Open Carbon Nanotubes Research Articles

Angular variation of the transmission coefficient of electrons from open carbon nanotubes by a 3-dimensional WKB method

We have calculated by a 3-dimensional (3D) WKB method the transmission coefficient T( θ) of open carbon nanotubes (CNTs) as a function of the angle θ and the geometry of the tube, namely the height h, the radius R and the thickness w of a CNT. For the potential barriers we have used the results of our previous work [G.C. Kokkorakis, J.A. Roumeliotis, J.P. Xanthakis, J. Appl. Phys. 95 (2004) 1468]. For the calculation of T( θ) we have used the method of Das and Mahanty [Phys. Rev. B 36 (1987) 898], which is a 3D generalization of the traditional one-dimensional (1D) WKB method. Due to the open shape of the tube and its nanometric scale we have found considerable deviations of T( θ) from the 1D WKB theory. In particular, the paths of the emitted electrons are highly curved at certain directions θ. In addition T( θ) has contributions predominately from the outer wall of a CNT due to the asymmetry of the barrier potential. These two factors will definitely affect the direction of the emitted current. Such a phenomenon does not occur with closed CNTs. Furthermore, we have established that an increase in the magnitude of the T( θ) (by the use e.g., of a higher tube or higher applied voltage) will result in a T( θ) which will be more asymmetric with respect to angle.

Study of Xe and Kr adsorption on open single-walled carbon nanotubes using molecular dynamics simulations

We have used molecular dynamics (MD) simulation to study the adsorption isotherms of noble gases on open ended single-walled carbon nanotubes (SWNTs). Adsorption isotherms of xenon at several temperatures between 95 and 130 K, and of krypton between 75 and 95 K on (10,10) tubes have been measured. Adsorption coverage, isosteric heat, and binding energy were calculated at various temperatures and pressures. It was shown that adsorption occur both inside and outside an open ended SWNTs. The interior coverage of 0.06 Xe–C and 0.08 Kr–C were reported at saturation conditions. The monolayer coverage on the exterior surface is 0.23 Xe–C and 0.25 Kr–C. These values are in good agreement with experimental measurements and theoretical predictions.

Polysulfone and functionalized carbon nanotube mixed matrix membranes for gas separation: Theory and experiment

Novel nanocomposite membranes containing single walled carbon nanotubes inside a polysulfone matrix have been prepared and characterized. The carbon nanotubes were functionalized with long chain alkyl amines to facilitate dispersion in the polymer. Both permeabilities and diffusivities of the membranes increased with increasing weight fraction of carbon nanotubes at 4 atm. Experimental sorption isotherms of H 2, O 2, CH 4 and CO 2 were consistent with isotherms predicted by atomistic simulations. Comparison of CO 2 adsorption of closed and open nanotubes with simulations of closed and partially open carbon nanotube bundles suggests that HiPco nanotubes are approximately 47% open.

Purification and Opening of Carbon Nanotubes Using Steam

Purification and opening of carbon nanotubes has been carried out by treatment of as-made single-wall carbon nanotubes (SWNTs) with pure steam at 1 atm pressure. Treated samples have been characterized by high-resolution transmission electron microscopy and IR and Raman spectroscopy. Comparison between the steam purification and the standard nitric acid purification treatment shows that steam is less aggressive toward damage to the tubular nanotube wall structure and forms fewer functional groups.

High resolution NMR of water absorbed in single-wall carbon nanotubes

1H MAS NMR study of water absorbed in open single-wall carbon nanotubes (SWCNTs) of average diameter 1.0 nm, synthesized with a Fe catalyst, was performed. Magic angle spinning significantly increases the spectral resolution and the spectrum shows that there are at least two distinct chemical shift regions for the water proton. From temperature dependent measurements, we assign the two chemical shift ranges to water absorbed inside and outside the nanotube. The spectra as a function of the water content show that the first water molecules that are absorbed by the dry material are absorbed inside the tubes.

