Young-Wan Kwon

Dimesogenic compounds consisting of cholesterol and azobenzene‐based moieties: dependence of liquid crystal properties on spacer length and fluorination of the terminal chain

The liquid crystalline (LC) properties of two series of non‐symmetric dimesogenic compounds consisting of cholesterol and azobenzene‐based moieties interconnected by ω‐oxyalkanoyl spacers of varying length are compared: one series (AOC‐n) has an octyloxy chain attached to the azobenzene mesogen unit while the other (AOCF‐n) has a perfluoroheptylmethyloxy chain. In general, compounds bearing the fluorinated alkoxy chain exhibited LC properties over a much broader temperature range than those with the alkoxy chain. In addition, the AOC‐n series exhibited the chiral smectic C (SmC*), smectic A (SmA) and cholesteric (N*) phases depending on the length of the central spacer, whereas the AOCF‐n series favoured the formation of only the SmA phase with the N* phase completely suppressed. Both series showed an odd–even dependence of the isotropization temperature on spacer length.

Raman spectroscopic study of plasma-treated salmon DNA

In this research, we studied the effect of plasma treatment on the optical/structural properties of the deoxyribonucleic acid (DNA) extracted from salmon sperm. DNA-cetyltrimethylammonium (CTMA) films were obtained by complexation of DNA with CTMA. Circular dichroism (CD) and Raman spectra indicated that DNA retained its double helical structure in the solid film. The Raman spectra exhibited several vibration modes corresponding to the nuclear bases and the deoxyribose-phosphate backbones of the DNA, as well as the alkylchains of CTMA. Dielectric-barrier-discharge (DBD) plasma treatment induced structural modification and damage to the DNA, as observed by changes in the ultraviolet-visible absorption, CD, and Raman spectra. The optical emission spectra of the DBD plasma confirmed that DNA modification was induced by plasma ions such as reactive oxygen species and reactive nitrogen species.

Memory effects in polymer stabilized ferroelectric liquid crystals, and their dependence on the morphology of the constituent molecules

The origin of distinct memory states (multistable, bistable, monostable) was verified in thiol-ene polymer stabilized ferroelectric liquid crystal system. The stabilization is obtained by the polymerized ene in the interlayer space. The multistability and the resolution of memory states has strong correlation with the dichroic ratio of thiol. As the thiol lengthens, thiols are more intersticed between ferroelectric liquid crystal molecules in the layer and the multistability and the resolution were also enhanced, consequently. In addition, it was found that the binding of thiol to the ene through polymerization was essential for the realization of multistable memory states. The cone mode viscosity was decreased as the thiol lengthens, and this also seems to promote multistability and resolution of memory.

Dimesogenic liquid crystal consisting of cholesterol and Schiff base moieties: dependence of LC properties on the spacer length and fluorination of the alkoxy tails

The liquid crystalline properties of two series of non‐symmetric liquid crystal dimers consisting of cholesterol and Schiff base moieties interconnected by ω‐oxyalkanoyl spacers of varying length are compared: one series (SBOC‐ n ) carry the octyloxy tail on the Schiff base mesogen, and the other (SBOF‐ n ) a perfluoroheptylmethyloxy tail. In general, compounds with the fluorinated alkoxy tail exhibited mesophases over a much wider temperature range than those with the alkoxy tail. The latter series favoured the formation of more diverse mesophases than the former. SBOC‐4, ‐5 and ‐7, and SBOF‐4, ‐5 and ‐10 formed the chiral smectic C phase.

Singly and Doubly Occupied Higher Quantum States in Nanocrystals

Filling the lowest quantum state of the conduction band of colloidal nanocrystals with a single electron, which is analogous to the filling the lowest unoccupied molecular orbital in a molecule with a single electron, has attracted much attention due to the possibility of harnessing the electron spin for potential spin-based applications. The quantized energy levels of the artificial atom, in principle, make it possible for a nanocrystal to be filled with an electron if the Fermi-energy level is optimally tuned during the nanocrystal growth. Here, we report the singly occupied quantum state (SOQS) and doubly occupied quantum state (DOQS) of a colloidal nanocrystal in steady state under ambient conditions. The number of electrons occupying the lowest quantum state can be controlled to be zero, one (unpaired), and two (paired) depending on the nanocrystal growth time via changing the stoichiometry of the nanocrystal. Electron paramagnetic resonance spectroscopy proved the nanocrystals with single electron to show superparamagnetic behavior, which is a direct evidence of the SOQS, whereas the DOQS of the two- or zero-electron occupied nanocrystals in the 1Se exhibit diamagnetic behavior. In combination with the superconducting quantum interference device measurement, it turns out that the SOQS of the HgSe colloidal quantum dots has superparamagnetic property. The appearance and change of the steady-state mid-IR intraband absorption spectrum reflect the sequential occupation of the 1Se state with electrons. The magnetic property of the colloidal quantum dot, initially determined by the chemical synthesis, can be tuned from diamagnetic to superparamagnetic and vice versa by varying the number of electrons through postchemical treatment. The switchable magnetic property will be very useful for further applications such as colloidal nanocrystal based spintronics, nonvolatile memory, infrared optoelectronics, catalyst, imaging, and quantum computing.

