Exotic Geometrical and Electronic Properties in Hydrogenated Graphyne

Koo, Jahyun,Hwang, Ho Jun,Huang, Bing,Lee, Hunpyo,Lee, Hosik,Park, Minwoo,Kwon, Yongkyung,Wei, Su-Huai,Lee, Hoonkyung American Chemical Society 2013 JOURNAL OF PHYSICAL CHEMISTRY C - Vol.117 No.23

<P>On the basis of first-principles calculations, we present exotic geometrical and electronic properties in hydrogenated graphyne, a 2D material of sp–sp<SUP>2</SUP> hybrid carbon networks. Hydrogen atoms adsorbed onto sp-bonded carbon atoms can form both sp<SUP>2</SUP>- and sp<SUP>3</SUP>-hybridized bonds and can exist in three different geometries: in-plane, out-of-plane, and oblique-plane; this is in sharp contrast to hydrogenated graphene, which has only one hydrogenation geometry. The band gaps of hydrogenated graphyne can vary by ∼3 eV as the geometry changes. We also find that change in the hydrogen concentration allows a large band-gap tuning of ∼5 eV. Unlike hydrogenated graphene, in which H atoms show a tendency to cluster, H atoms tend to be dispersed in graphyne, making band-gap tuning feasible. These exotic properties in hydrogenated graphyne indicate that the band gap of hydrogenated graphyne can be tailored for new device applications. Furthermore, the composite of fully hydrogenated graphyne is C<SUB>1</SUB>H<SUB>1.75</SUB>, which has a hydrogen-to-carbon ratio greater than that of graphane (C<SUB>1</SUB>H<SUB>1</SUB>). This large hydrogen capacity (∼13 wt % H) suggests that graphyne also can be used as a high-capacity hydrogen storage material.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpccck/2013/jpccck.2013.117.issue-23/jp402286f/production/images/medium/jp-2013-02286f_0010.gif'></P>

R347C Polymorphisms in ADRA1A Genes and Mirtazapine Treatment Response in Koreans with Major Depression

Koo, Jahyun,Lee, Min-Soo,Ham, Byungju,Won, Eun-Soo The Korean Society of Biological Psychiatry 2015 생물정신의학 Vol.22 No.4

Objectives Adrenergic alpha 1 and 2 receptors work as pathways to control the serotonergic neuron moderation and mirtazapine acts as antagonist of these receptors. The adrenoreceptor alpha 1a (ADRA1A) gene, which encodes adrenergic alpha 1 receptor, has Arg-347Cys genetic polymorphism and the polymorphism has strong relationship with many neuro-psychiatric diseases. In this study, we explored the relationship between ADRA1A R347C polymorphism and mirtazapine treatment response in Koreans with major depression. Methods 352 patients enrolled in this study, and the symptoms were evaluated by 17-item Hamilton Depression Rating (HAMD-17) scale. After 1, 2, 4, 8, and 12 weeks of mirtazapine treatment, the association between ADRA1A R347C polymorphism and remission/response outcomes was evaluated. Results Treatment response to mirtazapine was significantly better in T allele carriers than C allele homozygotes after 12 weeks of mirtazapine monotherapy. The percentile decline of HAMD-17 score in T allele carriers was larger than that of C allele homozygotes. ADRA1A R347C genotypes were not significantly associated with remission. Conclusions The result showed that treatment response to mirtazapine was significantly associated with ADRA1A R347C genetic polymorphism. T allele carriers showed better treatment response than C allele homozygotes. It can be supposed that T allele carriers have a trend of better treatment response to mirtazapine monotherapy.

