Hyeon Su Jeong

Accurate measurement of specific tensile strength of carbon nanotube fibers with hierarchical structures by vibroscopic method

The specific strength of a carbon nanotube (CNT) fiber can be estimated to be much higher than its real value when the linear density of the fiber is measured using the vibroscopic method. This is because CNT fibers are not made of a single fiber, as assumed in the standard ASTM procedure, but rather have a hierarchical structure composed of CNTs and CNT bundles. Based on careful investigation, a new procedure using the vibroscopic method is proposed to drastically reduce the probability of erroneous results and provide a more reliable tool to investigate the mechanical properties of one-dimensional nanostructured fibers.

Nitrogen-Doped Single Graphene Fiber with Platinum Water Dissociation Catalyst for Wearable Humidity Sensor

Humidity sensors are essential components in wearable electronics for monitoring of environmental condition and physical state. In this work, a unique humidity sensing layer composed of nitrogen-doped reduced graphene oxide (nRGO) fiber on colorless polyimide film is proposed. Ultralong graphene oxide (GO) fibers are synthesized by solution assembly of large GO sheets assisted by lyotropic liquid crystal behavior. Chemical modification by nitrogen-doping is carried out under thermal annealing in H2 (4%)/N2 (96%) ambient to obtain highly conductive nRGO fiber. Very small (≈2 nm) Pt nanoparticles are tightly anchored on the surface of the nRGO fiber as water dissociation catalysts by an optical sintering process. As a result, nRGO fiber can effectively detect wide humidity levels in the range of 6.1-66.4% relative humidity (RH). Furthermore, a 1.36-fold higher sensitivity (4.51%) at 66.4% RH is achieved using a Pt functionalized nRGO fiber (i.e., Pt-nRGO fiber) compared with the sensitivity (3.53% at 66.4% RH) of pure nRGO fiber. Real-time and portable humidity sensing characteristics are successfully demonstrated toward exhaled breath using Pt-nRGO fiber integrated on a portable sensing module. The Pt-nRGO fiber with high sensitivity and wide range of humidity detection levels offers a new sensing platform for wearable humidity sensors.

High-strength carbon nanotube/carbon composite fibers via chemical vapor infiltration

In this study, we have developed an efficient and scalable method for improving the mechanical properties of carbon nanotube (CNT) fibers. The mechanical properties of as-synthesized CNT fibers are primarily limited by their porous structures and the weak bonding between adjacent CNTs. These result in inefficient load transfer, leading to low tensile strength and modulus. In order to overcome these limitations, we have adopted chemical vapor infiltration (CVI) to efficiently fill the internal voids of the CNT fibers with carbon species which are thermally decomposed from gas phase hydrocarbon. Through the optimization of the processing time, temperature, and gas flow velocity, we have confirmed that carbon species formed by the thermal decomposition of acetylene (C2H2) gas successfully infiltrated into porous CNT fibers and densified them at relatively low temperatures (650-750 °C). As a result, after CVI processing of the as-synthesized CNT fibers under optimum conditions, the tensile strength and modulus increased from 0.6 GPa to 1.7 GPa and from 25 GPa to 127 GPa, respectively. The CVI technique, combined with the direct spinning of CNT fibers, can open up a route to the fast and scalable fabrication of high performance CNT/C composite fibers. In addition, the CVI technique is a platform technology that can be easily adapted into other nano-carbon based yarn-like fibers such as graphene fibers.

Polymer-Layer-Free Alignment for Fast Switching Nematic Liquid Crystals by Multifunctional Nanostructured Substrate

A novel polymer-layer-free system for liquid-crystal alignment is demonstrated by various shaped indium tin oxide (ITO) patterns. Liquid crystals are aligned along the ITO line pattern and secondary sputtering lithography can change the shape of the ITO line pattern. Different shapes can control the direction and size of the pretilt angle. This effect eliminates defects and reduces the response time.

Significantly Increased Solubility of Carbon Nanotubes in Superacid by Oxidation and Their Assembly into High-Performance Fibers

This study demonstrates that small amount of oxygen incorporated into carbon nanotubes (CNTs) during the purification process greatly increases their solubility in chlorosulfonic acid (CSA). Using as-purchased and unpurified CNT powders, the optimal purification process is established to significantly increase the solubility of CNTs in CSA, and spin CNT fibers with high mechanical strength (0.84 N tex-1 ) and electrical conductivity (1.4 MS m-1 ) from the CNT liquid crystal dope with high concentration of CNTs in CSA. The knowledge obtained here may guide development of a way to dissolve extremely long CNTs at high concentration and thereby to enable production of CNT fibers with ultimate properties.