Electrowetting in Carbon Nanotubes

We demonstrate reversible wetting and filling of open single-wall carbon nanotubes with mercury by means of electrocapillary pressure originating from the application of a potential across an individual nanotube in contact with a mercury drop. Wetting improves the conductance in both metallic and semiconducting nanotube probes by decreasing contact resistance and forming a mercury nanowire inside the nanotube. Molecular dynamics simulations corroborate the electrocapillarity-driven filling process and provide estimates for the imbibition speed and electrocapillary pressure.

Endohedral Condensation and Higher Exohedral Coverage of Kr on Open Single-Walled Carbon Nanotubes at 77 K

An isotherm quasi-discontinuity, or a near-vertical step, at 177 microTorr indicative of Kr condensation inside open single-walled carbon nanotubes (SWNT) has been observed at 77 K. The isotherm shows double adsorption-branch structure attributed to the existence of two endohedral phases of confined Kr. Three well-pronounced steps corresponding to the formation of various exohedral phases are present in the high-density (low-pressure) branch. The desorption branch exhibits three rounded steps assigned to higher order exohedral coverage.

An investigation of lithium intercalation into the carbon nanotubes by a.c. impedance

The closed and open carbon nanotube electrodes have been studied as a function of frequency and the open circuit cell potential. A comparison of these spectra reveals different behaviors depending on the form of the carbon nanotubes: for the closed carbon nanotubes the impedance spectra consists of only one arc in the high frequency, for the open carbon nanotubes the impedance spectra consists of two separated semicircles in the high frequency. Analysis based on plausible equivalent circuit models for the carbon nanotubes lead to evaluation the kinetic parameters for the various physicochemical processes occurring at the electrode/electrolyte interface.

External and internal wetting of carbon nanotubes with organic liquids

Individual multiwalled carbon nanotubes were controllably wetted by polyethylene glycol, glycerol, andwater. A Wilhelmy force balance approach was used to calculate contact angles at the nanotube-polyethyleneglycol and nanotube-glycerol interfaces, allowing examination of the contact angle dependence on the nano-tube diameter. Water, however, exhibited a significantly larger interaction with the nanotube, which could onlybe explained by allowing for internal wetting of the open carbon nanotube structure. This internal wetting angleis smaller than the external one.DOI: 10.1103/PhysRevB.71.115443 PACS numberssd: 68.08.Bc, 81.05.Tp, 81.07.DeI. INTRODUCTION

Molecular Dynamics Study of the Destruction of Single-Walled Carbon Nanotubes under Tension

The deformation and destruction of single-walled open carbon nanotubes are investigated in the framework of the molecular dynamics model of charges localized at bonds, which takes into account the atomic (ionic) and electronic degrees of freedom. This approach allows one to study excited electronic states induced by electronic transitions and to investigate both the ionic and electronic subsystems simultaneously. The structural transformations of nanotubes with (10, 2) chirality under tension are investigated, and the tensile stress-strain diagrams of these nanotubes in the temperature range 100–1000 K are calculated. It is established that, at low and moderate temperatures, the destruction of the nanotubes has a similar nature: the interatomic covalent bonds are broken in the normal cross section of the nanotube. At high temperatures, the nanotube undergoes a scission along the stretching axis.

Fabrication temperature effect of the field emission from closed and open tip carbon nanotube arrays fabricated on anodic aluminum oxide films

We have studied the field emission from highly ordered closed and open tip carbon nanotubes (CNTs) fabricated, in the range of 550–750°C, on porous anodic aluminum oxide films. The field-emission efficiency of the closed and open tip CNTs showed the opposite behavior as the fabrication temperature increased. The former decreased, whereas the latter increased with an increase in the fabrication temperature. From the HRTEM analysis, as the fabrication temperature increased, the crystallinity of the inner walls was improved, whereas that of the outer walls deteriorated due to the formation of a composite of carbon and alumina. This could be a direct cause of the opposite field-emission behavior of the closed and open tip CNTs with an increasing fabrication temperature, because for the open tips, the field emission could take place from all the walls including the inner walls but for the closed tips, only from the outer walls. This is supported by the observation that the field emission of the closed tip CNTs fabricated at a high temperature was greatly improved by milling the surface of the tips, slightly, with ion bombardment. Our results suggest a way to fabricate a high efficiency CNT field-emitter array.

