Yue Tian

Optical transition, electron-phonon coupling and fluorescent quenching of La2(MoO4)3:Eu3+ phosphor

La2(MoO4)3 phosphors with various Eu3+ concentrations were prepared via a facile co-precipitation process. The crystal structure and morphology of the phosphors were characterized by means of XRD and field emission scanning electron microscope. The crystal unit cell parameters a, b, and c for the monoclinic phase La2(MoO4)3 were calculated to be 16.989, 11.927, and 16.086 A, respectively. The average size of the phosphor particles was estimated to be around 88.5 nm. The Huang–Rhys factor was derived from the phonon sideband spectra to be 0.073. The self-generated quenching process of Eu3+ was explained based on Auzel’s model, and the intrinsic radiative transition lifetime for 5D0 level was confirmed to be 0.99 ms. A new approach for calculating the Judd–Ofelt parameters was developed, meanwhile the Judd–Ofelt parameters Ωλ (λ = 2, 4, 6) of Eu3+ in La2(MoO4)3 phosphors were confirmed to be 10.70 × 10−20, 1.07 × 10−20, and 0.56 × 10−20 cm2, respectively. Finally, the optimal doping concentration for achiev...

Self-assembled 3D flower-shaped NaY(WO4)2:Eu3+ microarchitectures: Microwave-assisted hydrothermal synthesis, growth mechanism and luminescent properties

Three dimensional (3D) flower-shaped microarchitectures of NaY(WO4)2 were synthesized via a microwave-assisted hydrothermal process in the presence of trisodium citrate (Na3Cit) and a post-calcination process. The effects of reaction conditions on the morphology of precursor microstructures were studied. It was found that Na3Cit, as the chelating agent and shape modifier, plays a key role in the microstructure growth. A possible growth mechanism for the flower-shaped microarchitectures was proposed. The as-formed precursor can completely transform into NaY(WO4)2 with its original flower-shaped morphology via a heat treatment process. The concentration and temperature quenching behaviors of Eu3+ fluorescence in the flower-shaped NaY(WO4)2 were studied, and the optimal doping concentration was confirmed, meanwhile the activation energy was obtained. Judd-Ofelt parameters Ωλ (λ = 2, 4 and 6) of Eu3+ in the flower-shaped NaY(WO4)2 phosphor were obtained by using the emission spectrum of Eu3+, moreover the radiative transition properties were analyzed.

Excitation pathway and temperature dependent luminescence in color tunable Ba5Gd8Zn4O21:Eu3+ phosphors

A series of polycrystalline Ba5Gd8Zn4O21:Eu3+ phosphors were synthesized by a solid state reaction for the first time. The crystal structures were examined by means of X-ray diffraction. The luminescence spectra and decay curves were investigated as a function of Eu3+ concentration and temperature. It was found that the luminescent color of phosphor can be adjusted from white to red with the increase of Eu3+ concentration. Interestingly, emission spectra with different 5D0/5D1,2,3 ratios were observed when excited at 276 (corresponding to the O2− → Eu3+ charge transfer band) and 395 nm (f–f transition 7F0 → 5L6). The energy transfers between Eu3+ ions in Ba5Gd8Zn4O21 have been confirmed to be resonant type via exchange interaction for the 5D0 level and electric dipole–dipole interaction for 5DJ (J = 1–3) levels. Temperature dependent measurements revealed that the crossover process is the main mechanism for quenching luminescence of the 5D0 level of Eu3+ in the Ba5Gd8Zn4O21:Eu3+ phosphors. Finally, Judd–Ofelt parameters of Eu3+ in the Ba5Gd8Zn4O21 phosphors were calculated by a facile method in the framework of the J–O theory, in which the refractive index of Ba5Gd8Zn4O21 was deduced to be about 1.95.

Controllable synthesis and luminescent properties of three-dimensional nanostructured CaWO4:Tb3+ microspheres.

