Enhanced ultraviolet up-conversion emissions of Tm/Yb codoped YF3 nanocrystals

Tm³⁺/Yb³⁺ codoped rod-like YF₃ nanocrystals were synthesized through a facile hydrothermal method. After annealing in an argon atmosphere, the nanocrystals emitted bright blue and intense ultraviolet (UV) light under a 980-nm continuous wave diode laser excitation. Up-conversion emissions centered at ∼291 nm (¹I₆ → ³H₆), ∼347 nm (¹I₆ → ³F₄), ∼362 nm (¹D₂ → ³H₆), ∼452 nm (¹D₂ → ³F₄), ∼476 nm (¹G₄ → ³H₆), ∼642 nm (¹G₄ → ³F₄), and ∼805 nm (³H₄ → ³H₆) were recorded using a fluorescence spectrophotometer. Especially, enhanced UV emissions were studied by changing Yb³⁺/Tm³⁺ doping concentrations, the annealing temperatures, and the excitation power densities. A possible mechanism, energy transfer-cross relaxation-energy transfer (ET-CR-ET), was proposed based on a simple rate-equation model to elucidate the process of the enhanced UV emissions.     

Synthesis and upconversion luminescence properties of YF3:Yb/Tm octahedral nanocrystals

YF₃:Yb³⁺(20%)/Tm³⁺(2%) octahedral nanocrystals were synthesized by a microemulsion method with NH₄HF₂. Pumped with a 980-nm diode laser, the nanocrystals emitted weak blue and intense ultraviolet light. Especially, unusual ³P₂ → ³H₆ (∼265 nm) and ³P₂ → ³F₄ (∼309 nm) emissions, coming from a five-photon excitation process, were observed. The emissions from ¹D₂ and ¹I₆ were much stronger than those from ¹G₄ and ³H₄. The upconversion mechanism was discussed in detail.     

Synthesis and spectral properties of Eu-doped YF3 nanobundles

YF₃:Eu³⁺ nanobundles were synthesized by a facile microemulsion method. Analysis of X-ray diffraction, scanning electron microscope, and transmission electron microscopy reveals that each nanobundle consists of numerous nanowhiskers with a mean length of ∼500 nm and a mean diameter of ∼2 nm. Under 393-nm excitation, the luminescence was dominated by ⁵D₀ → ⁷F₁ transition, indicating the inversion symmetry of Eu³⁺ site. The luminescence intensity increased with increasing Eu³⁺ concentration, up to about 30 mol%, and then decreased abruptly. The peak positions and spectral shapes of emissions were independent of Eu³⁺ concentration. Finally, the critical distance of energy transfer was calculated.     

Preparing nanometer scaled Tb-doped Y2O3 luminescent powders by the polyol method

Sub-micrometer Tb-doped Y₂O₃ luminescent powders were prepared from nitrate precursors using the polyol method. Just after precipitation, the powders consist of agglomerates with a spherical shape and a size ranging between 400 and 500 nm. Each agglomerate is composed of ultra-small crystallites (from 3 to 6 nm) of a bcc oxide phase whose luminescence presents original features in comparison with bulk materials. Powders were further calcinated at different temperatures and for annealing below 900 °C, highly crystalline samples with the classical green ⁵D₄→⁷F₅ luminescent transitions of Tb³⁺ ions are obtained. For optimized annealing temperatures, sintering between the agglomerates is avoided and a sub-micrometric powder with a narrow size distribution and a high luminescence is obtained.     

Hydrothermal synthesis and luminescent properties of LuBO3:Tb microflowers

Hexagonal vaterite-type LuBO₃:Tb³⁺ microflower-like phosphors have been successfully prepared by an efficient surfactant- and template-free hydrothermal process directly without further sintering treatment. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectrometry, transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), photoluminescence (PL) and cathodoluminescence (CL) spectra as well as kinetic decays were used to characterize the samples. The as-obtained phosphor samples present flowerlike agglomerates composed of nanoflakes with thickness of 40 nm and high crystallinity in spite of the moderate reaction temperature of 200 °C. The reaction mechanism has been considered as a dissolution/precipitation mechanism; the self-assembly evolution process has been proposed on homocentric layer-by-layer growth style. Under ultraviolet excitation into the 4f⁸→4f⁷5d transition of Tb³⁺ at 248 nm (or 288 nm) and low-voltage electron beam excitation, LuBO₃:Tb³⁺ samples show the characteristic green emission of Tb³⁺ corresponding to ⁵D₄→⁷F_{6, 5, 4, 3} transitions with the ⁵D₄→⁷F₅ transition (542 nm) being the most prominent group, which have potential applications in fluorescent lamps and field emission displays.     

