Two Distinct Phases of Bilayer Graphene Films on Ru(0001)

0.79 MB·English

by  Marco Papagno

TwoDistinctPhasesofBilayerGrapheneFilmsonRu(0001)
MarcoPapagno1,2∗,DanielaPacile´1,2,DineshTopwal3,4,PaoloMoras1,PolinaMakarovnaSheverdyaeva1,
FabianDonatNatterer5,AnneLehnert5,StefanoRusponi5,QuentinDubout5,FabianCalleja5,Emmanouil
Frantzeskakis5,Ste´phanePons5,6,JunFujii7,IvanaVobornik7,MarcoGrioni5,CarloCarbone1,andHaraldBrune5
1 IstitutodiStrutturadellaMateria,ConsiglioNazionaledelleRicerche,Trieste,Italy
2 DipartimentodiFisica,Universita´dellaCalabria,87036ArcavacatadiRende(Cs),Italy
3 InternationalCenterforTheoreticalPhysics,Trieste,Italy
3
4 InstituteofPhysics,SachivalayaMarg,Bhubaneshwar,751005,India
1
0 5InstituteofCondensedMatterPhysics(ICMP),EcolePolytechniqueFe´de´raledeLausanne(EPFL),CH-1015Lausanne,Switzerland
2
6 InstitutdesNanoSciencesdeParis(INSP),Universite´PierreetMarieCurie(UPMC),CNRSUMR7588,France
n
a 7 CNR-IOM,TASCLaboratory,Basovizza,Trieste,Italy
J

8

]
l
l
a
ABSTRACT:Bycombiningangle-resolvedphotoemissionspectroscopyandscanningtunnelingmicroscopywereveal
h
-
the structural and electronic properties of multilayer graphene on Ru(0001). We prove that large ethylene exposure
s
e
m allows to synthesize two distinct phases of bilayer graphene with different properties. The first phase has Bernal AB
. stackingwithrespecttothefirstgraphenelayer,displaysweakverticalinteractionandelectrondoping. Thelong-range
t
a
m orderedmoire´ patternmodulatesthecrystalpotentialandinducesreplicasoftheDiracconeandminigaps. Thesecond
- phasehasAAstackingsequencewithrespecttothefirstlayer,displaysweakstructuralandelectronicmodulationand
d
n
p-doping. ThelinearlydispersingDiracstaterevealsthenearly-freestandingcharacterofthisnovelsecondlayerphase.
o
c The peculiar electronic properties of graphene result from its two-dimensional (2D) honeycomb crystal lattice. [1–6] This
[

2Dstructuremakeseverycarbonatomasurfaceatomandthusstronglyexposedtothesurroundingenvironment. Thesimple
1
v
act of resting on a substrate may modify the electronic band structure and cause a precipitous decline of its charge carrier
0
1 mobility. [5, 8–10] For example, when graphene is placed on top of another graphene layer in a Bernal (ABAB) stacking the
6
1 couplingbetweentheplanesmodifiesthelineardispersionofthemasslesschargecarriersintoahyperbolicdispersionofmassive
.
1 chiralfermions.[11,12]Thegraphenelayerscanbepartiallydecoupledbymisorientingthembymorethan20◦ [13]orbyan
0
3 AAstackingsequence.[14,15]Inthesecases,thebandstructurehasbeenpredictedtobethesuperpositionoftwosingle-layer
1
: bandsoffreestandinggrapheneandonlystateswithlineardispersionclosetotheFermilevelinproximitytotheKpointsare
v
i
X present.
r Yet another way to preserve the electronic properties of graphene is to adsorb it on a material that has only very weak
a
interactionwithgraphene.Recently,graphenesupportedonahexagonalboronnitridesubstrate[16]showedelectricalproperties
comparabletothatoffreestandinggraphene. OnSiC(0001)[7,17,18]andonNi[19–21]thefirstgraphenelayersaturatesthe
substratebondsandallowsthesynthesisofsubsequentgraphenelayerswithnearly-freestandingcharacter.
