Effect of annealing temperature on the energy transfer in Eu-doped ZnO nanoparticles by chemical precipitation method

Eu-doped ZnO nanoparticles were synthesized by the chemical precipitation method and the annealing temperature effect on the structures and photoluminescence (PL) properties of the nanoparticles were briefly investigated. The X-ray diffraction and energy dispersive spectroscopy results indicated that the Eu³⁺ was successfully incorporated into the crystal lattice of ZnO host when the annealing temperature was fixed at 400 °C, but the Eu³⁺ ions were partly precipitated from the host with the annealing temperature increasing. The as-obtained ZnO: Eu nanocrystals composed of nanoparticles had an average size of 10 nm, and the valence states of europium ions in the nanocrystals were determined as tervalent. PL spectroscopy indicated that the characteristic red emissions of Eu³⁺ ions were attributed to the ⁵D₀ → ⁷F₀, ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ transitions, respectively. Moreover, the annealing temperature was found to have effect on the red emission of Eu³⁺ ions. That is to say, the energy transfer in the doped nanocrystals could be adjusted by different annealing temperatures.     

Behaviour of the EuD0 → F0 transition in CaMoO4 powellite type ceramics under Ar and Pb ions implantation

In this work, the Eu³⁺⁵D₀ → ⁷F₀ transition is used as a structural probe to follow the Eu³⁺ environment modification in powellite CaMoO₄ under irradiation or when its composition is varying. Six ceramics with compositions ranging from Ca_0.99Eu_0.01MoO₄ to Ca_0.76Sr_0.1Na_0.07 Eu_0.01La_0.02Nd_0.02Pr_0.02 MoO₄ were synthesized and each composition has a specific Eu³⁺ luminescence signal. The ⁵D₀ → ⁷F₀ transition appeared to be a very sensitive structural probe. Even if there is only one structural site in powellite for europium, the Eu³⁺⁵D₀ level position changes with the modification of the Eu³⁺ cationic neighbours. Low ⁵D₀ values are observed for environments containing trivalent rare earth elements when high ⁵D₀ values are observed for Na⁺ rich environments. Under 8 MeV Ar ions irradiation, the Eu³⁺⁵D₀ → ⁷F₀ transition is not really affected. Under 108 MeV Pb ions irradiation there is an homogenization of the Eu³⁺⁵D₀ → ⁷F₀ transition from the different irradiated samples revealing a reorganization of the crystalline structure.     

Cathodoluminescence emission spectra of trivalent europium-doped yttrium oxysulphide

Europium-doped yttrium oxysulphide phosphor (Y₂O₂S:Eu³⁺) was prepared. Y₂O₂S:Eu³⁺ was found to have a hexagonal structure with lattice parametersa=0.3776 nm andc=0.6538 nm. The Cathodoluminescence emission spectra of Y₂O₂S:Eu³⁺ were recorded between 400 and 635 nm at both 77 and 293 K. Most of the numerous emission lines were assigned on the basis of known energy levels of Eu³⁺ ions, crystal field splittings, and selection rules. More attention was paid to the high-intensity emission lines in the spectral red region, because this phosphor can be used as an efficient red-emitting phosphor in colour televisions. The highly intense lines at 611.3, 616.9 and 626.5 nm are attributed to the transitions from⁵ D ₀ to one of the Stark components of the⁷ F ₂ level. The other transitions from⁵ D ₁ ⁵ D ₂ and⁵ D ₃ to⁷ F _J (J=0, 1–5) were found and assigned.     

Luminescence properties of Eu ions in K2YF5 crystals

Luminescence properties of Eu³⁺ ions (0.1, 1.0 and 6.0 mol%) in K₂YF₅ crystals are investigated by the site-selective laser-excitation spectroscopy. The excitation spectrum of the ⁵D₀ emission exhibits two strong ⁷F₀ → ⁵D₀ excitation lines together with four weaker lines. Six different emission spectra are obtained under the excitation at each excitation line indicating that there existed at least six different sites for Eu³⁺. The relative intensity of the excitation lines is dependent on the Eu³⁺ concentrations. The ⁵D₀ emissions of Eu³⁺ show two types of decay behavior with lifetimes of 0.5 and 5.0 ms at 15 K. It is suggested that the fast and slow decays correspond to isolated and paired Eu³⁺ ions in K₂YF₅. The results are discussed in relation with one dimensional chain structure of the K₂YF₅ lattice.     

