Cover Picture: Fabrication and Electrical and Mechanical Properties of Carbon Nanotube Interconnections (Adv. Funct. Mater. 11/2005)

The fabrication of carbon nanotube (CNT) structures, including simple tube–tube connections, crossed junctions, T‐junctions, zigzag structures, and even nanotube networks, has been achieved by cutting and soldering CNTs using electron‐beam‐induced deposition of amorphous carbon (a‐C), as detailed in the work of Peng and co‐workers on p. 1825. These CNT structures have been constructed with a high degree of control, and it is found that the electric conductance and mechanical strength of the junctions can be improved by the deposition of a‐C and by increasing the contact area of the junctions. Individual carbon nanotubes (CNTs) have been cut, manipulated, and soldered via electron‐beam‐induced deposition of amorphous carbon (a‐C) and using a scanning tunneling microscope inside a transmission electron microscope. All CNT structures, including simple tube–tube connections, crossed junctions, T‐junctions, zigzag structures, and even nanotube networks, have been successfully constructed with a high degree of control, and their electrical and mechanical properties have been measured in situ inside the transmission electron microscope. It is found that multiple CNTs may be readily soldered together with moderate junction resistance and excellent mechanical resilience and strength, and the junction resistance may be further reduced by current‐induced graphitization of the deposited a‐C on the junction.     

Fabrication and Electrical and Mechanical Properties of Carbon Nanotube Interconnections

Individual carbon nanotubes (CNTs) have been cut, manipulated, and soldered via electron‐beam‐induced deposition of amorphous carbon (a‐C) and using a scanning tunneling microscope inside a transmission electron microscope. All CNT structures, including simple tube–tube connections, crossed junctions, T‐junctions, zigzag structures, and even nanotube networks, have been successfully constructed with a high degree of control, and their electrical and mechanical properties have been measured in situ inside the transmission electron microscope. It is found that multiple CNTs may be readily soldered together with moderate junction resistance and excellent mechanical resilience and strength, and the junction resistance may be further reduced by current‐induced graphitization of the deposited a‐C on the junction.     

Tailoring the Mechanical Properties of High‐Aspect‐Ratio Carbon Nanotube Arrays using Amorphous Silicon Carbide Coatings

The porous nature of carbon nanotube (CNT) arrays allows for the unique opportunity to tailor their mechanical response by the infiltration and deposition of nanoscale conformal coatings. Here, we fabricate novel photo‐lithographically defined CNT pillars that are conformally coated with amorphous silicon carbide (a‐SiC) to strengthen the interlocking of individual CNTs at junctions using low pressure chemical vapor deposition (LPCVD). We further quantify the mechanical response by performing flat‐punch nanoindentation measurements on coated CNT pillars with various high‐aspect‐ratios. We discovered new mechanical failure modes of coated CNT pillars, such as “bamboo” and brittle‐like composite rupture as coating thickness increases. Furthermore, a significant increase in strength and modulus is achieved. For CNT pillars with high aspect ratio (1:10) and coating thickness of 21.4 nm, the compressive strength increases by an order of magnitude of 3, towards 1.8 GPa (from below 1 MPa for uncoated CNT pillars) and the elastic modulus increases towards 125 GPa. These results show that our coated CNT pillars, which can serve as vertical interconnects and 3D super‐capacitors, can be transformed into robust high‐aspect‐ratio 3D‐micro architectures with semiconductor device compatible processes.     

衬底温度对制备出CNT膜的影响

在陶瓷衬底上通过磁控溅射方法镀上金属钛层,用含铁杂质的氧化硅对钛层进行抛光,通过微波等离子体化学气相沉积（MPCVD）法在不同的温度下短时间里制备出CNT膜。利用扫描电子显微镜、拉曼光谱,X射线衍射,分析了薄膜的结构和表面形貌。仔细研究不同温度下制备的CNT膜,得出衬底温度400℃时制备的碳膜是以非晶碳为主,600℃时置备的碳膜是良好的碳纳米管膜,800℃制备的碳纳米管膜的缺陷变得很多,以碳纳米链为主。最后得出了温度对催化活性有很大影响的结论。     

原位电子束诱导沉积制备碳纳米结构

以多壁碳纳米管为基本材料,利用电子束诱导沉积的方法进行了纳米结构加工、修饰研究.电子束诱导沉积实现了二个碳纳米管端部之间的牢固焊接,实现了纳米材料间的几乎无损伤连接.原位测量表明多壁碳纳米管间的连接为欧姆接触.进一步对碳纳米管施加外电场可以使端部碳原子间的π键打开,外部碳原子经电子束诱导沉积在碳纳米管的端部,并定向生长成非晶态碳纳米线.由于碳纳米管和纳米线结合处的.键作为绝缘界面,形成了电子输运的势垒,所得到的碳纳米管-纳米线复合结构具有整流特性.利用电子显微镜进行纳米材料的结构加工、修饰,具有选择位置精确、可实时监测、对纳米材料几乎无损伤、重复性和可靠性高,以及加工尺度可人为控制的特点.     

