| Summary: | 博士 === 國立臺灣大學 === 應用力學研究所 === 100 === 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.
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