Capped Carbon Nanotubes Research Articles

Rectifying performance and negative differential behavior in graphite—chain—carbon nanotube junctions

In this paper, the (5, 5) capped carbon nanotubes (CNTs) in contact with different lengths of sp monoatomic chains grown on the surface of graphite substrate are fabricated and its electronic transport properties sandwiched between CNT and graphite electrodes are investigated. The first-principles calculations based on nonequilibrium Green's function in combination with density-functional theory show that their rectifying performance and negative differential resistance behavior are observed under very low biases and obviously are enhanced when the length increases. From our analysis, the charge transfer, transmission spectra, projected density of states and evolutions of molecular orbitals are responsible for these phenomena.

Field Emission Simulations of Carbon Nanotubes and Graphene with an Atomic Model

Field Emission Simulations of Carbon Nanotubes and Graphene with an Atomic Model Most models of field emission from carbon nanotubes (CNTs) and graphene sheets consider the geometry of the emitter tip as a smooth spherical surface. In these models, indicated with the terms simplified geometry (SG), the electronic distribution is not taken into account. However, the electronic distribution is concentrated around the atoms nuclei and their bonds forming hexagonal patterns that affect the field enhancement and largely determines the field emission. In this paper we evaluate the field emission in a SG-model and compare it with the emission in a ball-stick model that better represents the atomic structure at the surface of the emitter. We found that the emission current from open and capped carbon nanotubes and for graphene sheets is typically 10 times larger in the ball-stick geometry than in the SG. Furthermore, different morphologies of the CNT highlight the sensitivity of the emission current.

Microscopic mechanisms of vertical graphene and carbon nanotube cap nucleation from hydrocarbon growth precursors

Controlling and steering the growth of single walled carbon nanotubes is often believed to require controlling of the nucleation stage. Yet, little is known about the microscopic mechanisms governing the nucleation from hydrocarbon molecules. Specifically, we address here the dehydrogenation of hydrocarbon molecules and the formation of all-carbon graphitic islands on metallic nanoclusters from hydrocarbon molecules under conditions typical for carbon nanotube growth. Employing reactive molecular dynamics simulations, we demonstrate for the first time that the formation of a graphitic network occurs through the intermediate formation of vertically oriented, not fully dehydrogenated graphitic islands. Upon dehydrogenation of these vertical graphenes, the islands curve over the surface, thereby forming a carbon network covering the nanoparticle. The results indicate that controlling the extent of dehydrogenation offers an additional parameter to control the nucleation of carbon nanotubes.

Charge distribution of metallic single walled carbon nanotube–graphene junctions

We report numeric and analytic calculations of the electrostatic properties for armchair carbon nanotube–graphene junctions. Using a semi-empirical method we first demonstrate that the equilibrium distance between a carbon nanotube and a graphene sheet varies with respect to the diameter of the carbon nanotube. We find significantly reduced values compared to AB-stacked graphene sheets in graphite, while even smaller value is found for a fullerene C60 implying a dimensionality dependence of the equilibrium distance between graphene and the other sp2 carbon allotropes. Then, we use conformal mapping and a charge–dipole model to study the charge distribution of the carbon nanotube–graphene junctions in various configurations. We observe that the charges are accumulated/depleted at and near the vicinity of the junctions and that capped carbon nanotubes induce a significantly smaller charge concentration at their ends than the open-end nanotubes. We demonstrate that the carbon nanotube influence on the graphene sheet is limited to only few atomic rows. Such an influence strongly depends on the distance between carbon nanotube and the graphene sheet and scales with the carbon nanotube radius, while the potential difference does not modify the length over which the charge concentration is disturbed by the presence of the tube. By studying the potential landscape of carbon nanotube–graphene junctions, our work could be used as a starting point to model the charge carrier injection in these unconventional systems.

