분자간 거리가 PEDOT의 광전자 특성에 미치는 영향

이승현,김경호,강현구,박지상 한국물리학회 2021 새물리 Vol.71 No.12

We investigated both the atomic and the electronic structures of poly(3,4-ethylene dioxythiophene) (PEDOT) by performing first-principles calculations and the optical properties by performing experiments. In our first-principles calculation, the use of different exchange-correlation potentials and van der Waals corrections optimize the distance between molecules differently, directly influencing the calculated band gap of PEDOT. In order to experimentally obtain the optical band gap, high-quality semiconducting PEDOT thin films were prepared using vapor-phase polymerization and chemical reduction process. The bandgap of PEDOT was then extracted from the absorption edge in the measured extinction spectrum. Direct comparison between the measured and calculated bandgap shows that the hybrid density functional theory improves the agreement with the experiments. 본 연구에서는 poly(3,4-ethylenedioxythiophene) (PEDOT) 의 원자 구조와 전자 구조를 여러제일원리 계산 방법론을 이용하여 계산하고, 광학적 특성을 실험을 통해 확인하였다. 제일 원리 계산결과 교환-상관 퍼텐셜과 판데르발스 상호작용 기술 방식에 따라 분자 사이의 거리가 다르게 기술되며, 결과적으로 계산된 PEDOT의 밴드갭에 직접적인 영향을 미친다는 것을 발견했다. 광학적 밴드갭을실험적으로 얻기 위해, 고품질 반도체 PEDOT 박막을 기상 중합방법과 화학적 환원 공정을 통해 제작했다. PEDOT의 밴드갭은 측정된 흡광 스펙트럼의 흡수 끝 파장으로부터 추출되었다. 측정된 밴드갭과 계산된 밴드갭의 직접 비교를 통해 하이브리드 밀도범함수 이론 계산이 실험값과 더 일치하는 밴드갭을 기술하는것을 확인할 수 있었다.

X-doped (X=Ru, P, Si) γ-Al2O3 상의 Cu 흡착 제일원리 계산 연구

이은혜,지현진,최은영,이정훈,조장현 한국수소및신에너지학회 2022 한국수소 및 신에너지학회논문집 Vol.33 No.1

Copper (Cu)-based catalysts have been widely used in a methanol steam reforming (MSR) reaction for hydrogen production for air-independent propulsion (AIP) applications and their good catalytic activities have attracted much attention. However, the agglomeration of the catalytic active site Cu causes deteriorating the catalytic performance and suppression of Cu agglomeration is a crucial issue in the AIP applications that the MSR system is typically operated at 250-300℃ for a long time. R. Sakai et al. recently showed a computational study on the anchoring effect that reduces an agglomeration of active sites by doping in a supporter. In order to present the anchoring effect on γ-Al2O3 supported Cu-based catalysts, in this study, the adsorption energies of Cu on X-doped (X=ruthenium, phosphorus, silicon) γ-Al2O3 were calculated and Cu adsorption energy decreased due to a change of the electronic structure originated from doping, thereby proving the anchoring effect.

