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Now showing items 1 - 16 of 106

  • Real-Time "Eye-Writing" Recognition Using Electrooculogram

    Lee, Kwang-Ryeol   Chang, Won-Du   Kim, Sungkean   Im, Chang-Hwan  

    Eye movements can be used as alternative inputs for human-computer interface (HCI) systems such as virtual or augmented reality systems as well as new communication ways for patients with locked-in syndrome. In this study, we developed a real-time electrooculogram (EOG)-based eye-writing recognition system, with which users can write predefined symbolic patterns with their volitional eye movements. For the "eye-writing" recognition, the proposed system first reconstructs the eye-written traces from EOG waveforms in real-time; then, the system recognizes the intended symbolic inputs with a reliable recognition rate by matching the input traces with the trained eye-written traces of diverse input patterns. Experiments with 20 participants showed an average recognition rate of 87.38% (F1 score) for 29 different symbolic patterns (26 lower case alphabet characters and three functional input patterns representing Space, Backspace, and Enter keys), demonstrating the promise of our EOG-based eye-writing recognition system in practical scenarios.
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  • Accelerated Data-Driven Accurate Positioning of the Band Edges of MXenes

    Mishra, Avanish   Satsangi, Swanti   Rajan, Arunkumar Chitteth   Mizuseki, Hiroshi   Lee, Kwang-Ryeol   Singh, Abhishek K.  

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  • Accelerated Data-Driven Accurate Positioning of the Band Edges of MXenes

    Mishra, Avanish   Satsangi, Swanti   Rajan, Arunkumar Chitteth   Mizuseki, Hiroshi   Lee, Kwang-Ryeol   Singh, Abhishek K.  

    Functionalized MXene has emerged a promising class of two-dimensional materials having more than tens of thousands of compounds, whose uses may range from electronics to energy applications. Other than the band gap, these properties rely on the accurate position of the band edges. Hence, to synthesize MXenes for various applications, a prior knowledge of the accurate position of their band edges at an absolute scale is essential; computing these with conventional methods would take years for all the MXenes. Here, we develop a machine learning model for positioning the band edges with GW level of accuracy having a minimum root-mean-squared error of 0.12 eV. An intuitive model is proposed based on the combination of Perdew-Burke-Ernzerhof band edge and vacuum potential having a correlation of 0.93 with GW band edges. These models can be utilized to identify MXenes for a desired application in an accelerated manner.
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  • Monolayer BC2: an ultrahigh capacity anode material for Li ion batteries

    Das, Deya   Hardikar, Rahul P.   Han, Sang Soo   Lee, Kwang-Ryeol   Singh, Abhishek K.  

    There is great interest in developing promising candidate materials for high-capacity, low cost, environmentally friendly, longer cycle life anodes for lithium ion batteries. Due to better Li adsorption properties than graphene, boron doped graphene has been considered to be an attractive anode material for Li-ion batteries. Using first principles density functional theory calculations, we investigate the effect of increasing boron concentration on the gravimetric capacity of monolayered boron doped carbon sheets. The calculations are performed for uniformly boron doped carbon sheets, BCx (x =3D 7, 5, 3, 2 and 1) as well as their non-uniformly doped counterparts, which are found to be energetically preferable for x =3D 5, 2 and 1. Our results indicate pronounced enhancement in gravimetric capacity with increasing concentration of B, up to x =3D 2. The storage capacity of the uniformly doped BC2 turns out to be the highest ever reported for B doped graphene sheets, which is 1.9 times (1667 mA h g(-1)) that of the previously reported value for BC3 (J. Phys. Chem. Lett., 2013, 4, 1737-1742). This dramatic increase in the capacity of uniformly doped BC2 occurs because of the availability of significantly more empty states above the Fermi level compared to the other BCx sheets. Moreover, the diffusion energy barriers and open circuit voltage are found to be lower in uniformly doped BC2, leading to better Li kinetics. For x =3D 1, Li binds very strongly to the uniformly doped BC and higher diffusion energy barriers are found for non-uniformly doped BC, rendering them ineffective as anode materials. Our study reveals that BC2 is the most promising candidate as an anode material for Li ion batteries owing to its high Li storage capacity combined with low diffusion barrier and low open circuit voltage.
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  • Real-time ‘eye-writing’ recognition using electrooculogram (EOG)

