Mechanical Properties Of Defects In Two-dimensional Materials

The discovery of graphene has made two-dimensional materials become a hot topic focused and lasted in both scientific research and industrial applications since2004.Owing to the recent rapid development of state-of-the-art experimental preparation,fabrication and characterization in nanoscale,it is predicted that there would be 500 kinds of two-dimensional materials in the world,forming an emerging material category.On account of their unique low-dimensional structural characterristics,two-dimensional materials always hold some fascinating physical properties,such as mechanical,thermal,electrical,magnetical and optical characteristics,which can make breakthrough in areas of environment,energy,electrical engineering,aerospace,biology,medicine and etc.In this dissertation,comprehensive joint research efforts through both theoretical analysis and computer simulations will be highly concentrated on the mechanical properties of defects in single-atomic-thicktwo-dimensional materials represented by topological defects(TDs)in graphene.In-depth investigations are focused on the dual roles of TDs in graphene which originate from local stress buildup and geometrical effects in two-dimensional manifold.Furthermore,some reasonable prospects for future practical applications are presented which is based on the aforementioned understandings on the mechanical properties of two-dimensional material.The detailed content is summarized and organized as follows:Topological effects of TDs in graphene.Stress field distribution of some TDs in graphene is obtained by the recombination of both the structures of elementary defects and their stress field.Built upon them establish the stress pileup models of 5|7dislocation and 5|8|5 dislocation dipoles.We propose the pseudo(inverse)Hall-Petch effects in the brittle fracture behavior of polycrystalline graphene which the in-plane tensile strength is decreased(increased)with the grain size.Moreover,triple grain boundaryjunctions serve as the nucleate centers for cracks and crack propagation is typified by both intergranular and transgranular modes.Geometrical effects of TDs in graphene.Before a systematic discussion on the bulking induced by local stress buildup of defects,the fracture behavior and sizeeffects of supported graphene under pressure is explored in advance.Then a comprehensive and richly detailed analysis of geometrical effects which are always overlooked but rather influential during nanoindentation is presented.These findings could provide a particular guidance on the precise mechanical characterization and measurement of ultrathin specimen.Mechanical properties of defects in curved surface.According to the Griffiththeoryof brittlefracture,unzipping of carbon nanotubes(CNTs)induced by oxidization and hydrogenation is explored by our theoretical models.Both diameter and chirality of CNTs is under consideration.Applications and prospects of two-dimensional materials.Based on the above-mentioned understandings of two-dimensional materials,we propose three future applications of two-dimensional materials:(1)nanoporous graphene membrane in forward osmosis;(2)ultrafast water transport through nanostrand-channelled graphene oxide membranes;(3)graphene-coated lipid membrane as biomedical materials.