Density Functional Theory를 이용한 Si (001) 표면 위의 In 나노선 성장 연구

김대현,김대희,서화일,김영철,Kim, Dae-Hyun,Kim, Dae-Hee,Seo, Hwa-Il,Kim, Yeong-Cheol 한국재료학회 2009 한국재료학회지 Vol.19 No.3

Density functional theory was utilized to investigate the growth of an indium nanowire on a Si (001) buckled surface. A site between the edge of two Si dimers is most favorable when the first In atom is adsorbed on the surface at an adsorption energy level of 2.26 eV. The energy barriers for migration from other sites to the most favorable site are low. When the second In atom is adsorbed next to the first In atom to form an In dimer perpendicular to the Si dimer row, the adsorption energy is the highest among all adsorption sites. The third In atom prefers either of the sites next to the In dimer along the In dimer direction. The fourth In atom exhibited the same tendency showed by the second atom. The second and fourth In adsorption energy levels are higher than the first and third levels as the In atoms consume the third valence electron by forming In dimers. Therefore, the In nanowire grows perpendicular to the Si dimer row on the Si (001) surface, as it satisfies the bonding of the three valence electrons of the In atoms.

리튬 이온 전지용 리튬 코발트 산화물 양극에서의 삽입 전압과 리튬 이온 전도

김대현,김대희,서화일,김영철,Kim, Dae-Hyun,Kim, Dae-Hee,Seo, Hwa-Il,Kim, Yeong-Cheol 한국전기화학회 2010 한국전기화학회지 Vol.13 No.4

본 연구는 밀도 범함수 이론을 이용하여 Li이온전지에 사용되는 Li코발트 산화물에서의 Li이온 삽입 전압과 전도에 관한 것이다. Li이온은 Li코발트 산화물 원자구조의 각 층을 1개씩 채우거나 한 층을 다 채우고 다음 층을 채울 수 있다. 평균 삽입 전압은 3.48V로 동일하나, 전자가 후자보다 더 유리하였다. 격자상수 c는 Li농도가 0.25보다 작을 때는 증가하였으나, 0.25보다 클 때는 감소하였다. Li농도가 증가하면, Li코발트 산화물에서의 Li이온 전도를 위한 에너지 장벽은 증가하였다. Li이온전지가 방전 중 출력 전압이 낮아지는 현상은 Li농도 증가에 따른 삽입 전압의 감소와 전도 에너지 장벽의 증가로 설명할 수 있었다. We performed a density functional theory study to investigate the intercalation voltage and lithium ion conduction in lithium cobalt oxide for lithium ion battery as a function of the lithium concentration. There were two methods for the intercalation of lithium ions; the intercalation of a lithium ion at a time in the individual layer and the intercalation of lithium ions in all the sites of one layer after all the sites of another layer. The average intercalation voltage was the same value, 3.48 V. However, we found the former method was more favorable than the latter method. The lattice parameter c was increased as the increase of the lithium concentration in the range of x < 0.25 while it was decreased as increase of the lithium concentration in the range of x > 0.25. The energy barrier for the conduction of lithium ion in lithium cobalt oxide was increased as the lithium concentration was increased. We demonstrated that the decrease of the intercalation voltage and increase of the energy barrier as the increase of the lithium concentration caused lower output voltage during the discharge of the lithium ion battery.

$HfCl_4$와 Si (001) 표면에 결합된 두 개의 수산화기와의 상호작용: 제일원리 연구

김대현,김대희,서화일,김영철,Kim, Dae-Hyun,Kim, Dae-Hee,Seo, Hwa-Il,Kim, Yeong-Cheol 한국반도체디스플레이기술학회 2009 반도체디스플레이기술학회지 Vol.8 No.2

Density functional theory was used to investigate the adsorption and reaction of $HfCl_4$ with two hydroxyls on Si (001)-$2{\times}1$ surface in atomic layer deposition (ALD) process. We prepared a reasonable Si substrate which consisted of six inter-dimer dissociated $H_2O$ molecules and two intra-dimer dissociated $H_2O$ molecules. The $HfCl_4$must react with two hydroxyls to be a bulk-like structure. When $HfCl_4$ was adsorbed on a hydroxyl, there was energy benefit of -0.55 eV. Though there was energy loss for $HfCl_4$ to react with H of hydroxyl, thermal energy of ALD chamber would be enough to pass the energy barriers. There were five reaction pathways for $HfCl_4$ to react with two hydroxyls; inter-dimer, intra-dimer, cross-dimer, inter-row, and cross-row. Inter-row, inter-dimer and intra-dimer were relatively favorable among the five reaction pathways based on the energy difference. The electron densities between O and Hf in these three reactions were higher than the others and they had shorter Hf-O and O-O bond lengths than the other two reaction pathways.

