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Choi, Jaeho,Le, Quyet Van,Hong, Kootak,Moon, Cheon Woo,Han, Ji Su,Kwon, Ki Chang,Cha, Pil-Ryung,Kwon, Yongwoo,Kim, Soo Young,Jang, Ho Won American Chemical Society 2017 ACS APPLIED MATERIALS & INTERFACES Vol.9 No.36
<P>It was demonstrated that organolead halide perovskites (OHPs) show a resistive switching behavior with an ultralow electric field of a few kilovolts per centimeter. However, a slow switching time and relatively short endurance remain major obstacles for the realization of the next generation memory. Here, we report a performance-enhanced OHP resistive switching device. To fabricate topologically and electronically improved OHP thin films, we added hydroiodic acid solution (for an additive) in the precursor solution of the OHP. With drastically improved morphology such as small grain size, low peak-to-valley depth, and precise thickness, the OHP thin films showed an excellent performance as insulating layers in Ag/CH3NH3PbI3/Pt cells, with an endurance of over 103 cycles, a high on/off ratio of 10(6), and an operation speed of 640 mu s and without electroforming. We suggest plausible resistive switching and conduction mechanisms with current voltage characteristics measured at various temperatures and with different top electrodes and device structures. Beyond the extended endurance, highly flexible resistive switching devices with a minimum bending radius of 5 mm create opportunities for use in flexible and wearable electronic devices.</P>
Choi, Jaeho,Jo, Wonhee,Lee, Seung Yeol,Jung, Yeon Sik,Kim, Shin-Hyun,Kim, Hee-Tak American Chemical Society 2017 ACS NANO Vol.11 No.8
<P>Springtails, insects which breathe through their skins, possess mushroom-shaped nanostructures. As doubly re-entrant geometry, in the mushroom-head enhances the resistance against liquid invasion, the springtails have robust, liquid-free omniphobic skins. Although omniphobic surfaces are promising for various applications, it remains an important challenge to mimic the structural feature of springtails. This paper presents a pragniatic method to create doubly re-entrant nanostructures and robust superomniphobic surfaces by exploiting localized photofluidization of azopolymers. Irradiation of circularly polarized light reconfigures azopolymer micropillars to have a mushroom-like head with a doubly re-entrant nanogeometry through protrusion and inward bending of polymer film from the top edge. The light-driven reconfigured micropillars facilitate the pining of triple line as springtails do. In particular, the unique geometry exhibits superomniphobicity even for liquids whose equilibrium contact angles are almost zero in the presence of a practical level of external pressure. In addition, the simple fabrication process is highly reproducible, scalable, and compatible with various substrate materials including flexible polymeric film. Our results suggest that our photofluidization technology will provide a practical route to develop robust superomniphobic surfaces.</P>
Jaeho Choi 한국영어어문교육학회 2013 영어어문교육 Vol.19 No.3
This study explores what factors influence a conversation between native and non-native English speakers. In second language acquisition (SLA) studies, a conversation between a native speaker (NS) and a non-native speaker (NNS) has been researched in terms of the effects on second language acquisition and learning. Focused on conversational interactions, interactionsists understand a conversation as an opportunity for negotiating meaning, which promotes L2 acquisition. On the other hand, communication strategy (CS) research has examined NNS's strategic language use for communicational purposes. While these approaches have investigated the nature of NS-NNS conversation and L2 learning, socio-affective and contextual factors have not been the focus of the research. However, with the increasing interest in the social and contextual factors, there is a growing need to expand conversation research to include these factors. Therefore, this study examines comprehensively conversational interactions, communication strategies, and intra- and inter-personal factors with three dyads of NS-NNS conversation participants. Six informal free conversations and interviews with all participants were conducted, and all conversations and interviews were transcribed for analysis. The results of the analysis indicate different communication performance across conversational dyads and interrelationship between conversational interactions, CSs, and intro- and inter-personal factors.
A Three-dimensional Modeling of Microscale Thin-film Initiator with Boron/titanium Multilayers
Jaeho Choi,Kyoungjin Kim 한국추진공학회 2020 Journal of Propulsion and Energy Vol.1 No.1
A novel integration attempt of nanoscale bimetallic multilayers into microscale thin-film explosive initiator has been proposed to improve initiator ignition performance. In order to study the ignition behaviors of boron (B) and titanium (Ti) multilayers by RC initiation circuit and bridge ohmic heating, this investigation introduced a three-dimensional computational model that includes thermal and atomic species diffusion in B/Ti multilayers as well as electric potential analysis in thin-film initiator coupled with RC circuit equation. Computational results found significant three-dimensional effects by spatially non-uniform bridge ohmic heating and lateral bridge heat loss. B/Ti reaction ignition and propagation proceeds quite differently from previous one-dimensional approximation model. More realistic present model predicts longer delay of bimetallic reaction ignition and much higher electrical energy requirement, when compared to existing one-dimensional model.
Jaeho Choi 대한금속·재료학회 2021 METALS AND MATERIALS International Vol.27 No.6
Identifying and evaluating the diffuse interface between the users’ material perceptions and the designer’s intended productpersonalities is a key aspect of decision making in user centred design. As such, acquiring reliable information on the materialperceptions of users is vital for studying material selection using the diffuse interface. To verify the reliability of materialperception data and its relevance to materials, two surveys on material perceptions were conducted using two differentgroups. The data were analysed by priority fluctuation, the p value of a hypothesis test of the difference in two proportions,and the Pearson coefficient and p value of a correlation analysis. The results revealed that conducting multiple surveys usingstatistically validated adjective pairs and merging the data can provide more objective material perception data of a largenumber of participants. Moreover, by creating a large database of statistically validated adjective pairs, designers will haveaccess to a variety of material perception data to enable more appropriate material selection to meet the diverse needs ofusers. Suitable material can be selected by employing the Euclidean distance between users’ common perceptions of materialsand the designer’s intended product personalities.
( Jaeho Choi ) 글로벌영어교육학회(구 호남영어교육학회) 2019 Studies in English education Vol.24 No.1
Study abroad (SA) research has evidenced the effects of SA in expediting second language acquisition. However, the effects of SA have been discussed mostly in terms of linguistic knowledge, lacking attention on affective aspects of language learning and the types of SA programs. This study examined the effects of SA experience on learners’ motivation and affectivity, the types of learning materials, and study hours between different types of SA experience groups. In total, 167 college students majoring in English education participated in the study and completed a survey. The analysis of survey data yielded evidence of the effects of SA experience on affective aspects: anxiety, competence, and motivation. The types of SA experience, which were classified depending on the length of stay, did not have significant effects except on competence. The results imply that SA experience influences affective aspects of language learning regardless of the length of stay. The findings were presented with discussion and suggestions for the future studies.