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Now showing items 17 - 32 of 106

  • Extreme water repellency of nanostructured low-surface-energy non-woven fabrics

    Shin, Bongsu   Lee, Kwang-Ryeol   Moon, Myoung-Woon   Kim, Ho-Young  

    We report the extreme water repellent nature of non-woven fabrics of PET (polyethyleneterephthalate) whose fiber surfaces are nanotextured with oxygen plasma and coated with a low-surface-energy nanofilm. The surface effectively suppresses vapor condensation and repels condensed water droplets in addition to exhibiting a high contact angle and a low contact angle hysteresis with a millimetre-sized water drop. We also show that the surface maintains its superhydrophobicity after water-vapor condensation and after oil-wetting due to high-aspect-ratio nanohairs on the fibers. The superior water-repellent ability of the plasma treated non-woven fabric can be exploited in a variety of industrial applications including water harvesting and fuel cell water management even under oily contaminations.
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  • Atomistic Origin of Phase Stability in Oxygen-Functionalized MXene:A Comparative Study

    Mishra, Avanish   Srivastava, Pooja   Carreras, Abel   Tanaka, Isao   Mizuseki, Hiroshi   Lee, Kwang-Ryeol   Singh, Abhishek K.  

    Oxygen-functionalized MXene, M2CO2 (M =3D group III-V metals), are emergent formidable two-dimensional (2D) materials with a tantalizing possibility for device applications. Using first-principles calculations, we perform an intensive study, on the stability of fully O-functionalized (M2CO2) MXenes. Depending on the position of O atoms, the M2CO2 can a.) O atom occupies a site which is exactly on the top of the metal exist in two different structural phases. On one side of MXene, the atom from the opposite side. On the other side, the O atom can occupy either the site on the top of the metal atom of the opposite side (BB' phase) or on the top of the C atom (CB phase). We find that for M =3D Sc and Y the CB phase is stable, whereas for M =3D Ti, Zr, Hf, V, Nb, and Ta the stable phase is BB'. The electron localization function, the atom-projected density of states, the charge transfer, and the Bader charge analyses provide a rational explanation for the relative stability of these two phases and justify the ground state structure by giving information about the preferential site of adsorption for the O atoms. We also calculate the phonon dispersion relations for both phases of M2CO2. The BB'-Sc2CO2 and the CB-Ti2CO2 are found to 'be dynamically unstable. Finally, we find that the instability of BB'-M2CO2 (M =3D Sc and Y) originates from the weakening of M-C interactions, which manifest as a phonon mode with imaginary frequency corresponding to the motion of C atom in the a-b plane. The insight into the stability of these competing structural phases of M2CO2 presented in this study is an important step in the direction of identifying the stable phases of these 2D materials.
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  • Mechanistic Insight into the Chemical Exfoliation and Functionalization of Ti3C2 MXene

    Srivastava, Pooja   Mishra, Avanish   Mizuseki, Hiroshi   Lee, Kwang-Ryeol   Singh, Abhishek K.  

    MXene, a two-dimensional layer of transition metal carbides/nitrides, showed great promise for energy storage, sensing, and electronic applications. MXene are chemically exfoliated from the bulk MAX phase; however, mechanistic understanding of exfoliation and subsequent functionalization of these technologically important materials is still lacking. Here, using density-functional theory we show that exfoliation of Ti3C2 MXene proceeds via HF insertion through edges of Ti3AlC2 MAX phase. Spontaneous dissociation of HF and subsequent termination of edge Ti atoms by H/F weakens Al-MXene bonds. Consequent opening of the interlayer gap allows further insertion of HF that leads to the formation of AlF3 and H-2, which eventually come out of the MAX, leaving fluorinated MXene behind. Density of state and electron localization function shows robust binding between F/OH and Ti, which makes it very difficult to obtain controlled functionalized or pristine MXene. Analysis of the calculated Gibbs free energy (Delta G) shows fully fluorinated MXene to be lowest in energy, whereas the formation of pristine MXene is thermodynamically least favorable. In the presence of water, mixed functionalized Ti3C2Fx(OH)(1-x) (x ranges from 0 to 1) MXene can be obtained. The Delta G values for the mixed functionalized MXenes are very close in energy, indicating the random and nonuniform functionalization of MXene. The microscopic understanding gained here unveils the challenges in exfoliation and controlling the functionalization of MXene, which is essential for its practical application.
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  • Interfacial reaction-dominated full oxidation of 5 nm diameter silicon nanowires

