Probing electrical transport in individual carbon nanotubes and junctions

The electrical transport properties of individual carbon nanotubes (CNTs) and multi-terminal junctions of CNTs are investigated with a quadraprobe scanning tunneling microscope. The CNTs used in this study are made of stacked herringbone-type conical graphite sheets with a cone angle of ～20° to the tube axis, and the CNT junctions have no catalytic particles in the junction areas. The CNTs have a significantly higher resistivity than conventional CNTs with concentric walls. The straight CNTs display linear current-voltage (I-V) characteristics, indicating diffusive transport rather than ballistic transport. The structural deformation in CNTs with bends substantially increases the resistivity in comparison with that for the straight segments on the same CNTs, and the I-V curve departs slightly from linearity in curved segments. The junction area of the CNT junctions behaves like an ohmic-type scattering center with linear I-V characteristics. In addition, a gating effect has not been observed, in contrast to the case for conventional multi-walled CNT junctions. These unusual transport properties can be attributed to the enhanced inter-layer interaction in the herringbone-type CNTs.     

Flexible High‐Conductivity Carbon‐Nanotube Interconnects Made by Rolling and Printing

Applications of carbon nanotubes (CNTs) in flexible and complementary metal‐oxide‐semiconductor (CMOS)‐based electronic and energy devices are impeded due to typically low CNT areal densities, growth temperatures that are incompatible with device substrates, and challenges in large‐area alignment and interconnection. A scalable method for continuous fabrication and transfer printing of dense horizontally aligned CNT (HA‐CNT) ribbon interconnects is presented. The process combines vertically aligned CNT (VA‐CNT) growth by thermal chemical vapor deposition, a novel mechanical rolling process to transform the VA‐CNTs to HA‐CNTs, and adhesion‐controlled transfer printing without needing a carrier film. The rolling force determines the HA‐CNT packing fraction and the HA‐CNTs are processed by conventional lithography. An electrical resistivity of 2 mΩ · cm is measured for ribbons having 800‐nm thickness, while the resistivity of copper is 100 times lower, a value that exceeds most CNT assemblies made to date, and significant improvements can be made in CNT structural quality. This rolling and printing process could be scaled to full wafer areas and more complex architectures such as continuous CNT sheets and multidirectional patterns could be achieved by straightforward design of the CNT growth process and/or multiple rolling and printing sequences.     

A multi-objective optimization approach for design of blast-resistant composite laminates using carbon nanotubes

A reliable process for the design of blast-resistance composite laminates is needed. We consider here the use of carbon nanotubes (CNTs) to enhance the mechanical properties of composite interface layers. The use of CNTs not only enhances the strength of the interface but also significantly alters stress propagation in composite laminates. A simplified wave propagation simulation is developed and the optimal CNT content in the interface layer is determined using multi-objective optimization paradigms. The optimization process targets minimizing the ratio of the stress developed in the layers to the strength of that layer for all the composite laminate layers. Two optimization methods are employed to identify the optimal CNT content. A case study demonstrating the design of five-layer composite laminate subjected to a blast event is used to demonstrate the concept. It is shown that the addition of 2% and 4% CNTs by weight to the epoxy interfaces results in significant enhancement of the composite ability to resist blast.     

Synthetic Butterfly Scale Surfaces with Compliance‐Tailored Anisotropic Drop Adhesion

Many natural surfaces such as butterfly wings, beetles' backs, and rice leaves exhibit anisotropic liquid adhesion; this is of fundamental interest and is important to applications including self‐cleaning surfaces, microfluidics, and phase change energy conversion. Researchers have sought to mimic the anisotropic adhesion of butterfly wings using rigid surface textures, though natural butterfly scales are sufficiently compliant to be deflected by capillary forces exerted by drops. Here, inspired by the flexible scales of the Morpho aega butterfly wing, synthetic surfaces coated with flexible carbon nanotube (CNT) microscales with anisotropic drop adhesion properties are fabricated. The curved CNT scales are fabricated by a strain‐engineered chemical vapor deposition technique, giving ≈5000 scales of ≈10 µm thickness in a 1 cm² area. Using various designed CNT scale arrays, it is demonstrated that the anisotropy of drop roll‐off angle is influenced by the geometry, compliance, and hydrophobicity of the scales; and a maximum roll‐off anisotropy of 6.2° is achieved. These findings are supported by a model that relates the adhesion anisotropy to the scale geometry, compliance, and wettability. The electrical conductivity and mechanical robustness of the CNTs, and the ability to fabricate complex multidirectional patterns, suggest further opportunities to create engineered synthetic scale surfaces.     

