Governing the Metal-Molecule Interface: Towards New Functionality in Single-Molecule Junctions

Single-molecule junctions, in which a single-molecule bridges a gap between metal electrodes, have attracted significant attention due to their potential applications in ultrasmall electronic devices and their unique structure. Single-molecule junctions are one-dimensional nanomaterials having two metal-molecule interfaces. Thus, unconventional properties and functionalities that would not be observed in other phases (e.g., isolated molecules and bulk crystals) are expected to appear in these nanomaterials. Despite interest in these expected unconventional properties, several issues have been noted with the investigation and practical application of the unique properties of single-molecule junctions. To explore new functionality, we have investigated single-molecule junctions using a combined approach comprising fabrication, characterization, and measurement. First, we have explored a new generation of the metal-molecule interfaces formed by direct p-bonding. The interfaces made by the direct p-bonding have increased electronic conductance at the singlemolecule junction, reaching the theoretical limit, 1 G(0)f (2e(2)/h), which is the conductance of typical metal monoatomic contacts. Secondly, we have developed new characterization techniques combined with a variety of spectroscopic methods to observe a single-molecule confined between metal electrodes. This has allowed us to reveal structural and electronic details of single-molecule junctions, such as the number of molecules, molecular species, interface-structure, electronic structure, and dynamics. Based on the development of the metal-molecule interface structures and the combined spectroscopic characterization techniques, we have searched for new single-molecule junction functionality. By controlling the metal-molecule interface structures, single-molecular switching functionality with multiple conductance states and a programmable singlemolecule junction with various electronic functionalities have been realized. Our newly developed interface structure, characterization technique, and the functionality of the singlemolecule junction opens the door for future research in the field of single-molecule junctions.