Structure of tetracene films on

PHYSICALREVIEWB74,205326͑2006͒

Structureoftetracenefilmsonhydrogen-passivatedSi(001)studied

viaSTM,AFM,andNEXAFS

ni,,,

*

DepartmentofPhysicsandGuelph-WaterlooPhysicsInstitute,UniversityofGuelph,Guelph,OntarioN1G2W1,Canada

͑Received7June2006;revisedmanuscriptreceived7September2006;published20November2006

͒

Scanningtunnelingmicroscopy͑STM͒,atomicforcemicroscopy͑AFM͒,andnear-edgex-rayabsorption

finestructure͑NEXAFS͒havebeenusedtostudythestructureoftetracenefilmsonhydrogen-passivated

Si͑001͒.STMimagingofthefilmswithnominalthicknessofthreemonolayers͑3ML͒exhibitsthecharac-

teristic“herringbone”molecularpackingknownfromthebulkcrystallinetetracene,showingstandingmol-

ensionsandorientationoftheherringbonelatticeindicateacommensurate

strespondingAFMimagesillus-

tratethatatandabovethethirdlayerofthefilms,theislandsareanisotropic,incontrastwiththesubmonolayer

fractals,arizationdependent

NEXAFSmeasurementsindicatethattheaveragemoleculartiltinganglewithrespecttothesurfacefirst

increaseswiththefilmthicknessupto3ML,thenstabilizesatavalueclosetothebulktetracenecase

binedresultsindicateadistinctgrowthmorphologicalchangethatoccursaroundafew

monolayersofthickness.

DOI:10.1103/PhysRevB.74.205326PACSnumber͑s͒:,68.37.Ϫd,,

UCTION

Drivenbytheirpotentialapplicationsinfutureorganic

electronics,smallaromaticmoleculesareattractingincreas-

ingattention.

1–3

Theyareofinterestinfundamentalstudies

oftheorganicelectronicsbecausemanyofthesemolecules

couldformhighpurityandwell-orderedcrystallinestruc-

turesbysublimationtechniques.

3

Tetraceneisaplanararo-

maticmolecule͑C

18

H

12

͒consistingoffourfused-benzene

rings͓Fig.1͑a͔͒.Thebulkcrystallinestructureoftetraceneis

layeredherringbonemolecularpackingonthe͑001͒plane

͓Fig.1͑b͔͒.

4

Tetracenethinfilmspreparedwithvacuumsub-

limationhavealreadybeenusedtofabricatefield-effecttran-

sistors͑FETs͒,

5

andtetracenefilmshavealsodisplayedim-

pressivepropertiesinorganiclight-emittingFETs.

6

Inorder

toachieveoptimalmaterialpropertiesforthesedevices,the

controlofthefilmgrowthbasedonagoodunderstandingis

r,researchontetracenethinfilmsisfar

ticalstudiesonanotherpolyacene,

pentacene,

7

indicatethatthebulkcrystalline͑001͒facethas

thelowestsurfaceenergy,andiftheinteractionbetweenthe

moleculeswithinalayerismuchstrongerthanthatbetween

themoleculeandsubstrate,asthetypicalcaseforthefilm

growthoninertsubstrates,thefilmdepositedisexpectedto

exhibitthe͑001͒orientation.

7,8

Giventhesimilaritiesinpen-

taceneandtetracenecrystalstructures,theaboveconclusions

saimedonincreas-

ingthemobilityintetracenethinfilmsonsilicondioxide

reportedthatthefilmgrowthisinfavorofformingthree-

dimensional͑3D͒islandswithagranularstructure,andthat

themobilitylimitationisprimarilycausedbythefilmgrowth

properties.

9

Similarstructuralresultswereindicatedbygraz-

ingincidencex-raydiffractionstudythatpolycrystalline

structureswithpolymorphsofdifferentuprightorientations

ofthemoleculeswerefound.

10

Untilrecentlythelayered

morphology,similartothoseinthepentacenefilm

1098-0121/2006/74͑20͒/205326͑9͒

growth,

11–13

hasbeenrealizedintetracenefilmgrowthona

solidsubstrate.

14

A“layer-by-layer”growthwasdemon-

stratedforthetetracenefilmsonhydrogen-passivated

Si͑001͒-2ϫ1substrateswithinoptimalrangeofdeposition

rates,andthatgoodfilmconnectivityandsignificantlyim-

provedgrainsizewereobtained.

