On Photoinduced Transformations in Plasmonic Hot Spots

Plasmonic nanostructures can concentrate incident light in nanometer-sized volumes, called hot spots. The highly localized surface plasmon resonance can trigger unusual processes involving molecules adsorbed in such a hot spot. These include enhanced optical responses, photocatalytic reactions and thermal desorption. In this thesis, these phenomena are investigated at the level of a single plasmonic hot spot, using tip-enhanced Raman spectroscopy (TERS). In TERS, a laser-illuminated plasmonic tip is placed on the surface of a sample to obtain its enhanced vibrational spectrum. Thanks to the tight confinement of the light at the apex of the tip, a strong, localized spectroscopic signal can be obtained from the molecules residing directly below the tip. In the last decade, TERS has shown great potential for surface analysis of inorganic nanostructures (graphene, MoS2) and organic monolayers (2D polymers, thiolate SAMs). However, until now the technique has been limited in its applications, due to issues with the stability of the spectroscopic signal. In the present thesis we explain those instabilities with processes of photoinduced damage occurring in the TERS hot spot during the measurement. Specifically, we attribute the appearance of fluctuating peaks to plasmon-assisted photocatalytic reactions of the analyte, and the intensity fluctuations to a dynamic rearrangement of the crystalline facets of the plasmonic metal. Both of these processes have been discovered only in the last decade and could not be predicted by the inventors of TERS. They are detrimental for TERS, but highly relevant for other applications of plasmonic nanostructures. Hence, beyond providing mechanistic insight, this thesis proposes concrete solutions both to mitigate these effects and to forge them into useful photochemistry.