Theoretical study of the toroidal forms of carbon and related endohedral complexes with lithium

The atomic and electron structures of toroidal carbon molecules (C₂₄₀ and two C₁₂₀ isomers) and related endohedral complexes with lithium (Li₂@C_n and Li₄@C_n) were theoretically studied using both nonempirical (3–21G basis set) and semiempirical (MNDO) calculation schemes. For the metal-containing compounds, the behavior of lithium atoms embedded into internal cavities of the carbon framework was studied using methods of molecular dynamics. It is demonstrated that the structure of electron levels of metal-containing carbon complexes exhibits an embedded state in the forbidden band, which appears due to the presence of electrons accepted from metal atoms. The position of this embedded state and the bandgap width depend both on the initial carbon structure and on the amount of metal atoms incorporated.     

A comparison of electronic properties of various modifications of graphite

We have investigated various electronic band structures for different graphite modifications using the extended tight binding method. The specific crystal structures studied are the two naturally occurring graphite modifications (Bernal and rhombohedral), and a hypothetical configuration where the carbon atoms in consecutive layers are directly above each other. The latter structure has not been observed in pure graphite, but it is the backbone structure for different stages of intercalated graphite, e.g. Li-graphite compounds LiC_n. On comparing band structures for various graphite modifications we find important differences in the π-bands close to the Fermi energy, the region dominating transport and low energy excitation properties.     

Electronic transport properties of graphene/Al2O3 (0001) interface

The electronic structure and transport properties of a single layer of graphene (Gr) on α-Al₂O₃ surface are studied using the density functional theory (DFT). We present three models that take into account the atom at the termination of the alumina surface: a) Al atoms, with the center of the Gr hexagon directly over an Al atom; b) Al atoms, with a carbon directly positioned above an Al atom; c) oxygen atoms. Two processes of geometric optimization were used: (i) All the atoms of the supercell were allowed to move in accordance with the BFGS quasi-Newton algorithm; (ii) The atoms of the three topmost layers of the α-Al₂O₃ (0001) slab, including the C atoms, were allowed to move, whereas the atoms of the remaining layers were frozen in their respective atomic bulk positions. The first two models preserve qualitatively the electronic structure of the pristine Gr using the geometric optimization process (i) whereas, in the third model this structure was lost due to a significant charge transfer between the carbon and oxygen atoms irrespective of the optimization procedure. However, models (a) and (b) with the optimization (ii) reveal a p-type semiconducting behavior.     

Structure of the amorphous phase of pyrolisates of lanthanum diphthalocyanine according to X-ray scattering data

Data on X-ray diffraction in lanthanum diphthalocyanine pyrolysates synthesized at temperatures of 800–1800°С demonstrate the formation of an amorphous carbon phase with embedded lanthanum atoms. Low-temperature pyrolysis (800–900°С) creates layered carbon structures. Due to annealing at 1000°С, carbon integrates into globules whose number of atoms is m ~ 100. Such structures with gyration radii of R _g ~ 0.4–0.5 nm on the order of the precursor molecule size are synthesized in the temperature range of 1000–1800°С, and are stable in terms of size and mass. In this case, their density approaches that of graphite.     

Percolation mechanism of the diffusion of impurity atoms in dense surface layers

The molecular dynamics method is used to study the migration of an impurity atom on an unfilled square lattice. The calculations are performed on a lattice containing 2¹² × 2¹⁴ sites at various values of the ratio p of the frequencies of jumps impurity and lattice atoms and various relative concentrations of vacancies ? _V . In the limit of vanishingly small concentrations of vacancies, ? _V ? 1, the present simulation results are in agreement with our previous analytical results. With increasing ? _V , the diffusion coefficient of impurity atoms predicted by the simulations exceeds the result of the analytical theory, a behavior that can be explained by the growing influence of vacancy clusters, voids on the surface, in which the impurity atom can travel long distances. This is most clearly seen in the case of highly mobile impurity atoms (p ? 1), where within the characteristic time of displacement of impurity atoms, lattice atoms remain practically immobile, and the problem appears to be closely related to the percolation problem. In this case, up to ? _V ≈ 0.3, the diffusion coefficient is independent of p; then, such a dependence appears, and the diffusion coefficient increases sharply with ? _V .     

