Self-assembled materials can form patterns that could be useful in optical devices

According to a team of MIT researchers, self-assembled materials called block copolymers, known to form a variety of predictable regular patterns, can now be transformed into much more complex models that can open up new areas of material design.

New discoveries appear in the log Nature CommunicationsYi Ding, Professor of Materials Science and Engineering, Alfredo Alexander-Katz and Caroline Ross, as well as three others.

"It's a discovery that was somehow fortuitous," Alexander-Katz said. "Everyone thought it was not possible," he says, describing the team's discovery of a phenomenon that allows polymers to self-assemble in different patterns of regular symmetric networks.

Self-assembling block copolymers are materials whose chain-like molecules, initially disordered, organize spontaneously into periodic structures. The researchers found that if there was a repeating pattern of lines or pillars created on a substrate, and a thin film of the block copolymer formed on that surface, the patterns of the substrate would be duplicated in the self-assembled material. . But this method could only produce simple patterns such as grids of points or lines.

In the new method, there are two different and incompatible models. One comes from a set of columns or lines etched on a substrate material and the other is an inherent pattern created by the self-assembling copolymer. For example, there may be a rectangular pattern on the substrate and a hexagonal grid that the copolymer forms by itself. The resulting block copolymer scheme could be expected to be misclassified, but this is not what the team discovered. Instead, "it was something much more unexpected and complicated," says Ross.

It turned out to be a subtle but complex order: entangled areas forming slightly different but regular patterns, of a type similar to quasicrystals, which do not repeat in exactly the same way than normal crystals. In this case, the patterns are repeated, but over longer distances than in ordinary crystals. "We take advantage of the molecular processes to create these patterns on the surface" with the block copolymer, Ross says.

This potentially opens the door to new ways to make devices with custom-made features for optical systems or for "plasmonic devices" in which electromagnetic radiation resonates with electrons in a perfectly adapted way, the researchers say. Such devices require very precise positioning and symmetry of the patterns with nanometric dimensions, which this new method can achieve.

Katherine Mizrahi Rodriguez, who worked on the project as an undergraduate, explains that the team has prepared many of these block copolymer samples and studied them under a scanning electron microscope. Yi Ding, who worked on this topic for her doctoral dissertation, "started looking again and again to see if there were any interesting trends," she says. "It is at this moment that all these new discoveries have somehow evolved."

The strange patterns that result are "the result of the frustration between the pattern that the polymer would like to form and the model," says Alexander-Katz. This frustration leads to a break in the original symmetries and to the creation of new subregions containing different types of symmetries, he says. "It's the nature of the proposed solution." In trying to match the relationship between these two models, there comes out a third thing that breaks everyone's patterns. " They describe the new models as a "super-network".

After creating these new structures, the team then developed models to explain the process. Co-author Karim Gadelrab Ph.D. 19 states that "the modeling work has shown that the emerging models are in fact thermodynamically stable and have revealed the conditions under which the new models would be formed".

Ding said "We understand the system perfectly in terms of thermodynamics" and the process of "self-assembly" allows us to create fine patterns and to access new symmetries difficult to manufacture. "

He says this removes some existing limitations in the design of optical and plasmonic materials, and "creates a new way" for material design.

So far, the team's work has been limited to two-dimensional surfaces, but they hope to extend the process into the third dimension, says Ross. "Three-dimensional manufacturing would change the game," she says. Current manufacturing techniques for microdevices build them one layer at a time, but "if you could build whole objects in 3D at once," this would potentially make the process much more efficient.

This story is republished with the permission of MIT News (web.mit.edu/newsoffice/), a popular site that covers the news of MIT's research, innovation, and teaching. .