Atomic-scale simulations of graphene synthesis and its applications for electronic and energy materials

Abstract

국문요지 그래핀 합성공정과 전자 및 에너지소재로의 응용에 대한 원자단위 시뮬레이션

윤경한

이 논문에서는 원자단위 시뮬레이션 기법을 활용하여 그래핀의 합성공정과 응용분야에 대한 연구를 수행한 결과가 논의된다. 양자 홀 효과로 인한 이례적으로 우수한 전기적 특성을 보일 것으로 기대되었던 그래핀은 구조적 특성 상 합성이 매우 어려워 이론적으로만 그 성능을 예측하는 것에 불과하였다. 하지만, 2004년 흑연으로부터 그래핀을 분리하는 방법이 발견되면서 그 동안 예측되어 왔던 특성들이 실험적으로 확인되며, 수많은 후속 연구가 수행되어 왔다. 즉, 그래핀은 높은 전하 이동도 (15000 m2V-1S-1), 단위 질량 당 넓은 면적(2630 m2/g), 우수한 열전도 특성(5000 wm-1K-1), 그리고 높은 탄성 계수(1 Tpa)와 같은 우수한 기계적 특성 등으로 인해 매우 다양한 분야에 응용 가능성을 두고 있는 소재로 각광받아왔다. 무엇보다도, 소자의 소형화에 따라 발생하는 다양한 문제점을 해결 할 수 있는 대체 소재로서 주목 받아왔다고 볼 수 있다. 특히, 최근에 나노 스케일 단위로 소자가 소형화 됨에 따라, 측정 기술에 있어서 원자단위 관측이 필수적으로 자리매김하고 있는데, 시뮬레이션 기법은 이러한 원자단위 측정기술의 한 방법으로서, 대상 시스템의 안정한 구조와 그 구조에 따른 전자구조를 분석함으로써 표면적으로 발현되는 물성의 기본 원리를 이해함에 매우 효과적인 측정 기술이라고 볼 수 있으며, 이미, 다양한 원자단위 시뮬레이션 기법을 통해 그 효과가 많은 사례에서 입증되고 있다. 본 연구에서는, 다양한 원자단위 시뮬레이션 기법 중, 고전역학을 기본으로 뉴턴의 운동 방정식을 계산함으로써 시간에 따른 원자의 안정한 상대적 위치를 확인 할 수 있는 분자동역학 시뮬레이션과 양자역학을 기초로 하여 시스템의 에너지 및 그에 따르는 전자구조를 확인할 수 있는 제일원리 계산 기법이 활용되었다. 분자동역학 시뮬레이션과 제일원리 계산은 수 나노 스케일 이하의 아주 작은 단위에서 소재를 관찰 할 수 있는 장점이 있으므로, 그래핀과 같은 나노소재의 연구에 있어서 매우 적합한 시뮬레이션 기법으로 활용되고 있다. 위의 실험 방법을 토대로 본 연구에서는, 그래핀의 합성공정에 대한 연구와 그래핀의 다양한 소자로의 응용 시 발현되는 특성에 그 초점을 맞추었다. 분자동역학 시뮬레이션 기법의 장점을 이용하여, 화학적으로 박리된 산화 그래핀의 환원 과정을 모사함으로써, 산화 그래핀의 표면에 결함으로 존재하는 산소 작용기의 환원 메커니즘을 관찰하고 분석하였으며, 제일원리 계산을 활용하여 그래핀이 전이 금속(Mn, Fe, 그리고 Co)과 같은 강자성 원자들과 결합하였을 때 시스템에서 나타나는 자기적 특성 변화에 대하여 분석하였다. 또한, 질소가 도핑된 그래핀이 이차전지 전극으로 사용될 때, 질소 결함이 전지 성능에 미치는 영향을 전극 표면에서 분자들이 화학 반응할 때 발생하는 자유에너지 변화를 계산 및 분석하여 각각의 서로 다른 결함의 역할을 구분하였다. 기본적으로 전기적 특성이 우수한 순수한 그래핀은 밴드갭이 없는 반 금속 상태의 소재이므로, 밴드갭을 확장하여 트랜지스터의 채널 소재로 응용코자 많은 시도를 하고 있다. 반면에 실험적으로 더욱 수월하게 합성할 수 있는 다양한 수정된 그래핀을 활용하는 방안 또한 병행하여 연구되는 시점에서 그래핀이라는 새로운 소재를 가장 효과적으로 활용할 수 있는 방법을 다양한 관점에서 발견해 나아감에 이 연구에 의의를 둔다. |Abstract

Atomic-scale Simulations of Graphene Synthesis and Its Applications for Electronic and Energy Materials

Kyung-Han Yun Dept. of Materials Science and Engineering The Graduate School Hanyang University

