Enhanced luminescence properties of YBO3:Eu phosphors by Li-doping

Different concentrations of Li-doped YBO₃:Eu³⁺ phosphors have been prepared by the conventional solid state reaction method and were characterized by X-ray diffraction, field emission scanning electron microscopy, photoluminescence excitation and emission measurements. An intense reddish orange emission is observed under UV excitation and the emitted radiation was dominated by an orange peak at 594 nm resulted from the ⁵D₀ → ⁷F₁ transitions of Eu³⁺ ions. The brightness of the YBO₃:Eu³⁺ phosphor was found greatly improved with Li-doping accompanied by slight improvement in the purity of the color which might be attributed to improvement in crystallinity, grain sizes and creation of oxygen vacancies with Li-doping. The observed results have been discussed in comparison with similar reported works.     

Synthesis and upconversion luminescence properties of Yb/Er codoped BaGd2(MoO4)4 powder

Synthesis and upconversion luminescence properties of the new BaGd₂(MoO₄)₄:Yb³⁺,Er³⁺ phosphor were reported in this paper. The phosphor powder was obtained by the traditional high temperature solid-state method, and its phase structure was characterized by the XRD pattern. Based on the upconversion luminescence properties studies, it is found that, under 980 nm semiconductor laser excitation, BaGd₂(MoO₄)₄:Yb³⁺,Er³⁺ phosphor exhibits intense green upconversion luminescence, which is ascribed to ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transition of Er³⁺. While the observed much weaker red emission is due to the non-radiative relaxation process of ⁴S_3/2 → ⁴F_9/2 and ⁴F_9/2 → ⁴I_15/2 transition originating from the same Er³⁺. The concentration quenching effects for both Yb³⁺ and Er³⁺ were found, and the optimum doping concentrations of 0.5 mol% Yb³⁺ and 0.08 mol% Er³⁺ in the new BaGd₂(MoO₄)₄ Gd³⁺ host were established.     

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.     

Optimum IV-group ion concentrations in ZnGa2O4−xMx (M=S, Se, Te) phosphors for enhancement of luminescence

The ZnGa₂O_4−xM_x (M=S, Se, and Te ) samples with varying S, Se, and Te concentrations are synthesized through solid-state reactions. The X-ray diffraction patterns of ZnGa₂O_4−xM_x (M=S, Se, and Te) show that the positions of the (4 0 0) diffraction peak gradually shift to lower angles due to the doping of VI-group ions (S, Se, and Te) with larger ionic radius than oxygen. For ZnGa₂O_4−xS_x samples, the solubility limit is found to be about x=0.30. The cathodoluminescence measurements on ZnGa₂O_4−xM_x samples show that the optimized S, Se, and Te concentrations with the highest cathodoluminescence intensities are 0.10, 0.05, and 0.03, respectively. The luminous intensity of ZnGa₂O_3.95Se_0.05 is four times higher than that of ZnGa₂O₄. Thus, ZnGa₂O_3.95Se_0.05 can be a promising candidate phosphor for FED applications.     

Synthesis and luminescence properties of novel LiSrPO4:Dy phosphor

Novel LiSrPO₄:Dy³⁺ phosphors for white light-emitting diodes (w-LEDs) were synthesized by the conventional solid-state reaction. X-ray powder diffraction (XRD) analysis confirmed the phase formation of LiSrPO₄:Dy³⁺ materials. Luminescence properties results showed that the phosphor could be efficiently excited by the UV–vis light region from 250 to 460 nm, and it exhibited blue (483 nm) and yellow (574 nm) emission corresponding to ⁴F_9/2 → ⁶H_15/2 transitions and ⁴F_9/2 → ⁶H₁₃/₂ transitions, respectively. The luminescence intensity of LiSrPO₄:xDy³⁺ phosphor firstly increased and then decreased with increasing Dy³⁺ concentration, and reached the maximum at x = 0.03. It was found that concentration quenching occurred as a result of dipole-dipole interaction according to the Dexter's theory. The decay time was also determined for various concentrations of Dy³⁺ in LiSrPO₄.     

