Aromatic structure degradation of single layer graphene on an amorphous silicon substrate in the presence of water, hydrogen and Extreme Ultraviolet light

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ContentslistsavailableatScienceDirect

Applied

Surface

Science

j o ur na l ho me pa g e :w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c

Full

Length

Article

Aromatic

structure

degradation

of

single

layer

graphene

on

an

amorphous

silicon

substrate

in

the

presence

of

water,

hydrogen

and

Extreme

Ultraviolet

light

B.K.

Mund

a,∗

_,

_J.M.

_Sturm

a

_,

_C.J.

_Lee

a,b

_,

_F.

_Bijkerk

a

a_Industrial_Focus_Group_XUV_Optics,_MESA+_Institute_of_{Nanotechnology,}_University_of_Twente,_Enschede,_The_Netherlands

b_Quantum_Transport_in_Matter_Group,_MESA+_Institute_of_{Nanotechnology,}_University_of_Twente,_Enschede,_The_Netherlands

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received28July2017

Receivedinrevisedform8September2017

Accepted12September2017

Availableonline14September2017

Keywords:

Singlelayergraphene

Reﬂectionabsorptioninfraredspectroscopy

Water

Temperatureprogrammeddesorption

ExtremeUltravioletlight

Hydrogen

a

b

s

t

r

a

c

t

InthispaperwestudythereactionofwaterandgrapheneunderExtremeUltraviolet(EUV)irradiationand inthepresenceofhydrogen.Inthiswork,singlelayergraphene(SLG)onamorphousSiasanunderlying substratewasdosedwithwater(0.75mL)andexposedtoEUV(=13.5nm,92eV)withpartialpressures ofH2inthebackground.Theresultsshowthatthearomaticstructureofgraphene,whenexposedto

EUVandH2,breaksdownintoarylketonesandenolsof1,3di-ketone.Infrared(IR)spectroscopyshows

thatSLGoxidizes,withincreasingH2pressureleadingtothegrainboundaryedgesofgrapheneforming

ketonesandcarboxylicacids.InsituandpostexposureanalysesalsorevealthatEUVexposurereduces thesp2_content_of_the_graphene_layer,_with_the_sp3 _content_increasing,_resulting_in_a_more_defective

graphenelayer.

1. Introduction

Graphene,atwodimensionalhexagonallypackednetworkof

covalentlyboundcarbonatoms,hasanumberofuniquephysical,

thermalandchemicalproperties[1–8].Itisknowntobe

imper-meabletogases[9],andhasbeentheoreticallyshowntoactasa

diffusionbarrierevenagainstmolecularhydrogen[10].Graphene

canbegrownonanindustrialscaleviachemicalvapordeposition

andtransferredontobothﬂat[11]andarbitraryproﬁlesubstrates

[12],broadeningitsscopeforpotentialapplications.

In the optical regime, single layer graphene combines the

highly desirable properties of beingsimultaneously conductive

and transparent inthe visible [13], and EUV [14] wavelengths.

Thesepropertiesmake ita promising candidatefortransparent

conductingelectrodes,requiredfortouchscreens,and,potentially

EUVadaptive optics.Furthermore,thehightransparencyinthe

EUVrangeisattractivebecausemostopticsrequireaprotective

toplayertoprotectthemfromthehighlyreactiveenvironment,

inducedbyradiation[15].

∗ Correspondingauthor.

E-mailaddress:b.k.mund@utwente.nl(B.K.Mund).

