Photoluminescence properties of Eu3+-activated CaMoO4 phosphors for WLEDs applications and its Judd–Ofelt analysis

A series of scheelite-type Eu³⁺-activated CaMoO₄ phosphors were synthesized by the nitrate–citrate gel combustion method. All the compounds crystallized in the tetragonal structure with space group I4 ₁ /a (No. 88). FESEM results reveal the spherical-like morphology. The CaMoO₄ phosphor exhibited broad emission centered at 500 nm under the excitation of 298 nm wavelength, while Eu³⁺-activated CaMoO₄ shows an intense characteristic red emission peak at 615 nm at different excitation wavelengths, due to ⁵D₀ → ⁷F₂ transition of Eu³⁺ ions. The intensities of transitions between different J levels depend on the symmetry of the local environment of Eu³⁺ ions and were estimated using the Judd–Ofelt analysis. The high asymmetric ratio revealed that Eu³⁺ occupies sites with a low symmetry and without an inversion center. The CIE chromaticity co-ordinates (x, y) were calculated from emission spectra, and the values were close to the NTSC standard. Therefore, the present phosphor is highly useful for LEDs applications.     

Synthesis and properties of LnBaFeCoO5 + δ (Ln = Nd, Sm, Gd)

LnBaFeCoO_{5 + δ} (Ln = Nd, Sm, Gd) layered oxides have been synthesized and their crystal structure, thermal stability, thermal expansion, electrical conductivity, thermoelectric power, and magnetic susceptibility have been studied. The oxides have a tetragonal structure (sp. gr. P4/mmm) with unit-cell parameters a = 0.3909(2) nm and c = 0.7695(6) nm for Ln = Nd (δ = 0.65), a = 0.3908(3) nm and c = 0.7662(6) nm for Ln = Sm (δ = 0.37), and a = 0.3908(2) nm and c = 0.7613(6) nm for Ln = Gd (δ = 0.37). The LnBaFeCoO_{5 + δ} compounds are antiferromagnetic p-type semiconductors. With decreasing Ln³⁺ ionic radius, their electrical conductivity and linear thermal expansion coefficient decrease and their thermoelectric power and antiferromagnetic ordering temperature increase. Near 518–653 K, the linear thermal expansion coefficient of the LnBaFeCoO_{5 + δ} oxides increases from (12.9–16.6) × 10^?6 to (19.3–26.5) × 10^?6 K^?1, which is due to the release of weakly bound oxygen from the oxides. We have determined parameters of charge transport in the [Fe(Co)O₂] layers in the crystal structure of the LnBaFeCoO_{5 + δ} phases.     

Au-modified silicon nanowires for surface-enhanced fluorescence of Ln3+ (Ln = Pr, Nd, Ho, and Er)

The interaction of the surface plasmons of gold nanoparticles on silicon nanowires with fluorophores, lanthanide ions (praseodymium ions, Pr³⁺, neodymium ions Nd³⁺, holmium ions Ho³⁺, and erbium ions Er³⁺) was investigated. In the presence of Au/Si nanomaterials, the fluorescence peaks were significantly enhanced, which resulted in about 2 orders of magnitude enhancement. The photoluminescence studies revealed that the enhanced fluorescence originates from the local field enhancement around Ln³⁺ ions, caused by the electronic plasmons resonance of the gold nanoparticles. Results showed that this Au/Si nanostructure had larger enhancement factor than that caused by unsupported Au nanoparticles. These results might be explained by the local field overlap originated from the closed and fixed gold nanoparticles on silicon nanowires.     

Synthesis and luminescent properties of BaLn2(1−x)ZnO5:2xTb3+ (Ln = Y, Gd) nanophosphors

