| Hexagonal two-dimensional (2D) materials, such as graphene, boron nitride (BN) nanosheet and CBN hybrid structure, have promising applications in spin electronics, electron optics, gas separation, energy reservation and sensor due to their unique struc-tures and physical properties. Recently, the synthesis, characterization, and properties as well as manipulation of these novel2D materials have received much research atten-tion. In this dissertation for Ph. D degree, we investigate the electronic structures and magnetism of graphene nanoribbons (GNRs), BN nanotubes (BNNTs) and C4N3nan-otubes (CNNTs) by performing extensive first-principles calculations, and try to tune their properties by introducing line-defect, doping, edge modification and transverse electrical field. This dissertation contains the following chapters.In Chapter1, we briefly introduce density functional theory (DFT), including Born-Oppenheimer approximation, one-electron approximation, the first and second Hohenberg-Kohn theorem, Kohn-Sham equation, various functionals for exchange and correlation, as well as the recent progress of DFT. At lase, we also present introduce several popular DFT-based packages and their powerful functions.In Chapter2, we briefly review the progress of the hexagonal2D materials. After introducing the synthesis, characterization, and properties of atomically thin layers, i.e. graphene and BN nanosheet, we give a brief review on the assembled graphene/h-BN hybrid structures, and the corresponding challenges and promising applications.In Chapter3, we investigate the electronic, mechanical and magnetic properties of GNRs with extended line-defect. The calculated results show that the band gaps of the GNRs can be effectively tuned by line-defect, which depend on both their widths and the position of defect. The band-gap reduction is attributed to the defect-induced impurity states. In particular, the metallic line-defect embedded zigzag GNRs are ferromagnetic ground state, while those semiconducting ones are antiferromagnetic. Upon the line-defect embedded in armchair GNRs, the band gaps vary periodically with the increasing widths.In chapter4, we examine the stability, electronic structures and magnetic properties of single-walled C4N3nanotubes (S WCNNTs) by performing extensive spin-polarized density functional theory calculations and molecular dynamics simulations. Theoreti-cal results clearly reveal that all examined SWCNNTs are stable at room temperature, Armchair (n,n)(n=4-10), Zigzag (n,0)(n=7-10), and two Helical SWCNNTs have fer-romagnetic ground states. These Armchair SWNNTs are metal-free half-metals, Heli-cal SWCNNTs are bipolar magnetic semiconductors (BMSs), while Zigzag SWCNNTs show size dependency, in which the semiconductor-to-metal transition can be realized by increasing of its radius. Moreover, the total magnetic moments of SWCNNTs can be tuned by changing the number of primary C4N3unit cells.In chapter5, we explore the electronic and magnetic properties of graphene flake-doped single-walled boron nitride nanotubes (BNNTs) based on extensive first-principles calculations. We find that the band structures of these doped BNNTs can be effectively engineered by altering the shape and size of the graphene flakes. Moreover, the Lieb theorem still works and the BNNTs with embedded triangle graphene flakes are ferro-magnetic due to the full spin polarization. All BNNTs embedded with the triangular graphene flakes with relatively small sizes are typical BMSs, which can be easily tuned into half-metals by carrier doping, opening the door to their promising applications in spintronic devices.In chapter6, we investigate the electronic and magnetic properties of the line-defect embedded Zigzag GNRs with asymmetric edges with H atoms. We find that the line-defect embedded zigzag GNRs are typical BMSs, and their electronic and magnetic properties obviously depend on the position of the line-defect. When the line-defect approaches the central region, the metallic property with the Dirac point feature will be tuned into the half-metallicity. Moreover, the energy differences between the anti-ferromagnetic and the ferromagnetic states as well as their energy gaps decrease with increasing of the width of nanoribbons.In chapter7, we examine the BN nanoribbons embedded with triangular graphene flakes (CBNNRs) under transverse electrical fields. We predict that these CBNNRs are BMSs. The transverse electrical field could engine the CBNNRs from BMSs to the spin gapless semiconductor (SGS) as well as the half-metal. The gap of Zigzag CBNNRs exhibit a linear response to the electric field due to the first-order Stark effect, while for these armchair CBNNRs, band gap varies quadratically because of the second-order Stark effect. Moreover, the Lieb theorem still works, and the magnetic moments of these CBNNRs are determined by the size of the embedded triangular graphene falkes. Keywords:first-principles, grapahene nanoribbon, BN nanotube, C4N3nanotube, elec-tronic structure, magnetism, doping, line-defect, bipolar magnetic semiconductor, half-metallicity... |
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