144 research outputs found
Red-emitting fluorescent Organic Light emitting Diodes with low sensitivity to self-quenching
International audienceConcentration quenching is a major impediment to efficient organic light-emitting devices. We herein report on Organic Light-Emitting Diodes (OLEDs) based on a fluorescent amorphous red-emitting starbust triarylamine molecule (4-di(4'-tert-butylbiphenyl-4-yl)amino-4'-dicyanovinylbenzene, named FVIN), exhibiting a very small sensitivity to concentration quenching. OLEDs are fabricated with various doping levels of FVIN into Alq3, and show a remarkably stable external quantum efficiency of 1.5% for doping rates ranging from 5% up to 40%, which strongly relaxes the technological constraints on the doping accuracy. An efficiency of 1% is obtained for a pure undoped active region, along with deep red emission (x=0.6; y=0.35 CIE coordinates). A comparison of FVIN with the archetypal DCM dye is presented in an identical multilayer OLED structure
Radiative properties of stellar plasmas and open challenges
The lifetime of solar-like stars, the envelope structure of more massive
stars, and stellar acoustic frequencies largely depend on the radiative
properties of the stellar plasma. Up to now, these complex quantities have been
estimated only theoretically. The development of the powerful tools of helio-
and astero- seismology has made it possible to gain insights on the interiors
of stars. Consequently, increased emphasis is now placed on knowledge of the
monochromatic opacity coefficients. Here we review how these radiative
properties play a role, and where they are most important. We then concentrate
specifically on the envelopes of Cephei variable stars. We discuss the
dispersion of eight different theoretical estimates of the monochromatic
opacity spectrum and the challenges we need to face to check these calculations
experimentally.Comment: 6 pages, 5 figures, in press (conference HEDLA 2010
Effect of third- and fourth-order moments on the modeling of Unresolved Transition Arrays
The impact of the third (skewness) and fourth (kurtosis) reduced centered
moments on the statistical modeling of E1 lines in complex atomic spectra is
investigated through the use of Gram-Charlier, Normal Inverse Gaussian and
Generalized Gaussian distributions. It is shown that the modeling of unresolved
transition arrays with non-Gaussian distributions may reveal more detailed
structures, due essentially to the large value of the kurtosis. In the present
work, focus is put essentially on the Generalized Gaussian, the power of the
argument in the exponential being constrained by the kurtosis value. The
relevance of the new statistical line distribution is checked by comparisons
with smoothed detailed line-by-line calculations and through the analysis of
2p-3d transitions of recent laser or Z-pinch absorption measurements. The issue
of calculating high-order moments is also discussed (Racah algebra, Jucys
graphical method, semi-empirical approach ...).Comment: submitted to High Energy Density Physic
Design and validation of a foldable and photovoltaic wide-field epiretinal prosthesis
Retinal prostheses have been developed to fight blindness in people affected by outer retinal layer dystrophies. To date, few hundred patients have received a retinal implant. Inspired by intraocular lenses, we have designed a foldable and photovoltaic wide-field epiretinal prosthesis (named POLYRETINA) capable of stimulating wireless retinal ganglion cells. Here we show that within a visual angle of 46.3 degrees, POLYRETINA embeds 2215 stimulating pixels, of which 967 are in the central area of 5 mm, it is foldable to allow implantation through a small scleral incision, and it has a hemispherical shape to match the curvature of the eye. We demonstrate that it is not cytotoxic and respects optical and thermal safety standards; accelerated ageing shows a lifetime of at least 2 years. POLYRETINA represents significant progress towards the improvement of both visual acuity and visual field with the same device, a current challenging issue in the field
Shape optimization for the generalized Graetz problem
We apply shape optimization tools to the generalized Graetz problem which is a convection-diffusion equation. The problem boils down to the optimization of generalized eigen values on a two phases domain. Shape sensitivity analysis is performed with respect to the evolution of the interface between the fluid and solid phase. In particular physical settings, counterexamples where there is no optimal domains are exhibited. Numerical examples of optimal domains with different physical parameters and constraints are presented. Two different numerical methods (level-set and mesh-morphing) are show-cased and compared
Thermal and Optical Characterization of Undoped and Neodymium-Doped Y3ScAl4O12 Ceramics
Y3–3xNd3xSc1Al4O12 (x = 0, 0.01, and 0.02) ceramics were fabricated by sintering at high temperature under vacuum. Unit cell parameter refinement and chemical analysis have been performed. The morphological characterization shows micrograins with no visible defects. The thermal analysis of these ceramics is presented, by measuring the specific heat in the temperature range from 300 to 500 K. Their values at room temperature are in the range 0.81–0.90 J g1–K–1. The thermal conductivity has been determined by two methods: by the experimental measurement of the thermal diffusivity by the photopyroelectric method, and by spectroscopy, evaluating the thermal load. The thermal conductivities are in the range 9.7–6.5 W K–1 m–1 in the temperature interval from 300 to 500 K. The thermooptic coefficients were measured at 632 nm by the dark mode method using a prism coupler, and the obtained values are in the range 12.8–13.3 × 10–6 K–1. The nonlinear refractive index values at 795 nm have been evaluated to calibrate the nonlinear optical response of these materials.This work is supported by the Spanish Government under projects MAT2011-29255-C02-01-02, MAT2013-47395-C4-4-R, and the Catalan Government under project 2014SGR1358. It was also funded by the European Commission under the Seventh Framework Programme, project Cleanspace, FP7-SPACE-2010-1-GA No. 263044
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