Byung Chun Choi

Low-frequency dielectric relaxation and ac conduction of SrBi2Ta2O9 thin film grown by pulsed laser deposition

Bi-excess SrBi2Ta2O9 (SBT) thin films on Pt/Ti/SiO2/Si substrate were prepared by pulsed laser deposition technique. The SBT structure was characterized by x-ray diffraction studies. The ferroelectric properties were confirmed by P–E hysteresis loops at different applied electric fields. The dielectric constant and the ac conductivity of the Pt/SBT/Pt capacitor were investigated in the frequency range from 0.01 Hz to 100 kHz and in the temperature range from 25 to 400 °C. The thermal activation energy of 0.90 eV is observed in the frequency dependent dielectric constant. The activation energy for conduction process is calculated as 0.91 eV from the slope of ac conductivity at the lowest frequency. The low-frequency dielectric relaxation and the ac conductivity of Bi-excess SBT thin film are discussed in relation to the electrical conduction of SBT/Pt junction.

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.

Chemical bond properties and charge transfer bands of O2−–Eu3+, O2−–Mo6+ and O2−–W6+ in Eu3+-doped garnet hosts Ln3M5O12 and ABO4 molybdate and tungstate phosphors

Charge transfer (CT) energy from the ligand to the central ions is an important factor in luminescence properties for rare earth doped inorganic phosphors. The dielectric theory of complex crystals was used to calculate chemical bond properties. Combining the photoluminescence and the dielectric theory of complex crystals, the CT bands of O(2-)-Eu(3+), O(2-)-Mo(6+) and O(2-)-W(6+) for Eu(3+)-doped inorganic phosphors have been investigated experimentally and theoretically. Taking Eu(3+)-doped Ln3M5O12 (Ln = Y, Lu and M = Al, Ga), Gd3Ga5O12, MMoO4 (M = Ca, Sr, Ba) and MWO4 (M = Ca, Sr, Ba) as typical phosphors, we investigated the effects of the cation size on the CT bands and chemical bond properties including the bond length (d), the covalency (fc), the bond polarizability (αb) and the environmental factor (he) of O(2-)-Eu(3+), O(2-)-Mo(6+) and O(2-)-W(6+), respectively. For systematic isostructural Ln3M5O12 (Ln = Y, Lu and M = Al, Ga) phosphors, with the increasing M ion radius, the bond length of Ln-O decreases, but fc and αb increase, which is the main reason that the environmental factor increased. For the isostructural MMoO4:Eu, with the increasing M ion radius, the Mo-O bond length increases, but fc and αb decrease, and thus he decreases. However, in the compound system MWO4:Eu (M = Ca, Ba) with the increasing M ion radius, the O-W bond length increases, but fc and αb increase, and thus he increases and the O-W CT energy decreases. Their O(2-)-Eu(3+), O(2-)-Mo(6+) and O(2-)-W(6+) CT bands as well as their full width at half maximum (FWHM) were directly influenced by he. And with the increasing he, CT bands of O-Eu or O-Mo or O-W decrease and their FWHM increases. These results indicate a promising approach for changing the material properties, searching for new Eu(3+) doped molybdate, tungstate or other oxide phosphors and analyzing the experimental result.

Wide-Band Excited Y6(WMo)0.5O12:Eu Red Phosphor for White Light Emitting Diode: Structure Evolution, Photoluminescence Properties, and Energy Transfer Mechanisms Involved

Y6(WMo)(0.5)O12 activated with Eu(3+) ions was investigated as a red-emitting conversion phosphor for white light emitting diodes (WLEDs). The phosphors were synthesized by calcining a citrate-complexation precursor at different temperatures. The photoluminescence properties of the phosphors and the energy transfer mechanisms involved were studied as a function of structure evolution. It was found that the host lattices were crystallized in a cubic or a hexagonal phase depending on the synthesis conditions. Although all the phosphors showed intensive red emission under an excitation of near-UV or blue light due to energy transfer from the host lattices to Eu(3+) ions, the photoluminescence spectra and temporal decay features were found to vary significantly with the structure and crystallinity of the host lattice. The mechanisms of the energy transfer from the host lattices to Eu(3+) ions and energy quenching among Eu(3+) ions were discussed on the basis of structure evolution of the host lattice. Phosphors calcined at 800 and 1300 °C were suggested to be promising candidates for blue and near-UV light excited WLEDs, respectively.

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.

Tunable white-light emission in single-phase Ca 9 Gd(VO 4 ) 7 :Tm 3+ , Eu 3+

A series of Tm3+ and/or Eu3+ doped Ca9Gd(VO4)7 single composition phosphors were synthesized by a solid state reaction method, and their luminescence properties were investigated. Tm3+ and Eu3+ co-doped Ca9Gd(VO4)7 phosphors showed a blue with the peak at 477 nm and red with the stronger peak at 620 nm dual emission bands under the UV excitation, which originates from the f-f transitions of Tm3+ and Eu3+ ions, respectively. The energy transfer from O2--V5+ CT (charge transfer) energy to Tm3+ and Eu3+ ions as well as the energy transfer from Tm3+ to Eu3+ ions were investigated. The photoluminescence intensity ratio of blue and red emission could be tuned by adjusting the concentration of Tm3+ and Eu3+ ions and as a result the emission color varies from blue to white to red. The white-light emission is realized in single phased phosphor of Ca9Gd(VO4)7:Tm3+, Eu3+ by combining the Tm3+-emission and the Eu3+-emission.