Luminescence and structural properties of Eu3+ doped BaY2ZnO5 for LED solid-state lighting application

Eu³⁺ doped BaY_2(1?x)ZnO₅ phosphor was successfully synthesized by a single step solution combustion process. The crystal structure and particle morphology were investigated by X-ray diffraction (XRD), scanning electron microscopy and transmission electron microscopy. The XRD results suggest that BaY₂ZnO₅ crystallizes in a single phased orthorhombic structure with space group Pbnm at 1,100 °C. The phosphor can be effectively excited by near-UV light, emitting intense red luminescence (628 nm) corresponding to the hypersensitive ⁵D₀ → ⁷F₂ transition of Eu³⁺ ions, located at low-symmetry site with no inversion center in BaY₂ZnO₅ crystal lattice. Fluorescence decay analysis was carried out to understand the energy transfer mechanism and quenching behavior of luminescence of Eu³⁺ ions in the BaY₂ZnO₅ phosphor. The BaY₂ZnO₅: Eu³⁺ emission (λ_ex = 395 nm) could be tuned from blue to white and red light by varying the Eu³⁺ ions concentration, making this phosphor as a promising candidate for LEDs application.     

Investigation of spectral properties of Eu<Superscript>3+</Superscript> and Tb<Superscript>3+</Superscript> doped strontium zirconium trioxide orthorhombic perovskite for optical and sensing applications

In this paper, the structural, morphological and spectral properties of Eu³⁺ and Tb³⁺ doped strontium zirconium trioxide perovskite phosphors are reported. The samples were synthesized by solid state reaction route with different doping concentrations of Eu³⁺ and Tb³⁺ ions. These synthesized phosphors were characterized by PXRD for structural analysis. The phosphors report orthorhombic structure with average crystallite size of 48 nm. FESEM and HRTEM analysis were done here for topographical and morphological studies. Also, the FTIR spectra of synthesized samples were investigated for functional group analysis. Photoluminescence and thermoluminescence spectra of synthesized samples were studied. On subjecting to 230 nm excitation, the phosphors give three distinct emissions of 596, 610 and 690 nm in the visible region corresponding to ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₄ of Eu³⁺ ions. The synthesized samples were also subjected to CIE and Afterglow decay analysis. The average decay lifetime is recorded as 56.24 ns confirming the luminescence decay characteristics of short duration. In TL analysis of these phosphors, second-order kinetics with low activation energy varying from 0.50002 to 0.65668 eV is reported. The enhanced optical characteristics of prepared perovskite phosphor substantiate it as a proficient alternative for photovoltaic, optical and sensing applications.     

Effect of symmetry reduction on the electronic transitions in polytypic GdAl3(BO3)4:Eu:Tb crystals

The existence of a recently described monoclinic phase (C2/c, Z = 8) (Beregi et al., 2012) in addition to the well-known Huntite type rhombohedral (R32) polytypic modification of the GdAl₃(BO₃)₄ (GAB) crystal at room temperature provides a unique possibility to investigate the incorporation of rare earth dopants into slightly modified crystal lattice by spectroscopic methods. In these characteristic GAB structures the dopant ions, e.g. Tb³⁺ or Eu³⁺, possess slightly different neighbor geometries and local symmetries. The Tb³⁺: ⁷F₆ → ⁵D₄ and Eu³⁺: ⁷F_0,1,2 → ⁵D_0,1,2 electronic transitions were successfully identified in the absorption spectra using polarization, concentration and temperature dependent measurements in both polytypic modifications. The positions of the investigated Tb lines are shifted by up to 10 cm⁻¹ due to symmetry changes. In addition, some of the Eu lines show splittings of about 4–30 cm⁻¹ as a consequence of the change of the local environment. From the room temperature absorption measurements some of the low energy crystal field levels of ⁷F and ⁵D states of the Eu³⁺ ions were successfully determined for both modifications.     

