Researchers have developed a molecular motor inspired by the Archimedes screw, an ancient device traditionally used to move water. The electrically driven machine is only a few millionths of a millimeter in size and can rotate in either direction depending on the polarity of the applied voltage.
The motor was created from a specially designed spiral molecule developed by a research group at the University of Copenhagen. Sulfur atoms allow it to attach to gold surfaces. At the Weizmann Institute of Science, researchers placed individual molecules between two gold tips sharpened to atomic dimensions and adjusted the distance between the electrodes to stretch or compress the molecular structure.
When voltage was applied, the molecule began to rotate. Reversing the voltage reversed the direction of rotation. The researchers describe this as the first demonstration of a molecular motor that is driven by electrical current, connected to two metal electrodes and capable of controlled directional rotation.
The rotation occurs because electrons passing through the spiral transfer angular momentum to the molecule. Once the voltage passes a certain threshold, the movement becomes continuous rotation. The spiral’s structure determines how it responds to the electron flow, in a process comparable to the effect of different thread directions in conventional screws.
The experiments also revealed that the rotating molecule conducts electricity far more efficiently than when it is stationary. To study this effect, researchers at the University of Augsburg used quantum-mechanical simulations on the LiCCA supercomputer.
The calculations showed that conductivity varies strongly with the molecule’s angle and can increase by as much as two orders of magnitude, or 100 times, during rotation. The simulations indicated that the molecule avoids some of the configurations that conduct electricity best when it is at rest. Rotation makes those arrangements accessible.
Researchers from the University of Augsburg, the University of Copenhagen, the Weizmann Institute of Science and Huazhong University of Science and Technology combined experimental measurements with theoretical modeling to explain the result. The model reproduced both the observed rotation and the accompanying increase in electrical current.
The findings could contribute to research into molecular electronics, spintronics and microfluidic systems. They also provide a way to study how motion and electrical conduction interact at the atomic scale, offering a possible operating principle for machines at the limits of miniaturization.
The study, “Current-Driven Molecular Machine with Rotation-Enhanced Conductance,” was published in Nature. The work at the University of Augsburg was supported by the German Research Foundation, with computational research carried out using the university’s LiCCA supercomputer.
Sources: ScienceDaily.