Quasicrystals have teased and intrigued scientists for three decades. Now, this already strange group of materials has a bizarre new member: a two-dimensional quasicrystal made from self-assembling organic molecules.
This odd quasicrystal is flat, made from a single layer of molecules with five-sided rings. The molecules form groups within the layer as weak hydrogen bonds link them together. These molecular groups are assembled in a way that forces other molecules in the layer into shapes including pentagons, stars, boats, and rhombi. If this were a regular old crystal, you’d expect to see these groups and shapes repeated over and over throughout the layer in a predictable way. But in this quasicrystal, you’ll see the same shapes over and over in the layer, but not in any organized pattern.
The things that set these quasicrystals apart from all the others, scientists say, are its organic materials and self-assembling parts.
“They’re markedly different from just about everything else out there,” said physical chemist Alex Kandel, whose lab at the University of Notre Dame described the material today in Nature. Previously known quasicrystals are mostly metallic, and tied together by strong ionic bonds rather than the weaker hydrogen bonds that can be found in complex organic molecules like DNA.
As their name suggests, quasicrystals have a structure that’s part crystalline, part disorganized. In other words, they are something in between a structure with repeating, symmetric units, and one with completely random building blocks. Their atomic units are locally symmetric, but are not regularly repeated over longer distances. Because of these arrangements, quasicrystals are slippery and have been used in things like non-stick frying pans.
The first quasicrystal of any sort was also accidentally made in the lab, in 1982, by materials scientist Daniel Schechtman who won a Nobel Prize for the discovery in 2011. Up until that point, scientists thought the semi-organized structure of quasicrystals was an impossibility. Now, we know that’s not true. Not only can quasicrystals be grown in the lab, they can also grow in nature. In 2012, Princeton University physicist Paul Steinhardt showed that quasicrystals found in eastern Russia had fallen to Earth in a meteorite.
Kandel’s group discovered the organic quasicrystal accidentally. Instead of trying to make the thing, they were actually hoping to study how electrons are distributed in ferrocenecarboxylic acid, the molecule the quasicrystal is built from. To do that, the team needed to build a stable, linear group of molecules. But when the scientists tried, they produced a two-dimensional quasicrystal instead.
“The first images were quite a shock,” Kandel said. “Certainly, 2-D quasicrystals aren’t easy to make, which is why we’re only seeing very recent reports of them now, some 30-odd years after the first quasicrystalline materials were discovered.”
Wolf Widdra of Germany’s Martin Luther University, who made the first 2-D quasicrystal, reported in October 2013, is a bit skeptical of the new research. He doesn’t think there’s enough evidence yet to prove quasicrystal structure over a large enough area.
There is also disagreement among scientists about what it means to be self-assembling. Widdra thinks the term could be applied to all quasicrystal structures, not just this new one. Kandel argues that structures assembled by way of strong chemical bonds — like the other quasicrystals — aren’t actually self-assembled. Those strong chemical bonds, he says, overwhelm the forces holding individual building blocks together and leave the material no choice but to form. In this new quasicrystal, those building blocks are joined by weak hydrogen bonds.
“Self-assembly is interesting precisely because the forces that drive organization are weaker than the forces responsible for the individual structure,” Kandel said.
By Nadia Drake