ZnO is a semiconductor that is promising for use in optoelectronics applications. Nanoscale structures of ZnO also exhibit piezoelectricity. Graphene, on the other hand, merges excellent electrical properties with high optical transparency and flexibility, making it an ideal material in a variety of applications, including as a substrate for the growth of well-defined nanostructures.
Scanning electron microscopy image of a hybrid nanowire–nanowall structure
Kim and his colleagues used chemical vapor deposition to grow ZnO nanowalls and nanowires on graphene using gold particles as catalysts. They showed that by varying the amount of gold initially deposited on the graphene, it is possible to tune the growth mode of ZnO, enabling the researchers to produce shapes from vertical nanowires to nanowire–nanowall hybrids and isolated nanowalls (see image). The length of wires and height of walls all increased with growth time.
Detailed observations of the various nanostructures under different growth conditions allowed the researchers to propose a growth mechanism for their process — an essential step in designing the fabrication of ZnO/graphene structures with desired morphologies.
The team also demonstrated a piezoelectric current generator based on a hybrid nanowire–nanowall structure on graphene. They used a gold-coated polyetherosulfone film as a top electrode, while the graphene acted as the bottom electrode. By applying pressure pulses from the top, the researchers observed voltage and current pulses due to the piezoelectricity of the ZnO. The signal was smaller than predicted, which the researchers explain as due to partial screening of the generated current by free carriers in the nanowalls.
Nanogenerators are not the only use for the composite materials that can be produced by this growth technique. "Producing three-dimensional nanoscale building blocks through the integration of semiconducting nanostructures with two-dimensional graphene layers is very promising for the realization of graphene-based novel functional nanodevices including, nanoscale solar cells and light-emitting diodes," says Kim.