Sangkyu Lee

Refractive index engineering of transparent ZrO2–polydimethylsiloxane nanocomposites

Transparent ZrO2–polydimethylsiloxane (PDMS) nanocomposites with high refractive index were prepared by dispersing ZrO2nanoparticles in a PDMS matrix via ligand molecule engineering. The ligand molecule consists of a diamine head group that adsorbs strongly onto the ZrO2nanoparticles, and a siloxane tail group with a double-tailed structure that can be easily stretched within the PDMS matrix and yields a greater steric hindrance compared to single-tailed structure. The transmission electron micrograph shows that the designed Si-based ligand molecule has a role to play in keeping ZrO2nanoparticles fully-dispersed even after the ZrO2nanoparticles were transferred into the PDMS matrix. UV-vis spectra indicate that ZrO2–PDMS nanocomposite had a high transparency of 93.3% in the whole visible range. The refractive index of the ZrO2–PDMS nanocomposite could be varied from 1.39 to 1.65 simply by increasing the ZrO2 content from 0 to 20.8% v/v.

Highly Cyclable Lithium-Sulfur Batteries with a Dual-Type Sulfur Cathode and a Lithiated Si/SiOx Nanosphere Anode.

Lithium-sulfur batteries could become an excellent alternative to replace the currently used lithium-ion batteries due to their higher energy density and lower production cost; however, commercialization of lithium-sulfur batteries has so far been limited due to the cyclability problems associated with both the sulfur cathode and the lithium-metal anode. Herein, we demonstrate a highly reliable lithium-sulfur battery showing cycle performance comparable to that of lithium-ion batteries; our design uses a highly reversible dual-type sulfur cathode (solid sulfur electrode and polysulfide catholyte) and a lithiated Si/SiOx nanosphere anode. Our lithium-sulfur cell shows superior battery performance in terms of high specific capacity, excellent charge-discharge efficiency, and remarkable cycle life, delivering a specific capacity of ∼750 mAh g(-1) over 500 cycles (85% of the initial capacity). These promising behaviors may arise from a synergistic effect of the enhanced electrochemical performance of the newly designed anode and the optimized layout of the cathode.

Nanoscale, Electrified Liquid Jets for High-Resolution Printing of Charge

Nearly all research in micro- and nanofabrication focuses on the formation of solid structures of materials that perform some mechanical, electrical, optical, or related function. Fabricating patterns of charges, by contrast, is a much less well explored area that is of separate and growing interesting because the associated electric fields can be exploited to control the behavior of nanoscale electronic and mechanical devices, guide the assembly of nanomaterials, or modulate the properties of biological systems. This paper describes a versatile technique that uses fine, electrified liquid jets formed by electrohydrodynamics at micro- and nanoscale nozzles to print complex patterns of both positive and negative charges, with resolution that can extend into the submicrometer and nanometer regime. The reported results establish the basic aspects of this process and demonstrate the capabilities through printed patterns with diverse geometries and charge configurations in a variety of liquid inks, including suspensions of nanoparticles and nanowires. The use of printed charge to control the properties of silicon nanomembrane transistors provides an application example.

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.

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.

TiO2 nanotube branched tree on a carbon nanofiber nanostructure as an anode for high energy and power lithium ion batteries

The inherently low electrical conductivity of TiO2-based electrodes as well as the high electrical resistance between an electrode and a current collector represents a major obstacle to their use as an anode for lithium ion batteries. In this study, we report on high-density TiO2 nanotubes (NTs) branched onto a carbon nanofiber (CNF) “tree” that provide a low resistance current path between the current collector and the TiO2 NTs. Compared to a TiO2 NT array grown directly on the current collector, the branched TiO2 NTs tree, coupled with the CNF electrode, exhibited ∼10 times higher areal energy density and excellent rate capability (discharge capacity of ∼150 mA·h·g−1 at a current density of 1,000 mA·g−1). Based on the detailed experimental results and associated theoretical analysis, we demonstrate that the introduction of CNFs with direct electric contact with the current collector enables a significant increase in areal capacity (mA·h·cm−2) as well as excellent rate capability.

