Carbon Nanotube Junctions Research Articles

Functionalized horizontally aligned CNT array and random CNT network for CO2 sensing

Horizontally aligned and density-controlled single-wall carbon nanotubes (CNT) represent attractive building blocks for nanoelectronics. In this paper, horizontally aligned CNT arrays and random CNT network are synthesized by thermal CVD at 925 °C in an ethanol atmosphere. With appropriate functionalization applied to the aligned CNTs, a high sensitivity of 8.48% for ambient CO2 gas concentration of 500 ppm is achieved. In addition, these aligned CNT array sensors show much faster response and recovery time than a random CNT network. Moreover, good selectivity against NO2 and NH3, and good repeatability are demonstrated. These results pave the way for a deeper understanding of the physical and electrical properties of single-wall CNT and inter-tube junctions, which could be helpful in designing and optimizing as-grown single-wall CNT for gas sensor and other nanoelectronic devices.

Pyrolytic-carbon coating in carbon nanotube foams for better performance in supercapacitors

Abstract Nowadays, the wide-spread adoption of supercapacitors has been hindered by their inferior energy density to that of batteries. Here we report the use of our pyrolytic-carbon-coated carbon nanotube foams as lightweight, compressible, porous, and highly conductive current collectors in supercapacitors, which are infiltrated with chemically-reduced graphene oxide and later compressed via mechanical and capillary forces to generate the active electrodes. The pyrolytic carbon coatings, introduced by chemical vapor infiltration, wrap around the CNT junctions and increase the surface roughness. When active materials are infiltrated, the pyrolytic-carbon coatings help prevent the π-stacking, enlarge the accessible surface area, and increase the electrical conductivity of the scaffold. Our best-performing device offers 48% and 57% higher gravimetric energy and power density, 14% and 23% higher volumetric energy and power density, respectively, and two times higher knee frequency, than the device with commercial current collectors, while the “ true-performance metrics” are strictly followed in our measurements. We have further clarified the solution resistance, charge transfer resistance/capacitance, double-layer capacitance, and Warburg resistance in our system via comprehensive impedance analysis, which will shed light on the design and optimization of similar systems.

Fingerprinting seamless single-walled carbon nanotube junctions via the migration of encapsulated N2molecules from bottom to top: are arrays of VA-SWNTs continuous?

Structure control such as diameter changes along single-walled carbon nanotubes (SWNTs) can be achieved in arrays of vertically aligned (VA-) SWNTs by switching the feedstock during growth. The local nature of the macroscopic transition from one diameter to another is then questioned as one can either envisage seamless transitions or discontinuous individual SWNTs. Here, we demonstrate that encapsulated molecules can serve as markers to doubtlessly identify seamless interconnections in macroscopic samples. A migration of nitrogen molecules inside the continuous SWNTs is observed using bulk scale measurements on double-layered SWNT arrays synthesized from different carbon/nitrogen feedstocks. The existence of N2 molecules at the top of the SWNT arrays proves that there are continuous SWNTs throughout the double layered arrays with different diameters.

An empirical force field for the simulation of the vibrational spectroscopy of carbon nanomaterials

An empirical force field for carbon based upon the Murrell-Mottram potential is developed for the calculation of the vibrational frequencies of carbon nanomaterials. The potential is reparameterised using data from density functional theory calculations through a Monte-Carlo hessian-matching approach, and when used in conjunction with the empirical bond polarisability model provides an accurate description of the non-resonant Raman spectroscopy of carbon nanotubes and graphene. With the availability of analytical first and second derivatives, the computational cost of evaluating harmonic vibrational frequencies is a fraction of the cost of corresponding quantum chemical calculations, and makes the accurate atomistic vibrational analysis of systems with thousands of atoms possible. Subsequently, the non-resonant Raman spectroscopy of carbon nanotubes and graphene, including the role of defects and carbon nanotube junctions is explored.

Estimation of adsorption energy for water molecules on a multi-walled carbon nanotube thin film by measuring electric resistance

Gas sensors based on carbon nanotube (CNT) films have attracted attention owing to their low power consumption. For further development of these sensors, we need to understand the surface interaction of the films with gas molecules. In our previous research, we investigated the influence of water molecules on the electrical conductance of multi-walled CNT films and explained this phenomenon using a two-layer adsorption model. This work motivated us to measure the adsorption energy of CNT-H2O. In this study, we focused on the first-layer adsorption and investigated the sheet resistance to water vapor pressure at various temperatures using the transmission line method (TLM). The results were fitted to Langmuir adsorption model and the adsorption equilibrium constant was determined. The temperature dependence of the sheet resistance followed a model of fluctuation induced tunneling (FIT), in which the energy barrier at the CNT junction is regarded as the main factor influencing the electrical conductance of the CNT film. The sheet resistance and equilibrium constant decreased as temperature increased. This result was consistent with the adsorption phenomenon. Finally, the adsorption energy was determined to be 0.22–0.31 eV, which is larger than the previously calculated value. It was also reported that the adsorption energy of the gas molecules in the interstitial site between two carbon nanotubes was larger than that on the CNT surface. These results indicate that the CNT junction plays a key role in the detection of gas molecules.