14

TheSi͑001͒-2ϫ1surface

usedinthislatterstudyprovidesastructurallywell-defined

͓Fig.2͑a͔͒substratetemplate,

15

whilethehydrogenpassiva-

tion͓Fig.2͑b͔͒removessurfacedanglingbonds

17

resulting

inaninertsubstrate,leadingtoawell-suitedsurfaceforfun-

damentalstudiesontheorganicthinfilmgrowth.

PreviousAFMresultsindicatedthatforatetracenefilm

depositedonH/Si͑001͒themoleculesareinanupright

standingconfiguration,

14

similartothatofpentacenesin

weakmolecule-substrateinteractioncases.

7,8,18

However,

morecompletestructuralcharacterizationsofthetetracene

FIG.1.͑a͒Schematicsideviewofasingletetracenemolecule

͑C

18

H

12

͒s

showninshadedblack,andHatominlightgray.͑b͒Schematictop

view͑alongthemoleculelongaxis͒ofasingleablayeroftetracene

initsbulkcrystallinestructurewithtwomoleculesintheunitcell.

©2006TheAmericanPhysicalSociety205326-1

ALREVIEWB74,205326͑2006͒

taxialdomainsandstabilizesaroundthebulktetracenevalue

ciatethemolecular-levelstructural

informationtoadistinctgrowthmorphologicalchangethat

occursaroundafewmononlayersofthickness.

MENTALDETAILS

preparationandSTM

lSTMimages͑150Åϫ150Å͒of͑a͒aclean

Si͑001͒surface͑filledstate,atabiasof−2.0V͒and͑b͒

monohydride-terminatedSi͑001͒͑emptystate,+2.0V͒,withatun-

hsurfaces,thealternating2ϫ1

and1ϫ2domainsareenergeticallydegenerate,withdimerrows

runningalongthe͓110͔and͓−110͔directions,

imageofthemonohydrideSi͑001͒hasthetunnelingintensitymini-

mumsatthetroughsbetweenthedimerrows,asindicatedbythe

darklinesin͑b͒,clearlydifferentfromthatforanempty-stateim-

ageofacleanSi͑001͒under+2.0V͑seeRef.16͒.

fiucturalpropertiesof

interestincludethein-planeunitcelldimensionsandpos-

sibleregistrationwiththesubstratelatticeandthemolecule

tiltingangles,andinadditiontheevolutionofthesestruc-

turalparameterswiththefipaper,we

reportacombinedmultitechniqueinvestigationonthese

structuralcharacterizationsforthetetracenefi

thatthefilmofafewmonolayerscoveragepossessesappar-

entepitaxialdomainsonthehydrogen-passivatedSi͑001͒

substratewithabulk-kindherringbonelatticeandacompres-

,

theaveragemoleculartiltinganglewithrespecttothesur-

facefirstincreaseswiththecoverageinformingtheexpi-

Thefilmgrowthandinsituscanningtunnelingmicros-

copy͑STM͒studieswereconductedinatwo-chamberultra-

highvacuum͑UHV͒STMandgrowthsystembuiltinour

laboratory.

19

Thetwochamberswereisolatedbyagatevalve

withbasepressureϳ1ϫ10

−10

TorrfortheSTMchamber

andϳ3ϫ10

−9

Torrforthegrowthchamber,respectively.

Samplescanbetransferredbetweenthetwochambersin

vacuumwithatransferarm.

IntheSTMchamber,Si͑001͒waferswerecleanedby

ubstratewasresistivelyheated

andthetemperaturewasmonitoredbyaC-typethermo-

situsubstratecleaningprocedureinvolvesdegassingthesili-

conwaferat970Kandflashingat1470Kforϳ1min,dur-

ingwhichtimethechamberpressureremainedinthe

10

−10

Torrrange.

Atomichydrogenpassivationwascarriedoutwiththe

siliconsubstrateatϳociatingmolecularhy-

drogen͑H

2

͒gasviaatungstenfilamentheatedatϳ1800K,

wegeneratedatomichydrogeninsidetheSTMchamberby

fillingH

2

gasintothechamberatapressureofϳ1

ϫ10

−6

Torrfor30mininordertoformmonohydride

H/Si͑001͒-͑2ϫ1͒surface.

20

Thesubstratewasplaced

ϳ8cmawayfromthetungstenfihehydrogen

passivation,thesubstratewasradiationcooledandtrans-

ferredtothefilmgrowthchamber.

Tetracene͑98%,Sigma-Aldrich͒wasevaporatedinthe

growthchamberwiththesamesetupandprocedurede-

scribedpreviously.

