ZnO Nanowire Array Piezoelectric Nanogenerator

1. ZnONanowire Array Piezoelectric Nanogenerator Jung Hwan Woo

2. Outline • Introduction

3. Piezoelectricity • Piezo- : derived from Greek meaning press or squeeze • Electricity generation by mechanical stress • Reversible • Direct PZ effect and converse PZ effect • First demonstration by Pierre Curie and Jacques Curie in1880 in the course of studying for pyroelectricity • Governing equation: • E = ε/d where E : electric field ε : strain d : PZ coefficient http://resources.edb.gov.hk/physics/articleIE/smartmaterials/SmartMaterials_e.htm http://en.wikipedia.org/wiki/Piezoelectricity

4. Piezoelectric Generator • Harvests energy using PZ effect • Example • “Crowd Farm” – electricity generation from human footsteps in places such as train stations1 • Light flickering shown on YouTube 1Wright, Sarah H (2007-07-25). "MIT duo sees people-powered "Crowd Farm"". MIT news. Massachusetts Institute of Technology http://web.mit.edu/newsoffice/2007/crowdfarm-0725.html

5. Nanotechnology Application • Biological application: • In-situ real-time monitoring nanosensors and nanorobots • Allows self-powered • Piezoelectric nanogenerator using zinc oxide nanowire arrays • Advantageous properties of ZnO • Piezoelectric • Semiconductive • Biologically safe and biocompatible • Diverse and abundant • Nano-applications • NWs, Nanobelts, Nanosprings, nanorings, nanobows, nanohelices • Schottky barrier between the metal and ZnO

6. Fabrication of Nanoarrays • ZnONanowires grown on c plane-oriented α-Al2O3 using Au as a catalyst by vapor-liquid-solid (VLS) process • All ZnO NWs are vertically aligned • Single crystal and uniform • NW grows along the [0001] direction • {0 1 -1 0} surface on the side • Au evaporates during the growth or removed by the tip of the atomic force microscope (AFM) during scanning • Nanowires are placed vertically on Ag plate and grounded SEM (left) and TEM (right) image Wang. Z. L., et. al.

7. Measurement Method • ZnOnanowire moved by AFM tip and strained • Tensile on outer surface • Compressive on inner surface • AFM tip is Pt covered Si cone at 70° • Electric field along the z-direction within the NW is created due to the PZ effect • PZ field direction is parallel to the z axis at the outer surface and anti parallel to the z axis at the inner surface AFM scan over the nanowire Wang. Z. L., et. al.

8. Results • Simulated strain contour and Schottky effect Wang. Z. L., et. al.

9. Results • Measured PZ effect • Average peak height: ~6 to 9 mV • Peak density: ~8/µm2 (captured by AFM) • Nanowire density: ~20/µm2 • Efficiency: ~8/20 ≈ 40% (with scanning AFM tip) Wang. Z. L., et. al.

10. Results • Power generation • ~0.5 pW per tip x 20/µm2 ≈10 pW/µm2 • A NW array of size 10 µm x 10 µm would yield appreciable power for a nano-device (~ 1 nW)

11. Changing the Material • Direct-write piezoelectric polymeric nanogenerator with high energy conversion efficiency • In place of ZnO, Polyvinylidene fluoride (PVDF) used • Advantages • Higher flexibility • Reduced physical resistance • Good PZ • Chemical stability • Disadvantages • α-, β-, and γ-crystalline phases exist • β-PVDF is used for best PZ effect • Must be mechanically stretched and electrically poled

12. Changing the material • Method • Directly written by near-field electrospinning (NFES) to produce nanofibers on substrate with in situ mechanical stretching and electrical poling Chang, C., et. al.

13. Changing the material • Results • 5-30 mV at 0.5-3 nA => 2.5-90 pW • Energy conversion efficiency • ~12.5% average • ~21.8% peak Chang, C., et. al.

