1. ZnO Nanowire 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 in 1880 in the course of studying for pyroelectricity
Governing equation:
E = ε/d where E : electric field
ε : strain
d : PZ coefficient

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 ( ). "MIT duo sees people-powered "Crowd Farm"". MIT news. Massachusetts Institute of Technology

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
ZnO Nanowires 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
{ } 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 ZnO nanowire 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),
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,

15. Questions?

16. G6 Rebuttal: 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 (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. G1 Piezoelectric Nanogenerator Review
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.
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 ZnO nanowires they produce a peak voltage of 1.26 V.
"Self-powered nanowire devices " Sheng Xu, Yong Qin, Chen Xu, Yaguang Wei, Rusen Yang and Zhong Lin Wang, Nat. Nanotechnol. Online

22. Review of Piezoelectricity by Alfredo BobadillaG2
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 ZnO nanowires, 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 ZnO nanowire more efficient for harvesting energy or for controlling the ZnO nanowire synthesis, i.e. crystal directions, to be able to use other vibrational modes.
Alfredo D. Bobadilla

24. Review: ZnO Nanowire Array Piezoelectric Nanogenerator
By Mary Coan
3/12/10
Review: ZnO Nanowire Array Piezoelectric 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
ZnO nanowires 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. G4 Review Piezoelectricity
Diego A Gomez-Gualdron

28. Applied mechanical stress
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
A common material in nanotechnology applications, ZnO, has piezoelectric properties that makes it a suited candidate for harvesting energy at the nanoscale

29. ZnO nanowires on a silver (Ag) plate
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
AFM tip
Wang. Z. L., et. al.
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 (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. Review of Piezoelectricity by Norma Rangel

33. ZnO Nanowire 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 ZnO nanowires 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.