Three-dimensional (3D) nanostructured CaWO(4):Tb(3+)microspheres assembled by submicrospindles were synthesized via a mild sonochemical route from an aqueous solution of CaCl(2), TbCl(3) and Na(2)WO(4) with the aid of surfactant Polyglycol 600 (PEG-600). The crystal structure and morphology of the as-prepared products were characterized by using X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). Rietveld refinement was carried out on the XRD data. The results showed that the CaWO(4):Tb(3+)nanoparticles can be formed without ultrasonic irradiation or addition of PEG-600. With continuously increasing irradiation time the submicrospindles and microspheres could be self-assembled. The central diameter and length of the submicrospindles are around 190 and 500 nm, respectively. The 3D CaWO(4):Tb(3+)nanostructured microspheres with diameter of 2-4 μm were assembled by the submicrospindles. A possible formation mechanism for the 3D-structured CaWO(4):Tb(3+)microspheres was proposed. The Photoluminescent (PL) properties of Tb(3+) ions in the nanostructured CaWO(4) microspheres were studied. The energy transfer processes in CaWO(4):Tb(3+)microspheres were analyzed. The electric dipole-dipole energy transfers related to (5)D(3) level were studied by inspecting the fluorescence decay of (5)D(3) level. The energy transfer critical distance was estimated.

Hydrothermal synthesis and tunable luminescence of persimmon-like sodium lanthanum tungstate:Tb3+, Eu3+ hierarchical microarchitectures

Persimmon-like NaLa(WO(4))(2) microarchitectures were prepared via hydrothermal process with using trisodium citrate (Na(3)Cit) as chelated reagent and characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), photoluminescence (PL), and fluorescent dynamics. The influences of Na(3)Cit concentration, organic additivities, and reaction time on the morphologies of NaLa(WO(4))(2) phosphor were studied. The results revealed that Na(3)Cit species had double functions of strong ligand and structure-directing reagent that could efficiently control the formation of persimmon-like NaLa(WO(4))(2) microarchitectures. The possible mechanism for the growth of persimmon-like NaLa(WO(4))(2) microarchitectures was attributed to the Ostwald ripening mechanism. The energy transfer from Tb(3+) to Eu(3+) in the persimmon-like NaLa(WO(4))(2) phosphors was observed. The energy transfer efficiencies and emission colors can be tuned by changing the concentration of Eu(3+). Finally, it was deduced that the electric dipole-dipole interaction (D-D) is the main mechanism for energy transfer between Tb(3+) and Eu(3+) in the persimmon-like NaLa(WO(4))(2) phosphor.

Fluorescence quenching of 5DJ (J = 1, 2 and 3) levels and Judd–Ofelt analysis of Eu3+ in NaGdTiO4 phosphors

NaGdTiO4 phosphors with various Eu3+ concentrations were synthesized by a solid-state reaction. The crystal structure and fluorescence properties were characterized by means of x-ray diffraction and fluorospectroscopy, respectively. An effective energy transfer from the titanate matrix to the activator Eu3+ was observed. An intense red emission originating from 5D0 ? 7F2 of Eu3+ was observed while excited at the charge transfer bands or f?f absorption bands of Eu3+. The fluorescence quenching of 5DJ (J = 1, 2 and 3) levels were studied based on the Van Uitert model. Electric dipole?dipole interaction was confirmed to be the main mechanism of the fluorescence quenching of 5DJ levels, which is responsible for the cross-relaxation between Eu3+ ions. The full optical transition intensity parameters ?? (? = 2, 4, 6) of Eu3+ in the NaGdTiO4 phosphors were calculated in the framework of Judd?Ofelt theory to be 6.024 ? 10?20, 1.512 ? 10?20 and 0.374 ? 10?20?cm2.

