Scientists have developed a remarkable new molecular motor that spins when electricity passes through it, marking a major milestone in the quest to build functional machinery at the absolute smallest possible scale. Inspired by one of the oldest inventions in human history, this tiny machine can even reverse its direction of rotation simply when the electrical voltage is switched.
The achievement bridges ancient engineering concepts with the cutting-edge realm of quantum mechanics and molecular electronics. While molecular machines are widely viewed as a promising next frontier in scientific miniaturization—with nature already providing intricate examples inside living cells where biological molecular motors use chemical energy to produce vital movement—recreating these systems artificially has been an uphill battle for researchers over the past several decades. Progress in this complex field was famously recognized with the 2016 Nobel Prize in Chemistry, and this latest development pushes the boundaries even further by demonstrating direct electrical control over a single-molecule device.
An Ancient Invention Inspires a Molecular Motor
Scientists first achieved significant breakthroughs in developing molecular motors powered directly by electricity back in 2011. However, those earlier experimental designs operated almost exclusively on flat surfaces and heavily relied on the ultra-fine tip of a scanning tunneling microscope to drive their mechanical movement, limiting their versatility and integration potential.
The new motor takes a radically different approach, cleverly borrowing its foundational mechanical design from the Archimedes screw. Widely regarded as one of the oldest continuous machines in recorded history, the traditional Archimedes screw was historically utilized to move water uphill by relying on a rotating internal spiral to transport material smoothly along an incline.
Researchers successfully adapted that time-tested physical principle down to the molecular scale. Instead of moving water through a large physical channel, their innovative design allows individual electrons to travel directly through a custom-built, spiral-shaped molecule. As the electrical current passes through, it forces the molecular structure to rotate continuously, effectively translating subatomic particle flow into macroscopic mechanical motion at the nanolevel.
A Tiny Molecular Screw Suspended Between Gold Tips
At the heart of this technological feat is a specially designed spiral molecule created by the research group of Professor Michael Pittelkow at the University of Copenhagen. Measuring only a few millionths of a millimeter in length, the molecule is engineered with sulfur atoms on its extremities, allowing it to anchor securely onto gold surfaces.
To test and observe the motor in action, researchers led by Professor Oren Tal and Dr. Anil Kumar Singh at the Weizmann Institute of Science in Israel positioned individual molecules precisely between two gold tips that had been sharpened down to atomic dimensions. By carefully adjusting the physical separation between these microscopic electrodes, the research team could dynamically stretch or compress the molecular screw.
When the scientists applied a precise electrical voltage across the junction, sensitive measurements revealed that the molecule began rotating on its axis. Even more crucially, the team discovered they could instantly reverse its rotational direction simply by altering the polarity of the applied voltage.
This accomplishment represents the first successful demonstration of a molecular motor that is driven entirely by electrical current, firmly connected at both ends to metal electrodes, and capable of rotating in a fully controllable and reversible direction.
How Electricity Makes a Single Molecule Spin
The primary driving force behind this continuous mechanical movement comes directly from electrons traveling through the helical structure of the spiral molecule. As these subatomic particles flow from one electrode to the other, they transfer angular momentum to the molecular framework, producing physical rotation in a manner remarkably similar to how moving air turns the blades of a wind turbine.
The specific geometry and shape of the molecular spiral dictate exactly how it reacts to this continuous flow of electrons, much like the functional difference between screws featuring right-handed or left-handed threads. Once the applied voltage exceeds a precise threshold, the erratic thermal jiggling transitions into smooth, continuous rotation.
However, as the experiments progressed, the researchers observed another intriguing and unexpected physical effect. The rotating molecule conducted electricity much more efficiently than it did when it remained stationary, raising profound theoretical questions about the complex electromechanical interplay happening inside the tiny machine.
Scientists Discover a Surprising 100-Fold Electrical Effect
To investigate this puzzling behavior, Professor 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, turned to advanced quantum mechanical simulations executed on the university’s LiCCA supercomputer.
Their rigorous calculations revealed an intimate, unexpected relationship between the molecule’s physical rotation and its fundamental ability to carry electrical current. The molecule’s electrical conductivity changes dramatically depending on its precise rotational angle, surging upward by as much as two orders of magnitude—a staggering 100-fold increase—while the machine is actively spinning.
The root cause of this phenomenon proved to be particularly surprising. Some of the specific molecular orientations and spatial positions that allow electricity to pass through most easily are also configurations that the molecule naturally tends to avoid when it is at rest. The continuous mechanical rotation forces the structure through these otherwise inaccessible states, unlocking pathways of high conductivity.
"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," explains Léon Martin, highlighting how molecular motion actively shapes electronic transport rather than merely consuming energy.
International Collaboration Solves the Mystery
To fully validate these computational insights, the Augsburg team collaborated closely with theoretical physicists Per Hedegård of the Niels Bohr Institute in Copenhagen and Jing-Tao Lü of the Huazhong University of Science and Technology in Wuhan.
By integrating the supercomputer simulation results, the international collective developed a simplified theoretical model that successfully recreated both the physical rotation observed in the physical experiments and the dramatic accompanying surge in electrical current.
This close synthesis of empirical laboratory measurements and theoretical calculations proved essential to deciphering the underlying mechanism. Because researchers could directly measure only the current passing through the external circuit, advanced simulations were required to visualize the invisible dynamics occurring within the atomic-scale nanocontact.
"In the experiment, you only see the current. Our calculations allow us to glimpse what is happening inside the nanocontact. It was only through close collaboration between experiment and theory across several countries that the mystery of the current increase could be solved," notes Yuta Ito, a PhD student originally from Japan who contributed to the theoretical modeling.
A New Frontier for Molecular Electronics
The successful creation and operation of this electrically driven molecular motor could eventually pave the way for revolutionary advancements in molecular electronic components and spintronics—a rapidly evolving field that harnesses electron properties extending well beyond simple electrical charge. Furthermore, the technology may eventually offer novel possibilities for advanced microfluidic systems designed to manipulate extremely minute quantities of liquids with ultra-precise control.
Beyond these tangible long-term applications, the motor provides researchers with an unprecedented tool to investigate fundamental physical processes operating at the ultimate atomic scale. Its unique capability to convert electrical energy into controlled mechanical rotation, while simultaneously allowing that physical movement to directly modulate electrical current, establishes an entirely new operating principle for machinery built at the physical limits of miniaturization.
"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," concludes Professor Pauly.
The groundbreaking research conducted at the University of Augsburg was supported by funding from the German Research Foundation (DFG) through Collaborative Research Center SFB 1585, with computational workloads carried out on the LiCCA supercomputer. Additional institutional support across the global collaboration was provided by the European Research Council under Horizon 2020, the Israel Science Foundation, the Minerva Foundation, the Villum Foundation, the Novo Nordisk Foundation, the Independent Research Fund Denmark, and national science funding programs in China.