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Surprise Discovery Shows 'Artificial Atoms' Rapidly Self-Assembling Into Complex Structures

A new front opens in materials science.

by Michael Byrne
Aug 1 2017, 12:00pm

Humans have been building things out of other things since the Stone Age, but it's really been the past 200 years that things have gotten wild. Just think of all of the alloys, plastics, rubbers, silicon substrates, liquid crystals, and hacked DNA molecules. It would have been easy enough to just declare ourselves gods of materials science and take a vacation, but researchers want to go deeper. As deep as it gets, really—hacking together atoms themselves and then making entirely new sorts of structures from those meta-atoms.

And that's where we're at. Really, we've been in the midst of a metamaterials revolution for about the past decade—with atomically thin graphene at the heart of much of said revolution—and it likely hasn't even peaked. Consider a new paper out Monday in Nature describing the rapid crystallization of superlattice nanostructures from engineered "artificial atoms." While growing such materials from the ground up has usually been understood as a prohibitively slow process, the new work speeds things up by orders of magnitude—from days to seconds.

Depending on the composition, superlattices can offer properties ranging from incredible strength to finely tunable conductivity. Some superlattices are good venues for observing and exploiting quantum effects.

An artificial atom isn't one specific thing so much as it is a loose term to describe an assemblage of particles that together behaves a lot like an atom. In particular, artificial atoms should be able to combine with other artificial atoms into structures resembling molecules. Here, the artificial atoms take the form of tiny crystals of palladium, a rare silvery-white metal. Each nanocrystal/artificial atom consists of only 100 to 10,000 actual atoms, which is still small enough to experience quantum effects.

A superlattice is basically just a crystal where proper atoms have been swapped out for artificial atoms. Like normal crystals, superlattices can be grown over a period of time as nanocrystals in a solution, lose energy, and fall into the ordered patterns of the crystal structure. It's been assumed that this period of time must be fairly long, but what the researchers behind the current paper discovered is that this is not necessarily the case at all.

This was more or less an accidental discovery. All the researchers wanted to do was watch how the crystals form, which they accomplished by adding an observation window to a small tangerine-sized reaction chamber. What they saw—with help from a probing X-ray beam—was that the superlattices were self-assembling in a matter of seconds. Where the beam should have reflected back a specific pattern representing mostly individual particles, the actual pattern returned indicated the presence of full-one lattice structures.

"So something weird is happening," the pair of grad students conducting the experiment texted their advisor, according to a press release.

"Once we understood this system, we realized this process may be more general than we initially thought," Liheng Wu, first author of the study and a chemical engineering researcher at Stanford University, offered in a statement. "We have demonstrated that it's not only limited to metals, but it can also be extended to semiconducting materials and very likely to a much larger set of materials."

In other words, it may be possible to make lots of exciting new materials using the same rapid process.