Lee Sung-Hoon

Using density functional theory calculations, we investigate the origin of the insulating phase and metal-insulator transition (MIT) in octahedral tantalum disulfide (1T-TaS_{2}), a layered van der Waals material with a prominent two-dimensional (2D) charge density wave (CDW) order. We show that the MIT is driven not by the 2D order itself, but by the vertical ordering of the 2D CDWs or the 3D CDW order. We identify two exceptionally stable 3D CDW configurations; one is insulating and the other

Using density-functional theory we identify a new low-energy structure for GaAs(001) in an As-poor environment. The discovered geometry is qualitatively different from the usual surface-dimer based reconstructions of III-V semiconductor (001) surfaces. The stability of the new structure, which has a c(8x2) periodicity, is explained in terms of bond saturation and favorable electrostatic interactions between surface atoms. Simulated scanning tunneling microscopy images are in good agreement with

Chiral edge states are the hallmark of two- and three-dimensional topological materials, but their one-dimensional (1D) analog has not yet been found. We report that the 1D topological edge states, solitons, of the charge density wave system of indium atomic wires self-assembled on a silicon surface have chirality. The system is described by a coupled double Peierls-dimerized atomic chain, where the interchain coupling induces dynamical sublattice symmetry breaking. This changes its topological

We investigate using first-principles calculations the atomic structure of the orthorhombic phase of Ta(2)O(5). Although this structure has been studied for decades, the correct structural model is controversial owing to the complication of structural disorder. We identify a new low-energy high-symmetry structural model, where all Ta and O atoms have the correct formal oxidation states of +5 and -2, respectively, and the experimentally reported triangular lattice symmetry of the Ta sublattice ap

Organic–inorganic perovskites are promising materials for improving the efficiency of solar cells, but there are still uncovered issues on the understanding of their electronic band structures. Using first-principles calculations, we investigate the electronic band features of organo-lead iodide perovskites and present the efficient model to predict the band gap variation based on the orbital interaction scheme. The orbital interaction between Pb and I atoms can be controlled through the structu

Using first-principles calculations of graphene having high-symmetry distortion or defects, we investigate band gap opening by chiral symmetry breaking, or intervalley mixing, in graphene and show an intuitive picture of understanding the gap opening in terms of local bonding and antibonding hybridizations. We identify that the gap opening by chiral symmetry breaking in honeycomb lattices is an ideal two-dimensional (2D) extension of the Peierls metal-insulator transition in 1D linear lattices.

Density functional theory calculations are used to study the initial-stage adsorption of ${\mathrm{O}}_{2}$ molecules on the $\mathrm{Si}(111)\ensuremath{-}(7\ifmmode\times\else\texttimes\fi{}7)$ surface. Contrary to experimental suggestions, we find no evidence of metastable ${\mathrm{O}}_{2}$ molecular states on this surface, i.e., ${\mathrm{O}}_{2}$ molecules dissociate spontaneously without any barrier. Our electronic and vibrational analysis reveals that the resulting atomic-oxygen products

We have studied the energetics and bonding geometry of the ammonia adsorbed Si(100) surface by performing density-functional total-energy calculations within generalized gradient approximation. Ammonia molecules are found to adsorb on the down atoms of buckled Si dimers with an adsorption energy of 1.2 eV, but there exists a low-energy (of about 0.6 eV) activation pathway to the more stable dissociative chemisorption where ${\mathrm{NH}}_{2}$ (H) bonds to the down (up) Si atom. This energy diagr

Density functional calculations are used to examine the chemical and structural origin of O $1s$ core-level shifts measured on the initial oxidation stage of $\mathrm{Si}(111)\ensuremath{-}(7\ifmmode\times\else\texttimes\fi{}7)$. Our analysis of metastable core-level peaks leads to a conclusive identification of the long-sought metastable oxidation species as a tetrahedral ${\mathrm{SiO}}_{4}$ unit, formed by two successive ${\mathrm{O}}_{2}$ adsorptions on a Si adatom. The origin of a higher-bi

We report first-principles calculations on the antiferromagnetic spin ordering in graphene under biaxial strain. Using hybrid functional calculations, we found that the semimetallic graphene sheets undergo a transition to antiferromagnetic insulators at a biaxial strain of 7.7$%$ and that the band gap rapidly increases after the onset of this transition before reaching 0.9 eV at a biaxial strain of 12$%$. We examined the competition of the antiferromagnetic spin ordering with two-dimensional Pei

We present a simple surface modification method for enhancing the frictional properties on soft, viscoelastic tissue of large intestine by integrating micropatterned structures with controlled shape and geometry. The micropatterned end-effecter (EE) was fabricated onto micromachined EE body (20 mm long, 2 mm diameter cylinders) in the forms of line, box, pyramid, and bottle shape by utilizing capillary molding technique with UV-curable poly(urethane acrylate) (PUA) polymer. To evaluate the frict

We have used the pseudopotential density-functional total-energy scheme to study the atomic and electronic structure of the Si(111)2\ifmmode\times\else\texttimes\fi{}1 surface: we examine and compare in detail both the standard Pandey's \ensuremath{\pi}-bonded chain (PC) model and the alternative three-bond scission (TBS) model including their variations. We find that while the PC model is acceptable both in energetics and in spectroscopic examination, the TBS model is energetically unfavored (0

Abstract Surface reconstruction plays a vital role in determining the surface electronic structure and chemistry of semiconductors and metal oxides. However, it has been commonly believed that surface reconstruction does not occur in van der Waals layered materials, as they do not undergo significant bond breaking during surface formation. In this study, we present evidence that charge density wave (CDW) order in these materials can, in fact, cause CDW surface reconstruction through interlayer c

Abstract Vertical charge order shapes the electronic properties in layered charge density wave (CDW) materials. Various stacking orders inevitably create nanoscale domains with distinct electronic structures inaccessible to bulk probes. Here, the stacking characteristics of bulk 1 T ‐TaS 2 are analyzed using scanning tunneling spectroscopy (STS) and density functional theory (DFT) calculations. It is observed that Mott‐insulating domains undergo a transition to band‐insulating domains restoring

연구배경: 노인들의 일상생활에 가장 큰 영향을 주는 것이 치매이며, 사회적 고립이나 고독감이 치매 발병과 연관이 있다는 연구들이 있다. 독거노인은 외로움의 고위험군이며 독거노인에 대한 외로움과 인지기능의 상관관계에 대한 국내 연구는 없었다. 방법: 서울시 성북구 노인 복지관에 등록되어 있는 노인가구 중 독거노인 95명을 대상으로 2006년 4월에 방문조사하고, 1년 후 탈락한 11명을 제외한 84명에 대해 다시 방문 조사를 하였다. 교육받은 3명의 연구원들이 연구에 맞게 고안된 설문지를 이용해 인구학적 특성, 외로움 척도검사, 사회적 지지도, 하세가와 치매척도, 도구적 일상생활활동평가, 우울증 선별검사를 시행하여 초기 조사시와 추적조사시의 치매 선별검사 점수의 변화와 다른 요인들 간의 연관성을 조사하였다. 결과: 조사 대상자들의 평균 연령은 78.3세의 고령으로 대부분이 여성노인(90.5%)이었고, 저학력(무학; 69%)의 특징을 나타냈다. 외로움 척도 점수는 초기조사와 추적조사에