ASnO$_{3}$/A$'$SnO$_{3}$ (A, A$'$ = Ba, Sr, Ca) 초격자 구조에서의 강유전상에 관한 제일원리 계산

송나영,김주영,김복기 한국물리학회 2016 새물리 Vol.66 No.9

We have studied the physical properties of ASnO$_{3}$/A$'$ SnO$_{3}$ cation-ordered superlattices by using first-principles calculations. Artificially-stacked superlattices with a perovskite unit, such as ASnO$_{3}$/A$'$SnO$_{3}$, may be hybrid improper ferroelectrics. The tilting of the octahedron is the primary order parameter in hybrid improper ferroelectrics whereas the polarization is a secondary order parameter. We use first-principles calculations to optimize the structure of superlattices of ASnO$_{3}$/A$'$SnO$_{3}$ with A or A$'$ = Ba, Sr, and Ca and with various cation-ordering patterns such as the [001], [110], and [111] directions. We observe a spontaneous phase transition from a tetragonal nonpolar phase (high-symmetry phase) to an orthorhombic polar phase (low-symmetry phase) and perform a group theoretical mode analysis to elucidate the phase transition. We also present a detailed spontaneous polarization analysis for the [001] and the [111] directions in the low-symmetry structure. 우리는 본 논문에서 ASnO$_3$/A$'$SnO$_3$ 초격자 구조에 대해 제일원리 전자구조 계산 결과를 논의하고자 한다. ASnO$_3$/A$'$SnO$_3$ 구조와 같이 두 층을 원자 레벨로 인위적으로 쌓아 올린 페로브스카이트 구조는 복합 부적절 강유전성 (hybrid improper ferroelectrics, HIF) 특징이 나타날 수 있다. 본 물질은 산소 팔면체의 회전을 일차 질서 변수로 가지며 분극이 이차 질서 변수인 전형적인 복합 부적절 강유전성의 특징을 가진다. 박막의 시료제작에서 방향성을 고려했을 때 이 초격자 구조는 [001] 이외에도 [110], [111]의 가능성이 있으며, 세 가지 방향성을 고려하고 ASnO$_3$/A$'$SnO$_3$ 구조의 A-와 A$'$-site에 Ba, Sr, 그리고 Ca 원자를 치환하여 구조를 얻었다. 본 물질은 무극성 상을 나타내는 공간군(high-symmetry)에서 극성 상을 나타내는 공간군 (low-symmetry)로 상 전이가 발생하며 우리는 이때 작용하는 모드를 그룹 이론을 통해서 분석해 보았다. 또한 [001]과 [111] 방향의 구조에 대해 자발 분극의 크기에 관한 연구도 수행하였다.

제일원리 계산을 통한 리튬금속전지용 Co-N- 복합 도핑 탄소체에서의 리튬 금속 증착 매커니즘 분석

장홍준(Hongjun Chang),남윤신(Younshin Nam),문장혁(Janghyuk Moon) 대한기계학회 2021 대한기계학회 춘추학술대회 Vol.2021 No.11

Lithium metal offers high energy density for lithium ion batteries owing to large gravimetric capacity (3860 mAh g<SUP>-1</SUP>) and low oxidation reduction potential (-3.04 V). Although Li has low redox potential and high specific energy density, Li dendrite growth results in capacity degradation generating dead Li during repeated cycles. To solve unwanted Li growth, 3D host framework such as carbon fiber and porous carbon framework were considered as alternative anode electrode. Among them, ZIF derived porous carbon structure shows higher Li storing and stable cycle performance. It contains a lot of N and Co consisting graphitic carbon structure. To understand mechanisms of Li storing and roles of doping atoms in porous carbon framework, we performed atomic modeling using first principle calculations. Li formation energies in graphite structure were calculated with various Li cluster configurations and these were compared with the values in N-doped or Co-N- doped graphite structures. We elucidated that electron charge transfer from Li to Co plays important role of Li uniform growth. Our atomic model can give a good insight of design guidance for Li metal host frameworks.

흡착원자의 덮임율에 따른 Ag/Si(111)√3X√3의 구조 변화

정호진,정석민,Jeong, Ho-Jin,Jeong, Suk-Min 한국진공학회 2008 Applied Science and Convergence Technology Vol.17 No.5