Another surface that strongly modifies graphene properties is Ru(0001). In this case, the first graphene layer relieves the
strain due to the lattice mismatch between graphene and the substrate by developing a (23×23) moire´ structure. [22] Core
levelspectroscopy,[23]scanningtunnelingmicroscopy(STM)[22,24–28]experimentsanddensityfunctionaltheorycalcula-
2
FIG.1:(Coloronline)(a)SecondderivativeoftheARPESintensitymapalongtheΓKdirectionofgrapheneforthelow-exposuresample.π
1
andπ labeltheelectronicstatesofthefirstmonolayerandofthecorrugatedsecondmonolayergraphene,respectively. Thebroadfeature
2C
enclosedbywhitecrossesisascribedtotheπ∗ statestronglymodifiedbythesubstrateinteraction. Duetomarginalelectrondopingofthe
1
corrugatedlayerweobservealsotheπ∗ state. (b)Rawand(c)secondderivativeARPESintensitymapsforthehigh-exposuresample. π
2C 2F
identifiesanewelectronicstateofthesecondmonolayer.(d)Energydistributioncurvesfromthespectraenclosedbytheredrectangleinpanel
(b).∆labelstheelectronicgapbetweentheπ andπ∗ states.
2C 2C
tions[24,27–31]haverevealedspatialinhomogeneitiesinthechargedensityreflectingregionswithvaryinggraphene-substrate
hybridization. Angle-resolvedphotoemissionspectroscopy(ARPES)studiesdisplayed asingleandbroad π bandwitha rela-
tively large gap. [31] The first graphene layer saturates the metal bonds and acts as a template for the growth of a subsequent
graphenelayerwithfreestandingcharacter[32,33]whichdisplaysasharpandlinearlydispersingDiraccone.[34]
Here,weprovideevidenceofanovelsecondlayerphaseonRu(0001),whicharisesafterexposuretomorethan105Lethylene
thatcoexistswiththehithertoreportedphase. WecompareARPESdataofthetwosecondlayerphasesandfindthatthenew
phasegivesrisetoasecondlinearlydispersingπbandwhichisshiftedwithrespecttotheoneofthefirstphaseduetosignificant
p-doping. OurSTMdatarevealthatthetwosecondlayerphaseshavedifferentstackingsequenceandmoire´ amplitude.
3
FIG.2:(Coloronline)(a)Constantenergymapsofgraphenestatesand(b)close-upoftheKpointforthe102LsampleatE−E =0.Panels
F
(c)and(d)displayunderlyingbandsforE −E =-0.3eVand-0.8eV,respectively. (e)Fermisurfaceofgraphenebands(f)andzoom-in
F
aroundtheKpointforthe105 Lsample. Panels(g)and(h)showtheconstantenergymapsmeasuredatE−E =-0.3eVand-0.8eV.Red
F
linesin(a)and(e)markthelimitsofthefirstBrillouinzoneofgraphene.
RESULTSANDDISCUSSION
Figure1(a)displaysthesecondderivativeoftheARPESintensitymapalongtheΓKdirectionofthegrapheneBrillouinzone
after exposure to 102 L ethylene. We reproduce the previously observed relatively broad π and π∗ bands and the sharp π
1 1 2C
and π∗ states. In addition, we identify replica bands associated to the π and σ states. In accordance with literature, [34]
2C 2C
we assign the bands as follows. The monolayer interacts strongly with the underlying substrate and displays a parabolic like
dispersion of the π state [31, 34, 35] (π band). This band has the minimum at E − E =-10.1 eV at the Γ point and the
1 F
maximumatE −E (cid:39)-4eVattheKpoint. Thestronginteractionwiththesubstratealsomodifiestheπ∗ intoadiffuseand
F 1
faint band (enclosed by the white crosses in Figure 1(a)) in proximity to the Fermi level. The first layer saturates the metal
orbitalsandallowsthegrowthofasecondgraphenelayerwhichexhibitsasharpandn-dopedπ Diraccone, withtheDirac
2C
pointat0.45±0.05eVbelowE ,alongwithreplicaandσ bands.[32–35]AswillbecomeclearfromtheSTMdatapresented
F
below,werefertothislayerascorrugatedlayer,thereforethelabelC.
