The Influence On Electronic Transport Properties Of Graphene Spirals Via Topological Manipulation

The successful discovery and fabrication of the lower-dimension materials intensely promotes the achievements and development of the miniaturization,integration and variation of electronic devices. Due to the limit of the dimension, lots of unusual quantum characters make lower-dimension materials different from the macroscopic three-dimension systems obviously, thus presenting some unexpected properties in physics and chemistry and laying the foundation for the development of the multi-functional quantized electronic devices. As a significant member of the lower-dimension materials, Graphene possesses a perfect lattice structure and excellent electronic characters, which is expected to replace the traditional silicon-based devices extremely. Utilizing various approaches to design the geometric structures of graphene can realize the controllable modulation of the electronic structure. Recently, graphene spirals(GS), another formation of graphene, is a π-electron structure with spiral curved surface and attracts enormous attention to investigate its electronic structures and the potential for electronics and spintronics. Because not only intra-layer interaction similar to monolayer graphene but also inter-layer interaction similar to graphite have been revealed in GSs. The unique geometric structure and electronic properties of GS provides more possibilities to fabricate and design multi-functional electronic devices. It is meaningful to investigate the physical mechanisms of the electron transport behaviors under various conditions in GS.In this thesis, with periphery doping, axial stress and periphery isomerism effects,the variation of the electronic structures and the transport properties is investigated in detail. Furthermore, the transport mechanisms of graphene as electronics and spintronics are expounded clearly and the influence of the inter-layer interaction on the electronic properties of one-dimension GSs is also analyzed. The detail contents can be divided into the following parts.Based on density functional theory in combination unequilibrium Green’s function technique, the electronic transport properties of GS as molecular electronics are studied with sulfurs periphery doping. Through the anchoring groups, GS is coupled to the gold electrodes, establishing the heterogeneous molecular junction devices. Researches show that the tunneling current flowing through the origin GS heterojunction is primarily contributed by the carriers inter-layer tunneling pattern under applied biases, and there does not appear spiral current along the spiral structure. With the sulfurs doping, the spiral path is opened for carriers tunneling along the outer or inner edge of the spiral heterojunctions, thus inducing the appearance of the spiral current. Simultaneously, thesulfurs doping can enhance the strength of the total tunneling current under lager applied biases. In addition, the I-V curves show an interesting nonlinear property of the negative differential resistance, NDR. The analysis above provides the theoretical prediction for GS heterojunction devices.On the other hand, GS can be utilized to design the spatial spintronics as well. By introducing carbon-hexagon at the periphery of GS to establish triangular structure,various spin-polarized edge states can be generated in isomerous GSs. Under applied biases, the spin-polarized transport character of the original GS is mainly contributed by the edge states of ZGNRs. The spin-polarized tunneling current is still contributed by the inter-layer tunneling pattern and tunneling strength is small relatively. When the triangular structure is introduced at periphery, the spin-polarized transport properties of GSs are enhanced obviously. It is observed that the triangular structures drive the spin-polarized carriers scattering to the central spiral region. For some isomerous GSs,the spiral current can be generated and transport along periphery structures. Additionally,the spin-polarized transport properties of isomerous GSs depend on the numbers and relative positions of the introduced carbon-hexagons.With axial stress and periphery isomerism effects, the infinite one-dimension GSs can achieve the transition from semi-conductor to metallicity. Generally, under equilibrium state, the GSs with spin-polarized edge structures can not present magnetic features, because the inter-layer interaction inhibits the generation of the spin-polarized phenomenon. With the axial stress increase, the inter-layer interaction will be minished gradually, thus inducing the spin-polarized to occur finally, where the phase transition is from non-spin-polarized metallicity to spin-polarized metallicity, and further to spin-polarized semi-conductor. Furthermore, axial stress can induce a certain isomerous GSs converting from indirect-gap semi-conductor to direct-gap semi-conductor.Additionally, the GSs with time-reversal symmetry show Rashba spin-orbital splitting effect at the(38) point of Brillouin zone.