Nanotube Fragments Research Articles

Single-electron tunneling and Coulomb blockade in carbon-based quantum dots

Single-electron tunneling (SET) and Coulomb blockade (CB) phenomena have been widely observed in nanoscaled electronics and have received intense attention around the world. In the past few years, we have studied SET in carbon nanotube fragments and fullerenes by applying the so-called “Orthodox” theory [28]. As outlined in this review article, we investigated the single-electron charging and discharging process via current-voltage characteristics, gate effect, and electronic structure-related factors. Because the investigated geometric structures are three-dimensionally confined, resulting in a discrete spectrum of energy levels resembling the property of quantum dots, we evidenced the CB and Coulomb staircases in these structures. These nanostructures are sufficiently small that introducing even a single electron is sufficient to dramatically change the transport properties as a result of the charging energy associated with this extra electron. We found that the Coulomb staircases occur in the I–V characteristics only when the width of the left barrier junction is smaller than that of the right barrier junction. In this case, the transmission coefficient of the emitter junction is larger than that of the collector junction; also, occupied levels enter the bias window, thereby enhancing the tunneling extensively.

Evidence of multi-walled carbon nanotube fragmentation induced by sonication during nanotube encapsulation via bulk-suspension polymerization

The synthesis of multi-walled carbon nanotube/polystyrene composites, with nanotube concentrations of 0.04, 0.08 and 0.16 wt%, was carried out by in situ bulk-suspension polymerization with the assistance of sonication. By using this method both encapsulation and exfoliation of the nanotubes into the polymer host were achieved. Evidence of significant nanotube fragmentation was found by scanning electron microscopy; the cause of such fragmentation was attributed to the induction of strong cavitation due to the application of ultrasound during the synthesis. Infrared spectroscopy showed no evidence of the formation of covalent bonds between the nanotubes and the polystyrene during the process of synthesis. The thermal stability was not improved by the inclusion of the nanotubes, it was attributed to the low nanotube concentrations; however, composites glass transition temperature showed improvements.

Adsorption of Molecular Hydrogen in a Graphene-Carbon Nanotube System

In this theoretical study, the H2 adsorption is considered in a novel system formed by a (6,6) carbon nanotube fragment and a planar graphene layer portion, when they are separated 4.72 a.u., with both subsystems inside a cubic supercell box of 25 a.u. side. There is greater H2 adsorption inside the carbon nanotube than in the interstitial space between the graphene layer and the carbon nanotube. The results are compared with the way the H2 molecule is adsorbed upon a lonely graphene layer and inside or outside the (6,6) carbon nanotube. It is studied if in the interstitial space close to the middle point between the wall and the graphene layer the hydrogen molecule could be adsorbed with a greater binding energy that in any other case. This was not possible for the selected supercell, but there are given some suggestions to be explored (using a nanotube with smaller radii or increasing the size of the supercell), that perhaps can optimize the binding energy for H2 adsorption. A general result is that the size of the cubic supercell can be selected to confine not only hydrogen inside the carbon nanotube but also in the interstitial space between the carbon nanotube wall and the graphene layer. It is believed that the studied system or a modification of this could sooner or later be used in a competitive way in comparison with other H2 storage materials respect to the hydrogen adsorption and desorption process.

Effect of Quenching on Properties of TiO<sub>2</sub> Nanotube Arrays

To increase the performance of TiO2 nanotube array electrodes in solar cells,we prepared self-organized TiO2 nanotube arrays on a titanium substrate in 0.5%(w,mass fraction) NH4F/glycerol by anodic oxidization at a constant potential. The electrodes were then quenched in water at different temperatures. These quenched TiO2 nanotube array electrodes were then characterized by X-ray diffraction(XRD) ,field emission scanning electron microscopy(FESEM) ,X-ray photoelectron spectroscopy(XPS) and cyclic voltammetry(CV) . Experimental results indicated that the quenching process produced many surface defects and also resulted in fragmentation of the TiO2 nanotubes. We found that the sample quenched in water at 0 ℃ contained the more Ti3+ surface defects,OH groups and nanotube fragments. These properties improved its photoelectrochemical performance significantly. This sample resulted in 96.2% photodegradation rate of methyl orange after irradiation for 40 min.