155Gd Mössbauer spectroscopy measurements on Gd2T17C x,with T=Fe,Co, Ni andx varying from 0.6 to 1.2

¹⁵⁵Gd Mössbauer measurements were performed on Gd₂T₁₇C _x with T=Fe, Co, and Ni andx varying from 0.6 to 1.2. The crystal structures of these compounds are either the Th₂Zn₁₇ or the Th₂Ni₁₇ structure type. There is only partial occupation of the site available to the carbon atoms, leading to several crystallographically non-equivalent gadolinium surroundings. These various Gd sites show different quadrupole interactions. Doping with carbon causes an increased Curie temperature for the Gd₂Fe₁₇C _x compounds. The randomness of the occupation by carbon has been studied.     

Simulations of structures of amorphous SixC1−x films

Amorphous Si_xC_1−x films possess the potential to improve wear performance in humid atmospheres and at higher temperatures. But some experimental work on the films showed that silicon contents greatly influenced their microstructures and mechanical properties. Therefore, simulations of molecular dynamics were carried out to predict structures of the Si_xC_1−x films at different silicon contents. The results show that the sp³/sp² ratio of all the films increases, but the stiffness of the films is decreasing with an increase in silicon contents. Moreover, silicon atoms are almost surrounded by carbon atoms, which is in agreement with the experiments.     

The magnetic and hyperfine properties of iron in silicon carbide

The magnetic and hyperfine properties of iron impurities in 3C- and 6H- silicon-carbide are calculated using the abinitio method of full-potential linear-augmented-plane-waves. The iron atoms are introduced at substitutional carbon, Fe _C , and silicon, Fe _Si , sites as well as at the tetrahedral interstitial sites with four nearest neighbours carbon atoms, Fe _I (C), and four nearest neighbours silicon atoms, Fe _I (Si). The effect of introducing vacancies at the neighbours of these sites is also studied. Fe atoms with complete neighbors substituted at Si or C sites are found to be nonmagnetic, while Fe atoms at interstitial sites are magnetic. Introduction of a vacancy at a neighboring site reverse the picture.     

Direct experimental study of the equilibrium diffusion of carbon atoms between the (100)Mo surface and the bulk

A direct experimental study of the diffusion of carbon atoms between the (100)Mo surface and the bulk has been carried out at process temperatures in the range 1400–2000 K, and the total balance of carbon atoms in the system has been determined. The difference in the activation energies of carbon dissolution and precipitation ΔE=E _S 1?E _{1 S} has been found under conditions of a dynamic equilibrium between both processes. This difference determines the temperature dependence of the degree of surface enrichment with carbon in reference to the bulk. The activation energy of the dissolution of carbon atoms has been determined in special experiments (E _S 1=3.9 eV), and the activation energy of the precipitation of carbon atoms E _{1 S} has been calculated (E _{1 S} =1.9 eV), which turns out to be close to the energy of carbon bulk diffusion in molybdenum.     

Ordering in the ζ-Ta<Subscript>4</Subscript>C<Subscript>3−<Emphasis Type="Italic">x</Emphasis></Subscript> carbide phase

The structure of the ζ-Ta₄C_3?x nonstoichiometric trigonal (rhombohedral) carbide, which is formed in the tantalum-carbon system, has been analyzed by neutron diffraction, x-ray diffraction, and metallography. The ordered distributions of carbon atoms and structure vacancies have been experimentally determined and the distribution function of carbon atoms over the nonmetallic-sublattice sites, where ordering occurs, has been calculated. The parameters of the unit cell of the ζ-Ta₄C_3?x (TaC_0.67) trigonal (space group \(R\overline 3 m\)) carbide are found to be a_H = 0.3123 nm and c_H = 3.0053 nm. It has been shown that the metallic close packed sublattice of the ζ-Ta₄C_3?x carbide is formed by alternating blocks, where metal atoms are located both in the fcc and hcp sublattices of the TaC_y cubic and Ta₂C hexagonal carbides, respectively.     

Chloride perovskite layer compounds of [NH3-(CH2)n-NH3]MnCl4 formula

[NH₃-(CH₂)_n-NH₃]MnCl₄ compounds with n = 2, …, 5 are chloride perovskite layer structures. The room temperature phase of members with odd numbers of carbon atoms is orthorhombic, whereas even numbers lead to monoclinic structures. Structural phase transitions were found in all compounds with n > 2. The magnetic behaviour is similar to the (C_nH_2n+1NH₃)₂MnCl₄-family.     

