Molecular junctions by joining single-walled carbon nanotubes

Healing of broken multiwalled carbon nanotubes using very low energy electrons in SEM: a route toward complete recovery

We report the healing of electrically broken multiwalled carbon nanotubes (MWNTs) using very low energy electrons (3-10 keV) in scanning electron microscopy (SEM). Current-induced breakdown caused by Joule heating has been achieved by applying suitably high voltages. The broken tubes were examined and exposed to electrons of 3-10 keV in situ in SEM with careful maneuvering of the electron beam at the broken site, which results in the mechanical joining of the tube. Electrical recovery of the same tube has been confirmed by performing the current-voltage measurements after joining. This easy approach is directly applicable for the repairing of carbon nanotubes incorporated in ready devices, such as in on-chip horizontal interconnects or on-tip probing applications, such as in scanning tunneling microscopy.

Electronic transport properties of carbon nanotoroids

We investigate the electronic transport properties of carbon nanotori covalently connected to external electrodes made up of carbon nanotubes of various chiralities. The study is based on computing ballistic transport characteristics within the framework of Green's function theory using a simple π-orbital tight-binding model. The calculations focus on the effect of the relative angle made by the electrodes as they are placed at different positions along the nanoring. The conductance behavior is found to depend on the details of the atomic structure of the torus but also on the positions of the electrodes. Our findings are rationalized using an elementary quantum mechanical interference model, which reproduces well the main features of the numerical data.

The creation of nanojunctions

This review describes recent progress in creation of nanojunctions between individual nanoobjects. The accomplishments of various strategies used for nanojunction creation are highlighted and the corresponding challenges are discussed. The possible ongoing development for the creation of device-oriented nanojunctions is speculated upon.

Effect of topological defects on graphene geometry and stability

The effect of two basic topological defects, mitosis and the Stone-Wales defect, is studied in the graphene structure. The topological rules of the curvatures due to the occurrence of the defects in different arrangements are determined. Despite the fact that the causes and the probability of these topological defects are not known today, this theoretical work studies the distortions caused by the defects geometry and stability of the graphene structure.

Signal transmission, conversion and multiplication by polar molecules confined in nanochannels

The mechanism of signal transmission, conversion and multiplication at molecular level has been of great interest lately, due to its wide applications in nanoscience and nanotechnology. The interferences between authentic signals and thermal noises at the nanoscale make it difficult for molecular signal transduction. Here we review some of our recent progress on the signal transduction mediated by water and other polar molecules confined in nanochannels, such as Y-shaped carbon nanotubes. We also explore possible future directions in this emerging field. These studies on molecular signal conduction might have significance in future designs and applications of nanoscale electronic devices, and might also provide useful insights for a better understanding of signal conduction in both physical and biological systems.

Transport in carbon nanotube field-effect transistors tuned using low energy electron beam exposure

We have studied the effect of low energy (30 keV) electron beam exposure on carbon nanotube field-effect transistors, using an electron beam lithography system to provide spatially controlled dosage. We show that reversible tuning of the transport behavior is possible when a backgate potential is applied during exposure. n-type behavior can be obtained by electron beam exposure of a device with positive gate bias, while ambipolar behavior can be obtained via negative gate bias. The observed transport behavior is relatively stable in time. We propose possible mechanisms for the observed phenomena and suggest directions for further research.

Measuring point defect density in individual carbon nanotubes using polarization-dependent X-ray microscopy

The presence of defects in carbon nanotubes strongly modifies their electrical, mechanical, and chemical properties. It was long thought undesirable, but recent experiments have shown that introduction of structural defects using ion or electron irradiation can lead to novel nanodevices. We demonstrate a method for detecting and quantifying point defect density in individual carbon nanotubes (CNTs) based on measuring the polarization dependence (linear dichroism) of the C 1s --> pi* transition at specific locations along individual CNTs with a scanning transmission X-ray microscope (STXM). We show that STXM can be used to probe defect density in individual CNTs with high spatial resolution. The quantitative relationship between ion dose, nanotube diameter, and defect density was explored by purposely irradiating selected sections of nanotubes with kiloelectronvolt (keV) Ga(+) ions. Our results establish polarization-dependent X-ray microscopy as a new and very powerful characterization technique for carbon nanotubes and other anisotropic nanostructures.

