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.     

Photoluminescence properties of Sr3(PO4)2: Eu2+, Dy3+ double-emitting blue phosphor for white LEDs

Double-emitting blue phosphor Sr₃(PO₄)₂: Eu²⁺, Dy³⁺ was synthesized by solid state reaction under H₂ atmosphere. XRD exhibited the pure hexagonal phase of the prepared phosphor. The photoluminescence results showed that all samples had intense broad absorption band between 250 and 450 nm, which matched well with the near-UV (350–420 nm) emission band of InGaN-based chips. The emission spectrum of Sr₃(PO₄)₂: Eu²⁺, Dy³⁺ consisted of two broad bands, peaked at 485 nm and 410 nm, which originated from two luminescent centers, related to 4f⁶5d¹ → 4f⁷ transition of Eu²⁺ in six-coordinated Sr(I) and ten-coordinated Sr(II) sites respectively. The intensity ratio of two emission bands could be easily tuned by adjusting Dy³⁺ co-doping content, which resulted in color-tunable luminescence in bluish green region to purplish blue region.     

Synthesis and efficient blue-emitting of Eu2+-activated borate fluoride BaAlBO3F2

Eu²⁺-doped borate fluoride BaAlBO₃F₂ was synthesized by the conventional high temperature solid state reaction. The crystal phase formations were confirmed by X-ray powder diffraction (XRD) measurements and the structure refinement. The photoluminescence (PL) excitation and emission spectra, and the decay curves were investigated. Eu²⁺-doped BaAlBO₃F₂ phosphor can be efficiently excited by near-UV light and presents narrow blue luminescence band centered at 450 nm. The maximum absolute quantum efficiency (QE) of BaAlBO₃F₂:0.05Eu²⁺ phosphor was measured to be 76% excited at 398 nm light at 300 K. The thermal stability of the blue luminescence was evaluated by the luminescence decays as a function of temperature. The phosphor shows an excellent thermal stability with high thermal activation-energy on temperature quenching effects because of the rigid crystal lattices.     

Preparation and luminescence properties of yellow long-lasting phosphor Ca2ZnSi2O7:Eu2+, Dy3+

Long-lasting phosphors Ca₂ZnSi₂O₇:Eu²⁺, Dy³⁺ are prepared by solid-state reaction method assisted with different fluxes. Broadband emission, peaked at 580 nm and originating from 4f to 5d transition of Eu²⁺, is observed. The emission intensities of the phosphors can be enhanced by 4.84 and 7.73 times with the introduction of H₃BO₃ and CaF₂, respectively. Moreover, their afterglow times are also respectively prolonged to 11 h and 12 h. The yellow afterglow can be excited by both ultraviolet and visible light, thus permitting its application in both room and outdoor environment. In terms of the crystal structure and trap feature of the phosphors added with different fluxes, the mechanism for the improved luminescence and afterglow properties is discussed.     

Single-component and white light-emitting phosphor BaAl2Si2O8: Dy3+, Eu3+ synthesis,luminescence, energy transfer,and tunable color

A series of Dy³⁺ - Eu³⁺ co-doped BaAl₂Si₂O₈ phosphors were prepared via the conventional solid-state reaction method. Their crystal structure, luminescent characteristic and lifetime were investigated. The optimum doping concentrations of Dy³⁺and Eu³⁺ are both 0.05 for Dy³⁺ or Eu³⁺ singly doped BaAl₂Si₂O₈. Furthermore, BaAl₂Si₂O₈: 0.05Dy³⁺ and BaAl₂Si₂O₈: 0.05Eu³⁺ emits yellow and red light. The emission color of BaAl₂Si₂O₈: Dy³⁺, Eu³⁺ could be tuned from yellow to white due to the energy transfer. This energy transfer from Dy³⁺ to Eu³⁺ was confirmed and investigated by photoluminescence spectra and the decay time of energy donor Dy³⁺ ions. With constantly increasing Eu³⁺ concentration, the energy transfer efficiency from Dy³⁺ to Eu³⁺ in BaAl₂Si₂O₈ host increased gradually and reached as high as 81%, the quantum yield was about 47.43%. BaAl₂Si₂O₈: Dy³⁺, Eu³⁺ phosphors can be effectively excited by UV (about 348 nm) light and emit visible light from yellow to white by altering the concentration ratio of Dy³⁺ and Eu³⁺, indicating that the phosphors have potential applications as a white light-emitting phosphor for display and lighting.     