Lanthanide dopant-induced phase transition and luminescent enhancement of EuF3 nanocrystals

A novel strategy is proposed to prepare hexagonal EuF3 nanocrystals by doping lanthanide ions (Ln3+) with large ionic radius (such as La3+, Ce3+, Pr3+, Nd3+ and Sm3+). Complete phase transition from orthorhombic to hexagonal phase can be achieved with the increase of ionic radius or doping concentration of Ln3+ ions. Meanwhile, the size and self-assembly degree of the product can be reduced dramatically. The possible growth and phase transformation mechanism of EuF3 nanocrystals was proposed. Finally, Eu3+ ion was used as the structural probe to investigate the effect of phase transition on the luminescence. It was found that the emissions of the 5D0 level of Eu3+ are greatly enhanced with the phase transition from orthorhombic to hexagonal phase. The luminescent enhancement is attributed to the larger radiative transition rate of the 5D0 level in the environment with lower symmetry, which can be confirmed by Judd–Ofelt (J–O) theory.

Dependence of upconversion emission intensity on Yb3+ concentration in Er3+/Yb3+ co-doped flake shaped Y2(MoO4)3 phosphors

Yttrium molybdate phosphors with fixed Er3+ and various Yb3+ concentrations were synthesized via a co-precipitation method. The crystal structure and the morphology of the phosphor were characterized by means of x-ray diffraction and field-emission scanning electron microscopy. Under 980?nm excitation, red and green upconversion emissions centred at 660, 553 and 530?nm were observed. Quantitative analyses on the dependence of upconversion emission intensity on the working current of a laser diode (LD) indicated that two-photon processes are responsible for both red and green upconversion emissions in both cases of low and high Yb3+ concentrations. The relationship between the emission intensity ratio of 2H11/2 ?4I15/2 to 4S3/2 ?4I15/2 and the working current of the LD was studied for the samples doped with low and high Yb3+ concentrations. Finally, a set of rate equations was established based on the possible upconversion mechanism, and an empirical formula was proposed to describe the Yb3+ concentration dependence of upconversion emission intensity; the empirical formula fits well with the experimental data.

Effect of RO （R=Ca, Sr, Ba, Zn or Pb） component on spectroscopic properties of Eu3＋ in RO-B2O3 glasses

Abstract Eu 3+ -doped binary borate glasses with different metal oxide components RO (R=Ca, Sr, Ba, Zn or Pb) were prepared by melt-quenching technique. The fluorescent spectral properties of Eu 3+ in these glasses were experimentally studied. The analysis on the phonon sidebands (PSBs) indicated that RO component did not cause obvious change of the electron-phonon coupling constant (EPC). By inspecting the optical absorption edges it was found that RO could greatly affect the band gap energy, and the glass with PbO component revealed the smallest band gap energy, the glasses with ZnO, BaO and SrO showed similar band gap energy. The optical transition intensity parameters of Eu 3+ in all studied glasses were calculated, it was found that for each sample its value of Ω 2 was larger than that of Ω 4 and Ω 6 , and the sample with PbO component exhibited the smallest Ω 2 , but the Ω λ values for ZBE, CBE, BBE and SBE were very similar. These results might be helpful for the design of borate glasses.

Composition-dependent optical transitions and 2.0 μm emission properties of Ho3+ in xGeO2-(80–x)TeO2-9.5ZnF2-10NaF-0.5Ho2O3 glasses

Abstract Ho3+ doped glasses with various compositions xGeO2-(80-x)TeO2-9.5ZnF2-10NaF-0.5Ho2O3 (x=0, 10, 20, 30, 40, 50, 60, 70 and 80) were prepared by a melt-quenching technique. The composition-dependent optical transition properties of Ho3+ were investigated in the framework of Judd-Ofelt theory. The emission cross sections for 5I7→5I8 transition of Ho3+ in these glasses were calculated according to McCumber theory, and their compositional dependence was discussed. A simple method was proposed in calculating the emission cross sections of 5I7→5I8 transition, and the calculation results were compared with those derived from McCumber theory. Finally, the optical gain coefficient spectra for 5I7→5I8 transition were also analyzed.