Controlling edge morphology in graphene layers using electron irradiation: from sharp atomic edges to coalesced layers forming loops

Recent experimental reports indicate that Joule heating can atomically sharpen the edges of chemical vapor deposition grown graphitic nanoribbons. The absence or presence of loops between adjacent layers in the annealed materials is the topic of a growing debate that this Letter aims to put to rest. We offer a rationale explaining why loops do form if Joule heating is used alone, and why adjacent nanoribbon layers do not coalesce when Joule heating is applied after high-energy electrons first irradiate the sample. Our work, based on large-scale quantum molecular dynamics and electronic-transport calculations, shows that vacancies on adjacent graphene sheets, created by electron irradiation, inhibit the formation of edge loops.

Charge transport through single molecules, quantum dots and quantum wires

We review recent progress in the theoretical description of correlation and quantum fluctuation phenomena in charge transport through single molecules, quantum dots and quantum wires. Various physical phenomena are addressed, relating to cotunneling, pair-tunneling, adiabatic quantum pumping, charge and spin fluctuations, and inhomogeneous Luttinger liquids. We review theoretical many-body methods to treat correlation effects, quantum fluctuations, non-equilibrium physics, and the time evolution into the stationary state of complex nanoelectronic systems.

Defect-driven room-temperature coalescence of double-walled carbon nanotubes

Room-temperature (RT) coalescence of double-walled carbon nanotubes has been observed for the first time. A combined pre-treatment of localized electron irradiation, Joule heating, and electromigration leads to the formation of large vacancy clusters, which can survive for tens of seconds during surface reconstruction. The dangling bonds of the edge atoms are highly reactive and thus promote the coalescence even at RT.

Morphology controlled surface-assisted self-assembled microtube junctions and dendrites of metal free porphyrin-based semiconductor

Solution-vapor annealing of drop-cast thin films of meso-5,10,15,20-tetra-n-decylporphyrin H(2)T(C(10)H(21))(4)P deposited on SiO(2) substrate and quartz leads to the formation of well-defined self-assemblies. Their self-assembling properties in n-hexane vapor and chloroform vapor were comparatively investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) technique, and IR and UV-vis spectroscopy. Intermolecular pi-pi interaction in cooperation with the van der Waals interaction of metal free porphyrin and solvent-solute interaction leads to the formation of microleaves and microtube dendrites in n-hexane vapor and chloroform vapor, respectively. Electronic absorption spectroscopic data on the self-assembled microstructures reveal the J-aggregate nature in both the microleaves and microtube dendrites. However, the difference in the shift of the Soret and Q bands for the two kinds of aggregates relative to corresponding solution absorption bands indicates the dependence of the solvent-porphyrin molecular interaction during the annealing self-assembly process, which counterbalances the intermolecular interactions, particularly the hydrophobic interaction between side chains. IR and XRD results clearly reveal the higher molecular ordering nature of microtube dendrites than that of microleaves, further confirming the effect of the solvent on tuning the intermolecular interaction and in turn the molecular packing mode in aggregates of porphryin compounds. The present results appear to represent the first example of orderly micrometer-sized tube junctions and dendrites of porphyrin prepared through a self-assembly process, providing an effective and new method toward the synthesis of complicated nanotubular structures. In addition, micrometer-sized leaves and tube dendrites were revealed to show good semiconductor features. Highly reproducible and sensitive gas response characteristics have also been observed in these microstructures.

