Luminescence efficiency and nonradiative transitions in lanthanide formates

Absolute quantum yields and luminescence lifetimes were measured for Eu, Tb, Gd, formates and Y formate activated by Eu³⁺ as functions of temperature, deuterium substitution in ligands, and Eu³⁺ concentration in Y formates. Radiative and nonradiative transition probabilities in the lanthanide formates were determined using the quantum yield φ and luminescence lifetime τ. The probabilities of radiative transitions in Eu³⁺ were also found by employing corrected luminescence spectra. Relaxation mechanisms of electronic excitation energy in the studied crystals were discussed.     

Intensity and bandwidth of multiphonon vibronic transitions of rare-earth ions in crystals

The theory of multiphonon vibronic coupling to electronic transitions is applied in analysing fluorescence spectra of Eu²⁺ in BaFCI, which consist of the 4f⁷(⁶P_7/2,) → 4f⁷(⁸S_7/2) and 4f⁶5d → 4f⁷ transitions, and the 4f⁷-4f⁶5d excitation spectrum of Ce³⁺ in YPO⁴. The 4f electrons are weakly coupled to lattice vibration modes so that only weak one- and two-phonon sidebands are observable in the 4f-4f optical transitions, whereas the electron-phonon coupling is significantly stronger for a 5d electron. Accordingly, intensive multiphonon vibronic transitions overwhelmingly dominate the 4f⁶5d → 4f⁷ spectrum. It is shown that the extended Judd-Ofelt theory for weak vibronic coupling in the framework of the M-process is equivalent to the Huang-Rhys theory for the δ-process. In the analysis of experimental data, contributions from local ligand modes and lattice acoustic modes are separated, and the coupling strength is evaluated, in terms of the Huang-Rhys parameter S, for the 4f-4f and 5d-4f vibronic transitions.     

Fluorinated Eu‐Doped SnO2 Nanostructures with Simultaneous Phase and Shape Control and Improved Photoluminescence

Fluorinated Eu‐doped SnO₂ nanostructures with tunable morphology (shuttle‐like and ring‐like) are prepared by a hydrothermal method, using NaF as the morphology controlling agent. X‐ray diffraction, field‐emission scanning electron microscopy, high‐resolution transmission electron microscopy, X‐ray photoelectron spectroscopy, and energy dispersive spectroscopy are used to characterize their phase, shape, lattice structure, composition, and element distribution. The data suggest that Eu³⁺ ions are uniformly embedded into SnO₂ nanocrystallites either through substitution of Sn⁴⁺ ions or through formation of Eu‐F bonds, allowing for high‐level Eu³⁺ doping. Photoluminescence features such as transition intensity ratios and Stark splitting indicate diverse localization of Eu³⁺ ions in the SnO₂ nanoparticles, either in the crystalline lattice or in the grain boundaries. Due to formation of Eu‐F and Sn‐F bonds, the fluorinated surface of SnO₂ nanocrystallites efficiently inhibits the hydroxyl quenching effect, which accounts for their improved photoluminescence intensity.     

High-temperature thermoelectric properties of Ca3Co4O9+δ with Eu substitution

We have prepared polycrystalline Ca_3−xEu_xCo₄O_9+δ (x=0, 0.15, 0.3 and 0.45) samples using a sol-gel process followed by SPS sintering and investigated the Eu substitution effects on their high-temperature thermoelectric properties. With the Eu substitution, both the electrical resistivity and thermopower increase monotonously. This could be attributed to the decrease of hole concentrations by substitution of trivalent Eu³⁺ for divalent Ca²⁺. The Eu substituted samples (x=0.15, x=0.3) have lower thermal conductivity than Ca₃Co₄O_9+δ due to their lower electronic and lattice thermal conductivity. The dimensionless figure of merit ZT reaches 0.3 at 1000 K for the sample of Ca_2.7Eu_0.3Co₄O_9+δ.     

