Jiayue Sun

Luminescent properties of LiBaPO4:RE (RE = Eu2+, Tb3+, Sm3+) phosphors for white light-emitting diodes

Multicolor emitting phosphors LiBaPO4:RE (RE = Eu2+, Tb3+, Sm3+) reported as interesting down-conversion luminescent materials in white light-emitting diodes (w-LEDs) have been prepared by a conventional solid-state reaction. The excitation and emission spectra indicate that these phosphors can be effectively excited by the near-UV light, and emit blue, green, and red light, respectively. The photoluminescence properties, decay times, and Commission Internationale de I’Eclairage (CIE) chromaticity indexes are determined for various concentrations of the activators Eu2+, Tb3+, and Sm3+ in LiBaPO4 host. Furthermore, the temperature-dependent luminescence of these phosphors was investigated. The emission intensities of Tb3+- and Sm3+-doped LiBaPO4 increased unexpectedly with increasing temperature. This interesting phenomenon is proposed to be the result of two ground states in the configurational coordination diagram. The present investigation suggests that LiBaPO4:RE (RE = Eu2+, Tb3+, Sm3+) were potential candidates for application in w-LEDs.

Luminescence Properties of Ba2Mg ( BO3 ) 2 : Eu2 + Red Phosphors Synthesized by a Microwave-Assisted Sol-Gel Route

Fine-sized red-emitting Ba 2-x Mg(BO 3 ) 2 :xEu 2+ phosphors were synthesized via a microwave-assisted sol-gel route. The crystallization process, structure, and morphology of the as-synthesized phosphor were characterized by thermogravimetry-differential thermal analysis, X-ray diffraction, and scanning electron microscopy analysis, respectively. The luminescence properties were characterized by diffuse reflection spectra (UV-visible) and photoluminescence spectra. The as-prepared phosphors had good excitation characteristics in the region of 320-450 nm, which perfectly matched the emission wavelength of near-UV light emitting diodes. Intense red emission peaking at 608 nm was obtained upon 365 nm excitation with the chromaticity coordinates of (0.585, 0.402). The energy transfer among the nearest-neighbor ions resulted in the concentration quenching of Ba 2 Mg(BO 3 ) 2 :Eu 2+ phosphor, and the critical distance was determined to be about 17.32 A.

Hydrothermal synthesis of SrF2:Yb3+/Er3+ micro-/nanocrystals with multiform morphologies and upconversion properties

Abstract Micro- and nanocrystals cubic-phase SrF 2 :Yb 3+ /Er 3+ upconversion luminescence phosphors were synthesized via a facile hydrothermal route in the presence of different surfactants. The samples were characterized with X-ray diffraction (XRD), Fourier transform infrared spectra (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and upconversion emission spectra. As-prepared products showed a variety of morphologies, such as cubic-shaped microcrystal, hierarchical structure microspheres, spherical-shaped nanocrystals and nanosheets. The intrinsic structural feature of cubic-phase SrF 2 and two important external factors, namely, the surfactants in the reaction solution and fluoride sources, were responsible for shape determination of SrF 2 :Yb 3+ /Er 3+ . The possible formation mechanisms for products with various architectures were presented. A systematic study on the photoluminescence of Yb 3+ /Er 3+ -doped SrF 2 samples with cubic shape, microspheres, spherical and nanosheets shapes showed that the optical properties of these products were strongly dependent on their morphologies and size.

Preparation and photoluminescence properties of Dy3+-doped Ba3Lu(PO4)3 phosphors

Ba3Lu(PO4)3:Dy3+ phosphors were successfully synthesized by conventional solid state reaction and its photoluminescence properties were investigated by X-ray diffraction (XRD), photoluminescence spectra and diffuse reflectance spectra, respectively. Results showed that the phosphor could be excited by the ultraviolet visible light in the region from 320 to 460 nm, and it showed two dominant emission bands peaking at 484 nm (blue light) and 575 nm (yellow light) which originated from the transitions of 4F9/2→6H15/2 and 4F9/2→6H13/2 of Dy3+, respectively. According to Dexters theory, the concentration quenching mechanism occurred via the electric dipole-dipole interaction. The lifetime values of Dy3+ ions at different concentrations (x=0.01, 0.03, 0.05, 0.08 and 0.10) were determined to be about 0.8556, 0.7595, 0.6864, 0.6059 and 0.5456 ms.

Upconversion luminescence of Yb3+/Ho3+/Er3+/Tm3+ co-doped KGd(WO4)2 powders

Different lanthanide ions (Yb3+/Ho3+/Er3+/Tm3+) codoped KGd(WO4)2 phosphors were prepared by high-temperature solid-state reaction. The upconversion luminescence properties of two-ion and three-ion co-doped KGd(WO4)2 phosphors were investigated in detail. The concentration quenching effect of the two-ion co-doped KGd(WO4)2 phosphors was studied, and the optimum concentration of Ho3+, Er3+ and Tm3+ are 2 mol.%, 2 mol.% and 3 mol.%, respectively. The Yb3+/Ho3+/Tm3+ co-doped KGd(WO4)2 sample is the best white upconversion luminescence material among three-ion co-doped KGd(WO4)2 phosphors. The CIE coordinates for the samples were calculated, and chromaticity coordinates were close to white light regions. The upconversion luminescence mechanism of the samples was discussed. The upconversion luminescence of Ho3+ and Er3+ was both produced through two-photon process. The emissions of Ho3+ at 546 and 654 nm originated from the transition of Ho3+ (5S2→5I8 and 5F5→5I8), while those of Er3+ at 530 and 551 nm were attributed to the transition of Er3+ (2I11/2→4I15/2 and 4S3/2→4I15/2). The upconversion luminescence mechanism of Tm3+ was a three-photon process. The blue emission of Tm3+ was due to the transition of Tm3+ (1G4→3H6).

