Bias Voltage Induced Surface-Enhanced Raman Scattering Enhancement on the Single-Molecule Junction

We have studied surface-enhanced Raman scattering (SERS) of aminobenzenethiol (ABT) and benzenedithiol (BDT) single molecules bridging Au electrodes (single-molecule junction) at different bias voltages. The SERS intensity of the ABT single-molecule junction increased with the bias voltage, and the non-totally symmetric b₂ mode appeared at a high bias voltage. Meanwhile, the SERS intensity did not change with the bias voltage in the case of the BDT single-molecule junction. The bias voltage-induced SERS intensity and appearance of the b₂ mode for the ABT single-molecule junction can be explained by the resonance effect. The energy difference between the metal occupied state and the lowest unoccupied molecular orbital (LUMO) of ABT decreased with an increase in the bias voltage. The charge transfer resonance taking place between the metal occupied state and the LUMO was, thus, allowed at higher bias voltages, which caused the enhancement of SERS intensity and appearance of the b₂ mode.