Spin Transport Properties Of Graphene Nano-Structures

We study the spin transport properties of zigzag graphene nanoribbions (ZGNRs) by using density fuction theory (DFT) and nonequilibrum Green's technique, combining withLandau-B u ttiker approach. The influence of symmetric effect, spin polarized effect and edge doped effect to transmission and conductance are systematically investigated. And we have found some meaning fruits such as spin-polarized current, negative differential resistance (NDR).In this thesis, we firstly review the discovery, experimental preparation and novel electronic properties of graphene. Then we introduce the theories of first principle and nonequilibrum Green's technique. Based on this, the works during the master period are presented in two parts.(1) We investigate the electronic properties of zigzag graphene nanoribbions (ZGNRs). The results show that when spin polarization is considered, the nanoribbion present semiconductor behavior, which is different from the conductor behavior in spin-unpolarized system. It is consistent of the latest experimental reports. Then, we list the work of Joonho Park et al.. They investigated effects of boron (B) atom on the electrionc and spin transport properties of a graphene nanoribbon device. In the case of a center-located impurity, the new spin states near the Fermi level responsible for the spin-polarized current. Besides, we also investigate the electronic properties of Beryllium (Be) impurity edge doped ZGNRs. It is found the impurity atom can suppress the local magnetism and the electronic structure of perfect nanoribbion. Several new local states is formed, and the two spin components electrons present conductor and semi-conductor properties.(2) The spin dependant electronic transport is studied. Here, the symmetric and impurity doped are considered. The results show that the symmetric plays an important role in spin transport property of ZGNRs. The asymmetric ZGNR displays linear transport behavior. However, the symmetric ZGNR displays nonlinear transport behavior, when the bias is lower than 1.5 V, NDR behavior is found for one spin component. Besides, we investigated the impurity doped ZGNR. It is found the two spin components behave differently in its transmission property, especially near Fermi energy. By solving the energy spectrum of molecular self-consistent Hamiltonian (MPSH), we found the degeneracy between two spin components is broken, thus spin-polarized current will be generated under external bias. The impurity also modulate the NDR behavior. A physical analysis of these results are given.