Scientists have developed a molecular motor that rotates when an electric current passes through it and reverses direction when the voltage polarity is changed. Inspired by the ancient Archimedes screw, the device connects a single spiral molecule to two metal electrodes and demonstrates a way to produce controlled motion at the smallest scale. The research also revealed that rotation can increase the molecule’s electrical conductivity by a factor of 100.
An ancient design for molecular machines
Molecular machines are considered a promising frontier in miniaturization. Living cells already contain molecular motors that use energy to produce movement, and researchers have spent decades developing artificial versions of these biological systems. Advances in the field were recognized with the 2016 Nobel Prize in Chemistry.
Scientists developed the first molecular motor driven directly by electricity in 2011. Those early devices operated on a surface and depended on the tip of a scanning tunneling microscope to drive their movement.
The new motor uses a different design, based on one of the oldest machines in recorded history: the Archimedes screw. Traditionally used to move water upstream, the device uses a rotating spiral to transport material.
At the molecular scale, the spiral instead provides a path for electrons. When an electric current passes through the molecule, the flow of electrons causes the molecular structure to rotate.
A molecular screw between gold electrodes
The motor’s core is a specially designed spiral molecule created by Professor Michael Pittelkow’s research group at the University of Copenhagen. The molecule is only a few millionths of a millimeter in size and can attach to a gold surface through its sulfur atom.
At the Weizmann Institute of Science in Israel, researchers led by Professor Oren Tal and Dr. Anil Kumar Singh placed individual molecules between two gold tips sharpened to atomic dimensions. By precisely changing the distance between the electrodes, they could stretch and compress the molecular screw.
When the researchers applied a voltage, measurements showed that the molecules began to rotate. Changing the voltage polarity reversed the direction of rotation.
The work is the first demonstration of a molecular motor driven by an electrical current, closely connected to two metal electrodes and capable of rotating in a controllable direction.
How the electric current produces rotation
The driving force comes from electrons moving through the helical molecule. As the electrons travel through the structure, they transfer angular momentum to it, causing rotation in a process comparable to moving air turning a wind turbine.
As with right-handed and left-handed threads, the shape of the spiral determines how it responds to the flow. Once the applied voltage exceeds a certain threshold, the motor enters continuous rotation.
The experiments also revealed that rotating molecules conduct electricity much more efficiently than stationary ones, raising questions about the processes taking place inside the molecular machine.
Rotation increases conductivity 100-fold
To investigate this unexpected behavior, Professor Fabian Pauly and PhD students Leon Martin and Yuta Ito from the Department of Theoretical Physics I at Augsburg University used quantum-mechanical simulations on the LiCCA supercomputer.
The calculations showed a close relationship between the molecule’s rotation and its ability to conduct current. Conductivity changes substantially with the molecule’s angle, increasing by two orders of magnitude—a factor of 100—as the molecule rotates.
The explanation was unexpected. Some molecular positions that allow electricity to pass most efficiently are also positions that the molecule naturally avoids when it is stationary. Rotation makes those highly conductive arrangements accessible.
“What is surprising is that when the molecule is at rest, it is precisely those positions that conduct electricity best. Only rotation makes these positions accessible,” Leon Martin explains.
The finding shows that molecular motion does more than dissipate electrical energy. Movement itself can strongly affect how electricity flows through the machine.
Combining experiments and theory
The researchers collaborated with theoretical physicists Per Hedegard of the Niels Bohr Institute in Copenhagen and Jingtao Lu of the Huazhong University of Science and Technology in Wuhan.
Using the simulation results, the scientists developed a simplified theoretical model that reproduced both the rotation and the associated increase in current observed in the experiments.
The combination of experimental measurements and theoretical calculations was essential for identifying the mechanism. Although the researchers could measure the current flowing through the molecules, simulations were needed to determine what was happening at this scale.
“In experiments, we only see the current. Our calculations give us a glimpse of what is happening inside the nanocontact. It is only through close cooperation between experiment and theory across several countries that we can solve the mystery of the current increase,” says Yuta Ito, a doctoral student from Japan.
Potential applications in molecular electronics
The result could eventually contribute to molecular electronic components and spintronics-related technologies, which use electron properties beyond charge. It may also support microfluidic systems designed to manipulate extremely small volumes of fluid.
More broadly, the motor gives scientists a new way to study physical processes at the atomic scale. Converting electricity into controlled rotation—and using that rotation to influence electrical current—introduces new principles for designing machines at the limit of miniaturization.
“We showed that electric current can be used to generate directional rotation, and that this rotation controls the electric current. This is a new operating principle for machines at the limit of miniaturization,” Pauly explains.
The research at the University of Augsburg was funded by the German Research Foundation (DFG) through Joint Research Center (SFB) 1585, project number 492723217. The computational study was carried out using the LiCCA supercomputer at the University of Augsburg under DFG project number 499211671.
Additional support for the collaborating institutions came from the European Research Council (ERC, Horizon 2020, grant 864008), the Israel Science Foundation (2129/23), the Minerva Foundation, the Vilam 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.
Source: www.sciencedaily.com
