Engineering the Flow of Energy and Charge Carriers in Hybrid Plasmonic Systems and Catalysts

Abstract

The emission of greenhouse gases due to the combustion of fossil fuels (i.e., coal, oil, and natural gas) is unanimously recognized as the primary driver of climate change. One attractive option for shifting away from a fossil fuel-based economy involves utilizing the sun's energy to produce electricity and chemicals renewably. In this context, nanostructures of plasmonic metals (Ag, Au, or Cu) have emerged as a popular platform to improve and enable solar energy conversion technologies because of their ability to harvest solar energy via the creation of surface plasmons. These plasmons decay to form energetic charge carriers (i.e., electron-hole pairs) within the nanostructures that can potentially be stored as electricity or used to drive chemical reactions. In practice, these processes often require the extraction of the excited charge carrier from the plasmonic nanostructure into another material (e.g., a molecule, semiconductor, or another metal) before they lose energy. Unfortunately, the ultrafast recombination rates of charge carriers within metal nanostructures significantly limits the efficiency of this extraction process. This drawback limits the number of viable applications for plasmonic nanomaterials.

This dissertation presents a combined experimental/theoretical approach to study whether energy/charge can be extracted from plasmonic materials by interfacing them with non-plasmonic materials to form "hybrid plasmonic" nanostructures. The first part of this dissertation sheds light on the factors governing the flow of energy and charge in hybrid plasmonic nanomaterials. We demonstrate that coating plasmonic nanostructures with thin layers of non-plasmonic materials can result in localized charge carrier formation (absorption) in the non-plasmonic component. Our results suggest that the plasmonic component preferentially dissipates light energy through the formation of charge carriers directly in the non-plasmonic component, effectively bypassing the charge recombination process. We reveal that two primary factors govern the energy transfer process: (1) the intensity of the electric field generated by the plasmonic nanostructure and (2) the availability of direct electronic excitations in the non-plasmonic material. We use these results to develop a unifying physical framework leading to molecular control of charge carrier generation in all hybrid plasmonic systems.

The second part of this dissertation investigates how the efficiency of this energy/charge localization process is affected in the limit of strong light-matter coupling. The strong coupling of optical absorbers (e.g., molecules or semiconductors) to plasmonic nanostructures fundamentally alters the physical properties of the coupled system via the formation of hybrid light-matter states. Our results show that the light absorption efficiencies in strongly coupled absorbers surpass those weakly coupled. These efficiencies are highest in configurations where the strongly coupled molecules interact directly with the incoming photon flux. We use these results to propose design principles for engineering nanostructured systems that allow for high efficiencies of charge carrier localization into strongly coupled absorbers. We also show that the strong coupling of molecular absorbers to optical cavities can improve their charge transport properties. The collective work presented in this dissertation provides a comprehensive framework for designing hybrid plasmonic materials applicable to solar energy conversion technologies related to mitigating climate change.