A molecular motor developed by a team of researchers from Switzerland consists of just 16 atoms.
The world’s smallest molecular motor consists of a single acetylene rotor anchored to a chiral atomic cluster provided by a palladium-gallium surface that acts as a stator. Image credit: Empa.
“This brings us close to the ultimate size limit for molecular motors,” said co-author Dr. Oliver Gröning, head of the Functional Surfaces Research Group at Empa – Swiss Federal Laboratories for Materials Science and Technology.
The team’s molecular motor measures less than one nanometer and consists of a stator and a rotor, i.e. a fixed and a moving part.
The rotor — a symmetrical acetylene (C2H2) molecule — rotates on the surface of the stator. It can take up six different positions.
“For a motor to actually do useful work, it is essential that the stator allows the rotor to move in only one direction,” Dr. Gröning said.
The stator has a basically triangular structure and consists of six palladium (Pd) and six gallium (Ga) atoms.
The rotor can rotate continuously, although the clockwise and counterclockwise rotation must be different.
“The motor therefore has 99% directional stability, which distinguishes it from other similar molecular motors,” Dr. Gröning explained.
“In this way, the molecular motor opens up a way for energy harvesting at the atomic level.”
The tiny motor can be powered by both thermal and electrical energy.
“The thermal energy provokes that the directional rotary motion of the motor changes into rotations in random directions — at room temperature, for example, the rotor rotates back and forth completely randomly at several million revolutions per second,” the researchers said.
“In contrast, electrical energy generated by an electron scanning microscope, from the tip of which a small current flows into the motors, can cause directional rotations.”
“The energy of a single electron is sufficient to make the rotors continue to rotate by just a sixth of a revolution. The higher the amount of energy supplied, the higher the frequency of movement — but at the same time, the more likely the rotor is to move in a random direction, since too much energy can overcome the pawl in the ‘wrong’ direction.”
According to the laws of classical physics, there is a minimum amount of energy required to set the rotor in motion against the resistance of the chute; if the supplied electrical or thermal energy is not sufficient, the rotor would have to stop.
Surprisingly, the scientists were able to observe an independently constant rotation frequency in one direction even below this limit — at temperatures below 17 Kelvin (minus 256 degrees Celsius, or minus 428.8 degrees Fahrenheit) or an applied voltage of less than 30 millivolts.
“The motor could enable us to study the processes and reasons for energy dissipation in quantum tunneling processes,” Dr. Gröning said.
The team’s motor is described in a paper in the Proceedings of the National Academy of Sciences.
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Samuel Stolz et al. Molecular motor crossing the frontier of classical to quantum tunneling motion. PNAS, published online June 15, 2020; doi: 10.1073/pnas.1918654117
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