Junghyun Choi

Electrode dependence of resistance switching in polycrystalline NiO films

We investigated resistance switching in top-electrode/NiO∕Pt structures where the top electrode was Au, Pt, Ti, or Al. For Pt∕NiO∕Pt and Au∕NiO∕Pt structures with ohmic contacts, the effective electric field inside the film was high enough to induce trapping or detrapping at defect states and thus resistance switching. For a Ti∕NiO∕Pt structure with well-defined Schottky contact at Ti∕NiO interface accompanied by an appreciable voltage drop, the effective electric field inside the NiO film was not enough to induce resistance switching. For an Al∕NiO∕Pt structure with a low Schottky barrier at the Al∕NiO interface, resistance switching could be induced at a higher voltage since the voltage drop at the Al∕NiO interface was not negligible but small.

3D Cross-Linked Nanoweb Architecture of Binder-Free TiO2 Electrodes for Lithium Ion Batteries

The nanoweb structure of TiO2 anode, cross-linked between electrospun nanofibers, is directly fabricated on the current collector by utilizing the fluidity of low glass transition temperature polymer, poly(vinyl acetate), at room temperature. This characteristic enables us to fabricate the nanoweb structure by direct electrospinning on the current collector, followed by uniaxial pressing. This proposed structure facilitates electron transport through the direct conducting pathways between TiO2 active materials and current collector as well as provides strong adhesion strength to the current collector without polymeric binders. Consequently, we could achieve stable cycle performance up to 100 cycles and the excellent rate capability of ∼60% at high rate charge/discharge condition of 10 C.

Miniaturized Flexible Electronic Systems with Wireless Power and Near-Field Communication Capabilities

Advances in materials and device architectures for these systems will create opportunities for increasing the range of capabilities, expanding the modes of use, improving the robustness/reliability, reducing the size/weight, and lowering the cost. The cellular phone platform will likely remain a key element in the broader technology landscape, as in currently available wrist band and watch style devices that measure body processes and communicate data to the phone. [ 2,3 ] Recent research demonstrates much different types of integration strategies compared to those of these existing systems, in which the wearable devices take the form of temporary transfer tattoos. The result is greatly improved contact with the body and corresponding increases in the diversity and accuracy of information that can be collected from integrated sensors. [ 2,4,5 ] Here, an overarching goal is to engineer the physical properties, and in particular the elastic modulus and elastic stretchability, to match those of the epidermis, as a way to reduce irritation and discomfort at the skin interface and to improve the robustness of the bonding. [ 2,4,6 ]

Synergistic Ultrathin Functional Polymer-Coated Carbon Nanotube Interlayer for High Performance Lithium–Sulfur Batteries

Lithium-sulfur (Li-S) batteries have been intensively investigated as a next-generation rechargeable battery due to their high energy density of 2600 W·h kg(-1) and low cost. However, the systemic issues of Li-S batteries, such as the polysulfide shuttling effect and low Coulombic efficiency, hinder the practical use in commercial rechargeable batteries. The introduction of a conductive interlayer between the sulfur cathode and separator is a promising approach that has shown the dramatic improvements in Li-S batteries. The previous interlayer work mainly focused on the physical confinement of polysulfides within the cathode part, without considering the further entrapment of the dissolved polysulfides. Here, we designed an ultrathin poly(acrylic acid) coated single-walled carbon nanotube (PAA-SWNT) film as a synergic functional interlayer to address the issues mentioned above. The designed interlayer not only lowers the charge transfer resistance by the support of the upper current collector but also localizes the dissolved polysulfides within the cathode part by the aid of a physical blocking and chemical bonding. With the synergic combination of PAA and SWNT, the sulfur cathode with a PAA-SWNT interlayer maintained higher capacity retention over 200 cycles and achieved better rate retention than the sulfur cathode with a SWNT interlayer. The proposed approach of combining a functional polymer and conductive support material can provide an optimiztic strategy to overcome the fundamental challenges underlying in Li-S batteries.

Patterned oxide semiconductor by electrohydrodynamic jet printing for transparent thin film transistors

This paper explores transport in transparent thin film transistors formed using a liquid precursor to indium zinc oxide, delivered to target substrates by electrohydrodynamic jet (e-jet) printing. Under optimized conditions, we observe field effect mobilities as high as 32 cm2V−1s−1, with on/off current ratios of 103 and threshold voltages of 2 V. These results provide evidence that material manipulated in fine-jet, electric field induced liquid flows can yield semiconductor devices without any adverse effects of residual charge or unintentional doping. E-jet printing methods provide levels of resolution (∼1.5 μm) that provide a path to printed transistors with small critical dimensions.

