Laser synthesized gold nanoparticles for high sensitive strain gauges

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ContentslistsavailableatScienceDirect

Sensors

and

Actuators

A:

Physical

j ou rn a l h o m e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / s n a

Laser

synthesized

gold

nanoparticles

for

high

sensitive

strain

gauges

Salamat

Burzhuev,

Aykutlu

Dâna,

Bülend

Ortac¸

∗

UNAM-InstituteofMaterialsScience&Nanotechnology,BilkentUniversity,Ankara06800,Turkey

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received17April2013

Receivedinrevisedform23August2013 Accepted23August2013

Available online 7 September 2013

Keywords: Laserablation Goldnanoparticles Goldnanoparticleﬁlms Quantumtunnelingeffect Straingauges

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Wedemonstratehighstrainsensitivitypropertyofgoldnanoparticle(Au-NP)thinfilmsfabricatedon flexiblepolydimethylsiloxane(PDMS)substrates.Thisbehaviorisattributedtoquantumtunnelingeffect thatishighlydependentonnanoparticleseparation.Au-NPsweresynthesizedinwaterbynanosecond laserablationmethod.Thecleansurfaceprovidinghightunnelingdecayconstant,sizeoftheAu-NPs andAu-NPsaggregateclustersofferadvantagesforhighsensitivitystrainsensor.WepreparedAu-NPs filmsonflexiblePDMSsubstratebyusinghands-ondrop-castmethod.Toobtainhighgaugefactor(g factor),weinvestigatedthenanoparticlesconcentrationonthesubstrate.Laser-generatedAu-NPsfilms demonstratedgfactorof∼300forhigherthan0.22%strainand∼80forthestrainlowerthan0.22%strain, whichisfavorablycomparabletoreportedsensitivitiesforstrainsensorsbasedonAu-NPs.Mechanical characterizationsfortheprolongedworkingdurationssuggestlongtermstabilityofthestrainsensors. Wediscussseveralmodelsdescribingconductanceoffilmsinlowandhighstrainregimes.

1. Introduction

Straingauges,whichconvertstraininputtoresistancechange, ﬁndwideapplicationintransductionofmechanicalsignals.Strain gauges basedonmetaland semiconductor ﬁlmsarecommonly usedinsensing.Thesensitivityofastraingaugeisexpressedby thegfactor,whichcanbecalculatedbythefollowingequation: g₌ R

Rε (1)

whereRisthechangeinresistanceuponappliedstrain;Risthe initialresistance;andεistheappliedstrain.Theconventionalmetal ﬁlmstrainsensorshavelonglifetimesandarerelativelyrobustto environmentaleffectssuchaschangesintemperatureand humid-ity.Ontheotherhand,thesesensorspresentlowsensitivitydue tothelimitedgfactors.Semiconductorbasedstrainsensorshave fundamentallydifferentoperationmechanism,namely modiﬁca-tionofbandstructure,effectivemassandmobility.Moreover,they aremoresensitive,butmayrequiretemperaturecompensation. Inaddition,thesensitivityofthesesensorsdecreaseunderhigh strainsandabreakdownstrain∼0.6%resultinginlimitedoperation range[1,2].

Thinfilmsbasedonnanomaterialshavefoundwideapplications in fields of optics, biology and detection. Metallic nanoparti-cles have gained significant attention mainly because of their uniqueoptical,electricalandcatalyticproperties.Intheliterature,

∗ Correspondingauthor.Tel.:+903122903526. E-mailaddress:ortac@unam.bilkent.edu.tr(B.Ortac¸).

theconductionbehaviorbetweenchemicallysynthesized nanopar-ticles has been intensively studied [3,4]. The conduction of n-alkanelthiol-stabilizedAu-NPfilms wasshownto have expo-nentialdecaydependencetothelengthofligands’alkanethiolate chains. This effect wassuggested to beattributed to quantum tunneling effect [3,4]. The quantum tunneling effect is highly dependentonwidthofthepotentialbarrier.Thus,itwas antici-patedthattheAu-NPsfilmscanbeusedashighly-sensitivestrain gauges.Inprinciple,suchfilmscanbeusedinothertypesofsensors. Forinstance,onecanmeasurethechangeconductancetoobserve theeffectofchemicalmodificationorattachmentofbiomolecules, whichoccursduetochangesinthetunnelingbarrierwidthand height.

