Synthesis and spectroscopic characterization of the K4BaSi3O9:Eu3+

K₄BaSi₃O₉:Eu³⁺ polycrystals were synthesized by solid state method. X-ray powder diffraction measurements confirmed structure of the samples. The excitation and the emission spectra of orthorhombic K₄BaSi₃O₉ doped with Eu³⁺ were investigated. The excitation spectrum exhibits a broad band with maximum at 220 nm corresponding to the charge transfer (CT) transition between O²⁻ and Eu³⁺ ions and smaller 4f–4f transitions. The emission of investigated phosphor was excited at 395 nm and has quantum efficiency (QE) equal 27%. The emission maximum at 616.5 nm was assigned to the ⁵D₀ → ⁷F₂ transition of Eu³⁺ ions. The luminescence decay profiles as well as the thermal quenching were measured and analyzed. K₄BaSi₃O₉:Eu³⁺has high temperature quenching of the emission T_0.5 = 335 °C.     

Luminescent materials derived from the surface-modification of Ln3+-doped zeolite L with a silylated terpyridine

Herein we report luminescent materials of Ln³⁺ (Ln = Eu or Tb) doped disc shaped zeolite L crystals (Eu³⁺/ZLD, Tb³⁺/ZLD) modified with a silylated terpyridine (Terpy-Si). The modified crystals show bright red emission and green emission under UV-light irradiation due to the energy transfer from the Terpy-Si to the Eu³⁺ and Tb³⁺ ions. The obtained materials were characterized with FT-IR, SEM, XRD and elemental analysis. Luminescence spectroscopy was used to study the luminescence properties of the modified Eu³⁺(Tb³⁺)/ZLD crystals. The formation of europium(III) and terbium(III) Terpy-Si silicon complexes and energy transfer from Terpy-Si to Eu³⁺ ions and Tb³⁺ have been confirmed by luminescence spectroscopy.     

Spectroscopic investigations on Eu3+ ions in Li–K–Zn fluorotellurite glasses

Eu³⁺ ions incorporated Li–K–Zn fluorotellurite glasses, (70 − x)TeO₂ + 10Li₂O + 10K₂O + 10ZnF₂ + xEu₂O₃, (0 ⩽ x ⩽ 2 mol%) were prepared via melt quenching technique. Optical absorption from ⁷F₀ and ⁷F₁ levels of the Eu³⁺-doped glass has been studied to examine the covalent bonding characteristics, energy band gap and Judd–Ofelt intensity parameters. The emission spectra (⁵D₀ → ⁷F_0,1,2,3,4) of the glasses were used to estimate the luminescence enhancement, asymmetric environment in the vicinity of Eu³⁺ ions, stimulated emission cross section and branching ratios. The phonon side band mechanism of ⁵D₂ level of the Eu³⁺ ions in the prepared glass was examined by considering the excitation and Raman spectra. The radiative lifetime calculated using Judd–Ofelt parameters was compared with the experimental lifetime to estimate the quantum efficiency of ⁵D₀ level of Eu³⁺ ions in Li–K–Zn fluorotellurite glass.     

VUV spectroscopy of complex fluoride systems Na0.4(Y1−xREx)0.6F2.2 (RE3+ = Nd3+, Tm3+)

Emission and excitation spectra as well as luminescence decay kinetics of complex non-stoichiometric fluoride crystals Na_0.4(Y_1−xNd_x)_0.6F_2.2 (x = 0.005, 0.05, 0.2, 1) and Na_0.4(Y_1−xTm_x)_0.6F_2.2 (x = 0.0005, 0.01, 0.05, 0.1) have been studied in the VUV spectral range at liquid-helium (T ∼ 10 K) temperatures. It has been shown that these crystals show intense broad-band VUV luminescence due to the interconfiguration 5d-4f transitions in Nd³⁺ and Tm³⁺ ions. Remarkable concentration quenching is observed for Nd³⁺ 5d-4f luminescence whereas fast (spin-allowed) 5d-4f luminescence of Tm³⁺ shows no concentration quenching for the studied doping level up to 10%. The spin-allowed 5d-4f luminescence of Tm³⁺ in these crystals was found to be rather weak compared to spin-forbidden 5d-4f luminescence because of efficient nonradiative relaxation from higher-energy 5d states of Tm³⁺ to the lowest-energy 5d level responsible for spin-forbidden 5d-4f luminescence. The studied fluoride systems can be considered as promising active media for the development of VUV solid state lasers with optical pumping.     

