Photoreduction of Europium (III) in Sol-Gel Derived Inorganic-Organic Hybrid Materials

Eu²⁺-doped inorganic-organic hybrid materials, which are potentially suitable for a tunable laser in the near ultra violet and blue region, were prepared through the photoreduction of Eu³⁺ ions in the materials under the irradiation of the fourth harmonic wave light (266 nm) of the Nd:YAG laser. The hybrid materials doped with Eu³⁺ ions were prepared from Si(OCH₃)₄, CH₃Si(OCH₃)₃, EuCl₃ and chloropropyltrimethoxysilane (CPTM). After the prehydrolized silica sol was added to the Eu³⁺-containing solution, Eu³⁺-doped transparent inorganic-organic hybrid material was obtained by drying at 50°C. The emission peak around 450–475 nm due to the charge transfer transition (5d-4f) of Eu²⁺ ions increased with the laser irradiation time. Eu³⁺ ions were effectively photoreduced to Eu²⁺ ions in pore-free materials prepared at high CPTM to Eu³⁺ ratios. Eu²⁺ ions were generated by the photodecomposition of the bond between Eu³⁺ and Cl (Cl^– or Cl(CH₂)₃ in CPTM).     

Y2O3:Eu (5 mol%) with Ag nanoparticles prepared by citrate precursor

Y₂O₃:Eu³⁺ (5 mol% Eu³⁺) and Y₂O₃:Eu³⁺ (5 mol% Eu³⁺) containing 1 mol% of Ag nanoparticles were prepared by heat treatment of a viscous resin obtained via citrate precursor. TEM and EDS analyses showed that Y₂O₃:Eu³⁺ (5 mol% Eu³⁺) is formed by nanoparticles with an average size of 12 nm, which increases to 30 nm when Ag is present because the effect of metal induced crystallization occurs. Ag nanoparticles with a size of 9 nm dispersed in Y₂O₃:Eu³⁺ (5 mol% Eu³⁺) were obtained and the surface plasmon effect on Ag nanoparticles was observed. The emission around 612 nm assigned to the Eu³⁺ (⁵D₀→⁷F₂) transition enhanced when the Ag nanoparticles were present in the Y₂O₃:Eu³⁺ luminescent material.     

Vacuum Ultraviolet Excited Photoluminescence Properties of Y2O2S：Eu^3＋,Bi^3＋ Phosphor

As an Hg-free lamp using phosphor, the Bi^3＋ and EH^3＋ co-doped Y2O2S phosphors were prepared and their luminescence properties under vacuum ultraviolet（VUV） excitation were investigated. The VUV photoluminescent intensity of Y2O2S：Eu^3＋ was weak, however, considerably stronger red emission at 626 nm with good color purity was observed in Y2O2S：Eu^3＋,Bi^3＋ systems. Investigation on the photoluminescence reveals that the strong VUV luminescence of Y2O2S：Eu^3＋,Bi^3＋ at 147 nm is mainly because the Bi^3＋ acts as a medium and effectively performs the energy transfer process： Y^3＋-O^2-→Bi^3＋→Eu^3＋, while the intense emission band at 172 nm is attributed to the absorption of the characteristic ^1So-^1P1 transition of Bi^3＋ and the direct energy transfer from Bi^3＋ to Eu^3＋. The Y2O2S：Eu^3＋,Bi^3＋ shows excellent VUV optical properties compared with the commercial （Y,Gd）BO3：Eu^3＋. Thus, the Y2O2S：Eu^3＋,Bi^3＋ can be a potential red VUV-excited candidate applied in Hg-free lamps for backlight of liquid crystal display.     

Luminescence and energy transfer in a highly symmetrical system: Eu2Ti2O7

Luminescence and energy transfer properties of Gd₂Ti₂O₇: Eu and Eu₂Ti₂O₇ are reported. Transfer between unperturbed (intrinsic) Eu³⁺ ions and perturbed (extrinsic) Eu³⁺ ions has been observed. At low temperatures the emission spectra of Eu₂Ti₂O₇ are dominated by trap emission, due to direct energy transfer from the intrinsic Eu³⁺ ions to the extrinsic Eu³⁺ ions. Above 10 K energy migration among the Eu³⁺ ions to quenching centers occurs. The interaction between the Eu³⁺ ions is probably exchange in character. The nature of the extrinsic Eu³⁺ ions has been elucidated.     

