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Kwac, Lee-Ku,Kim, Hong-Gun,Lee, Hyung-Joo Elsevier 2018 Infrared physics & technology Vol.93 No.-
<P><B>Abstract</B></P> <P>Optimum conditions of Ga<SUB>x</SUB>In<SUB>1-x</SUB>P/GaAs<SUB>1-y</SUB>P<SUB>y</SUB> strain compensation structure were investigated to improve excessive compressive strain in In<SUB>0.15</SUB>Ga<SUB>0.85</SUB>As based lighting-emitting diodes with 1000 nm emission wavelength. The GaAs<SUB>1-y</SUB>P<SUB>y</SUB> material, where only tensile strain is present, was used as a quantum barrier for compensating compressive strain of a In<SUB>0.15</SUB>Ga<SUB>0.85</SUB>As quantum well. The Ga<SUB>x</SUB>In<SUB>1-x</SUB>P strain material, which has both tensile and compressive strain, was used as a strain tuning barrier in order to improve unbalanced strain in In<SUB>0.15</SUB>GaAs/GaAsP<SUB>0.09</SUB> MQWs. Subsequent photoluminescence (PL) measurements, indicated that the GaAsP<SUB>0.09</SUB> tensile strain barrier was effective in compensating compressive strain of the In<SUB>0.07</SUB>GaAs quantum wells. Furthermore, noticeable PL intensity was observed from In<SUB>0.15</SUB>GaAs/GaAsP<SUB>0.09</SUB> MQWs with the conditioned Ga<SUB>0.53</SUB>In<SUB>0.47</SUB>P strain tuning barrier. A fabricated IR-LED containing In<SUB>0.15</SUB>GaAs based MQWs with optimized Ga<SUB>0.53</SUB>In<SUB>0.47</SUB>P/GaAsP<SUB>0.09</SUB> strain compensation barriers displayed 110% higher light output-power than a conventional LED. This result suggests that using of GaAsP<SUB>0.09</SUB> and Ga<SUB>0.53</SUB>In<SUB>0.47</SUB>P as tensile strain barriers effectively compensates excessive compressive strain of In<SUB>0.15</SUB>GaAs based MQWs with a 1000 nm emission wavelength.</P> <P><B>Highlights</B></P> <P> <UL> <LI> GaInP/GaAsP stain compensation barrier improved excessive compressive strain of InGaAs/GaAs hetero-structure. </LI> <LI> Output power of 1000 nm IR-LEDs was greatly enhanced by optimized GaInxP1-x/GaAs1-yPy strain barrier. </LI> </UL> </P>
Kwac, Kijeong,Lee, Ji Hoon,Choi, Jang Wook,Jung, Yousung American Chemical Society 2016 The Journal of Physical Chemistry Part C Vol.120 No.7
<P>There are a growing number of reports suggesting that the specific surface area in graphitic materials is not a critical parameter to determine the CO, capture capacity, but rather the pore size and its geometry are more relevant, yet a detailed theoretical and quantitative understanding that could facilitate further developments for the pore size effects is presently lacking. Using the thermodynamic continuum model combined with electronic structure calculations, we identify the critical size of pores in graphitic materials for enhanced carbon dioxide (CO2) uptake as well as its selectivity relative to N-2. We find that there exists a value of pore size which is most optimal in the CO, capture capacity as well as CO2/N-2 selectivity at a given pressure and temperature, supporting the previous experimental observations regarding critical parameters determining the CO2 adsorption capacity of porous carbon materials. The calculated results emphasize the importance of graphitic pore size from 8 to10 angstrom in CO, capture and selectivity against N-2.</P>
Multilayer Two-Dimensional Water Structure Confined in MoS<sub>2</sub>
Kwac, Kijeong,Kim, In,Pascal, Tod A.,Goddard, William A.,Park, Hyung Gyu,Jung, Yousung American Chemical Society 2017 JOURNAL OF PHYSICAL CHEMISTRY C - Vol.121 No.29
<P>The conflicting interpretations (square vs rhomboidal) of the recent experimental visualization of the two-dimensional (2D) water confined in between two graphene sheets by transmission electron microscopy measurements, make it important to clarify how the structure of two-dimensional water depends on the constraining medium. Toward this end, we report here molecular dynamics (MD) simulations to characterize the structure of water confined in between two MoS2 sheets. Unlike graphene, Water spontaneously fills the region sandwiched by two MoS2 sheets in ambient conditions to form planar multilayered water structures with up to four layer. These 2D water molecules form a specific pattern in which the square ring structure is formed by four, diamonds, via H-bonds, while each diamond shares a corner in a perpendicular manner, yielding an intriguing isogonal tiling structure. Comparison of the water structure confined in graphene (flat uncharged surface) vs MoS2 (ratchet-profiled charged surface), demonstrates that the polarity (charges),of the surface can tailor the density of:confined water, which in turn can directly determine the planar ordering of the multilayered water molecules in graphene or MoS2. On the other hand, the intrinsic surface profile (flat vs ratchet-profiled) plays a minor role in determining the 2D water configuration.</P>
Effect of GaP barrier on efficiency enhancement of 860-nm vertical-cavity surface-emitting laser
Kwac, Lee-Ku,An, Won-Chan,Kim, Hong-Gun,Park, Gwang-Hoon,So, Jin-Su,Jang, In-Kyu,Lee, Hyung-Joo Elsevier 2019 Infrared physics & technology Vol.96 No.-
