Narrow Single-walled Carbon Nanotubes Research Articles

Structural model of oxidatively unzipped narrow single-walled carbon nanotubes

In this study, we present a synthetic method for the preparation of oxo-graphene nanoribbons (oxo-GNRs) by oxidative cleavage of C–C bonds of enriched (6,5)-single-walled carbon nanotubes (SWCNTs). We applied a mild oxidation protocol to oxidize SWCNTs involving sulfuric acid as dispersion medium and potassium permanganate as oxidant. The obtained oxo-GNRs exhibit stunning fluorescence properties with emission maxima of about 300 nm and 400 nm, respectively, depending on oxidation time. The analyses reveal that functional oxo-groups are predominantly located at the rims of oxo-GNRs. Next to lactones, 1,3-ketones are identified, which can be further reacted with hydrazine, most likely forming pyrazoline groups. Based on our data, we propose a chemical structure model for the oxo-GNRs. The model is based on an intact inner π-system, responsible for photoluminescence properties and oxo-functionalized rims of GNRs.

Competition of quantum effects in H 2/D 2 sieving in narrow single-wall carbon nanotubes

Nanoporous materials have the potential to be used as molecular sieves to separate chemical substances in a mixture via selective adsorption and kinetic sieving. The separation of isotopologues is also possible via the so-called quantum sieving effect: the different effective size of isotopologues due to their different Zero Point Energy (ZPE). Here we compare the diffusion coefficients of Hydrogen and Deuterium in (8,0) Single Walled Carbon Nanotubes obtained with quantum dynamics simulations. The diffusion channels obtained present important contributions from resonances connecting the potential wells. These resonances, which are more important for H than for D , increase the low-temperature diffusivity of both isotopologues, but prevent the inverse kinetic isotope effect reported for similar nanostructured systems.

Effect of temperature and diameter of narrow single-walled carbon nanotubes on the viscosity of nanofluid: A molecular dynamics study

Equilibrium molecular dynamics simulations by all-atom model have been employed to investigate the adsorption properties and temperature dependence of the shear viscosity of ethanol molecules confined in narrow single-walled carbon nanotubes (SWCNTs). Since the properties of the narrow tubes (diameters < 7 Å) are strongly influenced by their curvature, the narrow SWCNTs with diameters ranging from 0.54 to 1.08 nm and the length of 2.5 nm in ethanol reservoir at 273.0, 298.0 and 323K and 1 bar have been studied. The results indicate that endo-adsorption occurs in nanotubes with the diameter larger than 0.81 nm and as a result ethanol nano-wires can be formed inside nanotubes.Calculations of shear viscosity of the nanofluid using Green–Kubo method reveal that the relative viscosity of the confined ethanol decreases with increasing temperature.All the CNT@ethanol fluids have lower viscosity than the base liquid ethanol at all temperatures and the viscosity increases with the increase of CNT radius and endo-adsorption.Chirality of the CNTs has minor effects on shear viscosity and endo-adsorption and the main factor is CNTs diameter.The results of the present work provide deep understanding of the viscosity changes with temperature at the nanofluids and can describe the mechanisms for base fluid property modifications caused by adding nanoparticles. These results are applicable in design and preparation of diesohols and fuel-grade ethanol.

Flue gas adsorption by single-wall carbon nanotubes: A Monte Carlo study.

Adsorption of flue gases by single-wall carbon nanotubes (SWCNT) has been studied by means of Monte Carlo simulations. The flue gas is modeled as a ternary mixture of N2, CO2, and O2, emulating realistic compositions of the emissions from power plants. The adsorbed flue gas is in equilibrium with a bulk gas characterized by temperature T, pressure p, and mixture composition. We have considered different SWCNTs with different chiralities and diameters in a range between 7 and 20 Å. Our results show that the CO2 adsorption properties depend mainly on the bulk flue gas thermodynamic conditions and the SWCNT diameter. Narrow SWCNTs with diameter around 7 Å show high CO2 adsorption capacity and selectivity, but they decrease abruptly as the SWCNT diameter is increased. For wide SWCNT, CO2 adsorption capacity and selectivity, much smaller in value than for the narrow case, decrease mildly with the SWCNT diameter. In the intermediate range of SWCNT diameters, the CO2 adsorption properties may show a peculiar behavior, which depend strongly on the bulk flue gas conditions. Thus, for high bulk CO2 concentrations and low temperatures, the CO2 adsorption capacity remains high in a wide range of SWCNT diameters, although the corresponding selectivity is moderate. We correlate these findings with the microscopic structure of the adsorbed gas inside the SWCNTs.