14

Thedepositionrateusedwas

1nm/min,whichwasmonitoredbyaquartzcrystalmi-

filmgrowthwasdonewiththesubstrate

filmgrowth,thesamplewasthen

transferredbacktotheSTMchamberforimaging.

Thehome-madeSTM͑Ref.19͒usesaquadranttube

scannerandtwoinchwormsfortheZandXcoarsemotion.

Swartzentruber

21

typeofcontrolelectronicsanddataacqui-

STMimages

presentedherewereobtainedinaconstant-currentmode.

Thesamplebiasusedinimagingtetracenefilmwas+2.4V

andtunnelingcurrent20pA.

Aftersubstratepreparationandfilmevaporationinsidethe

UHVsystemasdescribedabove,thesamplecanberemoved

scaletopographicimageswereobtainedbyanexsituatomic

forcemicroscopy͑DigitalInstrumentsDimension™3100͒in

tappingmodewithSitips,andtheimageswereanalyzed

usingsoftwareWSxM.

22

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STRUCTUREOFTETRACENEFILMSONHYDROGEN-…

PHYSICALREVIEWB74,205326͑2006͒

Polarizationdependentcarbon1snearedgex-rayabsorp-

tionfinestructure͑NEXAFS͒͑Ref.23͒characterizationwas

performedexsituonthetetracenefilmswiththreedifferent

thicknesses͑1.2ML,3ML,and15ML͒thatwereprepared

usingthegrowthsystemandproceduredescribedabove.

Multiplesamplesforeachtetracenefilmthicknesswereused

andconsistentresultsamongthedifferentbatchofsamples

AFSexperimentwascarriedatthe

SGMbeamline͑11ID-2͒attheCanadianLightSource͑CLS͒

inSaskatoon,lectronyieldwasusedfor

themeasurementsunderdifferentbeamincidenceangles͑

␪

͒

fluorescenceyieldwas

monitoredsimultaneouslyusingamultichannelplatedetec-

torandmainlyusedforsignaldiagnosticpurposeinthis

metryformanipulatingsampleorienta-

tionwithrespecttothedirectionoftheincomingsynchrotron

x-raybeamwasthesameasthatinRef.23andtheNEXAFS

SGMsolid-statechambervacuumwasbetweenlow10

−8

to

low10

−9

rtoavoidthe

effectofbeamdamageonthesample,eachsamplingarea

wasusedforonepairofthemeasurementsattwoincident

angles͑i.e.,

␪

=90°and10°͒

themeasurements,theCLSstorageringcurrentwasbetween

150mAto200mA,andthemonochromatorentranceand

exitslitsweresettobe5

␮

mand25

␮

m,

monochromatorenergyscalewascalibratedbymeasuring

COgasphaseC1s→

␲

*

resonanceenergyat287.40eV

͑Ref.24͒duringthesameexperimentalrunusingagascell

connectedtothesolidstatechamberontheSGMbeamline.

Thebeamlineopticaltransmissionfunctionwasmeasured

usingaphotodiodeatnormalincidenceandthebackground

dataforthetetracenefilmsamplesweremeasuredfroma

Si͑001͒databackgroundcorrectionandnor-

malizationwerecarriedoutusingthesamemultiplestep

procedureasthatinRef.25.

S

-spaceimaging(STMandAFM)

FIG.3.͑a͒AFMphaseimage͑100

␮

mϫ100

␮

m͒showssub-

monolayertetracenefractals͑darkfeature͒.͑b͒Azoom-intopo-

graphicimage͑20

␮

mϫ20

␮

m͒oftheareainthewhiteframeof

͑a͒.͑c͒Alineprofiletakenfromthewhitebarmarkedbytwo

trianglesin͑b͒showstheheightofϳ1nmforthelayer.

Figure3displaysthesurfacemorphologyofatetracene

submonolayerfilmonH-terminatedSi͑001͒preparedasde-

ctedfromourpreviouswork,

14

thefilmislandsappearasfractalsunderthegrowthcondi-

turedimensioniscloseto100microns͓Fig.

3͑a͔͒,muchlargerthanthevalueweobtainedpreviously

withaH-terminatedsubstratepreparedusingawetchemistry

method.

