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국소적 원자 궤도 기저에서 시간 의존 Kohn-Sham 방정식을 풀기 위한 자체 일관된 Crank-Nicolson 방법론
방준혁 한국물리학회 2020 New Physics: Sae Mulli Vol.70 No.8
The excited state dynamics of electrons in materials is crucial for understanding various phenomena in nature and applications. However, time-dependent density functional theory, which is a first-principles method for studying electronic dynamics, requires extensive computational power, which makes theoretical study difficult. In this work, we studied how to improve the computational speed of TDDFT. A Crank-Nicolson approximation for a time evolution operator of the time-dependent Kohn-Sham equation preserves unitarity and preforms a high-speed calculation in a localized atomic orbital basis. By including a self-consistent loop in the Crank-Nicolson approximation, we were able to increase the calculational speed by more than tenfold while maintaining the calculational accuracy and stability. In this way, we made possible the application of the first-principles method to the study of excited state dynamics on a long time scale. 물질 내 전자의 들뜬 상태 동역학은 다양한 자연 현상뿐만 아니라 여러 응용 연구를 이해하는데 중요한역할을 한다. 하지만 전자 동역학 제일원리 이론 분석 방법인 시간의존 밀도 범함수 이론(time-dependent density functional theory, TDDFT) 은 실제 시스템에 적용을 하기 어려울 정도로 많은 계산량이필요하며, 들뜬 상태 동역학 이론 연구를 어렵게 만든다. 본 연구에서는 이런 TDDFT의 계산 속도를높이는 방법에 대해여 연구하였다. TDDFT의 시간의존 Kohn-Sham 방정식에 대한 time evolution 연산자를 계산하는 Crank-Nicolson 근사 방법은 연산자의 unitarity 특성을 유지시켜주며, 국소적 원자궤도 기저에서 빠른 계산을 수행할 수 있게 만든다. Crank-Nicolson 근사 방법과 함께 자체 일관된 전자밀도 조건을 이용할 경우, 계산의 정확성과 안정성을 유지하면서 계산시간을 10배 이상 향상 시킬 수있음을 확인하였다. 이를 통해 상대적으로 긴 시간 영역에서 일어나는 전자 동역학 현상을 제일원리방법으로 분석 할 수 있게 만들었다.
MAPbI3가 PbI2로 열화되는 중간 과정에 관한 연구
방준혁 한국물리학회 2023 새물리 Vol.73 No.11
Perovskite solar cells greatly enhance energy conversion efficiency and are attracting a lot of attention as next-generation solar cells. More efforts are being made to commercialize perovskites due to the possibility of various applications along with the price aspect, which are difficult for current silicon solar cells. However, low reliability and device degradation are major issues that restrict further application. In this work, the degradation process of MAPbI3 perovskite was studied by first-principles calculations. MAPbI3 transforms into a stable PbI2 material by the release of MA molecules and I atoms to the outside. The findings revealed that MA molecules and I atoms can diffuse into MAPbI3 and be released during the degradation process. On this basis, we inferred that PbI2 enclosed in MAPbI3 can have a significant influence on device degradation and discussed that it can delay device degradation by reducing the enclosed PbI2 in MAPbI3.
Atomic Structure and Diffusion of Hydrogen in ZnO
방준혁,장기주 한국물리학회 2009 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.55 No.1
Using first-principles theoretical calculations, we investigate the atomic structure and the diffusion pathway and barrier of hydrogen in ZnO. We find that an interstitial H is energetically more stable at a bond-centered position than at an anti-bonding site of an O atom. We examine the stable geometry of a defect complex consisting of hydrogen and a Zn vacancy and find that the O-H bond is oriented along the c-axis in the wurtzite structure. The energy barrier for the migration of an interstitial H is around 0.4 – 0.5 eV while that for a substitutional H at an O site is about 1.7 eV, indicating that the substitutional H is thermally more stable. Kinetic Monte Carlo simulations confirm that the substitutional H diffuses out at annealing temperatures higher than that for the interstitial H.
Mesoporous activated carbons for high power density supercapacitors
방준혁,김병주 한국공업화학회 2018 한국공업화학회 연구논문 초록집 Vol.2018 No.0
Sueprcapacitors, also known as electric double-layer capacitors (EDLCs) are electrical storage devices, which have a both high energy efficiency and power density. They can complement or replace secondary batteries in electrical storage and other applications, when high power delivery is needed. In this work, we conducted phosphoric acid activation of biomass precursors to manufacture highly mesoporous activated carbons. An effort to observe and compare the delivery of the power output characteristics was achieved with those of commercial activated carbons. The pore characteristics were analyzed at N2/77K isothermal adsorption using the Brunauer-Emmett-Teller and Barrett-Joyner-Halenda equations. In addition, the electrochemical characteristics were analyzed by means of an electrolyte of 1M SBPBF4/propylene carbonate, using a specific capacitance versus current density, cyclic voltammetry, and electrochemical impedance spectroscopy.