Intense red-emitting phosphors for LED solid-state lighting

The phosphors Gd_2−xEu_x(MoO₄)₃ (x = 0.20, 0.40, 0.60, 0.80, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0), Gd_0.8−xY_xEu_1.2(MoO₄)₃ (x = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8) and Gd_0.2Y_0.6−xEu_1.2Sm_x(MoO₄)₃ (x = 0.02, 0.024, 0.028, 0.032, 0.036, 0.04) were prepared by solid-state reaction technique at 950 °C. The presence of the Y³⁺ and Sm³⁺ ions strengthen and broaden the absorption of the phosphors at ∼400 nm. The intense red-emitting phosphor Gd_0.2Y_0.572Eu_1.2Sm_0.028(MoO₄)₃ with orthorhombic structure was obtained. Both Eu³⁺ and Sm³⁺ f-f transition absorptions are observed in the excitation spectra, the main emission line is at 616 nm (⁵D₀ → ⁷F₂ transition of Eu³⁺) and the chromaticity coordinates (x = 0.66, y = 0.33) is very close to the NTSC standard values (x = 0.67, y = 0.33). It is considered to be an efficient red-emitting phosphor for GaN-based light emitting diode (LED).     

Facile 1,4-dioxane-assisted solvothermal synthesis,optical and electrochemical properties of CeO2 microspheres

CeO₂ microspheres, with an average diameter of 1 μm, have been successfully prepared through a facile 1,4-dioxane-assisted solvothermal synthesis, where 1,4-dioxane is acted as a capping agent for the first time. By varying the solvent ratio, the formation mechanism for the CeO₂ microspheres has been proposed on an isotropic growth through the capping function of 1,4-dioxane. The strong UV–vis spectra result indicates that the CeO₂ microspheres are suitable as an UV blocking and shielding material. The cyclic voltammetric measurements of hemoglobin immobilized on CeO₂-Nafion modified glassy carbon electrode reveal a typical characteristic of surface-controlled thin-layer electrochemical behavior of the CeO₂ microspheres.     

New red phosphor for near-ultraviolet light-emitting diodes with high color-purity

New red phosphors, Na₅Eu(MoO₄)₄ doped with boron oxide were prepared by the solid-state reaction. Their structure and photo-luminescent properties were investigated. With the introduction of boron oxide, the red emission intensity of the phosphors under 395 nm excitation is strengthened, with high color-purity (x = 0.673, y = 0.327). The single red light-emitting diode was obtained by combining InGaN chip with the red phosphor, bright red light can be observed by naked eyes from the red light-emitting diodes under a forward bias of 20 mA.     

Enhancement of red emission intensity of Ca9Y(VO4)7:Eu phosphor via Bi co-doping for the application to white LEDs

The present investigation aims at the luminescence properties of Ca₉Y(VO₄)₇:Eu³⁺, Bi³⁺ red phosphor materials. The red emission at 613 nm originating from ⁵D₀–⁷F₂ transition of Eu³⁺ in Ca₉Y(VO₄)₇ is enhanced strongly with Bi³⁺–V⁵⁺ couple as the sensitizer, under excitation either into the ⁵L₆ state or the ⁵D₂ state. The energy transfer from Bi³⁺–V⁵⁺ to Eu³⁺ is discussed. For a fixed Eu³⁺ concentration, there is an optimal Bi³⁺ concentration with 15 mol%, at which the maximum luminescence intensity of Eu³⁺ is achieved. The red emission of Ca₉Y(VO₄)₇:0.8Eu³⁺, 0.15Bi³⁺ (under 395 nm and 465 nm excitations) is stronger than that of commercial Y₂O₃:Eu³⁺ phosphor (under 395 nm and 467 nm excitations). Based on the ratios of the red emission at 613 nm to orange one at 592 nm, it is considered that the symmetry of Eu³⁺ site decreases with doping of Bi³⁺, leading to more opposite parity components. Lifetime and diffuse reflection spectra measurements indicate that the red emission enhancement is due to the enhanced transition probabilities from the ground state to ⁵L₆ and ⁵D₂ states of Eu³⁺ in the distorted crystal field. Therefore the present material is a promising red-emitting phosphor for white-light diodes with near-ultraviolet/blue GaN-based chips.     