14

InFig.3͑b͒acloseAFMexaminationrevealsde-

tailsofafractalandalsopresentsthemorphologyofthe

substratewithvisiblesingleatomic-layerstepsthatareori-

sno

clearindicationofinfluencefromthesesingle-atomic-layer

stepsinthedevelopmentofthefractalbranches:branch

splittingandextendingoccuronbothlocalhillsandvalleys

ghtsofthesesubmonolayerislands

aremeasuredusingtheAFMimages,avaluearoundϳ1nm

istypicallyobtained͓Fig.3͑c͔͒,whichissmallerthanthe

lengthofthetetracenemolecule͑Fig.1͒.Thisapparentlayer

thicknesssuggeststhatthemoleculesareessentiallyinan

uprightstandingconfigurationinthesubmonolayerfilmbut

likelytiltedawayfromsurfacenormaldirection.

Tocharacterizetheresultingstructureofthefractalfilms

atthemolecularscale,effortshavebeenmadetouseour

STMtoimagesampleswitheithersubmonolayercoverage

orwithacoveragethathasthetopsurfaceislandsbeingthe

secondmolecularlayer͓i.e.,thenominalthicknessisϳ1.2

ML͑Ref.14͔͒.Sofarnoorderedstructureshavebeenim-

agedfromthefiot

excludethepossibilitythattheabsenceofmolecularfeatures

intheSTMimageofthefilmsisaresultoftheweak

molecule-substrateinteractionsuchthatthesoftfilmbe-

comessusceptibletodeformationsinducedbytheSTM

therhand,theSTMresults

couldalsoindicateapossibilitythatthereisasignificant

205326-3

ALREVIEWB74,205326͑2006͒

ge͑85Åϫ85Å͒ofϳ3MLtetracenefilmon

H/Si͑001͒ngularlatticemimicsthetetracene“her-

ringbone”rdinatesin-

-

entationoftheherringbonelatticeappearstofollowthesubstrate

lattice.

densityofdifferentmoleculartiltingdomainsthatheavily

ethe

fractalformationislargelybasedondiffusion-limitedaggre-

gation͑DLA͒͑hitandstick͒mechanism,

11,14

theas-grown

firstlayercouldbehighlydisordered.

SuccessfulSTMimagingofthemolecularpackinghas

beenrealizedwhenwedepositthreemonolayer͑ML͒͑nomi-

nalthickness͒4showsa

typicalSTMimageofsuchfiracteristic“her-

ringbone”molecularpackingknownfromthebulkcrystal-

linetetraceneisclearlyvisible,withtheintensityprotrusions

ensityprotrusions

formrows,withonerowbeingbrighterthanitsadjacent

rowinanalternatingpattern,andtheunitcellappearsrect-

nSTMimagesofmultipleindependent

samples,wecalibratetheherringbonelattice͑e.g.,Fig.4͒as

witha=7.3±0.6Åandb=5.5±0.6Å,closelyresembling

theabunitcelldimensionsofbulkcrystallinetetracene͑Fig.

1͒r

words,theSTMimageinFig.4providesdirectevidencethat

thefilmiscrystallizedwithstandingtetracenemoleculeson

theabplaneontheinertsubstrateused.

AnotherstrikingfeatureoftheSTMimage͑Fig.4͒isthat

thebrightrows͑i.e.,alongthebaxis͒appearmatchingthe

esamesampleandimag-

ingorientationsettingsasthatusedforthesiliconsubstrate

͓Fig.2͑a͔͒,theb-axisorientationimagedforthetetracene

filminFig.4appearspreciselyalongthe͓110͔directionas

markedinFig.2͑a͒anySTMimages

obtained,theorientationsofthebrightrows͑alongbaxisin

Fig.4͒areeitherasthatshowninFig.4orappearingor-

filmlatticeorientationalignedtothe

crystallinesubstrateindicatesthatthemoleculescanrecog-

nizeandlockintosomespecificstructuresitesonthesub-

strateatthisfilmcoverage.

Itisplausiblethattheherringbonelatticeobserved͑Fig.

4͒hasacommensuratestructuralrelationshiptothesubstrate

tiontotheorientationalignmentbetweenthe

molecularstructureandthecrystallinesubstrateasmen-

tionedabove,thecalibratedspacingbetweenthebrightrows

͑i.e.,thelatticeconstantmeasuredalongaaxis͒alsomatches

wellwiththedistancebetweenthesubstratemonohydride

dimerrows͑7.68Å͒withintheuncertaintyofthemeasure-

hebaxis͑i.e.,brightrow͒directioninFig.4,

thelatticematchingisnotobvious,asneithersubstratedimer

spacing͑3.84Å͒nordimerrowspacing͑7.68Å͒matchthe

r,themultipleofb͑e.g.,5timesaslarge͒in

principlecouldleadtoasuperlatticetomatchthesubstrate

͑as5ϫ6.03Å=8ϫ3.84Å=4ϫ7.68Å͒.Thoughthereisno

visiblesuperlatticemodulationofimagecontrastindicated

fromourSTMdata,theexistenceofthesuperlatticecannot

beexcluded,asthemodulationcausedbytheinterfacecould

besmearedoutwhenimagedfromafewmolecularlayers

above.

