Smallest Atomically Precise Molecular Machine

Molecular motors are biological or artificial molecular machines that perform a rotary motion. They can, for example, be driven by light. Usually, these machines move according to classical kinetics. Molecular motors that move via a quantum tunneling effect have remained elusive. In quantum tunneling, small (often subatomic) particles move or “tunnel” through an energy barrier and appear on the other side without moving over the barrier. This is impossible in classical mechanics, but happens with a certain probability for objects governed by quantum mechanics.

Oliver Gröning, Swiss Federal Laboratories for Materials Science and Technology (Empa), Dübendorf, Switzerland, and colleagues have investigated an artificial molecular motor that consists of an acetylene (C₂H₂) rotor anchored to a chiral atomic cluster on a PdGa(111) surface. According to the team, the system is the smallest atomically precise molecular machine so far. The molecular motor was prepared by adsorbing acetylene on the surface and observed using scanning tunneling microscopy (STM).

The team found that the acetylene rotors adsorb on top of Pd₃ triangles on the surface in three symmetrically equivalent orientations. This symmetry is broken by the Ga and Pd atoms surrounding the triangle, resulting in a chiral surface. This chiral surface defines a unique sense of rotation. Usually, this is achieved by using a chiral rotor, but this inverted approach allows the use of acetylene as a simple, very small rotor.

The rotation of the acetylene was monitored under the STM tip. The team observed that thermally activated motion was nondirected, with increasing frequencies at higher temperatures. However, at very low temperatures, the rotation becomes unidirectional and the rotation frequency becomes temperature-independent. According to the researchers, this indicates that the movement is caused by quantum tunneling, i.e., does not require overcoming the usual energy barrier for the rotation. Due to its very small size, the rotor’s movement crosses from a classical to a quantum tunneling regime.