Researchers find this sheet—a single molecule thick—produces electricity when flexed.
by Shalini Saxena
Certain materials exhibit what are called piezoelectric characteristics, meaning they develop electric charges when stretched or compressed. In general, the piezoelectric materials we use are large crystals. But researchers have predicted that a substance that forms single-atom-thick molecules—MoS2—would be strongly piezoelectric. And now researchers have studied these effects experimentally, demonstrating that the number of layers and their orientation have a big impact on the substance’s piezoelectric characteristics.
To get enough material to work with, MoS2 layers were flaked off onto a flexible substrate and electrical contacts deposited at the MoS2 interface. The piezoelectric response of the material was studied by application of a strain, which causes a strain-induced polarization of charges at the sample edges. These drive the flow of electrons into an external circuit for measurement. Upon relaxation of the strain, the polarization of charges is diminished, causing the electrons to flow back to their original distribution.
Because of the way the flakes were created, each sample had a different number of layers. When a sample had odd numbers of MoS2 layers, stretching and releasing produced exactly the behavior we just described: oscillating piezoelectric voltage and current outputs were observed.
In contrast, flakes with even numbers of MoS2 layers produced no output. For a single-layer of MoS2, voltage and current outputs increased with increasing strain. Single layers exhibited a decrease in current and an increase in voltage when resistance was cranked up. Finally, cyclic strain and release studies demonstrated single-layers were stable for an extended period of use.
Further studies demonstrated that the piezoelectric characteristics of flakes with odd numbered layers of MoS2 decreased rapidly as the layer number went up (from 1 to 5 layers), and disappeared completely in the bulk material. Computational studies have shown that this disappearance is due to the atomic layers having random orientations, causing them to ultimately cancel one another out.
To further test whether this material would be useful for nanoelectronics, the researchers built integrated arrays of single-layer MoS2. MoS2 flakes arranged in parallel exhibited an increase in current with increasing number of single-layer flakes. MoS2 flakes arranged in series exhibited an increase in voltage with increasing number of single-layer flakes. These studies experimentally demonstrate that single layers of MoS2 are a promising candidate for powering nanoelectronics, particularly for stretchable electronics such as wearable technologies or electromechanosensing technologies.
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