Synthesis and Luminescence Properties of Gd_2O_3∶Eu^(3+) Phosphors

Gd 2O 3∶Eu 3+ phosphors were prepared by urea homogeneous precipitation with different surfactant and sol-gel method. XRD patterns show that all the obtained samples are in cubic Gd 2O 3, and the results of FTIR and fluorescent spectra conformed that OP is a good surfactant for preparing the Gd 2O 3∶Eu 3+ phosphors. The SEM photographs show that the particles prepared by urea homogeneous precipitation method are all spherical and well-dispersed, and grain morphology can be controlled by different surfactant. XRD and SEM indicate that the particle sizes prepared by sol-gel method are in the range of 5～30 nm, and the grain sizes increase with increasing of heated temperatures. Luminescence spectra indicat that the main emission peaks of all samples are at 610 nm, the intensities are different from samples prepared with different surfactant and the luminescence intensities increase with increasing of annealed temperatures.

￣(151)Eu-MOSSBAUER SPECTRA AND XPS INVESTIGATION OF BaFBr:Eu PHOSPHOR

Eu Mossbauer spectra of BaFBr:Eu phosphor are reported for the first time.The results show that f(Eu(2+) )<f(Eu(3+)) or <x2>Eu(2+)><x2>Eu(3+),which indicates that the attraction between phonons and Eu(2+) is stronger than that between phonons and Eu(3+),this may cause a faster increase of f(Eu=(2+)vs f(Eu(3+)) when temperature decreases as observed in the experiments.It is also found from Mossbauer investigation that when temperature decreases,the relative content of Eu(2+) decreases, while that of Eu(3+) increases.The temperature dependent Mossbauer spectra provide evidence for electron exchange or hopping between Eu(2+) or Eu(3+) sites in BaFBr lattice. The Mossbauer results are discussed and compared with the results of XPS and Photoluminescence studies.

Luminescence Characterization and Synthesis of γ-LiAlO_2:Eu^(3+) by Gel Combustion

γ-LiAlO2：Eu3＋（Eu3＋：LAO） phosphor was obtained by gel combustion method using LiNO3,Al（NO3）3·9H2O,Eu（NO3）3·6H2O and citric acid as raw materials.The structure,morphology and luminescence were characterized by means of X-ray diffraction （XRD）,scanning electron microscopy （SEM）,photoluminescence （PL）.The results demonstrated that the phosphor was pure-phase of flaky tetragonal crystal system with a mean size of around 1 μm.The strongest excitation peak was at 254 nm,belonging to the broadband excitation and the maximum emission peak was at 613 nm,corresponding to the 5D0→7F2 transition of Eu3＋.Luminous intensity is closely related to the calcination temperature and doping concentration of Eu3＋.

Optical Spectroscopy Properties of KMgF_3 :Eu^(2+) Nanocrystals and Powder Synthesized by Microemulsion and Solvothermal Process

Nanocrystals and powders of KMgF3 doped with Eu^2＋ were synthesized by the microemulsion method and the selvothermal process, respectively. The emission and excitation spectra of KMgF3： Eu^2＋ phosphors were measured and compared with those of the samples synthesized through a solid state reaction, Bridgman-Stockbarger method, and mild hydrothermal technique. The KMgF3： Eu^2＋ samples synthesized by means of the microemulsion method and the solvothermal process show only a sharp emission peak located at 360 nm in the emission spectra, which arises from the f→f（ ^6P7/2→^8S7/2 ） transition of Eu^2＋ The broad emission bands appear at 420 nm, which arises from Eu^2＋←O^2- cannot be observed（ in the mild hydrothermal and single crystal samples, the emission peak at 420 nm besides the emission of Eu^2＋ at 360 nm is observed）. In the excitation spectrum of the KMgF3 ： Eu^2＋ samples synthesized by the microemulsion method and the solvothermal process, the excitation peaks show an intensive blue shift. The blue shift can be attributed to the lower oxygenic content in the KMgF3 ： Eu^2＋ samples synthesized by the microemulsion method and the selvothermal process.

