Time-resolved fluorescence measurements on Eu3+- and Eu2+-doped glasses

Various fluoride, phosphate and borosilicate glasses with known properties and global structure have been doped with Eu³⁺ (4f⁶) at doping concentrations between 10¹⁸ and 10²¹ cm^?3. By applying reductive melting conditions Eu³⁺ could partially be transformed to Eu²⁺ (4f⁷). Four fluoroaluminate glasses with varying phosphate content between 0 and 20 mol% (FPx), a pure phosphate glass (P100) and two borosilicate glasses with low (DURAN^®-like) and high optical basicity (NBS1) have been used for these investigations. The time-resolved fluorescence in the visible range has been studied for both ions. Although static and dynamic fluorescence of Eu³⁺ and Eu²⁺ are dramatically different (f–f-transitions for Eu³⁺; d–f-transitions for Eu²⁺), glass structure changes have a similar influence on the dynamic fluorescence behavior of both ions. A strongly ionic surrounding due to fluorine ligands as in fluoroaluminate glass samples provides the longest fluorescence lifetime (about 7 ms for Eu³⁺; about 1.3 μs for Eu²⁺). Increasing phosphate content decreases the fluorescence lifetime due to more oxygen ligands. Interesting differences have been found for the two borosilicate glasses due to the difference in their optical basicity (Na₂O/B₂O₃ ratio). Measurements indicate a homogeneous distribution of europium ions in most FP samples. NBS1 measurements suggest that two different local europium sites are formed. For Duran-like samples only one specific europium site was found, although these samples show phase separation at high doping concentrations into a SiO₂-rich phase and borate- and europium-rich droplets. Fluorescence quenching due to energy transfer from Eu²⁺ to Eu³⁺ could be found for co-doped samples; Eu³⁺-doped samples show no fluorescence quenching.     

Controlled chemical co-precipitation and solid phase synthesis,microstructure and photoluminescence of La3PO7:Eu3+ phosphors

Eu³⁺-doped oxy-phosphate (La₃PO₇:Eu³⁺) with monoclinic phase has been synthesized via solid phase and co-precipitation methods. The products present various regular morphologies after high temperature thermal treatment, such as bulk with holes, leaf-like flake, nano-rod and so on. All the phosphors exhibit the characteristic fluorescence of Eu ion, the electric dipole transition ⁵D₀→⁷F₂ of Eu³⁺ ions is dominant indicating that the sites of Eu³⁺ ions in La₃PO₇ have no the inversion center. Furthermore, the intensity of ⁵D₀→⁷F₂ increases due to the introducing of surfactant and increasing of the calcination temperature.     

Characterization of Eu3+-doped fluorophosphate glasses for red emission

Fluorescence spectra and decay curves of the ⁵D₀ level for different concentrations of Eu³⁺ (4f⁶) ions in K–Ba–Al fluorophosphate glasses have been measured at room temperature and are analyzed. The Judd–Ofelt intensity parameters Ω₂ and Ω₄ have been determined from the intensity ratios of emission peaks corresponding to ⁵D₀ → ⁷F_J (J = 2 and 4) to ⁵D₀ → ⁷F₁ transitions for 1.0 mol% glass. The intensity parameters thus obtained are in turn used to calculate the radiative properties of the fluorescent levels of Eu³⁺ ions. Second and fourth rank crystal-field parameters have been evaluated by assuming a C_2V site symmetry for the local environment of Eu³⁺ ions to estimate the crystal-field strength experienced by Eu³⁺ ions in the present host. The decay profiles of the ⁵D₀ → ⁷F₂ transition of Eu³⁺ ions in the present glasses are found to be single exponential for all the studied Eu³⁺ ion concentrations. A marginal increase in lifetime of the ⁵D₀ level has been noticed with Eu³⁺ ion concentration up to 2.0 mol% and then the lifetime marginally decreases for higher Eu³⁺ ion concentrations.     

