Researchers from Columbia Engineering and the Georgia Institute of Technology reported the first experimental observation of piezoelectricity and the piezotronic effect in an atomically thin material, molybdenum disulfide (MoS2). In essence, the discovery opens the doors for an electric generator and mechanosensation devices that are optically transparent, extremely light, and very bendable and stretchable.
The findings were published this week in Nature, where the two research groups demonstrate the mechanical generation of electricity from the two-dimensional (2D) MoS2 material. The piezoelectric effect in this material had previously been predicted theoretically.
Piezoelectricity is a well-known effect in which stretching or compressing a material causes it to generate an electrical voltage (or the reverse, in which an applied voltage causes it to expand or contract). But for materials of only a few atomic thicknesses, no experimental observation of piezoelectricity has been made, until now. The observation reported today provides a new property for two-dimensional materials such as molybdenum disulfide, opening the potential for new types of mechanically controlled electronic devices. Commenting on the discovery's potential applications in a news release, James Hone, professor of mechanical engineering at Columbia and co-leader of the research said, "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."
How the technology works
According to the report, there are two keys to using molybdenum disulfide for generating current: using an odd number of layers and flexing it in the proper direction. The material is highly polar, but an even number of layers cancels out the piezoelectric effect. The material's crystalline structure also is piezoelectric in only certain crystalline orientations. For the Nature study, researchers placed thin flakes of MoS2 on flexible plastic substrates and determined how their crystal lattices were oriented using optical techniques. They then patterned metal electrodes onto the flakes. Next, measurement electrodes were then installed on samples, and researchers measured current flows as the samples were mechanically deformed. Researchers also monitored the conversion of mechanical to electrical energy, and observed voltage and current outputs. Learn more in the full news release.