Luminescence characterization of (Ca1−xZnx)Ga2S4:Eu phosphors for a white light-emitting diode

We investigated the luminescence properties of (Ca_1−xZn_x)Ga₂S₄:Eu²⁺ phosphor as a function of Zn²⁺ and Eu²⁺ concentrations. The luminescence intensity was markedly enhanced by increasing the mole fraction of Zn²⁺ at Ca²⁺ sites. Lacking any Zn²⁺ ions, CaGa₂S₄:0.01Eu²⁺ converted only 18.1% of the absorbed blue light into luminescence. As the Zn²⁺ concentration increased, the quantum yield increased and reached a maximum of 24.4% at x = 0.1. Furthermore, to fabricate the device, the optimized green-yellow (Ca_0.9Zn_0.1)Ga₂S₄:Eu²⁺ phosphor was coated with MgO. White light was generated by combining the MgO-coated phosphor and the blue emission from a GaN chip.     

The influence of auxiliary codopants on persistent phosphor Sr2P2O7:Eu2+,R3+ (R = Y,La, Ce,Gd, Tb and Lu)

We investigate the persistent luminescence in europium-doped strontium pyrophosphate upon codoping with auxiliary rare earth ions. The persistent phosphors are synthesized via solid-state reaction method under flowing N₂ + H₂. Under UV irradiation, broadband emission persistent luminescence located at 420 nm is observed in all of these phosphors at room temperature. The effects of auxiliary rare earth ions on Sr₂P₂O₇:Eu²⁺ are discussed according to the decay curves and thermoluminescence spectra. Sr₂P₂O₇:Eu²⁺,Lu³⁺ shows the best performance, while and La³⁺ and Ce³⁺ codoped samples are the weakest. The influence of auxiliary codopants is discussed in terms of ionic potential and ionic radius. We derive an empirical formula based on the experimental results.     

Persistent luminescence of Eu and Dy doped barium aluminate (BaAl2O4:Eu,Dy) materials

Polycrystalline Eu²⁺ and Dy³⁺ doped barium aluminate materials, BaAl₂O₄:Eu²⁺,Dy³⁺, were prepared with solid state reactions at temperatures between 700 and 1500 °C. The influence of the thermal treatments on the stability, homogeneity and structure as well as to the UV-excited and persistent luminescence of the materials was investigated by X-ray powder diffraction, SEM imaging and infrared spectroscopies as well as by steady state luminescence spectroscopy and persistent luminescence decay curves, respectively. The IR spectra of the materials prepared at 250, 700, and 1500 °C follow the formation of BaAl₂O₄ composition whereas the X-ray powder diffraction of compounds revealed how the hexagonal structure was obtained. The morphology of the materials at high temperatures indicated important aggregation due to sintering. The luminescence decay of the quite narrow Eu²⁺ band at ca. 500 nm shows the presence of persistent luminescence after UV irradiation. The dopant (Eu²⁺) and co-dopant (Dy³⁺) concentrations affect the crystallinity and luminescence properties of the materials.     

X-ray absorption study of rare earth ions in Sr2MgSi2O7:Eu,R persistent luminescence materials

The valence of the europium dopant and selected rare earth co-dopants (Ce³⁺, Dy³⁺, and Yb³⁺) in the Sr₂MgSi₂O₇:Eu²⁺,R³⁺ persistent luminescence materials were studied by room temperature XANES measurements. The results indicated the co-existence of both divalent and trivalent europium in all the studied materials. The relative amount of Eu³⁺ was observed to increase upon increasing exposure to X-rays, as expected by the persistent luminescence mechanism. This suggests a simultaneous filling of oxygen vacancies initially created by the reducing preparation conditions. For the Dy and Yb co-dopants, only trivalent species were observed. On the other hand, traces of tetravalent cerium were present in the Eu,Ce co-doped materials.     

Photoluminescence of Ca(Ba)Ga2S4:RE poly- and nanocrystals

The photoluminescence and photoluminescence excitation spectra and luminescence decay kinetics of CaGa₂S₄:Eu²⁺ bulk crystals and powders ranging in particle size from 100 to 600 nm have been studied in the temperature range 77–300 K. The results indicate that the full width at half maximum of the photoluminescence band of the CaGa₂S₄:Eu²⁺ nanopowders is about twice that of the bulk crystals. Analysis of the photoluminescence spectra shows that the energy position of the emission band is almost independent of the particle size, temperature, and excitation intensity in the ranges 77–300 K and 10^?3 to 10⁶ W/cm², respectively. The shape of the photoluminescence band is well represented by a Gaussian. The excited state lifetime of the Eu²⁺ ion is ～1000 ns as evaluated from the exponential portion of the luminescence decay curve.     

