Abstract & Overview
One of the central problems of molecular electronics is to understand electron conduction properties when a functional molecule is interfaced with external electrodes and put under external bias and gate potentials. These properties are influenced by the molecule-electrode interaction as well as by the structure of the functional region of the device. In this thesis, we investigate from first-principles the transport properties of a number of molecular-scale systems, and try to relate the observed features to both the atomic and electronic structure. We start with a detailed analysis of transport through carbon atomic wires, and find that the equilibrium conductance is sensitive to charge transfer doping, and that the I-V characteristics exhibit negative differential resistance at high bias due to a shift of conduction channels relative to the states of the electrodes. Using a Sc3N C80 metallofullerene device, we address several general questions about quantum transport through molecular systems and provide strong evidence that transport in such molecular devices is mediated by molecular electronic states which have been renormalized by the device environment. The possibility of inducing nuclear dynamics in single-molecule Au-C 60-Au transistors via inelastic, resonance-mediated tunneling current is examined using a method based on the combination of a theory of current-triggered dynamics[1] and our nonequilibrium Green's function approach of computing electron transport properties. We investigate several single molecule field-effect transistors consisting of conjugated molecules in contact with metallic electrodes. The source-drain current is found to be sensitive to the external gate potential and the molecular structure; with modulations of the current as large as several thousand fold. Given a proposed operation principle, we obtain quantitative results on the rectification properties for an organic molecule rectifying diode. The I-V characteristic shows clear rectification behavior, and is explained from the simple picture of shifting of molecular levels due to substituents and an externally applied bias voltage. Finally, we report a formulation combining density functional theory with the Keldysh nonequilibrium Green's function, for calculating quantum mechanical forces under external bias and during electron transport. We present an example force calculation consisting of a single atom point contact.