Figures1(b)and1(c)showrawandsecondderivativeARPESmapsrecordedafterexposingthe102Lsurfacetofurther105L
ethylene. Inadditiontothefirstandcorrugatedsecondmonolayerbands,wefindanewπstate,whichwecallπ . Fstandsfor
2F
flatlayer,seebelow. ThemainfeaturesofthisstatearebettercapturedinFigure1(d)whichdisplaysenergydistributioncurves
(EDCs)oftheregionmarkedbytheredrectangleinFigure1(b). Theπ statedispersesupwardparalleltotheπ stateand
2F 2C
4
crosses the Fermi level before the maximum is reached reminiscent of p-doping. By extrapolation of its linear dispersion we
estimatethemaximumofthisbandat0.60±0.05eVabovetheFermilevel. TheconstantdE/dk gradientconfirmsthenearly-
freecharacterofthelayersandyieldsanelectrongroupvelocityofv = (1.0±0.1)×106 m/sforbothbands. Furthermore,
g
EDCspectrahighlightanelectronicgap∆betweentheπ andπ∗ statesof200±30meVattheKpoint(bluecurve). This
2C 2C
gapisalsoobservedinthe102Lsampleatvariancewithearliermeasurements.[34,35]
Figure2presentsconstantenergymapsclosetotheKpointofthegrapheneBrillouinzoneandacquiredonthelowexposure
(Figures2(a)-2(d))andhighexposuresamples(Figures2(e)-2(h)).TheFermisurfaceofthe102LsampledisplaystheDirac-like
electronpocketoftheπ statecenteredattheKpoint, andsurroundedbyhexagonallyarrangedreplicas(Figures2(a)-2(b)).
2C
The excellent long-range order of the surface allows to observe up to the third replica and to accurately measure the spacing
betweenthereplicas,amountingto0.24±0.02A˚−1.TheelectrondensityestimatedfromtherelativesizeofthegrapheneFermi
surfacewithrespecttothesurfaceofBrillouinzoneofgraphiteis0.9×1013cm−2. ForE−E =-0.3eV(Figure2(c)),graphene
F
bandsarepoint-like,andthendisplayholepocketsforE−E <−0.6eV(Figure2(d)).SimilarlytographeneonSiC(0001),[5]
F
thesereplicascouldbeinducedbydiffractioneffectsduetothemoire´ofthefirstgraphenelayer,[34]orasdiscussedlaterinthe
text,tothesuperlatticepotentialofthesecondgraphenelayer.
The Fermi surface of the 105 L sample in Figures 2(e) and 2(f) shows an additional Dirac-like hole pocket due to the π
2F
bandcoexistingwiththebandsofthecorrugatedlayer. Thechargecarrierconcentrationestimatedfromtherelativesizeofthe
π holepocketis2.5×1013 holespercm−2. Withdecreasingenergies(Figures2(g)-2(h))theπ statespreadsoutink-space
2F 2F
withoutaffectingtheπ state.
2C
Itisevidentfromcomparisonoftheconstantenergymapsofbothsamplesthatthefeaturesofthe102Lsampleappearatthe
sameenergiesandpositionsink-spacealsoonthe105 Lsample. Thereforetheadditionalπ featureexhibitedbythe105 L
2F
samplecoexistsbutdoesnotinteractwiththeπ features.Thisisanotabledifferencewithrespecttomultilayergraphenewhere
2C
thenumberofπbandsincreaseswiththenumberoflayers,buttheirdispersiongetsmodifiedbyinterlayercoupling.[5,34]The
absenceofthisinterlayercouplinginthepresentcasestronglysuggeststhecoexistenceoftwosecondlayerphasesratherthan
thecoexistenceofsecondandathirdmonolayer.ThisconjectureisunequivocallyprovenbythefollowingSTMresultsrecorded
onasampleforwhichidenticalARPESresultsastheonesshowninFigures2(e)-(h)havebeenobtained. Figure3(a)showsan
STMimageofthe105 Lsample. TheRusurfaceisentirelycoveredbyawell-orderedgraphenefirstlayer. Thislayerdisplays
the(23×23)moire´pattern[22,23,31,32,34]resultingfromthelatticemismatchbetweenRuandgraphene.[22,26,36]Partof
thefirstlayeriscoveredbysecondlayergraphene. Onlargescaleimagesweclearlydiscerntwotypesofsecondlayerbytheir
differentmoire´ corrugation. Apatchofthesecondlayertypealreadyreportedinliterature[33]isvisibleinthelowerleftpart
oftheimage. Itdisplaysamoire´ patternwithamaximumcorrugationof1.10±0.05A˚.Itisthereforereferredtoascorrugated
(C)layerandcovers12±2%ofthesurface. ThesecondtypeofthesecondlayerisvisibleontherighthandsideofFigure3(a).
Itcoversthe25±5%ofthefirstlayer. Panel(c)showsthatthe2Flayerisessentiallyflatcomparedtothefirstmonolayer. The
STMimageinFigure3(d)showstheweakmoire´ contrastofthe2Flayer. Theapparentheightamplitudesofthemoire´ patterns
ofthefirst,and2Flayersare1.30±0.05,and0.05±0.02A˚,respectively.