    Lee, Kwang-Ryeol   Chang, Won-Du   Kim, Sungkean   Im, Chang-Hwan  

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  • Corrosion behavior of diamond-like carbon film induced by Al/Ti co-doping

    Xu, Xiaowei   Zhou, Yong   Liu, Linlin   Guo, Peng   Li, Xiaowei   Lee, Kwang-Ryeol   Cui, Ping   Wang, Aiying  

    Diamond-like carbon films with Al and Ti dopants (Al/Ti-DLC) were fabricated by a hybrid ion beam deposition system combined with a special design of splicing targets, in which the Al/Ti ratio was changed from 4.0 to 1.1 by tailoring the Al content. The structural evolution and corrosion behavior of Al/Ti-DLC films in 3.5 wt% NaCl solution were systematically investigated by comparing with the 316L SS and pure DLC film. Results revealed that compared with the pure DLC and 316L SS substrate, the Al/Ti-DLC film exhibited enhanced corrosion resistance by reducing the porosity and increasing the barrier for both the charge transfer and the diffusion of corrosive ions into the substrate. In particular, the anti-corrosion behavior of DLC film highly depended on the Al/Ti ratio, which decreased gradually with the Al/Ti ratio ranged from 4.0 to 1.1. Structural and EIS analysis indicated that this was ascribed to both the increased residual stress and sp(2) hybridized fraction, which induced not only the formation and propagation of cracks during the corrosion process but also the high electric conductivity and low diffusion barrier of corrosive ions, accelerating the corrosion processes at the film/substrate interface.
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  • Pressure induced manifold enhancement of Li-kinetics in FCC fullerene

    Das, Deya   Han, Sang Soo   Lee, Kwang-Ryeol   Singh, Abhishek K.  

    The reduction of the diffusion energy barrier for Li in electrodes is one of the required criteria to achieve better performances in Li ion batteries. Using density functional theory based calculations, we report a pressure induced manifold enhancement of Li-kinetics in bulk FCC fullerene. Scanning of the potential energy surface reveals a diffusion path with a low energy barrier of 0.62 eV, which reduces further under the application of hydrostatic pressure. The pressure induced reduction in the diffusion barrier continues till a uniform volume strain of 17.7% is reached. Further enhancement of strain increases the barrier due to the repulsion caused by C-C bond formation between two neighbouring fullerenes. The decrease in the barrier is attributed to the combined effect of charge transfer triggered by the enhanced interaction of Li with the fullerene as well as the change in profile of the local potential, which becomes more attractive for Li. The lowering of the barrier leads to an enhancement of two orders of magnitude in Li diffusivity at room temperature making pressurized bulk fullerene a promising artificial solid electrolyte interface (SEI) for a faster rechargeable battery.
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  • Composite oxygen-barrier coating on a polypropylene food container

    Kim, Seong Jin   Song, Eunkyung   Jo, Kyoungsik   Yun, Taekyung   Moon, Myoung-Woon   Lee, Kwang-Ryeol  

    The composite oxygen-barrier coating of plasma polymerized-hexamethyldisiloxane (pp-HMDSO) and silicon oxide (SiOx) on polypropylene (PP) was performed to lower the oxygen transmission rate. A pp-HMDSO interlayer was employed to cover PP pores at a relatively high porosity and provide better a thermal and mechanical buffer between brittle SiOx and soft and porous PP. There is an optimum thickness for the pp-HMDSO and SiOx films due to competition between the accumulated strain energy by compression in the composite layers and the interfacial adhesion strength. By choosing the optimal thickness of the layers, the oxygen transmission rate was improved from 7.42 x 10(-4) for a pristine PP container to 2.6 x 10(-5) cm(3)/m(2)-day-Pa with 8 +/- 2 nm pp-HMDSO and 24 +/- 6 nm SiOx layers. (C) 2013 Elsevier B.V. All rights reserved.
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  • Thermal stability of superhydrophobic, nanostructured surfaces

    Cha, Sung-Chul   Her, Eun Kyu   Ko, Tae-Jun   Kim, Seong Jin   Roh, Hyunchul   Lee, Kwang-Ryeol   Oh, Kyu Hwan   Moon, Myoung-Woon  