Influence of the electrokinetic behaviors of abrasive ceria particles and the deposited plasma-enhanced tetraethylorthosilicate and chemically vapor deposited Si 3 N 4 films in an aqueous medium on chemical mechanical planarization for shallow trench isolation

The effects of the electrokinetic behavior of abrasive ceria particles suspended in an aqueous medium and the deposited plasma-enhanced tetraethylorthosilicate (PETEOS) and chemical vapor deposition (CVD) Si 3 N 4 films onchemical mechanical planarization (CMP) for shallow trench isolation were investigated. The colloidal characteristics of ceria slurries, such as their stability and surface potential, in acidic, neutral, and alkaline suspensions were examined to determine the correlation between the colloidal properties of ceria slurry and CMP performance. The surface potentials of the ceria particles and the PETEOS and CVD Si 3 N 4 films in an aqueous suspending medium were dependent on the pH of the suspending medium. The differences in surface charges of ceria particles and the PETEOS and CVD Si 3 N 4 films have a profound effect on the removal rate and oxide-to-nitride selectivity of CMP performance.

Effect of poly(acrylic acid) and poly(vinyl alcohol) on the solubility of colloidal BaTiO3 in an aqueous medium

The influence of poly(acrylic acid) (PAA) and poly(vinyl alcohol) (PVA) on Ba dissolution from the BaTiO 3 -aqueous solution interface was investigated. Incongruent dissolution of Ba impacts the colloidal stability, microstructure, and electrical properties of BaTiO 3 and related perovskite dielectric materials used in the manufacture of ceramic capacitors. The solubility characteristics of BaTiO 3 were influenced significantly by the presence of PAA and PVA. PAA, which forms weak monodentate complexes with Ba 2 + , acted as both a passivating and a sequestering agent, depending on pH. Both PAA and PVA provided some degree of passivation in the acidic pH region. Above pH 8, where BaTiO 3 solubility decreases sharply, PVA had a moderate passivating effect, whereas solubility was enhanced by PAA with a positive linear dependence on concentration. The adsorptive and electrokinetic behavior of colloidal BaTiO 3 with respect to PAA and PVA are correlated with the observed passivating and sequestering properties of these polymers.

Stackable, three dimensional carbon–metal oxide composite for high performance supercapacitors

We report a stackable all-solid-state supercapacitor based on three dimensional PPy–NiO/RGO–CNT electrodes. The electrodes were prepared by growing NiO nanowalls (NWs) on both sides of a freestanding RGO–CNT film with the hydrothermal method, followed by coating with conductive polypyrrole. The double-sided architecture of the NiO NWs on a carbon-based current collector suggests that the novel approach addresses the issue of low areal capacitance in supercapacitors. Furthermore, conductive polypyrrole renders fast electron transport through the two-dimensional structure of NiO NWs. With our 3D-designed electrodes, we successfully fabricated a stackable all-solid-state supercapacitor and thus achieved a high areal capacitance of 1948 mF cm−2.

Effect of Physicochemical Properties of Solvents on Microstructure of Conducting Polymer Film for Non-Volatile Polymer Memory

The effect of physicochemical properties of solvents on the microstructure of polyvinyl carbazole (PVK) film for non-volatile polymer memory was investigated. For the solubilization of PVK molecules and the preparation of PVK films, four solvents with different physicochemical properties of the Hildebrand solubility parameter and vapor pressure were considered: chloroform, tetrahydrofuran (THF), 1,1,2,2-tetrachloroethane (TCE), and N,N-dimehtylformamide (DMF). The solubility of PVK molecules in the solvents was observed by ultraviolet-visible spectroscopy. PVK molecules were observed to be more soluble in chloroform, with a low Hildebrand solubility parameter, than solvents with higher values. The aggregated size and micro-/nano-topographical properties of PVK films were characterized using optical and atomic force microscopes. The PVK film cast from chloroform exhibited enhanced surface roughness compared to that from TCE and DMF. It was also confirmed that the microstructure of PVK film has an effect on the performance of non-volatile polymer memory.