Electrospinning preparation and photoluminescence properties of poly (methyl methacrylate)/Eu3+ ions composite nanofibers and nanoribbons

Nanofibers and nanoribbons of poly (methyl methacrylate) (PMMA)/Eu³⁺ ions composites with different concentration of Eu³⁺ ions were successfully prepared by using a simple electrospinning technique. From the results of scanning electron microscopy and energy-dispersive X-ray spectroscopy, we found that the morphology of the as-electrospun PMMA/Eu³⁺ ions composites could be changed from fiber to ribbon structure by adjusting the concentration of Eu³⁺ ions in the electrospun precursor solution. The coordination between the Eu³⁺ ions and PMMA molecules were investigated by Fourier transform infrared spectroscopy and differential thermal analysis. The photoluminescence (PL) properties of the as-electrospun PMMA/Eu³⁺ ions composites were studied in comparison to those of the Eu(NO₃)₃ powder. It was showed that the ⁵D₀–⁷F_J (J = 0, 1, 2, 3, 4) emission appeared in the PL spectra of the as-electrospun PMMA/Eu³⁺ ions composites, whereas the ⁵D₀–⁷F₀ emission was completely absent in the PL spectra of Eu(NO₃)₃ powder due to the different local environments surrounding Eu³⁺ ions. It was interesting to note that the intensity ratios of the electric–dipole and magnetic–dipole transitions for the PMMA/Eu³⁺ ions composites could be enhanced significantly by increasing electrospinning voltage.     

Hydrothermal synthesis and luminescent properties of GdVO4 : Eu3+ nanophosphors

Eu³⁺-doped GdVO₄ has been synthesised via hydrothermal method by altering the hydrothermal temperature, reaction time and surfactant. The microstructure and morphology information of the phosphors were investigated via the techniques of X-ray powder diffraction and scanning electronic microscopy, which show that the phosphors wear tetragonal phase and the products present various regular morphologies under different reaction conditions such as bulk and nanoparticle. Moreover, the morphologies of the products have been controlled by altering reaction temperature. In addition, the surfactant was also included to control the morphologies of the products and the phosphors present different morphologies. All the phosphors exhibit the characteristic fluorescence of Eu ion (⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₂). The electric dipole transition ⁵D₀ → ⁷F₂ of Eu³⁺ is dominant indicating that most sites of Eu³⁺ ions in GdVO₄ have no inversion centre. Furthermore, we found that the reaction time and the morphologies have great influence on optical properties.     

Structure and luminescence properties of Eu/Tb codoped oxyfluoride glass ceramics containing Sr2GdF7 nanocrystals

Eu/Tb codoped transparent oxyfluoride borosilicate glass ceramics containing Sr₂GdF₇ nanocrystals were fabricated under a reductive atmosphere and the conversion of Eu³⁺ ions to Eu²⁺ ions was observed. The Sr₂GdF₇ nanocrystals with an average size of 32 nm were homogeneously precipitated in the oxyfluoride borosilicate glass matrix, which could be evidenced by X-ray diffraction (XRD), transmission electron microscopy (TEM) and energy-dispersive X-ray (EDX) spectroscopy. The enhancement of photoluminescence emission intensity, reduction of the relative emission intensities between ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₁, and long fluorescence lifetimes of Eu²⁺, Eu³⁺, and Tb³⁺ ions revealed that more rare earth ions were partitioned into the low phonon energy environment Sr₂GdF₇ nanocrystals. Under ultraviolet excitation, pure and bright white light emission was obtained in the oxyfluoride borosilicate glass ceramic, which may be a potential blue, green and red-emitting phosphor for white LEDs.     

