Effects of Na2CO3 flux addition on the structure and photoluminescence properties for Eu-doped YVO4 phosphor

The luminescence properties of (Y_0.9Eu_0.1)VO₄ phosphor with Na₂CO₃ flux prepared using the solid-state reaction were investigated. The XRD patterns show that all of the peaks are attributed to the YVO₄ phase. The best crystallinity was obtained with 2 wt% Na₂CO₃ flux addition. The surface morphology of (Y_0.9Eu_0.1)VO₄ phosphor changed from fluffy to a bar shape structure after Na₂CO₃ flux addition due to the tetragonal crystal system of YVO₄. The calcined powders emit bright red luminescence centered at 618 nm due to the ⁵D₀→⁷F₂ electric dipole transition under an excitation wavelength of 318 nm; its intensity was increased about 15% with 2 wt% Na₂CO₃ flux addition. Red shift behavior was observed for the charge transfer state (CTS) absorption, which was due to the grain size of (Y_0.9Eu_0.1)VO₄ phosphor increasing with increasing flux content. For 2 wt% Na₂CO₃ flux addition, the red emission of the (Y_0.9Eu_0.1)VO₄ phosphor had CIE chromaticity coordinates of (0.66, 0.34), which are very close to the NTSC system standard red chromaticity coordinates of (0.67, 0.33).     

Self-assembled CaMoO4 and CaMoO4:Eu hierarchical superstructures: Facile sonochemical route synthesis and tunable luminescent properties

Tetragonal CaMoO₄ and CaMoO₄:Eu³⁺ with various novel three-dimensional (3D) hierarchical architectures were successfully synthesized via a facile, efficient sonochemistry process in the absence of any surfactant or template. XRD, EDS, FE-SEM, and photoluminescence (PL) were employed to characterize the as-obtained products. It was found that morphology modulation could be easily realized by changing pH value of the precursor. The pH value of the precursor not only affected the substructures of the hierarchical structures, but also determined the size distributions of the final products. The formation mechanism for different hierarchical architectures was proposed on the basis of time-dependent experiments. The luminescence spectra showed that CaMoO₄:Eu³⁺ phosphors can be effectively excited by the near ultraviolet (UV) (396 nm) and blue (466 nm) light, and exhibited strong red emission around 615 nm, which was attributed to the Eu³⁺⁵D₀→⁷F₂ transition. Compared with Y₂O₃:Eu³⁺ phosphor, CaMoO₄:Eu³⁺ is much more stable, efficient and suitable, therefore, this phosphors could be a promising red component for possible applications in the field of LEDs.     

Synthesis and photoluminescence properties of NaPbB5O9:Dy phosphor materials for white light applications

Given the recent increased interest in phosphor materials and their applications, we analyzed a new NaPbB₅O₉:Dy³⁺ phosphor material with different concentrations of Dy³⁺. In particular, we investigated the crystal structure, morphology, and luminescence properties of these materials. X-ray diffraction analyses confirmed the formation of NaPbB₅O₉:Dy³⁺ phosphor powder. The functional groups present in the phosphor materials were examined by Fourier transform infrared spectroscopy. Scanning electron microscope images showed that the size of the grains was in the micrometer range. Photoluminescence spectra were recorded at different excitation wavelengths for the phosphor materials and we analyzed the variation in the intensity of the emission bands with different concentrations of Dy³⁺ ions. The color co-ordinates were calculated and used to characterize the color of the phosphor. We found that the emission colors of the Dy³⁺-doped NaPbB₅O₉ powders depended on the Dy³⁺ ion doping concentration and the excitation wavelength.     

Photoluminescence properties of BaMgAl10O17:Eu phosphor prepared by the flux method

BaMgAl₁₀O₁₇:Eu²⁺ phosphors were synthesized by the flux method. When the appropriate amounts of fluxes are added, the synthesis temperature reduced by at least 200 °C compared with the conventional solid-state reaction method. SEM images demonstrated that addition of the flux in the process of phosphor synthesis benefitted the size and morphology of BaMgAl₁₀O₁₇:Eu²⁺ phosphor particles. Photoluminescence measurements under VUV excitation indicated that the luminescent intensity of the phosphor enhanced by adding the flux system (BaF₂+Li₂CO₃). Addition of the flux system can not only enhance the luminescence efficiency and improve the stability, but also control the morphology and grain size of the phosphor. Replacement of Ba²⁺ by Li⁺ could generate traps, which result in slightly longer decay time.     

