James Swensen

Increased mobility from regioregular poly(3-hexylthiophene) field-effect transistors

Relatively high mobilities, μ=0.2 cm2 V−1 s−1 in the accumulation mode and μ=0.17 cm2 V−1 s−1 in the depletion mode, are reported for regioregular poly(3-hexylthiophene) (RR-P3HT) in field-effect transistors (FETs). Significantly higher mobility is obtained from FETs in which the RR-P3HT film is applied by dip-coating to a thickness of only 20−40 A. These observations suggest that structural order of the semiconducting polymer at the interface between the semiconducting polymer and the SiO2 gate insulator is of paramount importance for achieving high carrier mobility. Heat treatment under nitrogen at 160 °C for 3 min increases the on/off ratio of the FET.

Continuous, Real-Time Monitoring of Cocaine in Undiluted Blood Serum via a Microfluidic, Electrochemical Aptamer-Based Sensor

The development of a biosensor system capable of continuous, real-time measurement of small-molecule analytes directly in complex, unprocessed aqueous samples has been a significant challenge, and successful implementation has been achieved for only a limited number of targets. Toward a general solution to this problem, we report here the Microfluidic Electrochemical Aptamer-based Sensor (MECAS) chip wherein we integrate target-specific DNA aptamers that fold, and thus generate an electrochemical signal, in response to the analyte with a microfluidic detection system. As a model, we demonstrate the continuous, real-time (approximately 1 min time resolution) detection of the small-molecule drug cocaine at near physiological, low micromolar concentrations directly in undiluted, otherwise unmodified blood serum. We believe our approach of integrating folding-based electrochemical sensors with miniaturized detection systems may lay the groundwork for the real-time, point-of-care detection of a wide variety of molecular targets.

Light emission from an ambipolar semiconducting polymer field-effect transistor

Ambipolar light-emitting field-effect transistors are fabricated with two different metals for the top-contact source and drain electrodes; a low-work-function metal defining the channel for the source electrode and a high-work-function metal defining the channel for the drain electrode. A thin film of polypropylene-co-1-butene on SiNx is used as the gate dielectric on an n++-Si wafer, which functioned as the substrate and the gate electrode. Transport data show ambipolar behavior. Recombination of electrons and holes results in a narrow zone of light emission within the channel. The location of the emission zone is controlled by the gate bias.

Integrated Microfluidic Electrochemical DNA Sensor

Effective systems for rapid, sequence-specific nucleic acid detection at the point of care would be valuable for a wide variety of applications, including clinical diagnostics, food safety, forensics, and environmental monitoring. Electrochemical detection offers many advantages as a basis for such platforms, including portability and ready integration with electronics. Toward this end, we report the Integrated Microfluidic Electrochemical DNA (IMED) sensor, which combines three key biochemical functionalities--symmetric PCR, enzymatic single-stranded DNA generation, and sequence-specific electrochemical detection--in a disposable, monolithic chip. Using this platform, we demonstrate detection of genomic DNA from Salmonella enterica serovar Typhimurium LT2 with a limit of detection of <10 aM, which is approximately 2 orders of magnitude lower than that from previously reported electrochemical chip-based methods.

Encapsulation of organic light-emitting devices using a perfluorinated polymer

Films of Cytop™, a perfluorinated polymer, are spin cast as a single barrier layer for evaluation of barrier properties on organic light-emitting devices and on Ca thin films. Cytop™ is water repellant, resulting in encapsulated organic light-emitting field effect transistors and organic light-emitting diodes (OLEDs), which remain active even after immersion into water or exposure to water droplets on the Cytop™ surface. OLEDs encapsulated with Cytop™ exhibit up to five times longer continuous operation under identical environmental and driving conditions compared with devices that are not encapsulated with Cytop™.

