Influence of NiCr/Au electrodes and multilayer thickness on the electrical properties of PANI/PVS ultrathin film grown by Lbl deposition.

MaterialsScienceandEngineeringB177 (2012) 359–366

ContentslistsavailableatSciVerseScienceDirect

Materials

Science

and

Engineering

B

j o ur n a l ho me p a g e :w w w . e l s e v i e r . c o m / l o c a t e / m s e b

Influence

of

NiCr/Au

electrodes

and

multilayer

thickness

on

the

electrical

properties

of

PANI/PVS

ultrathin

film

grown

by

Lbl

deposition

M.C.

Santos

a,b

,

M.L.

Munford

a

_,

_R.F.

_Bianchi

b,∗,1

a_{DepartmentofPhysics,FederalUniversityofVic¸osa,CEP36570-000,Vic¸osa,MG,Brazil}

b_{LaboratoryofPolymersandElectronicPropertiesofMaterials,DepartmentofPhysics,FederalUniversityofOuroPreto,CEP35400-000,OuroPreto,MG,Brazil}

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received17August2011

Receivedinrevisedform8December2011 Accepted23December2011

Available online 9 January 2012 Keywords: Compleximpedance Semiconductingpolymers Interface Surface Device

a

b

s

t

r

a

c

t

Inthepresentwork,weconcentrateonthestudyofeffectsofmetallicelectrodes,multilayerthickness andtemperatureinacanddcelectricalconductivityofpolyaniline/poly(vinylsulfonicacid)(PANI/PVS) ultrathinfilms.Thepolymersystemwasobtainedfromlayer-by-layer(Lbl)self-assemblytechniqueon aglasssubstratewithanelectrodearrayofadhesionlayerofNiCr(20nm)coveredwithAu(180nm).We observedasignificantandabruptincreaseinthevalueofdcconductivityandachangeofacconductivity behaviorofNiCr/Au–PANI/PVS–NiCr/AustructurewhenthethicknessofPANI/PVSsystemreachesthe Aulayer.TheseeffectswereascribedtotheidealcontactofAu–PANI/PVSandtherelativehighinterfacial contactresistancebetweenPANI/PVSandNiCr,thusreducingtheparallelresistanceofNiCr/Au–PANI/PVS interfaciallayerinanidealparallelplatecapacitorstructure.AtomicForceMicroscopyimagesconfirm thisassumption.Furthermore,theacconductivityofAu–PANI/PVS–Austructurewastypicalofsolid disorderedmaterials.Amodelbasedoncarrierhoppinginamediumwithrandomlyvaryingenergy barrierswaspresentedfortheacconductivityofthepolymersystem,whichalsoencompassesthehigh dielectricconstantofPANI/PVSblendedfilms,theneutralcontactAu–PANI/PVS,andtheelectrical resis-tanceofNiCr–PANI/PVSinterfaciallayer.Themodelallowedseparatingtheinterfaceandthebulkeffects intheelectricalresponseofNiCr/Au–PANI/PVS–NiCr/Austructureandinadditionthehighest activa-tionenergy(35MeV)correlatedwithanoptimizationofhoppingdistance(30nm)forcarriersjumpsin PANI/PVSsystem.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Asignificantapplicationgrowthin theareaoforganic semi-conductorsoverthepastfewyearshasbeenwellrecognizedby the scientific community [1–4]. The advancement is especially noticeable in the electrical response of organic electronic sys-temsuchaselectronictongue,flexiblechemicalsensors,organic fieldeffecttransistorsandthinfilmorganicphotovoltaicdevices. Manyapplicationsoftheorganicsemiconductorsrequirethe con-trolledassemblyofpolymerfilmsasmultilayerstructuresonsolid substrates.Thisisthecase ofchemical sensorsbased on nano-structurepolymerfilmspreparedbylayer-by-layer(Lbl)technique

∗ Correspondingauthorat:CampusMorrodoCruzeiro,CEP35400-000,Ouro Preto,MG,Brazil.Tel.:+553135591742;fax:+553135591667.

