Enhanced output voltage generation via ZnO Nanowires (50nm): Effect of diameterthinning on voltageenhancement.

Mansoor Ahmad1*, MuhammadAzhar Iqbal2, Janice Kiely3,Richard

Luxton3, MusarratJabeen4

1Department of Physics, University of Sargodha Lahore Campus, Lahore,Pakistan

2University of management and Technology, Lahore,Pakistan

3Institute of Bio-Sensing Technology, University of the West of England, Bristol,UK

4Department of physics, University of the Punjab, Lahore,Pakistan.

Abstract

50nm ZnO nanowires were grown on indium tin oxide (ITO) coated polyethylene terephthalate (PET) substrates by adapting facile aqueous growth technique usinglow temperature and vacuum conditions. Prior to growth of ZnO nanowires, purehexagonal wurtzitestructuredseedlayerwasgrownonflexiblesubstrates.Surfacemorphologyof nanostructure has been examined by scanning electron microscopy (SEM).Vertical growthorientationhasbeenevidencedinXRDpatterns.Minuteexternalmechanical force (~50nN) has produced periodic voltage peaks. 2.5nm and 7.5nm thick sputteredPtelectrode have been tested to obtain output voltages. 50nm ZnO nanowires hasproduced a maximum output voltage of 2.717volts having an output power density of

397.1mW/cm2. By squeezing the diameter, we have reduced reverse leakagecurrentthrough nanowires and enhanced outputvoltage.

Keywords: ZnO nanowires, Piezoelectric Potential, schottky contact, EnergyHarvester.

1.Introduction:

After carbon nanotubes [1-3] and silicon nanowires [4-6], ZnO nanostructureshave attracted researchers due to its wide range of applications in electronics andoptoelectronic industry. ZnO is an important semiconductor material having a direct bandgap of 3.34eV; dueto itswidebandgapithasgotnumerousapplicationsinoptics,sensors,spintronics,actuatorsand biomedicalsciences[7].ZnOpossesseswurtziteandblendstructureswhichZn+2andO-2are arrangedinlayerbylayermanneralongverticalaxis,duetothelackofcentralsymmetryinits

structures it exhibits piezoelectric properties that can be used in all mechanical energyharvesting devices,currentmodulateddevices,MEMSbasedsensorsandinsurfaceacousticwaves.The Production of Piezoelectric potential is vital in nanogenerators and in strain sensorsand piezoelectricpotentialcanbecreatedorcontrolledbyaSchottkycontactinbetweenmetaland semiconductor. Earlier in our work, we have used Au sputtered top electrode to work asschottky contact with ZnO nanowires but in this study we have explored the role of Pt sputteredelectrode [8-10]. ZnO nanostructures also exhibit robust properties which make them promisingcandidate to be used in mechanical-electrical energy conversion devices [11]. ZnO, due toits semiconductingandpiezoelectricproperties,itisconsideredhighlyfavorableinUVlasering [12], UV sensors [13], light emitting diodes [14], gas sensors [15, 16] and solar cells[17].

ZnOnanostructuresbecamemoreprominentduetotheiruseastinytransducerswhich canbeimplantedinnanoscaleelectronic,optoelectronicandin-vivobiomedicalsystems;the bestwaytousethesenanoscaletransducersistotriggerthemwithexternalambientenergy. Interestingly,harvestingofenergycanbeselectedaccordingtotheapplicationforinstance; human body has lot of mechanical systems that can impart their mechanical energy tonanoscale systems implanted in human body, due to their dimensions in nanoscale billions oftiny transducers can be accumulated on a single substrate of one centimeter. In our case piezoelectricproperty of ZnO was used to convert external force of 50nN to produce an output voltage of1.34 volts, each nanowire grown on a substrate covering an area of 1 cm2 worked as nanotransducers [8,18-20].TherearefewmethodsreportedforthegrowthZnOnanowiresforinstanceSol-jel [9,21],spraypyrolysis[22],chemicalbathdeposition[23]buthydrothermalgrowthwasfound cheapestfromallandmostimportantlyitdoesnotrequirehighvacuumandhightemperature conditions[8].Thetechniquehasgotasignificantfeaturethatitprovidesmorphologyanddensity control of ZnO nanowires without anycatalyst.

2.Materials andMethod:

All Reagents used were of analytical grade of 98% purity with no furtherpurification.

