Plasmonically enhanced molecular junctions for investigation of atomic-scale fluctuations in self-assembled monolayers

Molecular junctions represent a fascinating frontier in the realm of nanotechnology and are one of the smallest optoelectronic devices possible, consisting of individual molecules or a group of molecules that serve as the active element sandwiched between conducting electrodes. As devices approach the molecular scale, quantum mechanical effects become dominant, leading to a host of novel properties that do not exist in larger-scale devices. This thesis delves into electrically integrated and plasmonically enhanced molecular junctions, which are instrumental in understanding interactions at the metal-molecule interfaces. These junctions combine the optical capabilities of high field confinement (and enhancement) and high radiative efficiency, with the electrical capabilities of molecular transport. They can probe the electronic structure and dynamics of the molecules within the junction, offering a view of the electronic transitions, molecular vibrations, conformational changes in the molecules, charge transfer, and quantum transport properties. Their potential in pioneering nanoscale optoelectronic applications, such as ultrafast electronics and nanosensing, is significant. However, the complexity involved in creating scalable and robust molecular junctions at ambient operating conditions poses a substantial challenge. In this thesis, we present the utilization of a self-assembled molecular junction equipped with a nanoparticle bridge to explore the correlated fluctuations in conductance and the light emission induced by inelastic electron tunneling at room temperature. Unlike large-area SAM junctions, both the electrical conductance and light emission are remarkably sensitive to atomic-scale fluctuations, even though hundreds of molecules are present in the junction. This phenomenon mirrors the behavior observed in picocavities in Raman scattering and the luminescence blinking seen in photo-excited plasmonic junctions. Moving localization of these point-like emitters (identified as the movement of gold atoms at the surface) is observed in the light emission spectra and is supported by the conductance data. The research conducted for this thesis demonstrates a scalable molecular junction platform that facilitates both optical and electrical interrogation at the atomic level.