Chi Jung Kang

Resistive Switching Behavior in Organic–Inorganic Hybrid CH3NH3PbI3−xClx Perovskite for Resistive Random Access Memory Devices

The CH3 NH3 PbI3- x Clx organic-inorganic hybrid perovskite material demonstrates remarkable resistive switching behavior, which can be applicable in resistive random access memory devices. The simply designed Au/CH3 NH3 PbI3- x Clx /FTO structure is fabricated by a low-temperature, solution-processable method, which exhibits remarkable bipolar resistive switching and nonvolatile properties.

Scanning probe microscopy study of microcells from the organ surface Bonghan corpuscle

Microcells from organ surface Bonghan corpuscles [B. H. Kim, J. Acad. Med. Sci. DPR Kor. 90, 1 (1963)] of mammals have been studied by using optical microscopy, transmission electron microscopy and immunohistochemistry. In order to further investigate their physical and electrical properties at better resolution, many different modes of scanning probe microscopy were used in this research. Their surface morphology was studied by topography imaging and error-signal imaging of atomic force microscopy and their mechanical properties were investigated by force modulation microscopy. Electrostatic force microscopy was also used for their electrical characterization.

Digital versus analog resistive switching depending on the thickness of nickel oxide nanoparticle assembly

The thickness-dependent digital versus analog resistive switching of nickel oxide (NiOx) nanoparticle assemblies was investigated in a Ti/NiOx/Pt structure. The NiOx nanoparticles were chemically synthesized with ∼5 nm diameter. The Ti/NiOx/Pt structure with assembly thickness of ∼60 nm exhibited the digital-type bipolar resistive switching. However, the assembly with a thickness of ∼90 nm presented analog resistive switching with gradually decreasing resistance when sweeping −V while increasing resistance after applying +V. Repeating −V pulses decreased the resistance sequentially, but the high resistance was restored sequentially by repeating +V pulses, which is analogous to the potentiation and depression of adaptive synaptic motion.

A Nanopore Structured High Performance Toluene Gas Sensor Made by Nanoimprinting Method

Toluene gas was successfully measured at room temperature using a device microfabricated by a nanoimprinting method. A highly uniform nanoporous thin film was produced with a dense array of titania (TiO2) pores with a diameter of 70∼80 nm using this method. This thin film had a Pd/TiO2 nanoporous/SiO2/Si MIS layered structure with Pd-TiO2 as the catalytic sensing layer. The nanoimprinting method was useful in expanding the TiO2 surface area by about 30%, as confirmed using AFM and SEM imaging. The measured toluene concentrations ranged from 50 ppm to 200 ppm. The toluene was easily detected by changing the Pd/TiO2 interface work function, resulting in a change in the I–V characteristics.

Multimode threshold and bipolar resistive switching in bi-layered Pt-Fe2O3 core-shell and Fe2O3 nanoparticle assembly

The bias-polarity dependent multimode threshold and bipolar resistive switching characteristics in bi-layered Pt-Fe2O3 core-shell and γ-Fe2O3 nanoparticles assembly were investigated. The Ti/Pt-Fe2O3-core-shell-nanoparticles (∼20 nm)/γ-Fe2O3-nanoparticles (∼40 nm)/Pt structure exhibited a threshold switching upon applying −V at Ti electrode. However, the filaments were formed at +V and subsequently ruptured at −V, featured to be bipolar switching. After rupturing filaments, it returned to threshold switching mode. The presence of core-shell nanoparticles facilitates the threshold switching either by temporary formation of filaments or enhanced charge transport. Also, the oxygen reservoir role of Ti electrode was essential to form stable filaments for bipolar switching.

A novel structured polysilicon thin-film transistor that increases the on/off current ratio

We propose a novel polycrystalline silicon thin-film transistor (TFT) structure to reduce leakage current effectively by employing the offset region near the drain and extended gate electrodes. In the proposed devices, we have employed a novel gate insulator structure, which forms the offset region and the extended gate electrodes. According to the experimental results, the leakage current of the proposed TFT is reduced by more than two orders of magnitude, compared with that of conventional TFTs, while the ON current is reduced very little. This is verified by using a device simulator whereby the electron concentration in the offset region increases under the ON state and decreases under the OFF state, due to the extended gate electrodes and offset region.

Investigation of analog memristive switching of iron oxide nanoparticle assembly between Pt electrodes

The analog memristive switching of iron oxide (γ-Fe2O3) nanoparticle assembly was investigated. The γ-Fe2O3 nanoparticles were chemically synthesized with ∼10 nm in diameter and assembled to be a continuous layer as a switching element in Pt/nanoparticles/Pt structure. It exhibited the analog switching that the resistance decreased sequentially as repeating −V sweeps and pulses while increased as applying +V. The capacitance-voltage curves presenting hysteresis with flatband voltage shift and distortion of their shapes with respect to the applied voltage supported the redistribution of space charges in nanoparticle assembly that might induce resistive switching. The polarity-dependent analog resistance change proportional to pulse voltage, time, and number of pulses was analogy to potentiation and depression of adaptive synaptic motion.

