C.H. Kam

Luminescence of Eu3+ and Tb3+ doped Zn2SiO4 nanometer powder phosphors

Abstract We report, for the first time to the best of our knowledge, the luminescent properties of Tb 3+ and Eu 3+ doped Zn 2 SiO 4 phosphors prepared by a sol–gel process. The phosphors have shown prominent luminescence in green and red, respectively, due to the electronic transitions of 5 D 0 → 7 F 2 (Eu 3+ ) and 5 D 4 → 7 F 5 (Tb 3+ ). Structural characterization of these luminescent materials was carried out with XRD and TEM. Luminescence spectra were analyzed by measuring the excitation, photoluminescence spectra and lifetimes of the emission transitions of Eu 3+ ( 5 D 0 → 7 F J , J =1–5) and Tb 3+ ( 5 D 4 → 7 F J , J =1–6). Our experimental results have shown that these phosphors have comparable luminescence properties as those currently used in TVs and CRTs.

Optical and mechanical properties of TiO2/SiO2/organically modified silane composite films prepared by sol-gel processing

Abstract Thin films of TiO 2 /SiO 2 /organically modified silane (ORMOSIL) composite material were prepared by the sol–gel spin coating technique. Acid catalyzed solutions of y-glycidoxypropyltrimethoxysilane mixed with tetraethoxysilane and tetrapropylorthotitanate were used as precursors. Ellipsometry, thermal gravimetric analysis (TGA), differential thermal analysis (DTA), UV–visible spectroscopy (UV–vis), Fourier transform infrared (FTIR) spectroscopy, and Raman spectroscopy have been used to investigate the optical and structural properties of the composite films. Ellipsometry results showed that the film became thinner as the titanium content increased. A refractive index in the range of 1.52–1.61 at the optical wavelength of 633 nm was obtained by varying the molar ratio between silica and titanium. UV–vis transmission spectroscopy results showed that low absorption and high transparency (in the visible range) waveguide films could be obtained at a low temperature. Raman spectroscopy results indicated that the composite film with high titanium content contained amorphous carbon phases after being baked in air directly at a temperature of 500°C or higher. The hardness and Youngs modulus of the films were characterized by a nanoindenter and they showed a dependence on the heat-treatment temperature as well as the titanium molar fraction. A film hardness of 9.0 GPa was observed in a high titanium content film heat-treated at 800°C.

Photoluminescence of ZnS:Mn embedded in three-dimensional photonic crystals of submicron polymer spheres

ZnS:Mn has been in-filled in photonic crystals of submicron polymer spheres. The effect of the photonic band gap on the photoluminescence (PL) properties of ZnS:Mn has been investigated. Because of the overlap of the transmission dip of the photonic crystal and the photoluminescence band of ZnS:Mn, both suppression and enhancement in the PL of the phosphor have been observed. A strong dependence of the fluorescence lifetime on the emission wavelength in the range of the stop band has been found. This strong dependence is believed to arise from the very low photon density of state within the stop band of the ZnS:Mn in-filled photonic crystal as result of a high dielectric contrast between ZnS:Mn and the polystyrene spheres.

Optical and microstructural properties of sol-gel derived titania/organically modified silane thin films

Abstract Waveguide thin films with thickness more than one and a half micron were prepared by the sol–gel spin coating technique and low temperature heat treatment from relatively high titanium content organically modified silanes (ORMOSIL). The films were analyzed by X-ray diffraction (XRD), ellipsometry, atomic force microscopy (AFM), thermal gravimetric analysis (TGA), UV-visible spectroscopy (UV-vis), and Fourier transform infrared spectroscopy (FTIR) to study their optical and microstructural properties. It was observed that the film becomes thinner as the titanium content increases. A change of the refractive index through the range 1.44–1.55 at the optical wavelength of 633 nm was achieved by varying the molar ratio between ORMOSIL and titanium. UV-vis transmission spectroscopy and AFM results showed that macro-pore free and high transparency (in the visible range) waveguide films could be obtained at both low and high temperatures. While at low temperatures, our simpler recipe produced as good films as previously reported, it was noted that purely inorganic and crack-free silica-titania films with a thickness of about 0.5 μm could also be obtained when the film was baked at a temperature of 500°C or higher. Meanwhile, our films obtained after a high temperature heat treatment showed a hardness that is dependent on the heat treatment temperature. A hardness as high as 10.0 GP was achieved at a heat treatment temperature of 1000°C and we believe that this is a result of the formation of TiC structure.

