Molecular Bending at the Nanoscale Evidenced by Tip-Enhanced Raman Spectroscopy in Tunneling Mode on Thiol Self-Assembled Monolayers

The sharp and intense Raman features associated with hexil azobenzenethiol self-assembled monolayers on gold(111) make these molecules ideal probes for tip-enhanced Raman spectroscopy (TERS) investigations. Spectroscopic and electrical measurements at the molecular junction were performed by combining TERS with scanning tunneling microscopy. The azobenzene TERS signal was monitored while changing the tip-surface distance and/or increasing the bias applied to the tip. The TERS signal decreases with increasing bias faster than what would be expected if only the effect of the tip distance from the monolayer and the related drop of the optically enhancing near-field intensity were considered. We develop a simple geometrical model accounting for the molecular bending induced by the increasingly higher local field close to the tip apex, which brings an additional modification of the TERS intensity and is able to accurately describe the experimental data.