Vertical dielectric screening of few-layer van der Waals semiconductors

Koo, Jahyun,Gao, Shiyuan,Lee, Hoonkyung,Yang, Li The Royal Society of Chemistry 2017 Nanoscale Vol.9 No.38

<P>Vertical dielectric screening is a fundamental parameter of few-layer van der Waals two-dimensional (2D) semiconductors. However, unlike the widely-accepted wisdom claiming that the vertical dielectric screening is sensitive to the thickness, our first-principles calculation based on the linear response theory (within the weak field limit) reveals that this screening is independent of the thickness and, in fact, it is the same as the corresponding bulk value. This conclusion is verified in a wide range of 2D paraelectric semiconductors, covering narrow-gap ones and wide-gap ones with different crystal symmetries, providing an efficient and reliable way to calculate and predict static dielectric screening of reduced-dimensional materials. Employing this conclusion, we satisfactorily explain the tunable band gap in gated 2D semiconductors. We further propose to engineer the vertical dielectric screening by changing the interlayer distance<I>via</I>vertical pressure or hybrid structures. Our predicted vertical dielectric screening can substantially simplify the understanding of a wide range of measurements and it is crucial for designing 2D functional devices.</P>

The Effects of the SNS Activities of Politicians on Political Efficacy and the Intention to Participate in Voting

Jahyun Koo,Joonho Kim 한국경영정보학회 2016 Asia Pacific Journal of Information Systems Vol.26 No.1

Microstructural discovery of Al addition on Sn–0.5Cu-based Pb-free solder design

Koo, Jahyun,Lee, Changsoo,Hong, Sung Jea,Kim, Keun-Soo,Lee, Hyuck Mo Elsevier 2015 JOURNAL OF ALLOYS AND COMPOUNDS Vol.650 No.-

<P><B>Abstract</B></P> <P>It is important to develop Pb-free solder alloys suitable for automotive use instead of traditional Sn–Pb solder due to environmental regulations (e.g., Restriction of Hazardous Substances (RoHS)). Al addition has been spotlighted to enhance solder properties. In this study, we investigated the microstructural change of Sn–0.5Cu wt.% based Pb-free solder alloys with Al addition (0.01–0.05 wt.%). The small amount of Al addition caused a remarkable microstructural change. The Al was favored to form Cu–Al intermetallic compounds inside the solder matrix. We identified the Cu–Al intermetallic compound as Cu<SUB>33</SUB>Al<SUB>17</SUB>, which has a rhombohedral structure, using EPMA and TEM analyses. This resulted in refined Cu<SUB>6</SUB>Sn<SUB>5</SUB> networks in the Sn–0.5Cu based solder alloy. In addition, we conducted thermal analysis to confirm its stability at a high temperature of approximately 230 °C, which is the necessary temperature range for automotive applications. The solidification results were substantiated thermodynamically using the Scheil solidification model. We can provide criteria for the minimum aluminum content to modify the microstructure of Pb-free solder alloys.</P> <P><B>Highlights</B></P> <P> <UL> <LI> We observed dramatic microstructure-change with Al additions. </LI> <LI> We defined Cu<SUB>33</SUB>Al<SUB>17</SUB> IMC with Al additions using TEM analysis. </LI> <LI> We investigated grain refinement with Al additions using EBSD. </LI> <LI> We discussed the refinement based on Scheil solidification model. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>The minor Al additions refined eutectic Cu<SUB>6</SUB>Sn<SUB>5</SUB> IMC networks on the Sn–0.5Cu based solder alloys. The microstructure was dramatically changed with the minor Al addition.</P> <P>[DISPLAY OMISSION]</P>

Private Equity as an Alternative Corporate Restructuring Scheme: Does Private Equity Increase the Operating Performance of PE-Backed Firms?

KOO, JAHYUN Korea Development Institute 2016 KDI Journal of Economic Policy (KDI JEP) Vol.35 No.3

There has been a surge of interest in private equity as an alternative corporate restructuring scheme to complement the current institutional forms such as workouts and court receivership. By empirically examining whether private equity in Korea can improve investee companies, we find that while private equity in Korea did not sacrifice the long-term growth potential of investee firms, it did not improve their profitability (e.g. ROA, ROE, and ROS) or growth (e.g. sales growth) either. Both the negative correlation between business performance and firm age and our empirical results showing that young firms were favored by private equity for investment imply that Korean private equity may perform as growth capital, similar to venture capital rather than as buyouts for corporate restructuring.