A statistical-based material and process guidelines for design of carbon nanotube field-effect transistors in gigascale integrated circuits

Carbon nanotube field-effect transistors (CNFETs) show great promise as building blocks of future integrated circuits. However, synthesizing single-walled carbon nanotubes (CNTs) with accurate chirality and exact positioning control has been widely acknowledged as an exceedingly complex task. Indeed, density and chirality variations in CNT growth can compromise the reliability of CNFET-based circuits. In this paper, we present a novel statistical compact model to estimate the failure probability of CNFETs to provide some material and process guidelines for the design of CNFETs in gigascale integrated circuits. We use measured CNT spacing distributions within the framework of detailed failure analysis to demonstrate that both the CNT density and the ratio of metallic to semiconducting CNTs play dominant roles in defining the failure probability of CNFETs. Besides, it is argued that the large-scale integration of these devices within an integrated circuit will be feasible only if a specific range of CNT density with an acceptable ratio of semiconducting to metallic CNTs can be adjusted in a typical synthesis process.     

Direct Measurements of the Mechanical Strength of Carbon Nanotube–Poly(methyl methacrylate) Interfaces

Understanding the interfacial stress transfer between carbon nanotubes (CNTs) and polymer matrices is of great importance to the development of CNT‐reinforced polymer nanocomposites. In this paper, an experimental study is presented of the interfacial strength between individual double‐walled CNTs and poly(methyl methacrylate) (PMMA) using an in situ nanomechanical single‐tube pull‐out testing scheme inside a high‐resolution electron microscope. By pulling out individual tubes with different embedded lengths, this work reveals the shear lag effect on the nanotube–polymer interface and demonstrates that the effective interfacial load transfer occurs only within a certain embedded length. These results show that the CNT–PMMA interface possesses an interfacial fracture energy within 0.054–0.80 J/m² and a maximum interfacial strength within 85–372 MPa. This work is useful to better understand the local stress transfer on nanotube–polymer interfaces.     

Model-based design of low-temperature carbon nanotube synthesis via catalytic oxidation for supercapacitor application

Novel electrochemical double layer capacitors with carbon nanotube (CNT) electrode, often referred to as supercapacitors, have a potential to bridge a power and energy gap between traditional dielectric capacitors and chemical batteries. However, their future is uncertain because current fabrication technologies involve difficult-to-control post-growth manipulations of CNTs. This paper addresses this problem by introducing model-based design of low-temperature CNT synthesis that is suitable for in-situ fabrication of CNT-based supercapacitor electrode. The insight to the surface kinetics during low-temperature CNT synthesis via catalytic oxidation was obtained via coupled Molecular Dynamics and Quantum Semiempirical Hamiltonian simulations. It was determined that the presence of oxygen on the surface of catalyst increases, by several times, the time necessary for the decomposition of hydrocarbons as well as shifts the reaction zone from the surface of catalyst to the catalyst underlayer. Theoretical trends were confirmed by CNT growth experiments. A contact between conducting CNTs and zinc oxide binding layer was analyzed in detail since its properties strongly affect the performance of CNT electrode. It was demonstrated that the formed CNT-zinc oxide interface was free from unbonded oxygen atoms and/or clusters of zinc atoms and was weakly affected by defects in CNTs.     

Microstructure and mechanical properties of CNT/Ag nanocomposites fabricated by spark plasma sintering

