Site selection spectroscopy of SiO2:Eu3+ gels

Silica gels doped with Eu³⁺ ions were studied at temperatures between 10 K and 300 K by site selection spectroscopy in samples heated up to 200°C. The ⁵D₀ ⁷F₀ transition shows internal structures due to the different environments of the europium ions. Lifetimes, energy levels and homogeneous linewidths are site dependent. In the wet gel the Eu³⁺ ions prefer a liquid-like environment and only when the liquid is removed by heat treatment, the ion is linked more strongly to the silica network.     

Tunable luminescence and energy transfer process between Tb3+ and Eu3+ in GYAG:Bi3+, Tb3+, Eu3+ phosphors

A series of Eu³⁺ ions co-doped (Gd_0.9Y_0.1)₃Al₅O₁₂:Bi³⁺, Tb³⁺ (GYAG) phosphors have been synthesized by means of solvothermal reaction method. The XRD pattern of GYAG phosphor sintered at 1500 °C confirms their garnet phase. The luminescence properties of these phosphors have been explored by analyzing their excitation and emission spectra along with their decay curves. The excitation spectra of the GYAG:Bi³⁺, Tb³⁺, Eu³⁺ phosphors consists of broad bands in the shorter wavelength region due to 4f⁸ → 4f⁷5d¹ transition of Tb³⁺ ions overlapped with 6s² → 6s¹6p¹ (¹S₀ → ³P₁) transition of Bi³⁺ ions and the charge transfer band of Eu³⁺–O^2?. The present phosphors exhibit green and red colors due to ⁵D₄ → ⁷F₅ transition of Tb³⁺ ions and ⁵D₀ → ⁷F₁ transition of Eu³⁺ ions, respectively. The emission was shifted from green to red color by co-doping with Eu³⁺ ions, which indicate that the energy transfer probability from Tb³⁺ to Eu³⁺ ions are dependent strongly on the concentration of Eu³⁺ ions.     

Reduction of Eu3 + to Eu2 + in Al-Codoped Silica Glasses Fabricated by the Sol-Gel Technique and CO2-Laser Processing

The spectroscopic properties of europium in aluminium codoped silica glasses produced by the sol-gel technique have been studied with respect to the dopant concentrations and the thermal processing applied to the samples. After thermal annealing at temperatures up to 950_°C the bright red fluorescence around 613 nm characteristic for the trivalent europium ions (Eu^{3 +}) has been observed. The lifetime was measured to be 0.1–2.4 ms depending on dopant concentrations and thermal treatment. Subsequent CO₂-laser processing in air (short time remelting) gave rise to a bright blue fluorescence consisting of two broad bands, lying around 450 and 490 nm, with their peak position depending on the ratio between the aluminium and europium concentrations. The fluorescence lifetimes were found to be shorter than 1 s. This blue fluorescence is attributed to the divalent europium ion (Eu^{2 +}), leading to the conclusion that the CO₂-laser processing of europium doped alumina-silica glasses resulted in the reduction of the trivalent to the divalent europium ion. Laser processing could therefore be a valid alternative to conventional thermal annealing for the generation of Eu^{2 +} in alumina-silica glasses.     

Synthesis and Luminescence of Eu3+-Doped Narsarsukite Prepared by the Sol-Gel Process

The sol-gel synthesis and structural characterisation of narsarsukite powders are reported. Samples doped with different Eu³⁺ amounts (Ti/Eu = 20 and 200), calcined at 800°C, have been characterised by powder XRD, ²³Na, ²⁹Si MAS NMR, Raman and luminescence spectroscopies. Eu³⁺-doped narsarsukite displays a high and stable room-temperature luminescence. The presence in narsarsukite of two different Eu³⁺ local environments is inferred based both on the distinct ⁵D₀ ⁷F₀ lines observed and on the local field splitting of the ⁷F_1,2 levels. For low lanthanide contents, the Eu³⁺ ions are essentially localised in a centrosymmetric environment characterized by a low-energy ⁵D₀ ⁷F₀ line and a relatively long ⁵D₀ lifetime (3.56–3.96 ms). In contrast, at high lanthanide contents the Eu³⁺ ions are also present in a second local site with a less covalent first coordination shell. This corresponds to a high-energy ⁵D₀ ⁷F₀ line and a short ⁵D₀ lifetime (0.84–0.99 ms). Therefore, it is likely that Eu³⁺ ions substitute for both Ti⁴⁺ and Na⁺, although the former ions are preferentially replaced at low Eu³⁺ content.     

