Revealing the Conformational Dynamics in a Single-Molecule Junction by Site- and Angle-Resolved Dynamic Probe Method.

Single-molecule junctions have been extensively studied because of their high potential for future nanoscale device applications as well as their importance in basic studies for molecular science and technology. However, since the bonding sites at an electrode and the molecular tilt angles, for example, cannot be determined experimentally, analyses have been performed assuming the structures of such interactive key factors, with uncertainties and inconsistencies remaining in the proposed mechanisms. We have developed a methodology that enables the probing of conformational dynamics in single-molecule junctions simultaneously with the direct characterization of molecular bonding sites and tilt angles. This technique has revealed the elemental processes in single-molecule junctions, which have not been clarified using conventional methods. The mechanisms of the molecular dynamics in 1,4-benzenedithiol and 4,4'-bipyridine single-molecule junctions, which, for example, produce binary conductance switching of different types, were clearly discriminated and comprehensively explained.

Mechanically activated switching of Si-based single-molecule junction as imaged with three-dimensional dynamic probe.

Understanding and extracting the full functions of single-molecule characteristics are key factors in the development of future device technologies, as well as in basic research on molecular electronics. Here we report a new methodology for realizing a three-dimensional (3D) dynamic probe of single-molecule conductance, which enables the elaborate 3D analysis of the conformational effect on molecular electronics, by the formation of a Si/single molecule/Si structure using scanning tunnelling microscopy (STM). The formation of robust covalent bonds between a molecule and Si electrodes, together with STM-related techniques, enables the stable and repeated control of the conformational modulation of the molecule. By 3D imaging of the conformational effect on a 1,4-diethynylbenzene molecule, a binary change in conductance with hysteresis is observed for the first time, which is considered to originate from a mechanically activated conformational change.