1

Largethermoelectricfigureofmeritingraphenelayered

devicesatlowtemperature

DanielOlaya1,MikelHurtado-Morales1,2,DanielGómez1,OctavioAlejandro

Castañeda-Uribe3,Zhen-Yu.Juang4,5,YennyHernández1*

1NanomaterialsLaboratory,DepartmentofPhysics,UniversidaddelosAndes,Bogotá

111711,Colombia2DeparmentofElectronicEngineering,UniversidadCentral,Calle21#4–40,Bogotá–

Colombia.3DepartmentofBiomedicalEngineering,UniversidadManuelaBeltrán,Avenidacircunvalar

60–00,Bogotá–Colombia.4DepartmentofElectrophysics,NationalChiaoTungUniversity,Hsinchu30010,Taiwan

5SulfurScienceTechnologyCo.,Ltd.,Taipei10696,Taiwan.

Keywords:Thermoelectrics,grapheneheterostructures.

Abstract

Nanostructuredmaterialshaveemergedasanalternativetoenhancethefigureofmerit(ZT)

ofthermoelectric(TE)devices.Grapheneexhibitsahighelectricalconductivity(in-plane)that

is necessary for a high ZT; however, this effect is countered by its impressive thermal

conductivity. In this work TE layered devices composed of electrochemically exfoliated

graphene (EEG) and a phonon blocking material such as poly (3,4-

ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), polyaniline (PANI) and gold

nanoparticles(AuNPs)attheinterfacewereprepared.Thefigureofmerit,ZT,ofeachdevice

wasmeasuredinthecross-planedirectionusingtheTransientHarmanMethod(THM)and

complementedwithAFM-basedmeasurements.TheresultsshowremarkablehighZTvalues

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(0.81 < ZT < 2.45) that are directly related with the topography, surface potential,

capacitancegradientandresistanceofthedevicesatthenanoscale.

Introduction

TE materials have attracted the attention of the automotive, aerospace, medical and

electronic industries due to their ability to transform waste heat into electricity by the

Seebeckeffect1.Theperformanceofthesematerialsisdeterminedbytheirfigureofmerit

(ZT=S2sT/k),whichisafunctionoftheSeebeckcoefficient(S),theelectricalconductivity

(s),thethermalconductivity(k)andthetemperature(T).Thefigureofmeritofconventional

TEmaterials(highlydopedsemiconductors)isabalancebetweentheelectricalconductivity

andthethermalconductivity(connectedbytheWiedemann-FranzLaw)2.Thisbalanceleads

to ZT values not greater than the unit3, which limits the use of TE materials in power

generationandenergyharvestingapplications.ZTisfurtherlimitedinthepresenceofsmall

temperaturegradients2andTEmaterialsforapplicationswherethisisthecaseareyettobe

developed.

An alternative for increasing ZT is the use of low dimensionalmaterials as proposed by

DresselhausandHicks4,5.This improvement isdue toanenhancedSeebeckcoefficient,a

dimension-dependent electronic density of states and a low thermal conductivity due to

phonon scattering at the interfaces6. These findings lead to the development of

nanostructuredTEmaterialswithZTvaluesupto2.4(ataworkingtemperatureof~1000

K)7.

Carbonmaterialshavethebroadestrangeofthermalconductivityvaluesreported8.These

valuesgofrom0.01W/mKforamorphouscarbonupto2500W/mKfordiamond.Inthecase

ofgraphite,itexhibitsahighanisotropyinitsthermalconductivity(k)inthecross-planeand

in-plane directions with values at around 10 W/mK and 2000 W/mK respectively. This

anisotropyisalsoobservedintheelectricalconductivity(s)ofgraphitewithvaluesat3x102

S/mand2x105S/minthecross-planeandin-planedirectionsrespectively.

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Lowdimensionalcarbon-basedmaterials,suchasgrapheneandcarbonnanotubes(CNTs)

presentgoodelectricalandthermalconductivities(in-plane),whichimposesanobstaclefor

theiruseasTEmaterials.Multilayerstackingof twodimensional (2D)materialshasbeen

proposedasanefficientroutetowardstheenhancementofthermoelectricproperties9,10.In

particular, solution exfoliated graphene films11-13 display low thermal conductivity in the

cross-planedirection14 resembling the reportedvalue forgraphite8.Conductingpolymers

suchasPEDOT:PSSandPANIhavebeenusedforTEstudiesduetotheirpreferentialelectrical

conductivity along the polymer chain direction, their low thermal conductivity and their

measurableSeebeckresponse15.Compositematerialsofthesepolymerswithgraphene16,17

andcarbonnanotubes18 (CNT)havebeenrecentlypreparedandthishas ledto improved

values of S and s, which increases directly the power factor. Additionally, theoretical

calculationsofgoldnanopillarspatternedongraphenepredictedthepresenceofalargein-

planeSeebeckcoefficientforsuchstructure19.

