Direct Determination of the Energy Required to Operate a Single Molecule Switch

Using a noncontact atomic-force and scanning-tunneling microscope in ultrahigh vacuum, we have measured the switching energy of a single molecule switch based on the rotation of a di-butyl-phenyl leg in a Cu-tetra-3,5 di-tertiary-butyl-phenyl porphyrin. The mechanics and intramolecular conformation of the switched leg is controlled by the tip apex of the noncontact atomic-force microscope. The comparison between experimental and calculated force curves shows that the rotation of the leg requires an energy less than $100\times {10}^{-21}\mathrm{J}$, which is 4 orders of magnitude lower than state-of-the-art transistors.