Photoluminescence degradation mechanism of BaMgAl_(10)O_(17):Eu^(2+) phosphor by vacuum ultraviolet irradiation

A real high power vacuum ultraviolet light source is applied to the investigation on the vacuum ultraviolet irra- diation degradation of BaMgAl10O17：Eu2＋ phosphor. The degradations of emission intensity and color quality of the sample are clearly observed after irradiation. It reveals that the oxidation of Eu2＋ during irradiation is partly respon- sible for the degradations. The excitation and absorption spectra show that some traps generated during irradiation have negative influence on the luminescence of sample and these traps have been identified as positively charged oxygen vacancies by positron annihilation. The investigations on host emission and decay curve further confirm that these oxygen vacancies are involved in the perturbation of energy transfer from the host to Eu2＋ and finally result in the degradation.

Uncommon Emissions from Higher Excited State ~5D_J of Eu^(3+) in Rare-Earth Sr_2CeO_4 Host

Undoped and Eu3+-doped Sr2CeO4 luminescent materials were prepared by sol-gel method. The structure and uncommon photoluminescence of Sr2CeO4∶Eu3+ phosphors were investigated in detail by powder X-ray diffraction (XRD), Raman spectrum, and photoluminescence spectrum, respectively. The XRD results demonstrate that the as-prepared Sr2CeO4 phosphor is single phase and well crystallized. For Sr2CeO4∶Eu3+ phosphor, its excitation spectrum consists of a broad intense band from host and Eu3+-O2-charge transfer and a number of small peaks from Eu3+ ion. The broad emission band originated from Sr2CeO4 host and Eu3+ emission lines in the blue, green, and red regions coexist. Not only the characteristic transition lines from the lowest excited 5D0 level of Eu3+ but also those from higher energy levels 5DJ (J=1, 2) of Eu3+ ions are observed. These unusual luminescence properties result from the low vibration energy of Sr2CeO4 host-lattice and different energy transfer process from host to activator.

Synthesis and luminescence properties of red-emitting M_2Si_5N_8:Eu^(2+)-based(M=Ca,Sr,Ba) phosphors by a simple nitrate reduction

M2SisN8：Eu2＋-based （M=Ca, Sr, Ba） red-emitting phosphors are fabricated at relatively low temperature （1200℃） and atmos- pheric pressure using a simple solid-state reaction process. Several processing parameters are systematically investigated to optimize the phosphors structural characterization and photoluminescence performance, including the amount of europium and the properties of the precursor materials. The as-prepared M2SisNs：Eu2＋-based （M=Ca, Sr, Ba） phosphors are orange in color and are intensively emitted in the red region of 580-670 nm under 465 nm excitation.

Effect of gadolinium incorporation on optical characteristics of YNbO_4:Eu^(3+) phosphors for lamp applications

Eu3＋-doped （Y,Gd）NbO4 phosphor was synthesized by solid-state reaction for possible application in cold cathode fluorescent lamps. A broad absorption band with peak maximum at 272 nm was observed which was due to the charge transfer between Eu3＋ ions and neighboring oxygen anions. A deep red emission at the peak wavelength of 612 nm was observed which could be attributed to the 5D0→7F2 transition in Eu3＋ ions. The highest luminance for Y1-x-yGdyNbO4：Eux3＋ under 254 nm excitation was achieved at Eu3＋ concentration of 18 mol.% （x=0.18） and Gd3＋ concentration of 8.2 mol.% （y=0.082）. The luminance of Y0.738Gd0.082NbO4：Eu3＋0.18 was higher than that of a typical commercial phosphor Y2O3：Eu3＋ and the CIE chromaticity coordinate was （0.6490, 0.3506）, which was deeper than that of Y2O3：Eu3＋. The particle size of the synthesized phosphors was controlled by the NaCl flux and particle size as high as 8 μm with uniform size distribution of particles was obtained.

