Carbon Nanotube Junctions Research Articles

Influence of dispersion/mixture time on mechanical properties of Al–CNTs nanocomposites

The influence of dispersion/mixture time on microstructural evolution and mechanical properties of aluminum matrix nanocomposites reinforced by carbon nanotubes (Al–CNTs) were investigated in this study. The nanocomposites were produced by conventional powder metallurgy routes. The microstructure was characterized by scanning electron microscopy (SEM), electron backscattered diffraction (EBSD) and high-resolution transmission electron microscopy (HRTEM). Microstructural analysis shows clusters of carbon nanotubes (CNTs) mainly at the grain junctions, but also CNTs well dispersed and embedded in the aluminum matrix. A strong strengthening effect of the CNTs on the aluminum matrix was measured for the nanocomposites produced with 15min of dispersion/mixture time. For shorter times than 15min, it is not possible to obtain a dispersion, which can lead to a reinforcement of the aluminum matrix, while longer dispersion/mixture times cause a decrease in mechanical strength due to the damage caused by the increase in the number of defects and junctions of CNTs.

Enhancing conductivity of metallic carbon nanotube networks by transition metal adsorption

The conductivity of carbon nanotube thin films is mainly determined by carbon nanotube junctions, the resistance of which can be reduced by several different methods. We investigate electronic transport through carbon nanotube junctions in a four-terminal configuration, where two metallic single-wall carbon nanotubes are linked by a group 6 transition metal atom. The transport calculations are based on the Green's function method combined with the density-functional theory. The transition metal atom is found to enhance the transport through the junction near the Fermi level. However, the size of the nanotube affects the improvement in the conductivity. The enhancement is related to the hybridization of chromium and carbon atom orbitals, which is clearly reflected in the character of eigenstates near the Fermi level. The effects of chromium atoms and precursor molecules remaining adsorbed on the nanotubes outside the junctions are also examined.

Nonlocal vibration of Y-shaped CNT conveying nano-magnetic viscous fluid under magnetic field

This study deals with the vibration and stability analysis of a Y-shaped single-walled carbon nanotube (SWCNT) embedded in visco-Pasternak foundation and conveying nano-magnetic viscous fluid (NMF) based on nonlocal elasticity theory and Euler–Bernoulli beam model. The fluid is two-phases due to the existence of magnetic nanoparticles which its volume fraction is much little in comparison with the base fluid where the influence of 2D magnetic field is taken into account. Also, Knudsen number is used to correct the velocity profile of fluid. The Galerkin method is applied to solve the equation of motion which is obtained by employing Hamilton’s principle. The detail parametric study is conducted, focusing on the combined effects of carbon nanotube and Y-shaped junction fitted at the downstream end, fluid velocity, Knudsen number and elastic medium. The results indicate that increasing the angle between centerline of the CNT and the downstream elbows decreases stability of system.

Molecular Dynamics Study on the Generation of Single-Walled Carbon Nanotubes Junction by Direct C<sub>60</sub> Bombardment

Carbon nanotubes have been considered as promising materials for applications of nanodevices. As building blocks, carbon nanotube junctions formed by carbon-carbon covalent bonds are desired nanostructures for carbon nanotube based materials, however the formation of the junctions made of C-C bonds is still quite challenging. In this paper, a molecular dynamics study on the generation of single-walled carbon nanotubes junction by direct C60bombardment is conducted. Results show that carbon nanotube junctions can be formed by direct C60bombardment, however the structure of the formed junction is similar as the riveted structure in some case and the C60bombardment may lead to uncontrollable bonding disorder in the joints.