Optical processes of red emission from Eu doped GaN

A single crystalline Eu-doped GaN was grown by gas-source molecular beam epitaxy and photoluminescence (PL) properties were studied. The PL spectra show red-emission at 622 nm originating from intra 4f–4f transition of Eu³⁺ ion without band-edge emission of GaN. The peak shift of the red-emission with the temperature variation from 77 K to room temperature is less than 1.6 meV, and thermal quenching of the luminescence was found to be small compared with the band-to-band transition. Fourier transform infrared spectra showed an absorption peak at about 0.37 eV, which may be due to a deep defect level. The intensity of the red luminescence and the defect-related absorption peak increased with increasing Eu concentration, and a close correlation in the intensity was observed between them. These results suggest that the deep defect level plays an important role in the radiative transition of Eu³⁺ ion in GaN and the optical process for the luminescence at 622 nm was discussed with the relation to the defect.     

The effect of Eu3+ concentration on the Y2O3 host lattice obtained from citrate precursors

This work presents a thermal decomposition study of the precursor resin prepared from the citrate precursor along with structural features and optical properties materials composed by Y₂O₃ and Eu³⁺ containing Y₂O₃ in 0.5, 3, 5 and 7 mol%. The microcrystallite sizes were estimated from the Scherrer equation. The structural and optical properties revealed that the addition of 5 mol% of Eu³⁺ to the Y₂O₃ matrix gave rise to the highest relative emission intensity which was evidenced by the luminescence intensity. The lifetime of the 0.5 mol% Eu³⁺-doped sample suggested two different symmetry sites for Eu³⁺ ions because two different lifetime values were acquired for this sample, while for phosphors doped with 3 or 5 mol% of Eu³⁺ ions only one similar lifetime was observed. When the concentration of Eu³⁺ is 0.5 and 3 mol%, the luminescence intensity is weak due to the low probability of the O^2? - Eu³⁺ charge transfer transition. On the other hand, when the concentration of the Eu³⁺ ions is 7 mol%, a quenching effect is evidenced.     

Thermal stability and annealing behavior of photoluminescence from Eu doped YAG

Eu₂O₃ doped Y₃Al₅O₁₂ (YAG) crystals have been grown using a floating zone technique and evaluated thermal stability and annealing behavior of PL for a fluorescence thermo-sensor application. Color of the crystals grown varies from deep purple to colorless with O₂ concentration of the growth atmosphere and annealing in air. Photoluminescence (PL) peaking at λ = 590, 607, 624, 647 and 709 nm due to Eu³⁺ ions are observed from the crystals under UV excitation. Anomalous temperature dependence of PL intensity, which is observed in as-grown crystals, is improved greatly by annealing through the heat cycle. From annealing behavior of optical absorption spectra, residual Eu²⁺ ions are suggested to be responsible for the de-coloration and the improvement of anomalous temperature dependence of Eu doped YAG crystals.     

UV–VUV spectroscopy of rare earth doped persistent luminescence materials

The electronic and defect energy level structure of polycrystalline SrAl₂O₄:Eu²⁺,R³⁺ persistent luminescence materials were studied with thermoluminescence and UV–VUV synchrotron radiation emission and excitation spectroscopy. Theoretical calculations using the density functional theory (DFT) were carried out simultaneously with the experimental work. The experimental band gap energy (E_g) value of 6.6 eV agrees very well with the DFT value of 6.4 eV. The 4f⁷ → 4f⁶5d¹ excitation bands of Eu²⁺ were found rather similar irrespective of the R³⁺ co-dopant. The trap level energy distribution depended strongly on the R³⁺ co-dopant except for the shallowest trap energy above the room temperature remaining the same, however. The different processes in the mechanism of persistent luminescence from SrAl₂O₄:Eu²⁺,R³⁺ was constructed and discussed.     

