Molecule Walks Like a Human

September 27, 2005

A research team, led by UC Riverside’s Ludwig Bartels, is the first to design a molecule that can move in a straight line on a flat surface. It achieves this by closely mimicking human walking. The “nano-walker” offers a new approach for storing large amounts of information on a tiny chip and demonstrates that concepts from the world we live in can be duplicated at the nanometer scale – the scale of atoms and molecules.

The molecule – 9,10-dithioanthracene or “DTA” – has two linkers that act as feet. Obtaining its energy from heat supplied to it, the molecule moves such that only one of the linkers is lifted from the surface; the remaining linker guides the motion of the molecule and keeps it on course. Alternating the motions of its two “feet,” DTA is able to walk in a straight line without the assistance of nano-rails or nano-grooves for guidance.

The researchers will publish their work in next month’s issue of Physical Review Letters.

“Similar to a human walking, where one foot is kept on the ground while the other moves forward and propels the body, our molecule always has one linker on a flat surface, which prevents the molecule from stumbling to the side or veering off course,” said Bartels, assistant professor of chemistry and a member of UCR’s Center for Nanoscale Science and Engineering. “In tests, DTA took more than 10,000 steps without losing its balance once. Our work proves that molecules can be designed deliberately to perform certain dynamic tasks on surfaces.”

Bartels explained that, ordinarily, molecules move in every unpredictable direction when supplied with thermal energy. “DTA only moves along one line, however, and retains this property even if pushed or pulled aside with a fine probe.” Bartels said. “This offers an easy realization of a concept for molecular computing proposed by IBM in the 1990s, in which every number is encoded by the position of molecules along a line similar to an abacus, but about 10 million times smaller. IBM abandoned this concept, partly because there was no way to manufacture the bars of the abacus at molecule-sized spacing.

“DTA does not need any bars to move in a straight line and, hence, would allow a much simpler way of creating such molecular memory, which would be more than 1000 times more compact than current devices.”

The UCR research team is now trying to build a molecular ratchet, which would convert random thermal oscillation into directed motion. “It would be similar to an automatic watch that rewinds itself on the arm of the bearer – except that it would be just one nanometer in diameter,” Bartels said.

A nanometer is one billionth of a meter. A nanometer is to a meter what an inch is to 15,783 miles, more than half the distance around the Earth’s equator.

Bartels was assisted in the study by Ki-Young Kwon, Kin L. Wong and Greg Pawin of UCR; and Sergey Stolbov and Talat S. Rahman of Kansas State University. The US Department of Energy funded the research. Additional support came from the Petroleum Research Fund and the Air Force Office of Scientific Research. The San Diego Supercomputer Center provided computational resources.

Sorce: University of California, Riverside