Resistive switching characteristics of maghemite nanoparticle assembly on Al and Pt electrodes on a flexible substrate

Resistive switching characteristics of maghemite (γ-Fe2O3) nanoparticle assembly were investigated in structures of top-electrode (Al,Pt)/γ-Fe2O3-NPs (~ 30 nm-thick)/bottom electrode (Al,Pt) on a flexible polyethersulfone substrate. The assembled NP layer with Al electrodes showed both unipolar and bipolar switchings with abrupt resistance change in multiple levels associated with formation and sequential rupture of conducting filaments, which is ascribed to Fe enrichment by the interfacial reaction. On the other hand, the NP layer with Pt electrodes exhibited memristive switching with hysteresis in current–voltage characteristics dependent on bias polarity, gradually changing the resistance with respect to bias conditions, and preserved resistance until a new state was developed by subsequent biasing.

Conductance Control in Stabilized Carotenoid Wires

A study of molecular electronics of organic nanowires has been a popular research topic in the past decade not only because of a booming atmosphere of nanoscience and engineering but also a need for the smaller, faster, and flexible substitutes for the conventional metallic wires. The selfassembly of organic molecules containing a terminal thiol group on a gold substrate allows the measurement of electric conductance of the organic molecules using conducting Atomic Force Microscopy (c-AFM). Reproducible measurement of molecular conductance is possible by throughbond contacts (e.g., S Au bond) between the organic molecules and metal electrodes. However, materialization of high and controllable conductance in organic nanowires up to the level of metallic ones has been an elusive dream. Nevertheless, several nanometer-sized organic molecules such as p-phenylene-ethylene oligomers, p-phenylene– ethynylene oligomers, and especially carotenoids showed somewhat promising properties as a conducting molecular wire. It was pointed out that the existence of conjugated unsaturated carbon carbon bonds, reflecting delocalized pelectron system, was essential for high conductance of organic molecular wires. Carotenoids, natural products known as a strong antioxidant as well as a harmless red pigment, also play an important role of transferring electrons in biological processes such as visual action and photosynthesis. The potential of these ideal molecular wires was not fully elucidated but rather limitedly investigated under carefully controlled conditions mainly due to their thermal and photochemical instabilities. The molecular conductance of the carotene dithiol 1 with typical nine C=C bond conjugation (Figure 1), submerged in toluene under argon atmosphere was only 0.28 nS (nano-Siemens) even though it was still 6–7 orders of magnitude higher than that of n-alkane of equivalent chain length. The molecular conductance of the carotenoid wires decreased exponentially with a small decay constant (b = 0.22 ) as the number of double bonds in conjugation increased. Our experience in synthesizing carotenoids guided us to design the conceptually new and seemingly stable carotenoids to challenge the possibility of improving and diversifying the molecular conductance of carotenoids of the same size, so that the electric circuits of the stable carotenoid wires with various conductances (or resistances) may be realized in practical sense. The instability of carotenoids is an inevitable consequence of their antioxidant activities in quenching singlet oxygen and scavenging reactive radical species. The synergistic protective effect of carotenoids in combination with vitamin E containing the aromatic phenyl group against photodynamic cell damage has been reported. The enhanced antioxidant efficiency of carotenoids was explained by the prevention of free radical-mediated carotenoid degradation or repair of the semi-oxidized carotenoid molecules by vitamin E. We thus devised the novel carotenoid wires 2 so as to provide the labile conjugated polyene chain with stability as well as various conductances by attaching the aromatic phenyl groups containing the para-substituent X (OMe, Me, H, and Br) of diverse electronic natures to the polyene chain at C-13 and C-13’ (Figure 1). The stability of carotenoids 2 by the phenyl groups might be expected from the repulsive steric interactions with attacking nucleophiles and/or the reversible trapping of incoming radicals (e.g., reactive oxygen species) that would cause fragmentations of the conjugate polyene chain. The benzene rings containing a para[a] J. Maeng, S. B. Kim, E. Choi, S.-Y. Jung, I. Hong, S.-H. Bae, J. T. Oh, B. Lim, J. W. Kim, Prof. Dr. S. Koo Department of Chemistry; Department of Nano Science and Engineering Myong Ji University, San 38-2 Nam-Dong, Yongin, Kyunggi-Do, 449-728 (Korea) Fax: (+82) 31-335-7248 E-mail : sangkoo@mju.ac.kr [b] N. J. Lee, Prof. Dr. C. J. Kang Department of Physics, Department of Nano Science and Engineering Myong Ji University, San 38-2 Nam-Dong, Yongin, Kyunggi-Do, 449-728 (Korea) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201000700.

Series DNA Amplification Using the Continuous-Flow Polymerase Chain Reaction Chip

We proposed a continuous-flow polymerase chain reaction (PCR) chip that can be used for series DNA amplification. The continuous-flow PCR chip has several advantages such as fast thermal cycling, series of amplifications, cost-effective fabrication, portability, and fluorescence detection. The continuous-flow PCR chip is composed of two parts namely poly(dimethylsiloxane) (PDMS) microchannel for sample injection and indium–tin-oxide (ITO) heater/glass chip for thermal cycling. The fabricated microchannel width and depth are 250 and 200 µm, respectively. Also, the total working length of the PDMS microchannel is 1340 mm which is equivalent for 20 cycles of amplification. A 2:2:3 microchannel length ratio for three different temperature zones namely denaturation, annealing, and extension was assigned, respectively. Upon the operation of the fabricated continuous-flow PCR chip, the amplification of plasmid DNA pKS-GFP with 720 base pairs and PG-noswsi with 300 base pairs were found successfully with a total reaction time of 15 min.