# Synopsis: Spinning on a gold atom

The rotation of individual large molecules adsorbed onto a gold surface has been observed with a scanning tunneling microscope.

In biological systems, molecules convert chemical energy into mechanical motion—the source of movement in living organisms. Such molecular motors could be assembled into nanoscale machines, provided we can control their motion and harness them into large-scale arrays on surfaces.

Li Gao and scientists at the Institute of Physics and the Institute of Chemistry in Beijing, in collaboration with the University of Liverpool, have constructed an array of anchored single-molecule rotors on a gold surface. In a paper appearing in Physical Review Letters, they have found that single $\left(t-\text{Bu}{\right)}_{4}-\text{ZnPc}$ (tetra-tert-butyl zinc phtalocyanin) molecules on a reconstructed gold surface possess a well-defined axis of rotation, and that these molecules also form large-scale ordered arrays.

The group discovered the dynamic behavior of the adsorbed molecule using scanning tunneling microscopy. Instead of seeing the cross-shaped $\left(t-\text{Bu}{\right)}_{4}-\text{ZnPc}$ molecule, the authors observed a structure reminiscent of a folding fan, which they identified as the time-averaged image of the molecule rotating at high frequency. With evidence that the molecules are rotating, a combination of imaging experiments and density functional calculations establishes the center of rotation as an adsorbed gold atom at an elbow site in the reconstructed surface. A nitrogen atom in the molecule forms a bond with the gold atom, which serves as the pivot for the molecule’s rotation. – Daniel Ucko

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