Transcript The mechanism of ZnO nanogenerator

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
Aligned ZnO NWs grown on Al2O3
- n-type ZnO nanowire grown on Al2O3 substrate
- generating electricity by deforming NW with AFM tip
Science, 312 (2006) 242-246.
Output voltage from aligned ZnO nanowires
- Sharp output voltage
- Peak corresponds to maximum deflection of NW
Discharge occurs when tip contacts with compressed side
Science, 312 (2006) 242-246.
Mechanism of ZnO Nanogenerator
Transport is governed by
metal-semiconductor Schottky
barrier for PZ ZnO NW
VL=Vm-VS
Electron affinity of ZnO: 4.5 eV
Work function of Ag: 4.2 eV
Work function of Pt: 6.1 eV
Science, 312 (2006) 242-246.
The difference of Ohmic and Schottky
- No output signal form Al-In-coated Si tip (ohmic contact
with 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
Nanogenerator immersed in an
ultrasonic bath
Schematic view and SEM images of the nanogenerator
Direct-Current Nanogenerator Driven by
Ultrasonic Waves
Wang et al Science 2007, 316 p102
Power generation mechanisms
SEM cross-section view of the nanogenerator
Equivalent circuit
Schematic view of the discharging
mechanisms
Power generation
Current generated as a function of time
Device size: 2mm2
Power generated: 1pW
Estimated power per NW: 1-4 fW
Power density after optimization (109
active NW per cm2): 1-4 µW/ cm2
Current, bias and resistance of the
generator as a function of time
Applications: transistors and LED
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
A generator providing 10 to 50nW is required to
power such a cross NW FET
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
Sensor nodes (motes) applications:
•Structural monitoring of buildings
•Military tracking
•Personal tracking and record system
(Health)
Basic wireless sensor arrangement
Powering motes:
• Sensor 12µW quiescent power
• ADC 1µW for 8 bit sampling
• Transmitter 0.65µW for 1kbps
Energy Harvesting From Human and
Machine Motion for Wireless Electronic
Devices
Mitcheson et al, proceedings of the IEEE,
Vol 96, N.9, 2008
MEMS accelerometers already
used for various applications
Piezoelectric transducer for energy harvesting
Test: 608 Hz resonant operation 1g acceleration
0.89V AC peak–peak generated
2.16 µW power output
Fang HB et al, Microelectronics Journal
37 (2006) 1280–1284
Mitcheson et al, proceedings of the IEEE,
Vol 96, N.9, 2008
Electrostatic transducer for energy harvesting
Assembled JFET
SEM images of the generator integrated with a FET
schematic view of a constant
charge electrostatic transducer
Mitcheson et al, proceedings of
the IEEE, Vol 96, N.9, 2008
electret: permanent charge buried in the
dielectric layer
Generates 100 µW/cm3 from a vibration source
of 2.25 m/s2 at 120 Hz
S. Roundy, P. K. Wright, and J. M. Rabaey,
Energy Scavenging for Wireless Sensor
Networks, 1st ed. Boston, MA: Kluwer
Academic, 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:
Intrinsic time
<<
constant
τL ~ 10-2 ps
Gosele et al. Adv. Mater. 20, 4021 (2008)
Load time
constant
RL = 500MΩ
CL > 5pF
<< τL ~ 1s
Rectification of a Schottky diode
V ~ kBT/q ~ 25meV  quasi-ohmic
To get rectification:
V >> Vbi ~ 0.3-0.8V
Wang’s data: output ~ 10mV
Gosele et al. Adv. Mater. 20, 4021 (2008)
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 resistance
Load 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 levels
0.3V:
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
I. Time constant
?
The nanogenerator model
II. Rectification
There is a lot of more work to be
done…