Hyun Kyoung Yang

Crystal structure, electronic structure, and optical and photoluminescence properties of Eu(III) ion-doped Lu6Mo(W)O12.

Lu(6)WO(12) and Lu(6)MoO(12) doped with Eu(3+) ions have been prepared by using a citrate complexation route, followed by calcination at different temperatures. The morphology, structure, and optical and photoluminescence properties of the compounds were studied as a function of calcination temperature. Both compositions undergo transitions from a cubic to a hexagonal phase when the calcination temperature increases. All the compositions have strong absorption of near-UV light and show intense red luminescence under a near-UV excitation, which is related to the transfer of energy from the host lattices to dopant Eu(3+) ions. Density functional theory calculations have also been performed. The calculation reveals that hexagonal Lu(6)WO(12) and Lu(6)MoO(12) are indirect bandgap materials, and the near-UV excitations are due to the electronic transitions from the O-2p orbitals to W-5d and Mo-4d orbitals, respectively. The lattice parameters and bandgap energies of hexagonal Lu(6)WO(12) and Lu(6)MoO(12) were determined.

Hydrothermal synthesis and enhanced photoluminescence of Tb3+ in Ce3+/Tb3+ doped KGdF4 nanocrystals

RE3+ (RE3+ = Ce3+, Tb3+) doped and undoped KGdF4 nanocrystals have been synthesized by a citric acid assisted hydrothermal method. The nanocrystals crystallize in cubic phase with spherical morphology and an average diameter of 100 nm. The optical properties of Ce3+ and/or Tb3+ doped KGdF4 nanocrystals were characterized. The results reveal that the existence of Ce3+ (sensitizer) can dramatically enhance the photoluminescence emission intensity of Tb3+ (activator) in the co-doped samples due to an efficient energy transfer from Ce3+ to Tb3+. The energy transfer efficiency of Ce3+–Tb3+ was evaluated. The critical energy transfer distance between Ce3+ and Tb3+ was calculated by methods of concentration quenching and spectral overlapping. Experimental analysis and theoretical calculations reveal that the dipole–quadrupole interaction should be the dominant mechanism for the Ce3+–Tb3+ energy transfer.

Investigation of the structure and photoluminescence properties of Eu3+ ion-activated Y6WxMo(1 − x)O12

A series of Eu3+ ion-activated Y6WxMo(1 − x)O12 polycrystalline powders were synthesized using a solid-state reaction. The crystal structure, ultraviolet-visible and photoluminescence spectra of these compounds were characterized. The structural parameters, electronic structure and orbital population of Y6WO12 and Y6MoO12 were determined by means of density functional theory calculation. The calculated structural parameters agreed well with the experimental values. Both electronic structures and ultraviolet-visible spectra indicated that the band-gap of Y6WO12 is larger than that of Y6MoO12. The WO6 or MoO6 groups in the host lattices could be efficiently excited by near-UV or violet light, and then transferred the energy to the activator Eu3+ ions, resulting in red light dominated emission. It was shown that the ratio of W to Mo in the host lattice had an impact on the luminescence intensity, the purity of the emission light and the decay lifetime strongly. Compounds Y6WxMo(1 − x)O12:Eu could be red-phosphor candidates for WLED devices.

Controlled Fabrication and Shape-Dependent Luminescence Properties of Hexagonal NaCeF4, NaCeF4:Tb3+ Nanorods via Polyol-Mediated Solvothermal Route

Hexagonal monodisperse NaCeF(4) and NaCeF(4):Tb(3+) nanorods have been successfully synthesized by a polyol-mediated solvothermal route with ethylene glycol (EG) as solvent. The crystalline phase, size, morphology, and luminescence properties were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and photoluminescence (PL) spectra as well as dynamic decays. The experimental results indicate that the content of NH(4)F and NaNO(3) are crucial in controlling product morphology and size. Nanorods with different aspect ratios could be controllably obtained under settled conditions. Shape-dependent luminescence and energy transfer routes from Ce(3+) to Tb(3+) in NaCeF(4):Tb(3+) nanorods were observed by the modified local crystal field environment around rare earth ions. The 4f-5d transitions of Ce(3+) ions have much higher sensitivity to the anisotropic shape of samples than that of Tb(3+) ions.

Tunable photoluminescence properties of Eu(II)- and Sm(III)-coactivated Ca 9 Y(PO 4 ) 7 and energy transfer between Eu(II) and Sm(III)

A single-phased tunable color conversion phosphor for WLED was prepared by coactivating Ca9Y(PO4)7 with Eu2+ and Sm3+ ions. The structure, UV-visible and photoluminescence spectra of the phosphor were studied as a function of annealing atmosphere and concentration ratio of Eu2+/Sm3+. The coexistence of Eu2+ and Sm3+ was achieved by annealing the phosphor in a reducing atmosphere. The as-obtained phosphors showed a blended emission of blue-green and orange-red light upon near-UV excitation. White light emission was realized by adjusting the concentration ratio. Energy transfer and dominant interaction between Eu2+ and Sm3+ was also studied.