Synthesis and photoluminescence properties of ZnTiO_3:Eu^(3+) red phosphors via sol-gel method

ZnTiO3：Eu3＋ phosphors were synthesized with different concentrations of Eu3＋ doping through sol-gel method. The samples were calcined at different temperatures for 2 h in air. The synthesized powders were characterized by X-ray diffraction（XRD）, scanning electron microscopy-energy dispersive spectroscopy（SEM-EDS）, transmission electron microscopy（TEM）, Raman and photoluminescence spectroscopy. The XRD results showed that the Zn Ti O3：Eu3＋ phosphors doped with different concentrations of Eu3＋ ions calcined at 600 oC were of single phase, which indicated that the Eu3＋ ions had been successfully incorporated into the Zn Ti O3 host lattice and did not destroy the lattice structure of Zn Ti O3 host. The Raman spectrum, SEM and TEM also proved that the doping of Eu3＋ did not change the lattice structure of hexagonal Zn Ti O3 host. The photoluminescence（PL） of Eu3＋ ions with the main emission peak at 614 nm was observed to increase with Eu3＋ concentrations from 0.5 mol.% to 2.0 mol.% and decreased when the concentration was increased to 2.5 mol.%. The decrease in the PL intensity at higher Eu3＋ concentrations could be associated with concentration quenching effect. The CIE1931 chromaticity diagram（x, y） of Zn Ti O3：2.0 wt.%Eu3＋ phosphors were located in the red region（x=0.652, y=0.347）. The luminescence properties suggested that Zn Ti O3：Eu3＋ phosphors might be regarded as a potential red phosphor candidate for light emitting diodes（LEDs）.

Enhancing the photoluminescence intensity of CaTiO_3:Eu^(3+) red phosphors with magnesium

Red phosphors MgxCa1-xTiO3：Eu3＋ （0〈x〈0.5） were synthesized by solid-state reaction method. The crystalline structure and morphology of the as-prepared samples were confirmed by X-ray diffraction （XRD） and scanning electron microscopy （SEM）. The luminescence property was measured using photoluminescence excitation and photoluminescence emission spectra. Results showed that spherical particles appeared in the phosphor bodies and size of the phosphor particles were tmiformly distributed in the range of 600-800 nm when the Mg2＋ concentration was about 40 mol.%. It could readily be seen that the strongest PL emission was located at 617 nm monitored at 398 nm, which well matched with the near ultraviolet （NUV, 395400 nm） GaN-LEDs. More importantly, PL emission intensity （617 nm） of phosphor Mg0.nCa0.6TiO3：0.03Eu3＋ was 4.26 times of that of phosphor CaTiO3：0.03Eu3＋ Based on these results, it implied that the PL intensity of phosphorCaTiO3：0.03Eu3＋ could be significantly enhanced by introducing Mg2＋ into CaTiO3 host lattices and the phosphor Mg0.4Ca0.6TiO3：0.03Eu3＋ might be the promising red-emitting phosphor in making tricolor phosphor converted white-LEDs.

Study on white-light emission and light-emitting mechanism of hydrothermally synthesized YVO_4:1 mol.%Dy^(3+),x mol.%Eu^(3+) phosphor powders

White light-emitting YVO4:1 mol.%Dy3+,x mol.%Eu3+ phosphor powders with order morphology and well crystallization were hydrothermally synthesized at 180°C. The microstructure, white-light emission, and light-emitting mechanism of the powders were carefully studied using X-ray diffractometry, scanning electron microscopy and photoluminescence spectra. The excitation and emission spectra of the phosphor powders indicated the coexistence of efficient energy transfer from Eu3+ to Dy3+ and inefficient en-ergy transfer from Dy3+ to Eu3+ besides the energy transfer from VO43– to Eu3+. Increasing the Eu3+ concentration initially enhanced and then weakened the luminescent intensity of Dy3+. The white-light emissions of YVO4:1 mol.%Dy3+,xmol.%Eu3+ phosphor pow-ders were both related to the energy transfer between VO43– and Dy3+/Eu3+, as well as between Eu3+ and Dy3+. The inefficient energy transfer from Dy3+ to Eu3+ was first found.

