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Enhanced optical response of hybridized VO2/graphene films.
Kim, Hyeongkeun,Kim, Yena,Kim, TaeYoung,Jang, A-Rang,Jeong, Hu Young,Han, Seung Ho,Yoon, Dae Ho,Shin, Hyeon Suk,Bae, Dong Jae,Kim, Keun Soo,Yang, Woo Seok RSC Pub 2013 Nanoscale Vol.5 No.7
<P>Application of graphene as transparent electrodes is an active research area due to its excellent electrical and optical properties. Vanadium dioxide (VO2) is an attractive material since it is a thermochromic material that undergoes a structural phase transition when heat is applied. The phase transition results in the change of electrical and optical characteristics. We report optical characteristics of hybrid materials of graphene and VO2. We observed a 12% improvement in infrared transmittance with VO2 films deposited on graphene sapphire substrates compared to that of bare sapphire substrates. We also found that the phase transition temperature decreases as the number of graphene layers on the substrates increases. In the case of VO2 films on the substrate that was coated with four layers of graphene, the mean phase transition temperature was lowered to 56 C.</P>
Flow-dependent directional growth of carbon nanotube forests by chemical vapor deposition
Kim, Hyeongkeun,Kim, Keun Soo,Kang, Junmo,Park, Young Chul,Chun, Kyoung-Yong,Boo, Jin-Hyo,Kim, Young-Jin,Hong, Byung Hee,Choi, Jae-Boong IOP Pub 2011 Nanotechnology Vol.22 No.9
<P>We demonstrated that the structural formation of vertically aligned carbon nanotube (CNT) forests is primarily affected by the geometry-related gas flow, leading to the change of growth directions during the chemical vapor deposition (CVD) process. By varying the growing time, flow rate, and direction of the carrier gas, the structures and the formation mechanisms of the vertically aligned CNT forests were carefully investigated. The growth directions of CNTs are found to be highly dependent on the nonlinear local gas flows induced by microchannels. The angle of growth significantly changes with increasing gas flows perpendicular to the microchannel, while the parallel gas flow shows almost no effect. A computational fluid dynamics (CFD) model was employed to explain the flow-dependent growth of CNT forests, revealing that the variation of the local pressure induced by microchannels is an important parameter determining the directionality of the CNT growth. We expect that the present method and analyses would provide useful information to control the micro- and macrostructures of vertically aligned CNTs for various structural/electrical applications. </P>
김형근(Hyeongkeun Kim),박영철(Young-Chul Park),강준모(Junmo Kang),최성(Sung Choi),김근수(Keun Soo Kim),김영진(Young-Jin Kim),홍병희(Byung Hee Hong),최재붕(Jae-Boong Choi) 대한기계학회 2010 대한기계학회 춘추학술대회 Vol.2010 No.11
Graphene, a sheet consisting of sp²-bonded carbon atoms, is an attracting material for both scientists and industrial researchers because of its outstanding electrical and mechanical properties. Recently, the large-scale synthesis methods based on chemical vapor deposition (CVD) have been discovered, enabling the future application of graphene films as transparent electrodes. In this study, we measured the optical properties of graphene films through a various surface treatments such as multiple stacking, UV exposure, chemical doping and organic/inorganic hybrid. The optical properties of graphene samples were measured by imaging spectroscopic ellipsometry.
Kim, Yena,Kim, Hyeongkeun,Kim, Tae Young,Rhyu, Se Hyun,Choi, Dong Soo,Park, Won Kyu,Yang, Cheol-Min,Yoon, Dae Ho,Yang, Woo Seok Elsevier 2015 Carbon Vol.81 No.-
<P><B>Abstract</B></P> <P>It has become critically important to develop reliable method to transfer chemical vapor deposited (CVD) graphene from its growth substrate to the target substrate without leaving undesired polymer residues on the graphene surface. Here, we have found that for the two different transfer method – <I>wet transfer</I> using poly(methyl methacrylate) (PMMA) and <I>dry transfer</I> using polydimethylsiloxane (PDMS) as support layer, the amount of polymer residues and other impurities left on the graphene surface varies depending on the solvent used to remove those polymers. The exposure of the graphenes to different organic solvents such as acetone and chloroform resulted in different amount of polymer residues and impurities present on the graphene surface, which impact the electronic structure of the transferred graphene. It was found that the graphene obtained using the dry transfer method and acetone as solvent with a 2D to G (<I>I</I> <SUB>2D</SUB>/<I>I</I> <SUB>G</SUB>) intensity ratio of 4.58 and a 2D peak full width-half maximum (FWHM) of 24.66, which was higher than that using the wet transfer method and chloroform as solvent. These results showed that graphene was less affected by the polymer residues and impurities or the dry transfer method rather than the wet transfer method. In addition, using acetone rather than chloroform as solvent in the dry transfer method led to less contaminated graphene.</P>
Bio-inspired robot swarm control algorithm for dynamic environment monitoring
Kim, Kyukwang,Kim, Hyeongkeun,Myung, Hyun Techno-Press 2018 Advances in robotics research Vol.2 No.1
To monitor the environment and determine the source of a pollutant gradient using a multiple robot swarm, we propose a hybrid algorithm that combines two bio-inspired algorithms mimicking chemotaxis and pheromones of bacteria. The algorithm is implemented in virtual robot agents in a simulator to evaluate their feasibility and efficiency in gradient maps with different sizes. Simulation results show that the chemotaxis controller guided robot agents to the locations with higher pollutant concentrations, while the pheromone marked in a virtual field increased the efficiency of the search by reducing the visiting redundancy. The number of steps required to reach the target point did not increase proportionally as the map size increased, but were less than those in the linear whole-map search method. Furthermore, the robot agents could function with simple sensor composition, minimum information about the map, and low calculation capacity.
