New nanoscale architectures could produce materials with improved electronic, optical and mechanical properties
(News from Nanowerk) Scientists at Brookhaven National Laboratory have developed a new way to guide the self-assembly of a wide range of new nanoscale structures 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 constructing fanciful microscopic fiefdoms, scientists are exploring how these new shapes might affect a material’s functions.
The team from the Center for Functional Nanomaterials (CFN) at Brookhaven Lab describe their new approach to controlling self-assembly in a paper just published in Nature Communication (“Initiation of self-assembly pathways by stacking block copolymers”). Preliminary analysis shows that different shapes have radically different electrical conductivity. The work could help guide the design of custom surface coatings with optical, electronic, and mechanical properties suitable for use in sensors, batteries, filters, and more.
“This work opens the door to a wide range of possible applications and opportunities for scientists from academia and industry to partner with CFN experts,” said Kevin Yager, project and group leader. CFN electronic nanomaterials. “Scientists interested in studying optical coatings, battery electrodes, or solar cell designs could tell us the properties they need, and we can select the right structure from our library of exotically shaped materials to meet their their needs.”
To craft the exotic materials, the team relied on two longstanding areas of expertise at CFN. The first is the self-assembly of materials called block copolymers, including how various forms of processing affect the organization and rearrangement of these molecules. Second, a method called infiltration synthesis, which replaces rearranged polymer molecules with metals or other materials to make shapes functional and easy to view in three dimensions using a scanning electron microscope.
“Self-assembly is a really nice way to create structures,” Yager said. “You design the molecules, and the molecules spontaneously organize themselves into the desired structure.”
In its simplest form, the process begins with the deposition of thin films of long-chain molecules called block copolymers onto a substrate. The two ends of these block copolymers 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 rearrange themselves to be as far apart as possible while still being connected.
This spontaneous reorganization of the chains thus creates a new structure with two chemically distinct domains. Scientists then infuse one of the domains with a metal or other substance to create a replica of its shape and completely burn off the original material. The result: a piece of metal or oxide shaped with dimensions measuring just a billionth of a meter 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,” Yager said. “But the downside is that this process tends to form only simple shapes – flat, sheet-like layers called nanoscale lamellae or cylinders.”
Scientists have tried different strategies to go beyond these simple arrangements. Some have experimented with more complex branched polymers. Others have used microfabrication methods to create a substrate with tiny posts or channels that guide where the polymers can go. But fabricating more complex materials and the tools and models to guide nano-assembly can be both laborious 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 some really interesting, exotic structures,” Yager said.
Stacking and quenching
The CFN method relies on the deposition of thin layers of layered block copolymer.
“We take two of the materials that naturally want to form very different structures and literally place them on top of each other,” Yager said. By varying the order and thickness of layers, their chemical composition, and a range of other variables, including annealing times and temperatures, scientists have generated more than a dozen scale-exotic structures. nanoscale that had never been seen before.
“We found that both materials don’t really want to be layered. As they anneal, they want to mix,” Yager 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 and rapidly cooling the material, quenching it, “you can pull out transient structures and get other interesting shapes,” Yager said.
Scanning electron microscope images revealed that some structures, such as “parapets” and “aqueducts”, have composite characteristics derived from the order and reconfiguration preferences of the stacked copolymers. Others have criss-cross patterns or slats with a patchwork of holes that resemble neither the preferred configurations of starting materials nor any other self-assembled material.
Through detailed studies exploring imaginative combinations of existing materials and studying their “processing history”, CFN scientists have generated a set of design principles that explain and predict what structure will form under a certain set of conditions. They used computer molecular dynamics simulations to better understand the behavior of molecules.
“These simulations allow us to see where individual polymer chains go as they rearrange,” Yager said.
And, of course, scientists are thinking about how these unique materials might be useful. A material with holes can function as a membrane for filtration or catalysis; one with parapet-like pillars on top could potentially be a sensor due to its large surface area and electronic connectivity, Yager suggested.
The first tests, included in the Nature Communications article, were for electrical conductivity. After forming a set 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.
“These are the same starting molecules, and we turn them all into zinc oxide. The only difference between one and the other is how they are locally connected to each other at the nanoscale,” Yager said. “And that turns out to make a huge difference in the electrical properties of the final material. In a sensor or an electrode for a battery, this 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 beautiful 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 approach, we have a lot of control,” Yager said. “We can control what the structure is (thanks to this newly developed method), and also what material it is made of (thanks to our expertise in synthesis by infiltration). We look forward to working with CFN users on what this approach can lead to. »