The synthesis of BaLn_2(1?x)ZnO₅:2xTb³⁺ (Ln = Y, Gd) nanophosphors using solution combustion method with an aim to study the effect of sintering temperature and Tb³⁺ ions concentration on the luminescent properties has been investigated. Under UV excitation, BaY_2(1?x)ZnO₅ and BaGd_2(1?x)ZnO₅ nanoparticles exhibit apparent characteristic green emission from ⁵D₄ state to ⁷F_6?3 states of Tb³⁺ ions with the strongest at 544 nm. Luminescence concentration quenching could be observed when the Tb³⁺ ions contents were more than 4 mol% in both nanophosphors. The luminescence decay curves suggest monoexponential behavior of both nanophosphors. X-ray diffraction results confirmed the single-phased orthorhombic structure of both powders belonging to space group Pbnm. TEM analysis indicates the spherical morphology of nanoparticles with average grain size in the range of 85–95 nm. BaY_2(1?x)Tb_2x ZnO₅ and BaGd_2(1?x)Tb_2x ZnO₅ nanophosphors may have potential applications in field emission displays based on their uniform shape, low-cost synthetic route, and diverse luminescent properties.     

Sol–gel auto-combustion synthesis and luminescence properties of RVO4: Ln3+ (R=La,Gd; Ln=Sm,Er, Ho,Yb/Er) microcrystals

RVO₄: Ln³⁺ (R=La, Gd; Ln=Sm, Er, Ho, Yb/Er) microcrystals were successfully synthesized by a facile and rapid sol-gel method using glycine as the chelating agent. The crystalline structure, morphology and luminescence properties of obtained products were investigated in detail. The peaks of X-ray diffraction (XRD) patterns were well-indexed to the standard RVO₄ patterns, indicating that the Ln³⁺ ions were well doped into the crystalline lattices and had not changed the crystalline structure. The luminescence properties of the samples are systematically studied. The typical peaks of the emission spectra were sharp and intense, revealing energy was transferred efficiently from VO₄ ^3- to Ln³⁺ after excitation. The schematic diagram for energy transfer between the host matrix and doped lanthanide ions were also discussed. The chromaticity coordinates of RVO₄: Ln³⁺ microcrystals were calculated. This work reveals that the rare-earth vanadates are potential candidates as excellent host matrices of phosphors.     

Emission (Gd3+ and Sm3+) and ESR (Gd3+) studies of La1−xLnxB3O6 (Ln = Gd,Sm; 0 ≤ x ≤ 0.2 for Gd; 0 ≤ x ≤ 0.1 for Sm) phosphors

Polycrystalline powders of rare-earth doped La_1?xGd_xB₃O₆ (0?≤?x?≤?0.2) and La_1?xSm_xB₃O₆ (0.0?≤?x?≤?0.1) phosphors were successfully prepared by a B₂O₃ flux method. All the phosphor samples are well characterized by powder X-ray diffraction (XRD), infrared (IR), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) methods and fluorescence lifetime of Sm³⁺ ion. The XRD patterns show that La_1?xM_xB₃O₆ (M?=?Gd and Sm) adopt monoclinic with the I2/a space group. The SEM–EDS results confirmed the doping of Gd and Sm into LaB₃O₆ lattice. The IR and Raman spectra of these solid solutions gave distinctive bands corresponding to planar BO₃ and tetrahedral BO₄ groups. The photoluminescence (PL) spectra of La_1?xGd_xB₃O₆ gave a strong emission band, ⁶P_J?→?⁸S_7/2, at 310 nm. The PL spectra of La_1?xSm_xB₃O₆ phosphor showed orange-red emission at 598 nm when excited using light of wavelength of 402 nm. The results were obtained by the transition ⁴G_5/2?→?⁶H_7/2 of Sm³⁺ ions. The influence of dopant concentration on the emission profiles was studied. The ESR spectra of La_1?xGd_xB₃O₆ (x?=?0.02) gave a typical U-spectrum and spin-Hamiltonian parameters are deduced.

     

Preparation and thermophysical properties of nano-sized Ln2Zr2O7 (Ln?=?La, Nd, Sm, and Gd) ceramics with pyrochlore structure

Rare earth zirconates (Ln₂Zr₂O₇, Ln = La, Nd, Sm, and Gd) with pyrochlore structure were prepared by hydrothermal method with polyethylene glycol as surfactant. X-ray diffraction, thermogravimetric analysis/differential scanning calorimetry, Fourier transform infrared spectroscopy, Raman spectroscopy, and field emission scanning electron microscopy were utilized to characterize the phase structure, thermal decomposition, and morphology of the products. Qualitative analysis indicates that the as-prepared zirconates are pyrochlore-type structures. The specific surface area, lattice parameter, and average crystallite size of the as-prepared products are closely related to the ionic radius. The activation energy of crystal growth shows an increasing trend with the decrease in ionic radii. The sintering behavior of compacted body was also investigated, revealing that the sintering-resistance properties of Ln₂Zr₂O₇ are descending as the order of La₂Zr₂O₇, Nd₂Zr₂O₇, Sm₂Zr₂O₇, and Gd₂Zr₂O₇.     