Low-temperature precipitation in NaCl: Eu++ monocrystals

It has been shown that the macroscopic features of the as grown NaCl crystals containing up to about 140 m ppm of Eu⁺⁺ ions agree well with the main characteristics of the Na₂EuCl₄ phase present in these crystals. Thermal decomposition of the NaCl:Eu⁺⁺ solid solution leads to the formation of the mentioned Na₂EuCl₄ phase at temperatures not very much higher than 273 K. The kinetics of this process is governed by the pipe diffusion which leads to the growth of the small impurity aggregates already present in quenched samples.     

Impurity trapped exciton states related to rare earth ions in crystals under high hydrostatic pressure

Emission related to rare earth ions in solids takes place usually due to 4f ⁿ → 4f ⁿ and 4f ^n?15d ¹ → 4f ⁿ internal transitions. In the case of band to band excitation the effective energy transfer from the host to optically active impurity is required. Among other processes one of the possibilities is capturing of the electron at excited state and hole at the ground state of impurity. Localization of electron or hole at the dopand site creates a long range Coulomb potential that attracts the second carrier which then occupies the localized Rydberg-like states. Such a system can be considered as impurity trapped exciton. Usually impurity trapped exciton is a short living phenomenon which decays non-radiatively leaving the impurity ion in the excited state. However, in several compounds doped with Eu²⁺ the impurity trapped exciton states become stable and contribute to the radiative processes though anomalous luminescence that appears apart of the 4f ⁷ → 4f ⁷ and 4f ⁷5d ¹ → 5f ⁷ emission. In this contribution pressure effect on energies of the 4f ^n?15d ¹→5f ⁿ transitions in Ln doped oxides and fluorides as well as influence of pressure on the energy of impurity trapped exciton states is discussed. The latest results on high pressure investigations of luminescence related to Pr³⁺, and Eu²⁺ in different lattices are reviewed.     

Spectral analysis of a novel Eu2+ activated SiO2–BaO glass ceramics phosphor under UV-LED excitation

A novel Eu²⁺ activated 60SiO₂–40BaO (mol%) glass ceramics phosphor was prepared and the optical properties were investigated. X-ray diffraction (XRD) and Raman spectra confirmed the formation of Ba₂Si₃O₈ nano-crystals in the glass matrix. The Eu²⁺ activated glass ceramics exhibited broad emission band centered at 518 nm due to the 4f⁶5d¹→4f⁷ transition of Eu²⁺. Compared with the glass, the emission intensity of Eu²⁺ activated glass ceramics was much stronger, and the peak wavelength shifted toward shorter wavelength. The photoluminescence excitation (PLE) spectra of the glass ceramics showed an overlap with the main emission region of an ultraviolet light-emitting diode (UV-LED). According to the photoluminescence (PL) spectra, the CIE chromaticity coordinates of the Eu²⁺ activated glass and glass ceramics were calculated. The results indicated that the Eu²⁺ activated glass ceramics containing Ba₂Si₃O₈ nano-crystals can be used as a potential green emitting phosphor under UV-LED excitation.     

Intense luminescence of amorphous Eu2O3 prepared by aqueous sol–gel method

Amorphous Eu₂O₃ was prepared by an aqueous sol–gel method. Emission due to the ⁵D₀ → ⁷F_J (J = 0, 1, 2) transitions of Eu³⁺ ions were observed. The dominant transition was the ⁵D₀ → ⁷F₂ red emission of Eu³⁺. The properties of the as-prepared samples were different with changes in the annealing temperature. To investigate the luminescence properties of the amorphous Eu₂O₃, the temperature-dependent photoluminescence (PL) spectra of samples annealed at 600 °C were measured in the temperature range 77–300 K. PL peak positions were unchanged with the change of temperature.     

