Stockage de l’hydrogène dans les nanotubes de carbone à paroi unique : nouvelle méthode de préparation par excitation laser

A new mode for hydrogen adsorption and storage in single-wall carbon nanotubes is used, on the basis of laser excitation. Remember that this method has been useful to obtain, in the case of the fullerene C₆₀, many complex C₆₀-atoms or C₆₀-molecules, where atoms or molecular particles are trapped inside the C₆₀-molecules. We think this method might be important to store many hydrogen molecules inside carbon nanotubes.     

Studies in crystal structure and luminescence properties of Eu-doped metal tungstate phosphors for white LEDs

The correlation between the crystal structure and luminescent properties of Eu³⁺-doped metal tungstate phosphors for white LEDs was investigated. Red-emitting A_4−3x(WO₄)₂:Eu_x³⁺ (A=Li, Na, K) and B_(4−3x)/2(WO₄)₂:Eu_x³⁺ (B=Mg, Ca, Sr) phosphors were synthesized by solid-state reactions. The findings confirmed that these phosphors exhibited a strong absorption in the near UV to green range, due to the intra-configurational 4f-4f electron transition of Eu³⁺ ions. The high doping concentration of Eu³⁺ enhanced the absorption of near UV light and red emission without any detectable concentration quenching. Based on the results of a Rietveld refinement, it was attributed to the unique crystal structure. In the crystal structure of the Eu³⁺-doped metal tungstate phosphor, the critical energy transfer distance is larger than 5 Å so that exchange interactions between Eu³⁺ ions would occur with difficulty, even at a high doping concentration. The energy transfer between Eu³⁺ ions, which causes a decrease in red emission with increasing concentration of Eu³⁺, appears to be due to electric multi-polar interactions. In addition, the Eu-O distance in the host lattice affected the shape of emission spectrum by splitting of emission peak at the ⁵D₀→⁷F₂ transition of Eu³⁺.     

Low temperature quenching and high efficiency Tm, La or Tb co-doped CaSc2O4:Ce phosphors for light-emitting diodes

Low temperature quenching and high efficiency CaSc₂O₄:Ce³⁺ (CSO:Ce³⁺) phosphors co-doped with Tm³⁺, La³⁺ and Tb³⁺ ions were prepared by a solid state method and the phase-forming, morphology, luminescence and application properties of these phosphors were investigated. The results showed that co-doping of Tm³⁺, La³⁺ and Tb³⁺ ions can improve the luminescence properties and decrease temperature quenching of CSO:Ce³⁺ phosphor remarkably. High efficiency green-light-emitting diodes were fabricated with the prepared phosphors and InGaN blue-emitting (∼460 nm) chips. The good performances of the green-light-emitting LEDs made from co-doped CSO:Ce³⁺ phosphors confirm the luminescence enhancement and indicate that Tm³⁺, La³⁺ and Tb³⁺ co-doped CSO:Ce³⁺ phosphors are suitable candidates for the fabrication of high efficiency white LEDs.     

Mn, Cr-co-doped MgAl2O4 phosphors for white LEDs

The Mn-, Cr-doped and Mn, Cr-co-doped MgAl₂O₄ powders have been synthesized via a gel-solid reaction method. Energy transfer from Mn²⁺ to Cr³⁺ has been observed for the first time in the co-doped MgAl₂O₄ phosphors. When excited with blue light with a wavelength of 450 nm at room temperature, both green emission from Mn²⁺ around 520 nm and red emission from Cr³⁺ around 675and 693 nm were generated. Moreover, the color of the emission can be modified by controlling the doping concentrations of Mn²⁺ and Cr³⁺. Therefore, MgAl₂O₄: Mn²⁺, Cr³⁺ could be used as a single-phased phosphor for white LED with a blue LED chip. The energy transfer in terms of Mn²⁺ to Cr³⁺ is determined by means of radiation and reabsorption.     

Eu-activated Ba2Mg(BO3)2 yellow-emitting phosphors for near ultraviolet-based light-emitting diodes

A series of Eu²⁺-activated Ba₂Mg(BO₃)₂ yellow phosphors were prepared by a high temperature solid-state reaction. The phosphor emits intense yellow light under near ultraviolet excitation. Large Stokes shift can be attributed to the asymmetric nature of the Eu site and the lack of rigidity in the host. The concentration self-quenching mechanism of Ba₂Mg(BO₃)₂:Eu²⁺ is d-d interaction and the critical transfer distance is calculated to be about 12.29 Å. Prototype light-emitting diodes were fabricated by coating the Ba₂Mg(BO₃)₂:0.07Eu²⁺ phosphor onto ∼370 nm-emitting InGaN chips. The LEDs exhibit intense yellow-emitting under a forward bias of 20 mA. The results indicate that Eu²⁺-activated Ba₂Mg(BO₃)₂ is a candidate as a yellow component for fabrication of near-UV white light-emitting diodes.     

