Specifying the geometric and electronic structures of a metal–molecule interface at the single-molecule level is crucial for the improvement of organic electronics. A single-molecule junction (SMJ) can be used to investigate interfaces because it can be regarded as an elementary unit of the interface structure. Although considerable efforts have been made to this end, the detection of structural changes in SMJs associated with metal–molecule interactions remains challenging. In this study, we detected the surface-enhanced Raman scattering (SERS) signal originating from the metal–molecule interaction change induced by a local structural change in a C60 SMJ. This junction has attracted wide attention owing to its unique electronic and vibronic properties. We fabricated a C60 SMJ using a lithographically fabricated Au electrode and measured the SERS spectra along with the current–voltage (I–V) response. By continuous measurement of SERS for the C60 SMJ, we obtained SERS spectra dependent on the local structural change. The analysis of the I–V response revealed that the vibration energy shift originates from the change in the local structure for different Au–C60 interactions. Based on the discrimination of the states in accordance with the Au–C60 interaction, we found that the probability of SERS for geometry with a large Au–C60 interaction was enhanced.

We have investigated the effect of the bias voltage on the 1,4-benzenethiol (BDT) single-molecule junction using the surface-enhanced Raman scattering (SERS). By measuring the bias voltage dependence of SERS for the identical sample, we can clearly observe the blue shift of the vibrational mode at 355 cm–1 by the application of the bias voltage. This measurement is possible by using a highly stable nanogap electrode which is fabricated with lithographic techniques. Meanwhile, the energy of other vibrational modes does not change with the bias voltage. The change in the bond strength can be explained by the charge transfer induced by the application of the bias voltage on the single-molecule junction. The theoretical calculation result reveals that the effect of bias voltage is a result of the rearrangement of the π system. The 355 cm–1 mode is an out-of-plane mode of the aromatic ring. In the BDT single-molecule junction, the π system is delocalized on the sulfur atom. This delocalization is suppressed by the application of the bias voltage, and thus, the aromatic ring is more “benzene-like” and the out-of-plain vibration shifts to higher energy.

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 b 2 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 b 2 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 b 2 mode.