Initially ITO coated PET(10Ω/Sq) substrates were washed and dried in air thencleaned ultrasonically with deionised water and acetone respectively for 15 minutes each and thendried. Prior to ZnO nanowires growth pure wurtzite structured seed layer was grown on PETsubstrate. 10mM solution of zinc acetate dihydrate [Zn (CH3COO)2.2H2O] was used to grow a seedlayer, to obtain pure wurtzite structure of seed layer process was repeated two to three time. Seed layerannealingwascarriedoutat60oCfor2hours,growthorientationwascloselyrelatedtoseed layerannealing.Seedlayerannealingwasfoundquitecriticalstepingrowthofnanowires[24- 25]. Nutrient solution (500mL) of HMTA [C6H12N4] and Zinc nitrate hexahydrate[ZnO3.6H2O]was prepared to grow ZnO Nanowires of controlled diameter. Annealed substrateswere immersedinnutrientsolutionupsidedownfortwohours.Growthtimewasfoundcrucialfor controlled diameter growth of NW however density of the wires were controlled bythe concentration of HMTA and [Zn(NO3)2.6H2O].

Our previous work led us to grow controlled diameter growth of nanowires which enabled usto studytheeffectofdiameteronoutputvoltagegeneration[8,26].ITOcoatedPETsubstrates acted as base electrode and upper electrode has been sputtered by using sputtering unitEmscope SC500.

Ar was present in the chamber exerting an internal pressure of 0.1torr; 2KV voltagewas usedtoinitiatechargeirradiationprocessinthechamber,wehavesputtered2.5nmand7.5nm thickPtlayersontopofZnOnanowires.Extremelylightplasticrollerisusedtoapply~50nN force on top of nanowires to produce piezoelectric potential. Structure of the energy harvesteris showninlaterpartofthepaper,lowerpartconsistofPETsubstrateonwhichZnOnanowires were grown, which acted as base electrode, the intermediate part comprises ofZnO nanostructureandupperpartissputteredPtelectrode.Plasticrollerwasrolledontoobtainthe outputvoltagevaluesrecordedbyPicoscope5204.Plasticrollerwasnotattachedonupper electrode; it was rolled on upper electrodeexternally.

3.Results andDiscussions:

SEMimagesinfig.1(a)showuniformgrowthofZnOnanowiresonITOcoatedPET substrate, density of nanowires on substrate were controlled by adjusting the concentrationof nutrient solution. Dense packed structure of nanowires eliminated one fabricating step, whichmade iteconomicaltechnique,otherwisepolymermatrixwouldhavebeenrequiredtofillintermediate spaceinbetweennanowiressothatitcouldnotbeshortduringPtcoating.Fig.1(b,c)haveshown thediameterrangewasabout50nm,diametercontrolwasachievedbycloselymonitoredgrowth time in the nutrient solution. Our earlier reported results [8] were closely in agreement withour recent results, we have obtained improved morphology and density control by controllingsynthesis parameters such as growth time in solution and more dense structure was achieved by increasingthe concentrationratioofHMTA[C6H12N4]andzincnitratehexahydrate[ZnO3.6H2O]insolution. Narrow opening near the tips of ZnO nanowires were desired to get sufficient bendingfor piezoelectricpotential.Ourreportedvalueswerecloselyinagreementwithreportedvalues[27].Fig2.represented well oriented growth of ZnO nanowires along c-axis, all peaks were matchedfrom ZnO(JCPDScardno36-1451).Sharppeakalong(002)directionhasgivenevidencethatmostof nanowires were grown vertically upward from substrate however some weak peaksalong (101),(102),(110)and(100)showingotherorientationaswellbutnotofmuchsignificancewhich wasalsoevidencedinreportedvalue[28].Fig.3showstheschematicconfigurationofenergy harvester that we have used in our study. Bottom substrate acted as one conducting electrode,centralverticalstructureofZnONWandupperelectrodewasPtsputteredelectrodeandminuteexternal forceof~50nNwasappliedbyanexternalrollertoprovideexternalmechanicalenergy.Wehave usedPtelectrodeinsteadofAuelectrodetoimproveourresults,asAuhasaworkfunctionof (5.1eV-5.4eV) while Pt has a work function of (6.1eV) which is far greater than the electronaffinity of ZnO of (4.5eV) which means by doing so we have reduced the chances of reverse leakagecurrent tominimumandconsequentlyincreasingoutputvoltageacrossexternalcircuitthroughpicoscope 5204.Picoscopewasfoundsuitableinstrumenttomeasureminutevoltagetoitsperfectionalong with other features like output power density harmonic distortion band width andetc.

