2D sensors could be embedded into devices

What if a sensor sensing a thing could be part of the thing itself? US researchers believe they have a solution to do just that, having developed a method to make atom-flat sensors that seamlessly integrate with devices to report on what they perceive.

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

Now, engineers from Rice University have a new way to keep the materials and their associated circuitry, including electrodes, intact as they’re moved to curved or other smooth surfaces. Their work has been published in the journal ACS Nano.

The Rice team tested the concept by making a 10 nm-thick indium selenide photodetector with gold electrodes and placing it onto an optical fibre. Because it was so close, the near-field sensor effectively coupled with an evanescent field — the oscillating electromagnetic wave that rides the surface of the fibre — and detected the flow of information inside.

The benefit is that these sensors can now be embedded into such fibres where they can monitor performance without adding weight or hindering the signal flow. And there are several interesting possibilities for applying 2D devices in real applications, as noted by Rice materials scientist Jun Lou.

“For example, optical fibres 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,” Lou said. “If you have these sensors at different locations, you can sense the damage to the fibre.”

Lou noted that labs have become 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. But 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), described by Lou as a “sacrificial layer” which can be used as a device fabrication platform and easily etched away before transfer to the target. PMGI appears to work for any 2D material, as the researchers experimented successfully with 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.

Image caption: Rice University engineers have developed a method to transfer complete, flexible, two-dimensional circuits from their fabrication platforms to curved and other smooth surfaces. Such circuits are able to couple with near-field electromagnetic waves and offer next-generation sensing for optical fibres and other applications. Image credit: Zehua Jin/Rice University.

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