A one-step process for creating self-assembled metamaterials

US researchers have discovered a one-step process for creating materials with unique properties, called metamaterials — a breakthrough that could lead to the creation of ‘built-to-order’ nanostructures for use in electronics and optical devices. Their work has been published in the journal Nanoscale.

Metamaterials are typically made in the lab to provide specific physical, chemical, electrical and optical properties otherwise impossible to find in naturally occurring materials. These materials can have unique properties that make them ideal for a variety of applications, from optical filters and medical devices to aircraft soundproofing and infrastructure monitoring. Usually these nanoscale materials are painstakingly produced in a specialised cleanroom environment over days and weeks in a multistep fabrication process.

The new breakthrough was sparked when researchers from the University of Minnesota, Twin Cities were studying a thin-film material called strontium stannate, or SrSnO₃. During their research, they noticed the surprising formation of checkerboard patterns at the nanoscale, similar to the metamaterial structures fabricated in the costly, multistep process.

“At first we thought this must be a mistake, but soon realised that the periodic pattern is a mixture of two phases of the same material with different crystal structures,” said Minnesota’s Bharat Jalan, senior author of the study and an expert in material synthesis. “After consulting with colleagues at the University of Minnesota, University of Georgia and City University of New York, we realised that we may have discovered something quite special that can potentially have some unique applications.”

The material had spontaneously organised into an ordered structure as it changed from one phase to another. During what is called a ‘first-order structural phase transition’ process, the material moved into a mixed phase in which some parts of the system completed the transition and others did not.

“These nanoscale periodic patterns are the direct consequence of the first-order structural phase transition in this material,” said Minnesota’s Professor Richard James, a co-author on the study. “For the first time, our work enables a host of possibilities for utilising reversible structural phase transformations with nanoelectronic and photonic systems.”

The team successfully demonstrated a process for what is believed to be the first-ever, self-assembled, tuneable nanostructure to create metamaterials in just one step. They were able to tune the ability to store electrical charge property within a single film using temperature and laser wavelength, and effectively created a variable photonic crystal material with 99% efficiency.

Using high-resolution electron microscopes, the researchers confirmed the unique structure of the material. According to study co-author Professor Andre Mkhoyan, also from Minnesota, “We observed that the boundaries between these crystallographic phases were sharply defined at the atomic scale, which is remarkable for a self-assembled process.”

The researchers are now looking to future applications for their discovery in optical and electronic devices — applications that Jalan had previously not even considered.

“When we started this research … we were driven by the fundamental study of the physics of the material,” Jalan said. “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.”

Image caption: While studying a thin-film material called strontium stannate (SrSnO₃), University of Minnesota researchers noticed the surprising formation of checkerboard patterns at the nanoscale similar to structures fabricated in costly, multistep processes. Image credit: Jalan Group, University of Minnesota.

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