The development of Ce3+-activated (Gd,Lu)3Al5O12 garnet solid solutions as efficient yellow-emitting phosphors

Abstract

Ce³⁺-activated Gd₃Al₅O₁₂ garnet, effectively stabilized by Lu³⁺ doping, has been developed for new yellow-emitting phosphors. The powder processing of [(Gd_1?xLu_x)_1?yCe_y]₃Al₅O₁₂ solid solutions was achieved through precursor synthesis via carbonate precipitation, followed by annealing. The resultant (Gd,Lu)AG:Ce³⁺ phosphor particles exhibit typical yellow emission at ～570 nm (5d–4f transition of Ce³⁺) upon blue-light excitation at ～457 nm (the ²F_5/2–5d transition of Ce³⁺). The quenching concentration of Ce³⁺ was determined to be ～1.0 at% (y = 0.01) and the quenching mechanism was suggested to be driven by exchange interactions. The best luminescent [(Gd_0.9Lu_0.1)_0.99Ce_0.01]AG phosphor is comparative to the well-known YAG:Ce³⁺ in emission intensity but has a substantially red-shifted emission band that is desired for warm-white lighting. The effects of processing temperature (1000–1500 °C) on the spectroscopic properties of the phosphors, especially those of Lu³⁺/Ce³⁺, were thoroughly investigated and discussed from the centroid position and crystal field splitting of the Ce³⁺ 5d energy levels.     

Shaped single crystal growth and scintillating application of Yb:(Gd,Lu)3(Ga,Al)5O12 solid solutions

Shaped single crystals of (Yb_0.05Lu_xGd_0.95−x)Ga₅O₁₂ (0.0x0.9) and Yb_0.15Gd_0.15Lu_2.7(Al_xGa_1−x)O₁₂ (0.0x1.0) were grown by the modified micro-pulling-down method. Continuous solid solutions with garnet structure and a linear compositional dependency of crystal lattice parameter in the system Yb:(Gd,Lu)₃(Ga,Al)₅O₁₂ are formed. Measured optical absorption spectra of the samples show 4f–4f transitions related to Gd³⁺ ion at 275 and 310 nm, and also an onset of charge transfer transitions from oxygen ligands to Gd³⁺ or Yb³⁺ cations below 240 nm. A complete absence of Yb³⁺ charge transfer luminescence under X-ray excitation in any of the investigated samples was explained by the overlapping of charge transfer absorption of Yb³⁺ by that of Gd³⁺ ions. For specific composition of Lu_1.5Gd_1.5Ga₅O₁₂ an intense defect––host lattice-related emission, which achieve of about 40% integrated intensity compared with Bi₄Ge₃O₁₂, was found.     

Controlled processing of (Gd,Ln)2O3:Eu (Ln = Y,Lu) red phosphor particles and compositional effects on photoluminescence

Synthesis of (Gd_0.95−xLn_xEu_0.05)₂O₃ (Ln = Y and Lu, x = 0–0.95) powders via ammonium hydrogen carbonate (AHC) precipitation has been systematically studied. The best synthesis parameters are found to be an AHC/total cation molar ratio of 4.5 and an ageing time of 3 h. The effects of Y³⁺ and Lu³⁺ substitution for Gd³⁺, on the nucleation kinetics of the precursors and structural features and optical properties of the oxides, have been investigated. The results show that (i) different nucleation kinetics exist in the Gd–Y–Eu and Gd–Lu–Eu ternary systems, which lead to various morphologies and particle sizes of the precipitated precursors. The (Gd,Y)₂O₃:Eu precursors display spherical particle morphologies and the particle sizes increase along with more Y³⁺ addition. The (Gd,Lu)₂O₃:Eu precursors, on the other hand, are hollow spheres and the particle sizes increase with increasing Lu³⁺ incorporation, (ii) the resultant oxide powders are ultrafine, narrow in size distribution, well dispersed and rounded in particle shape, (iii) lattice parameters of the two kinds of oxide solid solutions linearly decrease at a higher Y³⁺ or Lu³⁺ content. Their theoretical densities linearly decrease with increasing Y³⁺ incorporation, but increase along with more Lu³⁺ addition and (iv) the two kinds of phosphors exhibit typical red emissions at ∼613 nm and their charge-transfer bands blue shift at a higher Y³⁺ or Lu³⁺ content. Photoluminescence/photoluminescence excitation intensities and external quantum efficiency are found to decrease with increasing value of x, and the fluorescence lifetime mainly depends on the specific surface areas of the powders.     