Inthiswork,anapproachtoenhancethecross-planefigureofmeritofgraphene-basedTE

materialsisproposed.Solutionprocessingmethodswereusedtofabricatelayereddevices

basedonelectrochemicallyexfoliatedgraphene(EEG)andinterlayerconductingmaterials,

suchasPEDOT:PSS,PANIandAuNPs.TheTEperformanceofthefabricateddevicesinterms

ofthefigureofmerit,Seebeckcoefficientandelectricalconductivityischaracterizedinthe

cross-plane direction by means of THM20. In addition, the devices are structurally and

electricallycharacterizedatthenanoscalebyAFM.Thelocalmapsoftopography,surface

potential,capacitancegradientandresistancearemeasuredtostudytheinfluenceofthe

nanostructuredmaterialsintheoverallTEbehaviorofthedevices.

ZTmeasurementoflayereddevicesviaTHM

Layered materials of graphene and a conductive interlayer material such as AuNPs,

PEDOT:PSSandPANI,werepreparedinaconfiguration3:2followingthecoatingprocedures

describedinthemethodssection(Figure1).Thethermoelectriccharacterizationwascarried

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outbymeansofTHMwithcurrentpulsesof1mswhichallowedstatisticalanalysis(7cycles)

oftheextracteddata(Figure2a).WithinTHM,thevoltagedropisdividedintworegimes:a

rapidvoltagedropduetotheresistance,VR,followedbyaslowdeclineduetotheSeebeck

voltage,VS,asthethermalgradientacrossthedevicedissipates(Figure2b).Theturningpoint

betweenthetworegimeswasdeterminedusingalinearandanexponentialregressionfor

eachone.TheratiobetweenVSandVRisequivalenttotheZTfigure21(ZT=VS/VR)andcanbe

calculatedasafunctionoftemperature(Figure2c).

Figure1.DiagramofalayeredTEdevice(configuration3:2).

ThelayereddevicesbuiltwithPEDOT:PSS,PANIandAuNPshaveaverageZTvaluesof2.45,

1.47 and 0.81 respectively (Figure 2c). The ZT figure of the EEG – Polymer structures is

unprecedentedforsolutionprocessedgraphene16,17andwhencomparedwithBi2Te3based

TEmaterialsworkingatthesametemperaturerange22.TheZTwithAuNPsattheinterfaceis

in good agreement with the report by Juang and coworkers on CVD graphene – AuNPs

heterostructures23. The dotted line in Figure 2c represents the ZT figure of a device

composed purely by SiO2 and silver contacts as a reference for the contribution of the

substrateandtheelectricalcontacts.

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The semiconducting behavior of the layered devices was corroborated by plotting the

maximumvoltagemeasuredasafunctionoftemperature(FigureS1).Thismeasurementwas

coupled with a comparison of the electrical resistance in the in-plane and cross-plane

configurationstofurthercorroboratetheanisotropyofourdevices(FigureS1).

Figure 2. (a) Representative figure of a THM measurement (b) Intercept between the

resistiveandtheSeebeckregime(c)and(d)ZTandsforEEG–AuNPs(blacksquare),EEG–

PANI(redsquare)andEEG–PEDOT:PSS(bluesquare)devicesasafunctionoftemperature

(configuration3:2).

Influenceoftheinterlayermaterial

One of the goals when choosing the interlayer material was to enhance the electrical

conductivityacrosstheheterostructureincomparisontoadevicebuiltpurelywithEEG.For

thisreasonandfortheirTEresponse15,conductingpolymerssuchasPEDOT:PSSandPANI

were chosen. The performance of such polymers in a cross-plane configuration was

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measuredbyTHMasacomparisonwiththeEEGheterostructures.TheZTfigureofthese

deviceswas~1whichislowerwhencomparedtotheheterostructures(FigureS2).Figure2d

shows the internal conductivityof the layereddevicesasa functionof temperature. It is

importanttonotethateventhoughthedevicewithAuNPsdisplayshigherconductivitythan

theonewithPEDOT:PSSitsZTfigureiswhatultimatelywoulddeterminetheefficiencyof

thedevice.ThedottedlineinFigure2drepresentstheaverageconductivityofanEEGdevice

incomparisonwiththeimprovementoftheelectricalconductivityduetothepresenceof

theinterlayermaterials.

