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Received5Aug2015|Accepted10Nov2015|Published14Dec2015
DOI:10.1038/ncomms10177
OPEN
Ultra-narrowmetallicarmchairgraphenenanoribbons
AminaKimouche1,MikkoM.Ervasti2,RobertDrost1,SimoHalonen3,AriHarju2,PekkaM.Joensuu3,JaniSainio1&PeterLiljeroth1
Graphenenanoribbons(GNRs)—narrowstripesofgraphene—haveemergedaspromisingbuildingblocksfornanoelectronicdevices.Recentadvancesinbottom-upsynthesishaveallowedproductionofatomicallywell-de?nedarmchairGNRswithdifferentwidthsanddoping.WhileallexperimentallystudiedGNRshaveexhibitedwidebandgaps,theorypredictsthateverythirdarmchairGNR(widthsofN?3mt2,wheremisaninteger)shouldbenearlymetallicwithaverysmallbandgap.Here,wesynthesizethenarrowestpossibleGNRbelongingtothisfamily(?vecarbonatomswide,N?5).WestudytheevolutionoftheelectronicbandgapandorbitalstructureofGNRsegmentsasafunctionoftheirlengthusinglow-temperaturescanningtunnellingmicroscopyanddensity-functionaltheorycalculations.AlreadyGNRswithlengthsof5nmreachalmostmetallicbehaviourwithB100meVbandgap.Finally,weshowthatdefects(kinks)intheGNRsdonotstronglymodifytheirelectronicstructure.
ofAppliedPhysics,AaltoUniversitySchoolofScience,POBox15100,Aalto00076,Finland.2COMPCentreofExcellence,Departmentof
AppliedPhysics,AaltoUniversitySchoolofScience,POBox11100,Aalto00076,Finland.3DepartmentofChemistry,AaltoUniversitySchoolofChemicalTechnology,POBox16100,Aalto00076,Finland.CorrespondenceandrequestsformaterialsshouldbeaddressedtoP.L.(email:peter.liljeroth@aalto.?).
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raphenenanoribbons(GNRs)areanewclassofmaterialsthathavepromisingapplicationsinnext-generationnanoelectronicandoptoelectronicdevices1–3.These
systemshavebeenthoroughlystudiedtheoreticallyatvariouslevelsofsophistication4–9.Accordingtothesestudies,theelectronicandmagneticpropertiescanbetunedbythenanoribbonchemicalstructureandedgegeometry,rangingfromsemiconductors7,8tospin-polarizedhalf-metals6.Zigzagedgestructureispredictedtoresultinspin-polarizededgestates,withpotentialapplicationsinspintronics4,6,10.Ontheotherhand,inarmchairGNRs,quantumcon?nementopensabandgapthatsensitivelydependsontheribbonwidth,allowinginprincipleforthedesignofGNR-basedstructureswithtuneableproperties.Thearmchairribbonscanbegroupedintothreefamilies,thatis,N?3p,N?3pt1andN?3pt2,wherepisanintegerandNthenumberofcarbonatomsalongtheGNRwidth4,11.N?3pandN?3pt1familieshavewidebandgapsthatscaleinverselywiththeribbonwidth.Incontrast,simplemodelspredictthefamilyN?3pt2tobemetallicwithzerobandgap4,12.Moresophisticatedmodelstakingintoaccount,forexample,edgerelaxationpredictnon-vanishingbutsmallbandgaps8,13.ThesenarrowarmchairGNRswithnarroworvanishingbandgapswouldformidealmolecularwirestobeusedasinterconnectsinmolecularscalecircuitry.However,thisrequiresatomic-levelcontroloftheedgestructure,whichisfarbeyondtheexistingtop–downapproachessuchaselectronbeamlithography.Recently,tremendousprogresshasbeenmadeinbottom-upchemicalsynthesisofGNRsfrommolecularprecursors14–23.Thesynthesisoftheseribbonsproceedsintwothermallyactivatedsteps14.Firststepisthecleavageofhalogensfromtheprecursorsandtheformationofacovalentlycoupledpolymerthroughradicaladditionreaction.Thelinearchainisconvertedintographeneinthesecondstepthatinvolvescyclodehydrogenationatahighertemperature.Bychangingthemonomerdesign,thefabricationofawiderangeofGNRsincludingdifferentwidthsanddopingcanbeachieved.