YAG:Ce3+, Gd3+ nano-phosphor for white light emitting diodes

YAG:Ce³⁺, Gd³⁺ nano-phosphors were synthesised by the glycothermal method. The X-ray diffraction measurements showed that the samples can be well-crystallised at 600°C. The transition electron microscope showed that the particles have sizes mostly in the range between 35 and 100?nm. The YAG:Ce nano-phosphor had a wide emission band ranging from blue to yellow with a peak at 532?nm, due to the transition from the lowest 5d band to ²F_7/2, ²F_5/2 states of the Ce³⁺ ion. Red-shift of emission peak wavelength from 532 to 568?nm was achieved doping Gd³⁺ ions into the YAG:Ce³⁺ to substitute some Y³⁺ ions. White light emitting diodes (LEDs) were obtained by combining blue LED chip (InGaN-based 460?nm emitting) with (Y_{2.94? x} Ce_0.06Gd _x )Al₅O₁₂ phosphor. As x has the value of 0.8, an intense white LED with good colour rendering of 86 was obtained.     

Composite phosphor films based on spherical Lu2O3:Eu3+ nanoparticles

A nanopowder of Lu₂O₃:Eu³⁺ (C _Eu = 5 at.%) was obtained by coprecipitation with urea (NH₂)₂CO from aqueous solutions. Using this nanopowder, compact Lu₂O₃:Eu³⁺ films with thicknesses within 20–200 μm and a relative density up to 65% of the theoretical limit were deposited using the spin-coating and painting techniques. The films were characterized by scanning electron microscopy, X-ray diffraction, and X-ray luminescence (XRL) measurements. It is established that the XRL intensity depends on the phosphor/organic binder ratio and thickness of the film. The most intense XRL and most homogeneous structure are observed for 20-μm-thick Lu₂O₃:Eu³⁺ films.     

Sol-Gel Synthesis and Photoluminescence of RE<Subscript>x</Subscript>Lu<Subscript>2-<Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>3</Subscript>:Eu<Superscript>3+</Superscript> (RE = Y,Gd) Nanophosphors

RE_xLu_2-x O₃:Eu³⁺ (RE = Y, Gd) nanophosphors with x = 0.4, 1.0, and 1.6 were synthesized using rare-earth salicylate coordination polymers as precursors for the luminescent species RE_xLu_2-x O₃:Eu³⁺ (RE = Y, Gd) composed of polyethylene glycol both as a dispersing medium and fuel. Their particle sizes were around 100 nm as determined by scanning electron microscopy and x-ray diffraction.__________From Neorganicheskie Materialy, Vol. 41, No. 6, 2005, pp. 706–710.Original English Text Copyright © 2005 by Zhou, Yan.This article was submitted by the authors in English.     

Optical spectroscopy of Ce3+ ions in Gd3(AlxGa1−x)5O12 epitaxial films

Epitaxial films of Ce-doped Gd₃(Al_xGa_1−x)₅O₁₂ with x = 0.00, 0.22, 0.31, 0.38 formula units were grown using liquid-phase epitaxy method, and their optical properties were studied. The emission of Ce³⁺ ions can be observed only when Al³⁺ ions are incorporated into the garnet structure, resulting in a shift of the 5d Ce³⁺ states from the conduction band to the bandgap. It is shown that the shift is caused by the cumulative effect of gradual low-energy shift of the lowest 5d level of Ce³⁺ and the raise of the garnet bandgap energy with increasing Al³⁺ concentration.     

On the Ce3+ → Cr3+ energy transfer in Lu3Al5O12 garnets

Two series of Lu₃Al₅O₁₂:Cr³⁺ and Lu₃Al₅O₁₂:0.5% Ce³⁺,Cr³⁺ luminescent materials were prepared by a sol–gel combustion method. All samples were characterized by powder X-ray diffraction (XRD), infrared (IR) and photoluminescence (PL) measurements. Moreover, luminous efficacies (LE), CIE 1931 colour points, and quantum efficiencies (QE) were calculated and discussed. Luminescence measurements indicated that Ce³⁺ ions located at Lu³⁺ site transfers absorbed energy to Cr³⁺ ions located at Al³⁺ site. However, with increasing Cr³⁺ concentration the total light output of Lu₃Al₅O₁₂:0.5% Ce³⁺,Cr³⁺ phosphors decrease.     

