Raman Studies of Monolayer Graphene The Substrate Effect

.C2008,112,10637–1064010637

RamanStudiesofMonolayerGraphene:TheSubstrateEffect

YingyingWang,

†

ZhenhuaNi,

†

TingYu,

†

ZeXiangShen,*

,†

HaominWang,

‡

YihongWu,

‡

WeiChen,

§

andAndrewThyeShenWee

§

DiVisionofPhysicsandAppliedPhysics,SchoolofPhysical&MathematicalSciences,NanyangTechnological

UniVersity,Singapore637371,DepartmentofElectricalandComputerEngineering,NationalUniVersityof

Singapore,4EngineeringDriVe3,Singapore117576,andDepartmentofPhysics,NationalUniVersityof

Singapore,2ScienceDriVe3,Singapore,117542

ReceiVed:January29,2008;ReVisedManuscriptReceiVed:April23,2008

Graphenehasattractedalotw,

micromechanicalcleavage(MC)ofgraphitehasbeenusedtoproducehigh-qualitygraphenesheetsondifferent

nderstandingofthesubstrateeffectisimportantforthepotentialdevicefabricationof

reporttheresultsoftheRamanstudiesofmicromechanicallycleavedmonolayergraphene

onstandardSiO

2

(300nm)/Si,singlecrystalquartz,Si,glass,polydimethylsiloxane(PDMS),

datasuggeststhattheRamanfeaturesofmonolayergrapheneareindependentofthesubstrateused;inother

words,theeffectofsubstrateontheatomic/electronicstructuresofgrapheneisnegligibleforgraphenemade

therhand,epitaxialmonolayergraphene(EMG)onSiCsubstrateisalsoinvestigated.

SignificantblueshiftofRamanbandsisobserved,whichisattributedtotheinteractionofthegraphenesheet

withthesubstrate,resultinginthechangeoflatticeconstantandalsotheelectronicstructure.

uction

Grapheneisthetwo-dimensional(2D)buildingblockfor

tsfirstdiscoveryin2004,

1

graphene

hasattractedmajorinterest,andtherearemanyongoingefforts

indevelopinggraphenedevicesbecauseofitshighcharge

mobilityandcrystalquality.

2–4

Ramanspectroscopyhashistoricallybeenusedtoprobe

structuralandelectroniccharacteristicsofgraphitematerials,

providingusefulinformationonthedefects(D-band),in-plane

vibrationofsp

2

carbonatoms(G-band),aswellasthestacking

orders(2D-band).

5

TheG-bandofgraphitematerialsisadoubly

degenerate(TOandLO)phononmode(E

2g

symmetry)atthe

Brillouinzonecenter,

6

whereastheD-bandisduetophonon

branchesaroundtheKpointandrequiresadefectforits

activation.

5

Theevolutionofthe2D-bandfordifferentgraphene

sheetshasbeenusedfordetermininggraphenethicknessaswell

asforprobingelectronicstructuresthroughthedoubleresonance

process.

7,8

Thesymmetricandsharp2D-band(∼30cm

-1

)can

beusedasadetectorformonolayergraphene.

7,8

Eventhe

electronorholedopingcanbemonitoredbyRamanmeasure-

ment,whichisreflectedinthestiffeningandsharpeningofthe

G-band.

9,10

Tillnow,mostoftheRamanstudieswerecarriedouton

graphenesheetsfabricatedbymicromechanicalcleavage(MC)

andtransferredtoSisubstratewithappropriatethicknessofSiO

2

cappinglayer(∼300nm).

7,8,11,12

Additionally,therehavebeen

studiesofgrapheneondifferentsubstrates,suchasindiumtin

oxide(ITO),

13

sapphire,glass,

14

r,therole

ofinteractionbetweensubstrateandthegraphenesheetsin

decidingtheRamanfeatureshasnotbeensufficientlyinvesti-

*Correspondingauthorphone:(+65)63168855;fax:(+65)67941325;

e-mail:zexiang@.

