The Simulation And Physical Properties Of Novel Device Based On Graphene Like Materials

Graphene was mechanically exfoliated in 2004 by Geim, which has remarkable mechanical, electrical, and thermal properties. The 2D graphene and the Q1 D graphene nanoribbons have been applied in designing nanoelectronic and spintronics devices for their special electronic transport properties such as the high mobility and the magnetic edge effects. Field effect transistors based on graphene nanoribbons have already been fabricated. With the advances in experiments, carbon atomic chains have been successfully obtained from graphene using a high energy electron beam by several groups. Carbon atomic chains are regarded as extremely narrow GNRs, which can be used as transport channels or as onchip interconnects for molecular electronic or spintronic nanodevices. Different geometries connecting carbon chains to graphene nanoribbons show distinguished physical characteristics. In this thesis, we have investigated the effect of connecting geometry on electron transport through graphene nanoribbons connected by carbon chains. Since doping is an important method functionalizing semiconductor materials, we have further studied the electron transport on edge-doped graphene nanoribbons connected by carbon chains. Silicene is another novel two-dimensional honeycomb structure material formed by silicon atoms. Compared with the planar graphene, silicene has a low-buckled honeycomb structure and is expected more compatible with the traditional semiconductor industry. In this thesis, we predicted that doped zigzag silicene nanoribbons can be half-metallic. The main aspects concerned in this thesis include:(1) The electron transport between two zigzag graphene nanoribbons(ZGNRs) connected by carbon atomic chains has been investigated by the non-equilibrium Green’s function method combined with the density functional theory(DFT). The symmetry of the orbitals in the carbon chain selects critically the modes and energies of the transporting electrons. The electron transport near the Fermi energy can be well-manipulated by the position and the number of carbon chains contacting the nanoribbons. In symmetric ZGNRs connected by a central carbon chain, a square conductance step appears at the Fermi energy because the antisymmetric modes below it are not allowed to go through the chain. If a carbon chain connected at the side of ZGNRs, there are conductance peaks both below and above the Fermi energy. These modes can additionally contribute to the conductance, due to the asymmetry of wavefunction. By choosing a proper geometry configuration, we can realize Ohmic contact, current stabilizer, or the negative differential resistance phenomenon in the devices.(2) We demonstrate that giant current and high spin rectification ratios can be achieved in atomic carbon chain devices connected between two symmetric ferromagnetic zigzag graphene nanoribbon electrodes. The spin dependent transport simulation is carried out by the density functional theory combined with the non-equilibrium Green’s function method. It is found that the transversal symmetries of the electronic wave functions in the nanoribbons and the carbon chain are critical to the spin transport modes. In the anti-parallel magnetization configuration of two electrodes, the spin-up(down) current is prohibited under the positive(negative) voltage bias, which results in a spin rectification ratio of order 104. While, to the parallel configuration, pure spin current is observed in both linear and nonlinear regions. When edge carbon atoms are substituted with boron atoms to suppress the edge magnetization in one of the electrodes, we obtain a diode with current rectification ratio over 106.(3) The spin-dependent electronic structures of aluminum-(Al) doped zigzag silicene nanoribbons(ZSi NRs) are simulated by density functional theory calculations. When ZSi NRs are substitutionally doped by a single Al atom on different sites in every three primitive cells, they become half-metallic in some cases, a property that can be used in spintronic devices. More interestingly, spin-down electrons can be transported at the Fermi energy when the Al atom is placed on the sub-edge site. In contrast, spin-up electrons can be transported at the Fermi energy when the ZSi NRs are doped on sites near their center. The magnetic moment on edge is considerably suppressed if the Al atom is doped on edge or near-edge sites. When two or more Si atoms are replaced by Al atoms, in general the halfmetallic behavior is replaced by a metallic, spin gapless semiconducting or semiconducting one. When a line of six Si atoms, along the ribbon’s width, are replaced by Al atoms, the spin resolution of the band structure is suppressed and the system becomes nonmagnetic.