Flexible, high mobility short-channel organic thin film transistors and logic circuits based on 4H-21DNTT.

The low mobility and large contact resistance in organic thin-film transistors (OTFTs) are the two major limiting factors in the development of high-performance organic logic circuits. Here, solution-processed high-performance OTFTs and circuits are reported with a polymeric gate dielectric and 6,6 bis (trans-4-butylcyclohexyl)-dinaphtho[2,1-b:2,1-f]thieno[3,2-b]thiophene (4H-21DNTT) for the organic semiconducting layer. By optimizing and controlling the fabrication conditions, a high saturation mobility of 8.8 cm2 V-1 s-1 was demonstrated as well as large on/off ratios (> 106) for relatively short channel lengths of 15 µm and an average carrier mobility of 10.5 cm2 V-1 s-1 for long channel length OTFTs (> 50 µm). The pseudo-CMOS inverter circuit with a channel length of 15 µm exhibited sharp switching characteristics with a high signal gain of 31.5 at a supply voltage of 20 V. In addition to the inverter circuit, NAND logic circuits were further investigated, which also exhibited remarkable logic characteristics, with a high gain, an operating frequency of 5 kHz, and a short propagation delay of 22.1 µs. The uniform and reproducible performance of 4H-21DNTT OTFTs show potential for large-area, low-cost real-world applications on industry-compatible bottom-contact substrates.

Fast electrochemistry of conductive polymer nanotubes: synthesis, mechanism, and application.

Conductive polymers exhibit several interesting and important properties, such as metallic conductivity and reversible convertibility between redox states. When the redox states have very different electrochemical and electronic properties, their interconversion gives rise to changes in the polymers' conformations, doping levels, conductivities, and colors, useful attributes if they are to be applied in displays, energy storage devices, actuators, and sensors. Unfortunately, the rate of interconversion is usually slow, at best on the order a few hundred milliseconds, because of the slow transport of counterions into the polymer layer to balance charge. This phenomenon is one of the greatest obstacles toward building rapidly responsive electrochemical devices featuring conductive polymers. One approach to enhancing the switching speed is decreasing the diffusion distance for the counterions in the polymer. We have found that nanotubular structures are good candidates for realizing rapid switching between redox states because the counterions can be readily doped throughout the thin nanotube walls. Although the synthesis of conductive polymer nanotubes can be performed using electrochemical template synthesis, the synthetic techniques and underlying mechanisms controlling the nanotube morphologies are currently not well established. We begin this Account by discussing the mechanisms for controlling the structures from hollow nanotubes to solid nanowires. The applied potential, monomer concentration, and base electrode shape all play important roles in determining the nanotubes' morphologies. A mechanism based on the rates of monomer diffusion and reaction allows the synthesis of nanotubes at high oxidation potentials; a mechanism dictated by the base-electrode shape dominates at very low oxidation potentials. The structures of the resulting conductive polymer nanotubes, such as those of poly(3,4-ethylenedioxythiophene) (PEDOT) and polypyrrole, can be characterized using scanning electron microscopy and transmission electron microscopy. We also discuss these materials in terms of their prospective use in nanotube-based electrochemical devices. For example, we describe an electrochromic device incorporating PEDOT nanotubes that exhibits an ultrafast color switching rate (<10 ms) and strong coloration. In addition, we report a supercapacitor based on PEDOT nanotubes that can provide more than 80% of its own energy density, even at power demands as high as 25 kW/kg.

Poly(3,4-ethylenedioxythiophene) nanotubes as electrode materials for a high-powered supercapacitor.

We report the fast charging/discharging capability of poly(3,4-ethylenedioxythiophene) (PEDOT) nanotubes during the redox process and their potential application to a high-powered supercapacitor. PEDOT nanotubes were electrochemically synthesized in a porous alumina membrane, and their structures were characterized using electron microscopes. Cyclic voltammetry was used to characterize the specific capacitance of the PEDOT nanotubes at various scan rates. A type I supercapacitor (two symmetric electrodes) based on PEDOT nanotube electrodes was fabricated, and its energy density and power density were evaluated by galvanostatic charge/discharge cycles at various current densities. We show that the PEDOT-nanotube-based supercapacitor can achieve a high power density of 25 kW kg(-1) while maintaining 80% energy density (5.6 W h kg(-1)). This high power capability is attributed to the fast charge/discharge of nanotubular structures: hollow nanotubes allow counter-ions to readily penetrate into the polymer and access their internal surfaces, while the thin wall provides a short diffusion distance to facilitate the ion transport. Impedance spectroscopy shows that nanotubes have much lower diffusional resistance to charging ions than solid nanowires shielded by an alumina template, providing supporting information for the high charging/discharging efficiency of nanotubular structures.

