Chang-won Lee

Interdimensional universality of dynamic interfaces

Despite the complexity and diversity of nature, there exists universality in the form of critical scaling laws among various dissimilar systems and processes such as stock markets, earthquakes, crackling noise, lung inflation and vortices in superconductors. This universality is mainly independent of the microscopic details, depending only on the symmetry and dimension of the system. Exploring how universality is affected by the system dimensions is an important unresolved problem. Here we demonstrate experimentally that universality persists even at a dimensionality crossover in ferromagnetic nanowires. As the wire width decreases, the magnetic domain wall dynamics changes from elastic creep in two dimensions to a particle-like stochastic behaviour in one dimension. Applying finite-size scaling, we find that all our experimental data in one and two dimensions (including the crossover regime) collapse onto a single curve, signalling universality at the criticality transition. The crossover to the one-dimensional regime occurs at a few hundred nanometres, corresponding to the integration scale for modern nanodevices.

High‐Performance Nanowire Oxide Photo‐Thin Film Transistor

A gate-modulated nanowire oxide photosensor is fabricated by electron-beam lithography and conventional dry etch processing.. The device characteristics are good, including endurance of up to 10(6) test cycles, and gate-pulse excitation is used to remove persistent photoconductivity. The viability of nanowire oxide phototransistors for high speed and high resolution applications is demonstrated, thus potentially expanding the scope of exploitation of touch-free interactive displays.

Coverage Control of DNA Crystals Grown by Silica Assistance

The impetus behind the current interest in combining DNA materials with conventional nanotechnologies, such as nanoelectronics, biosensors, and nanophotonics, emanates from an ambition to exploit its remarkable properties. One of these properties is self-assembly that is driven by the thermodynamics of sticky end hybridization and makes structural DNA nanotechnology a prime candidate for bottom-up fabrication schemes in these fields. However, unless self-assembled DNA nanostructures can be fabricated on solid surfaces to at least the degree of accuracy of existing top-down methods, it will be unfeasible to replace it with existing technologies. An intermediate step toward this goal has been to merge the two approaches such that DNA nanostructures are self-assembled onto lithographically patterned substrates. Previous works have been successful at depositing self-assembled DNA nanostructures on patterned substrates and controlling the spatial orientations of tailored DNA origami motifs at specifically designated sites. All these approaches have used random depositions (or similar methods) of preformed DNA structures onto lithographically patterned substrates. What has been lacking in literature is a method of precisely controlling the coverage of DNA structures on various substrates, that is, the percentage of the surface covered by crystals, especially on silica (SiO2), which is crucial if DNA is to be universally employed in electronics. We provide a solution to this problem by introducing a new surface-assisted fabrication method, termed the silica-assisted growth (SAG) method, to selfassemble DNA nanostructures on SiO2 surfaces. The novel fabrication technique presented herein bears two important distinctions from previous studies. Firstly, direct annealing on the substrates allows for very accurate control of the amount of DNA structures that self-assemble on the substrate, that is, the coverage. Secondly, because of electrostatic interactions with the silica surface, structures grown by this method show drastic topological changes that lead to previously unreported novel structures. The pretreatment process of SiO2 substrates and the various DNA structures grown on them are shown in Figure 1. Silanol groups on the SiO2 surface become deprotonated once the substrates are treated in a 10 TAE/Mg buffer (see the Experimental section for details) since the pH of this buffer exceeds the isoelectric point of SiO2. [13] This approach allows Mg ions to bind to the substrate surface, which in turn binds the negatively charged DNA backbones (Figure 1a). To demonstrate the SAG method, four different types of DNA nanostructures were prepared. 8 helix tubes (8 HT) and 5 helix ribbons (5 HR) were constructed from single-stranded tiles (SST), while double-crossover (DX) crystals and DX crystals with biotin modifications were fabricated from DX tiles (see Figure S1–S3 in the Supporting Information). The schematic diagrams of the various DNA structures are illustrated in Figure 1b–f and their corresponding AFM images on SiO2 substrates are shown in Figure 1g–p (Figure 1g–k show structures made from the free solution annealing method deposited onto SiO2 for imaging and Figure 1 l–p show structures made using the SAG method where the structures are annealed directly on the substrate, see Figure S4 in the Supporting Information). For the 8 HT, there is a dramatic difference between the structure formations of the free solution annealing method and the SAG method. Caused by a local minimum in the free energy landscape, monodisperse 8 HT structures on SiO2 fabricated from the free solution annealing method are stable, which can be clearly seen in Figure 1g. In this case, the structures have already been formed in solution before they are deposited onto the substrate. Meanwhile, in the case of SAG, the charges of the Mg ions bound on the substrate surface interact with the DNA strands to prevent the formation of tubes. Through these interactions, an acute topological change of the structures occurs, allowing SSTs to bind edgewise and to remain in a single-layer state (Figure 1 l) with some of the tiles overlapping along their boundaries (Figure 1 l, bright regions). To the best of our knowledge, this is the first observation of 2D crystals arising from SST motifs. Another type of 1D structure, the 5 helix ribbon, was also successfully fabricated using both the free solution annealing and SAG methods as can be seen in Figure 1h and m, respectively. The substrate acts as a catalyst to reduce the amount of energy needed for DNA structures to form, resulting in large-scale structure formations on the substrates. In the case of DX crystals, two-tile units of DXmonomers were used as building blocks to fabricate periodic arrays. One [*] J. Lee, J. Kim, Prof. S. H. Park Sungkyunkwan Advanced Institute of Nanotechnology (SAINT) and Department of Physics, Sungkyunkwan University Suwon 440-746 (Korea) E-mail: sunghapark@skku.edu S. Kim, Prof. Y. Roh School of Information and Communication Engineering, Sungkyunkwan University Suwon 440-746 (Korea) E-mail: yhroh@skku.edu

Tunable Metamaterials for Controlling THz Radiation

Remarkable progress in terahertz (THz) sources and detectors is followed by the necessity of manipulating of terahertz radiation. Since natural materials can not perform efficient interaction with THz radiation, artificial structures called metamaterials are designed to overcome “THz gap” in this area. A variety of tunable metamaterials using different methods of control are presented and discussed in this review paper.