The luminescent properties of lanthanide ions activated BaBPO5 in VUV-Vis range

The VUV-Vis spectroscopic properties of barium borophosphate BaBPO₅ doped with lanthanide ions are investigated. The host absorption band is found to be around 150-160 nm, the charge transfer band of Eu³⁺ is observed at about 264 nm, the f-d transitions of Ce³⁺, Eu²⁺, and Tb³⁺ in the host lattice are depicted. The partial reduction of Eu³⁺ in air atmosphere is reported.     

Synthesis and photoluminescence characteristics of (Sr, Ca)3B2O6:Eu for application in white light-emitting diodes

A series of yellow-green (Sr, Ca)₃B₂O₆:Eu phosphors have been synthesized using precursors prepared via a facile sol-gel route. The solid-solution phases crystallized to materials with the formula of Sr_3−x−yCa_xEu_yB₂O₆ with varied Ca²⁺ and Eu²⁺ contents. The emission peak centered at 540 nm under near-UV excitation exhibited a broad-band distribution in the range of 450-650 nm. The dependences of the luminescence intensity on the contents of Ca²⁺ substitution and Eu²⁺ dopant were also investigated. The composition in the host lattice sensitively affected the chromaticity index. Sr_1.21Ca_1.7Eu_0.09B₂O₆ (SCB:0.09Eu) was shown to possess the highest intensity and broadest emission band. Calcining temperature was shown to greatly influence the luminescent properties of SCB:0.09Eu. It is concluded that SCB:0.09Eu can be used as an efficient yellow-green phosphor for white light-emitting diodes (white LEDs) applications.     

超声喷雾共沉淀法制备的Lu3Al5O12∶Eu3+ 纳米粉体发光特性

采用新型超声喷雾共沉淀法技术,以Lu₂O₃、Eu₂O₃、Al(NO₃)₃·9H₂O为原料,制备了不同浓度Eu³⁺离子掺杂的Lu₃Al₅O₁₂纳米粉体.用X射线粉末衍射表征了获得纳米粉体的相,用扫描电镜观察了纳米粒子的形貌.测定了粉体的激发光谱、⁷F₀-⁵D₂声子边带谱与发射光谱.研究了不同高温烧结温度与Eu³⁺掺杂浓度对纳米粒子的发光强度与粒子形貌的影响规律.研究表明,当烧结温度高于900 ℃时,粉体发光强度明显增强,并且随着煅烧温度的增加,发光强度有所增强.Eu³⁺离子的最佳掺杂浓度为5~7 mol%.根据稀土离子Eu³⁺光学跃起矩阵元的特点,从发射光谱获得Eu³⁺光学跃起的J-O参量Ω₂与Ω₄.在Eu³⁺掺杂浓度均为5 mol%时,其强度参量达最小,电-声子耦合最强.然后随着掺杂浓度的进一步提高,强度参量略有增加,电-声子耦合减弱.说明Eu-O键强增加,共价性增强,Eu³⁺的局域环境对称性降低.Ω₂值低于Eu³⁺在玻璃与晶体基质中的情况,这是由于纳米粒子中存在着大量的缺陷以及晶体的结构畸变导致纳米粒子的对称性下降所致.     

Optical properties of Eu(fod)3-doped polymers using supercritical carbon dioxide

X-ray diffraction pattern of solvated Eu(fod)₃(H₂O)_1,5(i-PrOH)_0,5 form of Eu(fod)₃ complex (fod=6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionate) is presented. The photoluminescence (PL) spectra of Eu³⁺ ions in polycrystalline powder Eu(fod)₃ and also in Eu(fod)₃-doped polymethylmethacrylate (PMMA), polypropylene (PP) and polydimethylsiloxane (PDMS) were investigated. It is revealed that the matrix influences the temperature quenching of PL intensity in the range 20-100 °C. A polycrystalline powder Eu(fod)₃ and Eu(fod)₃-doped PP are the most effective materials for PL quenching. It is shown that water molecules in the first coordinating sphere of Eu³⁺ ions increase PL intensity temperature quenching.     

