Uniformly grown large-area ZnO ultrathin sheet on SiO2.
Uniformly grown large-area ZnO ultrathin sheet on SiO2.

Thin layers of zinc oxide (ZnO) just a nanometer thick, laid down on silicon dioxide (SiO2), show record levels of piezoelectricity, eight times that achieved in bulk material, according to new research [Mahmood et al., Materials Today (2020), https://doi.org/10.1016/j.mattod.2020.11.016].

Piezoelectricity is produced in certain materials under mechanical strain and arises from asymmetry in the material’s atomic structure. Many wurtzite-type crystals show piezoelectricity but only a few have been grown in single or few layer structures, which increases the effect. ZnO is one such wurtzite crystal, which shows promising levels of piezoelectricity in bulk form. According to researchers from RMIT University, University of New South Wales, and Deakin University in Australia, the effect can be boosted in thin-layer ZnO, which has been difficult to produce until now.

“We have been working on liquid metals and synthesizing atomically thin crystals using liquid metal processes for a while,” says Kourosh Kalantar-Zadeh of the University of New South Wales, who led the work.

When ZnO melts (at around 420°C), a very thin oxide layer forms naturally on top. The surface oxide layer is not tightly stuck to the liquid metal, so it can be easily transferred onto another substrate like SiO2.

“We used this elegant liquid-metal based synthesis process for creating highly crystalline ZnO layers,” explains Kalantar-Zadeh, “[which] we predicted could offer strong piezoelectric properties.”

The team wasn’t disappointed. Their density functional theory (DFT) simulations of ultrathin ZnO layers on SiO2 substrates indicating that a reduction in thickness would boost piezoactivity were borne out with measured levels of piezoelectricity reaching 8 times that of the bulk material in a 1.1 nm (or 5 unit cell thick) layer. But the researchers also showed that there are factors other than layer thickness in play, namely the interaction of the layers with the substrate.

“We serendipitously realized that the bottom of the layer of ZnO can establish covalent bonds with the substrate, which adds to the asymmetry of the system and produces a stronger than predicted piezoelectric layer.”

This new means of producing very thin layers of highly crystalline ZnO is not only simple, but can produce large areas of highly controllable thickness, point out the researchers. The simple approach could also be readily applied to other materials.

“This is a great demonstration for the development of future piezoelectric devices,” adds first author Nasir Mahmood of RMIT University. “The high piezoelectric coupling coefficient and ease of fabrication allow for very sensitive acceleration sensors (for air bags or smart phones), electronic filters, or large scale piezogenerators producing energy from motion.”

The researchers are now exploring the approach for ‘smart’ footpaths that harvest energy from human footfall, he told Materials Today.