Optical and scintillation properties of Nd-doped complex garnet

Nd 1% doped complex garnet scintillators were prepared by Furukawa and their optical and scintillation properties were investigated on a comparison with previously reported Nd-doped YAG. Chemical compositions of newly developed complex garnets were Lu₂Y₁Al₅O₁₂, Lu₂Y₁Ga₃Al₂O₁₂, Lu₂Gd₁Al₅O₁₂, Lu₂Gd₁Ga₃Al₂O₁₂, Gd₁Y₂Al₅O₁₂, Gd₁Y₂Ga₃Al₂O₁₂, and Gd₃Ga₃Al₂O₁₂. They all showed 50–80% transmittance from ultraviolet to near infrared wavelengths with several absorption bands due to Gd³⁺ or Nd³⁺ 4f–4f transition. In X-ray induced radioluminescence spectra, all samples exhibited intense lines at 310 nm due to Gd³⁺ or 400 nm due to Nd³⁺ depending on their chemical composition. Among them, the highest scintillation light yield was achieved by Lu₂Y₁Al₅O₁₂. Typical scintillation decay times of them resulted 1.5–3 μs. Thermally stimulated glow curve after 1 Gy exposure and X-ray induced afterglow were also investigated.     

Luminescence properties of Y0.95? x M x Eu0.05B1? y R y O3 (M?=?Ca, Sr, Ba, Zn, Al, 0?≤ x ≤?0.1; R?=?Si, P, 0?≤ y ≤?0.1) in UV and VUV regions

The phosphors Y_0.95−x M _x Eu_0.05B_1−y R _y O₃:Eu³⁺ (M = Ca, Sr, Ba, Zn, Al, 0 ≤ x ≤ 0.1; R = Si, P, 0 ≤ y ≤ 0.1) are successfully synthesized by solid-state reaction. All the solid samples are identified as isomorphs. Their luminescent properties are studied under UV and VUV excitation. With the incorporation of metallic or nonmetallic cations, the chromaticity of YBO₃:Eu³⁺ is remarkably improved as well as the luminescent intensity increased. When 10% of Al³⁺ is doped into YBO₃ host lattice, the best ratio of the red emission at 610 nm to the orange one at 591 nm is obtained and the luminescent intensity increased simultaneously. The reason is analyzed, and can be explained by the decrease of the symmetry around Eu³⁺ site in YBO₃ host lattice.     

Composition engineering of single crystalline films based on the multicomponent garnet compounds

The paper demonstrates our last achievement in development of the novel scintillating screens based on single crystalline films (SCF) of Ce doped multicomponent garnets using the Liquid Phase Epitaxy (LPE) method. We report in this work the optimized content and excellent scintillation properties of SCF of Lu_3-xGd_xAl_5-yGa_yO₁₂, Lu_3-xTb_xAl_5-yGa_yO₁₂ and Tb_xGd_xAl_5-yGa_yO₁₂ garnet compounds grown by the LPE method from PbOB₂O₃ based melt-solution onto Gd₃Al_2.5Ga_2.5O₁₂ and YAG substrates.We also show that the Tb_1.5Gd_1.5Al_2.5Ga_2.5O₁₂:Ce SCF possess the highest light yield (LY) in comparison with all ever grown garnet SCF scintillators. Namely, the LY of these SCF exceeds by 3.8 and 1.85 times the LY values of the best samples of YAG:Ce and LuAG:Ce SCF scintillators, respectively. The SCF samples of the mentioned compounds show low thermoluminescence in the above room temperature range and relatively fast scintillation decay time t_1/e in the 180–200 ns range.     

Luminescence properties of Eu ions doped in Y2−xGdxO3 thin films grown by pulsed laser deposition method

Luminescence properties of Y_2−xGd_xO₃:Eu³⁺ (x = 0 to 2.0) thin films are investigated by site-selective laser excitation spectroscopy. The films were grown by pulsed laser deposition method on SiO₂ (100) substrates. Cubic phase Y₂O₃ and Gd₂O₃ and monoclinic phase Gd₂O₃ are identified in the excitation spectrum of the ⁷F₀ → ⁵D₀ transition of Eu³⁺. The emission spectra of the ⁵D₀ → ⁷F_J (J = 1 and 2) transition from individual Eu³⁺ centers were obtained by tuning the laser to resonance with each excitation line. The excitation line at around 580.60 nm corresponds to the line from Eu³⁺ with C₂ site symmetry of cubic phase. New lines at 578.65 and 582.02 nm for the C_S sites of Gd₂O₃ with monoclinic phase are observed by the incorporation of Gd in Y₂O₃ lattice. Energy transfer occurs between Eu³⁺ ions at the C_S sites and from Eu³⁺ ions at the C_S sites to those at the C₂ site in Y_2−xGd_xO₃.     

