Probing Photophysical and Photochemical Processes via Nano-Confined Localized Surface Plasmons

Conventional spectroscopic techniques are limited by the optical diffraction limit to about half wavelength and therefore offer about 200 nm х 200 nm microscopic zone for working in the visible light range. Tip-enhanced Raman spectroscopy (TERS) emerges as an advanced analytical technique, where the localized surface plasmons (LSPs) at the plasmonically active tip apex are utilized to interrogate the local chemical environment of surface adsorbed molecules with sub-nanometer scale precision. When illuminated with light, the intense LSP field confined at the tip apex enhances the traditionally weak Raman scattering process with an enhancement factor by a few orders for the surface system that resides underneath the tip. In this dissertation, a topographical and chemical analysis of two regioisomers (positional isomers) was achieved with the spatial resolution down to 8 Å, which has a wide range of applications in various fields of surface science & nanotechnology. In addition, the molecule-substrate interactions were chemically addressed at the single-molecule level by employing different surfaces. Strong surface interactions at surface convert the flexible molecular structure distorted, which has been verified by TERS. Expanding upon these works, the chemical information available through TERS allows a chemical modification process to be investigated with single-molecule sensitivity. Furthermore, with the confinement of light at the tip apex, the highly energetic LSPs generated at the tip apex can be specifically utilized to drive certain chemical reactions which is unmanageable via conventional chemistry. For the first time, specific chemical bond dissociation inside a single molecule has been achieved by the nano-confined LSPs.