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.     

Intelligently Actuating Liquid Crystal Elastomer‐Carbon Nanotube Composites

Strategies for obtaining materials that respond to external stimuli by changing shape are of intense interest for the replacement of traditional actuators. Here, a strategy that enables programmable, multiresponsive actuators that use either visible light or electric current to drive shape change in composites comprising carbon nanotubes (CNTs) in liquid crystal elastomers (LCEs) is presented. In the nanocomposites, the CNTs function not only in the traditional roles of mechanical reinforcement and enhancers of thermal and electrical conductivity but also serve as an alignment layer for the LCEs. By controlling the orientation, location, and quantity of layers of CNTs in LCE/CNT composites, programmed, patterned actuators are built that respond to visible light or electrical current. Photothermal LCE/CNT film actuators undergo fast shape change, within 1.2 s using 280 mW cm^?2 light input, and complex, programmed localized deformations. Furthermore, twisting LCE/CNT composite films into a fiber increases uniaxial muscle stroke and work capacity for electrothermal actuation, thereby enabling about 12% actuation strain and 100 kJ m^?3 of work capacity in response to an applied DC voltage of 15.1 V cm^?1.     

A Pyrenylpropyl Phosphonic Acid Surface Modifier for Mitigating the Thermal Resistance of Carbon Nanotube Contacts

Efforts to utilize the high intrinsic thermal conductivity of carbon nanotubes (CNTs) for thermal transport applications, namely for thermal interface materials (TIMs), have been encumbered by the presence of high thermal contact resistances between the CNTs and connecting materials. Here, a pyrenylpropyl‐phosphonic acid surface modifier is synthesized and applied in a straight forward and repeatable approach to reduce the thermal contact resistance between CNTs and metal oxide surfaces. When used to bond nominally vertically aligned multi‐walled CNT forests to Cu oxide surfaces, the modifier facilitates a roughly 9‐fold reduction in the thermal contact resistance over dry contact, enabling CNT‐based TIMs with thermal resistances of 4.6 ± 0.5 mm² K W^?1, comparable to conventional metallic solders. Additional experimental characterization of the modifier suggests that it may be used to reduce the electrical resistance of CNT‐metal oxide contacts by similar orders of magnitude.     

High‐Conductivity Polymer Nanocomposites Obtained by Tailoring the Characteristics of Carbon Nanotube Fillers

We present a detailed study of the influence of carbon nanotube (CNT) characteristics on the electrical conductivity of polystyrene nanocomposites produced using a latex‐based approach. We processed both industrially‐produced multi‐wall CNT (MWCNT) powders and MWCNTs from vertically‐aligned films made in‐house, and demonstrate that while the raw CNTs are individualized and dispersed comparably within the polymer matrix, the electrical conductivity of the final nanocomposites differs significantly due to the intrinsic characteristics of the CNTs. Owing to their longer length after dispersion, the percolation threshold observed using MWCNTs from vertically‐aligned films is five times lower than the value for industrially‐produced MWCNT powders. Further, owing to the high structural quality of the CNTs from vertically‐aligned films, the resulting composite films exhibit electrical conductivity of 10³ S m^?1 at 2 wt% CNTs. On the contrary, composites made using the industrially‐produced CNTs exhibit conductivity of only tens of S m^?1. To our knowledge, the measured electrical conductivity for CNT/PS composites using CNTs from vertically‐aligned films is by far the highest value yet reported for CNT/PS nanocomposites at this loading.     

