Geometric and electronic structures of monolayer hexagonal boron nitride with multi-vacancy

Kim Do-Hyun,Kim Hag-Soo,Song Min Woo,Lee Seunghyun 나노기술연구협의회 2017 Nano Convergence Vol.4 No.13

Hexagonal boron nitride (h-BN) is an electrical insulator with a large band gap of 5 eV and a good thermal conductor of which melting point reaches about 3000 °C. Due to these properties, much attention was given to the thermal stability rather than the electrical properties of h-BN experimentally and theoretically. In this study, we report calculations that the electronic structure of monolayer h-BN can be influenced by the presence of a vacancy defect which leads to a geometric deformation in the hexagonal lattice structure. The vacancy was varied from mono- to tri-vacancy in a supercell, and different defective structures under the same vacancy density were considered in the case of an odd number of vacancies. Consequently, all cases of vacancy defects resulted in a geometric distortion in monolayer h-BN, and new energy states were created between valence and conduction band with the Fermi level shift. Notably, B atoms around vacancies attracted one another while repulsion happened between N atoms around vacancies, irrespective of vacancy density. The calculation of formation energy revealed that multi-vacancy including more B-vacancies has much lower formation energy than vacancies with more N-vacancies. This work suggests that multi-vacancy created in monolayer h-BN will have more B-vacancies and that the presence of multi-vacancy can make monolayer h-BN electrically conductive by the new energy states and the Fermi level shift.

서울 오피스 시장의 임대료조정메커니즘: 자연공실률과 실질임대료 관계를 중심으로

김경민,박정수 국토연구원 2009 국토연구 Vol.62 No.-

Natural vacancy rate, defined as a vacancy rate at equilibrium, has explanatory power in rent adjustment process. The estimation of natural vacancy rate for Seoul office market, however, is not figured out yet. In this venue, the study aims to 1) estimate natural vacancy rate of Seoul office market and 2) find rent adjustment process with excess vacancy. Using quarterly dataset since 1994, major findings are summarized as follows. First, there is an intertemporal variation in the natural vacancy rate of Seoul office market. If planned mega projects and/or office buildings in Seoul metropolitan area are delivered on time, the natural vacancy rate of Seoul office market after 2014 will be quite different comparing with the current one. Second, excess vacancy and real rent show negative correlation, confirming the existence of rent adjustment process. The last but not least, rental adjustment parameter falls into a range of 0.22 to 0.25. It implies that 1%point increase in excess vacancy will lead to 0.22~0.25% decline in real rent quarterly. 오피스 자산을 주 투자대상으로 하는 간접투자기구와 오피스 직접투자의 증가에 따라 서울 오피스 임대료의 변화와 조정메커니즘에 대한 관심이 고조되고 있다. 임대료조정메커니즘의 규명을 위해 외국의 연구들은 자연공실률을 이용하였다. 하지만 서울 오피스 시장의 경우, 임대료 조정 메커니즘을 설명하기 위한 자연공실률 추정치를 찾아보기 힘든 실정이다. 본 연구는 1994년 이후의 분기별 자료를 이용하여 1) 서울오피스시장 자연공실률과, 2) 초과공실을 이용한 서울 오피스 시장 임대료조정메커니즘을 규명하였다. 논문의 결론은 다음과 같다. 첫째, 서울 오피스 시장 자연공실률은 연도별 차이(intertemporal variation)가 존재한다. IMF 이전과 이후 자연공실률 간 차이에서 보듯이 초대형 오피스 건물들이 들어설 2014년 이후에는 서울 오피스 시장은 새로운 차원의 공실률이 나타날 것으로 여겨진다. 둘째, 초과공실률(시장공실률이 자연공실률보다 높은 경우)과 실질임대료 상승률과는 음의 관계다. 초과공실이 존재하면 실질임대료는 하락하며, 반대의 경우–초과공실이 존재하지 않으면 실질임대료는 상승한다. 셋째, 실질임대료 조정계수는 0.22~0.25 사이로 나타났으며, 이는 시장공실률이 자연공실률보다 1%포인트 높은 경우, 실질 임대료는 0.22~0.25% 정도 하락한다.