Phonon scattering at SWCNT–SWCNT junctions in branched carbon nanotube networks

In this research article, we analyze phonon scattering in branched single-walled carbon nanotube (SWCNT) networks with SWCNT–SWCNT T- and X- junctions using the wave packet method. Five phonon branches including the longitudinal acoustic, twisting, transverse acoustic, radial breathing, and flexural optical modes are selected to study energy reflection, ramification, and transmission through T- and X-junctions with (6,6) and (4,4) SWCNTs. The results of the simulations indicate that the diameter of SWCNTs affects phonon scattering at carbon nanotube junctions; T-junctions of (6,6) SWCNTs transmit energy more efficiently when compared to T-junctions with (4,4) SWCNTs. In addition, T-junctions of both (6,6) and (4,4) SWCNTs transmit vibrational energy more efficiently when compared to X-junctions in the same phonon frequency range—for example, in the case of the longitudinal acoustic branch, the average energy transmission at T-junctions for low-frequency phonons (lower than 6 THz) was found to be 1.8–2.4 times higher [for the case of (6.6) and (4,4) SWCNTs, respectively] when compared to the X-junctions. It is also observed that energy transmission at the T-junctions shows a dependency on the phonon group velocity with the higher group velocity phonons showing higher energy transmission; however, for the case of the X-junctions, there is little or no correlation observed between the group velocity and energy transmission indicating a complete energy redistribution of the incoming phonons at the junction. Moreover, for the SWCNT–SWCNT branched networks, the energy ramification at the T-junctions was found to be very similar to that at the X-junctions for both (6,6) and (4,4) SWCNTs indicating transverse thermal transport at the X-junctions to be as efficient as the T-junctions.

Sensing single molecules with carbon–boron-nitride nanotubes

We investigate the molecular sensing properties of carbon nanotube–boron nitride–carbon nanotube (CNT–BN–CNT) junctions.

Photochemical generation of bis-hexahapto chromium interconnects between the graphene surfaces of single-walled carbon nanotubes

The linkage of single-walled carbon nanotube junctions by the photochemistry of organometallic chromium reagents produces dramatic increases in network conductivity.

Development of nano and micro SQUIDs based on Al tunnel junctions

Superconducting quantum interference devices (SQUIDs) with nano (micro)-meter dimensions are called nano (micro)-SQUIDs. The high sensitivity for flux and position of nano (micro)-SQUIDs can be applied to detect local magnetic fields induced by vortices and the magnetization of mesoscopic superconductors. Nano-SQUIDs based on carbon-nanotube junctions and niobium weak junctions are well known. However, such nano-SQUIDs are not suitable for large-scale integrated circuits and mass production. Therefore, we employ a combination of lithography using the Niemeyer-Dolan technique and the inductively coupled plasma reactive-ion etching technique to fabricate nano-SQUIDs. Here, we report the fabrication of nano (micro)-SQUIDs based on superconducting aluminum tunnel junctions and their application for vortex formation into mesoscopic chiral superconducting Sr2RuO4[1-3].

In search of molecular scale devices: Theoretical study of linearly fused straight single-walled carbon nanotube junctions based on the pentagon/heptagon pair defects

The structural and electronic properties of topological defect of straight intramolecular junctions between two zigzag single-walled carbon nanotubes (SWCNT-IMJ) were studied theoretically. The interfacial junctions can be constructed by fusing two zigzag tubes that differ in helicities and diameters. In the present study, one segment is kept constant by using the (5,0) nanotube, while varying another segment from (6,0) to (10,0). Practically, these (5,0)//(m,0) structures are composed of one or more pentagon/heptagon pairs as “defect” functioning to fuse other perfect hexagonal lattices. It was found that, at the interfacial junctions the 5/7 pair defects induce modified rehybridization of the carbon–carbon networks, which directly affect the geometric and electronic properties. Consequently, the electronic structures are dependent on the variation of diameter and length of carbon nanotubes. Analyses of the electronic structures reveal that the HOMO and LUMO levels exhibit the even–odd “quantum size” oscillation versus varying nanotube chiralities, even with increasing length of nanotubes. The energy gap that indicates metallic or semi-conducting behavior depends on correlation between the tubes indices and tubule length. The NBO analysis has revealed that topological pair defects variation could be an important role for altering the electronic characteristics on these intramolecular junctions. By screening all structures based on their electronic structure diagrams and orbital visualization, we have found that the straight (5,0)//(7,0) junction should be suitable as a molecular rectifying diode. However, other junction structures can also be useful with appropriate tuning of the bias voltage. The outcome of this study would be of help as a knowledge base in the field of carbon nanomaterials molecular electronics.