The line profile in Figure 3(b) demonstrates that 2C and 2F further distinguish themselves by their apparent heights with
respecttothemonolayerof3.3±0.1A˚ and2.1±0.1A˚,respectively. TheapparentheightofaRu(0001)stepis2.2±0.1A˚,
5
inagreementwiththeliteraturevalueof2.14A˚.Thevaluemeasuredfor2Ccorrespondstothedistanceof3.3A˚ betweenthe
atomiclayersingraphitewhichisBernal,i.e.,ABstacked.[33,37]TheABstackingbreaksthegraphenesublatticesymmetry
whiletheAAstackingdoesnot.Thereforehigh-resolutionSTMdatacandiscernthetwostackings,asevidencedbyFigures3(e)
and (f) with their line profiles. In Figure 3(e) the protrusions mark the centers of the C rings and the two carbon atoms per
6
graphene unit cell are imaged as dark spots; The line profile taken along the blue line and displayed in Figure 3(g) evidences
adifferenceinapparentheightof25±6pmbetweenthesetwoatoms(verticaldottedlineinFigure3(g)). Thisliftingofthe
degeneracybetweentheAandBsublatticestakesplaceinalltheinvestigatedregionsofthecorrugatedlayerandisresponsible
forthebandgap∆betweentheπ andπ∗ statesobservedinFigure1.
2C 2C
Theapparentheightbetweenthefirstandthe2Flayerisconsiderablysmallerthantheinterlayerdistanceingraphite. However,
in a previous STM work an apparent height of 2.5 A˚ has been observed between the first and the second graphene layer on
SiC. [38] High resolution transmission electron microscope studies reported a distance of 2.2 A˚ between two graphene layers
grown on the (111) surface of diamond. [39] This relatively small interlayer distance has been explained by an AA(cid:48) stacking,
wherealternateplanesaretranslatedbyhalfthehexagonalcellwidth.Inourcaseashiftbetweenadjacentlayerscanbeexcluded
sinceadetailedanalysisoftheatomicpositionsoffirstandflatsecondlayershowsthattheyareinregistry.[40]
TheAAstackingbetweenthefirstandthe2Flayerisfurthersupportedbythewithintheerrorbaridenticalapparentheight
ofthetwographenesublatticesreportedinFigure3(f). Catomsareagainimageddark. Theapparentheightdifferencebetween
the two C sublattices in 2F layer has a mean value of 4±7 pm (vertical bar in Figure 3(h)). Therefore the STM data reveal a
sublatticedegeneracyforthe2Fidenticaltofreestandinggraphenewhilethisdegeneracyisclearlybrokeninthe2C.
Wenotethattheapparentheightof2.1±0.1A˚ forthe2Fphase,ismuchsmallerthanthetheoreticalvalueexpectedforAA
stackingof3.66A˚.[41]However, sinceSTMprobesthelocaldensityofstates(LDOS),themeasuredapparentheightcanbe
significantlydifferentfromtheinterlayerdistanceifthelayersaredifferentlydoped. Thoughthespotsizeofthephotonbeam
does not allow us measuring the LDOS of the two bilayers separately, ARPES measurements reveal a different charge carrier
concentrationforthetwobilayersduetoadifferentchargetransfertowardthefirstgraphenelayer.Thisdifferentdopingbetween
thelayersaffectstheSTMmeasurementsandmayresultinthemuchlowerapparentheightofthe2Flayerwithrespecttothe
2Clayer.
Thereasonswhythissecondtypeofsecondgraphenelayerforms,andwhyitneedshigherethyleneexposurestoformcanbe
attributedtothefollowing. ThedifferenceinenergybetweentheABandAAstakingisonly20meV/atom,[39,41]withAB
stackingbeingmorestable. Thisdifferenceissufficientlysmall,suchthattheAAstackingcanbestabilizedbykineticeffects
whicharemoredominantunderCVDgrowthwithhighersupersaturation.
