Title: Environmental Control of Charge Transport through Single-Molecule Junctions
Description: Metal-molecule-metal junctions have become a widely used test-bed for the study of nanoscale electronic phenomena. Single-molecule junctions in particular have provided a deeper understanding of charge transport across interfaces, and single-molecule electronic components have been proposed as a successor for silicon technology. This thesis presents an experimental approach for controlling the electronic properties of single-molecule junctions by manipulating the environment about the junction. With this tunable functionality, we are able to demonstrate single-molecule variants of transistors and diodes. We begin our work by probing charge transport through single-oligomers of commonly used molecules in organic electronic devices. We focus on these systems due to their narrow band gaps, giving them the potential for exhibiting high molecular conductances. Single-molecule junctions are formed using the Scanning Tunneling Microscope-based break junction (STM-BJ) technique. We first consider a family of oligothiophenes, ranging in length from 1 to 6 units.

We find that this family of molecules exhibits an anomalous conductance decay with molecular length; this is mainly due to conformational effects. These conformational effects also result in very broad conductance distributions, further preventing oligothiophenes from being useful in molecular electronic devices. However, we find that thiophene dioxides are particularly well-suited for single-molecule devices, primarily due to exceptionally narrow band gaps. Oligothiophene dioxides also constitute a unique system where the dominant conductance orbital changes with molecular length. Specifically, we find that the shorter oligomers have transport dominated by the highest occupied molecular orbital (hole-type transport), but longer oligomers have transport dominated by the lowest unoccupied molecular orbital (electron-type transport). We next demonstrate a method for gating single-molecule junctions. In order to over- come the difficulty of lithographically defining a gate electrode in close enough proximity to the molecular junction so that the gate voltage impacts the electrostatics of the junction, we turn to measurements in electrolytic solutions.

Ions in these solutions form compact layers of charge at metal surfaces, and these electric double layers can be controlled by the gate electrode; such electrolytic gating results in high gating efficiencies. Using this technique, we show that we are able to continuously modulate the conductance of non-redox active molecular junctions. Using ionic environments, we next develop a new technique for creating a single-molecule diode. Performing break junction measurements in electrolytic solutions without the presence of a gate electrode, we show that we still have control of the junction’s electrostatic environment. In particular, if the source and drain electrodes are of considerably different areas, we find that we asymmetrically control this environment. Using this technique, we demonstrate single-molecule diodes created from otherwise symmetric molecular junctions. Combining this with measurements on thiophene dioxide oligomers, we show single-molecule diodes with the highest reported rectification ratios to date.

This technique has the potential for application in nano-scale systems beyond single-molecule junctions. These results constitute another step toward the development of single-molecule devices with commercial applications. Finally, the methods presented in this thesis offer further insights into the electronic structure of molecular junctions. We show that we can assess energy-level alignment at metal molecule interfaces– this alignment is a crucial parameter controlling the proper- ties of the interface. We also demonstrate that we can probe large regions ( 2eV) of the transmission function which governs charge transport through the junction. By being able to control level alignment, we are also able to offer preliminary studies on single-molecule junctions in the resonant transport regime. Combined, the results presented in this thesis grant new insights into electron transport at the nanoscale and provide new routes for the development of functional single-molecule devices.