Room Temperature Persistent Spectral Hole-Burning in Sol-Gel-Derived Glasses Doped with Eu3+ Ions

Persistent spectral hole burning was investigated for the Eu³⁺ ions-doped glasses prepared by a sol-gel method. For the glasses containing OH bonds, persistent spectral hole is burned by the laser-induced rearrangement of the OH bonds surrounding the Eu³⁺ ions, which is thermally unstable to erase up to 200 K. On the other hand, the Eu³⁺-doped Al₂O₃-SiO₂ glasses which are heated under H₂ gas or irradiated with X-ray exhibit room temperature PSHB. The depth of the burnt hole increases as the Al₂O₃ content increases. The hole-formation could be explained by a model of the excitation of the Eu³⁺ ions and subsequent electron transfer with the excited [Eu³⁺]^– or oxygen-defect centers in the Al—O bonds. The burnt holes are more stable compared with those burned by the rearrangement of the OH bonds.     

Thermal study in Eu3+-doped boehmite nanofibers and luminescence properties of the corresponding Eu3+:Al2O3

Eu³⁺-doped boehmite nanofiber materials with different Eu³⁺ concentrations were synthesized without any surfactant, and followed by a series of characterizations. It was found that the boehmite nanofibers became coarser with the increase of Eu³⁺ concentration, which resulted in a gradual decrease of their specific surface areas. Moreover, the thermal stability of the boehmite nanofibers was studied by thermogravimetry–differential scanning calorimetry. All materials showed the phase transition from γ-Al₂O₃ to other forms. Yet the transition temperature was increased with the increase of Eu³⁺ concentration. The Eu³⁺-doped boehmite nanofibers with the maximum Eu³⁺ concentrations showed the best thermal stability. Photoluminescence spectra showed that the 2 mol% of doping concentration of Eu³⁺ ions in Eu³⁺:Al₂O₃ nanofiber was optimum.     

Reddish-orange,neutral and warm white emissions in Eu3+, Dy3+ and Dy3+/Eu3+ doped CdO-GeO2-TeO2 glasses

Eu³⁺, Dy³⁺ and Dy³⁺/Eu³⁺ doped CdO-GeO₂-TeO₂ glasses were prepared using the melt-quenching process and analyzed by X-diffraction, Raman spectroscopy, excitation and emission spectra, and emission decay time profiles. The lack of X ray diffraction peaks revealed that all samples are amorphous. Vibrational modes associated with TeOTe and GeOGe related bonds and molecular oxygen were detected by Raman spectroscopy. The luminescence characteristics were studied upon excitations that correspond with the emission of InGaN (370–420 nm) based LEDs. The Eu³⁺ singly doped glass displayed reddish-orange global emission, with x = 0.601 and y = 0.349 CIE1931 chromaticity coordinates, upon 393 nm excitation. Neutral emission with x = 0.373 and y = 0.412 CIE1931 chromaticity coordinates and correlated color temperature (CCT) of 4400 K, was achieved in the Dy³⁺ singly doped glass excited at 388 nm. The Dy³⁺/Eu³⁺ co-doped glass exhibited warm, neutral and soft warm white emissions with CCT values of 3435, 4153 and 2740 K, under excitations at 382, 388 and 393 nm, respectively, depending mainly on the Dy³⁺ and Eu³⁺ relative excitation. The Dy³⁺ excitation bands observed in the Dy³⁺/Eu³⁺ glass by monitoring the 611 nm Eu³⁺ emission, suggest that Dy³⁺ → Eu³⁺ energy transfer takes place, despite the fact that the Dy³⁺ emission decays in the Dy³⁺ and Dy³⁺/Eu³⁺ doped glass, remain without changes. The shortening of Eu³⁺ decay in presence of Dy³⁺ was attributed to an Eu³⁺ → Dy³⁺ non-radiative energy transfer process, which according with the Inokuti-Hirayama model might be dominated through an electric quadrupole-quadrupole interaction, with efficiency and probability of 5.5% and 51.6 s⁻¹, respectively.     