†

NanyangTechnologicalUniversity.

‡

DepartmentofElectricalandComputerEngineering,NationalUniver-

sityofSingapore.

§

DepartmentofPhysics,NationalUniversityofSingapore.

gated,anddifferentconclusionsweredrawnbydifferentgroups.

Clearunderstandingofthesubstrateeffectisimportantfor

potentialapplicationsanddevicefabricationofgraphene.

Therefore,inthisworkwecarryoutsystematicalRamanstudy

ofmonolayergrapheneproducedbyMCondifferentsubstrates:

standardSiO

2

(300nm)/Si,quartzsinglecrystal,Si,glass,

PDMS,ngmonolayergrapheneforourstudy

objectisfirstduetothefactthatitcanbeunambiguously

identifiedbyRamanspectroscopyfromthecharacteristic2D-

,comparedwithgrapheneofafewlayers,

whicharealsousedtostudythesubstrateeffectbyothergroup,

14

monolayergrapheneismoresensitivetotheinteractionbetween

comparedtheRaman

featuresofmonolayergrapheneontheabove-mentionedsub-

strateswiththoseofepitaxialmonolayergraphene(EMG)grown

onSiCsubstrate,forwhichwebelievethereisamuchstronger

erimental

resultsshowthattheweakinteraction(VandeWaalsforce)

betweenthegraphenesheetsandsubstratespreparedbyMC

arenotstongenoughtoaffecttheatomicstructureofgraphene

rEMGonSiCsubstratedoweobserveastrong

interactionbetweenEMGandSiC,whichchangestheatomic

andelectronicstructuresandconsequentlytheRamanfeatures

ofgraphene.

mentalMethods

ThegraphenesampleswerepreparedbyMC

1

andwere

transferredtodifferentsubstrates:standardsubstrateSiwafer

witha∼300nmSiO

2

cappinglayer,quartzsinglecrystal,Si,

glass,NiFe,andpolydimethylsiloxane(PDMS).TheEMG

samplesusedinthisexperimentwereepitaxiallygrownonthe

n-typeSi-terminated6H-SiC(0001)usingthetechniquethat

hasbeenreportedindetailbefore.

15–18

ThethicknessofEMG

isidentifilievedthatbelowtheEMGthere

isaninterfacialcarbonlayer/bufferlayerthatiscovalently

bondedtotheSiCsubstrate.

19,20

BecausethecharacteristicSTM

10.1021/jp8008404CCC:$40.752008AmericanChemicalSociety

PublishedonWeb06/26/2008

.C,Vol.112,No.29,2008

Figure1.(a)Opticalimageofgraphenesheetsonquartz,theredcircle

indicatesthelocationofmonolayergraphene.(b)Ramanimageplotted

circleshowsthepositionof

monolayergraphene.

imagesoftheinterfacialcarbonlayerandthesinglelayer

graphenearequitedifferent,theappearanceofEMGcanbe

determinedbymonitoringthephaseevolutionfromtheinter-

faciallayertographenebySTMduringthethermalannealing

ofSiCinultrahighvacuum(UHV)condition.

21

TheRaman

spectraandRamanimageswerecarriedoutwithaWITEC

CRM200Ramansystemwith532nm(2.33eV)excitationand

laserpoweratsamplebelow0.1mWtoavoidlaser-induced

erspotsizeatfocuswasaround500nmin

diameterwitha100×opticallens(NA)0.95).Thecontrast

spectraofgraphenewereobtainedbythefollowingcalculation:

C(λ))(R

0

(λ)-R(λ))/R

0

(λ),whereR

0

(λ)isthereflection

spectrumfromsubstrate,andR(λ)isthereflectionspectrum

fromgraphenesheet,whichisilluminatedbynormalwhite

light.