Preparation and luminescence properties of Mn-doped ZnGa2O4 nanofibers via electrospinning process

One-dimensional Mn²⁺-doped ZnGa₂O₄ nanofibers were prepared by a simple and cost-effective electrospinning process. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric and differential thermal analysis (TG-DTA), scanning electron microscopy (SEM), energy-dispersive X-ray spectrum (EDS), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), photoluminescence (PL) and cathodoluminescence (CL) spectra as well as kinetic decays were used to characterize the samples. SEM results indicated that the as-formed precursor fibers and those annealed at 700 °C are uniform with length of several tens to hundred micrometers, and the diameters of the fibers decrease greatly after being heated at 700 °C. Under ultraviolet excitation (246 nm) and low-voltage electron beams (1–3 kV) excitation, the ZnGa₂O₄:Mn²⁺ nanofibers presents the blue emission band of the ZnGa₂O₄ host lattice and the strong green emission with a peak at 505 nm corresponding to the ⁴T₁–⁶A₁ transition of Mn²⁺ ion.     

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.     

Novel red phosphors Na2CaSiO4:Eu for light-emitting diodes

Novel red phosphors Na₂CaSiO₄:xEu³⁺ were synthesized using high temperature solid-state reaction and their luminescence characteristics were investigated for the first time. The excitation spectra indicate that the Na₂CaSiO₄:xEu³⁺ phosphors can be effectively excited by ultraviolet (393 nm) light. The emission spectra of Na₂CaSiO₄:xEu³⁺ phosphors invariably exhibit four peaks assigned to the ⁵D₀-⁷F_J (J = 1, 2, 3 and 4) transitions of Eu³⁺ under 393 nm excitation. The Commission Internationale de l’Eclairage (CIE) chromaticity coordinates and quantum efficiency (QE) are (0.66, 0.34) and 58.9%, respectively. The good color saturation and high quantum efficiency indicate that Na₂CaSiO₄:Eu³⁺ phosphors are potential candidate for light-emitting diodes.     

Near-ultraviolet excitable Ca4Y6(SiO4)6O: Ce,Tb white phosphors for light-emitting diodes

The present investigation aims to demonstrate the potentiality of Tb³⁺ and Ce³⁺ co-doped Ca₄Y₆(SiO₄)₆O phosphors. By incorporation of Ce³⁺ into Ca₄Y₆(SiO₄)₆O: Tb³⁺, the excitation band was extended from short-ultraviolet to near-ultraviolet region. The energy transfer from Ce³⁺ to Tb³⁺ in Ca₄Y₆(SiO₄)₆O host was investigated and demonstrated to be a resonant type via a dipole–dipole mechanism with the critical distance of 10.2 Å. When excited by 352 nm, Ca₄Y₆(SiO₄)₆O: Ce³⁺, Tb³⁺ exhibited a brighter and broader violet-blue emission (421 nm) from the Ce³⁺ and an intense green emission (542 nm) from the Tb³⁺. Combining the two emissions whose intensities were adjusted by changing the doping levels of the co-activator, an optimized white light with chromaticity coordinates of (0.278, 0.353) is generated in Ca₄Y₆(SiO₄)₆O: 2% Ce³⁺, 8% Tb³⁺, and this phosphor could be potentially used in near-ultraviolet light-emitting diodes.     

Up-conversion luminescence in Er/Yb-codoped PbTiO3 perovskite obtained via Pechini method

Efficient infrared-to-visible conversion by Er³⁺/Yb³⁺-codoped PbTiO₃ perovskite will be reported. The process is observed under 980 nm laser diode (GaAs:Si) excitation and results in the generation of green (∼555 nm) and red (∼655 nm) emission. The main mechanism that allows for up-conversion is attributed the energy transfer among Yb and Er ions in excited states. The power up-conversion efficiency for red emission is predominant in this material. The results illustrate the large potential of this new class of material for photonic applications involving optoelectronics devices.     

Observation on long afterglow of Tb in CaWO4

The Tb³⁺ doped CaWO₄ phosphors are synthesized via high temperature solid state reaction. The X-ray diffraction shows that small amount of Tb³⁺ does not have a significant influence on the structure of CaWO₄. A broad absorption band of the WO₄²⁻ group is observed from photoluminescence and the energy transfer from WO₄²⁻ group to Tb³⁺ ions induces the f-f transition. The cross-relaxation between two adjacent Tb³⁺ ions weakens ⁵D₃-⁷F_j transitions and enhances the ⁵D₄-⁷F_j transitions, leading to a green long afterglow of the phosphors. The thermoluminescence curves centered around 75 °C reveal the trap depth for afterglow generation is about 0.74-0.77 eV. The optimum Tb³⁺ concentration for afterglow properties is about 1%. A deep hole trap is induced when Tb³⁺ concentration exceeds 1% and it suppresses the thermoluminescence and the decay properties.     