    Kim, Ilsoo   Park, Tae-Eon   Lee, Ki-Young   Ha, Ryong   Kim, Byung-Hyun   Chung, Yong-Chae   Lee, Kwang-Ryeol   Choi, Heon-Jin  

    While almost all Si nanostructures, including Si nanowires (SiNWs), Si nanocrystals, and Si nanotrench-like structures on a supra-or sub-10 nm scale exhibit self-limiting oxidative behavior, herein we report full oxidation of SiNWs 5 nm in diameter. We investigated the oxidative behavior of SiNWs with diameters of 5 nm and compared our findings with those for SiNWs with diameters of 30 nm. Single-crystalline SiNWs 5 and 30 nm in diameter were grown by a chemical vapor deposition (CVD) process using Ti as a catalyst. The SiNWs were then oxidized at 600-1000 degrees C for 30 min to 240 min in O-2. The thicknesses of the resulting oxide layers were determined by transmission electron microscopy (TEM). As expected, the SiNWs 30 nm in underwent self-limiting oxidation that was parabolic in nature. However, under the same conditions, the SiNWs 5 nm in diameter underwent full oxidation that was linear in nature. Atomic-scale molecular dynamic simulations revealed that the compressive stress in the oxide layer, which is generated owing to the increase in the volume of the oxide formed, decreased in the case of the SiNWs 5 nm in diameter. It is likely that this decrease in the compressive stress results in a lowering of the energy barrier for the diffusion of oxygen into the oxide layer, leading to the full oxidation of the SiNWs 5 nm in diameter. It is also responsible for the oxidation in the case of SiNWs 5 nm in diameter being interfacial reaction-dominated as opposed to the diffusion dominated-oxidation typical for SiNWs. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4764004]
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  • Tilted Janus polymer pillars RID A-4224-2010 RID E-1242-2011

    Moon, Myoung-Woon   Cha, Tae-Gon   Lee, Kwang-Ryeol   Vaziri, Ashkan   Kim, Ho-Young  

    Asymmetric adhesion is used by many insects and gecko lizards, allowing them to move on nearly any surface - horizontal, tilted or vertical. The feet of many of these creatures is covered with intricate fibrillar structures that are responsible for their superb manoeuvring ability. Among these creatures, gecko lizards have one of the most efficient and interesting adhesion devices consisting of finely angled arrays of branched fibers (setae). Here, we developed a method to create tilted Janus (two-face) micropillars on the surface of an elastomeric polymer to mimic the geometry of a gecko's footpad. The method combines soft lithography to create straight micropillars and ion beam irradiation to tilt the straight micropillars in a controlled fashion. A set of experiments were performed to measure the adhesion and friction characteristics of the fabricated tilted micropillars. Our experiments showed that the friction force along the tilting direction is approximately three times higher than the friction force associated with the sliding against the tilting direction of tilted micropillars due to the difference in the contact area during sliding of a glass ball.
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  • High aspect ratio wrinkles on a soft polymer RID E-1242-2011 RID A-4224-2010

    Ahmed, Sk. Faruque   Rho, Geon-Ho   Lee, Kwang-Ryeol   Vaziri, Ashkan   Moon, Myoung-Woon  