Pristine and functionalized capped carbon nanotubes under electric fields

The structural and electronic properties of end‐capped carbon nanotubes (CCN) functionalized by carboxyl, amine, amide, and hydroxyl chemical groups under the action of electric fields are investigated through first‐principles simulations based on density functional theory. Changes in the original properties of the functionalized capped nanotubes are observed due to the application of external electrical fields and the responses for different intensities are shown to be nonlinear. It is demonstrated that there are optimum values for the electric field to obtain the most stable binding energies for the studied systems. In all cases, the energy levels are rearranged to stabilize the systems and electric dipole moments are induced toward the incident electric field. It is also shown that these electric dipole moments generate polarized nanotubes and the previously weak physical interactions between molecules with the tube surface would become stronger and, by binding chemically, allow the manipulation of the intrinsic properties of the nanotubes.The structural and electronic properties of end‐CCN pristine and functionalized by carboxyl, amine, amide, and hydroxyl chemical groups under the action of electric fields are investigated through first‐principles simulations based on the density functional theory.

Distinct Mechanisms of DNA Sensing Based on N‐Doped Carbon Nanotubes with Enhanced Conductance and Chemical Selectivity

N-doped capped carbon nanotube (CNT) electrodes applied to DNA sequencing are studied by first-principles calculations. For the face-on nucleobase junction configurations, a conventional conductance ordering is obtained where the largest signal results from guanine according to its high highest occupied molecular orbital (HOMO) level, whereas for the edge-on counterparts a distinct conductance ordering is observed where the low-HOMO thymine provides the largest signal. The edge-on mode is shown to operate based on a novel molecular sensing mechanism that reflects the chemical connectivity between N-doped CNT caps that can act both as electron donors and electron acceptors and DNA functional groups that include the hyperconjugated thymine methyl group.

Electronic Properties of Capped Carbon Nanotubes under an Electric Field: Inhomogeneous Electric-Field Screening Induced by Bond Alternation

We study the electronic properties of capped carbon nanotubes under an electric field by investigating their electrostatic potentials, total energies, and energy gaps under a parallel electric field, based on the density functional theory with effective screening medium method. We find that, in the capped carbon nanotubes, screening against the external electric field strongly depends on local atomic arrangement due to the inhomogeneous charge distribution arising from its bond alternation induced by the pentagonal rings in the cap region. In the case of armchair nanotubes, we find that the relative permittivity and energy gap between the highest occupied and the lowest unoccupied states oscillate in triple periodicity in their units with respect to the length. The electric field induces the charge redistribution in which the charge accumulation and depletion only occur around the pentagonal rings at or vicinity of the top/bottom of the nanotubes.

The first-principles study of oscillating rectifying performance in nanoribbon-chain-carbon nanotube junctions

Electronic transport properties of armchair graphene nanoribbon and capped carbon nanotube junctions, covalently bridged by carbon atomic chains with different numbers of carbon atoms, are investigated. The first-principles calculations based on non-equilibrium Green's functions with the density-functional theory show that their I-V characteristics display odd-even effects and rectifying behaviors show obvious oscillations, namely, different bond patterns for even- and odd-numbered carbon chains affect the contact bonds, charge transfer, density of states, evolutions of molecular orbitals, and rectifying performance.

Pull-out simulations of a capped carbon nanotube in carbon nanotube-reinforced nanocomposites

Systematic atomic simulations based on molecular mechanics were conducted to investigate the pull-out behavior of a capped carbon nanotube (CNT) in CNT-reinforced nanocomposites. Two common cases were studied: the pull-out of a complete CNT from a polymer matrix in a CNT/polymer nanocomposite and the pull-out of the broken outer walls of a CNT from the intact inner walls (i.e., the sword-in-sheath mode) in a CNT/alumina nanocomposite. By analyzing the obtained relationship between the energy increment (i.e., the difference in the potential energy between two consecutive pull-out steps) and the pull-out displacement, a set of simple empirical formulas based on the nanotube diameter was developed to predict the corresponding pull-out force. The predictions from these formulas are quite consistent with the experimental results. Moreover, the much higher pull-out force for a capped CNT than that of the corresponding open-ended CNT implies a significant contribution from the CNT cap to the interfacial properties of the CNT-reinforced nanocomposites. This finding provides a valuable insight for designing nanocomposites with desirable mechanical properties.