Preparation and Performance of TiO2-ZnO/CNT Hetero-Nanostructures Applied to Photodegradation of Organic Dye

Water pollution by organic compounds is one of the major challenges faced by industries that use dyeing processes. Thus, some methods were developed for degrading dyes in wastewaters, including heterogeneous photocatalysis by semiconductor oxides. However, these oxides have limited photocatalytic activity due to the fast recombination of photogenerated electron-hole pairs. The aim of this study is the use of a carbon nanotube (CNT) and TiO2-ZnO oxide junction from modified sol-gel method to promote the degradation of organic dye through the photocatalytic activity of these nanocomposites. TiO2-ZnO/CNT nanocomposites were studied by X-ray diffraction (XRD), transmission electron microscope (TEM), scanning electron microscope (SEM) and photocatalytic decomposition of organic dyes. The results of photocatalysis show up to 60% the efficiency of the samples in the removal of dye.

Molecular dynamics study on thermal transport at carbon nanotube interface junctions: Effects of mechanical force and chemical functionalization

Classical molecular dynamics (MD) simulations are performed in this work to investigate the interfacial thermal transport across stacked carbon nanotube (CNT) junctions. Various approaches are implemented to increase the thermal conductance (G) between CNTs. Effects of crossing angle, contact area, bonding strength, external force and hydrocarbon functionalization on G are investigated. A remarkably increase of thermal conductance is achieved by connecting two CNTs with hydrocarbon chain linkers CH2. The predicted G changes from 229 pW/K without linkers to 4901 pW/K with an optimized linker number, which increases by a factor of 20. Meanwhile, thermal conductance is found to increase monotonically with contact area but decrease inversely with crossing angle. The van der Waals (vdW) bonding strength has similar effects with applied external force on thermal conductance, both of which facilitate the interfacial thermal transport by enhancing the contact pressures. Synthesized relationship of internal coupling strength, external force and final separation distance between CNTs is explored to illustrate the variation of thermal conductance and intermolecular potential energy.

Thermal conductance of carbon nanotube contacts: Molecular dynamics simulations and general description of the contact conductance

The contact conductance of carbon nanotube (CNT) junctions is the key factor that controls the collective heat transfer through CNT networks or CNT-based materials. An improved understanding of the dependence of the intertube conductance on the contact structure and local environment is needed for predictive computational modeling or theoretical description of the effective thermal conductivity of CNT materials. To investigate the effect of local structure on the thermal conductance across CNT-CNT contact regions, nonequilibrium molecular dynamics (MD) simulations are performed for different intertube contact configurations (parallel fully or partially overlapping CNTs and CNTs crossing each other at different angles) and local structural environments characteristic of CNT network materials. The results of MD simulations predict a stronger CNT length dependence present over a broader range of lengths than has been previously reported and suggest that the effect of neighboring junctions on the conductance of CNT-CNT junctions is weak and only present when the CNTs that make up the junctions are within the range of direct van der Waals interaction with each other. A detailed analysis of the results obtained for a diverse range of intertube contact configurations reveals a nonlinear dependence of the conductance on the contact area (or number of interatomic intertube interactions) and suggests larger contributions to the conductance from areas of the contact where the density of interatomic intertube interactions is smaller. An empirical relation accounting for these observations and expressing the conductance of an arbitrary contact configuration through the total number of interatomic intertube interactions and the average number of interatomic intertube interactions per atom in the contact region is proposed. The empirical relation is found to provide a good quantitative description of the contact conductance for various CNT configurations investigated in the MD simulations and is suitable for incorporation into mesoscopic models capable of predicting the effective thermal transport properties of CNT materials.

Highly deformable thermal interface materials enabled by covalently-bonded carbon nanotubes

The exceptional thermal conductivity of individual carbon nanotubes have rarely materialized in bulk materials mainly due to the large thermal contact resistance between carbon nanotubes (CNTs). This can be attributed to weak van der Waals bonding at the CNT junctions where the outstanding phonon transport along the strong covalent bonding on the graphitic layer is largely impeded. In bulk materials, however, it has been extremely difficult to achieve covalently bonded junctions between CNTs. Here we report polymer composites consisting of sponge-like CNT structures whose junctions between CNTs are covalently bonded, resulting in a high thermal conductivity and a low Young’s modulus, which are hard to achieve at the same time. The low modulus allows thermal interface material (TIM) to easily deform to make the surfaces of heat sink/source fully in contact, which is essential for TIM. Our facile scalable preparation process also makes our composite very attractive as TIM as well as provides insight to better utilize high thermal conductivity of CNTs.