Figure5showsanAFMimageofa3MLtetracenefilm

onaH-terminatedSi͑001͒geaddslarge-

scalemorphologyinformationtowhatwelearnedfromthe

rofiletakenatthewhite

bar͑AB͒isshownatthebottomofFig.5,indicatingthe

heightofϳ1.4nmforeverylayerincludingtheburiedfirst

layeratthefirasttotheheight

measuredonthefractalislands͑Fig.1͒,thefirstlayermol-

eculesinthemultilayerregionsmusthavehadtheirorienta-

tionsandpackingrearrangedtoincreasethetiltangleand

eprofileinFig.

5alsoallowsacleardefinitionofthelayers,welabelthem

ontheimagewiththesecondlayermarkedas“2,”andthe

thirdlayeras“3,”eneverobtainanywell-

definedsurfacestructurewithSTMwhenthefilmswerepre-

paredwithonlythefirsttwomonolayerspresented,themo-

lecularlyresolvedSTMimages͑e.g.,Fig.4͒mostlikely

reflectthestructureofthethirdorhigherlayerinthesam-

heless,allthemolecularlyresolved

STMimagesconsistentlyshowthesurfacesymmetryasthat

inFig.4andorientationregistrationtothesubstratelattice

discussedabove.

TheAFMimageshowninFig.5indicatesthatthefilm

hasadramaticstructurechangeandishighlycrystallizedon

,theearlythirdlayer

islands͑Fig.5͒presenttwo-dimensional͑2D͒dendriteswith

preferredbranchorientationsappearingperpendicularto

eachother,inclearcontrasttothesubmonolayerfractalmor-

phology͑Fig.2͒thathaverandomlyorientatedbranches.

Second,thereisasignificantchangeonthenucleationden-

sityontopofthethirdlayer:thedensityofthefourthlayer

islandsismuchhigherthanthatofthepreviouslayers͑

Fig.

6͒,exhibitingalowersurfacediffusivityonthethirdlayer.

Third,thestep-edgenucleationappearseffectiveoncethe

thirdlayerisformed͑e.g.,asmarkedbyanarrowinFig.5͒

thatdoesnotoccurinthelowerlayers,indicatingastructure

changethatcausesEhrlich-Schwoebelstepbarrierformedat

y,theformationofhighlycrys-

tallizedlayersisconsistentwithourSTMresultsthatawell-

-

paredtothefilmmorphologyinFig.6withthatinFig.3,it

205326-4

STRUCTUREOFTETRACENEFILMSONHYDROGEN-…PHYSICALREVIEWB74,205326͑2006͒

caleAFMimage͑50

␮

mϫ45

␮

m͒ofa3ML

tetracenefilmonH/Si͑001͒.Mostoftheanisotropicislandswith

ond

andthethirdlayersaremarkedby2and3intheimage,respec-

rdinatesatthelowerleftcornerindicatethatofthe

substratelattice.

ge͑25

␮

mϫ45

␮

m͒ofa3MLtetracenefilm

onH/Si͑001͒withanisotropictetraceneislandsonthethirdand

rofiletakenatthewhitebar͑AB͒isshownat

thebottom,indicatingtheheightofϳ1.4nmforeachlayerand

suggestingthesecond,third,andfourthlayersasmarkedby2,3,

and4intheupperimage,rdinatesindicatethat

wmarksastep-edge

nucleationoccurringonathirdlayerisland.

isconcludedthatagrowthmodechangehasoccurredbe-

tweenthesecoveragestages.

Inmostcases,theorthogonalbrancheddendrites͑i.e.,the

crosslikeislandsinFigs.5and6͒appeartohavetheirtwo

orthogonalbranchesorientedalongthesubstratedomaindi-

rections͑Fig.6͒,i.e.,alongsubstrate͓110͔and͓−110͔direc-

tions,,somedendriticislandsrotatedap-

proximately45°inazimuthangleawayfromthesubstrate

latticedirectionsarealsoobserved,asshowninthemiddle

ndenttowhichwaythedendriticis-

landsareorientedwithrespecttothesubstratelattice,they

alwayshavetheirtwomainbranchaxesappearingmutually

ructurecharacterforthedendritesisnot

onlyconsistentwithwhatwehavediscussedthatthesethird

layerislandsarehighlycrystallized,butalsoindicatingthat

theirformationismainlydrivenbymolecularself-

tthatthemajorityofthesesurfaceis-

landsfollowthesubstratelatticeorientations͑Fig.6͒sug-

geststheexistenceofanon-negligibleinfluenceofthe

substratelatticeinthefilmnucleationandgrowth,presum-

ablythecommensurateregistrationismoreenergeticallyfa-

vorable.

