Archimedes’ Screw Made From A Single Molecule

Eddie Gonzales Jr. – MessageToEagle.com – Molecular machines are regarded as the next step in miniaturization. Their models can be found in living cells, where molecular motors convert energy and generate movement.

The Archimedes screw is used to transport water uphill; an international team of physicists has now applied this principle at the molecular level. Image credit: Oren Tal / Anil Kumar Singh

Scientists have been working for decades to artificially replicate such systems. The 2016 Nobel Prize in Chemistry was awarded for this research.

A model from antiquity

Molecular motors powered directly by an electric current have existed since 2011. Until now, however, they have only functioned as molecules on a surface, driven by the tip of a scanning tunneling microscope.

For their new machine, the researchers drew inspiration from one of humanity’s oldest documented machines: the Archimedes screw, which transports water uphill. The researchers applied the Archimedes screw principle at the molecular level. Instead of water, electrons flow through a helical structure, setting it in rotation.

A molecular screw between two gold tips

At the heart of the machine is a helical molecule, which was produced specifically for the study by Prof. Michael Pittelkow’s team at the University of Copenhagen. The molecule, which measures just a few millionths of a millimeter, can be bound to gold surfaces via sulfur atoms. At the Weizmann Institute of Science in Israel, the team led by Prof. 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.

First molecular motor with controllable direction of rotation

Measurements then showed that the molecule rotates when a voltage is applied. The direction of rotation was unambiguous and could be reversed by changing the voltage polarity. The study thus demonstrates, for the first time, a current-driven molecular motor with a controllable direction of rotation that is firmly attached to two metal electrodes.

The movement is driven by the electrons themselves. As they flow through the helical molecule, they transfer angular momentum to it, much like the wind drives a wind turbine. The direction depends on the molecule’s coil, just as with a screw with a right- or left-hand thread. Above a certain voltage, this movement transitions into continuous rotation.

Augsburg calculations unravel the mechanism

But why does a rotating molecule conduct electricity significantly better than a stationary one? Prof. Dr. Fabian Pauly and his PhD students Léon Martin and Yuta Ito at the Chair of Theoretical Physics I at the University of Augsburg investigated this question using quantum mechanical simulations on the LiCCA supercomputer. Their calculations showed that the molecule’s rotation is closely linked to its conductivity. Conductivity depends heavily on the molecule’s angle of rotation and can increase by up to two orders of magnitude as it rotates.

“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.

International collaboration for research success

Building on these findings, theoretical partners Per Hedegård of the Niels Bohr Institute in Copenhagen and Jing-Tao Lü of Huazhong University of Science and Technology in Wuhan developed a simplified model that reproduced 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, a PhD student originally from Japan.

Implications for future applications

The discovery holds potential for molecular electronic components, spintronics applications, and microfluidic systems. Furthermore, such miniature machines offer new opportunities to study physical processes at the atomic scale.

“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,” explains Pauly.

Source – via EurekAlert 

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Written by Eddie Gonzales Jr. – MessageToEagle.com Staff Writer