Ultra‐Lightweight and Highly Adaptive All‐Carbon Elastic Conductors with Stable Electrical Resistance

The rapid development of wearable electronics needs flexible conductive materials that have stable electrical properties, good mechanical reliability, and broad environmental tolerance. Herein, ultralow‐density all‐carbon conductors that show excellent elasticity and high electrical stability when subjected to bending, stretching, and compression at high strains, which are superior to previously reported elastic conductors, are demonstrated. These all‐carbon conductors are fabricated from carbon nanotube forms, with their nanotube joints being selectively welded by amorphous carbon. The joint‐welded foams have a robust 3D nanotube network with fixed nodes and mobile nanotube segments, and thus have excellent electrical and mechanical stabilities. They can readily scale up, presenting a new type of nonmetal elastic conductor for many possible applications.     

Carbon‐Nanotube‐Templated,Sputter‐Deposited,Flexible Superconducting NbN Nanowire Yarns

Flexible superconducting yarns consisting of sputter‐deposited NbN nanowires on highly aligned carbon nanotube (CNT) array sheets are reported. In the microscopic view, the NbN nanowires are formed on top of individual CNT fibrils, and the superconductivity property of the twist‐spun NbN–CNT yarn system is comparable to that of a typical NbN thin film on a normal solid substrate. Because of its intrinsic porosity, the system exhibits superior mechanical flexibility with a small bending radius. It also remains a superconducting state even when subjected to severe mechanical deformations, primarily due to the proximity superconductivity through carbon nanotube bundles. The results demonstrate the possibility of fabricating flexible superconducting yarns in a conventional thin‐film deposition process, using ultraflexible free‐standing CNT sheets as a template. In addition, preliminary tests on reducing the normal‐state resistance toward superconducting cable applications are presented.     

Wide Range Control of Microstructure and Mechanical Properties of Carbon Nanotube Forests: A Comparison Between Fixed and Floating Catalyst CVD Techniques

Vertically aligned carbon nanotube (CNT) forests may be used as miniature springs, compliant thermal interfaces, and shock absorbers, and for these and other applications it is vital to understand how to engineer their mechanical properties. Herein is investigated how the diameter and packing density within CNT forests govern their deformation behavior, structural stiffness, and elastic energy absorption properties. The mechanical behavior of low‐density CNT forests grown by fixed catalyst CVD methods and high‐density CNT forests grown by a floating catalyst CVD method are studied by in situ SEM compression testing and tribometer measurements of force‐displacement relationships. Low‐density and small‐diameter CNT columns (fixed catalyst) exhibit large plastic deformation and can be pre‐deformed to act as springs within a specified elastic range, whereas high‐density and large‐diameter CNT columns (floating catalyst) exhibit significant elastic recovery after deformation. In this work the energy absorption capacity of CNT forests is tuned over three orders of magnitude and it is shown that CNT forest density can be tuned over a range of conventional foam materials, but corresponding stiffness is ～10× higher. It is proposed that the elastic behavior of CNT forests is analogous to open‐cell foams and a simple model is presented. It is also shown that this model can be useful as a first‐order design tool to establish design guidelines for the mechanical properties of CNT forests and selection of the appropriate synthesis method.     

Aligned Carbon Nanotube–Based Flexible Gel Substrates for Engineering Biohybrid Tissue Actuators

Muscle‐based biohybrid actuators have generated significant interest as the future of biorobotics but so far they move without having much control over their actuation behavior. Integration of microelectrodes into the backbone of these systems may enable guidance during their motion and allow precise control over these actuators with specific activation patterns. Here, this challenge is addressed by developing aligned carbon nanotube (CNT) forest microelectrode arrays and incorporating them into scaffolds for cell stimulation. Aligned CNTs are successfully embedded into flexible and biocompatible hydrogels exhibiting excellent anisotropic electrical conductivity. Bioactuators are then engineered by culturing cardiomyocytes on the CNT microelectrode‐integrated hydrogel constructs. The resulting cardiac tissue shows homogeneous cell organization with improved cell‐to‐cell coupling and maturation, which is directly related to the contractile force of muscle tissue. This centimeter‐scale bioactuator has excellent mechanical integrity, embedded microelectrodes, and is capable of spontaneous actuation behavior. Furthermore, it is demonstrated that a biohybrid machine can be controlled by an external electrical field provided by the integrated CNT microelectrode arrays. In addition, due to the anisotropic electrical conductivity of the electrodes provided by aligned CNTs, significantly different excitation thresholds are observed in different configurations such as the ones with electrical fields applied in directions parallel versus perpendicular to the CNT alignment.     

