Patterns Formed by Self-Assembling Materials May be Useful in Optical Systems

Block copolymers are self-assembling materials that are capable of forming a wide range of regular and predictable patterns. Now, according to MIT researchers, these materials can be developed into much more intricate patterns that may pave the way for new fields of materials design.

Scanning electron microscope images of the crystal structure of the block copolymer material, illustrating their unusual quasicrystal symmetries. Regions with different symmetry properties are highlighted in different colors, and examples of the different patterns, which resemble some ancient tiling patterns, are shown in the accompanying diagrams. (Image credit: MIT)

The latest findings have been reported in the journal Nature Communications, in a paper written by Yi Ding, a postdoc at MIT; Alfredo Alexander-Katz and Caroline Ross, professors of materials science and engineering; and three others.

This is a discovery that was in some sense fortuitous. Everyone thought this was not possible.

Alfredo Alexander-Katz, Professor, Department of Materials Science and Engineering, MIT

Alexander-Katz described the team’s finding of a phenomenon that enables the polymers to self-assemble in patterns deviating from normal symmetrical arrays.

The chain-like molecules of these self-assembling block copolymers are initially disordered and they suddenly organize themselves into periodic structures.

Earlier, scientists had discovered that when a repeating pattern of pillars or lines was produced on a substrate, followed by the formation of a thin film of the block copolymer on that surface, then the patterns from that substrate would be replicated in the self-assembled material. However, this technique is only capable of creating simple patterns, for example, grids of lines or dots.

Moreover, there are two different, mismatched patterns in the latest technique. One pattern is from a group of lines or posts etched on a substrate material, while the other is an innate pattern produced by the self-assembling block copolymer.

For instance, the substrate may contain a rectangular pattern and also a hexagonal grid that is formed by the copolymer itself. Therefore, the arrangement of the resulting block copolymer could be anticipated to be poorly ordered; however, that is not what the researchers found. Rather, “it was forming something much more unexpected and complicated,” stated Ross.

There emerged a subtle yet intricate kind of order—interlocking regions that created regular but somewhat different patterns, similar to quasicrystals. Such crystals do not necessarily repeat the way ordinary crystals do. Here, the patterns certainly repeat but over lengthier distances than in normal crystals. “We’re taking advantage of molecular processes to create these patterns on the surface” using the block copolymer material, Ross added.

According to the researchers, these findings can present new opportunities for creating devices with customized properties for optical devices or for “plasmonic devices” in which resonation occurs between electrons and electromagnetic radiation in accurately tuned ways. Such kinds of devices need highly accurate symmetry and positioning of patterns with nanoscale dimensions, something that can be achieved by the latest technique.

According to Katherine Mizrahi Rodriguez, who worked on the latest project as an undergraduate, many of these block copolymer samples were prepared and examined under a scanning electron microscope. Yi Ding, who worked on this for his doctoral dissertation, “started looking over and over to see if any interesting patterns came up,” she added. That’s when all of these new findings sort of evolved.”

The ensuing odd patterns are “a result of the frustration between the pattern the polymer would like to form, and the template,” explained Alexander-Katz. It was that frustration which caused the original symmetries to break down and led to the development of new sub-regions with varied kinds of symmetries inside them, he added.

That’s the solution nature comes up with. Trying to fit in the relationship between these two patterns, it comes up with a third thing that breaks the patterns of both of them.

Alfredo Alexander-Katz, Professor, Department of Materials Science and Engineering, MIT

The latest patterns are described as a “superlattice” by the research team. After producing these innovative structures, the researchers subsequently developed models to describe the process.

The modeling work showed that the emergent patterns are in fact thermodynamically stable, and revealed the conditions under which the new patterns would form.

Karim Gadelrab, Co-Author and PhD, Department of Materials Science and Engineering, MIT

Ding added, “We understand the system fully in terms of the thermodynamics,” and the self-assembling technique “allows us to create fine patterns and to access some new symmetries that are otherwise hard to fabricate.”

According to him, this eliminates certain prevalent limitations with regards to the design of plasmonic and optical materials, and thus “creates a new path” in the field of materials design.

To date, the work performed by the researchers has been limited to 2D surfaces; however, in ongoing studies, they are planning to apply this process to 3D surfaces as well, informed Ross. “Three-dimensional fabrication would be a game changer,” she added.

She further stated that present fabrication methods intended for microdevices build them up in a layer-by-layer fashion, but “if you can build up entire objects in 3D in one go,” that can possibly render the process relatively more efficient.

Such findings “open new pathways to generate templates for nanofabrication with symmetries not achievable from the copolymer alone,” stated Thomas P. Russell, the Silvio O. Conte Distinguished Professor of Polymer Science and Engineering at the University of Massachusetts, Amherst, who was not part of the study. He added that it “opens the possibility of exploring a large parameter space for uncovering other symmetries than those discussed in the manuscript.”

Russell concluded that “The work is of the highest quality,” and adds “The pairing of theory and experiment is quite powerful and, as can be seen in the text, the agreement between the two is remarkably good.”

The Office of General Sciences of the U.S. Department of Energy funded the study. Graduate student Hejin Huang was also a part of the team.


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