Lithium insertion into multi-walled raw carbon nanotubes pre-doped with lithium

Raw carbon nanotubes are synthesized by the electric arc method and pre-doped with lithium. Electrochemical intercalation of lithium into the raw carbon nanotubes pre-doped with lithium is investigated. The results show that the irreversible and reversible capacity of the sample is about 500 and 200 mAh g −1, respectively. The reversible capacity is higher than that of the raw carbon nanotubes and the open carbon nanotubes. The main reason is that LiNO 3 absorbed by the carbon nanotubes is decomposed into LiNO 2 at high temperature, and the lithium nitrite can form a film before the electrode contacts the electrolyte. The Li-containing compound film on the electrode surface can prevent the solvent to decompose into other products, so little charge is consumed in the electrolyte decomposition process. Complementary characterization techniques (elemental analysis, X-ray diffraction, scanning electron microscopy, constant-current charge–discharge tests) are used for a full structural characterization of the carbon nanotubes pre-doped with lithium and the electrochemical intercalation of lithium.

Design of single-walled carbon nanotubes with a large two-photon absorption cross section

Resonant enhancements of the two-photon absorption cross sections $\ensuremath{\delta}$ have been investigated for different types of seven single-walled carbon nanotubes. The obtained results show that the $\ensuremath{\delta}$ values increase as degree of curvature decreases, that is, $\ensuremath{\delta}$ values increase as $n$ increase for $(n,n)$ tubes ($n=3$,4,5). The $\ensuremath{\delta}$ values linearly vary with the lengths for the $(n,m)$ of $m\ensuremath{\ne}n$ tubes. The origination of a large two-photon absorption cross section has been discussed and it is ascribed to the dispersion effect of tube radius and large $\ensuremath{\pi}$-electron delocalizations in fine open carbon nanotubes.

Enhancement factor of open thick-wall carbon nanotubes

We have calculated the electric field around and on the surface of an open thick-wall carbon nanotube (CNT) of height h, external radius R, and wall thickness w. To accomplish that we simulate the CNT as a vertical array of touching toroids, each of external radius R and cross section radius w/2, and then we express the problem in toroidal coordinates. From our calculations we obtain the enhancement factor γ as a function of h, R, and w. By fitting to our numerical results we obtain an empirical but simple formula for γ, which extrapolates to that of a closed CNT in the limiting case of w=R.

Enhancements of third-order nonlinear optical response in the triplet excited state of finite open single-walled carbon nanotubes

Spin multiplicity enhancements of third-order nonlinear polarizabilities have been investigated at the excited state for finite single-walled carbon nanotubes (5,0) and (6,0) in terms of the time-dependent density functional theory combined with the sum-over-states method. The time-dependent density functional theory at the B3LYP/3-21G level is employed to calculate the transition moments and the excited state energies of optimized tubes. The obtained results show that the third-order polarizabilities of γ at the excited triplet state are enhanced about from 2 to 3 orders of magnitude and the estimated third-order optical susceptibility of χ(3) is as large as 10−8 esu magnitude order. The electron transitions among the three triplet states make significant contributions to third-order polarizabilities of the (5,0) nanotube, and the electron transition between the two triplet states makes main the contribution to those of the (6,0) nanotube.

