Potential of Nanogenerator Adv. Func Mater., 2008 (18) 1-15.
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Transcript of Potential of Nanogenerator Adv. Func Mater., 2008 (18) 1-15.
Potential of Nanogenerator
Adv. Func Mater., 2008 (18) 1-15.
Outline
Proof of principle of ZnO nanowires power generation triggered by an AFM tip (Wang et al, Science 2006)
Nanoscale generator (Wang et al, Science 2007) and potential applications
Controversy regarding the power generation mechanism
- n-type ZnO nanowire grown on Al2O3 substrate- generating electricity by deforming NW with AFM tip
Aligned ZnO NWs grown on Al2O3
Science, 312 (2006) 242-246.
Output voltage from aligned ZnO nanowires
Science, 312 (2006) 242-246.
- Sharp output voltage- Peak corresponds to maximum deflection of NW
Discharge occurs when tip contacts with compressed side
Electron affinity of ZnO: 4.5 eV
Work function of Ag: 4.2 eV
Work function of Pt: 6.1 eV
VL=Vm-VS
Mechanism of ZnO Nanogenerator
Transport is governed by metal-semiconductor Schottky barrier for PZ ZnO NW
Science, 312 (2006) 242-246.
The difference of Ohmic and Schottky
- No output signal form Al-In-coated Si tip (ohmic contactwith ZnO NW)
Adv. Func Mater., 2008 (18) 1-15.
ZnO Nanogenerator structure
Zig-Zag Pt coated Si electrode plays the role of an array of AFM tips
Device embedded in a polymer protecting layer
Schematic view and SEM images of the nanogenerator
Nanogenerator immersed in an ultrasonic bath
Direct-Current Nanogenerator Driven by Ultrasonic WavesWang et al Science 2007, 316 p102
Power generation mechanisms
Schematic view of the discharging mechanisms
Equivalent circuit
SEM cross-section view of the nanogenerator
Power generation
Device size: 2mm2 Power generated: 1pW
Current, bias and resistance of the generator as a function of time
Current generated as a function of time
Estimated power per NW: 1-4 fWPower density after optimization (109 active NW per cm2): 1-4 µW/ cm2
Applications: transistors and LED
A generator providing 10 to 50nW is required to power such a cross NW FET
a. Gate dependent IV characteristics of a cross NW FET b. SEM image of a cross NW junction, scale bar is 1µm Huang Y. et al, Science 2001 284 p1313
Current and emission intensity of a carbon nanotubes film as a function of gate voltage (Vd was 1V)Chen J. et al, Science 2005, 310, p1171
µW power level needed for a CNT LED
Applications: wireless sensors
Energy Harvesting From Human and Machine Motion for Wireless Electronic DevicesMitcheson et al, proceedings of the IEEE, Vol 96, N.9, 2008
Sensor nodes (motes) applications:
•Structural monitoring of buildings
•Military tracking
•Personal tracking and record system (Health)
Powering motes:• Sensor 12µW quiescent power• ADC 1µW for 8 bit sampling• Transmitter 0.65µW for 1kbps
MEMS accelerometers already used for various applications
Basic wireless sensor arrangement
Piezoelectric transducer for energy harvesting
Mitcheson et al, proceedings of the IEEE, Vol 96, N.9, 2008
Test: 608 Hz resonant operation 1g acceleration0.89V AC peak–peak generated2.16 µW power output
Fang HB et al, Microelectronics Journal 37 (2006) 1280–1284
Electrostatic transducer for energy harvesting
Assembled JFET
Generates 100 µW/cm3 from a vibration source of 2.25 m/s2 at 120 Hz
electret: permanent charge buried in the dielectric layer
SEM images of the generator integrated with a FET
schematic view of a constant charge electrostatic transducerMitcheson et al, proceedings of the IEEE, Vol 96, N.9, 2008
S. Roundy, P. K. Wright, and J. M. Rabaey,Energy Scavenging for Wireless SensorNetworks, 1st ed. Boston, MA: KluwerAcademic, 2003.
Argument against Wang
Advanced Materials 20, 4021 (2008)
Origin of the piezoelectric voltage
Strain displacive charge
Displacive charge voltage For ideal insulator:
Generation of piezoelectric charge can be considered equivalent to the generation of a potential
Gosele et al. Adv. Mater. 20, 4021 (2008)
Model of ZnO Piezoelectric Generator
For semiconducting ZnO:
Gosele et al. Adv. Mater. 20, 4021 (2008)
Load time constant RL = 500MΩ CL > 5pF τL ~ 1s
Intrinsic time constant
τL ~ 10-2 ps
<<
<<
Rectification of a Schottky diode
Gosele et al. Adv. Mater. 20, 4021 (2008)
V ~ kBT/q ~ 25meV quasi-ohmic To get rectification:V >> Vbi ~ 0.3-0.8V
Wang’s data: output ~ 10mV
Voltage argument
Wang et al’s previous opinion: Piezoelectric voltage is 0.3V (calculation)High contact resistance leads to low output of 10 mV (experiment)
Gosele et al ruled out the possibility of a high contact resistanceLoad resistor is 500 MΩ no way for a contact resistance higher than 500 MΩ
Wang et al. Nano Lett. 7, 2499 (2007)Gosele et al. Adv. Mater. 20, 4021 (2008)
Voltage argument
Wang et al’s new model:10 mV: difference of Fermi levels0.3V:real Schottky diode driving voltage
If Wang’s new model is true,0.3V is still a small voltage to rectify the piezoelectric signal…Wang et al. Adv. Mater. 20, 1 (2008)
Wang et al. Nano Lett. 8, 328 (2008)
Unknowns behind the nanogenerator
There is a lot of more work to be done…
I. Time constant
II. Rectification
The nanogenerator model ?