Preparation and photoluminescent properties of doped nanoparticles of ZnS by solid-state reaction

Nanometer-sized Eu³⁺-doped ZnS and Mn²⁺-doped ZnS particles were prepared by solid-state method at low temperature. The structures and properties of those materials were characterized by X-ray diffraction (XRD) and photoluminescent spectroscopy techniques. The XRD patterns reveal that the doped ZnS nanoparticles belong to zinc-blende structure. The concentration of doping ions has little effect on the sizes of the doped ZnS nanoparticles, which mainly depends on the temperature of preparation. The emission peaks from the ⁵D₀→⁷F_J (J=1, 2, and 4) electronic energy transitions of Eu³⁺ were observed in the emission spectra of the ZnS:Eu³⁺ nanoparticles. The intensity ratio of the two peaks from the ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ transitions indicates that more Eu³⁺ ions occupy the sites with no inversion symmetry. For the ZnS:Mn²⁺ nanoparticles, an orange emission from the ⁴T₁→⁶A₁ transition of Mn²⁺ is present, indicating that the doping ions occupy the positions of the ZnS lattices. Meanwhile, UV-induced luminescence enhancement was observed for the ZnS:Mn²⁺ nanoparticles, the possible reason of which is discussed primarily.     

Photoluminescence properties of thenardite activated with Eu

Na₂SO₄:Eu phosphors were prepared by heating pure natural thenardite with EuF₃ at 900 °C for 20 min in air. The photoluminescence (PL) and excitation spectra of as-prepared and γ-ray-irradiated phosphors were observed at 300 K. The PL spectrum under 394 nm excitation consisted of strong narrow bands with peaks at 579, 592, 616, 652, 697 and 741 nm, assigned to the ⁵D₀→⁷F_J (J=0, 1, 2, …, 5) transitions, respectively, within Eu³⁺. The PL spectrum under 340 nm excitation consisted of a broad Eu²⁺ band with a peak at 435 nm. The excitation spectrum obtained by monitoring the violet luminescence consisted of a weak band with a peak at approximately 261 nm and a broad Eu²⁺ band with a peak at approximately 338 nm. The relative efficiency of the violet luminescence of the γ-ray-irradiated phosphor at the exposure of 46 kGy increased up to 3.0 times that of the unirradiated phosphor. The enhancement of violet luminescence by γ-ray irradiation was ascribed to the conversion of Eu³⁺ to Eu²⁺ in Na₂SO₄.     

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.     

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.     

Synthesis and photoluminescent properties of YVO4:Eu nano-crystal phosphor prepared by Pechini process

A sol-gel technique emphasizing the Pechini process has been employed for the preparation of nano-crystal Eu³⁺-doped YVO₄ phosphor. The precursor powders were heated at 800 °C for 3 h to obtain good crystallinity with better luminescence. XRD results indicate that the second phase is not presented when the Eu³⁺ ion concentration is increased up to 50 mol%. The absorption and photoluminescent (PL) studies indicated that the energy is absorbed first by the host and then transferred to the emitting level of the Eu³⁺ ions. Excitation at 318 nm in terms of Eu³⁺ concentrations in YVO₄ powders shows that the YVO₄ phosphors display bright red luminescence at about 618 nm belonging to the ⁵D₀→⁷F₂ electric dipole transition, and a weak band in the orange region of the ⁵D₀→⁷F₁ transition at 594 nm. In addition, the time-resolved ⁵D₀→⁷F₂ transition presents a single-exponential decay behavior, revealing the decay mechanism of the ⁵D₀→⁷F₂ transition is a single decay component between Eu³⁺ ions only. The saturation of the emission intensity excited by the CTS when the Eu³⁺ concentration is 10 mol%. The concentration quenching is active when the Eu³⁺ concentration is larger than 10 mol%, and the critical distance is about 5.75 Å.     

Synthesis and luminescent properties of nanoparticles LaSrAl3O7:Eu, Tb

Phosphors of nanoparticles LaSrAl₃O₇:RE³⁺(REEu, Tb) have been prepared by a sol–gel method. The structure and luminescent properties of LaSrAl₃O₇:Eu³⁺ and LaSrAl₃O₇:Tb³⁺ phosphors were characterized by X-ray diffraction and atomic force microscopy (AFM), photoluminescence excitation and emission spectra were utilized. From X-ray diffraction (XRD) patterns, it is indicated that the phosphor LaSrAl₃O₇ forms without impurity phase at 900 °C. From atomic force microscopy (AFM) images, it is shown that the crystal size of the phosphores are about 60–80 nm. Upon excitation with UV irradiation, it is shown that there is a strong emission at around 617 nm corresponding to the forced electric dipole ⁵D₀–⁷F₂ transition of Eu³⁺, and at around 545 nm corresponding to the ⁵D₄–⁷F₅ transition of Tb³⁺. The dependence of photoluminescence intensity on Eu³⁺(or Tb³⁺) concentration and annealing temperature were also studied in detail.     

