Multilayer Two-Dimensional Water Structure Confined in MoS₂

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>

Protein Structure Prediction Using an Associated Memory Hamiltonian and All-Atom Molecular Dynamics Simulations

Kwac, Ki-Jeong,Wolynes, Peter G. Korean Chemical Society 2008 Bulletin of the Korean Chemical Society Vol.29 No.11

The optimum condition of Ga_xIn_1-xP/GaAs_1-yP_y strain compensation for excessive strained In_0.15GaAs MQWs in 1000 nm infrared light-emitting diode

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>

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>

Computational Analysis of Pressure-Dependent Optimal Pore Size for CO₂ Capture with Graphitic Surfaces

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>

탄소 복합재의 인장거동에 따른 기계적 특성에 관한 연구

곽이구(Kwac, Lee Ku),김홍건(Kim, Hong Gun),오승규(Oh, Seung Gyu),한웅(Han, Woong) 대한기계학회 2012 대한기계학회 춘추학술대회 Vol.2012 No.5-3

Protein Structure Prediction Using an Associated Memory Hamiltonian and All-Atom Molecular Dynamics Simulations

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.

초정밀 가공기의 실시간 운동오차 및 열변형오차 보상

곽이구(Lee-Ku Kwac),김재열(Jae-Yeol Kim),김홍건(Hong-Gun Kim) 한국생산제조학회 2006 한국생산제조학회지 Vol.15 No.4

Recently, demand the ultra precision product which is increasing rapidly is used extensively frontier industry field such as semi-conductor, computer, aerospace, precision machine. Ultra precision processing is the portion that is very needed to NT in the field of mechanical engineering. The latest date, together with radical advancement of electronic and photonics industry, necessity of ultra precision processing is on the increase for the manufacture of various kernel parts those are connected with these industrial fields. Specially, require motion accuracy of high resolution of nm order in stroke of hundreds millimeters according as diameter of processing object great and processing accuracy rises. In this case ,the response speed absolute delay because inertial mass of moving part is very large. Therefore, real time motion error compensation becomes very hardly. In this paper, we used ultra precision cutting unit(UPCU) to cope such problem. a UPCU is designed and tested to obtain sub-micrometer from accuracy in diamond turning of flat surfaces. The thermal growth spindle error is compensated for real time using a UPCU driven by piezoelectric actuator along with a laser encoder displacement sensor.

폐타이어 고무분말과 코드-고무 스크랩을 이용한 철재 롤코일 선박운송용 Dunnage 안정성 해석

곽이구(Lee-Ku Kwac),김홍건(Honggun Kim),김항우(Hang-Woo Kim),한재호(Jae-Ho Han),김재열(Jae-Yeol Kim) 한국생산제조학회 2006 한국생산제조학회지 Vol.15 No.6

When ironwork, especially steel roll coil, is transported to customers, land transportation and sea transportation are usually used. To transport steel roll coil fast and safe without damaging it, it is necessary that the steel roll coil has to be in stable condition. These days, apitong, which is all imported from overseas, is being used to support the steel roll coil, but because of apitong's rigidity, it damages the coil and when the coil is damaged, it is hard to fix. Due to the fact that recovering damage of the coil is almost impossible, we have to find the new type of dunnage that can substitute the apitong. In this paper, the arrays and the kinds of reinforcements, and rectangular type and trapezoid of dunnage will be talked about. The phenomenon of rolling and the impact when the carrier start moving and stop will be talked about as well. Therefore, we are going to develop a dunnage that does not damage ironwork and has better recovery and softness than existing apitong dunnage.