Finally,wediscussthenatureofthereplicabandsobservedfortheπ state. Figure4(a)showthefirstderivativeoftheπ
2C 2C
banddispersionclosetotheKpointforthe102 Lsample. Replicasmayarisebyumklappscatteringfromthemoire´ patternof
thefirstgraphenelayer.[34]Notethatgraphenestatesalsodisplaybreaksinthedispersionatthebandscrossing(highlighted
by red arrows) which reveal energy minigaps, similarly to graphene on Ir. [42–44] Indeed, final state scattering enhances the
photoemissionintensityofthebandsandmayfavortheresolutionofthethirdorderreplicasbyARPES,however,theumklapp
process cannot induce minigaps. These gaps are certainly due to a periodic perturbation to the crystal potential felt by the
electronsintheinitialstate. ThisissupportedbytheSTMmeasurementpresentedinFigure4(b)revealingthatthecorrugated
6
layerexhibitsalong-rangemoire´ patternofperiod∼2.7nm.[40]
CONCLUSION
In summary, we show evidence of a new phase of bilayer graphene on the Ru(0001) surface. Following massive ethylene
exposure, ARPES spectra reveal a new π state coexisting with the known ones which is reminiscent of a different interaction
with the first graphene layer. STM investigations show that the Ru surface is fully covered by the first graphene layer which
producesastronglyinteractingπ band. Ontopofthislayer,wefindtwophasesofsecondgraphenelayer. Oneofthetwo(2C)
1
exhibits Bernal stacking with respect to the first layer. This layer displays weak vertical interaction and a moire´ pattern with
marginallong-rangeorderwhichmodulatesthecrystalpotential. Thislayergeneratesasharpn-dopedDiraccone(π ),replica
2C
bandsandminigaps. Theotherphase(2F)exhibitsasingleDiracp-doped(π )statewithnearlyfreestandingcharacter. STM
2F
resultsshowthatthissecondlayerphaseisarrangedinanAAstackingsequencewithrespecttothefirstlayerandhasarelatively
weakstructuralandelectronicmodulation. Wethusprovethatthefirstlayercanbeusedasatemplatetogrowdifferentphases
ofgraphenebilayers.
METHODS
Measurementswereperformedinthreedifferentultrahighvacuum(UHV)chambers. ThepresentedARPESmeasurements
wereperformedattheVUV-PhotoemissionbeamlineoftheElettraSynchrotronFacilityinTriesteat120K,withaphotonenergy
of 100 eV, and an energy resolution of 30 meV. The presented STM experiments were carried out at the Ecole Polytechnique
Fe´de´raleinLausanne(EPFL)withahome-builtlowtemperaturescanningtunnelingmicroscopeoperatingat0.4Kandusedfor
thepresentstudyat4K.[45]AdditionalARPESandSTMmeasurementswerecollectedattheAPE-IOMbeamlineofElettra.
This beamline has the STM and ARPES spectrometer connected in situ, so that complementary characterization of the same
surfacescanbeperformed. Atthisendstation,theprimaryenergyhasbeensetto65eVwhereastheotherparametershavebeen
keptidentical,andthehome-builtSTMhasbeenoperatedat300K.AllexperimentswereperformedunderUHVconditionwith
abasepressurebelow1×10−10Torr.TheRu(0001)crystalwascleanedbyrepeatedcyclesofAr+sputtering(E =1200eV)and
annealingat1500K.Theorderandcleannessofthesamplewasmonitoredbylow-energyelectrondiffractionandphotoemission
spectroscopyatVUVandbySTMmeasurementsatEPFLandAPE.AgraphenebilayerwasgrownbyexposingtheRu(0001)
surfaceheldatatemperatureof1600Kto102 L(1.9×10−6 Torrfor53s)ofethylene(C H ).[46]Themultiphasegraphene
2 4
bilayers were obtained by additional exposure to 105 L (1.9×10−4 Torr for 530 s) ethylene with the Ru crystal at 1600 K.
Samples that have been prepared and characterized at Elettra have been transported to Lausanne and imaged in STM with
atomicresolutionafteraflashto800Kinapressureof1×10−10Torr.
Acknowledgment.ThisworkhasbeensupportedbytheEuropeanScienceFoundation(ESF)undertheEUROCORESProgram
EuroGRAPHENE,bytheSwissNationalScienceFoundation,andbytheItalianMinistryofEducation,University,andResearch
(FIRBFuturoinRicerca,PLASMOGRAPHProject).
∗e-mail: marco.papagno@fis.unical.it
7
REFERENCESANDNOTES
[1] Berger,C;Song,Z.;Li,X.;Wu,X.;Brown,N.;Naud,C.;Mayou,D.;Li,T.;Hass,J.;Marchenkov,A.N.;etal.ElectronConfinement
andCoherenceinPatternedEpitaxialGraphene.Science2006,312,1191-1196.
[2] Novoselov,K.S.;Geim,A.K.;Morosov,S.V.;Jiang,D.;Zhang,Y.;Dubonos,S.V.;Grigorieva,I.V.;Firsov,A.A.ElectricFieldEffect
inAtomicallyThinCarbonFilms.Science2004,306,666-669.