The improvement of moisture resistance and thermal stability of Ca3SiO4Cl2:Eu phosphor coated with SiO2

Green-emitting phosphors Ca₃SiO₄Cl₂:Eu²⁺ were prepared by the high temperature solid-state method. Sol-gel process was adopted to encapsulate the as-prepared phosphors with tetraethylorthosilicate (TEOS) as silicon coating reagent. Fluorescence spectrometer, scanning electron microscopy (SEM) and powder X-ray diffraction (XRD) patterns were employed to characterize the emission spectra, the surface morphologies and the phase structures, respectively. The chemical stability testing was operated by the method of soaking the phosphors in deionized water and roasting them at different temperatures. The results indicated that the surfaces of the green phosphors were evenly coated by SiO₂ and the phase structure of the coated phosphors remained the same as the uncoated samples. The luminance centre of Eu²⁺ did not shift after surface treatment and the luminance intensity of coated phosphors was lower than that of the uncoated samples. The results demonstrated that the water-resistance stability of the coated phosphor was improved to some degree because the pH value and the luminance intensity variation were both smaller than the uncoated phosphor after steeping within the same time. Moreover, the thermal stability of coated phosphors was enhanced obviously compared to the original samples based on the temperature dependent emission spectra measurement.     

Curvature Estimates for the Level Sets of Solutions to the Monge-Amp`ere Equation detD2u = 1

For the Monge-Ampere equation det D2u = 1, the authors find new auxiliary curvature functions which attain their respective maxima on the boundary. Moreover, the upper bounded estimates for the Gauss curvature and the mean curvature of the level sets for the solution to this equation are obtained.     

Ag-catalyzed Synthesis of Tb(BO2)3 Nanorods

Well-crystallized with excellent luminescent properties, Tb(BO₂)₃ nanorods were first suc-cessfully synthesized by a simple solid-state method with Ag as catalyst. The result of X-ray diffraction showed that the Tb(BO₂)₃ nanorods could be well-crystallized at 700 oC. As-prepared straight nanorods of Tb(BO₂)₃ had the typical diameters in the range of 100-200 nm, the thickness of 30-50 nm and the lengths up to 3 μm by transmission elec-tron microscopy and the corresponding selected area electron diffraction indicated that the nanorod calcined at 700 oC was single-crystalline. Based on the fact that Ag nanoparti-cles attached to the tips and middles of the Tb(BO₂)₃ nanorods, a growth model of the Tb(BO₂)₃ nanorods was proposed. Photoluminescence spectra under excitation at 369 nm showed that these Tb(BO₂)₃ phosphors had a green emission at 546 nm, which is ascribed to ⁵D₄→⁷F₅ transition. The effect of calcining temperature on the structures, morphologies, and luminescent properties of Tb(BO₂)₃ phosphors were studied.     

Photoluminescence and energy transfer in Tb/Mn co-doped ZnAl2O4 glass ceramics

We report on Tb³⁺ as efficient sensitizer for red photoemission from Mn²⁺-centers in ZnO-B₂O₃-Al₂O₃-Si₂O-Na₂O-SrO glasses and corresponding gahnite glass ceramics. In comparison to singly or co-doped glasses, the glass ceramics exhibit significantly increased emission intensity. Structural considerations, ESR, and dynamic luminescence spectroscopy indicate partial incorporation of Mn²⁺ as well as Tb³⁺ into the crystalline phase, the former on octahedral Zn²⁺-sites. Interionic distance and charge transfer probability between both species depend on crystallization conditions. This enables control of the energy transfer process and, hence, tunability of the color of photoemission by simultaneous emission from Tb³⁺ and Mn²⁺ centers. Concentration quenching in Mn²⁺-singly doped materials was found at a critical dopant concentration of about 1.0 mol%. The energy transfer process was studied in detail by dynamic as well as static luminescence spectroscopy. Spectroscopic results suggest the application of the studied materials as single or dual-mode emitting phosphor for luminescent lighting.     

ESR study of monoalkylzinc-2,2′-bipyridine complexes RZn · Bipy

Paramagnetic species are formed when LiBipy is treated with various alkylzinc halides at ?100°C and the ESR spectra are discussed. The observation that the half-life times of the radical species appear to depend only on the nature of the alkyl group bound to zinc proves that monoalkylzinc-2,2′-bipyridine complexes are formed. The nature of the bonding between zinc and 2,2′-bipyridine is discussed.