The equilibrium structures of the Li[C n ]1 (n = 7–12) complexes and their alternation depending on n

The equilibrium geometric configurations of the Li[C n ]1 (n = 7–12) complexes, where [C n ]1 is a cylindrical hydrocarbon containing the simplest zigzag nanotube fragment, were determined by the density functional theory method with the PBE0 exchange-correlation functional. Analytic molecular orbital (MO) estimates were obtained for isolated [C n ]1 hydrocarbons in the Huckel approximation. The appearance of nonbonding MOs for hydrocarbons with even n was demonstrated. Equilibrium structure types were found to alternate as n increased. This alternation correlated with the behavior of the frontier orbitals of the [C n ]1 hydrocarbon. At odd n, the Li atom was situated near the boundary of the π electron density of the bracelet, and the complex had C s symmetry. Complexes with even n had the C 2v point group, and lithium was situated in the inner cylinder cavity above the center of one of benzene rings.

Theoretical impacts of terminal atoms (C, B, N, and P) on fragments of single-walled hetero carbon nanotubes

Effects of terminal atoms on the energies and geometrical parameters of carbon nanotube (CNT) are reported. A fragment of (18,0) zigzag CNT (C 108H 36), with a length of two hexagonal structures, is compared and contrasted with its N-, (B-, N-), and P-substituted analogs: C 72N 36, C 72N 18B 18, and C 72P 36, respectively, at AM1-RHF level. Terminating the CNT with P-atoms (C 72P 36) increases its relative conductivity and reactivity, through decreasing its band gap, and imposing conspicuous deformations on its edge honeycomb structures, puckering the P-atoms outward, and forming a wheel rim configuration. In contrast, no significant twist or deformation is observed on hexagonal rings of neither C 72N 36 nor C 72N 18B 18. These CNTs appear with a cylindrical geometry and show relatively higher planarity and band gaps (Δ E HOMO-LUMO) than their unsubstituted C 108H 36 analog. Having C 2 symmetry, CNT, N-CNT, and P-CNT appear practically nonpolar, whereas the asymmetric (B-, N-)CNT emerges with a dipole moment of 1.0 D.

Structural peculiarities of in situ deformation of a multi-walled BN nanotube inside a high-resolution analytical transmission electron microscope

Multi-cycle bending deformation of an individual boron nitride (BN) nanotube is performed inside a 300 kV high-resolution analytical transmission electron microscope (TEM) using a piezo-driven scanning tunneling microscope (STM)–TEM holder. Phenomenological peculiarities of the deformation are recorded under the detailed control of nanotube morphology, atomic structure, chemistry and electrical transport during all stages of deformation. Interestingly, it is found that the nanotube layers are very flexible in the vicinity of a relatively rigid nanotube fragment filled with BN matter and may withstand 20 or even more reverse bending cycles. At the same time, the filled part is gradually graphitized such that the normal to the BN graphitic layers is perpendicular to both the nanotube axis and the bending axis.

On the influence of diameter and length on the properties of armchair single-walled carbon nanotubes: A theoretical chemistry approach

Open-ended fragments of armchair single-walled carbon nanotubes ( n, n) with n = 3, 4, 5, and 6, have been modeled, with increasing lengths from 0.6 to 3 nm long. The geometries of all the studied fragments have been fully optimized. The influence of diameter and length on different electronic properties has been analyzed. These properties are electronegativity, ionization potential, electron affinities, and hardness, and all of them have been expressed as functions of the frontier orbitals. The binding energies per C atom have been calculated, using an expression that improves the previously reported ones. Their absolute values were found to steadily increase with tubes length and diameter, which allows extrapolations to obtain BE /C atom for tubes of infinite length. The extrapolated values are 8.45, 8.65, 8.74, and 8.79 eV for armchair nanotubes with n = 3, 4, 5, and 6, respectively.

