Carbon Nanotube Junctions Research Articles

Electrical transport through carbon nanotube junctions created by mechanical manipulation

Using an atomic force microscope we have created nanotube junctions such as buckles and crossings within individual single-wall metallic carbon nanotubes connected to metallic electrodes. The electronic transport properties of these manipulated structures show that they form electronic tunnel junctions. The conductance shows power-law behavior as a function of bias voltage and temperature, which can be well modeled by a Luttinger liquid model for tunneling between two nanotube segments separated by the manipulated junction.

Manipulation and Imaging of Individual Single-Walled Carbon Nanotubes with an Atomic Force Microscope

The tip of an atomic force microscope is used to create carbon nanotube junctions by changing the position and shape of individual single-walled carbon nanotubes on a SiO2 surface. With this manipulation technique, we are able to bend, buckle, cross (see Figure), and break nanotubes, and to unravel a nanotube “crop circle” into a single tube. Tapping-mode atomic force microscopy measurements of the height of a carbon nanotube on the surface always yield values smaller than the nanotube diameter. Variation of the scan parameters shows that this is due to a tapping deformation by the tip. The tapping deformation of manipulated nanotube crossings and buckles is discussed as well.

Influence of structural defects on Fresnel projection microscope images of carbon nanotubes: Implications for the characterization of nanoscale devices

We report first quantum theoretical simulations of Fresnel type electronic diffraction images of carbon nanotubes junctions. These three-dimensional simulations are used to interpret fine features of experimental Fresnel projection microscope (FPM) images. The relevance of the FPM to observe nanodevices made from carbon nanotubes is then discussed.

Relation between transmission rates and the wave functions in carbon nanotube junctions

Electron transmission and wave functions through junctions with a pair of a pentagonal defect and a heptagonal defect connecting two metallic carbon nanotubes are analyzed by the analytical calculation with the effective-mass equation. The energy region $|E|<{E}_{c}$ is considered where the channel number is kept to two. Close relation between the transmission rate and the wave function is found; the transmission rate is given by the inverse squared absolute value of the wave function. The dependence of the transmission rates on the energy and on the size of the junction is clearly explained by the nature of the wave function. Though the wave function and the transmission rate calculated by the tight-binding model agree well with the corresponding analytical results by the effective-mass approximation, the discrepancy becomes considerable when $|E|\ensuremath{\simeq}{E}_{c}.$ To study the origin of this discrepancy, an efficient numerical calculation method is developed with a generalized transfer matrix for the tight-binding model. Their numerical results are compared with the corresponding analytical ones and the results show that the origin of the discrepancy comes from the evanescent waves with the longest decay length in the tube parts.

Transport through crossed nanotubes

In order to study the electronic properties of single-walled carbon nanotube junctions we have fabricated several devices consisting of two crossed nanotubes with electrical leads attached to each end of each nanotube. We correlate the properties of the junctions with the properties of the individual nanotubes: metal–metal, metal–semiconductor, and semiconductor–semiconductor junctions are all formed. We find that metal–metal SWNT junctions exhibit surprisingly high conductances of 0.1– 0.2 e 2/h . Semiconductor–semiconductor junctions also show significant linear response conductance. Metal–semiconductor junctions behave as p-type Schottky diodes. All the junction types can reliably pass currents of hundreds of nanoamps.

Degeneracy and repulsion between bands of periodic carbon nanotube junctions

The band structures of the periodic nanotube junctions are investigated by the use of effective mass theory (k·p approximation) and the tight binding model. The periodic junctions are constructed by the periodic introduction of defect pairs, consisting of a pentagonal defect and a heptagonal defect, into the carbon nanotube. We treat the periodic junctions whose unit cell is composed by two kinds of metallic nanotubes. The discussed energy region is near the undoped Fermi level where the channel number is kept to 2, so there are two bands. The degeneracy and repulsion between the two bands are determined only from symmetries.