Preparation, characterization and luminescence of La2TeO6 phosphor doped with Eu

Phosphors of La₂TeO₆ doped with Eu³⁺ ions have been synthesized by the oxidation of the corresponding rare-earths oxytellurides of formula La_2−xEu_xO₂Te (x = 0.02, 0.06, and 0.1) at 1050 K. Powder X-ray diffraction confirms that the as prepared materials consist of the orthorhombic La₂TeO₆ as main phase. The photoluminescence (PL) of red-emitting La_2−xEu_xTeO₆ powder phosphors is reported. The emission spectrum, exhibits an intense emission peak due to ⁵D₀ → ⁷F₂ transition at 616 nm, which indicates that the Eu³⁺ ion occupies a non-centrosymmetric site in the host lattice. These materials could find application for use as lamp phosphors in the red region.     

Novel red phosphors Na2CaSiO4:Eu for light-emitting diodes

Novel red phosphors Na₂CaSiO₄:xEu³⁺ were synthesized using high temperature solid-state reaction and their luminescence characteristics were investigated for the first time. The excitation spectra indicate that the Na₂CaSiO₄:xEu³⁺ phosphors can be effectively excited by ultraviolet (393 nm) light. The emission spectra of Na₂CaSiO₄:xEu³⁺ phosphors invariably exhibit four peaks assigned to the ⁵D₀-⁷F_J (J = 1, 2, 3 and 4) transitions of Eu³⁺ under 393 nm excitation. The Commission Internationale de l’Eclairage (CIE) chromaticity coordinates and quantum efficiency (QE) are (0.66, 0.34) and 58.9%, respectively. The good color saturation and high quantum efficiency indicate that Na₂CaSiO₄:Eu³⁺ phosphors are potential candidate for light-emitting diodes.     

Effect of activator distribution on photo- and X-ray excited luminescence properties of ZnS:Cu,Cl phosphors

ZnS:Cu,Cl phosphors were prepared by conventional solid state reaction with the aid of NaCl-MgCl₂ flux. The copper activator was introduced into the phosphor precursors by three different methods: co-precipitated with ZnS (CP), wet-coated onto ZnS powders (WC), and simply mixed with ZnS in a mortar (SM). The samples were characterized by X-ray powder diffraction, photoluminescence spectra and X-ray excited luminescence spectra. The results show that both photo- and X-ray excited luminescence intensities of the as-prepared ZnS:Cu,Cl phosphors are in the decreasing order of CP > WC > SM. The different copper activator distribution in the phosphors resulting from the different methods was the main reason responsible for the different luminescence intensity, and uniform distribution is beneficial to the luminescence of the phosphors.     

Synthesis and efficient blue-emitting of Eu2+-activated borate fluoride BaAlBO3F2

Eu²⁺-doped borate fluoride BaAlBO₃F₂ was synthesized by the conventional high temperature solid state reaction. The crystal phase formations were confirmed by X-ray powder diffraction (XRD) measurements and the structure refinement. The photoluminescence (PL) excitation and emission spectra, and the decay curves were investigated. Eu²⁺-doped BaAlBO₃F₂ phosphor can be efficiently excited by near-UV light and presents narrow blue luminescence band centered at 450 nm. The maximum absolute quantum efficiency (QE) of BaAlBO₃F₂:0.05Eu²⁺ phosphor was measured to be 76% excited at 398 nm light at 300 K. The thermal stability of the blue luminescence was evaluated by the luminescence decays as a function of temperature. The phosphor shows an excellent thermal stability with high thermal activation-energy on temperature quenching effects because of the rigid crystal lattices.     

Visible quantum cutting via downconversion in a novel green-emitting K2Gd(WO4)(PO4):Tb phosphor

A novel phosphor K₂Gd(WO₄)(PO₄):Tb was prepared via a solid-state reaction. The crystal structure of K₂Gd(WO₄)(PO₄) as a new host matrix for luminescence was defined to be the orthorhombic system with space group Ibca (73). Visible quantum cutting under Tb³⁺ 4f⁸–4f⁷5d¹ excitation and host excitation in K₂Gd(WO₄)(PO₄):Tb³⁺ via a downconversion was identified. In order to rationalize the quantum cutting effect, the proper mechanism was proposed. According to calculations, the quantum efficiency was up to 183.2% and 176.4% under excitation at 235 nm and 150 nm, respectively. When compared with Zn₂SiO₄:Mn²⁺ (P1-G1S), KGWP:0.5Tb³⁺ is slightly less bright over 450–650 nm but has a shorter decay time.     

Investigation of Na3GdP2O8:Tb as a potential green-emitting phosphor for plasma display panels

The photoluminescent properties of a series of Tb³⁺-doped Na₃GdP₂O₈ phosphors excitable by vacuum ultraviolet and ultraviolet light are reported. The host related absorption, f-f and f-d transitions of Gd³⁺ and Tb³⁺, and charge transfer of O²⁻ → Gd³⁺ and O²⁻ → Tb³⁺ are assigned. Under 147 nm light excitation, Na₃GdP₂O₈:Tb³⁺ phosphors show efficient green emissions with a dominant peak at 545 nm. The optimal sample Na₃Gd_0.4Tb_0.6P₂O₈ shows a shorter decay time and a comparable brightness when compared with the commercial Zn₂SiO₄:Mn²⁺ green phosphor. These results demonstrate that it is a potential candidate for plasma display panels application.     