Perfectsinglelayergraphene(SLG)isknownforitslow

chem-icalreactivityduetothedenselypackednatureofsp2_hybridized

carbonatoms[16].Unfortunately,duringchemicalvapor

deposi-tionandgraphenetransfer,defectsaregenerated[17],whichcan

behaveasinitiationpointsforgraphenetoreactwithits

environ-ment[18].Nevertheless,graphenehasbeenproposedasauseful

materialinchemicallyharshenvironments,suchasDeep

Ultra-violet,X-raysystemsandExtremeUltraviolet(EUV)Lithography

systems.EUVlithography(EUVL)systemsrepresentaparticular

challenge:EUVLsystemsoperateatawavelengthof13.5nm,in

vacuum,andthemainopticalcomponentsareexpectedtolastfor

thelifetimeofthesystem(>10years).Theopticsandbackground

gases,however,areexposedtoionizingradiation,creatingarich

environmentfor surfacechemistrythatmaymodifythesurface

of theoptics [19–21].Toreduce theinﬂuenceofresidual gases

—mainlywaterandhydrocarbons—thepressureisincreasedto

afewPa,byaddinghydrogen[22].Thebalancebetween

hydro-genasareducingagentandwaterasanoxidizingagentallowsa

dynamicequilibriumbetweencompetingprocessestobemanaged

[23].However,theconditionsunderwhichsuchabalancecanbe

achievedvariesfrommaterialtomaterial.Beforegraphenecanbe

usedinsuchasystem,itiscriticaltounderstanditsphysicaland

chemicalstability,andtodeterminethereactionpathwaysthatare

mostfavorableundervariouspartialpressuresofH2andH2O.

http://dx.doi.org/10.1016/j.apsusc.2017.09.098

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1034 B.K.Mundetal./AppliedSurfaceScience427(2018)1033–1040

Sincegrapheneisaoneatomthicklayerofcarbonatoms,

chem-icalreactivity is highlydependenton theunderlyingsubstrate.

Currenttechniques,suchasin-situRamanspectroscopy,canbe

usedtostudythegenerationofdefectsingraphene.Duetothe

zero-bandgapnatureofgraphene,theRamanscatteringcrosssectionis

large,makingitthepreferredmannertocharacterizegraphene.

However,for most othermaterials,the Ramanscattering cross

sectionis much smaller, making it difﬁculttoidentify detailed

modiﬁcationstothestructureandmolecularcompositionofthe

graphene.Furthermore,eventhoughthenormalmodesofwater

arebothRamanandIRactive,Ramanspectroscopyisless

sensi-tivetochangesinhydrogenbondedwaternetworks[24].Finally,

Ramanspectroscopyissusceptible toﬂuorescencefrom(inour

case)theamorphousSisubstrate,whichisstrongerthantheRaman

signalbyafactorof106_–108_._Therefore,_infrared_spectroscopy_is

thepreferablemethodtostudysurfacechemistryofwaterandits

interactionwithgraphene.

Previousresearchhasshownthatdefectsaregeneratedin

sin-glelayergrapheneduringEUVexposure[25,26].Here,wepresent

aninvestigationintothereactionsofgrapheneonanamorphous

SilayerinanEUVenvironment.Fortheseexperiments,the

sur-faceischaracterizedin-situusingReﬂectionAbsorptionInfrared

Spectroscopy(RAIRS)duringEUVexposure,andtemperature

pro-grammed desorption (TPD) spectroscopy before and after EUV

exposure.RAIRSisusedtoobservechangesinmolecular

orien-tationofwaterandchemicalstructureofgrapheneandrevealing

thepathwaybywhichgrapheneoxidizesinthepresenceofwater

andhydrogen.Ontheotherhand,TPDprovidesaquantitative

mea-sureofthenumberofmoleculesthatdesorbfromthesurfaceata

giventemperature,providinganaccuratemeasureofthedifferent

adsorbedmoleculesandtheirbindingenergies.

X-rayphotoelectronspectroscopy(XPS) measurementswere

carriedoutex-situ,beforeandafterEUVexposure, to

quantita-tivelyestimatetheelementalcompositionandthechemicalstate

ofthesurface.Thiswasusedtoconﬁrmtheendresultofgraphene

oxidation.