<P><B>Abstract</B></P> <P>The use of a GaP barrier in a vertical-cavity surface-emitting laser (VCSEL) and its effect on output power at 860 nm were investigated. The GaP barrier was found to have higher conductivity from current–voltage measurements. This material is suitable for its use as a current spreading layer in the VCSEL based on (Al)GaAs. Experimental measurements show that the output power from the VCSEL effectively increased as the thickness of the GaP barrier increased, eventually saturating for a specific thickness value. The calculated real emission area and optical properties show that the current spreading effect in the GaP barrier is limited by the current window. Among different VCSEL chips, the highest output power of 26.5 mW was obtained using an optimal 3-µm-thick GaP barrier, which is an increase of 43% compared with the power output from the conventional structure. Nearly equal output powers were observed for structures with 3- and 6-μm GaP barriers. The results show that the GaP barrier used as the current spreading layer is essential for improving the output power from the VCSEL at 860 nm. The optimal thickness of the GaP barrier in the VCSEL strongly depends on the aperture diameter (real emission area), which can be determined from the current window.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Highly conductive GaP layer improved 860 nm GaAs based vertical cavity surface emitting laser (VCSEL). </LI> <LI> Optimal thickness of GaP layer is effective in obtaining maximum emission area of GaAs based VCSEL. </LI> <LI> Output power of 860 nm VCSEL was greatly enhanced by using GaP layer with optimized thickness. </LI> </UL> </P>
Kijeong Kwac,Peter G. Wolynes 대한화학회 2008 Bulletin of the Korean Chemical Society Vol.29 No.11
We tried to predict the tertiary structure of the 63-residue-long alpha-helical protein, 1r69, from the amino acid sequence with the assumption that the locations of α-helical residues are known. We applied two approaches. One approach is to implement all-atom molecular dynamics (MD) simulations of segments of the target protein and use the snapshot structures of these simulations as memory sets in the associated memory Hamiltonian, which uses coarse-grained model of protein structure and describes the effect of solvent by a water-mediated long range interaction potential. The other approach is to implement all-atom MD simulations with implicit water model applying additional biasing potential functions to reduce the radius of gyration and induce the formation of secondary structures for the helical residues. In the coarse grained model of the associated memory Hamiltonian we tried two different sets of memory to see the effect of the local structural signals in the memory set. We found that the predicted results strongly depend on the structures used in the memory set. The predicted results from the associated memory Hamiltonian give a structure with RMSD value of 1.977 Å with respect to the native structure. The predicted results from the biased all atom MD simulation method give a structure with RMSD value of 2.971 Å.
ANALYSIS OF A LARGE INJECTION-MOLDED BODY USING KNOWLEDGE-BASED FLOW PROCESS TECHNOLOGY
L. K. KWAC,H. K. LEE,김홍건 한국자동차공학회 2012 International journal of automotive technology Vol.13 No.5
A knowledge-based flow process is presented for large injection-molded body technology (LIMBT). Injection molding of a large body is a difficult technique because of the many factors and their interactions during the molding process. The proposed flow process can support LIMBT through integration of CAE(Computer-Aided Engineering), CAI (Computer- Aided Inspection), and monitoring systems. CAE and DOE (Design of Experiment) are used to construct an optimal mold design in terms of gates and runners and to identify working conditions for the molding process. CAI and monitoring systems with temperature sensors and pressure sensors can be used to inspect the physical molding process and the molded parts. The flow process of a large body is systematically planned and constructed using a knowledge-based flow process with DOE and computer-aided technologies. The proposed flow process is implemented for the molding process of an automobile front bumper fascia.
상용차 시트용 평행사변형구조 마그네틱 현가기구의 설계 및 성능평가
곽이구(Lee Ku Kwac),김홍건(Hong Gun Kim),신희재(Hee Jae Shin),정영배(Young Bae Jung),송정상(Jung Sang Song) 대한기계학회 2015 大韓機械學會論文集A Vol.39 No.4
상용차 운전자들은 운전시간이 길고, 엔진폭발에 의한 아이들 진동 등으로 인하여 일반인보다 많은 피로감을 느낀다. 본 논문에서는 아이들 진동에 의해 상용차 운전석에 전달되는 진동을 줄여 승차감을 향상시키기 위하여, 마그네틱 스프링을 이용한 시트현가장치를 제안하였다. 또한 최적설계를 통해 평행사변형 마그네틱 현가기구를 설계하였으며 이 제안된 시트를 장착한 차량의 진동 및 성능을 연구하였다. 그 결과 진동 전달률은 코일스프링 현가기구가 0.990, 평행사변형 마그네틱 현가기구는 0.823로 우수한 성능을 보여주었다. Commercial drivers feel tired more than the general public, because their driving times are long and they experience more idle vibration. In this study, we developed a nonlinear model of a magnetic, linear spring seat suspension to determine the optimal design to improve ride comfort. The resonant frequency for the optimal design of the suspension was found to be 3.5 Hz, and the stiffness was analyzed through displacement-load experiments. Additionally, the vibration transmissibility was analyzed by the suspension stiffness, and the existing coil spring type vibration transmissibility was found to be 0.99. A parallelogram type magnetic spring was determined to result in a better performance than the existing spring with a vibration transmissibility of 0.823.