Theoretical Studies On Conductance And Rectifying Behavior Of Triangular Graphene Nanoflake And Aromatic Molecules

With the development of molecular electronics and continuous minimization of electronic devices, a developing trend is to construct functional devices using of single molecule or molecular clusters instead of silicon-based semiconductor materials. Among them, molecular rectifiers, as one of the most important components in the logic circuit, receive extensive attention of the research workers. The studies in the aspect of theory and experiments have made gratifying progress in recent years. In this thesis, on the basis of nonequilibrium Green's function method combined with the density functional theory, we theoretically design a series of molecular devices that possess high conductance and rectification performances. And the asymmetric electrodes effects, asymmetric coupling groups effects as well as molecular-electrode contact configurations effects on the transport performances of molecular devices are systematically investigated.1. Theoretial studies on the conductance and rectification performances of triangular graphene nanoflakeOne kind of carbon-based molecular rectifiers, a zigzag-edged trigonal graphene nanoflake (ZTGNF) is sandwiched between two asymmetric zigzag graphene nanoribbon (zGNR) and armchair graphene nanoribbon (aGNR) electrodes by carbon atomic chains, is constructed. Here, the effects of the width of the aGNR electrode and the number of anchoring carbon atomic chains (CACs) on the rectifying performance are studied. The theoretical calculation results show these two factors' influence. Especially, the molecular device displays giant rectification ratios in the order of 104 when two CACs are used as the anchoring group between the ZTGNF and the right aGNR electrode, which has significant improvement as compared with previously investigated results for such a molecule coupled to bulk metal electrodes.2. The effects of the aromaticity of molecules on their conductivityHere we construct two series of single-molecule junctions with one of cyclopentadiene, furan and thiophene by varying molecule-electrode contact configurations, and then calculate their currents using nonequilibrium Green's function method combined with the density functional theory. The contact configurations between molecule and electrodes are modeled and focused on. It is demonstrated that the conductance of such three molecular wires is negatively associated with their aromaticity for both molecule-electrode contact configurations considered, which agrees with the Chen's experimental findings. And it is revealed that this is due to variation of alignments between the frontier molecular orbital and Fermi energy of electrodes due to different aromaticity of the molecule.This thesis consists of five chapters. In the first chapter, the background and recent developments of molecular electronics, the molecular rectifiers as well as on the carbon-based molecular diode are introduced. The density functional theory which is widely used in the calculation of the electronic structures in multi-body systems and the nonequilibrium Green's function method which is used to calculate the transport properties of molecular devices is presented in the second chapter. And the self-consistently calculations and corresponding results obtained by applying the above methods are also presented in this chapter. In the next two chapters, we present our main theoretical results based on the above theory. The conductance and rectification performances of triangular graphene nanoflake are analyzed in the third chapter. The influences of the width of the electrode and number of the coupling groups are emphatically studied. The fourth chapter gives the effects of the aromaticity of molecules on their conductivity under different molecular-electrode contact configurations. In the end, the fifth chapter draws a conclusion for the whole work and views the future development of the molecular electronics.