Walking molecules

Ludwig Bartels and his colleagues at the University of California at Riverside, guided by theorist Talat Rahman of Kansas State University, have created a molecule—called 9,10-dithioanthracene (DTA)—with two thiol “legs” configured in such a way that only one “foot” at a time can rest on the substrate. Activated by either heat or the nudge of a scanning tunneling microscope tip, DTA will pull up one foot, put down the other, and thus walk in a straight line across a flat surface. The planted foot not only supports the body of the molecule but also keeps it from veering off course. In tests on a standard copper surface, such as the kind used in manufacturing microchips, the molecule has taken 10 000 unidirectional steps without faltering. The researchers now plan to develop a DTA-based molecule that can convert thermal energy into directed motion like a molecular-sized ratchet. (K.-Y. Kwon et al., Phys. Rev. Lett. 95, 166101, 2005.)