First-principles study of charge transport in molecular wires and field effect devices

In this work, we present theoretical analysis of charge transport at the molecular scale. We use a state-of-the-art theoretical tool to investigate a number of key issues of molecular electronics, paying particular attention to quantitative comparison with experimental data which have been confirmed by different labs. Our analysis allows us not only to understand the data, but also to make quantitative predictions.; We have calculated the length dependence of resistance for molecular wires, including oligophenylene thiol and alkanethiol molecules. Our results are in excellent agreement with the corresponding measured data. Our analysis provides a good understanding of charge conduction mechanism in these molecular wires. This is the first time in molecular conduction research that a parameter-free modeling agrees so well with real data. We have also studied the conformational dependence of current of a biphenyldithiol molecule in terms of the dihedral angle variations. The charge current can be tuned by this parameter, and the ratio of tuning can be as large as several hundred fold. A physical picture emerges from our analysis.; We have investigated the momentum filtering effect due to molecule orbitals. This study allows us to understand why some incoming Bloch states can conduct through the molecule, while others cannot. By adding different end-groups to the molecule, we found that conduction channels can be varied. We have also studied the gate potential control of electric current. The gate voltage shifts the resonance state of the molecule thereby inducing a current modulation. We found that the gating efficiency strongly depends on the geometry of the gate electrode. The current through a biphenyl dimethanethiol molecule is found to be switchable by applying gate voltages, and the on/off current ratio can be substantial.; Finally, using carbon nanotubes with substitutional nitrogen, we clearly demonstrate that conventional equilibrium conductance analysis was not enough to describe the whole transport features in molecular devices. A nitrogen doped zigzag nanotube showed that even a single atom substitution has increased the current flow and, for small radii tubes, narrowed the current gap. Periodical substitution makes zigzag semiconducting tubes metallic, a prediction which has been confirmed by a subsequent experiment. For an armchair metallic nanotube, doping with a single impurity reduces current. The physics of these behaviours has been addressed.