Probing of inversion symmetry site in Eu-doped GdPO4 by luminescence study: Concentration and annealing effect

Eu³⁺-doped gadolinium orthophosphate (GdPO₄) (Eu³⁺ at%=0, 2, 5, 7, 10, 15, 20 and 30) nanoparticles have been prepared by ethylene glycol route and subsequently heated at 500 and 900 °C. The crystallite size increases with increasing heat-treatment temperature. Luminescence study shows that magnetic dipole transition (⁵D₀→⁷F₁) is prominent over the electric dipole transition (⁵D₀→⁷F₂), which has been attributed to occupancy of inversion symmetry site by more Eu³⁺ ions in Eu³⁺-doped GdPO₄. The luminescence intensity is enhanced as heat-treatment temperature increases from 500 to 900 °C due to the improved crystallinity. Optimum luminescence is observed for 5–7 at% Eu³⁺ in GdPO₄ nanoparticles. Above this concentration, luminescence intensity decreases due to concentration quenching effect. This is supported by lifetime study.     

Synthesis, characterization and optical properties of water-soluble ZnS:Mn nanoparticles

ZnS nanoparticles with Mn²⁺ doping (0.5-20%) have been prepared through a simple chemical method, namely the chemical precipitation method. The structure of the nanoparticles has been analyzed using X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and UV-vis spectrometer. The size of the particles is found to be 3-5 nm range. Photoluminescence spectra were recorded for undoped ZnS nanoparticles using an excitation wavelength of 320 nm, exhibiting an emission peak centered at around 445 nm. However, from the Mn²⁺-doped samples, a yellow-orange emission from the Mn²⁺⁴T₁-⁶A₁ transition is observed along with the blue emission. The prepared Mn²⁺-doped sample shows efficient emission of yellow-orange light with the peak emission 580 nm with the blue emission suppressed. The maximum PL intensity is observed only at the excitation energy of 3.88 eV (320 nm). Increase in stabilizing time up to 48 h in de-ionized water yields the enhancement of emission intensity of doped (4% Mn²⁺) ZnS. The correlation made through the concentration of Mn²⁺ versus PL intensity resulted in opposite trend (mirror image) of blue and yellow emissions.     

Optical absorption and photoluminescence studies of Eu-doped phosphate and fluorophosphate glasses

Phosphate (P₂O₅+K₂O+BaO+Al₂O₃+Eu₂O₃) and fluorophosphate (P₂O₅+K₂O+BaO+BaF₂+Al₂O₃+Eu₂O₃) glasses with different Eu³⁺ ion concentrations have been prepared and characterized through optical absorption, photoluminescence and decay times. An intense red luminescence is observed from the ⁵D₀ emitting level of Eu³⁺ ions in these glasses. The relative luminescence intensity ratio of ⁵D₀→⁷F₂→⁵D₀→⁷F₁ transitions has been evaluated to estimate the local site symmetry around the Eu³⁺ ions. The emission spectra of these glasses show a complete removal of degeneracy for the ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ transitions. Second and fourth rank crystal-field (CF) parameters have been calculated together with the CF strength parameter by assuming the C_2v symmetry for the Eu³⁺ ions in both the phosphate and fluorophosphate glasses. Judd-Ofelt parameters have been evaluated from the luminescence intensity ratios of ⁵D₀→⁷F_J (J=2, 4 and 6) to ⁵D₀→⁷F₁ transitions. These parameters have been used to derive radiative properties such as transition probabilities, branching ratios, radiative lifetimes and peak stimulated emission cross-sections for the ⁵D₀→⁷F_J transitions. Decay curves of the ⁵D₀ level of Eu³⁺ ions in these two Eu³⁺:glass systems have been measured by monitoring the ⁵D₀→⁷F₂ transition (611 nm) at room temperature. The experimental lifetime of the ⁵D₀ level in the title glasses is found to be higher than Eu³⁺-doped niobium phosphate glasses. The analysis indicates that the lifetime of the ⁵D₀ level is found to be less sensitive to the Eu³⁺ ion concentration and addition of BaF₂ has no significant effect on the optical properties of Eu³⁺-doped phosphate glasses.     