제일원리계산 방법을 이용하여 Ag/Si(111)$\sqrt{3}{\times}\sqrt{3}$(이후로 $\sqrt{3}-Ag$로 표시) 표면에 은 원자가 추가로 흡착된 표면의 원자구조와 에너지를 조사하였다. 은의 덮임율을 0.02 ML에서 0.14 ML로 변화시켜가며 구조변화를 살펴보았다. 흡착된 은 원자들은 대부분 $\sqrt{3}-Ag$ 표면의 은 원자층의 작은 삼각형(ST)의 중간에 자리 잡았다. 특이한 것은 은 원자들은 은 원자층 보다 아래로 내려간다는 것이다. 은 흡착원자(adatom)의 덮임율이 증가함에 따라 adatom들은 클러스터를 만들려는 경향을 보였다. 은 흡착원자들이 모인 클러스터의 에너지를 계산해 보면 흡착원자가 세 개일 때 가장 안정됨을 알 수 있었다. 이 삼원자 클러스터를 구성 단위로 하여 $\sqrt{21}{\times}\sqrt{21}$ 구조의 원자구조를 결정할 수 있었다. 각 덮임율에서 가장 에너지가 낮은 구조들에 대한 STM 영상을 시뮬레이션 해 보면 은 원자는 찬 상태에서 어둡게 보였다. 이는 은 원자가 기판으로 전하를 제공해 줌을 의미한다. 그리고 원자층의 구조변화가 STM 실험에서 보이는 미세한 특성까지도 잘 설명하였다. Using a first-principles total-energy method, we investigate structural and energy changes on Ag/Si(111)$\sqrt{3}{\times}\sqrt{3}$($\sqrt{3}-Ag$ hereafter) as the number of the additional Ag adatoms increases. The Ag coverage varies from 0.02 to 0.14 ML. Most Ag adatoms occupy the ST site, which is the center of small triangles of the substrate Ag layer that is composed of small and large triangles. One of the interesting adsorption features is that the adatoms immerse below the substrate layer. The total energy calculations show that the clusters become the most stable when the number of Ag atoms is three. This three-Ag cluster becomes the building block of the $\sqrt{21}{\times}\sqrt{21}$ phase that shows a large surface conductivity. The simulated STM images show that the adatoms look dark in filled-state images while bright in empty-state images. This suggests that the adatoms donate their charge to the substrate. The simulated STM images agree well with the experimental images.

제일원리 계산법을 이용한 비정질 실리콘 내에서의 리튬 이온 확산 예측

문장혁(Janghyuk Moon),조경재(Kyeongjae Cho),조맹효(Maenghyo Cho) 대한기계학회 2012 대한기계학회 춘추학술대회 Vol.2012 No.11

We have studied the lithium absorption in crystalline silicon with the strain effects on unit cell using density functional theory calculation. We have concluded that the dependences of the lithium diffusion were on the local volume and environmental. In the various strained cells, the effect of the lattice deformation about migration barriers for the motion of the lithium atom has been fit on the linear regression equation based on the volume of silicon surrounding lithium impurity and the migration distance of lithium atom. This result has applied to the calculation of diffusion coefficient in amorphous silicon which was generated by annealing from crystalline structure at 3000K. The migration barriers and attempt frequency, by Arrhenius formula, of lithium in the amorphous silicon structures has been determined by local environment using the linear regression equation. Then, the statistical method, kinetic Monte Carlo method, has been demonstrated for the diffusion coefficient of lithium. Finally, we have parameterized in terms of the amorphous effects into Arrhenius diffusion formula. This study have supported that the diffusion of lithium in amorphous silicon is faster than that in crystalline silicon.

알루미늄 도핑된 스피넬 구조에서의 얀-텔러 효과의 이해

김두호(Duho Kim),임진명(Jin-Myoung Lim),황태순(Taesoon Hwang),조경재(Kyeongjae Cho),조맹효(Maenghyo Cho) 대한기계학회 2015 대한기계학회 춘추학술대회 Vol.2015 No.11

Through first-principles calculations and experimental observations, we present the fundamental understanding of mechanism of the suppression of Jahn-Teller effects using Al doping in the cubic spinel oxide (LiMn2O4). The Al-doped spinel exhibits better cycle stability as compared with the pristine spinel. Considering partial density of states (PDOS) and band fillings of the two oxides from the first-principles calculations, the pristine spinel is entirely suffered from Jahn-Teller distortion by Mn3+, indicating the anisotropic electronic structure. On the other hand, the Al-doped spinel shows suppressed Jahn-Teller effects, presenting the isotropic electronic structure. These findings based on the mechanistic understanding of Jahn-Teller effects are expected to be a basic solution and design new cathodes for the electrochemically enhanced properties in Li-ion batteries.