White-light-emitting long-lasting phosphorescence in Dy-doped SrSiO3

We report on a luminescent phenomenon in Dy³⁺-doped SrSiO₃ long-lasting phosphor. After irradiation by a 254-nm UV lamp for 5 min, the Dy³⁺-doped SrSiO₃ phosphor emits white light-emitting long-lasting phosphorescence for more than 1 h even after the irradiation source has been removed. Photoluminescence, long-lasting phosphorescence and thermoluminescence (TL) spectra are used to explain this phenomenon. Photoluminescence spectra reveal that the white light-emitting long-lasting phosphorescence originated from the two mixtures of Dy³⁺ characteristic luminescence, the 480-nm blue emission (⁴F_9/2→⁶H_15/2) and the 572-nm yellow emission (⁴F_9/2→⁶H_13/2). TL spectra shows that the introduction of Dy³⁺ ions into the SrSiO₃ host produces a highly dense trapping level at 377 K (0.59 eV), which is responsible for the long-lasting phosphorescence at room temperature. A possible mechanism of the long-lasting phosphorescence based on the experimental results is proposed. It is considered that the long-lasting phosphorescence is due to persistent energy transfer from the electron traps to the Dy³⁺ ions, which creates the persistent luminescence of Dy³⁺ to produce the white light-emitting long-lasting phosphorescence.     

Enhanced quantum yield of yellow photoluminescence of Dy ions in nonlinear optical Ba2TiSi2O8 nanocrystals formed in glass

Transparent crystallized glasses consisting of nonlinear optical Ba₂TiSi₂O₈ nanocrystals (diameter: ∼100 nm) are prepared through the crystallization of 40BaO-20TiO₂-40SiO₂-0.5Dy₂O₃ glass (in the molar ratio), and photoluminescence quantum yields of Dy³⁺ ions in the visible region are evaluated directly by using a photoluminescence spectrometer with an integrating sphere. The incorporation of Dy³⁺ ions into Ba₂TiSi₂O₈ nanocrystals is confirmed from the X-ray diffraction analyses. The total quantum yields of the emissions at the bands of ⁴F_9/2→⁶H_15/2 (blue: 484 nm), ⁴F_9/2→⁶H_13/2 (yellow: 575 nm), and ⁴F_9/2→⁶H_11/2 (red: 669 nm) in the crystallized glasses are ∼15%, being about four times larger compared with the precursor glass. It is found that the intensity of yellow (575 nm) emissions and the branching ratio of the yellow (575 nm)/blue (484 nm) intensity ratio increase largely due to the crystallization. It is suggested from Judd-Ofelt analyses that the site symmetry of Dy³⁺ ions in the crystallized glasses is largely distorted, giving a large increase in the yellow emissions. It is proposed that Dy³⁺ ions substitute Ba²⁺ sites in Ba₂TiSi₂O₈ nanocrystals.     

Photoluminescence properties of a novel phosphor, Na3La9O3(BO3)8:RE(RE=Eu,Tb)

The new oxyborate phosphors, Na₃La₉O₃(BO₃)₈:Eu³⁺ (NLBO:Eu) and Na₃La₉O₃(BO₃)₈:Tb³⁺ (NLBO:Tb) were prepared by solid-state reactions. The photoluminescence characteristics under UV excitation were investigated. The dominated emission of Eu³⁺ corresponding to the electric dipole transition ⁵D₀→⁷F₂ is located at 613 nm and bright green luminescence of NLBO:Tb attributed to the transition ⁵D₄→⁷F₅ is centered at 544 nm. The concentration dependence of the emission intensity showed that the optimum doping concentration of Eu and Tb is 30% and 10%, respectively.     

