Bilayer C60 Polymer/h-BN Heterostructures: A DFT Study of Electronic and Optic Properties.

Interest in fullerene-based polymer structures has renewed due to the development of synthesis technologies using thin C60 polymers. Fullerene networks are good semiconductors. In this paper, heterostructure complexes composed of C60 polymer networks on atomically thin dielectric substrates are modeled. Small tensile and compressive deformations make it possible to ensure appropriate placement of monolayer boron nitride with fullerene networks. The choice of a piezoelectric boron nitride substrate was dictated by interest in their applicability in mechanoelectric, photoelectronic, and electro-optical devices with the ability to control their properties. The results we obtained show that C60 polymer/h-BN heterostructures are stable compounds. The van der Waals interaction that arises between them affects their electronic and optical properties.

Diamane-like Films Based on Twisted G/BN Bilayers: DFT Modelling of Atomic Structures and Electronic Properties.

Diamanes are unique 2D carbon materials that can be obtained by the adsorption of light atoms or molecular groups onto the surfaces of bilayer graphene. Modification of the parent bilayers, such as through twisting of the layers and the substitution of one of the layers with BN, leads to drastic changes in the structure and properties of diamane-like materials. Here, we present the results of the DFT modelling of new stable diamane-like films based on twisted Moiré G/BN bilayers. The set of angles at which this structure becomes commensurate was found. We used two commensurate structures with twisted angles of θ = 10.9° and θ = 25.3° with the smallest period as the base for the formation of the diamane-like material. Previous theoretical investigations did not take into account the incommensurability of graphene and boron nitride monolayers when considering diamane-like films. The double-sided hydrogenation or fluorination of Moiré G/BN bilayers and the following interlayer covalent bonding led to the opening of a gap up to 3.1 eV, which was lower than the corresponding values of h-BN and c-BN. The considered G/BN diamane-like films offer great potential in the future for a variety of engineering applications.

Ultra-Low Thermal Conductivity of Moiré Diamanes.

Ultra-thin diamond membranes, diamanes, are one of the most intriguing quasi-2D films, combining unique mechanical, electronic and optical properties. At present, diamanes have been obtained from bi- or few-layer graphene in AA- and AB-stacking by full hydrogenation or fluorination. Here, we study the thermal conductivity of diamanes obtained from bi-layer graphene with twist angle θ between layers forming a Moiré pattern. The combination of DFT calculations and machine learning interatomic potentials makes it possible to perform calculations of the lattice thermal conductivity of such diamanes with twist angles θ of 13.2∘, 21.8∘ and 27.8∘ using the solution of the phonon Boltzmann transport equation. Obtained results show that Moiré diamanes exhibit a wide variety of thermal properties depending on the twist angle, namely a sharp decrease in thermal conductivity from high for "untwisted" diamanes to ultra-low values when the twist angle tends to 30∘, especially for hydrogenated Moiré diamanes. This effect is associated with high anharmonicity and scattering of phonons related to a strong symmetry breaking of the atomic structure of Moiré diamanes compared with untwisted ones.

Moiré Diamones: New Diamond-like Films of Semifunctionalized Twisted Graphene Layers.

We proposed novel carbon nanostructures based on a twisted few-layered graphene with one side passivated by hydrogen or fluorine: Moiré diamones on graphene. The presence of a dangling bond at the bottom layer of diamones leads to the appearance of spin density localization, which can be tuned by the variation of the twist angle with the following formation of Moiré diamones. The spin-polarized nature of electronic density distribution was obtained and discussed in detail on the basis of ab initio calculations. Such a feature makes Moiré diamones a promising key element in the field of controllable spintronic devices.

Mechanical Engineering Effect in Electronic and Optical Properties of Graphene Nanomeshes.

Here, we present an ab initio study of ways for engineering electronic and optical properties of bilayered graphene nanomeshes with various stacking types via mechanical deformations. Strong evolution of the electronic structure and absorption spectra during deformation is studied and analyzed. The obtained results are of significant importance and open up new prospects for using such nanomeshes as materials with easily controlled properties in electronic and optoelectronic nanodevices.