제일원리 계산을 통한 알칼리 금속과 흑연의 전기화학 반응 메커니즘 분석

황태순(Taesoon Hwang),임진명(Jin-Myoung Lim),조경재(Kyeongjae Cho),조맹효(Maenghyo Cho) 대한기계학회 2016 대한기계학회 춘추학술대회 Vol.2016 No.12

Graphite has been used as a Li-ion battery (LIB) anode due to large capacity and good rate capability comparing to other LIB anodes and graphite also has been continuously studied for application to the other Alkali-ion battery anode. Especially, although interest in graphite for Na ion battery anode recently increases owing to low cost of Na metal, Na ion intercalation into graphite is inactive unlike Li and K. In this study, through first-principle calculations, intercalation mechanism of Alkali metals into the graphite was investigated. Alkali-metals, such as Na and K, show different electrochemical properties comparing to Li. Both Na and K increase the interlayer distance of graphite much larger than that of Li because Na and K have larger ionic radius. Particularly, intercalation of Na ion into enlarged graphite is thermodynamically unstable, even if intercalation of K ion into enlarged graphite is thermodynamically stable. Therefore, in addition to strain effect of graphite, Na intercalation into graphite has a great effect on electrochemical interaction between Na and graphite. For these reasons, this electrochemical interaction could be origin of inactive

제일원리 계산법을 이용한 실리콘 음극 소재의 그래핀 코팅 효과 분석

문장혁(Janghyuk Moon),조경재(Kyeongjae Cho),조맹효(Maenghyo Cho) 대한기계학회 2013 대한기계학회 춘추학술대회 Vol.2013 No.12

Computational study on the effect of graphene coating to Si anode material is performed by using density functional theory calculations. We construct the atomic model to examine interactions between amorphous silicon and graphene during lithiation. The lithiation of Si anode increases the mechanical contact force between outer graphene layer and amorphous silicon and the shear resistance is also increased. To explain the interaction between graphene and silicon we examine the charge distribution of silicon and graphene considering lithiation insertion. The number of density, electro field distribution and electric potential are also calculated. Charge-non polar interaction between Li-ion and graphene increase the contact energy between graphene-silicon. To calculate the contact force, we simplified interaction atomic force model. In this theoretical study, potential improvement of cyclability and improved mechanical properties of graphene coating for Si anode have been investigated.

제일원리에 기반한 자기 힘 이론 연구

김도훈,이형준,한명준 한국자기학회 2022 韓國磁氣學會誌 Vol.32 No.4

본 논문에서는 1980년 대 처음으로 제시된 이래 꾸준히 발전하고 있는 소위 ‘자기 힘 이론(magnetic force theorem; magnetic force theory; magnetic force response theory)’을 소개한다. 자기 힘 이론은 제일원리 전자구조 계산법과 결합되어 다양한 물질들에서 나타나는 복잡한 자성을 이해하는 데 유용한 도구가 되어왔다. 특별히 최근에는 비등방적 상호작용으로의 확장과 강상관 효과를 기술하는 다양한 이론 접근법들과의 결합에 힘입어 그 응용 범위가 더욱 넓어지고 있다. 여기에서는 자기 힘 이론의 원리에 대해 간략히 기술하고 몇 가지 응용 사례들을 살펴봄으로써 이 계산법이 갖는 주요한 특징과 장점들에 대해 논의할 것이다. 또한 해당 분야의 국내외 연구 현황과 관련 코드들도 함께 소개한다.