E-mailaddresses:bianchi@eecs.berkeley.edu,bianchi@eecs.berkeley.edu (R.F.Bianchi).

1 _{Temporaryaddress:DepartmentofElectricalEngineeringandComputer}

Sci-ences,UniversityofCaliforniaatBerkeley,2299Piedmont-Berkeley,CA,Brazil. Tel.:+15103252126.

[5]. This technique is promising because it can lead to nanos-tructuressimilartothosebuiltwiththeLangmuir–Blodgett(LB) technique[6],butyethastheadvantageofrequiringmuchsimpler experimentalprocedures[7,8].Amongthesemiconducting poly-mersusedtofabricateLblfilms,polyaniline(PANI)appearsasa potentialcandidatefromthetechnologicalstandpointonaccount of itslow costand ability tobehaveeither asa semiconductor or asa metal[9]. Meanwhile,although there arestudies about theroleoffilmthicknessonthemorphology[10]andelectrical response[11,12]ofultrathinPANIfilms,anoverall understand-ingofthecharge carriertransport mechanisminPANILblfilms hasbecomeakeytothedevelopmentofdeviceswithspecificand optimizedpropertiesfororganicsensors[13].Asaconsequence, a detailedinvestigationofelectrical propertiesofLbl polymeric device requiresthe knowledgeof severalfactors, including,for example,theinfluenceofpolymerbulk,polymermorphologyand polymer–electrodeinteractions[14–16]insuchawaythatquantify theinfluenceofthesecharacteristicsontheelectricalpropertiesof thedeviceisimportantandalsoextremelycomplicated.Inorder toovercomethesedifficulties,impedancespectroscopyand com-plexconductivitymeasurementsappearsasa powerfultool for 0921-5107/$–seefrontmatter © 2012 Elsevier B.V. All rights reserved.

Fig.1.Schematicillustrationof(a)interdigitatedmicroelectrodesonglasssubstrate.(b)PANI/PVSfilmsontoNiCr–Auelectrodescoveredbypolymerfilm.(c)Onlyonepair ofinterdigitatedmicroelectrodewithitsdimensions,whereitslength(L1),interdigitatedspacing(y)andNiCr(h1)andAu(h2)thicknessesare,5mm,10␮m,20nmand

180nm,respectively.(d)Pictureofthepolymersystemusedtocarryouttheelectricalmeasurements. studying the electrical mechanism of organic systems, since it

allowsustodistinguishfrombulktointerfaceinfluences,aswell astoobtainpolymerbulkandpolymer–electrodeinterface contri-butionstoelectricalcurrent[14–17].However,inordertodescribe suchmeasurementsofpolymerfilms,manyauthorsusually repre-sentedtheirelectricalcharacteristicsasacircuitexpressionbased onresistor–capacitor(RC) circuits [18,19],where both polymer layerandpolymer–electrodeinterfacesaredescribedbyasingle dielectricrelaxationtimes,ratherthanamorerealistic distribu-tionexpectedforsoliddisordermaterials[20].Althoughinmostof thecaseitisingoodagreementwithexperimentaldata,itisnot sufficienttoexplain–withgreataccuracy–theeffectofcarriers hoppingonthepolymersystems.

Inthispaperweinvestigatedthedcandacelectrical behav-ioroffilmscontainingalternatedlayersofPANIandtheanionic polyelectrolyte–poly(vinylsulfonicacid)(PVS),depositedonan electrodearrayofadhesionlayerofNiCrcoveredwithAu(NiCr/Au), aimingatunderstandingtheinfluenceoffilmthicknessandthe metallicelectrodes onthe electricalproperties ofpolymer bulk andmetal–polymer interfaciallayers.Onemodel basedon ran-dom free energy barrier model (RFEB) [21,22]is presented for theacconductivityof thepolymer system,whichencompasses thedisorderedand dielectric propertiesof Lblfilms, as wellas theNiCr–PANI/PVSinterfacialeffects, which allowedseparating theinterface and the bulk effects in the electrical response of NiCr/Au–PANI/PVS–NiCr/Austructure.