VUV–UV luminescence of Ce3+, Tb3+, Eu3+, and Dy3+ doped GdOCl

The vacuum ultraviolet spectroscopic properties of GdOCl:Re³⁺ (Re³⁺ = Ce³⁺, Tb³⁺, Eu³⁺, and Dy³⁺) are investigated in detail for the first time. The host absorption band is determined to be around 179 nm, and the f–d transition bands as well as the charge transfer bands are assigned. Upon 179 nm excitation, Re³⁺ (Re³⁺ = Ce³⁺, Tb³⁺, Eu³⁺, Dy³⁺) ions shown their characteristic emissions. Energy transfers from Gd³⁺ to Re³⁺ ion were observed. A broad band ranging from 350 to 400 nm corresponding to the d–f transition of Ce³⁺ is observed. Eu³⁺ has typical red emission with the strongest peak at 620 nm; Tb³⁺ shows characteristic transition of ⁵D_3,4 → ⁷F_j, and its spin-forbidden and spin-allowed f–d transitions in VUV region are calculated with Dorenbos’ equations, these calculated values agree well with the experimental results. Dy³⁺ presents yellow emission (⁴F_9/2 → ⁶H_13/2) with the strongest peak at 573 nm.     

Preliminary spectroscopic properties of K4SrSi3O9 doped with Eu3+

K₄SrSi₃O₉ doped with Eu³⁺ has been for the first time obtained by the conventional solid-state reactions. The X-ray powder diffraction confirmed that prepared materials were single phase. The excitation, emission spectra and decay time of the emitting level have been measured. The influence of Eu³⁺ concentration on luminescence properties has been analyzed. The luminescence intensity of this material at 100 °C is equal to 94% of emission intensity recorded at room temperature, and makes the material a good candidate for commercial phosphors.     

The effect of Eu3+ concentration on the Y2O3 host lattice obtained from citrate precursors

This work presents a thermal decomposition study of the precursor resin prepared from the citrate precursor along with structural features and optical properties materials composed by Y₂O₃ and Eu³⁺ containing Y₂O₃ in 0.5, 3, 5 and 7 mol%. The microcrystallite sizes were estimated from the Scherrer equation. The structural and optical properties revealed that the addition of 5 mol% of Eu³⁺ to the Y₂O₃ matrix gave rise to the highest relative emission intensity which was evidenced by the luminescence intensity. The lifetime of the 0.5 mol% Eu³⁺-doped sample suggested two different symmetry sites for Eu³⁺ ions because two different lifetime values were acquired for this sample, while for phosphors doped with 3 or 5 mol% of Eu³⁺ ions only one similar lifetime was observed. When the concentration of Eu³⁺ is 0.5 and 3 mol%, the luminescence intensity is weak due to the low probability of the O^2? - Eu³⁺ charge transfer transition. On the other hand, when the concentration of the Eu³⁺ ions is 7 mol%, a quenching effect is evidenced.     

Photoluminescence mechanisms of color-tunable Sr2CeO4: Eu3+, Dy3+ phosphors based on experimental and first-principles investigation

We report single-phased color-tunable phosphors (Sr₂CeO₄: Eu³⁺, Dy³⁺) synthesized by a polymer-network gel method for UV–LED. The photoluminescence properties and possible energy transfer mechanisms of Eu³⁺ and Dy³⁺ in Sr₂CeO₄ were investigated by experiments and first principles calculations. The results show that the ⁵D₀ → ⁷F₂ emission of Eu³⁺ is enhanced by the increase in the amount of Eu³⁺ ions and Eu³⁺ substitution makes more stable defect than Dy³⁺ substitution does. The photoluminescence mechanism of Sr_1.994−xEu_xDy_0.006CeO₄ can be explained by the energy transfer model with the consideration of the defect conditions in the crystals.     

Optical spectroscopy,thermal quenching and electron–vibrational interaction of the octahedrally coordinated Eu2+ ion in CaAl2B2O7 and BaAl2B2O7

The spectroscopic properties of the six-fold coordinated Eu²⁺ in CaAl₂B₂O₇ and BaAl₂B₂O₇ are investigated and reported in this paper. Relevant optical parameters such as the Stokes shift, crystal-field splitting, the centroid shift and the red shift of the Eu²⁺ 4f⁶5d¹ electronic configuration are calculated from the experimental data. Further, the Huang–Rhys factor and the effective phonon energy, which are responsible for the electron–vibrational interaction, are theoretically estimated for the Eu²⁺ ion in CaAl₂B₂O₇ and BaAl₂B₂O₇. The low quenching temperature of the luminescence in these materials is tentatively connected with the thermally induced promotion of the Eu²⁺ 4f⁶5d¹ electron to the host lattice conduction band (photoionization). Evidence for the limited solubility of Eu²⁺ in BaAl₂B₂O₇ is presented. A comparative study of the Eu²⁺ luminescence in M²⁺Al₂B₂O₇ (M²⁺ = Ca²⁺, Sr²⁺, Ba²⁺) family of materials is also presented.     

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.     