Effects of water-channel attractions on single-file water permeation through nanochannels

Single-file transportation of water across narrow nanochannels such as carbon nanotubes has attracted much attention in recent years. Such permeation can be greatly affected by the water-channel interactions; despite some progress, this issue has not been fully explored. Herein we use molecular dynamics simulations to investigate the effects of water-channel attractions on occupancy, translational (transportation) and orientational dynamics of water inside narrow single-walled carbon nanotubes (SWNTs). We use SWNTs as the model nanochannels and change the strength of water-nanotube attractions to mimic the changes in the hydrophobicity/polarity of the nanochannel. We investigate the dependence of water occupancy inside SWNTs on the water-channel attraction and identify the corresponding threshold values for drying states, wetting-drying transition states, and stably wetting states. As the strength of water-channel attractions increases, water flow increases rapidly first, and then decreases gradually; the maximal flow occurs in the case where the nanochannel is predominately filled with the 1D water wire but with a small fraction of ‘empty states’, indicating that appropriate empty-filling (drying-wetting) switching can promote water permeation. This maximal flow is unexpected, since in traditional view, the stable and tight hydrogen-bonding network of the water wire is the prerequisite for high permeability of water. The underlying mechanism is discussed from an energetic perspective. In addition, the effect of water-channel attractions on reorientational dynamics of the water wire is studied, and a negative correlation between the flipping frequency of water wire and the water-channel attraction is observed. The underlying mechanism is interpreted in term of the axial total dipole moment of inner water molecules. This work would help to better understand the effects of water-channel attractions on wetting properties of narrow nanochannels, and on single-file water permeation through nanochannels, which might be helpful in designs of high-flux nanochannels.

From dimers to collective dipoles: Structure and dynamics of methanol/ethanol partition by narrow carbon nanotubes.

Alcohol partitioning by narrow single-walled carbon nanotubes (SWCNTs) holds the promise for the development of novel nanodevices for diverse applications. Consequently, in this work, the partition of small alcohols by narrow tubes was kinetically and structurally quantified via molecular dynamics simulations. Alcohol partitioning is a fast process in the order of 10 ns for diluted solutions but the axial-diffusivity within SWCNT is greatly diminished being two to three orders of magnitude lower with respect to bulk conditions. Structurally, alcohols form a single-file conformation under confinement and more interestingly, they exhibit a pore-width dependent transition from dipole dimers to a single collective dipole, for both methanol and ethanol. Energetic analyses demonstrate that this transition is the result of a detailed balance between dispersion and electrostatics interactions, with the latter being more pronounced for collective dipoles. This transition fully modifies the reorientational dynamics of the loaded particles, generating stable collective dipoles that could find usage in signal-amplification devices. Overall, the results herein have shown distinct physico-chemical features of confined alcohols and are a further step towards the understanding and development of novel nanofluidics within SWCNTs.

On the Energetics of Ions in Carbon and Gold Nanotubes

We investigate the insertion of halide and alkali atoms into narrow single-walled carbon nanotubes with diameters <9 Å by density functional theory; both chiral and non-chiral tubes are considered. The atoms are stored in the form of ions; the concomitant charge transfer affects the band structure and makes originally semiconducting tubes conducting. The electrostatic interaction between a charge and the walls of the tube is explicitly calculated. The insertion energies and the positions of the ions are determined by a competition between electrostatic energy and Pauli repulsion. For comparison, we consider ions in gold nanotubes. Alkali ions follow the same principles in gold as in carbon tubes, but chloride is specifically adsorbed inside gold tubes.