Density Functional Calculations of the 13C NMR Chemical Shifts in (9,0) Single‐Walled Carbon Nanotubes.

The electronic structure and (13)C NMR chemical shift of (9,0) single-walled carbon nanotubes (SWNTs) are investigated theoretically. Shielding tensor components are also reported. Density functional calculations were carried out for C(30)-capped and H-capped fragments which serve as model systems for the infinite (9,0) SWNT. Based on the vanishing HOMO-LUMO gap, H-capped nanotube fragments are predicted to exhibit metallic behavior. The (13)C chemical shift approaches a value of approximately 133 ppm for the longest fragment studied here. The C(30)-capped SWNT fragments of D(3d)/D(3h) symmetry, on the other hand, are predicted to be small-gap semiconductors just like the infinite (9,0) SWNT. The differences in successive HOMO-LUMO gaps and HOMO and LUMO energies, as well as the (13)C NMR chemical shifts, converge slightly faster with the fragment's length than for the H-capped tubes. The difference between the H-capped and C(30)-capped fragments is analyzed in some detail. The results indicate that (at least at lengths currently accessible to quantum chemical computations) the H-capped systems represent less suitable models for the (9,0) SWNT because of pronounced artifacts due to their finite length. From our calculations for the C(30)-capped fragments, the chemical shift of a carbon atom in the (9,0) SWNT is predicted to be about 130 ppm. This value is in reasonably good agreement with experimental estimates for the (13)C chemical shift in SWNTs.

Density functional calculations of the 13C NMR chemical shifts in (9,0) single-walled carbon nanotubes.

The electronic structure and (13)C NMR chemical shift of (9,0) single-walled carbon nanotubes (SWNTs) are investigated theoretically. Shielding tensor components are also reported. Density functional calculations were carried out for C(30)-capped and H-capped fragments which serve as model systems for the infinite (9,0) SWNT. Based on the vanishing HOMO-LUMO gap, H-capped nanotube fragments are predicted to exhibit "metallic" behavior. The (13)C chemical shift approaches a value of approximately 133 ppm for the longest fragment studied here. The C(30)-capped SWNT fragments of D(3d)/D(3h) symmetry, on the other hand, are predicted to be small-gap semiconductors just like the infinite (9,0) SWNT. The differences in successive HOMO-LUMO gaps and HOMO and LUMO energies, as well as the (13)C NMR chemical shifts, converge slightly faster with the fragment's length than for the H-capped tubes. The difference between the H-capped and C(30)-capped fragments is analyzed in some detail. The results indicate that (at least at lengths currently accessible to quantum chemical computations) the H-capped systems represent less suitable models for the (9,0) SWNT because of pronounced artifacts due to their finite length. From our calculations for the C(30)-capped fragments, the chemical shift of a carbon atom in the (9,0) SWNT is predicted to be about 130 ppm. This value is in reasonably good agreement with experimental estimates for the (13)C chemical shift in SWNTs.

Electrophoretic analysis and purification of fluorescent single-walled carbon nanotube fragments.

Arc-synthesized single-walled carbon nanotubes have been purified through preparative electrophoresis in agarose gel and glass bead matrixes. Two major impurities were isolated: fluorescent carbon and short tubular carbon. Analysis of these two classes of impurities was done. The methods described may be readily extended to the separation of other water-soluble nanoparticles. The separated fluorescent carbon and short tubule carbon species promise to be interesting nanomaterials in their own right.

Side-wall opening of single-walled carbon nanotubes (SWCNTs) by chemical modification: a critical theoretical study.

