Chan Eon Park

Highly Stretchable Resistive Pressure Sensors Using a Conductive Elastomeric Composite on a Micropyramid Array

A stretchable resistive pressure sensor is achieved by coating a compressible substrate with a highly stretchable electrode. The substrate contains an array of microscale pyramidal features, and the electrode comprises a polymer composite. When the pressure-induced geometrical change experienced by the electrode is maximized at 40% elongation, a sensitivity of 10.3 kPa(-1) is achieved.

Effect of Selenophene in a DPP Copolymer Incorporating a Vinyl Group for High‐Performance Organic Field‐Effect Transistors

A new polymeric semiconductor, PDPPDTSE, is reported which is composed of a diketopyrrolopyrrole moiety and selenophenylene vinylene selenophene, with a high field-effect mobility achieved through intermolecular donor-acceptor interactions. The field-effect mobility of OFET devices based on PDPPDTSE by spin-casting is 4.97 cm(2) V(-1) s(-1) , which is higher than predecessor polymeric semiconductors.

Low-voltage pentacene field-effect transistors with ultrathin polymer gate dielectrics

Organic field-effect transistors (OFETs) for low-voltage operation have been realized with conventional polymer gate dielectrics such as polyimides and cross-linked poly-4-vinyl phenols (PVPs) by fabricating ultrathin films. These ultrathin polymers (thickness ∼10nm) have shown good insulating properties, including high breakdown fields (>2.5MV∕cm). With ultrathin dielectrics, high capacitances (>250nF∕cm2) have been achieved, allowing operation of OFETs within −3V. Pentacene OFETs with ultrathin PVP dielectrics exhibit a mobility of 0.5cm2∕Vs, an on-off ratio of 105, and a small subthreshold swing of 174mV∕decade when devices are operated at −3V.

Investigation of crystallinity effects on the surface of oxygen plasma treated low density polyethylene using X-ray photoelectron spectroscopy

Plasma treated polymer surface shows rapid hydrophobic recovery after plasma treatment. Surface rearrangement and the hydrophilicity change of low density polyethylene (LDPE) films with different degrees of crystallinity after oxygen plasma treatment were investigated using X-ray photoelectron spectroscopy (XPS) and water contact angle measurements. Since the mobility of polymer chain was restricted in the crystalline region, the chain movement of high crystalline polymer would be reduced. Water contact angles of quenched and annealed LDPE just after plasma treatment were almost same and oxygen contents of quenched LDPE was slightly higher than that of annealed LDPE. However, annealed LDPE showed slower contact angle raise and kept higher oxygen concentration than quenched LDPE with increasing aging time. By deconvolution of carbon XPS spectra, it was observed that major group contributed to decrease of oxygen contents was the hydroxyl groups and hydroxyl groups of quenched LDPE decayed faster than annealed LDPE while other oxygen species kept constant or decreased slightly. Increasing crystallinity reduce the surface rearrangement of polymer chain after plasma treatment.

Low-operating-voltage pentacene field-effect transistor with a high-dielectric-constant polymeric gate dielectric

Low-operating-voltage organic field-effect transistor has been realized by using the cross-linked cyanoethylated poly(vinyl alcohol) (CR-V) as a gate dielectric. The cross-linked CR-V dielectric was found to have a high dielectric constant of 12.6 and good insulating properties, resulting in a high capacitance (92.9nF∕cm2 at 20Hz) for a dielectric thickness of 120nm. A pentacene field-effect transistor fabricated with the cross-linked CR-V dielectric was found to exhibit a high carrier mobility (0.62cm2∕Vs), a small subthreshold swing (185mV∕decade), and little hysteresis at low operating voltages (⩽−3V).

Al2O3/TiO2 nanolaminate thin film encapsulation for organic thin film transistors via plasma-enhanced atomic layer deposition.