Water-mediated signal multiplication with Y-shaped carbon nanotubes

Molecular scale signal conversion and multiplication is of particular importance in many physical and biological applications, such as molecular switches, nano-gates, biosensors, and various neural systems. Unfortunately, little is currently known regarding the signal processing at the molecular level, partly due to the significant noises arising from the thermal fluctuations and interferences between branch signals. Here, we use molecular dynamics simulations to show that a signal at the single-electron level can be converted and multiplied into 2 or more signals by water chains confined in a narrow Y-shaped nanochannel. This remarkable transduction capability of molecular signal by Y-shaped nanochannel is found to be attributable to the surprisingly strong dipole-induced ordering of such water chains, such that the concerted water orientations in the 2 branches of the Y-shaped nanotubes can be modulated by the water orientation in the main channel. The response to the switching of the charge signal is very rapid, from a few nanoseconds to a few hundred nanoseconds. Furthermore, simulations with various water models, including TIP3P, TIP4P, and SPC/E, show that the transduction capability of the Y-shaped carbon nanotubes is very robust at room temperature, with the interference between branch signals negligible.

Selective generation of single-walled carbon nanotubes with metallic, semiconducting and other unique electronic properties

As-synthesized single-walled carbon nanotubes (SWNTs) are mixtures of semiconducting and metallic species and separation of the two is of crucial importance for many applications. In this article, the methods employed for the enrichment of semiconducting and metallic SWNTs are presented, along with possible procedures to prepare either of the species selectively. Equally important are the methods for chirality selection. The discovery of metal-semiconductor transitions in SWNTs induced by interaction with electron donor and acceptor molecules is not only of academic interest, but may also find applications. Synthesis of Y-junction SWNTs with unique electronic properties at the junction is yet to be fully accomplished.

Ultrathin graphitic structures and carbon nanotubes in a purified synthetic graphite

A new class of carbon structure is reported, which consists of microscale graphitic shells bounded by curved and faceted planes containing two to five layers. These structures were originally found in a commercial graphite produced by the Acheson process, followed by a purification treatment. The particles, which could be several hundreds of nanometres in size, were frequently decorated with nanoscale carbon particles, or short nanotubes. In some cases, nanotubes were found to be seamlessly connected to the thin shells, indicating that the formation of the shells and that of the nanotubes are intimately connected. The structures are believed to form during a purification process which involves passing an electric current through the graphite in the presence of a reactive gas. In support of this, it is shown that similar particles can be produced in a standard carbon arc apparatus. With their extremely thin graphene walls and high surface areas, the new structures may have a range of useful properties.

Structural transformations in graphene studied with high spatial and temporal resolution

Graphene has remarkable electronic properties, such as ballistic transport and quantum Hall effects, and has also been used as a support for samples in high-resolution transmission electron microscopy and as a transparent electrode in photovoltaic devices. There is now a demand for techniques that can manipulate the structural and physical properties of graphene, in conjunction with the facility to monitor the changes in situ with atomic precision. Here, we show that irradiation with an 80 kV electron beam can selectively remove monolayers in few-layer graphene sheets by means of electron-beam-induced sputtering. Aberration-corrected, low-voltage, high-resolution transmission electron microscopy with sub-ångström resolution is used to examine the structural reconstruction occurring at the single atomic level. We find preferential termination for graphene layers along the zigzag orientation for large hole sizes. The temporal resolution can also be reduced to 80 ms, enabling real-time observation of the reconstruction of carbon atoms during the sputtering process. We also report electron-beam-induced rapid displacement of monolayers, fast elastic distortions and flexible bending at the edges of graphene sheets. These results reveal how energy transfer from the electron beam to few-layer graphene sheets leads to unique structural transformations.

Enhancement of tunneling density of states at a junction of three Luttinger liquid wires

We study the tunneling density of states (TDOS) for a junction of three Tomonaga-Luttinger liquid wires. We show that there are fixed points which allow for the enhancement of the TDOS, which is unusual for Luttinger liquids. The distance from the junction over which this enhancement occurs is of the order of x=v/(2omega), where v is the plasmon velocity and omega is the bias frequency. Beyond this distance, the TDOS crosses over to the standard bulk value independent of the fixed point describing the junction. This finite range of distances opens up the possibility of experimentally probing the enhancement in each wire individually.