Magnetic Properties of Intrinsic Vacancies on the GaN(10¯10) Surface: a Density-functional-theory Study

Jung-Min Hyun,Hanchul Kim 한국물리학회 2016 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.68 No.3

We report density functional theory calculations on the vacancy-induced magnetic properties of the wurzite GaN(10¯10) surface. Among various intrinsic vacancies on the surface, we focus on the monatomic Ga and N vacancies . We have found two stable geometries of a surface Ga vacancy and one stable geometry of a surface N vacancy. Two stable surface Ga vacancy structures are shown to have magnetic moments of 3 μB, which is the same as for the bulk Ga vacancy. Differently from the bulk N vacancy carrying null magnetic moment, the monatomic surface N vacancy has a finite magnetic moment of 1 μB. These results support the experimental claim that the roomtemperature ferromagnetism observed in inorganic nanoparticles is due to surface vacancies and their complexes. We also present simulated scanning tunneling microscope images for comparison with those obtained in future experimental studies.

Investigation of the interstitial oxygen behaviors in vanadium alloy: A first-principles study

Xingming Zhang,Yifan Li,Qiaoling He,Ruilian Li,Lei Deng,Liang Wang,Xunlin Liu,Jianfeng Tang,Huiqiu Deng,Wang-Yu Hu 한국물리학회 2018 Current Applied Physics Vol.18 No.2

This study aims at characterizing the interstitial Oxygen (O) behaviors in the Vanadium (V) Alloy by means of the first-principles calculations. For this, the interations between vacancy (Vac) and O interstitil atom are studied in detail to obtain the binding energies and stable structures of the complexes. It can be seen that monovacancy binding with two O atoms occupied the opposing octahedral stie are particularly stable, and is liable to form VacO2 cluster in the V alloys. According to the mass action analysis, the predicted temperature dependence of the concentration for VacOn complexes are presented. Apart from monovacancy, we also consider the trapping behavior of vacancy cluster on the O atoms. The results also prove that one vacancy can trap two O atoms in the V alloys. Based the diffusion theory, we obtain the diffusion coefficients as a function of temperature with or without the vacancy effect in the V alloys. The predicted O diffusion coefficients in defect-free V alloys from our first-principles calculations are in excellent ageement with the experimental data, meanings that the vacancy-limited mechanism does not play the key role for O diffusion in V alloys. Regarding the interactions between vacancy, solutes and O atom, combining with the diffusion barriers of O affected by vacancy and solute, we infer the formation mechanism of the precipitates in the V alloys.

Geometric shapes and vacancy rates for the modulation of the electronic and transport properties in MoS2 nanoflakes

You Suejeong,Kim Heesang,Kim Nammee 한국물리학회 2023 Current Applied Physics Vol.46 No.-

The electronic and transport properties of MoS2 nanoflakes are investigated using a six-band tight-binding model. The energy band structures are modified by changing the shape, size, vacancy rate, and vacancy distribution of the flakes, resulting in additional energy states in the energy bandgap region. For large flakes, the effects of their geometric shape are found to be negligible, because shape effects on the density of states (DOS) decrease with increasing flake size. The positions and DOS of these additional in-gap states are strongly related to the vacancy rate as well as to the distribution of vacancies. The number of in-gap states and the magnitude of the density of these states are both proportional to the vacancy rate. However, if the rate is high enough for vacancy clusters to form, the transmission is somewhat degraded at certain incident energies, owing to backscattering and capture of carriers by the clusters. Since the current transmission is determined primarily by the detailed energy band structure, the in-gap states due to vacancies enable the realization of low-energy currents in devices based on low-dimensional materials.

Kinetic lattice Monte Carlo simulation study on vacancy diffusion in germanium

강정원,권오근,Young Gyu Choi 한국물리학회 2009 Current Applied Physics Vol.9 No.2

We report kinetic lattice Monte Carlo simulations of vacancy-assisted self-diffusion in germanium below the melting point. We show that effective migration energies differ greatly from the migration energy of a free vacancy, which was calculated for a system with a low vacancy concentration. Vacancies are agglomerated in clusters and free vacancies are dissociated from clusters at temperatures above 657 ℃, and the nucleation temperatures of vacancy clustering in germanium are estimated to be 617–657 ℃. At temperatures below 617 ℃, there are a few remaining free vacancies for low concentrations, and in that case the effective migration energy of 0.17 eV closely coincides with the migration energy derived from previous theoretical calculations for low vacancy concentrations. The effective migration energy of 1.17 ± 0.1 eV above 657 ℃ also closely coincides with that measured in previous experiments.

