Credit: © 2006 Nature Materials

Molecules designed to mimic the functions of macroscopic machines are often studied as large ensembles in which their individual characteristics are not observed directly. In this way, the precise atomic-scale motions of the components of such systems remain hidden amongst averaged overall behaviour.

Now, researchers in France and Germany have looked at a molecular 'rack-and-pinion' device in which a single molecule (the pinion) — reminiscent of a six-toothed cog — can be moved along the edge of a molecular island (the rack) comprising many copies of the same compound. Christian Joachim and colleagues1 from CEMES-CNRS in Toulouse and the Freie Universität Berlin used a scanning tunnelling microscope (STM) to move a molecular cog along the serrated edge of a self-assembled island on a Cu surface. The STM tip was placed above the centre of a single cog, pinning the molecule to the surface while still allowing it to rotate about its central axis. As the tip was displaced along the island's border, the cog moved with it to the next position along the 'rack'.

One tooth of the cog had a different structure to the others so could be distinguished during imaging and from this it could be seen that the displacements along the 'rack' resulted in successive 60° rotations of the cog.