Thetwoapparentlyorthogonalorientationsofthe2Dden-

dritesshownonthethirdlayerofthefilmarebelievedtobe

theherringbonelattice͓010͔and͓100͔directions͑i.e.,along

thedirectionsofbaxisandaaxis,respectively,inFig.1͒.

Theoreticalresultsonpentacenecrystalshowthatthemo-

lecularbindingenergies͑E

b

͒atthein-planefacetsareinthe

followingrelation:

7

E

b

͑010͒ϾE

b

͑100͒ϾE

b

͑otherfacets͒.

Thecorrespondinggrowthvelocitiesofthe͑010͒and͑100͒

stepsarethusrelativelyhigherthantheotherfacetsteps,

7

i.e.,thegrowthprobabilityalongthebaxisisthehighestand

theaaxisthesecondhighest͓seeFig.1͑b͔͒.Ifasimilar

bindingenergyrelationisapplicabletothecaseoftetracene

duetothestructuralsimilaritybetweenthetwopolyacenes,

thenthepreferredislandbranchdevelopmentsobserved͑Fig.

5͒canbeexplainedbytherelativelyhighergrowthprobabil-

ityalongthebaxisandaaxis͑whichareclosetoorthogonal

toeachother͒,withthelongerdimensionbeingalongtheb

axis.

B.X-rayabsorptionspectroscopy(NEXAFS)

Toexaminetheinteriorstructureofthefilmandhowit

evolveswiththefilmmorphologychangediscussedabove,

polarizationdependentcarbon1snearedgex-rayabsorption

finestructure͑NEXAFS͒͑Ref.23͒

thisapplicationtheC1s→

␲

*

photoionizationresonancein-

tensity͑I

␲

*

͒ofthecarbonatomsinthesampleismeasured,

whichvarieswiththerelativeorientationbetweenthepolar-

izationoftheincomingx-rayphoton

␧

andthetransition

dipolemoment͑P͒ofthemoleculesfollowingtherelation-

shipI

␲

*

ϰ͉

␧

·P͉

2

.Theorientationoftransitiondipolemoment

205326-5

ALREVIEWB74,205326͑2006͒

thefisconfirmedbyour

NEXAFSmeasurements.

26

Inthissimplifiedhighsymmetry

case,onlytwoNEXAFSmeasurementsatdifferentx-ray

incidentangles͑

␪

͒areneeded

22

fordeterminingtheaverage

moleculetiltingangle͑

␣

inFig.7͒.Theresonanceintensity

cannowbedescribedbythefollowingexpression:

I

␲

*

͑

␪

,

␣

͒ϰ

1

p

1+

͑3cos

2

␪

−1͒͑3cos

2

␣

−1͒

2

3

+

1−p

sin

2

␣

,

2

ͫͬ

͑1͒

ticviewoftheset-upgeometryfortheNEXAFS

enemoleculeisrepresentedbytherectangular

lebetweenthemoleculartransitiondipolemo-

ment͑P͒͑orientednormaltotheplaneofaromaticrings͒andthe

substratesurfacenormal͑Z͒isthetiltingangle

␣

measuredby

NEXAFS.X-rayincidenceangle͑

␪

͒withrespecttothesubstrate

surfaceisadjustedbyrotatingaroundR͑x-raydirectionisstation-

aryinexperiment͒.Thelinearpolarizationdirectionofthexrayis

indicatedbythevector

␧

.

Pforthe

␲

*

statesofthearomaticringsisalongthenormal

orebymeasuringI

␲

*

atdiffer-

entx-raypolarizations͑varying

␧

͒theorientationofPand

hencethatofthemoleculewithrespecttothesubstratecan

urrentcasethemolecularorientation

obtainedrepresentsanaverageoverallthetetracenemol-

eculelayers.

Theangularrelationshipsamongthevariousquantities

involvedintheNEXAFSexperimentareillustratedinFig.7.