2. Experimentalprocedures

PANI was synthesized according to the route described by Chiang and MacDiarmid [23] while PVS used as the polyanion waspurchasedfromAldrichCo.PANIwasdissolvedindimethyl acetamide(DMAc)ataconcentrationof10mg/mLbystirringthe solutionandthenwassonicatedfor12h.Thesolutionwasthen filteredwithaceramicfilterwith8␮mporestoremovethetrace ofundissolvedPANIparticles.Later,thePANIsolutionwasdiluted withaqueoussolution1:9andthepHwasadjustedto3.0bythe additionofa1MHCl.Theexperimentalproceduresforfilm fabrica-tionareessentiallyasdescribedelsewhere.[7]Forthebuildingof

multilayersthroughlayer-by-layertechnique,PANIwasalternated withPVS,wherethelatterwasdilutedinultrapurewater, with 100␮Lofa25wt%aqueoussolutionofPVS(Aldrich)beingmixed with24mLofaqueoussolutionwithpH4.0.Therinsingsolution waswaterwithitspHadjustedto2.5with1MHCl.The multilay-erswereassembledbythealternatingimmersionofthesubstrate inthePANIandPVSsolutionsfor5mineachatroomtemperature (25◦C).Aftereachdeposition,thesubstrateswererinsedfor20s inanaqueouswashingsolutionwithsubsequentdryingunderan airflow.FilmsofPANI/PVSwereadsorbedonBK7opticalglass. Thehydrophilicsubstrateswereobtainedusingthephysic treat-mentUV–ozonecleaning.[24]TheUV–ozonecleaningconsisted ofexposingthesampletoultravioletradiationfromagermicidal lampinatmosphere.

The PANI/PVS adsorption was monitored by measuring the UV–vis spectra with a Spectrophotometer UV-1650PC Shi-madzu. For the electrical measurements, PANI/PVS films with different number of bilayers (n) were deposited onto NiCr adhesion line array completely covered by Au [25] previously evaporated on BK7 optical glass substrates using photolithog-raphy procedure, where its length (L1), interdigitated spacing

(y) and NiCr (h1) and Au (h2) thicknesses provided by the

manufacturerarearound10␮m,5mm,20nmand180nm, respec-tively.

Fig. 1 shows the schematic illustration of the samples. A rough estimation of a single bilayer (h) was obtained by measuring the thickness of a 40 bilayers film on BK7 sub-strate using a Veeco Dektak 150 Surface Profilometer. Atomic Force Microscopy (AFM) images were obtained with NTEGRA Probe NanoLaboratory. Current vs. voltage (I vs. V) measure-mentswereobtainedwitha6517AElectrometerHigh/Resistance MeterKeithleyinthe−1Vto1Vvoltagerange, whilecomplex impedance,Z*(f)=Z(f)+iZ(f),and thusalternating conductivity, *(f)=_(f)+i_{(f)=(L1/y.n.h)(1/Z*(f)), measurements were}

car-riedoutusinga1260SolartronFrequencyResponseAnalyserinthe 1Hzto2MHzfrequency(f)rangeandvoltageamplitudeequalto 100mVin200–325Ktemperaturerange.Fig.1dshowsthepicture ofthepolymersystemusedtocarryouttheelectrical measure-ments.

M.C.Santosetal./MaterialsScienceandEngineeringB177 (2012) 359–366 361

Fig.2. Imagesofatomicforcemicroscopy(AFM)oftheinterdigitated microelec-trodescoveredwithPANI/PVSsystem,where L1,L2 andL3 correspondtothe

interdigitatedspacing,microelectrodeswidthandtheedgesbetweenthe metal-licelectrodeandPANI/PVSfilmsurface,respectively.(a)Thesuperficialstructureof theinterdigitatedmicroelectrodeswith13bilayersofPANI/PVSsystem.Panel(b) and(c)showthesidewallprofileofthemetallicelectrodeandPANI/PVSregions representedbyhatchedarea.