11

ForthecontrastandRamanimage,thesamplewas

placedonanx-ypiezostageandscannedundertheillumination

anandreflectionspectrafrom

gemovement

anddataacquisitionwerecontrolledusingScanCtrlSpectros-

copyPlussoftwarefromWITecGmbH,alysis

wasdoneusingWITecProjectsoftware.

sandDiscussion

Figure1ashowstheopticalimageofgraphenesheetson

phenesheetsshowdifferentcontrast

regions,whichcanbeunderstoodashavingdifferentthickness.

Theredcircleindicatestheareaofmonolayergraphene,which

isconfirmedbytheverysharp2D-band(∼30cm

-1

).ARaman

Wangetal.

Figure2.(a)Thecontrastimageofgraphenesheetsonquartzsubstrate.

(b)Contrastspectraofgraphenewithdifferentthicknessesonquartz

substrate.

anspectraofmonolayer,bilayer,threelayers,and

fourlayersgrapheneonquartz(a)andSiO

2

(300nm)/Sisubstrate(b).

Theenlarged2D-bandregionswithcurvefitarealsoshowninpanels

candd.

imageobtainedusingtheintensityoftheG-bandisshownby

olayergraphenehasthelowestG-band

intensity(appearingthedarkest,markedbytheredcircle).As

.C,Vol.112,No.29,200810639

peandpositionof2D-bandchangedramatically

fromonetofourlayers,asshowninthecurvefitofFigure3,

2D-bandinbilayer,three,andfourlayers

graphenecanberesolvedintotwoormorecomponents

,

whereas

monolayergraphenehasasinglecomponent.

7,8

Accordingto

thisgraph,itcanalsobeseenthatthesymmetricandsharp

2D-band(∼30cm

-1

)isthebestindicatorformonolayer

graphenemadebyMCondifferentsubstrates.

Figure4showstheRamanspectraofmonolayergraphene

ondifferentsubstrates,frombottomtotop,PDMS,NiFe,glass,

Si,quartz,andSiO

2

(300nm)/SisubstrateaswellastheRaman

-bandand

2D-bandpositionandtheirfullwidthathalf-maximum(fwhm)

see

thatG-bandposition(1581(1cm

-1

)andfwhm(15.5(1

cm

-1

)aresimilarforgrapheneonSiO

2

(300nm)/Si,quartz,

Si,glass,NiFe,lldifferencein

theG-bandpositiononthesesubstratesarewithintherangeof

fluctuation(1580-1588cm

-1

)byunintentionalelectronorhole

dopingeffectreportedbyCasiraghietal.

22

formorethan40

graphenesamplesonSiO

2

/ore,ourobserva-

tionindicatesthattheinteractionbetweenmicromechanically

cleavedgraphenesheetsanddifferentsubstratesisnotstrong

ultsareinline

withCalizoetal.

14

whosuggestedthattheweaksubstrateeffect

canbeexplainedbythefactthatG-bandismadeupofthe

long-wavelengthopticalphonons(TOandLO),

5

andtheoutof

planevibrationsingraphenearenotcoupledtothisin-plane

vibration.

23

Ontheotherhand,forgraphenegrownonSiC

substrate,itcanbeseenthattheintensityratioofthyeG-and

2D-bandsofEMGdiffersalotfromthoseofmonolayer

er,significantblueshiftsofthe

G-band(10cm

-1

)andthe2D-band(∼39cm

-1

)ofEMGare

mightbesomeelectrondopingtransferredfromtheunderlying

SiCtoEMG(duetothecovalentbonding),

24,25

howeverit

own

thatthedependenceofdopingonshiftinthe2D-bandisvery

weakandisroughly∼10-30%comparedtothatofG-band;

9,26,27

therefore,the39cm

-1

2D-bandshiftistoolargetobeachieved

byelectron/holedopings.

9,10

Here,thissignificantblueshiftof

Ramanbandscanbeunderstoodbythestraineffectcausedby

nEMGandtheSiCsubstrate,thereisan

interfacialcarbonlayer/bufferlayer,whichhasagraphene-like

honeycomblatticethatiscovalentlybondedtotheSiC

substrate.

