Herbert Shea

Single- and multi-wall carbon nanotube field-effect transistors

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.

Carbon nanotubes : nanomechanics, manipulation, and electronic devices

Carbon nanotubes are novel materials with unique electrical and mechanical properties. Here we present results on their atomic structure and mechanical properties in the adsorbed state, on ways to manipulate individual nanotubes, on their electrical properties and, finally, on the fabrication and characteristics of nanotube-based electron devices. Specifically, atomic force microscopy (AFM) and molecular mechanics simulations are used to investigate the effects of van der Waals interactions on the atomic structure of adsorbed nanotubes. Both radial and axial structural deformations are identified and the interaction energy itself is obtained from the observed deformations. The conditions under which the structure of a nanotube will adjust to the topography of the substrate are defined. We show that the strong substrate–nanotube interaction allows the manipulation of both the position and shape of individual nanotubes at inert surfaces using the AFM. AFM manipulation is then utilized to position individual nanotubes on electrical pads so that their electrical characteristics can be evaluated. We demonstrate the operation of a field-effect transistor based on a single semiconducting nanotube and of a single-electron transistor using a nanotube bundle as Coulomb island. Finally, conducting nanotubes are employed as tips for AFM lithography.

Rings of single-walled carbon nanotubes

Among the most studied processes of self-organization, are the coiling and ring formation of biopolymers such as DNA and proteins. These processes are complex, involving several different types of interaction. We have found that single-walled carbon nanotubes (SWNTs), which are renowned for their extremely high flexural rigidity, , can also be induced to organize themselves into rings or coils, with high yields of up to 50%. But unlike coils of biopolymers, in which hydrogen bonding and ionic interactions are usually involved, coils of nanotubes can be stabilized by van der Waals forces alone.

1100 x 1100 port MEMS-based optical crossconnect with 4-dB maximum loss

We present a microelectromechanical systems-based beam steering optical crossconnect switch core with port count exceeding 1100, featuring mean fiber-to-fiber insertion loss of 2.1 dB and maximum insertion loss of 4.0 dB across all possible connections. The challenge of efficient measurement and optimization of all possible connections was met by an automated testing facility. The resulting connections feature optical loss stability of better than 0.2 dB over days, without any feedback control under normal laboratory conditions.

Beam-steering micromirrors for large optical cross-connects

This paper describes Si-micromachined two-axis beam-steering micromirrors and their performance in 256 /spl times/ 256- and 1024 /spl times/ 1024-port large optical cross-connects (OXCs). The high-reflectivity wavelength-independent mirrors are electrostatically actuated; capable of large, continuous, controlled, dc tilt in any direction at moderate actuation voltages; and allow setting times of a few milliseconds. Packaged two-dimensional (2-D) arrays containing independently addressable identical 256 and 1296 mirrors are used to build fully functional bitrate and wavelength-independent single-stage, low-insertion-loss, single-mode fiber OXC fabrics.

Versatile Soft Grippers with Intrinsic Electroadhesion Based on Multifunctional Polymer Actuators

A highly versatile soft gripper that can handle an unprecedented range of object types is developed based on a new design of dielectric elastomer actuators employing an interdigitated electrode geometry, simultaneously maximizing both electroadhesion and electrostatic actuation while incorporating self-sensing. The multifunctionality of the actuator leads to a highly integrated, lightweight, fast, soft gripper with simplified structure and control.

Effects of electrical leakage currents on MEMS reliability and performance

Electrostatically driven MEMS devices commonly operate with electric fields as high at 10/sup 8/ V/m applied across the dielectric between electrodes. Even with the best mechanical design, the electrical design of these devices has a large impact both on performance (e.g., speed and stability) and on reliability (e.g., corrosion and dielectric or gas breakdown). In this paper, we discuss the reliability and performance implications of leakage currents in the bulk and on the surface of the dielectric insulating the drive (or sense) electrodes from one another. Anodic oxidation of poly-silicon electrodes can occur very rapidly in samples that are not hermetically packaged. The accelerating factors are presented along with an efficient early-warning scheme. The relationship between leakage currents and the accumulation of quasistatic charge in dielectrics are discussed, along with several techniques to mitigate charging and the associated drift in electrostatically actuated or sensed MEMS devices. Two key parameters are shown to be the electrode geometry and the conductivity of the dielectric. Electrical breakdown in submicron gaps is presented as a function of packaging gas and electrode spacing. We discuss the tradeoffs involved in choosing gap geometries and dielectric properties that balance performance and reliability.

1296-port MEMS transparent optical crossconnect with 2.07 petabit/s switch capacity

A 1296-port MEMS transparent optical crossconnect with 5.1dB/spl plusmn/1.1dB insertion loss at 1550 nm is reported. Measured worst-case optical crosstalk in a fabric was n38 dB and nominal switching rise/fall times were 5 ms. A 2.07 petabit/s switch capacity was verified upon cross-connecting a forty-channel by 40 Gb/s DWDM data stream through a prototype fabric.

Rollable Multisegment Dielectric Elastomer Minimum Energy Structures for a Deployable Microsatellite Gripper

Debris in space presents an ever-increasing problem for spacecraft in Earth orbit. As a step in the mitigation of this issue, the CleanSpace One (CSO) microsatellite has been proposed. Its mission is to perform active debris removal of a decommissioned nanosatellite (the CubeSat SwissCube). An important aspect of this project is the development of the gripper system that will entrap the capture target. We present the development of rollable dielectric elastomer minimum energy structures (DEMES) as the main component of CSOs deployable gripper. DEMES consist of a prestretched dielectric elastomer actuator membrane bonded to a flexible frame. The actuator finds equilibrium in bending when the prestretch is released and the bending angle can be changed by the application of a voltage bias. The inherent flexibility and lightweight nature of the DEMES enables the gripper to be stored in a rolled-up state prior to deployment. We fabricated proof-of-concept actuators of three different geometries using a robust and repeatable fabrication methodology. The resulting actuators were mechanically resilient to external deformation, and display conformability to objects of varying shapes and sizes. Actuator mass is less than 0.65 g and all the actuators presented survived the rolling-up and subsequent deployment process. Our devices demonstrate a maximum change of bending angle of more than 60° and a maximum gripping (reaction) force of 2.2 mN for a single actuator.

Standards for dielectric elastomer transducers

Dielectric elastomer transducers consist of thin electrically insulating elastomeric membranes coated on both sides with compliant electrodes. They are a promising electromechanically active polymer technology that may be used for actuators, strain sensors, and electrical generators that harvest mechanical energy. The rapid development of this field calls for the first standards, collecting guidelines on how to assess and compare the performance of materials and devices. This paper addresses this need, presenting standardized methods for material characterisation, device testing and performance measurement. These proposed standards are intended to have a general scope and a broad applicability to different material types and device configurations. Nevertheless, they also intentionally exclude some aspects where knowledge and/or consensus in the literature were deemed to be insufficient. This is a sign of a young and vital field, whose research development is expected to benefit from this effort towards standardisation.