Nak-Jin Choi

Ultrasensitive and Highly Selective Graphene-Based Single Yarn for Use in Wearable Gas Sensor

Electric components based on fibers or textiles have been investigated owing to their potential applications in wearable devices. High performance on response to gas, drape-ability and washing durability are of important for gas sensors based on fiber substrates. In this report, we demonstrate the bendable and washable electronic textile (e-textile) gas sensors composed of reduced graphene oxides (RGOs) using commercially available yarn and molecular glue through an electrostatic self-assembly. The e-textile gas sensor possesses chemical durability to several detergent washing treatments and mechanical stability under 1,000 bending tests at an extreme bending radius of 1 mm as well as a high response to NO2 gas at room temperature with selectivity to other gases such as acetone, ethanol, ethylene, and CO2.

Nonstoichiometric Co-rich ZnCo2O4 Hollow Nanospheres for High Performance Formaldehyde Detection at ppb Levels

Since metal oxide semiconductors were investigated as chemiresistors, rapid advances have been reported in this field. However, better performance metrics are still required, such as higher sensitivity and selectivity levels for practical applications. To improve the sensing performance, we discuss an optimal composition of the active sensing material, nonstoichiometric Co-rich ZnCo2O4, prepared by the partial substitution of Co(2+) into Zn(2+) in Co3O4 without altering a hollow sphere morphology. Remarkably, this Co-rich ZnCo2O4 phase achieved detection limits for formaldehyde as low as 13 ppb in experimental measurements and 2 ppb in theory, which were the lowest values ever reported from actual measurements at a working temperature of 225 °C. It was also unprecedented that the selectivity for formaldehyde was greatly enhanced with respect to the selectivity levels against other volatile organic compounds (VOCs). These excellent sensing performances are due to the optimal composition of the Co-rich ZnCo2O4 material with a proper hole concentration and well-organized conductive network.

A statistical method for determining intrinsic electronic transport properties of self-assembled alkanethiol monolayer devices

We present a statistical method to investigate the electronic transport of molecular devices. Electrical characterizations are performed with subsequent statistical analysis on 6745 molecular devices with nanometer-scale junction diameter. The comprehensive temperature-variable current-voltage measurements are also performed to elucidate the dominant charge conduction mechanism responsible for intrinsic molecular transport properties. The entity of data acquired represents a reliable basis for statistical analysis, which consequently provides an objective criterion to determine the most probable transport characteristics of molecular devices.

Application of ionic polymer-metal composites for auto-focusing compact camera modules

Step-motor, piezo, liquid lens and voice coil motor (VCM) have been thought as good candidates for actuators in auto-focusing compact camera module (CCM). Currently, VCMs take possession of big place in auto-focusing CCM market. However, VCMs have limitations in developing thin, low-power CCMs. Therefore, ionic polymer-metal composites (IPMCs) could be thought as one of the best candidates in developing auto-focusing CCM due to their well-known characteristics such as low-power consumption and large displacement. It is required that fast bending response (20 μm/20 ms) and large blocking force (800 mgf) should be achieved for the practical applications of IPMCs in developing auto-focusing CCM. Here, we present the method for increasing IPMCs bending response and displacement by anisotropic plasma treatment. Furthermore, we demonstrate the fabrication of a prototype of CCM actuated by IPMC and its remarkable low power consumption.

Statistical representation of intrinsic electronic tunneling characteristics through alkyl self-assembled monolayers in nanowell device structures

Systematic electronic transport measurements in nanometer-scale junctions containing self-assembled monolayers of alkyl molecules are reported using nanowell device structures. The comprehensive temperature-variable current-voltage characterizations and statistical analysis for the acquired transport data show that direct tunneling indeed can be assigned as the dominant charge transport mechanism of the alkyl monolayers in a voltage range ⩽±1V. The intrinsic tunneling characteristics of alkyl molecular junctions are examined by excluding other parasitic conduction mechanisms by the data analyses and statistically defining representative data. The demonstrated intrinsic tunneling characteristics are well consistent with numerous previous reports for alkyl-based monolayers. The current characteristics are temperature independent and exponentially depend on the molecular length. The tunneling decay coefficient is determined as 0.83–0.73A−1 in the bias range from 0.1to1.0V and is independent of temperature. T...