Transport properties of carbon atomic wire in the environment of H2O molecules: An ab initio study

The transport properties of carbon atomic wire in the environment of H₂O molecules are studied by the non-equilibrium Green function method based on density functional theory. In particular, the carbon wire with seven atoms sandwiched between the Al(1 0 0) electrodes is considered. It is found that the transport properties are sensitive to the variation of the number and the position of the H₂O molecule adsorbed on the carbon wire. To our surprise, with different positions of a single H₂O molecule on the carbon wire, the equilibrium conductance shows an evident odd–even oscillatory behavior. For example, the equilibrium conductance of the carbon wire becomes bigger when the H₂O is adsorbed on the odd-numbered carbon atoms; an opposite conclusion is obtained for the H₂O adsorbed on the even-numbered carbon atoms. For the cases of two H₂O molecules, the equilibrium conductance varies largely and the contribution of the third eigenchannel becomes larger in some special configurations. The calculated current–voltage curves show different behavior with the variation of the positions of the H₂O molecules. In certain cases, large negative differential resistance (NDR) is shown, while in other cases, it only slightly deviates from the linear behavior. The above behavior is analyzed via the charge transfer and the density of states (DOS) and reasonable explanations are presented.     

Modeling electronic,mechanical, optical and thermal properties of graphene-like BC6N materials: Role of prominent BN-bonds

We model monolayer graphene-like materials with BC₆N stoichiometry where the bonding between the B and the N atoms plays an important role for their physical and chemical properties. Two types of BC₆N are found based on the BN bonds: In the presence of BN bonds, an even number of π-bonds emerges indicating an aromatic structure and a large direct bandgap appears, while in the absence of BN bonds, an anti-aromatic structure with an odd-number of π-bonds is found resulting a direct small bandgap. The stress-strain curves shows high elastic moduli and tensile strength of the structures with BN-bonds, compared to structures without BN-bonds. Self-consistent field calculations demonstrate that BC₆N with BN-bonds is energetically more stable than structures without BN-bonds due to a strong binding energy between the B and the N atoms, while their phonon dispersion displays that BC₆N without BN-bonds has more dynamical stability. Furthermore, all the BC₆N structures considered show a large absorption of electromagnetic radiation with polarization parallel to the monolayers in the visible range. Finer detail of the absorption depend on the actual structures of the layers. A higher electronic thermal conductivity and specific heat are seen in BC₆N systems caused by hot carrier–assisted charge transport. This opens up a possible optimization for bolometric applications of graphene based material devices.     

The phase diagrams of a finite superlattice with disordered interface: A Monte Carlo study

Monte Carlo Simulation (MCS) has been used to study critical and compensation behavior of a ferrimagnetic superlattice on a simple cubic lattice. The superlattice consists of k unit cells, where the unit cell contains L layers of spin −1/2 A atoms, L layers of spin −1 B atoms and a disordered interface in between that is characterized by a random arrangement of A and B atoms of A_pB_1−p type and a negative A-B coupling. We investigate the finite and the infinite superlattices and we found that the existence and the number of the compensation points depend strongly on the thickness of the superlattice (number of unit cells).     

Diboride bifullerenes and binanotubes

The structures and energy characteristics of a new class of nanotubes and fullerenes formed by binary layers of a trigonal network of metal atoms M and a graphite-like network of boron atoms are considered by the example of magnesium and zirconium diborides. It is shown that, contrary to the familiar carbon monolayer nanotubes, the dependence of the strain energy of diboride bitubulene on its diameter D deviates from the 1/D ² law because of the “crustlike” shape of a free fragment of the MB₂ bilayer, while bitubulenes with axis parallel to the M-M bonds have the optimum shape at a given diameter. Such bitubulenes are expected to be metallic because of the specific features of the band structure of bilayered diborides.     