DespiteseveralkindsofarmchairGNRshavingbeensynthesizedviaspeci?cmolecularprecursors2,16–19,21–26,thestudiedwidthshavebeenratherlimited.MostofthembelongtothewidebandgapN?3pt1familyandnarrowbandgap,near-metallicbehaviourhasnotbeenveri?edexperimentally.WefocusontheN?3pt2familyandtargetthenarrowestmemberofthisfamilywithN?5.Thesynthesisstartswithadibromoperylenemolecule,whichundergoesdehalogenationandcyclodehydrogenationstepstoyieldatomicallyperfectN?5armchairGNRs.Thiswidthispredictedtobemetallicwithinanearest-neighbourtight-bindingmodel4,7.Morerealisticcalculationspredictthepresenceofabandgap,butitshouldremainmuchsmallerthanthatfoundinarmchairGNRsoftheotherfamilies7.Wecon?rmthesetheoreticalpredictionsandmeasureexperimentallyaB100meVbandgapinlongGNRsusinglow-temperaturescanningtunnellingmicroscopy(STM).Thisnear-metallicregimeisalreadyreachedinGNRsofsixperylenemonomerunits,thatis,withlengthslongerthan5nm.Inaddition,westudytheeffectoftheribbonlengthontheelectronicstructureandmapoutthelocaldensityofstates(LDOS)inrealspaceusingscanningtunnellingspectroscopy.Theexperimentalresultsarecorroboratedbydensity-functionaltheory(DFT)calculationsthatareusedtoidentifythe?ngerprintsofmolecularorbitalsasafunctionoftheGNRlength.Finally,weshowthatdefects(kinks)intheGNRsdonotdestroythenear-metallicbehaviour.OurresultsdemonstratethatthisarmchairGNRsubfamilycanbeusedasmolecularwiresthatshouldexhibitmetallicbehaviouratroomtemperature.ThissuggestthattheseGNRswouldformidealinterconnectsinmolecularscaleelectroniccircuitry.
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Results
GrowthofN?5armchairGNRs.WegrewarmchairGNRsinultrahighvacuum(UHV)usingtheon-surfacepolymerizationwithdibromoperyleneC20H10Br2(DBP)asthemolecularpre-cursor(Fig.1a)23.Thisprocessyieldsverynarrow5-GNRswitharmchairedgestructure.DetailsofthesamplepreparationcanbefoundintheMethodssection.TheresultingGNRswerecharacterizedbylow-temperatureSTMatT?5K.WeusedamixtureofprecursormonomerswitheitherparallelorantiparallelpositionsofBratoms(3,9-DBPor3,10-DBP).RegardlessofthepositionsoftheBratoms,fullyconjugatedribbonsareformed.AtypicaloverviewSTMscanisshowninFig.1b.TheseribbonsalignandassemblealongorperpendiculartothedirectionoftheherringbonereconstructionoftheAu(111)substrate.MostribbonsareisolatedandeasilymanipulatedalongtheribbonaxisduetotheweakinteractionwiththeunderlyingAu(111)substrate.WhilemostoftheGNRsarestraight,thereisasigni?cantnumberofnon-straightGNRs.Thesekinkedribbonsareformedbytwo(ormore)straightsegmentsconnectingduringtheinitialpolymerizationstepwithanangleof30°.Thestructureandelectronicpropertiesofthesekinkedribbonswillbediscussedindetaillater.StatisticsontheribbonlengthsandthefractionofkinkedribbonsaregiveninSupplementaryFig.1.Figure1cshowsahigh-resolutionSTMimagewithGNRswithseveraldifferentlengths.Theshortestribbonsweobserveconsistoftwomonomers,amediumlengthribbonof5-monomerunitsishighlightedwithanoverlaidmodelstructure.Wecaneasilydeterminethenumberofmonomerunitsbycountingthenumberofbenzenerings(lobes)alongthearmchairedgesoftheGNRs.EvolutionofGNRorbitalsasafunctionoflength.Wehaveperformeddetaileddifferentialconductance(dI/dV)spectroscopy
a
Br
Br(Br)
b
Polymerization at 200 °CCyclodehydrogenation at 320 °C
c
Figure1|Bottom-upsynthesisofN?5armchairGNRs.(a)ReactionschemeofthepolymerizationoftheDBPprecursortoatomicallywellde?nedN?5armchairGNRs.(b)OverviewSTMimageafter
cyclodehydrogenationat320°C,showingstraightandkinkedGNRs
(V?500mV,I?50pA;scalebar,10nm).(c)Zoomed-inSTMtopographyofdifferentribbonlengths(V?300mV,I?50pA;scalebar,2nm).