    Instability of a stiff thin film attached to a compliant substrate often leads to the emergence of exquisite wrinkles with length scales that depend on the system geometry and applied stresses. These patterns have vast potential applications including in tissue engineering, flexible electronics and the semiconductor industry. However, one of the limiting factors in the usage of these patterns is the low amplitude/wavelength ratio that can be achieved using the current surface engineering techniques. Here, we present an effective method that allows the creation of wrinkles with an amplitude/wavelength aspect ratio as large as 2.5 on a soft polymer. In this method, first, the surface of a poly(dimethylsiloxane) (PDMS) is pre-patterned using an Ar ion beam. Then, an amorphous carbon film gets deposited on the pre-patterned polymeric surface using glancing angle deposition (GLAD). We show that the amplitude of the created patterns can be varied between several nm to submicron size by changing the carbon deposition time, allowing us to harness patterned polymeric substrates for a variety of applications. Specifically, we demonstrate a potential application of the high aspect ratio wrinkles for changing the surface optical band gap.
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  • Magnetic Properties of Vanadium-Doped Silicon Carbide Nanowires RID E-1242-2011

    Seong, Han-Kyu   Park, Tae-Eon   Lee, Seung-Cheol   Lee, Kwang-Ryeol   Park, Jae-Kwan   Choi, Heon-Jin  

    This study reports the magnetic properties of vanadium (V) doped single crystalline silicon carbide nanowires. The first principle calculation indicated that the V-doped cubic SiC phase can exhibit half-metallic ferromagnetic properties that are essential for the realization of spintronic devices. Based on this calculation, V-doped SiC nanowires were fabricated in a chemical vapor deposition process. The single crystalline beta-SiC nanowires, which are doped with ca. 4 at.% of V, had diameters of < 100 mn and a length of several mu m. High-resolution transmission electron microscopy observations revealed vanadium carbide (VC) phases in the nanowires, even at this low concentration of dopants. Magnetic characterization implies that the nanowires are a mixture of the diamagnetic phase of VC and ferro- or paramagnetic phases of V-doped SiC. These results suggest that the doping of transition metal having high solubility to the SiC phase can lead to the realization of dilute magnetic semiconductor behavior at very low temperature.
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  • The Schottky barrier modulation at PtSi/Si interface by strain and structural deformation

    Srivastava, Pooja   Shin, Mincheol   Lee, Kwang-Ryeol   Mizuseki, Hiroshi   Kim, Seungchul  

    We show, using density functional theory (DFT) calculations, that the Schottky barrier height (SBH) at the PtSi/Si interface can be lowered by uniaxial strain applied not only on Si but also on PtSi. The strain was applied to the (001) direction of Si and PtSi, which is normal for the interface. The SBH of the hole is lowered by 0.08 eV under 2% of tensile strain on Si and by 0.09 eV under 4 % of compressive strain on PtSi. Because the SBH at PtSi/Si contact is approximately 0.2 eV, this amount of reduction can significantly lower the resistance of the PtSi/Si contact; thus applying uniaxial strain on both PtSi and Si possibly enhances the performance of Schottky barrier field effect transistors. Theoretical models of SB formation and conventional structure model are evaluated. It is found that Pt penetration into Si stabilizes the interface and lowers the SBH by approximately 0.1 eV from the bulk-terminated interface model, which implies that conventionally used bulk-terminated interface models have significant errors. Among the theoretical models of SB formation, the model of strong Fermi level pining adequately explains the electron transfer phenomena and SBH, but it has limited ability to explain SBH changes induced by changes of interface structure. (C) 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.
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  • Liquid spreading on superhydrophilic micropillar arrays RID A-4224-2010 RID E-1242-2011

    Kim, Seong Jin   Moon, Myoung-Woon   Lee, Kwang-Ryeol   Lee, Dae-Young   Chang, Young Soo   Kim, Ho-Young  

    When a drop is deposited on a superhydrophilic micropillar array, the upper part of the drop (referred to as the bulk) collapses while the bottom part penetrates into the gaps of the array, forming a fringe film. Here we quantify the early stage dynamics of this process using a combination of experiment and theory. We show that the circular front of the fringe film spreads like t(1/2), t being time, when coupled to the bulk flow. However, the film is found to advance like t(1/3) through faceted zippering in the absence of the bulk. We then show that the spreading of the bulk and the entire drop footprint follows a power law (t(1/4)) that is different from Washburn's law. This work can be a starting point to completely understand the spreading of liquids on superhydrophilic surfaces and opens questions specific to superwetting behaviour including the criteria to determine whether the fringe film will expand through lateral zipping or advance radially outwards.
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  • Biomedical applications of diamond-like carbon coatings: a review.