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.     

A red europium (III) ternary complex for InGaN-based light-emitting diode

One kind of europium (III) ternary complex was synthesized, and its photoluminescence properties were investigated. This complex exhibits broad excitation band in near-UV range, and strong red emission which is due to the ⁵D₀–⁷F_j transitions of Eu³⁺ ions. The luminescence quantum yield for the Eu³⁺ complex is 0.17. Thermogravimetric analysis confirms a high thermal stability of the complex with a decomposition temperature of 344 °C. All the characteristics indicate that the Eu³⁺ complex is a highly efficient red phosphor suitable to be excited by near UV light. An intense red light-emitting diode was fabricated by combining the europium (III) ternary complex with a ~395 nm-emitting InGaN chip.     

SnO2:Eu nanoparticles dispersed in silica: A low-temperature synthesis and photoluminescence study

SnO₂:Eu and SnO₂:Eu nanoparticles dispersed in silica matrix were prepared at a relatively low temperature of 185 °C in ethylene glycol medium. For as-prepared SnO₂:Eu nanoparticles there exists a weak energy transfer from the SnO₂ host to the Eu³⁺ ions. However, the energy transfer can be significantly improved by dispersing the Eu³⁺-doped SnO₂ nanoparticles in silica matrix. Effective shielding of surface Eu³⁺ ions in SnO₂:Eu nanoparticles from the stabilizing ligand by silica matrix is the reason for the improved extent of energy transfer. Increase in asymmetric ratio of luminescence (ratio of the intensity of the electric dipole allowed transition, ⁵D₀→⁷F₂, to magnetic dipole allowed transition, ⁵D₀→⁷F₁) for SnO₂:Eu nanoparticles dispersed in silica compared to that of SnO₂:Eu nanoparticles, has been attributed to the distorted environment around surface Eu³⁺ ions brought about by the presence of both tin and silicon structural units. ¹¹⁹Sn and ²⁹Si MAS NMR studies on this sample confirmed that there is no interaction between the tin and silicon structural units even after heating the samples at 900 °C.     

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.     

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.     

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.     

Optical and structural properties of Eu-diffused and doped ZnO nanowires

Single crystal ZnO nanowires diffused with europium (Eu) from a solid source at 900 °C for 1 h or doped with Eu during growth have been characterized. The ZnO nanowires were grown by chemical vapor deposition on Si substrates employing Au as a catalyst. The diameter of the resulting nanowires was 200 nm with a length of 1 μm. Photoluminescence spectra excited by a He–Cd laser at room temperature showed the green luminescence at 515 nm in Eu-diffused nanowires. A small red shift of near-band-edge emission of ZnO nanowires was observed in the diffused wires, but sharp emission from Eu³ ions was not present. Transmission electron microscopy shows crystalline Eu₂O₃ formation on the diffused nanowire surface, which forms a coaxial heterostructure system. When Eu was incorporated during the nanowire growth, the sharp ⁵D_O–⁷F₂ transition of the Eu³⁺ ion at around 615 nm was observed.     

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.     

Broadband,site selective and time resolved photoluminescence spectroscopic studies of finely size-modulated Y2O3:Eu3+ phosphors synthesized by a complex based precursor solution method

Undoped and Eu³⁺-doped cubic yttria (Y₂O₃) nanophosphors of good crystallinity, with selective particle sizes ranging between 6 and 37 nm and showing narrow size distributions, have been synthesized by a complex-based precursor solution method. The systematic size tuning has been evidenced by transmission electron microscopy, X-ray diffraction, and Raman scattering measurements. Furthermore, size-modulated properties of Eu³⁺ ions have been correlated with the local structure of Eu³⁺ ion in different sized Y₂O₃:Eu³⁺ nanophosphors by means of steady-state and time-resolved site-selective laser spectroscopies. Time-resolved site-selective excitation measurements performed in the ⁷F₀ → ⁵D₀ peaks of the Eu³⁺ ions at C₂ sites have allowed us to conclude that Eu³⁺ ions close to the nanocrystal surface experience a larger crystal field than those in the nanocrystal core. Under the site-selective excitation in the ⁷F₀ → ⁵D₀ peaks, energy transfer between the sites has also been observed.     

