Large-Amplitude, High-Frequency Single-Molecule Switch

We use a scanning tunneling microscope to form and electrically interrogate metal - molecule - metal junctions. To form such junctions, molecules must be functionalised with suitable contact groups (e.g. thiols, thioethers, 4-pyridyls, amines) at each extremity. We show here that 2-(methylthio)thiophene units not only act as contact groups, but can reversibly switch between a monodentate configuration (MeS-only) and a bidentate configuration (MeS- and thienyl S) upon junction compression; as the junction is compressed the electrical conductance increases greatly with the increased molecule-contact interaction. This means that such molecules show a large-amplitude mechanical switching behavior; we also show that this is reversible and that switching can occur at a rate of at least 10 kHz. Control molecules with MeSC6H5 contact groups do not show this behavior. This, together with detailed theoretical and transport calculations on the compressed and extended molecular junctions, supports our contention that it is the thienyl S that is involved in the switching mechanism.