AVS 57th International Symposium & Exhibition
    Surface Science Tuesday Sessions
       Session SS-TuP

Paper SS-TuP2
MORTON S. TRAUM AWARD FINALIST: Understanding and Controlling Rotation at the Single-Molecule Level: Turning Rotors into Motors

Tuesday, October 19, 2010, 6:00 pm, Room Southwest Exhibit Hall

Session: Surface Science Poster Session
Presenter: H.L. Tierney, Tufts University
Authors: H.L. Tierney, Tufts University
A.D. Jewell, Tufts University
A.E. Baber, Tufts University
E.V. Iski, Tufts University
E.C.H. Sykes, Tufts University
Correspondent: Click to Email
While molecular machines driven by chemical, light or thermal energies can be found throughout nature, little progress has been made toward creating synthetic counterparts. The gap between nature and nanotechnology remains due to the limited fundamental understanding of the transfer of energy to mechanical motion at the nanoscale. Understanding and actuating the rotation of individual molecules on surfaces is a crucial step towards the development of nanoscale devices such as fluid pumps, sensors, delay lines, and microwave signaling applications. Towards this end we have used a group of small molecules in order to understand the fundamental aspects of molecular rotation. Thioethers constitute a simple, robust system with which molecular rotation can be actuated thermally, mechanically and electrically, and can be studied using scanning tunneling microscopy (STM) as a function of molecular chemistry and proximity of neighboring molecules. Interestingly, the thermal onset to rotation was found to be nearly identical for studied thioether molecules with alkyl tails of two carbons or more. It is proposed that this plateau in thermal onset was due to an interplay between degrees of freedom in the alkyl tail vs. the S-metal bond length. While small amounts of thermal energy are capable of inducing rotation, thermodynamics dictates that thermal energy alone cannot be used to perform useful work in the absence of a temperature gradient. Therefore, for molecules to meet their full potential as components in molecular machines, methods for coupling them to external sources of energy that selectively excite the desired motions must be devised. To this end, we have studied using an electrical current to rotate individual dibutyl sulfide molecules on command. For these studies the source of energy is supplied via high energy electrons from the STM tip. By monitoring the rate of rotation as a function of tunneling electron energy (action spectroscopy) we have demonstrated that the rotors can be driven electrically via a mechanism that involves excitation of a C-H stretch. Finally, using theoretical methods the minimum energy adsorption site was determined and the mechanism of rotation was elucidated for the simplest thioether, dimethyl sulfide. These theoretical results indicate that the rotation of a small, simple molecule is actually rather complex; as the CH3 groups of dimethyl sulfide rotate around the Au−S bond, the central S atom precesses around a surface Au atom.