Stretchable Electrode Based on Laterally Combed Carbon Nanotubes for Wearable Energy Harvesting and Storage Devices

Carbon nanotubes (CNTs) are a promising material for use as a flexible electrode in wearable energy devices due to their electrical conductivity, soft mechanical properties, electrochemical activity, and large surface area. However, their electrical resistance is higher than that of metals, and deformations such as stretching can lead to deterioration of electrical performances. To address these issues, here a novel stretchable electrode based on laterally combed CNT networks is presented. The increased percolation between combed CNTs provides a high electrical conductivity even under mechanical deformations. Additional nickel electroplating and serpentine electrode designs increase conductivity and deformability further. The resulting stretchable electrode exhibits an excellent sheet resistance, which is comparable to conventional metal film electrodes. The resistance change is minimal even when stretched by ≈100%. Such high conductivity and deformability in addition to intrinsic electrochemically active property of CNTs enable high performance stretchable energy harvesting (wireless charging coil and triboelectric generator) and storage (lithium ion battery and supercapacitor) devices. Monolithic integration of these devices forms a wearable energy supply system, successfully demonstrating its potential as a novel soft power supply module for wearable electronics.     

Carbon Nanotube Based Multifunctional Ambipolar Transistors for AC Applications

Field‐effect transistors (FETs) fabricated on large diameter carbon nanotubes (CNTs) present typical ambipolar transfer characteristics owing to the small band‐gap of CNTs. Depending on the DC biasing condition, the ambipolar FET can work in three different regions, and then can be used as the core to realize multifunctional AC circuits. The CNT FET based circuits can work as a high‐efficiency ambipolar frequency doubler in the ambipolar transfer region, and also can function as in‐phase amplifier and inverted amplifier in the linear transfer region. Due to current saturation of the CNT FET, an AC amplifier with a voltage gain of 2 is realized when the device works in the linear transfer region. Achieving an actual amplification and frequency doubling functions indicates that complicated radio frequency circuits or systems can be constructed based on just one kind of device: ambipolar CNT FETs.     

Substrate-Versatile Direct-Write Printing of Carbon Nanotube-Based Flexible Conductors,Circuits, and Sensors

Manufacturing of printed electronics relies on the deposition of conductive liquid inks, typically onto polymeric or paper substrates. Among available conductive fillers for use in electronic inks, carbon nanotubes (CNTs) have high conductivity, low density, processability at low temperatures, and intrinsic mechanical flexibility. However, the electrical conductivity of printed CNT structures has been limited by CNT quality and concentration, and by the need for nonconductive modifiers to make the ink stable and extrudable. This study introduces a polymer-free, printable aqueous CNT ink, and, via an ambient direct-write printing process, presents the relationships between printing resolution, ink rheology, and ink-substrate interactions. A model is constructed to predict printed feature sizes on impermeable substrates based on Wenzel wetting. Printed lines have conductivity up to 10 000 S m⁻¹. The lines are flexible, with <5% change in DC resistance after 1000 bending cycles, and <3% change in DC resistance with a bending radius down to 1 mm. Demonstrations focus on i) conformality, via printing CNTs onto stickers that can be applied to curved surfaces, ii) interactivity using a CNT-based button printed onto folded paper structure, and iii) capacitive sensing of liquid wicking into the substrate itself. Facile integration of surface mount components on printed circuits is enabled by the intrinsic adhesion of the wet ink.     

Integration and electrical characterization of carbon nanotube via interconnects

Carbon nanotubes (CNTs) are considered a promising material for interconnects in the future generations of microchips because of their low electrical resistance and excellent mechanical stability. In particular, CNT-based contacts appear advantageous when compared with current tungsten or copper technologies and could therefore find an application as metal contacts interconnecting the transistors with the back end of line of the microchip. In this work, the integration of vertical CNT bundles in sub-micron contact holes is evaluated at wafer scale and the major integration challenges encountered in the practical realization of the process are discussed. Nickel PVD films were used to selectively grow CNT into the contact holes at temperatures as low as 400 °C, which is the thermal budget available for contacts. The height of the contacts and the length of the CNT are controlled by a chemical mechanical polishing step (CMP) after embedding the CNT into SiO₂. Ti/Au metal pads are then formed onto the CNT bundles by PVD and lift-off. The integrated CNT are electrically characterized and an annealing treatment was found to improve the CNT-via resistance. As the electrical properties of the CNT can be evaluated, the structure and the process presented constitute a test vehicle for the development of high-quality CNT-contacts.     