Thermal and optical investigation of EuF3-doped lead fluorogermanate glasses

Europium-doped lead germanate and lead fluorogermanate glasses are studied by using differential thermal analysis, X-ray diffraction, photoluminescence and fluorescence lifetimes measurements of the ⁵D_j, j = 0, 1, 2 levels. PbF₂ addition increases the thermal stability of the lead germanate glass, while Eu³⁺ ions promote the crystallization of β-PbF₂:Eu³⁺ nano-crystals embedded in a glassy matrix. In the lead fluorogermanate glasses, Eu³⁺ ions exhibit a strong affinity for F⁻ ions although oxygen ions are much more numerous. It appears that luminescence concentration quenching is not important, while cross relaxation is very efficient in the glasses. The results allow to propose for these glasses a molecular model in which small fluorine rich island, incorporating the Eu³⁺ ions in low symmetry sites, are separated from each other by chains of germanate (GeO₄)⁴⁻ ions linked together.     

Preparation and phase characterization of the SrCl2?EuCl2 and SrCl2?EuCl3 systems

The SrCl₂ EuCl₂ and SrCl₂ EuCl₃ systems were investigated over the full composition ranges by the Guinier powder X-ray diffraction technique. In the strontium chlorideeuropium dichloride system the solubility from the both ends of reactants was found when fast cooling of the melt was used. In the strontium chloride-europium trichloride system a solid solution for the composition region up to 20 mol per cent of EuCl₃ and two new intermediate chlorides, Sr₄EuCl₁₁ and Sr₉Eu₅Cl₃₃, were observed. The compounds crystallize in orthorhombic symmetry with lattice parameters of 7.220(2), 35.15(1), 6.790(4) and hexagonal 12.854(4) and 24.702(8), respectively.     

Thermal Expansion of EuF<Subscript>2 + <Emphasis Type="Italic">x</Emphasis></Subscript> Single Crystals and Their Thermal Stability

Thermal expansion of an EuF_2.136 nonstoichiometric crystal with the fluorite structure type (Eu _0.864 ²⁺ Eu _0.136 ³⁺ F_2.136, lattice parameter 5.82171(5) Å) has been experimentally investigated in the temperature range of 9–500 K. The coefficient of thermal expansion is α = 15.8 × 10^–6 K^–1 at T = 300 K. The observed anomalies in the behavior of the coefficient of thermal expansion at T > 400 K are related to the oxidation processes with partition of Eu²⁺ ions. It is established by differential scanning calorimetry that the onset temperature of EuF_{2 + x} oxidation in air is 430 K and that this process occurs in three stages. X-ray diffraction analysis shows that the oxidation is accompanied by the formation of a phase mixture based on two modifications of the Eu _{1– y} ³⁺ Eu _y ²⁺ F_3–y solid solution with the structure types of tysonite (LaF₃), orthorhombic β-YF₃ phase, and europium oxyfluorides of variable composition EuO_1–xF_{1 + 2x}, with dominance of the latter.     

Liquid Crystalline and Luminescent Behavior of Lanthanide Complexes Composed of Terbium or Europium and Dendritic Amphiphile

Luminescent lanthanide (Tb³⁺, Eu³⁺) complexes were prepared by reaction of Tb³⁺ or Eu³⁺ with dendritic amphiphile (3,4,5-tris(dodecyloxy)benzoate), then their liquid crystalline and luminescent behavior were studied. The complexes exhibited thermotropic liquid crystalline mesophases with columnar hexagonal structure. The complexes emitted the characteristic luminescence of the core metals. Additionally, in the Eu³⁺ complex, the ratio of the intensity of 614 nm to the intensity of 585 nm (I₆₁₄/I₅₈₅) remarkably decreased around the transition temperature from crystal to mesophase, suggesting that the coordination environment of Eu³⁺ gets more symmetrical due to the phase transition.     

