Scientists have developed an electrically driven molecular motor that can rotate in either direction when the polarity of the applied voltage is changed. Inspired by the ancient Archimedes screw, the device uses a single spiral molecule connected between two gold electrodes. The research also revealed that rotation can increase the molecule’s electrical conductivity by a factor of 100, providing new insight into how motion and current interact at the smallest scale.
An ancient design at molecular scale
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. Progress in the field was recognized with the 2016 Nobel Prize in Chemistry.
Scientists developed the first molecular motor driven directly by electricity in 2011. Those early designs 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 the Archimedes screw, one of the oldest machines in recorded history. Traditionally, the device uses a rotating spiral to move water upstream. At the molecular scale, the spiral instead provides a path for electrons, and the flow of current causes the molecule to rotate.
A molecular screw between gold electrodes
The motor’s central component 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. Reversing the voltage polarity reversed the direction of rotation.
The experiment is the first demonstration of a molecular motor driven by an electrical current, tightly connected to two metal electrodes and capable of rotating in a controllable direction.
How current drives rotation
The driving force comes from electrons moving through the helical molecule. As the electrons travel through it, they transfer angular momentum to the molecular structure, causing it to rotate in a process comparable to moving air turning a wind turbine.
As with right-handed and left-handed threads, the spiral’s shape determines how it responds to the flow of electrons. Once the applied voltage exceeds a certain threshold, the motor enters a regime of continuous rotation.
The experiment also revealed that rotating molecules conduct electricity much more efficiently than stationary ones, prompting further investigation into the mechanism.
Rotation produces a 100-fold change in conductivity
Professor Fabian Pauly and PhD students Leon Martin and Yuta Ito, from the Department of Theoretical Physics I at Augsburg University, investigated the unexpected effect using quantum mechanical simulations on the LiCCA supercomputer.
Their calculations showed a close relationship between the molecule’s rotation and its ability to conduct current. Conductivity varied sharply with the molecule’s angle, increasing by two orders of magnitude—a factor of 100—as the molecule rotated.
The explanation was unexpected. Some molecular configurations that conduct electricity most efficiently are also configurations that a stationary molecule naturally avoids. 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,” explains Leon Martin.
The finding indicates that molecular motion does more than dissipate electrical energy. Movement itself can substantially influence how electricity flows through a molecular 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 related increase in current observed in the experiments.
The combination of measurements and theoretical calculations was essential to identifying the mechanism. Although the researchers could measure the current flowing through the molecules, simulations were needed to determine what was happening at the nanoscale.
“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 properties of electrons beyond their charge. It may also have applications in microfluidic systems designed to manipulate extremely small volumes of fluid.
The motor also 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 limits 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.
Funding and collaboration
The research at the University of Augsburg was funded by the German Research Foundation (DFG) through the 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