Walking small. DTA (inset) makes its way across a row of atoms in this illustration.

One (Very) Small Step

Chemists have designed a molecule that walks in a straight line across a metallic surface by shuffling from one chemical "foot" to another. Those miniscule steps may be a stride toward a new approach to chemistry on surfaces.

Molecules on a crystalline surface often jiggle about, driven by thermal energy. Researchers have succeeded in directing wandering molecules by creating surfaces with stripes of atoms or with "step edges"--seams along which the surface of the crystal steps up by one layer of atoms. The molecules move back and forth along the stripes or edges a bit like a trolley car guided by its track. The new molecule can travel in a straight line in any one of three directions on a more symmetrical surface, report chemists Ki-Young Kwon and Ludwig Bartels of the University of California, Riverside, and colleagues.

Known as 9,10-dithioanthracene (DTA), the molecule looks a bit like an ant missing all but two of its legs. It consists of a string of three hexagonal rings of carbon and hydrogen atoms with a sulfur atom attached to either side of the middle ring. The researchers placed the molecule on a crystalline copper surface, in which the copper atoms arrange themselves in a triangular array, like so many pennies snuggled side by side. The surface can be seen as many parallel rows of atoms; although, because of the triangular symmetry, those rows can be taken as running in any one of three directions--just like the rows of a Chinese checkerboard. The molecule prefers to line up with the rows, so it chooses one of the three orientations. The sulfur atoms straddled two neighboring rows of copper atoms, as an ant's feet might straddle a pine needle.

Then it walks. Using a scanning tunneling microscope, the researchers observed the molecule skittering back and forth along the rows of atoms. Detailed computer simulations showed that the molecule pivots around one sulfur atom and then the other to create a waddling motion, the researchers report in an upcoming issue of Physical Review Letters. Such motion might someday be used to deliver reagents and control chemical reactions on a surface, Bartels speculates, and the multidirectional approach may provide more flexibility than one-way-only techniques.

"It's one of those beautiful results that you wouldn't imagine, but that you can understand afterwards," says Tony Heinz, a physicist at Columbia University in New York City. Paul Weiss, a chemist and physicist at Pennsylvania State University in University Park, says the result shows proper design of molecule and surface can give researchers an extra degree of control over the movement of molecules. But, he adds, "We are a long way from doing anything practical" with such phenomena.

Related sites
See the molecule in action
Bartels's group
Weiss's group
Heinz's Web page