Some Theoretical Researches Of Strained Graphene

Since discovered in2004, graphene and graphene nanoribbons have shown a series of peculiar physical properties. This has attracted extensive attention from both experimental and theoretical physicists. Meanwhile, people have been trying to manipulate the electronic band structure and transport properties in graphene nanoribbons by doping, defect, adsorption, gating, edge modification, strain etc. in order to explore novel physical phenomenon and properties with application potential. In addition, graphene is up to now the strongest material under tensile strain and its unique mechanical property may be applicable to nanoelectromechanical systems and nanometer sensor. On the other hand, grapheme in devices is unavoidable under various tensile and compressive strains. In this case, it is scientifically and functionally valuable to study the electronic structure and transport properties of graphene and graphene nanorribons under strain.In this dissertation, we at first introduce the discovery, the experimental preparation, and the novel electronic properties of grapheme. Then we briefly describe the quantum transport theory based on the Green’s function method. After the background, we then present in detail the research work during my graduate study, which is summarized into the following two parts.(1)Employing the tight-binding model (with only the nearest neighbor coupling), we study the electronic band structure of intrinsic graphene and graphene nanoribbons, and the effect of strain on the band structure and the edge states. Our result show:(a) A uniaxial strain can open an energy gap when the strain exceeds30%, but for the biaxial strain case energy gap opens with strain less than20%.(b)The strain will not open energy gap in ZGNRs. But it will affect the edge states of zigzag graphene nanoribbons.(c)The transversal and longitudinal strain can make the three types of AGNRs realize metal-semiconductor-metal transition. Tension and compression of the same direction show the asymmetry. Tension and compression of the different direction also show the asymmetry.(2) The quantum conductance of AGNRs and ZGNRs are systematically studied under a uniaxial strain. A tension (compression) in the transverse direction or a compression (tension) in the longitudinal direction increases (decreases) the conductance of ZGNR in the high-energy region. Overall, a transversal strain can tune the quantum conductance of ZGNRs more efficiently than a longitudinal one. In metallic AGNRs under strain, a conductance gap usually emerges near the Fermi energy as well as a corresponding energy gap. Interestingly, in the conductance spectrum, valleys appear and shift to lower energy under the transversal compression due to the variation of energy band. Similar to ZGNRs, the transversal strain in AGNRs is also more efficient to tune the quantum conductance.