Electron transport across molecular junctions: Effect of junction geometry and molecule functionality

Molecular electronic devices provide an alternative to the current semiconductor based electronic devices enabling a higher device density on integrated circuits. Despite extensive experimental characterization studies, the relationship between the junction structure and the electron transport mechanisms is still not clear. The aim of this dissertation was therefore, to examine the effects of variations in the design parameters and chemical functionality on the device electrical characteristics by using density functional theory and non-equilibrium Green's function techniques.; Electronic structure methods probe the interfacial electronic structure and associated electron transport mechanism and can therefore, aid in guiding device design. The intermolecular interactions and orientations of the adsorbed molecular junctions vary with changes in structural parameters such as surface coverage and electrode spacing. These changes lead to shifting of the molecular energy states with respect to the electrode Fermi levels resulting in current flow variations. The electron transport within Au(111)-octane dithiolate-Au(111) junction occurs via conduction through occupied molecular energy states for low molecular coverage and moderate electrode spacing. The electron transport mechanisms also depend on the chemical functionality included in the molecular junction. The conduction through tunable molecules containing delocalized stilbene substituted octahydrosilsesquioxane (POSS) core is examined for potential application as rectifying devices. Both occupied and unoccupied hybrid molecular energy states emerge for Au(111)-stilbene substituted POSS-Au(111) system, which are accessible at biases equal or higher than the range of 2.5 to 3 V. Thus, the electron transport characteristics are sensitive to the molecular configuration and position of substitution, which can be altered depending on the electrical behavior required. Devices exhibiting negative differential resistance (NDR) are employed for memory storage applications. A ruthenium metal ion attached to delocalized terpyridyl ligands and bonded to Pt(111) electrodes demonstrates NDR behavior with a peak to valley ratio in the range 8.5 to 9.5. The electrons are transmitted through unoccupied and delocalized molecular states close to the metal Fermi level such that under high enough bias, these molecular states are inaccessible due to coupling changes within the junction.; In conclusion, the electrical characteristics for a molecular device are dictated by the chemical functionality of the molecule bonded to the metal electrodes and the substituents attached. The electron flow can be further modified by variations in the device structure parameters and the molecular configurations, which realign the molecular energy states with respect to the reference metal Fermi levels.