2. Experimental

Singlelayergraphenewasgrownbychemicalvapordeposition

onapolycrystallinecopperfoil(purity99.9%,AlfaAesar).The

cop-perfoilwasintroducedintoafurnaceat1100◦Cwithagasﬂowof

100sccmofCH4,500sccmofArand6sccmofH2leadingtosingle

layergraphenebeinggrownonbothsidesoftheCufoil.The

sin-glelayergraphene[10mm×10mm]wasthentransferredusing

thewettransfermethod[27]ontoanamorphousSisurface.The

graphenelayerwastransferredusingaPMMAsupportlayer,which

wasremovedbyannealingat350◦CwithArandH2for∼3h.

Thesubstrateonto whichthegraphenewastransferredwas

basedonaSiwafer,whichhadaMolayer(9nmthick)deposited

onit,followedby22nmofamorphoussilicon.Bothlayerswere

depositedbysputterdeposition,andthethicknesseswereknown

via depositioncalibration based on X-ray reﬂectivity

measure-ments.Theamorphoussiliconwasnotprotectedfromatmosphere

afterremovalfromthedepositionchamber,thus,thetop∼1–2nm

isoxidized.Themolybdenumlayerisnecessarytoreﬂectinfrared

radiation, allowing reﬂection absorption infrared spectroscopy

(RAIRS)tobeperformedinsitu.

Allsurfacechemistryexperimentswereperformedinthesame

experimentalsetup[28,29]sequentiallywithoutbreakingvacuum.

Thechamber’sbasepressureismaintainedat5×10−9mbar.

Dur-ingexperiments,hydrogenisintroducedtothechamber,increasing

the background pressure up to 1_×10−5mbar. The chamber is

equippedwithRAIRS,TPDspectroscopy,surfacecleaningfacilities,

surfacedosing,andattachedtoanEUVsource.

Table1

Experimentalconditionsofthesurfaceandchamberwithinitialwaterdose,partial

pressureofH2andtheﬁnalwatercoverageafterexposure.

Experiment InitialWater Dose(mL) Partial pressure(H2) FinalWater coverage(mL) NoEUV 0.75mL 2×10−9_mbar _0.75 EUV 0.75mL 3×10−9_mbar _0.75 EUV 2.25mL 1×10−7_mbar _3.00 EUV+H2 0.75mL 1×10−7mbar 0.98 EUV+H2 0.75mL 1×10−6mbar 1.42 EUV+H2 0.75mL 1×10−5mbar 3.56

RAIRSspectraaremeasuredatgrazingincidenceusinganFTIR

spectrometer (Bruker Vertex 70V), equipped with a liquid N2

cooleddetector.Eachspectrumissummedover256scanswith

aresolutionof4cm−1,withbackgroundscansbeingrecordedat

thelowestpossiblestablesampletemperatureof80K.Toobtain

TPDspectra,thesampletemperatureisrampedfrom80Kto450K

atarateof1K/sec.Thesampleisplacednormaltotheentranceof

aconethatisattachedtoadifferentiallypumpedquadrupolemass

spectrometer(QMS—HidenAnalytical).Theconehasanentrance

apertureof4mm,located∼2mmawayfromthesample.The

tem-peratureismeasuredusingathermocoupleattachedtothesample

withaMoclamp.Topreventdamagetographene,thesample

tem-peraturewaslimitedto450K,whichissufﬁcienttoremovewater,

CO,andhydrogen.Basedonexperimentsonaruthenium(0001)

surfaceusingthesamechamber,weshowthat thebackground

depositionrateofallchambercontaminantsisverylow(<0.005

monolayers(ML)perhour)[19].

Deionizedwater,cleanedusingthefreeze-pump-thawprocess,

isdosedonthesamplethrougharetractablequartztubeconnected

toa pinhole.Surface coverageis calibratedagainstTPD spectra

obtainedfromacleanRusurface[28].