Concentration-dependent luminescence of Tb ions in high calcium aluminosilicate glasses

This study deals with the results on the concentration-dependent fluorescence properties of Tb³⁺-doped calcium aluminosilicate (CAS) glasses of composition (100−x)(58SiO₂–23CaO–5Al₂O₃–4MgO–10NaF in mol%)-x Tb₂O₃ (x=0, 0.25, 0.5, 1, 2, 4, 8, 16, 24, 32, 40 in wt%). The FTIR reflectance spectra suggested the role of dopant ions as network modifiers in the glass network. The fluorescence spectra of low Tb³⁺-doped glasses have revealed prominent blue and green emissions from ⁵D₃ and ⁵D₄ excited levels to ⁷F_j ground state multiplet, respectively. The glass with 2 wt% of Tb₂O₃ has exhibited maximum intensity of blue emission from ⁵D₃ level, while green emission from ⁵D₄ level has increased linearly up to 24 wt% and showed reduction in the rate of increase for higher Tb₂O₃ concentrations. The concentration quenching of blue emission (⁵D₃→⁷F_j) is attributed mainly to the resonant energy transfer (RET) assisted cross-relaxation (CR) among the excited and nearest neighbour unexcited Tb³⁺ ions in the glass matrix. The decline in rate of increase of green emission (⁵D₄→⁷F_j) at higher concentrations has been explained due to a possible occurrence of cooperative energy transfers leading to 4f⁸→4f⁷5d transition interactions. The blue and green emission decay kinetics have been recorded to compute the excited level (⁵D₃ and ⁵D₄) lifetimes, which confirmed the Tb³⁺ concentration quenching of the blue emission in these glasses.     

Synthesis, luminescence and crystallographic structure of Eu-doped garnet-type yafsoanite Ca3Te2(ZnO4)3

A red-emitting phosphor of Eu³⁺-doped calcium–tellurium–zinc oxide, Ca₃Te₂(ZnO₄)₃, with a garnet-type structure was synthesized by high temperature solid-state reactions. This phosphor exhibited a strong red emission. The photoluminescence excitation spectrum showed that Ca₃Te₂(ZnO₄)₃:Eu³⁺ can be effectively excited by UV–visible light. The property of long-wavelength excitation for this material has a benefit as a red phosphor in application of white light-emitting diodes. The colour coordinates were calculated. The excitation and emission spectra and luminescence decay curves were obtained using a pulsed, tunable, narrowband dye laser. Crystallographic sites and charge compensation mechanism of Eu³⁺ ions were discussed. The emission line from Eu³⁺ in intrinsic crystallographic site in the lattice was located at 579.56 nm. The emission line from Eu³⁺ in another disturbed site, which is created by the defects created by the charge-compensation, was located at 580.88 nm. The disordered crystallographic sites of Eu³⁺ are benefit for their strong red luminescence corresponding to the ⁵D₀→⁷F₂ transition.     

Surface modification and fluorescent properties of Mn2+-doped and undoped ZnS nanoparticles

Abstract

ZnS nanoparticles and Mn²⁺-doped ZnS nanoparticles were prepared by a reverse micelle reaction system. In addition, ZnS and Mn²⁺-doped ZnS nanoparticles were modified with poly(vinyl alcohol) (PVA) and 1-dodecanethiol (C₁₂H₂₅SH). The average particle size of the ZnS sample is determined around 2.3 nm by using the well-known Scherrer equation, which is in accordance with the results obtained from UV–vis and TEM analysis. Fluorescence intensity of the Mn²⁺-doped ZnS nanoparticles increases with increasing Mn²⁺ content compared with undoped ZnS nanoparticles, and coating PVA can also make fluorescence intensity increase. Different Zn²⁺/S^2- or C₁₂H₂₅SH/Zn²⁺ can affect intensity of PL emission peak and its position, which is discussed in this paper.     

