The field of nanophotonics has realized rapid growth over the past several decades, as novel nanoscale materials are consistently being developed and researched for a wide variety of promising light-driven applications. Plasmonics is a particularly fascinating subset of nanomaterials research, owing to the unique ability of metallic nanostructures to interact with light from areas larger than their physical size, effectively focusing it to dimensions below the diffraction limit. This interaction, called a localized surface plasmon, arises when the electric field of light induces a coherent oscillation of the conduction electrons in the metal. Tuning the geometry and near-field environment of metallic nanostructures allows for controlled light scattering and absorption across a broad spectral range. Moreover, the strong spatial confinement of electric fields near the metal surface can remarkably enhance a host of molecular processes, motivating the development of plasmonic nanomaterials for single molecule sensing, photocatalysis and photoelectrochemical devices. This thesis will focus on two sets of interactions between plasmonic metal nanostructures and molecules in their local environment. Using spectroscopic and electrochemical techniques, the experimental far-field responses are correlated to the calculated near-field properties of the metal nanostructures. This is followed by a demonstration that molecules themselves may sustain plasmon resonances through active electrochemical charging.
In the first part of this thesis, the near-field coupling of plasmons and molecular excitons are studied at the single-particle level. Polarization-dependent hyperspectral dark field microspectroscopy is used to probe the far-field scattering response of plasmonic dimers, which is influenced by strong near-field coupling to molecular J-aggregates located in the dimer junction. The coupling strengths are quantified and a rigorous theoretical investigation reveals that the plexcitonic coupling is dependent on the intensity of the plasmonic field enhancement in the dimer junction, which can be tuned by varying the polarization of incident light. These nanostructures represent a class of hybrid plasmonic materials that show reversible, all-optical spectral modulation, a necessary feature for a number of applications ranging from ultrafast optical switches to tactical obscurants.
The next part of this thesis investigates the factors influencing the efficiency and kinetics of plasmonic hot carrier- driven redox reactions in an photoelectrochemical cell. Multi-layered Au-SiO2-Au nanoparticles, called nanomatroyshkas (NMs), serve as nano-electrodes due to their interesting optical properties. Strong light absorption by the NM electrodes leads to the formation of energetic hot electron-hole pairs that can be utilized to drive chemical reactions of surface adsorbates. The photooxidation of citrate by plasmonic hot holes and subsequent reduction of water by hot electrons on the surface of the NM electrodes is studied as a function of excitation wavelength, electrode potential and incident laser power. A qualitative system for optimizing plasmon-enabled photoelectrochemical reactions is presented by considering the interplay between plasmonic absorption and the energy alignment of hot carriers with molecules at the metal-solution interface. This result is an important step toward the ultimate goal of designing optimized nanomaterials for efficient photoelectrochemical devices. In the last part of this thesis, a new class of carbon-based plasmonic materials is proposed by reconsidering our understanding of the nature of optical transitions in charged polycyclic aromatic hydrocarbons (PAHs). 2D- graphene doped with charge carriers has been shown to support a surface plasmon resonance that is optically- and electrically-tunable in the mid-infrared (IR) and terahertz regimes. This study confirms theoretical predictions that spatial confinement of graphene to its smallest dimensions, that is, to individual PAH molecules containing only a few tens of atoms, can result in visible optical transitions when the molecules are charged with single electrons. A custom spectroelectrochemical setup is built to study the visible resonances of a series of PAH molecules in their reduced state, sustained by the absence of water or oxygen in a non-aqueous electrochemical cell. Time-dependent density functional theory (TDDFT) calculations provide insight into the origins of the broad, intensely-absorbing experimental optical spectra, which are concluded to be a superposition of light-induced electronic and vibronic transitions that are dipolar in nature. While efforts to fully understand these polarizable molecular transitions are ongoing, a number of research possibilities and potential applications arise from the addition of PAH molecules to the nanophotonics toolbox.