Synthesis, crystal growth, phase transformation and photoluminescence properties of GdVO4:Eu3+ micro-rods by a high-energy ball milling method

GdVO4 micro-rods were synthesized using a high-energy ball (HEB) milling method without other chemicals or solvents. The powder samples were sintered in the air at 350, 400, 450, 500, 550 and 600 °C for 5 h to form polycrystalline GdVO4 micro-rods. The effects of the processing parameters on the crystallinity, morphology and structure were examined by X-ray diffraction, field emission-scanning electron microscopy and TEM. The results showed that the rate of GdVO4 rod formation from NaGdV2O7 increased with increasing sintering temperature. The luminescent properties have been carried out by the measurement of their excitation and emission spectra. Overall, high-energy ball (HEB) milling is a promising method for preparing GdVO4 with a rod-like morphology.

Li-DOPING EFFECT ON THE CATHODOLUMINESCENT PROPERTIES OF Y2O3:Eu3+ PHOSPHORS

Influence of lithium doping on the crystallization, the surface morphology, the chemical states and the luminescent properties of Y2O3:Eu3+ phosphors was investigated. The structural, surface morphology characteristics and chemical states of the phosphors were analyzed by using X-ray diffraction (XRD), scanning electron microscope (SEM), and X-ray photoelectron spectroscopy (XPS), respectively. The crystallinity, the surface morphology, and the cathodoluminescence (CL) of phosphors highly depended on the Li doping. The relationship between the crystalline and morphological structures and the luminescent properties was studied, and Li+ doping affected not only the crystallinity but also the luminescent brightness of Y2O3:Eu3+ phosphors. In particular, the incorporation of the Li+ ion into the Y2O3 lattice could induce remarkable increase in the CL intensity. The enhanced photoluminescence brightness with Li doping may result both from the improved crystallinity leading to higher oscillating strengths for the optical transitions, and the increased surface area due to the larger particle sizes. The strongest emission intensity was observed with Li doped Y2O3:Eu3+ ceramics whose brightness was increased by a factor of 1.8 in comparison with that of Y2O3:Eu3+ ceramics.

Evolution of CaGd2ZnO5:Eu3+ nanostructures for rapid visualization of latent fingerprints

Nanostructured inorganic materials are potential building blocks for advanced materials and devices with desired functions. Latent fingerprint technology requires cost-effective and high-quality luminescent nanomaterials to identify an individual with sufficient information. We report the synthesis and growth mechanism of novel nanostructured CaGd2ZnO3(CO3)2·H2O and CaGd2ZnO5 materials through a urea- and EDTA-mediated hydrothermal process. The conversion from the carbonate to the oxide form is explained using physicochemical measurements. Furthermore, when doped with Eu3+ ions, CaGd2ZnO5 nanostructures exhibit superior luminescent properties to replace conventional red phosphors for latent fingerprint technology. In contrast to previously reported individual fluorescent nanoparticles, CaGd2ZnO5:5Eu3+ nanostructures provide three levels of identification with high-quality imaging on various hydrophilic and hydrophobic substrates. These properties demonstrate that CaGd2ZnO5:Eu3+ nanorod bundles are a promising luminescent material for solid-state lighting and latent fingerprint technologies.

Enhancement of the luminescent characteristics of Li-doped CaTiO3?:?Pr3+ thin films grown by pulsed laser deposition

The influence of Li doping on the crystallization, surface morphology and luminescent properties of CaTiO3 : Pr3+ films has been investigated. The films were grown on Al2O3 (0 0 0 1) substrates using the pulsed laser deposition method under different oxygen pressures. The substrate temperature was fixed at 700 °C and the range of oxygen pressure was 100–400 mTorr. The crystallinity and surface morphology of the films were investigated using x-ray diffraction and atomic force microscopy, respectively. The emitted radiation at 322 nm excitation was dominated by a red emission peak at 613 nm radiated from the transition of 1D2 → 3H4 of Pr3+ ions. In particular, the incorporation of Li+ ions into the CaTiO3 lattice could induce the increase in the intensity of the photoluminescence. The enhanced luminescence may result not only from the improved crystallinity but also from the reduced internal reflections caused by rougher surfaces. The luminescent intensity and surface roughness exhibited similar behaviour as a function of the oxygen pressure.

Effect of Yb3+ Concentrations on the Upconversion Luminescence Properties of ZrO2:Er3+,Yb3+ Phosphors

ZrO2:Er3+ and ZrO2:Er3+,Yb3+ phosphors were synthesized via a solvothermal reaction method. For low concentrations of Yb3+, the crystalline structure changed from the tetragonal to monoclinic phase as sintering temperature increased. As the Yb3+ concentration increased to a value above 0.05 mol, ZrO2 phosphors displayed a very stable tetragonal phase. The green and red upconversion emissions of ZrO2:Er3+,Yb3+ phosphors were measured under the excitation with a 975 nm continuous wave diode laser, and the pump power dependence of upconversion intensity was investigated. As the Yb3+ concentration increased from 0 to 0.05 mol, the red upconversion emission intensity increased more rapidly than the green emission intensity. This is attributed to the energy transfer (4I11/2→4I15/24I13/2→4F9/2) between Er3+ ions and the energy back transfer [4S3/24I13/2(Er3+)→2F7/22F5/2(Yb3+)] between the Er3+ and Yb3+ ions. In this case, the pump power dependence of red emission intensity changed from quadratic to linear as Yb3+ concentration increased.