Optical investigation of Eu:PbF2 ceramics and transparent glass-ceramics

This paper reports an optical investigation of glass-ceramics formed by annealing glasses with compositions 50 GeO₂-40 PbO-10 PbF₂-x EuF₃, x = [0.5; 1; 1.5; 2] and polycrystalline ceramics with composition 100 PbF₂, y EuF₃, with y = 5, 10, 15 and 20. For each material, the photoluminescence spectrum and the photoluminescence lifetimes of the ⁵D₀, ⁵D₁ and ⁵D₂ Eu³⁺ levels are measured. Occurrence of Eu³⁺:β-PbF₂ nanocrystallites in the glass-ceramics is confirmed and total ceramisation requires more than 10% of EuF₃ with respect to PbF₂ in the starting glass.In the Eu³⁺:β-PbF₂ ceramics and glass-ceramics, Eu³⁺ ions replace Pb²⁺ in their regular cubic site, but they interact together to form dimers and higher nuclearity clusters. These two species are easily distinguished according to their photoluminescence decay rate. For the EuF₃ rates investigated here, there are no isolated Eu³⁺ ions in the PbF₂ lattice.A preliminary investigation of the optical properties of co-doped Gd³⁺:Eu³⁺:β-PbF₂ ceramics was also performed. It shows that mixed Gd³⁺-Eu³⁺ dimers and clusters are formed, and that efficient Gd³⁺ → Eu³⁺ energy transfer occurs in these ceramics. The Pb²⁺ ions of the lattice may also be involved in the energy transfer process.     

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.     

Branch-shaped NaGdF4:Eu3+ nanocrystals: Selective synthesis,and photoluminescence properties

The branch-shaped NaGdF₄:Eu³⁺ nanocrystals (NCs) were synthesized by using polyvinylpyrrolidone (PVP) as a capping agent in ethylene glycol (EG) solution. The NCs were readily dispersed into water or ethanol to form a relatively stable suspension, which may facilitate their applications in biological fields. Meanwhile, the crystal structures of the NCs were tunable from the mixture of the α-(cubic) and β-(hexagonal) phases to the pure β-phase by varying the F^?/Ln³⁺ molar ratio or the reaction temperature. The pure β-phase NCs were obtained at relatively high F^?/Ln³⁺ molar ratio and reaction temperature. In addition, the Eu³⁺-doping concentration—dependent optical properties of the NaGdF₄:Eu³⁺ NCs were investigated in detail. The result shows that the emissions from high energy level transitions (e.g., ⁵D₁, ⁵D₂, and ⁵D₃) are significantly impaired with increasing the Eu³⁺-doping concentration due to the cross-relaxation process, and the emission at 612 nm is predominant since the doped Eu³⁺ ions locate in the crystal fields without inversion center.     

Synthesis and optical properties of a lithium niobiosilicate glass doped with europium

The sol–gel method was used to prepare a lithium niobiosilicate glass doped with europium ions, 91SiO₂–4Li₂O–4Nb₂O₅–1Eu₂O₃ (% mole). The dried gel was heat-treated between 500 °C and 750 °C and glass ceramics were obtained. X-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman studies show that crystallization starts at temperatures above 600 °C. The LiNbO₃ and EuNbO₄ nanocrystals were found in the heat-treated samples as corroborated by the structural, morphological and optical properties. Room temperature photoluminescence (PL) studies allow us the identification of multiple Eu-related optical centres and the strongest red ⁵D₀ → ⁷F₂ intraionic transitions occur in the samples heat-treated at 600 °C where the LiNbO₃ crystal phase was detected.     