14. References • Wang, Z. L., Song, J., Piezoelectric Nanogenerators Based on Zinc Oxide Nanowire Arrays. Science, 2006, 312 (5771), 242-246. • Chang, C., Tran, V. H., Wang, J., Fuh, Y., Lin, L., Direct-Write Piezoelectric Polymeric nanogenerator with High Energy Conversion Efficiency. Nano Letters, 2010, 10, 726-731.

15. Questions?

16. G6Rebuttal: Piezoelectricity Jung Hwan Woo

17. Comments • Thank you all for constructive comments. I will work harder to provide future research suggestions and to include more current research being carried out. I will also include suggestions to how the experiment is performed to improve the results.

18. Q&A • Q: Why so little advance in the topic from 2006-2010 (judging for the presentation)? There is a fundamental obstacle? Have other nanoarrays be tested? Is this the best alternative to harvest energy at the nanoscale? • A: I believe that the limitation comes from the nature of the material. The piezoelectricity can only yield little power per area. Increase in the dimension of the array could yield higher power but it will eliminate these generators for nanotechnological use. However, it is certain that this method is one of the better ways to harvest energy for nanorobots as the external power source is completely eliminated. • Q: Is the only work done? Are there many other people working on this? Was the presented work a breakthrough? • A: This topic is a hot research topic these days. There are many more papers out there, though the Science paper was a bright original idea that suggested new strategy in powering nano-devices.

19. Q&A • Q: Could you better explain why current is measured when AFM tip exerts stress on nanowires and how it is measured? • A: For a free-standing piezoelectric nanowire under stress, one side will be compressively strained and the other tensilely stressed. This produces a flow of electron in a clockwise (or counter-clockwise) direction in a 2D model. If there is a source of electron, in this case the AFM tip, electrons from this source will be pulled to the same direction as the electron movements due to the electric field. At the same time, AFM could also measure the current out of the AFM tip.

20. G1Piezoelectric NanogeneratorReview Edson P. Bellido Sosa

21. The presenter describe the basic concepts about piezoelectricity and how this phenomenon is being used at macro-scale. He also described why ZnO is being used for this application. He explained the synthesis process and the experimental setup. He also discuss the use of Polyvinylidene fluoride for nano-piezoelectric applications. Since the power obtained form this source is low and the energy consumption of nano-electronic devices is in the same range. From my point of view this technology will have a impact in how we will get energy for future nano-devices. And open the possibility of the fabrication of self powered nano-electronic devices. http://spie.org/Images/Graphics/Newsroom/Imported/101/101_fig2.jpg From my point of view the presentation was good and the topic was very interesting, maybe he could have given more examples of devices being fabricated currently. The group of Dr. Zhong Lin Wang, creator of this technology, is working extensively in this area and have recently published a paper in “Nature Nanotechnology” where they use this technology to power up a pH sensor and a UV sensor and using a lateral integration of 700 rows of ZnOnanowiresthey produce a peak voltage of 1.26 V. "Self-powered nanowire devices " ShengXu, Yong Qin, Chen Xu, Yaguang Wei, Rusen Yang and Zhong Lin Wang, Nat. Nanotechnol. Online

22. G2 Review of Piezoelectricity by Alfredo Bobadilla

23. Piezoelectricity lecture’s review The introduction part illustrated very well the basic working principle of a piezoelectric material. Nevertheless it was not illustrated the physical aspect behind it, i.e. how the mechanical flexion or the change in geometry cause the electric field. I think the video shown to illustrate an experiment didn’t have enough image quality, it was not possible to appreciate what was happening there. It was not explained how the AFM work, and being the AFM the scientific instrument used to make the experiment with ZnOnanowires, it was not clear enough how the experiment was performed. So it was not mentioned what’s the challenge or technical difficulties when performing that experiment. No suggestion was given on what additional work is necessary to make the ZnOnanowire more efficient for harvesting energy or for controlling the ZnOnanowire synthesis, i.e. crystal directions, to be able to use other vibrational modes. Alfredo D. Bobadilla