Roles of doping ions in persistent luminescence of SrAl<Subscript>2</Subscript>O<Subscript>4</Subscript>:Eu<Superscript>2+</Superscript>, RE<Superscript>3+</Superscript> phosphors

The polycrystalline Eu²⁺ and RE³⁺ co-doped strontium aluminates SrAl₂O₄:Eu²⁺, RE³⁺ were prepared by solid state reactions. The UV-excited photoluminescence, persistent luminescence and thermo-luminescence of the SrAl₂O₄:Eu²⁺, RE³⁺ phosphors with different composition and doping ions were studied and compared. The results showed that the doped Eu²⁺ ion in SrAl₂O₄:Eu²⁺, Dy³⁺ phosphors works as not only the UV-excited luminescent center but also the persistent luminescent center. The doped Dy³⁺ ion can hardly yield any luminescence under UV-excitation, but can form a electron trap with appropriate depth and greatly enhance the persistent luminescence and thermo-luminescence of SrAl₂O₄:Eu²⁺. Different co-doping RE³⁺ ions showed different effects on persistent luminescence. Only the RE³⁺ ion (e.g. Dy³⁺, Nd³⁺), which has a suitable optical electro-negativity, can form the appropriate electron trap and greatly improve the persistent luminescence of SrAl₂O₄:Eu²⁺. Based on above observations, a persistent luminescence mechanism, electron transfer model, was proposed and illustrated.     

Persistent luminescence of SrAl2O4: Eu,Dy, Cr phosphors in the tissue transparency window

We report on the persistent luminescence of SrAl₂O₄: Eu²⁺, Cr³⁺ phosphor centered at 760 nm. The phosphor was prepared by sol-gel-combustion method. Persistent luminescence from Cr³⁺ lasted for hundreds of seconds, comparable to the long afterglow from Eu²⁺ ions in the visible region based on the continuous energy transfer from Eu²⁺ ions to Cr³⁺ ions. The introduction of Dy³⁺ ions into the phosphor further prolonged the afterglow time of Eu²⁺ and Cr³⁺ ions through the depth control of the charge traps. The optimum doping concentrations for Eu²⁺, Cr³⁺ and Dy³⁺ were 1%, 2% and 1.5%, respectively.     

Synthesis and photoluminescence properties of a new red emitting phosphor: Ca3(VO4)2:Eu; Mn

A new red emitting phosphor, Ca₃(VO₄)₂:Eu³⁺; Mn²⁺, was synthesized by a citric acid sol-gel combustion method and characterized by XRD, TEM and photoluminescence (PL) spectra. The red emission located at about 613 nm was ascribed to ⁵D₀-⁷F₂ transition of Eu³⁺. And the red luminescence intensity changed with annealing temperature and concentration of Eu³⁺. The effect of the co-doped Mn²⁺ was also investigated systematically.     

Nanocomposite BaSO4/Y2O3:Eu powders prepared by heterogeneous nucleation processing

Forming core–shell-structured phosphor particles is an effect way to improve the properties of the rare-earth-doped inorganic luminescent systems, as well as to achieve a reduction in the amount of expensive rare earth metal. Heterogeneous nucleation processing is a commonly used method to prepared core–shell-structured particles. A nanocomposite BaSO₄/Y₂O₃:Eu³⁺ powder was prepared by coating BaSO₄ submicrospheres with nano-Y₂O₃:Eu³⁺ particles via heterogeneous nucleation processing. Thermogravimetric analysis and differential scanning calorimetry (TGA/DSC) were utilized to reveal the mechanism of the homogenous precipitation reaction process. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS) were utilized to characterize the BaSO₄/Y₂O₃:Eu³⁺ core–shell-structured phosphor particles. By controlling the hydrolysis of urea, BaSO₄ particles are well coated with the shell of Y₂O₃:Eu³⁺, and the nucleation of coating materials is predominantly heterogeneous rather than homogeneous. Photoluminescence spectra were utilized as well. The BaSO₄/Y₂O₃:Eu³⁺ particles show a red emission corresponding to ⁵D₀–⁷F₂ of Eu³⁺ under the excitation of ultraviolet.     