Synthesis,Persistent Luminescence,and Thermoluminescence Properties of Yellow Sr3SiO5:Eu2+,RE3+ (RE=Ce,Nd, Dy,Ho, Er,Tm, Yb) and Orange‐Red Sr3−xBaxSiO5:Eu2+, Dy3+ Phosphor

Sunlight‐excitable orange or red persistent oxide phosphors with excellent performance are still in great need. Herein, an intense orange‐red Sr_3?xBa_xSiO₅:Eu²⁺,Dy³⁺ persistent luminescence phosphor was successfully developed by a two‐step design strategy. The XRD patterns, photoluminescence excitation and emission spectra, and the thermoluminescence spectra were investigated in detail. By adding non‐equivalent trivalent rare earth co‐dopants to introduce foreign trapping centers, the persistent luminescence performance of Eu²⁺ in Sr₃SiO₅ was significantly modified. The yellow persistent emission intensity of Eu²⁺ was greatly enhanced by a factor of 4.5 in Sr₃SiO₅:Eu²⁺,Nd³⁺ compared with the previously reported Sr₃SiO₅:Eu²⁺, Dy³⁺. Furthermore, Sr ions were replaced with equivalent Ba to give Sr_3?xBa_xSiO₅:Eu²⁺,Dy³⁺ phosphor, which shows yellow‐to‐orange‐red tunable persistent emissions from λ=570 to 591 nm as x is increased from 0 to 0.6. Additionally, the persistent emission intensity of Eu²⁺ is significantly improved by a factor of 2.7 in Sr_3?xBa_xSiO₅:Eu²⁺,Dy³⁺ (x=0.2) compared with Sr₃SiO₅:Eu²⁺,Dy³⁺. A possible mechanism for enhanced and tunable persistent luminescence behavior of Eu²⁺ in Sr_3?xBa_xSiO₅:Eu²⁺,RE³⁺ (RE=rare earth) is also proposed and discussed.     

Effect of Sr2+ doping on the luminescence properties of YVO4:Eu3+,Sr2+ particles prepared by a solvothermal method

Well-dispersed Eu³⁺ and Sr²⁺ co-doped YVO₄ luminescent particles (YVO₄:Eu³⁺,Sr²⁺) on the submicron scale were prepared by a facile solvothermal method at low temperature. The effect of Sr²⁺ doping on the luminescence of YVO₄:Eu³⁺,Sr²⁺ particles was investigated by fixing the Eu³⁺ doping concentration at 7 mol%. It was found that the luminescence intensity of the as-prepared YVO₄:Eu³⁺,Sr²⁺ particles increased with the Sr²⁺ doping concentration x to reach a two-fold enhancement when x = 5 %, and then decreased for higher x. We also investigated the effect of thermal annealing on the luminescence properties of the YVO₄:Eu³⁺ and YVO₄:Eu³⁺,Sr²⁺ particles. A remarkable enhancement in their luminescence properties was observed after annealing at 900 °C in air for 30 min. It was showed that the annealed YVO₄:Eu³⁺,Sr²⁺ particles exhibited a two-fold stronger emission than the annealed YVO₄:Eu³⁺. This work indicates that Sr²⁺ doping is beneficial to the luminescence enhancement for both the as-prepared and annealed YVO₄:Eu³⁺,Sr²⁺ particles.     

In situ analysis for spontaneous reduction of Eu<Superscript>3+</Superscript> in LiCl pyroprocessing media at 923 K

The spontaneous reduction of Eu³⁺ to Eu²⁺ was examined when EuCl₃ was added into a pyroprocessing media of LiCl molten salt at 923 K. The amount of Eu²⁺ was calculated by measuring the total charge consumed to oxidize Eu²⁺ ions to Eu³⁺ ions. The concentration ratio of Eu²⁺ to Eu³⁺ was estimated to be about 0.40 in the media. In addition, it is confirmed that the reduction of Eu³⁺ to Eu²⁺ is caused by the oxidation power of Cl⁻ to Cl₂. The coexistence of Eu³⁺ and Eu²⁺ in the LiCl molten salt system was examined by UV–Visible and luminescence spectroscopy. The molar absorptivities of Eu³⁺ and Eu²⁺, calculated from UV–Visible absorption spectra, were 423 and 1954 M⁻¹ cm⁻¹, respectively.     