Synthesis and spectroscopic characterization of the K4BaSi3O9:Eu3+

K₄BaSi₃O₉:Eu³⁺ polycrystals were synthesized by solid state method. X-ray powder diffraction measurements confirmed structure of the samples. The excitation and the emission spectra of orthorhombic K₄BaSi₃O₉ doped with Eu³⁺ were investigated. The excitation spectrum exhibits a broad band with maximum at 220 nm corresponding to the charge transfer (CT) transition between O²⁻ and Eu³⁺ ions and smaller 4f–4f transitions. The emission of investigated phosphor was excited at 395 nm and has quantum efficiency (QE) equal 27%. The emission maximum at 616.5 nm was assigned to the ⁵D₀ → ⁷F₂ transition of Eu³⁺ ions. The luminescence decay profiles as well as the thermal quenching were measured and analyzed. K₄BaSi₃O₉:Eu³⁺has high temperature quenching of the emission T_0.5 = 335 °C.     

Luminescence concentration quenching and site-occupancy of Eu2+ ions in Na2Ca2Si3O9 phosphors derived from 45S5 glass-ceramics

The phosphors of Na₂Ca_2-2xEu_2xSi₃O₉ (x = 0, 0.03, 0.05, 0.07, 0.09) were first synthesized by sol-gel method. The crystal phase formations of the phosphors were detected by X-ray powder diffraction (XRD) measurements and the structure refinement. The photoluminescence spectra, the concentration quenching, the luminescence decay curves and the luminescence color chromaticity were measured, respectively. The excitation spectra indicate that the phosphors can be effectively excited by near UV-LED chips. Two kinds of Eu²⁺ sites centered at 545 nm and 505 nm were discussed by analyzing the spectra, concentration-dependent luminescence intensity and lifetimes. This is a potential tool for monitoring the bioactivity of 45S5 glass-ceramics in situ.     

Photoluminescence of Eu3+-doped LaPO4 nanocrystals synthesized by combustion method

Eu³⁺-doped LaPO₄ nanocrystals were synthesized for the first time by a combustion method with urea as a fuel calcined at 700 °C. The diffraction profile of the obtained sample was indexed as a monoclinic monazite-structure by X-ray diffraction (XRD) data. The obtained nanocrystals appeared to be short rod-like with diameters of 5–10 nm and lengths of 20–70 nm. The luminescence intensities of Eu³⁺-doped LaPO₄ nanocrystals were found to be strongly dependent on the quantities of urea added and the concentration of Eu³⁺.     

Powder synthesis and white light-emitting properties of Eu3+ co-doped K2CaP2O7:Dy3+ phosphors with energy transfer between Dy3+ and Eu3+

A series of white-emitting K₂CaP₂O₇:Dy³⁺ and K₂CaP₂O₇:Dy³⁺, Eu³⁺ phosphors were synthesized via a solid-state method, and Eu³⁺ was co-doped in K₂CaP₂O₇:Dy³⁺ to improve its white light performance. The influences of preparation temperature and Dy³⁺/Eu³⁺ concentration on the crystal structure and photoluminescence characteristics were investigated. XRD results indicate that K₂CaP₂O₇:Dy³⁺ samples prepared above 700 °C matches the standard K₂CaP₂O₇ phase. Under excitation of 349 nm, K₂CaP₂O₇:Dy³⁺ phosphor exhibited characteristic emission peaks at 487 nm (blue) and 579 nm (yellow), and white emission was realized through combining these blue and yellow emissions. After co-doping Eu³⁺ ions, the co-luminescence of Dy³⁺/Eu³⁺ with energy transfer between Dy³⁺and Eu³⁺ were demonstrated. The chromaticity of white light was controlled by changing the ratio of Dy³⁺/Eu³⁺ concentrations, which lead to a warm white light. Therefore, the results indicate that K₂CaP₂O₇:Dy³⁺, Eu³⁺ powders have a potential application in w-LEDs as single-component white-emitting phosphor.     

Photoluminescent properties of Eu3+ doped electrospun CeO2 nanofibers

In this study, CeO₂ nanofibers and that doped with Eu³⁺ were prepared via a facile electrospinning route and annealed at different temperatures ranging from 500 to 900 °C. Their structures were investigated using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Photoluminescence properties of the nanofibers were studied in detail. It was found that the nanofibers with Eu% concentration of 0.67 mol.% and annealed at 700 °C exhibited the highest intensities of the luminescence peaks between 550 and 650 nm.     