Sr2CeO4:Eu and Sr2CeO4:5 mol% Eu, 3 mol% Dy microphosphors: Wet chemistry synthesis from hybrid precursor and photoluminescence properties

In this article, Sr₂CeO₄:x mol% Eu³⁺ and Sr₂CeO₄:5 mol%Eu³⁺, 3 mol% Dy³⁺ phosphors were synthesized from assembling hybrid precursors by wet chemical method. As-prepared samples present uniform grain-like morphology and the particle size is about 0.2 μm. The luminescence spectra of Sr₂CeO₄:x mol% Eu³⁺ have been measured to examine the influence of the intensity of red emission lines for Eu³⁺ on the concentration of Eu³⁺, showing that the intensity of the red emission increases with an increase of the concentration from 1 to 5 mol%. Additionally, from the emission spectra of Sr₂CeO₄:5 mol%Eu³⁺, 3 mol% Dy³⁺ phosphors, the characteristic lines of Dy³⁺ have also been observed. This result indicates that there also exists an energy transfer process between Sr₂CeO₄ and Dy³⁺.     

Synthesis and luminescence properties of YVO4:Eu,Bi phosphor with enhanced photoluminescence by Bi doping

YVO₄:Eu³⁺,Bi³⁺ phosphors have been prepared by the high-temperature solid-state (HT) method and the Pechini-type sol-gel (SG) method. Spherical SiO₂ particles have been further coated with YVO₄:Eu³⁺,Bi³⁺ phosphor layers by the Pechini-type SG process, and it leads to the formation of core-shell structured SiO₂/YVO₄:Eu³⁺,Bi³⁺ phosphors. Therefore, the phase formations, structures, morphologies, and photoluminescence properties of the three types of as-prepared YVO₄:Eu³⁺,Bi³⁺ phosphors were studied in detail. The average diameters for the phosphor particles are 2-4 μm for HT method, 0.1-0.4 μm for SG method, and 0.5 μm for core-shell structured SiO₂/YVO₄:Eu³⁺,Bi³⁺ particles, respectively. Photoluminescence spectra show that effective energy transfer takes place between Bi³⁺ and Eu³⁺ ions in each type of as-prepared YVO₄:Eu³⁺,Bi³⁺ phosphors. Introduction of Bi³⁺ into YVO₄:Eu³⁺ leads to the shift of excitation band to the long-wavelength region, thus the emission intensities of ⁵D₀-⁷F₂ electric dipole transition of Eu³⁺ at 615 nm upon 365 nm excitation increases sharply, which makes this phosphor a suitable red-emitting materials that can be pumped with near-UV light emitting diodes (LEDs).     

Growth and optical properties of Eu/Li-co-doped SrB4O7 single crystals

This paper reports the growth and optical properties of Eu²⁺/Li⁺-co-doped SrB₄O₇ single crystals. High-quality Eu,Li:SrB₄O₇ crystals without macro-defects or cracks were grown using the top-seeded solution growth (TSSG) method. The absorption and luminescent properties were measured and different spectra were observed in the as-grown crystals. As the doping amount of lithium increases, the absorption peak at 300 nm becomes stronger and the emission peak shifts to a longer wavelength. This phenomenon could be attributed to the doping lithium ions, which might affect the electric field distribution in the lattice structure.     

Determination of kinetic parameters of thermally stimulated luminescence of Cu-doped BaSO4

Studies of thermally stimulated luminescence (TSL) of doped BaSO₄ with Cu activator have been carried out. The polycrystalline sample of Cu-doped BaSO₄ is prepared by recrystallisation method. The characterization and elemental analysis of BaSO₄:Cu compound are carried out by X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDX). The compound has orthorhombic structure at room temperature. The TSL studies of Cu-doped BaSO₄ sample show two glow peaks at 160 and 227 °C. Comparison of TL intensity of the most intensive glow peak of Cu- and Mn-doped BaSO₄ compounds with that of undoped BaSO₄ shows that addition of Cu and Mn impurity in BaSO₄ compound enhances the TL intensity by about 9 and 3 times, respectively. Among the studied samples, namely undoped, Cu- and Mn-doped BaSO₄, Cu-doped BaSO₄ is found to be the most sensitive. The trap parameters namely order of kinetics (b), activation energy (E), frequency factor (s) and Balarin parameter (γ) associated with the most intensive glow peak, i.e. the 227 °C glow peak of BaSO₄:Cu phosphor were determined by using isothermal decay method and glow curve shape (Chen's) method and these parameters are in good agreement.     

