International Journal of Nanoscience

Single-walled carbon nanotubes (SWNTs) are grown by thermal chemical vapor deposition at 900°C using methane. Application of an electric field (0.4 V/μm) in situ during the growth process results in directed growth of SWNTs on a horizontal plane bridging a distance as long as 25 μm. This approach is useful in the fabrication of nanotube based transistors.

Controlled growth and precise placement of carbon nanotubes (CNTs) are two vital capabilities for the practical implementation of CNT device designs. This article presents an overview of our research activities aimed at using CVD template growth to both structure and precisely align CNTs into functional system building blocks. Fabrication results on highly-ordered CNT arrays and Y-junction CNTs are described along with an outline of their novel physical properties that could lead to unique device applications.

Carbon nanotubes were functionalized by polystyrene copolymers, poly(styrene-co-hydroxymethylstyrene) and poly(styrene-co-aminomethylstyrene), under esterification and amidation reaction conditions, respectively. The polymer-attached carbon nanotubes are soluble in common organic solvents, forming colored homogeneous solutions. Results from the characterization of the functionalized carbon nanotubes are presented and discussed.

A newly designed gas phase thermal decomposition reactor, ohmically heating the catalytic sites, has been used to synthesize multiwall carbon nanotubes (MWCNTs) on carbon paper and stainless steel screen. Co-Ni catalyst particles were dispersed by a silane intermediate layer adsorbed onto the carbon fibers or the stainless steel threads of the supports. MWCNTs were obtained on both substrates by a tip grown mechanism. They are about 20 μm in length and 15–50 nm in diameter. A methanol pretreatment of the carbon fibers significantly increased the density of the tubes on the carbon paper, but the same treatment had a negative effect on stainless steel. The MWCNTs, which adhere firmly to the carbon paper and the stainless steel screen, may find applications as electrodes in fuel cells, sensors and in photonics.

Electric-arc discharge deposition (EADD) is used to produce CNT products. The design and electric scheme are discussed in details, as well as all technological features. The results of the outputs of all technological schemes are discussed on the basis of thermal analysis (TGA and DTA), supported by TEM and fullerene-dissolving tests. It is established that CNTs mainly form as compact cathode deposits ("stubs"). Conditions of stub preferential growth are experimentally defined. These conditions have narrow range of parameters that may cause some difficulties in scaling of CNT production.

We summarize recent results on the impact of defects on the electronic properties of single-wall carbon nanotubes. We probe the influence of defects on electron transport in CNFETs by combined scanning gate microscopy (SGM) and scanning impedance microscopy (SIM). Depletion surface potential of individual defects is quantified from the SGM-imaged defect radius as a function of tip bias voltage. This provides a measure of the Fermi level at the defect with zero tip voltage. In the "off" state, transport is first dominated by barriers at depleted defects. It becomes diffusive as the CNFET is turned on, and finally is quasi-ballistic in the regime of "degenerate electrostatic doping". Metallic nanotubes with good contacts show a metal-to-insulator crossover as the gate voltage is varied. In the metallic state we see quantitative agreement with the "twiston" scattering picture.

We discuss the fabrication of gold and carbon nanotubes prepared by the template method. The gold nanotubes are prepared via electroless deposition of Au onto the pore walls of a nanoporous polycarbonate filtration membrane. Carbon nanotube membranes (CNMs) are prepared by doing chemical vapor deposition of carbon within the pores of a microporous alumina template. The pores in these filtration membranes act as templates for the nanotubes. We have shown that by controlling the Au deposition time, Au nanotubes that have effective inside diameters of molecular dimensions (<1 nm) can be prepared. These membranes are a new class of molecular sieves and can be used to separate molecules based on size. Furthermore, we describe fundamental investigations of electro osmotic flow (EOF) in carbon nanotube membranes. In addition, an electrochemical derivatization method was used to attach carboxylate groups to the nanotube walls; this enhances the anionic surface charge density, resulting in a corresponding increase in the EOF rate in the CNM.

The magnetic birefringence of the water suspension of materials containing up to 5% of multi-walled carbon nanotubes (MWNTs) was investigated in this work. The observed magneto-optic effect may be considered as a superposition of competitive effects. The negative one is due to the presence of graphite nanoclusters in the suspension, and the positive effect is due to the stick-like nanostructures such as MWNTs and their aggregates. It was found that the speed of sedimentation for nanotubes is less than it for graphite-like particles. As a result, the enrichment process takes place.

The oxidation of p-toluidine in acetonitrile by hydrogen peroxide is a very slow reaction which produces 4,4'-dimethylazoxybenzene as the product. The presence of functionalized multiwalled carbon nanotubes (CNT) in the above reaction produces 4,4'-dimethylazobenzene. The reactions have been followed by GC/MS, which show the following mass nos.: 226,92 (without CNT) and 210,120,92,28 (with CNT). UV-VIS absorption spectroscopy shows maximum corresponding to 4,4'-dimethylazobenzene at 444 nm. The rate of formation of 4,4'-dimethylazobenzene has been determined as 0.0093 s^-1.

