Nano-motor of just 16 atoms runs at the boundary of quantum physics
Researchers at Empa and EPFL have created one of the smallest motors ever made. It’s composed of just 16 atoms, and at that tiny size it seems to function right on the boundary between classical physics and the spooky quantum realm.
Like its macroscopic counterparts, this mini motor is made up of a moving part (the rotor) and a fixed part (the stator). The stator in this case is a cluster of six palladium atoms and six gallium atoms arranged in a rough triangular shape. Meanwhile, the rotor is a four-atom acetylene molecule, which rotates on the surface of the stator. The whole machine measures less than a nanometer wide.
The molecular motor can be powered by either thermal or electrical energy, although the latter was found to be much more useful. At room temperature, for example, the rotor was found to rotate back and forth at random. But when an electric current was applied using an electron scanning microscope, the rotor would spin in one direction with a 99-percent stability.
This, the team says, makes it far more practical than previous molecular motors. Ultimately, it could be used not only for moving tiny machines around, but also for energy harvesting on the nanoscale.
But there are a few other strange quirks of the new motor. It’s made to spin in one direction the same way that a regular motor would, using a ratcheting scheme. Normally, this is done using a gear with sloped teeth and a pawl, which slides along the flat side of the teeth but can’t climb back up the steep side, forcing one-way movement.
In this case though, the molecular motor works backwards. Somehow, the rotor prefers to move against the grain, climbing the steep side and ignoring the flat route. As counterintuitive as it seems, the effect is basically the same, so the rotor still turns in one direction.
Another oddity is that the molecular motor seems to break a law of classical physics. As we innately understand at the macro scale, a minimum amount of energy is required for a movement to overcome resistance – on a bicycle, for instance, you can’t just stop pedaling and expect to ride uphill.
But somehow, that’s basically what this mini motor was doing. The researchers found that the rotor moved even under tiny amounts of thermal or electrical energy – far less than “should” be required to get it spinning. That means temperatures below -256 °C (-248.8 °F) or an applied voltage of under 30 millivolts.
Instead, what seems to be happening is a phenomenon known as quantum tunneling. Essentially, particles have regularly been observed ”tunneling through” barriers that they don’t have the energy to overcome normally. Back to the bike analogy, this isn’t so much like gliding to the top of the hill without pedaling as it is just teleporting to the other side of it.
But even this explanation raises further questions. Quantum tunneling is thought to be frictionless, but if that was the case the rotor would spin in any direction randomly. The fact that it prefers one direction with 99-percent probability suggests that energy is being lost during this process.
"The motor could enable us to study the processes and reasons for energy dissipation in quantum tunneling processes," says Oliver Gröning, lead researcher on the study.
The study was published in the journal Proceedings of the National Academy of Sciences. An animation of the rotor can be seen in the video below.