Luminescence properties of Ca0.65La0.35F2.35:Yb, Er with enhanced red emission via upconversion

Yb³⁺/Er³⁺ codoped Ca_0.65La_0.35F_2.35 materials with intense red emission via upconversion were prepared by a high temperature solid-state method. Based on the upconversion luminescence properties investigations, it was found that, under 980 nm excitation, Ca_0.65La_0.35F_2.35:20 mol.%Yb³⁺, xEr³⁺ showed intense red upconversion luminescence, which was ascribed to ⁴F_9/2 → ⁴I_15/2 transition of Er³⁺, although both green and red emissions could be detected. It was also found that the green and red emissions originated the two photon processes, and the ground-state absorption (GSA), excited-state absorption (ESA) and energy transfer (ET) processes between Er³⁺/Yb³⁺ ions and Er³⁺/Er³ ions were involved in the enhanced red emission mechanism.     

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.     

Hydrothermal synthesis and luminescence properties of octahedral LiYbF4: Er microcrystals

LiYbF₄: Er³⁺ octahedral microcrystals have been successfully prepared through a facile hydrothermal method assisted with EDTA (ethylenediaminetetraacetic acid). X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FTIR), thermogravimetric and differential scanning calorimeters (TG-DSC), photoluminescence (PL) spectra are used to characterize the samples. Under 976 nm excitation, the upconversion (UC) luminescence emission spectra of LiYbF₄: Er³⁺ microcrystals show the characteristic Er³⁺ emissions. The results show that the infrared light emissions at 792 nm of ⁴I_9/2 → ⁴I_15/2 are dominantly strong unusually, while the green emissions at 526 and 545 nm assigned to ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2, respectively, and the red emission at 667 nm of ⁴F_9/2→⁴I_15/2 are relatively weaker. Most importantly, the samples show more efficient luminescence with further heat treatment.     

Structural study and luminescence of TlSrLa(AsO4)2

This work presents the crystal structure and luminescent properties of TlSrLa(AsO₄)₂. In this phase Tl⁺ ions are located in large tunnels delimited by chains of alternating (AsO₄) and (Sr,La)O₈ polyhedra. Thallium atoms are eightfold coordinated with C₁ symmetry. Large TlO distances are observed revealing a low stereochemical activity of the 6s² lone pair. Excitation and emission spectra of Tl⁺ in TlSrLa(AsO₄)₂ showed broad bands at lower energy than those observed in previous works. Excitation spectra are decomposed into multiple Gaussian bands and a theoretical analysis is made to explain the number of observed components. Two Gaussian components are revealed for emission spectra.     

Luminescence of SrGdGa3O7:RE(RE = Eu, Tb) phosphors and energy transfer from Gd to RE

Eu³⁺- and Tb³⁺-activated SrGdGa₃O₇ phosphors were synthesized by the solid-state reaction and their luminescence properties were investigated. Sr(Gd_1 − xEu_x)Ga₃O₇ and Sr(Gd_1 − xTb_x)Ga₃O₇ formed continuous solid solution in the range of x = 0-1.0. Unactivated SrGdGa₃O₇ exhibited a typical characteristic excitation and emission of Gd ion. The SrGdGa₃O₇:xEu³⁺ and SrGdGa₃O₇:xTb³⁺ phosphors also showed the well-known Eu³⁺ and Tb³⁺ excitation and emission. The energy transfer from Gd³⁺ to Eu³⁺ and Tb³⁺ were verified by photoluminescence spectra. The dependence of photoluminescence intensity on Eu³⁺ and Tb³⁺ concentration were also studied in detail and the photoluminescence (PL) intensity of SrGdGa₃O₇:Eu and SrGdGa₃O₇:Tb were compared with commercial phosphors, Y₂O₃:Eu and LaPO₄:Ce,Tb. The luminescence decay measurements showed that the lifetimes of Eu³⁺ and Tb³⁺ were in the range of microsecond. The energy transfer from Gd³⁺ to Tb³⁺ was also observed in decay curve.     