On the Edge of Bilayered Graphene: Unexpected Atomic Geometry and Specific Electronic Properties.

The tendency of bilayered graphene edges to connect with each other allows to create hollow sp2-hybridized material with specific electronic properties. However, the unknown geometry of the formed edges hinders further investigation. Here we show that a closed bigraphene edge can be represented as a connection of generally misoriented graphene domains with topological defects and can be further described by grain boundary theory. The energy dependence of closed edges of commensurate twisted bilayered graphene is derived for any twist angle and edge orientation. Our findings allow to predict what particular edge types appear in the bilayered graphene holes and explain the structure of the connected bilayered graphene edges, which are often observed in the experiment.

Fluorinated graphene nanoparticles with 1-3 nm electrically active graphene quantum dots.

A new approach to creating a new and locally nanostructured graphene-based material is reported. We studied the electric and structural properties of partially fluorinated graphene (FG) films obtained from an FG-suspension and nanostructured by high-energy Xe ions. Local shock heating in ion tracks is suggested to be the main force driving the changes. It was found that ion irradiation leads to the formation of locally thermally expanded FG and its cracking into nanoparticles with small (∼1.5-3 nm) graphene quantum dots (GQD), embedded in them. The bandgap of GQD was estimated as 1 -1.5 eV. A further developed approach was applied to correct the functional properties of printed FG-based crossbar memristors. Dielectric FG films with small quantum dots may offer prospects in graphene-based electronics due to their stability and promising properties.

One-atom-thick 2D copper oxide clusters on graphene.

The successful isolation and remarkable properties of graphene have recently triggered investigation of two-dimensional (2D) materials from layered compounds; however, one-atom-thick 2D materials without bulk layered counterparts are scarcely reported. Here we report the structure and properties of novel 2D copper oxide studied by experimental and theoretical methods. Electron microscopy observations reveal that copper oxide can form monoatomic layers with an unusual square lattice on graphene. Density functional theory calculations suggest that oxygen atoms at the centre of the square lattice stabilizes the 2D Cu structure, and that the 2D copper oxide sheets have unusual electronic and magnetic properties different from 3D bulk copper oxide.

Bilayered graphene/h-BN with folded holes as new nanoelectronic materials: modeling of structures and electronic properties.

The latest achievements in 2-dimensional (2D) material research have shown the perspective use of sandwich structures in nanodevices. We demonstrate the following generation of bilayer materials for electronics and optoelectronics. The atomic structures, the stability and electronic properties of Moiré graphene (G)/h-BN bilayers with folded nanoholes have been investigated theoretically by ab-initio DFT method. These perforated bilayers with folded hole edges may present electronic properties different from the properties of both graphene and monolayer nanomesh structures. The closing of the edges is realized by C-B(N) bonds that form after folding the borders of the holes. Stable ≪round≫ and ≪triangle≫ holes organization are studied and compared with similar hole forms in single layer graphene. The electronic band structures of the considered G/BN nanomeshes reveal semiconducting or metallic characteristics depending on the sizes and edge terminations of the created holes. This investigation of the new types of G/BN nanostructures with folded edges might provide a directional guide for the future of this emerging area.

Heterostructures based on graphene and MoS2 layers decorated by C60 fullerenes.

Here we present a comprehensive investigation of various novel composite structures based on graphene (G) and molybdenum disulphide (MoS2) monolayers decorated by C60 fullerenes, which can be successfully applied in photovoltaics as a solar cell unit. Theoretical studies of the atomic structure, stability and electronic properties of the proposed G/C60, MoS2/C60 and G/MoS2/C60/G nanostructures were carried out. We show that making the G/MoS2/C60/G heterostructure from the 2D films considered here will lead to the appearance of particular properties suitable for application in photovoltaics due to the broad energetic region of high electronic density of states.

Phase diagram of quasi-two-dimensional carbon, from graphene to diamond.

