Kai Nip, Staff Photographer

In a study published in the journal ACS Applied Materials and Interfaces on Nov. 16, Yale researchers presented their discovery of a novel approach to replicating surfaces at the atomic level.

Led by graduate student Zheng Chen GRD ’20 and Professor of Mechanical Engineering and Materials Science and Department Chair Udo Schwarz, researchers at the School of Engineering and Applied Science developed a new nanoimprinting process that significantly expands the technology’s versatility, with potential for a wide range of applications in nanomanufacturing.

“Nanostructured surfaces are usually produced by nature or artificially using a clean room, which can cost millions of dollars,” Schwarz said. “Nanoimprinting provides a super precise and really cheap alternative for replicating surfaces down to the atomic level.”

Nanoimprinting allows scientists to replicate details of a surface that can be smaller than one 10-billionth of a meter. It has a wide range of applications, including high-density data storage, holograms and water filtration, according to the paper.

Structures at the atomic scale can be replicated using bulk metallic glasses, or BMGs, which are metallic alloys, in a process called thermoplastic forming, or TPF, in which a sheet is heated and stretched across a BMG mold to obtain an imprint of a given surface. BMGs are particularly useful because of their unique chemical and physical properties, including great strength at low temperatures and high flexibility at high temperatures. However, only a few BMG-forming alloys can be used for atomic-scale imprinting, rendering it impractical for applications in many fields.

“Given the absence of intrinsic dimensional limitations, [bulk] metallic glasses can be used to conduct atomic-scale replication that has been realized earlier via thermoplastic forming, which, however, has limited selections of substrates and metallic glasses,” Chen said. “With the method based on magnetron sputtering, we enlarged the range of alloys and substrates that are able to achieve atomic-scale replication.”

Magnetron sputtering is the collision process between gas ions and target atoms within the BMG, according to the paper. This collision causes the target atoms to be ejected from the surface and travel across a vacuum until they reach the surface of interest, where they come together to form a thin film. This process essentially replicates the original surface down to the atomic level, creating an accurate imprint. 

By implementing magnetron sputtering, Schwarz’s team was able to replicate atomic-scale features in a cost-effective, practical and highly versatile manner, all while maintaining the accuracy and precision of classical nanoimprinting methods.

Furthermore, the novel method articulated in the paper allows for not only a wide range of alloys to be used as targets, but also a large surface area to be replicated, which means that the process can be easily scaled up.

“Dr. Schwarz’s method of sputtering a BMG thin film onto a mold enables a very accurate manufacturing technique for nanometer sized parts in nanoscale electronic, biomedical, and other devices,” Corey O’Hern, the director of undergraduate studies in the mechanical engineering department, wrote in an email to the News. “Dr. Schwarz’s innovative method will revolutionize the field of nanomanufacturing, making it cost effective to build parts with sub-nanometer scale precision.”

Schwarz’s work described in the paper builds on another paper he published in APL Materials on Nov. 4, which showed that using metallic glasses allows for subatomic accuracy when replicating surface features.

While crystals are made up of atoms arranged in a specific, rigid pattern, atoms in glasses are more free to adjust to different conformations, according to Schwarz’s earlier paper. When metallic glasses are heated, the atoms become even more free to move around and thus conform to whatever surface features are being replicated.

The discoveries made by Schwarz and his team in these two papers have potential for countless applications, especially when replicating nature’s “beautifully precise” work, according to Schwarz. One avenue that he finds particularly promising is using nanoimprinting to create a plastic overlay for solar panels — an idea for which he is currently writing a proposal.

“The ideal location for solar panels is in deserts because they have an almost infinite amount of sunshine during the day,” Schwarz said. “However, one big problem is the sand. When a sandstorm occurs, layers of sand form on the solar panels and reduce their efficiency. One way we can get rid of the sand is by producing plastic layers with anisotropic friction that will essentially cause the sand to roll off if the panels are shaken. These panels can be easily and cheaply produced with nanoimprinting.”

Anisotropic friction refers to the uneven frictional force of a surface. These forces vary depending on the direction of sliding. For example, roof shingles are made up of overlapping elements all pointing in one direction, resulting in sliding that is more favorable in one direction than another. Anisotropic friction is what allows snakes to travel through the sand — something that Schwarz drew inspiration from and hopes to replicate on the solar panels.

This idea for solar panels is just one of many possible applications for nanoimprinting. In the future, Schwarz is optimistic that scientists will take advantage of this novel process to do other exciting things in their fields.

The Yale School of Engineering and Applied Science is located at 10 Hillhouse Avenue.

Veronica Lee | veronica.lee@yale.edu