Thermal conductance of the junction between single-walled carbon nanotubes

The thermal conductances of the carbon nanotube (CNT) junctions that would be found in a CNT aerogel are predicted using molecular dynamics simulations. At a temperature of 300 K, the thermal conductance of a perpendicular junction converges to 40 pW/K as the CNT lengths approach 100 nm. The key geometric parameter affecting the thermal conductance is the angle formed by the two CNTs. At pressures above 1 bar, the presence of a surrounding gas leads to an effective increase in the junction thermal conductance by providing a parallel path for energy flow.

Carbon nanotube gas sensor array for multiplex analyte discrimination

The lack of selectivity toward a particular analyte has always been the primary concern regarding CNT-based gas sensors. For that reason, in here we present a gas discrimination strategy that focuses on the electrode–CNT junction. The junction is shown to play a key role in the sensing mechanism. Resistive gas sensors based on horizontal CNT arrays have been fabricated using various designs and different top-contacting metals: Pt, Pd and Au. Arrays of devices have been exposed to a series of gases to monitor their resistive response. It was found for our system that the sensor response does not significantly change as a function of the device design or the available CNT sensing area in between the anchoring electrodes. On the contrary, responses to gases are observed to be specific to each sensor electrode metal. Exposure of locally passivated devices (for which distinct areas have been covered) to NO2, H2 and NH3 highlights different sensing mechanisms for each gas. Multiplex gas discrimination for room temperature can be achieved by strategically choosing the right metal/CNT combination in a complete sensor system.

Mechanical and electrical properties of electrospun PVDF/MWCNT ultrafine fibers using rotating collector.

Poly(vinylidene fluoride) (PVDF) ultrafine fibers with different proportions of multi-walled carbon nanotube (MWCNT) embedded have been fabricated using a modified electrospinning device with a rotating collector. With the increasing of MWCNT content, the β phase was noticeable enhanced, and the fibers became more elastic, which was manifested by Young's modulus decreased drastically. Furthermore, with adding the amounts of MWCNTs, the density of carbon nanotube (CNT)-CNT junctions among the fibers increased accordingly. When the MWCNT content was of 1.2 wt.%, a stable three-dimensional conducting network was formed. After this percolation threshold, the density of CNT-CNT junctions among the fibers tended to be a constant quantity, leading to a stabilized conductivity consequently. It is hoped that our results can be helpful for the fabrication of flexible devices, piezoelectric devices, force transducer, and so on.PACS81.05.Qk; 81.16.-c

Low-frequency noise in carbon nanotube network thin-film transistors

Bottom-gate single-walled carbon nanotube (SWCNT) network thin-film transistors (TFTs) are fabricated and their low-frequency noise characteristics evaluated. Clear 1/f (f: frequency) and (IDS: drain current) dependences are observed in the drain current noise power densities. The A/R (A: noise coefficient and R: resistance) was proportional to (LG: gate length). The experimental results for TFTs are discussed on the basis of Hooge’s empirical law. A simple model is proposed to explain why the A/R in our TFTs are smaller than those reported in individual CNT field-effect transistors (FETs). These results demonstrate that the 1/f noise in SWCNT TFTs originates not with the CNT–CNT junctions but rather with the CNTs themselves.

Different sensing mechanisms in single wire and mat carbon nanotubes chemical sensors

Chemical sensing properties of single wire and mat form sensor structures fabricated from the same carbon nanotubes (CNT) materials have been compared. Sensing properties of CNT sensors were evaluated upon electrical response in the presence of five vapours as acetone, acetic acid, ethanol, toluene, and water. Diverse behaviour of single wire CNT sensors was found, while the mat structures showed similar response for all the applied vapours. This indicates that the sensing mechanism of random CNT networks cannot be interpreted as a simple summation of the constituting individual CNT effects, but is associated with another robust phenomenon, localized presumably at CNT–CNT junctions, must be supposed.