Shape controlled synthesis,characterization and photoluminescence properties of YVO4:Dy3+/Eu3+ phosphors

Raisin-like, grape-like and box-like nanocrystalline samples of YVO₄:Dy³⁺(2 at.wt%)/Eu³⁺(4 at.wt%) have been synthesized by different preparative routes. The raisin-like nanocrystalline sample has been synthesised by a facile hydrothermal process, whereas a surfactant-assisted synthesis led to the formation of grape-like and box-like nanocrystalline samples, under different reaction conditions. The resulting products were characterised by XRD, TEM, FTIR and photoluminescence spectroscopy. One significant outcome of the photoluminescence study involved a detailed observation of the variations in the relative intensity of the ⁵D₀–⁷F₂ transition with respect to the ⁵D₀–⁷F₁ transition of Eu³⁺ in YVO₄:Eu³⁺, and the relative intensity of ⁴F_9/2–⁶H_13/2 transition with respect to the ⁴F_9/2–⁶H_15/2 transition of Dy³⁺ inYVO₄:Dy³⁺. The observations in either case were subsequently explained on the basis of change in the local symmetry of the dopant ion, due to the change in the host morphology. The presented results strongly indicate that the local environment around the dopant ion changes with the change in the host morphology, which in turn influences the chromaticity of the prepared phosphors (both YVO₄:Eu³⁺ and YVO₄:Dy³⁺).     

Luminescence behavior of Dy3+ ions in lead borate glasses

Dy-doped lead borate glasses were studied. The luminescence spectra showed two characteristic bands at 480 and 573 nm due to ⁴F_9/2–⁶H_15/2 (blue) and ⁴F_9/2–⁶H_13/2 (yellow) transitions of Dy³⁺. The yellow/blue luminescence of trivalent dysprosium was analyzed as a function of the B₂O₃/PbO ratios, the activator (Dy³⁺) and the PbX₂ (X = F, Cl, Br) content.     

Strong luminescence and efficient energy transfer in Eu3+/Tb3+-codoped ZnO nanocrystals

Single crystalline Eu³⁺/Tb³⁺-codoped ZnO nanocrystals have been synthesized by using a simple co-precipitation method. Successful doping is realized so that strong green and red luminescence can be efficiently excited by ultraviolet and near ultraviolet radiation, demonstrating an efficient energy transfer from ZnO host to rare earth ions. The energy transfer from the ZnO host to Tb³⁺ in ZnO: Tb³⁺ samples and ZnO host to Eu³⁺ in the ZnO: Eu³⁺ samples under UV excitation are investigated. It is found that the red ⁵D₀ → ⁷F₂ emission of Eu³⁺ ions decreases with increasing temperature but the green ⁵D₄ → ⁷F₅ emission of Tb³⁺ ions increases with increasing temperature, implying a different energy transfer processes in the two samples. Moreover, energy transfer from Tb³⁺ ions to Eu³⁺ ions in ZnO nanocrystals is also observed by analyzing luminescence spectra and the decay curves. By adjusting the doping concentration, the Eu³⁺/Tb³⁺-codoped ZnO phosphors emit green and red luminescence with chromaticity coordinates near white light region, high color purity and high intensity, indicating that they are promising light-conversion materials and have potential in field emission display devices and liquid crystal display backlights.     

Formation and spectral probing of transparent oxyfluoride glass-ceramics containing (Eu2+, Eu3+:BaGdF5) nano-crystals

In the present study, we report the formation of transparent glass-ceramics containing BaGdF₅ nanocrystals under optimum ceramization of SiO₂–BaF₂–K₂O–Sb₂O₃–GdF₃–Eu₂O₃ based oxyfluoride glass and the energy transfer mechanisms in Eu²⁺ → Eu³⁺ and Gd³⁺ → Eu³⁺ has been interpreted through luminescence study. The modification of local environment surrounding dopant ion in glass and glass ceramics has been studied using Eu³⁺ ion as spectral probe. The optimum ceramization temperature was determined from the differential scanning calorimetry (DSC) thermogram which revealed that the glass transition temperature (T_g), the crystallization onset temperature (T_x), and crystallization peak temperature (T_p) are 563 °C, 607 °C and 641 °C, respectively. X-ray diffraction pattern of the glass-ceramics sample displayed the presence of cubic BaGdF₅ phase (JCPDS code: 24-0098). Transmission electron microscopy image of the glass-ceramics samples revealed homogeneous distribution of spherical fluoride nanocrystals ranging 5–15 nm in size. The emission transitions from the higher excited sates (⁵D_J, J = 1, 2, and 3) as well as lowered asymmetry ratio of the ⁵D₀ → ⁷F₂ transition (forced electric dipole transition) to that of the ⁵D₀ → ⁷F₁ transition (magnetic dipole) of Eu³⁺ in the glass-ceramics when compared to glass sample demonstrated the incorporation of dopant Eu³⁺ ions into the cubic BaGdF₅ nanocrystals with higher local symmetry with enhanced ionic nature. The presence of absorption bands of Eu²⁺ ions and Gd³⁺ ions present in the glass matrix or fluoride nanocrystals in the excitation spectra of Eu³⁺ by monitoring emission at 614 nm indicated energy transfer from (Eu²⁺ → Eu³⁺) and (Gd³⁺ → Eu³⁺) in both glass and glass-ceramics samples.