Large thermoelectric figure of merit in graphene layered ...
Transcript of Large thermoelectric figure of merit in graphene layered ...
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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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