho3+ co-activated Sr4Al14O25: Eu2+, Dy3+ phosphors

We report on the photoluminescence properties of Sr4Al14O25:Eu2+,Dy3+ (SAED) phosphors modified by adding Ho3+ in excess and partially substituting Dy3+ with Ho3+ (SAEDHs). Samples were produced using an established solid-state synthesis method known to consistently yield long-persistent afterglow phoshors. Substitutions were performed in stoichiometrically modified host lattices (Al/Sr = 3.2, 3.5, 3.7). Partially substituting Dy3+ with Ho3+ resulted in a remarkable increase in photoluminescence (PL) intensity, Ho3+ co-doping caused a shift in the main emission peaks between 494 and 497 nm and introduced new peaks as well. Important from the security application point of view, the PL intensity was significantly improved under both purple (400 nm) and blue (445 nm) illumination. Color shift of the powders from green to a yellowish and pale pink shade resulted from the Ho3+ addition. Afterglow (AG) properties and thermoluminescence (TL) response decreased non-gradually as Dy3+ was substituted by Ho3+, due to possible charge transfer effects caused by Ho3+. AG and TL properties remained more intact in the presence of Ho3+ when the Al/Sr ratio differed from the ideal 3.5 ratio. Rearrangements of trap energy levels, and energy transfer induced by Ho3+ co-doping were analysed by TL. Dy3+/Ho3+ co-dopants and Al/Sr stoichiometric ratios asymmetrically influenced the trapping/detrapping and energy transfer processes of the SAEDH phosphors. The developed new materials meet the general characteristics of long-persistent phosphors: charging by sunlight, efficient light conversion, chemical stability, all-night afterglow observable by the human eye.

Role of long persistence phosphors on their enhancement in performances of photoelectric devices: In case of dye-sensitized solar cells

To enhancing enhanced sunlight harvesting, long persistence phosphor (LPP) materials were often incorporated into photoanodes of photoelectric devices. However, the role of LPP layer on the photoelectric enhancements is not yet clear. Here, the authors have systematically studied the effect of green LPP (SrAl2O4: Eu3+) and its thickness on photoelectric behaviours of dye-sensitized solar cells (DSSCs). Results showed that the P25/LPP DSSCs generates power conversion efficiency (PCE) of 7.16% at the optimal LPP thickness, which is 24.3% higher than that of the P25 ones. Series of analysis indicate that the enhancements in short-circuit current density and PCE could mainly be due to the LPP’s function of back scattering (containing down-conversion effect and enhancement in light absorption) of incident sunlight. Moreover, the enhanced carriers’ lifetime and open-circuit voltage are mainly due to the LPP layer’s afterglow effect. In addition, the P25/LPP DSSCs can still generate an output power density (82.15 μW cm−2) with a high PCE value of 46.94% in dark.

Biodegradable long-persistent luminescent films based on PHB/PHBV as matrix and sunlight conversion applications

Fully biodegradable and environmentally-friendly luminescent films were fabricated by doping the long-persistent luminescence inorganic phosphors containing europium ions into poly(3-hydroxybutyrate) (PHB) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) matrix. The resulting films were characterized by Fourier transform infrared spectroscopy (FT-IR), photoluminescence (PL), and thermogravimetric analysis (TGA). The FT-IR results were confirmed that the inorganic phosphors were uniformly dispersed in the PHB and PHBV matrix. The emission spectra of the yellow PHB film with 15% phosphor and the blue PHBV film with 20% phosphor exhibited the highest emission intensities. Compared with the yellow and red fluorescent films, the afterglow decay curves indicated that the blue films could keep the best luminescent performance for a long time. On the basis of TGA cures, all the luminescent films exhibited good thermal stability. As far as we known, this was the first example that doping long-persistent luminescent phosphors into biodegradable polymer matrix to fabrication luminescent films for sunlight conversion.

Polyoxometalate modified all-weather solar cells for energy harvesting

With the reduction of available non-renewable energy sources, the full use of renewable energy is particularly important, especially solar energy. The traditional dye-sensitized solar cells can only work during daytime, but never at night. In this work, H3 [α-PW12O40]·21H2O (PW12) is first introduced to the all-weather solar cell as the electronic transmission medium for decreasing the electron recombination caused by the weak fluorescence of the long persistence phosphors (LPPs) at night. Meanwhile, we fabricated an all-weather solar cell for the first time by adding a smaller amount of long persistence phosphors (LPPs) to the photo-anode, which can absorb the solar energy at daytime then transfer the energy to fluorescence at night, so the dye (N719) can absorb the fluorescence produced by LPP to continually generate electricity for achieving the aim of all-weather power generation. In general, our all-weather DSSC yields the maximum photoelectric conversion efficiency (PCE) 6.9% at daytime. The values of Jsc, Voc and Pmax are 0.28 mA cm−2, 500 mV and 79.8 μW cm−2 at night.