The high-resolution nanostructuring of Si wafer surface with 10 nm scale using a combined ion bombarding technique and chemical reaction

We describe a highly efficient technique for nanostructuring silicon (Si) wafer surfaces with high-resolution (< 15 nm) and high aspect ratio (20) structures without any deposition processes. Our strategy is based on advanced secondary sputtering lithography (SSL), which combines physical and chemical plasma etching during an ion bombardment process. Compared with general SSL techniques using Ar gas only, the reactive radicals assisted the SSL and promoted the Si etching rate to simultaneously deposit the etched Si materials onto the side surface of a pre-patterned polymer. In addition, various three-dimensional Si nanostructure shapes could be developed simply by controlling the pre-patterned polymer, thereby providing a simple and versatile approach to customizing this technique.

Carboxymethlyated cellulose nanofibrils(CMCNFs) embedded in polyurethane foam as a modular adsorbent of heavy metal ions

Polyurethane (PU) foam was utilized as an efficient and durable template to immobilize surface-functionalized nanocellulose, carboxymethylated cellulose nanofibrils (CMCNFs), to address some of the challenges for the application of nanocellulose to industrial water purification, such as its agglomeration, difficulties in separation from effluent, and regeneration. The composite foams exhibited well dispersed CMCNFs in PU matrices with open pore structure; the hydrogen bonds result in the enhancement of mechanical strength, which is another requirement of ideal adsorbents for wastewater treatment. The composite foams show high adsorption capacity and the potential for recyclability. The combination of optimal surface modification of nanocellulose with isolation and immobilization in durable PU foam achieved an efficient and cost-competitive bio-sorbent for heavy metal ions.

Mussel‐Inspired Defect Engineering of Graphene Liquid Crystalline Fibers for Synergistic Enhancement of Mechanical Strength and Electrical Conductivity

Inspired by mussel adhesive polydopamine (PDA), effective reinforcement of graphene-based liquid crystalline fibers to attain high mechanical and electrical properties simultaneously is presented. The two-step defect engineering, relying on bioinspired surface polymerization and subsequent solution infiltration of PDA, addresses the intrinsic limitation of graphene fibers arising from the folding and wrinkling of graphene layers during the fiber-spinning process. For a clear understanding of the mechanism of PDA-induced defect engineering, interfacial adhesion between graphene oxide sheets is straightforwardly analyzed by the atomic force microscopy pull-off test. Subsequently, PDA could be converted into an N-doped graphitic layer within the fiber structure by a mild thermal treatment such that mechanically strong fibers could be obtained without sacrificing electrical conductivity. Bioinspired graphene-based fiber holds great promise for a wide range of applications, including flexible electronics, multifunctional textiles, and wearable sensors.

Fabrication of a high-performance thin film polarizer using lyotropic chromonic liquid crystals using a high-resolution nanoscale template

Lyotropic chromonic liquid crystals (LCLCs) have attracted attention for their potential applications as thin-film polarizers. Although it is well reported that template-assisted method can efficiently align LCLCs for thin-film polarizers, the polarizer performance using the technique has not been well demonstrated. In this study, we report for the first time that the high-resolution (∼10 nm) pattern was found to be able to overcome the issue of dead space in the channel, and the LCLC columns were well aligned along the nanoscale array. The nanoscale array templates were fabricated by secondary sputtering lithography, and the chromonic nematic phase of the LCLC materials were prepared by dissolving the hydrogen chloride salt of bis-(N,N-diethylaminoethyl)-perylene-3,4,9,10-tetracarboxylic diimide in deionized water. A polarizing performance was observed (79.1% DOP and 29.5% TT at 475 nm) using a nanoscale array template with a high resolution (∼10 nm) and a high aspect ratio (>30). To further enhance polarizer performance, the combination of a nanoscale array template and mechanical shearing was exploited. With this combined route, an LCLC polarizer exhibiting a total transmission of 40.6% and a degree of polarization of 99.5% was obtained. Compared to mechanical shearing, nanoscale array templates were found to dominantly align highly mobile LCLCs.