2,000-year-old Archimedes’ screw inspires motor that spins and boosts electric current

A machine inspired by an ancient water pump can now operate at the scale of a single molecule. A team of international researchers has built a tiny motor that uses an electric current to drive a screw-shaped molecule into rotation, while the movement itself can increase the current flowing through it by up to 100 times. This discovery establishes a new way to couple mechanical motion and electricity at the smallest scales, opening new possibilities for molecular electronics. Scientists have spent decades trying to build artificial molecular machines inspired by the motors that power movement inside living cells. However, making a molecule rotate predictably under an electric current while maintaining electrical contact with metal electrodes has remained a challenge. The new device demonstrates voltage-controlled rotation with 100 percent directional preference, using a principle that dates back more than 2,000 years. “We have shown that an electric current can be used to generate a directed rotation and that this rotation, in turn, controls the current. This is a new operating principle for machines at the very limits of miniaturization,” Fabian Pauly, one of the study authors and a professor at the University of Augsburg in Germany, said. The Archimedes screw gets a molecular makeover The researchers took inspiration from the Archimedes’ screw, an ancient device with a helical blade inside a cylinder that lifts water (or solids) when rotated. In the new machine, electrons flowing through a similarly twisted structure generate the torque that makes it rotate. Instead of relying on an external mechanical tool to move the molecule, the current itself powers the motion. The motor’s central component is a dithioacetyl-functionalized (13) helicene molecule, synthesized specifically for the study by Michael Pittelkow’s team at the University of Copenhagen. Measuring just a few nanometres, the molecule can attach to gold surfaces through sulfur atoms. At the Weizmann Institute of Science in Israel, researchers led by Oren Tal and Anil Kumar Singh connected individual molecules between two extremely fine gold tips. These electrodes provided a path for electric current through the molecule. By adjusting the distance between the tips, the team could stretch or compress the molecular structure, changing the voltage at which the current enhancement began. When the researchers applied a voltage, the molecule underwent cyclic motion with a clear directional preference. Reversing the voltage’s polarity reversed the direction of rotation. This gave the team control over the motor’s movement without physically manipulating it. The motion is driven by current-induced forces: as electrons pass through the helical molecule, they transfer angular momentum to it. The molecule’s handedness helps determine the direction of rotation, much as the thread of a conventional screw determines how it advances. The researchers demonstrated that the direction could be switched by reversing the voltage polarity. Other researchers have also developed DNA-based molecular nanomachines, although they use a different source of energy to generate motion. The spinning molecule opens a better path for electrons The study authors also observed a surprising electrical effect: once the molecule began moving, the current could rise sharply. To understand why, they used quantum-mechanical simulations on the LiCCA supercomputer. Their calculations showed that the molecule conducts electricity very differently depending on its rotational angle. As it rotates, it can access configurations that allow electrons to flow much more easily, increasing conductance by up to two orders of magnitude, or roughly 100 times. “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,” Léon Martin, one of the researchers and a PhD student at the University of Augsburg, said. Next, the researchers developed a simplified model that reproduced the observed rotation and current increase. Their calculations also predict that, above a threshold voltage, the molecule can transition from stepwise movement to continuous rotation. Such calculations are part of a wider effort to understand why molecular simulations remain challenging. “In the experiment, you only see the current. Our calculations allow us to glimpse what is happening inside the nanocontact. Yuta Ito, one of the study authors, said. A motor, not yet a miniature machine shop The findings could help researchers develop molecular electronic components, spintronic devices that use electron spin to process information, and tiny systems for manipulating fluids. They also offer a way to investigate how mechanical motion and electrical transport interact at the molecular scale. However, the device remains a laboratory prototype based on an individual molecule held between gold electrodes. Its performance in practical devices has not yet been established, and the proposed applications remain possibilities rather than demonstrated uses. Further research will be needed to determine how the motor behaves under different operating conditions and whether the principle can be incorporated into functional molecular technologies. The study is published in the journal Nature. 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