Molecular dynamics investigation of carbon nanotube junctions in non-aqueous solutions

The properties of liquids in a confined environment are known to differ from those in the bulk. Extending this knowledge to geometries defined by two metallic layers in contact with the ends of a carbon nanotube is important for describing a large class of nanodevices that operate in non-aqueous environments. Here we report a series of classical molecular dynamics simulations for gold-electrode junctions in acetone, cyclohexane and N,N-dimethylformamide solutions and analyze the structure and the dynamics of the solvents in different regions of the nanojunction. The presence of the nanotube has little effect on the ordering of the solvents along its axis, while in the transversal direction deviations are observed. Importantly, the orientational dynamics of the solvents at the electrode–nanotube interface differ dramatically from that found when only the electrodes are present.

Joint measurement of current-phase relations and transport properties of hybrid junctions using a three junctions superconducting quantum interference device

We propose a scheme to measure both the current-phase relation and differential conductance dI/dV of a superconducting junction, in the normal and the superconducting states. This is done using a dc Superconducting Quantum Interference Device with two Josephson junctions in parallel with the device under investigation and three contacts. As a demonstration, we measure the current-phase relation and dI/dV of a small Josephson junction and a carbon nanotube junction. In this latter case, in a regime where the nanotube is well conducting, we show that the non-sinusoidal current phase relation we find is consistent with the theory for a weak link, using the transmission extracted from the differential conductance in the normal state. This method holds great promise for future investigations of the current-phase relation of more exotic junctions.

Structural deformation and infrared sensor response of ultralong carbon nanotubes

We investigated the changes in the first- and second-order Raman spectra of suspended crossed ultralong carbon nanotube (CNT) junctions using different laser excitation energies. The CNT junctions were in situ fabricated by growing CNTs in two perpendicular directions using chemical vapor deposition (CVD) technique. Raman spectra substantiated the structural deformation by the compression between CNTs in the junction. I–V curves of crossed CNT junctions showed the linear behavior. These crossed CNT–CNT junctions have higher current values than individual CNTs. The coexisting suspended and unsuspended CNTs on the substrate showed higher sensitivity to infrared (IR) radiation but longer response time than those with only suspended ones or CNT junctions.

An investigation on piezoresistive behavior of carbon nanotube/polymer composites: II. Positive piezoresistive effect

Due to the diversity of carbon nanotubes (CNTs), polymers, and the preparation processes of the composites, CNT-filled polymeric composites present various piezoresistive properties. One puzzling issue is the concurrence of a negative piezoresistive effect and a positive piezoresistive effect in composites with different polymer matrixes. In this paper, we present a microscopic view of the nature of the positive piezoresistive effect and its dependence on the polymer matrix types based on the model in our previous study, in which the piezoresistive behavior was tailored by a parameter—the average junction gap variation (AJGV)—describing the statistical property of the CNT conductive network. The microscopic movement process of CNTs embedded in a polymer matrix was analyzed and then the Poisson’s ratio of the polymer matrix was determined as a key factor that is in charge of negative or positive piezoresistive properties. The obstacle effect of polymer chains on the movement of CNTs was also found to be responsible for the positive piezoresistive effect as it affects the AJGV in compressive strain. Based on numerical simulations of CNT network deformation with different Poisson’s ratios and minimum junction gaps caused by the obstacle effect, the positive piezoresistive effect was found resulted from the obstacle effect on CNT junction gap variations that exceeds the initial value of the AJGV caused by the CNT network deformation, and only occur under the precondition of the polymer matrixes with a large Poisson’s ratio close to 0.5. The conclusions were then verified experimentally using composites with two kinds of polymer matrixes with significantly different Poisson’s ratios.

The Effect of Carbon Layer Thickness on the Electrical and Optical Properties of Si - CNT Junction

The Effect of Carbon Layer Thickness on the Electrical and Optical Properties of Si - CNT Junction

Barrier Height at the Graphene and Carbon Nanotube Junction

Graphene/carbon nanotube (CNT) junction barrier height was investigated using all-carbon field-effect transistor structure with graphene and single-walled CNT (SWCNT) network as source (S)/drain (D)/gate electrodes and as channel, respectively. SWCNT network channel was formed by dielectricphoresis process at the prepatterned graphene S/D electrodes. By analyzing the measured current-voltage characteristics by the diode circuit model, the Schottky barrier height at the graphene and CNT junction was found to be approximately 0.5 eV.