Contact Geometry Selection in the Formation Process of a Single-Molecule Junction Elucidated by Surface-Enhanced Raman Scattering Spectroscopy and <i>I–V</i> Measurements: Deliberate Control of Molecule–Electrode Interface for Molecular Electronics

A primary objective in the field of nanomaterials, particularly in the domain of nanoelectronic materials, is to exercise precise control over the contact geometry of metal–molecule interfaces at the single-molecule level. This is due to the fact that these interfaces have a significant impact on the physical properties of nanomaterials. Single-molecule junctions (SMJs) can be regarded as a model system to study the structure–property relationship at the single-molecule level because they can be considered the elementary units of the metal–molecule–metal interfaces. Nowadays, unique and efficient properties have been observed in SMJs. Although beneficial and distinctive properties have been exhibited by SMJs, fabricating an SMJ with the desired structure remains a notable challenge. Herein, we demonstrate the selective fabrication of two conductive states based on a formation process, focusing on the Raman shift of the CN stretching mode in 1,4-phenylene diisocyanide (PDI) SMJs, which sensitively reflects the contact geometry with which PDI is bonded to the electrode. PDI SMJ formation is studied during the breaking and making of a nanocontact. The breaking process produces a low-conductive state, while the making process generates a high-conductive state. Synchronized surface-enhanced Raman scattering and current–voltage measurements reveal the conductance-dependent Raman shift of the CN stretching mode. Density functional theory analysis of the vibrational shift shows that the parallel and tilted configurations correspond to the low- and high-conductive states observed in the breaking and making processes, respectively. A rearrangement of the atomic configuration on the electrode surface affects the preferred junction configuration. Consequently, we selectively fabricate two conductive states, as revealed by a spectroscopic technique, which contributes to the precise control of the physical property of the nanoelectronic materials by the fabrication of the metal–molecule interface geometry at the single-molecule level. This development paves the way to the realization of reliable molecular electronics.