Aniline is oxidized to nitrosobenzene as the initial product, which undergoes further oxidation to nitrobenzene. The nitrosobenzene formation is catalyzed by functionalized multiwalled carbon nanotubes (CNT) followed by a coupling reaction between nitrosobenzene and aniline to produce azobenzene. This coupling requires close proximity of the reactants. It proceeds rapidly resulting in the UV-VIS absorption spectrum showing maxima at 327 nm and 425 nm. The nitrosobenzene yield in the presence of CNTs is controlled by the amount present in the medium. As the reaction is not catalyzed by unfunctionalized CNTs or graphitic particles, the uniqueness of the functionalized multiwalled CNTs in this catalysis suggests a nanodimensional reaction pathway.

Inorganic fullerene-like MoS₂ nanotubes synthesized by a gas–solid reaction were used as the electrode materials of hydrogen and lithium storage in aqueous and nonaqueous electrolytes, respectively. Very interesting results such as reversible hydrogen reduction/oxidation and lithium insertion have been obtained.

Very recently, narrow nanotubes have been observed with diameters of 5 or even 4 Å. In this report we survey calculations that have so far been performed on narrow model nanotubes, namely capped by fragments of D_2d and D_6hC₃₆ fullerene cages or by fragments of C₃₂ and C₁₆quasi-fullerene cages with two four-membered rings, or finally by a fragment of dodecahedral C₂₀. The computations can reproduce the observed diameters of the narrow nanotubes. The results also indicate that fragments of C₃₂, used as caps instead of C₃₆, can lead to quite competitive energetics. Thus, a novel possibility that some of the narrow nanotubes can contain four-membered rings at their tips seems plausible. The present paper also surveys computational study of oxygen additions to the narrow nanotubes, i.e., a problem frequently studied with fullerenes. Both thermodynamic enthalpy changes and kinetic activation barriers for oxygen addition to selected bonds are computed and analyzed. The lowest isomer (thermodynamically the most stable) is never of the 6/6 type, i.e., the thermodynamically most convenient structures are produced by oxygen additions to the nanotube tips. The computations show that narrow nanotubes should be relatively prone to additions of oxygen.

The symmetry of single-wall carbon and inorganic tubes is reviewed. For the carbon nanotubes it is used to get the full set of quantum numbers, in the efficient precision (combined density functional and tight-binding methods) calculation of electronic bands and their complete assignation, to obtain the selection rules for optical transitions and the momenta matrix elements for the Bloch eigen-states. The optical characteristics are thoroughly found, and discussed.

The encaging of H₂ or D₂ molecules flux by carbon closed nanotube is considered. The investigation is based on the computer simulation of the process. Several equations of particle motion were solved numerically by the use of high-speed computer. The nanotube was represented by carbon closed tube consisting of 1572 atoms. The interatomic forces were supposed to be defined from combined potential. The interaction of all participating atoms with each others was taken into account. The energy of incident molecules varied from 25 to 175 eV/atom. The direction of molecular beam was chosen to be orthogonal to the tube axis. The statistical validity was provided by 1000 modeled trajectories of incident molecules for each value of initial energy. The variety of final situations were considered. The encaging events were registered at the low energies. They consisted about 15% at energy values up to 80 eV for H and up to 60 eV for D. At higher energies the encaging probability diminished monotonically. The adsorption probability falls from 80% at the 25-to-90-eV energy interval to 10% at 150 eV for H and at 120 eV for D and then monotonically to zero. The penetration of H atoms through two walls of carbon nanotube grows from zero to 10% at 80 eV, then reaches 50% at 110 eV. At 140 eV it is close to 90%. The corresponding values of energy for D atoms are less by factor 1.2 or so. The molecule dissociation probability was not more than 10% for the total energy interval under investigation. The investigation allows to conclude that the carbon closed nanotube is able to serve as the light atom trap at the incident molecule energies up to about 100 eV/atom.

An atomistic capacitance is derived for a single-wall carbon nanotube in a nano-electromechanical device. Multi-scale calculation is performed using a continuum model for the geometrical capacitance, and statistical and quantum mechanical approaches for the quantum capacitance of the nanotube. The geometrical part of the capacitance is studied in detail using full three-dimensional electrostatics. Results reported in this paper are useful for compact modeling of the electronic and electromechanical nanotube devices.

A novel carbon nanotube electronic device is proposed: a light-controlled electronic switch. A charged molecule has been designed for a noncovalent modification of the nanotube. Photon absorption by the molecule results in nanomechanical switching of the charge position which controls the electron/hole transport through the nanotube.

The atomic and electronic structures of semiconductor heterostructures, including steps, misfit dislocations and interface disorder, are studied by using the density-functional tight-binding (TB) method. Atomic structures of misfit dislocations both edge type 1/2 <110> (001) and 60° dislocations in the semiconductor heterostructures, like Si-Ge superlattices and GaAs/Si, InAs/GaAs(001), InP/GaAs(001) systems, are studied by using order of N [O(N)] calculational method. The path probability method (PPM) in the statistical physics is used to study the influence of the interface disorder on the electronic properties of the semiconductor heterostructures. It is shown that the junction relaxation influences quite significantly the electronic and mechanical properties of semiconductor heterostructures. The critical layer thickness h_c for the generation of misfit dislocations depends significantly on the interface disorder (increase of h_c) at the semiconductor heterostructures.