Molecular-scale devices from first principles

摘要: Electronic structure calculations are becoming more widely applied to complex and realistic materials systems and devices, reaching well into the domain of nanotechnology, with applications that include metal-molecule junctions, carbon-nanotube field effect transistors, and nanostructured metals or semiconductors. For such complex systems, characterizing the properties of the elementary building blocks becomes of fundamental importance. In this thesis we employ first-principles calculations based on density-functional theory (DFT) to investigate fundamental properties of molecular-scale devices. We focus initially on the constituent components of these devices (polymers, metal surfaces, carbon nanotubes), following with studies of entire device geometries (nanotube/metal interfaces). We first study a proposed molecular actuating system in which the interaction between oligothiophenes is the driving force behind an electromechanical response. The oligothiophenes are attracted to each other through p-stacking interactions driven by redox reactions. We show that counterions strengthen this interaction further through enhanced screening of the electrostatic repulsion. Many molecular scale devices require contact with a metallic conductor, we also study the fundamental properties of metal surfaces in the slab-supercell approximation; in particular layer relaxation, surface energy, work function, and the effect that slab thickness has on these properties. The surfaces of interest are the low index, (111), (100), and (110) surfaces of Al, Au, Pd, and Pt and the close packed (0001) surface of Ti.