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.     

Novel Nonconductive Adhesives/Films With Carbon Nanotubes for High-Performance Interconnects

Novel nonconductive adhesives/films (NCAs/NCFs) with carbon nanotubes (CNTs) for high-performance interconnects were developed. A small amount of CNTs inside NCAs/NCFs could increase the thermal conductivities and at the same time decrease the coefficient of thermal expansion (CTE) for high thermomechanical reliability of the NCAs/NCFs' interconnect joints. Thermal mechanical analyzer measurements showed that the CTE value of the 0.03 wt% CNT filled NCAs/NCFs was significantly decreased. Current–voltage characterizations showed that the current carrying capabilities of the CNT (0.03 wt%) filled NCAs/NCFs were increased by 14% compared to the unfilled NCAs/NCFs due to the more efficient thermal dissipation and higher electrical conductivity by CNTs.      

Carbon Nanomaterials for Next-Generation Interconnects and Passives: Physics, Status, and Prospects

This paper reviews the current state of research in carbon-based nanomaterials, particularly the one-dimensional (1-D) forms, carbon nanotubes (CNTs) and graphene nanoribbons (GNRs), whose promising electrical, thermal, and mechanical properties make them attractive candidates for next-generation integrated circuit (IC) applications. After summarizing the basic physics of these materials, the state of the art of their interconnect-related fabrication and modeling efforts is reviewed. Both electrical and thermal modeling and performance analysis for various CNT- and GNR-based interconnects are presented and compared with conventional interconnect materials to provide guidelines for their prospective applications. It is shown that single-walled, double-walled, and multiwalled CNTs can provide better performance than that of Cu. However, in order to make GNR interconnects comparable with Cu or CNT interconnects, both intercalation doping and high edge-specularity must be achieved. Thermal analysis of CNTs shows significant advantages in tall vias, indicating their promising application as through-silicon vias in 3-D ICs. In addition to on-chip interconnects, various applications exploiting the low-dimensional properties of these nanomaterials are discussed. These include chip-to-packaging interconnects as well as passive devices for future generations of IC technology. Specifically, the small form factor of CNTs and reduced skin effect in CNT interconnects have significant implications for the design of on-chip capacitors and inductors, respectively.      

High Electromechanical Response of Ionic Polymer Actuators with Controlled‐Morphology Aligned Carbon Nanotube/Nafion Nanocomposite Electrodes

Recent advances in fabricating controlled‐morphology vertically aligned carbon nanotubes (VA‐CNTs) with ultrahigh volume fraction create unique opportunities for markedly improving the electromechanical performance of ionic polymer conductor network composite (IPCNC) actuators. Continuous paths through inter‐VA‐CNT channels allow fast ion transport, and high electrical conduction of the aligned CNTs in the composite electrodes lead to fast device actuation speed (>10% strain/second). One critical issue in developing advanced actuator materials is how to suppress the strain that does not contribute to the actuation (unwanted strain) thereby reducing actuation efficiency. Here, experiments demonstrate that the VA‐CNTs give an anisotropic elastic response in the composite electrodes, which suppresses the unwanted strain and markedly enhances the actuation strain (>8% strain under 4 V). The results reported here suggest pathways for optimizing the electrode morphology in IPCNCs using ultrahigh volume fraction VA‐CNTs to further enhanced performance.     