[3] Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A. Two-
dimensionalGasofMasslessDiracFermionsinGraphene.Nature2005,438,197-200.
[4] Zhang, Y.; Tan, Y.W.; Stormer, H.L.; Kim, P.ExperimentalObservationoftheQuantumHallEffectandBerrysPhaseinGraphene.
Nature2005,438,201-204.
[5] Ohta,T.;Bostwick,A.;Seyller,T.;Horn,K.;Rotenber,E.ControllingtheElectronicStructureofBilayerGraphene.Science2006,313,
951-954.
[6] Novoselov,K.S.;Jiang,Z.;Zhang,Y.;Morozov,S.V.;Stormer,H.L.;Zeitler,U.;Maan,J.C.;Boebinger,G.S.;Kim,P.;Geim,A.K.
Room-TemperatureQuantumHallEffectinGraphene.Science2007,315,1379-1379.
[7] Bostwick,A.;Ohta,T.;Seyller,T.;Horn,K.;Rotenberg,E.QuasiparticleDynamicsinGraphene.Nat.Phys.2007,3,36-40.
[8] Oostinga,J.B.;Heersche,H.B.;Liu,X.;Morpurgo,A.F.;Vandersypen,L.M.K.Gate-InducedInsulatingStateinBilayerGraphene
Devices.NatureMat.2007,7,151-157.
[9] Feldman,B.E.;Martin,J.;Yacobi,A.Broken-SymmetryStatesandDivergentResistanceinSuspendedBilayerGraphene.NaturePhys.
2009,5,889-893.
[10] Rutter,G.M.; Jung,S.; Klimov,N.N.; Newell,D.B.; Zhitenev,N.B.; Stroscio,J.A.MicroscopicPolarizationinBilayerGraphene.
NaturePhys.2011,7,649-655.
[11] Geim,A.K.;Novoselov,K.S.TheRiseofGraphene.Nat.Mater.2007,6,183-191.
[12] McCann,E.AsymmetryGapintheElectronicBandStructureofBilayerGraphene.Phys.Rev.B2006,74,161403.
[13] Luican,A.;Li,G.;Reina,A.;Kong,J.;Nair,R.R.;Novoselov,K.S.;Geim,A.K.;Andrei,E.Y.Single-layerBehaviorandItsBreakdown
inTwistedGrapheneLayers.Phys.Rev.Lett.2011,106,126802.
[14] Mak,K.F.;Shan,J.;Heinz,T.F.ElectronicStructureofFew-LayerGraphene: ExperimentalDemonstrationofStrongDependenceon
StackingSequence.Phys.Rev.Lett.2010,104,176404.
[15] Lobato,I.;Partoens,B.MultipleDiracParticlesinAA-StackedGraphiteandMultilayersofGraphene.Phys.Rev.B2011,83,165429.
[16] Dean,C.R.;Young,A.F.;Meric,I.;Lee,C.;Wang,L.;Sorgenfrei,S.;Watanabe,K.;Taniguchi,T.;Kim,P.;Shepard,K.L.;etal.Boron
NitrideSubstratesforHigh-QualityGrapheneElectronics.NatureNanotech.2010,5,722-726.
[17] Ouerghi, A.; Marangolo, M.; Belkhou, R.; ElMoussaoui, S.; Silly, M.G.; Eddrief, M.; Largeau, L.; Portail, M.; Fain, B.; Sirotti, F.
EpitaxialGrapheneon3C-SiC(111)Pseudosubstrate:StructuralandElectronicProperties.Phys.Rev.B2010,82,125445.
[18] Hass,J.;deHeer,W.A.;Conrad,E.H.TheGrowthandMorphologyofEpitaxialMultilayerGraphene.J.Phys.:Condens.Matter2008,
20,323202.
[19] Varykhalov, A.; Sa´nchez-Barriga, J.; Shikin, A. M.; Biswas, C.; Vescovo, E.; Rybkin, A.; Marchenko, D.; Rader, O. Electronic and
MagneticPropertiesofQuasifreestandingGrapheneonNi.Phys.Rev.Lett.2008,101,157601.
[20] Yu, Q. K.; Lian, J.; Siriponglert, S.; Li, H.; Chen, Y. P.; Pei, S. S. Electrical Transport Properties of Wafer-Fused p-GaAs/n-GaN
Heterojunctions.App.Phys.Lett.2008,93,112103.
