Effect of purification on the electron structure and field emission characteristics of a carbonaceous material containing single-wall carbon nanotubes

A carbonaceous material containing single-wall carbon nanotubes (SWNTs) has been synthesized by arc-discharge evaporation of graphite with a catalytic additive of nickel and cobalt powders. The synthesized SWNTs were purified from an amorphous carbon component (soot) and the catalyst particles by boiling in nitric acid. A comparison of the X-ray fluorescence spectra measured before and after this treatment showed that acid etching significantly decreased the content of soot in the material. The material enriched with SWNTs is characterized by a reduced threshold for the appearance of the field emission current, which is explained by a decrease in the screening effect of soot. The current-voltage characteristics of SWNTs exhibit a hysteresis, which is suggested to be due to the adsorption of molecules and radicals on the surface and at the ends of carbon nanotubes.     

Synthesis and investigation of the electronic structure of molybdenum tetrasulfide and its lithium intercalates

An X-ray amorphous phase of molybdenum tetrasulfide with the analytical formula MoS₄ has been sythesized. Quantum chemical modeling of the suggested local structure of MoS₄ and EHT calculation of the electronic structure of the basic (Mo₂S₄)⁴⁺ fragment are reported. The electronic structure of molybdenum tetrasulfide and its lithium intercalates was investigated by X-ray emission and X-ray photoelectron spectroscopy. It is shown that the change in the electronic structure of the starting molybdenum tetrasulfide to four lithium atoms per formula unit in the intercalate may be considered in the rigid band model. Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 4, pp. 727–734, July–August, 1996. Translated by L. Smolina     

Investigation of electronic interactions in solid hydrogen fluoride

Intermolecular interactions in solid hydrogen fluoride are studied by the combined quantum chemical and X- ray diffraction method. The structure of crystalline HF is modeled by (HF)_n chains (n =2, 3,...,20, by an (HF)₄₅ cluster consisting of five (HF)₉ chains, and by an (HF)₁₀₈ cluster consisting of twelve (HF)₉ chains with nearly zero dipole moment. The quantum chemical calculations of the clusters are performed by the semiempirical AM1 method, which is most suitable for electronic structure investigations of hydrogen fluoride, as shown by comparing the X- ray experimental and theoretical spectra. The theoretical X- ray spectra are also compared with the experimental FK_α spectra of gaseous and solid hydrogen fluoride. For more detailed studies of electronic interactions in crystalline HF, fragrnent analysis of MOs of the clusters with respect to the MOs of the central molecule is carried out. Translated fromZhumal Strukturnoi Khimii, Vol. 38, No. 4, pp. 686–695, July–August, 1997.     

MNDO calculations of the electronic structure of lead β-diketonates

Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturmoi Khimii, Vol. 35, No. 6, pp. 186–190, November–December, 1994.     

Spontaneous symmetry breaking during the switching of a buckled graphene membrane

Switching between the states of a buckled graphene membrane for non-volatile memory applications has been studied. The structure of a zigzag graphene strip bent by a force applied to the central region has been investigated by means of the density functional theory with the use of a B3LYP/6-31G approximation. The initial state of the buckled graphene membrane has the geometry of a segment of a half of a (20,0) carbon nanotube with a length of 5 hexagons. The force has been simulated by sequential displacement of the central atoms of the strip toward the fixed edges of the structure. The dependences of the deformation energy and internal forces on the positions of central atoms have been found. Spontaneous breaking of the membrane symmetry decreasing the energy threshold between the states has been discovered.     

