Electric field effect in bilayered graphene nanomeshes.

The effect of applying an external electric field on the electronic properties of nanoporous bilayered graphene is reported. Such an effect was demonstrated on bilayered graphene structures with various types of stacking and relative arrangements of nanopores. The direct-indirect band gap transformation combined with significant changes of electronic band structure behavior was predicted. The obtained effects are of significant importance for further engineering the optical properties of such materials and open new prospects for using nanoporous bilayered graphene in electronic and optoelectronic device applications.

Nanostructuring few-layer graphene films with swift heavy ions for electronic application: tuning of electronic and transport properties.

The morphology and electronic properties of single and few-layer graphene films nanostructured by the impact of heavy high-energy ions have been studied. It is found that ion irradiation leads to the formation of nano-sized pores, or antidots, with sizes ranging from 20 to 60 nm, in the upper one or two layers. The sizes of the pores proved to be roughly independent of the energy of the ions, whereas the areal density of the pores increased with the ion dose. With increasing ion energy (>70 MeV), a profound reduction in the concentration of structural defects (by a factor of 2-5), relatively high mobility values of charge carriers (700-1200 cm2 V-1 s-1) and a transport band gap of about 50 meV were observed in the nanostructured films. The experimental data were rationalized through atomistic simulations of ion impact onto few-layer graphene structures with a thickness matching the experimental samples. We showed that even a single Xe atom with energy in the experimental range produces a considerable amount of damage in the graphene lattice, whereas high dose ion irradiation allows one to propose a high probability of consecutive impacts of several ions onto an area already amorphized by the previous ions, which increases the average radius of the pore to match the experimental results. We also found that the formation of "welded" sheets due to interlayer covalent bonds at the edges and, hence, defect-free antidot arrays is likely at high ion energies (above 70 MeV).

The possible formation of a magnetic FeS2 phase in the two-dimensional MoS2 matrix.

The possibility of a FeS2 phase formation in the 2D MoS2 structure was investigated by an ab initio DFT approach. Various concentrations of FeS2 in MoS2 have been analyzed, and it is shown that the energy favorable structures of the Mo1-xFexS2 composition are in-plane hybrid phases, FeS2 and MoS2 domains. After increasing the Fe/Mo concentration ratio up to 0.68, a complete transformation of the whole structure is predicted. We have found that the introduction of only a small amount of Fe atoms leads to a change in the electronic and magnetic properties of the film. An increase of the FeS2 nucleus size leads to the nearly monotonous increase of the magnetic moment governed by the exponential law.

Bilayered graphene as a platform of nanostructures with folded edge holes.

The stability and the electronic properties of new AB-stacking and moiré bilayer graphene superlattices with closed edge nanoholes are studied using DFT calculations. The closing of the edges is made of C-C bonds that form after folding the borders of the holes. Superlattices with periodic hexagonal symmetry are considered in more detail. The electronic band structure of the nanomeshes has metallic to semiconductor characteristics, depending not only on the size of the holes and the distance between them, but also on the hole shape. Bilayered graphene is considered as a platform for the engineering of nanostructures with folded edge holes.

MoS2 decoration by Mo-atoms and the MoS2-Mo-graphene heterostructure: a theoretical study.

Here we propose a completely new covalent heterostructure based on graphene and self-decorated MoS2 monolayers. Detailed investigation of the decoration process of the MoS2 surface by Mo adatoms was performed using first principles DFT methods. Comparison between valence-only and semicore pseudopotentials was performed to correctly describe the interaction between Mo adatoms and the MoS2 surface. It was found that self-decoration by Mo atoms is favorable from an energetic point of view. We studied in detail various decoration paths of Mo atoms on the MoS2 surface. The strong variation of electronic properties after the decoration of MoS2 was found. The impact of the presence of Mo adatoms on the electronic properties of the graphene/MoS2 heterostructure was shown.

Transport investigation of branched graphene nanoflakes.

We present a theoretical study of current-voltage characteristics of different junctions of graphene nanoribbons. We considered isolated Y- and T-junctions of graphene nanoribbons (GNRs) with various geometry parameters and a graphene Y-junction in the graphane sheet. Our ab initio calculations based on the nonequilibrium Green's functions formalism displayed the influence of the geometry parameters of different ribbons on the I-V curves e.g. the shifting of zero voltage regions. We showed that not only the shape of the structure, but also the arrangement of electrodes attached to the structure will lead to changes in the transport properties.

