Yosuke Nakai

The new red-emitting phosphor of oxyfluoride Ca 2 RF 4 PO 4 :Eu 3+ (R=Gd, Y) for solid state lighting applications

The novel red-emitting phosphors of Eu3+-activated Ca2RF4PO4:Eu3+ (R=Gd, Y) prepared by a solid-state reaction have been evaluated as a candidate for white solid state lighting. The detailed luminescence properties, e.g., the excitation spectra, the luminescence spectra and quantum efficiency under the excitation of near-UV, and decay lifetimes were reported. The phosphors can be efficiently excited by near UV light and exhibit a dominant emission peaked at 611 nm (5D0-7F2) with CIE coordinates of (x=0.661, y=0.333). The thermal stabilities were investigated from the luminescence intensities, color purity and the decay curves by increasing temperature. The luminescence parameters related to white LEDs applications were compared to some red phosphors and discussed in details. The red-emitting Ca2RF4PO4:Eu3+ (R=Gd, Y) may be potentially useful in the fabrication of white LEDs.

Luminescence properties and site occupations of Eu3+ ions doped in double phosphates Ca9R(PO4)7 (R = Al, Lu)

Eu3+ doped red-emitting phosphors of double phosphates Ca9R(PO4)7 (R = Al, Lu) were synthesized by a general high temperature solid-state reaction. The phosphors were characterized by X-ray powder diffraction (XRD). The detailed luminescence properties, e.g., the emission spectra under the excitation of UV light, the photoluminescence excitation spectra and decay lifetimes were reported. The phosphors can be efficiently excited by near UV light to realize an intense red luminescence (613 nm) corresponding to the electric dipole transition 5D0 → 7F2 of Eu3+ ions. The luminescence properties and the potential applications were analyzed. These phosphors were investigated by the site-selective emission spectra and the fluorescence decay curves in the 5D0 → 7F0 region using a pulsed, tunable, narrowband dye laser. It is suggested that Eu3+ ions have three different crystallographic sites doped in Ca9Al(PO4)7, and five sites in Ca9Lu(PO4)7 host. The site assignments of Eu3+ ions in Ca9R(PO4)7 (R = Al, Lu) were discussed on the base of both optical spectroscopy results and structural analysis.

Photoluminescence of bis(8-hydroxyquinoline) zinc (Znq2) and magnesium (Mgq2)

Absorption and photoluminescence (PL) spectra, PL quantum efficiency, and PL lifetime have been investigated on bis(8-hydroxyquinoline) zinc (Znq2) and magnesium (Mgq2) in solutions and powder. Znq2 and Mgq2 have the lowest-energy absorption band at 376 and 396 nm in acetonitrile solution, respectively, and emission band with peak at 555 and 480 nm. The PL quantum efficiency is 0.03 and 0.45 for Znq2 and Mgq2 in the solution, respectively, while 0.45 and 0.36 in powder. Unlike the case of powders, two PL lifetimes are obtained in solutions. The longer lifetime is attributed to molecule having interaction with its neighboring molecule, while the shorter one to the isolated single molecule.

Relationships between main-chain chirality and photophysical properties in chiral conjugated polymers

A series of R- and S-binaphthyl-containing polyfluorenes, bearing different contents and types of axial chirality in the main chain, have been synthesized through Suzuki polycondensation to investigate the influence of the covalently incorporated chirality on the photophysical properties of chiral conjugated polymers. The experimental measurements obtained by UV-Vis and photoluminescence (PL) spectroscopy, cyclic voltammetry, and circular dichroism spectroscopy reveal that the chiral copolymers possess high thermostability, high luminescence efficiency, reversible electrochemical properties, and intrachain transferred dichroism. Surprisingly, the R-chiral polymers exhibit better spectral thermostability, stronger suppressing ability upon the β-phase formation of the main-chain, and higher PL quantum efficiency than S-chiral polymers in solid films. The theoretical insights obtained by either ab initio density functional theory (DFT) calculations or molecular dynamics (MD) simulations suggest that these differences probably resulted from the more planar chain conformation of S-chiral polymers, which leads to stronger interchain interaction and an increased tendency to form inefficient and unstable excimers or quenchers. The different effects of enantiomers on the photophysical properties of chiral conjugated polymers may provide an import update in the understanding of chiro-optics.

Temperature effect on emission spectra and fluorescence lifetime of the 4I13=2 state of Er3+-doped Gd2SiO5 crystal

By measuring the emission spectra and the fluorescence lifetime of the 4I13=2 state of Er3+ ions in Gd2SiO5 crystal at different temperatures, the effects of temperature on the spectra and the lifetime of the 4I13=2 state are investigated. When the temperature increases, the emission line width for the 4I13=2 -> 4I15=2 transition is broadened, and the main emission lines at 1 596, 1 609, and 1 644 nm shifte toward shorter wavelengths. The measured lifetime of the 4I13=2 state decreases from 13.2 to 8.4 ms with temperature increase from 13 to 300 K, which is mainly due to the temperature dependence of multiphonon relaxation between the 4I13=2 and 4I15=2 states and the changing population distribution among the Stark levels within the 4I13=2 state. The experimental results imply that low temperature condition is better for the ~1.6- \mu m laser output.

Temperature dependence of Bi4Ge3O12 photoluminescence spectra

Bi4Ge3O12 single crystals were obtained using Czochralski growth method. Photoluminescence spectra were analyzed versus temperature from 12 to 295 K. Besides the previously observed emission bands at 610 and 820 nm, the new emission band at 475 nm was found by a careful temperature dependence measurement in the present study. The influence of basic and defect structure on the shape and position of the spectra versus temperature was discussed.

Thermal and spectroscopic properties of Er3+: Lu2SiO5 and characteristics of its in-band pumped laser

We investigated the thermal properties and Stark-level energies of Er3+:Lu2SiO5 crystal and its in-band laser-pumped CW laser. The thermal conductivity, which was calculated by the thermal diffusion coefficient and the specific heat, decreased from 5.3 to 3.2 Wm−1 k−1 with the temperature increasing from 298.15 to 573.15 K. From the low temperature absorption spectra at 10 K and photoluminescence spectrum at 12 K, the Stark level energies of Er3+ were obtained for the ground state and many excited states. Under pumping by a 1536 nm laser with 4.4 W power, a 1596 nm CW laser with maximum output power of 610 mW was obtained, corresponding to a slope efficiency and optical efficiency of 16.3 % and 13.9 %, respectively.