Luminescence Spectra of Eu3+-Activated Potassium Lanthanum and Potassium Gadolinium Phosphate Vanadates

The effects of the Eu³⁺concentration and PO₄/VO₄ratio on the photoluminescence (PL) of K₃R_{1 – y} Eu _y (PO₄) _x (VO₄)_{2 – x} (R = La, Gd) phosphate vanadates were studied over wide composition ranges. The energies of the Stark components of the ⁷ F _j(j= 0, 1, 2) levels were determined, and the crystal-field parameters were evaluated. The concentration quenching of Eu³⁺luminescence was examined. The PL intensity was found to increase with Eu³⁺content up to y= 0.3. At higher Eu³⁺contents, the PL intensity decreases, without, however, complete luminescence quenching even at y= 1. K₃Gd_0.7Eu_0.3(VO₄)₂and K₃Gd_0.7Eu_0.3(PO₄)_0.3(VO₄)_1.7were found to exhibit the brightest photoluminescence.     

Energy transfer and radiative recombination processes in (Gd, Lu)3Ga3Al2O12:Pr3+ scintillators

(Gd_xLu_3−x)Ga₃Al₂O₁₂:0.3 at.%Pr (x = 0.025, 0.05, 0.1, 0.2, 0.4, 0.6) (GLGAG:Pr) polycrystalline powders are prepared by solid-state reaction method. To better understand the luminescence mechanism, the optical diffuse reflectance, photoluminescence emission and excitation, X-ray excited luminescence spectra and decay kinetics of GLGAG:Pr were investigated in detailed, allowing the determination of energy transfer from 5d state of Pr³⁺ to 4f state of Gd³⁺, and the non-radiative relaxation from 5d to 4f state of Pr³⁺. Besides, the former process plays more negative role in the emission quenching of GLGAG:Pr than later one. Pr³⁺ ion is regarded as an ineffective activation ion in Gd-based multicomponent aluminate garnets. In addition, the wavelength-resolved thermoluminescence spectra of GLGAG:Pr were studied after UV and X-ray irradiation. It is revealed that the localized recombination processes from electron traps to lower lying 4f levels of Pr³⁺ occurs without populating the higher 5d levels of Pr³⁺.     

Multifunctional broad-band excited Eu3+-activated fluorescent materials for potential warm white light-emitting diodes (w-LEDs) and temperature sensor applications

Discovery of novel multifunctional fluorescent materials are of great concern to the development of disciplinary crossing and integration. In this work, we prepared a broad-band Eu³⁺-activated Lu₂MoO₆ multifunctional material for potential warm white light-emitting diodes (w-LEDs) and temperature sensor applications. Lu_1.9MoO₆:0.1Eu³⁺ phosphor was synthesized by high-temperature solid-state reaction method, and the structure characterization was investigated for the first time. The excitation spectrum of this phosphor exhibits the intense broad band ranging from 250?nm to 440?nm assigned to the O^2?-Mo⁶⁺ charge transfer band (CTB) transition, and a strong red emission centered at 610?nm corresponding to the ⁵D₀?→?⁷F₂ transition of Eu³⁺ ions were detected under 365?nm excitation. Importantly, warm white light composite material was hybridized via blending commercial green, blue and the present red phosphors, which under 365?nm excitation exhibited a high color rendering index (CRI) of 78 at a correlated color temperature of 4271?K with CIE coordinates of x?=?0.351, y?=?0.308. Furthermore, a strong temperature sensitization phenomenon was found, where the results show that the emission intensities and chromaticity coordinates are sensitive to the temperature, which can provide guidance for the potential application in temperature sensing.     

Thermoelectric power in Gd3+-substituted Cu-Cd ferrites

Polycrystalline Cd _x Cu_1−x Fe_2−y Gd _y O₄ ferrites fory=0·0 and 0·1 were prepared by ceramic technique. X-ray diffractograms of powder samples show cubic symmetry withx⩾0·2 fory=0·0 and 0·1, while compositions withx=0·0 fory = 0·0 and 0·1 are tetragonal. The thermopower measurements for Gd³⁺-undoped ferrites in the temperature range 300 K to 788 K shown-type conductivity forx⩾0·2. The substitution of Gd³⁺ changedn-type conductivity of the compositions top-type. The mobilities calculated show decreasing trend on Gd³⁺ substitution. The values of activation energy ΔE and drift mobilityE _d suggest polaron formation in substituted samples. The conduction mechanism is explained on the basis of localized model and formation of Gd³⁺+Fe²⁺ stable pairs at B site and Cu¹⁺+Fe³⁺ at A site.     

