A. G. Kudashov

Arrays of Carbon Nanotubes Aligned Perpendicular to the Substrate Surface: Anisotropy of Structure and Properties

Arrays of multiwalled carbon nanotubes (CNTs) aligned perpendicular to the substrate surface have been produced by the pyrolysis of a mixture of hydrocarbons with ferrocene as the source of the catalyst. Based on scanning electron microscopy data, the mechanism of formation of aligned CNT arrays is proposed. The potential of angle-resolved X-ray spectroscopy for determining the degree of disorder of graphite layers in CNTs is demonstrated. Aligned CNT arrays are characterized by anisotropy of magnetic properties due to encapsulation of metallic rods in the inner cavities of nanotubes.

Gas-phase synthesis of nitrogen-containing carbon nanotubes and their electronic properties

Nitrogen-containing carbon nanotubes are synthesized using a gas-phase reaction. The synthesis of nitrogen-doped carbon nanotubes from 100 to 500 Å in diameter is accomplished through pyrolysis of acetonitrile (CH3CN) at a temperature of 800°C. Cobalt and nickel metallic particles formed upon thermal decomposition of a mixture of maleate salts are used as catalysts. The materials synthesized are investigated by scanning and transmission electron microscopy. Analysis of the x-ray photoelectron spectra demonstrates that the content of nitrogen atoms in three nonequivalent charge states is approximately equal to 3%. A comparison of the CKα x-ray fluorescence spectrum of the carbon nanotubes synthesized through electric-arc evaporation of graphite and the x-ray fluorescence spectrum of the nitrogen-containing carbon nanotubes prepared by catalytic decomposition of acetonitrile indicates that, in the latter case, the spectrum contains a certain contribution from the sp3 states of carbon atoms. The temperature dependences of the electrical conductivity for different types of multi-walled carbon nanotubes are compared. The difference observed in the temperature dependences of the electrical conductivity is associated with the presence of additional scattering centers in nitrogen-containing carbon nanotubes.

Magnetic properties of Fe3C ferromagnetic nanoparticles encapsulated in carbon nanotubes

The low-temperature dependences of magnetic characteristics (namely, the coercive force Hc, the remanent magnetization Mr, local magnetic anisotropy fields Ha, and the saturation magnetization Ms) determined from the irreversible and reversible parts of the magnetization curves for Fe3C ferromagnetic nanoparticles encapsulated in carbon nanotubes are investigated experimentally. The behavior of the temperature dependences of the coercive force Hc(T) and the remanent magnetization Mr(T) indicates a single-domain structure of the particles under study and makes it possible to estimate their blocking temperature TB = 420–450 K. It is found that the saturation magnetization Ms and the local magnetic anisotropy field Ha vary with temperature as ∼T5/2.

Electronic Structure and Field‐Emission Properties of Nitrogen‐Doped Carbon Nanotubes

Abstract Nitrogen‐doped carbon nanotubes have been synthesized by thermal decomposition of acetonitrile vapor over Ni/Co catalysts with varied ratio of metals. X‐ray photoelectron spectroscopy revealed that nitrogen atoms are embedded into tube walls in two different forms at least: (1) three‐coordinated nitrogen and (2) pyridinic‐like nitrogen. The ratio between these forms depends on the catalyst composition. Electronic structure of nitrogen‐doped carbon nanotubes was examined by means of photoemission, X‐ray photoelectron, X‐ray emission and X‐ray absorption spectroscopy. Field electron emission characteristics of samples produced were correlated with nitrogen content. The experimental data was interpreted using results of quantum‐chemical semiempirical AM1 calculations on models of nitrogen‐containing carbon tubes. Incorporation of three‐coordinated nitrogen was found from calculation of the models using the transfer matrix approximation to improve the current‐voltage characteristics of carbon nanotubes.

Phase states and magnetic properties of iron nanoparticles in carbon nanotube channels

The structure, phase composition, and magnetic properties of carbon nanotubes filled with iron nanoparticles and obtained by thermolysis of a mixture of ferrocene and C60 fullerene or ferrocene and orthoxylene at a temperature of 800°C are investigated. Electron microscopy, X-ray diffraction, and Mössbauer spectroscopy data lead to the conclusion that carbon nanotubes are multilayer systems partially filled with iron nanoparticles and/or nanorods. Metallic inclusions in nanotube channels form α-Fe, γ-Fe, and Fe3C phases. The concentration of each phase in the samples is determined. It is shown that 10–20-nm iron clusters in nanotubes exhibit magnetic properties typical of bulk phases of iron. High elasticity of carbon nanotube walls facilitates stabilization of the high-temperature γ-Fe phase; the relative concentration of this phase in a sample can be increased by lowering the concentration of ferrocene in the initial reaction mixture.

