246 research outputs found

    Stretching and breaking of chemical bonds, correlation of electrons, and radical properties of covalent species

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    Chemical bonds are considered in light of correlation of valence electrons that is strengthened when the bond is dissociated. In the framework of the unrestricted Hartree-Fock single-reference version of the configuration interaction theory, effectively unpaired electrons lay the foundation of the electron correlation measure in terms of total number of the electrons (molecular chemical susceptibility). graphs and their singularities with respect to the interatomic distance allow presenting a quantitative description of stretching and breaking of chemical bonds. The approach validity is demonstrated on a large number of bonds of different order and chemical composition.Comment: 24 pages, 17 figures, 4 tables, Advances in Quantum Chemistry, vol. 70, 201

    Electronic Structure, Electron-Phonon Coupling, and Multiband Effects in MgB2

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    We review the current situation in the theory of superconducting and transport properties of MgB2. First principle calculations of of the electronic structure and electron-phonon coupling are discussed and compared with the experiment. We also present a brief description of the multiband effects in superconductivity and transport, and how these manifest themselves in MgB2. We also mention some yet open questions.Comment: Physica C, in pres

    Thermal properties of graphene

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    The two-dimensional (2D) monolayer structure of carbon atoms were initially considered as unstable. The 2D materials have recently been discovered and many researchers have started analyzing these materials. Graphene, a two-dimensional allotrope of graphite with sp2 bonded carbon atoms, is arranged in honeycomb structure. Graphene has excellent thermal conductivity and can be considered as a potential material for applications in the electronics industry where heating of materials is a serious concern. In this study, thermal properties of p and n doped graphene nanosheets and nanoribbons are studied as function of percentage composition of the dopants and the direction of dissipation of heat flux. Phonon dispersion spectra are presented for these structures using Materials Studio. Non- Equilibrium Molecular Dynamics simulation has been implemented for the calculations. Structures of doped graphene are modeled using Density Functional Theory to study the phonon dispersion. The specific heat of pristine and doped graphene structures are reported

    Synthesis and analysis of carbon nanowalls and their raman spectroscopy

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    Growth of Carbon Nanowalls has been carried out by Radio Frequency Chemical Vapor Deposition (RF-PECVD) on various substrates. Hydrogen was used as an active gas for the growth where as Ethanol bubbled with Argon was used as carbon source for formation of Carbon Nanowalls (CNW). The growth was then confinned by Raman Spectroscopy and Scanning Electron Microscope (SEM) images. Also the need of using catalyst in CNW has been analyzed. The intensities of D and G bands obtained from Raman spectrum have been related to the length of Graphene walls, to study the CNW\u27s length on different substrates. The growth was carried on three substrates; Silicon wafer, Silicon wafer coated with Nickel Formate Dihydrate (NFD) as a catalyst.and Pure Nickel foil. It has been found that the catalyst does not play much important role in the growth of Carbon Nanowalls. Also it has been determined that Graphene sheets are the basic building blocks for nanowalls and the Graphene walls obtained in our experiment comprise only of one to two Graphene sheets. The morphologies of the growth of Carbon Nanowalls have been studied on the three substrates using SEM images. The growth of CNW in case of Silicon substrate and Silicon substrate having catalyst was found to be only in that direction and places, where the gases were passing these substrates. But in case of Nickel foil CNWs were found over the whole substrate that might be resulting because of Nickel itself acting as an active catalyst for the whole surface

    Stability and precipitation of diverse nanoparticles

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    Nanotechnology is a rapidly growing industry that is exploiting the novel characteristics of materials manufactured at the nanoscale. Carbon based nanomaterials such as Carbon Nanotubes (CNTs) and Detonation Nanodiamond (DND) possess unique properties and find a wide range of industrial applications. With the advent of mass production of such materials, there is a possibility of contamination of water resources. Depending on the surface properties and structures, they might aggregate and settle down, or be dispersed and transported by the water. Therefore, there is a need to develop an understanding of the fate of such materials in aqueous media. The understanding and effect of solution chemistry is a key to predicting their deposition, transport, reactivity, and bioavailability in aquatic environments. The colloidal behavior of organic dispersed CNTs and water dispersed DNDs is investigated. The aggregation behavior of these two colloidal systems is quite different from that of hydrophilic, water soluble functional ized CNTs (F-CNTs). The values of the Fuchs stability ratio or the critical coagulant concentration are determined experimentally using time-resolved dynamic light scattering and are used to predict the stability of such systems. It is found that the aggregation behavior of the organic dispersed, antisolvent precipitated system does not follow the conventional Derjaguin—Landau—Verwey—Overbeek (D LVO) theory. But they stabilize in the long term, which is attributed to the supersaturation generated by different solubility of a solute in the solvent/antisolvent. Based on particle size distribution, zeta potential as well as the aggregation kinetics, the water dispersed DNDs are found to be relatively stable in aqueous solutions, but aggregate rapidly in presence of mono and divalent salts. Also, the formation of carboxylic groups on the DND surface does not alter colloidal behavior as dramatically as it does for other nanocarbons especially carbon nanotubes. Formation of colloidal dispersions via precipitation processes has been widely used in the chemical and pharmaceutical industries. The synthesis of micro-particles for hydrophobic drugs is effectively carried out via anti-solvent precipitation method. The formation of small particles in the precipitation method is strongly influenced by colloidal interactions, and therefore, dependent on the properties of the particles and the liquid. The effect of solvent on the colloidal stability of the micro-drug particles is studied in detail. It is found that the organic solvent plays an important role on particle formation, polymorphism and stability of micron scale drug particles in aqueous media. Also, the supersaturation can be varied by using different solvents and the physicochemical characteristics of the suspension can be altered, which affects stability. Understanding of the colloidal stability and the aggregation kinetics has great importance not only for fundamental researches, but also for their applications

    The Electron-Phonon Interaction in the Presence of Strong Correlations

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    We investigate the effect of strong electron-electron repulsion on the electron-phonon interaction from a Fermi-liquid point of view: the strong interaction is responsible for vertex corrections, which are strongly dependent on the vFq/ωv_Fq/\omega ratio. These corrections generically lead to a strong suppression of the effective coupling between quasiparticles mediated by a single phonon exchange in the vFq/ω≫1v_Fq/\omega \gg 1 limit. However, such effect is not present when vFq/ω≪1v_Fq/\omega \ll 1. Analyzing the Landau stability criterion, we show that a sizable electron-phonon interaction can push the system towards a phase-separation instability. A detailed analysis is then carried out using a slave-boson approach for the infinite-U three-band Hubbard model. In the presence of a coupling between the local hole density and a dispersionless optical phonon, we explicitly confirm the strong dependence of the hole-phonon coupling on the transferred momentum versus frequency ratio. We also find that the exchange of phonons leads to an unstable phase with negative compressibility already at small values of the bare hole-phonon coupling. Close to the unstable region, we detect Cooper instabilities both in s- and d-wave channels supporting a possible connection between phase separation and superconductivity in strongly correlated systems.Comment: LateX 3.14, 04.11.1994 Preprint no.101
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