In this thesis, molecular dynamics simulations and density functional theory calculations are performed to explore the graphene synthesis process and the properties when applied to the electronic and energy materials. Since the innovative method to exfoliate graphene monolayer from bulk graphite was introduced and thereby the predicted extraordinary electronic property was proved in practice by measuring the half-integer quantum-hall effect of graphene, graphene has attracted much research interests to mainly semiconductor industry. Along with the high carrier mobility, the other unique properties such as large specific surface area (2630 m2/g),high Young’s modulus (1 Tpa), and high thermal conductivity (5000 wm-1K-1) has led the direction of research on graphene to the various applications for decade, as well as semiconductors. Particularly, graphene has received great expectation as a new alternative materials solving a scale-down problems in nanodevices. Accompanying the attempts to miniaturize the devices into nanoscale, the techniques of observing and analyzing the properties of materials in atomic scales become more necessary. Here, the simulation technique is one of the powerful methods to observe and comprehend the phenomena inside the materials in atomic scales. Among various simulation methods, molecular dynamics (MD) simulations are based on the Newtonian equation of motion that describes the atomic behavior with time flow. Hence, through molecular dynamics simulations, it is possible to observe the kinetics of atoms in nanosecond scale and thereby analyze accurately the mechanism of transformation of materials. In addition to the observation of atomic behavior, the electronic structures corresponding to the certain system can be calculated by density functional theory (DFT) calculations. Here, DFT has been the most successful computational physics and quantum chemistry to describe properties of condensed matter systems, which include not only standard bulk materials but also complex materials, molecules, particles, or interfaces. Particularly, in research on nanomaterials such as graphene and its combined system with other materials, MD and DFT simulation techniques have played an important role. In this dissertation, MD simulations were performed to observe the graphene synthesis process in atomic scale. The reduction mechanism of functional groups from a chemically exfoliated graphene oxide (GO) was systematically investigated by modeling a heat annealing process. The application parts are divided into spintronics and energy materials. In applications to spintronics, the magnetic properties of transition metals (Mn, Fe, and Co)/graphene system were investigated via DFT calculations. The effect of the external electric field on magnetic properties were observed and analyzed by observing the charge distributions and the electronic structures when single transition metal atoms adsorbed on graphene surface. In applications to the energy materials, distinction of effects of different defective sites on nitrogen-doped graphene (N-GNS) was classified via DFT calculations, when N-GNS is used for secondary battery electrode. The nitrogen doping technique is quite effective method to generate active sites on graphene surface. These active sites act as a catalytic media for oxygen reduction reaction in fuel cell based secondary battery systems such as lithium-oxygen batteries. Through comparing the charge transferring and energetics, it was found which type of N-defects (graphitic, pyridinic, and pyrrolic types) play an advantageous role in use for lithium-oxygen battery electrode.; Abstract

Atomic-scale Simulations of Graphene Synthesis and Its Applications for Electronic and Energy Materials

Kyung-Han Yun Dept. of Materials Science and Engineering The Graduate School Hanyang University

In this thesis, molecular dynamics simulations and density functional theory calculations are performed to explore the graphene synthesis process and the properties when applied to the electronic and energy materials. Since the innovative method to exfoliate graphene monolayer from bulk graphite was introduced and thereby the predicted extraordinary electronic property was proved in practice by measuring the half-integer quantum-hall effect of graphene, graphene has attracted much research interests to mainly semiconductor industry. Along with the high carrier mobility, the other unique properties such as large specific surface area (2630 m2/g),high Young’s modulus (1 Tpa), and high thermal conductivity (5000 wm-1K-1) has led the direction of research on graphene to the various applications for decade, as well as semiconductors. Particularly, graphene has received great expectation as a new alternative materials solving a scale-down problems in nanodevices. Accompanying the attempts to miniaturize the devices into nanoscale, the techniques of observing and analyzing the properties of materials in atomic scales become more necessary. Here, the simulation technique is one of the powerful methods to observe and comprehend the phenomena inside the materials in atomic scales. Among various simulation methods, molecular dynamics (MD) simulations are based on the Newtonian equation of motion that describes the atomic behavior with time flow. Hence, through molecular dynamics simulations, it is possible to observe the kinetics of atoms in nanosecond scale and thereby analyze accurately the mechanism of transformation of materials. In addition to the observation of atomic behavior, the electronic structures corresponding to the certain system can be calculated by density functional theory (DFT) calculations. Here, DFT has been the most successful computational physics and quantum chemistry to describe properties of condensed matter systems, which include not only standard bulk materials but also complex materials, molecules, particles, or interfaces. Particularly, in research on nanomaterials such as graphene and its combined system with other materials, MD and DFT simulation techniques have played an important role. In this dissertation, MD simulations were performed to observe the graphene synthesis process in atomic scale. The reduction mechanism of functional groups from a chemically exfoliated graphene oxide (GO) was systematically investigated by modeling a heat annealing process. The application parts are divided into spintronics and energy materials. In applications to spintronics, the magnetic properties of transition metals (Mn, Fe, and Co)/graphene system were investigated via DFT calculations. The effect of the external electric field on magnetic properties were observed and analyzed by observing the charge distributions and the electronic structures when single transition metal atoms adsorbed on graphene surface. In applications to the energy materials, distinction of effects of different defective sites on nitrogen-doped graphene (N-GNS) was classified via DFT calculations, when N-GNS is used for secondary battery electrode. The nitrogen doping technique is quite effective method to generate active sites on graphene surface. These active sites act as a catalytic media for oxygen reduction reaction in fuel cell based secondary battery systems such as lithium-oxygen batteries. Through comparing the charge transferring and energetics, it was found which type of N-defects (graphitic, pyridinic, and pyrrolic types) play an advantageous role in use for lithium-oxygen battery electrode.