화학기상증착 그래핀의 전기적 특성 최적화 연구

학위논문 (박사)-- 서울대학교 대학원 자연과학대학 화학부, 2017. 8. 홍병희.After the first artificially-isolated single-atom-thick material, which is graphene, the new era of two-dimensional materials has begun. With its extraordinary physical and chemical properties, graphene has drawn considerable attention as a subject of the fundamental research by condensed matter physicists. Thereafter, the realization of large scale graphene synthesis on catalytic substrates, such as Ni, Fe, and especially Cu, by chemical vapor deposition (CVD) makes researchers in numerous fields rush into graphene study. Though large-scale graphene films synthesized by the CVD method shows tremendous potential of applications for transparent electrodes with high electrical, optical properties and mechanical stability, the CVD-grown graphene films still have higher sheet resistance, compared to the exfoliated graphene. One of the limiting factors to degrade graphene quality is polycrystallinity and grain boundary of graphene, which originated from the randomly oriented nucleation and growth of graphene islands. Another factor is the ruga morphology of graphene, which is induced by the different thermal expansion coefficient between graphene and catalyst. The cooling down process makes catalyst surface corrugated and it leads to quasi periodic nanoripple arrays of graphene after graphene transfer onto target substrates. The rippled graphene gives rise to flexural phonon scattering, which play a limiting role in charge mobility and sheet resistance. The present thesis addresses the synthesis of graphene by CVD on copper, which has become the most popular catalyst for graphene growth. A significant study on the graphene growth and property control is reported in the present thesis. This dissertation provides the details of my work on all projects related to synthesize and characterization of the graphene synthesis by CVD on copper and its application for field effect transistor. Especially, we will discuss the improvement of the sheet resistance of graphene by controlling synthesis condition. We also controlled the work function of graphene films via vapor phase doping process. First, tension-controlled graphene growth is presented herein. The methods for optimizing the crystalline orientations of Cu foils by tension control are explained in detail. In addition, the optimized Cu foils allow the growth of larger single-crystalline graphene with higher charge carrier mobility. Second, we demonstrated tuning the cooling rate can control the electrical properties of CVD-grown graphene. A higher cooling rate gives rise to large suspended graphene formation, which in turn results in reduced ripple density and its heights after transfer onto SiO2/Si substrates. Finally, an improved graphene doping method is also described. we have investigated the effect of the dopant structure and number of amines group in dopant. The doping concentration was stronger as increasing amino group in linear ethylene amine structure. However, the branched structure showed weaker doping, although it has the most amino functional groups in the series of ethylene amine dopants. In conclusion, we believe that our findings will provide crucial ideas to design a system for the CVD growth of high-quality graphene films on roll-to-roll Cu foils which would be of great importance for the continuous mass-production of graphene films for practical applications in the future.Chapter 1. Introduction 1 1.1. General introduction 1 1.2. Graphene synthesis 4 1.2.1. Mechanical exfoliation 4 1.2.2. Chemical exfoliation 6 1.2.3. Chemical vapor deposition (CVD) 8 1.3. Defects of CVD graphene 11 1.3.1. Grain boundaries 11 1.3.2. Corrugated morphology 13 1.4. Doping of graphene 16 1.5. Scope of the Thesis 18 1.6. References 20 Chapter 2. Tension-controlled single-crystallization of copper foils for high-quality graphene synthesis 23 2.1. Introduction 23 2.2. Experimental 27 2.3. Results and Discussion 31 2.4. Conclusions 38 2.5. References 39 Chapter 3. Controlling the ripple density and heights: a new way to improve the electrical performance of chemical vapor deposition grown graphene 45 3.1. Introduction 45 3.2. Experimental 48 3.3. Results and Discussion 50 3.4. Conclusions 61 3.5. References 62 Chapter 4. Stable n-type doping of graphene via high-molecular-weight ethylene amines 66 4.1. Introduction 66 4.2. Experimental 69 4.3. Results and Discussion 71 4.4. Conclusions 79 4.5. References 80 List of Publications 84 Abstract in Korean 86Docto

influences of cognitive and vocabulary retrieval ability on speech rate and pause characteristics in elderly

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멸종위기식물 매화마름의 생태적 특성 : 강화도 매화마름을 대상으로

Thesis(masters) --서울대학교 대학원 :생명과학부,2009.2.Maste