Comparison of structure and conductivity of multiwall carbon nanotubes obtained over Ni and Ni/Fe catalysts

Abstract Multiwall carbon nanotubes (MWNT) were produced by pyrolysis of acetonitrile (CH3CN) on metallic particles of Ni and Ni/Fe at 850°C. The special program for statistical treatment of electron micrograph images was developed. Research of diameter distribution of MWNT grown over different catalysts was carried out. Two kinds of carbon nanotubes with different diameter and microstructure are formed on Ni catalyst. The MWNT with smaller diameter and cylindrical packing of layers were found to have the higher conductivity.

The field emission properties of carbon nanotubes and SiC whiskers synthesized over Ni particles deposited in ion tracks in SiO2

Carbon nanotubes (CNTs) and SiC whiskers are synthesized by the pyrolysis of acetonitrile vapor over Ni nanoparticles deposited in the pores obtained by etching heavy ion tracks in dielectric SiO2 layers on single crystal silicon substrates. The structures obtained are studied by scanning electron microscopy and Raman spectroscopy, and their field emission characteristics are measured. The formation of CNTs or SiC whiskers as a result of catalytic hydrocarbon pyrolysis is found to depend on the occupation of ion tracks by nickel clusters. It is shown that the threshold of electron emission appearance is 1 V/µm for both types of the samples and that differences in the shape of current-voltage characteristics are explained by differences in the electronic structure of CNTs and SiC whiskers.

Influence of the inhomogeneity of local magnetic parameters on the curves of magnetization in an ensemble of Fe3C ferromagnetic nanoparticles encapsulated in carbon nanotubes

Methods have been proposed and tested for analyzing local magnetic parameters in a system of single-domain ferromagnetic nanoparticles using their magnetization curves. The magnetic inhomogeneity in ensembles of Fe3C nanoparticles encapsulated in carbon nanotubes has been investigated. It has been established that the Fe3C nanoparticles encapsulated in carbon nanotubes are characterized by two-modal distribution functions of the local magnetic anisotropy energy. The particle distribution over the blocking temperature is reconstructed from the experimental temperature dependence of the coercive force. The allowance made for the inhomogeneity of the local magnetic parameters of the Fe3C nanoparticles, which were studied by the proposed methods, explains the discrepancy between the magnetic anisotropy energy determined by the method of the magnetization approaching saturation and the magnetic anisotropy energy estimated from the coercive force of single-domain nanoparticles.

Synthesis of CNx nanotubes using catalysts prepared from zinc and nickel bimaleates

Nitrogen-containing carbon nanotubes have been prepared via acetonitrile (CH3CN) pyrolysis at 850°C catalyzed by nanoparticles produced by the thermal decomposition of zinc and nickel bimaleates and their solid solutions. The synthesized samples have been characterized by transmission electron microscopy, x-ray diffraction, and x-ray photoelectron spectroscopy. The results demonstrate that increasing the zinc content of catalyst nanoparticles reduces the yield of carbon nanotubes and increases the nitrogen content of the material. The high synthesis temperature gives rise to zinc vaporization, which influences the growth process, increasing the nanotube diameter, reducing the wall thickness, and lowering the structural perfection of the graphite layers.

Fluorination of multiwall nitrogen-doped carbon nanotubes

A film of oriented nitrogen-doped multiwall carbon nanotubes was grown on a silicon substrate as a result of the thermolysis of an acetonitrile + ferrocene mixture. The fluorination of the film by BrF3 vapor at room temperature removed the substrate; however, the vertical orientation of the nanotubes was not destroyed. Analysis of micrographs of a fluorinated sample obtained with a high-resolution transmission electron micro-scope showed that only the surface walls of the nanotubes were fluorinated. The fluorine concentration of the product as determined from X-ray photoelectron spectroscopy was about 16%. A comparison of the N1s spectra of the starting and fluorinated samples showed that the nitrogen atoms of CNx nanotubes changed their electronic state as a result of fluorination. Matching of the X-ray photoelectron spectroscopic data with the results of quantum-chemical calculations for fragments of fluorinated nitrogen-doped nanotubes showed that fluorine atoms preferred to attach to pyridine-like nitrogen atoms or to carbon atoms in the ortho or meta positions relative to a nitrogen atom.