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A generator in just two dimensions: researchers at Columbia University and Georgia Tech have demonstrated the world's thinnest electric generator, a film of molybdenum disulfide a few atoms thick.

The generator has no moving parts. Instead, it is a piezoelectric material that produces current when stretched or compressed. This is the first time anyone has shown the piezoelectric effect in atomically thin films, although the property has been predicted.

The work underscores how materials properties change at the nanoscale, since bulk molybdenum disulfide does not exhibit piezoelectric properties.

Molybdenum disulfide is part of a family of transition metal dichalcogenides. Theorists predicted that they would all have piezoelectric properties when made into atomically thin films. Such films are often called 2-D materials. The best known example of a 2-D film is graphene, which researchers are investigating for electronics uses. Their nanoscale thickness of 2-D films often gives them unusual properties.

In addition to its ability to generate current, 2-D molybdenum disulfide has other intriguing properties. In bulk, molybdenum disulfide is an opaque and brittle ceramic. Its 2-D counterpart is optically transparent and very bendable and stretchable. This would enable 2-D films to generate power without cracking or breaking.

"This material--just a single layer of atoms--could be made as a wearable device, perhaps integrated into clothing, to convert energy from your body movement to electricity and power wearable sensors or medical devices, or perhaps supply enough energy to charge your cell phone in your pocket," said James Hone, a Columbia professor of mechanical engineering.

Georgia Tech materials scientist Zhong Lin Wang believes 2-D films could power entire atomically thick nanosystems by harvesting mechanical energy from the environment. These could be used for power sources, and also as sensors in robots, MEMS, and human-machine interfaces. Wang and Hone collaborated on the study.

Of course, the work is far from commercial. -Instead of creating oriented films, researchers examined a variety of atomically thin molybdenum disulfide flakes they had created to find the right combination of orientation and properties. They then installed electrodes to measure current. They found that the output voltage actually reversed sign when they changed the direction of the applied strain.

The 2-D film, Hone explained, is "an elegant example of how the world becomes different when the size of material shrinks to the scale of a single atom."

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Title Annotation:TECH BUZZ
Author:Brown, Alan S.
Publication:Mechanical Engineering-CIME
Date:Jan 1, 2015
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