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Gear Heads



A combined computational and experimental study of self-assembled silver-based structures known as superlattices has revealed an unexpected behavior: arrays of gear-like molecular-scale machines that rotate in unison when pressure is applied to them.

The superlattice structures, self-assembled from smaller clusters of silver nanoparticles and organic-protecting molecules, form in layers, with the hydrogen bonds between their components serving as “hinges” to facilitate the rotation. Movement of the “gears” is related to another unusual property of the structures: Increased pressure on the superlattice softens it, allowing subsequent compression to be done with less force.

Materials containing the gear-like nanoparticles – each composed of nearly 500 atoms – might be useful for molecular-scale switching, sensing, and even energy absorption. The complex superlattice structure is among the largest solid ever mapped in detail using a combination of X-ray and computational techniques.

“As we squeeze on this material, it gets softer and softer and suddenly experiences a dramatic change,” said Regents Professor Uzi Landman, the F.E. Callaway professor in the Georgia Tech School of Physics. “When we look at the orientation of the microscopic structure of the crystal in the region of this transition, we see that something very unusual happens. The structures start to rotate with respect to one another, creating a molecular machine with some of the smallest moving elements ever observed.”

The gears rotate as much as 23 degrees and return to their original position when the pressure is released. Supported by the Air Force Office of Scientific Research and the Department of Energy’s Office of Basic Energy Sciences, the research was reported in the journal Nature Materials. The University of Toledo also contributed to the project.

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