A potential Eu-activated Ca10K(PO4)7 red phosphor for white light-emitting diodes

New red Ca₁₀K(PO₄)₇:Eu³⁺, K⁺ phosphors were synthesized by solid state reaction and their photoluminescence properties as well as those by co-doping Mo⁶⁺ under near ultraviolet excitation were investigated. From the excitation spectra monitored at 611 nm, it can be seen that the strongest excitation peak is situated at 393 nm, well matching with the emission wavelength of near-ultraviolet chips for white LEDs. Upon 393 nm excitation, the brightness of Ca₉K(PO₄)₇:0.5Eu³⁺, 0.5 K⁺ with the optimal Eu³⁺-doping concentration is about 2.3 times stronger than that of the commercial red Y₂O₃:Eu³⁺ phosphor. The introducing of Mo⁶⁺, which results in a possible variety for the excited energy level of the host, can enhance the brightness of Eu³⁺ to be maximized by about 15%. The CIE chromaticity coordinates of Ca₉K(PO₄)₇:0.5Eu³⁺, 0.5 K⁺ are calculated to (0.654, 0.345), which are close to the (0.67, 0.33) standard of the National Television System Committee. All the above results indicate Eu³⁺-activated Ca₁₀K(PO₄)₇ is a potential candidate for white LEDs.     

Synthesis and calcination temperature dependent photoluminescence properties of novel bromosilicate phosphors

Novel Eu²⁺-doped bromosilicate phosphors, (CaO-CaBr₂-SiO₂):0.03Eu²⁺, were prepared by the traditional solid-state method under a different calcination temperature. The as-prepared (CaO-CaBr₂-SiO₂):0.03Eu²⁺ phosphors obtained under various reaction temperatures all indicated a broad excitation in the near ultraviolet region (350-450 nm). The phosphor system exhibits bluish-green light with a peak wavelength at 498 nm when the calcination temperature is below 800 °C, while it shows a strong blue emission light with a peak wavelength at 474 nm as the calcination temperature is above 800 °C, and it also gives an interesting greenish-yellow long-lasting phosphorescence with a peak wavelength at 540 nm in the dark.     

Luminescence properties and energy transfer of a novel bluish-green tunable NaBa4(BO3)3:Ce3+, Tb3+ phosphor

A series of single-phased emission tunable NaBa₄(BO₃)₃:Ce³⁺, Tb³⁺ phosphors were synthesized by solid-state reaction. The crystal structure, photoluminescence properties, concentration quenching and energy transfer of NaBa₄(BO₃)₃:Ce³⁺, Tb³⁺ were systematically investigated. The wavelength-tunable bluish-green light can be realized by coupling the emission bands centered at 425 and 543 nm ascribed to the contribution from Ce³⁺ and Tb³⁺, respectively. The energy transfer from Ce³⁺ to Tb³⁺ in NaBa₄(BO₃)₃ host was studied and demonstrated to be a resonant type via a dipole–dipole interaction mechanism. The energy transfer efficiency (Ce³⁺ → Tb³⁺) obtained by decay curves was consistent with the result calculated by the emission intensity, which gradually increased from 0% to 84.5% by increasing the Tb³⁺ doping content from 0 to 0.45. The results indicate that the NaBa₄(BO₃)₃:Ce³⁺, Tb³⁺ phosphors have potential applications as an ultraviolet-convertible phosphor due to its effective excitation in the ultraviolet rang.     

High pressure optical studies of α-ZnAl2S4:Cr

The effect of hydrostatic pressure (varying up to 91 kbar) at room temperature emission spectra and fluorescence lifetime of the Cr³⁺ R1 line (²E → ⁴A₂ transition) in tiospinel ZnAl₂S₄ was studied for the first time. The studies performed reveal the shift of the R1 line with increasing pressure. Such a red shift was accompanied by a significant change of the fluorescence lifetime. Both trends were successfully explained by a simple model, which resulted in adequate agreement between the calculated and the observed.     

