With the frontiers of electronics racing towards foldable, portable devices, there is an earnest search for flexible electronic components. To this end, Lih-Juann Chen and co-workers from the National Tsing Hua University in Taiwan1 have grown zinc oxide nanorods on paper. These highly ordered single-crystal nanorods with tunable properties may find application in a wide variety of electrical goods.

At first glance, it may seem that paper has undesirable properties for such applications; it is insulating, rough and flammable. However, it also has many attractive attributes; it is cheap and readily available as well as highly flexible and environmentally benign.

Chen explains that zinc oxide (ZnO) was chosen because it is a cheap, non-hazardous, multifunctional semiconducting material with a wide direct band gap and large excitation energy, as well as being biocompatible and displaying piezoelectric properties allowing it to respond electrically to mechanical stress.

Fig. 1: Scanning electron microscopy image of ZnO nanowires grown on a paper substrate.© 2010 A. Manekkathodi

The researchers deposited several thin ZnO films onto paper by spin-coating using a zinc acetate solution. On further treatment with zinc nitrate and hexaethylene tetramine solution, they obtained a highly uniform coverage of standing ZnO nanorods (Fig. 1). The characteristics of these nanostructures could be controlled by altering the growth temperature and the composition of the spin-coating solution.

The team also applied a coating of a pyrene-based polymer to protect the paper from real-world hazards like solvents, acids, moisture, fungi and bacteria. Before growing the nanorods, the paper could be coated with a metal or semiconductor to give a flexible structure with the desired conducting characteristics. This additional treatment step also helped smooth out the surface and promote uniform nanorod growth.

To demonstrate that their platform can perform as an electrical component, the team prepared an organic/inorganic hybrid junction and a photodiode device that acted as a switch when exposed to ultraviolet light. They also demonstrated their platform’s flexibility — the electrical conductivity across a paper-based zinc oxide diode was maintained even when the sheet was bent and twisted. “We expect that the paper platform could be used for low-cost, flexible display devices such as light-emitting diodes, dye-sensitized solar cells, and various sensing device applications,” said Chen.