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Disordered Materials (disordered + material)
Selected AbstractsA Concerted Approach for the Determination of Molecular Conformation in Ordered and Disordered MaterialsCHEMISTRY - A EUROPEAN JOURNAL, Issue 22 2007Jan Sehnert Abstract We present the successful application of a concerted approach for the investigation of the local environment in ordered and disordered phases in the solid state. In this approach we combined isotope labeling with computational methods and different solid-state NMR techniques. We chose triphenylphosphite (TPP) as an interesting example of our investigations because TPP exhibits two crystalline modifications and two different amorphous phases one of which is highly correlated. In particular we analyzed the conformational distribution in three of these phases. A sample of triply labeled 1-[13C]TPP was prepared and 1D MAS as well as wide-line 13C NMR spectra were measured. Furthermore we acquired 2D 13C wide-line exchange spectra and used this method to derive highly detailed information about the phenyl orientation in the investigated TPP phases. For linkage with a structure model a DFT analysis of the TPP molecule and its immediate environment was carried out. The ab initio calculations of the 13C chemical shift tensor in three- and six-spin systems served as a base for the calculation of 1D and 2D spectra. By comparing these simulations to the experiment an explicit picture of all phases could be drawn on a molecular level. Our results therefore reveal the high potential of the presented approach for detailed studies of the mesoscopic environment even in the challenging case of amorphous materials. [source] Temperature-Resolved Local and Macroscopic Charge Carrier Transport in Thin P3HT Layers,ADVANCED FUNCTIONAL MATERIALS, Issue 14 2010Patrick Pingel Abstract Previous investigations of the field-effect mobility in poly(3-hexylthiophene) (P3HT) layers revealed a strong dependence on molecular weight (MW), which was shown to be closely related to layer morphology. Here, charge carrier mobilities of two P3HT MW fractions (medium-MW: Mn,=,7,200 g mol,1; high-MW: Mn,=,27,000 g mol,1) are probed as a function of temperature at a local and a macroscopic length scale, using pulse-radiolysis time-resolved microwave conductivity (PR-TRMC) and organic field-effect transistor measurements, respectively. In contrast to the macroscopic transport properties, the local intra-grain mobility depends only weakly on MW (being in the order of 10,2 cm2 V,1 s,1) and being thermally activated below the melting temperature for both fractions. The striking differences of charge transport at both length scales are related to the heterogeneity of the layer morphology. The quantitative analysis of temperature-dependent UV/Vis absorption spectra according to a model of F. C. Spano reveals that a substantial amount of disordered material is present in these P3HT layers. Moreover, the analysis predicts that aggregates in medium-MW P3HT undergo a "pre-melting" significantly below the actual melting temperature. The results suggest that macroscopic charge transport in samples of short-chain P3HT is strongly inhibited by the presence of disordered domains, while in high-MW P3HT the low-mobility disordered zones are bridged via inter-crystalline molecular connections. [source] The effect of disorder on the chemical reactivity of an organic solid, tetraglycine methyl ester: Change of the reaction mechanismJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 2 2002Evgenyi Shalaev Abstract Many drugs undergo chemical changes in the solid state, and understanding chemical reactivity of organic crystals is a critical factor in the drug development process. In this report, the impact of milling on the thermal chemical reactivity of an organic solid, tetraglycine methyl ester, was studied using DSC, isothermal calorimetry, chemical analysis (HPLC and insoluble residue determination), and powder X-ray diffraction. Significant changes in both X-ray diffraction patterns and DSC curves were detected after very brief milling (5 s). The changes were interpreted as the formation of a disordered phase. The disordered phase was tentatively identified as a crystal mesophase that combines properties of both crystalline (i.e., long-range order) and amorphous (i.e., glass transition) states. In the disordered material, the reaction mechanism changed from the methyl transfer reaction, which was observed in the intact crystal, to a polycondensation reaction when the reaction was performed at 165°C. Such changes in the reaction mechanism occurred in materials milled for >,30 s. © 2002 Wiley-Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 91:584,593, 2002 [source] ta-C/Si heterojunction diodes with apparently giant ideality factorsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 2 2010Marc Brötzmann Abstract A common feature of many wide band gap heterojunction diodes is an unexplained large ideality factor n > 2. In this context we investigate the diode characteristics of heterojunction diodes consisting of a crystalline semiconductor material such as Si covered with a thin semiconducting film of amorphous or disordered material. As thin amorphous film we use tetrahedral amorphous carbon (ta-C). These heterojunctions exhibit a pronounced rectifying behavior, low saturation current and low parasitic currents. Moreover, we observe an apparently giant ideality factor reaching values of n > 75. As a consequence, the turn on voltage is around 3 , 10 V and the I-V curves can be measured for bias up to 40 V without reaching saturation or electrical breakdown. We present a quantitative model for the unusual diode characteristics of these Metal , Amorphous Semiconductor , Semiconductor diodes (MASS-diodes). We demonstrate that the I-V characteristics of the heterojunctions are well described by a serial arrangement of an ideal Schottky-diode, a Frenkel-Poole type resistance and an Ohmic contact resistance, emulating a p-n- or Schottky diode characteristic with giant ideality factor and referred to as the FPID-model. We propose that heterojunctions exhibiting apparently large ideality factors n , 2 may possess an interfacial disordered or amorphous layer with Frenkel-Poole conduction properties. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] The role of doping type in setback layers on wafer-fused AlGaAs/GaAs/GaN HBTsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 9 2008Chuanxin Lian Abstract The role of doping type in the setback layer of wafer fused AlGaAs/GaAs/GaN HBTs has been investigated. The calculation assuming an ideal GaAs/GaN interface indicates that the current blocking effect due to the conduction band discontinuity between GaAs and GaN can be effectively alleviated by inserting a lightly doped GaAs setback layer with either n-type or p-type doping. And the n-type setback is expected to work better than the p-type setback for the same doping concentration. In experiments encouraging current gain of , 8 was observed in wafer fused HBTs containing 30 nm GaAs setback layers of either n-type or p-type doping, substantially improved from the HBTs without any setback, but still much smaller than annealed as-grown AlGaAs/GaAs/GaAs HBTs. It was proposed that the non-ideal fused interface with a thin disordered material and interface charges/traps dominates the device performance, and that likely both types of fused HBTs (setback doping concentration is , 1 × 1017 cm,3) have similar band diagrams after taking into account impurity redistribution near the interface. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] The magnetic moments and their long-range ordering for Fe atoms in a wide variety of metallic environmentsINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 15 2010A. Ayuela Abstract The magnitudes of Fe magnetic moments, together with their long-range ordering when appropriate, are compared and contrasted in a variety of metallic environments. Thus, Fe, in its stable body-centered-cubic (bcc) phase under ambient conditions, is considered under pressures p, which can yield different crystal structures at high p, including fcc. The modification of the ferromagnetism in bcc Fe is surveyed as one passes through a bc-tetragonal lattice to the fcc form. In the latter, evidence is presented, both from theory and experiment, that the ordering is antiferromagnetic in character. Then, binary metallic alloys with Fe atoms as the majority component are considered, Fe,Co and Fe,Ga being focal points in both ordered and disordered materials. Finally, some discussion is given, involving again both experiment and theory, of the possible spin polarization of neighboring Cs atoms when Fe impurity atoms are inserted into the low conduction electron density characterizing this heavy alkali metal under ambient conditions. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [source] Broadband dielectric spectroscopy on the molecular dynamics in different generations of hyperbranched polyesterJOURNAL OF APPLIED POLYMER SCIENCE, Issue 4 2009Gamal Turky Abstract Dielectric spectroscopy (10,2 Hz to 106 Hz) was employed to investigate the molecular dynamics of hyperbranched polyesters where the number of the generation is systematically varied from 2 to 5. As a first result, the dielectric properties depends strongly on the generation of the hyperbranched polymers. For higher generations (3 to 5) at temperatures below Tg two relaxation processes are observed, a ,-process at lower temperatures and a ,-process at higher ones. The apparent activation energies are around 100 kJ/mol which seems to be too high for truly localized processes. For the Generation 2, only the ,-process is observed. For all investigated polymers the dielectric ,-relaxation could not be observed because of strong conductivity effects. Therefore, the conductivity is systematically analyzed which obeys the peculiarities found to be characteristic for semiconducting disordered materials. Especially, the Barton/Nakajima/Namikawa relationship is found to be valid. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009 [source] Proceedings of the 11th International Conference on Phonon Scattering in Condensed Matter (Phonons2004)PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 11 2004Alexander Kaplyanskii The 11th International Conference on Phonon Scattering (Phonons2004) was held in St. Petersburg, Russia, 25,30 July 2004. Both theoretical and experimental results on studies in phonon physics and related phenomena were presented and discussed. Main topical areas were: Phonons in nanostructures, coherent phonons, phononic crystals and superlattices, ultrafast acoustics, solitons and nonlinear phenomena, electron,phonon interaction, phonons in glasses and disordered materials, phonon transport and imaging, quantum fluids, lattice dynamics, and Raman and neutron scattering. [source] Maximum entropy method and charge flipping, a powerful combination to visualize the true nature of structural disorder from in situ X-ray powder diffraction dataACTA CRYSTALLOGRAPHICA SECTION B, Issue 2 2010Ali Samy In a systematic approach, the ability of the Maximum Entropy Method (MEM) to reconstruct the most probable electron density of highly disordered crystal structures from X-ray powder diffraction data was evaluated. As a case study, the ambient temperature crystal structures of disordered ,-Rb2[C2O4] and ,-Rb2[CO3] and ordered ,-K2[C2O4] were investigated in detail with the aim of revealing the `true' nature of the apparent disorder. Different combinations of F (based on phased structure factors) and G constraints (based on structure-factor amplitudes) from different sources were applied in MEM calculations. In particular, a new combination of the MEM with the recently developed charge-flipping algorithm with histogram matching for powder diffraction data (pCF) was successfully introduced to avoid the inevitable bias of the phases of the structure-factor amplitudes by the Rietveld model. Completely ab initio electron-density distributions have been obtained with the MEM applied to a combination of structure-factor amplitudes from Le Bail fits with phases derived from pCF. All features of the crystal structures, in particular the disorder of the oxalate and carbonate anions, and the displacements of the cations, are clearly obtained. This approach bears the potential of a fast method of electron-density determination, even for highly disordered materials. All the MEM maps obtained in this work were compared with the MEM map derived from the best Rietveld refined model. In general, the phased observed structure factors obtained from Rietveld refinement (applying F and G constraints) were found to give the closest description of the experimental data and thus lead to the most accurate image of the actual disorder. [source] Effect of electric field on diffusion in disordered materialsANNALEN DER PHYSIK, Issue 12 2009F. Jansson Abstract The effect of electric field on diffusion of charge carriers in disordered materials is studied by Monte Carlo computer simulations and analytical calculations. It is shown how an electric field enhances the diffusion coefficient in the hopping transport mode. The enhancement essentially depends on the temperature and on the energy scale of the disorder potential. It is shown that in one-dimensional hopping the diffusion coefficient depends linearly on the electric field, while for hopping in three dimensions the dependence is quadratic. [source] | ||||||||||||||||