First - Principles Study On Electronic Structure And Magnetoelectric Effect Of Graphene - Like System

Graphene has aroused great interest since its successful isolation in experiments, because of its excellent physical and chemical properties, such as Dirac fermion carriers, very high carriers mobility at room temperature, very high thermal conductivity and Hall effects at room temperature, etc. Nowadays, graphene nanoribbons (GNRs) with width varying from several tens to few nanometers were achieved in experiments. Moreover, room temperature transistors have been built from the ribbons with very narrow widths. Many interesting behaviors have been achieved in zigzag GNRs, such as energy band-gap engineering, spin gapless semiconductors, half metals, etc. Compared to zigzag GNRs, the studies of armchair GNRs are quite few, which are usually non-magnetic semiconductors due to the lack of edge states located around the Fermi level. They are, however, energetically and thermodynamically more stable than zigzag GNRs. Therefore, exploring possible magnetism in armchair GNRs and related nanoribbons is full of importance.Inspired by the d0magnetism discovered in graphene, researchers examined the electronic states in other low-dimensional systems, such as BN and BCN-hybrid systems, whose single-atom layers or nanotube structures have already been synthesized in experiments. For zigzag BN nanoribbons, the theoretical investigation indicated that when the B edge, but not the N edge, was passivated, the ribbon showed a giant spin splitting. When B, C, and N atoms are mixed in certain percentages, the BCN-hybrid systems can be fabricated. Different from armchair graphene nanoribbons, armchair BCN-hybrid nanoribbons are found to present magnetism along the edges of the nanoribbons if B and N atoms are unpaired in the nanoribbons. The spin polarization in these armchair nanoribbons is ascribed to the appearance of the unsaturated electronic states in the systems. Intriguing spin-polarized bands, including magnetic semiconductors, half metals, and magnetic metals, are obtained in the armchair nanoribbons with both the edges composed of C and N atoms. Such kind of magnetism can be called d0-magnetism or sp-magnetism due to no traditional magnetic metal ions contained in the ribbons. The BCN system exhibit d0magnetic properties even in the absence of the doping of magnetic atoms, moreover, the system exhibit a variety of excellent electronic properties, for instance, the spin gapless semiconductor, half-metal, magnetic semiconductors, according to whether the B atoms and N atoms match each other. By applying an electric field vertical nanoribbons, the spin gapless semiconductor and half-metal can be converted each other. It provides a new scheme to carry out potential applications in future spintronics.The transition-metal dichalcogenide semiconductor MoS2has attracted great interest because of its distinctive electronic, optical, and catalytic properties, as well as its importance for dry lubrication. The bulk MoS2crystal, an indirect-gap semiconductor, is built up of van der Waals bonded S-Mo-S units. Each of these stable units (referred to as a MoS2monolayer) consists of two hexagonal planes of S atoms and an intermediate hexagonal plane of Mo atoms coordinated through ionic-covalent interactions with the S atoms in a trigonal prismatic arrangement. Because of the relatively weak interactions between these layers and the strong intralayer interactions, the formation of ultrathin crystals of MoS2by the micromechanical cleavage technique is possible。In Chapter1of this thesis, the lattice structures, electronic structures, and syntheis methods of graphene and GNRs are first briefly introduced. Then, some background of the BCN-hybrid single layers and BCN-hybrid nanoribbons are presented. Finally, we introduced the background of magnetoelectric effect in two-dimensional and one-dimensional systems.In Chapter2, as the theory foundation of the first-principles calculations, density functional theory (DFT) is presented. The pseudopotential method is also referred to simplify the DFT calculations. The Van der Waals correlation energy is introduced, which can improve the tranditional DFT results for the systems with long-range interactions. In Chapter3, we study magnetism and electronic structures of armchair BCN-hybrid nanoribbons based on density functional theory. Different from armchair grapheme nanoribbons, armchair BCN-hybrid nanoribbons are found to present magnetism along the edges of the nanoribbons if B and N atoms are unpaired in the nanoribbons. Intriguing spin-polarized bands, including magnetic semiconductors, half metals, and magnetic metals, are obtained in the armchair nanoribbons with both the edges composed of C and N atoms. The spin polarization in these armchair nanoribbons is ascribed to the appearance of the unsaturated electronic states in the systems. The magnetic metallicity can be tuned further to half metallicity by adsorbing O atoms at appropriate positions in the ribbons. The electronic structures of the nanoribbons without spin polarization are also analyzed. Our studies provide understanding of the magnetism mechanisms and the electronic properties and most importantly, how to achieve half metallicity in low-dimensional BCN-hybrid systems.In Chapter4, magnetism and magnetoelectric effect of armchair BC4N nanoribbons are explored from densityfunctional theory.100%spin polarization presents in the N-rich nanoribbons due to sigmadangling bond states induced by the unpaired N atoms. In the nanoribbons with "paired" B and N atoms, however, striking magnetism is also found to survive, rationalized by a unique zigzag-like pairing pattern. The ribbons can present remarkable edge magnetoelectric effects, for which the edge magnetoelectric coefficient is defined and calculated. The obtained d0magnetism and the large magnetoelectric effect are helpful to explore possible spintronic applications in BCN.In Chapter5, bulk MoS2, a prototypical layered transition-metal dichalcogenide,are explored from densityfunctional theory. It is an indirect band gap semiconductor similar to previously reported. Reducing its slab thickness to a monolayer, MoS2undergoes a transition to the direct band semiconductor. We further studied the impact of stress on the electronic properties of MoS2. When the stress reaches a certain degree in the c direction, MoS2changes to metal from the semiconductor. In Chapter6, a brief summary of the thesis is presented.。...