비화학양론적 Na⁺β-alumina를 위한 Mg 원자의 치환: 제일원리 계산

김대현,김대희,정용찬,서화일,김영철,Kim, Dae-Hyun,Kim, Dae-Hee,Jeong, Yong-Chan,Seo, Hwa-Il,Kim, Yeong-Cheol 한국재료학회 2010 한국재료학회지 Vol.20 No.2

$Na^+$ ion conductivity can be improved by the substitution of an Mg atom for an Al atom to form a nonstoichiometric $Na^+$ $\beta$-alumina. We performed a first principles study to investigate the most stable substitution site of an Mg atom and the resulting structural change of the nonstoichiometric $Na^+$ $\beta$-alumina. Al atoms were classified as four different layers in the spinel block that are separated by conduction planes in the nonstoichiometric $Na^+$ $\beta$-alumina. The substitution of an Mg atom for an Al atom at a tetragonal site was more favorable than that at an octahedral site. The substitution in the spinel block was more favorable than that close to the conduction plane. This result was well explained by the volume changes of the polyhedrons, by the standard deviation of the Mg-O distance, and by the comparison with bulk MgO structure. Our result indicates that the most preferable site for the Mg atom was the tetrahedral site at the spinel block in the nonstoichiometric $Na^+$ $\beta$-alumina.

아파트 단지 옥외공간 변천 및 요인 연구

김대현,김대수,신지훈,김순분,Kim, Dae-Hyun,Kim, Dae-Soo,Sin, Ji-Hoon,Kim, Soon-Boon 한국조경학회 2005 韓國造景學會誌 Vol.32 No.6

The purposes of this study were to classify the changing periods of apartment outdoor space in Korea and to clearly understand the specific characteristics of each period. The study has been done through on-site observation of apartment complexes, and through the analysis of housing estate planning techniques, rules and regulations affecting outdoor spaces during the rapidly developing era. The results of this study shows that three major periods with distinct characteristics were found, which were as follows: 1) The first well defined period spans from 1960 to 1990. In this period, apartment construction was rapid, and as a result, quality of housing environments were sacrificed. As little attention was given to the landscapes in apartment complex, they were constructed very simple and identical in outdoor space of apartment complex. This period can be referred to as the 'commonly universal period in apartment outdoor space'. 2) The second period spans from 1991 to 1996, during which time a housing construction project for two million residences was completed in order to significantly stimulate the provision of houses to the general public. As a result, regulations were relaxed causing housing environments to deteriorate. Although apartments were constructed very close together, there was some diversification and experimentation in outdoor styles. This period can be referred to as the 'congestion period of apartment outdoor space'. 3) The third period spans from 1997 to the present. During this time the number of unsold apartments has grown considerably throughout the country. Many different apartment complex marketing strategies have been employed to reduced the glut of unsold apartments. This experimental trend has continued and been well-received since the IMF period. The distinguishing characteristics of this period are the introduction of a rich natural landscape, the increase in communications among residents, the human oriented design, and the refined and elegant design for the outdoor street furnitures. As a result, this period can be named the "differentiation period of apartment outdoor space".

DEMS와 H-terminated Si (001) 표면의 상호작용: 제일원리연구

김대현,김대희,박소연,서화일,이도형,김영철,Kim, Dae-Hyun,Kim, Dae-Hee,Park, So-Yeon,Seo, Hwa-Il,Lee, Do-Hyoung,Kim, Yeong-Cheol 한국세라믹학회 2009 한국세라믹학회지 Vol.46 No.4

We performed a density functional theory study to investigate the interaction of DEMS (diethoxymethylsilane) with the H-terminated Si (001) surface. The optimum structure of DEMS was first calculated by a first principles study. The dissociation probability of the O-C bond of DEMS was higher than the other seven bonds based on the bond energy calculation. When the fragmented DEMS groups reacted with the H-terminated Si (001) surface, it was the most favorable among the eight reactions to form a bond between the Si atom on the surface and the O atom of a fragmented DEMS group (($C_2H_5O$)Si($CH_3$)(H)-O-) by forming a $C_2H_6$ as by-product.