Carbon nanotube/silver (CNT/Ag) nanocomposites include CNT volume fraction up to 10?vol.% were prepared by chemical reduction in solution followed by spark plasma sintering. Multiwalled CNTs underwent surface modifications by acid treatments, the Fourier transform infrared spectroscopy data indicated several functional groups loaded on the CNT surface by acid functionalisation. The acid-treated CNTs were sensitised and activated. Silver was deposited on the surface of the activated CNTs by chemical reduction of alkaline silver nitrate solution at room temperature. The microstructures of the prepared CNT/Ag nanocomposite powders were investigated by high-resolution scanning electron microscopy (HRSEM), transmission electron microscopy and X-ray powder diffraction analysis. The results indicated that the produced CNT/Ag nanocomposite powders have coated type morphology. The produced CNT/Ag nanocomposite powders were sintered by spark plasma sintering. It was observed from the microstructure investigations of the sintered materials by HRSEM that the CNTs were distributed in the silver matrix with good homogeneity. The hardness and the tensile properties of the produced CNT/Ag nanocomposites were measured. By increasing the volume fraction of CNTs in the silver matrix, the hardness values increased but the elongation values of the prepared CNT/Ag nanocomposites decreased. In addition, the tensile strength was increased by increasing the CNTs volume fraction up to 7.5?vol.%, but the sample composed of 10?vol.% CNT/Ag was fractured before yielding.     

Effect of density variation and non-covalent functionalization on the compressive behavior of carbon nanotube arrays

Arrays of aligned carbon nanotubes (CNTs) have been proposed for different applications, including electrochemical energy storage and shock-absorbing materials. Understanding their mechanical response, in relation to their structural characteristics, is important for tailoring the synthesis method to the different operational conditions of the material. In this paper, we grow vertically aligned CNT arrays using a thermal chemical vapor deposition system, and we study the effects of precursor flow on the structural and mechanical properties of the CNT arrays. We show that the CNT growth process is inhomogeneous along the direction of the precursor flow, resulting in varying bulk density at different points on the growth substrate. We also study the effects of non-covalent functionalization of the CNTs after growth, using surfactant and nanoparticles, to vary the effective bulk density and structural arrangement of the arrays. We find that the stiffness and peak stress of the materials increase approximately linearly with increasing bulk density.     

Force output, control of film structure, and microscale shape transfer by carbon nanotube growth under mechanical pressure

We demonstrate that a film of vertically aligned multiwall carbon nanotubes (CNTs) can exert mechanical energy as it grows, and in our experiments the average force output is approximately 0.16 nN per CNT, for CNTs having an outer diameter of 9 nm and five walls. The film thickness after a fixed growth time and the alignment of CNTs within the film decrease concomitantly with increasing pressure which is applied by placing a weight on the catalyst substrate prior to growth, and CNTs grown under applied pressure exhibit significant structural faults. The measured mechanical energy density of CNT growth is significantly less than the energies of primary steps in the CNT formation process yet, based on the film volume, is comparable to the energy density of muscle and based on the volume of CNTs is comparable to hydraulic actuators. We utilize this principle to fabricate three-dimensional structures of CNTs which conform to the shape of a microfabricated template. This technique is a catalytic analogue to micromolding of polymer and metal microstructures; it enables growth of nanostructures in arbitrarily shaped forms having sloped surfaces and nonorthogonal corners and does not require patterning of the catalyst before growth.     

Interfacial stress transfer of fiber pullout for carbon nanotubes with a composite coating

An analytical approach has been established to evaluate the interfacial stress transfer characteristics of single- and multi-walled carbon nanotubes (CNTs) with composite coatings by means of fiber pullout model. According to the present model, the effects of several parameters such as coating thickness, layer numbers and dimension of CNTs on interfacial stress transfers were investigated and analyzed. The results suggested that the maximum interfacial shear stress occurred at the pullout end of CNTs and decreased with increasing coating thickness as well as CNT wall thickness (layer numbers). Moreover, the distribution of the interfacial shear and coating axial stress along the CNT length was found to be largely affected by the friction coefficient in the interface between the CNT and the coating layer.     

Aligned carbon nanotube stacks by water-assisted selective etching

Well-aligned, high-purity carbon nanotube (CNT) stacks of up to 10 layers fabricated in one batch process have been formed by water-assisted selective etching of carbon atoms. Etching takes place at the CNT caps as well as at the interface between CNTs and metal catalyst particles. This simple process generates high-purity CNTs and opens the CNT ends by removing the nanotube caps. High-resolution transmission electron microscopy indicates that the process does not damage CNT wall structures. A mechanism for stacked growth of CNT layers is proposed.     

Nanomechanics of imperfectly straight single walled carbon nanotubes under axial compression by using molecular dynamics simulation

The buckling characteristics of several curved forms of single walled carbon nanotubes (SWCNTs) were studied in this work via molecular dynamics simulation method. The structural morphology of the CNT was modified to induce curvature along the tube axis. The nature of mechanical properties of these classes of CNTs under compression deviates from the ideal ‘perfectly straight’ CNTs. We found that the inclusion of curvature along the tube axis can significantly impact the performance of the carbon nanotube under compression. These curvilinear CNTs, when combined with other CNTs to form a bundle, will have a greater weakening effect on the mechanical performance of the CNT bundle.     