19,20

Suchbondingwouldchangeitslatticeconstant

ore,thelattice

anspectraofmonolayergrapheneondifferent

substratesaswellthatofepitaxialmonolayergrapheneonSiC.

theG-bandintensityincreasesalmostlinearlyasthelayer

increases,

8,11

weareabletoidentifythethicknessofmultilayer

cknessof

graphenesheetsarefurtherconfirmedbyreflectionandcontrast

microscopy.

11

Thereflectionandcontrastmicroscopywas

successfullyusedtodeterminethenumberofgraphenelayers

(lessthan10)onSiO

2

/trastspectraC(λ)

areobtainedbythecalculationshownineq1,

C(λ))

R

0

(λ)-R(λ)

R

0

(λ)

(1)

whereR

0

(λ)isthereflectionspectrumfromsubstrate,andR(λ)is

thereflngraphene

sheets,thecontrastvaluechangesalmostlinearlywiththe

2ashowsthecontrastimageof

trastofgraphene

isnegativebecausethereismorereflectionfromgraphenethan

2bshowsthecontrastspectraofgraphenewith

trastspectraarealmostflatinthe

rangeof450-600nmandthecontrastvaluesare-0.068(one

layer),-0.125(twolayers),-0.181(threelayers),and-0.247

(fourlayers),whichchangesalmostlinearlywiththenumber

oflayers.

PanelsaandbofFigure3,representivelyshowtheRaman

spectraofmonolayer,bilayer,threelayers,andfourlayers

grapheneonquartzsubstratesaswellasonthestandardSiO

2

(300nm)/anfeaturesof

differentlayersofgrapheneonthosetwosubstratesarequite

TABLEI:TheG-bandand2D-bandPositionandTheirfwhmforGraphene/GraphiteonDifferentSubstrates

a

substrate

SiC

SiO

2

/Si

SiO

2

/Si

22

Quartz

Si

PDMS

Glass

Glass

14

NiFe

GaAs

14

Sapphire

14

Graphite

a

G-bandposition(cm

-1

)

1591.5

1580.8

1580-1588

1581.9

1580

1581.6

1582.5

1580

1582.5

1580

1575

1580.8

G-bandfwhm(cm

-1

)

31.3

14.2

6-16

15.6

16

15.6

16.8

35(split)

14.9

15

20

16.0

2D-bandposition(cm

-1

)

2710.5

2676.2

2674.6

2672

2673.6

2672.8

2678.6

2D1:2675.4

2D2:2720.8

2D-bandfwhm(cm

-1

)

59.0

31.8

29.0

28.3

27

30.8

31.4

41.4

35.6

Resultsfromrefs14and22arealsoincluded.

.C,Vol.112,No.29,2008

mismatchbetweengraphenelatticeandinterfacialcarbonlayer

maycauseacompressivestressonEMG,hencetheshiftofthe

G-bandRamanpeakfrequencies.

21

GrapheneondifferentsubstratessuchasITO,

13

sapphire,and

glass

14

rast

totheirresults,wedidnotobservethesplitorlargered/blue

shiftoftheRamanG-bandofgrapheneondifferentsubstrates

madebyMC,

14

partiallyduetothedifferentstartingmaterials

sibilityofformingbonds

betweenmicromechanicallycleavedgrapheneandsubstrate

isquitelowassuchbondsareonlypossibleathigh-temperature

growth.

24,25,28

sions

Insummary,throughourRamanstudiesofmonolayer

grapheneproducedbyMCondifferentsubstrates—standard

SiO

2

(300nm)/Si,quartz,Si,glass,NiFe,andPDMS–wecould

knowtheweakinteraction(VandeWaalsforce)between

graphenesheetsandthesubstratesplayanegligiblerolein

G

grownonSiCsubstrateshowsstrongblueshiftofG-band,which

canbeunderstoodbythestraineffectcausedbythecovalent

bondingbetweenSiCsubstrateandepitaxialgraphene,resulting

inthechangesthelatticeconstantofgraphene,andhencethe

Ramanfeatures.

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JP8008404