Conducting polymers (CPs) find applications in energy conversion and storage, sensors, and biomedical technologies once processed into thin films. Hydrophobic CPs, like poly(3,4-ethylenedioxythiophene) (PEDOT), typically require surfactant additives, such as poly(styrenesulfonate) (PSS), to aid their aqueous processability as thin films. However, excess PSS diminishes CP electrochemical performance, biocompatibility, and device stability. Here, we report the electrosynthesis of PEDOT thin films at a polarized liquid|liquid interface, a method nonreliant on conductive solid substrates that produces free-standing, additive-free, biocompatible, easily transferrable, and scalable 2D PEDOT thin films of any shape or size in a single step at ambient conditions. Electrochemical control of thin film nucleation and growth at the polarized liquid|liquid interface allows control over the morphology, transitioning from 2D (flat on both sides with a thickness of <50 nm) to “Janus” 3D (with flat and rough sides, each showing distinct physical properties, and a thickness of >850 nm) films. The PEDOT thin films were p -doped (approaching the theoretical limit), showed high π–π conjugation, were processed directly as thin films without insulating PSS and were thus highly conductive without post-processing. This work demonstrates that interfacial electrosynthesis directly produces PEDOT thin films with distinctive molecular architectures inaccessible in bulk solution or at solid electrode–electrolyte interfaces and emergent properties that facilitate technological advances. In this regard, we demonstrate the PEDOT thin film’s superior biocompatibility as scaffolds for cellular growth, opening immediate applications in organic electrochemical transistor (OECT) devices for monitoring cell behavior over extended time periods, bioscaffolds, and medical devices, without needing physiologically unstable and poorly biocompatible PSS.

The mechanism of radio-frequency current collapse in GaN–AlGaN heterojunction field-effect transistors (HFETs) was investigated using a comparative study of HFET and metal–oxide–semiconductor HFET current–voltage (I–V) and transfer characteristics under dc and short-pulsed voltage biasing. Significant current collapse occurs when the gate voltage is pulsed, whereas under drain pulsing the I–V curves are close to those in steady-state conditions. Contrary to previous reports, we conclude that the transverse electric field across the wide-band-gap barrier layer separating the gate and the channel rather than the gate or surface leakage currents or high-field effects in the gate–drain spacing is responsible for the current collapse. We find that the microwave power degradation in GaN–AlGaN HFETs can be explained by the difference between dc and pulsed I–V characteristics.

Highly conducting polypyrrole (PPY) films, doped with various anions [pTS À , ClO 4 À , and NO 3 À and mixed electrolyte system (pTS À þ ClO 4 À )], have been electrochemically synthesized in aqueous solution at $275 K in an inert atmosphere. PPY exhibits metallic order dc conductivity at room temperature and shows variation of conductivity with respect to time of polymerization. Effect of dopant anion on growth mechanism of PPY is evident from its surface morphology. X-ray photoelectron spectroscopy (XPS), used to examine the surface composition and doping level of various PPY films, confirms the anionic doping into the polymer backbone. Both XPS and ultraviolet-visible spectroscopy give evidence of formation of polarons and bipolarons. The temperature (4.2-320 K)-dependent dc conductivity data of these PPY films have been explained by Mott's 3D variable-range hopping conduction model. Mott's parameters have been estimated, and structural disorder with doping is correlated for all the samples. Mott's criterion for distant hopping sites prevails in case of moderately doped samples (PPY3, PPY4, and PPY5), whereas the hopping to nearest neighbor sites is found more suitable in case of highly doped samples (PPY1 and PPY2). The origin of these changes is due to the modification in the molecular structure of PPY, which is governed by different growth mechanisms for organic (pTS À ) and inorganic (ClO 4 À and NO 3 À ) counter anions. V C 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 50: 347-360, 2012

We report on AlGaN/GaN metal oxide semiconductor heterostructure field effect transistor (HFET) over SiC substrates with peripheries from 0.15 to 6 mm. These multigate devices with source interconnections were fabricated using a novel oxide-bridging approach. The saturation current was as high as 5.1 A for a 6 mm wide device with a gate leakage of 1 A/cm 2 for 1.5 m gate length in a 5 m source-drain opening. The cutoff frequency of around 8 GHz was practically independent of the device periphery. Large-signal output rf-power as high as 2.88 W/mm was measured at 2 GHz. Both the saturation current and the rf-power scaled nearly linearly with the gate width.