Computational and experimental studies show that the superlattice structures, which are 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 material: increased pressure on the superlattice softens it, allowing subsequent compression to be done with significantly 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 believed to be among the largest solids ever mapped in detail using a combined X-ray and computational techniques.
"As we squeeze on this material, it gets softer and softer and suddenly experiences a dramatic change," said Uzi Landman, a Regents' and F.E. Callaway professor in the School of Physics at the Georgia Institute of Technology. "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. Gears in alternating layers move in opposite directions, said Landman, who is director of the Center for Computational Materials Science at Georgia Tech.
Supported by the Air Force Office of Scientific Research and the Office of Basic Energy Sciences in the Department of Energy, the research was reported April 6 in the journal Nature Materials. Researchers from Georgia Tech and the University of Toledo collaborated on the project.