Single-component light-emitting electrochemical cell fabricated from cationic polyfluorene: Effect of film morphology on device performance

Planar light-emitting electrochemical cells (LECs) were prepared by drop casting the conjugated polyelectrolyte poly(9,9-bis[6′-(N,N,N,-trimethylammonium)hexyl]fluorene-co-alt-1,4-phenylene) bromide (PFN+Br−) onto substrates patterned with micrometer-sized interelectrode (Au) gaps. When operated at room temperature (RT), such LECs exhibit a gap-size-dependent turn-on voltage for light emission, but when operated at 140°C, the turn-on voltage is essentially gap-size independent and close to the band gap of PFN+Br−. This temperature dependence is consistent with operation below and above the melting temperature (Tm=130°C) of PFN+Br− and is consequently a signature of anion mobility. The RT morphology of PFN+Br− is dependent on the film preparation process: slow evaporation of the solvent via drop casting produces a partially crystalline film, while a fast evaporation via spin casting produces a glassy film. The glassy spin-cast film can, however, be transformed into a partially crystalline film by slow cool...

Light emission from an ambipolar semiconducting polymer field effect transistor: Analysis of the device physics

Light emitting field-effect transistors (LEFETs) were fabricated with a low work function metal (calcium) and a high work function metal (gold) as the source and drain electrodes. The gold electrode serves as the source for holes into the π band and the drain for electrons from the π* band; the calcium electrode serves as the source for electrons into the π* band and the drain for holes from the π band. For 65V<VG<103V, the LEFET operates in the ambipolar regime. The emission zone has been spatially resolved (as it is moved across the channel by sweeping the gate voltage) using confocal microscopy; the full width at half maximum is 2μm. At the gate voltage extremes (VG=0 or VG=150V), the electron (hole) density extends all the way across the 16μm channel such that the electron (hole) accumulation layer functions as the cathode (anode) for a light-emitting diode, with opposite carrier injection by tunneling; i.e., the carrier densities are sufficiently high that the accumulation layer functions as a low re...

Toward improved and tunable polymer field-effect transistors

We report an electrochemical method to improve charge injection in thin-film field-effect transistors fabricated with semiconducting polymers. By having ions, which are mobile only at elevated temperatures, in the active material [a mixture of a soluble poly(para-phenylene vinylene) copolymer, a crown ether and a LiCF3SO3 salt] we create electric double layers at the drain/source Au electrode interfaces by applying a low voltage (V=2 V) at T=85 °C for a short time (t∼1–5 min). After cooling to room temperature under applied voltage, we demonstrate significantly improved transistor performance. In addition, we present evidence of reversible electrochemical doping in this active material.

Light amplification in polymer field effect transistor structures

The amplified spontaneous emission (ASE) of optically pumped films of poly(2-(2′,5′-bis(octyloxy)benzene)-1,4-phenylenevinylene (BOP-PPV) was studied in structures comprising a gate electrode, a thin film of gate insulator material (SiO2) and the polymer film as luminescent semiconducting layer (i.e. a field effect transistor without the source and drain electrodes). The influences of different gate electrodes on the threshold and the wavelength of the amplified emission were measured for variable thickness of the gate insulator. An exponential increase in ASE threshold (It) with decreasing separation between electrode and polymer layer was observed. In structures with 200 nm SiO2 gate insulator, It=300 kW/cm2 with an n-Si gate electrode and 200 kW/cm2 with Au electrode (100 nm thick). Compared to the same polymer film on pure SiO2 (It=2 kW/cm2), this increase results from waveguide losses in the nearby gate electrode. With an indium–tin–oxide (ITO) gate electrode (140 nm thick) on glass, again with a 200...

Optical amplification of the cutoff mode in planar asymmetric polymer waveguides

Modes with low threshold for optical gain were observed at wavelengths close to the cutoff in experiments probing the amplified spontaneous emission of light-emitting polymer thin films. The polymer was the semiconductor layer in a multilayer semiconductor–insulator–metal structure that simulates the one-dimensional waveguide characteristics in the channel of a field-effect transistor. The “cutoff” mode propagates at the polymer/gate-insulator interface, has an optical gain threshold of approximately 10 kW/cm2, and is not influenced by absorption of the gate electrode. The wavelength of the amplified emission tracks the cutoff wavelength of the asymmetric double-waveguide structure and the cutoff mode is, therefore, tunable in wavelength. Our results suggest that the light-emitting field-effect transistor architecture is a promising route for the construction of an injection laser.