8
[21] Reina,A.;Jia,X.T.;Ho,J.;Nezich,D.;Son,H.B.;Bulovic,V.;Dresselhaus,M.S.;Kong,J.LargeArea,Few-LayerGrapheneFilms
onArbitrarySubstratesbyChemicalVaporDeposition.NanoLett.2009,9,30-35.
[22] Martoccia,D.; Willmott,P.R.; Brugger,T.; Bjo¨rck,M.; Gu¨nther,S.; Schlepu¨tz,C.M.; Cervellino,A.; Pauli,S.A.; Patterson,B.D.;
Marchini,S.;etal.GrapheneonRu(0001):a25×25Supercell.Phys.Rev.Lett.2008,101,126102.
[23] Preobrajenski,A.B.;Ng,M.L.;Vinogradov,A.S.;Ma˚rtensson,N.ControllingGrapheneCorrugationonLattice-MismatchedSubstrates.
Phys.Rev.B2008,78,073401.
[24] Wang,B.;Bocquet,M.L.;Marchini,S.;Gu¨nther,S.;Wintterlin,J.ChemicalOriginofaGrapheneMoire´OverlayeronRu(0001).Phys.
Chem.Chem.Phys.2008,10,3530-3534.
[25] Marchini,S.;Gu¨nther,S.;Wintterlin,J.ScanningTunnelingMicroscopyofGrapheneonRu(0001).Phys.Rev.B2007,76,075429.
[26] Moritz,W.;Wang,B.;Bocquet,M.-L.;Brugger,T.;Greber,T.;Wintterlin,J.;Gu¨nther,S.StructureDeterminationoftheCoincidence
PhaseofGrapheneonRu(0001).Phys.Rev.Lett.2010,104,136102.
[27] Stradi,D.;Barja,S.;D´ıaz,C.;Garnica,M.;Borca,B.;Hinarejos,J.J.;Sa´nchez-Portal,D.;Alcam´ı,M.;Arnau,A.;Va´zquezdeParga,A.
L.;etal.F.RoleofDispersionForcesintheStructureofGrapheneMonolayersonRuSurfaces.Phys.Rev.Lett.2011,106,186102.
[28] Feng, W.; Lei, S.; Li, Q.; Zhao, A. Periodically ModulatedElectronic Properties of the Epitaxial Monolayer Graphene on Ru(0001).
JPCC2011,115,24858-24864
[29] Iannuzzi,M.; Hutter,J.ComparativeStudyoftheNatureofChemicalBondingofCorrugatedGrapheneonRu(0001)andRh(111)by
ElectronicStructureCalculations.Surf.Sci.2011,605,1360-1368.
[30] Jiang,D.;Du,M.-H.;Dai,S.FirstPrinciplesStudyoftheGraphene/Ru(0001)Interface.J.Chem.Phys.2009,130,074705.
[31] Brugger, T.; Gu¨nther, S.; Wang, B.; Dil, J. H.; Bocquet, M.-L.; Osterwalder, J.; Wintterlin, J.; Greber, T. Comparison of Electronic
StructureandTemplateFunctionofSingle-layerGrapheneandaHexagonalBoronNitrideNanomeshonRu(0001).Phys.Rev.B2009,
79,045407.
[32] Sutter,P.W.;Flege,J.I.;Sutter,E.A.EpitaxialGrapheneonRuthenium.Nat.Mat.2008,7,406-411.
[33] Sutter,E.;Acharya,D.P.;Sadowski,J.T.;Sutter,P.ScanningTunnelingMicroscopyonEpitaxialBilayerGrapheneonRuthenium(0001).
App.Phys.Lett.2009,94,133101.
[34] Sutter, P.; Hybertsen, M.S.; Sadowski, J.T.; Sutter, E.ElectronicStructureofFew-LayerEpitaxialGrapheneonRu(0001).NanoLett.
2009,9,2654-2660.
[35] Enderlein, C.; Kim, Y. S.; Bostwick, A.; Rotenberg, E.; Horn, K. The Formation of an Energy Gap in Graphene on Ruthenium by
ControllingtheInterface.NewJ.Phys.2010,12,033014.
[36] Va´zquezdeParga,A.L.;Calleja,F.;Borca,B.;Passeggi,M.C.G.;Jr.;Hinarejos,J.J.;Guinea,F.;Miranda,R.PeriodicallyRippled
Graphene:GrowthandSpatiallyResolvedElectronicStructure.Phys.Rev.Lett.2008,100,056807.
