Byung Kee Moon

Enhanced luminescence of pulsed-laser-deposited Y2O3:Eu3+ thin-film phosphors by Li doping

Y2O3:Eu3+ and Li-doped Y2O3:Eu3+ luminescent thin films have been grown on sapphire substrates using pulsed-laser deposition. The films grown under different deposition conditions have been characterized using microstructural and luminescent measurements. The photoluminescence (PL) brightness data obtained from Li-doped Y2O3:Eu3+ films grown under optimized conditions have indicated that sapphire is a promising substrate for the growth of high-quality Li-doped Y2O3:Eu3+ thin-film red phosphor. In particular, the incorporation of Li+ ions into Y2O3 lattice could induce a remarkable increase of PL. The highest emission intensity was observed with LiF-doped Y1.84Li0.08Eu0.08O3, whose brightness was increased by a factor of 2.7 in comparison with that of Y2O3:Eu3+ films. This phosphor may promise for application to the flat panel displays.

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.

Excitation induced efficient luminescent properties of nanocrystalline Tb3+/Sm3+:Ca2Gd8Si6O26 phosphors

The cathodoluminescence and the excitation induced photoluminescence properties have been investigated for the nanocrystalline Tb3+/Sm3+:Ca2Gd8Si6O26 phosphors prepared by a solvothermal reaction method. The XRD patterns confirm their hexagonal structure. The green, orange and white emissions have been obtained by exciting at 275, 378, and 405 nm wavelengths, respectively. The corresponding CIE chromaticity coordinates are found to be in close proximity to the standard points in their respective regions. The cathodoluminescence at low accelerating voltage has also covered the entire visible region, resulting in white emission. These luminescent powders are expected to find potential applications in the development of LEDs and FEDs.

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.

Gd3 + Sensitization Effect on the Luminescence Properties of Tb3 + Activated Calcium Gadolinium Oxyapatite Nanophosphors

The solvothermal synthesis and structural characterization of silicate based oxyapatite Tb 3+ activated Ca 2 Gd 8 Si 6 O 26 (CGS) nanophosphors have been reported. The structure of these phosphors was elucidated by the powder x-ray diffraction (XRD) and further characterized by scanning electron microscopy. The XRD results revealed that the obtained Tb 3+ :CGS shows the characteristic peaks of oxyapatite in a hexagonal lattice structure. The photoluminescence (PL) properties were studied with variations of Tb 3+ concentration and sintering temperature. Under 275 nm excitation, both Tb 3+ ( 5 D 3,4 → 7 F J=6.5.4.3 ) and Gd 3+ ( 6 P 7/2 → 8 S 7/2 ) characteristic emissions associated with 4f-4f transitions have been observed, and when the concentration of Tb 3+ increases above 1 mol % the 5 D 3 emission intensity decreases due to cross relaxation. The Gd 3+ emission intensity decreases with increasing Tb 3+ concentration and the PL intensity of Tb 3+ at 378 nm excitation was much weaker than the obtained intensity with excitation at 275 nm, suggesting that the efficient energy transfer occurred from Gd 3+ to Tb 3+ ions in CGS host lattice. The decay curves of the 5 D 4 level show that the lifetime decreases with increasing crystallite size and concentration of Tb 3+ ions. These luminescent powders are expected to find potential applications such as white light emitting diodes and optical display systems.

Novel rare-earth-free yellow Ca5Zn3.92In0.08(V0.99Ta0.01O4)6 phosphors for dazzling white light-emitting diodes

White light-emitting diode (WLED) products currently available on the market are based on the blue LED combined with yellow phosphor approach. However, these WLEDs are still insufficient for general illumination and flat panel display (FPD) applications because of their low color-rendering index (CRI < 75) and high correlated color temperature (CCT = 6000 K). Although near-ultraviolet (UV) LED chips provide more efficient excitation than blue chips, YAG:Ce3+ phosphors have very weak excitation in the near-UV spectral region. Hence, there is an increasing demand for novel yellow phosphor materials with excitation in the near-UV region. In this work, we report novel self-activated yellow Ca5Zn3.92In0.08(V0.99Ta0.01O4)6 (CZIVT) phosphors that efficiently convert near-UV excitation light into yellow luminescence. The crystal structure and lattice parameters of these CZIVT phosphors are elucidated through Rietveld refinement. Through doping with In3+ and Ta5+ ions, the emission intensity is enhanced in the red region, and the Stokes shift is controlled to obtain good color rendition. When a near-UV LED chip is coated with a combination of CZIVT and commercial blue Ba0.9Eu0.1MgAl10O17 phosphors, a pleasant WLED with a high CRI of 82.51 and a low CCT of 5231 K, which are essential for indoor illumination and FPDs, is achieved.

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.