Far from being simple conductors for transporting charge in future nanoelectronic devices, nanowires exhibit unique characteristics that could be also be exploited for other applications. An example of such multifunctional properties is provided by Wei Pan and colleagues at Tsinghua University in Beijing, China who find that aluminium-doped zinc oxide (AZO) nanowires exhibit both photoconductivity and voltage–induced bistable switching behaviour.1

Fig. 1: Electron micrograph of the nanowires grown by electrospinning enabling growth of nanowires many centimetres long.

The AZO nanowires were grown by electrospinning. This process works by using a needle to deliver a precursor solution of chemicals over two conductors, from which the nanowires subsequently grow; here a mixture of zinc acetate, aluminium nitrate, and poly(vinyl alcohol) was used. Application of a high voltage (20 kV) between the needle and two filaments of copper (separated from each other by a gap of 1 cm, and 20 cm beneath the syringe) caused the solution to form a long thin jet, which resulted in the formation of multiple nanowires of around 200 nm diameter between the filaments. Subsequent heating at 550ºC removed the solvent and induced calcination, leading to the formation of narrower (around 60 nm in diameter) polycrystalline AZO nanowires (Fig.1).

Electrical measurements of these nanowires showed their conductivity to change substantially during exposure to visible light—by around 20 times compared to their conductivity in the dark—providing the first report of photoconductive response by zinc oxide nanowires to light with energy much less than the bandgap of zinc oxide. Such behaviour could be useful for fabricating nanoscale photoswitches for optoelectronic applications.

But perhaps more surprisingly, they found that when a large voltage of ~100 V was applied in one direction to the nanowires, they switched to a low resistance state and when a similar voltage was applied in the opposite direction, they switched to a high resistance state—with no change occurring at voltages below those at which the wires were observed to switch. Such persistent bistable switching could provide the basis for non-volatile nanowire memories.

Although both photoconductivity and non-volatile memory effects have been observed previously in other material systems, an attractive advantage of their demonstration in electrospun AZO nanowires is the versatility of the electrospinning approach itself. “Electrospinning is a very convenient and cheap process for making polycrystal nanowires with extremely high aspect ratio and specific surface area,” says Pan who leads the group. “It enables arrays of long nanowires to be grown much more easily than is possible with more conventional techniques such as physical or chemical vapour deposition.”