Argon ions induced thermoluminescence properties of Ba0.12Sr0.88SO4 phosphor

Thermoluminescence properties of barium strontium mixed sulfate have been studied by irradiation with Argon ions. The sample was recrystallized by chemical co-precipitation techniques using H₂SO₄. The X-ray diffraction study of prepared sample suggests the orthorhombic structure with average grain size of 60 nm. The samples were irradiated with 1.2 MeV Argon ions at fluences varying between 10¹¹ and 10¹⁵ ions/cm². The argon ions penetrate to the depth of 1.89 μm and lose their energy mainly via electronic stopping. Due to ion irradiation, a large number of defects in the sample are formed. Thermally stimulated luminescence (TSL) glow curves of ion irradiated Ba_0.12Sr_0.88SO₄ phosphor exhibit broad peak with maximum intensity at 495 K composed of four overlapping peaks. This indicates that different sets of traps are being activated within the particular temperature range each with its own value of activation energy (E) and frequency factor (s). Thermoluminescence (TL) glow curves were recorded for each of the ion fluences. A linear increase in intensity of TL glow peaks was found with the increase in ion dose from 59 kGy to 5.9 MGy. The kinetic parameters associated with the prominent glow peaks were calculated using glow curve deconvolution (GCD), different glow curve shape and sample heating rate methods.     

Combustion synthesis and photoluminescence of Eu, Dy-doped La2Zr2O7 nanocrystals

A facile and energy saving sol-gel combustion method has been used to prepare La₂Zr₂O₇ nanocrystallines. The pyrochlore La₂Zr₂O₇ nanocrystals have been obtained at a relatively low temperature with the grain size ranging from 45 to 70 nm. Eu³⁺ and Dy³⁺ have been introduced into the La₂Zr₂O₇ crystal structure, respectively, and the intense photoluminescence was observed. The relative intensity of electric dipole transition and magnetic dipole transition is considered for luminescence emission both of Eu³⁺ and Dy³⁺. The dependence of luminescence intensity on dopant concentration and the effect of Dy³⁺ co-doping on Eu³⁺ luminescence are also discussed.     

Studies on the influence of lithium incorporation in the photoluminescence of Y2O3:Eu thin films

Nanostructured europium-doped yttrium oxide thin films with lithium as a co-dopant were prepared using pulsed laser ablation technique. X-ray diffraction studies of the films indicated amorphous nature of the as deposited films and a transformation to crystalline phase with increase of annealing temperature. In this transformation, lithium co-doped films showed early crystallization. Lithium substitution resulted not only in enhancement of photoluminescence at 612 nm, resulting from ⁵D₀-⁷F₂ transition within europium, but also found to reduce the required processing temperature for intense photoemission. The deviation observed in the value of lattice constant of films annealed at different temperatures is found to be sensitive to annealing temperature. In the light of this, the dependence of photoluminescence intensity on the magnitude of lattice imperfection is also discussed. The morphology and transmittance of the films are also found to be sensitive to annealing process and lithium doping.     

Microwave-assisted synthesis and optical property of CdMoO4 nanoparticles

Microwave-assisted synthesis is a novel method used to synthesize CdMoO₄ nanoparticles in propylene glycol. The effects of reaction time and microwave power on phase, morphologies, and optical properties of CdMoO₄ nanoparticles were studied, using X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), and UV-visible spectroscopy. The present analyses proved that these crystalline powders were scheelite-type tetragonal structured CdMoO₄, with the crystallite size of 14-20 nm, and 4.51-4.73 eV band gaps, controlled by the synthetic conditions.     