TheuseofAu-NPsinstrainsensinghaspreviouslybeenreported inliterature.Herrmannetal.demonstratedthehighcapabilityof Au-NP filmstosensesmall linear strains[5].The Au-NPstrain gaugeswereshowntobenearlytwoordersofmagnitudesensitive thantheconventionalstraingauges[5].Vossmeyeretal. demon-stratedlowersensitivityAu-NPstrainsensorsfabricatedonflexible substratesbydepositing12-dodecylamine-stabilizedAu-NPsvia layer by layer self-assembly on oxidized flexible polyethylene. This technique provides high adhesion of Au-NPs on the sub-stratebecauseoflinkercompound,1,9-nonanedithiol,whichalso increasesthemechanicalrobustnessofsensors[6].Ressieretal. developedstop-and-goconvectiveself-assemblymethodto fabri-catefew micrometerwiresbasedonAu-NPs,anddemonstrated thatmonolayerwiresofAu-NPshavehigherstrainsensitivity–g factorof132–thanmultilayerwires[7–9].

PresenceofchemicalstabilizersonAu-NPsurfacespotentially decreasestrainsensitivity.Thinﬁlmsbasedonmetalnanoparticles

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havingcleansurfaceswerepredictedtohaveimprovedgfactors[5]. Recently,Tanneretal.demonstratedthatPlatinum(Pt) nanopar-ticlecoatingsobtainedbysputtering in vacuumpresent highg factors(700).Theyalsostudiedhow surfacedensity affectsthe sensitivityofsuchdevices [10,11].Nanoparticles freeof chemi-calstabilizers canbeobtainedbylaserablationmethod[12,13]. Uniquelaser-matterinteractionpropertiesopenthedoortothe generationofavarietyofnanoparticles[14].Comparedwithother nanoparticlesynthesismethods,laserablation,especiallyin liq-uids,isa versatilemethodforgeneratingcolloidal,highly pure, andsurfactant-freenanoparticles.Inthispaper,wehavestudied theconduction propertiesof thin ﬁlmsof Au-NPs,prepared by nanosecondlaser ablationin deionizedwater. Au-NPthinﬁlms wereobtainedbysimplydrop-castingAu-NPcolloidalsolutionon thePDMSsubstrates.

2. Experimentaldetails

2.1. Au-NPsynthesisbylaserablationinliquid

Au-NPswereobtainedbylaserablationindeionizedwater.Gold block(99.999%,KurtJ.Lesker)wascleanedbysoniﬁcationin ace-toneprior tolaserablationwithout anyadditionalpuriﬁcation. ThegenerationofcolloidalnanoparticlesfromAublockwas car-riedoutbyusingacommercialpulsedND:YLFlaser(wavelength: =527nm,16Waveragepower,pulseduration t=100ns,pulse energyE=16mJfor1kHz).ThecleanedAublockwasplacedina glassvesselcontaining23.5mLofpuredeionizedwater.Thelaser beamwasfocusedonthetargetbyusingaplano-convexlenswith focallengthof50mm.Theheightofliquidlayeroverthegold tar-getis∼5mm.Thelaserablationwascarriedoutfor∼10min,and thelaserbeamisscannedoverthetargetsurface.Duringthelaser ablation,theformationofcolloidalnanoparticlesolutionwith dis-persedAu-NPsinliquidmediawasobservedasacolorchangeofthe deionizedwater.Afterthelaserirradiation,thecoloroftheAu-NPs solutionbecamedark-red.

ThestructureofAu-NPsgeneratedbynanosecondpulsedlaser ablationwerestudiedbyTEM(TEMmodelFEI– TecnaiG2F30) system.Sample for TEManalysiswaspreparedbydrop-casting solutionsontocarbon-coatedgrid.RepresentativeTEMimageof theAu-NPsisshowninFig.1,showingwelldispersed, spherical-likeNPs.SizedistributionwasmeasuredfromTEMimagewhere 100particles’approximatediametersweremeasured.Size distri-butionofAu-NPsisgiveninFig.1inset.Anaveragenanoparticles sizeof13nmisseen,withadistributioncoveringtherangefrom2 to25nm.