Single-walled carbon nanotubes (SWCNTs) have unique electronic, mechanical, and structural characteristics; consequently, promising applications derived from these materials, such as chemical sensors or nanometer-scale electronic devices, can be expected. Structurally altered nanotubes with appropriate addends should facilitate their use by improving solubility, processability, and ease of dispersion, as well as by providing sites for chemical attachment to surfaces and polymer matrices. Avexing problem is ascertaining the detailed structures of nanotube derivatives after their preparation. The characterization of functionalized SWCNTs is difficult; all experimental attempts to determine the precise location and mode of addition of newly attached groups have failed. SWCNT adducts with possible three-membered rings (3MRs) that result from oxygen, methylene, and NH additions are simple but very important side-wall functionalized derivatives. Oxidation reactions are used widely to purify nanotubes, and the electrical properties of carbon nanotubes are extremely sensitive to oxygen exposure. Methylene and NH adducts are the prototypes of the recently synthesized covalently bonded dichlorocarbene and nitrene adducts. The available theoretical studies on the structures of nanotube oxide and dichlorocarbene adducts that involve either armchair or zigzag tube models, employed rather unsatisfactory methodology (see below). Owing to the large size of nanotubes, carefully chosen truncated models, appropriate for the problem being investigated, are required. One approach uses small nanotube fragments to simulate a full nanotube, but carries out computations at a relatively high level. The other approach uses the ONIOM technique, which treats part of the system at a high theoretical level but the rest of the system at a lower level. This strategy allows larger systems to be simulated at a practical computational cost. Thus, a recent ONIOM(B3LYP/6-31G*:AM1) study employed a C16 fragment (Figure 1a) for the high-level computation to

Polyaniline and Carbon Nanotubes Based Composites Containing Whole Units and Fragments of Nanotubes

Using surface-enhanced Raman scattering (SERS) and Fourier transform infrared (FTIR) spectroscopy, we show that composites based on polyaniline (PANI) and single-walled carbon nanotubes (SWNTs) are different when they are prepared by two different methods: (1) by adding dispersed SWNTs to the polymer solutions and (2) by chemical polymerization of aniline in the presence of SWNTs. The difference originates from the irreversible chemical transformation of SWNTs in the polymerization medium. The synthesis medium used for the preparation of PANI transforms SWNTs into fragments of shorter length like closed-shell fullerenes. This explains the similarity of SERS and FTIR spectra of the composites PANI/SWNTs and PANI/C60 chemically prepared. All compounds exhibit an absorption band at 1144 cm-1 in their FTIR spectra, increasing with the carbon nanoparticules content, as a signature of a charge transfer between the constituents. Besides, the FTIR spectrum of the compounds obtained by adding SWNTs to the polymer...

In situ electrical measurements and manipulation of B/N-doped C nanotubes in a high-resolution transmission electron microscope.

B/N-doped multiwalled C nanotubes were electrically probed by means of a tungsten needle attached to a piezo-driven stage of a high-resolution transmission electron microscope holder. Two-terminal transport measurements were performed in a 'W needle-nanotube-ground' circuit. The I-V curves were recorded in situ while viewing the nanotubes in the imaging mode of the microscope. This allows us to trace nanotube array morphological changes under applied voltage (up to 50 V). Specific manipulation with nanotube assemblies was found to be possible under applied electrical field: attachment of a tiny nanotube bundle to the W needle and extraction of a given nanotube fragment from an entangled complex bunch were achieved. The electrically-probed B/N-doped C nanotubes exhibited alternating B-rich and C-rich B-C-N domains within tubular layers, as revealed by elemental mapping during energy-filtered TEM (Omega filter). At room temperature the nanostructures displayed resistivity (rho) of approximately 1.8 x 10(-5) omegam and linear I-V curves. The key role of a given contact between the probing needle and a nanotube during electrical measurements was particularly verified.