Visible quantum cutting through downconversion in Eu3+-doped K2GdZr(PO4)3 phosphor

K₂Gd_1?xZr(PO₄)₃:Eu_x³⁺ (0.02 ≤ x ≤ 0.1, x is in mol.%) were prepared by solid-state reaction method and their photoluminescence properties were investigated in ultra-violet (UV) and vacuum ultra-violet (VUV) region. The phenomenon of visible quantum cutting through downconversion was observed for the Gd³⁺–Eu³⁺ couple in this Eu³⁺-doped K₂GdZr(PO₄)₃ system. Visible quantum cutting, the emission of two visible light photons per absorbed VUV photon, occurred upon the 186 nm excitation of Gd³⁺ at the ⁶G_J level via two-step energy transfer from Gd³⁺ to Eu³⁺ by cross-relaxation and sequential transfer of the remaining excitation energy. The results revealed that the efficiency of the energy transfer process from Gd³⁺ to Eu³⁺ in the Eu³⁺-doped K₂GdZr(PO₄)₃ system could reach to 155% and K₂GdZr(PO₄)₃:Eu³⁺ was effective quantum cutting material.     

A novel strategy to synthesize Gd2O2S:Eu3+ luminescent nanobelts via inheriting the morphology of precursor

Gd₂O₃:Eu³⁺ nanobelts were fabricated by calcination of the electrospun PVP/[Gd(NO₃)₃ + Eu(NO₃)₃] composite nanobelts. For the first time, Gd₂O₂S:Eu³⁺ nanobelts were successfully prepared via inheriting the morphology and sulfurization of the as-prepared Gd₂O₃:Eu³⁺ nanobelts precursor using sulfur powders as sulfur source by a double-crucible method we newly proposed. X-ray diffraction analysis indicated that Gd₂O₂S:Eu³⁺ nanobelts were pure hexagonal in structure with space group P \( \bar{3} \) m1. Scanning electron microscope analysis results showed that the width and thickness of the Gd₂O₂S:Eu³⁺ nanobelts were ca. 2.1 μm and 129 nm, respectively. Under the excitation of 330-nm ultraviolet light, Gd₂O₂S:Eu³⁺ nanobelts emitted red emissions of predominant peaks at 628 and 618 nm which were attributed to the ⁵D₀ → ⁷F₂ energy levels transitions of the Eu³⁺ ions. It was found that the optimum doping molar concentration of Eu³⁺ ions in Gd₂O₂S:Eu³⁺ nanobelts was 5 %. Possible formation and sulfurization mechanisms of Gd₂O₂S:Eu³⁺ nanobelts were also proposed. This new sulfurization technique is of great importance, not only can inherit the morphology of rare earth oxides, but also can fabricate pure-phase rare earth oxysulfides at low temperature compared with conventional sulfurization method.     

Preparation of Eu3+-Y3+ co-doping red-emitting phosphors for white-light emitting diodes (W-LEDs) application and investigation of their optical characteristics

Eu³⁺-Y³⁺ co-doping Ca_0.54Sr_0.34?1.5x Eu_0.08Y _x (MoO₄) _y (WO₄)_1?y (x = 0.01, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14, 0.16, 0.18, 0.20. y = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0), have been prepared and their luminescent properties are investigated to seek a new red-emitting phosphor for ultraviolet-light emitting diodes chips. In absorption efficiency, luminescent intensity and chromaticity coordinates, Ca_0.54Sr_0.22(MoO₄)_0.2(WO₄)_0.8:0.08Eu³⁺, 0.08Y³⁺ phosphor is better than commercial Y₂O₂S:Eu³⁺ phosphor excited by 390?C405 nm light emitting diodes chip.     

Ln<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>:Mn<Superscript>4+</Superscript> (Ln = Y and Lu): non-rare-earth doped red phosphor for improving light properties of white light emitting diodes

We fabricated a series of Y₃Al₅O₁₂:Mn⁴⁺ and Y_1?yLu_yAl₅O₁₂:Mn⁴⁺ phosphors by a solid state reaction. The phase and the optical properties of the synthesized phosphors were investigated. Under the excitation at 465 nm, Y₃Al₅O₁₂:Mn⁴⁺ phosphors show emission bands locating at deep red regions, which is induced by the spin- and parity-forbidden ²E_g → ⁴A_1g transitions of Mn⁴⁺. The substitution of Y³⁺ by Lu³⁺ decreases the lattice parameter and thus strengthens the crystal field strength, which gives rise to the blue shift of emission band for Y_1?yLu_yAl₅O₁₂:Mn⁴⁺ phosphors. Due to the compensation of red light by Y₃Al₅O₁₂:Mn⁴⁺ or Y_1?yLu_yAl₅O₁₂:Mn⁴⁺ phosphor, the values of correlated-color-temperature for fabricated LEDs are decreased, which leads to the suitable application for them in indoor illumination.