Band Structures of Periodic Carbon Nanotube Junctions and Their Symmetries Analyzed by the Effective Mass Approximation

The band structures of the periodic nanotube junctions are investigated by the effective mass theory and the tight binding model. The periodic junctions are constructed by introducing pairs of a pentagonal defect and a heptagonal defect periodically in the carbon nanotube. We treat the periodic junctions whose unit cell is composed by two kinds of metallic nanotubes with almost same radii, the ratio of which is between 0.7 and 1 . The discussed energy region is near the undoped Fermi level where the channel number is kept to two, so there are two bands. The energy bands are expressed with closed analytical forms by the effective mass theory with some assumptions, and they coincide well with the numerical results by the tight binding model. Differences between the two methods are also discussed. Origin of correspondence between the band structures and the phason pattern discussed in Phys. Rev. B {\bf 53}, 2114, is clarified. The width of the gap and the band are in inverse proportion to the length of the unit cell, which is the sum of the lengths measured along the tube axis in each tube part and along 'radial' direction in the junction part. The degeneracy and repulsion between the two bands are determined only from symmetries.

Analysis of quantum conductance of carbon nanotube junctions by the effective-mass approximation

The electron transport through the nanotube junctions which connect the different metallic nanotubes by a pair of a pentagonal defect and a heptagonal defect is investigated by Landauer's formula and the effective mass approximation. From our previous calculations based on the tight binding model, it has been known that the conductance is determined almost only by two parameters,i.e., the energy in the unit of the onset energy of more than two channels and the ratio of the radii of the two nanotubes. The conductance is calculated again by the effective mass theory in this paper and a simple analytical form of the conductance is obtained considering a special boundary conditions of the envelop wavefunctions. The two scaling parameters appear naturally in this treatment. The results by this formula coincide fairly well with those of the tight binding model. The physical origin of the scaling law is clarified by this approach.

Carbon Nanotube Based Molecular Electronic Devices

Complex three-point junctions of single-walled carbon nanotubes are proposed as building blocks of nanoscale electronic devices. Both T- and Y-junctions, made up of tubes with differing diameters and chiralities, are studied as prototypes. All the proposed complex junctions have been found to be local minima of the total energy on relaxation with a generalized tight-binding molecular dynamics scheme.

Fullerene-derived molecular electronic devices

The carbon nanotube junctions have recently emerged as excellent candidates for use as the building blocks in the formation of nanoscale electronic devices. While the simple joint of two dissimilar tubes can be generated by the introduction of a pair of heptagon-pentagon defects in an otherwise perfect hexagonal graphene sheet, more complex joints require other mechanisms. In this work we explore structural and electronic properties of complex three-point junctions of carbon nanotubes using a generalized tight-binding molecular-dynamics scheme.

Carbon Nanotubes: Molecular Electronic Components

ABSTRACT: The carbon nanotube junctions have recently emerged as excellent candidates for use as the building blocks in the formation of nanoscale molecular electronic networks. While the simple joint of two dissimilar tubes can be generated by the introduction of a pair of heptagon‐pentagon defects in an otherwise perfect hexagonal graphene sheet, more complex joints require other mechanisms. In this work we explore structural characteristics of complex 3‐g38 point junctions of carbon nanotubes using a generalized tight‐binding molecular‐dynamics scheme. The study of pi‐electron local densities of states (LDOS) of these junctions reveal many interesting features, most prominent among them being the defect‐induced states in the gap.

Conductance of Carbon Nanotube Junctions in Magnetic Fields

The conductance of a junction connecting two carbon nanotubes of differing diameter is calculated in magnetic fields perpendicular to the tube axis. The relative importance of mixing between different valleys exhibits a characteristic change depending on the effective strength of the magnetic field in the two tubes. The conductance depends only on the component of the magnetic field in the direction of the pentagonal and heptagonal rings.

Electronic transport in carbon nanotube junctions

Features of the ballistic electron transport through a network of carbon nanotubes provide an important basis for the design of future ‘nanotube devices’. As the first step toward the elucidation of these features, we studied reflection and transmission of electron waves at a number of junctions of two metallic carbon nanotubes with different diameters. Geometrical structures of the junctions are determined by the relative arrangement of a pair of a five membered ring and a seven membered ring on the development map. The transmission coefficients are calculated by a tight binding scattering wave method for more than eight hundreds of different junctions, and a remarkable scaling law was found. The conductance σ calculated by the Landauer's formula depends only on i R , where R and i are the circumference of the thinner tube and the length of the junction, respectively. For the larger i R , σ depends as ( i R ) −3 .