3. Resultsanddiscussion

Inordertoconfirmthedimensionsofthemicroelectrodearray providedby themanufacturer,Fig.2shows AFMimagesofthe interdigitated microelectrodes covered with PANI/PVS system, whereL1,L2andL3correspondtotheinterdigitatedspacing,

micro-electrodeswidthandtheedgesbetweenthemetallicelectrodeand PANI/PVSfilmsurface,respectively.Fig.2ashowsthesuperficial topographyoftheinterdigitatedmicroelectrodeswith13bilayers ofPANI/PVSsystem,whileFig.2bandcshowsthesidewallprofile ofthemetallicelectrodeandPANI/PVSregions.Moreover,the side-wallprofileobtainedfromtheAFMimagesallowedustoobserve thatL3∼0.3␮mismuchsmallerthanL1(10␮m).ThisvalueofL3

Fig.3.Absorptionat900nmandestimatedthicknessaccordingtothenumberof bilayers(n).TheinsetshowstheUV–visabsorptionspectraforPANI/PVSLblsystem withnvaryingfrom1to10.

isnotanimageartifact,becausetheAFMtipemployedhasacone sideangleof∼20◦_{whichcanreadsuchstepedgeangle(∼45}◦_)of

thesamplewithouttouchingthetipside.Itisworthnotingthat thisresultisimportanttoproposehereinarealisticandideal par-allelplatecapacitorstructuretostudytheelectricalresponseof NiCr/Au–PANI/PVS–NiCr/Austructure.

Fig.3showstheabsorptionandtheestimatedthicknessofthe polymerfilmsaccordingtothenumberofbilayers(n).Thelinear increaseatthemaximumabsorption(ca.900nm)withnindicates thatthesameamountofmaterialwasprobablyadsorbedineach depositionstep.TheinsetinFig.3showstheUV–visabsorption spectraforPANI/PVSLblsystemwithnvaryingfrom1to10.Itis observedanabsorptionbandatca.900nm,whichischaracteristic ofPANIdopedstate,whiletheestimatedthicknessesofamultilayer films(fromn=1to10)wereobtainedfrommeasuringthethickness ofthefilmwith40bilayers(seeSection2)andtheextrapolationof thelineardependenceoftheabsorptioncurveasshowninFig.3. Thevalueofasinglebilayeraround3.5nmisinagreementwith literatureforothersemiconductingpolymersLblfilms[26].

Fig.4showstheelectricalresistance(R0)vs.numberofPANI/PVS

bilayers(n)obtainedfromNiCr/Au–PANI/PVS–NiCr/Austructure. TheR0valueswereobtainedfromcurrentvs.voltagecurveswhich

presentedaperfectohmicandsymmetricbehaviorforforwardand reversebiasmodes,asshownininsetinFig.4.ItisobservedthatR0

hasitsvaluechangeddrasticallyandabruptlywhenthethickness ofthefilmsreachedtheAucontactorwasnearlytheNiCrlayer

Fig.4.Electricalresistance(R0)andrealcomponentofthecompleximpedanceat

lowfrequencies(Zdc)vs.numberofPANI/PVSbilayers(n).TheinsetshowstheI

vs.VmeasurementsobtainedwithaPANI/PVSLblfilm(n=13)andtheschematic illustrationofonepairoftheinterdigitatedmicroelectrodescoveredwithPANI/PVS system,whereh1andh2correspondtoNiCrandAulayerthicknesses,respectively.

Fig.5.Argandcomplexplaneobtainedfromsampleswith(a)1,(b)4and(c)13 bilayers.

(h1=20nm),i.e.,n=5,insuchawaythatR0remainsalmost

con-stantforn≤4anddecreasedasthenumberofbilayersincreasesfor n≥5.Thiseffectwasattributedtothehigherelectricalresistance betweentheNiCrlayerandthePANI/PVSsystem,incomparison totheAulayer,exhibitingaresistanceintherangefrom7×107_

(n=1)to1×104_(n=13).

