Aug 7, 2014
Decoupling the piezotronic effect from the piezoresistive
The piezoresistive effect is used in strain sensors and has seen application in artificial skin, soft robotics and wearable electronics. The recently discovered piezotronic effect also shows strain sensitivity but the two are always intertwined in current devices. Reporting in Nanotechnology, researchers at the University of Minnesota have applied the classic four-point measurement to separate and compare the two effects. The piezotronic effect at the contact is found to be dominant in response to strain and is therefore expected to produce ultra-sensitive devices.
Zinc oxide (ZnO) nanowires have inherent features that make them a promising material for strain sensing: nanoscale size, biocompatibility, abundance, and ultra-high sensitivity. However, their physics are far more complicated than the conventional metal-wire strain gauge.
Piezoresistive and piezotronic effects
As a semiconductor, the strain-induced band structure shift changes their resistivity – known as the piezoresistive effect. Since it is also a piezoelectric material, the strain-induced polarization charge changes the barrier height at the ZnO-metal interface - the ‘piezotronic effect’. Both effects affect the current transport. It is necessary to decouple and compare them quantitatively for better fundamental understanding and practical device design.
Modifying a standard technique
Four-point measurement is a standard technique to eliminate the contact resistance during the resistivity measurement. In a ZnO nanowire device, the contact resistance represents the piezotronic effect, while the resistivity represents the piezoresistive effect. In light of this, Rusen Yang and his student Ren Zhu put multiple electrodes on a single nanowire, and modify the four-point measurements to separate the two effects.
The measurement shows that the resistivity of nanowires increases under a tensile strain. The contact barrier may become higher or lower, depending on the position of the electrode on the nanowire. Most importantly, the change of the barrier height is more effective in tuning the resistivity than the change of the resistivity. It means that the piezotronic effect dominates the current transport in a zinc oxide nanowire. This finding allows researchers to simplify the model of a nanowire strain sensor and to design ultra-sensitive devices with the piezotronic effect.
The researchers presented their work in the journal Nanotechnology 25 345702 (IOPselect article).
Applicability of the classic elastic models at the nanoscale (June 2014)
How does size affect the piezoelectric properties of ZnO nanowires? (Oct 2013)
ZnO nanowire nanogenerators inspire self-powered active gas sensor (July 2013)
About the author
Ren Zhu is a PhD candidate at the University of Minnesota – Twin cities in the department of Mechanical Engineering, supervised by Rusen Yang. His research focuses on the growth of piezoelectric nanomaterials and their application for energy harvesting and sensing.