Microstructural, dielectric and spectroscopic properties of Li2O-Nb2O5-ZrO2-SiO2 glass system crystallized with V2O5

Li₂O-Nb₂O₅-ZrO₂-SiO₂ glasses mixed with different concentrations of V₂O₅ were crystallized. The samples were characterized by XRD, SEM and DTA techniques. The SEM pictures indicated that the samples contain well defined and randomly distributed crystal grains. The X-ray diffraction studies have revealed the presence of several crystalline phases in these samples. Optical absorption, ESR and photoluminescence spectral studies on these samples have indicated that a considerable proportion of vanadium ions do exist in V⁴⁺ state in addition to V⁵⁺ state and the redox ratio seems to be increasing with increase in the concentration of crystallizing agent V₂O₅. The infrared spectral studies have pointed out the existence of conventional SiO₄, ZrO₄, NbO₆, VO structural units in the glass ceramic network. The study of dielectric properties suggested a decrease in the insulating character of the glass ceramics with increase in the crystallizing agent. A.C. conductivity in the high temperature region seems to be connected mainly with the polarons involved in the process of transfer from V⁴⁺↔V⁵⁺ ions.     

Low temperature synthesis of Mn0.4Zn0.6In0.5Fe1.5O4 nanoferrite for high-frequency applications

In the present study, nanoferrite of composition Mn_0.4Zn_0.6In_0.5Fe_1.5O₄ has been synthesized by co-precipitation method. Decomposition of residue at a temperature as low as 200 °C gives the ferrite powder. The ferrite has been, finally, sintered at 500 °C. The structural studies have been made by using X-ray diffraction (XRD) technique and scanning electron microscopy (SEM), which confirm the formation of single spinel phase and nanostructure. The dc resistivity is studied as a function of temperature and values found are more than twice those for the samples prepared by the other chemical methods. It is found that the resistivity decreases with increase in temperature. The initial permeability value is found to be higher as compared to the other chemical routes. The initial permeability value is found to increase with increase in temperature. At a certain temperature called Curie temperature, it attains a maximum value, after which the initial permeability decreases sharply. Even at nanolevel, appreciable value of initial permeability is obtained and low magnetic losses make these ferrites especially suitable for high-frequency applications. The particle size is calculated using Scherrer's equation for Lorentzian peak, which comes out between 35 and 49 nm. Possible mechanisms contributing to these processes have been discussed.     

Density functional theory study on two-peak emission of Eu activators in Sr2SiO4

The supercells of pure and Eu-doped Sr₂SiO₄ were theoretically analyzed by density functional theory (DFT) calculations to investigate the typical two-peak emission of Sr₂SiO₄:Eu²⁺, which originates from two different Sr²⁺ (or Eu²⁺) sites in the Sr₂SiO₄ host structure. The Perdew-Wang generalized-gradient approximation (GGA) functional and the double numerical plus d-functions (DND) basis set with effective core potentials (ECP) were employed in the calculations of electronic properties. The electron transfer between Eu²⁺ ions placed at two different crystallographic Sr²⁺ sites was understood based on the accurate assignment of deconvoluted peaks of the two-peak emission to their corresponding crystallographic sites. This study ought to be instructive as a basic guideline to improve the color chromaticity of Sr₂SiO₄:Eu²⁺ for use in white light emitting diodes (WLEDs).     

Preparation and luminescent properties of BaCa2Si3O9:Eu

A blue emitting phosphor of the triclinic BaCa₂Si₃O₉:Eu²⁺ was prepared by the combustion-assisted synthesis method and an efficient blue emission ranging from the ultraviolet to visible was observed. The luminescence and crystallinity were investigated using luminescence spectrometry and X-ray diffractometry (XRD), respectively. The emission spectrum shows a single intensive band centered at 445 nm, which corresponds to the 4f⁶5d¹→4f⁷ transition of Eu²⁺. The excitation spectrum is a broad extending from 260 to 450 nm, which matches the emission of ultraviolet light-emitting diodes (UV-LEDs). The critical quenching concentration of Eu²⁺ in BaCa₂Si₃O₉:Eu²⁺ phosphor is about 0.05 mol. The corresponding concentration quenching mechanism is verified to be a dipole-dipole interaction. The CIE of the optimized sample Ba_0.95Ca₂Si₃O₉:Eu_0.05²⁺ was (x, y)=(0.164, 0.111). The result indicates that BaCa₂Si₃O₉:Eu²⁺ can be potentially useful as a UV radiation-converting phosphor for white light-emitting diodes (LEDs).     

