Design of Thin Films and Monolayers for the Control of Light-matter Coupling

This thesis describes the design of films to control light-matter coupling. Such coupling can occur between localised surface plasmon polaritons in metal nanostructures and molecular excitons. Weak coupling enhances the excitons’ spontaneous emission rate, while strong coupling yields two new hybrid states that combine both light and matter properties. Plasmon-exciton interactions are explored for various applications, including electronics, sensing, and control of chemical reactivity.

This work forms part of a broader programme which aims to produce programmable photonic materials that exploit strong light-matter coupling to improve exciton dynamics. These materials include pigment-polymer complexes (PPC) – dye-functionalised polymers, and alpha-helical barrels – synthetic peptides with a dye-binding pore.

In this thesis, the development of nanostructured materials important for measuring exciton dynamics in PPC was investigated. A fast and simple nanofabrication technique capable of creating features as small as 14 nm, termed tribochemical nanolithography, has been established. Aminosiloxane films with photoremovable nitrophenyl protecting groups were modified using an atomic force microscope. The linewidths of features written using gold-coated probes were greater at loads above 3 micronewton compared to uncoated ones, attributed to an enhanced deprotection rate arising from weak coupling. To form PPC films, bromine initiators were first introduced to the nanopatterns, followed by the growth of polymer brushes via surface-initiated atom transfer radical polymerisation, and finally, the attachment of a fluorescent cyanine dye.

The feasibility of binding alpha-helical barrels to silver surfaces was demonstrated. Silver is a plasmonic material that couples strongly to many dyes but is susceptible to oxidation during prolonged air exposure. Thiol films deposited on silver were found to remain largely stable under various storage conditions for 48 hours. 11-amino-1-undecanethiol hydrochloride films were chemically derivatised and then attached with histidine-tagged alpha-helical barrels, reaching a limiting total film thickness of 4.9 nm, consistent with alpha-helical barrels monolayer coverage.