Scientists have developed a surprising new molecular motor that spins when electricity passes through it. Inspired by an ancient invention, the little machine can even reverse its direction when the electrical voltage is changed. The achievement marks an important step towards creating functional machines at the smallest possible scale. Molecular machines are widely considered a promising next frontier in miniaturization. Nature already provides examples within living cells, where molecular motors use energy to produce movement. Researchers have spent decades trying to artificially recreate these remarkable biological systems, and advances in this field were recognized with the 2016 Nobel Prize in Chemistry. An ancient invention inspires a molecular motor Scientists first developed molecular motors driven directly by electricity in 2011. However, those previous designs operated on surfaces and relied on the tip of a scanning tunneling microscope to power their motion. The new engine takes a different approach and borrows its basic design from the Archimedean screw, one of the oldest machines in history. This simple device, traditionally used to move water uphill, relies on a rotating spiral to transport material. The researchers adapted that principle to the molecular scale. Instead of moving water, its design allows electrons to travel through a spiral-shaped molecule, causing it to spin as the electric current passes. A small molecular screw suspended between gold spikes The central component of the motor is a specially designed spiral molecule created by the research group of Prof. Michael Pittelkow at the University of Copenhagen. The molecule, which measures just a few millionths of a millimeter, can bind to gold surfaces through sulfur atoms. At Israel’s Weizmann Institute of Science, researchers led by Prof. Oren Tal and Dr. Anil Kumar Singh placed individual molecules between two gold spikes sharpened to atomic dimensions. By carefully changing the spacing between these electrodes, the team was able to stretch or compress the molecular screw. When the scientists applied an electrical voltage, measurements revealed that the molecule began to spin. Even more importantly, they could reverse their direction simply by changing the polarity of the voltage. This represents the first demonstration of a molecular motor driven by electric current, firmly connected to two metal electrodes and capable of rotating in a controllable direction. How electricity spins a single molecule The driving force comes directly from the electrons traveling through the spiral molecule. As they move, they transfer angular momentum to the molecular structure, producing rotation in the same way that moving air spins a wind turbine. The shape of the spiral determines how it responds to this flow, similar to the difference between screws with right-hand or left-hand threads. Once the applied voltage exceeds a certain threshold, the motion becomes continuous rotation. But the experiments revealed another intriguing effect. The rotating molecule conducted electricity much more efficiently than when it was stationary, raising an important question about what was happening inside the little machine. Scientists discover surprising hundred-fold electrical effect To investigate this unexpected behavior, Prof. Dr. Fabian Pauly and his doctoral students Léon Martin and Yuta Ito, working at the Chair of Theoretical Physics I at the University of Augsburg, used quantum mechanics simulations on the LiCCA supercomputer. Their calculations revealed a close relationship between the molecule’s rotation and its ability to carry electric current. The conductivity changes dramatically depending on the angle of the molecule, increasing up to two orders of magnitude, or 100 times, during rotation. The reason turned out to be particularly surprising. Some of the molecular positions that allow electricity to pass more easily are also positions that the molecule naturally avoids when not in motion. Rotation makes those highly conductive arrangements accessible. “The surprising thing is that it is precisely the positions that the molecule avoids at rest that conduct electricity best. Only rotation makes these positions accessible,” explains Léon Martin. The finding demonstrates that molecular movement does more than simply consume electrical energy. The movement itself can strongly influence how electricity flows through the machine. International collaboration solves mystery The researchers then worked with theoretical physicists Per Hedegård from the Niels Bohr Institute in Copenhagen and Jing-Tao Lü from Huazhong University of Science and Technology in Wuhan. Using the simulation results, the scientists developed a simplified theoretical model that successfully recreated both the rotation observed in the experiments and the subsequent increase in electrical current. This combination of experimental measurements and theoretical calculations was essential to understanding the mechanism. Although the researchers could measure the current passing through the molecule, they needed simulations to determine what was happening at such an extraordinarily small scale. “In the experiment, only the current is seen. Our calculations give us a glimpse of what happens inside the nanocontact. Only through close collaboration between experiment and theory in several countries could the mystery of the current increase be solved,” says Yuta Ito, a doctoral student originally from Japan. A new frontier for molecular electronics The achievement could eventually contribute to the development of molecular electronic technologies and components related to spintronics, which uses properties of electrons beyond their electrical charge. It may also offer possibilities for microfluidic systems, which manipulate extremely small amounts of fluids. Beyond these potential applications, the engine provides scientists with a new way to investigate physical processes at the atomic scale. Its ability to convert electricity into controlled rotation, while allowing that rotation to influence electrical current, introduces a new principle for designing extraordinarily small machines. “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,” explains Pauly. The work at Augsburg University received funding from the German Research Foundation (DFG) through the Collaborative Research Center (SFB) 1585 (project number 492723217). The computational research was carried out on the LiCCA supercomputer at the University of Augsburg (DFG project number 499211671). Additional support for collaborating institutions came from the European Research Council (ERC, Horizon 2020, Grant 864008), the Israel Science Foundation (2129/23), the Minerva Foundation, the Villum Foundation, the Novo Nordisk Foundation, the Danish Independent Research Fund (DFF), the National Key Research and Development Program of China, and the National Natural Science Foundation of China.