Solvothermal synthesis and luminescent properties of YAG:Tb nano-sized phosphors

Nano-sized Tb-doped YAG phosphor particles were synthesized by a mixed solvo-thermal method using stoichiometric amounts of inorganic aluminum and yttrium salts. The formation of YAG:Tb was investigated by means of XRD and IR spectra. The pure crystalline-phase YAG was prepared under moderate synthesis conditions (300 °C and 10 MPa), indicating that ethanol partly replaces water as the solvent, thus favoring the formation of YAG. TEM images showed that YAG:Tb phosphor particles sintered at 300 °C were basically of spherical shape, with good dispersion about a particle size of around 80 nm. The crystalline YAG:Tb showed green emission with ⁵D₄−⁷F₆ (544 nm) as the most prominent group. The PL intensity and crystallinity of YAG:Tb phosphors increases with increasing synthesis temperature, and reaches maximum brightness at 300 °C, which is lower than that exhibited by a commercial product.     

The photoluminescent properties of Eu in MgO-Ga2O3-SiO2 nanocrystalline glass-ceramic

The MgO-Ga₂O₃-SiO₂ glass-ceramic (GC) containing MgGa₂O₄ nanocrystals and glasses doped with Eu³⁺ ions were prepared by the sol-gel method. The down-conversion and up-conversion luminescence (UCL) properties were studied. The results indicated that the relative intensity of f-f transitions of Eu³⁺ decreased in contrast with that of charge transfer (CT) absorption with the increase in heating temperature. Using a Xe lamp and 800 nm femtosecond (fs) laser excitation, strong red luminescence of Eu³⁺ in MgO-Ga₂O₃-SiO₂ glasses and GC was observed.     

Optical gain due to the Eu transition in the alloy of Ca1-xEuxGa2S4

Room temperature photoluminescence quantum efficiency of the alloy of Ca_1−xEu_xGa₂S₄ was measured as a function of x, and was found to be nearly unity under excitation at peak wavelength of excitation spectrum (510 nm) in the x range of 0.01≤x≤0.2. At larger x values, it tends to decrease, but still as high as 30% for stoichiometric compound EuGa₂S₄. Taking these backgrounds into account, pump-probe experiments were done with Ca_1−xEu_xGa₂S₄ for searching optical gain at x=0.2. The optical gain of nearly 30 cm⁻¹ was confirmed to exist, though the pumping induced transient absorption which seems to limit the higher gain was found.     

Comparative structural and photoluminescent study of Eu-doped La2O3 and La(OH)3 nanocrystalline powders

Eu³⁺-doped La₂O₃ nanocrystalline powder was prepared by polymer complex solution method and further used for preparation of Eu³⁺-doped La(OH)₃. Structural and optical characterization was carried out by powder X-ray diffraction and photoluminescent spectroscopy. XRD measurements confirmed the formation of hexagonal La₂O₃ and its recrystallization into La(OH)₃ in a humid atmosphere. Excitation spectra show redshift of host lattice and charge transfer emission bands in La(OH)₃ while bands that correspond to Eu³⁺f–f transitions are placed at same wavelengths in both samples. Photoluminescence spectra recorded over the temperature range from 10 K to 300 K show that intensities of emission lines in Eu³⁺-doped La₂O₃ do not depend on temperature as much as in La(OH)₃ sample. Observed dominant ⁵D₀→⁷F₂ and markedly visible ⁵D₀→⁷F₀ emissions in doped La₂O₃ indicate that Eu³⁺ ion is located in a structural site without an inversion center. On the other hand, in Eu³⁺-doped La(OH)₃⁵D₀→⁷F₀ transition is barely visible while ⁵D₀→⁷F₂ is not prominent, and with temperature drop three ⁵D₀→⁷F_J (J=1, 2, 4) transitions become almost of the same intensity. In both La₂O₃ and La(OH)₃ structures Eu³⁺ ion replaces La³⁺ in non-centrosymmetric C_3v and C_3h crystallographic sites, respectively, and difference in symmetry of the crystal field around europium ion is explained by comparing shape and volume of these sites. Decay times of the ⁵D₀- level recorded over the temperature range 10−300 K revealed that emission lifetime values in La₂O₃ (~0.7 ms) are almost two times higher than in La(OH)₃ (~0.4 ms), and unlike in La₂O₃, lifetime in La(OH)₃ is temperature dependent.