    The thermal stability of superhydrophobic, nanostructured surfaces after thermal annealing was explored. Flat surfaces coated with hydrophobic diamond-like carbon (DLC) via plasma polymerization of hexamethyldisiloxane (HMDSO) showed a gradual decrease in the water contact angle from 90 degrees to 60 degrees while nanostructured surfaces maintained superhydrophobicity with more than 150 degrees for annealing temperatures between 25 and 300 degrees C. It was also found that surfaces with nanostructures having an aspect ratio of more than 5.2 may maintain superhydrophobicity for annealing temperatures as high as 350 degrees C; above this temperature, however, the hydrophobicity on surfaces with lower aspect ratio nanostructures gradually degraded. It was observed that regardless of the aspect ratios of the nanostructure, all superhydrophobic surfaces became superhydrophilic after annealing at temperatures higher than 500 degrees C. (C) 2012 Elsevier Inc. All rights reserved.
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  • Li diffusion through doped and defected graphene

    Das, Deya   Kim, Seungchul   Lee, Kwang-Ryeol   Singh, Abhishek K.  

    We investigate the effect of nitrogen and boron doping on Li diffusion through defected graphene using first principles based density functional theory. While a high energy barrier rules out the possibility of Li-diffusion through the pristine graphene, the barrier reduces with the incorporation of defects. Among the most common defects in pristine graphene, Li diffusion through the divacancy encounters the lowest energy barrier of 1.34 eV. The effect of nitrogen and boron doping on the Li diffusion through doped defected-graphene sheets has been studied. N-doping in graphene with a monovacancy reduces the energy barrier significantly. The barrier reduces with the increasing number of N atoms. On the other hand, for N doped graphene with a divacancy, Li binds in the plane of the sheet, with an enhanced binding energy. The B doping in graphene with a monovacancy leads to the enhancement of the barrier. However, in the case of B-doped graphene with a divacancy, the barrier reduces to 1.54 eV, which could lead to good kinetics. The barriers do not change significantly with B concentration. Therefore, divacancy, B and N doped defected graphene has emerged as a better alternative to pristine graphene as an anode material for Li ion battery.
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  • Experimental study of drop spreading on textured superhydrophilic surfaces

    Kim, Seong Jin   Kim, Jungchul   Moon, Myoung-Woon   Lee, Kwang-Ryeol   Kim, Ho-Young  

    The spreading dynamics of a drop deposited on a superhydrophilic micropillar array are qualitatively different from those on a smooth surface in that a thin fringe layer rapidly wicks into microstructures while the bulk collapses in the center. Here we experimentally measure the temporal evolution of the shapes of water and silicone oil drops on the superhydrophilic surfaces, and find various power laws that describe the spreading dynamics. The bulk radius increases initially but shrinks in the late stages for drainage of its volume by wicking. The fringe film tends to grow diffusively for the entire timespan of spreading with the effective diffusivity being a function of surface tension, pillar height, viscosity, and surface roughness. The entire footprint is shown to grow like t(1/4), t being time, for the entire time range, which allows us to estimate the time for complete drop spreading. (C) 2013 AIP Publishing LLC.
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  • Atomistic simulations of diamond-like carbon growth

    Joe, Minwoong   Moon, Myoung-Woon   Lee, Kwang-Ryeol  

    Diamond-like carbon (DLC) films are composed of carbon bonds with different hybridizations, including sp(2), sp(3) and even sp(1). Understanding the atomic bonding structure is essential to understanding the properties and optimizing the process parameters of the films. Because of the limited analytical tools for characterizing the atomic bonding structure in amorphous materials, computational research at the atomistic scale could provide significant insight into the structure-property relationships in diamond-like carbon films and has been applied to understanding the various phenomena occurring during DLC film growth. The contributions of the atomistic simulations and electronic structure calculations pertain mainly to three important issues: (i) the sp(3) bond formation and stress generation mechanisms, (ii) the stress reduction mechanism by metal incorporation, and (iii) the impact angle-dependent surface smoothening/roughening mechanisms. (C) 2011 Elsevier B. V. All rights reserved.
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  • Nanoporous structures of polyimide induced by Ar ion beam irradiation