Photoluminescence and dielectric properties of LnTiTaO<Subscript>6</Subscript> (Ln = Ce,Pr, Sm) polycrystals

Polycrystalline LnTiTaO₆ (Ln = Ce, Pr, Sm) compounds were prepared in the solid state ceramic route. The powders were calcined between 1250 °C and 1300 °C and sintered between 1480 °C and 1520 °C. The materials were characterized by XRD and SEM. The dielectric properties were measured in the frequency range 1 kHz–5 MHz. The absorption spectra and photoluminescence spectra of the samples were analysed. The materials are useful as gain media in micro lasers because of the optical absorption and photoluminescence.     

Controllable synthesis and luminescence property of LnPO<Subscript>4</Subscript> (Ln = La,Gd, Y) nanocrystals

Lanthanide orthophosphate LnPO₄ (Ln = La, Gd, Y) nanocrystals with different crystalline structures and morphologies were synthesized successfully by a hydrothermal method under mild conditions. The obtained LaPO₄ have a monoclinic or hexagonal structure with varying the temperature. However, as to GdPO₄ and YPO₄, the temperature has no obvious effect on their structures. By tuning the amount of cetyltrimethylammonium bromide (CTAB), the morphology and size of all samples can be controlled easily and effectively. Noticeably, the structure of YPO₄ can also be controlled by using the CTAB. A systematic study of the photoluminescence in Ln³⁺-doped lanthanide phosphates shows that the luminescent properties of these nanophosphors are strongly dependent on their crystal structures and morphologies.     

Photoluminescence properties of β-SiAlON:Yb2+, a novel green-emitting phosphor for white light-emitting diodes

Abstract

We have synthesized Yb²⁺-activated Si_6?zAl_zO_zN_8?z (0.05z2.3, 0.03 mol% Yb²⁺0.7 mol%) green phosphors by solid-state reaction at 1900 °C for 2 h under a nitrogen pressure of 1.0 MPa. Phase purity, photoluminescence and its thermal quenching were investigated. A single phase was obtained for all values of z and Yb²⁺ concentration. A distinct emission band was observed at 540 nm originating from the 5d–4f electronic transition in Yb²⁺ under 480 nm excitation. The photoluminescence properties mainly depended on the Yb²⁺ concentration and chemical composition of the matrix. The resultant phosphor showed high thermal stability, that is, the emission intensity at 150 °C was about 82% of that measured at room temperature. The experimental results indicate that β-SiAlON:Yb²⁺ is a potential green phosphor for white light-emitting diodes (LEDs), which use blue LEDs as the primary light source.     

Structural and Magnetic Properties of RMO3 (R = Pr,Nd and M = Fe,Co) Perovskites

Journal of Superconductivity and Novel Magnetism - In this work, the RMO3 (R?=?Pr, Nd and M?=?Fe, Co) perovskites had been synthesized by hydrothermal method. The structural...     

Rare earth niobium oxynitrides, LnNbON2−δ (Ln = Y, La, Pr, Nd, Gd, Dy): Synthesis, structure and properties

Ammonolysis of rare earth niobates of the type LnNbO₄ (Ln = Y, La, Pr, Nd, Gd, Dy) yields oxynitrides of different structures. When Ln = La, Nd and Pr, the structure is that of an orthorhombic perovskite of the general formula LnNbON₂. As the size of the rare earth decreases, the oxynitride has a nitrogen-deficient defect fluorite (Ln = Pr, Nd, Gd), or pyrochlore (Ln = Y) structure. The IR spectra of the oxynitrides and the corresponding oxides are significantly different. Thermogravimetric analysis suggests the formation of an intermediate phase wherein the N₂ molecule is attached to the oxide lattice above 400 °C and decomposes to give the oxide on heating in an oxygen atmosphere. Raman spectra of the intermediate phases show evidence for the NN stretching vibration. Gadolinium niobium oxynitride is found to be paramagnetic.     