Synthesis and photoluminescence characteristics of doped ZnS nanoparticles

Free-standing powders of doped ZnS nanoparticles have been synthesized by using a chemical co-precipitation of Zn²⁺, Mn²⁺, Cu²⁺ and Cd²⁺ with sulfur ions in aqueous solution. X-ray diffraction analysis shows that the diameter of the particles is ∼2–3 nm. The unique luminescence properties, such as the strength (its intensity is about 12 times that of ZnS nanoparticles) and stability of the visible-light emission, were observed from ZnS nanoparticles co-doped with Cu²⁺ and Mn²⁺. The nanoparticles could be doped with copper and manganese during the synthesis without altering the X-ray diffraction pattern. However, doping shifts the luminescence to 520–540 nm in the case of co-doping with Cu²⁺ and Mn²⁺. Doping also results in a blue shift on the excitation wavelength. In Cd²⁺-doped ZnS nanometer-scale particles, the fluorescence spectra show a red shift in the emission wavelength (ranging from 450 nm to 620 nm). Also a relatively broad emission (ranging from blue to yellow) has been observed. The results strongly suggest that doped ZnS nanocrystals, especially two kinds of transition metal-activated ZnS nanoparticles, form a new class of luminescent materials. Received: 16 October 2000 / Accepted: 17 October 2000 / Published online: 23 May 2001     

Yellow-orange light emission from Mn2+-doped ZnS nanoparticles

ZnS nanoparticles with Mn²⁺ doping (1–2.5%) have been prepared through a simple soft chemical route, namely the chemical precipitation method. The nanostructures of the prepared undoped ZnS and Mn²⁺-doped ZnS:Mn nanoparticles have been analyzed using X-ray diffraction (XRD), Scanning electron microscope (SEM), transmission electron microscope (TEM) and UV–vis spectrophotometer. The size of the particles is found to be in 2–3 nm range. Room-temperature photoluminescence (PL) spectrum of the undoped sample only exhibits a blue-light emission peaked at ∼365 nm under UV excitation. However, from the Mn²⁺-doped samples, a yellow-orange emission from the Mn^{2+ 4}T₁–⁶A₁ transition is observed along with the blue emission. The prepared 2.5% Mn²⁺-doped sample shows efficient emission of yellow-orange light with the peak emission at ∼580 nm with the blue emission suppressed.     

Photophysics of the Lanthanide Complexes with Conjugated Carboxylic Acids by Low Temperature Fluorescent Spectroscopy

In the context, some lanthanide (Eu³⁺, Tb³⁺ and Sm³⁺) complexes with conjugated carboxylic acids (pyridine-carboxylic acids derivatives) have been synthesized and characterized. The low temperature fluorescent spectra for these complexes have been measured at nitrogen atmosphere (77 K), indicating that the central Ln³⁺ ions locate in an equivalent coordination environment with low symmetry for most of these lanthanide complexes belonging to dimeric or polymeric structure. Therefore, the electronic dipole transition (supersensitive transition) (⁵D₀ → ⁷F₂ for Eu³⁺, ⁵D₄ → ⁷F₆ for Tb³⁺, ⁴G_5/2 → ⁶H_9/2 for Sm³⁺) and magnetic dipole transition (⁵D₀ → ⁷F₁ for Eu³⁺, ⁵D₄ → ⁷F₅ for Tb³⁺, ⁴G_5/2 → ⁶H_5/2 for Sm³⁺) show the regular change in the corresponding split number of fluorescent spectra, which can be realized to predict the fine structure of lanthanide complexes.     

Studies in crystal structure and luminescence properties of Eu-doped metal tungstate phosphors for white LEDs

