Arash Takshi

Artificial muscle technology: physical principles and naval prospects

The increasing understanding of the advantages offered by fish and insect-like locomotion is creating a demand for muscle-like materials capable of mimicking natures mechanisms. Actuator materials that employ voltage, field, light, or temperature driven dimensional changes to produce forces and displacements are suggesting new approaches to propulsion and maneuverability. Fundamental properties of these new materials are presented, and examples of potential undersea applications are examined in order to assist those involved in device design and in actuator research to evaluate the current status and the developing potential of these artificial muscle technologies. Technologies described are based on newly explored materials developed over the past decade, and also on older materials whose properties are not widely known. The materials are dielectric elastomers, ferroelectric polymers, liquid crystal elastomers, thermal and ferroelectric shape memory alloys, ionic polymer/metal composites, conducting polymers, and carbon nanotubes. Relative merits and challenges associated with the artificial muscle technologies are elucidated in two case studies. A summary table provides a quick guide to all technologies that are discussed.

Depletion width measurement in an organic Schottky contact using a metal-semiconductor field-effect transistor

Although the capacitance measurement is a common method to obtain the depletion width in a Schottky contact, the method is challenging in an organic Schottky junction since the capacitance is a combination of the capacitances associated with the trapped charges, bulk semiconductor, and the depletion region. The authors have implemented a metal-semiconductor field-effect transistor structure in order to estimate the depletion width in an organic Schottky contact. In the transistor the depletion width is calculated from the drain current at a small drain-source voltage. The result indicates a nonquadratic relation between the voltage and the depletion width.

Large photocurrent response and external quantum efficiency in biophotoelectrochemical cells incorporating reaction center plus light harvesting complexes.

Bacterial photosynthetic reaction centers (RCs) are promising materials for solar energy harvesting, due to their high ratio of photogenerated electrons to absorbed photons and long recombination time of generated charges. In this work, photoactive electrodes were prepared from a bacterial RC-light-harvesting 1 (LH1) core complex, where the RC is encircled by the LH1 antenna, to increase light capture. A simple immobilization method was used to prepare RC-LH1 photoactive layer. Herein, we demonstrate that the combination of pretreatment of the RC-LH1 protein complexes with quinone and the immobilization method results in biophotoelectrochemical cells with a large peak transient photocurrent density and photocurrent response of 7.1 and 3.5 μA cm(-2), respectively. The current study with monochromatic excitation showed maximum external quantum efficiency (EQE) and photocurrent density of 0.21% and 2 μA cm(-2), respectively, with illumination power of ∼6 mW cm(-2) at ∼875 nm, under ambient conditions. This work provides new directions to higher performance biophotoelectrochemical cells as well as possibly other applications of this broadly functional photoactive material.

Simulation of a Low-Voltage Organic Transistor Compatible With Printing Methods

The use of printing methods to deposit organic semiconductors promises to enable low-cost electronics. However, printing processes deposit thick and amorphous semiconductor layers that result in poorly performing organic field-effect transistors (OFETs) that generally are not appropriate for incorporation into commercially viable circuits. Another undesirable property of OFETs is their high operating voltage (~40 V). Organic metal-semiconductor FETs (OMESFETs) are proposed as alternatives to OFETs for use with printing methods. OMESFETs operate at low voltages (~5 V) and are expected to show better on/off current ratios than OFETs in a thick-film semiconductor. Simulations of OFETs and OMESFETs are performed assuming regioregular poly (3-hexylthiophene) (rr-P3HT) as the amorphous semiconductor layer with localized states close to the band edge. The results of the simulations show a current ratio of 104 in the OMESFET and of 700 in the OFET for a 400-nm-thick semiconductor layer. Because the OMESFET operates in the depletion mode, versus the accumulation mode in the OFET, the calculated mobility in the OMESFET is two orders of magnitude smaller than that in the OFET. Simulations suggest that the OMESFET design offers performance advantages over printable OFETs, where low-voltage operation is demanded.