ThechamberisattachedtoaXeplasmadischargeEUVsource

(PhilipsEUVAlphaSource2)witharepetitionrateof500Hz.The

sourceisﬁlteredbyreﬂectionfromaMo/Simultilayermirror(55%

reﬂectivityat13.5nm)andtransmissionthroughaSi/Mo/Zr

mem-brane(35%transmissionat13.5nm)[30].Thisresultsin apeak

reﬂectivityof19%at13.5nmwithaFWHMof0.2nmandabroad

reﬂectivitypeakof9%at21.5nmwithaFWHMof3.1nm[14,21,31].

TheEUVbeamhasanaverageintensityof35–55mW/cm2_,_and_a

proﬁlethatisapproximatelyGaussianwithafullwidthhalf

max-imum(FWHM)of3mm.Overthecourseoftheexperiments,the

EUVpulseﬂuencevariedfrom90to110␮J/cm2_.

Thegeneralexperimentalprocedureconsistedof:thegraphene

was ﬁrst cooled to 80K, and a reference RAIRS spectrum was

obtained.Thesamplesurfacewasdosedwithwater,andthe

cham-berwasﬁlledtoachosenbackgroundpressureofhydrogen.The

samplewasthenexposedtoEUVandRAIRSspectrawereobtained

beforeEUVexposure,andevery10minduringtheexposure.After

EUVexposure,aTPDspectrumwasobtained.Therangeofexposure

conditionsaresummarizedinTable1.

AfterTPD/RAIRSexperimentshadbeencompleted,exsituX-ray

photoelectronspectra(XPS)weremeasuredusingmonochromatic

Al-K␣radiation,employingaThermoFisherThetaprobe

instru-mentwithabeamspotsizeof1mmindiameter.Parallelangle

resolvedXPSspectraweremeasuredinananglerangefrom26◦to

80◦,thedisplayedspectracorrespondtoatake-offangleof34◦.

3. Resultsanddiscussion

ThegrapheneonSisamplewasexposedtoEUVwithadditional

H2partialpressuresof0mbar,at10−7mbar,10−6mbar,10−5mbar

(seeninTable1).Alltheexposureswere1hlong.Thecumulative

EUVdoseforthisexperimentwascalculatedtobe0.32–0.39J/cm2_,

(3)

Fig.1.RAIRSspectraoftheC OstretchpeakofCO2,enolsandarylketoneasseenonthesurfaceofthesinglelayergraphene.Waterdoseis0.75mLunlessotherwise

speciﬁed.Anewbackgroundspectrumistakenbeforeeachexposure,directlybeforedosingH2Otothesurface.

anincrease intemperatureduringEUV irradiation),eliminating

thermodynamically driven processes from consideration. Thus,

photonandphotoelectron-drivenprocessesareresponsibleforthe

observedchangesinsurfacestructureandcomposition.

Fortheseexperiments,RAIRSOHpeaks,suchasthelibration

modes(750–950cm−1)andbendingmodes(1500–1700cm−1)are

tooweakforthewatercoverageused.OHstretchingmodesfrom

water(3000–3700cm−1)werevisible,butdidnotprovideany

sig-niﬁcantinformationaboutthestructureandinteractionofwater

onthesurfaceandarethereforenotshown[32].

3.1. FormationofketonesandadsorbedCO2onthesurfaceof

singlelayergraphene

AsdetailedinGerakinesetal.[33],theformationofCO2is

evi-denced byvibrational modesin the2200–2400cm−1 region.In

Fig.1,anasymmetric(as)C Ostretchpeakispresentat2343cm−1

and2373cm−1(peakI1,I2),indicatingthepresenceofCO2.When

onlywateris presentonthesurface,intheabsenceofEUV,an

inversepeakisseenforI2,indicatingremovalofCO2fromthe

sur-face.Subsequently,athigherpartialpressures(10−6,10−5mbar)of

H2inthepresenceofEUV,thepeakreappears,indicatingCO2asa

possibleendproductofcarbonoxidation.

