Advancing Future Flexible Wearable Electronics Through Liquid Metal Synthesis

July 13, 2020 by Gary Elinoff

Research aims to harness the piezoelectric properties of unique ultra-thin materials.

Atomically-thin tin-monosulfide (SnS) manifests the piezoelectric effect, which can convert mechanical forces into electrical energy. They are also physically flexible. This combination of properties points to employing this material for the conversion of random human motion into tiny amounts of electricity, a process called nano generation.

Researchers at Fleet, an Australian collaboration, including the University of New South Wales and RMIT University, report their progress in overcoming the considerable difficulties involved in synthesizing atomically-thin, monolayer SnS.

Fleet stands for future low-energy electronics technology. Appropriately, they see one of the research's eventual goals as the development of flexible nanogenerators to be worn by people to power wearable or internal self-powered biosensors.

Atomically-thin SnS has long been believed to exhibit strong piezoelectric properties. However, up until now, exploiting this potential has been hampered by producing large enough batches of highly crystalline monolayer SnS.

Electrical microgeneration by means of SnS thin films
Electrical microgeneration by means of SnS thin films. Image credited to Fleet


Liquid Metal Synthesis

The results of the study were published in the journal Nature Communications. The method employed in the synthesis involves melting tin (Sn) in the presence of hydrogen sulfide (H2S) gas.

Then, van der Waals exfoliation of the SnS formed on the surface of tin is performed. The technique is equally applicable to other monolayer group IV monochalcogenides, which are also expected to exhibit high piezoelectric properties. These include GeS, GeSe, and SnS. It was confirmed that the monolayer of SnS has high carrier mobility and piezoelectric coefficient.

This means that any applied strain will result in higher peak values of generated voltage and loading power than any other known 2D nanogenerator.

Synthesizing an atomically-thin layer of SnS.
Synthesizing an atomically-thin layer of SnS. Image credited to Fleet


Fabrication of the Piezoelectric Nanogenerator

The SnS monolayers were delaminated onto either smooth fluorphlogopite mica sheets measuring 50 × 20 mm or onto polydimethylsiloxane (PDMS). In order to ensure flexibility, the thickness of the mica substrate was reduced to 20 μm.

Next came the deposition of 10/100 nm Cr-Au electrodes. The width of the electrodes were 2 mm, with a separation of 40 μm between them. Devices featuring both two-electrode and multi-electrode devices were built. The multi-electrode devices consisted of 15 pairs of 100 μm finger width. The silver paste was used to connect electrical wires to the electrodes' contact pads.

The final step was the application of an encapsulation layer of PDMS was applied to the top of the device for encapsulation for the purposes of passivation and to support the thin mica sheets.

For the development of flexible nanogenerators, the process involved a PDMS substrate with a layer of polyamide optionally utilized for encapsulation.


Potential Implications of the Research

The bane of wearables has been providing them with power. Batteries take up size, weight, and all too soon, they have to be replaced.

To get around this difficulty, designers have tried many ingenious means, including the electrical power of near field radio waves. Most interestingly, it exploits lactate, the energy-rich compound found in human sweat. The wearable needs to be impractically close to the radio source to utilize its radiant energy, and people don't always sweat.

This development is relevant not only for wearables but also for implantabiles because there is motion inside the human body and outside. And of course, replacing a battery inside a living person involves surgery, the need for which would be prevented by a piezoelectric nanogenerator.

Devices utilizing this new technology also have implications for the IoT, where battery replacement is also an issue. Any type of mechanical perturbation can be exploited by a piezoelectric nanogenerator to create a remote source of electricity that can be stored in a capacitor, waiting for a remote sensor to make its report.