Fig.1. TEMimageofAu-NPs.Theinsetshowsthehistogramofsizedistribution calculatedfromTEMimages.

2.2. Strainsensorfabrication

ThinﬁlmsofAu-NPshavebeenproducedbydrop-castingon severalsubstrates.It wasobserved that,uniformityoftheﬁlms werepooronSiO2andsiliconsubstratesduetotheirhydrophilic

surfaces.Au-NPsonglassandsiliconwererandomlydistributed andintensivelylocatedattheedgeoftheinitialAu-NPdroplet’s contactarea.Ontheotherhand,PDMSsubstratepresentslow sur-faceenergy,thusahydrophobicsurface.Thehighcontactangle andlowsurfaceenergyenablestouniformlyprecipitateparticles and highly concentratethem onthe surface. Afterdroplet was completelydried,adarkbluespotwasformedonthesurfaceof PDMS.Au-NPthinﬁlmsobtainedonPDMSwereuniformly dis-tributed(Fig.2a).Contactswereobtainedusingashadowmaskand a350␮mgapwasformedbycoating80nmPtﬁlmusingsputtering (PECSGatan682)(Fig.2b).

Scanningelectronmicroscope(SEM)(FEINovaNanoSEM600) wasusedtofurtherinvestigatethelocalizationofAu-NPsonthe PDMS substrate. In theSEM image, aggregated clusters canbe

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Fig.3.SEMimageofAu-NPsthinﬁlmonPDMSsubstrate.

realized,throughwhichconductiontakesplace(Fig.3).Also,PDMS surfaceisnotfullycoveredbyAu-NPsandtherearecracks(Fig.3). ThesemayincreaseresistanceofAu-NPsﬁlm.

2.3. Mechanicalcharacterization

Au-NPﬁlmonthePDMSsubstratewascharacterizedby apply-inglinearstrainandobservingchangeinresistanceasshownin Fig.4.Inthisﬁgure,Au-NPstrainsensorisschematicallyshown, whereAu-NPsandaggregatesofAu-NPsaremoveapartwhile ten-silestrainisapplied;resultinginthechangeofresistance(Fig.4). AggregationoflasergeneratedAu-NPscanbeobservedinFig.3.A

lineartranslationstagefittedwithapiezoelectricactuator (Thor-labsAE0203D08F)wasusedtocharacterizethestrainresponseof Au-NPsfilms.Instaticmeasurements,usingafixedbiasof1V cur-rentthroughthefilmswasmonitoredusinga precisioncurrent meter (Keithley4200-SCS). Dynamicstrainresponse of the Au-NPsthinfilmstrainsensorwasmonitoredusingatransimpedance amplifier (Stanford Research systems SR-570) and digital oscil-loscope(TektronixTDS1012B),Piezoelectricactuatorwasdriven withasquarefunctionoffrequency1Hzandpeak-to-peak ampli-tude of 5␮m. Dynamic measurements were performed under 0.25Vbiasappliedtothefilm.

3. Resultsanddiscussion

Theconcentrationofthenanoparticlesonthesubstratestrongly affectstheconductivityandstrainsensitivityoftheAu-NPfilm.The concentrationofgoldintheAu-NPsinwaterobtainedby∼10min laserablationwasapproximately1.08mM.GoldmassinAu-NP col-loidalsolutionwascalculatedbyweighingthegoldblockbefore and aftertheablationprocess. Thenthemolarity valueofgold inAu-NPsolutionwascalculatedinterms ofmM.Thiscolloidal solutionwasdilutedseveraltimestoobtainsolutionswith dif-ferentconcentrations.300␮lofdilutedsolutionswereappliedto PDMSsubstratesanddriedatambientconditionstoformspots withtypicaldiameterof5mm.Filmspreparedwith>0.9mm col-loidalsolutionswereveryconductiveanddidnotrespondtostrain; while,filmspreparedwith<0.72mMsolutionswerehighly resis-tive(resistance>200M)andsensitivetostrain.Itwasobserved thatfilmsthathavehigherresistanceweremoresensitivetostrain. OptimalconcentrationforAu-NPsstraingaugesisfoundtobeabout ∼0.81mMthatyieldshighsensitivityandmeasurableresistance. Laser-synthesizednanoparticlesolutionstendedtoaggregatedue toabsenceofchemicalstabilizersresultingindifferentfilm forma-tionafterseveraldays.