First-principles calculations of BC4N nanostructures: stability and electronic structure

In this work, we apply first-principles methods to investigate the stability and electronic structure of BC₄N nanostructures which were constructed from hexagonal graphite layers where substitutional nitrogen and boron atoms are placed at specific sites. These layers were rolled up to form zigzag and armchair nanotubes, with diameters varying from 7 to 12 Å, or cut and bent to form nanocones, with 60^° and 120^° disclination angles. The calculation results indicate that the most stable structures are the ones which maximize the number of B–N and C–C bonds. It is found that the zigzag nanotubes are more stable than the armchair ones, where the strain energy decreases with increasing tube diameter D, following a 1/D ² law. The results show that the 60^° disclination nanocones are the most stable ones. Additionally, the calculated electronic properties indicate a semiconducting behavior for all calculated structures, which is intermediate to the typical behaviors found for hexagonal boron nitride and graphene.     

Possibilities of C 1s XPS and N(E) C KVV Auger spectroscopy for identification of inherent peculiarities of diamond growth

The interaction of C-atoms and CH_n-radicals with uncleaned and argon cleaned silicon substrate and with diamond surface after H-treatment have been studied in situ by XPS and Auger spectroscopy. It was found the formation of a new chemical surface state of carbon atoms in the case of carbon atoms and radicals interaction with cleaned silicon. The same chemical state was revealed on the H-treated diamond surface. Graphite-like structure of carbon atoms was observed on the surface of unlearned silicon and H-treated diamond after interaction with carbon atoms and radicals. N(E) C KVV Auger spectrum for the new chemical state of carbon atoms significantly differs from typical spectra for sp²- and sp³-bonded carbon materials. The high energy part of this spectrum was interpreted under the hypothesis of sp³-bonded carbon atoms but with shifted fermi level position.     

Magnetic properties in different fullerenes Xn nano-structures: Monte Carlo study

The aim of this work is to study the magnetic properties in deferent fullerenes X_n like nano-structures, where the symbol X can be assigned to any magnetic atom and “n” is the number of these magnetic atoms. Our study is based on the fullerenes: C₂₀, C₆₀ and C₇₀, formed by atoms with mixed spins σ = 1 and S = 1/2. We focus our interest on the examination of the influence of the coupling exchange interactions between atoms, the external magnetic and the crystal fields. Also, we present and discuss the nature of the corresponding ground state phase diagrams. The effect of the compensation and critical temperatures is also presented for different magnetization profiles by using the Monte Carlo simulations. The behavior of the total magnetizations as a function of the physical system parameters is also analyzed and discussed, in this study.     

An Ab Initio Study of the Structural and Electronic Properties of the Low-Defect TiC(110) Surface Simulating Oxygen Adsorption after Exposure to Laser Plasma

An ab initio simulation of the adsorption of atomic oxygen on the low-defect titanium carbide (110) surface reconstructed by laser radiation was performed. The relaxed atomic structures of the (110) surface of the O/Ti _x C _y system with Ti and C vacancies observed during the thermal treatment were studied in terms of the density functional theory. DFT calculations of their structural, thermodynamic, and electronic properties were performed. The bond lengths and adsorption energies were determined for various reconstructions of the atomic structure of the O/Ti _x C _y (110) surface. The effects of the oxygen adatom on the band and electronic spectra of the O/Ti _x C _y (110) surface were studied. The effective charges on the titanium and carbon atoms surrounding the oxygen atom in various reconstructions were determined. The charge transfer from titanium to oxygen and carbon atoms was found, which is determined by the reconstruction of the local atomic and electronic structures and correlates with chemisorption processes. The potential mechanisms of laser nanostructuring of the titanium carbide surface were suggested.     

Substitutional impurity in the graphene quantum dots

The process of formation of the localized defect states due to substitutional impurity in sp²-bonded graphene quantum dot is considered using a simple tight-binding-type calculation. We took into account the interaction of the quantum dot atoms surrounding the substitutional impurity from the second row of elements. To saturate the external dangling sp² orbitals of the carbon additionally 18 hydrogen atoms were introduced. The chemical formula of the quantum dot is H₁₈C₅₁X, where X is the symbol of substitutional atom. The position of the localized levels is determined relative to the host-atoms (C) ε_p energies. We focused on the effect of substitutional doping by the B, N and O on the eigenstate energies and on the total energy change of the graphene dots including for O the effect of lattice distorsion. We conclude that B, N, and O can form stable substitutional defects in graphene quantum dot.