Experimental studies on the conductive properties of molecular systems led to the discovery and characterization of two essential elements of molecular electronics: a molecular

wire and a single electron switch. The synthesis and cyclic current versus voltage (I/V) measurements of intrinsically conductive linear chain [-Ag-S-BP-S-]n polymers determined their relatively large conductivity and suggested that mobile Ag adatoms on the roughened Ag surface catalyzed the polymerization. On the other hand, the single electron switch was determined to be controlled by the spin-vibronic density of an odd electron on a single molecule. Analysis of the two-level telegraphic conductance switching produced effective wiring diagrams to control the switch’s frequency, amplitude, polarity, and duty-cycle. Furthermore, it gave insight into the molecule’s governing vibronic Hamiltonian.

Experimental techniques with the sensitivity to measure properties of individual molecules present an opportunity to investigate and understand fundamental chemical and physical processes. One such technique, scanning tunneling microscopy (STM), has proven itself to be the best tool for probing the properties of single molecules, enabling researchers to measure a molecule’s electronic and vibrational properties with angstrom-level precision. Combining STM with optical vibrational spectroscopy allows the technique to measure molecular properties, bonding dynamics, optical phenomena, and light-matter interactions that are invisible to bare STM. Developing and perfecting the coupling of lasers with the STM tunneling junction can lead to time-resolving the technique, enabling sub-molecular topographic imaging with femtosecond time resolution. In this work, I detail the results of single molecule investigations with STM. In particular, I highlight the progress I have made in improving ultra-high vacuum STM by combining it with tip-enhanced Raman spectroscopy (TERS). The improvements made to TERS-STM focus on a variety experimental considerations including optimizing laser-tip beam alignment through ray-tracing simulations and drastically improving the performance and reliability of TERS tips through the introduction of new tip manufacturing and preparation techniques. The efficacy of these improvements are demonstrated by the many high-resolution STM measurements including (1) the discovery and characterization of a single-election switch involving the spin flip of an unpaired electron in a single molecule, (2) the observation via TERS-STM of one molecule undergoing a single reaction as well as an analysis of intensity and spectral fluctuations common to TERS, and (3) the demonstration and analysis of angstrom-resolved TERS-STM and its significance to our understanding of single-molecule physics and the tip-enhanced Raman effect.