We explore how a few-layer graphene can undergo phase transformation into thin diamond film under reduced or no pressure, if the process is facilitated by hydrogenation of the surfaces. Such a "chemically induced phase transition" is inherently nanoscale phenomenon, when the surface conditions directly affect thermodynamics, and the transition pressure depends greatly on film thickness. For the first time we obtain, by ab initio computations of the Gibbs free energy, a phase diagram (P, T, h) of quasi-two-dimensional carbon-diamond film versus multilayered graphene. It describes accurately the role of film thickness h and shows the feasibility of creating novel quasi-two-dimensional materials. Further, the role of finite diameter of graphene flakes and possible formation of the diamond films with the (110) surface are described as well.

Theoretical study of elastic properties of SiC nanowires of different shapes.

The atomic structure and elastic properties of silicon carbide nanowires of different shapes and effective sizes were studied using density functional theory and classical molecular mechanics. Upon surface relaxation, surface reconstruction led to the splitting of the wire geometry, forming both hexagonal (surface) and cubic phases (bulk). The behavior of the pristine SiC wires under compression and stretching was studied and Young's moduli were obtained. For Y-shaped SiC nanowires the effective Young's moduli and behavior in inelastic regime were elucidated.

Theoretical study of atomic structure and elastic properties of branched silicon nanowires.

The atomic structure and elastic properties of Y-shaped silicon nanowires of "fork"- and "bough"-types were theoretically studied, and effective Young moduli were calculated using Tersoff interatomic potential. The oscillation of fork Y-type branched nanowires with various branch lengths and diameters was studied. In the final stages of the bending, the formation of new bonds between different parts of the wires was observed. It was found that the stiffness of the nanowires is comparable with the stiffness of Y-shaped carbon nanotubes.

Structure and layer interaction in carbon monofluoride and graphane: a comparative computational study.

Carbon monofluoride (CF)(n) and graphane are two very different materials from the practical point of view, but the basic chemical motifs of these materials are closely related: both can be described as two-dimensional polycyclic (fluoro-/hydro-)carbons. However, the actual experimental data on the structure of these materials is ambiguous ((CF)(n)) or scarce (graphane). Herein, we report a detailed computational study of structure of (CF)(n) and graphane, both in a monolayer configuration and in three-dimensional stacked arrangements. A crucial point in achieving a proper description of layer interactions is the use of a nonlocal density functional to describe long-range dispersion attraction from first principles. We find strong qualitative and quantitative similarities between the two materials in both conformational energetics (including a "gauche-chair" conformational motif not considered in previous studies) and layer stacking arrangements. A molecular mechanics force field is derived for (CF)(n) that performs exceptionally well at reproducing our quantum chemical results and fits into a very general OPLS/AA molecular mechanics framework. The combined results of quantum chemical calculations and classical molecular dynamics simulations using the new force field suggest a pathway to explain the too-small experimental in-plane lattice constant values observed in these materials, as well as the variation of interlayer distance in (CF)(n), on the common basis of conformational disorder.

Atypical quantum confinement effect in silicon nanowires.

The quantum confinement effect (QCE) of linear junctions of silicon icosahedral quantum dots (IQD) and pentagonal nanowires (PNW) was studied using DFT and semiempirical AM1 methods. The formation of complex IQD/PNW structures leads to the localization of the HOMO and LUMO on different parts of the system and to a pronounced blue shift of the band gap; the typical QCE with a monotonic decrease of the band gap upon the system size breaks down. A simple one-electron one-dimensional Schrodinger equation model is proposed for the description and explanation of the unconventional quantum confinement behavior of silicon IQD/PNW systems. On the basis of the theoretical models, the experimentally discovered deviations from the typical QCE for nanocrystalline silicon are explained.

Carbon nanotube "T Junctions": formation pathways and conductivity.

Using tight-binding molecular dynamics we simulate the formation of single wall carbon nanotube T junctions via the fusing of two nanotubes. We propose energetically efficient pathways for this process in which all atoms maintain their sp(2) arrangements throughout. Recent experimental advances have greatly increased the plausibility of synthesizing T junctions as proposed in the simulations. We further report I-V characteristics of the formed junctions.