Numerical Simulation Study Of Electronic Transport Properties In Graphene With Topological Defects

In2004, the two dimensional (2D) plane graphene was manufactured in laboratory for the first time, which immediately stimulated a great interest of researchers in condensed matter physics because of its unique geometry structure and amazing physical and chemical properties, which make it to be a promising nanostructure material used in the field of materials science, electronics and nanotechnology.Pristine graphene, however, is a zero-gap semiconductor, which brings a great difficulty to control graphene electron and limits its potential applications in the field of nano-electronic devices. In order to replace the silicon and other semiconductors by graphene, it is needed to produce an energy-gap for it by different methods. One often used method is to produce some defects in graphene, which will have a big effect on its electronic transport properties in addition to generate an energy-gap, such as changing the moving direction of electron wave packet or flitering the electron’s valley degree of freedom and etc.At present, the best method to make the large-scale production of graphene is the epitaxial growth of it on a substrate. However, because of the substrate imperfections and the kinetic factors in the growth process, the produced graphene will be inevitably a polycrystalline, in which there would always exist some defects and grain boundaries. Right now, the grain boundaries and the line defect composed of an octagon and a pair of pentagons have been observed in experiments. Therefore, it will be fundamental importance and have potential applications to study theoretically the effects of these topological defects on the electronic structures and transport properties of graphene. In this thesis, based upon tight-binding Hamiltonian model and numerical simulation method of the wave packet dynamics (KPM approach), we have made a real-space simulation study of the transport properties of graphene electron, passing through the grain boundaries and line defect in the polycrystalline graphene.Firstly, we have studied the electron transport in graphene with a (3,3)｜(5,0) grain boundary. The obtained results from the wave packet dynamics simulation show that such a grain boundary can generate an energy-gap, which reaches its largest value of about2eV at the perpendicular incidence of electron wave packet in contrast to its lowest one of about leV at an incidence angle of almost90°. More importantly, it is found that in some scattering cases (especially at relatively high energy), there exist the interesting anomalous negative reflection and refraction of the wave packets. Then, we have studied the situation of the (2,1)|(2,1) grain boundary in graphene, finding that such a grain boundary can not generate an energy-gap. Because the Brillouin zones of graphene on two sides of the (2,1)|(2,1) grain boundary are symmetric to each other, the reflected and transmitted waves appear always in pairs, making the corresponding angles of reflection and transmission are equal.At last we have simulated the electron transport in the graphene with a line defect, paying more attention to the valley filtering property of the line defect. Our obtained results show that the graphene line defect is semitransparent for the graphene electron with valley degree of freedom, and the transmission probability depends not only the angle of incidence, but also the electron’s energy. A peak of transmission probability appears at a critical angle of incidence θc, and the transmission probability will drastically decrease to0when the absolute value of incidence angle|θ|is greater than|θc|, which is found to be caused by the nonlinear terms in the relation of the transmission probability with electron’s energy. Finally, our numerical simulation results further prove the practical applicability of the graphene line defect used as a valley pseudospin filter device in future.