Exotic nanoscale structures formed by layering block copolymers

Researchers at Brookhaven National Laboratory have introduced a way to guide the self-assembly of a wide range of novel nanoscale structures using simple polymers as starting materials. The researchers are exploring how these novel shapes might affect a material’s functions. A preliminary analysis, as reported in a recent issue of Nature Communications (https://doi.org/10.1038/s41467-022-34729-0), shows that different shapes have dramatically different electrical conductivity. The work could help guide the design of custom surface coatings with tailored optical, electronic, and mechanical properties for use in sensors, batteries, and filters, for example.

Layering block copolymers and heating them for different times resulted in a range of exotic nanoscale structures. This image shows scanning electron micrographs and cartoon representations of parapets (cylinders on top and lamellae on bottom), a Swiss-cheese-like patchwork of porous lamellae, and aqueducts (lamellae on top of cylinders). Metals or oxides formed in these shapes could have properties useful for sensors, membranes, transistors, for example. Image credit: Brookhaven National Laboratory.

In its simplest form, the process starts by depositing thin films of block copolymers onto a substrate. The two ends of these block copolymers are chemically distinct and want to separate from each other. Through annealing, the copolymers’ two ends rearrange to move as far apart as possible while still being connected. This spontaneous reorganization of chains creates a new structure with two chemically distinct domains. The researchers then infuse one of the domains with a metal or other substance to make a replica of its shape, and completely burn away the original material. The result: a shaped piece of metal or oxide that could be useful for semiconductors, transistors, or sensors.

“It’s a powerful and scalable technique. You can easily cover large areas with these materials,” says Kevin Yager, leader of the Electronic Nanomaterials Group at Brookhaven National Laboratory’s Center for Functional Nanomaterials. “But the disadvantage is that this process tends to form only simple shapes—flat sheetlike layers called lamellae or nanoscale cylinders.”

Scientists have tried different strategies to go beyond those simple arrangements. Some have experimented with more complex branching polymers. Others have used microfabrication methods to create a substrate with tiny posts or channels that guide where the polymers can go. But making more complex materials and the tools and templates for guiding nanoassembly can be both labor-intensive and expensive.

“What we’re trying to show is that there’s an alternative where you can still use simple, cheap starting materials, but get really interesting, exotic structures,” Yager says. Yager’s group relies on depositing block copolymer thin films in layers.

“We take two of the materials that naturally want to form very different structures and literally put them on top of one another,” he says. By varying the order and thickness of the layers, their chemical composition, and a range of other variables including annealing times and temperatures, the researchers generated more than a dozen novel exotic nanoscale structures.

“We discovered that the two materials don’t really want to be stratified. As they anneal, they want to mix,” Yager says. “The mixing is causing more interesting new structures to form.”

If annealing is allowed to progress to completion, the layers will eventually evolve to form a stable structure. But by stopping the annealing process at various times and cooling the material rapidly, quenching it, “you can pull out transient structures and get some other interesting shapes,” Yager says.

Scanning electron microscope images revealed that some structures, like “parapets” and “aqueducts,” have composite features derived from the order and reconfiguration preferences of the stacked copolymers. Others have crisscross patterns or lamellae with a patchwork of holes that are unlike either of the starting materials’ preferred configurations—or any other known self-assembled materials.

Through detailed studies exploring imaginative combinations of existing materials and investigating their “processing history,” the researchers generated a set of design principles that explain and predict what structure is going to form under a certain set of conditions. They used computer-based molecular dynamics simulations to get a deeper understanding of how the molecules behave.

“These simulations let us see where the individual polymer chains are going as they rearrange,” Yager says.

A material with holes might work as a membrane for filtration or catalysis; one with parapet-like pillars on top could potentially be a sensor because of its large surface area and electronic connectivity, Yager suggests.

The first tests, the researchers reported, focused on electrical conductivity. After forming an array of newly shaped polymers, the team used infiltration synthesis to replace one of the newly shaped domains with zinc oxide. When they measured the electrical conductivity of differently shaped zinc oxide nanostructures, they found huge differences.

“It’s the same starting molecules, and we’re converting them all into zinc oxide. The only difference between one and the other is how they’re locally connected to each other at the nanoscale,” Yager says. “And that turns out to make a huge difference in the final material’s electrical properties. In a sensor or an electrode for a battery, that would be very important.”

The scientists are now exploring the mechanical properties of the different shapes.

“The next frontier is multifunctionality,” Yager says. “Now that we have access to these nice structures, how can we choose one that maximizes one property and minimizes another—or maximizes both or minimizes both, if that’s what we want?”

Source: Brookhaven National Laboratory