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J. Phys. Chem. C 2010, 114, 3152–3155
Understanding the Rotational Mechanism of a Single Molecule: STM and DFT Investigations of Dimethyl Sulfide Molecular Rotors on Au(111) Heather L. Tierney,† Camilo E. Calderon,‡ Ashleigh E. Baber,† E. Charles H. Sykes,† and Feng Wang*,‡ Department of Chemistry, Tufts UniVersity, Medford, Massachusetts 02155, and Department of Chemistry, Boston UniVersity, Boston, Massachusetts 02215 ReceiVed: NoVember 21, 2009; ReVised Manuscript ReceiVed: January 11, 2010
Thioether molecules are an ideal surface-mounted rotor system that allows many fundamental properties of molecular rotors to be studied and quantified. Recent work on thioether molecular rotors has revealed that their rotation can be controlled thermally, electrically, and mechanically. This study combines scanning tunneling microscopy experiments and density functional theory calculations to investigate the rotational properties of a simple thioether molecule, dimethyl sulfide. Experimental studies showed a very low barrier to rotation for dimethyl sulfide (it rotated >103 Hz at 5 K) and this barrier was calculated theoretically. Also, using theoretical methods the minimum energy adsorption site was determined and the mechanism of rotation was elucidated. Our 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. Introduction While molecular machines driven by chemical, radiant, or thermal energy can be found throughout nature,1-3 little progress has been made toward creating synthetic counterparts.4,5 Molecular rotors and motors are crucial parts of many biological processes including cellular transport, cell division, muscle contraction, genetic transcription, and translation.1,2,6 The gap between nature’s machines and nanotechnology’s mimics remains large due to a limited fundamental understanding of rotational dynamics, structure-property relationships, and how energy is converted to mechanical motion at the nanoscale.2,3 Some impressive advances featuring ensembles of large organic molecules can be found in the literature detailing rotors in the aqueous phase,5,7-10 and relatively large surface-mounted rotors,11-19 but little has been done to study very simple molecular rotors in detail.20-25 Recent studies of surface-bound molecular rotors have used experimental techniques such as scanning tunneling microscopy (STM) to measure molecular rotational rates and energy barriers. The rotation of acetylene23,26,27 and cis-2-butene molecules has been examined with use of inelastic electron tunneling (IET) experiments.27 Dielectric response and STM measurements have helped to elucidate the rotational dynamics of chloromethyl- and dichloromethylsylil molecules on fused silica surfaces.28-31 Halogenated thiophenol molecules have also been studied on Cu(111) surfaces and have been seen to rotate even at low temperatures.32 Even a simple methane thiol (MeSH) molecule was observed to rotate on a Au surface with the S-Au bond as its axle.33 Although some theoretical studies25,29,33-35 have helped to explain the experimental observations, few were able to explain mechanisms of rotation for many of these complex rotor systems. * To whom correspondence should be addressed. E-mail: fengwang@ bu.edu. † Tufts University. ‡ Boston University.
This study examines the rotational properties of dimethyl sulfide (Me2S) on Au(111) with atomic-scale detail. Recently, thioether molecules on noble metal surfaces have been shown to be a stable and robust system with which to study fundamental properties of rotation.21,22,24,25 In these thioether systems, molecular rotation could be controlled thermally, electrically, or mechanically.21,24,25 Experiments showed that, while symmetric dialkyl sulfides with chains of 2-6 carbons all had similar rotational barriers, dimethyl sulfide (with only one carbon in each of its chains) had a very low activation energy for its rotation. This study combines STM experiments and DFT calculations to understand this very low barrier for rotation and reveal the mechanism by which Me2S rotates. Experimental Methods All STM experiments were performed in a low-temperature, ultrahigh vacuum (LT-UHV) microscope built by Omicron Nanotechnology. The Au(111) sample was purchased from MaTecK and was prepared by cycles of Ar sputtering (1.0 keV/ 14 µA) for 30 min followed by 2 min anneal periods up to 1000 K. Approximately 12 of these sputter/anneal cycles were performed upon receiving the crystal, followed by a further 2 sputter/anneal cycles between each STM experiment. After the final anneal, the crystal was transferred in less than 5 min in vacuum (
