Molecular and electronic structure of several heterofullerene BNC58 and B2N2C56 oligomers and [B2N2C56] n macromolecule

Molecular and electronic structure of heterofullerene BNC₅₈ (C _s) and B₂N₂C₅₆ (C _2h) monomers, B₂N₂C₁₁₆ and B₄N₄C₁₁₂ dimers, and B₆N₆C₁₆₈ trimer (the last three molecules withC _2h symmetry) was simulated by the MNDO method. Clusters BNC₅₈ and B₂N₂C₅₆ are formed by replacement of carbon atoms participating in one or two of the most distant oppositely lying (6,6)-type C−C bonds in fullerene C₆₀ by B and N atoms. In one of the two studied isomers of the B₂N₂C₁₁₆ dimer, the monomers are linked by the four-membered carbon cycle, while the heteroatoms form the most distant oppositely lying bonds of the dimer. In the other isomer of the B₂N₂C₁₁₆ dimer, as well as in the B₄N₄C₁₁₂ dimer and B₆N₆C₁₆₈ trimer, the monomers are linked by four-membered B₂N₂ cycles with alternation of the atoms. For all the systems studied, the optimum geometric parameters, heats of formation, ionization potentials, and atomic charges were calculated. Dimerization energies of heterofullerenes BNC₅₈ and B₂N₂C₅₆ lie in the range from 33 to 49 kcal mol⁻¹. It was found that the B₂N₂C₁₁₆ dimer, in which the monomers are linked by the four-mernbered carbon cycle, is the most stable system. In the case of B₂N₂C₅₆ trimerization, the energy gain (compared to the triple monomer energy) is about twice as large as the dimerization energy. Molecular structure of the quasi-linear [B₂N₂C₅₆) _n macromolecule was simulated, and extended Hückel calculations of its energy band structure by the crystal orbital method were performed. It was found that the electron energy spectrum is of semiconducting type (the band gap is equal to 1.27 eV. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 431–435, March, 1999.     

Bucky-corn: van der Waals composite of carbon nanotube coated by fullerenes

ABSTRACT

Can a C₆₀ layer cover a surface of single-wall carbon nanotube (SWCNT) forming an exohedral pure-carbon hybrid with only van der Waals interactions? The aim of the present paper is to address this question and to demonstrate that the fullerene shell layer in such a bucky-corn structure can be stable. Theoretical study of the structure, stability and electronic properties of bucky-corn hybrids is reported for the shell of C₆₀ and C₇₀ molecules on an individual SWCNT, C₆₀ dimers on an individual SWCNT as well as C₆₀ molecules on SWNT bundles. The geometry and total energies of the bucky-corn hybrids were calculated by the molecular dynamics method, while the density functional theory method was used to simulate the electronic band structures.     

Magnetism in quantum dots on graphene-graphane nanoribbons

The magnetic properties of quantum dots on a graphene nanoribbon are investigated in the context of the method of the generalized gradient approximation of spin-polarized density functional theory. It is shown that the antiferromagnetic ordering of magnetic moments is most stable for ribbons with periodic alternation of graphene-graphane nanosegments. When the graphene nanoribbon is separated by the rows of C atoms with hydrogen atoms adsorbed on them in the form of a two-periodic superlattice, the ferrimagnetic ordering in the coupled unequal quantum dots turns out to be most favorable.     

Mechanism of carbon onion transformation into diamond-like structure

The formation of diamond-like structure in the carbon onion core was modeled by the molecular-dynamics method. The stages of this transformation (formation of holes in the fullerene shells and their healing accompanied by a decrease in the interlayer spacing in onion; accumulation of “free” atoms in the core and formation of new shells by these atoms; and core transformation under the action of these modified shells and temperature) adequately describe the main features of the Banhart and Ajayan observations.     

Characteristic features of the sorption of light atoms on the surface of a single-layer carbon tubelene

The mechanisms of sorption of H, O, C, and Cl atoms on the surface of a single-layer carbon tubelene are studied, and a comparison is made with the case of sorption of these atoms on graphite. Three versions of the position of the adatoms above the surface were studied. A cyclic-cluster model and an appropriately modified MNDO computational scheme are used. The optimal geometry of the sorption complexes and the sorption energies are obtained. The high hydrogen accumulation efficiency in a material consisting of single-layer carbon nanotubes is explained. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 12, 799–804 (25 December 1997)     

Polymers from C60 barrelenes

Dimer, trimer, and chain structures consisting of covalently joined barrel-shaped C₆₀ monomers (barrelenes) with different types of bonds are modeled. The computed distances between the centers of the molecules within a barrelene chain (0.70–0.74 nm) and between the symmetry axes of the chains (0.63–0.77 nm) are comparable to the analogous distances on the surface of fullerites synthesized by Davydov’s group [V. A. Davydov, L. S. Koshevarova, A. V. Rakhmanina et al., JETP Lett. 63, 818 (1996)] and subjected to high pressures (∼10 GPa) and heating (∼700 K). The formation of special two-dimensional structures (triangles, squares, chains with a kink, stars), which are observed experimentally, is explained on the basis of the model, and 3D polymer structures consisting of C₆₀ barrelenes with density close to that of samples obtained earlier are constructed. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 9, 678–683 (10 May 1998)     

Metal-semiconductor (semimetal) superlattices on a graphite sheet with vacancies

It has been found that periodically closely spaced vacancies on a graphite sheet cause a significant rearrangement of its electronic spectrum: metallic waveguides with a high density of states near the Fermi level are formed along the vacancy lines. In the direction perpendicular to these lines, the spectrum exhibits a semimetal or semiconductor character with a gap where a vacancy miniband is degenerated into impurity levels.     

Tamm states of carbon nanotubes

The fine structure of the π-electron spectrum of semi-infinite carbotubulenes (cylindrical graphene tubes) is investigated. Together with closed short-diameter tubelenes, open nanotubes whose terminal fragments contain heteroatoms are studied. Algebraic equations are obtained for the one-electron energy levels corresponding to wave functions localized on a terminal fragment. It is shown that Tamm energy levels, lying in either the band gaps or the continuous spectrum (resonance states), can exist in the systems studied. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 8, 588–593 (25 April 1996)     

Diamond-like C2H nanolayer, diamane: Simulation of the structure and properties

We consider a new C₂H nanostructure based on bilayer graphene transformed under the covalent bond of hydrogen atoms adsorbed on its external surface, as well as compounds of carbon atoms located opposite each other in neighboring layers. They constitute a “film” of the 〈111〉 diamond with a thickness of less than 1 nm, which is called diamane. The energy characteristics and electron spectra of diamane, graphene, and diamond are calculated using the density functional theory and are compared with each other. The effective Young’s moduli and destruction thresholds of diamane and graphene membranes are determined by the molecular dynamics method. It is shown that C₂H diamane is more stable than CH graphane, its dielectric “gap” is narrower than the band gap of bulk diamond (by 0.8 eV) and graphane (by 0.3 eV), and is harder and more brittle than the latter.