The correlation between the crystal structure and luminescent properties of Eu³⁺-doped metal tungstate phosphors for white LEDs was investigated. Red-emitting A_4−3x(WO₄)₂:Eu_x³⁺ (A=Li, Na, K) and B_(4−3x)/2(WO₄)₂:Eu_x³⁺ (B=Mg, Ca, Sr) phosphors were synthesized by solid-state reactions. The findings confirmed that these phosphors exhibited a strong absorption in the near UV to green range, due to the intra-configurational 4f-4f electron transition of Eu³⁺ ions. The high doping concentration of Eu³⁺ enhanced the absorption of near UV light and red emission without any detectable concentration quenching. Based on the results of a Rietveld refinement, it was attributed to the unique crystal structure. In the crystal structure of the Eu³⁺-doped metal tungstate phosphor, the critical energy transfer distance is larger than 5 Å so that exchange interactions between Eu³⁺ ions would occur with difficulty, even at a high doping concentration. The energy transfer between Eu³⁺ ions, which causes a decrease in red emission with increasing concentration of Eu³⁺, appears to be due to electric multi-polar interactions. In addition, the Eu-O distance in the host lattice affected the shape of emission spectrum by splitting of emission peak at the ⁵D₀→⁷F₂ transition of Eu³⁺.     

Optical properties of annealed Mn-doped ZnS nanoparticles

Cysteine stabilized ZnS and Mn²⁺-doped ZnS nanoparticles were synthesized by a wet chemical route. Using the ZnS:Mn²⁺ nanoparticles as seeds, silica-coated ZnS (ZnS@Si) and ZnS:Mn²⁺ (ZnS:Mn²⁺@Si) nanocomposites were formed in water by hydrolysis and condensation of tetramethoxyorthosilicate (TMOS). The influence of annealing in air, formier gas, and argon at 200-1000 °C on the chemical stability of ZnS@Si and ZnS:Mn²⁺@Si nanoparticles with and without silica shell was examined. Silica-coated nanoparticles showed an improved thermal stability over uncoated particles, which underwent a thermal combustion at 400 °C. The emission of the ZnS@Si and ZnS:Mn²⁺@Si passed through a minimum in photoluminescence intensity when annealed at 600 °C. Upon annealing at higher temperatures, ZnS@Si conserved the typical emission centered at 450 nm (blue). ZnS:Mn²⁺@Si yielded different high intensity emissions when heated to 800 °C depending on the gas employed. Emissions due to the Mn²⁺ at 530 nm (green; Zn₂SiO₄:Mn²⁺), 580 nm (orange; ZnS:Mn²⁺@Si), and 630 nm (red; ZnS:Mn²⁺@Si) were obtained. Therefore, with a single starting product a set of different colors was produced by adjusting the atmosphere wherein the powder is heated.     

Synthesis of high quality and monodisperse CdS:Mn/ZnS and CdS:Mn/CdS core–shell nanoparticles

CdS:Mn²⁺/ZnS and CdS:Mn²⁺/CdS core–shell nanoparticles were synthesized in aqueous medium via chemical precipitation method in an ambient atmosphere. Polyvinylpyrrolidone (PVP) was used as a capping agent. The effect of the shell (ZnS and CdS) thickness on CdS:Mn²⁺ nanoparticles was investigated. Inorganically passivated core/shell nanocrystals having a core (CdS:Mn²⁺) diameter of 4 nm and a ZnS-shell thickness of ∼0.5 nm exhibited improved PL intensity. Optimum concentration of doping ions (Mn²⁺) was selected through optical study. For all the core–shell samples two emission peaks were observed, the first one is band edge emission in the lower wavelength side due to energy transfer to the Mn²⁺ ions in the crystal lattice; the second emission is characteristic peak of Mn²⁺ ions (⁴T₁ → ⁶A₁). The XRD, TEM and PL results showed that the synthesized core–shell particles were of high quality and monodisperse.     

Photoluminescence characteristics of ZnO doped with Eu powders

In this work, we have investigated the photoluminescence spectra of europium-doped zinc oxide crystallites prepared by a vibrating milled solid-state reaction method. X-ray diffraction, scanning electron microscopy, luminescence spectra and time-resolved spectra analysis were used to characterize the synthetic ZnO:Eu³⁺ powders. XRD results of the powders showed a typical wurtzite hexagonal crystal structure. A second phase occurred at 5 mol% Eu₂O₃-doped ZnO. The ⁵D₀-⁷F₁ (590 nm) and ⁵D₀-⁷F₂ (609 nm) emission characteristics of Eu³⁺ appeared after quenching with more than 1.5 mol% Eu₂O₃ doping. The Commission Internationale d’Eclairage (CIE) chromaticity coordinates of a ZnO:Eu³⁺ host excited at λ_ex=467 nm revealed a red-shift phenomenon with increase in Eu³⁺ ion doping. The lifetime of the Eu³⁺ ion decreased as the doping concentration was increased from 1.5 to 10 mol%, and the time-resolved ⁵D₀→⁷F₂ transition presents a single-exponential decay behavior.     

