Molecular crystal approach for pi-conjugated polymers: from PPP Hamiltonian to Holstein model for polaron states

Starting from the $\pi$-electron Pariser-Parr-Pople (PPP) Hamiltonian which includes both strong electron-phonon and electron-electron interactions, we propose some strongly correlated wave functions of increasing quality for the ground state of conjugated polymers. These wavefunctions are built by combining different finite sets of local configurations extended at most over two nearest-neighbour monomers. With this picture, the doped case with one additional particle is expressed in terms of quasi-particle. Thus, the polaron formation problem goes back to the study of a Holstein like model.Comment: 27 pages, 6 eps figs, Revtex; enlarged version. Submitted to Journal of Physics: Condensed Matte

A922 Sequential measurement of 1 hour creatinine clearance (1-CRCL) in critically ill patients at risk of acute kidney injury (AKI)

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Photopolymerization of Thin Polycrystalline Diacetylene Films and Quenching of the Precursor Excited State

International audienceMicrocrystalline films of the diacetylenes 4BCMU and 3BCMU were prepared by vacuum evaporation and polymerized by UV irradiation. A method for determining the absolute polymer content X in films has been developed. In 4BCMU the reaction rate is strongly decreased beyond X similar to 0.3. Existing models fail to fit the kinetics. A numerical model based on quenching of the monomer excited state, the initiation precursor, by energy transfer to a nearby existing chain is developed and quantitatively fits the kinetics. Such quenching may occur in any diacetylene solid, so that one should not assume that an apparent saturation of the absorbance of a film corresponds to complete polymerization. 3BCMU is less reactive and briefly compared to 4BCMU

Raman scattering studies of heavily doped microcrystalline porous silicon and porous silicon free-standing membranes

International audienceThe phonon confinement model is often used to analyze Raman scattering band shapes of porous silicon and hence to determine the size of crystallites embedded in the porous layer. We show in this paper, from back-scattering Raman spectra, the importance to avoid using resonant excitation which would select a particular size of nanocrystals. In nonresonant conditions, and working on free-standing porous silicon membranes, broader and more realistic size distributions are found. The disappearance of Fano effect evidences the loss of free carriers in the porous layer. Raman intensity angular variations in parallel and perpendicular polarization configurations are obtained and discussed. They show the presence of unexpected different constant offsets for these two configurations. By comparing with theoretical calculation we deduce that they sign the impact of the porous layer on the light propagation

Temperature dependence of spectroscopic properties of isolated polydiacetylene chains strained by their monomer single crystal matrix

International audienceThe temperature dependence of exciton absorption and Raman frequencies of polydiacetylene (PDA) isolated chains is studied for 3BCMU and 4BCMU chains dispersed in their respective monomer crystal matrix. While isolated 3BCMU chains are practically unstrained, and behave as typical high quality bulk PDA, isolated 4BCMU chains are under a compression that increases as T decreases and show a very different behaviour. A simple model of chain deformation and its effect on the exciton energy is proposed, which qualitatively accounts for all the results. (c) 2007 Elsevier B.V. All rights reserved

New anthracene-based semi-conducting polymer analogue of poly(phenylene sulfide): Synthesis and photophysical properties

International audienceA new anthracene-based polymer analogue of poly(phenylene sulfide) has been synthesized via Wittig polycondensation. The polymer is soluble and shows a good film quality. This organic material showed an amorphous behavior with a Tg of 70 °C. The absorption and fluorescence properties of the polymer were investigated. The HOMO/LUMO energy levels were estimated by cyclic voltammetry measurements. The PAnS thin film exhibits an optical gap of 2.56 eV and emits in orange region. The fluorescence quantum efficiency in dilute solution of PAnS was of 66%. A PAnS-based single-layer diode has been fabricated and shows relatively low turn-on voltage of 4.8 V

Method for determining the polymer content in non soluble polydiacetylene films – application to pentacosadiynoic acid

International audienceThe absorptivities of polydiacetylenes (PDA) used in Langmuir films or vesicles for the development of PDA sensors films or other applications such as in non linear optics or for field-effect transistors are not known, so polymer contents cannot be deduced from experimental spectra. Here we introduce a novel method, using Nuclear Reaction Analysis (NRA) that allows quantitative determination of the polymer content X proportion of monomers that have been incorporated into PDA chains. We apply it to pentacosadiynoic acid (PCDA) evaporated microcrystalline films. A calibration curve giving X as a function of the area under an absorption spectrum normalized to the monomer areal density is obtained for blue and red PCDA. The method is applicable to all kinds of films, and to other PDA, provided films with known molecular areal density are available. An example of application to a PCDA Langmuir film is given

Effects of thermal annealing on n-type GaAs : V grown by MOCVD

International audienceVanadium (V)-doped GaAs (GaAs:V) layers grown by metal-organic chemical vapour deposition under different V-doping levels (10(17)-10(19) cm(-3)) were annealed in an arsine-H-2 gas mixture up to annealing temperatures of 750 and 850 degrees C for 30 min. The effect of thermal treatments on their electrical and optical properties was studied by means of the Hall effect, deep level transient spectroscopy and photoluminescence (PL). Annealing at 750 degrees C induces a thermal conversion from the n- to p-type of weakly V-doped GaAs. The conductivity of highly V-doped materials remains n-type. All the V-doped samples convert from n- to p-type following annealing at 850 degrees C. A comparison between the PL spectra for materials annealed under different conditions underlines the important role of gallium vacancies and a possible V accumulation at the surface in the case of thermal conversion

Blue-luminescent poly(p-phenylenevinylene) derivatives Synthesis and effect of side-group size on the optical properties

International audienceNew conjugated polymers based on separated PPV-type chromophores and incorporating different types of solubilizing side-groups (ethoxy: P1, hexyloxy: P2, dodecyloxy: P3 and benzyloxy: P4) were synthesized via Wittig polycondensation, using a series of bisphenol A-derived di(triphenylphosphonium) salts as starting monomers. The polymers are soluble in common organic solvents and their structures were confirmed by 1H NMR, 13C NMR and FTIR spectroscopies. The optical properties of these materials were investigated by UV–vis absorption and fluorescence spectroscopies. In dilute solution, quasi-identical fluorescence spectra were obtained and all the polymers showed a blue emission (420, 445 nm) and a narrow spectrum. In thin solid films, the polymers show side-group-dependent optical behavior and, whereas the emission remains blue in the case of P2, P3 and P4, a green fluorescence was observed for the ethoxylated polymer P1. From cyclic voltammetry analysis, the electrochemical band gaps were estimated to be 2.99, 3.07, 3.15 and 3.06 eV for P1, P2, P3 and P4, respectively. Single-layer diode devices of the [indium tin oxide/polymer/aluminum] configuration have been fabricated and show relatively low turn-on voltages between 2.6 and 4.9 V