Molecular Ensemble Junctions: a combined experimental & theoretical investigation

This thesis presents an investigation of the electrical properties of about 70 different molecules in molecular tunneling junctions (MTJs). It begins by presenting the field of molecular electronics (ME) -- from the advent of quantum mechanics to the first conception of ideas and experiments in ME. Then it discusses different steps in the fabrication of self-assembled monolayers (SAMs) and their incorporation in large-area MTJ, followed by data acquisition and analysis of current-voltage characteristics. Further, the theoretical methodology, which uses density functional theory in conjunction with non-equilibrium Green's function approach to simulate charge transport probability through MTJs, is reported. After the introduction of several concepts, an in-depth study of quantum interference (QI) effect on a series of molecular wires, comprising backbones with different conjugation patterns, is reported. Continuing with the QI studies, having established the effect of functional groups on QI, another approach to modulate the tunneling probabilities in MTJs by switching QI ON and OFF in one of the two parallel intramolecular pathways is presented. Next, the thesis discusses the non-linear tunneling current decay in sigma-pi molecular structures (oligothiophene-terminated butanethiols) with increasing molecular lengths, explained using a two-barrier model. Finally, the final chapter describes a generalized single-level model that compares the degree of electronic coupling between molecules and electrodes across 40 different large-area EGaIn-based MTJs from different laboratories. Thus, starting from the introduction of several ME concepts, experiments, and simulations, this thesis enables the reader through investigations on a big library of molecules that serve manifold purposes in ME.