Structural,photoluminescent, and dielectric properties of Eu3+-doped Ba0.7Sr0.3TiO3 thin films

Eu³⁺-doped Ba_0.7Sr_0.3TiO₃ thin films were prepared by a chemical solution deposition method and characterized by X-ray diffraction, field emission scanning electron microscopy, photoluminescence and dielectric measurements. The thin films were well crystallized with a pure perovskite structure. A contraction of the unit cell was observed upon incorporation of Eu³⁺ ions below 2 mol%, while an expansion occurred as the Eu³⁺ concentration was further increased above 2 mol%, indicating that Eu³⁺ ions with different concentrations occupied different lattice sites. Photoluminescence spectra showed two prominent transitions of Eu³⁺ ions at 594 nm (⁵D₀ → ⁷F₁) and 618 nm (⁵D₀ → ⁷F₂) upon excitation at 395 nm (⁷F₀ → ⁵L₆). There existed two quenching concentrations at 2 mol% and 4 mol% due to different lattice sites of the Eu³⁺ ions. We also investigated the dielectric properties of the thin films. Our study suggests that Eu³⁺-doped Ba_0.7Sr_0.3TiO₃ thin films have potential applications in multifunctional optoelectronic devices.     

Luminescence spectroscopy of Eu3+ and Mn2+ ions in MgGa2O4 spinel

Photoluminescence and excitation spectra of the spinel-type MgGa₂O₄ with 0.5 mol. % Mn²⁺ ions and Eu³⁺ content from 0 to 8 mol. % have been investigated in this work at room temperature. Polycrystalline samples were synthesized via high-temperature solid-state reaction method. Photoluminescence spectra of all samples exhibit host emission presented by a broad “blue” band peaking ∼430 nm, which consists of at least three elementary bands that correspond to different host defects. Excitation of the host luminescence showed the broad band with a maximum at 360 nm. Characteristic bands of d–d transitions of Mn²⁺ ions and f–f transitions of Eu³⁺ ions together with charge-transfer bands (CTB) of these ions were also found on the excitation spectra. Mn²⁺ and Eu³⁺ co-doped samples emit in green and red spectral regions. Mn²⁺ ions are responsible for the green emission band at 505 nm (⁴Т₁→⁶А₁ transition). The studies of photoluminescence spectra of activated samples with different Eu³⁺ ions content show characteristic f–f luminesecence of Eu³⁺ ions. The maximum of Eu³⁺ emission was found at 618 nm (⁵D₀→⁷F₂) and optimal concentration of activator ions was around 4 mol. %.     

Role of Al or Eu on thermoluminescence of Al- and/or Eu-doped SiO2 crystals

Although SiO₂ crystals have been used in electroluminescence devices and thermoluminescence (TL) dosimeters, the emission mechanism of TL has not yet been clearly explained. Recently, as we could get amorphous and highly pure SiO₂ prepared by the sol-gel method, we have investigated the TL emission mechanism using Al³⁺- and/or Eu³⁺-doped SiO₂ crystalline samples prepared by the heat-treatment under much lower temperature that the melting point of SiO₂. The TL spectrum of the Eu³⁺-doped sample displayed several peaks, including two main peaks due to the electron transitions from ⁵D₂ to ⁷F₅ (ca. 570 nm) and from ⁵D₀ to ⁷F₂ (ca. 610 nm). As doping concentration increased, all the peak intensities reduced from maximum values except that due to the electron transition from ⁵D₀ to ⁷F₂. These observations are thought to result from a cross-relaxation process due to the lack of inversion symmetry at the Eu³⁺ site.     

