Picture the ultimate in miniaturization—functional machines built out of individual molecules, mere atoms in size. In a breakthrough development, researchers led by Christian Joachim from the A*STAR Institute of Materials Research and Engineering in Singapore1 have reported the invention of an essential component for single-molecule mechanical machines: a molecular gear that can be controllably rotated with a 100% rate of success.
Joachim and his colleagues built their molecular gear by placing a molecule called hexa-t-butyl-pyrimidopentaphenylbenzene (HB-NBP) onto a gold crystal. HB-NBP was specifically designed to mimic a real-world gear; it has six ‘teeth’ arranged symmetrically around a central benzene core. Each tooth contains a set of bulky, inert butyl molecules that raises the central core above the gold surface—making the gear easy to manipulate with the sharp tip of a scanning tunneling microscope (STM) (Fig. 1).
In order to behave like a real gear, however, the researchers needed some way to fix the molecule to the surface so that it could be rotated without moving laterally. “In our experiments, we tried to find the best pinning center for HB-NBP,” says Carlos Manzano, one of the co-authors of the study.
Impurity atoms, located in an area of the gold surface covered with zigzag crystal ridges, proved to be ideal pinning sites. When the gear was mounted onto one of these small protrusions, it sat balanced and immobilized above the surface. Gently nudging one of the gear’s teeth with the STM tip smoothly rotated the gear in place with no lateral displacement.
Each push of the STM tip rotated the gear in small increments of 30–60°; eventually, full rotation was possible in both clockwise and counterclockwise directions. By combining experimental observations with theoretical calculations, the researchers discovered that this unique step-by-step rotation was due to the surface environment surrounding the gear.
According to Manzano, when the HB-NBP gear is pushed onto the impurity atom, it is trapped in place by a potential energy barrier created by the zigzag surface ridges. Fortunately, the energy barrier could be overcome with a push of the STM tip, causing the gear to turn until it contacted the potential barrier again. In this manner, the researchers were able to deliberately control the gear’s rotation.
Having an infinitesimal version of a working gear promises to spark development of more complex devices, capable of converting energy into work on an atomic scale. “This gear could be an essential component in any envisioned mechanically driven molecular machinery,” says Manzano.
The A*STAR affiliated authors in this highlight are from the Institute of Materials Research and Engineering