In comparison to conventional electronic components, the nano-piezotronic devices operate much differently and exhibit unique characteristics.
In conventional field-effect transistors, for instance, an electrical potential – called the gate voltage – is applied to create an electrical field that controls the flow of current between the device’s source and its drain. In the piezotronic transistors developed by Wang and his research team, the current flow is controlled by changing the conductance of the nanostructure by bending it between the source and drain electrodes. The bending produces a “gate” potential across the nanowire, and the resulting conductance is directly related to the degree of bending applied.
“The effect is to reduce the width of the channel to carry the current, so you can have a 10-fold difference in the conductivity before and after the bending,” Wang explained.
Diodes, which restrict the flow of current to one direction, have also been created through nano-piezotronic mechanisms to take advantage of a potential barrier created at the interface between the electrode and the tensile (stretched) side of the nanowire by mechanical bending. The potential barrier created by the piezoelectric effect limits the follow of current to one direction.
Nanogenerators, which were announced in the April 14, 2006 issue of the journal Science, harvest energy from the environment around them, converting mechanical energy from body movement, muscle stretching, fluid flow or other sources into electricity. By producing current from the bending and releasing of zinc oxide nanowires, these devices could eliminate the need for batteries or other bulky sources for powering nanometer-scale systems.
Piezotronic nanosensors can measure nano-Newton (10 -9) forces by examining the shape of the structure under pressure. Implantable sensors based on the principle could continuously measure blood pressure inside the body and relay the information wirelessly to an external device similar to a watch, Wang said. The device could be powered by a nanogenerator harvesting energy from blood flow.
Other nanosensors can detect very low levels of specific compounds by measuring the current change created when molecules of the target are adsorbed to the nanostructure’s surface. "Utilizing this kind of device, you could potentially sense a single molecule because the surface area-to-volume ratio is so high,” Wang said.
In addition to Wang, the research team included J.H. Song, X.D. Wang, P.X. Gao, J.H. He, J. Zhou, N.S. Xu, L.J. Chen and J. Liu from Georgia Tech, the National Tsing Hua University in Taiwan and Sun Yat-Sen University in China.