Excitation-dependent variation in local symmetry in Ba2Mg(BO3)2 evidenced by Eu3+ luminescent structural probe

Eu³⁺ luminescence was studied in Ba₂Mg(BO₃)₂ by selectively substituting at Mg site. The parent host Ba₂Mg(BO₃)₂ and Ba₂Mg_0.9Eu_0.05Li_0.05(BO₃)₂ were synthesized by conventional solid state reaction method. Their isostructural nature was confirmed using powder X-ray diffraction technique. The photoluminescence excitation spectrum of Eu³⁺ exhibited a broad Eu³⁺O²⁻ charge transfer band with a maximum at 253 nm along with other excitation transitions. The emission characteristics of Eu³⁺ were found to be excitation wavelength-dependent. The equally intense magnetic and electric dipole transitions for excitation under longer wavelengths showed the presence of Eu³⁺ at a site with non-inversion symmetry. Excitation using 253 nm resulted in the predominant magnetic dipole transition revealing Eu³⁺ at a site with inversion symmetry. The difference in the relative intensities of magnetic and electric dipole transitions originates from the change in symmetry around Eu³⁺ in Ba₂Mg(BO₃)₂ under different excitations.     

Complexation of Eu3+ and Am3+ with soil humic acid extracted from Okchon Basin of Korean Peninsula

The complexation of Eu³⁺ and Am³⁺ ions with the humic acids has been investigated at various pH (4.0, 4.5, 5.0, 5.4) in 0.1M NaClO₄ solution using solvent extraction technique. Two humic acids are used in this study: humic acid extracted from the soil of Taejon on the Okchon Basin of Korea (TJHA) and commercially available one from Aldrich Chemical Co. (AHA). The total carboxylate group concentrations were determined to be 3.58 meq/g and 4.59 meq/g for Taejon and Aldrich humic acids, respectively. The conditional stability constants (log ₁ and log ₂), dependent on the pH of the solution, of the complexes of Eu³⁺ and Am³⁺ ions with the humic acids have been determined at the ionic medium of 0.1M NaClO₄. The values of stability constants with the degree of ionization of TJHA for Eu and Am complexes are quite well agreed with those of Lake Bradford humic acid (LBHA), indicating that structural characteristics of TJHA and LBHA may be quite similar to one another.     

Synthesis and spectral characteristics of Sr2Y8(SiO4)6O2: Eu polycrystals

Spectral-luminescent characteristics of Sr₂Y₈(SiO₄)₆O₂: Eu powder crystal phosphor with the apatite structure and high-intensity luminescence of Eu³⁺ ions have been studied. The charge state of europium in the samples has been characterized by means of X-ray L₃-adsorption spectroscopy. It was established that Eu³⁺ forms two types of optical centers. Besides, luminescence of Eu²⁺ions was found. Reduction Eu³⁺→Eu²⁺ was considered, which may be due to vacancy formation in the 4f crystal lattice position and to negative charge transfer by this vacancy to two ions. Thus, in the silicate lattice there exist inhomogeneously distributed oxygen-deficient centers, which are responsible for nonradiative transfer of excitation energy to Eu³⁺ and Eu²⁺ ions. To study electron-vibrational interactions in the crystal phosphor samples, their IR and Raman spectra were examined. In the luminescence spectrum of Eu²⁺, a series of low-intensity bands caused by interaction of the 4f⁶5d state of Eu²⁺ with silicate lattice vibrations was observed.     

Spectral Hole-Burning in Femtosecond Laser-Irradiated Eu<Superscript>3+</Superscript>-Doped Aluminosilicate Glasses

We studied the persistent spectral hole-burning (PSHB) of the Eu³⁺-doped Al₂O₃-SiO₂ glasses, prepared by a sol–gel process, exposed to femtosecond laser pulses. The spectral holes were burned in the excitation spectra of the ⁷F₀⁵D₀ transition of Eu³⁺ ion. The depth and width of the burned holes were 15% and 2.5 cm^–1 fwhm at 7 K, respectively. The burned hole is stable up to room temperature. Fluorescence line narrowing spectra showed that Eu³⁺ ions were located in two different sites.     

High sensitivity and multicolor tunable optical thermometry in Bi3+/Eu3+ co-doped Ca2Sb2O7 phosphors

