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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

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Potential of Nanogenerator

Adv. Func Mater., 2008 (18) 1-15.



Proof of principle of ZnOnanowires 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 ZnOnanowiregrown on Al2O3 substrate
  • generating electricity by deforming NW with AFM tip

Science, 312 (2006) 242-246.


Output voltage from aligned ZnOnanowires

  • 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 ZnONanogenerator


Transport is governed by metal-semiconductor Schottky barrier for PZ ZnO NW

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

ZnONanogenerator 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

Power generation mechanisms

SEM cross-section view of the nanogenerator

Equivalent circuit

Schematic view of the discharging mechanisms

power generation

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

Applications: transistors and LED

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

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

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

MEMS accelerometersalreadyused for various applications

  • 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

piezoelectric transducer for energy harvesting
Piezoelectric transducer for energy harvesting

Test: 608 Hz resonantoperation 1g acceleration

0.89V AC peak–peakgenerated

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 generatorintegratedwith a FET

schematicview of a constant charge electrostatictransducer

Mitcheson et al, proceedings of the IEEE, Vol 96, N.9, 2008

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,

EnergyScavenging for Wireless Sensor

Networks, 1st ed. Boston, MA: Kluwer

Academic, 2003.

electret: permanent charge buried in the dielectric layer

argument against wang
Argument against Wang

Advanced Materials 20, 4021 (2008)

origin of the piezoelectric voltage
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
Model of ZnO Piezoelectric Generator

For semiconducting ZnO:

Intrinsic time constant

τL ~ 10-2ps

Load time constant RL = 500MΩ

CL > 5pF

τL ~ 1s



Gosele et al. Adv. Mater. 20, 4021 (2008)

rectification of a schottky diode
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
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. NanoLett. 7, 2499 (2007)

Gosele et al. Adv. Mater. 20, 4021 (2008)

voltage argument19

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. NanoLett. 8, 328 (2008)

unknowns behind the nanogenerator
Unknowns behind the nanogenerator


I. Time constant

The nanogenerator model

II. Rectification

There is a lot of more work to be done…