Direct Experimental Evidence of Hot Carrier-Driven Chemical Processes in Tip-Enhanced Raman Spectroscopy (TERS)

Nanoscale localization of electromagnetic fields using metallic nanostructures can catalyze chemical reactions in their immediate vicinity. Local optical field confinement and enhancement is also exploited to attain single-molecule detection sensitivity in surface- and tip-enhanced Raman (TER) spectroscopy. In this work, we observe and investigate the sporadic formation of 4-nitrobenzenethiolate upon TER imaging of a 4-nitrobenzenethiol (4NBT) monolayer on Au(111). Density functional theory (DFT), finite-difference time-domain (FDTD), and finite element method (FEM) calculations together confirm that this chemical reaction does not occur as a result of thermal desorption of the molecule, which requires temperatures in excess of 2100 K at the tip-sample junction. Our combined experimental and theoretical analyses strongly suggest that the chemical transformations observed throughout the course of TERS mapping is not driven by plasmonic photothermal heating, but rather by plasmon-induced hot carriers.