The Study Of Edge Dynamics Of Graphene

Graphene is an allotrope of carbon which consists of one atomic carbon layer attracted tremendous attentions by both scientists and engineers since its first discovery in 2004. This is attributed to its extremely high carrier mobility, and thus it is considered as the ideal candidate toward the next-generation semiconductor devices. Furthermore, graphene has many unique properties, such as high thermal conductivity, high transparency and high Young's modulus, etc. However, two-dimensional graphene cannot be directly applied in semiconductor devices due to the zero-bandgap. One-dimensional graphene nanoribbon exhibit non-zero bandgap and hence can be used to fabricate semiconductor device with good performance owning to its high on-off ratio. But it is theoretically and experimentally proved that the bandgap of graphene nanoribbon is strongly dependent on the chirality of the edges, which means that the electronic behavior can be tuned by modifying its edge structure. Annealing is commonly used in semiconductor manufacturing process, therefore, investigation of thermal dynamic behavior of graphene edges becomes important. However, to date, the understanding of the graphene edges' thermal dynamics is still very limited. Furthermore, the study of structure modification of graphene edges under Ar plasma treatment is also deserved. At this background, I focused on the detailed investigation of thermal dynamics for edges of single- and bi-layer graphene, as well as the edge dynamics of graphene edges under Ar plasma treatment.Firstly, it is discovered that the armchair edges of single layer graphene is thermally stable. Even the annealing temperature is as high as 500℃, large portion of armchair segments can still keep remained. Meanwhile, the center region of single layer graphene is also thermally stable as no obvious appearance of defects.Secondly, it is found that the zigzag edges of single layer graphene is not as thermally stable as armchair edges because it is the atoms of zigzag edges prefer to transform to armchair segments during annealing. Even for annealing temperature as low as 200℃. small portion of zigzag edges is starting to rearrange/modify to form armchair segments along±30°with respect to the original edge direction. When the annealing temperature is 300℃, large amount of atoms of zigzag edges rearrange/modify to form armchair segments which is relatively thermal stable. Therefore, the modifications of edge structures by thermal annealing (zigzag segments rearrange in form of armchair segments) provide a flexible way to control the electronic properties of graphene and graphene nanostructures.Thirdly, based on the aforementioned thermal dynamic behaviour of single layer graphene edges, we found that the armchair edges of AB-stacked bilayer graphene present thermal dynamic behavior similar to that of single layer graphene, which means that the armchair edges of top and bottom graphene can be considered as two independent individual armchair edges. However, for zigzag edges of AB-stacked bilayer grpahene. the thermal dynamic behaviour shows obviously difference compare to that of single layer. In stead of rearrange to form armchair segments during annealing for zigzag edges of single layer graphene. the top and bottom zigzag edges of AB-stacked bilayer graphene prefer to form perfect closed structure hybridized by sp2 bonds. This is also supported by the theoretical calculation results very well as there is no energy barrier from the initial open edge state to closed edge state, moreover. the system energy undergoes more than 2 eV drop when the atoms on the top and bottom zigzag edges firstly bonded. Furthermore, it is theoretically predicted that AB-stacked bilayer graphene with closed zigzag edges present exotic electronic properties, such as pseudospin repulsion induced bandgap opening and charge separation. Therefore, the discovery of this new graphene-based form may illuminate a simple and easy way to engineer graphene electronics.Finally, the graphene edge structure evolution after Ar plasma treatment is investigated. It is found that the armchair edges present relatively good stability under Ar plasma treatment. Meanwhile, the zigzag edge atoms prefer to transform to armchair segments after Ar plasma treatment. This result indicates that the armchair segments are more stable under plasma radiation. This finding provides useful information for studying anti-radiation properties of graphene based nano-electronic devices.