Doping effects of Co on exo-hydrogenated narrow single-walled carbon nanotubes

Density functional theory (DFT) calculations with the generalized gradient approximation (GGA) were employed for a systematic study of electronic structure and morphologic characteristics of bare and exo-hydrogenated Co-doped single walled carbon nanotubes (CNTs). Two internally and one externally doping configurations for the cobalt adatoms were investigated. Binding energies, bond lengths and angles, under full and half converge of the adsorbed hydrogen atoms were calculated for both cases. Effect of hybridization between the Co-3d and the H-s orbitals showed that the exo-hydrogenated CNTs with full and half coverage cases would be stable in the internally doped Co atom systems; whereas, the stability of the hydrogenated systems under externally doped Co adatom was not trivial. In general, for the externally Co-doping, the Co atoms can act as additional adsorbents so the amount of total adsorbed hydrogens could be varied substantially; whereas, for the internally Co-doping the nature of the exo-hydrogenation (being atomic or molecular) shows nanotube's chirality dependent.

Ab initio systematic study of chirality effects on phonon spectra, mechanical and thermal properties of narrow single walled carbon nanotubes

Pseudo-potential method under frame work of density functional perturbation theory (DFPT) has been utilized to calculate the phonon spectrum, phonon DOS, specific heat capacity and mechanical properties of narrow armchair and zigzag single walled carbon nanotubes (SWCNTs). Our calculations show that although their Young modulus are about 1TPa, but armchair SWCNTs have greater compressive modulus than the zigzag tubes while the situation would be vice versa for tensile modulus. On the other hand, it was found out that specific heat capacity of narrow tubes shows chirality independence and decreases by increasing the radius of nanotube whereas, this is not the case for the wider ones. In addition, chirality dependence of the radial breadth mode (RBM) frequency of narrow SWCNTs has been shown.

Electron-beam engineering of single-walled carbon nanotubes from bilayer graphene

Bilayer graphene nanoribbons (BGNRs) with a predefined width have been produced directly from bilayer graphene using a transmission electron microscope (TEM) in scanning mode operated at 300kV. The BGNRs have been subsequently imaged in high-resolution TEM mode at 80kV. During imaging, the interaction of the electrons with the sample induces structural transformations in the BGNR, such as closure of the edges and thinning, leading to the formation of a single-walled carbon nanotube (SWCNT). We demonstrate using molecular dynamics simulations that the produced SWCNT is, in fact, a flattened SWCNT with elliptical circumference. Density functional theory calculations show that the band gap of the flattened semiconducting SWCNTs is significantly smaller than that of the undeformed semiconducting SWCNTs, and this effect is particularly profound in narrow SWCNTs.

Designing different nanomagnets via delocalization of cobalt magnetic moment inside narrow single walled carbon nanotubes

Density functional theory calculations were employed to study the effects of chirality and diameter of single walled carbon nanotubes (SWCNTs) on electronic, structural and magnetic properties of cobalt-doped (9,0), (5,5) and (5,0) nanotube systems. The (9,0) and (5,5) SWCNTs have similar diameters but different chiralities, whereas the (5,0) tube has a very small diameter. The Co-SWCNT systems are considered with four different possible arrangements, three of which are stable and only substitution of the Co with one of the carbon atoms on the surface of the SWCNTs is an exemption. Although the quasi-metallic band gap of the (9,0) SWCNT is eliminated by the cobalt doping process, metallic features of the (5,5) and (5,0) nanotubes remain unchanged. On the other hand, delocalization of the cobalt’s magnetization and inducement of a noticeable magnetization to the tubes provide a vast area of possible total magnetizations for the Co-SWCNT systems. The results are applicable to spintronics and useful for designing other nanomagnetic systems.

