Strain- and Adsorption-Dependent Electronic States and Transport or Localization in Graphene

The chapter generalizes results on influence of uniaxial strain and adsorption on the electron states and charge transport or localization in graphene with different configurations of imperfections (point defects): resonant (neutral) adsorbed atoms either oxygen- or hydrogen-containing molecules or functional groups, vacancies or substitutional atoms, charged impurity atoms or molecules, and distortions. To observe electronic properties of graphene-admolecules system, we applied electron paramagnetic resonance technique in a broad temperature range for graphene oxides as a good basis for understanding the electrotransport properties of other active carbons. Applied technique allowed observation of possible metal-insulator transition and sorption pumping effect as well as discussion of results in relation to the granular metal model. The electronic and transport properties are calculated within the framework of the tight-binding model along with the Kubo-Greenwood quantum-mechanical formalism. Depending on electron density and type of the sites, the conductivity for correlated and ordered adsorbates is found to be enhanced in dozens of times as compared to the cases of their random distribution. In case of the uniaxially strained graphene, the presence of point defects counteracts against or contributes to the band-gap opening according to their configurations. The band-gap behaviour is found to be nonmonotonic with strain in case of a simultaneous action of defect ordering and zigzag deformation. The amount of localized charge carriers (spins) is found to be correlated with the content of adsorbed centres responsible for the formation of potential barriers and, in turn, for the localization effects. Physical and chemical states of graphene edges, especially at a uniaxial strain along one of them, play a crucial role in electrical transport phenomena in graphene-based materials.Comment: 16 pages, 10 figure

Superconductivity of Carbon Materials - Unstable Phases

Different intercalated carbon systems: K$\text{}_{x}$C$\text{}_{60}$, Rb$\text{}_{x}$C$\text{}_{60}$ as well as highly oriented pyrolytic graphite + potassium with unstable structural and superconducting behavior were investigated with the EPR and magnetically modulated microwave absorption techniques. Three characteristic, well-separated EPR signals were observed for A$\text{}_{x}$C$\text{}_{60}$ (A = K or Rb) systems at the early stages of doping process. We ascribed these signals to C$\text{}_{60}^{+}$, C$\text{}_{60}^{1-}$, and C$\text{}_{60}^{3-}$ radicals. Evolution of the EPR spectrum characterizes different phases of K$\text{}_{x}$C$\text{}_{60}$ system including superconducting one. Two superconducting phases with T$\text{}_{c}^{(1)}$=(21±0.5) K (unstable) and T$\text{}_{c}^{(2)}$=(18.5±0.5) K (final, stable) were distinctly separated for K$\text{}_{x}$C$\text{}_{60}$ system. For C$\text{}_{60}$ intercalated by rubidium a similar evolution of EPR spectrum is observed. Prior to well-defined superconducting Rb$\text{}_{3}$C$\text{}_{60}$ phase was created, strong instabilities of the magnetically modulated microwave absorption signal were observed at the narrow part of the doping process. For potassium intercalated highly oriented pyrolytic graphite, EPR oscillations below 100 K were observed as well as Josephson hysteresis loops were registered at 5 K

Localized states in nanocarbons

Localization phenomenon is studied in different modern nanocarbon materials: pristine C60, C60-fullerides, carbon nanotubes and graphene-based structures in the form of activated carbon fibers built of quantum dot-like basic structural units. Two experimental methods are used to define the localization and population control of spins (charge carriers) in the nanocarbon materials – electron paramagnetic resonance (EPR) and direct current (d.c.) electrical conductivity measurements. Results are discussed in the frame of the possible applications of the aforementioned materials in the molecular electronics or spintronics

Size Modification of Nanographite System of Activated Carbon Fibers Studied by EPR

We report results of EPR measurements of activated carbon fibers. Experiments made for pristine activated carbon fibers and activated carbon fibers with adsorbed molecules (CCl$\text{}_{4}$, C$\text{}_{6}$H$\text{}_{5}$NO$\text{}_{2}$, and H$\text{}_{2}$O) confirmed the localized character of paramagnetic centers observed in the system. Pristine activated carbon fibers are characterized by single Lorentzian line. Broader component of EPR signal appears when guest molecules are adsorbed in nanopores. The strongest localization is observed for water-filled activated carbon fibers nanopores (with hydrophobic pore walls) where changes in distance between nanographite particles were monitored by the g-shift to higher values. This process is related to stronger spin-orbit interaction of electrons trapped at nanographite particles compressed by guest molecules

