AVS 53rd International Symposium
    Surface Science Monday Sessions
       Session SS1-MoM

Invited Paper SS1-MoM10
Diffusion of Adsorbates: on "Walking" and "Dancing" Molecules

Monday, November 13, 2006, 11:00 am, Room 2002

Session: Catalytic Chemistry of Hydrocarbons
Presenter: L. Bartels, University of California at Riverside
Authors: K.-Y. Kwon, University of California at Riverside
K.L. Wong, University of California at Riverside
G. Pawin, University of California at Riverside
X. Lin, University of California at Riverside
R. Frisbee, University of California at Riverside
L. Bartels, University of California at Riverside
Correspondent: Click to Email
We report on the diffusion of individual 9,10-dithioanthracene (DTA) and CO molecules on Cu(111). DTA adsorbs with the aromatic backbone lying flat on the substrate and with both sulfur atoms attached to it. In variable-temperature STM studies, we find that DTA possesses the notable property, that it diffuses exclusive in the direction of its aromatic ring system. It achieves this by moving its substrate linkers in an alternating fashion much resembling bipedal locomotion. Over a wide temperature range we neither find rotation of the molecule in the surface plane nor diffusion perpendicular to the aromatic system. Consequently, an individual molecule is confined to shuttle forward and backward along a straight line albeit the sixfold symmetry of the top substrate layer. We will discuss possible extensions of this work towards breaking of the forward/backward symmetry, transport, etc. DTA forms molecular rows rather than islands on Cu(111). Exchange of the thiol linkers by carbonyl causes these rows to form a mesh resembling a honeyecomb with regular "pore" sizes of ca. 50 Angstrom, much larger than the molecule itself. We contrast the behavior of DTA with that of CO on the same surface: CO diffusion was found to be generally isotropic, as expected, and depending on the local coverage. At a local coverage corresponding to one molecule per 20 substrate atoms, CO has a 3 times higher diffusion rate than an isolated molecule. The diffusion barrier for isolated CO molecules was found to be 75 ± 5 meV at a prefactor of 10 MHz. Time-lapsed STM measurements provide 2D trajectories of the motion of CO molecules and pairs of them. The latter exhibit dominantly tangential diffusion at close CO-CO proximity Taking into account the diffusion directions as well as the intermolecular distances, the 2D diffusion potential of a CO in the vicinity of another CO can be reconstructed.