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calculatedconductancemapsareshowninFig.2d,f.Theoccupiedstatesarefoundatà550and26mVreferredasHOMO-1andHOMO,respectively.TheorbitalcorrespondingtotheHOMOofanisolatedribbonisfoundatpositivebias,thatis,theribbonhasbecomepositivelychargedontheAu(111)substrate.We?ndthistransitiontooccurbetween4-and5-monomerlongribbons.Itisconsistentwiththeknownp-dopingofbulkgrapheneonagoldsubstrate27,28.Theunoccupiedstatesfoundat250mV,850mV,1.25Vand1.57VarereferredtoasLUMO,LUMOt1,LUMOt2andLUMOt3,respectively.Theseunoccupiedstatesfollowtheenergiesexpectedforone-dimensionalparticle-in-a-boxstatesofmasslessDiracfermions.Inadditiontotheseresonancescorrespondingtosinglemolecularorbitals,therearetwofurtherpeaks(peaks7and8inFig.2b)intheexperimentaldI/dVspectra.AccordingtotheDFTcalculations,thesecorrespondtoseveralcloselyspacedorbitals(SupplementaryFig.2).ThisisfurthercorroboratedbysimulatedLDOSmapsthatnicelyreproducethespatialfeaturesoftheexperimentalmaps.
Inadditiontoquantumcon?nement,whichmakestheelectronicstructureof?nitegrapheneribbonsverysensitivetothewidth,?nite-sizeeffectshavetobetakenintoaccounttodeterminethescalingoftheHOMO–LUMOenergygapwiththelengthoftheribbon.Here,wewillexaminehowthe?nite-sizeeffectsaffecttheenergiesofmolecularlevelsinthetransitionfromshorttolongribbons.ExperimentalorbitalenergieswithrespecttoFermilevelextractedfromdI/dVspectraaredepictedinFig.3afordifferentribbonlengths.Ascanbeseen,thegapbetweenthehighestoccupiedandthelowestunoccupiedmolecularorbitalsclosesquicklywithincreasingribbonlength.Asindicatedearlier,theHOMOlevelcrossestheFermilevelbetween4-and5-monomerlongribbons,thatis,theribbonbecomespositivelycharged(thisisillustratedinSupplementaryFig.3).Themiddleofthegapextrapolatesto0.20±0.02eV
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5468experimentsasafunctionoftheribbonlengthtoprobethecorrespondingchangesintheirelectronicstructure.Inthefollowing,wewillusetworibbonlengths(3and5monomerunits)tohighlighthowtheribbonelectronicstateschangewithlength.Acharacteristicspectrum(redcurve,Fig.2a)recordedclosetotheedgeofa3-monomerGNRexhibitsapronouncedshoulderatà1.02Vandaprominentpeakatà160mV(labelledstates1and2,respectively).ThesestatescorrespondtothehighestoccupiedmolecularorbitalsHOMO-1(state1)andHOMO(state2).Atpositivebias,weprobethelowestunoccu-piedstates.We?ndstatesat540mV(state3)and1.8V(state4),whichcorrespondtothelowestandsecondlowestunoccupiedmolecularorbitals(LUMOandLUMOt1).TheadditionalfeatureobservedinthedI/dVspectrumaround1.2VdidnotshowanycontrastinthedI/dVmapanditoriginatesfromthebackgroundsignal(blackline).