Chirality-dependent absorption and third-order polarizability spectra in open single-wall carbon nanotubes

The optimized configurations and electronic structures have been obtained based on the density-functional theory for finite open single-walled carbon nanotubes of (4,4) and (5,5). The calculated results show that the bond lengths, bond angles, and charge distributions at the tube ends are significantly different from those at the tube wall. Then, the electronic absorption spectra of the two tubes are calculated by the time-dependent density-functional theory. The evidences show agreement between the theoretical and available experimental results. A strong and wide absorption band occurs for the case of light polarized parallel to the tube axis; a weak absorption and large transparent range appear as the case of light polarized perpendicular to the tube axis. Finally, the dynamic third-order optical polarizabilities in the three optical processes are obtained in terms of the sum-over-states method combined with the above-mentioned method. It is found that the anisotropy of the polarizabilities is larger for the (4,4) than (5,5) tube, and the tube with a larger axial length and smaller diameter has a larger third-order optical polarizabilities at nonresonant frequencies. The largest third-order polarizability is in the direction of both the polarized light and basic light along with the tube axis, and two photon excitation states make main contributions to the polarizabilities of the (4,4) and (5,5) tubes.

Mechanical and electrostatic properties of carbon nanotubes under tensile loading and electric field

Coupled mechanical and electronic behaviours of single walled open carbon nanotubes (CNTs) under applied electric field and tensile loading are investigated by the use of quantum mechanics as well as quantum-molecular dynamics techniques based on the Roothaan–Hall equations and the Newton motion laws. Different failure mechanisms and mechanical properties are found for CNTs subjected to electric fields and that subjected to tensile load. The electric field induced breaking in CNT begins from the outmost carbon atomic layers while the tensile load breaks the nanotube near its middle at 300 K. Electronic polarization and mechanical deformation induced by an electric field can significantly change the electronic properties of a CNT. Under electric field, the CNT can be stretched but the toughness is much lower than that under mechanical loading. Applied tensile loading causes no electronic polarization even in a metallic tube but it indeed changes the energy gap of the tube, thus exhibits influence on field-emission properties of CNTs. When a tube is tensioned in an electric field, the critical tensile strength of the tube may decrease significantly with increasing intensity of electric field. The coupling of mechanical and electrical behaviours is an important characteristic of CNTs.

Highly ordered carbon nanotube arrays with open ends grown in anodic alumina nanoholes

Highly ordered multiwalled carbon nanotube arrays were fabricated by pyrolysis of acetylene within anodic alumina templates. Nanotubes are very uniform in diameter and open at both ends. High resolution transmission electron microscopy and electron diffraction analysis show that the carbon nanotubes are well graphitized. These standing and open carbon nanotubes are possible to offer a potential elegant technique for electron emitting devices, chemical functionalization and nanotube composites.

Three-dimensional simulations of field emission through an oscillating barrier from a (10,0) carbon nanotube

We present three-dimensional simulations of field emission through an oscillating barrier from an ideal open (10,0) carbon nanotube without adsorption by using a transfer-matrix methodology. By introducing pseudopotentials for the representations of carbon atoms and by repeating periodically a basic unit of the nanotube, band-structure effects are manifested in the distributions of energies. The total-energy distribution of the emitted electrons present features which are related essentially to the gap of the semiconducting (10,0) nanotube and to stationary waves in the structure. The current enhancement due to photon-stimulation reaches a saturation plateau for photon energies larger than 5 eV and decreases for high energies. Maximal enhancement is achieved at a photon energy around 8 eV, one electron being then emitted for every 20 photons crossing the nanotube.

Co–Mo catalyzed growth of multi-wall carbon nanotubes from CO decomposition

We investigated the growth of multi-wall carbon nanotubes (CNTs) catalyzed by SiO 2-supported Co–Mo bi-metallic catalyst in flowing CO at 700 °C. We found that both Co and Mo are present in catalytic particles at the tips of CNTs, but their compositions vary from one catalytic particle to another and significantly deviate from the initial mixing composition. The Co concentration and distribution in the catalytic particle of a CNT largely determines the length of the CNT. The CNT growth process is carbon adsorption on exposed area of a catalytic particle and subsequent precipitation at the CNT-catalyst interface or open CNT wall edges. The encapsulation of a catalytic particle was found to occur by the growth of the open-edged graphene walls around the particle. Two types of long CNTs were observed: one with their CNT walls ended at the CNT-particle interface, and the other with their CNT walls open to the environment. The former have diameters similar to their catalytic particle size while the latter have larger diameters.