Organic electronic devices require a passivation layer that protects the active layers from moisture and oxygen because most organic materials are very sensitive to such gases. Passivation films for the encapsulation of organic electronic devices need excellent stability and mechanical properties. Although Al2O3 films obtained with plasma enhanced atomic layer deposition (PEALD) have been tested as passivation layers because of their excellent gas barrier properties, amorphous Al2O3 films are significantly corroded by water. In this study, we examined the deformation of PEALD Al2O3 films when immersed in water and attempted to fabricate a corrosion-resistant passivation film by using a PEALD-based Al2O3/TiO2 nanolamination (NL) technique. Our Al2O3/TiO2 NL films were found to exhibit excellent water anticorrosion and low gas permeation and require only low-temperature processing (<100 °C). Organic thin film transistors with excellent air-stability (52 days under high humidity (a relative humidity of 90% and a temperature of 38 °C)) were fabricated.

Bending-stress-driven phase transitions in pentacene thin films for flexible organic field-effect transistors

The effects of bending strain on the structure and electrical characteristics of pentacene films in flexible devices were investigated. It was found that the volume fraction of bulk phase in the pentacene film increases from 10.7% to 27.7% under 1.1% of tensile strain but decreases to 3.5% under 1.0% of compressive strain. These bending-stress-driven phase transitions between the bulk phase and the thin-film phase in the pentacene film resulted in the changes in field-effect mobility, and were driven by the differences between the in-plane dimensions of the crystal unit cells of the two phases to reduce the external bending stress.

Effects of direct solvent exposure on the nanoscale morphologies and electrical characteristics of PCBM-based transistors and photovoltaics

We investigated the effects of direct solvent exposure on the properties of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) films and poly(3-hexylthiophene) (P3HT)/PCBM blend films employed as active layers in, respectively, organic field-effect transistors (OFETs) and organic photovoltaics (OPVs). The crystallinity, morphology, and OFET characteristics of the PCBM thin films were significantly influenced by direct exposure to solvent, especially to select alcohols. Control over the nanoscale morphology of the PCBM film, achieved via direct solvent exposure, yielded highly efficient poly(3-hexylthiophene) (P3HT)/PCBM OPVs with a short-circuit current density of 10.2 mA cm−2, an open-circuit voltage of 0.64 V, and a power conversion efficiency of 3.25% under AM 1.5 illumination with a light intensity of 100 mW cm−2. These results indicated that optimal phase separation in the P3HT/PCBM films could be obtained simply by exposing the active layer films for a few seconds to solvent.

Adhesion improvement of copper/epoxy joints

Copper oxides have been formed to improve the adhesive strength of copper/epoxy joints. Initial adhesive strength and durability of copper/epoxy joints were compared depending upon the type of oxides, black or red oxide. Although the initial adhesive strength of black oxide treated joints was worse than that of red oxide treated joints, the durability in acidic environment was better. In order to improve the durability of red oxide treated joints, 5-amino-indazole was applied to inhibit the corrosion of oxide layer in acidic medium. With the treatment of 5-amino-indazole, initial adhesive strength was increased by 50%, and durability was improved. The loci of failure for oxide treated joints were investigated with scanning electron microscope and X-ray photoelectron spectroscopy.

The effect of rubber particle size on toughening behaviour of rubber-modified poly(methyl methacrylate) with different test methods

Abstract In order to study the effect of particle size on the toughening behaviour of rubber-toughened poly(methyl methacrylate) (PMMA), a systematic model study has been carried out using core-shell type particles, made up of a poly(n-butyl acrylate) (PBA) core and a PMMA outer shell. Two different fracture tests, i.e. a three-point bending test for the evaluation of the fracture toughness (KIC) and an impact test were employed to study the toughening behaviour. In the case of the impact test, maximum impact strength was obtained for rubber particles with a 0.25 μm diameter regardless of the rubber phase contents, and only a modest improvement of the impact strength was obtained for blends containing 0.15 or 2 μm particles. For the three-point bending test, in contrast, even large particles such as the 2 μm particles provided a significant enhancement in the fracture toughness. The difference in the toughening behaviour due to particle size may be attributed to the test method dependence associated with a change in the deformation mechanism. In the three-point bending test, the deformation mechanism was found to be multiple crazing, whereas in the impact test, the shear yielding induced by cavitation of the rubber particles is predominant.