In Fig.5,itis clearlyseenthat thereis signiﬁcantchangein resistanceofAu-NPﬁlmwhilethestrainisbeingapplied,which

Fig.4. SchematicdemonstrationofAu-NPsstraingauge.PtcontactsareattheedgesofsensorandAu-NPsareshownasyellowballs.Notethat,dimensionsarenottoscale. Intheleftside,Au-NPfilmisintheinitialstatewherethereisnostrainandresistanceoffilmisR.Intherightside,strainisappliedtoAu-NPfilmthatincreasesdistances betweennanoparticlesandaggregatedclustersresultinginresistanceincreaseofR.(Forinterpretationofthereferencestocolourinthisfigurelegend,thereaderisreferred tothewebversionofthisarticle.)

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Fig.5.ResistancechangeresponseofAu-NPﬁlmswhileapplyingstrain.Fit1and2 arethecorrespondinglinearandexponentialfunctions.Also,metalfoilstraingauge responseisshownforcomparisonwhichhavegfactorof2(dashed-dottedline).

indicateshighersensitivityofAu-NPﬁlmthanconventionalmetal foilstraingauges.Previously,thestrainresponseofAu-NPstrain sensorisexplainedbyanexponentialdependenceas:

R/R=exp(gε)−1 (2)

where particles initially separated from each other [5]. Strain responseoftheAu-NPﬁlmwithinitialresistanceof 20.8Mis showninFig.6.ItisobservedthattheAu-NPstrainsensorresponse islinearundersmallstrains(<0.22%).Thegfactorisobtainedby ﬁttingafunctionoftheform:

R/R≈gε (3)

whichresultsing ∼_{= 80.2}undersmallstrains(Fig.5,fit1).Under higherstrains(>0.22%)Au-NPfilmresistanceisfittedwithan expo-nentialfunctionoftheform:

R/R−y1=exp(g(ε−x1))−1 (4)

where(x1y1)isApointinFig.5whichresultsinaneffectivegfactor

ofg ∼_{= 298}(Fig.5,ﬁt2).Gfactorisdescribedbyfollowingequation:

g=ˇ(d+l) (5) 5 10 15 20 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Δ

R/

R

Time (s)

5 6 7 8 9 0.0 0.2 0.4 0.6 0.8 1.0 1.2 ΔR/ R Time (s)

Fig.6.AlternatingstrainresponseofAu-NPsstrainsensor(at1Hzfrequency).

wheregisthegfactor;disthediameterofnanoparticleoraggregate ofnanoparticles;listhedistancebetweennanoparticles;ˇisthe tunnelingdecayconstant[5].Tunnelingdecayconstantdependson theheightofquantumbarrier.Underlowbiases,ˇ=2

2mϕ/2

wherem,ϕandareelectroneffectivemass,workfunctionand reducedPlanck’sconstant,respectively.

Assuming that tunneling undergoes in air, tunneling decay constant is ∼20nm−1 and average nanoparticle diameter is ∼13–15nm;so,gcanbeapproximatedasavalueof300.Aggregated clustersmightbealsoresponsiblefor highsensitivity ofAu-NP straingaugesinceeveryclustercanbeconsideredasa particle. Inthisscenario,aggregatesremainunstrainedwhilemostofthe deformationoccursbetweenthem.Asaresult,Eq.(5)canbe mod-iﬁedinsuchwaythatdcorrespondstoaverageclusterdiameter [5,11].