Superconductivity of Carbon Materials - Unstable Phases

Different intercalated carbon systems: K$\text{}_{x}$C$\text{}_{60}$, Rb$\text{}_{x}$C$\text{}_{60}$ as well as highly oriented pyrolytic graphite + potassium with unstable structural and superconducting behavior were investigated with the EPR and magnetically modulated microwave absorption techniques. Three characteristic, well-separated EPR signals were observed for A$\text{}_{x}$C$\text{}_{60}$ (A = K or Rb) systems at the early stages of doping process. We ascribed these signals to C$\text{}_{60}^{+}$, C$\text{}_{60}^{1-}$, and C$\text{}_{60}^{3-}$ radicals. Evolution of the EPR spectrum characterizes different phases of K$\text{}_{x}$C$\text{}_{60}$ system including superconducting one. Two superconducting phases with T$\text{}_{c}^{(1)}$=(21±0.5) K (unstable) and T$\text{}_{c}^{(2)}$=(18.5±0.5) K (final, stable) were distinctly separated for K$\text{}_{x}$C$\text{}_{60}$ system. For C$\text{}_{60}$ intercalated by rubidium a similar evolution of EPR spectrum is observed. Prior to well-defined superconducting Rb$\text{}_{3}$C$\text{}_{60}$ phase was created, strong instabilities of the magnetically modulated microwave absorption signal were observed at the narrow part of the doping process. For potassium intercalated highly oriented pyrolytic graphite, EPR oscillations below 100 K were observed as well as Josephson hysteresis loops were registered at 5 K

EPR and MMMA Study of C60\text{}_{60} upon K-Doping

EPR and ΜΜMΑ studies of C$\text{}_{60}$ upon K-doping have been performed. Two different and well separated EPR narrow lines were detected for C$\text{}_{60}^{1-}$ and C at temperatures below 100 K. Time dependent changes in the intensities of both and C$\text{}_{60}^{3-}$ lines were observed when the system undergoes an eutectoid transformation. The evolution of superconductivity with two T$\text{}_{c}$ related to different valences (v) of C$\text{}_{60}^{v}$ ion radicals have been found

EPR and Impedance Measurements of Graphene Oxide and Reduced Graphene Oxide

We report the observations of electron paramagnetic resonance and impedance measurements of graphene oxide and reduced graphene oxide performed in the wide temperature range in order to get insight into the electronic properties of graphene-based materials and the role of oxygen functionalities in the charge carrier transport phenomena. In such systems the strong spin localization, hopping charge carrier transport as well as the formation of adsorption layers are observed, all the phenomena changing significantly after the heavily oxidized graphene is reduced

Electric Conductivity of Carbon Nanoparticles Stimulated by Electric Field

Host-guest interactions can be the unique method of spin manipulation in nanoscale. Strong changes in spin localization are generated when potential barriers between nanographitic units of activated carbon fibers are modified by interaction with adsorbed molecules. Stronger modifications occur when dipolar guest molecules are stimulated with external electric field. We report experimental results which show the influence of electric field on the spin localization in activated carbon fibers

Effect of Ultrasound Irradiation on Graphite Crystals Dimension

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Pseudorotational Averaging of EPR Spectrum of Cu(II)O5\text{}_{5} Complex in YBa2\text{}_{2}Cu3\text{}_{3}O7−δ\text{}_{7-δ} in Low Temperatures

In low temperatures the condensation of oxygen was found to occur at 05 site in an elementary cell of YBa$\text{}_{2}$Cu$\text{}_{3}$O$\text{}_{7-δ}$ being the fifth ligand forming the CuO$\text{}_{5}$ complex around Cul copper in chain. This change of coordinantion from CuO$\text{}_{4}$ to CuO$\text{}_{5}$ is the origin of a pseudorotation related to a strong vibronic coupling of two distorted configurations: a tetragonal pyramid C$\text{}_{4v}$ and a trigonal bipyramid D$\text{}_{3h}$, which yields a pseudocubic EPR spectrum in low temperatures. The averaged spectroscopic splitting coefficient is related to a superposition of vibronically coupled orbital states |x$\text{}^{2}$ - y$\text{}^{2}$〉 and |3z$\text{}^{2}$ - r$\text{}^{2}$〉. The averaged spectrum was for the first time observed in low temperatures since the oxygen condensation in YBa$\text{}_{2}$Cu$\text{}_{3}$O$\text{}_{7-δ}$ at 05 site of the chain occur only when oxygen undergoing fast diffusion among the chains, gets localized with decreasing temperature. The activation energy of oxygen desorption from the 05 site is 36 K