We fabricated field-effect transistors based on individual single-and multi-wall carbon nanotubes and analyzed their performance. Transport through the nanotubes is dominated by holes and, at room temperature, it appears to be diffusive rather than ballistic. By varying the gate voltage, we successfully modulated the conductance of a single-wall device by more than 5 orders of magnitude. Multi-wall nanotubes show typically no gate effect, but structural deformations-in our case a collapsed tube-can make them operate as field-effect transistors. © 1998 American Institute of Physics. ͓S0003-6951͑98͒00143-0͔Carbon nanotubes ͑NTs͒ are a new form of carbon with unique electrical and mechanical properties.1 They can be considered as the result of folding graphite layers into carbon cylinders and may be composed of a single shell-single wall nanotubes ͑SWNTs͒, or of several shells-multi-wall nanotubes ͑MWNTs͒. Depending on the folding angle and the diameter, nanotubes can be metallic or semiconducting. Simple theory also shows that the band gap of semiconducting NTs decreases with increasing diameter. These predictions have been verified in recent scanning tunneling spectroscopy experiments. 2,3Their interesting electronic structure makes carbon nanotubes ideal candidates for novel molecular devices. Metallic NTs, for example, were utilized as Coulomb islands in single-electron transistors 4,5 and, very recently, Tans and coworkers built a molecular field-effect transistor ͑FET͒ with a semiconducting nanotube. 6In this letter, we report on the fabrication and performance of a SWNT-based FET and explore whether MWNTs can be utilized as the active element of carbon-based FETs. Despite their large diameter, we find that structurally deformed MWNTs may well be employed in NT-FETs. Based on the output and transfer characteristics of our NT devices, we evaluate their carrier density and discuss the transport mechanism.The SWNTs used in our study were produced by laser ablation of graphite doped with cobalt and nickel catalysts. For cleaning, the SWNTs were ultrasonically treated in an H 2 SO 4 /H 2 O 2 solution. MWNTs were produced by an arcdischarge evaporation technique 8 and used without further treatment. The NTs were dispersed by sonication in dichloroethane and then spread on a substrate with predefined electrodes. A schematic cross section of a NT device is shown in Fig. 1. They consist of either an individual SWNT or MWNT bridging two electrodes deposited on a 140 nm thick gate oxide film on a doped Si wafer, which is used as a back gate. The 30 nm thick Au electrodes were defined using electron beam lithography. For imaging, we used an atomic force microscope operating in the noncontact mode. The sourcedrain current I through the NTs was measured at room temperature as a function of the bias voltage V SD and the gate voltage V G . Figure 2͑a͒ shows the output characteristics I -V SD of a device consisting of a single SWNT with a diameter of 1.6 nm for several values of the gate voltage. At V G ϭ0 V, the I -V SD curv...

Advances in soft robotics, materials science, and stretchable electronics have enabled rapid progress in soft grippers. Here, a critical overview of soft robotic grippers is presented, covering different material sets, physical principles, and device architectures. Soft gripping can be categorized into three technologies, enabling grasping by: a) actuation, b) controlled stiffness, and c) controlled adhesion. A comprehensive review of each type is presented. Compared to rigid grippers, end-effectors fabricated from flexible and soft components can often grasp or manipulate a larger variety of objects. Such grippers are an example of morphological computation, where control complexity is greatly reduced by material softness and mechanical compliance. Advanced materials and soft components, in particular silicone elastomers, shape memory materials, and active polymers and gels, are increasingly investigated for the design of lighter, simpler, and more universal grippers, using the inherent functionality of the materials. Embedding stretchable distributed sensors in or on soft grippers greatly enhances the ways in which the grippers interact with objects. Challenges for soft grippers include miniaturization, robustness, speed, integration of sensing, and control. Improved materials, processing methods, and sensing play an important role in future research.

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Dielectric elastomer actuators (DEAs) are flexible lightweight actuators that can generate strains of over 100%. They are used in applications ranging from haptic feedback (mm-sized devices), to cm-scale soft robots, to meter-long blimps. DEAs consist of an electrode-elastomer-electrode stack, placed on a frame. Applying a voltage between the electrodes electrostatically compresses the elastomer, which deforms in-plane or out-of plane depending on design. Since the electrodes are bonded to the elastomer, they must reliably sustain repeated very large deformations while remaining conductive, and without significantly adding to the stiffness of the soft elastomer. The electrodes are required for electrostatic actuation, but also enable resistive and capacitive sensing of the strain, leading to self-sensing actuators. This review compares the different technologies used to make compliant electrodes for DEAs in terms of: impact on DEA device performance (speed, efficiency, maximum strain), manufacturability, miniaturization, the integration of self-sensing and selfswitching, and compatibility with low-voltage operation. While graphite and carbon black have been the most widely used technique in research environments, alternative methods are emerging which combine compliance, conduction at over 100% strain with better conductivity and/or ease of patternability, including microfabrication-based approaches for compliant metal thin-films, metal-polymer nano-composites, nanoparticle implantation, and reel-to-reel production of µm-scale patterned thin films on elastomers. Such electrodes are key to miniaturization, low-voltage operation, and widespread commercialization of DEAs.

Insects are a constant source of inspiration for roboticists. Their compliant bodies allow them to squeeze through small openings and be highly resilient to impacts. However, making subgram autonomous soft robots untethered and capable of responding intelligently to the environment is a long-standing challenge. One obstacle is the low power density of soft actuators, leading to small robots unable to carry their sense and control electronics and a power supply. Dielectric elastomer actuators (DEAs), a class of electrostatic electroactive polymers, allow for kilohertz operation with high power density but require typically several kilovolts to reach full strain. The mass of kilovolt supplies has limited DEA robot speed and performance. In this work, we report low-voltage stacked DEAs (LVSDEAs) with an operating voltage below 450 volts and used them to propel an insect-sized (40 millimeters long) soft untethered and autonomous legged robot. The DEAnsect body, with three LVSDEAs to drive its three legs, weighs 190 milligrams and can carry a 950-milligram payload (five times its body weight). The unloaded DEAnsect moves at 30 millimeters/second and is very robust by virtue of its compliance. The sub–500-volt operation voltage enabled us to develop 780-milligram drive electronics, including optical sensors, a microcontroller, and a battery, for two channels to output 450 volts with frequencies up to 1 kilohertz. By integrating this flexible printed circuit board with the DEAnsect, we developed a subgram robot capable of autonomous navigation, independently following printed paths. This work paves the way for new generations of resilient soft and fast untethered robots.