Onepossibleexplanationforthepresenceofmultipleregimes withdistincteffectivegfactorsissoughtbyconsideringthe con-ductionmechanismintheAu-NPfilm.Initially,byproperchoice ofNPconcentration,thefilmsarepreparednearthepercolation threshold,withalargenumberofparallelconductionpaths.Under theapplicationofasmallstrain(zone1inFig.5),thenumberof pathscontributingtoconductanceofthefilmsdiminishes, caus-ingalinearincreaseinresistancedescribedbyEq.(3).Asthepaths arebrokenatpointA,conductionthroughthepathsaredominated bytunneling(zone2inFig.5),whichexhibitsalargerresistance change.At thepoint A(x1,y1) quantum tunneling startsto play

majorrole:asa result,Eq.(2)shouldbecorrected toEq.(4) so thatexponentialdependencebeginsfromthepoint(x1,y1).

Modulation strain response of Au-NP strain gauge shows dynamicresponse ofresistance(Fig.6).Theinitialresistanceof thisstrainsensorusedforthisexperimentwas11.11M.G fac-torofAu-NPsensorusedinthisexperimentisabout118.Strain responsedidnotexhibitnoticeablechangeafterhoursof opera-tion.Also,reproducibleresponsescanbetakenevenafterweeksof initialpreparation.Forthefrequencieslargerthan1Hz,sensitivity ofthedevicestartstodegradebecauseoflowYoung’smodulusand viscoelasticpropertyofPDMS.

ResistanceofAu-NPfilmishighlydependentongapsbetween nanoparticles.Statisticalstudyisrequiredinordertounderstand howAu-NPfilmresistanceisdependentongapsbetweenAu-NPs. Wemadesimulationwhereeverygapisconsideredtobeeither seriesorparallelresistor(Fig.7).Weperformnumerical Monte-Carlosimulationstounderstandhow thegapsbetweenAu-NPs affecttheresistanceoftheAu-NPstrainsensor.Simmons’s gen-eralizedformulafortunnelingwasusedtoobtainresistancesof thesamples[15].Potentialdifferencebetweenthenanoparticles isassumedasthedistancebetweennanoparticlestimeselectric field(V=El)whereelectricfieldequalstoappliedbiasdividedby distancebetweentocontacts(E=Vbias/L).Directtunnelingshould

beobservedsince potentialdifferencebetweennanoparticlesis smallduetosmall(<1nm)distancesbetweennanoparticles.Inthe simulation,inputwasarandomGaussiandistributionofthegaps (700*10,0002Dnanoparticlesmatrix,performed1000times)and outputwasnumberofsamplesvs.resistanceofthesamples.Fig.7 isseemsasaGaussiandistribution,whichshowsthenumbervs. resistancesofsamples.Thegapbetweennanoparticlesis0.18nm withstandarddeviationof0.18nm.Inotherwords,the nanoparti-clesareveryclosetoeachother.Thisgraphprojectsthatdifferent substratesappearwhiledryingsameamountofAu-NPsonPDMS. Furthermore,lasersynthesizedAu-NPfilmspresentpromising propertiesforsensingapplicationsbecausethesesensorsareeasily fabricated onPDMS substrate and have high strain sensitivity. PDMS,havinglow Young’smodulus,is flexiblesubstrate which canbeadvantageousforsomeapplicationssuchaslowpressure sensing.ItiseasytoobtainuniformfilmsonPDMS;however,this

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21.0 21.2 21.4 21.6 21.8 22.0 22.2 22.4 50 100 150 200

Number of samples

Resistance

(MΩ)

R

11

R

12

R

1i

R

21

R

j1

R

ji

V

Fig.7.SimplecircuitmodelofAu-NPsthinﬁlmandhistogramofnumberofsamplesvs.resistanceofthesamples.

