The nano-propeller

For the first time, an electric motor has been made from a single molecule. At 1 nanometre long, that makes the organic compound the smallest electric motor ever.

Its creators plan to submit their design to Guinness World Records, but the teeny motor could also have practical applications, such as pushing fluid through narrow pipes in “lab-on-a-chip” devices.

Molecules have previously converted energy from light and chemical reactions into directed motion like rolling or flapping. Electricity has also set an oxygen molecule spinning randomly. But controlled, electrically-driven motion – necessary for a device to be classed as a motor – had not yet been observed in a single molecule.

To address this, E. Charles Sykes at Tufts University in Boston and colleagues turned to asymmetric butyl methyl sulphide, a sulphur atom with a chain of four carbons on one side and a lone carbon atom on the other. They anchored the molecule to a copper surface via the sulphur atom, producing a lopsided, horizontal “propeller” that is free to rotate about the vertical copper-sulphur bond (see diagram).


Record smashed

Above the molecule they placed a metal needle a few atoms wide at its tip. When they flowed a current from this tip, through the molecule, to the conductive copper below, the molecule converted the electrical energy into rotational energy. It bounced around in jittery hops about 50 times a second.

Because the propeller is asymmetrical, there are two ways it can be oriented with respect to the copper. In one orientation – but not the other – the molecule’s hops were not random but slightly biased towards rotating clockwise, allowing the researchers to classify it as a motor.

It is not clear why the bias occurs but Sykes suspects that an inherent asymmetry in the tip of the metal needle could explain why it only occurs in one molecular orientation.

Friction fighter

If accepted by Guinness, the motor will be a record smasher. The current world-record holder for the smallest electric motor is a giant by comparison, composed of two 200-nanometre-long carbon nanotubes. Current running through these nanotubes pushes drops of molten metal from the outside of one tube to the other.

Sykes hopes to harness his tiny motor to fight the friction that slows fluid flow in nanosized tubes.

Kevin Kelly of Rice University in Houston, Texas, who was not involved in the work, points out that if electrical energy transfer behaves differently depending on the shape of the molecules, this could have implications for the design of molecule-scale electrical circuits, which could be used in tiny sensors or computer chips.

Journal reference: Nature Nanotechnology, DOI: 10.1038/nnano.2011.142