[37] Sun, J.; Hannon, J. B.; Tromp, R. M.; Johari, P.; Bol, A. A.; Shenoy, V. B.; Pohk, K. Spatially-Resolved Structure and Electronic
PropertiesofGrapheneonPolycrystallineNi.ACSNano2010,4,7073-7077.
[38] Rutter,G.M.;Crain,J.N.;Jarvis,E.A.A.;Stiles,M.D.;Li,T.;First,P.N.;Stroscio,J.A.ImagingtheInterfaceofEpitaxialGraphene
withSiliconCarbideviaScanningTunnelingMicroscopy.Phys.Rev.B2007,76,235416.
[39] Lee,J.-K;Lee,S.-C;Ahn,J.-P.;Kim,S.-C.;Wilson,J.I.B.;John,P.TheGrowthofAAGraphiteon(111)Diamond.J.Chem.Phys.
2008,129,234709.
[40] Calleja,F.;Dubout,Q.;Sclauzero,G.;Pasquarello,A.;Etzkorn,M.;Lehnert,A.;Claude,L.;Gambardella,P.;Rusponi,S.;Brune,H.to
bepublished
[41] Charlier, J.-C.; Gonze, X.; Michenaud, J.-P. First-Principles Study of the Stacking Effect on the Electronic Properties of Graphite(s).
Carbon1994,32,289-299.
9
[42] Pletikosic´,I.;Kralj,M.;Pervan,P.;Brako,R.;Coraux,J.;N(cid:48)Diaye,A.T.;Busse,C.;Michely,T.DiracConesandMinigapsforGraphene
onIr(111).Phys.Rev.Lett.2009,102,056808.
[43] Rusponi,S.;Papagno,M.;Moras,P.;Vlaic,S.;Etzkorn,M.;Sheverdyaeva,P.M.;Pacile´,D.;Brune,H.;Carbone,C.HighlyAnisotropic
DiracConesinEpitaxialGrapheneModulatedbyanIslandSuperlattice.Phys.Rev.Lett.2010,105,246803.
[44] Papagno,M.;Rusponi,S.;Sheverdyaeva,P.M.;Vlaic,S.;Etzkorn,M.;Pacile´,D.;Moras,P.;Carbone,C.;Brune,H.LargeBandGap
OpeningbetweenGrapheneDiracConesInducedbyNaAdsorptionontoanIrSuperlattice.ACSNano2012,6,199-204.
[45] Claude L. Construction d’un Microscope a´ Effet Tunnel a´ Basse Tempe´rature et E´tudes d’Impurete´s Magne´tiques en Surfaces. Ph.D
ThesisNo3276,EcolePolytechniqueFe´de´raledeLausanne2005.
[46] Natterer,F.D.;Rusponi,S.;Papagno,M.;Carbone,C.;Brune,H.OptimizingLong-RangeOrder,BandGap,andGroupVelocitiesfor
GrapheneonClose-PackedMetalSurfaces..J.Phys.:Condens.Matter2012,24,314203.
10
FIG.3: (Coloronline)(a)ConstantcurrentSTMimageofgrapheneonRu(0001)(ethyleneexposure105L,V =-3V;I =75pA;T=4K;size
t t
2320A˚×283A˚).1,2C,and2F,standforfirstlayer,andsecondcorrugatedandflatlayer,respectively.(b)Apparentheightprofilealongthered
linedisplayedin(a).(c)ConstantcurrentSTMimageofasampleregionwithcoexistingfirstgraphenelayerand2Flayer(V =-3V;I =75pA;
t t
T=4K;size762A˚×762A˚).(d)STMimageofthe2Fisland(V =-1V;I =100pA;T=4K;size=391A˚×391A˚)showingaveryweakmoire´
t t
pattern. (e)High-resolutionSTMimagefor2C(V =-0.1V;I =200pA;T=4K;size25A˚ ×25A˚)and(f)2F(V =-0.V;I =100pA;T=4K;
t t t t
size25A˚ ×25A˚).NotethattheSTMtipgiveswithrespecttotheatomicgeometryareversedcontrastwithCatomsimagedasdarkspots.
Redhexagonsmarkthegraphenehoneycomblattice. (g)and(h)Apparentheightprofilesalongtheblueandgreenlinesdisplayedin(e)and
(f),respectively. Dottedandcontinuousverticalbarsin(g)and(h)highlightthedifferenceintheapparentheightbetweencarbonatomswith
differentsublatticesymmetryforthe2Cand2Flayers,respectively.