UV–VUV spectroscopy of rare earth doped persistent luminescence materials

The electronic and defect energy level structure of polycrystalline SrAl₂O₄:Eu²⁺,R³⁺ persistent luminescence materials were studied with thermoluminescence and UV–VUV synchrotron radiation emission and excitation spectroscopy. Theoretical calculations using the density functional theory (DFT) were carried out simultaneously with the experimental work. The experimental band gap energy (E_g) value of 6.6 eV agrees very well with the DFT value of 6.4 eV. The 4f⁷ → 4f⁶5d¹ excitation bands of Eu²⁺ were found rather similar irrespective of the R³⁺ co-dopant. The trap level energy distribution depended strongly on the R³⁺ co-dopant except for the shallowest trap energy above the room temperature remaining the same, however. The different processes in the mechanism of persistent luminescence from SrAl₂O₄:Eu²⁺,R³⁺ was constructed and discussed.     

Synthesis and spectral analysis of Yb/Tm/Ho-doped Na0.5Gd0.5WO4 phosphor to achieve white upconversion luminescence

Yb³⁺/Tm³⁺/Ho³⁺-doped Na_0.5Gd_0.5WO₄ phosphors were synthesized by the high-temperature solid-state method. Bright white luminescence upon 980 nm near-infrared excitation can be observed for the sample at the optimum chemical composition of Na_0.5Gd_0.5WO₄:10%Yb³⁺/1%Tm³⁺/0.4%Ho³⁺, which is produced via an upconversion (UC) process by tuning the dopant ions concentration. The measured white light consists of the blue, green, and red UC emissions which correspond to the transitions ¹G₄ → ³H₆ of Tm³⁺, ⁵F₄(⁵S₂) → ⁵I₈, and ⁵F₅ → ⁵I₈ of Ho³⁺ ions, respectively. The calculated color coordinates display that white light can be achieved in a wide range of dopant concentrations. The UC mechanisms were also proposed based on their spectral and pumping power dependence analyses.     

Enhanced luminescence by charge compensation in red-emitting Eu-activated Ca3Sr3(VO4)4

The effects of charge compensation on the luminescence behavior of a red-emitting phosphor, Ca₃Sr₃(VO₄)₄:Eu³⁺, were investigated. It has been observed that charge compensated by monovalent ions, especially Na⁺, shows greatly enhanced red emission under ultraviolet excitation. It is found that Na₂CO₃ addition acts as a fluxing agent and plays a role in charge compensation, which clearly improves the emission intensity of Eu³⁺-activated Ca₃Sr₃(VO₄)₄. Enhanced emission intensity of the corresponding charge compensated phosphors under ultraviolet radiation may find application in the production of red phosphors for white light-emitting diodes.     

Synthesis of CaAl2O4:Eu,Nd long persistent phosphor by combustion processes and its optical properties

Eu²⁺,Nd³⁺ co-doped calcium aluminate with high brightness and long persistent luminescence was prepared by the combustion method. The luminescent properties of CaAl₂O₄-based luminescent materials have been studied systematically. The phosphor powders were further investigated by X-ray diffractometer (XRD), photoluminescence excitation and emission spectra (PL) and brightness meter. The analytical results indicated that the phase of CaAl₂O₄ was formed when the initiating combustion temperature was 400 °C. The broad band UV excited luminescence of the CaAl₂O₄:Eu²⁺,Nd³⁺ was observed at the blue region (λ_max = 440 nm) due to transitions from the 4f⁶5d¹ to the 4f⁷ configuration of the Eu²⁺ ion. The decay time of the persistence indicated that the persistent luminescence phosphor has bright phosphorescence and maintains a long duration.     

Optical properties of Ho:SrMoO4 single crystal

Ho³⁺:SrMoO₄ single crystal was grown by the Czochralski method in N₂ atmosphere. The polarized absorption spectra, emission spectra and the lifetime decay curves were measured at room temperature. The Judd-Ofelt theory has been applied to analyze the absorption spectra. The spectroscopic parameters, including three intensity parameters, radiative transition rates, radiative lifetimes, fluorescent branching ratios and emission cross sections were obtained. The luminescence lifetime of the ⁵S₂ level was determined to be 4.40 μs.     

