The establishment of structure-property relationships is central to molecular science and this principle applies equally well to the developing field of molecular and organic/polymer electronics. My dissertation research centers on studying the structure-property relationship of charge transport in molecular wires. In particular it involves demonstrating a long-standing prediction of a mechanistic transition from tunneling to hopping as the lengths of molecular wires are varied. In analogous conceptual and technical frameworks, my research has continued to examine hopping conduction as a function of precisely controlled chain architectures. These experiments directly address variable rates of intrachain hopping conduction upon a change of structural entities, which will provide guidance in designing highly conductive molecular wires as well as understanding transport in bulk organic devices.
In this work, we provided direct evidence for a change in transport mechanism from tunneling to hopping in molecular junctions based on three conjugated systems; oligophenyleneimine (OPI) wires ranging in length from 1.5-7.3 nm, oligonaphthalene-fluoreneimine (ONI) wires from 2.3-10.1 nm, and oligotetrafulvalenepyromellitimide-imine (OTPI) wires from 2.5-20.2 nm. Our experimental approach involved contacting molecular wires that were grown from one electrode using controlled aryl imine addition chemistry; a metal-coated atomic force microscope tip is used to make the second contact. We showed that near 4 or 5 nm in length the mechanism of transport in the wires changes abruptly, as evidenced by striking changes in length, temperature and electric field dependence of the current-voltage (I-V) characteristics. For longer wires, we have been able to analyze the bias dependence to establish at least three different regimes of transport, and we were also able to estimate the single wire conductivity and the hopping energy for each wires, which shows the great sensitivity of hopping transport to the precisely controlled wire architectures in solid-states for the first time. Overall, these experiments open significant opportunities to probe the physical organic chemistry of molecular conduction, e.g. the roles of specific functional groups and bonding architectures on hopping transport in molecular wires.