24. By Mary Coan 3/12/10 Review:ZnONanowire ArrayPiezoelectric Nanogenerator

25. Review • Described what a Piezoelectric effect is • Governing equations and Definition • Gave examples of the applications for a Piezoelectric efect • Biological • Nanowire • Gave Advantages for ZnO • Fabrication • ZnOnanowires can have a PZ effect

26. Review • Gave advantages and disadvantages of PZ polymeric nanogenerator • Discussed materials, fabrication and results • Overall the presentation held a lot of information • Used images and movies to explain certain hard aspects • Could have transitioned better between topics • From ZnO wires to NanoFibers • Given background on Nanofibers

27. G4Review Piezoelectricity Diego A Gomez-Gualdron

28. Piezoelectricity Applied mechanical stress • Piezoelectric materials respond to mechanical stimuli by producing and instantaneous current (or voltage) due to polarization changes caused by the shape change Resulting voltage • The effect might potentially exploited to harvest energy in nanodevices. For instance, to power-up a nanorobot http://en.wikipedia.org/wiki/Piezoelectricity • A common material in nanotechnology applications, ZnO, has piezoelectric properties that makes it a suited candidate for harvesting energy at the nanoscale

29. A prototype device ZnO nanowires on a silver (Ag) plate • The schottky barrier behavior of the ZnO/metal systems makes the generated current unidirectional allowing to store energy. • The ZnO material is biocompatible allowing to use the material for nanodevices meant to be introduced to the body and work at in vivo conditions Performance: A scanning with an AFM tip is done along the surface. The mechanical stress on the ZnO nanowire originates from the tip movement and the voltage generated is measured with an incorporated voltameter that registered a production of 64 meV per squared micrometer Wang. Z. L., et. al. AFM tip Voltameter

30. Comments • It appears that the energy production per area is not high enough for the size/power-input required for the implementation of this arrangement as a primary energy source in a nanodevice. It is perhaps difficult to increase the performance of an individual nanowire without exploring a new material, but maybe another geometrical configurations of the nanowire arrays can lead to an efficiency increase • The mechanical stimulus produced by the AFM is perhaps not a good model of how the array would be stimulated in another environment (for instance by collisions against the blood vessel walls in a nanorobot). How the specific fashion the mechanical stress is applied affects performance?

31. Review • An interesting topic was presented. However there was little focus on the most current paper (nanoletters 2010), and rather was limited to an older one (science 2006). Even though the older paper was suitable to show the geometry of the nanoarray and how the device could work and its performance measured, there were many things left in the air. Why so little advance in the topic from 2006-2010 (judging for the presentation)? There is a fundamental obstacle? Have other nanoarrays be tested? Is this the best alternative to harvest energy at the nanoscale? • The presentation was too short because the critical analysis that follows to the presentation of the work was completely missing. The pros and cons of the work were not discussed. The main problems to solve were not pointed out. Other alternatives existent, in development, or proposed were not shown. In summary, the audience was presented a nice and interesting work , but was not told of how this work stands among other works in nanotechnology. Is the only work done? Are there many other people working on this? Was the presented work a breakthrough? • Other than the aforementioned points, it was a good presentation, but perhaps fell short in scope.

32. G5 Review of Piezoelectricity by Norma Rangel

33. ZnONanowire Array Piezoelectric Nanogenerator, by Jung Hwan Woo • Jung gave a very clear explanation of piezoelectricity, some background and presented a video where the actual concept of a piezoelectric generator was shown working and suggested an interesting application for generation of electricity from human footsteps in places where there is a big traffic such as train stations. • It was unclear for me how the current was measured in the ZnOnanowires from one side of wire to the other if the wires have a nanometer diameter and at the same type have both compressive and tensile stress.