Fig. 4 also shows the real component of the complex impedance of NiCr/Au–PANI/PVS–NiCr/Au structure at low fre-quencies,Zdc=Z*(f→0),asfunctionofn.Again,R0 andthus Zdc

remainsalmostconstantforn≤4anddecreaseforn≥5.Theinset inFig.4alsoshowstheschematicillustrationofapairof microelec-trodescoveredwithPANI/PVSsystemforn≤4andn≥5,whereh1

andh2correspondtoNiCrandAulayerthicknesses,respectively.

Fig. 5 shows the Argand complex plane obtained from NiCr/Au–PANI/PVS–NiCr/Austructurewithnequalto1,4and13. Allcurvesshowasemicircleatlowrealimpedancevalueswhilea secondsemicircleisobservedonlyforn=1,asoutlinedintheinset ofFig.5a,wheretheelectrodeeffectontheelectricalresponseofthe systemismorepronounced.Fig.5bandcshowsasinglesemicircle

Fig.6. Real_{(f)andimaginary}_{(f)componentsofacconductivityasafunction}

ofthefrequency(f)obtainedfromsampleswith(a)n=1,(b)n=3,(c)n=4,(d)n=5 and(e)n=13.Theinsetshowstheschematicillustrationofapairofmicroelectrodes coveredwithPANI/PVSfilmsfordifferentthickness.Thefulllinesrepresentthe theoreticalfitting.

M.C.Santosetal./MaterialsScienceandEngineeringB177 (2012) 359–366 363 indicatingthatonlyonetransport mechanismtakesplaceor,at

least,dominatesthetransportprocess,defininganimpedance pro-cessesmostprobablyassociatedtothedisorderedbulkconduction forn≥5.Itisrepresentedbyauniquerelaxationtimedistribution [27].Moreover,atlowfrequencies,thevalueobtainedfromZ is similartoR0,obtainedfromcurrentvs.voltagecurve.

Fig.6showsthereal_{(f)andimaginary}_{(f)componentsofac}

conductivityofNiCr/Au–PANI/PVS–NiCr/Austructureasafunction ofthefrequency(f)obtainedfromsampleswithn=1,3,4,5and 13.Fromtheelectricalresultsshowedinthisfigure,itisobserved thatatlowfrequenciestherealcomponentincreasedwithnand exhibitsa plateauatcertainfrequency. Thisfrequency, denom-inatedhereas thecriticalfrequency, fc,alsoincreasedabruptly

from800Hzforn=1toabout400kHzforn=13.Both_(f)and

_{(f)presentedatypicalelectrodeinfluenceatlowfrequencies}

(<10Hz)andbulkeffectsathigherfrequencies.However,thiseffect ismorepronouncedin_(f)atlowfrequencies.Thisisconsistent withtheresultsshowninArgandcomplexplane(Fig.5)andalso tothecompleximpedancemeasurementsofultrathinPANIfilms recentlycarriedoutinourpreviouswork[28].Uponincreasingthe frequency,␴_{(f)displayedastrongfrequency-dependence,}

obey-ingthepower-law␴_(f)∝fs_{(0≤s≤1).Thisisthetypicalbehavior}

expectedfordisorderedmaterials,inwhichthelossmechanisms presentawiderangeofpossiblerelaxationtimes[15,21].Theinset inFig.6showsaschematicillustrationofamicroelectrodecovered withPANI/PVSfilmsfordifferentthickness.

Fig.7showsthereal␴ _{(f)andimaginary␴}_{(f)components}

ofacconductivityasafunctionofthefrequency(f)obtainedfrom samplewithn=13fordifferenttemperatures.Inthiscase,itwas pointedout,inconnectionwithFigs.4and5thatthecontribution to␴_(f)and␴_{(f)arerelatedtobulkpropertiesofPANI/PVSsystem.}

