L. G. Bulusheva

Fluorographene: A Two-Dimensional Counterpart of Teflon

A stoichiometric derivative of graphene with a fluorine atom attached to each carbon is reported. Raman, optical, structural, micromechanical, and transport studies show that the material is qualitatively different from the known graphene-based nonstoichiometric derivatives. Fluorographene is a high-quality insulator (resistivity >10(12) Ω) with an optical gap of 3 eV. It inherits the mechanical strength of graphene, exhibiting a Youngs modulus of 100 N m(-1) and sustaining strains of 15%. Fluorographene is inert and stable up to 400 °C even in air, similar to Teflon.

Ab initio study of dielectric response of rippled graphene

Ab initio calculations of dielectric function and electron energy loss (EEL) function of periodically rippled armchair-edged graphene were performed in the framework of the random-phase approximation. The bending of graphene was found to remove restrictions on the electron transitions being forbidden in the flat graphene for certain light polarization. As a result, new peaks appear in the optical absorption spectrum and EEL spectrum of rippled graphene. Energy position, intensity, and width of the peaks are sensitive to the height of out-of-plane graphene bending that can be used in construction of graphene-based materials with variable transparency window.

Anisotropy of chemical bonding in semifluorinated graphite C2F revealed with angle-resolved X-ray absorption spectroscopy.

Highly oriented pyrolytic graphite characterized by a low misorientation of crystallites is fluorinated using a gaseous mixture of BrF(3) with Br(2) at room temperature. The golden-colored product, easily delaminating into micrometer-size transparent flakes, is an intercalation compound where Br(2) molecules are hosted between fluorinated graphene layers of approximate C(2)F composition. To unravel the chemical bonding in semifluorinated graphite, we apply angle-resolved near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and quantum-chemical modeling. The strong angular dependence of the CK and FK edge NEXAFS spectra on the incident radiation indicates that room-temperature-produced graphite fluoride is a highly anisotropic material, where half of the carbon atoms are covalently bonded with fluorine, while the rest of the carbon atoms preserve π electrons. Comparison of the experimental CK edge spectrum with theoretical spectra plotted for C(2)F models reveals that fluorine atoms are more likely to form chains. This conclusion agrees with the atomic force microscopy observation of a chain-like pattern on the surface of graphite fluoride layers.

Fluorinated cage multiwall carbon nanoparticles

Abstract Multiwall carbon nanoparticles from the inner part of a deposit grown onto a cathode in an arc-discharge condition were fluorinated at room temperature using volatile fluoride BrF 3 . The sample produced was characterized by means of X-ray diffraction (XRD), infrared (IR) and X-ray photoelectron spectroscopy (XPS). These methods point to the presence of covalent C–F bonds in the sample with retention of the sp 2 -hybridizated carbon atoms composing the cage tubular or quasi-spherical shells. Transmission electron microscopy (TEM) indicated a decrease of cage nanoparticles in the fluorinated material relative to the pristine sample that may be connected with unrolling the nanotubes during fluorination.

Anisotropic electromagnetic properties of polymer composites containing oriented multiwall carbon nanotubes in respect to terahertz polarizer applications

Polystyrene composites with 0.5 wt. % loading of oriented multiwall carbon nanotubes (MWCNTs) have been produced by forge rolling method. The composites showed anisotropy of transmission and reflection of terahertz radiation depending on sample orientation relative to the polarization of electromagnetic wave. The structural characteristics of composites (nanotube ordering, length, defectiveness) were estimated by fitting the theoretical dependencies calculated within the Clausius-Mossotti formalism for cylindrical particles to the experimental data. The presented model was used for prediction of electromagnetic response of composites containing oriented MWCNTs with various structural parameters in THz region.