Probing of surface Eu ions present in ZnO:Eu nanoparticles by covering ZnO:Eu core with Y2O3 shell: Luminescence study

Eu³⁺ doped ZnO nanoparticles are known to have significance extent of surface Eu³⁺ ions due to a large difference in ionic radii. Effect of such Eu³⁺ ions on the luminescence properties of ZnO:Eu nanoparticles has been understood from the luminescence studies of ZnO:Eu nanoparticles covered with Y₂O₃ shell. Based on the asymmetric ratio of luminescence and extent of energy transfer, it is established that when ZnO:Eu nanoparticles are covered with Y₂O₃ shell, a part of Eu³⁺ ions present with ZnO:Eu core migrate to Y₂O₃ shell and occupy Y³⁺ lattice positions.     

Luminescence and Tb-Ce-Eu ion energy transfer in single-crystalline films of Tb3Al5O12:Ce,Eu garnet

The paper is devoted to investigation of the processes of excitation energy transfer between the host cations (Tb³⁺ ions) and the activators (Ce³⁺ and Eu³⁺ ions) in single-crystalline films of Tb₃Al₅O₁₂:Ce,Eu (TbAG:Ce,Eu) garnet which is considered as a promising luminescent material for the conversion of LED's radiation. The cascade process of excitation energy transfer is shown to be realized in TbAG:Ce,Eu: (i) from Tb³⁺ ions to Ce³⁺ and Eu³⁺ ions; (ii) from Ce³⁺ ions to Eu³⁺ ions by means of dipole-dipole interaction and through Tb³⁺ ion sublattice.     

New promising phosphors Ba3InB9O18 activated by Eu/Tb

Borate Ba₃InB₉O₁₈ (BIBO) has been adopted as a host material for phosphors for the first time. Lanthanide ions (Eu³⁺/Tb³⁺)-doped BIBO phosphors have been synthesized by solid-state reaction and luminescent properties investigated under ultravoilet (UV) excitation. For red phosphor BIBO:Eu, dominant emission peaking at 590 nm was attributed to ⁵D₀→⁷F₁ transition of Eu³⁺, which confirmed that the local site of Eu³⁺ occupied by In³⁺ ion in BIBO crystal lattice is at inversion symmetry center. Optimum Eu³⁺ concentration of BIBO:Eu under UV excitation with 227 nm wavelength is around 40%. The green phosphor BIBO:Tb showed bright green emission at 550 with 232 nm light excited and optimal of Tb³⁺ concentration measured in BIBO is about 8%. The corresponding luminescence mechanisms of Ln-doped BIBO (Ln=Eu³⁺/Tb³⁺) were analyzed. The luminescent intensity of Tb³⁺ can be significantly improved by co-doping of Bi³⁺ in the BIBO:Tb lattice. The likely reason was proposed in terms of the different interactions of the host lattice with these ions, and of these ions with each other.     

Preparation and characterization of nanosized (Y,Bi)VO4:Eu and Y(V,P)O4:Eu red phosphors

Nanosized luminescent (Y,Bi)VO₄:Eu³⁺ and Y(V,P)O₄:Eu³⁺ were synthesized at low temperatures either by a coprecipitation method or by a hydrothermal method from aqueous solutions. The effect of Bi³⁺ ion or P⁵⁺ ion content in the lattice, annealing temperature effects on the crystal structure and the particle size, and the luminescence property of (Y,Bi)VO₄:Eu³⁺ and Y(V,P)O₄:Eu³⁺ nanoparticles were examined with a field-enhanced scanning electron microscopy, XRD, and a spectrofluorometer. The pristine YVO₄:Eu³⁺, (Y,Bi)VO₄:Eu³⁺, or Y(V,P)O₄:Eu³⁺ nanoparticles are 35-50 nm in size. The luminescence spectrum of the Eu³⁺ ion was used to probe its position in the crystal lattice. The dopant ions enter the same lattice sites in the nanocrystalline as in the corresponding bulk material, resulting similar spectral features between them. Photoluminescence intensity is weak for the pristine nanoparticles. Annealing the nanoparticles at temperatures up to 1000 °C results in the increased luminescence intensity (>80% of micrometer-sized phosphors) with the minimal particle growth and the improved particle crystallinity.     