substratehassomedrawbackssuchascreepdeformation,stress relaxation and high thermal coefficient. Depending on sensing applications,differentsubstrateswithhydrophobicsurface,which allowsuniformdepositionofnanoparticlesbydrop-castmethod, can beemployed todevelop Au-NPstrain gauge. For example, substrateswithhighYoung’smoduluscanincreaseoperation fre-quencyrangeofthedevice.Studiesondepositiontechniquesmay berequiredtoimproveadhesionofAu-NPsonthesubstratewhich may improve mechanical robustness. Coating Au-NP film with protectionlayercouldbealsoneededtoreduceunwanted envi-ronmentaleffectssuchasvariationsinhumidity.Smallercontacts canbeemployedfordecreasingnominalresistancewhichinturn willreducenoiseassociatedwithlowcurrentdetection.Inorderto reducevariationsingaugefactor,drop-castingoncompressively strainedsubstratecanbeused.Afterrelievingsubstrateitexpands andallconductingpathwillbebroken;asaresult,onlyoneregime (tunneling)willberesponsiblefor resistancechangein Au-NPs straingauges.ThiswillalsomakeAu-NPfilmsensitiveto compres-sivestrain.Forfurtherstudies;thereliability,therepeatabilityand theenvironmentaleffectssuchasgaugehysteresis,temperature dependence,longtermaging,compressionvs.tensiondeviation, sensitivityshiftwithnumberofcycles,etc.couldbeinvestigated forreal-worldapplicationsoftheAu-NPstaingauge.

4. Conclusion

Inconclusion,wehavedemonstratedthatlasergenerated Au-NPfilmsonPDMScanbefurtherusedforhighlysensitivestrain gauge. G factor of these films is found to be ∼300 for strains higherthan0.22%,whichishighestreportedsensitivityforAu-NPs strainsensors.HighsensitivitypropertiesoflasergeneratedAu-NP strainsensorswasattributedtocleansurfaces,sizeandaggregated clustersofAu-NPs.High stabilitywasalsostudiedbyself-made mechanicalcharacterizations.Forthefurtherinvestigation,the Au-NPfilmsyieldinterestingelectricalpropertiesthatmaybeusedin transitionvoltagespectroscopy.

Acknowledgments

StatePlanningOrganization(DPT)ofTurkeyisacknowledged forthesupportofUNAM-InstituteofMaterialsScienceand Nano-technology.Dr.Ortac¸acknowledgesthe‘IndustrialThesisProjects Programme’oftheMinistryofIndustryandTradeforfundingthe San-Tez(636.STZ.2010-1)project.

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Biographies

SalamatBurzhueviscurrentlyaM.S.studentatInstituteofMaterialsScience andNanotechnology,BilkentUniversity,Ankara,Turkey.HereceivedtheB.S.in PhysicsfromMiddleEastTechnicalUniversity,Ankara,Turkeyin2006.His cur-rentresearchinterestsincludeultrafastlasersanditsapplications;nanomaterials forsensordevelopment.

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Dr.AykutluDânareceivedhisB.S.degreeinElectricalEngineeringfromBilkent University,Ankara,Turkeyin1995,M.S.degreeElectricalEngineeringfrom Stan-fordUniversity,CA,USAin1998,andPh.D.degreeinElectricalEngineeringfromthe StanfordUniversityin2003.Heiscurrentlyworkingasaresearchassistantprofessor atInstituteofMaterialsScienceandNanotechnology,BilkentUniversity.His cur-rentresearchareascoverfundamentalandappliedstudiesofmicroandnanoscale opto-electronicdevicesandsystems;surfaceplasmonresonancebasedbiosensors; micromachinedsurgicaltools;modelling,design,fabricationandcharacterization ofsemiconductordeviceswithnovelnanostructuredfeatures;X-Rayphotoelectron spectroscopyforelectricalcharacterization;nanomechanics,forcemicroscopyand spectroscopy.

Dr.BülendOrtac¸receivedtheB.S.degreeinPhysicsfromtheKaradeniz Techni-calUniversity,Trabzon,Turkey,in1997,M.S.degreeinTeachingandDiffusionof SciencesandTechnologyfromENSCachanUniversity,Paris,France,in2000,and Ph.D.degreeinOptoelectronicsfromRouenUniversity,Rouen,France,in2004 respectively.InMars2005,hejoinedtheInstituteofAppliedPhysics, Friedrich-SchillerUniversity,Jena,Germany,asaPost-DoctoralAssociate.SinceNovember 2009,hehasbeenworkingasaresearchassistantprofessoratInstituteof Materi-alsScienceandNanotechnology,BilkentUniversity.Hiscurrentresearchinterests includethedevelopmentofpowerfulﬁberlasersinthecontinuous-waveregimeto pulsedregime(ns,psandfs)andthedemonstrationoflasersystemsforrealworld applications.