Radial collapse of carbon nanotubes for conductivity optimized polymer composites

The optimization of the electronic conduction of carbon nanotube polymer composites is studied by tuning the radial geometry of the carbon nanotubes in a compression cycle. We have investigated the structural evolution of multi-walled carbon nanotubes in a polyamide matrix as a function of applied high pressure. Combining high resolution electron microscopy and small angle neutron scattering experiments, we conclude that the nanotube radial cross-section is irreversibly deformed following applied pressures up to 5 GPa. Studying highly percolated composites we observe that the sample resistivity drastically decreases with pressure up to about 2 GPa with no further change up to the maximum 5 GPa applied pressure. An important hysteresis is observed upon decompression which leads to an enhanced electrical conductivity of the composite in all the studied compression cycles with maximum pressures ranging from 1 to 5 GPa. Modelling the radial collapse of single-walled carbon nanotubes shows that the modified radial geometry can considerably improve the electronic transport properties in contacted carbon nanotube junctions. Our results open opportunities for engineering nanotube composites by controlling the radial collapse.

Role of inter-tube coupling and quantum interference on electrical transport in carbon nanotube junctions

Due to excellent transport properties, Carbon nanotubes (CNTs) show a lot of promise in sensor and interconnect technology. However, recent studies indicate that the conductance in CNT/CNT junctions are strongly affected by the morphology and orientation between the tubes. For proper utilization of such junctions in the development of CNT based technology, it is essential to study the electronic properties of such junctions. This work presents a theoretical study of the electrical transport properties of metallic Carbon nanotube homo-junctions. The study focuses on discerning the role of inter-tube interactions, quantum interference and scattering on the transport properties on junctions between identical tubes. The electronic structure and transport calculations are conducted with an Extended Hückel Theory-Non Equilibrium Green's Function based model. The calculations indicate conductance to be varying with a changing crossing angle, with maximum conductance corresponding to lattice registry, i.e. parallel configuration between the two tubes. Further calculations for such parallel configurations indicate onset of short and long range oscillations in conductance with respect to changing overlap length. These oscillations are attributed to inter-tube coupling effects owing to changing π orbital overlap, carrier scattering and quantum interference of the incident, transmitted and reflected waves at the inter-tube junction.

Flexible infrared detectors based on p-n junctions of multi-walled carbon nanotubes.

Different types of multi-walled carbon nanotubes (CNTs), synthesized by chemical vapor deposition, are used to fabricate infrared (IR) detectors on flexible substrates based on CNT p-n junctions. It is found that this kind of detector is sensitive to infrared signals with a power density as low as 90 μW mm(-2) even at room temperature. Besides, unlike other devices, the detector with this unique structure can be bent for 100 cycles without any damage and its functionality does not degenerate once it recovers to the initial state. The results give a good reference for developing efficient, low-cost, and flexible IR detectors.

Mechanical behavior of carbon nanotube and graphene junction as a building block for 3D carbon nanostructures

The incorporation of defects in junction area of 1D and 2D carbon nanostructures has a major impact on properties of their 3D structures. In the present study, molecular dynamics simulation is utilized to examine the mechanical behavior of graphene sheet (GS) in carbon nanotube (CNT)-GS junctions. The tensile load was applied along the GS in connection with CNTs of different chiralities. The adaptive intermolecular reactive empirical bond order potential was chosen to model C-C interactions. It provided a reliable model for CNT, GS and their junctions. The results revealed that the connection of CNT to the GS with a hole could improve the mechanical properties of defective GS, which appeared to be independent of CNT type. It was found that the high strength C-C bonds postpone the crack propagation and motivates new crack nucleation. When a hole or CNT placed on the GS, it caused stress concentration, exactly along a line on its side. The lower mechanical properties were consequently associated with crack nucleation and propagation on both sides in a way that cracks encountered each other during the failure; while, the cracks in pristine GS propagate parallel to each other and could not encounter each other.

Optoelectronic Switching of a Carbon Nanotube Chiral Junction Imaged with Nanometer Spatial Resolution

Chiral junctions of carbon nanotubes have the potential of serving as optically or electrically controllable switches. To investigate optoelectronic tuning of a chiral junction, we stamp carbon nanotubes onto a transparent gold surface and locate a tube with a semiconducting-metallic junction. We image topography, laser absorption at 532 nm, and measure I-V curves of the junction with nanometer spatial resolution. The bandgaps on both sides of the junction depend on the applied tip field (Stark effect), so the semiconducting-metallic nature of the junction can be tuned by varying the electric field from the STM tip. Although absolute field values can only be estimated because of the unknown tip geometry, the bandgap shifts are larger than expected from the tip field alone, so optical rectification of the laser and carrier generation by the laser must also affect the bandgap switching of the chiral junction.