Fabrication of porous TiO2 nanofiber and its photocatalytic activity

Porous TiO₂-based nanofiber was fabricated via a combined electrospinning and alkali-dissolution method. TiO₂/SiO₂ composite nanofiber was firstly prepared by electrospinning and sintering, and then silica was leached out with alkaline solution from the bulk of TiO₂/SiO₂ composite nanofiber to produce porous microstructure. The thermal decomposition and phase structure of the composite nanofiber precursor was investigated with TG/DSC and XRD, and optimal sintering temperature was obtained. SEM-EDX and FT-IR characterization show that most silica can be dissolved out from the composite nanofiber and thus porous nanofiber with excellent microstructure can be spontaneously formed. The effect of composite nanofiber composition on porous microstructure was studied, and it is found that the composite nanofiber with 20wt% silica can produce better porous microstructure compared to those with 10wt% and 30wt% silica. Meanwhile, porous TiO₂ nanofiber with 20wt% silica shows higher degradation efficiency to Congo Red.     

A novel emulsifier-free emulsion route to synthesize ZnS hollow microspheres

ZnS hollow microspheres were prepared via a novel emulsifier-free emulsion route for the first time. The aqueous solution of zinc nitrate and the toluene solution of thioacetamide were emulsified under sonication to form water in toluene emulsion, and then the emulsion was processed at elevated temperature under sonication for 45 min, by which ZnS nanoparticles were generated and assembled spontaneously at the surface of water droplets; after the subsequent separation, wash and desiccation process, ZnS hollow microspheres were obtained. ZnS hollow microspheres were characterized by TEM, XRD, UV-vis and particle size analyzer; the effect of the reaction temperature on the products was investigated. The results showed that ZnS hollow microspheres were made up of ZnS nanoparticles and had cubic structures (β-ZnS); with the rise of reaction temperature, the ZnS hollow microspheres became smaller and more compact. This study provided a promising route for the preparation of a variety of hollow microspheres.     

A thermoluminescence study on the state of Cl in ZnS:Ag by electron beam

The degradation behavior of ZnS:Ag, Cl as a phosphor for CL by EB irradiation at 7 kV was examined by TL measurement. After EB irradiation, TL intensity decreased and the TL peak shifted to the lower temperature side. By comparing TL thermograms of mechanically damaged ZnS:Ag, Cl, ZnS:Cl with varying Cl concentration as well as ZnS:Ag, Al after EB irradiation, we conclude that the decrease in the effective concentration of Cl⁻, serving as active luminescence center, is responsible for the CL degradation of ZnS:Ag, Cl by EB irradiation.     

The photoluminescence properties of Eu,Bi co-doped yttrium oxysulfide phosphor under vacuum ultraviolet excitation

Y₂O₂S co-doped with Bi³⁺,Eu³⁺ phosphors were prepared and their luminescence properties under vacuum ultraviolet (VUV) excitation were investigated. Much stronger red emission for Y₂O₂S:Eu³⁺,Bi³⁺ were observed under 147 nm excitation than that for Y₂O₂S:Eu³⁺. Investigation on photoluminescence and calculation of electronic structure of Y₂O₂S:Eu³⁺,Bi³⁺ revealed that the Bi³⁺ acts as a medium in energy transfer process and the great VUV luminescence enhancement of Y₂O₂S:Eu³⁺ by doping Bi³⁺ is due to effective energy transfer process: charge transfer (CT) transition of Y³⁺-O²⁻ → Bi³⁺ → Eu³⁺. The Y₂O₂S:Eu³⁺,Bi³⁺ showed excellent optical properties when compared with the commercial (Y,Gd)BO₃:Eu³⁺. Thus, the Y₂O₂S:Eu³⁺,Bi³⁺ would be a promising VUV-excited red phosphor.     

Structural characterization and photocatalytic properties of novel Bi2YVO8

Bi₂YVO₈ was prepared by solid-state reaction for the first time. The structural and photocatalytic properties of Bi₂YVO₈ were studied. The results showed that this compound has the tetragonal crystal system with space group I4/mmm. The band gap of Bi₂YVO₈ was estimated to be about 2.09 eV by plotting (αhν)² versus hν and obtaining the hν axis intercept value according to Tauc's equation. For the photocatalytic water splitting reaction, H₂ or O₂ evolution was observed from pure water with Bi₂YVO₈ as the photocatalyst under ultraviolet light irradiation (wavelength = 390 nm). Degradation of aqueous methylene blue photocatalyzed by this compound was investigated under visible light irradiation. Bi₂YVO₈ showed higher photocatalytic activity compared to Bi₂YTaO₇, Bi₂InTaO₇ or TiO₂ (P-25). Complete removal of aqueous methylene blue was achieved after visible light irradiation for 170 min. The decrease of the total organic carbon and the formation of inorganic products such as SO₄²⁻ and NO₃⁻ revealed the continuous mineralization of aqueous methylene blue during photocatalytic reaction.