The photoluminescence properties of Y2O3:Eu prepared by surfactant assisted co-precipitation-molten salt synthesis

Y₂O₃:Eu³⁺ red phosphors were prepared by surfactant assisted co-precipitation-molten salt synthesis method. The effects of surfactant content and annealing temperature on the structure and luminescence were investigated by X-ray diffraction and fluorescence spectrophotometer. The use of surfactant reduces the impurities on the surface of particles and promotes the reaction. The color purity of as-prepared Y₂O₃:Eu³⁺ red phosphors is improved with the presence of surfactant. In the excitation spectra, two strong bands at 394 and 466 nm are attributed to ⁷F_0,1-⁵L₆, ⁷F_0,1-⁵D₂ transitions of Eu³⁺ ions respectively. With the excitation of 394 or 466 nm, the as-fabricated samples reveal excellent red emission as high as that of samples monitored by 254 nm. Thus, the Y₂O₃:Eu³⁺ is a promising red phosphor for ultraviolet-visible light-emitting diodes.     

Energy transfer from Eu2+ to trivalent rare earth ions in BaY2F8

An efficient energy transfer from divalent europium to trivalent rare earth ions appears in the luminescence spectrum of BaY₂F₈ when it is simultaneously doped with Eu²⁺ and Ln³⁺ ions (Ln = Tb, Ho, Er) as a consequence of a f-f emission of Eu²⁺.     

Preparation of KMgF3 and Eu-doped KMgF3 nanocrystals in water-in-oil microemulsions

In this study, KMgF₃:Eu²⁺ luminescent nanocrystals (NCs) were prepared in water/cetyltrimethylammonium bromide (CTAB)/2-octanol microemulsions. The KMgF₃:Eu²⁺ NCs were characterized by transmission electron microscopy (TEM), X-ray diffractometer (XRD), fluorescence spectrum, infrared spectroscopy (IR) and elementary analysis. The results showed that the size of the KMgF₃:Eu²⁺ NCs was hardly affected by water content and surfactant (CTAB) concentration. The emission spectrum showed that the position of the 362 nm peak is due to the K⁺ sites substituted Eu²⁺. Two emission peaks located at 589 and 612 nm can be attributed to Eu³⁺, which exist at two different types of Eu³⁺ centers: one is Eu³⁺ at a K⁺ site, the other is clustering of Eu³⁺ ions in the interstices of KMgF₃ host lattice.     

Green emission from Eu/Dy codoped SrO-Al2O3-B2O3 glass-ceramic by ultraviolet light and femtosecond laser irradiation

A spectroscopic investigation of Eu²⁺/Dy³⁺ codoped SrO-Al₂O₃-B₂O₃ glass-ceramic is presented. The sample exhibits green emission excited by ultraviolet (UV) light and near-IR femtosecond (fs) laser. The emission profile obtained by near-IR fs laser irradiation is similar to that by UV excitation, indicating that both of the emissions come from 5d → 4f transition of the Eu²⁺ ions. The relationship between the upconversion luminescence (UCL) intensity and pump power reveals a two-photon process in the conversion of near-IR radiation to the green emission. The possible mechanism of UCL from such glass-ceramic is proposed.     

Proprietes de luminescence des thiosilicates alcalins et alcalino-terreux

The emission characteristics of phosphors based on alkaline and alkaline earth thiosilicates as hosts lattice are described. The influence of cationic and anionic species and also the nature of the activator Eu²⁺ or Ce³⁺ have been discussed. Each phosphor emits in the visible region. Three of the more interesting phosphors are BaSi₂S₅: Eu²⁺, SrSi₂S₅: Eu²⁺ and Sr₂SiS₄: Eu²⁺ which emit in the green, the blue-green and yellow region of the spectrum.     

UV–VUV spectroscopy of rare earth doped persistent luminescence materials

The electronic and defect energy level structure of polycrystalline SrAl₂O₄:Eu²⁺,R³⁺ persistent luminescence materials were studied with thermoluminescence and UV–VUV synchrotron radiation emission and excitation spectroscopy. Theoretical calculations using the density functional theory (DFT) were carried out simultaneously with the experimental work. The experimental band gap energy (E_g) value of 6.6 eV agrees very well with the DFT value of 6.4 eV. The 4f⁷ → 4f⁶5d¹ excitation bands of Eu²⁺ were found rather similar irrespective of the R³⁺ co-dopant. The trap level energy distribution depended strongly on the R³⁺ co-dopant except for the shallowest trap energy above the room temperature remaining the same, however. The different processes in the mechanism of persistent luminescence from SrAl₂O₄:Eu²⁺,R³⁺ was constructed and discussed.