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Elena F. Sheka

Publications and source records attributed to Elena F. Sheka.

29 records · Page 2Linked to original sources
arXiv2014

Shungite in view of neutron scattering

Recently suggested new concept of shungite (Int. J. Smart Nano Mat. DOI: 10.1080/19475411.2014.885913) exhibits this carboneous raw material as a multi-level fractal structure of nanosize fragments of reduced graphene oxide (rGO). In view of the extraordinary importance of the rGO starting material for the current molecular graphene technology, the natural rGO deposits turns out to be quite challenging, making it highly necessary to prove the reliability of the proposed rGO concept of shungite. Once consistent with all the block of the available geological and physical-chemical data obtained during the last few decades, the concept nonetheless needs a direct confirmation in terms of the current graphene science. The first such acknowledgement has been received just recently when studying photoluminescence (PL) of shungite dispersions (arXiv:1308.2569v2 [cond-mat.mes-hall]). A close similarity of PL spectra of aqueous dispersion of shungite and those of synthetic graphene quantum dots of the rGO origin has been established. The current paper presents the next direct confirmation supplied with neutron scattering. Elastic neutron diffraction and inelastic neutron scattering have left no doubts concerning both graphene-like configuration and chemical composition of basic structural elements of shungite attributing the latter to rGO nanosize sheets with an average 11:1:3 (C:O:H) atomic content ratio. The experimental data are supplemented with quantum-chemical calculations that allowed suggesting a clear vision of the shungite structure at its first nanolevels.

cond-mat.mtrl-sci↗
arXiv2013

The uniqueness of physical and chemical natures of graphene: their coherence and conflicts

Molecular-crystalline duality of graphene ensures a tight alliance of its physical and chemical natures, each of which is unique in its own way. The paper examines the physical-chemical harmony and/or confrontation in terms of the molecular theory of graphene. Chemistry that is consistent with graphene physics expectations involves: small mass of carbon atoms, which provides a lightweight material; sp2 configuration of the atoms valence electrons, ensuring a flat 2D structure of condensed benzenoid units; high strength of C-C valence bonds responsible for exclusive mechanical strength. Chemistry that is in conflict with graphene physics expectations covers: radical character of graphene material; collective character of electronic system of graphene, preventing from localization of its response on any external impact; the molecular nature and topochemical character of graphene mechanics; the molecular nature of graphene magnetism. Each of the properties is a direct consequence of the odd electron correlation, on one hand, and ensures a high instability of graphene material, on the other. Electron correlation inhibition by deposition of graphene monolayers on substrates is seen as a promising way to solve the problem.

cond-mat.mes-hall↗
arXiv2013

Shungite as loosely packed fractal nets of graphene-based quantum dots

The current paper presents the first attempt to recreate the origin of shungite carbon at microscopic level basing on knowledge accumulated by graphene science. The main idea of our approach is that different efficacy of chemical transformation of primarily generated graphene flakes (aromatic lamellae in geological literature), subjected to oxidation/reduction reactions in aqueous environment, lays the foundation of the difference in graphite and shungite derivation under natural conditions. Low-efficient reactions in the case of graphite do not prevent from the formation of large graphite layers while high-efficient oxidation/reduction transforms the initial graphene flakes into those of reduced graphene oxide of ~1nm in size. Multistage aggregation of these basic structural units, attributed to graphene quantum dots of shungite, leads to the fractal structure of shungite carbon thus exhibiting it as a new allotrope of natural carbon. The suggested microscopic view has finds a reliable confirmation when analyzing the available empirical data.

cond-mat.mtrl-sci↗
arXiv2012

Molecular theory of graphene oxide

Applying to graphene oxides, molecular theory of graphene is based on the oxide molecular origin when it is considered as a final product in the succession of a graphene molecule polyderivatives related to a particular oxidation reaction. The graphene oxide structure is created in due course of calculations following the algorithms that take into account the graphene molecules natural radicalization, correlation of odd electrons, an extremely strong influence of structure on properties, a sharp response of the molecule behavior on small action of external factors. Taking together, the theory facilities has allowed for getting a clear, transparent and understandable explanation of hot points of the graphene oxide chemistry and suggesting reliable models of both chemically produced and chemically reduced graphene oxides.

cond-mat.mtrl-sci↗
arXiv2010

Structure-Sensitive Mechanism of Nanographene Failure

The response of a nanographene sheet to external stresses is considered in terms of a mechanochemical reaction. The quantum chemical realization of the approach is based on a coordinate-of-reaction concept for the purpose of introducing a mechanochemical internal coordinate (MIC) that specifies a deformational mode. The related force of response is calculated as the energy gradient along the MIC, while the atomic configuration is optimized over all of the other coordinates under the MIC constant-pitch elongation. The approach is applied to the benzene molecule and (5, 5) nanographene. A drastic anisotropy in the microscopic behavior of both objects under elongation along a MIC has been observed when the MIC is oriented either along or normally to the C-C bonds chain. Both the anisotropy and high stiffness of the nanographene originate at the response of the benzenoid unit to stress.

