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

Paper SS-TuA2
Tunneling and Guidance in the Diffusion of Polyatomic Molecules at a Metal Surface

Tuesday, October 19, 2010, 2:20 pm, Room Picuris

Session: Chemical Dynamics at Surfaces
Presenter: L. Bartels, University of California at Riverside
Authors: Z.H. Cheng, University of California at Riverside
E. Chu, University of California at Riverside
D.Z. Sun, University of California at Riverside
D.H. Kim, University of California at Riverside
Y.M. Zhu, University of California at Riverside
M. Luo, University of California at Riverside
G. Pawin, University of California at Riverside
K.L. Wong, University of California at Riverside
K.-Y. Kwon, University of California at Riverside
R. Carp, University of California at Riverside
M. Marsella, University of California at Riverside
L. Bartels, University of California at Riverside
Correspondent: Click to Email
Pentacene derivatives that have two and four carbonyl groups attached to them diffuse on a Cu(111) surface in a uniaxial fashion, i.e. exclusively along a substrate atomic row, despite the threefold symmetry of the substrate. In this, they resemble the behavior of dithioanthracene and anthraquinone, previously dubbed “molecular walkers”, which move across Cu(111) sequentially stepping the thiol/carbonyl linkers on each side of the molecule. This poses the question of how a fourfold substituted, i.e. quadrupedal, molecule can accomplish uniaxial motion: will it move its substrate linkers on opposite ends of the molecule at the same time resembling the gait of trotting, or will it instead move both linkers on one side at the same time, resembling the gait of pacing? Density functional theory (DFT) suggests the latter. Variable temperature scanning tunneling microscopy (STM) monitoring of the molecular motion reveals a striking difference between the diffusion prefactors of the quadrupedal and bipedal species, with the latter being very low. Wentzel Kramers Brillouin-based modeling of the motion of the substrate linkers in the calculated diffusion barriers suggest that the origin of this discrepancy lies in the relevance of the tunneling of the substrate linkers, allowing bipedal species, which only need to move one substrate linker at a time, to accomplish motion through tunneling of the linker, whereas quadrupedal species, whose diffusion requires concerted motion of two substrate linkers, cannot move in this way, resulting in significantly higher diffusion temperatures.