Impact of Morphology on Printed Contact Performance in Carbon Nanotube Thin‐Film Transistors

Silver nanoparticles (NPs) are the most widely used conductive material throughout the printed electronics space due to their high conductivity and low cost. However, when interfacing with other prominent printed materials, such as semiconducting carbon nanotubes (CNTs) in thin‐film transistors (TFTs), silver is suboptimal when compared to more expensive or less conductive materials. Consequently, there would be significant value to improving the interface of printed silver to CNT films. In this work, the impact of nanostructure morphology on the electrical properties of printed silver and nanotube junctions in CNT‐TFTs is investigated. Three distinct silver morphologies (NPs, nanoflakes – NFs, and nanowires – NWs) are explored with top‐ and bottom‐contact configurations for each. The NF morphology in a top‐contact configuration is found to yield the best electrical interface to CNTs, resulting in an average contact resistance of 1.2 MΩ ? µm. Beyond electrical performance, several trade‐offs in morphology selection are revealed, including print resolution and process temperature. While NF inks produce the best interfaces, NP inks produce the smallest features, and NW inks are compatible with low processing temperatures (<80 °C). These results outline the trade‐offs between silver contact morphologies in CNT‐TFTs and show that contact morphology selection can be tailored for specific applications.     

Structure and Electrical Properties of Ni Nanowire/Multiwalled‐Carbon Nanotube/Amorphous Carbon Nanotube Heterojunctions

A one‐dimensional heterojunction is fabricated and characterized. This heterojunction comprises a Ni nanowire, a multiwalled carbon nanotube (MWCNT), and an amorphous carbon nanotube (a‐CNT). The three components are in an end‐to‐end configuration, and form two MWCNT contacts, namely a Ni/MWCNT and an MWCNT/a‐CNT contact. The interfacial structures of the two contacts show that multiple outer walls in the MWCNT simultaneously contact the Ni nanowire and the a‐CNT, and can simultaneously participate in electrical transport. By investigating the electrical‐transport properties of the heterojunctions, the two contacts to the MWCNT in every heterojunction are found to behave as two diodes connected in series face‐to‐face, at least one of which exhibits the characteristics of a nearly ideal Schottky diode and obeys thermionic‐emission theory, wherein only the image force lowers the Schottky barrier. The appearance of this type of nearly ideal diode is attributed to the good contacts to the multiple outer walls of the MWCNTs realized by the heterojunctions' structures.     

Versatile,High‐Power,Flexible, Stretchable Carbon Nanotube Sheet Heating Elements Tolerant to Mechanical Damage and Severe Deformation

A macroscopic carbon nanotube (CNT) sheet‐based heating element having flexible, stretchable, and damage‐tolerant features, and wide applicability in harsh environments, is introduced. Because of the intrinsic connection of extremely flexible CNT bundles throughout the sample by van der Waals interactions without use of a binder, the electrical resistance variation of the CNT sheet on elastomer heating element as a function of strain is completely suppressed to some extent, even when stretched under up to 400% strain, which guarantees electrical stability under severe mechanical deformation. In addition, the spatial uniformity of the heat generated from the microaligned CNT bundles reduces the temperature variation inside the sample, which also guarantees thermal stability and operation at a higher average temperature. Such exceptional performance is achieved by the passivation of the elastomer layer on the CNT sheets. Furthermore, the mechanical robustness of this flexible, stretchable heating element is demonstrated by stable heater operation, even when the heating element is damaged. In addition, this design concept of CNT sheet on elastomer is extended to transparent flexible heaters and electric‐thermochromic windows.     