Tunable photoluminescence of NaYF4:Eu nanocrystals by Sr codoping

NaYF₄:Eu/Sr nanocrystals were synthesized by a hydrothermal method. Tunable photoluminescence of the NaYF₄:Eu nanocrystals was successfully achieved by codoping with Sr²⁺ ions. With increasing Sr²⁺ concentration, not only the X-ray diffraction peaks of the nanocrystals become broader, but also the positions of them shift toward larger lattice parameters. Eu³⁺ and Eu²⁺ have been found to coexist in an NaYF₄:Eu/Sr. The Eu³⁺/Eu²⁺ emission intensity ratio changed with the Sr²⁺ concentration and excitation wavelength. More interestingly, the spectral configurations of Eu²⁺ and Eu³⁺ also varied with the excitation wavelength, indicating that the nanocrystals have multiple luminescence centers or emitting states.     

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.     

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.     

Mechanistic Insight into Energy-Transfer Dynamics and Color Tunability of Na4CaSi3O9:Tb3+,Eu3+ for Warm White LEDs

In this work, a latent energy-transfer process in traditional Eu³⁺,Tb³⁺-doped phosphors is proposed and a new class of Eu³⁺,Tb³⁺-doped Na₄CaSi₃O₉ (NCSO) phosphors is presented which is enabled by luminescence decay dynamics that optimize the electron-transfer energy process. Relative to other Eu³⁺,Tb³⁺-doped phosphors, the as-synthesized Eu³⁺,Tb³⁺-doped NCSO phosphors show improved large-scale tunable emission color from green to red upon UV excitation, controlled by the Tb³⁺/Eu³⁺ doping ratio. Detailed spectroscopic measurements in the vacuum ultraviolet (VUV)/UV/Vis region were used to determine the Eu³⁺–O²⁻ charge-transfer energy, 4f–5d transition energies, and the energies of 4f excited multiplets of Eu³⁺ and Tb³⁺ with different 4f^N electronic configurations. The Tb³⁺→Eu³⁺ energy-transfer pathway in the co-doped sample was systematically investigated, by employing luminescence decay dynamics analysis to elucidate the relevant energy-transfer mechanism in combination with the appropriate model simulation. To demonstrate their application potential, a prototype white-light-emitting diode (WLED) device was successfully fabricated by using the yellow luminescence NCSO:0.03Tb³⁺, 0.05Eu³⁺ phosphor with high thermal stability and a BaMgAl₁₀O₁₇:Eu²⁺ phosphor in combination with a near-UV chip. These findings open up a new avenue to realize and develop multifunctional high-performance phosphors by manipulating the energy-transfer process for practical applications.     

Unique Spectral Overlap and Resonant Energy Transfer between Europium(II) and Ytterbium(III) Cations: No Quantum Cutting

Samples of the Ca₃Sc₂Si₃O₁₂ (CSS) host singly doped with Eu²⁺ or Yb³⁺, doubly doped with Eu²⁺ and Yb³⁺, and triply doped with Ce³⁺, Eu²⁺ and Yb³⁺ were synthesized by a sol–gel combustion process under reducing conditions. Unlike previous reports of Eu²⁺→Yb³⁺ energy transfer in other systems, the energy transfer is resonant in the CSS host and the transfer efficiency reaches 100 % for lightly doped samples. The transfer mechanism is multipolar rather than electron transfer for the sample compositions employed herein. The emission intensity of Yb³⁺ is further enhanced by co‐doping with Ce³⁺ in addition to Eu²⁺. The quantum efficiencies of the doped materials range between 9 % and 93 %.     

Silica-Coated CaF2:Eu3+ Nanoparticles Functionalized with Oxalic Acid for Bio-conjugation to BSA Proteins

A new method for silica‐coated CaF₂:Eu³⁺ core‐shell nanoparticles functionalized with oxalic acid for bio‐conjugation to bovine serum albumin (BSA) proteins has been developed. Moreover, CaF₂:Eu³⁺/SiO₂ core‐shell nanoparticles modified with oxalic acid are biocompatible and can be dispersed in water. As an organic functional molecule, oxalic acid is able to react with hydroxyl groups existed on the surface of SiO₂ layer by esterification reaction to form carboxylic acid for further bio‐conjugation with BSA. The final products were characterized by means of X‐ray diffraction (XRD), transmission electron microscope (TEM), field‐emission scanning electron microscopy (FE‐SEM), ultraviolet (UV) spectrophotometer, infrared (IR) spectrophotometer and photoluminescence (PL) spectra. XRD result confirmed the phase purity of CaF₂:10 mol% Eu³⁺ and CaF₂:10 mol% Eu³⁺/SiO₂ nanoparticles obtained from the quaternary reverse micelles of cetyltrimethylammonium bromide (CTAB), cyclohexane, n‐pentanol and water. Images of TEM and FE‐SEM showed that the average grain sizes of CaF₂:10 mol% Eu³⁺/SiO₂ and bio‐conjugation of CaF₂:10 mol% Eu³⁺/SiO₂ nanoparticles with BSA were about 17 nm. The patterns of UV and IR spectra showed that BSA was linked to CaF₂:10 mol% Eu³⁺/SiO₂ nanoparticles. In the emission spectrum of CaF₂:10 mol% Eu³⁺/SiO₂ conjugated by BSA nanoparticles, characteristic emission peaks of Eu³⁺ within the wavelength ranging from 500 to 700 nm were observed, which is corresponding to the transitions from the excited ⁵D₀ levels to ⁷F_J levels. This confirmed that the Eu³⁺ dopant ion is located in a Ca²⁺ crystal site with T_d symmetry. CaF₂:10 mol% Eu³⁺/SiO₂ conjugated by BSA nanoparticles remain stable in aqueous media within 15 d with pH ranging from 2 to 9. Therefore, these luminescent colloidal nanoparticles can be potentially employed as targeted fluorescent labels in biomedical research applications.     