Controlled electrochemical synthesis of conductive polymer nanotube structures.

We have investigated the electrochemical synthetic mechanism of conductive polymer nanotubes in a porous alumina template using poly(3,4-ethylenedioxythiophene) (PEDOT) as a model compound. As a function of monomer concentration and potential, electropolymerization leads either to solid nanowires or to hollow nanotubes, and it is the purpose of these investigations to uncover the detailed mechanism underlying this morphological transition between nanowire and nanotube. Transmission electron microscopy was used to characterize electrochemically synthesized PEDOT nanostructures and measure the extent of their nanotubular portion quantitatively. The study on potential dependency of nanotubular portion shows that nanotubes are grown at a low oxidation potential (1.2 V vs Ag/AgCl) regardless of monomer concentration. This phenomenon is attributed to the predominance of electrochemically active sites on the annular-shape electrode at the pore bottom of a template. The explanation was supported by a further electrochemical study on a flat-top electrode. We elaborate the mechanism by taking into account the effect of electrolyte concentration, temperature, and template pore diameter on PEDOT nanostructures. This mechanism is further employed to control the nanotube dimensions of other conductive polymers such as polypyrrole and poly(3-hexylthiophene).

Bias-free pneumatic sample injection in microchip electrophoresis.

We have developed a new microfluidic chip capable of accurate metering, pneumatic sample injection, and subsequent electrophoretic separation. The pneumatic injection scheme, enabling us to introduce a solution without sampling bias unlike electrokinetic injection, is based upon the hydrophobicity and wettability of channel surfaces. An accurately metered solution of 10 nL could be injected by pneumatic pressure into a hydrophilic separation channel through Y-shaped hydrophobic valves, which consist of polydimethylsiloxane (PDMS) and fluorocarbon (FC) film layers. We demonstrated the successful pneumatic injection of a red ink solution into the separation channel as a proof of the concept. A mixture of fluorescein and dichlorofluorescein (DCF) could be baseline-separated using a single power source in microchip electrophoresis.

On-line sample cleanup and chiral separation of gemifloxacin in a urinary solution using chiral crown ether as a chiral selector in microchip electrophoresis.

In chiral capillary electrophoresis of primary amine enantiomers using (+)-18-crown-6-tetracarboxylic acid (18C6H4) as a chiral selector, the presence of alkaline metal ions in the sample solution as well as in the run buffer is undesirable due to their strong competitive binding with 18C6H4. A channel-coupled microchip electrophoresis device was designed to clean up alkaline metal ions from a sample matrix for the chiral analysis of amine. In the first channel, the metal ions in the sample were monitored by indirect detection using quinine as a chromophore and drained to the waste. In the second separation channel, gemifloxacin enantiomers, free of the alkaline metal ions, were successfully separated using only a small amount of the chiral selector (50 microM 18C6H4).

Integration of on-column immobilized enzyme reactor in microchip electrophoresis.

A simple method integrating an immobilized enzyme reactor into a microchip electrophoresis device was developed. The enzyme immobilization into a microchip was performed by spotting and drying a drop of dissolved nitrocellulose (NC) on a glass substrate, and adsorbing enzyme on the reconstituted NC membrane. This enzyme-immobilized glass plate was assembled with a polydimethylsiloxane substrate on which the separation channel was fabricated. The advantage of this method is the ability to easily change the position and size of the reactor within the microchip electrophoresis device. A beta-galactosidase reaction was demonstrated with fluorescein di-beta-D-galactopyranoside using this integrated on-column enzyme reactor. A successful electrophoretic separation of its hydrolysis products, i.e., fluorescein mono-beta-D-galactopyranoside (FMG) and fluorescein, was achieved. Enzyme kinetics and inhibition of the beta-galactosidase using FMG and 2-phenylethyl beta-D-thiogalactoside, respectively, were also studied with microchip electrophoresis.

Chiral separation of gemifloxacin in sodium-containing media using chiral crown ether as a chiral selector by capillary and microchip electrophoresis.

Chiral crown ether, (+)-(18-crown-6)-tetracarboxylic acid (18C6H(4)), is an effective chiral selector for resolving enantiomeric primary amines owing to the difference in affinities between 18C6H(4) and each of the amine enantiomers. In addition to the destacking effect of sodium ion in the sample solution, the strong affinity of sodium ion to the polyether ring of crown ether is unfavorable to chiral capillary electrophoresis using 18C6H(4) as a chiral selector. In this report, the chiral separation of gemifloxacin dissolved in a saline sample matrix using 18C6H(4) was investigated. Adding a chelating agent, ethylenediaminetetraacetic acid (EDTA), to the run buffer greatly improved the separation efficiencies and peak shapes. The successful chiral separation of gemifloxacin in a urinary solution was demonstrated for both capillary and microchip electrophoresis.