Facile diameter control of vertically aligned, narrow single-walled carbon nanotubes

Here, we report a diameter-controlled synthesis of vertically aligned (VA) single-walled carbon nanotubes (SWNTs) via catalytic chemical vapor deposition (CVD), enabled by ultrathin iron (Fe) catalysts on alumina (Al2O3) and low acetylene (C2H2) partial pressure. A long forest of sub-3-nm SWNTs up to one millimeter in height could be obtained without addition of hydrogen or moisture, and precise control of the SWNT diameters was successfully established. Key for the efficient growth of such arrays of narrow SWNTs is threefold: (a) growth temperature low enough to suppress catalyst agglomeration and Ostwald ripening, (b) C2H2 partial pressure below a certain level to extend the catalyst lifetime, and (c) size-matching at nanometer scale between Fe catalyst seeds and Al2O3 support asperities in order to mitigate the surface migration and undesirable enlargement of catalyst particles. Our findings can contribute to the facile achievement of uniform, dense arrays of high quality VA-SWNTs with narrow diameter distributions desirable for advanced nanofiltration and electronic applications.

Molecular wire of urea in carbon nanotube: a molecular dynamics study

We perform molecular dynamics simulations of narrow single-walled carbon nanotubes (SWNTs) in aqueous urea to investigate the structure and dynamical behavior of urea molecules inside the SWNT. Even at low urea concentrations (e.g., 0.5 M), we have observed spontaneous and continuous filling of SWNT with a one-dimensional urea wire (leaving very few water molecules inside the SWNT). The urea wire is structurally ordered, both translationally and orientationally, with a contiguous hydrogen-bonded network and concerted urea's dipole orientations. Interestingly, despite the symmetric nature of the whole system, the potential energy profile of urea along the SWNT is asymmetric, arising from the ordering of asymmetric urea partial charge distribution (or dipole moment) in confined environment. Furthermore, we study the kinetics of confined urea and find that the permeation of urea molecules through the SWNT decreases significantly (by a factor of ∼20) compared to that of water molecules, due to the stronger dispersion interaction of urea with SWNT than water, and a maximum in urea permeation happens around a concentration of 5 M. These findings might shed some light on the better understanding of unique properties of molecular wires (particularly the wires formed by polar organic small molecules) confined within both artificial and biological nanochannels, and are expected to have practical applications such as the electronic devices for signal transduction and multiplication at the nanoscale.

Origin of Giant Ionic Currents in Carbon Nanotube Channels

Fluid flow inside carbon nanotubes is remarkable: transport of water and gases is nearly frictionless, and the small channel size results in selective transport of ions. Very recently, devices have been fabricated in which one narrow single-walled carbon nanotube spans a barrier separating electrolyte reservoirs. Ion current through these devices is about 2 orders of magnitude larger than predicted from the bulk resistivity of the electrolyte. Electroosmosis can drive these large excess currents if the tube both is charged and transports anions or cations preferentially. By building a nanofluidic field-effect transistor with a gate electrode embedded in the fluid barrier, we show that the tube carries a negative charge and the excess current is carried by cations. The magnitude of the excess current and its control by a gate electrode are correctly predicted by the Poisson-Nernst-Planck-Stokes equations.

Multicomponent ballistic transport in narrow single wall carbon nanotubes: Analytic model and molecular dynamics simulations

The transport of gas mixtures through molecular-sieve membranes such as narrow nanotubes has many potential applications, but there remain open questions and a paucity of quantitative predictions. Our model, based on extensive molecular dynamics simulations, proposes that ballistic motion, hindered by counter diffusion, is the dominant mechanism. Our simulations of transport of mixtures of molecules between control volumes at both ends of nanotubes give quantitative support to the model's predictions. The combination of simulation and model enable extrapolation to longer tubes and pore networks.

Quantum fluctuations increase the self-diffusive motion of para-hydrogen in narrow carbon nanotubes