Kinetic lattice Monte Carlo simulation study on vacancy diffusion in germanium

Kang, J.W.,Kwon, O.K.,Choi, Y.G. Elsevier 2009 Current Applied Physics Vol.9 No.2

We report kinetic lattice Monte Carlo simulations of vacancy-assisted self-diffusion in germanium below the melting point. We show that effective migration energies differ greatly from the migration energy of a free vacancy, which was calculated for a system with a low vacancy concentration. Vacancies are agglomerated in clusters and free vacancies are dissociated from clusters at temperatures above 657<SUP>o</SUP>C, and the nucleation temperatures of vacancy clustering in germanium are estimated to be 617-657<SUP>o</SUP>C. At temperatures below 617<SUP>o</SUP>C, there are a few remaining free vacancies for low concentrations, and in that case the effective migration energy of 0.17eV closely coincides with the migration energy derived from previous theoretical calculations for low vacancy concentrations. The effective migration energy of 1.17+/-0.1eV above 657<SUP>o</SUP>C also closely coincides with that measured in previous experiments.

Trap effect of vacancy on hydrogen diffusivity in bcc-Fe

Zhu, D.,Oda, T. North Holland Pub. Co 2016 JOURNAL OF NUCLEAR MATERIALS Vol.469 No.-

Evaluation of tritium effective diffusivity in structural materials with defects is essential to appropriately estimate the accumulation and leakage of tritium in fusion reactors. In the present work, we focus on the trap effect of vacancy on the diffusivity of hydrogen in bcc-Fe. The hydrogen effective diffusivities in different bcc-Fe systems with varied hydrogen/vacancy ratios (H/V ratios) are evaluated by using molecular statics (MS) and molecular dynamics (MD) simulations, and are compared with that in the system without vacancy. The hydrogen effective diffusivity is obviously decreased due to the vacancy trap effect. The trap effect is more profound in small H/V-ratio systems due to the dependence of V-H binding energy on the number of trapped H atoms. The trapped H atoms hardly contribute to hydrogen diffusivity because of the difficulty in detrapping and small mobility of V-H complex. This fact results in a good correlation between the effective diffusivity of hydrogen and the fraction of H atoms solute in the bulk without being trapped by vacancy.

A Study on the Vacancy and the Vacancy-interstitial Complex Diffusivities in Steel using Dilatometery (초)

박노근,장성욱,김지홍,박기훈,이재승,권순주 대한금속재료학회 ( 구 대한금속학회 ) 2008 대한금속재료학회 학술대회 개요집 Vol.2008 No.1

In-situ coalesced vacancies on MoSe₂ mimicking noble metal: Unprecedented Tafel reaction in hydrogen evolution

Lee, Junghyun,Kim, Changmin,Choi, KeunSu,Seo, Jihyung,Choi, Yunseong,Choi, Wooseon,Kim, Young-Min,Jeong, Hu Young,Lee, Jun Hee,Kim, Guntae,Park, Hyesung Elsevier 2019 Nano energy Vol.63 No.-

<P><B>Abstract</B></P> <P>Transition metal dichalcogenides (TMDs) have shown promising potential as electrocatalyst materials for the hydrogen evolution reaction (HER). However, the low catalytic activity in the basal planes of TMDs results in only limited HER activity, and several strategies to overcome this bottleneck have been proposed, involving various post-synthesis treatments such as introducing chalcogen vacancies or applying mechanical strain. Herein, we demonstrate in-situ modulation of chalcogen vacancy sites during the chemical vapor deposition synthesis of molybdenum diselenides (MoSe<SUB>2</SUB>) for application in the HER. We demonstrate for the first time that the Tafel reaction can be activated via in-situ vacancy-engineered MoSe<SUB>2</SUB>, resulting in improved onset potential and an exceptionally low Tafel slope, which exhibits one of the lowest values reported for TMDs to date in our knowledge. Density functional theory calculations revealed that vacancy coalescence in the MoSe<SUB>2</SUB> lattice reduced the hydrogen adsorption free energy and diffusion barrier to activate the Tafel reaction. Our approach could contribute to the development of high-performance TMDs-based electrocatalysts with relatively simple processability to make hydrogen production more viable.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Vacancy MoSe<SUB>2</SUB> was synthesized by hydrogen reactivity control during CVD process without any post-treatment. </LI> <LI> The <I>in-situ</I> synthesized vacancy-MoSe<SUB>2</SUB> exhibited outstanding HER performance with exceptionally low Tafel slope. </LI> <LI> Our results present a new perspective for developing high performance TMDs-based electrocatalysts. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>