Luminescent properties and application of Eu^(3+)-activated Gd_2(MoO_4)_3 red-emitting phosphor with pseudo-pompon shape for solid-state lighting

Eu3+-activated Gd2(MoO4)3 pseudo-pompon-like red-emitting phosphors were prepared by solid-state method. The structure, morphology, and luminescent properties of these powder samples were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and fluorescent spectrophotometry, respectively. The as-obtained phosphors were single crystalline phase with orthorhombic unit cell. The particles of the powder samples had the length of 5-12 m and width of 3-7 m with flake shape and large surface area, which is suitable for manufacture of white LEDs. The phosphor could be efficiently excited by the incident light of 348-425 nm, well matched with the output wavelength of near-UV (In,Ga)N chip, and re-emitted an intense red light peaking at 615 nm. By combing this phosphor with a ~395 nm-emitting (In,Ga)N chip, a red LED was fabricated, so that the applicability of this novel phosphor to white LEDs was confirmed. It is considered to be an efficient red-emitting conversion phosphor for solid-state lighting based on (In,Ga)N LEDs.

Luminescence properties of tunable red-green emitting phosphor Ba2Ca(BO3)2：Eu3＋,Tb3＋

A series of Eu^(3+) or Tb^(3+) doped Ba_2Ca(BO_3)_2 phosphors were synthesized by a high temperature solid state method, and the luminescence properties are investigated. Ba_2Ca(BO_3)_2:Tb^(3+) can show an obvious green emission, and the peak locates at 551 nm, which corresponds to the 5D34→7F5 transition of Tb^(3+). Ba_2Ca(BO_3)_2:Eu+ can present the characteristic emission of Eu^(3+), and the peak locates at 600 nm, which is ascribed to the 5D70→F2 transition of Eu^(3+). In order to achieve the emission-tunable phosphors, the Eu^(3+)/Tb^(3+) co-doped Ba_2Ca(BO_3)_2 are synthesized. When tuning the Eu^(3+) or Tb^(3+) concentration, Ba_2Ca(BO_3)_2:Eu^(3+), Tb^(3+) can both show the tunable emission, which may be induced by the energy transfer from Tb^(3+) to Eu^(3+).

Origin of the red luminescence in Sr_3Al_2O_6:Eu phosphor ——From the synergetic effects of Eu^(2+) and Eu^(3+)

In order to uncover the real origin of red luminescence from Sr_3Al_2O_6：Eu and the physical mechanisms that were involved in the dynamical process of luminescence, variant amount of Eu and Dy activated Sr_3Al_2O_6 phosphors were synthesized with the solid-state reaction and the combustion-assisted solid-state reaction, respectively, using the fine graphite powder or the mixed H^2＋N_2gases as a reducing agent. The phase was examined with XRD analysis and the photoluminescence properties were characterized by a fluorescence spectrometer. Although the phosphors possessed the same Sr_3Al_2O_6 phase, different emission colors（red or green） were obtained, relying on synthesis conditions. The simultaneous existence of Eu^2＋ and Eu^3＋ was not only observed in the emission and excitation spectra, but also identified with the near edge X-ray absorption fine structure spectroscopy（NEXAFS）.The mixed valence（higher than ＋2 and less than ＋3） of Eu may be related with the six different sites of Sr, whose effective valence ranged from ＋1.5058 to ＋2.2698, in the crystal lattice of Sr_3Al_2O_6 that could accommodate Eu. Moreover, the reduction of Eu^3＋ to forming Eu^2＋ depended on the amount of Eu^3＋ or Dy^3＋ doped, due to the different energy barrier in each site of Sr that Eu had to overcome. The residual Eu^3＋, similar to the doped Dy^3＋, played an important role in supplying the hole for Eu^2＋ to form a bound trap（Eu^2＋）＊ after excitation. During electron returning to the 4f^7 ground state of Eu^2＋, the red luminescence was radiated. Therefore, the synergetic effects of Eu^2＋ and Eu^3＋（Dy^3＋） produce red luminescence.