Preparation and luminescence properties investigation of Eu3+, Tb3+-doped LaNbO4:RE3+ (RE = Eu,Eu/Tb,Tb)

Journal of Materials Science: Materials in Electronics - In this work, doped and co-doped LaNbO4:RE3+ (RE?=?Eu, Eu/Tb, Tb) single crystals were prepared by a unique hydrothermal method...     

Ln2O2S:Yb,Pr(Ln=Y、La)纳米材料的上转换发光研究

研究乙醇辅助燃烧法制备的纳米Ln2O2S:Yb,Pr(Ln=Y、La)的上转换发光特性.Ln2O2S:Yb,Pr纳米上转换发光材料在980hm激光泵浦下,呈现明亮的蓝绿色发光.Ln2O2S:Yb,Pr的发射光谱峰值为514nm,而Ln2O2S:Yb,Pr的发射光谱峰值为508nm,属于Pr3+的3P0→H4跃迁.这是由于Pr3+离子的5d轨道与4f轨道很接近,其f-f跃迁受Pr3+离子周围晶场环境影响很大,其上转换发光光谱在不同基质中有较大不同.     

Synthesis and luminescence of BiPO4:Ln3+ (Ln = Ce3+, Tb3+) phosphors

BiPO₄:Ce³⁺ and BiPO₄:(Ce³⁺, Tb³⁺) powders were synthesized by the method of precipitation. The X-ray diffraction patterns show that BiPO₄:Ce³⁺ and BiPO₄:(Ce³⁺, Tb³⁺) samples have pure hexagonal phases. The transmission electron microscopy results show that the synthesized samples are nanoparticles. Ethylene glycol plays an important role in the formation of nanoparticles. The excitation spectrum of BiPO₄:Ce³⁺ sample shows the transition from the ground ² F _5/2 state to the excited 5d states of the Ce³⁺ ions. The emission spectrum exhibits a strong band centered at 352 nm originating from the 5d → 4f transitions of the Ce³⁺ ions. The emission spectrum of the BiPO₄:(Ce³⁺, Tb³⁺) sample contains both a weak emission band of the Ce³⁺ ions and strong green emission bands of the Tb³⁺ ions. The excitation and emission spectra show that there are energy transfers between Ce³⁺ and Tb³⁺ ions in the BiPO₄:(Ce³⁺, Tb³⁺) sample. The energy transfers between Ce³⁺ and Tb³⁺ ions improve the emission efficiency of BiPO₄:(Ce³⁺, Tb³⁺) sample.     

Structure,morphology, dielectric,and impedance properties of (1-x) (Al0.2La0.8TiO3) + (x) (CuTiO3) (x = 0.2–0.8) nanocomposites

Journal of Materials Science: Materials in Electronics - (1?x) (Al0.2La0.8TiO3) + (x) (CuTiO3) (x?=?0.2–0.8) [ALTCT] nanocomposites were prepared through hydrothermal...     

First-principles calculations of crystal structure,electronic structure and optical properties of Ba<Subscript>2</Subscript>RETaO<Subscript>6</Subscript> (RE = Y,La, Pr,Sm, Gd)

The crystal structure, electronic structure and optical properties of Ba₂RETaO₆ (RE?=?Y, La, Pr, Sm, Gd) have been studied by first-principles calculation. The calculated lattice parameters are in good agreement with the previously reported values. With increasing the atomic number of RE (i.e., the number of 4f electrons), the energy level of RE 4f bands becomes lower continuously. The relationship between the electronic structure and optical properties is explored based on first-principles calculation. The electron transitions between O 2p states, RE 4f states and Ta 5d states have a key effect on optical properties such as dielectric function, refractive index, absorption coefficient and reflectivity. The phase structures have great influence on the optical properties of Ba₂SmTaO₆ and Ba₂GdTaO₆, and the big variation of reflectivity induced from phase transition makes them have potential applications in the infrared radiation protection area.