Effect of nitrogen doping on the electromagnetic properties of carbon nanotube-based composites

Nitrogen-doped and pure carbon nanotube (CNT) based composites were fabricated for investigating their dielectric properties in static regime as well as electromagnetic response properties in microwave frequency range (Ka-band). Two classes of host matrix—polystyrene and phosphate unfired ceramics—have been used for composites fabrication. The study reveals miscellaneous effect of nitrogen doping on the dielectric permittivity, dc conductivity and electromagnetic interference shielding efficiency of CNT-based composites, produced with both polymer and ceramic matrices. The high-frequency polarizability, estimated for different-length CNTs, and static polarizability, calculated for nitrogen-containing CNT models using a quantum-chemical approach, show that this effect results from a decrease of the nanotube defect-free-length and deterioration of the polarizability with incorporation of nitrogen in pyridinic form.

Synthesis and structure of films consisting of carbon nanotubes oriented normally to the substrate

Films consisting of oriented carbon nanotubes (CNTs) are synthesized by thermal decomposition of a carboniferous compound + ferrocene mixture and studied by scanning electron microscopy. It is shown that the thickness and structure of the films depend on the synthesis conditions, specifically, on the way the reaction mixture is introduced. Multiple injection of the reaction mixture causes growth of multilayer CNT films, the number of layers in which equals the number of injections. The basic possibility of CNT continuous growth normally to the substrate surface is demonstrated. A mechanism behind the layered structure of the films consisting of oriented CNTs is suggested.

Bromination of graphene and graphite

We present a density functional theory study of low density bromination of graphene and graphite, finding significantly different behaviour in these two materials. On graphene we find a new Br2 form where the molecule sits perpendicular to the graphene sheet with an extremely strong molecular dipole. The resultant Br+-Br- has an empty pz-orbital located in the graphene electronic pi-cloud. Bromination opens a small (86meV) band gap and strongly dopes the graphene. In contrast, in graphite we find Br2 is most stable parallel to the carbon layers with a slightly weaker associated charge transfer and no molecular dipole. We identify a minimum stable Br2 concentration in graphite, finding low density bromination to be endothermic. Graphene may be a useful substrate for stabilising normally unstable transient molecular states.

Arrays of Carbon Nanotubes Aligned Perpendicular to the Substrate Surface: Anisotropy of Structure and Properties

Arrays of multiwalled carbon nanotubes (CNTs) aligned perpendicular to the substrate surface have been produced by the pyrolysis of a mixture of hydrocarbons with ferrocene as the source of the catalyst. Based on scanning electron microscopy data, the mechanism of formation of aligned CNT arrays is proposed. The potential of angle-resolved X-ray spectroscopy for determining the degree of disorder of graphite layers in CNTs is demonstrated. Aligned CNT arrays are characterized by anisotropy of magnetic properties due to encapsulation of metallic rods in the inner cavities of nanotubes.

Gas-phase synthesis of nitrogen-containing carbon nanotubes and their electronic properties

Nitrogen-containing carbon nanotubes are synthesized using a gas-phase reaction. The synthesis of nitrogen-doped carbon nanotubes from 100 to 500 Å in diameter is accomplished through pyrolysis of acetonitrile (CH3CN) at a temperature of 800°C. Cobalt and nickel metallic particles formed upon thermal decomposition of a mixture of maleate salts are used as catalysts. The materials synthesized are investigated by scanning and transmission electron microscopy. Analysis of the x-ray photoelectron spectra demonstrates that the content of nitrogen atoms in three nonequivalent charge states is approximately equal to 3%. A comparison of the CKα x-ray fluorescence spectrum of the carbon nanotubes synthesized through electric-arc evaporation of graphite and the x-ray fluorescence spectrum of the nitrogen-containing carbon nanotubes prepared by catalytic decomposition of acetonitrile indicates that, in the latter case, the spectrum contains a certain contribution from the sp3 states of carbon atoms. The temperature dependences of the electrical conductivity for different types of multi-walled carbon nanotubes are compared. The difference observed in the temperature dependences of the electrical conductivity is associated with the presence of additional scattering centers in nitrogen-containing carbon nanotubes.