Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Piezoelectric transducer for energy harvestingSlide 13Argument against WangOrigin of the piezoelectric voltageModel of ZnO Piezoelectric GeneratorRectification of a Schottky diodeVoltage argumentVoltage argumentUnknowns behind the nanogeneratorPotential of NanogeneratorAdv. Func Mater., 2008 (18) 1-15.OutlineProof 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 applicationsControversy regarding the power generation mechanism- n-type ZnO nanowire grown on Al2O3 substrate- generating electricity by deforming NW with AFM tipAligned ZnO NWs grown on Al2O3Science, 312 (2006) 242-246.Output voltage from aligned ZnO nanowiresScience, 312 (2006) 242-246.- Sharp output voltage- Peak corresponds to maximum deflection of NW Discharge occurs when tip contacts with compressed sideElectron affinity of ZnO: 4.5 eVWork function of Ag: 4.2 eVWork function of Pt: 6.1 eVVL=Vm-VSMechanism of ZnO NanogeneratorTransport is governed by metal-semiconductor Schottky barrier for PZ ZnO NWScience, 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 structureZig-Zag Pt coated Si electrode plays the role of an array of AFM tipsDevice embedded in a polymer protecting layerSchematic view and SEM images of the nanogeneratorNanogenerator immersed in an ultrasonic bathDirect-Current Nanogenerator Driven by Ultrasonic WavesWang et al Science 2007, 316 p102Power generation mechanismsSchematic view of the discharging mechanismsEquivalent circuitSEM cross-section view of the nanogeneratorPower generationDevice size: 2mm2 Power generated: 1pWCurrent, bias and resistance of the generator as a function of timeCurrent generated as a function of timeEstimated power per NW: 1-4 fWPower density after optimization (109 active NW per cm2): 1-4 µW/ cm2Applications: transistors and LEDA generator providing 10 to 50nW is required to power such a cross NW FETa. 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 p1313Current and emission intensity of a carbon nanotubes film as a function of gate voltage (Vd was 1V)Chen J. et al, Science 2005, 310, p1171µW power level needed for a CNT LEDApplications: wireless sensorsEnergy Harvesting From Human and Machine Motion for Wireless Electronic DevicesMitcheson et al, proceedings of the IEEE, Vol 96, N.9, 2008Sensor 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 1kbpsMEMS accelerometers already used for various applicationsBasic wireless sensor arrangementPiezoelectric transducer for energy harvestingMitcheson et al, proceedings of the IEEE, Vol 96, N.9, 2008Test: 608 Hz resonant operation 1g acceleration0.89V AC peak–peak generated2.16 µW power outputFang HB et al, Microelectronics Journal 37 (2006) 1280–1284Electrostatic transducer for energy harvestingAssembled JFETGenerates 100 µW/cm3 from a vibration source of 2.25 m/s2 at 120 Hzelectret: permanent charge buried in the dielectric layerSEM 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, 2008S. Roundy, P. K. Wright, and J. M. Rabaey,Energy Scavenging for Wireless SensorNetworks, 1st ed. Boston, MA: KluwerAcademic, 2003.Argument against WangAdvanced Materials 20, 4021 (2008)Origin of the piezoelectric voltageStrain displacive chargeDisplacive charge voltageFor ideal insulator: Generation of piezoelectric charge can be considered equivalent to the generation of a potentialGosele et al. Adv. Mater. 20, 4021 (2008)Model of ZnO Piezoelectric GeneratorFor semiconducting ZnO:Gosele et al. Adv. Mater. 20, 4021 (2008)Load time constant RL = 500MΩ CL > 5pF τL ~ 1sIntrinsic time constant τL ~ 10-2 ps<<<<Rectification of a Schottky diodeGosele et al. Adv. Mater. 20, 4021 (2008)V ~ kBT/q ~ 25meV quasi-ohmic To get rectification:V >> Vbi ~ 0.3-0.8VWang’s data: output ~ 10mVVoltage argumentWang 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 MΩWang et al. Nano Lett. 7, 2499 (2007)Gosele et al. Adv. Mater. 20, 4021 (2008)Voltage argumentWang et al’s new model:10 mV: difference of Fermi levels0.3V:real Schottky diode driving voltageIf 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 nanogeneratorThere is a lot of more work to be done…I. Time constantII. RectificationThe nanogenerator
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