Optical and structural properties of plasma-treated Cordyceps bassiana spores as studied by circular dichroism, absorption, and fluorescence spectroscopy

To understand the killing mechanism of fungal spores by plasma treatment, the optical, structural, and biological properties of the insect pathogenic fungus Cordyceps bassiana spores were studied. A nonthermal atmospheric-pressure plasma jet (APPJ) was used to treat the spores in aqueous solution. Optical emission spectra of the APPJ acquired in air indicated emission peaks corresponding to hydroxyl radicals and atomic oxygen. When the APPJ entered the aqueous solution, additional reactive species were derived from the interaction of plasma radicals with the aqueous solution. Fluorescence and absorption spectroscopy confirmed the generation of hydroxyl radicals and hydrogen peroxide in the plasma-activated water (PAW). Spore counting showed that plasma treatment significantly reduced spore viability. Absorption spectroscopy, circular dichroism (CD) spectroscopy, and agarose gel electrophoresis of the DNA extracted from plasma-treated spores showed a reduction in spore DNA content. The magnitude of the dip in the CD spectrum was lower in the plasma-treated spores than in the control, indicating that plasma treatment causes structural modifications and/or damage to cellular components. Tryptophan fluorescence intensity was lower in the plasma-treated spores than in the control, suggesting that plasma treatment modified cell wall proteins. Changes in spore viability and DNA content were attributed to structural modification of the cell wall by reactive species coming from the APPJ and the PAW. Our results provided evidence that the plasma radicals and the derived reactive species play critical roles in fungal spore inactivation.

Realization of grayscale memory operation in a step-growth based polymer-stabilized ferroelectric liquid crystal system

A grayscale memory states operation can be realized in which the liquid crystal directors are uniformly aligned in each state. This operation is realized by a specific morphology of polymers obtained through step-growth polymerization of a thiol–ene stabilizing the ferroelectric liquid crystal system. The continuous director rotation (CDR) of liquid crystal (LC) molecules as well as the grayscale memory behavior is observed where the LC directors are oriented uniformly in each state. The memory operation is due to the interaction of LC molecules with the thiols intercalated in the smectic layer with one end of each thiol bound to the polymerized ene in the inter-layer planes. The morphology of the polymer in the liquid crystal plays a crucial role in the existence of grayscale memory behavior, and the morphology depends strongly on the phase of the sample when the thiol-ene monomers are polymerized. A new mechanism is suggested for the CDR motion of the ferroelectric liquid crystal (FLC). [DOI: 10.1143/JJAP.45.5872]

Effects of reactive oxygen species on the biological, structural, and optical properties of Cordyceps pruinosa spores

Effects of reactive oxygen species (ROS) on the optical, structural, and biological properties of Cordyceps pruinosa spores were studied. Both the atmospheric pressure plasma jet (APPJ) and chemically induced ROS significantly reduced the viability of C. pruinosa spores. Changes in the peak intensity of fluorescence and the depth of the dip in the circular dichroism (CD) spectrum suggested that both the APPJ and chemical induction of ROS can cause structural alteration of the spore cell wall. Fluorescence spectra of propidium iodide-stained spores indicated that alteration of cell wall (and/or membrane) permeability is involved in the change of spore viability after plasma treatment. High-performance liquid chromatography analysis of C. pruinosa ethanol extracts showed that the APPJ and chemical induction of ROS decreased the amount of ergosterol in the spores, indicating that excessive oxidative stress destroys cellular antioxidant capacity. Absorption spectroscopy, CD spectroscopy and agarose gel electrophoresis of the DNA extracted from the plasma-treated spores showed that a decrease in the DNA content and DNA degradation can be caused by either of the two treatments. The nonthermal APPJ and chemical induction were used to generate ROS in an aqueous solution. Electron spin resonance spectra provided evidence that hydroxyl radicals and singlet oxygen exist in the plasma activated water (PAW). Overall, the decline in spore viability, in antioxidative capacity, and in DNA content can be attributed to structural alteration of the cell wall and cellular damage by reactive species originating from the APPJ and the PAW.

Electrical properties of nanofibers and structural characterization of DNA-Au(III) complexes

In order to realize deoxyribonucleic acid (DNA)-based molecular electronics, chemical modifications of DNA are needed to improve electrical conductivity. We developed a novel method utilizing the incorporation of Au(III) ions into DNA bases to alter their electronic properties. When Au(III) ions were incorporated proportionally into DNA bases, conductance increased up to an Au(III) content of 0.42 Au(III) ion/nucleotide. Surprisingly, electron paramagnetic resonance signals of Au(II) ions were detected at g ∼1.98, and the calculated spin number of Au(II) ions ranged from ∼10(13) to ∼10(15). The structural deformation of the DNA helix occurred when complexed with Au(III); simultaneously, the conductance of DNA-Au(III) complexes decreased when the content of Au(III) was higher than 0.42 atom/nucleotide. This observation implies that the maintenance of helical structure in the Au(III) doped state of DNA molecules is very important to the enhancement of the carrier mobility of DNA.

High electron mobility of β-HgS colloidal quantum dots with doubly occupied quantum states

Electron occupation of the lowest electronic state of the conduction band (1Se) of a semiconducting nanocrystal offers numerous opportunities to efficiently utilize the quantization of the colloidal quantum dot. The steady-state electron occupation of the 1Se gives rise to unprecedented electrical, optical, and magnetic properties. We report an electron mobility of ∼1.29 cm2 V−1 s−1 measured in a mercury sulfide (β-HgS) quantum dot field effect transistor (FET), demonstrating the best carrier mobility for the HgS colloidal nanocrystal solid. The high electron mobility of the HgS nanocrystals with the doubly occupied quantum state originates from the efficient ligand exchange from oleylamine to thiocyanate, better carrier hopping via shortened inter-dot-distance, and the packing of nanocrystals by optimized thermal annealing conditions.