Forx-raysincidentalongthesurfacenormal͑

␪

=90°͒,an

uprightstandingmoleculesuchastheoneschematically

showninFig.7wouldyieldstrong

␲

*

resonances,whilefor

grazing-incidencexrays͑

␪

=10°͒the

␲

*

resonanceintensity

attheangle

␣

inFig.7isthe

polarangleofPrelativetothesurfacenormalbutthereare

twowaysthemolecularplanecanbeorientedtoproduceit:

,asthecaseillustrated

inFig.7;orwhenthemoleculelongaxisliesintheplane

ude

thelattercasebecausetheSTMresultsshowninFig.4and

theAFMthicknessestimates͑Figs.3and5͒clearlyindicate

thatthroughallthegrowthstagesthemoleculesareina

ore,thebiggertiltingangle

␣

resultedfromNEXAFSmeasurementsindicatesmoreup-

rightstandingorientation.

Ingeneral,theresonanceintensityI

␲

*

mayalsodepend

ontheazimuthorientationofthedipoletransitionmoment

rwhenthe

substratelatticepossessesathreefoldorhigherorderofsym-

metry,theazimuthdependenceofI

␲

*

vanishes.

23

Con-

sideringthattheH/Si͑100͒-2ϫ1substratepossessestwode-

generatereconstructiondomains͑2ϫ1and1ϫ2͒orthogonal

toeachother,underthemacroscopicsamplingareacovered

byx-raybeam,thesubstratesurfacewouldbehaveasifthere

isa2ϫ2͑fourfold͒oreinourcasethe

NEXAFSofthetetracenefilmsshouldbeindependentto

wherep=͉E

ʈ

͉

2

/͉͑E

ʈ

͉

2

+͉E

Ќ

͉

2

͒isthedegreeoflinearpolariza-

tionintheplaneofthesynchrotronstorageringelectron

beamorbitandE

ʈ

andE

Ќ

aretheprojectionsofthesynchro-

tronx-rayinandoutofelectronorbitplane,

radiationfromaplanarundulatorasthecaseinthisstudy,the

polarizationisalwayslinearifobservedinthesamehorizon-

talplaneoftheinsertiondevicecenter.

27

Whenconsidering

variousfactors,theestimateduncertaintyof

␪

usingthecur-

rentCLSSGMbeamlinesetupis±3°,

26

sothedegreeof

linearpolarizationpisestimatedtobeintherangeof97%to

100%.Theresultsonusingeither100%or97%forpdiffer

lessthan2°andthisuncertaintywillbeincludedinthefinal

␣

angleerrorbartobereportedbelow.

Figure8showsthetypicalNEXAFSdataforthethree

tetracenefilmthicknesses,withbeamatnormalincidence

strongcontrastinthe

␲

*

resonanceintensityfromthetwo

beampolarizationsindicatesthatallthefilmsofdifferent

thicknessespossessahighdegreeofmolecularorientation

rallspectralfeaturesinFig.8areconsistent

withtheresultspreviouslyreportedbyYokoyamaetal.

28

for

thicktetracenefilmmeasuredatthemagicangle,butwith

moredetailresolvedbelowtheionizationpotential͑IP͒

aroundϳongestresonancepeakat285.7eV,

whichhasbeengenerallyassignedasaC1sto

␲

*

-orbitals

resonanceinpolyacenes,

29,30

hasitsenergylocationandin-

tensityunchangedthroughthefilmthicknessesmeasured.

Thisisactuallythecasefortheotherresolvedfeaturesbelow

IPaswell.

31

Thereforechemicallythemoleculesinallthe

filmsaresimilartoeachother,whichisconsistentwiththe

expectedchemicallyweakinfluenceofthefilm-substratein-

terfaceinteraction.

Quantitativeassessmentsonthemolecularorientationre-

quiremolecularorbitalidentificationsofobservedresonance

ticalstudybyĂgrenetal.

29

has

shownthatthediscreteresonancefeaturesintetracene

NEXAFSbelowtheionizationpotentialconsistpredomi-

nantlyinthesplitsingle

␲

*

statesoftheexcitedcarbonat-

eadingofthe

␲

*

resonancehasbeenmainly

attributedtoenergyandintensityvariationoftheresonance

causedbythecore-holeinteractioneffect͑finalstateeffect͒,

andinadditiontothesite-dependentcoreionizationenergy

͑initialstateeffect͒.

