Title: Three Tales of Two Theories
Description: Since the single-molecule diode was first envisioned by Aviram and Ratner in 1974, researchers have investigated how the electronic properties of molecules might be designed to achieve a variety of device functionality. However, although electron-phonon and electron-photon interactions have been studied in systems where the molecule is poorly electronically coupled to the environment, only a few experimental modalities exist for studying inelastic transport in two-terminal single-molecule junctions. Furthermore, each phenomena typically has a few possible mechanisms which must be distinguished. The objective of this dissertation is to expand the experimental tools available for probing inelastic transport in single-molecule junctions, with special attention to electron-photon interactions. Throughout the dissertation, we utilize the scanning tunneling microscope break-junction technique to form either tunnel junctions or single-molecule junctions. By repeatedly pushing and pulling a Au STM tip into a Au-coated mica substrate, a variety of junction geometries are sampled to give a distribution of device performances.

Transport and optical measurements are made while controlling the electrode displacement and junction bias independently, which permits flexible experimental design. The body of the dissertation is divided into three chapters, each chapter exploring a different phenomenon. In the first chapter, I study light emission from tunnel junctions driven at high bias. It was shown previously that electroluminescence from tunnel junctions can have photon energies exceeding the classical limit, so-called overbias emission. Multi-electron processes and blackbody radiation have been proposed as possible explanations for this extraordinary result. We demonstrate that the intensity of the overbias emission depends superlinearly on the junction conductance even at room temperature, which strongly supports the theory from multi-electron processes. Additionally, we show that blackbody radiation-like effects can be produced by multi-electron processes. In the second chapter, I demonstrate experimentally the enhanced conductance of single-molecule junctions under illumination.

Again, we consider two mechanisms for enhancement, namely photon-assisted tunneling and hot-electron distributions. By carefully comparing the two theories, we find that their steady-state signatures are nearly identical, but that the contribution from hot-electron distributions is larger in our system. This is confirmed by measuring a conductance enhancement at a polarization where photon-assisted tunneling is negligible. In the third chapter, I explore both junction rupture and nonlinear transport phenomena in single-molecule junctions around the resonant tunneling regime. Importantly, we develop nonlinear regression curve-fitting to allow straightforward extraction of key transport parameters on individual single-molecule junctions. We observe a strong correlation between the bias at which the junction ruptures and the level alignment of the dominant transport orbital, which suggests that, in the resonant tunneling regime, the tunneling electrons interact strongly with the nuclear degrees of freedom.

However, we also find that not all junctions rupture and those that sustain display negative differential resistance and hysteresis. We hypothesize that this nonlinear behavior is due to a change in the charge state of the molecule. We study the stability of this charge state and find that the dynamics of charging and discharging occur on millsecond timescales. Although the blocking-state and polaron models each predict parts of our data, neither are fully consistent with the experiments in their entirety. This reveals opportunities for further experimental and theoretical investigations into transport in the resonant tunneling regime.