WithanincreaseinH2concentration,abroadpeakstartstoform

at2454cm−1(peakJ1).Thiscorrespondstoliteraturereportsfora

C Ostretchforketo-enolformation,speciﬁcallytheC Ostretches

forarylketonesandtheenolformsofa1,3-diketone(asshownin

Fig.2)[34].Enolof1,3–diketonesaremorefavorableduetotheir

increasedstabilitywhenformingasixmemberedring—

hydro-genbondedinthiscase.AsH2pressureincreases,thisC Ostretch

formsa broadpeak at2477cm−1 (peakJ2), whichconﬁrms the

formationofketonesonthesurface[32].Inaddition,ablue-shift

isseenforpeakJasthehydrogenpressureisincreased,

indicat-ingmoreketo-enolvibrationsdetectedonthesurface[34].These

ketonesareformedduetographeneoxidizingandbreakingdown

intoarylandketo-enolstructures,duetoEUVinducedH2andH2O

dissociationonthesurface.Hradicals,formedbyEUV-induced

dis-sociationofH2,createdefectsitesinthegraphenelayer,which

are subsequently oxidizedby OH groups or Oformedby

EUV-inducedH2Odissociation.AsindicatedinTable1,exposuretoa

higherpressureofH2resultsinadditionalwaterbeingdepositedon

thesample,duetocontaminationofthegasline.Inordertocheck

whethertheformationofketo-enolspeciesisindeedrelatedtoEUV

inducedreactionsinthepresenceofH2andH2OandnottoEUV

exposurewithlargerH2Ocoverages,acontrolexperiment with

initiallyhigherdosedwatercoverage(2.25mL)andlowH2

pres-sure(1×10−7mbar)wascarriedout.Undertheseconditions(Fig.1,

3rdlinefromtop)noketo-enolformationisobserved,provingthat

presenceofH2pressures>1×10−6mbarandadsorbedwaterare

bothneededforEUV-inducedketo-enolformation.Assumingthat,

forthelowexposuresinthisstudy,themajorityofketone

forma-tionoccursalonggrainboundaries(seeFig.2),themostlikelyforms

areenolsof1,3-diketoneandhydroxylarylketones.

Keto-enolformationisonlyseenatahighhydrogenpressure

(10−5–10−6mbar),duetohigherdefectcreationbyEUV-induced

radicals,whichiscomparedinFig.3.AtaH2pressureof10−6mbar

and 10−5mbar, the height of peak J increases at a rate of

0.0015%/minand0.0132%/minrespectively.Thisincreaseinpeak

Fig.2.Graphenebreakingdownintohydroxylarylgroupsandsubsequentlyformingketones.Arylketonesandketo-enoltautomerismhavethesamestretchvibration

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Fig.3. Growthofketo-enolC Ostretch(peakJ)ascomparedtoEUVexposuretimeatdifferentH2pressures.Theverticalaxisindicatesthenegativepeakintensityinthe

RAIRStransmissionplot,soahighervalueindicateslargersurfacecoverage.

heightis8.8timestherateat10−6mbar,indicatingthatgraphene

beginsoxidizingrapidlywithahigherpartialpressureofH2.

Twolocationsaremostlikelyforoxidation,andtheformationof

enolsandketones:pointdefectsandgrainboundaries.Pointdefects

canbeeffectivelydeterminedusingRamanspectroscopy.Results

fromCancadoetal.[35]statethatID/IGratiosofgraphenecanbe

usedtospecifytheinterdefectdistance.Agraphenereferencelayer

producedwiththesamegrowthprocess,buttransferredontoaNi

surfaceinsteadofaSisurfacehasanID/IGof0.75,which

trans-latestoaninter-defectdistanceof14nm.Sincethetypicalgrain

sizeofourgrapheneis∼100nm,correspondingtoatypicalgrain

areaof∼8000nm2_,_this_inter-defect_distance_would_correspond_to

∼100pointdefectspergrain,oneorderlowerthantheoxidation

sitesavailablethroughgrainboundaries[36].Therefore,itislikely

thattheRAIRSspectralchangesaredominatedbyketone

forma-tionalongthegrainboundarieswhichleadstograinboundaries

unzippingathigherH2pressures,toformmoresitesforoxidation

[36].

