Ben Feringa and his research group at the university of Groningen have successfully managed to power a molecule using a scanning tunnel microscope tip across a surface.
The device is based on a model described back in 1999. The 1999 molecular motor consists of two parts: a fixed, static part that can serve as anchor and a unidirectional rotating part that can work as a screw or propeller. It gave rise to the idea that it could function as something like an outboard motor fixed to a larger unit.
However, in the current nanovehicle [Feringa et al., Nature, doi: 10.1038/nature10587] the motor is used as a wheel. The new molecule has a long mid-section with four rotating wheels at its corners. Or perhaps it would be better to call them paddles, as the extremities are not completely round.
The four-wheeled molecule receives its power to move from an STM tip that serves as something like a train’s overhead wire. An STM (scanning tunnel microscope) feels its way across a surface with a pointed wire without any actual ‘physical’ contact taking place – the very last bit is bridged by an electrical charge.
The energy from the STM tip ‘tunnels’ into the molecule, which then enters a higher energy level, leading to the wheels turning step by step; this process is analogous to energy transfer by photons.
In the Nature article STM images show the molecule moving across a copper surface. After ten steps it has moved 6 nanometres in a more or less straight line. To prove that it is indeed the wheels that are propelling it, the researchers show in follow-up experiments what happens when the wheels rotate in the wrong direction. The chemists studied molecules where the rear wheels turned in the opposite direction to the front ones, or where the left wheels rotated backwards while the right pair rotated forwards or vice versa. The resulting movements are exactly what you would expect – the molecule hardly shifts position.
These last experiments clearly prove that the movement is caused by the wheels rotating, according to the researchers in their conclusion. Perhaps the speed and performance leave something to be desired but the proof has been given: a single molecule with intrinsic motor functions is capable of turning energy delivered from outside into a movement in a single direction across a surface.