The Seebeck coefficient, extracted from the THMmeasurement (VS) and the measured

temperaturegradient,appeartobeindependentfromthenatureoftheinterlayermaterial

(Figure3a).Itisnoteworthy,however,thatthisresponseisenhancedwhencomparedtoa

pureEEGdevice(Figure3d).Meanwhile,thepowerfactor(S2s)differsbetweendevicesat

lowtemperatureduetothedifference inelectricalconductivity.Eventhough, theAuNPs

devicedisplaysahighervaluethanthatofthedeviceswithconductingpolymersatroom

temperature,as thetemperature increases, thisdifference isnotaspronouncedandthis

determines the recommended operating temperature for the TE device (Figure 3b).

Althoughkwasnotmeasureddirectlyitcanbecalculatedbydividingthepowerfactorand

theZTfiguretoillustratetheimportancenotonlyofhavinghighsbutalsolowkfortheTE

efficiencycalculation(Figure3b(inset)).Notethattheextractedvaluesforkarewithinthe

sameorderofmagnitudetothosereportedforgraphiteinthecross-planedirection8.

ItisinterestingtocomparethedeviceswithhighestZTandhighestpowerfactorwithanEEG

device.Ontheonehand,toexplainthehighZTfiguremeasuredfortheEEGdevice(Figure

3c)thereareacouplethingstoconsider:EEGhastypicalcrystalsizesof~5µm(FigureS3)

andthinfilmsaredepositedbyspraycoating,upto20nminthickness,toassurefullcoverage

ofthesubstrate.ThewrinklesandedgesofthedepositedEEGfilms(FigureS4)areknownto

containsharpfeaturesintheelectronicdensityofstates24whichplaysanincrementalrole

ontheSeebeckcoefficient25andintheelectricalconductivityinthecross-planedirection.

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On the other hand, the thermal conductivity across the EEG device is two orders of

magnitudelowerthanthehighestcalculatedvaluekforthelayereddevices;hencethehigh

ZTfigure(FigureS4).TheSeebeckcoefficientofthegraphenedeviceisinthesameorderof

magnitudeofthelayereddevices,howeveritspowerfactorissignificantlylower,whichcould

beattributedtoitslowelectricalconductivity(Figure3d).

Figure3.(a)and(b)Seebeckcoefficient,powerfactorandk(inset)forEEG–AuNPs(black

squares),EEG–PANI(redsquares),EEG–PEDOT:PSS(bluesquares)andEEG(greysquares)

devicesasa functionof temperature (c)ZT forEEG(greydiamonds),EEG–AuNPs (black

squares)andEEG–PEDOT:PSS(bluesquares)devicesasafunctionoftemperature(d)Power

factorandSeebeckcoefficientofEEGasafunctionoftemperature.

Nanoscalecharacterization

The nanoscale electrical characterization of the samples was divided into independent

measurementsofsurfacepotential,capacitancegradientandlocalresistance.Thesurface

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potential images (Figure 4b) show a heterogeneous distribution of potentials, which is

attributedtothedifferenceintheworkfunctionsoftheconstitutivematerialsofeachdevice

(EEG,conductingpolymerandAuNPs layers).Particularly, theEEG–PANIdeviceexhibits

extensiveequipotentialregions(yellowandturquoiseareas)thatworkaspotentialbarriers

thatblocktheelectrontransportinthecross-planedirection.

Figure4.(a)Topography,(b)Surfacepotential,(c)𝜕𝐶 𝜕𝑧(d)ConductingAFMofEEG–PANI,

EEG–PEDOT:PSSandEEG–AuNPs.

Thecapacitancegradientdenotestheratioofthevariationsbetweensamplecapacitance

andheight(𝜕𝐶 𝜕𝑧).Thehighhomogeneity(lowcontrast)ofthe𝜕𝐶 𝜕𝑧mapsinFigure4cis

a resultof theuniformity in thecapacitanceandheightof thetop layermaterialofeach

device.Thisindicatesahighqualityinthemulti-layeringprocessoffabrication.Moreover,

theestimationofthetotalequivalentcapacitancebasedonthe𝜕𝐶 𝜕𝑧measurementsof

eachdevicepairedwithimpedancespectroscopyanalysis26isacomplexprocedureanditis

outofthescopeofthispaper.

