US team generates electricity from 2D MoS2

First experimental observation of piezoelectricity in 2D material 

Researchers from Columbia Engineering and the Georgia Institute of Technology report today that they have made the first experimental observation of piezoelectricity and the piezotronic effect in a 2D material, MoS₂. The result, they say, is a unique electric generator and mechano-sensation device that is optically transparent, extremely light, and very bendable and stretchable.

In a paper published online October 15, 2014, in Nature, research groups from the two institutions demonstrate the mechanical generation of electricity from the 2D MoS₂ material. The piezoelectric effect in this material had previously been predicted theoretically.

MoS₂ is one of a new class of 2D semiconducting materials known as transition metal dichalcogenides. There is much interest in these materials as they can also emit light. 

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 picture above shows positive and negative polarized charges squeezed from a single layer of atoms of MoS2 as it is being stretched).

"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," says 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 2D materials," says 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 MoS₂, and demonstrating the piezoelectric effect in it adds a new facet to the material."

Hone and his research group demonstrated in 2008 that graphene 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 MoS₂ for generating current: using an odd number of layers and flexing it in the proper direction. The material is highly polar, but, Zhong Lin 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.

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," says Lei Wang.

To be piezoelectric, a material must break central symmetry. A single atomic layer of MoS2 has such a structure, and should be piezoelectric. However, in bulk MoS₂  successive layers are oriented in opposite directions, and generate positive and negative voltages that cancel each other out and give zero net piezoelectric effect. "This adds another member to the family of piezoelectric materials for functional devices," says Wenzhuo Wu.

2D materials can be stretched much farther than conventional materials, particularly traditional ceramic piezoelectrics, which are quite brittle.

"This is the first experimental work in this area and is an elegant example of how the world becomes different when the size of material shrinks to the scale of a single atom," Hone adds. "With what we're learning, we're eager to build useful devices for all kinds of applications."

Ultimately, Zhong Lin Wang notes, the research could lead to complete atomic-thick nanosystems that are self-powered by harvesting mechanical energy from the environment. This study also reveals the piezotronic effect in two-dimensional materials for the first time, which greatly expands the application of layered materials for human-machine interfacing, robotics, MEMS, and active flexible electronics.