Thesamewaythatthe␴_{(f)forsampleswithvariousn,atlow}

fre-quencies,wehaveobservedthattherealcomponentincreaseswith temperatureandexhibitsaplateauuptocriticalfrequency,while theimaginarycomponent␴_{(f)irrespectivelyofthetemperature,}

obeysapproximatelyalinearfunctioninthedoublelogarithm␴

(f)–fplot.TheKramers–Krönigrelations[29],inthefrequency rangecoveredbythepresentexperiments,donotfullydetermine _{(f)fromtheknowledgeof}_{(f),andviceversa.Inshort,this}

isbecauseKramers–Krönigrelations areintegral(nonlocal) and differentbehavior obtainedfordifferentparametersoutsidethe measuredfrequencyrangeofthereal(imaginary)partinfluences thebehavioroftheimaginary(real)partinsidethefrequency inter-val.Therelatedproblemofinferringtheimaginarypartfromthe measuredrealpartwasalreadydiscussedintheliterature[17,30]. Theeffectofthetemperatureon␴_{(f)isdisplayedinFig.8a,}

forthicker sample(n=13).Thisfigureshowsthestrongchange of ␴ _{(f) with temperature when f<fc and do not display a}

temperature-dependenceforf>fc.Webelievethattheelectronic

conductionisviahoppingmechanismorphononassisted tunnel-ingprocessamonglocalizedsites,whosetransitionratedepends on the spatial distance on the energy difference between two localizedsitesas described inRef [18].It canbeobserved that the effect of the temperature (in the range of 200–325K) is similar to that of increase of number of bilayers since the dc conductivityincreases withthe temperature,as it does withn (Fig.6).Thefcvariesfromapproximately3kHzat200Ktoabout

20kHz at325K.The dc conductivityisless than5×10−4(S/m) at 200K and ca. 3×10−3(S/m)at 325K. Therelation ␴ _(f)∝fs

holds above fc and s seems to be temperature independent

(s ∼_{= 0.7).}Amastercurve,showninFig.8b,wasobtainedby plot-tingtheresultsofFig.8ausingnormalizedscales _(f)/

dc and

f/(Tfcdc).FromFigs.6and8,itisalsoimportanttoremarkthat

thevalues of  _{(f) for n=13 are not thesame at 300K (room}

temperature) because these measurements were taken under vacuum.

Fig.7.Real_{(f)andimaginary}_{(f)componentsofacconductivityasafunctionof}

thefrequency(f)obtainedfromsamplewithn=13fordifferenttemperatureunder vacuum.Thefulllinesrepresentthetheoreticalfittings.

4. Theoreticalconsiderationsandfittings

Dyrehasproposedtherandomfreeenergybarriermodel(RFEB) forexplainingconductionindisorderedsystems.Accordingtothis model,thehoppingcarrierhastoovercomeenergybarriers ran-domly distributed betweena minimum Wmin and a maximum

energy Wmaxuniformly dispersedthroughthesample bulk.For

lowfrequenciesthethermalizedparticlehasenoughtimetomeet high-energybarriersandthelowestjumpingfrequency(min2fc)

[21,22] dominates theprocess, with min=0e(−Wmax/kT), where

k istheBoltzmann constantand Tis theabsolutetemperature. Thisgivesrisetothelowfrequencyplateauregion.Foran elec-tronichoppingprocess0dependsexponentiallyonthehopping

distance,r,betweenthesitesthroughtheoverlapintegralofthe localizedwavefunctions,by0=₀e−2˛r where₀ istheescape

frequencyand_˛isthereciprocaloftheelectrondecaydistance. Fromthemeanjumpingfrequency,Dyre’sexpression[17,22]for theacconductivityofdisorderedmaterials,*(ω)=_(ω)+i_(ω),

Fig.8.(a)RealcomponentsofthecomplexconductivityofPANI/PVSsystemat differenttemperatures:200,250,275,300and325K.(b)Mastercurveobtained fromtheresultsof(a)inwhicheach_{(f)wasdividedbythecorrespondentvalue}

dcandfwasdividedby(Tfcdc).Allmeasurementswerecarriedoutundervacuum.

is ∗

D(ω)=_ln(1+(iω/0iω/min

min)/1+(iω/max))−

0iω

minln(max/min)

(1) where max=0e(−Wmin/kT), ω is theangular frequency (ω=2f)

while0istheconductivityforω→0,obtainedofacconductivity

experimentalcurves.TherealpartofEq.(1)leadstoasecondhigh frequencyplateau,butifweassumewithDyrethatmaxmin,

onlytheoneatlowfrequencywillbepresent,andEq.(1)is simpli-fiedto ∗ D(ω)=0



iω/min ln(1+iω/min)