3.2. Saturationofenolformationonthesurface

Asdiscussed earlier, enols of 1–3 di-ketone are most likely

formedduetothepreferentialoxidationofgraphenealonggrain

boundaries.Furthersequentialexperimentswereconductedtotest

ifoxidationsaturates:e.g.,thatthegrainboundariesbecomefully

oxidizedandoxidationthenslows.ThiscanbeseeninFig.4where

thegrowthoftheC Ostretchfromenolsandarylketone(peakJ)for

differentexposuresisshown.Asnotedearlier,peakJﬁrstappears

whentheH2OcoveredsurfaceisexposedtoEUVandmolecularH2

at10−6mbar(traceI),indicatingthatgrapheneisstartingto

oxi-dize.Asmentionedpreviously,thispeakincreaseswithincreasing

mol.H2pressure(traceII).Followingtheseexposures,thesurface

Fig.4.Changeinketo-enolformation(peakJ)aftersubsequentexposuresofEUVandH2tothegraphenelayerat10−6and10−5mbar.Anewbackgroundspectrumistaken

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Fig.5.Changeinketo-enolformation(peakJ)overtimeaftersubsequentexposuresofEUVandH2tothegraphenelayerat10−6and10−5mbar.Anewbackgroundspectrum

istakenaftereachexposure.

Table2

Rateofgrowthofketo-enolformation(peakJ)asseeninFig.5.

Traces Transmission%

minute I:EUV+H2,10−6mbar 0.0015

II:EUV+H2,10−5mbar 0.0132

III:EUV+H2,10−6mbar 0.0008

IV:EUV+H2,10−5mbar 0.0023

wasagainexposedtoEUVandH2at10−6mbar(traceIII).Asseen

incurveIIIinFig.4,thereisnoindicationoffurther(outofplane)

enolorarylketoneformationonthesurfaceforthispartial

pres-sure.ThisisbecauseoxidationofthegraphenelayeratH2pressure

of10−6mbarhassaturated,andforfurtheroxidationtohappen,

theH2pressureneedstobeincreasedtocreateadditionaldefect

siteswherewatercanreact.Furthermore,atasubsequent

expo-sureatanH2partialpressureof10−5mbar(traceIV)enolandaryl

ketonegrowthismuchslowerthaninthepreviousexperimentat

thesamepressure(traceII),indicatingthattheoxidationprocess

issaturating.

Thisgrowthfor peakJcanbequantiﬁedinFig.5wherethe

change in keto-enol formation is measuredover thecourse of

onehourat differentEUVexposuresand H2 pressures.Asseen

inTable2,therateofgrowthofpeakJisshowntoreducewith

increasingmolecularH2pressureinthepresenceofEUV,

indicat-ingthatketo-enolformationsaturatesasexposuretimetoEUVand

H2increases.

3.3. Temperatureprogrammeddesorptionofwaterfrom

graphenesurface

InFig.6,thetemperatureprogrammeddesorptionofwaterfrom

thegraphene/Sisurfaceis shown.Whenthesurfaceis exposed

toEUV,thecoverageofwaterdoesnotchangesigniﬁcantly.An

increaseinwatercoverageisobservedforhigherhydrogen

pres-sures;0.23mLfor10−7mbarofH2,0.67mLfor10−6mbarofH2

and2.81mLfor10−5mbarofH2.Theamountofwaterdesorbedfor

increasingH2partialpressuresishighduetowatercontamination

fromtheH2line.

Also,twodistinctpeakscanbeseenfortheTPDspectrainFig.6,

indicating that water hastwo bindingmodes onthesubstrate.