Carbon Nanotube Wires Sheathed by Aramid Nanofibers

Coaxial fibers are the key elements in many optical, electrical, and biomedical applications. Recent success in materials synthesis has provided versatile choices for the core part, but the search of high‐performance sheath materials remains much less productive. These surface coatings are however as important as the core for their role as protection layers and interaction medium with the externals, thereby critically affecting the real performance of coaxial fibers. Here it is shown that aramid nanofibers (ANFs) with exceptional environmental stability and mechanical properties can be advanced coating materials for both wet‐ and dry‐spun carbon nanotube (CNT) wires. Co‐wet‐spinning ANFs with CNT aqueous dispersion can produce coaxial fibers with a compact sheath comprised of aligned ANFs, showing much enhanced mechanical properties by transferring stress to the sheath without sacrificing the conductivity. On the other hand, an immersion‐precipitation process is used to prepare a porous sheath made from randomly distributed nanofibers on dry‐spun CNT wires, which can be combined with ionic conductive gel electrolyte as a strong packaging layer for flexible solid‐state supercapacitors. The excellent intrinsic characteristics as well as variable ways of structural organizations make ANF‐based coatings an attractive tool for the design of multifunctional high‐performance hybrid materials.     

A Closer Look Inside Nanotubes: Pore Structure Evaluation of Anodized Alumina Templated Carbon Nanotube Membranes Through Adsorption and Permeability Studies

Although hollow nanostructures, such as nanotubes, represent a major portion of nanoscaled materials with a tremendously large application range, a detailed evaluation of their internal characteristics still remains elusive. Transmission electron microscopy is the most common analytical technique to examine the internal configuration of these structures, yet it can only provide evidence of a minimal portion of the overall material, thus, it cannot be accurately generalized. In the present paper, in addition to electron microscopy and other spot‐size analysis methods (X‐ray diffraction, Raman spectroscopy, etc.), a combination of techniques including adsorption, permeability, and relative permeability are employed in order to provide important insights into various crucial details of the overall internal surface and hollow‐space characteristics of carbon nanotube (CNT) arrays and membranes. The CNT arrays are fabricated using anodized alumina as a template in a flow‐through chemical vapor deposition (CVD) reactor. This is the first systematic approach for investigating the internal configuration of template‐based CNT arrays in detail. Key findings are made for the customized optimization of the resulting nanotube membranes for a variety of applications, including separations, nanofluidics and nanoreactors, biological capturing and purification, and controlled drug delivery and release.     

Visualizing Strain Evolution and Coordinated Buckling within CNT Arrays by In Situ Digital Image Correlation

Spatial mapping of strain fields within compressed carbon nanotube (CNT) array columns is achieved using digital image correlation (DIC) analysis of in situ scanning electron microscopy (SEM) image sequences. Full‐field displacement and strain maps are generated based upon the motion of the constituent CNTs, which serve as a traceable high‐contrast speckle pattern for DIC analysis. The deformation modes and CNT array buckling characteristics vary systematically as a function of column aspect ratio, including bending, crushing, and bottom‐up buckle accumulation behaviors. In spite of disparate appearing deformation modes, strain maps indicate that CNT array buckling consistently initiates at 5% local principal strain (?₂) for all columns. The ability to quantitatively assess the deformation modes and buckling behavior of CNT arrays at the nanoscale will enable their improved design for high‐strain electrical contacts, compliant thermal interfaces, force sensors, energy‐absorbing foams, or other applications.     

Stress Controllability in Thermal and Electrical Conductivity of 3D Elastic Graphene‐Crosslinked Carbon Nanotube Sponge/Polyimide Nanocomposite

Stress controllability in thermal and electrical conductivity is important for flexible piezoresistive devices. Due to the strength‐elasticity trade‐off, comprehensive investigation of stress‐controllable conduction in elastic high‐modulus polymers is challenging. Here presented is a 3D elastic graphene‐crosslinked carbon nanotube sponge/polyimide (G_w‐CNT/PI) nanocomposite. Graphene welding at the junction enables both phonon and electron transfer as well as avoids interfacial slippage during cyclic compression. The uniform G_w‐CNT/PI comprising a high‐modulus PI deposited on a porous templated network combines stress‐controllable thermal/electrical conductivity and cyclic elastic deformation. The uniform composites show different variation trends controlled by the porosity due to different phonon and electron conduction mechanisms. A relatively high k (3.24 W m^?1 K^?1, 1620% higher than PI) and suitable compressibility (16.5% under 1 MPa compression) enables the application of the composite in flexible elastic thermal interface conductors, which is further analyzed by finite element simulations. The interconnected network favors a high stress‐sensitive electrical conductivity (sensitivity, 973% at 9.6% strain). Thus, the G_w‐CNT/PI composite can be an important candidate material for piezoresistive sensors upon porosity optimization based on stress‐controllable thermal or electrical conductivity. The results provide insights toward controlling the stress‐induced thermal/electrical conductivities of 3D interconnected templated composite networks for piezoresistive conductors or sensors.     