High Weight Fraction Surfactant Solubilization of Single-Wall Carbon Nanotubes in Water

We report a simple process to solubilize high weight fraction single-wall carbon nanotubes in water by the nonspecific physical adsorption of sodium dodecylbenzene sulfonate. The diameter distribution of nanotubes in the dispersion, measured by atomic force microscopy, showed that even at 20 mg/mL ∼63 ± 5% of single-wall carbon nanotube bundles exfoliated into single tubes. A measure of the length distribution of the nanotubes showed that our dispersion technique reduced nanotube fragmentation.

Strength and Breaking Mechanism of Multiwalled Carbon Nanotubes Under Tensile Load

The tensile strengths of individual multiwalled carbon nanotubes (MWCNTs) were measured with a "nanostressing stage" located within a scanning electron microscope. The tensile-loading experiment was prepared and observed entirely within the microscope and was recorded on video. The MWCNTs broke in the outermost layer ("sword-in-sheath" failure), and the tensile strength of this layer ranged from 11 to 63 gigapascals for the set of 19 MWCNTs that were loaded. Analysis of the stress-strain curves for individual MWCNTs indicated that the Young's modulus E of the outermost layer varied from 270 to 950 gigapascals. Transmission electron microscopic examination of the broken nanotube fragments revealed a variety of structures, such as a nanotube ribbon, a wave pattern, and partial radial collapse.

An Alternative Structure for C576

Carbon nanotubes with rollup vector indices equal, i. e., (n, n) nanotubes, are calculated to be metallic electrical conductors. Several years ago, a toroidal fullerene structure, C576, was proposed and suggested to possess a small bandgap on the basis of its having approximately half of its atoms arranged as in a (4, 4) nanotube. a more recent calculation determined that this structure has a very large anisotropic ring-current diamagnetic susceptibility and is therefore likely to be a good electrical conductor. the present work proposes an alternative structure for C576 which is constructed entirely from six copies of a C96 fragment of a (4, 4) nanotube.

X-ray spectroscopic and quantum–chemical study of carbon tubes produced in arc-discharge

High-resolution CKα spectra have been obtained for samples containing carbon cage particles, synthesized using the arc-discharge graphite evaporation technique. The X-ray fluorescence spectrum of particles from the inner part of the cathode deposit was found to differ from that of the soot collected on the chamber walls. The dependence of the spectral profile on the geometry was investigated using quantum–chemical calculations of carbon nanotube fragments with different helical pitches. The spectrum of multiwall particles agrees best with the theoretical spectra of zig-zag tubes. The agreement between the spectra of tubular particles in the soot and the theoretical results is best for single-wall armchair tube structures. These studies are complemented by calculations of the frontier orbitals of nanotube fragments.

Stress-induced fragmentation of multiwall carbon nanotubes in a polymer matrix

We report the observation of single nanotube fragmentation, under tensile stresses, using nanotube-containing thin polymeric films. Similar fragmentation tests with single fibers instead of nanotubes are routinely performed to study the fiber-matrix stress transfer ability in fiber composite materials, and thus the efficiency and quality of composite interfaces. The multiwall nanotube-matrix stress transfer efficiency is estimated to be at least one order of magnitude larger than in conventional fiber-based composites.

High strain rate fracture and C-chain unraveling in carbon nanotubes

Nanotube behavior at high rate tensile strain (~ 1 MHz) is studied by molecular dynamics using a realistic many-body interatomic potential. The simulatins performed for single- and double-walled nanotubes of different helicities, and at different temperatures, show that nanotubes have an extremely large breaking strain. It decreases somewhat with increasing temperature and smaller strain rate, while the influence of helicity is very weak. At later stages of fracture, the nanotube fragments are connected by a set of unraveling monoatomic chains. The chains ‘compete’ with each other for carbon atoms popping out of the original tube segments. The interaction between chains eventually leads to a single chain, which grows up to hundreds of atoms in length before its breakage.