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Thin electric generator paves way for energy-harvesting devices

Posted: 16 Oct 2014     Print Version  Bookmark and Share

Keywords:electric generator  molybdenum disulphide  piezoelectric  wearables 

Researchers developed a unique electric generator and mechanosensation devices that are optically transparent, extremely light, and bendable and stretchable. This came after they had reported the first experimental observation of piezoelectricity and the piezotronic effect in an atomically thin material, molybdenum disulphide (MoS2).

In a paper published in the journal Nature, research groups from the two institutions demonstrate the mechanical generation of electricity from the two-dimensional (2D) MoS2material. 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 provides a new property for 2D materials such as MoS2, opening the potential for new types of mechanically controlled electronic devices.

Thin electric generator

Shown is a sample of the material that could be the basis for unique electric generator and mechanosensation devices that are optically transparent, very light, and extremely bendable and stretchable. (Source: Rob Felt)

"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, professor of mechanical engineering at Columbia and co-leader of the research.

"Proof of the piezoelectric effect and piezotronic effect adds new functionalities to these two-dimensional materials," said Zhong Lin Wang, Regents' Professor in Georgia Tech's School of Materials Science and Engineering and a co-leader of the research. "The materials community is excited about molybdenum disulfide, and demonstrating the piezoelectric effect in it adds a new facet to the material."

Hone and his research group demonstrated in 2008 that graphene, a 2D form of carbon, is the strongest material. He and Lei Wang, a postdoctoral fellow in Hone's group, have been actively exploring the novel properties of 2D materials like graphene and MoS2 as they are stretched and compressed.

Zhong Lin Wang and his research group pioneered the field of piezoelectric nanogenerators for converting mechanical energy into electricity. He and postdoctoral fellow Wenzhuo Wu are also developing piezotronic devices, which use piezoelectric charges to control the flow of current through the material just as gate voltages do in conventional three-terminal transistors.

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, Wang notes, so 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, Hone's team 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. In research done at Georgia Tech, Wang's group installed measurement electrodes on samples provided by Hone's group, then measured current flows as the samples were mechanically deformed. They monitored the conversion of mechanical to electrical energy, and observed voltage and current outputs.


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