Whenwater is ﬁrst dosed onthe cold surface, it forms an

H-bonded2Dicenetworkonthegraphenelayer[37].Asmorewateris

depositedontopofthesurface,a3Dicenetworkforms.The

desorp-tionpeakseenat∼139KinFig.6isduetoH2Omoleculesdesorbing

from3Dice networks.Followingthis, thegrapheneboundH2O

moleculesin2Dicenetworksarethenexttodesorbat∼160K.

For verylow coverage ofwater, whenthe surfaceis leftfor

120mininthechamberwithnowaterdosedonthesurface,Fig.7(a)

showsthatwatercoverageonthesurfaceincreaseswithtimedue

toresidualgases.Thisis alsoclearlynotedforlow coveragesin

Fig.7(b)aswell,whenasmallamountofwater(0.01mL)isdosed

onthesurface.Comparedtothecasewithnoinitialcoverage,the

amountofwaterinFig.7(b)increasesbyafactorof3.4and7.3for

60and120min,relativeto30minexposuretoresidualgas,

respec-tively.Incomparison,inFig.7(a),thewateronthesurfaceincreases

byafactorof0.1and0.6forthesameperiodsof60and120min,

indicatingthatwaterismorelikelytoattachtowatermolecules

thanitistothegraphenelayer.Additionally,thetwopeaksseen

forlowcoveragesinFig.7(a)illustratethata2Dwaternetwork

istheﬁrsttoform,withwatermoleculesattachingtothedefects

andgrainboundariesofgraphene,andthenwaterattachingtothe

defectboundwater.SimilarlyinFig.7(b),acommonleadingedgeis

seen,suchthatthedesorptionpeakshiftstoahighertemperature

forincreasingcoverage,indicatingpeaksthetwopeaksinFig.7(a)

convergingtoformasingularpeak.Consequently,thisbecomesthe

ﬁrstdesorptionpeakinFig.6.This0thorderdesorptionbehavior

ischaracteristicfordesorptionofwateradsorbedtootherwater

moleculesasshowninClayetal.[38]Sincegrapheneisknown

tobehydrophobicinnature[39],waterclustersaremorelikelyto

form,suchthat0thorderdesorptionofwatercanbeobserved,even

forsubmonolayercoverages.

3.4. X-rayphotoelectronspectroscopy

Fig.8showstheXPSspectraandﬁtfortheC1speakofgraphene

beforeandafteritisexposedtomultipledosesofH2O,EUVand

H2seeninFigs.1–7.Table3liststheatomicconcentrationofsp2,

sp3_and_C _O_bonds_along_with_the_full_width_half_maximum_of_the

ﬁttedsp2_peak_for_the_unexposed_graphene_sample_and_the

sam-pleexposedtoEUV.Notably,thesp2_bond_{concentration}_decreases

from13±1%to6±4%afterexposure,while,incontrast,thesp3

concentrationincreasesfrom5±1%to12±4%.Theincreaseinsp3

bondingisaconsequenceofcombinationofoxidationand

hydro-genation,asexpected.Itshouldbenotedthattheseparationofthe

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Fig.6.TPDspectraofH2OafterdepositionofwaterandexposuretoEUVandH2.Thenumbersrefertoadditionalwaterseenonthesurfaceascomparedtothesurface

withoutEUVandH2.Thespectrahavebeensmoothedover4values.

Fig.7. Controlexperimentsforresidualgasinchamberwith(a)lowcoverageand(b)coverage(0.01mL)withwateronthesurface.

Table3

AtomicconcentrationofC1sspectralcomponents,carbonthickness,nativeSiO2concentrationofthesubstrateofgraphenesamplesbeforeandafterexposuretomultiple

dosesofEUVandH2.TheCthicknessismeasuredusingangleresolvedXPS.

Sample Csp2_(atomic_%) _C_sp3_(atomic_%) _C _O_(atomic_%) _Si _O_(atomic_%) _O_1s_(atomic_%) _sp2_FWHM_(eV) _Carbon_thickness_(nm)

UnexposedgrapheneonSi 13±1 5±1 1.0±0.3 16±1 26±1 0.90±0.1 0.34 ExposedgrapheneonSi 6±4 12±4 7±2 15±1 26±1 0.9±0.2 0.40

exposedsample,whichhasbeenreﬂectedintheerrormarginsof

thequantiﬁcation.