cond-mat.mes-hall↗
arXiv2009

Stepwise Computational Synthesis of Fullerene C60 derivatives. 2.Hydrogenated Fullerenes from C60 to C60H60

Hydrogenation from C60 to C60H60 has been studied by unrestricted broken spin symmetry Hartree-Fock approach implemented in semiempirical codes based on AM1 technique. The calculations were focused on sequential addition of hydrogen molecules to the fullerene cage following the indication of the cage target atoms by the highest atomic chemical susceptibility calculated at each step. The obtained results are analyzed from energetic, symmetry, and the composition abundance viewpoints.

cond-mat.mes-hall↗
arXiv2009

Step-Wise Computational Synthesis of Fullerene C60 derivatives. 1.Fluorinated Fullerenes C60F2k

The reactions of fullerene C60 with atomic fluorine have been studied by unrestricted broken spin-symmetry Hartree-Fock (UBS HF) approach implemented in semiempirical codes based on AM1 technique. The calculations were focused on a sequential addition of fluorine atom to the fullerene cage following indication of the cage atom highest chemical susceptibility that is calculated at each step. The effectively-non-paired-electron concept of the fullerene atoms chemical susceptibility lays the foundation of the suggested computational synthesis. The obtained results are analyzed from energetic, symmetry, and the composition abundance viewpoints. A good fitting of the data to experimental findings proves a creative role of the suggested synthesis methodology.

cond-mat.mes-hall↗
arXiv2009

Graphene-carbon nanotube composites

The formation of graphen-nanotube composites addresses a few basic problems. First, both partners are good donors and acceptors of electrons, which significantly complicates the intermolecular interaction between them leading to a two-well shape of the ground state energy term. The second problem concerns odd-electron character of the components. Similarly to high aromatics and fullerenes, much larger C-C distances provide a considerable weakening of odd electrons interaction in nanotubes and graphene that necessitates taking the configurational interaction of odd electrons into account. Avoiding a severe complication, the broken spin-symmetry approach makes the problem feasible. Moreover, unrestricted broken-symmetry Hartree-Fock approach possesses a unique sensitivity in revealing enhanced chemical activity of the species caused by their partial radicalization in terms of atomic chemical susceptibility. The chemical susceptibility profiles along the tube and across their body as well as over graphene sheets form the ground of computational synthesis of graphen-nanotube composites in due course of the relevant addition reactions and make it possible to select two main groups of the composites, conditionally called hammer and cutting-blade structures. The final product will depend on whether both components of the composition are freely accessible or one of them is fixed. Thus, in diluted solutions where the first requirement is met, one can expect the formation of the multi-addend cutting-blade composites. Oppositely, when either nanotubes or graphene sheets are fixed on some substrates, the hammer composites will be formed. A particular "cradle" composite is suggested for an individual graphene sheet to be fixed by a pair of nanotubes.

cond-mat.mtrl-sci↗
arXiv2009

Broken Symmetry Approach and Chemical Susceptibility of Carbon Nanotubes

Constituting a part of odd electrons that are excluded from the covalent bonding, effectively unpaired electrons are posed by the singlet instability of the single-determinant broken spin-symmetry unrestricted Hartree-Fock (UBS HF) SCF solution. A correct determination of the total number of effectively unpaired electrons ND and its fraction on each atom NDA is well provided by the UBS HF solution. The NDA value is offered to be a quantifier of atomic chemical susceptibility (or equivalently, reactivity) thus highlighting targets that are the most favorable for addition reactions of any type. The approach is illustrated for two families involving fragments of arm-chair (n,n) and zigzag (m,0) single-walled nanotubes differing by length and end structure. Short and long tubes as well as tubes with capped end and open end, in the latter case, both hydrogen terminated and empty, are considered. Algorithms of the quantitative description of any length tubes are suggested.

cond-mat.mtrl-sci↗
arXiv2007

Electronic Structure of Single-Walled Carbon Nanotubes Governed by Odd-Electrons Interaction

For the first time, an approach is suggested for the quantitative description of the electronic structure of single-walled carbon nanotubes and the prediction of active sites for the tube controlled functionalization in view of the tube length and ends structure. The approach is based on the tube characteristic feature connected with odd electrons and is illustrated for a family involving fragments of arm-chair configured (4,4) single-walled nanotubes differing by length and end structure. Short and long tubes as well as tubes with cap end and open end, in the latter case, both hydrogen terminated and empty, are considered. Algorithms of the quantitative description of the tubes of any length are suggested. Calculations were performed in the framework of the single-determinant unrestricted Hartree-Fock approach.

cond-mat.mtrl-sci↗