A promising red phosphor MgMoO4:Eu for white light emitting diodes

Motivated by the need for new red phosphors for solid-state lighting applications Eu³⁺-doped MgMoO₄ was prepared by solid-state reaction and its excitation and emission spectra were measured at room temperature. In addition, the effects of firing temperature and Eu³⁺ doping concentration on the PL intensities were also investigated. Compared with Y₂O₂S:0.05Eu³⁺, the obtained Mg_0.80MoO₄:Eu³⁺_0.20 phosphor shows a stronger excitation band near 400 nm and intensely red-emission lines at 616 nm correspond to the forced electric dipole ⁵D₀ → ⁷F₂ transitions on Eu³⁺ under 394 nm light excitation. The CIE chromaticity coordinates (x = 0.651, y = 0.348) of Mg_0.80MoO₄:Eu³⁺_0.20 close to the NTSC (National Television Standard Committee) standard values, and therefore may find application on near UV InGaN chip-based white light emitting diodes.     

Preparation and characterization of rare earth orthoborates, LnBO3 (Ln = Tb, La, Pr, Nd, Sm, Eu, Gd, Dy, Y) and LaBO3:Gd, Tb, Eu by metathesis reaction: ESR of LaBO3:Gd and luminescence of LaBO3:Tb, Eu

Lanthanide orthoborates of composition LnBO₃ (Ln = Tb, La, Pr, Nd, Sm, Eu, Gd, Dy, Y) and LaBO₃:Gd, Tb, Eu have been prepared by metathesis reaction. This method provides a convenient route for the synthesis of orthoborates and its solid solutions at low temperatures. Powder X-ray diffraction and FT-IR spectroscopy were used to characterize these borates. Rare earth borates, (LnBO₃) are isomorphous with different forms of CaCO₃ depending on the radius of rare earth ion. LaBO₃, LaBO₃:Gd, Tb, Eu, PrBO₃, NdBO₃ crystallized in aragonite structure, SmBO₃ crystallized in H-form and TbBO₃, EuBO₃, GdBO₃, DyBO₃, YBO₃ crystallized in vaterite structure. The structural analysis of TbBO₃ was carried out. The morphology of these borates was obtained from Scanning electron microscopy. Spin-Hamiltonian parameters for Gd³⁺ are deduced from room temperature electron spin resonance spectrum of LaBO₃:Gd. The luminescence of LaBO₃:Tb, Eu gave characteristics peaks corresponding to Tb³⁺, Eu³⁺ respectively.     

Crystal growth and spectral properties of Pr:La2(WO4)3

Pr³⁺-doped La₂(WO₄)₃ single crystal with dimensions up to Ø 20 mm × 35 mm has been grown by the Czochralski method. The structure of the Pr³⁺:La₂(WO₄)₃ crystal was determined by the X-ray powder diffraction and the Pr³⁺ concentration in this crystal was determined. The absorption and fluorescence spectra of Pr³⁺:La₂(WO₄)₃ crystal were measured at room temperature, and the fluorescence lifetime of main emission multiplets were estimated from the recorded decay curves. The spectral properties related to laser performance of the crystal were evaluated.     

White-emitting phosphors Ca6La2Na2(PO4)6F2:Dy and luminescence enhancement through Ce → Dy energy transfer

Ce³⁺ and Dy³⁺ activated fluoro-apatite Ca₆La₂Na₂(PO₄)₆F₂ with chemical formulas Ca₆La_2−xLn_xNa₂(PO₄)₆F₂ (Ln = Ce³⁺, Dy³⁺) were prepared by a solid state reaction technique at high temperature. The vacuum-ultraviolet (VUV) and ultraviolet (UV) spectroscopic properties are investigated. The results indicate that Ce³⁺ ions show the lowest 5d excitation band at ∼305 nm and a broad emission band centered at ∼345 nm. Dy³⁺ ions exhibit intense absorption at VUV and UV range. White-emitting under 172 nm excitation is obtained based on two dominant emissions from Dy³⁺ ions centered at 480 and 577 nm. In addition, the energy transfer from Ce³⁺ to Dy³⁺ in the co-doped samples are observed and discussed.