The computational design of junctions between carbon nanotubes and graphene nanoribbons

Using first-principles density functional theory calculations, various junction models constructed from different carbon nanotube and graphene nanoribbon units via covalent linkage have been envisioned. These models consist of linear, T- and H-shaped junctions within the connection modes between carbon nanotube and graphene nanoribbon units. The electronic transport properties of different junctions have been systematically investigated by using the non-equilibrium Green's function. The simulation results suggested that the proposed models are promising for future applications in novel nanoelectronics.

Molecular dynamics study of damage production in single-walled carbon nanotubes irradiated by various ion species

The irradiation-induced damage production in single-walled carbon nanotubes (CNTs) by several types of ions is investigated using the molecular dynamics method with analytical potentials. We found that, in the incident energy range 25-1000 eV, the bonding action or the chemical effect of the ions could significantly enhance their damage capabilities to CNTs relative to that of non-bonding ions, and the dependence of damage yield on the ion mass is no longer monotonic. This is contrary to the previous viewpoint that the chemical aspect of the interaction is of no importance to the ion-induced defect production mechanism in CNTs. The bonding interaction of ions with CNTs also increases their implantation probabilities into CNTs. The chemical erosion effect of incident ions remarkably intensifies the sideward recoil from CNTs under irradiation while the downward recoil is still governed by the physical collision effect.

Guiding electrical current in nanotube circuits using structural defects: a step forward in nanoelectronics

Electrical current could be efficiently guided in 2D nanotube networks by introducing specific topological defects within the periodic framework. Using semiempirical transport calculations coupled with Landauer-Buttiker formalism of quantum transport in multiterminal nanoscale systems, we provide a detailed analysis of the processes governing the atomic-scale design of nanotube circuits. We found that when defects are introduced as patches in specific sites, they act as bouncing centers that reinject electrons along specific paths, via a wave reflection process. This type of defects can be incorporated while preserving the 3-fold connectivity of each carbon atom embedded within the graphitic lattice. Our findings open up a new way to explore bottom-up design, at the nanometer scale, of complex nanotube circuits which could be extended to 3D nanosystems and applied in the fabrication of nanoelectronic devices.

Field emission from a selected multiwall carbon nanotube

The electron field emission characteristics of individual multiwalled carbon nanotubes were investigated by a piezoelectric nanomanipulation system operating inside a scanning electron microscopy chamber. The experimental set-up ensures a precise evaluation of the geometric parameters (multiwalled carbon nanotube length and diameter and anode-cathode separation) of the field emission system. For several multiwalled carbon nanotubes, reproducible and quite stable emission current behaviour was obtained, with a dependence on the applied voltage well described by a series resistance modified Fowler-Nordheim model. A turn-on field of ∼30 V µm(-1) and a field enhancement factor of around 100 at a cathode-anode distance of the order of 1 µm were evaluated. Finally, the effect of selective electron beam irradiation on the nanotube field emission capabilities was extensively investigated.

Growth of Y-shaped Carbon Nanofibers from Ethanol Flames

Y-shaped carbon nanofibers as a multi-branched carbon nanostructure have potential applications in electronic devices. In this article, we report that several types of Y-shaped carbon nanofibers are obtained from ethanol flames. These Y-shaped carbon nanofibers have different morphologies. According to our experimental results, the growth mechanism of Y-shaped carbon nanofibers has been discussed and a possible growth model of Y-shaped carbon nanofibers has been proposed.

Quantum wire hybridized with a single-level impurity

We have studied low-temperature properties of interacting electrons in a one-dimensional quantum wire (Luttinger liquid) side-hybridized with a single-level impurity. The hybridization induces a backscattering of electrons in the wire which strongly affects its low-energy properties. Using a one-loop renormalization group approach valid for a weak electron-electron interaction, we have calculated a transmission coefficient through the wire, T(epsilon), and a local density of states, nu(epsilon) at low energies epsilon. In particular, we have found that the antiresonance in T(epsilon) has a generalized Breit-Wigner shape with the effective width Gamma(epsilon) which diverges at the Fermi level.

