Charge Transport in Molecular Junctions Beyond Tunneling

Abstract

Electrical behavior of thin layers of molecules sandwiched vertically between two carbon contacts is the subject of this thesis. The main focus was put on the effect of molecular structural variations on charge transport through the molecular layers. The charge transport mechanism was deduced by analyzing the dependence of current on voltage, thickness of the molecular layer and temperature. The effect of compositional asymmetry was investigated in bilayers made from two different molecules. Multilayers made from only one type of molecule by reduction of diazonium ions are compositionally symmetric and were found to exhibit symmetric current-voltage characteristics. Bilayers made from two different molecules also showed symmetric current-voltage behavior when the second layer of the bilayer was only a monolayer, attached via azide-alkyne click chemistry to a multilayer of ethynylbenzene, and the total thickness of the bilayer was less than 5 nm. The charge transport properties of thin bilayers were found to be consistent with tunneling charge transport mechanism. Bilayers consisting of multilayers of two different molecules were rectifiers, given the total thickness of the bilayer was more than 10 nm and the bilayer was composed of a multilayer of an electron acceptor molecule such as naphthalene diimide together with a multilayer of an electron donor molecule such as fluorene in a vertical stack. Reversing the order of the two multilayers in the bilayer resulted in reverse rectification direction, confirming the molecular origin of this rectification behavior. The rectification persisted even at low temperatures of liquid helium. Charge transport in thick molecular junctions was studies further in a series of phenylthiophene derivative molecular junctions with thickness of 2-16 nm. A multistep tunneling charge transport was suggested to be operative in these thick carbon-based molecular junctions.