Single Molecule Spins Like Archimedes’ Screw When Driven by Electric Current

An international team of physicists has built a molecular motor modelled on one of humanity’s oldest machines: the Archimedes screw. Instead of lifting water, the helical molecule uses a flow of electrons to set itself in rotation, and the direction of that rotation can be deliberately controlled by reversing the voltage’s polarity. The study, published in Nature, demonstrates for the first time a current-driven molecular motor with a controllable direction of rotation that is firmly attached between two metal electrodes.

Molecular machines are regarded as the next step in miniaturisation, and living cells already contain natural versions in which molecular motors convert energy into movement. Scientists have worked for decades to replicate such systems artificially, an effort recognised with the 2016 Nobel Prize in Chemistry. Electrically driven molecular motors have existed since 2011, but until now they have only functioned as molecules on a surface, pushed around by the tip of a scanning tunnelling microscope. The new device is different: the rotating molecule is clamped between two electrodes, and the current itself does the driving.

The helical molecule at the heart of the machine was produced specifically for the study by Professor Michael Pittelkow’s team at the University of Copenhagen. It measures just a few millionths of a millimetre and carries sulphur atoms that allow it to bind to gold surfaces. At the Weizmann Institute of Science in Israel, the group led by Professor Oren Tal and Dr Anil Kumar Singh stretched individual molecules between two atomically fine gold tips. By precisely adjusting the distance between the electrodes, the researchers could stretch or compress the molecular screw at will.

Measurements showed that the molecule rotates when a voltage is applied, and that the direction of rotation is unambiguous but can be reversed by switching the polarity of the voltage. The electrons themselves provide the drive: as they flow through the helical structure, they transfer angular momentum to it, much as wind drives a wind turbine. The handedness of the molecular coil determines the direction of rotation, just as a screw with a right- or left-hand thread turns one way or the other. Above a certain voltage, the movement transitions into continuous rotation.

A striking experimental observation was that the rotating molecule conducts electricity significantly better than a stationary one. Professor Fabian Pauly and his doctoral students Léon Martin and Yuta Ito at the University of Augsburg investigated this question using quantum mechanical simulations on the LiCCA supercomputer. Their calculations revealed that the molecule’s conductivity depends heavily on its angle of rotation and can increase by up to two orders of magnitude during rotation. “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.

Building on these findings, the theoretical partners Per Hedegård from the Niels Bohr Institute in Copenhagen and Jing-Tao Lü from Huazhong University of Science and Technology in Wuhan developed a simplified model that successfully reproduced both the experimentally observed rotation and the increase in current. “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,” says Yuta Ito.

The researchers see the result as a new operating principle for machines at the very limits of miniaturisation. “We have shown that an electric current can be used to generate a directed rotation and that this rotation, in turn, controls the current,” explains Pauly. Because the molecule’s rotation and its conductivity are so tightly coupled, the device links mechanical motion and electronic transport in a single molecular unit, a coupling that could prove valuable in future molecular electronic components.

The discovery holds potential for applications in molecular electronics, spintronics and microfluidic systems. Such miniature machines also offer new opportunities for researching physical processes on an atomic scale, where the interplay between current, structure and motion can be examined directly. The work, published in Nature, resulted from a collaboration spanning the University of Copenhagen, the Weizmann Institute of Science, the University of Augsburg, the Niels Bohr Institute and Huazhong University of Science and Technology, combining custom molecular synthesis, single-molecule electrical measurements and quantum mechanical theory.

Subject of Research: A helical single molecule stretched between gold electrodes rotates in a controllable direction when driven by an electric current, demonstrating the Archimedes screw principle at the molecular scale.

Article Title: Archimedes’ screw made from a single molecule

Article References: Archimedes’ screw made from a single molecule. Original publication (Bibliographic details unavailable.)

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Keywords: molecular motor, Archimedes screw, single molecule, electric current, helical molecule, gold electrodes, quantum mechanical simulation, conductivity, molecular electronics, spintronics, miniaturisation, Nature publication