A rapid growth of aligned carbon nanotube films and high-aspect-ratio arrays

The remarkable properties of carbon nanotubes (CNTs) make them attractive for microelectronic applications, especially for interconnects and nanoscale devices. In this paper, we report an efficient process to grow well-aligned CNT films and high-aspect-ratio CNT arrays with very high area distribution density (>1600 μm⁻²). Chemical vapor deposition (CVD) was invoked to deposit highly aligned CNTs on Al₂O₃/Fe coated silicon substrates of several square centimeter area using ethylene as the carbon source, and argon and hydrogen as carrier gases. The nanotubes grew at a high rate of ∼100 μm/min. for nanotube films at 800°C, while the nanotube arrays grew at ∼140 μm/min. even at 750°C, due to the base growth mode. The CNTs were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and x-ray photoelectron spectroscopy (XPS). The results demonstrated that the CNTs are of high purity and form densely aligned arrays with controllable size and height. The as-grown CNT structures have considerable potential for thermal management and electrical interconnects for microelectronic devices.     

Effects of Nanostructure and Coating on the Mechanics of Carbon Nanotube Arrays

Nanoscale materials are one of the few engineering materials that can be grown from the bottom up in a controlled manner. Here, the effects of nanostructure and nanoscale conformal coating on the mechanical behavior of vertically aligned carbon nanotube (CNT) arrays through experiments and simulation are systematically investigated. A modeling approach is developed and used to quantify the compressive strength and modulus of the CNT array under large deformation. The model accounts for the porous nanostructure, which contains multiple CNTs with random waviness, van der Waals interactions, fracture strain, contacts, and frictional forces. CNT array micropillars are grown and their porous nanostructure is controlled by the infiltration and deposition of thin conformal coatings using chemical vapor deposition. Flat‐punch nanoindentation experiments reveal significant changes in material properties as a function of coating thickness. The simulations explain the experimental results and show the novel failure transition regime that changes from collective CNT buckling toward structural collapse due to fracture. The compressive strength and the elastic modulus increase exponentially as a function of the coating thickness and demonstrate a unique dependency on the CNT waviness. More interestingly, a design rule is identified that predicts the optimum coating thickness for porous materials.     

Photosensitive Carbon Nanotube Paste Based on Acrylated Single‐Walled Carbon Nanotubes

A novel photosensitive carbon nanotube (CNT) paste based on an acrylated single‐walled carbon nanotube (ac‐SWNT), a cross‐linking agent, and a photoinitiator has been prepared. Unlike the conventional photosensitive CNT pastes reported to date, our photosensitive paste system does not use a polymeric binder for the photopolymerization following UV exposure because the ac‐SWNT itself has cross‐linkable groups. Therefore, the subsequent firing process can be performed at relatively low temperatures and the residue of the organic vehicle in the SWNT pattern is minimized after firing. The ac‐SWNT was synthesized from the reaction between carboxylated SWNT (ca‐SWNT) and methacryloyl chloride in the presence of base, and its structure was characterized by Fourier transform infrared, Raman, and X‐ray photoelectron spectroscopy. After UV exposure and development with N,N‐dimethyl formamide a pattern with a resolution of 8 µm was obtained from the photosensitive CNT paste, which was then fired at 300 °C to give a clear SWNT pattern. When the photosensitive CNT paste was used for the fabrication of a cathode emitter for field emission displays, the CNT pattern emitted electrons under an applied electrical field with emission characteristics comparable with those obtained with screen‐printing from conventional CNT pastes. Therefore, such a photosensitive paste for fabricating SWNT patterns can be used in the production of field‐emission displays and in future device integration requiring carbon nanotubes, because it provides large‐area patterning of SWNT with high stability and uniformity.     

Carbon‐Nanotube‐Reinforced Zr‐Based Bulk Metallic Glass Composites and Their Properties

In this paper, we systematically report the preparation of carbon‐nanotube (CNT)‐reinforced Zr‐based bulk metallic glass (BMG) composites. The physical and mechanical properties of the composites were investigated. Compressive testing shows that the composites still display high fracture strength. Investigation also shows that the composites have strong ultrasonic attenuation characteristics and excellent wave absorption ability. The strong wave absorption implies that CNT‐reinforced Zr‐based BMG composites, besides their excellent mechanical properties, may also have significant potential for applications in shielding acoustic sound or environmental noise.