There’s nothing worse than your cellphone running out of power when you’ve been relying on it to send that important email. Batteries these days still rely on technology that was developed over a century ago, and no matter how efficient they get at power storage, it seems that our devices are just a short step ahead of them in terms of power consumption. But what if you could charge your phone in your pocket just by moving around—in the same way as a Kinetic watch works? That dream is now closer to reality with the discovery of a new 2D piezoelectric material—the first time that a non-bulk material that can exploit piezoelectricity has been discovered.

The Piezoelectric Effect

The piezoelectric effect has been known for a long time, and essentially involves the generation of a voltage caused by the physical deformation of a material. That is, if you compress or stretch a material, electricity is produced. Since the discovery of piezoelectricity in 1880, the piezoelectric effect has been harnessed in bulk materials for many years, from hi-tech uses like ultrasonic imagery to more mundane applications like in cigarette lighters.

More recently, there has been rising interest in using piezoelectric materials at the nanoscale level to provide extremely low power consumption switches – but when the thickness of a material approaches a single layer of molecules, piezoelectric structures can become thermodynamically unstable because of the large surface energy. Recent work, however, has shown that piezoelectricity in single-layer films is no longer just theoretically possible.

Measuring it in Molybdenum

Molybdenum disulphide (MoS2) is an inorganic semiconductor that may well be a possible candidate for the post-silicon era. Researchers from the US Department of Energy’s Lawrence Berkeley National Laboratory performed an experiment where they demonstrated piezoelectricity in a free standing single layer of molybdenum disulphide. The researchers stretched a thin film of MoS2 on HSQ posts and clamped it in place using gold electrodes. Using a probe, the team indented the material and measured its piezoelectric coefficient. The size of the effect compares favourably with known piezoelectric bulk materials such as aluminium nitride and zinc oxide.

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However, the effect vanished when a second layer was overlaid on top of the first, which may be because the material is highly polar, meaning that the electricity generated in the second layer cancels the first layer out. An odd number of layers is required to obtain the effect, and can even increase it—but MoS2 is not piezoelectric in bulk form, meaning that an extremely careful manufacturing process is required so that exactly the right number of layers are placed on top of each other. The output voltage also reverses sign when the direction of the applied strain on the material is changed, so any generation of current would require the material to be stretched in the right direction.

Useful Materials

The study reveals a 2D piezoelectric effect for the first time, and in fact molybdenum disulphide is just one in an entire family of materials that are predicted to have similar piezoelectric properties. Crucially, 2D materials can be stretched much more than conventional materials, especially the rather brittle traditional ceramic piezoelectrics. Further research is required to determine which of these materials will be the most useful, but the initial signs are very promising.

The uses of this kind of material are manifold, and include the application of layered materials in human–machine interfaces, active flexible electronics, robotics, micro-electromechanical systems, and ultrasensitive biosensors; and of course, one day they could even produce enough energy to charge the smartphone in your pocket. The research could result in complete self-powered atom-thick nanosystems that can harvest mechanical energy from their environment to enable them to operate, though this is some distance away.

It’s important to note that any change to the fundamental materials used in either transistor chips or PCB circuit lines will have very different electromagnetic emission and interference properties, requiring some learning curve from electrical engineers responsible for EMI compliance testing. Regardless of the eventual outcome, the changes wrought by these new materials will keep a lot of people busy for a long time, and could lead to some exciting new developments.