(2) However,thecomplexconductivityshowedinFig.6alsoshows acapacitivebehavior[30].Thus,thetotalbulkconductivitymust includesthecontributionfromthedielectricpolarization,∗

P(ω),

5 0.7 5.0 0.6 1.4×10−3 _{4.0 8.7×10 20}

12 0.7 5.0 0.6 3.0×10−2 1.0 2.0×105 _0.9

13 0.7 5.0 0.6 4.0×10−2 1.0 3.0×105 _0.6

whichwillbewrittensimplyasiωε_ε0,whereε_{isthedielectric}

constantand ε0 isthevacuumpermittivity.Therefore,thetotal

bulkcomplexconductivity∗

B(ω)isobtainedasfollows[17,31,32]:

∗

B(ω)=D∗(ω)+iωεε0 (3)

ThecarriertransportmechanisminPANIisexplainedassuming thatthesampleisconsideredtobeconstitutedofmetallicregions sparselydistributed in adisorderedmedium, thatis, dispersive matrix.ItiswellknownthatdopedPANIexhibitsconductive pro-tonatedregions distributedinthebulk ofunprotonatedmatrix. Concerningthedisorderedmatrixwehaveassumedittosatisfy theRFEBmodelproposedbyDyre(Eq.(2))whichmustincluded thecontributionfrompolargroupsasdescribedinEq.(3).

Furthermore,thecontributionoftheinterfaceNiCr–polymer and Au–polymer in the electrical response of the system can-not be neglected.Therefore, based on theconfiguration of the metal–polymerinterfaceandthepolymerbulk,thetotalcomplex conductivityofthesystemisdescribedastwodistinctequivalent electricalcircuits:oneforn_≤4(Eq.(4))andotherforn_≥5(Eq.(5)). Eq.(4)representsparallelRCcircuitsforNiCr–polymerinterface andEq.(3)forpolymerproperties,whileEq.(5)representsthe par-allelcombinationsofRCcircuitsforNiCr–polymerandAu–polymer interfacesinserieswiththepolymerbulk.

(4)

(5) RN, CN and ˛N onEqs. (4) and (5) correspond to the

resis-tance,capacitanceand˛ parameterof NiCr–PANI/PVSinterface, RA correspondtotheresistanceofAu–PANI/PVS interface,L1 is

theinterdigitatedspacingandAisthecontactareabetweenthe polymerfilmandtheelectrodes(A=ynh).ForNiCr–PANI/PVS inter-face,whichhastypicallyfeatureinsulator,wereusedtheCole–Cole approach(ˇ=1)[33],while forAu–PANI/PVS interfacewhich is ohmiccontact(neutralcontact),wasusedasinglecontact resis-tance.

FulllinesinFigs.6and7representthetheoreticalfittingwiththe modelfromthissection,inwhich˛0estimatedfromthedc

mea-surementswasusedasinputparameter.Theparametersemployed aresummarized in Tables1 and 2for measurementsshown in Figs.6and7,respectively.

Table1showsthevalueofparametersRN,CNand˛NandRA

relatedtotheinterfacemetal–polymerand0,kPandminrelated

tothebulkresponseobtainedinthefittingof_(f)and_(f)for sampleswithn-bilayers.ItisobservedthatRNdecreaseswhileCN

M.C.Santosetal./MaterialsScienceandEngineeringB177 (2012) 359–366 365

Fig.9. Theratio0/min(a)forsampleswithnbilayersand(b)forsamplewith13

bilayersatdifferenttemperature.(c)ln(2min)vs.T−1fromdataofTable2.The

curvepresentsanactivationenergyWmax=(34±2)MeVwhenfittedbyArrhenius

process.Thedotlinesareusedonlytoguidetheeyes.