Development of zeolite-derived novel aluminosilicate phosphors

In this research, zeolite-derived aluminosilicate phosphors were synthesized through the ion exchange route. Red light-emitting property of Eu³⁺-doped aluminosilicate phosphors were discussed from a view point of the Eu content, heat-treatment condition and the oxidation state of Eu ions. The crystalline phase of the host aluminosilicates could be successfully controlled as designed based on the published NaAlO₂–SiO₂ binary phase diagram. Orange-red emission peaks derived from the ⁵D₀→⁷F_j (j=0, 1, 2, 3, 4) transition of Eu³⁺ were observed around 590–700 nm, and 4f⁶5d→4f⁷ transition of Eu²⁺ was observed at around 400–500 nm. The relative intensity I(⁵D₀→⁷F₂) of the dominant emission peak at 612 nm increased consistently with the Eu content. The results of the XANES spectroscopy analysis revealed that Eu²⁺ ion in the 1400 °C as heat-treated host aluminosilicate were successfully converted to Eu³⁺ by the additional annealing at 1100 °C. The Eu contents and heat-treatment conditions were determined to exhibit the best performance as a red phosphor, which were 10 wt% and 1500 °C, respectively     

Luminescence properties of Eu- and Mg-doped LiTaO3 obtained via the polymeric precursor method

This work reports the pure lithium tantalate (LiTaO₃), europium (III)-doped LiTaO₃ and magnesium (II)-europium (III)-doped LiTaO₃ preparation by the polymeric precursor method, using four different powered samples of Eu³⁺ ion concentrations 0.1 and 1 at%, observing their effect on the luminescent property of the material. Results indicated LiTaO₃ phase free of secondary phases at 650 °C and the photoluminescence (PL) emission spectra showed the characteristic sharp emission bands given by Eu³⁺ ions when they are excited at a wavelength of 399 nm. An increase of dopants led us to a non-homogeneous broadening and showed a slightly larger one when Mg was added. A displacement of the transition ⁵D₀→⁷F₀ to shorter wavelengths as function of Eu³⁺concentration was also noticed.     

Correlation of photoluminescence of (La,Ln) PO<Subscript>4</Subscript>:Eu<Superscript>3+</Superscript> (Ln = Gd and Y) phosphors with their crystal structures

Eu³⁺-doped (La, Ln) PO₄ (Ln = Gd and Y) phosphors were prepared by a facile co-precipitation method. Their structures and luminescent properties under UV excitation were investigated. Structural characterization of the nanostructured luminescence material was carried out with X-ray powder diffraction analysis. Scanning electron microscopy was carried out to understand the surface morphological features and grain sizes with 50–100 nm. It is found that (La, Gd) PO₄:Eu³⁺ phosphors have the same crystal structure as LaPO₄:Eu³⁺, which is monoclinic with a little different lattice parameters. In the case of (La, Y) PO₄:Eu³⁺ phosphors, however, the gradual change from monoclinic to tetragonal structure of host lattice was observed, as the amount of Y ion increased. From the photoluminescence spectra for (La, Ln) PO₄:Eu³⁺ (Ln = Gd and Y), the emission transition ⁵D₀ → ⁷F₁ has been found to be more prominent over the normal red emission transition ⁵D₀ → ⁷F₂. Furthermore, the size influence on the products was discussed. It was observed that the spectral features possess sharp and bright emission for potential applications on the monitors of the television and some other related electronic systems, in observing the images in orange–red color.     

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.     

Solvent effect in monoclinic to hexagonal phase transformation in LaPO4:RE (RE=Dy, Sm) nanoparticles: Photoluminescence study

Nanosized phosphor materials, LaPO₄:RE (RE=Dy³⁺, Sm³⁺) have been synthesized using water, dimethyl sulfoxide (DMSO), ethylene glycol (EG) and mixed solvents at a relatively low temperature of 150 °C. X-ray diffraction (XRD) study reveals that as-prepared nanoparticles prepared in DMSO and EG are well crystalline and correspond to monoclinic phase. In the mixed water-DMSO or water-EG solvents, XRD patterns are in good agreement with hexagonal phase, but transformed to monoclinic phase at higher temperature of 900 °C. TEM images show well-dispersed and rice-shaped nanoparticles of diameter 5-10 nm, length of 13-37 nm for Dy³⁺-doped LaPO₄ and diameter of 25-35 nm, length of 73-82 nm for Sm³⁺-doped LaPO₄. Dy³⁺-doped LaPO₄ shows two prominent emission peaks at 480 and 572 nm corresponding to ⁴F_9/2→⁶H_15/2 (magnetic dipole) and ⁴F_9/2→⁶H_13/2 (electric dipole) transitions, respectively. Similarly, for Sm³⁺-doped LaPO₄, three prominent emission peaks at 561, 597 and 641 nm were observed corresponding to ⁴G_5/2→⁶H_5/2, ⁴G_5/2→⁶H_7/2 (magnetic dipole) and ⁴G_5/2→⁶H_9/2 (electric dipole) transitions, respectively. The luminescence intensity of the sample prepared in EG is more than that of DMSO or mixed solvents. Enhancement of luminescence is also observed after heat-treatment at 900 °C due to removal of quencher such as water, organic moiety and surface defects/dangling bonds. The samples are re-dispersible in polar solvent and can be incorporated in polymer film.