Strong luminescence and efficient energy transfer in Eu3+/Tb3+-codoped ZnO nanocrystals

Single crystalline Eu³⁺/Tb³⁺-codoped ZnO nanocrystals have been synthesized by using a simple co-precipitation method. Successful doping is realized so that strong green and red luminescence can be efficiently excited by ultraviolet and near ultraviolet radiation, demonstrating an efficient energy transfer from ZnO host to rare earth ions. The energy transfer from the ZnO host to Tb³⁺ in ZnO: Tb³⁺ samples and ZnO host to Eu³⁺ in the ZnO: Eu³⁺ samples under UV excitation are investigated. It is found that the red ⁵D₀ → ⁷F₂ emission of Eu³⁺ ions decreases with increasing temperature but the green ⁵D₄ → ⁷F₅ emission of Tb³⁺ ions increases with increasing temperature, implying a different energy transfer processes in the two samples. Moreover, energy transfer from Tb³⁺ ions to Eu³⁺ ions in ZnO nanocrystals is also observed by analyzing luminescence spectra and the decay curves. By adjusting the doping concentration, the Eu³⁺/Tb³⁺-codoped ZnO phosphors emit green and red luminescence with chromaticity coordinates near white light region, high color purity and high intensity, indicating that they are promising light-conversion materials and have potential in field emission display devices and liquid crystal display backlights.     

Photoluminescence of Eu<Superscript>3+</Superscript> ion in SnO<Subscript>2</Subscript> obtained by sol–gel

Photoluminescence data of Eu-doped SnO₂ xerogels are presented, yielding information on the symmetry of Eu³⁺ luminescent centers, which can be related to their location in the matrix: at lattice sites, substituting to Sn⁴⁺, or segregated at particles surface. Influence of doping concentration and/or particle size on the photoluminescence spectra obtained by energy transfer from the matrix to Eu³⁺ sites is investigated. Results show that a better efficiency in the energy transfer processes is obtained for high symmetry Eu³⁺ sites and low doping levels. Emission intensity from ⁵D₀→⁷F₁ transition increases as the temperature is raised from 10 to 240 K, under excitation at 266 nm laser line, because in this transition the multiphonon emission becomes significant only above 240 K. As an extension of this result, we predict high effectiveness for room temperature operation of Eu-based optical communication devices. X-ray diffraction data show that the impurity excess inhibits particle growth, which may influence the asymmetry ratio of luminescence spectra.     

A novel single-phased white light emitting phosphor of Eu ions-doped Ca2LaTaO6

A novel single-phased white light emitting phosphor, Ca₂(La_1−xEu_x)TaO₆ (x = 0.001–0.7), was synthesized using a vibrating milled solid state reaction. The results indicate that the emission spectra of Ca₂LaTaO₆:Eu³⁺ samples exhibit a series of shaped peaks assigned to the ⁵D₀ → ⁷F_J (J = 0, 1, 2, 3, 4) transitions under an excitation of 395 nm. The characteristic peaks of Eu³⁺ ion intra-4f transitions from excited states to lower levels include ⁵D₃ → ⁷F_J (J = 1, 2, 3), ⁵D₂ → ⁷F_J (J = 0, 1, 2, 3), ⁵D₁ → ⁷F_J (J = 1,2,3), and ⁵D₀ → ⁷F_J (J = 0, 1, 2, 3, 4) transitions. The higher level ⁵D_J (J = 1, 2, 3) emission depends on the concentration of Eu³⁺ ions. The emission spectra shows a completely different ratio between the ⁵D_3,2,1 blue-green emission and ⁵D₀ red emission for lower and higher Eu³⁺ concentrations. For lower Eu³⁺ ion concentrations, the dominant transition of Ca₂(La_1−xEu_x)TaO₆ phosphors are ⁵D_3,2,1,0 → ⁷F_J emissions, whereas the dominant transitions are ⁵D₀ → ⁷F_J emissions for higher Eu³⁺ ion concentrations. The chromaticity coordinate of the Ca₂(La_1−xEu_x)TaO₆ phosphor varies with the Eu³⁺-doped concentrations from white, to reddish orange, and red. Thus, this type of phosphor may be potentially applicable as a white light emitting phosphor for ultraviolet light-emitting diodes.     