Optical studies on X and γ irradiated vacuum grown CsBrI:Eu2+ crystals

CsBr_0.9I_0.1:Eu²⁺ crystals were grown by Bridgman technique. Optical absorption spectrum of the unirradiated CsBr_0.9I_0.1:Eu²⁺ crystals show absorption bands at 270 nm and 340 nm. Irradiated CsBr_0.9I_0.1:Eu²⁺ shows single F band for F(Br⁻) and F(I⁻) centers at 730 nm. Conversion of Eu²⁺ to Eu³⁺ after irradiation is confirmed by optical absorption technique. Sharp and single Photoluminescence (PL) emission band is observed at 440 nm for CsBr_0.9I_0.1:Eu²⁺ crystals. Photostimulated Luminescence (PSL) emission band observed for CsBr_0.9I_0.1:Eu²⁺ crystals at 442 nm due to excitation at 730 nm shows that the F centers are photostimulable. PSL emission intensity increases linearly with irradiation dose up to 2.5 Krad.     

Structural and spectroscopic characteristics of Eu3+-doped tungsten phosphate glasses

Tungsten based phosphate glasses are interesting non-crystalline materials, commonly known for photochromic and electrochromic effects, but also promising hosts for luminescent trivalent rare earth ions. Despite very few reports in the literature, association of the host́s functionalities with the efficient emissions of the dopant ions in the visible and near-infrared spectra could lead to novel applications. This work reports the preparation and characterization of glasses with the new composition 4(Sb₂O₃)96−x(50WO₃ 50NaPO₃)xEu₂O₃ where x = 0, 0.1, 0.25, 0.5 and 1.0 mol%, obtained by the melt quenching technique. The glasses present large density (∼4.6 g cm⁻³), high glass transition temperature (∼480 °C) and high thermal stability against crystallization. Upon excitation at 464 nm, the characteristic emissions of Eu³⁺ ions in the red spectral region are observed with high intensity. The Judd–Ofelt intensity parameters Ω₂ = 6.86 × 10⁻²⁰, Ω₄ = 3.22 × 10⁻²⁰ and Ω₆ = 8.2 × 10⁻²⁰ cm² were calculated from the emission spectra and found to be higher than those reported for other phosphate glass compositions. An average excited state lifetime value of 1.2 ms, was determined by fitting the luminescence decay curves with single exponential functions and it is comparable or higher than those of other oxide glasses.     

Synthesis and luminescent properties of SrZnO2:Eu3+,M+ (M = Li,Na, K) phosphor

In this paper, a series of novel luminescent materials, SrZnO₂:Eu³⁺,M⁺ (M = Li, Na, K) have been synthesized by conventional solid-state reaction. X-ray diffraction (XRD) patterns and photoluminescence (PL) spectra were carried out to characterize their structural and luminescent properties. It was found that under ultraviolet excitation with a wavelength of 301 nm, SrZnO₂:Eu³⁺ gives a red luminescence that was attributed to the transitions from ⁵D₀ excited states to ⁷F_J (J = 0–4) ground states of Eu³⁺ ions. The feature and the high intensity of hypersensitive transition ⁵D₀ → ⁷F₂ indicate that Eu³⁺ prefers to occupy a low symmetry site. The incorporation of alkali metal ions greatly enhanced the luminescence intensity and slightly changed the excitation and emission peak position, probably due to the influence of the coordination conditions for Eu³⁺ ions.     