도시의 토지이용 형태별 경관특성과 유형 - 대전광역시를 사례로 -

김대현,김대수,주신하,오세래,Kim Dae-Hyun,Kim Dae-Soo,Joo Shin-Ha,Oh Se-Rae 한국조경학회 2005 韓國造景學會誌 Vol.33 No.4

Recently, as urban landscape is growing in importance, urban landscape planning is being actively performed. for this purpose, classification of the urban landscape is definitely required. Therefore, this research focuses on classifying urban landscape in Daejeon metropolis by dividing the urban land use pattern. This results are as follows. 1. Urban land use pattern is divided into 20 classes. The residential, commercial and industrial areas, the old market and the bus terminal are evaluated negatively, whereas the areas of school, water reservoir, neighborhood park and train station are appreciated as being positive in landscape characters. 2. As a result of a cluster analysis, urban landscape has five different landscape types. These are: landscapes of medium diversity lacking green area, landscapes of high diversity lacking green area, landscapes rich in green area and with medium diversity, landscapes rich in green area and with high diversity, and landscapes rich in green area and with low diversity. 3. In landscape characters of beauty and harmony, landscapes rich in green area and with medium diversity are more positively evaluated than those rich in green area and with low diversity. This point should be taken into account for planning the urban landscape.

TiO₂/ITO 나노구조체 광전극의 합성 및 염료감응 태양전지에의 적용

김대현,박경수,최영진,최헌진,박재관,Kim, Dae-Hyun,Park, Kyung-Soo,Choi, Young-Jin,Choi, Heon-Jin,Park, Jae-Gwan 한국세라믹학회 2011 한국세라믹학회지 Vol.48 No.1

A Sn-doped $In_2O_3$ (ITO) nanowire photoelectrode was produced using a simple metal evaporation method at low synthesis temperature (< $540^{\circ}C$). The nanowire electrodes have large surface area compared with that of flat ITO thin film, and show low electrical resistivity of $5.6{\times}10^{-3}{\Omega}cm$ at room temperature. In order to apply ITO nanowires to the photoelectrodes of dye-sensitized solar cell (DSSC), those surfaces were modified by $TiO_2$ nanoparticles using a chemical bath deposition (CBD) method. The conversion efficiency of the fabricated $TiO_2$/ITO nanostructure-based DSSC was obtained at 1.4%, which was increased value by a factor of 6 than one without ITO nanowires photoelectrode. This result is attributed to the large surface area and superior electrical property of the ITO nanowires photoelectrode, as well as the structural advantages, including short diffusion length of photo-induced electrons, of the fabricated $TiO_2$/ITO nanostructure-based DSSC.

IMF 기간 일간신문 분양광고에 나타난 아파트 단지 옥외공간의 차별화 내용 및 요소

김대현,Kim Dae-Hyun 한국조경학회 2004 韓國造景學會誌 Vol.32 No.4

During the IMF period the unsold of apartment housing was significantly increased due to financial problems. Apartment constructors had to severely struggle with fund problems and even some of them were bankrupted. Due to this fact, they had to find new marketing strategies to sell the apartment complexes even with low margins. One of the solutions was that housing companies could present a distinctive and outstanding outdoor space in apartment complexes. The purpose of this study was to investigate such differentiated marketing strategies by reviewing the apartment housing advertisements during the IMF period. The major findings through this study were as follows: 1. During the IMF period housing companies presented a distinctive and outstanding outdoor space in apartment complexes. In 1999, which was the peak of the IMF impact, there were frequent advertisements for apartment housing in daily newspapers. 2. Daily newspapers had been mostly used for advertising the apartments. A thorough review of them is very valuable in investigating differentiated marketing strategies of the outdoor space in apartment complexes. 3. The main theme of differentiation of outdoor space can be summarized as : Emphasis of the importance of healthy life, application of environment-friendly facilities, and use of traditional korean style elements in apartment complexes.

Cu-Fe계 합금의 강도 및 전기전도도에 미치는 Cr 원소첨가의 영향

김대현,이광학,Kim, Dae-Hyun,Lee, Kwang-Hak 한국재료학회 2010 한국재료학회지 Vol.20 No.2

This study looked at high performance copper-based alloys as LED lead frame materials with higher electrical-conductivity and the maintenance of superior tensile strength. This study investigated the effects on the tensile strength, electrical conductivity, thermal softening, size and distribution of the precipitation phases when Cr was added in Cu-Fe alloy in order to satisfy characteristics for LED Lead Frame material. Strips of the alloys were produced by casting and then properly treated to achieve a thickness of 0.25 mm by hot-rolling, scalping, and cold-rolling; mechanical properties such as tensile strength, hardness and electrical-conductivity were determined and compared. To determine precipitates in alloy that affect hardness and electrical-conductivity, electron microscope testing was also performed. Cr showed the effect of precipitation hardened with a $Cr_3Si$ precipitation phase. As a result of this experiment, appropriate aging temperature and time have been determined and we have developed a copper-based alloy with high tensile strength and electrical-conductivity. This alloy has the possibility for use as a substitution material for the LED Lead Frame of Cu alloy.