Toward the Ultra-incompressible Carbon Materials. Computational Simulation and Experimental Observation.

The common opinion that diamond is the stiffest material is disproved by a number of experimental studies where the fabrication of carbon materials based on polymerized fullerenes with outstanding mechanical stiffness was reported. Here we investigated the nature of this unusual effect. We present a model constituted of compressed polymerized fullerite clusters implemented in a diamond matrix with bulk modulus B0 much higher than that of diamond. The calculated B0 value depends on the sizes of both fullerite grain and diamond environment and shows close correspondence with measured data. Additionally, we provide results of experimental study of atomic structure and mechanical properties of ultrahard carbon material supported the presented model.

Band gaps in jagged and straight graphene nanoribbons tunable by an external electric field.

Band gap control by an external field is useful in various optical, infrared and THz applications. However, widely tunable band gaps are still not practical due to a variety of reasons. Using the orthogonal tight-binding method for π-electrons, we have investigated the effect of the external electric field on a subclass of monolayer chevron-type graphene nanoribbons that can be referred to as jagged graphene nanoribbons. A classification of these ribbons was proposed and band gaps for applied fields up to the SiO2 breakdown strength (1 V nm(-1)) were calculated. According to the tight-binding model, band gap opening (or closing) takes place for some types of jagged graphene nanoribbons in the external electric field that lies on the plane of the structure and perpendicular to its longitudinal axis. Tunability of the band gap up to 0.6 eV is attainable for narrow ribbons. In the case of jagged ribbons with armchair edges larger jags forming a chevron pattern of the ribbon enhance the controllability of the band gap. For jagged ribbons with zigzag and armchair edges regions of linear and quadratic dependence of the band gap on the external electric field can be found that are useful in devices with controllable modulation of the band gap.

Sharp variations in the electronic properties of graphene deposited on the h-BN layer.

Investigation of the complex structure based on the graphene monolayer and the twisted BN monolayer was carried out. Sharp variations in the electronic structure during the hydrogen adsorption at low concentration were observed. Upon increasing the hydrogen concentration on the structure surfaces more impurity levels were observed due to the addition of the hydrogen atoms without any dependence on the position of hydrogen atoms on graphene and BN surfaces. An investigation of the dependence of the band gap on the hydrogen concentration on the Moiré surface was made. Upon increasing the hydrogen concentration the value of the band gap increased up to 0.5 eV.

Simulation of geometrical and electronic structure of quasi-two-dimensional layer consisting of fullerenes D6h-C36.

This article describes a computer simulation of the geometrical and electronic structure of a quasi-two-dimensional carbon layer with a trigonal lattice consisting of fullerenes C36 (1) with topological symmetry D6h. Every polyhedral cluster 1 of this polymeric layer (2) is surrounded by six similar fullerenes and connected with every such a fullerene by two covalent bonds. Atomic coordinates of the repeating unit are estimated on the basis of MNDO/PM3 calculations of hydrocarbon molecule (D6h)-C132H48 (3). The carbon skeleton of 3 coincides with a sufficiently large fragment of the polymeric layer 2. The electronic spectrum of the quasi-two-dimensional layer 2 is calculated by the crystalline orbital method in the EHT approximation. The band gap in the electronic spectrum of 2 was found to be equal to 1.5 eV. The geometric and electronic structure of some oligomers of cluster C36, quasi-linear macromolecule [C36]n, and "hypergraphite" layer is also discussed.

Planar optical waveguides coupled by means of Bragg scattering.

A new analytic approach to the analysis of grating-assisted couplers is proposed and used for description of the noncollinear coupling of two slab waveguides with arbitrary mode polarizations (TE and TM), propagation directions, and phase velocities. This approach is based on the boundary-perturbation theory, the method of successive approximations, and the energy-conservation law, and does not use any overlap integrals. The specific case in which the converted mode propagates parallel to a periodic array boundary (extremely asymmetric coupling) is considered by means of an original simple analytic approach that allows for the diffractional divergence of the converted wave. Applicability conditions of the results obtained are derived in both the cases of conventional and extremely asymmetrical coupling. Comparison with the previous methods is carried out for the collinear coupling.