Synthesis and characteristic of the Fe3O4@SiO2@Eu(DBM)3·2H2O/SiO2 luminomagnetic microspheres with core-shell structure

The core-shell structured luminomagnetic microsphere composed of a Fe₃O₄ magnetic core and a continuous SiO₂ nanoshell doped with Eu(DBM)₃·2H₂O fluorescent molecules was fabricated by a modified Stöber method combined with a layer-by-layer assembly technique. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), confocal microscopy, photoluminescence (PL), and superconducting quantum interface device (SQUID) were employed to characterize the Fe₃O₄@SiO₂@Eu(DBM)₃·2H₂O/SiO₂ microspheres. The experimental results show that the microshpere has a typical diameter of ca. 500 nm consisting of the magnetic core with about 340 nm in diameter and silica shell doped with europium complex with an average thickness of about 80 nm. It possesses magnetism with a saturation magnetization of 25.84 emu/g and negligible coercivity and remanence at room temperature and exhibits strong red emission peak originating from electric-dipole transition ⁵D₀ → ⁷F₂ (611 nm) of Eu³⁺ ions. The luminomagnetic microspheres can be uptaken by HeLa cells and there is no adverse cell reaction. These results suggest that the luminomagnetic microspheres with magnetic resonance response and fluorescence probe property may be useful in biomedical imaging and diagnostic applications.     

Photoluminescence of new red phosphor SrZnO2:Eu

SrZnO₂:Eu³⁺ has been synthesized by solid-state reaction and its photoluminescence in ultraviolet (UV)-vacuum ultraviolet (VUV) range was investigated. The broad bands around 254 nm are assigned to CT band of Eu³⁺-O²⁻. With the increasing of Eu³⁺ concentration, Eu³⁺ could occupy different sites, which leads to the broadening of CT band. A sharp band is observed in the region of 110-130 nm, which is related to the host absorption. The phosphors emit red luminescence centered at about 616 nm due to Eu³⁺⁵D₀→⁷F₂ both under 254 and 147 nm, but none of Eu²⁺ blue emission can be observed.     

Synthesis, vacuum ultraviolet and near ultraviolet-excited luminescent properties of GdCaAl3O7: RE (RE=Eu, Tb)

Vacuum ultraviolet (VUV) excitation and photoluminescent (PL) properties of Eu³⁺ and Tb³⁺ ion-doped aluminate phosphors, GdCaAl₃O₇:Eu³⁺ and GdCaAl₃O₇:Tb³⁺ have been investigated. X-ray diffraction (XRD) patterns indicate that the phosphor GdCaAl₃O₇ forms without impurity phase at 900 °C. Field emission scanning electron microscopy (FE-SEM) images show that the particle size of the phosphor is less than 3 μm. Upon excitation with VUV irradiation, the phosphors show a strong emission at around 619 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 results reveal that both GdCaAl₃O₇:RE³⁺ (RE=Eu, Tb) are potential candidates as red and green phosphors, respectively, for use in plasma display panel (PDP).     

Catalytic hydrogenation of CO and CN bonds via heterolysis of H2 mediated by metal-sulfur bonds of rhodium and iridium thiolate complexes

Coordinatively unsaturated rhodium and iridium complexes having a bulky thiolate, [Cp∗M(PMe₃)(SDmp)](BAr^F₄) (1a: M = Rh; 1b: M = Ir; Dmp = 2,6-(mesityl)₂C₆H₃, Ar^F = 3,5-(CF₃)₂C₆H₃), catalyzed the hydrogenation of benzaldehyde, N-benzylideneaniline, and cyclohexanone, under 1 atm of H₂ at low temperatures. In these catalytic reactions, the M-H/S-H complexes [Cp∗M(PMe₃)(H)(HSDmp)](BAr^F₄) (2a: M = Rh; 2b: M = Ir) generated via H₂ heterolysis by 1a or 1b were suggested to transfer both M-H hydride and S-H proton to substrates. The catalytic reactions were terminated by the dissociation of H-SDmp from the metal centers of 2a and 2b that occurs at ambient temperature under H₂ atmosphere.     

Investigation on sensitized chemiluminescence systems and their mechanism for five fluoroquinolones

