First-Principles Investigation On Electronic Properties Of Graphene

Due to their amazing electronic structures, novel quantum properties, and wide po-tential applications, Recently, graphene and graphene-like systems have attracted much research attention. Here, we try to explore and manipulate the electronic properties of graphene by performing extensive first-principles calculations. This dissertation for Ph. D includes the following chapters.In Chapter1, we concisely review the basic ideas of density functional theory (DFT), several commonly used exchange-correlation functionals, and the flowchart of DFT calculations. At the end of this Chapter, several DFT-based packages used in this dissertation are briefly introduced.In Chapter2, we briefly introduce graphene including its amazing electronic prop-erties and the latest progresses. The motivations of this dissertation are shortly presented at the end of this chapter.In Chapter3, we explore the periodically modulated electronic properties of the epitaxial monolayer graphene (MG) on Ru(0001). Based on scanning tunneling mi-croscopy/spectroscopy (STM/STS) and first-principles calculations, we investigate the geometric and electronic properties of MG on Ru(0001) surface. Our theoretical results reveal that the periodic geometrical corrugation of MG originates from the strong chemi-cal bonding in combination with lattice mismatch between graphene and Ru(0001). The predicted values of local work function are close to the experimental results, and their spatial distribution are strong related to the presence of the periodic surface dipole mo-ment. These observations suggest that MG/Ru(0001) can be used as an ideal template for periodic nanostructures with various applications.In Chapter4, we demonstrate that the tunable molecule-substrate interaction offer-s’ possibility to realize a single cobalt phthalocyanine (CoPc) rectifier. When a MG is intercalated between CoPc and Ru(0001) substrate, CoPc molecule show a prominent rectifying effect. First-principles calculations clearly show that CoPc molecule couples with MG/Ru(0001) substrate mainly through the dz2orbital of Co atom, and the tun-neling between CoPc molecule and Ru(0001) substrate is mainly intermediated by the Co-dz2orbital. The resonant tunneling through this orbital at-0.35eV gives rise to the abrupt current enhancement and hence the rectifying effect. The simulated Ⅰ-Ⅴ curves using Tersoff-Hamann approximation reproduce the main feature of experimental mea-surements.In Chapter5, we focus on tuning the magnetism of defective graphene including graphene with single atomic vacancy (GSV) and hydrogen-absorbed GSV (H-GSV) by performing extensive spin-polarized DFT calculations. Our theoretical results show that carrier (hole and electron) doping can effectively tune the magnetic properties of GSV. The hole and electron doping effect on magnetic coupling is distinct different. The hole doping can obviously enhance the magnetic coupling of the GSV system, the magnetic coupling is depressed for the cases with electron doping. At the same time, we find the applied tensile strain on GSV can significantly enhance its magnetism. As for H-GSV systems, the electron and hole doping can effectively reduce the magnetic moments and couplings. Through analysing the calculated electronic structures, we find that the magnetism tunable mechanism is strong related to the localized sp2and quasi-localized pz-derived states around the Fermi level. These theoretical findings provide practical ways and useful insights to tune the magnetism of defective graphene.In Chapter6, the electronic properties ofhydrogenated Ge sheets (GeH) under me-chanical strains are explored by using DFT calculations with HES06hybrid functionals. We find that the energy gap of GeH can be effectively tuned by the applied strain. With increasing of compression strain, the semiconducting GeH sheet with direct band gap will change to the semiconductor with indirect band gap. While the GeH with direct band gap can be tuned to be metallic by increasing of tensile strain to be about10.0%. In addition, the optical absorption properties of GeH sheet can be modulated by the applied strain.