    Ahmed, Sk. Faruque   Lee, Kwang-Ryeol   Yoon, Ju-il   Moon, Myoung-Woon  

    The surface morphology evolution of polyimide (PI) that was treated with an Ar ion beam was explored using a hybrid ion beam system. A hole-like nanostructure formed on PI during the Ar ion beam treatment at a lower fluence, but PI formed 3D porous nanostructures with a mean diameter of similar to 90 nm at a higher fluence. The chemical binding energy and the composition of the Ar ion irradiated PI were analyzed using FT-IR and XPS spectra, which revealed that the polymer chain scissioning increased with increasing Ar ion treatment duration, i.e., fluence. The surface hardness and the elastic modulus of PI increased from 1.17 to 1.62 GPa and 4.06 to 5.41 GPa, respectively, with respect to the Ar ion beam treatment duration. (C) 2011 Elsevier B.V. All rights reserved.
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  • Indirect-to-Direct Band Gap Transition of Si Nanosheets:Effect of Biaxial Strain

    Kim, Byung-Hyun   Park, Mina   Kim, Gyubong   Hermansson, Kersti   Broqvist, Peter   Choi, Heon-Jin   Lee, Kwang-Ryeol  

    The effect of biaxial strain on the band structure of two-dimensional silicon nanosheets (Si NSs) with (111), (110), and (001) exposed surfaces was investigated by means of density functional theory calculations. For all the considered Si NSs, an indirect-to-direct band gap transition occurs as the lateral dimensions of Si NSs increase; that is, increasing lateral biaxial strain from compressive to tensile always enhances the direct band gap characteristics. Further analysis revealed the mechanism of the transition which is caused by preferential shifts of the conduction band edge at a specific k-point because of their bond characteristics. Our results explain a photoluminescence result of the (111) Si NSs [U. Kim et al., ACS Nano 2011,.5, 2176-2181] in terms of the plausible tensile strain imposed in the unoxidized inner layer by surface oxidation.
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  • Controlled Catalytic Properties of Platinum Clusters on Strained Graphene

    Kim, Gyubong   Kawazoe, Yoshiyuki   Lee, Kwang-Ryeol  

    We employed graphene under isotropic strain as the supporting material for Pt clusters (Pt-x, x = 1, 4, or 6) and studied the site-resolved molecular adsorption behaviors of H-2, CO, and OH on the clusters using ab initio calculations. It was shown that the applied strain enhances the binding of the Pt atom or clusters on the graphene, which lowers the average energy of the d electrons (d-band center). However, for the Pt-4 and Pt-6 clusters that form two Pt atomic layers on the graphene, only the d-band center of the bottommost Pt layer can be readily tuned by the external strain on graphene. However, the site-resolved calculations of molecular binding demonstrate that controlling the d-band center of the bottommost Pt atoms can be a substantial factor for all the catalytic activities of the Pt cluster. We also found that the stability of the Pt/graphene system was enhanced by applying strain to the graphene support.
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  • Surface energy tunable nanohairy dry adhesive by broad ion beam irradiation

    Rahmawan, Yudi   Kim, Tae-il   Kim, Seong Jin   Lee, Kwang-Ryeol   Moon, Myoung-Woon   Suh, Kahp-Yang  

    We present a simple and shape-controllable method to tune the surface energy and tilting angle of nanohairy structures using broad ion beam irradiation. First, the vertical nanohair arrays made of ultraviolet (UV)-curable polyurethane acrylate (PUA) material were prepared by replica molding. Subsequently, the nanohairs were bent to a wide range of angles from 0 (vertical) to 75 degrees (stooped hairs) by the oblique broad Ar ion beam. The surface energy was tuned with normal oxygen (O(2)) ion beam irradiation together with aging effect (47.1 to 71.9 mJ m(-2)). It was observed that the shear adhesion strength was significantly increased from 25.2 to 58.1 N cm(-2) on the hydrophilic test surface and from 22.8 to 45.7 N cm(-2) on the hydrophobic test surface, respectively, with the variation of surface energy from 47.1 to 71.9 mJ m(-2). In addition, the shear adhesion strength was increased from 58.1 to 124.1 N cm(-2) and from 45.7 to 110.6 N cm(-2) on hydrophilic and hydrophobic test surfaces, respectively, as the tilting angle was varied from 0 to 75 degrees at the highest surface energy of 71.9 mJ m(-2.) The measured adhesion data were compared with the theoretical adhesion forces based on Johnson-Kendall-Roberts (JKR), modified-JKR, peel zone (PZ), and Kendall peeling models, suggesting that the contact state of nanohairs is a mixture of tip and side contacts.
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