Prof. Michael Pittelkow’s team at the University of Copenhagen made a helical molecule designed around the principle of the Archimedes screw. Sulfur atoms in the spiral allow it to attach to gold surfaces.
At the Weizmann Institute of Science in Israel, Prof. Oren Tal and Dr. Anil Kumar Singh then placed individual molecules between two gold electrode tips sharpened to atomic dimensions. The molecule, only a few millionths of a millimeter across, was held in place through bonds between its sulfur atoms and the gold.
When the researchers applied voltage across this nanocontact, they measured the current flowing through it and found that the spiral began to turn. Switching the voltage polarity reversed the direction of rotation.
Above a certain voltage, the motion changed from steps into continuous rotation. The rotor showed perfect unidirectionality.
The change in motion also changed the electrical behaviour. During rotation, the molecule’s conductivity rose by up to 100 times compared with its stationary state.
The electrodes could be moved closer together or farther apart, compressing or stretching the molecular spiral to tune its behaviour.
The researchers say the effect follows from the molecule’s helical form. As electrons move through the structure, they transfer angular momentum to it.
Prof. Dr. Fabian Pauly and his PhD students Leon Martin and Yuta Ito at the University of Augsburg used quantum mechanical simulations on the LiCCA supercomputer to examine what was happening in the nanocontact.
The calculations showed that rotation lets the molecule take up highly conducting arrangements unavailable when it is at rest. Martin said the positions that conduct electricity best during rotation are the very positions the molecule avoids while stationary.
Ito said the experiment measures current, while the calculations allow researchers to see what is happening inside the nanocontact.
Per Hedegård of the Niels Bohr Institute in Copenhagen and Jing-Tao Lu of Huazhong University of Science and Technology in Wuhan also developed a simplified model that reproduced both the observed rotation and the rise in current.
Scientists first made molecular motors powered directly by electricity in 2011. Until now, such motors had operated as molecules on a surface, driven by the tip of a scanning tunneling microscope.
This experiment is the first demonstration of a molecular motor driven by electrical current, firmly connected to two metal electrodes and able to rotate in a controllable direction. The work offers a prototype for a new class of torque-driven molecular machines.
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