Quantum fluctuations significantly increase the self-diffusive motion of para-hydrogen adsorbed in narrow carbon nanotubes at 30 K comparing to its classical counterpart. Rigorous Feynman's path integral calculations reveal that self-diffusive motion of para-hydrogen in a narrow (6,6) carbon nanotube at 30 K and pore densities below ∼29 mmol cm(-3) is one order of magnitude faster than the classical counterpart. We find that the zero-point energy and tunneling significantly smoothed out the free energy landscape of para-hydrogen molecules adsorbed in a narrow (6,6) carbon nanotube. This promotes a delocalization of the confined para-hydrogen at 30 K (i.e., population of unclassical paths due to quantum effects). Contrary the self-diffusive motion of classical para-hydrogen molecules in a narrow (6,6) carbon nanotube at 30 K is very slow. This is because classical para-hydrogen molecules undergo highly correlated movement when their collision diameter approached the carbon nanotube size (i.e., anomalous diffusion in quasi-one dimensional pores). On the basis of current results we predict that narrow single-walled carbon nanotubes are promising nanoporous molecular sieves being able to separate para-hydrogen molecules from mixtures of classical particles at cryogenic temperatures.

Kinetics of water filling the hydrophobic channels of narrow carbon nanotubes studied by molecular dynamics simulations

The kinetics of water filling narrow single-walled carbon nanotubes was studied using molecular dynamics simulations. The time required to fully fill a nanotube was linear with respect to the tube length. We observed that water molecules could enter into nanotubes of different lengths, either from one end or from both ends. The probability of having a nanotube filled completely from both ends increased exponentially with the tube length. For short tubes, filling usually proceeded from only one end. For long tubes, filling generally proceeded from both tube ends over three stages, i.e., filling from one end, filling from both ends, and filling from both ends with the dipole reorientation of water molecules to give a concerted ordering within the fully filled tube. The water molecules in the partially filled nanotube were hydrogen bonded similarly to those in the fully filled nanotube. Simulations for the reference Lennard-Jones fluid without hydrogen bonds were also performed and showed that the filling behavior of water molecules can be attributed to strong intermolecular hydrogen bonding.

Different possible hydrogenation in narrow SWCNTs and their electronic characteristics

Density functional theory (DFT) calculations have been employed for a systematic study of electronic dispersion and morphologic characteristics (length and angle of chemical bonds) of bare and hydrogenated narrow zigzag single walled carbon nanotubes (SWCNTs), d < 7 Å. For these narrow tubes, the “band gap” and the bond character show a strong dependence on the diameter of the tubes. Our results show that exo-hydrogenation changes the band gap substantially. The gap shows essential dependence on the coverage of the exo-hydrogenation and increases with decreasing tube diameter. These properties can be useful in sensing and characterising applications. However, endo-hydrogenation in wider tubes occurs mainly by storing H 2 gas molecules inside them and their electronic structure changes slightly whereas for very narrow tubes, there is an interplay between the curvature and chirality effects. The nature of hydrogen adsorption in these cases can be a mixture of the internal C–H bonds and H 2 molecules and existence of free atomic hydrogen gas inside the tubes cannot be ruled out.

Thermoactivated transport of molecules H2 in narrow single-wall carbon nanotubes

By use both of the plane wave DFT and the empirical exp-6 Lennard-Jones potential methods we calculate the inner potential in narrow single-wall carbon nanotubes (SWCNT) (6, 0), (7, 0) and (3, 3) which affects the hydrogen molecules. The inner potential forms a goffered potential surface and can be approximated as V(z,r,φ)≈V0sin (2πz/a)+V(r). We show that in these SWCNTs transport of molecules is given mainly by thermoactivated hoppings between minima of the periodic potential along the tube axis. The rate hoppings is substantially depends on temperature because of thermal fluctuations of tube wall.

Structural correlation of band‐gap modifications induced in mercury telluride by dimensional constraint in single walled carbon nanotubes

AbstractA unique tubular structure for mercury telluride HgTe grown inside narrow single walled carbon nanotubes (SWNTs) is described with both Hg and Te in novel coordination geometries. Two unique projections were obtained by high resolution transmission electron microscopy from two separate encapsulated crystal fragments enabled reconstruction of a three dimensional atomic arrangement structurally consistent with both fragments. The observed structure was separately determined by quantum mechanical calculations that confirmed the global stability of the HgTe structure and demonstrated conclusively that it arises as a result of its constrained low‐dimensionality. We have also calculated that this structural change is correlated directly with a fundamentally modified electronic structure resulting in the transformation of HgTe from a bulk semimetal to a semiconductor when confined within a SWNT. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)