WeprobethesymmetriesofthedifferentmolecularorbitalsbymappingthespatiallyresolveddI/dVsignalattheenergiescorrespondingtotheresonances(Fig.2c).TheexperimentalmapsshowthatboththeHOMOandLUMOareextendedthroughtheribbon,incontrasttothelocalizedendstatesobservedonwiderarmchairGNRs2,19.TheLUMOt1hasonemorenodalplanecomparedwiththeLUMOasexpected.ThecorrespondingcalculatedLDOSmaps(Fig.2e)areinexcellentagreementwiththeexperimentalresults,clearlyreproducingtheoccupiedandunoccupiedstates.TheunoccupiedorbitalsyieldbroaderresonancesinthedI/dVspectraandtheirspatialmapsarenotaswellresolvedasfortheoccupiedstates.Thisislikelytobecausedbyshorterlifetimeofthetunnellingelectronsonthesestatesleadingtoincreasedlifetimebroadening.Nevertheless,theorbitalscanbeidenti?edbytheconductancemapping.
Figure2bshowsthedI/dVspectraacquiredatdifferentlocationsalonga5-monomerGNR.Thecorrespondingexperimentaland
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43dl/dV (a.u.)dl/dV (a.u.)12–1.0
0.01.0Bias (V)
2.0–1.00.0
1.0Bias (V)
2.0
cd
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Figure2|dI/dVspectroscopyandreal-spaceimagingoftheGNRwavefunctions.(a,b)dI/dVspectraacquiredonthree(a)and?ve(b)monomerGNR
withametallictip.LocationofthespectraaremarkedintheSTMtopographiesintheinsetsandtheblackcurveismeasuredonAu(111).Theredandbluecurvesareshiftedforclarity.(c-f)Experimentalconstant-heightdI/dVmaps(c,d)andthecorrespondingcalculatedLDOSmaps(e,f)forthree(c,e)and?ve(d,f)monomerGNRsatbiasvoltagescorrespondingtothepositionsmarkedwitharrowsina,b.
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versusgoldFermilevel,inagreementwiththedopingofbulkgrapheneonAu(111)27,28.Thereisnosigni?cantabruptshiftoftheorbitalenergiesduetothechargingoftheribbons,whichindicatesthatthechargingenergyissmallandcanbeneglected29.ThisalsoimpliesthattheSTMtransportgapisnearlyequaltothesingle-particleHOMO–LUMOgap.
WeplottheHOMO–LUMOgapasafunctionofthelengthoftheribboninFig.3balongwithvaluesfromtheDFTcalculationsonisolatedribbons.TheHOMO–LUMOgapdecreasesveryquicklywithincreasingribbonlength(scalingroughlyas1/L2)andisbelow200meValreadyfor5-monomerribbons(lengthofca.4nm).Itsaturatestovalueof0.10±0.02eVcorrespondingtoasmallbut?niteenergygapinlongribbons.ThespacingbetweentheHOMOandHOMO-1alsodecreases,butwithadifferentscaling(roughly1/L).Thisscalingindicatesthatthesestatescanbeviewedasquantumcon?nedlevelsofparticleswithlineardispersionasexpectedingraphene.TheHOMO-1–HOMOgapextrapolatesto0asonewouldexpectforthelevelspacingwithinasubbandinin?niteribbons.TheseexperimentalobservationsarewellreproducedbyDFTcalculations.
