Molecular Alligator Clips: A Study of Metal-Molecule Contacts for Molecular Electronics

Doctoral thesis, 2004

In this thesis we present a computational study of the elementary building blocks for molecular electronics, interconnected electronic devices with critical dimensions below 5 nm. Building electronic circuits on that scale requires stable connection between the various parts of the device and to the macroscopic metal electrodes, needed for power supply, support and integration with conventional electronics. Such a connection can be realized through conducting organic molecules, so-called molecular wires. We investigate the interface between sulphur-terminated molecules and gold crystalline electrodes, known as molecular alligator clips. We start with the elementary process of sulphur (and selenium) adsorption on Au(111) surface and hydrogenation of the sulphur film. We have performed ab initio computer simulations within the density functional theory (DFT) framework, focusing on coverage dependence of structure and energetics of adsorption. Transport properties and structure of a family of short sulphur-terminated conducting molecules have been studied with nonequilibrium Green’s functions technique, based on DFT description of the atomic system, including the adsorbed molecules and metal surfaces. We concentrate on simultaneously accurate descriptions of the structure and transport properties, both performed on the same level of theory. Dependence of the transport properties on the molecule and adsorption site, including adsorption near surface defects is studied. We find a strong influence of back-bond on the magnitude of the zero bias transmission. Surface defects can significantly affect transport properties of the molecule, however for stable configurations on the flat Au(111) surface the influence of the adsorption site is negligible, at least at low bias voltages. The latter observation is especially important as it allows considerable simplification of the computational description of the molecular adsorption, when its transport properties are of prime interest. The dependence of transport properties on the molecular length has been studied for dithiolated phenylene vinylene molecules. We find a systematic decrease of conductivity due to combined effect of decreasing hybridization of molecular orbitals and increase of interelectrode distance. We additionally report studies of the asymmetric resonance structure in transmission and the study on functionalization of the molecular wire, an attempt to design a molecular switch.