Researchers in Israel, Denmark and Germany report a single-molecule motor that rotates when current flows through it and reverses with the voltage, while its spinning raises conductivity by up to two orders of magnitude.
A research collaboration has reported a molecular motor that turns when electric current passes through it and reverses direction when the polarity of the voltage is switched. Its design borrows from the Archimedes screw, the ancient spiral device used to lift water: electrons travelling through a spiral-shaped molecule are said to transfer angular momentum to it and make it rotate. The molecule, only a few millionths of a millimetre long, was created by the group of Prof. Michael Pittelkow at the University of Copenhagen and attaches to gold through sulfur atoms. At the Weizmann Institute of Science in Israel, a team led by Prof. Oren Tal and Dr. Anil Kumar Singh held single molecules between two gold tips sharpened to atomic dimensions and showed that rotation became continuous once the voltage passed a threshold. Electrically driven molecular motors have existed since 2011, but those operated on surfaces and relied on the tip of a scanning tunnelling microscope; the new work is described as the first to show a current-driven motor firmly connected to two electrodes with a controllable direction of rotation.
The experiments also revealed an unexpected effect: the spinning molecule carried current far better than a stationary one. To explain it, a team led by Prof. Fabian Pauly at the University of Augsburg ran quantum-mechanical simulations on a supercomputer and found that conductivity depends strongly on the molecule's angle, rising by as much as two orders of magnitude, or about 100 times, during rotation. According to the researchers, some of the orientations that conduct electricity best are ones the molecule naturally avoids when at rest, and only rotation makes them accessible. Theorists Per Hedegård of the Niels Bohr Institute and Jing-Tao Lü of Huazhong University of Science and Technology then built a simplified model that reproduced both the rotation and the rise in current seen in the measurements. In the researchers' words, the work shows that an electric current can generate a directed rotation and that the rotation, in turn, controls the current.
The team suggests the result could eventually feed into molecular electronic components, spintronics, which exploits electron properties beyond charge, and microfluidic systems that handle extremely small volumes of liquid, and says it gives scientists a new way to study physical processes at the atomic scale. These are prospects rather than products: the demonstration involves individual molecules held between laboratory electrodes, and no device applications have been shown. This account is based on a university press release as republished by ScienceDaily, which does not name the journal in the text we read, and we have not independently verified the findings or seen the underlying paper, so details should be treated with that caution. The research drew funding from bodies including the German Research Foundation, the European Research Council and the Israel Science Foundation.