An example of concentration sensitive electron-phonon coupling in {(C4H9)4N}3 EuxY1−x(NCS)6 and a new hypothesis for self-quenching

The strong vibronic one-phonon side-bands of the ⁵D₀ → ⁷F₀ emission of Eu³⁺ in {(C₄H₉)₄N}₃ Eu_xY_1?x(NCS)₆ are used to compute the Huang-Rhys electron-phonon coupling factor (S₀) of Eu³⁺ with the mode at 35 cm^-1. Increasing concentration from 1 to 100 is found to lead to a doubling of the electron-phonon coupling strength. Generalization of such an effect is proposed as a new hypothesis for part of the self-quenching process of rare-earth ions.     

Effect of Ca and Sr alkaline earth ions on luminescence properties of BaAl12O19:Eu nanophosphor

Nanosized barium aluminate materials was doped by divalent cations (Ca²⁺, Sr²⁺) and Eu²⁺ having nominal compositions Ba_1−xMxAl₁₂O₁₉:Eu (M=Ca and Sr) (x=0.1-0.5), were synthesized by the combustion method. These phosphors were characterized by XRD, scanning electron microscopy-energy-dispersive spectrometry (SEM-EDS) and photoluminescence measurement. The photoluminescence characterization showed the presence of Eu ion in divalent form which gave emission bands peaking at 444 nm for the 320 nm excitation (solid-state lighting excitation), while for 254 nm it gave the same emission wavelength of low intensity (1.5 times) compared to 320 nm excitation. It was also observed that alkaline earth metal (Ca²⁺ and Sr²⁺) dopants increase the intensity of Eu²⁺ ion in BaAl₁₂O₁₉ lattice, thus this phosphor may be useful for solid-state lighting.     

Enhanced electroluminescence of Eu by Tb in complexes Tb1−xEux(TTA)3Dipy

A series of rare earth ternary compounds of Tb_1−xEu_x(TTA)₃Dipy (HTTA=thenoyltrifluoroacetone, Dipy=2,2′-dipyridyl) have been synthesized, and the characteristics of the compounds have been performed by DTA-TG, IR, UV and fluorescence spectroscopy. Photoluminescence measurements indicated that the complexes of Eu(III) emit strong red luminescence under UV radiation. IR spectra suggest that complexes have been successfully synthesized, and TG curves indicate that the complexes are stable up to a temperature of about 220 °C. The Eu complex was blended with poly(N-vinylcarbazole) (PVK) and spin coated into films, and electroluminescence devices with the structure of Indium Tin Oxide (ITO)/PVK:Tb_1−xEu_x(TTA)₃Dipy/BCP(2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline)/aluminum quinoline (AlQ)/Al were fabricated, the luminescence of Eu³⁺ complexes enhances after doping with Tb³⁺. Therefore, it may be an effective method to improve the EL intensity of the lanthanide complex.     

Stability of divalent/trivalent oxidation state of europium in some Sr-based inorganic compounds

This work investigates the stability of Eu²⁺ and Eu³⁺ in some Sr-based inorganic compounds. Generally reducing condition is adopted in order to obtain Eu²⁺, however, the Eu doped SrAl₂O₄/SrLaAlO₄ case indicates that for some compounds Eu³⁺ is stabilized even in reducing atmosphere. Bond valence method is applied to explain this phenomenon and it reveals that crystal structure also determines the valence state of europium cations along with reducing/oxidizing condition. An analysis of other Eu doped Sr-based materials is performed which shows the relationship between Eu²⁺/Eu³⁺ stability and the Global Instability Index (GII). This research provides a guideline for synthesizing specific novel Eu²⁺/Eu³⁺ phosphors.     