DetailedcomparisonbetweentheoryandexperimentshowsthatDFToverestimatestheexperimentalHOMO–LUMOgap.Thisislikelytobecausedbytheomissionofthesubstrate,theresultingchargingoftheribbonandthestrongmetallicscreening.Furthermore,theneutralisolatedribbonsarepredictedtohavesingletedgestatesforlengthsZ7monomerunits(SupplementaryFig.4andSupplementaryMethods).ThenarrowbandgappredictedearlierbyDFT7matchesverywellwithourmeasuredandcalculatedgaps.ThesevaluesdifferfromthosereportedrecentlybyZhangetal.18,whoobtainedabandgapof2.8eV.Theyusedanotherprecursormolecule(tetrabromonaphthalene),whichalsoresultsinN?5armchairGNRs.Comparisonwithourresultssuggeststhattheyhave
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identi?edthemostprominentfeaturesinthespectraastheconductionandvalencebandedges(seeforexamplethepeakataround2.0VinFig.2b).However,ourexperimentsshowthatthereareother,lowerlyingorbitalsandtheactualHOMO–LUMOgaponAu(111)reachesvalueofB100meVinlongribbons.Finally,themany-bodyphenomenapredictedforwiderarmchairribbons30,31orforzigzagribbons32–34arenotexpectedforsuchashortsegmentofzigzagedge.
KinkedGNRsformedbyconnectedstraightsegments.InadditiontostraightGNRs,alsokinkedribbonsareformed(seeFig.1a).Theseribbonstypicallyconsistoftwostraightsegmentsconnectedatanangleof150°correspondingtoarotationof30°(Fig.4a).Grainboundariesingrapheneareknowntoaccommodatecarbonpentagonandheptagondefects35,36.Consideringhowtwonanoribbonsmightconnect,thecreationofapentagondoesnotrequireaddingorremovingcarbonatomsfromtwostraightribbons.Inaddition,DFTcalculationsonGNRswithpentagondefectsreproducetheexperimentalresultsbetterthanstructureswithheptagons(seebelowandSupplementaryFig.5).
WehavemeasureddI/dVspectraaswellconductancemapsonkinkedribbonsandFig.4showsaribbonwhere4-monomerand5-monomerlongstraightsegmentsarejoined.Figure4bshowsadI/dVspectrumacquiredatthekinkexhibitingpeaksatà120and100mVandapronouncedshoulderat300mV.TheseenergiescorrespondtotheHOMOà1,HOMOandLUMO,respectively.SimulateddI/dVmapsforapentagonconnection(HOMOà1,HOMOandLUMOshowninFig.4d)matchtheexperimentalresultsreasonably.Theenergygapofthekinkedribbonisca.200meV,whichisclosetoastraight9-monomerribbonandsmallerthanexpectedforfour-or?ve-monomerlongGNRs.Inaddition,themolecularorbitalsremaindelocalizedover
a
Energy shift (eV)1.00.50.0–0.5–1.0–1.5
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HOMO-2HOMO-1HOMOLUMO
b
1.2ΔE (eV)0.80.40.0
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(LUMO-HOMO)EXP(LUMO-HOMO)DFT(HOMO-HOMO-1)EXP(HOMO-HOMO-1)DFT
681012Length (monomer unit)
Figure3|Energiesofmolecularorbitalsasfunctionofribbonlength.(a)Energiesofthedifferentmolecularorbitals(HOMO-2toLUMO)asafunctionoftheribbonlength.(b)Comparisonbetweenexperimentalandcalculatedenergygapsasafunctionoftheribbonlength.
acd
1b
dl/dV (a.u.)123KinkAu2–0.4–0.2
0.00.2Bias (V)
0.40.63Figure4|ElectronicstructureofakinkedGNRwithfour-and?ve-monomersegments.(a)STMtopographywithanoverlaidmodelshowsthe
connectiononthekinkbyapentagon.(b)dI/dVspectrumacquiredatthekink(redline),anAu(111)spectrum(blackline)isprovidedforcomparison.(c)Constant-heightdI/dVmapsatenergieslabelledbyarrowsinb.(d)CorrespondingsimulatedLDOSmaps.
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