Carbon Nanotubes: High Electromechanical Response of Ionic Polymer Actuators with Controlled‐Morphology Aligned Carbon Nanotube/Nafion Nanocomposite Electrodes (Adv. Funct. Mater. 19/2010)

Recent advances in fabricating controlled‐morphology vertically aligned carbon nanotubes (VA‐CNTs) with ultrahigh volume fraction create unique opportunities for markedly improving the electromechanical performance of ionic polymer conductor network composite (IPCNC) actuators. Continuous paths through inter‐VA‐CNT channels allow fast ion transport, and high electrical conduction of the aligned CNTs in the composite electrodes lead to fast device actuation speed (>10% strain/second). One critical issue in developing advanced actuator materials is how to suppress the strain that does not contribute to the actuation (unwanted strain) thereby reducing actuation efficiency. Here, experiments demonstrate that the VA‐CNTs give an anisotropic elastic response in the composite electrodes, which suppresses the unwanted strain and markedly enhances the actuation strain (>8% strain under 4 V). The results reported here suggest pathways for optimizing the electrode morphology in IPCNCs using ultrahigh volume fraction VA‐CNTs to further enhanced performance.     

The Fusion of Imidazolium‐Based Ionic Polymer and Carbon Nanotubes: One Type of New Heteroatom‐Doped Carbon Precursors for High‐Performance Lithium–Sulfur Batteries

Rational design of sulfur host materials with high electrical conductivity and strong polysulfides (PS) confinement is indispensable for high‐performance lithium–sulfur (Li–S) batteries. This study presents one type of new polymer material based on main‐chain imidazolium‐based ionic polymer (ImIP) and carbon nanotubes (CNTs); the polymer composites can serve as a precursor of CNT/NPC‐300, in which close coverage and seamless junction of CNTs by N‐doped porous carbon (NPC) form a 3D conductive network. CNT/NPC‐300 inherits and strengthens the advantages of both high electrical conductivity from CNTs and strong PS entrapping ability from NPC. Benefiting from the improved attributes, the CNT/NPC‐300‐57S electrode shows much higher reversible capacity, rate capability, and cycling stability than NPC‐57S and CNTs‐56S. The initial discharge capacity of 1065 mA h g^?1 is achieved at 0.5 C with the capacity retention of 817 mA h g^?1 over 300 cycles. Importantly, when counter bromide anion in the composite of CNTs and ImIP is metathesized to bis(trifluoromethane sulfonimide), heteroatom sulfur is cooperatively incorporated into the carbon hosts, and the surface area is increased with the promotion of micropore formation, thus further improving electrochemical performance. This provides a new method for optimizing porous properties and dopant components of the cathode materials in Li–S batteries.     

Electrically Addressable Hybrid Architectures of Zinc Oxide Nanowires Grown on Aligned Carbon Nanotubes

The fabrication and characterization of hybrid architectures of ZnO nanowires (ZNWs) grown on organized carbon nanotubes (CNTs), by a two‐step chemical vapor deposition (CVD) process involving CNT growth from a hydrocarbon source followed by ZNW growth using a Zn metal source, is reported. The ZNWs grow uniformly and radially from individual CNTs and CNT bundles, and the aligned morphology of the CNTs is not disturbed by the ZNW growth process. The nucleation and growth of ZnO crystals on CNTs are analyzed in relation to the classical vapor–solid mechanism. Importantly, the CNTs make uniform and distributed electrical contact to the ZNWs, with up to a 1000‐fold yield advantage over conventional ZNW growth on a flat substrate. Hybrid ZNW/CNT sheets are fabricated by scalable CVD, rolling, and printing methods; and their electrical properties, which are governed by transport through the anisotropic CNT network, are characterized. Functional interaction between the ZNWs and CNTs is demonstrated by photoconductive behavior and photocurrent generation of the hybrid material under UV illumination. There is significant future opportunity to extend these processing methods to fabricate other functional oxides on CNTs, and to build devices that harness the attractive properties of ZNWs and CNTs with high volumetric efficiency over large areas.     