Photoluminescence in MgxSr1−xAl2O4:Eu, Ndand electron-vibrational interaction in the Eu 5d states

Green phosphor compositions Mg_xSr_1−xAl₂O₄:Eu, Nd (with x=0.05-0.25) were prepared by solid state reaction method. The effect of Mg substitution on photoluminescence characteristics was investigated. The photoluminescence show intense green emission for MgSrAl₂O₄:Eu²⁺, Nd³⁺ with long persistence. This green emission corresponds to transitions from 4f⁶5d¹ to 4f⁷ of Eu²⁺ ion. Comparative analysis of the excitation and emission spectra were used to evaluate the crystal field splitting of the 5d states of Eu²⁺ and the parameters of electron-vibrational interaction, such as Huang-Rhys factor, effective phonon energy, and zero-phonon line position.     

Photoluminescence and thermoluminescence characterization of Eu- and Dy -activated Ca3(PO4)2 phosphor

Rare-earth-doped polycrystalline Ca₃(PO₄)₂:Eu, Ca₃(PO₄)₂:Dy and Ca₃(PO₄)₂:Eu,Dy phosphors prepared by a modified solid-state synthesis has been studied for its X-ray diffraction, thermoluminescence (TL) and photoluminescence (PL) characteristics. The PL emission spectra of the phosphor suggest the presence of Eu³⁺ ion in Ca₃(PO₄)₂:Eu and Dy³⁺ ion in Ca₃(PO₄)₂:Dy lattice sites. The TL glow curve of the Ca₃(PO₄)₂:Eu compounds has a simple structure with a prominent peak at 228 °C, while Ca₃(PO₄)₂:Dy peaking at 146 and 230 °C. TL sensitivity of phosphors are compared with CaSO₄: Dy and found 1.52 and 1.20 times less in Ca₃(PO₄)₂:Eu and Ca₃(PO₄)₂:Dy phosphors, respectively. The Ca₃(PO₄)₂:Eu,Dy phosphors shows switching behavior under two different excitation wavelengths and enhancement in PL intensity of Dy³⁺ ions were reported. The paper discusses the photoluminescence and thermoluminescence behavior of Eu³⁺ and Dy³⁺ ion in Ca₃(PO₄)₂ hosts, it may be applicable to solid-state lighting as well as thermoluminescence dosimetry applications.     

Synthesis and photoluminescence of Eu,Mg-alon phosphors by carbothermal reduction

Eu²⁺–Mg²⁺ co-doped alon phosphors were successfully prepared by carbothermal reaction at 1600 °C for 2 h and their structure and photoluminescence properties were studied. The lattice parameter increased with increasing Eu concentration, indicating the incorporation of Eu into alon crystal lattice. The pure alon phase was obtained when Eu concentration is below 0.2%. Compared with solid state reaction process, this method lead to less amounts of secondary phase, larger amounts of Eu²⁺ against Eu³⁺, smaller particle size, narrower particle size distribution and stronger luminescence intensity. The absolute emission intensity of obtained phosphor reached 82% relative to that of famous BAM: Eu²⁺ phosphor. The emission could be tuned widely from blue to green by varying carbon content.     

Photoluminescence studies on Eu and Ce-doped Li2SrSiO4

The luminescence of Li₂SrSiO₄: 0.01Eu, xCe (x=0.0025, 0.005, 0.0075, and 0.01) is studied as a potential ultraviolet light-emitting diode (UV-LED) phosphor that is capable of converting the ultraviolet emission of a UV-LED into white light with good luminosity. There are broad blue and yellow emissions peaked at 413 and 575 nm, respectively. The two emissions come from d-f transitions of Ce³⁺ and Eu²⁺, respectively. The emission intensity of Li₂SrSiO₄: 0.01Eu, xCe reaches its maximum at x=0.0075. The energy transfer from Ce³⁺ to Eu²⁺ is demonstrated to be the type of electric dipole-dipole interaction with considerable spectral overlap and nonradiative transition is calculated to dominate. The Commission International de I’Eclairage (CIE) chromaticity coordinates of Ce³⁺/Eu²⁺ substituted compounds is also discussed.