Synthesis of ultra-long super-aligned double-walled carbon nanotube forests.
Kim, Hyeongkeun,Kang, Junmo,Kim, Youngjin,Hong, Byung Hee,Choi, Jaeboong,Iijima, Sumio American Scientific Publishers 2011 Journal of Nanoscience and Nanotechnology Vol.11 No.1
<P>The pre-treatment (catalyst reduction with H2) time effect on the carbon nanotube (CNT) growth is reported. The total CNT height, the initial growth rate, the diameter, the number of walls, and the alignment in the CNT forests change with the catalyst reduction time. Densely packed, vertically super-aligned, double-walled CNT (DWCNT) forests with 9 mm height were synthesized in 10 hrs. We find that the density and the size of catalysts plays an important role in the alignment of the DWCNT forests, which is evidenced by atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy.</P>
Effects of catalyst on the super-growth of multi-walled carbon nanotubes.
Kim, Hyeongkeun,Chun, Kyoung-Yong,Choi, Jaeboong,Kim, Youngjin,Baik, Seunghyun American Scientific Publishers 2010 Journal of Nanoscience and Nanotechnology Vol.10 No.5
<P>We investigated the effects of catalyst (Fe)/buffer layer (Al2O3) on the vertical growth multiwalled carbon nanotubes (CNTs) synthesized by thermal chemical vapor deposition. The Fe was deposited onto Al2O3/SiO2/Si wafer by using e-beam evaporator. Ethylene was used as a feeding gas while argon was used as a carrier gas. Hydrogen was also introduced. The catalyst layers were optimized to control the length, diameter, purity, and alignment of CNTs. For the optimized conditions we produced vertically aligned CNTs of about 1 cm in length during 10 h. The effects of catalysts on nanotube characteristics were systematically analyzed using SEM, TEM, and Raman measurement.</P>
Hyeongkeun Kim,Cheesung Lee,최재붕,전경용,김영진,백승현 한국물리학회 2009 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.54 No.3
The effects of sandwich-like catalysts on the vertical growth of multiwalled carbon nanotubes (MWCNTs) are investigated. We achieved the synthesis of 2.2-mm-long MWCNT forests with carefully designed catalyst films by thermal chemical vapor deposition. Two types of sandwich-like catalysts were prepared by electron beam evaporation. Fe and Al2O3 layers were deposited on a SiO2 surface of 300 nm in thickness. Here, Fe and Al2O3 were used for the catalyst and the supporting buffer layer, respectively. C2H4 was used as a carbon source while Ar and H2 were used as carrier gases. The sandwich-like catalyst supported by a buffer layer of Al2O3 significantly enhanced the quality of the CNTs. The termination of MWCNT growth was found to depend on the position of the Fe layer. Also, the type of sandwich-like catalyst affects the crystalline perfection of the MWCNTs.
열화학기상증착법에 의해 성장된 탄소나노튜브의 기하학적 구조에 있어 촉매 두께가 미치는 영향
김형근(Hyeongkeun Kim),전경용(Kyoung-Yong Chun),최재붕(Jaeboong Choi),김영진(Youngjin Kim),백승현(Seunghyun Baik) 대한기계학회 2009 대한기계학회 춘추학술대회 Vol.2009 No.5
Vertically aligned carbon nanotube (CNT) arrays have been a hot topic in material research since ljima confirmed their structure and morphology in 1991. Here, we report the growth of carbon nanotubes (CNTs) by thermal chemical vapor deposition (CVD). We investigate the effect of catalyst thickness on the morphology of CNTs according to catalyst deposition methods. The diameters, density, and growth rate of the aligned CNTs were found to strongly depend on the catalyst thickness. The effect of different catalyst thickness on the characteristics of CNTs was systematically analyzed.
김형근(Hyeongkeun Kim),최재붕(Jaeboong Choi),홍병희(Byung Hee Hong),김영진(Youngjin Kim) 대한기계학회 2010 대한기계학회 춘추학술대회 Vol.2010 No.5
Recently, graphene has been extensively studied as a material for making future electronic device. In this work we report a chemical vapor deposition (CVD) synthesis of 30-inch scale monolayer graphene and roll-based layer-by-layer transfer onto polyethylene terephthalate (PET) substrates. Transmission electron microscopy (TEM), UV/Vis/IR spectroscopy, 4-probe electrical measurements and Raman spectroscopy studies indicate that these graphenes have highly-quality crystalline structures with the controllable number of layers.