Charge Transport of Alkanethiol Self-Assembled Monolayers in Micro-Via Hole Devices

In this paper we fabricated 13440 microscale via hole structure devices using different length of alkanethiol self-assembled monolayers and characterized their electronic transport properties. Statistically averaged transport parameters such as current density, transport barrier height, effective electron mass, and transport decay coefficient were obtained from the great number of these devices. The yield of working microdevices was found as 1.5%. Temperature variable current-voltage characteristics for alkanethiol micro-via hole devices showed typical tunneling behavior when properly fabricated.

Ionic Polymer-Metal Composites (IPMCs) Containing Cu/Ni Electrodes and Ionic Liquids for Durability

We describe the fabrication of ionic polymer-meal composites (IPMCs) containing Cu/Ni electrode as an electrode material and ionic liquid as an electrolyte. Cu/Ni is notorious for vulnerability to oxidation and acid. The authors have investigated best candidate of ionic liquids for this vulnerable electrode. This new IPMC shows increased displacement and blocking force compared to that of conventional IPMC containing Pt electrode and ionic liquid due to increased stiffness of resulting IPMC and size effect of mobile cations. In this research, the effect of ionic liquid was investigated by monitoring displacement and blocking force of IPMCs depending on the type of ionic liquids.

A model of the IPMC actuator using finite element method

Ionic polymer-metal composite (IPMC) is an attractive actuator among many electro-active polymers. In order to improve the performance of IPMC actuator, an IPMC actuator with the patterned surface was proposed. It is named the patterned IPMC actuator. In order to make use of its maximum effect, it is needed to establish a valid mathematical model. Among many models of IPMC actuator, the grey box modeling proposed by Kanno et al. was suited to model the patterned IPMC actuator. In this paper, we applied the grey box model based on Kannos model. Theoretical and experimental results demonstrate that the model is practical and effective enough in predicting the bending displacement partly.

Charge Storage Effect on In2O3 Nanowires with Ruthenium Complex Molecules

Charge storage effect on In2O3 nanowire field-effect transistors (FETs) is controlled by a chemical gate, ruthenium(II) terpyridine (RuII-tpy) complex molecules. In2O3 nanowire FETs functionalized with a self-assembled monolayer of the molecules exhibit large hysteretic characteristics with regard to source–drain current vs gate voltage characteristics. The devices are operated with reversible switching behavior at gate voltage cycles of writing, reading, erasing, and reading, and their retention time is in excess of 1000 s. These results reveal that the reversible chemical reaction (i.e., oxidation and reduction of the molecules) of RuII-tpy complexes produces a charging/discharging process of In2O3 nanowire FETs.

Ultrasensitive formaldehyde gas sensors based on a hollow assembly and its 3-dimensional network formation of single-crystalline Co3O4 nanoparticles

The detection of formaldehyde at a very low concentration is a significant research topic, because of its harmful impact on human health. In this current study, we have fabricated a hierarchical structure by the reasonable assembly of single-crystalline Co3O4 nanoparticles. A hollow morphology using sacrificial ZnO spheres could make a three-dimensional conducting network in a solid state. The resulting structure was quite active for formaldehyde sensing, and the detection limit was 50 ppb, which was nearly close to the record-high value among the other semiconducting materials. Such superior properties were attributed to the regular, hierarchically assembled structures with a small crystalline domain size, a thin hollow morphology with a large surface area, and a three-dimensional conductive network with a narrow diameter. We think that this hierarchical assembly can show great potential as a platform for improving human health through the monitoring of indoor environments.