Electron-hole asymmetry in two-terminal graphene devices

A theoretical model is proposed to describe asymmetric gate-voltage dependence of conductance and noise in two-terminal ballistic graphene devices. The model is analyzed independently within the self-consistent Hartree and Thomas-Fermi approximations. Our results justify the prominent role of metal contacts in recent experiments with suspended graphene flakes. The contact-induced electrostatic potentials in graphene demonstrate a power-law decay with the exponent varying from -1 to -0.5. Within our model we explain electron-hole asymmetry and strong Fabri-Perot oscillations of the conductance and noise at positive doping, which were observed in many experiments with submicrometer samples. Limitations of the Thomas-Fermi approximation in a vicinity of the Dirac point are discussed.Comment: 7 pages, 8 figure

The Structure and EPR Behavior of Nitroxide Biradical Containing Phosphorus Atom in the Bridge

Two short nitroxide biradicals of similar composition: (H5C6)O=P(OR6)2 (BP) and O=S(OR6)2 (BS), where OR6 is 1-oxyl-2,2,6,6-tetramethyl-4-oxypiperidine, have been studied by electron paramagnetic resonance spectroscopy, and X-ray structural analysis. Variations of the intramolecular electron spin exchange in the biradicals, dissolved in toluene, as a function of temperature were characterized by changes in the isotropic 14N hyperfine splitting (hfs) constant a, values of the exchange integral |J|, and compared with the X-ray structural data. Thermodynamic parameters of the conformational rearrangements were calculated. Geometry optimization and spin density distribution calculations of biradical BP were carried out on the DFT/UB3LYPPBE0/cc-pVdTZz and DFT/ROPBE/N07D levels of theory and compared with those of BS. Probable differences in biradicals behavior are discussed. © 2015, Springer-Verlag Wien

The Structure and EPR Behavior of Nitroxide Biradical Containing Phosphorus Atom in the Bridge

Two short nitroxide biradicals of similar composition: (H5C6)O=P(OR6)2 (BP) and O=S(OR6)2 (BS), where OR6 is 1-oxyl-2,2,6,6-tetramethyl-4-oxypiperidine, have been studied by electron paramagnetic resonance spectroscopy, and X-ray structural analysis. Variations of the intramolecular electron spin exchange in the biradicals, dissolved in toluene, as a function of temperature were characterized by changes in the isotropic 14N hyperfine splitting (hfs) constant a, values of the exchange integral |J|, and compared with the X-ray structural data. Thermodynamic parameters of the conformational rearrangements were calculated. Geometry optimization and spin density distribution calculations of biradical BP were carried out on the DFT/UB3LYPPBE0/cc-pVdTZz and DFT/ROPBE/N07D levels of theory and compared with those of BS. Probable differences in biradicals behavior are discussed. © 2015, Springer-Verlag Wien

Carbon doped titanium dioxide Visible light photocatalysis and EPR investigation

X-band EPR studies of a series of undoped and carbon-doped titania powders reveal the presence of a carbon-centered radical species. The concentration of spins is five to six orders of magnitude smaller than the total carbon content. Photoexcitation of the carbon-doped samples increases the corresponding EPR signal intensities both under ultraviolet and visible light irradiation. After illumination, the original intensity is restored within a few hours. Samples with higher carbon radical concentration reveal higher photocatalytic activity in degradation of 4-chlorophenol under visible light irradiation

Recharge processes of paramagnetic centers during illumination in nitrogen-doped nanocrystalline titanium dioxide

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Structure and Properties of a Biradical Containing Acetylene and Phenylene Groups in the Bridge

X-band EPR spectroscopy and X-ray analysis (XRA) were used to study structure and dynamics of the intramolecular electron spin exchange in nitroxide biradical B1, in which radical rings are linked by two acethylene groups (-C equivalent to C-) and two para-phenylene groups -(C6H4-) as well as of the corresponding radical R1. The existence of two (cys- and trans-) conformers with different values of the exchange integral of the biradical and conformational transitions between them have been revealed. The energetically most stable geometry of biradical B1 has been calculated using DFT method with its good agreement with the X-ray data

Structures and catalytic properties of Cu(II) complex with chelating fluorinated ligands

Structure of Cu(II) complex with chelating fluorinated ligands were studied at 298 K and 90 K using CW X-band electron paramagnetic resonance (EPR) spectroscopy and density functional theory (DFT). Spin-Hamiltonian and structural parameters have been determined and the latter were compared with the X-ray data showing good similarity. A phase transition occurred at 360 K in solid crystalline sample was observed by EPR and discussed. Qualitative kinetic analysis of the catalytic behavior of Cu(II) complex upon H2O2 decomposition in acetonitrile-aqueous solutions at 313 K was studied using EPR and UV–visible spectroscopy