The Raman scattering studies of multi-layer graphene obtained by high temperature annealing of carbon terminated face of 4H-SiC(000-1) substrates are presented. Intensity ratio of the D and G bands was used to estimate the average size of the graphene flakes constituting carbon structures. The obtained estimates were compared with flake sizes from atomic force microscopy data. We found that even the smallest structures observed by atomic force microscopy images are much bigger than the estimates obtained from the Raman scattering data. The obtained results are discussed in terms of different average flake sizes inside and on the surface of the multi-layer graphene structure, as well as different type of defects which would be present in the investigated structures apart from edge defects.
This paper is devoted to identification of the most important factors responsible for formation of magnetic moments at edges of graphene-like nanoribbons. The main role is attributed to the Hubbard correlations (within unrestricted Hartree-Fock approximation) and intrinsic spin-orbit interactions, but additionally a perpendicular electric field is also taken into account. Of particular interest is the interplay of the in-plane edge magnetism and the energy band gap. It is shown that, with the increasing electric field, typically the following phases develop: magnetic insulator (with in-plane spins), nonmagnetic narrow-band semiconductor, and nonmagnetic band insulator.
A novel sp^3-bonded nanosize domain, known as a diaphite which is an intermediate state between a graphite and a diamond, is generated by the irradiation of visible laser pulse onto a graphite crystal. The sp^3 structure is well stabilized by shear displacement between neighboring graphite layers. We theoretically study the interlayer sp^3 bond formation with frozen shear displacement in a graphite crystal, using a classical molecular dynamics and a semi-empirical Brenner potential. We show that a pulse excitation under the fluctuation of shearing motion of carbons in an initial state can generate interlayer sp^3 bonds which freeze the shear, though no frozen shear appears if there is no fluctuation initially. Moreover, we investigate a pulse excitation under the coherent shearing motion and consequently obtain that the sp^3-bonded domain with frozen shear is efficiently formed. We conclude that the initial shear is important for the photoinduced sp^3 nanosize domain formation.
We investigate theoretically the electronic properties of graphene functionalized with nitrogen and boron atoms substituted into the graphene monolayer. Our study is based on the ab initio calculations in the framework of the density functional theory. We predict the dependence of the energy band gap, binding energy per atom, and the shift of the Fermi level on the concentration of dopants. Moreover, we examine the influence of the distribution of B/N atoms on the specified properties.
"Graphene paper" prepared by new proprietary method involving high pressure and high temperature treatment in the reduction process show new possibilities in this area. Different phase content: multilayer and single layer graphene stacks recorded in this study for RGO samples are accompanied by the specific electric and optical parameters. We have found that process temperatures above 900°C play crucial role in structural and other properties. For the process temperature around 2000°C we found the onset of the graphitization in the samples.
We present the results of ab initio calculations of gas adsorption processes on graphene. Static density functional theory framework is used to obtain adsorption energies of several species on a Stone-Wales defected graphene monolayer. The Van der Waals interaction is taken into account by a semi-empirical correction. Sites closer to the defect are found to induce stronger adsorption compared to sites further away, where the graphene crystal structure is intact. The Car-Parrinello ab initio molecular dynamics simulations are performed at high temperatures. CH₃ is found to be stably physisorbed or chemisorbed at 300 K.
Due to its fascinating properties such as high surface area, very good electrical and thermal conductivity, excellent mechanical properties, optical and electrochemical properties, graphene may be the ideal material as a substrate of nanocomposites for applications in electronics. Graphene layer can be used as a conductive matrix allowing good contact between crystallites of nanomaterials. Despite pure graphene, its composites with other species can be of interest. In this paper the results of studies on the effect of methods and parameters of synthesis, for obtaining composites graphene/Fe₂O₃ on their structural properties and electrical properties are presented. A series of experiments was conducted using a commercially available graphene (Graphene Nanopowder AO-3) and iron nitrate. The materials were obtained using two pressure methods: pressure synthesis in the autoclave and synthesis in the microwave solvothermal reactor. The syntheses were carried out in a solution of ethanol. The specific surface area, helium density, morphology, phase composition, thermal properties and electric conductivity of the obtained composites were investigated.
Features associated with short and prolonged growth time in the chemical vapor deposition process of growth of graphene stacks on SiC (0001) substrate are reported. In particular general bimodal (as far as d_{002} interlayer spacing is concerned) distribution of graphene species across the surface of the samples is observed. It consists of thin few layer graphene coverage of most of the sample surface accompanied by thick graphite-like island distribution. It points to the two independent channels of graphene stacks growth with two characteristic growth rates.
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