    Roy, Ritwik Kumar   Lee, Kwang-Ryeol  

    Owing to its superior tribological and mechanical properties with corrosion resistance, biocompatibility, and hemocompatibility, diamond-like carbon (DLC) has emerged as a promising material for biomedical applications. DLC films with various atomic bond structures and compositions are finding places in orthopedic, cardiovascular, and dental applications. Cells grew on to DLC coating without any cytotoxity and inflammation. DLC coatings in orthopedic applications reduced wear, corrosion, and debris formation. DLC coating also reduced thrombogenicity by minimizing the platelet adhesion and activation. However, some contradictory results (Airoldi et al., Am J Cardiol 2004;93:474-477, Taeger et al., Mat-wiss u Werkstofftech 2003;34:1094-1100) were also reported that no significant improvement was observed in the performance of DLC-coated stainless stent or DLC-coated femoral head. This controversy should be discussed based on the detailed information of the coating such as atomic bond structure, composition, and/or electronic structure. In addition, instability of the DLC coating caused by its high level of residual stress and poor adhesion in aqueous environment should be carefully considered. Further in vitro and in vivo studies are thus required to confirm its use for medical devices.
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  • Functionalization effect on a Pt/carbon nanotube composite catalyst:a first-principles study

    Kim, Byung-Hyun   Lee, Kwang-Ryeol   Chung, Yong-Chae   Park, Mina  

    Chemical interactions between Pt and both pristine and defective carbon nanotubes (CNTs) that were functionalized with various surface functional groups, including atomic oxygen (-O), atomic nitrogen (-N), hydroxyl (-OH) and amine (-NH2) groups, were investigated through first-principles calculations. Our calculations suggest that the oxygen or nitrogen of the surface functional group can promote better structural stability of a Pt/CNT complex in terms of the binding energy enhancement between Pt and CNTs. Enhanced binding of the Pt/CNT complex would improve the long-term durability of the complex and thus enhance the catalytic activity of Pt catalysts supported on CNTs. Among the functional groups investigated, atomic nitrogen resulted in the most consistent increase in the Pt binding energies on pristine or defective CNTs. Moreover, atomic nitrogen decoration on the surface of CNTs rather than substitution into the CNTs appears to be more desirable. A d-band centre analysis and H-2 adsorption calculations also revealed that the catalytic activity of Pt can be improved via efficient functionalization of the CNT support.
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  • Mechanism of contact pressure-induced friction at the amorphous carbon/alpha olefin interface

    Li, Xiaowei   Wang, Aiying   Lee, Kwang-Ryeol  

    Combining an amorphous carbon (a-C) film with a lubricating oil can significantly improve the friction performance and lifetime of moving mechanical components. However, the friction mechanism is not well understood owing to a lack of information regarding the structure of the interface when exposed to high contact pressure. Here, we select linear alpha olefin, C5H10, as a lubricant and study the evolution of the structure of the a-C/C5H10/a-C sliding interface under contact pressure via reactive molecular dynamics simulation. Our results suggest that introducing C5H10 into the a-C/a-C interface reduces the friction coefficient by up to 93% compared with no lubricant, although the lubricating efficiency strongly depends on the contact pressure. In particular, increasing the contact pressure not only induces the binding of the lubricant with a-C, but also facilitates the dissociation of the C5H10 carboncarbon skeleton by specific scissions, which governs the friction behavior. These results disclose the underlying lubrication mechanism and could enable the development of new and effective lubricating systems with long lifetimes.
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  • Langmuir-Blodgett artificial solid-electrolyte interphases for practical lithium metal batteries