The novel chemiluminescence (CL) reaction systems were established for lomefloxacin (LMFX), ofloxacin(OFLX), norfloxacin (NFLX), gatifloxacin (GAFX) and enoxacin (ENX). The sensitized CL emission mechanism was investigated for the five systems by comparing the fluorescence emission with CL spectra. For LMFX-Ce(IV)-S₂O₃²⁻-H₂SO₄ and OFLX-Ce(IV)-S₂O₄²⁻-H₂SO₄ systems, the CL intensity is enhanced through intermolecular energy transfer from the excited SO₂^* to LMFX and OFLX. For NFLX-Ce(IV)-S₂O₄²⁻-HNO₃ system, the sensitized CL is based on intermolecular energy transfer from the excited SO₂^* to NFLX oxide. For Eu³⁺-GAFX-Ce(IV)-S₂O₄²⁻-HCl and Dy³⁺-ENX-Ce(IV)-S₂O₃²⁻-H₂SO₄ systems, the CL spectra are from the narrow characteristic emission at 590, 619 and 649 nm of Eu^3+* (⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₂, ⁵D₀ → ⁷F₃) and at 482 and 578 nm of Dy³⁺ (⁴F₉ → ⁶H_15/2, ⁴F₉ → ⁶H_13/2) through intermolecular energy transfer from the excited SO₂^* to GAFX and ENX, followed by intramolecular energy transfer from GAFX^* to Eu³⁺ and ENX^* to Dy³⁺. The conditions of CL emission were investigated and optimized. The proposed five enhanced CL systems have good linearity, higher sensitivity, precision and potential capability for residue analysis of studied analytes in foods and biological samples.     

Calorimetric study and thermal analysis of [Gd4/3Y2/3(Gly)6(H2O)4](ClO4)6·5H2O and [ErY(Gly)6(H2O)4](ClO4)6·5H2O (Gly: glycin)

Two solid-state coordination compounds of rare earth metals with glycin, [Gd_4/3Y_2/3(Gly)₆(H₂O)₄](ClO₄)₆·5H₂O and [ErY(Gly)₆(H₂O)₄](ClO₄)₆·5H₂O were synthesized. The low-temperature heat capacities of the two coordination compounds were measured with an adiabatic calorimeter over the temperature range from 78 to 376 K. [Gd_4/3Y_2/3(Gly)₆(H₂O)₄](ClO₄)₆·5H₂O melted at 342.90 K, while [ErY(Gly)₆(H₂O)₄](ClO₄)₆·5H₂O melted at 328.79 K. The molar enthalpy and entropy of fusion for the two coordination compounds were determined to be 18.48 kJ mol⁻¹ and 53.9 J K⁻¹ mol⁻¹ for [Gd_4/3Y_2/3(Gly)₆(H₂O)₄](ClO₄)₆·5H₂O, 1.82 kJ mol⁻¹ and 5.5 J K⁻¹ mol⁻¹ for [ErY(Gly)₆(H₂O)₄](ClO₄)₆·5H₂O, respectively. Thermal decompositions of the two coordination compounds were studied through the thermogravimetry (TG). Possible mechanisms of the decompositions are discussed.     

Composition dependent frequency upconversion of Er/Yb-codoped lead chloride tellurite glasses

The green and red upconversion luminescence of Er³⁺ in lead chloride tellurite glasses excited at 980 nm is investigated. Three intense emission bands centered at 530, 545, and 658 nm corresponding to the transitions ⁴S_3/2→⁴I_15/2, ²H_11/2→⁴I_15/2 and ⁴F_9/2→⁴I_15/2, respectively, were simultaneously observed at room temperature. With increasing PbCl₂ content, the intensity of green (530 nm) emissions increase slightly, while the green (545 nm) and red (658 nm) emissions increase significantly. The results indicate that PbCl₂ has more influence on the green (545 nm) and red (658 nm) emissions than the green (530 nm) emission. The dependence of upconversion intensities on excitation power and possible upconversion mechanisms are discussed and evaluated.     

Host-sensitized luminescence of Dy, Pr, Tb in polycrystalline CaIn2O4 for field emission displays

CaIn₂O₄:Dy³⁺/Pr³⁺/Tb³⁺ blue-white/green/green phosphors were prepared by the Pechini sol-gel process. X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), diffuse reflectance, photoluminescence (PL) and cathodoluminescence (CL) spectra as well as lifetimes were utilized to characterize the samples. The XRD results reveal that the samples begin to crystallize at 800 °C and pure CaIn₂O₄ phase can be obtained after annealing at 900 °C. The FE-SEM images indicate that the CaIn₂O₄:Dy³⁺, CaIn₂O₄:Pr³⁺ and CaIn₂O₄:Tb³⁺ samples consist of spherical grains with size around 200-400 nm. Under the excitation of ultraviolet light and low-voltage electron beams (1-5 kV), the CaIn₂O₄:Dy³⁺, CaIn₂O₄:Pr³⁺ and CaIn₂O₄:Tb³⁺ phosphors show the characteristic emissions of Dy³⁺ (⁴F_9/2-⁶H_15/2 and ⁴F_9/2-⁶H_13/2 transitions, blue-white), Pr³⁺ (³P₀-³H₄, ¹D₂-³H₄ and ³P₁-³H₅ transitions, green) and Tb³⁺ (⁵D₄-⁷F_6,5,4,3 transitions, green), respectively. All the luminescence is resulted from an efficient energy transfer from the CaIn₂O₄ host lattice to the doped Dy³⁺, Pr³⁺ and Tb³⁺ ions, and the corresponding luminescence mechanisms have been proposed.     