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To elucidate the relation of the cross-plane electronic transportwith the TE parameters

previouslydiscussed,conductiveAFMmeasurementswereconducted.InthecaseoftheEEG

–PANIdevicethedensityofcurrentpathsissignificantlylowerthantheothersampleswhich

correlateswell totheelectricalconductivitymeasurements.TheEEG–PEDOT:PSSdevice

displaysagreaterdensityofcurrentpathswhencomparedtotheEEG–AuNPsone.This

indicatesmoreprominentJouleheatingthatincreasesVsandthereforeZT.

Figure5.(a)ZTand(b)SeebeckcoefficientofEEG–PEDOT:PSS(4:3)(blacktriangles),EEG–

PEDOT:PSS (3:2) (blue triangles), EEG – PEDOT:PSS (2:1) (red triangles) as a function of

temperature (c)ZT ofEEG–PEDOT:PSS (3:2) (blue triangles)andCNT–PEDOT:PSS (3:2)

(whitetriangles)asafunctionoftemperature(d)PowerfactorandSeebeckcoefficientof

CNT–PEDOT:PSS(3:2)asafunctionoftemperature.

LayerdependenceandcomparisonwithCNT

Theeffectofthenumberoflayerswasalsostudiedinconfigurations2:1,3:2and4:3forthe

EEG-PEDOT:PSSlayereddevice(Figure5a).TheZTfigureforthe2:1and4:3deviceislower

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thanthe3:2configurationandthiscouldbeassociatedtoalowerSeebeckcoefficientinboth

cases(Figure5b).ThevalueofScanbeenhancedbyincreasingthenumberoflayers(2:1

and 3:2), however this enhancement is limited by the voltage output in the Seebeck

measurement(4:3).TheextractedvaluesofthethermalconductivitycomparedwiththeEEG

deviceindicatethattheinclusionofPEDOT:PSShasanasymmetriceffectwiththenumber

of layers (Figure S5) which differs from previous reports on graphene only multilayer

structures27.Ontheonehand,theinclusionofonelayerofPEDOT:PSS(2:1configuration)

has almost no effect onk. On the other hand, the 3:2 and 4:3 configurations displayed

increased and lowered k values respectively. The increased value of k for the 3:2

configuration could be attributed to the formation of electronic percolating pathswhich

enhancestheelectroniccontributiontothethermalconductivity.Incontrast,thelowered

valued of k for the 4:3 configuration could be associated to phonon blocking at the

interfaces.Fromthis,itcanbeconcludedthatacloseinterplaybetweenSandkwouldbe

keywhendesigningnewlowdimensionalTEheterostructures.

Additionally,aCNT-PEDOT:PSSdevice,inaconfiguration3:2,waspreparedwiththeinterest

of comparing itwith thegraphenebasedone.TheZT valuesmeasured in this casewere

significantlylower(Figure5c).ThelowefficiencyoftheCNTbaseddevicecouldbeattributed

tothelargeanisotropyontheelectricalandthermaltransportofCNTwhichispreferential

along the tube axis8 which in this case is oriented in-plane. Additionally, a low Seebeck

coefficientwasmeasured(cross-plane)incontrastwithreportsonlargeSeebeckcoeficcients

insemiconductingsinglewallnanotubefilms(inplane)28.Furtherexperimentaleffortsare

neededtodesigncarbonnanotube–graphenehierarchicalstructures.

Conclusion

Inthisworkanew,lowcost,methodtoproducehighperformanceTEcomponentsthatcould

beusedinmicro-electronicapplicationshasbeendemonstrated.Graphenebaseddevices,

preparedfromsolution,displayedlargefiguresofmerit(0.81<ZT<2.45)whencompared

to their counterparts within the same temperature range (<400K). The present study

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correlatesthethermoelectricperformanceofthedeviceswiththephysicalpropertiesofthe

interlayermaterial.Thisadvancementiscertainlyasteppingstonetowardstheengineering

ofnewadvancedlayeredTEmaterials.