Does Early Incubating Improve the Performance of Start-ups?: Evidence from TIPS in Korea

KOO, JAHYUN Korea Development Institute 2018 KDI Journal of Economic Policy (KDI JEP) Vol.40 No.2

Recently, the government has been pursuing various policies to create new industries and jobs through the invigoration of start-ups. For the sustained growth of start-ups, it is necessary to look not only at the supply of risk capital but also at policies for venture capital firms that nurture and foster start-ups. The purpose of this study is to estimate the effects of the nurturing and fostering role of risk capital, such as mentorship on the performance of start-ups, and to do this we analyzed the effects, as a newly introduced form of venture capital, of mentoring by an accelerator and investor ties on the performance outcomes of start-ups. We find that mentoring and investor ties for start-up enterprises positively influence follow-up investment in start-ups. In addition, this study finds that with a younger CEO of a start-up, it is more likely that the performance of the start-up will improve. Meanwhile, when examining increases in employment as a measure of the business performance of start-ups, mentoring and investor ties are found to have a positive effect on the increase of employment at start-ups. These results suggest that there is a need to promote policies that strengthen the mentoring role of venture capital in Korea's equity finance policies and in the government's SME support policies.

Wireless bioresorbable electronic system enables sustained nonpharmacological neuroregenerative therapy

Koo, Jahyun,MacEwan, Matthew R.,Kang, Seung-Kyun,Won, Sang Min,Stephen, Manu,Gamble, Paul,Xie, Zhaoqian,Yan, Ying,Chen, Yu-Yu,Shin, Jiho,Birenbaum, Nathan,Chung, Sangjin,Kim, Sung Bong,Khalifeh, Jawad Nature Publishing Group 2018 Nature medicine Vol. No.

Off-Plane Dielectric Screening of Few-Layer Graphdiyne and Its Family

Koo, Jahyun,Yang, Li,Lee, Hoonkyung American Chemical Society 2019 ACS APPLIED MATERIALS & INTERFACES Vol.11 No.3

<P>We performed first-principles calculations on few-layer graphdiyne (GDY) and its family, sp-sp<SUP>2</SUP> hybrid carbon atomic layers, for an off-plane, static dielectric screening. The vertical dielectric constants of semiconducting GDY structures are finite and independent of the thickness. However, unlike the widely accepted wisdom that the static metallic screening is infinite, those of metallic GDY structures are finite and dependent on their thickness. Furthermore, the vertical dielectric screening can be tuned by varying the interlayer distance. We also studied the dielectric properties of heterostructures of GDY/its family; the vertical dielectric constant has an equivalent value from the two distinct values of the two distinct monostructures. The dielectric screening behaviors are well described by the uniform dielectric slab model. In addition, the band gaps can be widely tuned from 0 to 0.8 eV, by varying the thickness and electric field. Our results provide a method for engineering the dielectric constant and band gap of GDY and its family for applications of supercapacitors and nanodevices.</P> [FIG OMISSION]</BR>

The Effects of the SNS Activities of Politicians on Political Efficacy and the Intention to Participate in Voting

Koo, Jahyun,Kim, Joonho The Korea Society of Management Information System 2016 Asia Pacific Journal of Information Systems Vol.26 No.1

Political activities through social networking service (SNS) are common in the political communication environment. The main aim of this study is to present logical and empirical evidence on the effects of politicians' SNS activities. To achieve this goal, this research analyzed the relationship between the SNS activities of politicians and the political efficacy and political participation of citizens through a survey of 644 people. The results of the survey analysis showed that politicians' SNS activities have a positive effect on both the internal and external efficacies of citizens and that these efficacies positively influence political participation. This finding means that the SNS activities of politicians promote political participation by increasing the political efficacy of citizens and states. The results also suggest that SNS, with its characteristics of communication and interaction, plays a crucial role as a political communication medium. Therefore, with SNS, a variety of information sources on politics can be offered, suggestions from citizens can be embraced, and such suggestions can be revealed to politicians as a means of promoting political efficacy and participation.