Figure.1(a,b,c)SEMimageof50nmZnOnanowiresgrownonPETsubstrateatlowandhighmagnificationrespectivelyand

(d) SEM image of 1µm long ZnOnanowires.

Figure.2 XRD pattern of ZnO nanowires grown on PET substrate annealed at 60oC for 30minutes.

The results described in the research article are an extension and improvement of ourpreviously reportedwork[8,23,and29],inwhichwehavesynthesized100nmZnOnanowiresandstudiedits outputvoltageusinggoldsputteredelectrode.Toimproveourresults,50nmZnOnanowireswere grown to achieve high voltage values by blocking the reverse leakage current through thenanowires. Allofpiezoelectricpotentialgeneratedbyminuteexternalforceof~50nNmustbeobtainedat outputstagesandbyreducingthediameterofnanowireswehavereducedthepossibilityofthe currenttopassthroughthechannelsavailableinnanowires.Asreductionofdiameterofnanowireshave minimized the conducting channels in nanowires which are multiples G = e2/2h [32]. We haveachieved high voltage as quantized energy channels have been reduced which caused aconsiderable increment of1.063volts.

Figure.3 Schematic diagram of energyharvester.

Figure.4(a)Outputvoltageusing2.5nmthickPtsputteredelectrode.Figure.4(b)Maximumoutputvoltage(2.717V)using7.5nm thickPtsputteredelectrode.Fig.4(c)Maximumoutputpowerdensity(397.1mW)of50nmZnOnanowiresgrownon PETsubstrate.

Fig.4 (a) shows periodic output voltage peaks of 1.071 volts with 2.5nm thickgold electrode, we have used specific thickness in our earlier reported results to ensure theproduction of piezoelectric potential and our earlier work has clearly indicated that 7.5nnm thick electrodehas produced maximum piezoelectric potential so we have used 7.5nm thickness withoutanalyzing

any intermediate thickness in between 2.5nm and 7.5nm. We have also obtained outputpower density of maximum generated voltage as it has the significant value 397mW/cm2.Periodic voltage peaks in voltage in fig.4 (a) has indicated formation of schottky contact between the tipof ZnO nanowire and Pt top electrode. Schottky contact at one regulates the flow of currentthrough external circuit while Ohmic contacts at both ends cannot generate power[20].

Earlier[29],wehavereportedvoltagegenerationviaZnOnanowireshavingdiameterrange

~350nm;piezoelectricpotentialproducedbythemwas1.654voltsbutinthisstudywehavereduced diameterupto~50nmandconsiderablyenhancedpiezoelectricpotentialwasachieved.Oneschottkycontact is essential for the piezoelectric potential; Au and Pt both have been used for the samepurpose [20,29-30].Infirststage,where2.5nmthicktoplayerisgrownhasgiventheevidenceofschottky contactbetweennanowiresandtopelectrodelikementionedinourearlierresults[8].Ausputtered electrode for schottky contact has been reported earlier [31] and ITO coated bottom electrode hasalso been used in the study which acted as bottom electrode however the previous study was focused onRF sputter coated seed layer and its effects on piezoelectric potential but top and bottomelectrode structure was quite close the experimental approach that we have discussed in our results. The evidence of schottky contact has also been reported for AFM contact mode, where investigationof piezoelectricvoltagehasbeencarriedout,AFMtiphasproducedminuteexternalpressureincontact modethatwehaveproducedbyanexternalroller[32],ascontactmodeAFMhasitsownlimitations and AFM tip has to be changed after every now and then that makes it more expensive and non-viable approach.

In this study we have transformed external mechanical energy in electric energy that can beused to power numerous nano electrical devices without batteries. Minute external force of ~50nNis applied by extremely light plastic roller to produce piezoelectric potential within nanowires.Applied mechanical force has produced strain 0.15% which is quite less than maximum tensile strain 6%which is theoretically predicted value for ZnO nanowire after which the wire gets fractured[33].

Wehaveusedminuteexternalforcetodeveloppiezoelectricpotentialinsidenanowires,inthe absenceofexternalforcethechargecentreofcationsandanionscoincidewitheachotherbutas external force is applied the charge centre disturbs and electric dipole is created whichresults piezoelectricpotential,piezopotentialisconservedinstrainednanowires.WeoptedVING(vertical nanowireintegratednanogenerator)formechanicalenergyharvesting,duetoitslowcostsynthesis whileLING(lateralintegratednanowirenanogenerator)involvesexpansivestepslikeAuorPtlayer needs to be sputtered on the substrates to act as electrodes then a mask is involved for patternedgrowth of ZnO nanowires with in sputtered electrodes and for mask patterning lithographical lift-offprocesses are also involved likewise NEG (Nanocomposite electric nanogenerator) involves filling requires filling solutions with in nanowire matrix which makes it more expensive and complicated to opt(34,35).