Currently, with increasing demand for non-contact fluorescence intensity ratio-based optical thermometry, a novel phosphor with high-efficiency, dual-emitting centers, and differentiable temperature sensitivity is more and more urgent to develop. In this work, an efficient dual-emitting center optical thermometry with high sensitivity and multicolor tunable in Ca₂Sb₂O₇:Bi³⁺, Eu³⁺ phosphor is firstly designed and successfully prepared. Under 330 nm excitation, the fabricated phosphor presents the featured and distinguishable emissions of Bi³⁺ and Eu³⁺ ions. The high efficiency energy transfer from Bi³⁺ to Eu³⁺ ions is proved and its corresponding mechanism belongs to dipole-dipole interaction. By modulating the ratio of Bi³⁺/Eu³⁺, the multicolor changes from blue to pink are realized. Based on the discriminative thermal quenching behavior between Bi³⁺ and Eu³⁺, the fluorescence intensity ratio of Eu³⁺ to Bi³⁺ in Ca₂Sb₂O₇ samples illustrates excellent optical thermometry performance from 298 to 523 K. The maximum absolute sensitivity (S_a) and relative sensitivity (S_r) reach as high as 0.2773 K^?1 at 523 K and 2.37% K^?1 at 448 K, respectively. Notably, the discriminated surrounding temperature can be directly confirmed by observing the emitting color from purple to orange-red with the temperature increase from 298 to 523 K. Furthermore, the as-prepared phosphor materials also demonstrate outstanding repeatability and excellent reversibility. These results exhibit that the designed Ca₂Sb₂O₇:Bi³⁺, Eu³⁺ phosphors have great promising applications in the field of non-contact optical temperature thermometry and thermochromic.     

Eu<Superscript>3+</Superscript>-fluorescence properties in nano-crystallized SnO<Subscript>2</Subscript>-SiO<Subscript>2</Subscript> glass-ceramics

Fluorescence and spectral hole burning properties of Eu³⁺ ions were studied in nanocrystals-precipitated SnO₂-SiO₂ glasses. The glasses were prepared to contain various amount of Eu₂O₃ using the sol-gel method, in which SnO₂ nanocrystals were precipitated by heating in air. In the glasses containing Eu₂O₃ less than 1%, the Eu³⁺ ions were preferentially doped in the SnO₂ nanocrystals and their fluorescence intensities were enhanced by the energy transfer due to the recombination of electrons and holes excited in SnO₂ crystals. The SnO₂ nanocrystals-precipitated glasses exhibited the persistent spectral holes with the depth of ∼25% of the total fluorescence intensities of the Eu³⁺ ions. With the increasing Eu₂O₃ concentration, the amount of SnO₂ nanocrystals decreased and the Sn⁴⁺ ions formed the random glass structure together with the silica network. This structure change induced the fluorescence intensities and the hole depth to decrease.     

Effect of SnO2 Nanocrystals on the Emission of Eu3+ Ions in Silica Matrix

Silica xerogels containing Eu³⁺ ions and SnO₂ nanocrystals were prepared in the sol‐gel process, and characterized by x‐ray diffraction (XRD) and photoluminescence spectra. Under the excitation at 393 nm, characteristic emission of Eu³⁺ ions at 614 nm was enhanced with increasing amount of SnO₂ nanocrystals. Moreover, when the Eu³⁺/SnO₂ co‐doped samples were excited at 345 nm, corresponding to the sideband of SnO₂ nanocrystals, the emission of Eu³⁺ ions at 614 nm was clearly observed, while no emission of Eu³⁺ ions for the Eu³⁺‐doped sample. It may be ascribed to the energy transfer from SnO₂ conduction band to Eu³⁺ conduction band. Further experimental results suggest that the energy transfer may be achieved through surface transition state.     

Control of particle size distribution of CdS quantum dots in gel matrix

A novel sol-gel process has been developed to prepare nano-sized CdS quantum dots to improve the nonlinear optical properties. A bifunctional ligand, 3-aminopropyl triethoxysilane H₂N(CH₂)₃Si(OC₂H₅)₃, was used to disperse the Cd²⁺ ions in the gel solution. The CdO and CdS particles were observed by transmission electron microscope (TEM). The size of CdS microcrystallites with concentrations up to 13 wt.% in SiO₂ gel matrix was found to be in the range of 2–4 nm with a very sharp size distribution. A well-defined absorption edge was observed in the absorption spectrum.     

Synthesis and optical properties of novel red phosphors YNbTiO6:Eu3+ with highly enhanced brightness by Li+ doping

Highly luminescent euxenite phased YNbTiO₆:Eu³⁺ and Li⁺-doped YNbTiO₆:Eu³⁺ red phosphors have been prepared through a facile sol–gel combustion process and investigated for the first time. The introduction of Li⁺ ions into YNbTiO₆:Eu³⁺ is able to result in significant changes of the crystallinity and particle size, and bring a clear red-shift of absorption edge. A dominant red emission peak at 611 nm due to the ⁵D₀ → ⁷F₂ transition of Eu³⁺ was observed from photoluminescence spectra of the YNbTiO₆:Eu³⁺ and Li⁺-doped YNbTiO₆:Eu³⁺ phosphors. In particular, the emission intensity of the optimal Li⁺-doped YNbTiO₆:Eu³⁺ was examined to be close to 400% of commercial Y₂O₃:Eu³⁺ phosphor. The mechanism of the enhanced emission by Li⁺ doping was discussed.     

