Oct 26, 2011
Short nanogenerators may be good enough
Nanogenerators based on arrays of vertically compressed ZnO nanowires (NWs) have been used to power PH sensors, LCD displays, wireless transducers and in vivo devices, with output voltages as high as a few volts. Pushed on by these achievements, researchers from the University of Rome Tor Vergata and the Georgia Institute of Technology have carried out a joint theoretical study featuring Finite Element (FE) analysis (using TiberCAD – a multiscale CAD tool for the simulation of nanoelectronic and optoelectronic devices) to investigate the electro-mechanical behaviour of a vertical compressed ZnO nanowire under open circuit condition. The aim is to understand how to design high-output nanogenerators and effective piezotronic devices.
As well as providing insight on the dependence of the output potential on doping, the team's calculations show that the potential drops mainly in the vicinity of the NW tip, across a region whose size increases as the doping decreases (see image). As a consequence, the output potential is essentially independent of the length of the NW.
Considering the case of a 1016 cm–3 doped NW, the output potential of about 42 mV is preserved even with very short NWs. When a force is applied to the NW, an effective negative charge is produced at the tip of the NW and electrons diffuse toward the base of the NW (which is kept to ground), creating a depletion region, which develops the output potential.
The work also highlights the importance of the dielectric material surrounding the nanowire on the output potential, which can be reduced significantly at low donor concentrations. These results are crucial for designing high-efficiency self-powered nanodevices. Furthermore, shorter NWs may easily be fabricated by wet-chemistry methods, which are fully compatible with low-cost, CMOS or MEMS processes.
The researchers presented their results in the journal Nanotechnology.
About the author
Giuseppe Romano is currently a postdoctoral researcher in the Department of Material Science and Engineering at Massachusetts Institute of Technology, US. His research is focused on energy conversion and storage at the nanoscale. Giulia Mantini is a Fulbright Scholar at the University of Rome Tor Vergata. Christian Falconi, Arnaldo D'Amico and Aldo Di Carlo are pofessors at the University of Rome Tor Vergata, Italy, with the Department of Electronics Engineering. Zhong Lin Wang is the High Tower Chair of the Department of Materials Science and Engineering at the Georgia Institute of Technology, US.