Green upconversion luminescence in Er3+:BaTiO3 films

Green upconversion emissions at 548 and 528 nm have been obtained from sol-gel derived nanocrystalline Er3+:BaTiO3 films under 980 nm excitation. The green emissions are attributed to the ground-state (4I15/2) -directed transitions from 2H11/2 (528 nm) and 4S3/2 (548 nm) of Er3+ ions. Analysis of the temporal evolution of the emission at 548 nm shows that both excited state absorption of individual ion and energy transfer between the two neighboring ions contribute to the upconversion emissions in Er3+ (3 mol %):BaTiO3 film. The lifetime of the dominant emission at 548 nm is about 90 μs.

Luminescent properties of rare-earth ion doped yttrium silicate thin film phosphors for a full-colour display

In this paper, we report our results on the sol–gel preparation and spectral properties of (RExY1−x)2SiO5 (RE3+ = Eu3+, Tb3+, Ce3+, x = 0.005, 0.01, 0.02, 0.03 and 0.05) thin films. Effects of annealing temperatures on the thin films structure and optical properties, including luminescence, decay kinetics and CIE colour coordinates have been investigated. Upon exposure to the UV source, films containing Eu3+ and Tb3+ have exhibited bright red- and green-phosphorescent colours, due to the prominent transitions of 5D0→7F2 at 613 nm of Eu3+ and 5D4→7F5 at 543 nm of Tb3+ arising because of the intra-4f n electronic transitions, respectively. The photoluminescence spectrum of (CexY1−x)2SiO5 film mainly shows a broader emission band at 488 nm, which originates from the 5d→4f electron transitions of Ce3+. Luminescence mechanisms of these phosphor thin films have been discussed in detail.

Emission properties of GdOBr: Ce3+ and Tb3+ phosphors

Abstract The present investigation aims to elucidate the photoluminescent behavior of luminescent powder phosphors of GdOBr:Ce 3+ and GeOBr:Tb 3+ prepared by a solid state reaction method. Their emission properties have been systematically investigated. Photoluminescence have revealed a bright and broader (320–480 nm) violet-blue color emission (FWHM=85 nm) from the Ce 3+ (5d( 2 D)→ 2 F 5/2 ) and an intense and sharp (543 nm) green color emission (FWHM=5 nm) from Tb 3+ ( 5 D 4 → 7 F 5 ) phosphors as well as three other weaker emission transitions 5 D 4 → 7 F 6, 4, 3 at 485, 582 and 619 nm. Under a UV source, a very bright green emission color has been observed from terbium phosphor and a strong bluish violet color from cerium phosphor. The emission color richness defining CIE coordinates have been evaluated for these blue-violet ( x =0.155, y =0.008) and green ( x =0.285, y =0.554) luminescent powder phosphors.

Enhancement of green emission from Tb3+:GdOBr phosphors with Ce3+ ion co-doping

Abstract A strong green emission (5D4→7F5) has been observed from Tb3+ (1.0 mol%) doped into GdOBr powder phosphors upon excitation with an UV-wavelength (226 nm). To this phosphor, different amounts of Ce3+ (from 0.1 to 1.5 mol%) have been added to further improve the green emission efficiency of Tb3+-doped GdOBr. It has been found that the optimum concentration, 0.2 mol% of Ce3+, results in a twofold enhancement of the green emission intensity. The green emission from Tb3+ could not be increased at higher Ce3+ concentrations due to the Ce3+-Ce3+ ion interactions, i.e., the concentration quenching effect. The mechanism involved in the energy transfer between the Ce3+ and Tb3+ has been explained with an energy level diagram.

Visible up-conversion luminescence in Er3+:BaTiO3 nanocrystals

Abstract Visible emissions at 548, 528 and 660 nm have been observed from the sol–gel derived nanocrystalline Er 3+ :BaTiO 3 powders under infrared excitations at both 980 and 810 nm. Combined with the energy level structure of Er 3+ and the decay kinetics of the green emissions, the up-conversion mechanism has been analyzed and explained. The bright green emission at 548 nm has been attributed to the ground state-directed transition from 4 S 3/2 , which is populated through excited state absorption (ESA) for 810 nm excitation, and through both ESA and energy transfer (ET) processes for 980 nm excitation.

Deposition and photoluminescence of sol-gel derived Tb3+:Zn2SiO4 films on SiO2/Si

Abstract Tb3+ doped Zn2SiO4 films have been deposited on SiO2 buffered Si wafers by sol–gel method. The structures of these films have been investigated with X-ray diffraction (XRD), atomic force microscopy (AFM), and scanning electron microscopy (SEM). The results revealed that these films were composed of nanometer-size grains with a Willemite structure and had smooth surfaces. Photoluminescence measurements of the films showed a strong emission from 5D4 to 7F5 at 544 nm. The blue emission from 5D3–7Fj was depressed because of cross-relaxation effect. The decay kinetics of the 5D4–7F5 green emission was studied and a best fitting was obtained by a double exponential function. The lifetime of the excited 5D4 state is estimated to be 5.2 ms.