Potential-Controlled Interfacial Charge Transfer and Chemical Enhancement of SERS Spectra in Plasmonic Molecular Junctions

Plasmonic molecular junctions can generate a concentrated and intense light field to achieve the application of surface-enhanced Raman spectroscopy (SERS) under visible light illumination. SERS can be used for probing the interfacial charge transfer and chemical reaction of noble metals in situ and in real time. The energy gap between the Fermi level and the frontier molecular orbitals determines the activity of interfacial charge transfer between the noble metal and molecules. In this paper, time-resolved SERS was used for studying the changes in the SERS spectra of 4-aminothiophenol and 4-mercaptobenzoic acid under the direct-current and alternating-current potential applied to the gold electrode. The momentary changes in the spectral intensity with the electrode potential can be clearly observed. Furthermore, we applied the pulsing and linear alternating-current potentials to the gold electrode. Excitingly, we observed that the spectral intensity also fluctuates impulsively and linearly with the applied electrode potential, respectively. Combined with density functional theory calculations, we illustrated the mechanism of interfacial charge transfer under the electrode potential, which laid a foundation for studying the mechanism of the single-molecule chemical reaction on noble metals. It also provides a method to monitor and modulate the interfacial charge transfer between the gold electrode and the molecules.