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.     

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.     

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.     

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.     

Electronic structure of monosubstituted benzenes and X-ray emission spectroscopy

The electronic structure of fluorobenzene was investigated by X-ray emission spectroscopy (using the F−Kα- and C−Kα-spectra) and quantum-chemical MNDO calculations. Molecular orbitals of fluorobenzene were compared with those of benzene and hydrogen fluoride. The P_π−p_π-interaction between the phenyl ring and the fluorine atom in the fluorobenzene molecule is weak for both the outer and inner π levels. For Part 2, see Ref. 1. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1454–1460, August, 1997.