Molecules Take a Walk

SURFACE DYNAMICS

Scientists have designed a molecule that walks across a surface in a straight line, putting one bond in front of the other. Such purposeful control of a molecules motion is vital for advancing fields such as molecular self-assembly, molecular machines, and computing.

The electrically charged tip of a scanning tunneling microscope serves as a sort of carrot, luring a single 9,10-dithioanthracene molecule across a copper surface. A thermal vibration can also act as a lure. The researchers—Ludwig Bartels, assistant professor of physical chemistry and chemical physics at the University of California, Riverside, graduate student Ki-Young Kwon, and colleagues there and at Kansas State University—report their findings in an upcoming issue of Physical Review Letters.

The molecule toddles over the copper atoms on thiol linker legs: As the body of the molecule pivots forward, one leg bonds to the surface while the other detaches. The group performed density functional theory calculations to help confirm the walking motion.

Flemming Besenbacher, head of the Interdisciplinary Nanoscience Center for the University of Aarhus and Aalborg University, both in Denmark, calls the study a nice piece of work, but notes that a number of researchers have explored one-dimensional diffusion of molecules on a surface (Nat. Mater. 2004, 3, 779). Those studies, however, have generally used surfaces with step edges or anisotropic surfaces, which help keep the molecule moving in one direction. The new research was done on an isotropic surface, where all directions look the same to the molecule.

The Bartels groups work adds to the already fertile field of manipulating and probing molecules on surfaces, says Robert J. Celotta, a physicist at the National Institute of Standards &Technology in Gaithersburg, Md., who recently studied the guided motion of a single cobalt atom across a copper surface (Science 2004, 306, 242). It represents a beautiful demonstration of the use of [these methods] to understand a very basic form of molecular locomotion.