Elena F. Sheka

Shungite as the natural pantry of nanoscale reduced graphene oxide

Shungite is presented as a natural carbon allotrope of a multilevel fractal structure that is formed by a successive aggregation of ~1 nm reduced graphene oxide nanosheets. Turbostratic stacks of the sheets of ~1.5 nm in thickness and globular composition of the stacks of ~6 nm in size determine the secondary and tertiary levels of the structure. Aggregates of globules of tens of nanometers complete the structure. Molecular theory of graphene oxide, supported by large experience gained by the modern graphene science, has led to the foundation of the suggested presentation. The microscopic view has found a definite confirmation when analyzing the available empirical appearance of shungite. To our knowledge, this is the first time a geological process is described at quantum level.

Stepwise computational synthesis of fullerene C60 derivatives. Fluorinated fullerenes C60F2k

Reactions of fullerene C60 with atomic fluorine are studied by the unrestricted broken spin symmetry Hartree-Fock (UBS HF) approach implemented in semiempirical codes based on the AMI technique. The calculations are focused on a successive addition of a fluorine atom to the fullerene cage following the indication of the highest chemical susceptibility of the cage atom, which is calculated at each step. The proposed computational synthesis is based on the effectively unpaired-electron concept of the chemical susceptibility of fullerene atoms. The obtained results are analyzed from the standpoints of energy, symmetry, and the composition abundance. A good fitting of the data to experimental findings proves a creative role of the suggested synthetic methodology.

Fractals of graphene quantum dots in photoluminescence of shungite

Viewing shungite as loosely packed fractal nets of graphene-based (reduced graphene oxide, rGO) quantum dots (GQDs), we consider photoluminescence of the latter as a convincing proof of the structural concept as well as of the GQD attribution to individual rGO fragments. We study emission from shungite GQDs for colloidal dispersions in water, carbon tetrachloride, and toluene at both room and low temperatures. As expected, the photoluminescence of the GQD aqueous dispersions is quite similar to that of synthetic GQDs of the rGO origin. The morphological study of shungite dispersions shows a steady trend of GQDs to form fractals and to drastically change the colloid fractal structure caused by the solvent exchange. Spectral study reveals a dual character of the emitting centers: individual GQDs are responsible for the spectra position while the fractal structure of GQD colloids ensures high broadening of the spectra due to structural inhomogeneity, thus causing a peculiar dependence of the photoluminescence spectra on the excitation wavelength. For the first time, photoluminescence spectra of individual GQDs were observed in frozen toluene dispersions, which paves the way for a theoretical treatment of the GQD photonics.

Fullerenes as polyradicals

We present the investigation of the electronic structure of X60 molecules (X=C, Si), containing 60 odd electrons with spin-dependent interaction between them. Conditions for the electrons to be excluded from the covalent pairing are discussed. A computational spin-polarized quantum-chemical scheme is suggested to evaluate four parameters—energy of radicalization, exchange integral, atom spin density, and squared spin— to characterize the effect quantitatively. A polyradical character of the species, weak for C60 and strong for Si60, is established.

A tricotage-like failure of nanographene

The response of a nanographene sheet to external stresses was considered in terms of a mechanochemical reaction. The quantum chemical realization of the approach was based on the 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 was observed when the MIC was oriented either along or normally to the C–C bond chain. Both the anisotropy and the high stiffness of the nanographene originate from the response of the benzenoid unit to stress.

Odd-electron molecular theory of graphene hydrogenation

This paper highlights the molecular essence of graphene and presents its hydrogenation from the viewpoint of the odd-electron molecular theory. This chemical transformation was performed computationally, using a particular algorithm, through the stepwise addition of either hydrogen molecules or hydrogen atoms to a pristine graphene molecule. The graphene was considered to be a membrane, such that either both sides or just one side of the membrane was accessible to adsorbate, and the atoms on the perimeter of the membrane were either fixed (fixed membrane) or free to move (free-standing membrane). The algorithm explored the spatial distribution of the number of effectively unpaired electrons NDA over the carbon skeleton of the molecule. The highest ranked NDA values were considered to indicate the target atoms at each reaction step. The dependence of the hydrogenation itself and the final graphene hydrides on external factors such as whether the membrane was fixed, if both sides or only one side of the membrane were accessible to hydrogen, and whether the hydrogen was in the molecular or atomic state. Complete hydrogenation followed by the formation of a regular chairlike graphane structure (CH)n was only found to be possible for a fixed pristine graphene membrane for which the basal plane is accessible to hydrogen atoms from both sides.

Structure-sensitive mechanism of nanographene failure

The quantum-mechanochemical-reaction-coordinate approach has disclosed atomically matched peculiarities that accompany the deformation-failure-rupture process occurring in nanographenes. The high stiffness of the graphene body is provided by the benzenoid unit. The anisotropy of the unit mechanical behavior in combination with different configurations of the unit packing with respect to the body C-C bond chains forms the ground for the structure-sensitive mechanism of the mechanical behavior that is drastically different for two different deformation modes. The zig-zag deformation mode is particularly manifested with the formation of one-atom chains. The approach allows tracing a deformation-stimulated change in the chemical reactivity of both the nanographene body and its individual atoms.

Computational synthesis of hydrogenated fullerenes from C60 to C60H60

Hydrogenation from C60 to C60H60 was studied by an unrestricted broken spin symmetry Hartree–Fock approach implemented in semiempirical codes based on the AM1 technique. The calculations focused on the successive addition of hydrogen molecules to the fullerene cage following the identification of the cage target atoms by calculating the highest atomic chemical susceptibility at each step. The results obtained are analyzed from energy, symmetry, and composition perspectives.

Neutron scattering study of reduced graphene oxide of natural origin

This paper presents a direct confirmation of graphene-like configuration and first suggests the chemical composition of basic structural elements of shungite attributing the latter to reduced graphene oxide nanosheets with an average 11:1:3 (C:O:H) atomic content ratio.

The nature of enhanced linear and nonlinear optical effects in fullerene solutions

The “blue” emission from fullerene C60 and its derivatives in frozen toluene solution is discovered and analyzed in the framework of the electromagnetic theory of enhanced optical effects. It is shown that the emission, combining enhanced spectra of Raman scattering and one-photon luminescence, is due to clustering of fullerene molecules in solution. Photoexcitation of charge-transfer excitons in clusters provides the polarization required for the enhancement. A direct relationship is established between the observed phenomenon and nonlinear optical properties of the medium. Empirical and computational tests are proposed to select matrices with various nonlinear optical properties.