TheconcentrationofC Ogroupsincreasesfrom1%to7%,

con-ﬁrmingthatgrapheneoxidizesinthepresenceofadsorbedwater,

andexposuretoEUVandH2.Itshouldbenotedthatthehighsp2

contentofpristinegrapheneresultsinatailofthesp2_peak

extend-inguptothebindingenergyrangewhereC Obondsaredetected

forexposedgraphene[40].Therefore,thequantiﬁedamountofC O

speciesmaybeoverestimatedfortheunexposedsample.

Finally,aslightincreaseincarbonthicknesswasobserved,due

tothedepositionofamorphoushydrocarbonsfromresidual

hydro-carbonsintheEUVsource,orduetosamplehandling[41,42].

4. Conclusions

Single layer graphene on an amorphous silicon substrate

exposed toEUV radiation inthe presenceof water and

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Fig.8.XPSspectraofgraphenecoveredSisamplebefore(top)andafter(bottom)

exposuretoEUVandH2.TheC1sspectraisrawdata,whilesp2,sp3andC Ocurves

areﬁtted.

throughtheformationofketoneandenolgroups,suchastheenol

formsof1,3-diketone,mostlikelyatthegrainboundariesand/or

pointdefects.ThisformationisnotedviaRAIRS,whichindicates

thatringstructuredegradationoccursthroughgrapheneoxidation,

duetothewaterpresentonthesurface.Additionally,thiscanbe

conﬁrmedbyXPSwhichshowsanincreaseinthebond

percent-ageofoxygenboundtocarbononthesurfaceafterEUVexposure.

Meanwhile,sp2_bonds_in_graphene_cleave_to_form_sp3_bonds,_also

conﬁrmedviaXPS,duetoEUVphotonsand/orhydrogenradicals

breakingsp2_bonds,_leading_to_graphene_becoming_more_defective.

Furthermore,therateoftheoxidationprocessatgivenwater

cov-erageandhydrogenpressureslowsdownandnearlysaturatesover

time,whilestillsp2_carbon_is_left_after_the_exposure._This_behavior

couldbeattributedtosaturationofthereactionoccurringatgrain

boundariesand pointdefects.Finally, thisworkshows thatthe

balanceofEUV-inducedphotochemistryofgrapheneinthe

pres-enceofwaterandhydrogenisverydifferentfromruthenium,an

importantreferencecappingmaterialforEUVoptics[43].Whilefor

rutheniumanincreasedpressureofH2leadsto(complete)

reduc-tionoftheoxidationoftheRuresultingfromEUVinducedreactions

ofwater,incaseofgraphenetheoxidationreactionisenhancedby

ahigherhydrogenpressure.Thisshowsthat abalancebetween

oxidationandreductionofagraphenecapcanmostlikelynotbe

obtained,suchthattheuseofgraphenewithalowdefectdensity

isofutmostimportance.

Acknowledgements

TheauthorswouldliketothankDr.RobbertvandeKruijsand

Mr.TheovanOijenforpreparationofsubstrate,Mr.EldadGrady

forhelpwithsynthesisofgrapheneand,Mr.LucStevensandMr.

GoranMilinkovicforhelpwithexperimentalmeasurements.This

researchissupportedbytheDutchTechnologyFoundationSTW

(projectnumber140930),whichispartoftheNetherlands

Orga-nizationforScientiﬁcResearch(NWO),andwhichispartlyfunded

bytheMinistryofEconomicAffairsaswellasASMLand ZEISS.

Wealsoacknowledgetheﬁnancialandfacilitarycontributionsby

ASML,ZEISS,PANalytical,andtheProvinceofOverijsselthrough

theIndustrialFocusGroupXUVOpticsattheMESA+Institute.

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