Scientists Build Nano Barriers, Water Channels, and Other Shapes – ScienceDaily


Scientists at the US Department of Energy’s (DOE) Brookhaven National Laboratory have developed a new method to direct the self-assembly of a wide range of novel nanostructures using simple polymers as starting materials. Under an electron microscope, these nanoscale structures look like tiny Lego building blocks, including parapets for miniature medieval castles and Roman aqueducts. But rather than building fanciful microscopic fiefdoms, scientists are exploring how these new shapes might affect the functions of matter.

A team from the Brookhaven Lab’s Center for Functional Nanomaterials (CFN) describe their new approach to controlling self-assembly in a paper recently published in Nature Communications. Elemental analysis shows that different shapes have significantly different electrical conductivities. The work can help guide the design of custom surface coatings with custom optical, electronic, and mechanical properties for use in sensors, batteries, filters, and more.

“This work opens the door to a wide range of potential applications and opportunities for scientists from academia and industry to partner with experts in CFN,” said Kevin Yager, project leader and CFN’s Electronic Nanomaterials Group. “Scientists interested in studying photonic coatings, battery electrodes, or solar cell designs can tell us the properties they need, and we can choose just the right structure from our library of exotic materials to meet their needs.”

automatic grouping

To make the exotic materials, the team drew on two areas of CFN’s longstanding expertise. The first is the self-assembly of materials called block copolymers — including how different forms of processing affect the organization and rearrangement of these molecules. The second is a method called infiltration synthesis, which replaces rearranged polymer molecules with metals or other materials to make shapes functional—and easier to visualize in three dimensions with a scanning electron microscope.

“Self-assembly is a really cool way to make structures,” said Yager. “You design the molecules, and you automatically organize the molecules into the desired structure.”

In its simplest form, the process begins with depositing thin films of long, chain-like molecules called block copolymers onto a substrate. The two ends of these polymer blocks are chemically distinct and want to separate from each other, like oil and water. When you heat these films through a process called annealing, the two ends of the copolymer realign so that they move as far apart as possible while still attached. Thus, this spontaneous reorganization of the chains creates a new structure with two chemically different domains. Scientists then embed one of the spheres with a metal or other material to make an exact copy of its shape, completely burning out the original material. The result: a piece of metal or oxide with dimensions not exceeding a billionth of a meter that could be useful for semiconductors, transistors or sensors.

“It’s a powerful and scalable technology,” Yager said. “You can easily cover large areas with this material.” “But the disadvantage is that this process tends to form only simple shapes — flat, plate-like layers called lamellae or nanocylinders.”

Scientists have tried different strategies to get around these simple arrangements. Some have experimented with more complex branched polymers. Others have used microfabrication methods to create a substrate with small columns or channels that guide where the polymers can go. But making more complex materials, tools and molds to guide nanoassembly can be labor intensive and expensive.

“What we’re trying to show is that there is an alternative where you can still use simple, cheap raw materials, but get really interesting and exotic structures,” Yager said.

Stacking and cooling

The CFN method is based on depositing thin copolymer films in layers.

“We take two materials that naturally want to form very different structures and we literally put them on top of each other,” Yager said. By varying the order and thickness of the layers, their chemical composition, and a host of other variables including annealing times and temperatures, the scientists have created more than a dozen exotic nanostructures that have not been seen before.

“We discovered that the two materials don’t really want to be layered. And when they harden, they want to blend,” Yaeger said. “Mixing causes new, more interesting 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 different times, cooling the material quickly, and quenching it, “you can pull out the transient structures and get some other interesting shapes,” Yager said.

Scanning electron microscopy images revealed that some structures, such as ‘septa’ and ‘channels’, have composite features derived from the ordering and recombination preferences of stacked polymers. Others have cross-stitch patterns or sheets with a patchwork of perforations that differ from the preferred configurations of starting materials – or any other self-assembled material.

Through detailed studies exploring imaginary combinations of existing materials and investigating the “history of processing,” CFN scientists have created a set of design principles that explain and predict the structure that will form under a given set of conditions. They used computer-based molecular dynamics simulations to gain a deeper understanding of how molecules behave.

“These simulations allow us to see where the individual polymer chains are going as they are rearranged,” Yager said.

promising applications

And, of course, scientists are thinking about how to make use of this unique material. A material with holes may act as a filtration or catalytic membrane; Yager suggested that one with barricade-like pillars on top could be a sensor because of its large surface area and electronic connectivity.

The first tests, included in Nature Communications Paper focusing on electrical conductivity. After forming a matrix of newly formed polymers, the team used infiltration synthesis to replace one of the newly shaped domains with zinc oxide. When they measured the electrical conductivity of zinc oxide nanostructures of different shapes, they found significant differences.

“They’re the same starting molecules, and we’re converting them all into zinc oxide. The only difference between one and the other is how they bond locally to each other at the nanoscale,” Yager said. “And this has been shown to make a huge difference in the electrical properties of the final material. In a sensor or battery electrode, that would be very important.”

Scientists are now exploring the mechanical properties of the different shapes.

“The next frontier is multifunctionality,” Yager said. “Now that we have access to these wonderful 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.”

“With this style, we have a lot of control,” said Yager. “We can control what the structure is (using this newly developed method), and also what material it is made of (using our expertise in infiltration synthesis). We look forward to working with CFN users on where this approach can lead.”

This research was funded by the Department of Energy’s Office of Science (BES). The experimental work was led by Sebastian Russell, a CFN postdoctoral fellow who now works in industry. Other co-authors include Masafumi Fukuto of Brookhaven Laboratory’s Synchrotron Light Source II (NSLS-II); CFN’s Chang Yong-nam, Sowon Bae, Nikhil Tiwale and Gregory Durk; and Ashwanth Subramanian of Stony Brook University (SBU). CFN and NSLS-II are DOE Office of Science user facilities. This work also made use of computational resources managed by the Center for Scientific Data and Computing, a component of the Brookhaven Lab’s Computational Science Initiative.



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