Preparation of crystalline fibres of codoped BaAl2O4:Eu2+:Cr3+

Phosphor material BaAl₂O₄:Eu²⁺, Cr³⁺ with varying concentrations of Cr co‐doping were prepared by solid‐state synthesis method. Crystalline fibres were obtained by controlled annealing temperature. Synthesized compositions were characterized for their phase and crystallinity by powder x‐ray diffraction. The crystalline morphology was investigated using SEM analysis. High resolution transmission electron microscopy (HRTEM) in image and diffraction modes was used to investigate the microstructure. The effect of Cr doping on quality and morphology of grown crystals was investigated. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Energy transfer from Bi3+ sensitizing the luminescence of Eu3+ in clusters embedded into sol–gel silica glasses

Y³⁺(La³⁺), Eu³⁺ and Bi³⁺ ions co-doped sol–gel silica glasses were synthesized. Photoluminescence spectra show that there is energy transfer from Bi³⁺ ions to the emission band of Eu³⁺ ions. The co-dopants Y³⁺ or La³⁺ have strong effects on the local structure and luminescence of Eu³⁺ ions. For 0.5 mol% Eu³⁺ ions doped glasses, the co-doping of 1 mol% Bi³⁺ and 1 mol% Y³⁺ is the most appropriate for the sensitization from Bi³⁺ to Eu³⁺. The sensitization effectiveness from Bi³⁺ ions to Eu³⁺ ions was studied by changing the amount of Bi³⁺ and Y³⁺, and clusters containing rare earth ions and Bi³⁺ ions dominate the energy transfer processes. The comparison of luminescent R-values (the intensity ratio of ⁵D₀ → ⁷F₂/⁵D₀ → ⁷F₁ in Eu³⁺ ions) between glasses containing La³⁺ and containing Y³⁺ verifies the formation of clusters in sol–gel glasses. As a favorable configuration for energy transfer, the accurate design and synthesis of clusters-contained glasses may provide a new kind of luminescent materials.     

Eu2+/Dy3+ co-doped white light emission glass ceramics under UV light excitation

In the present work, transparent Eu²⁺/Dy³⁺ co-doped white light-emitting glass ceramics containing CaF₂ were successfully prepared under reducing atmosphere. Their luminescence properties have been studied by excitation and emission spectra. A combination of blue, yellow and red emissions has emerged in Eu²⁺/Dy³⁺ co-doped glass and glass ceramics, which allows the observation of bright white light when the samples are excited by the ultraviolet light. Energy transfer (ET) from Eu²⁺ to Dy³⁺ is discovered by directly observing the luminescence intensity of Dy³⁺ in the Eu²⁺/Dy³⁺ co-doped glass which is much stronger than that in the Dy³⁺-doped glass. ET from Eu²⁺ to Dy³⁺ in glass is further confirmed by fluorescence studies performed on the samples with various activator (Dy³⁺) concentrations. The color of luminescence could be adjusted by varying the proportions of europium and dysprosium. The optimal composition generates white light with chromaticity coordinates (0.296, 0.311). The results indicate that Eu²⁺/Dy³⁺ co-doped glass ceramics containing CaF₂ nano-crystals is a potential material for white LED.     

Local field dependent fluorescence properties of Eu ions in a fluorometaphosphate laser glass

A fluorometaphosphate laser glass doped with Eu³⁺ ions has been synthesized and studied by broad band optical spectroscopy and time-resolved fluorescence line narrowing techniques in order to explore the local field dependent fluorescence properties of the lanthanide ions in this host. From the Raman and the vibronic spectra, various structural phosphate groups coupled to the Eu³⁺ ions have been identified. Local field dependent ⁵D₀ → ⁷F_J (J = 0-6) emission spectra and the lifetimes of the ⁵D₀ level have been measured under resonant excitation of the Eu³⁺ ions at different wavelengths within the ⁷F₀ → ⁵D₀ band at 16 K. From these data and using the Stark level positions of the ⁷F₁ and ⁷F₂ multiplets, a crystal-field analysis has been carried out assuming a C_2v orthorhombic local symmetry. The radiative Judd-Ofelt parameters have been calculated for the different local fields found in the glass and their relative variation has been discussed. The results obtained suggest the existence of a relatively narrow distribution of local fields generated by successive distortions of a unique kind of site for all the Eu³⁺ ions in this fluorometaphosphate glass.     

