Nature 479, 208–211 (2011)

It is relatively easy to make a molecule rotate on a surface with the help of thermal energy. Controlling the direction of rotation is more difficult, but has been achieved with rotary motors powered by light, chemical and electrical energy. Similar challenges are encountered when trying to control the translational motion of a molecule over a surface, though previous systems have been limited to those that diffused along the surface or were dragged by the tip of a scanning tunnelling microscope (STM). Syuzanna Harutyunyan, Karl-Heinz Ernst, Ben Feringa and colleagues have now shown that a molecule containing four rotary motors can be driven over a metal surface using electrons.

The researchers — who are based at the University of Groningen, the Swiss Federal Laboratories for Materials Science and Technology, and the University of Zurich — built a car-like molecule by attaching four unidirectional rotary units to a central axis. The molecules were deposited on a copper surface at 7 K and electrons supplied from the tip of an STM. The electrons induce conformational changes in the rotors and a paddlewheel-like motion that pushes the molecule forward.

The four-wheeled molecule can move 6 nm across the surface with ten doses of electrons and, by altering the direction of the rotary motion of individual motor units, can be made to follow linear or random trajectories.