First-principles Study On The Magnetism Of Graphene-like ⅢA-nitrides Structures

In recent years, as isostructure analogs of graphene, the low-dimensional nanostructures of ⅢA-nitrides have attracted extensive attentions due to the on-going researches on graphene. Although the geometry structures of low-dimensional ⅢAnitrides are very similar to that of graphene, their electronic properties are distinctly different. For instance, the band gap of pristine graphene is closed, however, the BN nanoribbons are wide band gap semiconductors. Impurity doping or atomic adsorption has been proved to be a simple and effective way to modify most of the low-dimensional materials, especially for metal atoms, some even can induce ferromagnetism. Using first-principles calculations in the framework of density functional theory, we have studied the effects of metal atoms and adsorption on the electronic structures and magnetic properties of BN, Ga N, Al N and In N low-dimensional nanostructures, and devote to research half-metallicity and magnetism in these systems. The main results of this thesis are listed below.(1) A detailed study on the electronic structures and magnetism of two-dimensional ⅢA-nitride(Al N, Ga N and In N) monolayer doped with Ag atoms. The results show that the ⅢA-nitride binary compounds doped with small concentrations of Ag atoms, all are spin-polarized and obtain 2.0 Bohr magnetons. Single layered Al N and Ga N doped with Ag atoms both demonstrate two possible ground states: ferromagnetism and antiferromagnetism. However, In N monolayer doped with Ag atoms only has ferromagnetic ground state with half-metallicity. Different doping concentration of Ag atoms result in various half-metal band-gaps. With the concentration of Ag atoms increases, the energy gap of corresponding system decreases. Above results show a rich variety of electronic and magnetic properties of ⅢA-nitride binary compounds doped with Ag, and on certain concentrations, room-temperature ferromagnetism is found in these systems, which provide a simple and effective method to design spin switching devices.(2) We investigate the electronic and magnetic properties of pristine and hydrogen-terminated zigzag Ga N nanoribbons(ZGa NNRs). When the nitrogen edge of the ZGa NNRs is passivated, regardless of the gallium edge, the ZGa NNRs are wide band-gap semiconductors. However, when the nitrogen edge is unpassivated, the ZGa NNRs have 100% spin polarization around the Fermi level and become halfmetals. It is the strong interaction between the N-2p orbitals and Ga-4p orbitals that leads to the half-metallic ferromagnetism. What’s more, with the ribbons width increases, the half-metallic gap of only gallium edge hydrogenated ZGa NNRs decreases monotonously in a wide range. The tunability of half-metallic gap for ZGa NNRs can be applied to electronic and spintronic devices with wide or specific energy gaps.(3) First-principles calculations have been used to research the electronic structure and magnetic properties of zigzag boron nitride nanoribbons(ZBNNRs) terminated/jointed by armchair dimer-Fe chains(respectively called Fe-terminated ZBNNRs and Fe-jointed ZBNNRs). The Fe-terminated ZBNNRs is a semiconductor for different ribbon widths, and the Fe-jointed ZBNNRs become half-metallic regardless of the ribbon width. The magnetism of both structures mainly stems from the Fe atoms. It is found that the self-metallicity of the Fe-jointed ZBNNRs results from the strong interaction between the 3d orbitals of Fe atoms and the 2p orbitals of N atoms. The stability of the Fe-jointed ZBNNRs under room temperature has been confirmed by molecular dynamics simulation. This kind of half-metal property means a selectivity for the two different electrons, it can be applied to spintronics devices. Other transition-metal jointed ZBNNRs are also studied, which can be metals, half-metals or semiconductors with different ground states.(4)We investigate the electronic structures and magnetic properties of Fe chain-embedded zigzag boron nitride nanoribbons(ZBNNRs) with different dimers(B2, N2, C2) in pentagon–octagon–pentagon line defects. The calculations show that Fe atoms spontaneously embed into the center of octagonal rings and form an atomic chain along the line defects. The ferromagnetic states are their ground state. The hydrogen-passivated systems with B2 or N2 dimers are semiconductors with small band gaps, while the C2 dimer result in half-metallic behavior. The strong interaction between the Fe-3d orbitals and the C-2p orbitals turns the ZBNNRs into half-metal from semiconductor. The half metallic ferromagnetism are also found in other transition-metals embedded ZBNNRs, depending on the types of metals and line defects. Our results provide a means to significantly reduce the band gap of ZBNNRs, and the half-metallic room-temperature ferromagnetism can be applied to devise spintronics devices.