Mongryong Lee

Liquid crystalline blue phase I observed for a bent-core molecule and its electro-optical performance

We investigated the liquid crystalline blue phase I (BP I) observed for a nematogenic achiral bent-core molecule doped with a small percentage of a chiral additive with a high twisting power. We also observed the electro-optical performance of BP I by incorporating in-plane electric field geometry.

Surface plasmon resonance biosensing based on target-responsive mobility switch of magnetic nanoparticles under magnetic fields

A novel surface plasmon resonance (SPR) detection technique based on the programmed assembly of superparamagnetic iron oxide nanoparticles (SPIONs) and the corresponding change in mobility under external magnetic fields was demonstrated. In this approach, SPIONs act as a magnetophoretic mobility switch undergoing aggregation only in the presence of target analytes. The aggregated SPIONs which were magnetically attracted to a metal film create a layer over the sensor surface with a refractive index contrast, resulting in a notable SPR angle change. The experimental results indicated that the concentrations of the reactants such as SA and the size of the SPIONs play important roles in achieving enhanced SPR sensing. As a result, this study illustrates the potential for sensitive and selective detection of target molecules without the requirement of immobilized receptors on the sensor surface.

Surface properties and liquid crystal alignment behavior of poly(2-hydroxyethyl methacrylate) derivatives with alkyl ester side chains.

Poly(2-hydroxyethyl methacrylate) derivatives with amphiphilic side chains composed of polar ester and non-polar alkyl groups (PHEMA#C, #=9, 11, 13, 15, and 17), where # is the number of carbon atoms in the alkyl side groups, were synthesized. In this paper, the influence of ester and alkyl groups on the molecular structure and wettability of the polymers were studied through varying # in the alkyl side groups. PHEMA#Cs with relatively longer alkyl side groups (#≥15) show bilayer lamellar structures with well aligned side chains giving rise to the very low surface energies, calculated from advancing contact angles, in the range of 22.7-22.8 mN/m. In contrast, PHEMA#Cs with shorter alkyl side groups with #≤13 have disordered structures on the polymer surfaces and stick-slip behavior was observed when water was used as the test liquid for the advancing contact angle measurements. Furthermore, the alignment behavior of nematic liquid crystal, 5CB on the PHEMA#C films could be correlated with the molecular structure and wettability of the polymers.

Liquid crystal alignment behaviors and wetting properties of polymer blend surfaces

AbstractBlend surfaces of poly(2-hydroxyethyl methacrylate) derivatives with different alkyl side chain lengths were prepared, and their wetting properties and liquid crystal (LC) alignment behaviors were studied as a function of blend ratio. Based on these results, we found that LC alignment behavior on the blend surfaces is predominantly affected by polymer regions with ordered alkyl side chains, whereas wettability is determined by polymer regions with disordered alkyl side chains. This difference is explained by the fact that wetting properties rely on the interaction at the three phase line between the liquid and polymer surfaces in contrast to LC alignment behavior, which is influenced by the entire polymer surface. We suggest that wettability has only limited utility to explain LC alignment behavior.

Surface characterization and liquid crystal alignment behavior of comb-like poly(oxyethylene)/poly(3-hexylthiophene) blend films.

The blend surfaces of poly[oxy(n-decylsulfonylmethyl)ethylene] (CH(3)-10SE) and poly (3-hexylthiophene) (P3HT) with different weight ratios were prepared by spin coating the polymer solution mixtures. In this study, their surface properties such as surface morphology, chemical composition, molecular structure, and wettability were systematically studied and correlated with liquid crystal (LC) alignment behaviors on the blend films. Therefore, we found that CH(3)-10SE part with a well-ordered side chain structure predominantly affects the both of wettability and LC alignment behavior of the blend films while there was no clear association between the wettability and the LC alignment behavior.

Temperature-Dependent Behaviours of Blue Phase I Observed for a Bent-Core Molecular System

Temperature-dependent behaviours, such as the Bragg reflection band and the EO performance, of the liquid-crystalline BP I observed for a nematogenic achiral bent-core molecule doped with a small amount of chiral additive was investigated. Our experimental results indicate that the physical behaviours of our sample were highly sensitive to temperature changes.

Fabrication of Broadband Cholesteric Liquid Crystal Films by Photopolymerization-Induced Phase Separation

We optimized fabrication conditions of cholesteric liquid crystal (CLC) films; 1) amount of photo-initiator, 2) exposure conditions. Based on these conditions, we could succeed in fabricating broadband CLC films with Δλ ≈ 250 nm. We also examined the spatial helical pitch gradient structure fabricated here using a field emission-scanning electron microscope (FE-SEM) at a freeze-fractured cross-section of the fabricated CLC films.

The effect of surface polarity of glass on liquid crystal alignment

ABSTRACT The effects of the surface polarity of a glass substrate on the orientation of nematic liquid crystals (LCs) were studied using the polarised optical microscope and Fourier-transform infrared spectroscopy. On the surface of oxygen plasma treated glass, a homeotropic alignment of LCs was induced for LCs with negative dielectric anisotropy. This suggests that vertical orientation of LCs could be induced on a polar glass substrate without using an LC alignment layer. Upon cooling towards the isotropic–nematic transition, E7 with positive dielectric anisotropy changes its LC arrangement to isotropic, homeotropic, planar orientations in order. The nematic LC anchoring transition of E7 was interpreted by considering the competition between van der Waals forces and dipole interactions that control the alignment of LC molecules on a polar glass surface. Graphical Abstract

Electrode configurations for pentacene-deposited organic thin film transistors

ABSTRACT When a pentacene OTFT is fabricated with a bottom gate structure, the top-contact configuration of gold electrodes is preferred because it results in better electrical performance than the bottom-contact configuration. Pentacene molecules are deposited only on the SiO2 dielectric layer in the top-contact configuration, and thus all pentacene molecules form a vertical orientation that induces efficient charge transport between molecules. However, in an OTFT device with the bottom-contact configuration, pentacene molecules adopt a random tilted orientation at the boundary of the gold electrode, resulting in poor charge transport between molecules.