Formation and spectral probing of transparent oxyfluoride glass-ceramics containing (Eu2+, Eu3+:BaGdF5) nano-crystals

In the present study, we report the formation of transparent glass-ceramics containing BaGdF₅ nanocrystals under optimum ceramization of SiO₂–BaF₂–K₂O–Sb₂O₃–GdF₃–Eu₂O₃ based oxyfluoride glass and the energy transfer mechanisms in Eu²⁺ → Eu³⁺ and Gd³⁺ → Eu³⁺ has been interpreted through luminescence study. The modification of local environment surrounding dopant ion in glass and glass ceramics has been studied using Eu³⁺ ion as spectral probe. The optimum ceramization temperature was determined from the differential scanning calorimetry (DSC) thermogram which revealed that the glass transition temperature (T_g), the crystallization onset temperature (T_x), and crystallization peak temperature (T_p) are 563 °C, 607 °C and 641 °C, respectively. X-ray diffraction pattern of the glass-ceramics sample displayed the presence of cubic BaGdF₅ phase (JCPDS code: 24-0098). Transmission electron microscopy image of the glass-ceramics samples revealed homogeneous distribution of spherical fluoride nanocrystals ranging 5–15 nm in size. The emission transitions from the higher excited sates (⁵D_J, J = 1, 2, and 3) as well as lowered asymmetry ratio of the ⁵D₀ → ⁷F₂ transition (forced electric dipole transition) to that of the ⁵D₀ → ⁷F₁ transition (magnetic dipole) of Eu³⁺ in the glass-ceramics when compared to glass sample demonstrated the incorporation of dopant Eu³⁺ ions into the cubic BaGdF₅ nanocrystals with higher local symmetry with enhanced ionic nature. The presence of absorption bands of Eu²⁺ ions and Gd³⁺ ions present in the glass matrix or fluoride nanocrystals in the excitation spectra of Eu³⁺ by monitoring emission at 614 nm indicated energy transfer from (Eu²⁺ → Eu³⁺) and (Gd³⁺ → Eu³⁺) in both glass and glass-ceramics samples.     

Structural and photophysical properties of lanthanide complexes with N-(diphenylphosphoryl)-4-methylbenzenesulfonamide

Lanthanide complexes with N-(diphenylphosphoryl)-4-methylbenzenesulfonamide (HPMSP) as new sensitizers of visible luminescence were obtained. The series of stable lanthanide complexes Na[Ln(PMSP)₄], where Ln = Eu³⁺, Gd³⁺, Tb³⁺ were characterized by X-ray diffraction, IR, absorption, emission, and excitation spectra at 295 and 77 K as well as luminescence decay times and intrinsic emission quantum yields. The Tb complex, exhibiting relatively efficient ligand-to-metal energy transfer and strong metal-centred emission, is a promising candidate for effective UV-to-visible energy converters. Temperature dependent quenching of sensitized ⁵D₀ europium emission and presence of ⁵D₁ emission are discussed.     

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.     

Synthesis and efficient blue-emitting of Eu2+-activated borate fluoride BaAlBO3F2

Eu²⁺-doped borate fluoride BaAlBO₃F₂ was synthesized by the conventional high temperature solid state reaction. The crystal phase formations were confirmed by X-ray powder diffraction (XRD) measurements and the structure refinement. The photoluminescence (PL) excitation and emission spectra, and the decay curves were investigated. Eu²⁺-doped BaAlBO₃F₂ phosphor can be efficiently excited by near-UV light and presents narrow blue luminescence band centered at 450 nm. The maximum absolute quantum efficiency (QE) of BaAlBO₃F₂:0.05Eu²⁺ phosphor was measured to be 76% excited at 398 nm light at 300 K. The thermal stability of the blue luminescence was evaluated by the luminescence decays as a function of temperature. The phosphor shows an excellent thermal stability with high thermal activation-energy on temperature quenching effects because of the rigid crystal lattices.     

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.     

Green EuAlO3:Eu2+ nanophosphor for applications in WLEDs

Green-emitting aluminate nanophosphors fabricated by a pressure-assisted combustion synthesis (P₀ = 1.4, 2.8 and 3.4 MPa) and annealed in pure H₂ is reported. The XRD analysis indicates the formation of the EuAlO₃ orthorhombic phase. Transmission electron microscopy (TEM) shows coalesced irregular grains with lengths in the range of 35–140 nm. In addition, the nanophosphor synthesized at an initial pressure of 3.4 MPa produced the highest green luminescence centered at 530 nm, which was associated to 4f⁷–4f⁶5d¹ allowed transitions of the Eu²⁺ located into the EuAlO₃:Eu²⁺ compound. A weak red emission peak corresponding to Eu³⁺ was also observed by cathodoluminescence. The CIE coordinates for green emission are x = 0.2613 and y = 0.3892, and the luminance produced from nanophosphors when they are excited with a commercial UV-396 nm LED (8.0 μW) was 285 ± 4.3 Cd/m².     

