Size matters in piezoelectric materials

Nanowires that produce current when bent and deformed can show huge improvements in efficiency as their diameters are shrunk. The findings will help advance research to power technology at the nanoscale.

Ravi Agrawal and Horacio Espinosa of Northwestern University in Illinois, US, have been investigating the properties of nanowires for several years: ’Nanowires are potential building blocks of future devices,’ Espinosa explains. ’However, before the construction of devices that use millions of nanowires takes over, the fundamental properties of an individual nanowire must be fully understood.’

Recently scientists have shown that at the nanoscale, conversion of mechanical energy to electrical energy (so-called piezoelectricity) is much higher than in bulk materials. However, Espinosa noticed that those who had measured this property in nanowires reported widely differing results, so he decided to use a computational approach to find out what was actually being generated.

Modelling gallium nitride and zinc oxide nanowires (known to be piezoelectric), Espinosa and Agrawal calculated that the piezoelectric coefficient of the wires can be improved two-fold if their diameter is reduced to less than 1nm, rather than 10s or 100s of nanometres that have been investigated in the past.  

For ZnO wires, the improvement in piezoelectric properties was limited to wires under 1.5nm in diameter, but GaN wires showed enhancement at diameters up to and over 2.5nm. ’We certainly expected a change, but maybe not that large,’ says Espinosa. 

’This is an important result in guiding the research in nanogenerators and piezotronics’ says Zhong Lin Wang of Georgia Institute of Technology, US, who first showed that zinc oxide nanowires could act as generators of electricity. He sees envisages future in which nanogenerators are used to power everything from implanted biosensors to mp3 players. 

’The results suggest that there is an advantage in reducing the size of the piezoelectric elements used in an energy harvester since the extracted electrical energy will be greater,’ says Espinosa. Both Espinosa and Wang hope that the work will inspire the design of new devices using these materials and their properties. For Espinosa though, he will continue to focus on discovering new phenomena in nanowires and nanostructures. 

Laura Howes