TINY iron fragments can shape-shift to get out of a jam. The metal nanocrystals have been glimpsed reshaping themselves to squeeze through tight spaces, without melting or compressing. That could be a boon to digital memory storage and to nanofabrication.

Solid chips of metal about 20 nanometres across will slide through carbon nanotubes when an electric current is switched on. The crystals move back and forth when the polarity of the current changes, allowing a chip to be precisely positioned inside a tube.

But not all carbon nanotubes are perfectly shaped. Physicists mass-produce the tubes by heating a cloud of carbon-containing gas in the presence of metals, which spur carbon atoms to assemble into nanotube structures. Sometimes the tubes have consistent diameters, but sometimes they are pinched in the middle, shrinking from 20 to just 5 nanometres wide (see diagram).

To stop nanocrystals getting stuck in tight spots, scientists have been sorting through the tiny tubes to weed out imperfect versions. But Sinisa Coh of the University of California, Berkeley, and colleagues saw something surprising when they used a high-resolution electron microscope to watch an iron crystal moving through a kinked nanotube. The solid crystal squeezed right through, even though it shouldn’t have been able to fit.


“The atoms are arranged in a regular periodic grid and are not jiggling around, as in a liquid,” says Coh. “We were puzzled.”

Using computer simulations, Coh figured out that the crystal was disassembling and re-assembling itself to get through the narrower gap. If the crystal was moving from left to right, some of the leftmost atoms on its surface would detach and leapfrog over the rest of the crystal, settling on its right side (Physical Review Letters, doi.org/mfp). Atoms that had previously been inside the crystal were now on the surface, where they repeated the process.

“As the crystal approaches a constriction, it essentially regrows inside the constriction as atoms are transported from the left surface towards the right surface,” says Coh. He says the current is probably pulling the individual atoms along electrostatically, and that this may be how nanocrystals move through all nanotubes.

As the crystal approaches a constriction, it regrows as atoms are transported from the left to the right

Though the flow is reminiscent of a liquid, the metal chip is still a solid as most of its atoms are stationary most of the time.

Metal chips that slide back and forth inside carbon nanotubes could form the 1s and 0s needed to act as archival storage devices. Because the chips are stable, these memories could last for more than a billion years. The new result means that such a device could work even with damaged tubes.

The peculiar locomotion could also help grow layered nanocrystals, which are used in electronics and drug delivery, says team member Alex Zettl, also at Berkeley. A piece of one type of metal could follow another through a kinked passage, prompting a layer of atoms from the trailing metal to coat the leading lump. Such nanocrystals have been made before but with limited control, says Zettl. “Our method could allow near atom-by-atom tailored construction.”

This article appeared in print under the headline “Nanocrystal morphs out of tight spots”