Novel highly luminescent europium dinitrosalicylates

A series of lanthanide dinitrosalicylates M₃Ln(3,5-NO₂-Sal)₃ · nH₂O (Ln = Eu, Gd; M = Li, Na, K, Cs) was synthesized. It was found that the luminescence efficiency of some M₃Eu(3,5-NO₂-Sal)₃ · nH₂O compounds was near to the high efficiency of europium dibenzoylmethanate with 1,10-phenanthroline, Eu(DBM)₃ · Phen. The luminescence excitation spectra, electron-vibrational luminescence spectra, and vibrational IR spectra were investigated. The energy of the lowest excited triplet state of the ligand was obtained from phosphorescence spectra of M₃Gd(3,5-NO₂-Sal)₃ · nH₂O, M(3,5-NO₂-HSal) · nH₂O, and M₂(3,5-NO₂-Sal) · nH₂O. The details of the structure of compounds were discussed. The influence of different M-cations on the Eu³⁺ luminescence efficiency and on the processes of excitation energy transfer to a Eu³⁺ ion was analyzed. The presence of large alkali metal cations in lanthanide dinitrosalicylates and an increase in the temperature weaken the network of hydrogen bonds and, to some extent, the “ligand–metal” bonds. This is a cause of a long-wavelength shift of the intraligand charge transfer (ILCT) band in Eu³⁺ excitation spectra arising at inclusion of Cs⁺ instead of Li⁺ cations in the crystal lattice and at the heating of compounds. A change of the energies of ligand electronic states at substitution of Li⁺ and Na⁺ for Cs⁺ can give a tenfold enhancement of the Eu³⁺ luminescence efficiency at 300 K.     

Time-dependent PSL studies on CsBrCl:Eu2+: a promising image screen phosphor

The time-resolved PSL studies of CsBr_1−xCl_x:Eu²⁺ system has been studied for different europium concentrations. An efficient PSL material for use in image plate should have very short PSL emission lifetime. PSL emission lifetime for stimulation at 650 and 630 nm (F(Br⁻)- and F(Cl⁻)-centers) for CsBrCl:Eu²⁺ is determined to be 0.69 μs. The lifetime of BaFBr:Eu²⁺, the commercially available image screen phosphor is 0.8 μs. The present observation support the use of CsBrCl:Eu²⁺ as an efficient X-ray image screen phosphor.     

Luminescent and thermal properties of a new green emitting Ca6Sr4(Si2O7)3Cl2:Eu2+ phosphor for near-UV light-emitting diodes

A novel green emitting phosphor, Eu²⁺-activated Ca₆Sr₄(Si₂O₇)₃Cl₂, was synthesized using the solid-state reaction and its temperature-dependent luminescence characteristic was reported for the first time. Crystallographic site-occupations of Eu²⁺ ions in this host were assigned and two distinguishable Sr²⁺ sites were confirmed. As the temperature increases, the emission lines of Ca₆Sr_3.99(Si₂O₇)₃Cl₂:0.01Eu²⁺ show an anomalous blue-shift along with the broadening bandwidth and decreasing emission intensity, which is ascribed in terms of the phonon-assisted back tunneling from the excited state of low-energy emission band to the high-energy emission band in the configuration coordinate diagram. Further, the luminescence quenching temperature, the activation energy for thermal quenching (ΔE), and the chromaticity coordinates were also investigated. In view of its preferable excitation spectrum profile, intense green emission peaking at 511 nm, and high thermal luminescence stability, the as-prepared phosphor is expected to find applications as a new green emitting phosphor for near-UV light emitting diodes.