Crystal and molecular structures of bis(2,2,6,6-tetramethyl-3-methylaminoheptan-5-onate) copper(II) and nickel(II)

Two bis-chelates M(tmih)₂ (M = Cu(II), Ni(II), tmih = (CH₃)₃C(NCH₃)CHCOC(CH₃)₃)^? are synthesized and their crystal structures are determined using XRD (Bruker APEX-II diffractometer with a CCD detector, λMoK _α, λCuK _α, graphite monochromator, T = 240(2) K and 296(2) K): Cu(tmih)₂ (I) (space group P2₁/c, a = 12.9670(8) Å, b = 18.4921(9) Å, c = 11.0422(6) Å, β = 93.408(4)°, V = 2643.1(3) ų, Z = 4) and Ni(tmih)₂ (II) (space group P2₁/c, a = 12.810(2) Å, b = 18.529(2) Å, c = 11.243(2) Å, β = 91.959(7)°, V = 2667.1(6) ų, Z = 4). The complexes are isostructural; the coordination polyhedron of metal atoms is a flattened tetrahedron formed from two O atoms (Cu-O of 1.901(2) Å, 1.892(2) Å, Ni-O of 1.845(2) Å, 1.833(2) Å) and two N atoms (Cu-N of 1.976(3) Å, 1.972(3) Å, Ni-N of 1.911(2) Å, 1.920(2) Å) of the ligand; the chelate OMN angles (M = Cu(II), Ni(II)) are in the 87.4–93.1° range; the OMO and NMN angles are 162.2° and 167.2° in I, 171.1° and 173.2° in II. The complexes have the molecular structures formed from isolated molecules bonded by van der Waals interactions. Using a quantum chemical hybrid M06 method, the structures of copper(II) chelates with the H, CH₃, CH₂CH₃, CH(CH₃)₂, and C(CH₃)₃ substituents at the nitrogen atom are calculated. Found that with a bulky substituent at the nitrogen atom, the formation of chelates is hindered due to the intraligand repulsion between the atoms of this substituent and the tert-butyl group.     

Effect of annealing on the optical absorption spectra of single-walled carbon nanotubes

The thermal stability of initial and purified samples of single-walled carbon nanotubes prepared through gas-phase disproportionation of carbon monoxide CO in the presence of iron particles under high pressure (the HiPCO method) is investigated using optical absorption spectroscopy and thermogravimetry. An analysis of the optical absorption spectra demonstrates that thermal oxidation of the initial material proceeds rather rapidly and uniformly owing to the catalytic effect caused by the presence of iron particles in the sample. The destruction of the carbon nanotubes contained in the as-prepared and purified samples begins at temperatures of ～250 and ～300°C, respectively. It is shown that single-walled metallic nanotubes undergo faster oxidation as compared to the single-walled semiconducting nanotubes.     

X-ray spectral and quantum chemical analyses of the electronic structure of poly(monofluorocarbon)

The electronic structure of poly(monofluorocarbon) has been studied by X-ray spectral and quantum chemical methods. Calculations were performed in terms of the MNDO method, with the fluorographite layer modeled by clusters of different sizes. The high-resolution CK_a and FK_a spectra have been obtained; the calculated spectra are consistent with the experimental ones. It has been shown that carbon and fluorine are bonded mainly through the σ bonds. The p orbitals of fluorine atoms that are perpendicular to the C-F bond are not involved in the chemical bond, while the transitions from the molecular orbitals consisting of these p orbitals are responsible for the main maximum in the FK_a spectrum. Deceased. Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 4, pp. 630–635, July–August, 1995. Translated by I. Izvekova     

Influence of defects in the carbon network on the static polarizability of fullerenes

The static polarizability of the C₆₀, C₇₀, C₈₀, and C₁₈₆ fullerenes has been calculated within the semiempirical MNDO approximation implemented in the MOPAC quantum-chemical program package. It is demonstrated that the results obtained are comparable with experimental data and the results of the ab initio B3LYP method using the 6–31G(d, p) basis set. The influence of topological defects (five-, seven-, and eight-membered rings), vacancies produced by removing pentagons, and nitrogen and boron atoms on the geometric parameters and the polarizability of the C₆₀, C₂₄₀, and C₅₄₀ fullerenes has been investigated by the MNDO method. It is revealed that the polarizability of the fullerene with topological defects is higher than the polarizability of the perfect icosahedral fullerene. The formation of vacancies in the carbon cage leads to a linear decrease in the polarizability of the fullerene and an increase in the specific polarizability. The polarizability of the heterofullerene with nitrogen or boron atoms spaced apart in the carbon cage is higher than that of the fullerene with heteroatoms located adjacent to each other.