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 ?