Tiny 2-D Drumheads Made From Black Phosphorus

Image: Philip Feng Research Group/Case Western Reserve University

A sheet of black phosphorus placed over a resonating cavity

They’re sure to rank among the world’s smallest trampolines.

A team led by Philip Feng, an assistant professor of electrical engineering and computer science at Case Western Reserve University in Cleveland, Ohio, has draped atom-thick layers of semiconducting material over cavities to create new kinds of resonators. Similar, albeit bigger, micromechanical systems are used today to make a variety of components, including reference oscillators for on-chip clocks, amplifiers, and frequency-selective devices. 

In a talk given last week at the IEEE International Electron Devices Meeting in San Francisco, Feng showed the results of his team’s study of drumheads made from molybdenum disulphide (MoS2) and black phosphorus, a structural variety of phosphorus that forms puckered sheets and has been gaining more and more attention as a potential 2-D material for future electronic devices.

Image: Philip Feng Research Group/Case Western Reserve University

Layers of molybdenum disulphide are draped over a cavity. Devices can be made with single or multiple layers.

These are not the world’s first drumhead resonators. The list of previously used materials includes silicon nitride, silicon carbide, diamond, and graphene, Feng says. But he notes that his team’s drumheads are the first to be made from 2-D materials that have semiconducting properties (graphene does not qualify, because in its natural form it’s always conductive).

This “immediately adds new possibilities and potential functions,” he says, because stretching a 2-D semiconducting material alters its bandgap. That’s the amount of energy needed to kick electrons up into a state where they can move freely through the material.

Since they can be stretched more, 2-D materials should produce resonators that operate in a wider range of frequencies than conventional crystal-based resonators made from silicon or diamond.   

To fabricate the devices, Feng’s group refined a transfer technique that begins with the fabrication of electrodes and circular “microtrenches.” The 2-D material is laid down over the trench, much “like covering a muffin tin with kitchen plastic wrap,” says Feng. The process, which the team has also used to make MoS₂ transistors, is outlined in a recent paper published in the Journal of Vacuum Science & Technology B.

At last week’s meeting, Feng showed that the vibrations of the resonators could be detected down to their noise limit—the random fluctuations caused by thermal noise. 

Although he doesn’t yet have specific new applications to point to, Feng says that work on 2-D resonators with bandgaps could open up a new avenue for coupling mechanical vibrations to electrical and optical behavior. “I’m not sure it’s going to turn out better than graphene,” he says, “but I do think it’s going to be different.”