Structural and spectroscopic investigations on Eu3+-doped alkali fluoroborate glasses

Eu³⁺-doped alkali fluoroborate glasses B₂O₃–XCO₃–NaF–Eu₂O₃ (where X = Li₂, Na₂, K₂, and Ca, Mg) have been prepared using the conventional melting technique and their structural and optical properties have been evaluated. The XRD pattern of the glasses confirmed the amorphous nature and the FTIR spectra reveal the presence of BO₃ and BO₄ units as their local structures along with the strong OH^? groups. From the absorption spectra the bonding parameters have been calculated and confirmed that the Eu–O bonds in the studied glasses are of covalent nature. Judd–Ofelt (JO) analysis has been carried out from the emission spectra. The JO parameters have been used to calculate transition probabilities (A), lifetime (τ_R) and branching ratios (β_R) and peak stimulated emission cross-section (σ_P^E) for the ⁵D₀ → ⁷F_J (J = 1, 2, 3 and 4) transitions of the Eu³⁺ ions. The decay from the ⁵D₀ level of Eu³⁺ ions in the title glasses has been measured and analysed. The lifetime of the ⁵D₀ level is found to be shorter than the reported glasses which may be due to the presence of OH^? groups.     

Local structure and photoluminescent characteristics of Eu<Superscript>3+</Superscript> in ZnO–SiO<Subscript>2</Subscript> glasses

The photoluminescence properties of xZnO–(100−x)SiO₂ (x = 0, 5, 10, 20) containing 1% Eu₂O₃ prepared by a sol–gel method were systematically investigated. The results indicated that the relative proportion of f–f transitions to charge transfer (CT) absorption decreased with the increase of ZnO concentration. The intensity of ⁵D₀–⁷F_J transitions of Eu³⁺ ions was enhanced with the increase of ZnO content due to local structure changes and decreased quantities of Eu³⁺ ions clusters. The results of fluorescence line narrow (FLN) spectra indicated that Eu³⁺ ions occupied one site in SiO₂ glass and two sites in ZnO–SiO₂ glasses. The second-order crystal field parameters were calculated. B₂₀ and B₂₂ for site 1 increased with excitation energy, while ones hardly changed for site 2.     

Synthesis and photoluminescence of the Y2O3:Eu3+ phosphor nanowires in AAO template

The monodisperse array and nanowires of Y₂O₃:Eu³⁺ phosphor were synthesized using anodic aluminum oxide (AAO) template by sol–gel method. Scanning electron microscope (SEM) images indicated that Y₂O₃:Eu³⁺ nanowires are parallelly arranged, all of which are in uniform diameter of about 50 nm. The high-magnification SEM image showed that each nanowire is composed of a lot of agglutinating particles. The patterns of selected-area electron diffraction confirmed that Y₂O₃:Eu³⁺ nanowires mainly consist of polycrystalline materials. Excitation and emission spectra of Y₂O₃:Eu³⁺/AAO composite films were measured. The characteristic red emission peak of Eu³⁺ ion attributed to ⁵D₀→⁷F₂ transition in Y₂O₃:Eu³⁺/AAO nanowires broadened its halfwidth.     

Structure and spectroscopic properties of (Y,Eu)(PO3)3 polyphosphate red phosphors

A series of orange-red emitting phosphor Y(PO₃)₃: xEu³⁺ (x = 0.1–1.0) was prepared by a solid-state reaction route. The phosphors were characterized by X-ray diffraction (XRD) and photoluminescence (PL) as well as decay lifetimes. Studies revealed the phase transfer from monoclinic to orthorhombic when Y³⁺ is totally replaced by Eu³⁺, and expansion of the unit cell occurs with increasing Eu³⁺ doped content. The PL spectra show that the phosphors Y(PO₃)₃: xEu³⁺ can be effectively excited by near ultraviolet (n-UV) light, and exhibit strong red-orange emission with no concentration quenching. The profile of PL spectra changes significantly at high Eu³⁺ content (x ≥ 0.80), which is due to the variation of preference for substitution of Eu³⁺. The luminescence due to the ⁵D₀ → ⁷F_J (J = 1, 2) transitions at 77 K exhibits its own spectral features for different crystallographic site. It is found that Eu³⁺ ions occupy the centers of octahedral polyhedron and form C_i/C₁ point group in Y(PO₃)₃.