Luminescent nano-composites generated from a spray

The spray-pyrolysis (SP) synthesis technique has been employed to obtain SiO₂:Eu³⁺ and γ-AlOOH:Eu³⁺. It was possible to obtain sub-micrometric spherical particles of SiO₂ with luminescent Eu³⁺ ions bonded to the silica surface or embedded in amorphous silica beads, by controlling the synthesis and annealing process. Boehmite γ-AlOOH doped with Eu³⁺ nanoparticles were synthesized by SP at moderate temperature (200 °C) with Eu³⁺ ions bonded to the surface hydroxyls of the boehmite nanocrystals. Luminescent nano-composites were obtained by controlled reaction of γ-AlOOH:Eu³⁺ nanocrystals with ASN (asparagine). In these nano-composites, the Eu³⁺ are held at the surface of the boehmite nanocrystals and partially shielded from interactions with additional luminescence quenchers (hydroxyl groups, water molecules).     

A facile route to synthesize luminescent YVO4:Eu3+ porous nanoplates

Porous nanoplates made of yttrium orthovanadate doped with europeum (YVO₄:Eu³⁺) have been successfully synthesized via a chemical co-precipitation method using NH₄VO₃, Y₂O₃, Eu₂O₃ and ethylene glycol. To investigate the effect of temperature on the pore size and morphology of the final product, the as-synthesized YVO₄:Eu³⁺ nanoplates were subjected to heat treatment at 300 and 600 °C for 2 h. The obtained samples were characterized by X-ray diffractometion, Fourier-transform infrared spectroscopy, transmission electron microscopy, high-resolution transmission electron microscopy and photoluminescence spectroscopy. The experimental results showed that the luminescent properties were significantly enhanced with increasing pore size of the YVO₄:Eu³⁺ porous nanoplates.     

Mössbauer study of europium in fluorozirconate glass

¹⁵¹Eu Mössbauer spectra of fluorozirconate glasses of nominal composition 0.61 ZrF₄ · (0.32 ? x) BaF₂ · x EuF₂ · 0.07 ThF₄ with x = 0.10 and 0.20 show that Eu²⁺ is present in sites with isomer shifts in the range ?15.5 to ?12.9 mm/s, corresponding to EuF bond lengths of 2.3–2.8 Å and a coordination number ranging from 8 to 12. The glasses contain a minor amount (～10%) of Eu³⁺ in well-defined sites. Effective Debye temperatures deduced from the recoilless fractions for the two ions are θ_D²⁺ = 145 K and θ_D³⁺ = 261 K. The differences in coordination and bonding indicate that trivalent eurpoium is a network former, whereas the divalent ions are network modifiers.     

Reduction and luminescence of europium ions in glass ceramics containing SrF2 nanocrystals

Reduction of Eu³⁺ → Eu²⁺ and luminescence of europium (Eu) ions in glass ceramics containing SrF₂ nanocrystals have been investigated. The formation of SrF₂ nanocrystals in glass ceramics was confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Blue luminescence of the Eu²⁺ ions was observed in the Eu doped glass ceramics which were prepared by the heat treatment of the glass in air atmosphere. The double-exponential decay curves of ⁵D₀ state of Eu³⁺ in the Eu doped glass ceramics indicated that there were two different surroundings of the Eu ions in the glass ceramics.     

Line width measurements of Cr in a zinc borate glass

The temperature dependence of the homogeneous line width (Γ_h) of the R-line of Cr³⁺ in 4ZnO·3B₂O₃ glass in the region between 15 K and room temperature has been measured. We found that Γ_h has a T ² dependence down to approximately 30 K. Below 30 K the data do not follow the power law, T ². The results are compared with those of Cr³⁺ in mullite and ED2 silicate glasses where the crossover was observed at about 80 K and with the data for Eu³⁺ in zinc borate and other glasses. The comparison indicates that the local environment affects Γ_h. In fact, the Cr³⁺ ions appear to form more defined complexes with the oxygen ligands than do the Eu³⁺ ions, which have a larger distribution of sites in the glass.