The effect of Eu-activated InVO4 phosphors prepared by sol-gel method

Indium orthovanadate (InVO₄) doped with Eu³⁺ ions had been synthesized by sol-gel method. The precursor of InVO₄:Eu³⁺ powders were heated at 950 °C for 6 h in air, and the crystal structure, surface morphologies and photoluminescence properties were also investigated. XRD patterns indicated that the crystallinity of InVO₄:Eu³⁺ powders decreases with increasing Eu³⁺ ion concentrations. From the SEM micrographs, the shapes of the InVO₄ particles are uniform and like pebbles. With increasing Eu³⁺ ion concentrations, the shapes of the InVO₄:Eu³⁺ particles become smaller and irregular. In the PL studies, the sharp excitation peaks between 300 and 600 nm correspond to the Eu³⁺ intra-4f transitions. Excitation at 326 nm in terms of Eu³⁺ concentrations in (In_1−xEu_x)VO₄ powders shows that the (In_1−xEu_x)VO₄ phosphors display bright red luminescence at about 615 nm belonging to the ⁵D₀ → ⁷F₂ electric dipole transition, and the time-resolved ⁵D₀ → ⁷F₂ transition presents a single exponential decay behavior. The concentration quenching is active when the Eu³⁺ concentration is larger than 30 mol%, and the critical distance is about 8.024 Å.     

Synthesis and annealing effects on the optical spectroscopy properties of red-emitting Gd(P<Subscript>0.5</Subscript>V<Subscript>0.5</Subscript>)O<Subscript>4</Subscript>: x at.% Eu<Superscript>3+</Superscript>

In this work, Gd(P_0.5V_0.5)O₄: x at.% Eu³⁺ phosphors with different dopant concentrations (x?=?1, 3, 5, 6, 7, 9) were synthesized through chemical coprecipitation method. The phosphors were characterized by XRD, SEM, infrared spectroscopy, photoluminescence excitation, emission spectra and CIE. The results of XRD indicate that the obtained phosphors have the tetragonal phase structure. Eu³⁺ emission transitions arise mainly from the ⁵D₀ level to the ⁷F_J (J?=?0, 1, 2, 3, 4) manifolds. The emission intensity and crystalline of Gd(P_0.5V_0.5)O₄:x at% Eu³⁺ powders are increasing with annealing temperature at 600, 800, 1000, 1100, and 1200 °C, respectively. The introduction of VO₄^3? can broaden the range of UV excitation spectrum wavelength and enhance the transition between ⁵D₀ → ⁷F₁ to ⁵D₀ → ⁷F₂ for long wavelength emission. And the most dominant emission peak of Eu³⁺ for ⁵D₀ → ⁷F₂ transition is closer to pure red light at 622 nm. The maximum emission intensity of the phosphors is the concentration of 6 at.% Eu³⁺ because of the distance of the neighbor Eu³⁺ ions reaching a certain critical value and the influence of multipolar interaction. Compared to commercial phosphors Y₂O₃:Eu³⁺ and (Y,Gd)BO₃:Eu³⁺, our work yielded a longer wavelength red light emission intensity and a higher proportion of red light to orange light. All our results indicate that color purity of this phosphor turns it into a promising red phosphor in ultraviolet-pumped light-emitting diodes.     

Influence of MO/MF2 modifiers (M = Ca,Sr, Ba) on spectroscopic properties of Eu3+ ions in germanate and borate glasses

Series of Eu³⁺-doped lead-free germanate and borate glasses were synthesized. The MO glass modifiers (M = Ca, Sr or Ba) were partially or totally substituted by MF₂ in chemical composition. In contrast to samples modified by CaO/CaF₂ or SrO/SrF₂, the germanate glass samples containing BaO and/or BaF₂ are fully amorphous, while the lead-free borate glasses are fully amorphous, independently from glass modifiers. Effect of glass modifiers on spectroscopic properties of Eu³⁺ were systematically investigated.For that reason, excitation and emission spectra of Eu³⁺ ions in examined systems were registered. Based on the emission spectra, ratio of integrated luminescence intensity of the ⁵D₀ → ⁷F₂ transition to that of the ⁵D₀ → ⁷F₁ transition (R factor) was calculated. Moreover, the luminescence decay curves were collected and the luminescence lifetimes of the ⁵D₀ excited state of Eu³⁺ ions were determined in function of MF₂ concentration.