Probing coverage-dependent adsorption configuration and on-surface dimerization by single-molecule tip-enhanced Raman spectroscopy

The photocatalytic dimerization reaction of p -nitrothiophenol (PNTP) to p,p' - dimercaptoazobenzene (DMAB) is a popular model system for plasmon-enhanced Raman spectroscopy studies, thanks to the tunability of reaction efficiencies by excitation lasers and substrate conditions. However, previous studies are usually based on molecular assemblies and how molecular adsorption configurations and local environment affect the dimerization reaction has not been well understood at the single-molecule level. In this work, we apply the single-molecule tip-enhanced Raman spectroscopy (TERS) technique to investigate coverage-dependent adsorption configurations and on-surface dimerization of PNTP molecules on the Ag substrate. For the “bilayer” molecular island on Ag(100), the PNTP molecules at the bottom appear almost close-packed and the molecules inside tend to tilt strongly towards the surface, which are not favorable for dimerization because of the steric hindrance effect. Whereas for the partially covered upper layer, either single up-standing PNTP molecules or molecular clusters are observed, with no sign of photocatalytic dimerization reactions because of the isolation from the metal substrate. By contrast, when the molecular coverage on Ag (100) is below one monolayer, PNTP molecules are found to be easily catalyzed by the Ag substrate, undergoing an on-surface dimerization to form the dimerized DMAB molecules. Similar dimerization also occurs for sub-monolayer covered PNTP molecules on the less-active Ag (111) substrate, with the obtained DMAB molecules forming a highly ordered assembly structure. These observations indicate that the bare metal surface at sub-monolayer coverages is important for molecules to move around and activate the on-surface dimerization through direct charge transfer from the metal substrate, which may be instructive for understanding surface chemistry and catalysis.