Modeling the mechanical behavior of carbon nanostructures

Abstract

Low-dimensional nanostructures are expected to have vast number of applications in the future. Particularly large amount of research has been invested in the atomthick carbon membrane called graphene, which has become popular due to its unique electronic and mechanical properties. This thesis presents studies of the mechanical and electromechanical properties of several different types of graphene nanostructures. In addition, short detours are performed in order to study the elasticity of gold nanostructures and topology effects in graphene nanoribbons. The research is performed by using several different simulation methods. In simulations the system parameters and environment can be chosen at will, giving large amount of control over the studied phenomena. This control, and the access to different system parameters, can give insight into system properties that are hard to deduce from experiments alone. The reliability of the simulations depends on the used methods that are thus chosen according to the level of desired accuracy. Large-scale deformations of graphene nanostructures are studied by classical force field methods. We present and explain edge rippling due to compression at graphene nanospiral perimeters when the nanospiral is elongated above a certain threshold. Further insight into the elastic behavior of these nanospirals is obtained by continuum elasticity modeling. For graphene nanoribbons we explain two previous experimental observations, an abrupt buckling under in-plane bending and the stability of curved graphene nanoribbon geometry on a smooth substrate. Buckling is predicted by simple model and is found to be due to the compression at the inner edge of the curved graphene nanoribbon. The stability of the curved geometry is shown to be due to registry effects between the graphene nanoribbon and the substrate. Moreover, intricate interlayer sliding patterns under peeling of multilayer graphene stacks are discussed and we show that such stacks are likely to recover after the peeling force is released. Via electronic structure calculations we find a connection between the graphene nanospiral elongation and electronic structure and show that for graphene nanospirals the interlayer interactions play major part in the electronic structure near the structural equilibrium. Moreover, for graphene nanoribbons we study the effect of Möbius topology by using the revised periodic boundary conditions in a novel way. By the introduced method we are able to impose Möbius topology into flat graphene nanoribbons enabling the study of the role of the topology alone. We conclude that the topology affects only graphene nanoribbons with small length-to-width ratios. Finally we consider the temperature dependence of the bending rigidity of a two-dimensional gold nanostructure realizable in suitably sized graphene pores. The underlying motivation for most of the performed studies is the connection between the mechanical deformations and the electronic structure, which is discussed qualitatively even for large systems, where explicit electronic structure calculations are not possible.