Electronic and electrical properties of silicon nanowire-single wall carbon nanotube junction as a nanoelectronic switch

In this work, we performed first principles calculations based on self-consistent charge density functional tight-binding to investigate how the overlapping length of two inter-shell structure effect on the electronic and electrical properties of silicon nanowire-single wall carbon nanotube junction. By computing of density of state, current–voltage curve, conductance and transmission probability, it is found that the binding energy, charge transferring between two structures and conductance changes with the overlapping length. Our results show that charge transferring between the two structures is not affected by overlapping length. Also, we found that the current increases by increasing the overlapping length. At finally, the conductance increase from the zero to the high amount of overlapping length as an off and on states of a nanometer switch.

Manipulating the magnetic state of a carbon nanotube Josephson junction using the superconducting phase

The magnetic state of a quantum dot attached to superconducting leads is experimentally shown to be controlled by the superconducting phase difference across the dot. This is done by probing the relation between the Josephson current and the superconducting phase difference of a carbon nanotube junction whose Kondo energy and superconducting gap are of comparable size. It exhibits distinctively anharmonic behavior, revealing a phase mediated singlet to doublet transition. We obtain an excellent quantitative agreement with numerically exact quantum Monte Carlo calculations. This provides strong support that we indeed observed the finite temperature signatures of the phase controlled zero temperature level-crossing transition originating from strong local electronic correlations.

Nanoscale Azide Polymer Functionalization: A Robust Solution for Suppressing the Carbon Nanotube–Polymer Matrix Thermal Interface Resistance

The large thermal resistance across the carbon nanotube (CNT)–polymer matrix interface is a limiting factor for achieving polymer composites with high thermal conductivities. Using equilibrium molecular dynamics simulations we show that an azide-terminated aromatic polymer HLK5 (C22H25O3N3) functionalized onto the CNT sidewall can efficiently decrease the thermal resistance between the nanotube and different types of polymer matrices (polystyrene, epoxy, and polyethylene). The HLK5 functionalization can also significantly decrease the CNT–CNT junction resistance. Compared with hydroxyl and octane functionalizations, the HLK5 one alters less the high intrinsic CNT thermal conductivity at the same surface coverage ratio. By revealing the important role played by the atomistic van der Waals interactions in attaining these key results, our study brings a new perspective in the nanoscale design of advanced CNT–polymer materials.

Solutions for Clamped Adhesively Bonded Single Lap Joint With Movement of Support End and Its Application to a Carbon Nanotube Junction in Tension

This article presents analytical solutions for a clamped-clamped adhesively bonded single lap joint with movement of supports and its application to studying the failure mechanism of carbon nanotube junctions in a tensile test. In the analytical model, the interface shear and normal stresses, movement of one support end, geometric nonlinearity, and the contact stresses between two cylinders are considered. Analytical solutions are derived for a clamped-clamped single lap joint with movement of one support end first, and then geometrically nonlinear finite element analysis is conducted to verify the present analytical solutions. An equivalent two-dimensional model is presented for a junction self-assembled by two carbon nanotubes, and the failure mechanism of the carbon nanotube junction is then studied by using the present analytical solutions. Structural performance of single lap joints with movement of support ends as boundary conditions is also investigated.

Local Electrical Investigations of Nitric Acid Treatment Effects on Carbon Nanotube Networks

Nitric acid is a commonly used chemical in preparing and functionalizing carbon nanotube (CNT) networks. It is also used to enhance the conductivity of CNT-based networks or films, which is usually attributed to p-type doping by NO3– ions. We used local electrical characterization methods based on atomic force microscopy to elucidate the effects of nitric acid treatments on sparse and random CNT networks. IV characteristics of CNT network devices before and after nitric acid treatments are obtained along with local electrical characteristics with both scanning gate microscopy and electrostatic force microscopy. Our results show both p-type doping of CNTs and reduction of CNT–CNT junction resistances after nitric acid treatments.

Solution-mediated selective nanosoldering of carbon nanotube junctions for improved device performance.

As-grown randomly aligned networks of carbon nanotubes (CNTs) invariably suffer from limited transport properties due to high resistance at the crossed junctions between CNTs. In this work, Joule heating of the highly resistive CNT junctions is carried out in the presence of a spin-coated layer of a suitable chemical precursor. The heating triggers thermal decomposition of the chemical precursor, tris(dibenzylideneacetone)dipalladium (Pd2(dba)3), and causes local deposition of Pd nanoparticles at the CNT junctions, thereby improving the on/off current ratio and mobility of CNT network devices by an average factor of ∼6. This process can be conducted either in air or under vacuum depending on the characteristics of the precursor species. The solution-mediated nanosoldering process is simple, fast, scalable with manufacturing techniques, and extendable to the nanodeposition of a wide variety of materials.