Intrinsic high-frequency characteristics of graphene layers

The experimental results for the high-frequency transport character- istics (from 0.5 to 110GHz) of graphene sheets are presented. Samples with graphene sheets of a different number of layers, as well as samples with- out graphene, were fabricated and compared by their room-temperature radio- frequency (RF) transmission properties. From RF two-port network experiments, the circuit parameters of resistance, inductance and capacitance were extracted using a lumped circuit model that consists of graphene, metal electrodes and con- tacts. Self-inductance was suppressed with decreasing number of layers, possibly due to minimized interlayer conduction or scattering. The graphene-electrode contact property shows dependence on the number of graphene layers. Our investigation may promote an understanding of the intrinsic graphene character- istics and graphene-electrode contact configuration in passive graphene devices for RF applications.

Enhanced Inhibitory Effect of Ultra-Fine Granules of Red Ginseng on LPS-induced Cytokine Expression in the Monocyte-Derived Macrophage THP-1 Cells

Red ginseng is one of the most popular traditional medicines in Korea because its soluble hot-water extract is known to be very effective on enhancing immunity as well as inhibiting inflammation. Recently, we developed a new technique, called the HAC-gearshift system, which can pulverize red ginseng into the ultra-fine granules ranging from 0.2 to 7.0 μm in size. In this study, the soluble hot-water extract of those ultra-fine granules of red ginseng (URG) was investigated and compared to that of the normal-sized granules of red ginseng (RG). The high pressure liquid chromatographic analyses of the soluble hot-water extracts of both URG and RG revealed that URG had about 2-fold higher amounts of the ginsenosides, the biologically active components in red ginseng, than RG did. Using quantitative RT-PCR, cytokine profiling against the Escherichia coli lipopolysaccharide (LPS) in the monocyte-derived macrophage THP-1 cells demonstrated that the URG-treated cells showed a significant reduction in cytokine expression than the RG-treated ones. Transcription expression of the LPS-induced cytokines such as TNF-α, IL-1β, IL-6, IL-8, IL-10, and TGF-β was significantly inhibited by URG compared to RG. These results suggest that some biologically active and soluble components in red ginseng can be more effectively extracted from URG than RG by standard hot-water extraction.

Modulation of the Dirac point voltage of graphene by ion-gel dielectrics and its application to soft electronic devices.

We investigated systematic modulation of the Dirac point voltage of graphene transistors by changing the type of ionic liquid used as a main gate dielectric component. Ion gels were formed from ionic liquids and a non-triblock-copolymer-based binder involving UV irradiation. With a fixed cation (anion), the Dirac point voltage shifted to a higher voltage as the size of anion (cation) increased. Mechanisms for modulation of the Dirac point voltage of graphene transistors by designing ionic liquids were fully understood using molecular dynamics simulations, which excellently matched our experimental results. It was found that the ion sizes and molecular structures play an essential role in the modulation of the Dirac point voltage of the graphene. Through control of the position of their Dirac point voltages on the basis of our findings, complementary metal-oxide-semiconductor (CMOS)-like graphene-based inverters using two different ionic liquids worked perfectly even at a very low source voltage (V(DD) = 1 mV), which was not possible for previous works. These results can be broadly applied in the development of low-power-consumption, flexible/stretchable, CMOS-like graphene-based electronic devices in the future.

Current-Induced Domain-Wall Motion in [CoFe/Pt]

We experimentally demonstrate the current-induced domain wall motion in [CoFe/Pt]5 nanowire with perpendicular magnetic anisotropy. By use of the scanning MOKE measurement, we observe that two domain walls are moved simultaneously by spin polarized current at the current density of 1.43 times 1012 A/m2. The domain wall speed is estimated to be about 1.5 m/s, determined by the domain wall displacement (750 nm) and the current pulse duration (500 ns). From in situ resistance measurement, the temperature of the nanowire rises up to 700 K when the domain wall moves at such the high current density.

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The design, synthesis, and supramolecular organization of a nanocomposite in which nanoscale excitonic interactions between quantum dots and the chiral polymer dramatically enhance the optical activity is reported. This material is highly suitable for application in the emerging field of chiral photonics.

Nanowire With Perpendicular Magnetic Anisotropy

We investigated the structural and electrical properties of polycrystalline NiO thin films on Pt electrodes formed by thermal oxidation. A Ni–Pt alloy phase was found at the interface, which could be explained by the oxidation kinetics and reactions of Ni, NiO, and Pt. An increase in the oxidation temperature decreased the volume of the alloy layer and improved the crystalline quality of the NiO thin films. Pt/NiO/Pt structures were fabricated, and they showed reversible resistance switching from a high-resistance state (HRS) to a low-resistance state (LRS) and vice versa during unipolar current–voltage measurements. The oxidation temperature affected (did not affect) the HRS (LRS) resistance of the Pt/NiO/Pt structures. This indicated that the transport characteristics of HRS and LRS should be different.