    Kim, Mun Sek   Ryu, Ji-Hyun   Deepika   Lim, Young Rok   Nah, In Wook   Lee, Kwang-Ryeol   Archer, Lynden A.   Cho, Won Il  

    Practical lithium metal batteries require full and reversible utilization of thin metallic Li anodes. This introduces a fundamental challenge concerning how to create solid-electrolyte interphases (SEIs) that are able to regulate interfacial transport and protect the reactive metal, without adding appreciably to the cell mass. Here, we report on physicochemical characteristics of Langmuir-Blodgett artificial SEIs (LBASEIs) created using phosphate-functionalized reduced graphene oxides. We find that LBASEIs not only meet the challenges of stabilizing the Li anode, but can be facilely assembled in a simple, scalable process. The LBASEI derives its effectiveness primarily from its ability to form a durable coating on Li that regulates electromigration at the anode/electrolyte interface. In a first step towards practical cells in which the anode and cathode capacities are matched, we report that it is possible to achieve stable operations in both coin and pouch cells composed of a thin Li anode with the LBASEI and a high-loading intercalation cathode.
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  • Nano-embossed structure on polypropylene induced by low energy Ar ion beam irradiation

    Ahmed, Sk. Faruque   Rho, Geon-Ho   Lee, Ji Yeong   Kim, Seong Jin   Kim, Ho-Young   Jang, Yong-Jun   Moon, Myoung-Woon   Lee, Kwang-Ryeol  

    The surface morphology evolution of polypropylene (PP) irradiated with an Ar ion beam was explored using a hybrid ion beam system. PP formed the emboss-like nanostructure of similar to 50 nm induced by the Ar ion beam treatment for shorter ion beam treatment times, while the emboss-like structure was transited into a 3-D long nanofiber-like nanostructure for longer treatment times. The Fourier transform infrared spectra revealed that the polymer chain cross-linking increased with increasing ion treatment time, while the Raman analysis showed that the conducting amorphous carbon increased on the surface of PP. The surface hardness and plane strain modulus of the PP decreased from 0.11 to 0.07 GPa and from 1.75 to 1.26 GPa, respectively, with respect to the Ar ion beam treatment time due to the surface nanostructures formed by the Ar ion beam. (C) 2010 Elsevier B.V. All rights reserved.
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    Lee, Kwang-Ryeol   Baik, Young-Joon   Eun, Kwang Yong   Han, Seunghee  

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  • Tuning the electronic,mechanical,thermal,and optical properties of tetrahexcarbon via hydrogenation

    Kilic, Mehmet Emin   Lee, Kwang-Ryeol  

    Recently, Tetrahexcarbon (TH-carbon), a new two-dimensional(2D) carbon allotrope, has been identified with an intrinsic direct bandgap, which makes it promising for practical applications in optoelectronic devices. Using first-principles calculations, we examined the possibility of manipulating the physical and chemical properties of TH-carbon sheet by controlled hydrogenation. We systematically studied pristine TH-carbon and its hydrogenated derivatives with various configurations such as single- and double-sided hydrogenation. Our study revealed their stability in energetic, dynamic, thermal, and mechanical aspects. Depending on the hydrogen coverage and configurations, we observed the tunability of the phononic and electronic bandgap, and the direct-indirect-direct bandgap transitions. These results suggest the plausibility of modulating its electronic properties by hydrogenation. The heat transport in TH-carbon is anisotropic. A significant decrease in thermal conductivity was observed in the fully hydrogenated TH-carbon. The thermal conductivity in TH-carbon can be controlled by the sp(3) C-H low conduction domains. A notable increase in specific heat capacity was observed in hydrogenated derivatives of TH-carbon, which would make them useful in nanoscale engineering of thermal transport. The hydrogenation was found to reduce the in-plane stiffness and Young's modulus, but increase the ultimate strength. These findings would provide important guidelines for practical applications of TH-carbon. (C) 2020 Elsevier Ltd. All rights reserved.
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