Revealing Intermolecular Interaction and Surface Restructuring of an Aromatic Thiol Assembling on Au(111) by Tip-Enhanced Raman Spectroscopy

Controlling the packing structure and revealing the intermolecular interaction of self-assembled monolayers (SAMs) on solid surfaces are crucial for manipulating its properties. We utilized tip-enhanced Raman spectroscopy (TERS) to address the challenge in probing the subtle change of the intermolecular interaction during the assembly of a pyridine-terminated aromatic thiol on the single crystal Au(111) surface that cannot produce enhanced Raman signal, together with electrochemical methods to study the charge transfer properties of SAM. We observed that the aromatic C=C bond stretching vibration can be a marker to monitor the strength of the intermolecular interaction of SAMs, because this Raman peak is very sensitive to the intermolecular pi-pi stacking. Our results indicate that the SAM experiences a surface restructuring after the formation of a densely packed monolayer. We propose that the intermolecular electrostatic repulsion governs the restructuring when the packing density is high. The correlated TERS and electrochemical studies also suggest that the intermolecular interaction may have some impact on the charge transfer properties of SAM. This study provides a molecular-level insight into understanding and exploiting the intermolecular interactions toward better control over the assembling process and tuning the electrical properties of aromatic thiols.