1to4,reachingconstantvaluesforn≥5.Moreover,theelectrical resistanceattributedtoNiCr–PANI/PVSinterfaceismuchhigher thanthevaluesobtainedfromtheinterfaceAu–PANI/PVS. Further-more,kPdecreaseswithnandtheobtainedvaluesaremuchhigher

thatthevaluesobtainedfromPANIfilmsintheliteratureindicating thatsomeinfluenceofPVSisattributedtoPANI/PVSsystem.This assumptionisstillunderinvestigation.Finally,theconductivity0

andtheminincreasewiththenumberofbilayers.However,the

ratio0/minremainsalmostconstant,asshowninFig.9a.A

sim-ilarresultwasobservedwhentemperatureistheexperimentally variedparameter,Fig.9b.Thisresultsuggeststhatthemobilityof chargecarriersincreaseswithnandT,whilethedensityofthese carriersisconstantasdescribedforotherpolyanilinesystem[17].

Table2

DataoffittingsbetweenEqs.(4)and(5)andexperimentalresultsofacconductivity forthesamplewith13bilayersatdifferenttemperatures.

T(K) RN(M) CN(␮F) ˛N 0D(S/m) kP(×103) min/2(Hz) RA(k) 200 0.90 3.0 0.60 4.6×10−4 _{2.4 8.0×10}3 _8.5 250 0.70 5.0 0.60 1.3×10−3 _{2.2 1.2×10}4 _6.0 275 0.50 7.0 0.60 1.5×10−3 _{2.1 1.3×10}4 _4.0 300 0.10 20 0.60 1.9×10−3 _{2.0 1.5×10}4 _1.8 325 0.09 30 0.60 2.7×10−3 2.0 1.8×104 _1.1

Table 2 shows values of the same set of parameters as in Table1obtainedfrommeasurementswiththe13bilayers sam-ple for differenttemperaturesfrom 200K to325K. Again,it is observedthatRN,RAdecreaseandCNincreasewiththe

temper-aturewhilekPand˛N remainsalmostconstant. Moreover,both

0andmindecayexponentiallywiththetemperature,atypical

behavioroftheArrheniusprocess,havingtheactivationenergy, Wmax=(34±2)MeV, as shown in Fig.9c.It wasobtained from

ln(2min)vs.T−1 curves. Indeed,weexpected 0 (=0/2) in

=_0e(−W/kT)toobeytherelation0=0e−2˛r,where0 isthe

escapefrequency.Assuming1012_{Hz for}

0 and0.25nm−1 for˛

[34]whichisthereciprocaloftheelectrondecaydistance,themean distancerisapproximately30nmforsamplewith13bilayers.

5. Conclusions

ThesidewallprofileobtainedfromtheAFMimages, allowed us to observe that L3 (∼0.3␮m) is much smaller than L1

(10_␮m) in such way that it was possible to propose a real-istic theoretical–experimental model to study the electrical response of NiCr/Au–PANI/PVS–NiCr/Au structure. The more important, unexpected, conclusion of this work is that the presence of NiCr electrode does not favor the electrical cur-rent in the NiCr/Au–PANI/PVS–NiCr/Au structure: the interface NiCr–PANI/PVSincreasessignificantlytheelectricalresistanceof thesystem.Thisfact,especiallyinassociationwiththeinfluence ofmorphology[11],explainswhythedcconductivityobtainedof NiCr/Au–PANI/PVS–NiCr/Austructureismuchlowerforfilmswith fewbilayers.Theglobalresultsuggestedthattheelectricalresponse oforganicdevicespreparedwithconjugatedpolymersonanarray ofNiCr/Auelectrodesisrelatednotonlytothebulkpropertiesof conjugatedpolymers,but alsoto theinterfacialmetal–polymer effects andpolymer thickness.Webelieve thatourresults sug-gestnewdirectionsforthefutureapplicationsofnanostructured conductingpolymermultilayers-basedelectronicdevicesinwhich NiCrprovidesgoodadhesiontotheglasssubstratewhileAuserves asanohmiccontact.

Acknowledgments

The financial support from MAPA/CNPq (Edital 64/2008), NANOBIOMED/CAPES,INEO/CNPQ(Proc.PDE200682/2011-2)and FAPEMIG.

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