石墨烯晶界的力學與熱學性質之研究

石墨烯是碳原子以sp2雜化軌域組成單層蜂巢狀晶格的新型二維材料，由於其優良的材料性質，在工業上各方面都極具應用潛力，因此被視為未來可能取代矽元素的明星材料。現今製備大面積石墨烯的主流方法，是以化學氣相沉積的方式，在高溫的銅或鎳基板上通入氫氣及甲烷，使碳原子得以附著在基板上形成石墨烯。然而以此法所製備出的石墨烯會伴隨許多缺陷、差排甚至是晶界的生成，因此研究晶界對於石墨烯物性的影響便是十分重要的問題。本論文以分子動力學模擬的方式，來研究具晶界之石墨烯的力學與熱學性質。我們以化學氣象沉積的概念，結合了沉積原子、模擬退火與結構優化，發展了混成分子動力學，並藉以找到了穩定的石墨烯晶界結構。依照晶界的錯向角，可將其歸類為鋸齒型、過渡型與扶手椅型三種晶界。藉由對晶界加入原子的方式，我們也研究了穩態與亞穩態晶界間的轉換與歸納出三類晶界遷移的機制。在機械性質方面，模擬的結果顯示，晶界的存在對於石墨烯的楊氏係數與剪切係數並不會有顯著的影響，然而其抗拉與抗剪強度卻會明顯降低。此外也發現差排密度較大的晶界，反而會具有較大的抗拉與抗剪強度。我們也研究了重位晶格與抗拉強度之間的關係，並對不同形態的晶界，分別提出了經驗公式來預測晶界的強度。在熱傳性質方面，較完美石墨烯而言，我們認為石墨烯晶界會有更高的熱調變性。藉由計算石墨烯晶界奈米帶的熱傳導係數後，發現在鋸齒型與扶手椅型晶界中，兩個差排密度最大的晶界奈米帶，其熱傳導係數反而較其它差排密度較小的晶界奈米帶來的高。此外，我們也提出了計算石墨烯熱容與熱傳導係數的解析公式，並以之求出各種結構的聲子平均自由路徑。最後我們也探討了差排偏振的動力學。差排在進行兩個偏振方向之間的轉換時，其能量小於鍵斷裂或是鍵旋轉數倍。藉由差排處在不同的偏振的情況，可以使晶界變為不同的構型。我們發現當晶界中差排偏振的情況越複雜、起伏程度越大，會使材料的抗壓縮能力變好，但熱傳能力則會變差。

關鍵字

石墨烯；晶界；分子動力學模擬

並列摘要

Graphene, a two-dimensional material consisting of sp2-hybridized carbon atoms with an one-atom-thick honeycomb crystal lattice, has outstanding electronic, thermal, and mechanical properties, yet mass fabrication of large-area, high-quality graphene films is not trivial. Recently, the growth of a single or few-layer graphene on Cu and Ni substrates through chemical vapor deposition has become one of the most promising approaches to fabricating graphene films. Nevertheless, the defects, dislocations and even grain boundaries are often introduced into graphene during chemical vapor deposition growth processes, potentially affecting the properties of graphene films in a significant way. In this dissertation, we investigate the structures of graphene with grain boundaries and their mechanical and thermal properties by using molecular dynamics simulations. Here we performed a series of hybrid molecular dynamics simulations, combining static relaxations and finite temperature molecular dynamics simulations in time scales of the order of 10 nanoseconds. In particular, we concern ourselves with symmetric tilt grain boundaries of graphene with a wide range of misorientation angles. We also investigated how these grain boundaries transfer from one configuration to another, or even migrate by adding carbon atoms at the grain boundaries. It is found that the graphene symmetric tilt grain boundary with a higher dislocation density has a higher tensile strength and a higher shear strength. This counter-intuitive result is attributed to the mutual cancelation of strain fields of grain boundary dislocations when they are close to each other. It is shown that the thermomutability of graphene grain boundary is higher than that of the pristine one. In addition, we have investigated the thermal transport properties of graphene nanoribbons with grain boundaries. It is found that the thermal conductivities of two defective ribbons, each with the highest dislocation density in its own category, are much higher than those of the ribbons with lower dislocation densities, due to the non-bended structure of the former defective ribbons. Furthermore, we performed temperature accelerated dynamics simulations to study the structure and dynamics of topological defects with out-of-plane bulges in graphene. It is shown that the graphene grain boundary with a more complex dislocation polarity distribution will increase the ability against the compressive deformation yet reduce the thermal conductivity.

並列關鍵字

Graphene ； Grain boundary ； Molecular dynamics simulation

參考文獻

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石墨烯晶界的力學與熱學性質之研究

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