Oscillatory metallic behaviour of carbon nanotube superlattices-an ab initio study

We study the structural, electronic and optical properties of the (n,n)/(2n,0);n = 3 and 6 superlattices of carbon nanotubes (CNs) by employing the first principles pseudo-potential method within density functional theory (DFT) in the generalized gradient approximation (GGA). There occur pentagon-heptagon defects along the circumference of the heterojunction of these superlattices. The role of the length of the superlattice unit cell on the electronic and optical properties has been investigated. The curvature effects on the various properties are also discussed. The heterojunctions of the small diameter n(3,3)/n(6,0) superlattices which possess a threefold rotational symmetry exhibit an oscillatory behaviour in terms of the fundamental energy bandgaps which vanish whenever the integer n is a multiple of 3. These results indicate that a similar oscillatory behaviour in the fundamental gap energy having a periodicity of 6 may be observed in the case of the large diameter n(6,6)/n(12,0) superlattices whose heterojunctions reveal a sixfold symmetry. The system energy of the 3(6,6)/3(12,0) superlattice shows a minimum. The electronic structure and optical absorption of a superlattice are quite different from those of its constituent carbon nanotubes. The present results obtained after employing all the s-, p- and d-orbitals of the atoms (although the d-orbital contributions are quite small) are quite different from the findings of earlier workers who have employed a phenomenological tight-binding formulation considering only one pi orbital or four orbitals. We find that most of the states are extended resonance states and are quite delocalized in contrast to the earlier finding of the occurrence of the completely localized states in sections of the constituent nanotubes. The metallic superlattices exhibit a high density of states (DOS) at the Fermi level (E(F)). For the large diameter n(6,6)/n(12,0) superlattices, the electron energy gap vanishes for n = 1 and 2 but increases up to a maximum value of 0.344 eV for n = 3 and decreases thereafter for larger n, a result which is in disagreement with earlier workers. These new facts have not been reported in the literature so far.

Plumbing carbon nanotubes

Since their discovery, the possibility of connecting carbon nanotubes together like water pipes has been an intriguing prospect for these hollow nanostructures. The serial joining of carbon nanotubes in a controlled manner offers a promising approach for the bottom-up engineering of nanotube structures--from simply increasing their aspect ratio to making integrated carbon nanotube devices. To date, however, there have been few reports of the joining of two different carbon nanotubes. Here we demonstrate that a Joule heating process, and associated electro-migration effects, can be used to connect two carbon nanotubes that have the same (or similar) diameters. More generally, with the assistance of a tungsten metal particle, this technique can be used to seamlessly join any two carbon nanotubes--regardless of their diameters--to form new nanotube structures.

Carbon nanotubes

Nanotubes, the last in the focus of scientists in a series of 'all carbon' materials discovered over the last several decades are the most interesting and have the greatest potential. This review aims at presenting in a concise manner the considerable amount of knowledge accumulated since the discovery of this amazing form of solid carbon, particularly during the last 15 years. The topics include methods of synthesis, mathematical description, characterization by Raman spectroscopy, most important properties and applications. Problems related to the determination of CNT properties, as well as difficulties regarding their applications, in particular scaling, which would lead to their utilization, are outlined.

Engineering of nanostructured carbon materials with electron or ion beams

Irradiating solids with energetic particles is usually thought to introduce disorder, normally an undesirable phenomenon. But recent experiments on electron or ion irradiation of various nanostructures demonstrate that it can have beneficial effects and that electron or ion beams may be used to tailor the structure and properties of nanosystems with high precision. Moreover, in many cases irradiation can lead to self-organization or self-assembly in nanostructures. In this review we survey recent advances in the rapidly evolving area of irradiation effects in nanostructured materials, with particular emphasis on carbon systems because of their technological importance and the unique ability of graphitic networks to reconstruct under irradiation. We dwell not only on the physics behind irradiation of nanostructures but also on the technical applicability of irradiation for nanoengineering of carbon and other systems.

