Bottom-up construction with a 2D twist could produce new materials

Tetrahedron-shaped nanoparticles are interesting enough on their own, but under the right circumstances, Rice University scientists have found that they do something remarkable.

While routinely checking a batch of tiny gold tetrahedra, Rice chemist Matthew Jones and graduate student Zhihua Cheng discovered that their microscopic particles had the unforeseen ability to organize themselves into 2Dchiral superstructures.

The discovery, which is detailed in a new study in Nature Communicationis likely the first known spontaneous self-assembly of a planar chiral structure, Jones said.

Chiral structures are opposite mirrors, similar shapes, like right and left hands, that cannot be superimposed. This is an important distinction in drug design, where chiral molecules can be therapeutic on the one hand and toxic on the other.

Tetrahedra themselves are not chiral, that is, they can be superimposed on their mirror images. This made it doubly surprising that they so easily fall into chiral forms in experiments when they evaporate on a surface, Jones said.

“It’s unexpected,” he said. “It’s very rare to see a chiral structure form when your building blocks aren’t chiral.”

Jones said the 2D superlattices created by tetrahedra could lead to advances in metamaterials that manipulate light and sound in useful ways. “There are a whole series of papers that predict that some of the most interesting properties of optical metamaterials appear in structures that have chirality at this length scale,” he said.

The chiral surfaces created at Rice are ultrathin assemblies of particles that incorporate left-handed and right-handed domains in equal numbers. This matters in how they deal with circularly polarized light, a useful tool in spectroscopy and plasmonics.

Jones said one way to build precise 2D structures is to start with a large piece of material and work from top to bottom, like a sculptor, removing unwanted pieces to arrive at the desired shape. Self-assembly is a bottom-up approach where a large structure, like a tree, grows from the assembly of countless small pieces. Bottom-up assembly is generally the faster and more efficient of the two approaches.

“Most of the time people use spherical particles in self-assembly, but you just can’t get that much complexity in terms of structure,” Jones said. “My group takes non-spherical particles and tries to make them fit together into more sophisticated structures.”

Having discovered a way to make well-formed gold nano-tetrahedra, Jones and Cheng put them in a solution and placed a droplet on a substrate. “We just let the droplet evaporate, and what we get are these amazing superlattices,” he said.

“There are two things that make them amazing,” he said. “The first is that they are exclusively two-dimensional, and the second, which is more interesting, is that they are chiral.”

Jones and Cheng initially thought the particles could grow in three dimensions, “but now we understand how they form such a complicated 2D structure that is two particles thick,” Jones said.

Cheng said, “At first, we didn’t expect them to come together at all. I just wanted to see that the particles were pure and uniform in size. When I saw the different chiral arrangements, I I was totally amazed that they fit together into such a beautiful structure!”

Jones said the particles take advantage of several phenomena when they are assembled, including van der Waals forces, the electrostatic repulsion between molecules on the surfaces of the tetrahedron, and the substrate on which the droplet is placed. “Over time, as the droplet evaporates, the particles change from mostly repulsive to strongly attractive, and that’s how they crystallize into superlattices,” he said. .

The hexagonal domains of the material form when the tetrahedra come together with their tips up or down. As the particles come together, their tips eventually meet, causing them to slide past each other a bit to keep coming closer together. This forces all the particles in the assembly hexagon to spin randomly one way or the other, forming left and right chiral domains.

Jones noted that there is a mathematical basis to the phenomenon that someone could possibly figure out.

“It’s only recently that the densest packing of spheres has been mathematically proven, so it may be some time before we can expect something similar for tetrahedra,” he said. he declares. “It’s very, very complicated.”

Jones said he sees the possibility “of one day assembling a material like this on the surface of a swimming pool” so that advanced metamaterial coatings can be applied to virtually any object simply by dipping it in through the surface of the liquid.

Jones is the Gene and Norman Hackerman Junior Chair, Norman Hackerman-Welch Young Investigator, and Assistant Professor of Chemistry, Materials Science, and Nanoengineering.

The Robert A. Welch Foundation (C-1954), David and Lucile Packard Foundation (2018-680049), and Rice supported the research.