Theoretical Study On Electron Transport Properties Of Low Dimensional Nanometer System

Miniaturization of traditional electronic devices has aroused the extensive attentions to the research of nano-and molecular devices. There are several researches in both experiment and theory. Many low dimensional nanosystems, e.g., organic molecule, nanotube, fullerene, graphene, single-molecule magnet, etc., can show some functional characteristics owned by the micro-scale electronic devices, such as field effect transistor, molecular switch, negative differentive resistance, spin filter and spin valve, etc. In this dissertation, by using nonequilibrium Green’s function method combined with density functional theory, we have studied the electronic transport characteristics of some low dimensional nanosystems, including the organic conjugated molecules, single-molecule magnet, and graphene nanoribbons with defect, edge protrusions and doping. It mainly consists of the following several parts.In Chapter Ⅰ, we briefly introduce the research background of nano-and molecular electronics. After a brief review of the birth of nano-and molecular electronics and the recent research on it, the thesis focuses on introducing several experimental techniques which promote the rapid development of nano-and molecular electronics and the relating researches. Then, we introduce the research progress in theoretical calculation and simulation. Last, we give the major research content in this dissertation.In Chapter Ⅱ, we introduce the theroretical method used in this dissertation. First, we introduce the electronic transport picture based on the Landauer-Biittiker formalism when investigating the quantum transport problems. Then, we review the nonequilibrium Green’s function method which is used to calculate the electronic transport, and density functional theory which is adopted to calculate the geometric and electronic structures of nano-and molecular systems. At the last, we introduce the first-principles technique of investigating the electronic transport, namely, combining the nonequilibrium Green’s function method with density functional theory.In Chapter Ⅲ, we investigate the electronic transport characteristic of a series of organic conjugated molecules comprised of porphyrin and benzene. It has been discussed how to tune the electronic transport by changing the intramolecular conformation and adding certain functional groups. The results demonstrate that the circuit can change from "ON" to "OFF" state when tuning the intramolecular conformation from coplanar to perpendicular conformation with high ON/OFF ratio, which can be enhanced by adding a substituent amino or nitro. It can show more interesting switching effect when changing the intramolecular conformation of the complexe comprised by porphyrin and two benzenes. It is suggested that such molecular wires, comprised by π conjugated molecule, usually can present the switching effect, and its ON/OFF ratio can be tuned by adding certain substituent groups.In Chapter Ⅳ, we investigate the spin-dependent transport properties of a single-molecule magnet Mn(dmit)2. We have discussed the electronic structures, spin polarization of current, and spin valve characteristics of its coplanar and perpendicular conformations. The current flowing through its coplanar conformation is high spin-polarized, up to a high efficiency of82%. The current is strongly suppressed when a ligand is rotated and perpendicular to the other. These results suggest that Mn(dmit)2is a potential candidate for spin filters or molecular switches.In Chapter Ⅴ, we investigate the electronic transport properties of zigzag graphene nanoribbons (GNRs) with two kinds of triangular defects. The results demonstrate that the current of the GNR with an upward-triangle defect can be larger than that of the perfect GNR due to the defect-induced symmetry breaking and more conductive channels. Dissimilarly, if the orientation of the triangle is changed rightward, the current is depressed much and shows negative differential resistance behavior. Our findings indicate that defect designs can be an efficient way to tune the electronic transport of GNR nanodevices, and we suggest such ZGNRs with right-triangle defect can be the candiates of negative differentive resistance devices.In Chapter Ⅵ, we investigate the electronic transport properties of ZGNRs with triangle protrusions at the edges. The results demonstrate the protrusion generally breaks down the edge state along the same edge, and then tunes the electronic transport. For the graphene nanoribbons having even number of zigzag chains, however, the protrusions can increase or decrease significantly the conductance with different relative position of the two protrusions, accompanied by negative differential resistance characteristics. The abnormal increase of the conductance is ascribed to the forming of a new Z-like conducting pathway as well as the ruining of the mirror symmetry of the ribbons. In terms of odd ZGNRs, the introduction of edge protrusions only suppresses current flow and linear Ⅰ-Ⅴ curves are achieved. These edge-modified ways make the graphene-based nanomaterials present more abundant electronic transport phenomena and can be useful for the design of future nanoelectronic devices.In Chapter VII, we investigate the electronic transport properties of several ZGNRs doped by B (or N) atoms orderly. The results indicate that B (or N) atoms doping can mostly increase the conductance of the graphene nanoribbons due to the B (or N) induced conductance channels. The higher the doping concentration, the current amplification factor is larger. For the nanojunctions with one row B (or N) atoms, the current amplification factor can be larger when the doping position is near to the center. The current-voltage curves show the negative differential resistive phenomenon for the case of B doping with low concentration and the case for N doping. More interestingly, the B or N doping can almost completely smear the even-odd effect on electronic transport of the ZGNRs. Our studies provide avenues to drastically improve the electronic transport of ZGNRs, helpful for the graphene applications.In Chapter Ⅷ, we give a brief summary and outlook of the dissertation.