Covalently Bonded Graphene–Carbon Nanotube Hybrid for High‐Performance Thermal Interfaces

The remarkable thermal properties of graphene and carbon nanotubes (CNTs) have been the subject of intensive investigations for the thermal management of integrated circuits. However, the small contact area of CNTs and the large anisotropic heat conduction of graphene have hindered their applications as effective thermal interface materials (TIMs). Here, a covalently bonded graphene–CNT (G‐CNT) hybrid is presented that multiplies the axial heat transfer capability of individual CNTs through their parallel arrangement, while at the same time it provides a large contact area for efficient heat extraction. Through computer simulations, it is demonstrated that the G‐CNT outperforms few‐layer graphene by more than 2 orders of magnitude for the c‐axis heat transfer, while its thermal resistance is 3 orders of magnitude lower than the state‐of‐the‐art TIMs. We show that heat can be removed from the G‐CNT by immersing it in a liquid. The heat transfer characteristics of G‐CNT suggest that it has the potential to revolutionize the design of high‐performance TIMs.     

Shaping Carbon Nanotubes and the Effects on Their Electrical and Mechanical Properties

A method is developed and shown to be able to shape a carbon nanotube (CNT) into a desired morphology while maintaining its excellent electrical and mechanical properties. Single, freestanding nanotubes are bent by a scanning tunneling microscopy probe, and their morphology is fixed by electron‐beam‐induced deposition (inside a transmission electron microscope) of amorphous carbon on the bent area. It is shown that the mechanical strength of the bent CNT may be greatly enhanced by increasing the amount of carbon glue or the deposition area, and the electrical conduction of the nanotube shows hardly any dependence on the bending deformation or on the deposition of amorphous carbon. Our findings suggest that CNTs might be manipulated and processed as interconnections between electronic devices without much degradation in their electrical conductance, and be used in areas requiring complex morphology, such as nanometer‐scale transport carriers and nanoelectromechanical systems.     

Carbon nanotubes for nanoscale low temperature flip chip connections

In this work we demonstrate a new approach for ultra fine flip chip interconnections based on carbon nanotubes as a wiring material. In contrast to other works we show patterned growth of multi walled CNTs on substrates with pre-structured bond pads including a complete metallization system for electrical characterization. Furthermore, we succeeded achieving a reliable flip chip connection between CNT-covered contact pads and metal pads at temperatures lower than 200 °C. Our goal is a reversible electrical and mechanical chip assembly with CNT bumps.For bonding experiments and electrical characterization a test structure with a damascene metallization including a layer stack of TiN/Cu/TiN was prepared. For CNT growth a thin nickel catalyst layer was selectively deposited with sputtering and a lift-off technique on the contact pads. The CNTs were grown by thermal CVD with ethylene as carbon source. CNT growth parameters like catalyst thickness, gas composition, growth time and temperature were optimized to get dense CNT growth. The metal bumps of the counter chip consist of electroless deposited Ni. With the selected layout we can obtain daisy chain and four-point measurements for lossless determination of single contact resistance. We have obtained reliable electrical contacts with relatively small resistance reaching values as low as 2.2 Ω. As CNT-quality is strongly dependent on the growth temperature we observed a strong change in resistivity of the flip chip connection as the growth temperature was varied. Reliability tests showed long time stability under thermal stress proving a reliable electrical contact between the contact pads. There is an appropriate potential for further optimization of the CNT bump resistance and applying this technology for IC-devices.     

镍/碳纳米管复合薄膜的制备及其性能的研究

结合电泳沉积碳纳米管和电镀金属镍的方法,在导电基片上成功制备了镍/碳纳米管复合薄膜.观察了薄膜的形貌和成份,测试了所制备的镍/碳纳米管复合薄膜的表面电阻和场发射特性,并与传统的电泳方法制备的碳纳米管薄膜进行了比较.实验结果表明,镍/碳纳米管复合薄膜表面碳纳米管大范围排布有序、密度可控;与传统电泳方法制备得到的碳纳米管薄膜相比,镍-碳纳米管复合薄膜具有更小的表面电阻和更好的场发射性能.镍层在薄膜中起到了固定碳纳米管、改善碳纳米管与基底以及碳纳米管与碳纳米管之间电接触的作用;由于镍在薄膜中含量较少(小于10%),薄膜整体仍体现出碳纳米管的优异性能.