Synthesis, characterization and optical spectroscopy of Eu doped titanate nanotubes

The synthesis of Eu³⁺ doped titania nanotubes was carried out via a hydrothermal method. X-ray diffraction and transmission electron microscope analyses showed that the nanotubes were formed by rolling multilayered titania structure with a length of up to 100 nm. The Eu³⁺-doped nanotubes exhibited strong emission lines associated with the ⁵D₀→⁷F_J (with J from 1 to 4) transition of Eu³⁺ and the differences between the luminescence properties of the precursor powders and the nanotubes were studied at low temperature.     

Photoluminescence properties of rare earth doped α-Si3N4

The photoluminescence properties of Eu²⁺, Ce³⁺ and Tb³⁺ doped α-Si₃N₄ have been studied and a possible structural model has been proposed on the basis of the Rietveld refinement of X-ray powder diffraction data. Nearly single phase rare earth doped α-Si₃N₄ was synthesized by a solid state reaction at 1600 °C in N₂-H₂ atmosphere starting from amorphous Si₃N₄ and rare earth oxides or nitrides. Because of small crystal field splitting of the 5d levels, the excitation and emission bands of Eu²⁺ and Ce³⁺ are positioned at higher energies as isolated ions in comparison with that in Ca-α-Sialon. Both Eu²⁺- and Ce³⁺-doped α-Si₃N₄ show blue band emission peaking at about 470 and 450 nm, respectively, under UV excitation. α-Si₃N₄:Tb³⁺ exhibits dominant green line emission mainly arising from ⁵D₄→⁷F_J (J=6-3) with weak ⁵D₃→⁷F_J (J=6-3) transitions of Tb³⁺ when excited by UV light. The thermal stability of α-Si₃N₄:Eu²⁺ is comparable with that of Ca-α-Sialon:Eu²⁺ and is much better than that of α-Si₃N₄:Ce³⁺.     

Synthesis and photoluminescent behavior of Eu3+-doped alkaline-earth tungstates

Eu³⁺-doped alkaline-earth tungstates MWO₄ (M=Ca²⁺, Sr²⁺, Ba²⁺) were prepared by a polymeric precursor method based on the Pechini process. The polymeric precursors were calcined at 700 °C for 2 h in order to obtain well-crystallized powders and then characterized by X-ray diffraction (XRD), thermogravimetric analysis (TG), scanning electron microscopy (SEM), Fourier-transform infrared (FTIR) spectroscopy and photoluminescence spectroscopy (PL). All prepared samples showed a pure crystalline phase with scheelite-type structure confirmed by XRD. It was noted that the charge-transfer band shifted from 260 to 283 nm when calcium is replaced by strontium. However, this band was not observed for Eu³⁺-doped barium tungstate. Upon excitation at 260 nm, the emission spectra are dominated by the red ⁵D₀→⁷F₂ transition at 618 nm. By analyzing of the emission lines, it was inferred that Eu³⁺ ions occupy low symmetry sites in the host lattice. It was also found that Eu³⁺-doped SrWO₄ displays better chromaticity coordinates and greater luminescence intensity than the other samples.     

Structure and photoluminescence of β-Ga2O3:Eu nanofibers prepared by electrospinning

Eu³⁺-doped β-Ga₂O₃ nanofibers were fabricated by electrospinning. The influence of Eu³⁺ concentration on the photoluminescence properties of the obtained nanofibers was investigated. The morphology and structure of β-Ga₂O₃:Eu³⁺ were characterized by field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD) and Raman spectra. The diameter of the Eu³⁺-doped β-Ga₂O₃ nanofibers was in the range of 180-300 nm. When the β-Ga₂O₃:Eu³⁺ nanofibers were excited by 325 nm wavelength, the main emission peak of the samples was 620 nm (⁵D₀→⁷F₂), which corresponded to a typical red emission (⁵D₀→⁷F_j (j = 1, 2, 3, 4) intra-4f transitions of Eu³⁺ ions). In addition, the concentration quench effect and energy transfer mechanism in β-Ga₂O₃:Eu³⁺ were also discussed.