Scientists have successfully developed a revolutionary molecular motor that spins when an electrical current passes through it. Inspired by one of the oldest mechanical inventions in human history, this microscopic machine can even reverse its direction of rotation simply when the applied electrical voltage is switched. The remarkable achievement marks a critical, forward-looking step toward the realization of working machines built at the absolute smallest possible scale, bridging the gap between ancient engineering principles and cutting-edge quantum mechanics.
For decades, molecular machines have been widely regarded by the scientific community as a deeply promising next frontier in the ongoing quest for extreme miniaturization. Nature has long provided extraordinary working examples of this inside living biological cells, where natural molecular motors harness chemical energy to produce vital mechanical movement. Researchers have spent generations attempting to artificially recreate these sophisticated biological systems in the laboratory, a monumental effort that was famously recognized with the awarding of the 2016 Nobel Prize in Chemistry.
An Ancient Invention Inspires a Modern Molecular Motor
The journey toward electrically driven nanotechnology has been gradual. Scientists first developed molecular motors powered directly by electricity back in 2011, yet those earlier foundational designs operated strictly on flat surfaces and relied heavily on the ultra-precise, cumbersome tip of a scanning tunneling microscope to physically drive their movement.
The newly engineered motor takes a radically different and more self-contained approach, cleverly borrowing its fundamental structural design from the Archimedes screw. Hailed as one of the oldest machines in all of recorded history, the traditional Archimedes screw has long been utilized to move water uphill through the simple mechanical action of a rotating internal spiral that transports material upward.
An international team of researchers successfully adapted that enduring engineering principle down to the true molecular scale. Instead of physically shifting water droplets, their innovative design allows individual electrons to travel directly through a specially engineered spiral-shaped molecule, causing the entire microscopic structure to rotate continuously as an electrical current flows through it.
A Tiny Molecular Screw Suspended Between Gold Tips
At the very heart of this technological breakthrough is a specialized spiral molecule synthesized meticulously by the research group led by Professor Michael Pittelkow at the University of Copenhagen. Measuring only a few millionths of a millimeter across, this fragile organic molecule is ingeniously designed to anchor securely to gold surfaces via built-in sulfur atoms.
Further advancing the experiment, researchers at the Weizmann Institute of Science in Israel—led by Professor Oren Tal and Dr. Anil Kumar Singh—positioned individual molecules delicately between two microscopic gold tips that had been meticulously sharpened down to atomic dimensions. By exercising extreme precision in altering the physical separation between these two microscopic electrodes, the research team could stretch or compress the molecular screw at will.
When the scientists applied a controlled electrical voltage across the system, sensitive measurements revealed that the molecule spontaneously began to rotate. More critically, the team discovered they could smoothly and predictably reverse the direction of this rotation simply by reversing the polarity of the applied voltage.
This experiment represents the first definitive demonstration of a molecular motor that is driven fundamentally by an electrical current, firmly connected to two metal electrodes at either end, and fully capable of rotating in a highly controllable, directional manner.
How Electricity Makes a Single Molecule Spin
The primary driving force behind this microscopic phenomenon stems directly from electrons traveling through the structural curves of the spiral molecule. As these subatomic particles move along their path, they transfer minute amounts of angular momentum directly to the molecular structure, generating continuous mechanical rotation in a manner remarkably similar to how moving air turns the blades of a massive wind turbine.
The specific geometry of the spiral dictates precisely how it reacts to this continuous flow of electrons, bearing a conceptual resemblance to the distinct physical differences found between mechanical screws with right-handed or left-handed threads. Once the applied voltage exceeds a precise operational threshold, the intermittent movement transitions into a smooth, continuous rotation.
However, as the experiments progressed, the research team observed another highly intriguing physical effect. The actively rotating molecule conducted electricity significantly more efficiently than it did when it was completely stationary. This unexpected behavior raised profound questions about the hidden mechanical and electrical dynamics occurring inside the tiny machine.
Scientists Discover a Surprising 100-Fold Electrical Effect
To thoroughly investigate this anomalous electrical behavior, Professor Dr. Fabian Pauly and his doctoral students, Léon Martin and Yuta Ito—working diligently at the Chair of Theoretical Physics I at the University of Augsburg—turned to advanced quantum mechanical simulations executed on the university’s high-performance LiCCA supercomputer.
Their rigorous computational calculations unveiled a startlingly close and intricate relationship between the physical rotation of the molecule and its underlying capacity to transport electrical current. The electrical conductivity changes dramatically depending on the exact rotational angle of the molecule, surging upward by as much as two full orders of magnitude—representing a staggering 100-fold increase—during active rotation.
The underlying reason for this dramatic shift proved to be deeply surprising. Certain specific molecular physical positions that allow electricity to flow through with the least amount of resistance are precisely the configurations that the molecule naturally tends to avoid when it is resting in a static state. The physical act of rotation forces the molecule out of its comfort zone, making these highly conductive spatial arrangements continuously accessible.
"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 the counterintuitive nature of quantum-scale mechanics.
This critical finding demonstrates conclusively that molecular motion accomplishes far more than merely consuming electrical energy. The physical movement itself actively dictates and strongly influences how electricity flows through the machine, establishing a dynamic feedback loop at the atomic scale.
International Collaboration Solves the Mystery
To fully validate these findings, the German research team joined forces 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.
Utilizing the comprehensive data generated by the quantum simulations, the multinational group of scientists developed a streamlined theoretical model that successfully and accurately recreated both the physical rotation observed in the laboratory experiments and the corresponding dramatic surge in electrical current.
This essential synergy between empirical experimental measurements and complex theoretical calculations proved indispensable to fully comprehending the underlying mechanism. Although researchers could readily measure the electrical current passing through the molecule in real time, they relied heavily on sophisticated computer simulations to visualize and understand what was truly happening at such an extraordinarily diminutive scale.
"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 participated in the Augsburg research.
A New Frontier for Molecular Electronics
The successful development of this electrical molecular motor could eventually pave the way for groundbreaking advancements in molecular electronic components and sophisticated technologies involving spintronics, a specialized field that harnesses fundamental properties of electrons well beyond their basic electrical charge. Furthermore, the breakthrough may unlock entirely new possibilities for advanced microfluidic systems, which are designed to manipulate extremely minute, microscopic quantities of liquids with absolute precision.
Beyond these tangible long-term technological applications, the motor provides the scientific community with an entirely novel experimental platform to investigate fundamental physical processes at the atomic scale. Its unique capability to seamlessly convert electrical energy into controlled mechanical rotation—while simultaneously allowing that rotation to directly modulate the electrical current—introduces an unprecedented operating principle for engineering functional machines at the absolute limits of human 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 foundational research conducted at the University of Augsburg received generous financial backing from the German Research Foundation through Collaborative Research Center 1585. The intensive computational research required to model the quantum mechanics was successfully executed using the university’s LiCCA supercomputer facility. Additional vital financial and institutional support for the collaborating research centers 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, as well as key national research and development programs and foundations in China.
