More than 2,000 years ago, the great polymath Archimedes devised a screw-shaped machine to lift water. Now, scientists have built a version so small that its moving part consists of a single molecule.
Instead of pumping water, this molecular screw spins when electricity passes through it. As it rotates, the molecule moves through positions that allow electrons to flow more easily, increasing the current by as much as a hundredfold. This could lead to a new generation of molecular-scale electronic switches, where the movement of a single molecule controls how much electricity flows through a circuit.
What’s more, the molecular motor’s direction of rotation can be reversed simply by changing the electrical current. Unlike earlier designs, it operates between two stationary electrodes, and its movement directly influences its electrical conductivity.
A new twist on Archimedes screw: Simulation of a helical molecule rotating between two gold electrodes under the influence of an electric current flowing through it. The molecule’s rotation, in turn, enhances its electrical conduction. Credit: Weizmann Institute of Science.
How to Make a Molecule Spin
The Archimedes screw works by rotating a spiral blade to push water uphill. The researchers borrowed its helical design but replaced the water with a stream of electrons.
Chemists led by Michael Pittelkow at the University of Copenhagen synthesized a spiral-shaped organic molecule called helicene. Its ends contain sulfur atoms that attach to gold, allowing scientists to suspend it between two extraordinarily fine gold electrodes.
At the Weizmann Institute of Science in Israel, researchers led by Oren Tal and Anil Kumar Singh assembled these molecular devices and cooled them to just 4.2 degrees above absolute zero.
When the researchers switched on the electricity, they noticed something unusual. Instead of flowing steadily, the current rose and fell in a repeating pattern. Each change suggested that the molecule had turned slightly, moving into a new position that allowed a different amount of electricity to pass through. By tracking these changes, the researchers could work out how the molecule was moving, even though it was far too small to watch directly.
Reversing the voltage reversed the sequence of jumps, indicating that the molecule had changed its direction of rotation.
×
Thank you! One more thing…
Please check your inbox and confirm your subscription.
The underlying mechanism resembles a wind turbine. As electrons flow through the spiral, they transfer angular momentum to the molecule, exerting a twisting force that drives rotation.
Scientists have built electrically driven molecular motors before, including one from 2011 that spun on a copper surface. Another 2013 experiment showed how to control rotation in either direction. But both these designs relied on scanning microscope tips to drive molecules attached to surfaces. The new motor hangs between fixed electrodes, with its rotation controlled by the direction of the current passing through it.
The Surprising Link Between Rotation and Electricity
At lower voltages, the molecule appears to rotate in discrete steps, briefly settling into stable positions. Above a certain threshold, the current suddenly increased by as much as a hundredfold.
Why? Quantum-mechanical calculations by collaborators at the University of Augsburg and others suggest that the molecule transitions from stepwise movement to continuous rotation. As it spins, it passes through positions that allow electrons to travel between the gold electrodes much more easily. The molecule effectively becomes a better electrical conductor while moving.
“The surprising thing is that it is precisely the positions that the molecule avoids when at rest that conduct electricity best. It is only the rotation that makes these positions accessible,” said León Martin, a doctoral researcher at the University of Augsburg.
The researchers also found that they could adjust when this dramatic increase occurred. By moving the gold electrodes less than a tenth of a nanometer, they could stretch or compress the molecular spiral, changing the electrical current needed to drive the faster motion.
Importantly, the surge wasn’t simply caused by the screw pumping electrons, as an Archimedes screw pumps water. The computer models showed that this effect was far too small to explain the observations. Instead, rotation changed how effectively electrons passed through the molecule.
The team also tested a comparable straight molecule. It showed neither the characteristic cyclic electrical signals nor the dramatic conductivity increase.
The researchers believe their design could eventually contribute to molecular electronics, devices that exploit electron spin, and microscopic systems that manipulate fluids. It seems like electricity can power molecular movement, and that movement can, in turn, control electricity.
The new findings were reported in the journal Nature.
