Turning Molecules into Motors

We have developed a nanoscale model system that has advanced understanding of the fundamental limits of thermally- and electrically-driven molecular rotation.(1-4) Using low-temperature scanning tunneling microscopy, we are able to record the dynamics of individual molecular rotors in real time at the atomic-scale. Small, simple molecules called thioethers function as molecular rotors and their simplicity has allowed us to devise methods for turning them into molecular motors.
Studying the rotation of molecules bound to surfaces offers the advantage that a single layer can be assembled, monitored and manipulated using the tools of surface science. Thioether molecules constitute a simple, robust system with which to study molecular rotation as a function of temperature, electron energy, applied fields, and proximity of neighboring molecules. We recently demonstrated that a butyl methyl sulphide (BuSMe) molecule adsorbed on a copper surface can be operated as a single-molecule electric motor.(1) Electrons from a scanning tunneling microscope are used to drive directional motion of the BuSMe molecule in a two terminal setup. Moreover, the temperature and electron flux can be adjusted to allow each rotational event to be monitored at the molecular-scale in real time. The direction and rate of the rotation are related to the chiralities of the molecule and the tip of the microscope (which serves as the electrode), illustrating the importance of the symmetry of the metal contacts in atomic-scale electrical devices.(2)
We have also used alloy surface reconstructions to direct the growth of ordered arrays of molecular rotors, just like placing cogs on a pegboard, and can mechanically “brake” them by altering their proximity to other molecules.(4)  These experiments are providing a framework for understanding coupled arrays of dipolar rotors, an important step towards tunable dielectrics and miniature signal delay lines. The work has also revealed novel methods for driving and monitoring the motion of arrays of motors, an area that may find application in the modulation of optical signals at microwave or THz frequencies. In summary, this simple molecular rotor system has enabled us to study many important fundamental aspects of molecular rotation with unprecedented resolution and to realize the first single molecule electric motor, which has captured the attention of the scientific community and the general public around the world.

1) "Experimental Demonstration of a Single-Molecule Electric Motor" H. L. Tierney, C. J. Murphy, A. D. Jewell, A. E. Baber, E. V. Iski, H. Y. Khodaverdian, A. F. McGuire, Nikolai Klebanov and E. C. H. Sykes - Nature Nanotechnology 2011, 6, 625-629. (link)

2) "Regular Scanning Tunneling Microscope Tips can be Intrinsically Chiral" H. L. Tierney, C. J. Murphy and E. C. H. Sykes - Physical Review Letters 2011, 106, 010801. (link)

3) "Time-resolved studies of individual molecular rotors" A. D. Jewell, H. L. Tierney, A. E. Baber, E. V. Iski, M. M. Laha and E. C. H. Sykes - Journal of Physics-Condensed Matter 2010, 22, 264006-264016. (link)

4) "Engineering Dislocation Networks for the Directed Assembly of Two-Dimensional Rotor Arrays" D. O. Bellisario, A. E. Baber, H. L. Tierney and E. C. H. Sykes - Journal of Physical Chemistry C 2009, 113, 5895-5898. (link)