The Modulation Of Electron Transport In Graphene Nanostructures

Graphene, an only one-atom thin sheet, was successfully made in 2004 by Geim and Novoselov et al.. This new kind of carbon nanomaterial, showing excellent physical properties in many areas, such as mechanics, thermodynamics, electronics and optics, has attracted much attention. Especially, it has now become a hotspot of nanomaterial due to its huge potential applications in electronics in recent years. The study of the electronic properties of graphene develops to be one of the most active frontiers in condensed matter physics. In this thesis, we study electron transport in graphene nanostructures by using the Green's function method. The modulating and controlling effects of both geometry and potential on the electron transport are explored, and some novel transport phenomena are discovered. The corresponding physical mechanisms of phenomena are exposed. By proposing new microscopic and nano electronic devices, we hope that our work will provide not only theoretical foundation but also models for building and fabricating graphene-based electronic devices.This thesis consists of six chapters.In the first chapter, we introduce the geometry structures of graphene and graphene nanostructures, and also their physical properties, fabrication methods and the potential applications.In the second chapter, the Green's function method is introduced, including its application to transport properties of simple graphene nanostructures.In the third chapter, we study transport properties of two L-shaped graphene junctions with different corner forms. The modulation of geometry and size of corners on transport properties is discussed. Our results indicate that the electronic properties of the system strongly depend on the corner form. The L-shaped graphene junctions transmit from metallic to semiconducting due to the variation of the inner corner form. However, resonant transmission is exhibited on the conductance profile of the L-shaped graphene junctions that change both inner and outer corner forms. The resonant peaks are induced by the quasi-bound states confined in the L-shaped graphene junctions.In the forth chapter, we study electron transport of the folded graphene nanoribbons. By comparing the transport properties of monolayer, bilayer and folded graphene nanoribbons, the folding effect on the electron transport in graphene nanoribbons is discussed. It is found that a metallic monolayer armchair graphene nanoribbon is still metallic after folding, but electronic properties of a semiconducting monolayer armchair graphene nanoribbon depend on the coupling strength and structural width. On the other hand, the conductance profiles of folded zigzag graphene nanoribbons are different from both the monolayer and bilayer zigzag graphene nanoribbons. Around the Fermi level, the conductance steps are even numbers of the basic conductance G₀, while other steps are odd numbers of G₀.In the fifth chapter, we study electron transport in graphene nanoribbons under a one-dimensional central potential. The results indicate that the energy gaps of armchair graphene nanoribbons vary with the strength of potential. An armchair graphene nanoribbon transits from metallic to semiconducting with the increase of potential modulation. A zigzag graphene nanoribbon will transfer from metallic to semiconducting as the potential strength exceeds a critical value.The last chapter presents the conclusion of this thesis and some prospects for forture investigation.