Numerical simulation of spin coating processes involving functionalised Carbon nanotube suspensions

This paper reports the numerical simulation of spin coating for functionalised Carbon Nanotube (CNT) suspensions. Spin coating is a process commonly used to deposit uniform thin films onto flat substrates by means of high rotation velocity and centrifugal force. The functionalised CNTs modelled in this study were chemically treated in a way such that aggregation was prevented through electrostatic repulsion between CNTs. The functionalised CNTs in the semi-dilute suspensions can be modelled as rigid fibres with their orientation dictated by the flow of the solvent. The evolution of CNT orientations was simulated using a pre-averaged kinetic theory with an appropriate rotary diffusion coefficient accounting for randomising events. A Natural Element strategy with an updated Lagrangian framework was implemented to solve the free-surface problem involving large domain deformation and to avoid numerical problems associated with Finite Element modelling. The model reported herein couples micro-scale CNT orientation with the macroscopic suspension kinematics and it offers important insights in relation to the final properties of spin-coated CNT films as well as the processing behaviour of CNT suspensions.     

Hydrothermal Coating of Patterned Carbon Nanotube Forest for Structured Lithium‐Ion Battery Electrodes

Controlling the arrangement and interface of nanoparticles is essential to achieve good transfer of charge, heat, or mechanical load. This is particularly challenging in systems requiring hybrid nanoparticle mixtures such as combinations of organic and inorganic materials. This work presents a process to coat vertically aligned carbon nanotube (CNT) forests with metal oxide nanoparticles using microwave‐assisted hydrothermal synthesis. Hydrothermal processes normally damage delicate CNT forests, which is addressed here by a combination of lithographic patterning, transfer printing, and reduction of the synthesis time. This process is applied for the fabrication of structured Li‐ion battery (LIB) electrodes where the aligned CNTs provide a straight electron transport path through the electrode and the hydrothermal coating process is used to coat the CNTs with conversion anode materials for LIBs. These nanoparticles are anchored on the surface of the CNTs and batteries fabricated following this process show a fourfold longer cyclability. Finally, this process is used to create thick electrodes (350 µm) with a gravimetric capacity of over 900 mAh g^?1.     

Template synthesis and growth mechanism of metal nanowire/carbon nanotube heterojunctions

Copper (Cu) and Cobalt (Co) with remarkable difference in the catalytic activity for the growth of carbon nanotubes (CNTs) have been used to prepare metal-nanowire/CNT heterojunctions. The ordered arrays of Cu nanowire/CNT (CuNW/CNT) and Co nanowire/CNT (CoNW/CNT) heterojunctions were prepared by combining electrochemical deposition and chemical vapor deposition. The interfaces between CNTs and Cu or Co nanowires have been examined and compared. At the interface of CuNW/CNT heterojunction, the tip of CuNW is encapsulated by carbon material (named "cap") and connected the CNT consisting of amorphous carbon (a-C). Two-segment CuNW/amorphous CNT (CuNW/a-CNT) hybrid nanostructure was obtained for the CuNW/CNT heterojunctions due to low catalytic activity of Cu. It is also interesting that a hollow gap was observed between the "cap" and the CuNW. By contrast with the case of Cu, multi-walled CNT (MWCNT) was achieved and no hollow gap was observed at the interface of CoNW/CNT heterojunctions. Three-segment CoNW/MWCNT/a-CNT hybrid nanostructure was observed for the CoNW/CNT heterojunctions because of high catalytic activity of Co. Because no stable copper carbides are observed, we infer that the growth mechanism of CuNW/CNT heterojunctions is different from that of CoNW/CNT. Possible growth models of CuNW/CNT and CoNW/CNT heterojunctions are proposed based on experimental results, respectively.     