Production and characterization of coaxial nanotube junctions and networks of CNx/CNT

Novel coaxial structures consisting of nitrogen-doped carbon nanotube (MWNTs-CNx) cores with external concentric shells of pure carbon were produced by the pyrolysis of toluene over Fe-coated MWNTs-CNx. These materials were thoroughly characterized by SEM, HRTEM, X-ray diffraction, and TGA; a possible growth scenario for their formation is also proposed. In addition, these coaxial structures were able to form 2D and 3D covalent networks that mainly exhibited T-, Y-, and on-type morphologies. The two-step technique presented here could be further developed to fully control the growth of these new coaxial structures, study of individual junctions, and it could be used to create periodic nanotube networks, in which the heterocable structure could find applications in nanoelectronics.

A Magnetism-Assisted Chemical Vapor Deposition Method To Produce Branched or Iron-Encapsulated Carbon Nanotubes

A magnetism-assisted chemical vapor deposition method was developed to synthesize branched or iron-encapsulated carbon nanotubes. In the process, the external magnetic field can promote the coalescence or division of the catalyst particles, causing the formation of branched or encapsulated nanostructures. This finding will extend the understanding of the chemical vapor deposition method in a magnetic field and promote the applications of branched or encapsulated nanostructures.

Sub-10 nm device fabrication in a transmission electron microscope

We show that a high-resolution transmission electron microscope can be used to fabricate metal nanostructures and devices on insulating membranes by nanosculpting metal films. Fabricated devices include nanogaps, nanodiscs, nanorings, nanochannels, and nanowires with tailored curvatures and multi-terminal nanogap devices with nanoislands or nanoholes between the terminals. The high resolution, geometrical flexibility, and yield make this fabrication method attractive for many applications including nanoelectronics and nanofluidics.

Covalent 2D and 3D networks from 1D nanostructures: designing new materials

We show extensive theoretical studies related to the generation and characterization of 2D and 3D ordered networks using 1D units that are connected covalently. We experimentally created multi-terminal junctions containing 1D carbon blocks in order to study the most common morphologies and branched structures that could be used in the theoretical design of network models. We found that the mechanical and electronic characteristics of ordered networks based on carbon nanotubes (ON-CNTs) are dominated by their specific super-architecture (hexagonal, cubic, square, and diamond-type). We show that charges follow specific paths through the nodes of the multi-terminal systems, which could result in complex integrated nanoelectronic circuits. The 3D architectures reveal their ability to support extremely high unidirectional stress when their mechanical properties are studied. In addition, these networks are shown to perform better than standard carbon aerogels because of their low mass densities, continuous porosities, and high surface areas.

Mechanical properties of super honeycomb structures based on carbon nanotubes

As a result of repeating carbon nanotube Y junctions periodically, super honeycomb structures have recently been proposed. In this paper, the mechanical properties of these structures are investigated by using the shell model of the finite element method. The study shows that the super honeycomb structures have great flexibility and outstanding capability in force transferring; the network configuration increases the ductility of the nanomaterials. Furthermore, it can be concluded that the equivalent tensile modulus and Poisson's ratio of super structures are dependent on the number of junctions in the width direction.

Molecular dynamics simulation of single wall carbon nanotubes polymerization under compression

Single wall carbon nanotubes (SWCNTs) often aggregate into bundles of hundreds of weakly interacting tubes. Their cross-polymerization opens new possibilities for the creation of new super-hard materials. New mechanical and electronic properties are expected from these condensed structures, as well as novel potential applications. Previous theoretical results presented geometric modifications involving changes in the radial section of the compressed tubes as the explanation to the experimental measurements of structural changes during tube compression. We report here results from molecular dynamics simulations of the SWCNTs polymerization for small diameter arm chair tubes under compression. Hydrostatic and piston-type compression of SWCNTs have been simulated for different temperatures and rates of compression. Our results indicate that large diameter tubes (10,10) are unlike to polymerize while small diameter ones (around 5 A) polymerize even at room temperature. Other interesting results are the observation of the appearance of spontaneous scroll-like structures and also the so-called tubulane motifs, which were predicted in the literature more than a decade ago.