Large apparent inductance in organic Schottky diodes at low frequency

A large low frequency inductance is found in a Schottky diode composed of regioregular poly(3-hexylthiophene) and aluminum. This apparent inductance is evident in response to both swept frequency sinusoidal, ramp and step voltage inputs above a threshold voltage. The constant slope of the current in response to a voltage step suggests an incredibly large inductance (a few hundred megahenry) in a device that is only 2000μm3 in size. A number of potential mechanisms including chemical reactions, barrier modulation, and memory effects are evaluated in order to find a suitable explanation for the inductive behavior. Similarity in the dc characteristics of the organic Schottky diode and organic bistable devices that are being applied as memory suggests that the current leads the voltage due to increments in tunneling current that occur as charges are gradually stored in localized states.

Multilayer Stretchable Conductors with a Large Tensile Strength

A metallic conductor layer on an elastomeric substrate has a limited stretchability. Application of a conductive rubber layer underneath the metal can enhance the flexibility and preserve the electrical connection even at high strains. Silicone and polydimethyl siloxane (PDMS) have been tested as elastic substrates. A repeatable resistance-strain behavior up to 100% strain is achieved for a multilayer conductor with PDMS substrate. It is found that this type of substrate has a large influence on the size of cracks on the metal layer.

Electrochemical Field-Effect Transistor Utilization to Study the Coupling Success Rate of Photosynthetic Protein Complexes to Cytochrome c

Due to the high internal quantum efficiency, reaction center (RC) proteins from photosynthetic organisms have been studied in various bio-photoelectrochemical devices for solar energy harvesting. In vivo, RC and cytochrome c (cyt c; a component of the biological electron transport chain) can form a cocomplex via interprotein docking. This mechanism can be used in vitro for efficient electron transfer from an electrode to the RC in a bio-photoelectrochemical device. Hence, the success rate in coupling RCs to cyt c is of great importance for practical applications in the future. In this work, we use an electrochemical transistor to study the binding of the RC to cytochrome. The shift in the transistor threshold voltage was measured in the dark and under illumination to estimate the density of cytochrome and coupled RCs on the gate of the transistor. The results show that ~33% of the cyt cs on the transistor gate were able to effectively couple with RCs. Due to the high sensitivity of the transistor, the approach can be used to make photosensors for detecting low light intensities.

Time Dependent Parallel Resistance in an Organic Schottky Contact

The DC characteristics of a Schottky contact between regioregular poly (3-hexylthiophene) and aluminum are studied in forward and reverse bias regimes. Current-voltage curves of the junction in reverse bias show a resistive path in parallel with the expected Schottky contact. This is the sign of a nonuniform junction between the metal and semiconductor that exhibits ohmic behavior in some regions. Reduction of this parallel resistance and degradation of the Schottky junction are observed over a period of two weeks. Accumulation of undesired ions in the polymer or diffusion of aluminum atoms into the semiconductor are two possible mechanisms which may explain the time dependent behavior of these Schottky junctions.

Low Voltage Polymer Transistors

The combination of polymer semiconducting inks and relatively high resolution printing processes promises to make integrated printed circuits commercially viable. These circuits can be flexible and capital costs of production are very low. One of the key challenges to implementation of this technology is that most transistors made using polymer semiconductors have high operating voltages, which is not ideal for low power, low cost, battery compatible technology. We report on an organic metal semiconductor field effect transistor which operates at 3.5 V. In the initial implementation of this transistor poly(3-hexylthiophene) is the semiconductor, with an aluminium layer used as the gate electrode. The on/off current ratio of this device is only 24.6, but simulations suggest that an improvement of up to three orders of magnitude is possible. The fabrication process requires one less step than other organic transistors. It also uses the gate to encapsulate the transistor, limiting exposure oxygen and moisture, which are known to degrade their performance.