Disorder effect on conductance in a doped C60 molecular bridge

In this work, we study electrical conductance in a C60 molecular nanobridge with randomly point dopants sandwiched between two (5,5) carbon nanotubes (CNTs) in two cases with opened end and closed end. Two different ways of coupling C60 fullerene through one and five carbon atoms to the uncap- and cap-edges are considered in each case. Our calculations are based on the Green's function technique in the tight-binding approximation. The CNT surface self-energy is also obtained, analytically. The effects of contacts, various strengths of random disorder, cage type, and the bond dimerization on the conductance are investigated. Our results indicate that the appearance of conductance resonances is a manifestation of resonant states of CNT caps, which lie within the molecular HOMO-LUMO gap and consequently they change by disorder concentration. By controlling the disorder concentration, bond dimerization, and contact geometries, this kind of system can explain the extended states from the localized states. The numerical results can serve as a base for developments in designing nanoelectronic devices.

Label-free molecular detection with capped carbon nanotubes

We present a theoretical proof-of-principle analysis of a device composed of mutually faced capped carbon nanotubes for the selective detection of hardly distinguishable gas molecules in binary mixtures (methanol/formaldehyde and H2O/H2S). Selectivity arises, on one side, from I–V responses of the sensor in the presence of different gases trapped between the nanotube caps. On the other side, the different dipole moments and polarizabilities of the molecules lead to different population probabilities in the sensing region, acting as a highly selective electrostatic trapping device. The selectivity can be tuned by the bias voltage and by the gap distance between the nanotubes.

Defect Healing and Enhanced Nucleation of Carbon Nanotubes by Low-Energy Ion Bombardment

Structural defects inevitably appear during the nucleation event that determines the structure and properties of single-walled carbon nanotubes. By combining ion bombardment experiments with atomistic simulations we reveal that ion bombardment in a suitable energy range allows these defects to be healed resulting in an enhanced nucleation of the carbon nanotube cap. The enhanced growth of the nanotube cap is explained by a nonthermal ion-induced graphene network restructuring mechanism.

Intrinsically low-resistance carbon nanotube-metal contacts mediated by topological defects

Abstract

Erratum to: Intrinsically low-resistance carbon nanotube-metal contacts mediated by topological defects — CORRIGENDUM/ERRATUM

In H.S. Kim et al.1, capped carbon nanotube models were mislabeled several times. On page 3, third paragraph, line 7, “41 kΩ for cap3” should read “41kΩ for cap6.” On page 5, second paragraph, lines 8–9, “why cap1 is a better metallic contact than cap6” should read “why cap6 is a better metallic contact than cap1.” The authors regret the error. In addition, the order of authors was incorrect as published. The correct author order is as follows: Han Seul Kim, Ga In Lee, Hu Sung Kim, Jeung Ku Kang, and Yong-Hoon Kim. The publisher regrets the error.

Field emission properties of capped carbon nanotubes doped by alkali metals: a theoretical investigation

The electronic structures and field emission properties of capped CNT55 systems with or without alkali metal atom adsorption were systematically investigated by density functional theory calculation. The results indicate that the adsorption of alkali metal on the center site of a CNT tip is energetically favorable. In addition, the adsorption energies increase with the introduction of the electric field. The excessive negative charges on CNT tips make electron emittance much easier and result in a decrease in work function. Furthermore, the inducing effect by positively charged alkali metal atoms can be reasonably considered as the dominant reason for the improvement in field emission properties.

Interplay of Wetting and Elasticity in the Nucleation of Carbon Nanotubes

We use molecular dynamics and simple thermodynamic arguments to model the interaction between catalyst nanoparticles and carbon nanotube caps, and we illustrate how the competition between cap strain energy and adhesion plays a role in the lifting of these caps from the catalyst surface prior to tube elongation. Given a particular cap structure, we show that there is a lower bound on the catalyst size from which the cap can lift. This lower bound depends on the cap's spontaneous curvature and bending rigidity, as well as the catalyst binding strength, and it explains the mismatch between single-walled carbon nanotube and catalyst diameters observed in prior experiments. These findings offer new insight into the nucleation of carbon nanotubes, and they may lead to the design of catalysts that can better control nanotube structure.

