Transition-metal activated phosphors are an important family of luminescent materials that can produce white light with an outstanding color rendering index and correlated color temperature for use in light-emitting diodes.

Energy migration (energy transfer among identical luminescence centers) is always thought to be related to the concentration quenching in luminescence materials. However, the novel Eu 3+ -doped Ba 6 Gd 2 Ti 4 O 17 phosphor seems to be an exception. In the series of Ba 6 Gd 2(1−x) Ti 4 O 17 :xEu 3+ (x = 0.1, 0.3, 0.5, 0.7, and 0.9) phosphors prepared and investigated, no concentration quenching is found. Detailed investigations of the crystal structure and the luminescence properties of Ba 6 Gd 2(1−x) Ti 4 O 17 :xEu 3+ reveal that the nonoccurrence of concentration quenching is related to the dimensional restriction of energy migration inside the crystal lattices. In Ba 6 Gd 2 Ti 4 O 17 , directly increasing the number of Eu 3+ ions to absorb as much excitation energy as possible allows to achieve a higher brightness. The highly Eu 3+ -doped Ba 6 Gd 2(1−x) Ti 4 O 17 :xEu 3+ (x = 0.9) sample can convert near-UV excitation into red light, whose Commission Internationale de l'Eclairage (CIE) coordinates are (0.64, 0.36) and the color purity can reach up to 94.4%. Moreover, warm white light with the CIE chromaticity coordinates of (0.39, 0.39), the correlated color temperature of 3756 K, and the color rendering index of 82.2 is successfully generated by fabricating this highly Eu 3+ -doped phosphor in a near-UV light-emitting diode chip together with the green YGAB:Tb 3+ and blue BAM:Eu 2+ phosphors. KEYWORDS: Eu 3+ -doped phosphor, Ba 6 Gd 2 Ti 4 O 17 host, nonconcentration quenching, energy migration, light-emitting diodes

Eu ion can be effectively sensitized by Ce ion through an energy-transfer chain of Ce-(Tb) -Eu, which has contributed to the development of white light-emitting diodes (WLEDs) as it can favor more efficient red phosphors. However, simply serving for WLEDs as one of the multicomponents, the design of the Ce-(Tb) -Eu energy transfer is undoubtedly underused. Theoretically, white light can be achieved with extra blue and green emissions released from Ce and Tb. Herein, the design of the white light based on these three multicolor luminescence centers has been realized in GdBO. It is the first time that white light is generated via accurate controls on the Ce-(Tb) -Eu energy transfer in such a widely studied host material. Because the thermal quenching rates of blue, green, and red emissions from Ce, Tb, and Eu, respectively, are well-matched in the host, this novel white light exhibits superior color stability and potential application prospect.

The structural peculiarities and electro-optic performance of liquid crystal (LC)-colloidal nanoparticle (NP)-polymer (P) composites formed by photoinduced phase separation are considered. We classify these materials under two groups according to two limiting cases of polymer morphology. The first group corresponding to small polymer concentration comprises LCs filled with NPs that are stabilized with a polymer network. It is found that, in addition to the light scattering caused by the LC orientational defects, the refractive index mismatch between LC and NP aggregates may significantly affect the electro-optic contrast and its angular characteristics. The second group is represented by polymer dispersed liquid crystals (PDLCs) filled with NPs. It is established that, in the process of photoinduced phase separation of the LC-NP-prepolymer mixture, the nanoparticles are mainly involved with the polymer, serving as building blocks for the polymer matrix. When the aggregation rate of the NPs is high or their size is large, the NPs enhance light scattering in the polymer. For low aggregation rate, NPs modify the effective refractive index and/or the absorption coefficient of the polymer phase without producing any noticeable optical inhomogeneity. Additionally, we found that TiO2 NPs may cause a photochromic effect, which manifests itself in color changes in the course of the photoinduced phase separation. For PDLCs with optically transparent polymer matrices modified by NPs, it is shown that doping with NPs can be used to control the refractive index ratio of the LC and polymer. In this way one can modify the contrast and substantially reduce the off-axis haze of the PDLC. The observed effects show LC-NP-P composites as materials of considerable promise for LCD and other electro-optic applications.

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A present chapter is focused on remarkable dielectric, electro-optical and micro-structural peculiarities of LC-CNTs dispersions, their correlation and mutual influence. It is mainly based on authors' original results obtained within recent years. The structure of this chapter is the following. The introductory part (section 1) gives short introduction to LC-CNTs composites and elucidates benefit of combination of LC and CNTs. It also outlines a field of questions further considered. A section 2 gives details of our samples and experimental methods. The next three sections correspondingly consider dielectric, electro-optical and structural peculiarities of LC-CNTs composites. Each of these topical sections starts with a short review and lasts with the authors' original results. The final, conclusion part (Section 6), summarizes most interesting properties of LC-CNTs suspensions, their application perspectives and mention some exciting problems for further investigations. www.intechopen.com Liquid crystal dispersions of carbon nanotubes: dielectric, electro-optical and structural peculiarities 453 power of 150 W. The concentration of CNTs, c, was varied in the range 0-2 wt %. Doping of CNTs has not influenced essentially the phase transition temperatures of LC-CNTs composites. 2.4 Cells The cells for electro-optical and dielectric measurements were made from glass substrates containing patterned ITO electrodes and aligning layers of polyimide. The polyimides AL2021 (JSR, Japan) and SE5300 (Nissan Chemicals, Japan) were used for homeotropic alignment of LC EBBA, MLC6608 and MLC6609, while the polyimide SE150 (Nissan Chemicals, Japan) was utilized for planar alignment of LC 5CB. The polyimide layers were rubbed by a fleecy cloth in order to provide a uniform planar alignment of LC in either fieldon state (EBBA, MLC6608 and MLC6609) or a zero field (5CB). The cells were assembled so that the rubbing directions of the opposite aligning layers were antiparallel. A cell gap was maintained by the glass spacers of appropriate size (16 m, if not otherwise stated). Finally, these cells were filled capillary with neat or CNTs doped liquid crystals heated to isotropic state. In some dielectric measurements the cells without alignment layers were utilized. The structure of LC-CNTs composites was monitored by observation of the filled cells placed between crossed polarizers, both by naked eye and in an optical polarizing microscope. 2.5 Electro-optical measurements The electro-optical measurements were carried out using the experimental setup described in (Koval'chuk et al., 2001a). The cell was set between two crossed polarizers so that the angle between the polarizer axes and the rubbing direction was 45°. The sinusoidal voltage 0-60 V (at frequency f=2 kHz) was applied to the cell. The voltage was stepwise increased from 0 to 60 V and then decreased back to 0; the total measuring time, i.e., time of voltage application, was about 1 min. The transmittance of the samples was calculated as =(I out /I in)*100%, where I in and ...

Energy transfer from Ce3+ to Sm3+ in orthosilicate host enhances the luminescence and shows promising application in white LEDs.

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