Encapsulation of S/SWNT with PANI Web for Enhanced Rate and Cycle Performance in Lithium Sulfur Batteries

Lithium-sulfur batteries show great potential to compete with lithium-ion batteries due to the fact that sulfur can deliver a high theoretical capacity of 1672 mAh/g and a high theoretical energy density of 2500 Wh/kg. But it has several problems to be solved in order to achieve high sulfur utilization with high Coulombic efficiency and long cycle life of Li-S batteries. These problems are mainly caused by the dissoluble polysulfide species, which are a series of complex reduced sulfur products, associating with shuttle effect between electrodes as well as side reactions on lithium metal anode. To alleviate these challenges, we developed a sulfur-carbon nanotube (S/SWNT) composite coated with polyaniline (PANI) polymer as polysulfide block to achieve high sulfur utilization, high Coulombic efficiency, and long cycle life. The PANI coated S/SWNT composite showed a superior specific capacity of 1011 mAh/g over 100 cycles and a good rate retention, demonstrating the synergic contribution of porous carbon and conducting polymer protection to address challenges underlying sulfur cathode.

Commissioning of the PLS-II

After 14 years of successful operation, the Pohang Light Source (PLS) has completed PLS-II upgrade to meet the increasing demand from the growing user community. The PLS-II upgrade has increased the beam energy from 2.5 GeV to 3 GeV; the number of insertion devices has been increased by a factor of two (20 IDs); and the beam current has been increased to 400 mA from 200 mA. The beam emittance has been reduced to below 10 nm while retaining the existing PLS tunnel as well as the existing injection system. During the six months of commissioning in the latter half of 2011, we have successfully achieved 14 insertion device operations and top-up operations with 100 mA beam current and 5.8 nm beam emittance. In this paper, we report the experimental results obtained from the PLS-II commissioning.

Enhancement of titanium nitride barrier metal properties by nitrogen radical assisted metalorganic chemical vapor deposition

Using metalorganic chemical vapor deposition assisted by a nitrogen radical irradiation generated by rf plasma, we have enhanced the quality and the step coverage of titanium nitride barrier metals for the contact holes with a high aspect ratio and a submicron radius. Electrical resistivity measurements show that the film resistivity improves by a factor of five as the proper nitrogen irradiation has been applied. The step coverage in a contact hole with 0.4 μm diam and 3:1 aspect ratio has been improved from 50% to 80% by applying nitrogen plasma, clearly demonstrating the effectiveness of this technique in the conformal deposition of barrier metals for the ultra‐large scale integration. The incident nitrogen radical is believed to play several roles, such as the enhancement of surface migration rate of molecules and the reduction of the amount of hydrocarbon incorporating into the film during the deposition.

AUF1 contributes to Cryptochrome1 mRNA degradation and rhythmic translation

In the present study, we investigated the 3′ untranslated region (UTR) of the mouse core clock gene cryptochrome 1 (Cry1) at the post-transcriptional level, particularly its translational regulation. Interestingly, the 3′UTR of Cry1 mRNA decreased its mRNA levels but increased protein amounts. The 3′UTR is widely known to function as a cis-acting element of mRNA degradation. The 3′UTR also provides a binding site for microRNA and mainly suppresses translation of target mRNAs. We found that AU-rich element RNA binding protein 1 (AUF1) directly binds to the Cry1 3′UTR and regulates translation of Cry1 mRNA. AUF1 interacted with eukaryotic translation initiation factor 3 subunit B and also directly associated with ribosomal protein S3 or ribosomal protein S14, resulting in translation of Cry1 mRNA in a 3′UTR-dependent manner. Expression of cytoplasmic AUF1 and binding of AUF1 to the Cry1 3′UTR were parallel to the circadian CRY1 protein profile. Our results suggest that the 3′UTR of Cry1 is important for its rhythmic translation, and AUF1 bound to the 3′UTR facilitates interaction with the 5′ end of mRNA by interacting with translation initiation factors and recruiting the 40S ribosomal subunit to initiate translation of Cry1 mRNA.

Control of the leakage current in SrTiO3 films by acceptor doping

Stoichiometric SrTiO3 (STO) films doped with Fe or Cr were prepared by r.f. magnetron sputtering technique. The effects of Fe or Cr doping in the SrTiO3 films were studied on the leakage current property which was discussed by defect chemistry. The experimental results can be explained by a model in which oxygen vacancies are the key defects responsible for the leakage current. Acceptor doping, with a small concentration of Fe or Cr, has led to a substantial improvement to 10−9 order in the leakage current density. Above the concentration of 0.01∼0.02 mol% Fe2O3, Cr2O3, however, as the concentration increased, the leakage current increased. These acceptors in Ti4+ site are expected to electrically compensate for donor species such as oxygen vacancies, thereby reducing the concentration of mobile carriers that contribute to electrical conduction. Consequently, acceptor doped STO films have been shown to be superior to undoped films for applications requiring high leakage resistance, such as dynamic random access memory capacitors.