Structural, spectroscopic and photoluminescence studies of LiEu(WO4)2−x(MoO4)x as a near-UV convertible phosphor

A series of lithium europium double tungsto-molybdate phosphors LiEu(WO₄)_2−x(MoO₄)_x (x=0, 0.4, 0.8, 1.2, 1.6, 2.0) have been synthesized by solid-state reactions and their crystal structure, optical and luminescent properties were studied. As the molybdate content increases, the intensity of the ⁵D₀→⁷F₂ emission of Eu³⁺ activated at wavelength of 396 nm was found to increase and reach a maximum when the relative ratio of Mo/W is 2:0. These changes were found to be accompanied with the changes in the spectral feature, which can be attributed to the crystal field splitting of the ⁵D₀→⁷F₂ transition. As the molybdate content increases the emission intensity of the 615 nm peak also increases. The intense red-emission of the tungstomolybdate phosphors under near-UV excitation suggests them to be potential candidate for white light generation by using near-UV LEDs. In this study the effect of chemical compositions and crystal structure on the photoluminescent properties of LiEu(WO₄)_2−x(MoO₄)_x is investigated and discussed.     

Novel powellite-based red-emitting phosphors: CaLa1−xNbMoO8:xEu for white light emitting diodes

We report the photoluminescence properties of a novel powellite-based red-emitting phosphor material: CaLa_1−xNbMoO₈:xEu³⁺ (0.01, 0.03, 0.05, 0.1) for the first time. The photoluminescence investigations indicated that CaLa_1−xNbMoO₈:xEu³⁺ emits strong red light at 615 nm originating from ⁵D₀→⁷F₂ (electric dipole transition) under excitation either into the ⁵L₀ state with 394 nm or the ⁵D₂ state with 464 nm, that correspond to the two popular emission lines from near-UV and blue LED chips, respectively. When compared with emission intensity from a CaMoO₄:Eu³⁺, the emission from CaLaNbMoO₈:Eu³⁺ showed greater intensity values under the same excitation wavelength (394 nm). The enhanced red emission is attributed to the enhanced f-f absorption of Eu³⁺. These materials could be promising red phosphors for use in generating white light in phosphor-converted white light emitting diodes (WLEDs).     

A novel red emitting phosphor Ca2SnO4: Eu

A novel red emitting phosphor, Eu³⁺-doped Ca₂SnO₄, was prepared by the solid-state reaction. X-ray powder diffraction (XRD) analysis confirmed the formation of Ca₂SnO₄: Eu³⁺. Field-emission scanning electron-microscopy (FE-SEM) observation indicated a narrow size-distribution of about 500 nm for the particles with spherical shape. Photoluminescence measurements indicated that the phosphor exhibits bright red emission at about 615 nm under UV excitation. The excellent luminescence properties make it possible as a good candidate for plasma display panels (PDP) application. Splitting of the ⁵D₀-⁷F_J transitions of Ca₂SnO₄: Eu³⁺ suggests that the Eu³⁺ ions occupied two nonequivalent sites in the crystallite. The luminescence lifetime measurement showed a bi-exponential decay, providing other evidence for the existence of two different environments for Eu³⁺ ions.     

Photoluminescence of Eu-doped triple phosphate Ca8MgR(PO4)7 (R=La, Gd, Y)

Eu³⁺-doped triple phosphate Ca₈MgR(PO₄)₇ (R=La, Gd, Y) was synthesized by the general high-temperature solid-state reaction. Excitation and emission spectra as well as luminescence decay were used to characterize the phosphors. Photoluminescence excitation and emission spectra showed that the phosphor could be efficiently excited by UV-vis light from 260 to 450 nm to give bright red emission assigned to the transition (⁵D₀→⁷F₂) at 612 nm. The richness of the red color has been verified by determining their color coordinates (X, Y) from the CIE standard.