New One-Step Process Helps Create Tunable, Self-Assembled Metamaterials

A research team, headed by scientists from the University of Minnesota Twin Cities, has identified a revolutionary one-step process for producing materials with exceptional properties, known as metamaterials.

The researchers’ results demonstrate the true possibility of developing analogous self-assembled structures with the ability to produce “built-to-order” nanostructures for extensive applications in optical and electronic devices.

The study was published and featured on the cover of a peer-reviewed scientific journal, Nano Letters, which is published by the American Chemical Society.

Metamaterials are essentially materials that are created in laboratory settings to provide certain chemical, physical, optical, and electrical characteristics that otherwise cannot be found in naturally occurring materials.

Materials like these can have special characteristics that render them perfect for a wide range of applications, such as medical devices, optical filters, infrastructure monitoring, aircraft soundproofing, etc. Most often, the nano-scale materials are meticulously created in dedicated cleanroom surroundings for days and even weeks using a multi-step fabrication procedure.

In the latest study, a research team from the University of Minnesota was analyzing a thin-film material, known as strontium stannate with the chemical formula SrSnO₃. At the time of the study, the researchers observed the unexpected formation of nanoscale checkerboard patterns that were analogous to the metamaterial structures produced in the expensive, multi-step process.

At first we thought this must be a mistake, but soon realized that the periodic pattern is a mixture of two phases of the same material with different crystal structures. After consulting with colleagues at the University of Minnesota, University of Georgia, and City University of New York, we realized that we may have discovered something quite special that can potentially have some unique applications.

Bharat Jalan, Study Senior Author and Shell Chair, Department of Chemical Engineering and Materials Science, University of Minnesota

Jalan is also an expert in material synthesis.

The novel material had spontaneously arranged into a well-ordered structure as it altered from phase to phase. During the so-called “first-order structural phase transition” process, the novel material changed to a mixed-phase, where certain portions of the system finished the transition, while others did not.

These nanoscale periodic patterns are the direct consequence of the first-order structural phase transition in this material. For the first time, our work enables a host of possibilities for utilizing reversible structural phase transformations with nanoelectronic and photonic systems.

Richard James, Study Co-Author and Aerospace Engineering and Mechanics Professor, University of Minnesota

James is also a Distinguished McKnight University Professor.

The researchers revealed a process for the world’s first tunable, self-assembled nanostructure to produce metamaterials in just a single step. The team effectively tuned the potential to preserve the electrical charge property inside a single film using laser wavelength and temperature. They successfully produced a variable photonic crystal material that has an efficiency of 99%.

With the help of high-resolution electron microscopes, the investigators demonstrated the exclusive structure of the material.

“We observed that the boundaries between these crystallographic phases were sharply defined at the atomic scale, which is remarkable for a self-assembled process,” stated Andre Mkhoyan, the co-author of the study, professor, and an expert in sophisticated electron microscopy.

Mkhoyan is also the Ray D. and Mary T. Johnson/Mayon Plastics Chair in the Department of Chemical Engineering and Materials Science at the University of Minnesota.

At present, the team is searching for upcoming applications for their discovery in electronic and optical devices.

When we started this research, we never thought about these applications. We were driven by the fundamental study of the physics of the material. Now, all of a sudden, we seem to have opened up a completely new area of research, which is driven by the possibility of many new and exciting applications.

Bharat Jalan, Study Senior Author and Shell Chair, Department of Chemical Engineering and Materials Science, University of Minnesota

Besides Jalan, the team also included researchers Abhinav Prakash, Ashley Bucsek, Tianqi Wang, Tristan K. Truttmann, Hwanhui Yun, K. Andre Mkhoyan, and Richard James from the University of Minnesota; researchers Alireza Fali and Yohannes Abate from the University of Georgia; researchers Michele Cotrufo and Andrea Alù from the City University of New York; and researchers Jong-Woo Kim and Philip J. Ryan from the Argonne National Laboratory.

The study was mainly funded by the Air Force Office of Scientific Research (AFOSR) and the National Science Foundation (NSF), with additional support from the University of Minnesota Institute on the Environment, the Norwegian Centennial Chair Program, and two Vannevar Bush Faculty Fellowships.

The study at the University of Minnesota, involving the characterization of thin films, was supported by the U.S. Department of Energy.

Part of the study was performed at the Minnesota Nano Center and Characterization Facility at the University of Minnesota, partly supported by the National Science Foundation. An additional study was finished at the Advanced Photon Source, an Office of Science User Facility operated by Argonne National Laboratory for the U.S. Department of Energy Office of Science.

Journal Reference

Prakash, A., et al. (2020) Self-Assembled Periodic Nanostructures Using Martensitic Phase Transformations. Nano Letters. doi.org/10.1021/acs.nanolett.0c03708.