A team working across Denmark, Israel, Germany and China has built an electrically driven motor from a single molecule, and reports that the device conducts electricity considerably better while it is rotating than while it is at rest. The work is published in Nature under the title “Current-Driven Molecular Machine with Rotation-Enhanced Conductance”.
The motor is a spiral-shaped molecule a few nanometres across, or a few millionths of a millimetre, attached to gold electrodes through sulfur atoms. Individual molecules were placed between two gold tips sharpened to atomic dimensions, and the gap between the tips was varied to stretch or compress the spiral.
Electrons flowing through the spiral transfer angular momentum to it, which makes it turn, in the way that wind turns a turbine. The design borrows from the Archimedes screw, the ancient device for moving water uphill. The spiral’s handedness governs its response, much as right- and left-handed screw threads differ, and reversing the voltage reverses the direction of rotation. Above a certain voltage, the motion becomes continuous rotation.
The spiral molecule was created by Professor Michael Pittelkow’s group at the University of Copenhagen. Professor Oren Tal and Dr Anil Kumar Singh ran the experiments at the Weizmann Institute of Science in Israel. Quantum mechanical simulations were carried out at the University of Augsburg by Professor Fabian Pauly with the doctoral students Léon Martin and Yuta Ito, on the LiCCA supercomputer, with theoretical work from the Niels Bohr Institute in Copenhagen and Huazhong University of Science and Technology in Wuhan.
The simulations indicate that conductance can increase by up to two orders of magnitude during rotation. The reason, the team says, is that some of the molecular angles that conduct best are positions the molecule normally avoids when it is sitting still. “It is only the rotation that makes these positions accessible,” Martin said.
The experiment itself could not show this directly. “In the experiment, you only see the current,” Ito said, explaining why the simulations were needed. The team built a simplified model that reproduced both the rotation and the increase in current.
Earlier electrically driven molecular motors, demonstrated in 2011, worked on surfaces and were driven by the tip of a scanning tunnelling microscope. This design is described as the first electrically driven molecular motor firmly connected to two metal electrodes and rotating in a controllable direction.
“This is a new operating principle for machines at the very limits of miniaturization,” Pauly said. The team suggests possible uses in molecular electronic components, in spintronics, in microfluidic systems handling very small volumes of fluid, and as a tool for studying physics at the atomic scale. The announcement came from the University of Augsburg and was carried by ScienceDaily. No rotation speed is reported, the voltage threshold is not specified, and the hundredfold figure comes from the simulations rather than the measurements.
The finding deserves to be stated plainly, because it inverts the arrangement every other machine has to live with. This motor is a better conductor when it is working than when it is idle. Its efficiency is not degraded by motion. Motion is where the efficiency comes from.
Our analysts have searched the wider economy for a precedent and cannot find one. Every other device in the building is at its best on the day it is installed and declines from there. A bearing wears. A kettle scales. A molecular Archimedes screw, apparently, limbers up.
The mechanism is the part that invites misuse. The positions that conduct best are positions the molecule will not adopt voluntarily. It has to be spun into them. The team’s own sentence is the one to keep: it is only the rotation that makes these positions accessible.
The Department of Unnecessary Research regards this as the most significant management finding of the year, and notes with satisfaction that it was obtained at a scale of a few millionths of a millimetre, which is where most management findings belong.
One caution should be recorded. The hundredfold improvement appears in the simulations rather than the measurements, because in the experiment you only see the current. The molecule is therefore known to be performing better. Nobody has yet been able to watch it do so.