Piezoelectric Nanogenerators Based on Zinc Oxide Nanowire Arrays Zhong Lin Wang1,2,3* and Jinhui Song1 14 APRIL 2006 VOL 312 SCIENCE Presented by Yiin-Kuen(Michael) Fuh

Outline Motivation & Background Experimental design Results Conclusion

Motivation & Background Motivation—self-powered device can greatly reduce the size of integrated nanosystems for optoelectronics, biosensors and more. Background— –1D ZnO nanomaterials exhibits both semiconducting and piezoelectric(PZ) properties for electromechanically coupled sensors and transducers. is relatively biosafe and biocompatible for biomedical applications. exhibits the most diverse and abundant configurations of nanostructures knownsuch as NWs, nanobelts (NBs),nanosprings, nanorings, nanobows and nanohelices (10). –The mechanism of the power generator relies on the coupling of piezoelectric and semiconducting properties of ZnO as well as the formation of a Schottky barrier between the metal and ZnO contacts.

Experimental design (A)Aligned ZnO NWs grown on - Al2O3 substrate. (B) TEM image showing the NW without an Au particle or with a small Au particle at the top. Each NW is a single crystal and has uniform shape. Inset at center: an electron diffraction pattern from a NW. Most of the NWs had no Au particle at the top. Inset at right: image of a NW with an Au particle (C) The base of the NW is grounded and an external load of RL is applied, which is much larger than the resistance RI of the NW. The AFM scans across the NW arrays in contact mode Experimental design for converting nanoscale mechanical energy into electrical energy by a vertical piezoelectric (PZ) ZnO NW.

Results-Electromechanically coupled discharging process observed in contact mode. (A) Topography image, NW density ~20/um 2 (B) Output voltage, V peak ~6-9mV (C) A series of line profiles of the voltage output signal when the AFM tip scanned. (D) Line profiles from the topography (red) and output voltage (blue) images across a NW. The peak of the voltage output corresponds approximately to the maximum deflection of the NW, indicating that the discharge occurs when the tip is in contact with the compressed side of the NW. (E)Vpeak of FWHM. (F) W elastic =W PZD +W vib., W PZD~ 0.5CV 2  =W PZD /W elastic ~17-30%

(A) Experimental setup. (B) Topography image (C) Output voltage. The voltage output contains no information but noise, proving the physical mechanism demonstrated Results-Electromechanically coupled discharging process observed in tapping mode.

Theory --Transport is governed by a metal- semiconductor Schottky barrier for the PZ ZnO NW (A) NW coordination system. (B) Longitudinal strain z distribution (NW of length 1 µm and an aspect ratio of 10). (C) induced electric field Ez distribution (D) Potential distribution due to PZ effect. Schottky rectifying behavior (E) is to separate and maintain the charges as well as build up the potential. The process in (F) is to discharge the potential and generates electric current. The PZ potential is built up in the displacing process (G), and later the charges are released through the compressed side of the NW (H). (I) Large Au particle: The charges are gradually "leaked" out, no accumulated potential will be created.

Conclusion Self-powering nanotechnology ? Estimated power~ 10 pW/µm 2 and much more power if drives resonantly! Use flexible substrate for scavenging energy produced by acoustic waves, ultrasonic waves, or hydraulic pressure/force or environment etc. for applications such as implantable biomedical devices, wireless sensors, and portable electronics Continued work published as “Direct-Current Nano generator Driven by Ultrasonic Waves ” Science 6 April 2007:Vol no. 5821, pp. 102 – 105 Nanogenerator is featured in the overview section in the NSF FY 2008 budget request to congress