Schottky contact at one end of the nanowire is essential for piezo potential, whichis created at the top of nanowires moreover 2.5nm thick Au and Pt electrode has been sputteredand corresponding output voltage has been analyzed Au having low work function than Pt has createdless piezoelectric potential similarly 5nm and 7.5nm thick Au and Pt electrodes thick electrodes havealsobeen tested in our previous work [8, 26]. Our previous results confer that maximum output voltagehas been achieved with 7.5nm thick sputtered electrode. In this study, we have only used 2.5nm thickPt electrode just to ensure the formation of schottky contact in between nanowires and top electrode.We have enhanced output voltage by thinning of diameter upto ~50nm which reduced the reverseleakage current through nanowires but piezoelectric potential without having metal contact is not possible aswe have discussed earlier and in numerous reported results [36-38] have verified that one end metalcontact is essential. Schottky contact at one end device prevents the electrons to flow through nanowiresZnO nanowire work as charge pump and electrons flow the through external circuit. As the external forceis applied, negative piezoelectric potential rises up at the top of nanowires relative to the bottom andso the Fermi level, consequently electron flow from top to bottom through external circuit and tendsto accumulate at bottom until the equilibrium is reached, when external force is removedpiezoelectric potential in nanowire vanishes electrons move from bottom electrode to top electrode viaexternal circuit and voltage peak in opposite direction is achieved. Periodic potential peaks exhibitsthe formation of schottky contact in between ZnO nanowires and top sputtered electrode. It’sa manifestation of piezotronic effect i.e. coupling piezoelectric and semiconducting properties[30].

Asmentionedaboveextremelylightplasticrollerisusedtoapply~50nNforceontop electrode for the production of piezoelectric potential. Thinning of diameter enhancesscattering phenomenonwithinnanowireswhichcausespoorconductivitythroughnanowires.Inourcase,high piezoelectricpotentialwasrequiredanditisachievedbyreducingreverseleakagecurrentthrough nanowires.Maximumpiezoelectricpotentialcouldbedeliveredatoutputstagesonlyifthereverse current through nanowires is reduced. Electrons suffer huge deflection to pass through the nanowirein small diameter ranges; 50nm ZnO nanowires have shown poor conductivity of reverse leakagecurrent which can be seen in fig.4 (a) exhibit periodic output voltage peaks with 2.5nm thick Pt electrodewhile fig.4(b)representingoutputvoltagepeakswith7.5nmthinkPtelectrodeandfig.4(c)showinghighoutput power density of 397.1mW/cm2 isachieved.

4.Conclusions:

By squeezing the diameter of ZnO nanowires up to 50nm, we have achieved highoutput voltageof2.717voltswithanoutputpowerdensityof397.1mW/cm2.Piezoelectricpotentialwas generated by applying minute external pressure. Due to high work function (φ = 6.1eV) of Pt, itwas used as top electrode. Low cost aqueous route has been adopted for the synthesis of ZnOnanowires,their morphology has been controlled by adjusting physical growth parameters and growth hasbeen carried out in a catalyst free environment. ITO coated PET substrates have acted as conductingbase electrode,centralnanostructurewasgrownintwosteps,infirststephexagonalwurtziteseedlayer wasgrownonPETsubstratesandinsecondstepverticalgrowthofnanowireshasbeencarriedout innutrientsolution.Picoscope5204hasbeenusetorecordoutputpotentialsofstructure.XRD pattern revealed vertical growth of nanowires along c-axis and surface morphology was evidencedin SEMimages.

Acknowledgements:

AuthorsarethankfultoHigherEducationCommission,Pakistanforprovidingfinancialsupport under International Research Support Initiative Program (Grant No IRSIP 21 Ps 04) to carryout research work and extremely thankful to Dept. of Applied Sciences, UWE Bristol andBristol Robotic Laboratory, UK for providing full support to avail lab. Equipment and researchfacilities.

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Highlights

  • 50nm ZnO nanowires wires on ITO coated PET substrates for mechanical energyharvesting.
  • Diameter of nanowires was reduced to stop reverse leakage current throughnanowires.
  • Pt sputtered electrode was used to produce a schottky contact between nanowires andtopelectrode.
  • High output piezoelectric potential (2.717V) having an output power density(397.1mW/cm2)

was achieved.