Catalytic-free growth of VACNTs for energy harvesting

Abstract

The present study introduces a process to grow micro-honeycomb (µ-HC) vertically aligned carbon nanotubes (VACNTs) using thermal chemical vapor deposition technique. Methane is used as a source of carbon and hydrogen gas as a reducing agent. Where, the fabricated µ-HC structure reported in literature involves complex synthesis process and requires a catalyst layer, the novelty of the process used here lies in the fact that no catalyst layer is used for the growth of CNT network, rather copper foil is used as a substrate. The in-situ cracking of CNTs due to water treatment leads to the formation of µ-HC CNT network, which is confirmed by Raman spectroscopy. Further scanning electron microscopy analysis shows that the length of developed µ-HC CNT is ～5?µm. Hexagonal µ-HC network shows more than 94% absorption in UV-Vis-NIR wavelength region. The designed process provides high-yield with a low-cost synthesis of vertically aligned CNTs having 3?D microarchitecture. The fabricated CNT network can be used as an electrode for supercapacitor, as an active layer in a photovoltaic cell and most of the energy harvesting devices.     

Single‐Carbon‐Nanotube Manipulations and Devices Based on Macroscale Anthracene Flakes

Because of the outstanding mechanical and electrical properties of carbon nanotubes (CNTs), a CNT‐based torsion pendulum is demonstrated to show great potential in nano‐electromechanical systems. It is also expected to achieve various manipulations for further characterization and increase device sensitivity using ultrlong CNTs and macroscale moving parts. However, the reported top‐down method limits the CNT performance and device size by introducing inevitable contamination and destruction, which greatly hinders the development of single‐molecule devices. Here, a bottom‐up method is introduced to fabricate heterostructures of anthracene flakes (AFs) and suspended CNTs, providing a nondamaging CNT mechanical measurement before further applications, especially for the twisting behavior, and providing a controllable and clean transfer method to fabricate CNT‐based electrical devices under ambient conditions. Based on the unique geometry of CNT/AF heterostructures, various complex manipulations of single‐CNT devices are conducted to investigate CNT mechanical properties and prompt novel applications of similar structures in nanotechnology. The AF‐decorated CNTs show high Young's modulus (≈1 TPa) and tensile strength (≈100 GPa), and can be considered as the finest and strongest torsional springs. CNT‐based torsion balance enables to measure fN‐level forces and the torsional spring constant is two orders of magnitude lower than previously reported values.     

Interface effects on the viscoelastic characteristics of carbon nanotube polymer matrix composites

It is generally known that load transfer from the polymer matrix to carbon nanotubes (CNTs) can be greatly hindered due to the pristine CNT surface condition. This imperfect condition can have a profound influence on the effectiveness of CNT reinforcement. In order to address this issue in the context of viscoelastic response, an effective medium theory is first presented, and then applied to study the effect of interfacial sliding on the time-dependent creep, stress relaxation, strain-rate sensitivity, and storage and loss moduli of a multi-walled CNT/polypropylene nanocomposite. We show that, without accounting for the imperfect load transfer at the interface, the predicted creep compliances are too stiff, but with the introduction of a weakened interface, the measured creep curves at various CNT loading can be well captured. Both stress relaxation and stress–strain relations are also found to greatly depend on the interface condition. Under low-frequency harmonic loading our calculations also reveal that the interface condition is further weakened and that a larger interface sliding parameter is required to reflect the measured storage and tangent moduli. We conclude that the viscoelastic characteristics of a CNT nanocomposite are very sensitive to the interface condition, and that continued improvement in surface functionalization is necessary to realize the full potential of CNT reinforcement.     

Percolation model of reinforcement efficiency for carbon nanotubes dispersed in thermoplastics

Considerable experimental work on carbon nanotube-reinforced composites has shown that the reinforcement efficiency of carbon nanotubes (CNTs) becomes lower than the theoretical expectation when CNT content reaches a critical value. This critical volume fraction (percolation threshold) is considered related to the formation of percolating network. In this work, a percolation model is proposed to describe the observed sharp decrease in the reinforcement efficiency of multiwalled CNTs (MWCNTs) dispersed in thermoplastics when the CNT content exceeds the percolation threshold. The percolation threshold is estimated via a numerical simulation of randomly curved CNTs according to the statistics on geometrical features of real CNTs. The percolation model, integrated into the Halpin–Tsai equations, is verified using the experimental data of various thermoplastic composites reinforced with MWCNTs. The developed mechanical model achieves a good agreement with the measured moduli of nanocomposites, and demonstrates an excellent prediction capability over a wide range of CNT content.