Researchers have discovered a two-dimensional material that could possibly generate electricity from motion thanks to its piezoelectric properties.
The one-atom thin layer of molybdenum disulphide (MoS2) used by researchers from Columbia Engineering and the Georgia Institute of Technology in an experiment described in the latest issue of Nature could potentially open new possibilities for manufacturing lightweight flexible electronics that would require no additional power source.
Although the piezoelectric effect in MoS2 has been predicted in theory, the latest study is the first to demonstrate it in practice.
“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.
Piezoelectricity is an 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). Until now, no one has observed the effect in materials only a few atoms thick. The most famous of such materials is graphene – the two-dimensional layer of carbon atoms hailed for its wonder properties.
“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.”
MoS2 doesn’t have piezoelectric properties in the bulk form but acquires them when reduced to a one atom thick layer.
When bend and flexed, the material generates electrical current. The researchers found they can stack the material into a larger structure but have to keep the number of layers odd, otherwise the positive and negative voltages generated by each layer would eliminate each other.
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 and measured current flows as the samples were mechanically deformed. They monitored the conversion of mechanical to electrical energy, and observed voltage and current outputs.
The researchers also noted that the output voltage reversed sign when they changed the direction of applied strain, and that it disappeared in samples with an even number of atomic layers, confirming theoretical predictions published last year. The presence of piezotronic effect in odd layer MoS2 was also observed for the first time.
“What’s really interesting is we’ve now found that a material like MoS2, which is not piezoelectric in bulk form, can become piezoelectric when it is thinned down to a single atomic layer,” Wang said.
“This adds another member to the family of piezoelectric materials for functional devices,” said Wenzhuo Wu.
The research could lead to complete atomic-thick nanosystems that are self-powered by harvesting mechanical energy from the environment.