Methods

Grapheneexfoliation,CNTdispersionandfilmformation

Graphenewaselectrochemicallyexfoliatedfromgraphitefoil(AlfaAesar,0.254mmthick,

99.8%)usingsulfuricacid(SigmaAldrich,99.999%)at0.1Mat10V.Theexpandedmaterial

wasfilteredusingVVPPfilters(poresize0.1µm),towashresidualacidonthesurface.The

filteredmaterialwasthendispersedin50mlofMilliporewaterviabathsonication(Branson

1800,90min)tofinishtheexfoliationprocedure.Thickergraphiteflakeswereremovedvia

centrifugation (HermleZ306,60minat3500RPM).Thisprocesswasconducted twice to

obtainstablegrapheneaqueousdispersionsat0.28mg/ml.Graphenefilmswereformedon

(1cmx1cm)SiO2substrates(thickness500µm)byspraycoating1.6mlofthedispersionat

100°Csubstratetemperature.

TopreparetheCNTfilms,anaqueoussolutionofsodiumcholate(SigmaAldrich)at0.2mg/ml

wasprepared.SubsequentlyCNT(NanoIntegris,13–18nmouterdiameter)weredispersed

viatipsonication(QSonica,50W,300s),followedby24hshelfdecantation.Thisprocesswas

conductedtwicetoobtainstableCNTdispersionsat0.2mg/ml.CNTfilmswereformedon

SiO2substratesbyspraycoating5mlat100°Csubstratetemperature.

AuNPs,PANIandPEDOT:PSSpreparationanddeposition

AuNPs (US ResearchNanomaterials Inc., 14 nm) and PEDOT:PSS (SigmaAldrich, 1.3wt%

dispersioninH2O)weredispersedusingasonictip(180s)inMilliporewaterat0.1mg/ml

and2mg/mlrespectively.PANI(SigmaAldrich,Mw>15.000)wasdispersedintolueneusing

a sonic tip (300 s) at 0.2mg/ml. AuNPs and PEDOT:PSSwere deposited via spin coating

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(Laurell,WS-650MZ)at3500RPMfor40s.PANIwasdepositedbyspraycoating1mlat60°C

substratetemperature.

Productionoflayereddevices

EEG and CNT deposition protocols were designed to attain continuous films. With the

previouslymentionedprotocolslayereddeviceswereproducedbyspraycoatingEEGorCNT

as thebottomandtop layers ina typicalconfiguration (3:2)with the interlayermaterials

depositedaspreviouslymentioned.Theaveragethicknesseswas~115nmand~105forEEG

andCNTdevicesrespectively(FigureS8).

AFM-BasedCharacterization

Nanoscale electrical characterization of the samples was conducted using atomic force

microscopy (Asylum Research MFP3D-BIO). The surface potential, capacitance gradient

(𝜕𝐶 𝜕𝑧)andlocalresistanceweremeasuredonthesurfacesamplebymeansofKelvinprobe

forcemicroscopy (KPFM), secondharmonic electrostatic forcemicroscopy (2ndHarmonic

EFM) and conductive atomic forcemicroscopy (C-AFM) respectively. Themeasurements

were conducted using an electrical conductive cantilever (AC240TM-R3) and applying a

voltagesignal(intherangeof0.02Vto0.5V)betweenthetipandthesample.Inadditionto

the electrical measurements the topography of the samples was also measured by the

conventionaltappingmodeofAFM.Alltheimagesweretakenatascanrateof0.5Hzanda

resolutionof512by512pixels.

THMforthemeasurementofthefigureofmeritZT

For THM, currents from 3 to 60 mA were injected across each device with a Keithley

sourcemeter(2450)whilethevoltageresponsewassensedacrossthemusingaTektronix

(TBS1152)oscilloscope(seeFigure1).TheZTfigurewasmeasuredatdifferenttemperatures

byplacingthesampleonaheatingelementcontrolledbyNationalInstrumentselectronics.

Measurementswere taken from room temperature up to 90°C (± 0.1 °C). The electrical

contactsweredepositedbythermalevaporationofsilver(100nm).

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THMallows thedeterminationof the Seebeck voltage,while the temperaturedifference

between the cold and hot side of each device is measured by thermocouples (National

Instruments, k-type). These two measurements are essential to calculate the Seebeck

coefficient. The internal electrical resistance is determined with the maximum voltage

generatedbytheinjectedcurrent.

Authorcontributions

Y.H.andZ-Y.J.conceivedtheexperiments.D.O.,M.H-M.andD.G.fabricatedthedevices.

D.O.andD.Gperformedtheexperiments.A.C.performedtheAFMmeasurements.Allthe

authors discussed the results. Y.H. and D. O. wrote the manuscript and all the authors

contributedtoit.

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