New way to help 2D sensors hug the curve

Engineers at Rice University, led by materials scientists Pulickel Ajayan and Jun Lou, have developed a method for making atom-flat sensors from two-dimensional (2D) materials and then transferring them to curved surfaces.

Electronically active 2D materials have been the subject of much research since the introduction of graphene in 2004. Even though they are often touted for their strength, they can be difficult to move to where they're needed without destroying them.

The Ajayan and Lou groups, along with the lab of Rice engineer Jacob Robinson, have now come up with a way to keep 2D materials and any associated circuitry, including electrodes, intact as they're moved to curved or other smooth surfaces. They report their work in a paper in ACS Nano.

The Rice team tested the concept by fabricating a 10nm-thick indium selenide photodetector with gold electrodes and then placing it onto an optical fiber. Because it was so close, the photodetector effectively coupled with an evanescent field – the oscillating electromagnetic wave that rides the surface of the fiber – and accurately detected the flow of information inside. The benefit of this approach is that these sensors can now be imbedded into such fibers, where they can monitor performance without adding weight or hindering the signal flow.

"This paper proposes several interesting possibilities for applying 2D devices in real applications," Lou said. "For example, optical fibers at the bottom of the ocean are thousands of miles long, and if there's a problem, it's hard to know where it occurred. If you have these sensors at different locations, you can sense the damage to the fiber."

Lou said that labs have gotten good at transferring the growing roster of 2D materials from one surface to another, but the addition of electrodes and other components complicates the process. "Think about a transistor," he said. "It has source, drain and gate electrodes, and a dielectric (insulator) on top, and all of these have to be transferred intact. That's a very big challenge, because all of those materials are different."

Raw 2D materials are often moved with a layer of polymethyl methacrylate (PMMA), more commonly known as Plexiglas, on top, and the Rice researchers made use of that technique. In addition, however, they needed a robust bottom layer that would not only keep the circuit intact during the move, but could also be removed before attaching the device to its target. (The PMMA is also removed when the circuit reaches its destination.)

The ideal solution was polydimethylglutarimide (PMGI), which can be used as a device fabrication platform and then easily etched away before transfer to the target. "We've spent quite some time to develop this sacrificial layer," Lou said. PMGI appears to work for any 2D material, with the researchers successfully demonstrating the method on molybdenum diselenide and other materials as well.

The Rice labs have only developed passive sensors so far, but the researchers believe their technique will make active sensors or devices possible for telecommunication, biosensing, plasmonics and other applications.

This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.