Enhanced electron emission from tetrahedral amorphous carbon capped carbon nanotube core-shelled structure

Core-shelled composites have been shown to enhance material properties, notably for chemical sensing and biomolecular applications. In this comprehensive study, we show that a simple core–shell structure, made up of tetrahedral amorphous carbon (ta-C) tip-capped onto an aligned dense array of carbon nanotubes (CNT), exhibits superior electron emission properties, having a distinct enhancement in the turn-on field of < 1 V/μm without any lithography patterning. This is compared to ~ 2 V/μm for pristine CNT samples. A KrF pulsed laser deposition process was used to deposit high (> 70%) sp 3 content non-hydrogenated ta-C films of varying thickness for the tip-capping process. Combining scanning electron microscopy images and electron emission testing results, our systematic study shows that first, the thickness of the ta-C can range from 20 to 100 nm and is sufficient to achieve the enhancement effect. However, thicker coatings > 200 nm will degrade the electron emission. Second, a simple tip-capping is sufficient to achieve the enhancement without the need to encapsulate the entire carbon nanotube.

Considering the effect of different arrangements of pentagons on density of states of capped carbon nanotubes

We have used a non-equilibrium surface Green's function matching formalism combined with a tight-binding Hamiltonian to consider the effect of different arrangements of pentagon rings on localization of density of states at the tip regions of semi-infinite capped carbon nanotubes. The transfer matrixes are obtained by an iterative procedure. The results demonstrate that the positions of the peaks near Fermi energy are remarkably affected by the relative locations of pentagons. It is observed that in thin nanotubes, carbon atoms belonging two neighboring pentagon rings have significant contribution in the localized states near fermi energy. From our calculations, it turns out that the metallic or semiconducting behavior of capped nanotubes in the tip regions depends on the metallic or semiconducting nature of their nanotube stems.

Designing patterns of the isomeric carbon nanotube caps

AbstractStructure of the carbon nanotubes (CNTs) caps controls the structure of their walls (zigzag, armchair, and chiral types), i.e., the isomerism of CNTs. The latter is crucial for the result of chemical modification of CNTs that is, for the localization of the introduced functional groups and, in consequence, for the distribution of metal and non‐metal nanoparticles generated on such functionalized CNTs to form specific composites. They evoke interest as catalysts for fuel cells, anodic material in lithium batteries, photovoltaics, and others. Feasible structures of CNT caps are presented and impact of particular cap structures upon the internal and external structure of multiwall CNTs and their symmetries are discussed. Modeling of CNT caps became available by means of the involvement of the HyperChem 7 software (Molecular Mechanics, MM+ approach). Performed computations showed, that the CNT cap has to contain a main pentagon almost always located on the top of the tip. Furthermore, five pentagons provide the link between the cap and CNT tubule. Their localization in the cap determines the CNTs isomerism.

Effect of the Metal−Substrate Interaction Strength on the Growth of Single-Walled Carbon Nanotubes

Single-walled carbon nanotubes are usually synthesized by catalytic growth driven by reaction of a precursor gas over metallic nanoparticles supported on a substrate. Here we use molecular dynamics simulations (MD) with the purpose of determining how the catalyst−substrate strength of adhesion influences the structure of the carbon networks synthesized on the catalyst surface. It is found that the strength of the catalyst/substrate interaction energies defines the shape of the catalyst particle. When these energies are attractive, the nanocatalyst height decreases due to enhanced wetting and in turn favors the lifting up of carbon nanotube caps during the synthesis process. In addition, the presence of an appropriate substrate may avoid catalyst poisoning. This effect may result from repulsion forces from the substrate toward catalyzed carbon atoms, which cause carbon atoms to diffuse to upper layers, thus keeping the catalyst−substrate interface exposed to continuous catalytic activity. However, too stro...