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.     

Magnetic properties and Eu Mössbauer effects of mixed valence europium copper sulfide, Eu2CuS3

Ternary europium copper sulfide Eu₂CuS₃ have been investigated by X-ray diffraction, ¹⁵¹Eu Mössbauer spectroscopy, magnetic susceptibility, magnetization, and specific heat measurements. In this compound, Eu²⁺ and Eu³⁺ ions occupy two crystallographically independent sites. The ¹⁵¹Eu Mössbauer spectra indicate that the Eu²⁺ and Eu³⁺ ions exist in the molar ratio of 1:1, and the Debye temperatures of Eu²⁺ and Eu³⁺ are 180 and 220 K, respectively. In its magnetic susceptibility, the divergence between the zero-field cooled and field cooled susceptibilities appears below 3.4 K. The specific heat has a λ-type anomaly at the same temperature. From the field dependence of magnetization at 1.8 K, the Eu²⁺ ion was found to be in the ferromagnetic state with the saturation magnetization M_S=6.7 μ_B.     

Controlled synthesis and luminescent properties of Eu(Eu), Dy-doped Sr3Al2O6 phosphors by hydrothermal treatment and postannealing approach

In this work, Sr₃Al₂O₆: Eu²⁺ (Eu³⁺), Dy³⁺ phosphors have been prepared by hydrothermal treatment and subsequently postannealing approach, using Sr(NO₃)₂, Al(NO₃)₃·9H₂O, and CO(NH₂)₂ as starting materials. The as-obtained phosphors were characterized by means of XRPD, FESEM, and PL techniques. In addition, many reaction parameters were studied in detail, including the initial mole ratios, hydrothermal reaction temperature, calcination temperature and calcination atmosphere. Remarkably, two scientific merits exist herein: Sr₃Al₂O₆: Eu²⁺ (Eu³⁺), Dy³⁺ phosphors can be selectively obtained in a reducing atmosphere (H₂/Ar, 20%+80%) and in air, respectively; adding certain amount of sodium citrate can alter the shape and size of Sr₃Al₂O₆: Eu²⁺ (Eu³⁺), Dy³⁺ phosphors in essence. Besides, the luminescent properties of Sr₃Al₂O₆: Eu²⁺ (Eu³⁺), Dy³⁺ phosphors were studied by excitation spectra, emission spectra and decay curves.     

Photoluminescence and structural characteristics of Lu2O3:Eu nanocrystallites in silica matrix

Two silica ceramics were obtained by mixing nanocrystalline Lu₂O₃:Eu³⁺ with silica sol using the sol-gel technique. The synthesis procedure for both samples differed in the pH of the sol and time of the sol condensation before substrates were mixed together, which implied their different optical properties. The first one has the same spectroscopic properties as Lu₂O₃:Eu³⁺ nanocrystallites with an exception of small lowering of the charge transfer (CT) band intensity. This feature is preserved up to about 950 °C. Above this temperature, nanocrystallites of Lu₂O₃:Eu³⁺ react with the silica matrix synthesis pyrosilicate (Lu₂Si₂O₇). The Eu³⁺ ions occupy only one crystallographic site in the crystal lattice for low concentration of the activator (1%) and two sites for higher concentration (10%). The second sample exhibits different Eu³⁺ emission than Lu₂O₃:Eu³⁺ nanocrystallites and, additionally, a broad band of the matrix originating at the green region of the spectrum. Sintering the sample at higher temperatures leads to disappearance of this broad emission and continuous changes of the Eu³⁺ emission because of the progressive conversion of the Lu₂O₃:Eu³⁺ to pyrosilicate. At 1300 °C for both samples, the reaction of synthesis lutetium pyrosilicate is completed. Structural characteristic of the samples is presented and correlate with the decay profile of the Eu³⁺ emission.     

