Adsorption sites of molecules critically determine the electric/photonic properties and the stability of heterogeneous molecule-metal interfaces. Then, selectivity of adsorption site is essential for development of the fields including organic electronics, catalysis, and biology. However, due to current technical limitations, site-selectivity, i.e., precise determination of the molecular adsorption site, remains a major challenge because of difficulty in precise selection of meaningful one among the sites. We have succeeded the single site-selection at a single-molecule junction by performing newly developed hybrid technique: simultaneous characterization of surface enhanced Raman scattering (SERS) and current-voltage (I-V) measurements. The I-V response of 1,4-benzenedithiol junctions reveals the existence of three metastable states arising from different adsorption sites. Notably, correlated SERS measurements show selectivity toward one of the adsorption sites: "bridge sites". This site-selectivity represents an essential step toward the reliable integration of individual molecules on metallic surfaces. Furthermore, the hybrid spectro-electric technique reveals the dependence of the SERS intensity on the strength of the molecule-metal interaction, showing the interdependence between the optical and electronic properties in single-molecule junctions.

Single molecular junctions, in which a single molecule bridges between metal electrodes, have attracted wide attention as novel properties can appear due to their peculiar geometrical and electronic characters. The single molecular junction has also attracted attention due to its potential application in ultrasmall single molecular electronic devices, where single molecules are utilized as active electronic components. Thus, fabrication of single molecular junctions as well as understanding and controlling their properties (e.g. conductance, optical and magnetic properties) have become long-standing goals of scientists and engineers. This review article focuses on the experimental aspects of single molecular junctions, with primary focus on the electron transport mechanism.

The formation of the single benzene molecule junction was investigated for Au and Ag electrodes by conductance measurements and inelastic tunneling electron spectroscopy at 10 K. While a single benzene molecule junction was hardly formed for the Au electrodes, a single benzene molecule junction was formed for the Ag electrodes. The single Ag/benzene/Ag junction showed a fixed conductance value of 0.24 G 0 (G 0 = 2e 2/h), indicating the formation of a well-defined single benzene molecule junction. By comparing with previously reported results for Pt electrodes, in which the single benzene molecule junction showed various conductance values, it was shown that the moderate metal−molecule interaction is favorable to obtain a well-defined single molecule junction.

An analysis combining SERS and current–voltage response measurements with DFT calculations has identified the molecular adsorption site in a single molecule junction.