Mechanics of defective graphene sheets.

摘要

Graphene has recently emerged as one of the most exciting material systems to study due to its high strength and rigidity, high room temperature electron mobility, and unconventional quantum Hall effect. Defects, such as vacancies and interstitials, act as stress concentrators and scattering centers, significantly degrading both the mechanical properties and the electronic properties of graphene. Understanding the nucleation and motion of these defects is not only scientifically interesting, but also practically important as it provides physical insights to the graphene production. The central theme of this research is to develop and apply multiscale simulation techniques to uncover the underlying atomistic mechanisms without the limitations to the system size and time scales inherent to the fully atomistic models. In reflecting these efforts, this dissertation will cover three unified projects on the mechanics of graphene.The first part presents molecular dynamics simulations of ion irradiation of the edges of a cantilevered graphene. We found that graphene underwent deflection where the magnitude and the direction of deflection depend on the kinetic energy of the impacting ions. Besides providing atomic details of the edge morphologies, the molecular dynamics simulations revealed that the deflection of the graphene is caused by a competing mechanism of production and annihilation of interstitial- and vacancy-like defects in the course of ion irradiations. As free edge plays a critical role in the electrical properties of graphene, the simulation results provide useful insight into the edge engineering. The second part of the thesis involves the characterization of thermal activation of brittle and ductile fracture in graphene by using a pathway sampling scheme (nudged elastic band method). With this scheme, the time scale of atomistic simulations is substantially extended. It is concluded that the fracture of graphene takes a special mode, featuring alternating bond breaking and bond rotation at the crack tip. The third part focuses on quantifying the strain-mediated kinetics of migration of various defects in graphene. Migration of double-vacancy defects under combined thermal and mechanical loadings was studied using the reaction pathway nudged elastic band calculations. It was revealed that the migration kinetics of the defects depends not only on the level of the applied strain, but also on the direction along which the strain is applied. The results from the empirical potentials are compared to those from quantum mechanical calculations.