Synthesis of well oil-dispersible BaYF5:Yb3+/Er3+ nanocrystals with green upconversion fluorescence

Abstract Nearly monodisperse, regular-shaped and well oil-dispersible tetragonal BaYF 5 :0.2Yb 3+ /0.02Er 3+ nanocrystals (NCs) were synthesized in water-ethanol-oleic acid-sodium oleate system. The as-obtained NCs exhibited bright upconversion (UC) fluorescence under the 980 nm excitation. Blue ( 2 H 9/2 - 4 I 15/2 ), green (( 2 H 11/2 , 4 S 3/2 )- 4 I 15/2 ) and red ( 4 F 9/2 - 4 I 15/2 ) transitions were observed. The results indicated that the relative intensity of green to red increased gradually with increasing power density, which were seldom in the previous work. Therefore, the UC properties and mechanism were studied in detail.

Characterization and luminescence properties of Sr3Gd(PO4)3: Sm3+ orange–red phosphor

Abstract. Reddish-orange emitting phosphors, Sr3Gd(PO4)3:  Sm3+, were successfully synthesized by a conventional solid-state reaction. The crystal structure of the phosphors was characterized by x-ray diffraction. The excitation spectra and emission spectra were utilized to characterize the luminescence properties of the as-prepared phosphors. The results show that the phosphor consisted of some sharp emission peaks of Sm3+ ions centered at 564, 600, 647, and 707 nm, respectively. The critical distance of Sr3Gd0.93(PO4)3: 0.07Sm3+ was calculated to be 19.18  Å and the lifetime value of the sample was 1.63 ms. The band gap of Sr3Gd(PO4)3 was estimated to be about 2.74 eV from the diffuse reflection spectrum. The optimum doping concentration is 7 mol. % and the quenching occurs via dipole–dipole interaction according to Dexter’s theory. The Commission Internationale de L’Eclairage value of Sr3Gd(PO4)3:  Sm3+ phosphors presented that it has high color purity. These results indicated that the Sr3Gd(PO4)3:  Sm3+ may be a promising reddish-orange emitting phosphor for cost-effective near ultraviolet white light-emitting diodes.

Yellow upconversion luminescence in Ho3+/Yb3+ co-doped Gd2Mo3O9 phosphor

Abstract The strong yellow upconversion (UC) light emission was observed in Ho 3+ /Yb 3+ co-doped Gd 2 Mo 3 O 9 phosphor under the excitation of 980 nm diode laser. The phosphors were synthesized by the traditional solid-state reaction method. The structures of the samples were characterized by X-ray diffraction (XRD). Under 980 nm excitation, Ho 3+ /Yb 3+ co-doped Gd 2 Mo 3 O 9 exhibited strong yellow UC emission based on the green emission near 541 nm generated by 5 F 4 , 5 S 2 → 5 I 8 transition and the strong red emission around 660 nm generated by 5 F 5 → 5 I 8 transition, which assigned to the intra-4f transitions of Ho 3+ ions. The doping concentrations of Ho 3+ and Yb 3+ were determined to be 0.01 mol Ho 3+ and 0.2 mol Yb 3+ for the strongest yellow emission. Then the dependence of UC emission intensity on excitation power density showed that the green and red UC emissions were involved in two-photon process. The possible UC mechanisms for the strong yellow emission were also investigated. The result indicated that this material was a promising candidate for the application in the yellow display field.

Synthesis and luminescence properties of Eu3+-doped Ba3Gd(PO4)3 phosphors for light-emitting diodes

Abstract. Reddish-orange emitting Eu3+-activated Ba3Gd(PO4)3 phosphors have been successfully synthesized by a conventional solid-state reaction. X-ray powder diffraction confirmed the phase formation. The photoluminescence excitation and emission spectra were investigated. The band gap of Ba3Gd(PO4)3 was estimated to be about 3.68 eV from the diffuse reflection spectrum. The temperature-dependent luminescence of Ba3Gd(PO4)3:Eu3+ was discussed, and the activation energy for thermal quenching was calculated as 1.74 eV. The fluorescence decay curve and the commission internationale de L’Eclairage value of Ba3Gd(PO4)3:Eu3+ phosphors were also investigated in detail. The phosphor presented high color purity. Ba3Gd(PO4)3:Eu3+ might be a promising reddish-orange emitting phosphor used in white light-emitting diodes.

Luminescence properties of a new red emitting Eu3+-doped alkaline-earth fluoborate phosphor: BaCa(1-2x)BO3F:xEu3+, xM+ (M=Li, Na, K)

Abstract A series of new red-emitting BaCa 1-2 x BO 3 F: x Eu 3+ , x M + (M=Li, Na, K) phosphors were synthesized by the solid-reaction method. X-ray diffraction (XRD), diffuse reflection (UV-vis) and photoluminescence spectra were utilized to characterize the crystallization process, structure and luminescence properties of the as-synthesized phosphors. The XRD results indicated that the sample began to crystallize at 800 °C, and single-phase BaCaBO 3 F was fully obtained after annealing at 1000 °C. The charge compensated behaviors were investigated in this paper by considering different cations like Li + , Na + and K + acting as the charge compensator. The as-prepared phosphors had better emission properties, and the two characteristic emission lines peaking at 590 and 615 nm could be obtained upon 394, 463 and 532 nm excitation with the chromaticity coordinates of (0.596, 0.391), which were due to 5 D 0 - 7 F 1 and 5 D 0 - 7 F 2 transitions of Eu 3+ ions. Further, the concentration quenching and corresponding luminescence mechanisms of BaCa 1-2 x BO 3 F: x Eu 3+ , x Na + phosphors were also discussed.