Synthesis and characterization of Eu<Superscript>3+</Superscript> doped Lu<Subscript>2</Subscript>O<Subscript>3</Subscript> nanophosphor using a solution-combustion method

Fine Eu³⁺-doped lutetium oxide (Lu₂O₃:Eu³⁺) nanophosphor were synthesized using a low-temperature solution-combustion method in a methyl-alcohol solution. The characteristics of the nanophosphors synthesized at various sintering temperatures with different Eu³⁺ concentrations were analyzed to determine the optimum synthesis conditions. Thermogravimetry/differential thermal analysis showed that Lu₂O₃:Eu³⁺ crystallizes completely when the dry powder is sintered at 500 °C. The Lu₂O₃:Eu³⁺ crystals had a cubic structure and monoclinic phase. The peak position of the luminescence spectrum did not differ with the concentration of Eu or the sintering temperature or atmosphere, whereas the luminescence intensity was strongly dependent on the concentration and sintering conditions.     

Rare earth complexes chemiluminescence catalyzed by gold nanoparticles for fast sensing of Tb3+ and Eu3+

Developing multiplex sensing technique is of great significance for fast sample analysis. However, the broad emissions of most chemiluminescence(CL) luminophores make the multiplex CL analysis be difficult. In this work, a simple and sensitive CL analytical method has been developed for the simultaneous determination of Tb³⁺and Eu³⁺thanking to their narrow band emission. The technique was based on a mixed CL system of periodate(IO₄^-)-hydrogen peroxide(...     

Research on Molecular Structure and Electronic Properties of Ln3+ (Ce3+, Tb3+, Pr3+)/Li+ and Eu2+ Co-Doped Sr2Si5N8 via DFT Calculation

We use density functional theory (DFT) to study the molecular structure and electronic band structure of Sr₂Si₅N₈:Eu²⁺ doped with trivalent lanthanides (Ln³⁺ = Ce³⁺, Tb³⁺, Pr³⁺). Li⁺ was used as a charge compensator for the charge imbalance caused by the partial replacement of Sr²⁺ by Ln³⁺. The doping of Ln lanthanide atom causes the structure of Sr₂Si₅N₈ lattice to shrink due to the smaller atomic radius of Ln³⁺ and Li⁺ compared to Sr²⁺. The doped structure’s formation energy indicates that the formation energy of Li⁺, which is used to compensate for the charge imbalance, is the lowest when the Sr2 site is doped. Thus, a suitable Li⁺ doping site for double-doped lanthanide ions can be provided. In Sr₂Si₅N₈:Eu²⁺, the doped Ce³⁺ can occupy partly the site of Sr₁²⁺ ([SrN₈]), while Eu²⁺ accounts for Sr₁²⁺ and Sr₂²⁺ ([SrN₁₀]). When the Pr³⁺ ion is selected as the dopant in Sr₂Si₅N₈:Eu²⁺, Pr³⁺ and Eu²⁺ would replace Sr₂²⁺ simultaneously. In this theoretical model, the replacement of Sr²⁺ by Tb³⁺ cannot exist reasonably. For the electronic structure, the energy level of Sr₂Si₅N₈:Eu²⁺/Li⁺ doped with Ce³⁺ and Pr³⁺ appears at the bottom of the conduction band or in the forbidden band, which reduces the energy bandgap of Sr₂Si₅N₈. We use DFT+U to adjust the lanthanide ion 4f energy level. The adjusted 4f-CBM of Ce_Sr1Li_Sr1-Sr₂Si₅N₈ is from 2.42 to 2.85 eV. The energy range of 4f-CBM in Pr_Sr1Li_Sr1-Sr₂Si₅N₈ is 2.75–2.99 eV and its peak is 2.90 eV; the addition of Ce³⁺ in Eu_Sr1Ce_Sr1Li_Sr1 made the 4f energy level of Eu²⁺ blue shift. The addition of Pr³⁺ in Eu_Sr2Pr_Sr2Li_Sr1 makes part of the Eu²⁺ 4f energy level blue shift. Eu²⁺ 4f energy level in Eu_Sr2Ce_Sr1Li_Sr1 is not in the forbidden band, so Eu²⁺ is not used as the emission center.