Piezoelectric materials generate a voltage when they are mechanically deformed, and can hence be used to scavenge energy that would otherwise be wasted from mechanical systems. If such generators could also be made transparent and flexible, they would enable novel applications such as self-powered touch screens, flags that generate power from wind, or artificial skin.

Past attempts at creating transparent and flexible piezoelectric devices, however, have been hampered by brittle electrodes. Now, Jae-Young Choi, Sang-Woo Kim and colleagues in Korea have constructed a flexible, transparent piezoelectric generator from a new, flexible contact material based on zinc oxide nanorods.1

Fig. 1: An artist’s rendition of zinc oxide nanorods, which generate a current under mechanical stress, between two flexible and transparent graphene electrodes.

The material used was graphene, the well-known two-dimensional lattice of carbon atoms, which the researchers prepared using chemical vapor deposition onto a four-inch wafer. After transferring the graphene to a flexible polymer substrate, zinc oxide piezoelectric nanorods (Fig. 1) were grown on the flexible graphene/polymer substrate (2×2 cm2) at a low temperature of 95 °C. Finally, a top graphene electrode was added to complete the device.

The graphene contacts were 75% transparent, and their electrical performance was not significantly affected on repeated bending into a circle of 4 mm radius. This high mechanical robustness was not only due to graphene’s intrinsic mechanical strength but also to its thinness, which lowered the bending-induced strain. In addition, the piezoelectric current from the device showed a high on–off ratio — a result of graphene’s high electron mobility.

The researchers also noted that the single-crystal zinc oxide nanorods seemed to grow epitaxially, meaning that their crystal structure was set by that of the graphene onto which they were grown. Such low-temperature epitaxial growth onto graphene substrates could be used with other materials and for other designs of flexible device.

The results may address a bottleneck in the development of fully flexible electronics, say Kim and Choi: “Even if most of the elements in an electronic device are flexible, if the power generator is not flexible, then the finished device will not be.” The research team plans to focus part of its future efforts on controlling the conductivity and work function of the graphene device — both of which are key to achieving high performance. This can be done, for example, by doping foreign atoms into the graphene lattice.