A molecular motor was developed that consists of only 16 atoms and rotates reliably in one direction. It could allow energy harvesting at the atomic level. The motor moves exactly at the boundary between classical motion and quantum tunneling. The motor measures less than one nanometer or around 100,000 times smaller than the diameter of a human hair.
In principle, a molecular machine functions in a similar way to its counterpart in the macro world: it converts energy into a directed movement. Like a large-scale motor, the 16-atom motor consists of a stator and a rotor, i.e. a fixed and a moving part. The rotor rotates on the surface of the stator and can take up six different positions.
Since the energy that drives the motor can come from a random direction, the motor itself must determine the direction of rotation using a ratcheting scheme; however, the atom motor operates opposite of what happens with a ratchet in the macroscopic world with its asymmetrically serrated gear wheel: While the pawl on a ratchet moves up the flat edge and locks in the direction of the steep edge, the atomic variant requires less energy to move up the steep edge of the gear wheel than it does at the flat edge. The movement in the usual blocking direction is therefore preferred and the movement in the running direction is much less likely. So, the movement is virtually only possible in one direction.
The researchers implemented this “reverse” ratchet principle in a minimal variant by using a stator with a basically triangular structure consisting of six palladium and six gallium atoms; the structure is rotationally symmetrical but not mirror-symmetrical. As a result, the rotor (a symmetrical acetylene molecule) consisting of only four atoms can rotate continuously, although the clock-cule) consisting of only four atoms can rotate continuously, although the clockwise and counterclockwise rotation must be different.
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. 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. 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.