29,30

SimilarconclusionasthatofĂgren

chedinarecentNEXAFSstudyofepitaxial

growthpentacenefilmsonCu͑110͒surfacebySöhnchenet

al.,

32

wherethediscreteresonancefeaturesbelowtheioniza-

tionpotentialwerefoundtohaveanalmostpure

␲

*

charac-

205326-6

STRUCTUREOFTETRACENEFILMSONHYDROGEN-…PHYSICALREVIEWB74,205326͑2006͒

suredaveragemoleculetiltingangle͑

␣

͒asa

functionofthefiesegmentslinkingthepoints

areonlyforvisualaidpurpose.

onthiscurvewithanexperimentallymeasuredresonance

intensityratiothecorrespondingangle

␣

-

titativeanalysisofthedatainFig.8leadstotheresultsof

␣

varyingasthefilmthickness,whichissummarizedinTable

IandplottedinFig.9.

SIONS

K-edgeNEXAFSdatafortetracenefilmson

H/Si͑001͒.Thefilmthicknessesareasmarkedinthefi

datawithxrayatnormalincidence͑

␪

=90°͒andgrazing-incidence

͑

␪

=10°͒areshownbythesolidlinesanddottedlines,respectively.

Datafor3MLand15MLareshiftedverticallyforclarity.

ntheseresults,wetreatalltheresonancefeatures

belowIPinFig.8asdueto

␲

*

contributions͑I

␲

*

͒,andthe

insignificantintensitywith

␴

*

characterfromthetetracene

endatoms

29

inthisspectralregionisneglected.

26

Withthese

spectralassignments,theclearpolarizationdependenceof

the

␲

*

statesinFig.8indicatesthatthemoleculesinthese

fiaunder-

neaththe

␲

*

resonanceswasintegratedfrom

284.2eVto291.0eVtoobtainthemeasuredresonancein-

tensitiesatthetwopolarizationanglesandhencetheexperi-

mentalI

␲

*

͑

␪

=10°͒/I

␲

*

͑

␪

=90°͒ratios͑TableI͒.Usingthe

expression͑1͒,thetheoreticalI

␲

*

͑

␪

=10°͒/I

␲

*

͑

␪

=90°͒ratio

asafunctionofmolecularorientationangle

␣

isplotted,and

inationoftheaveragemoleculetiltingangle

͑

␣

͒betweenthemoleculartransitiondipolemomentandthesub-

stratenormal.

Experimental

I͑

␪

=10°͒/I͑

␪

=90°͒

13.9%

20.6%

46.7%

Thickness͑ML͒

15ML

3ML

1.2ML

␣

78°±4°

74°±4°

65°±3°

Someinsightsaboutthestructuralevolutioninthetet-

racenefilmgrowthonH/Si͑100͒-2ϫ1boreoutbycompar-

,the

laterallatticeorientationanddimensionsgivenbySTM,the

interlayerseparationormonolayerheightandthemorphol-

ogymeasuredbyAFM,andthemoleculartiltinganglesasa

functionofthefilmthicknessbyNEXAFS.

Forbulktetracenecrystal,theunitcelloftheherringbone

-

specttotheabplanenormal,thesetwomoleculeshavetheir

molecularplanenormalorientedatanglesof81.9°and

70.1°,respectively.

4

Therefore,theaveragetiltingangleof

thetwomolecular-planenormalsis76°forthebulktetracene

crystal.

33

Inotherwords,ifmeasuredbyNEXAFS,theav-

eragetiltingangle͑

␣

͒forthebulkcasewouldbe76°.

TheNEXAFSdataclearlyindicatethatthemeasured

␣

of

thetetracenefilmchangeswithincreasingthickness͑TableI

andFig.9͒.Forthe1.2MLfilm,theaveragetiltingangleis

65°±3°,wellbelowthebulkcase͑76°͒.Consideringthe

smallerheight͑ϳ1nm͒obtainedfromAFMfortheuncov-

eredfirstmonolayerandthecorrespondingresultofnonor-

deredstructuresimagedfromSTM,itisnosurprisethatthe

filmexhibitsadifferentmoleculartiltinganglefromthatof

ainedlowertiltinganglein

conjunctionwiththeAFMandSTMdatasuggeststhatthe

filmissignificantlydisorderedwithanaveragemolecular

densitylowerthanthatinthebulkcrystal.

At3MLnominalcoverage,NEXAFSresultsindicatethat

theaveragetiltinganglehasincreasedsignificantlyto

74°±4°,AFMalso

indicatesignificantstructuralchangesatthisfilmthickness:

themolecularlyresolvedSTMimagesexhibitorderedsur-

facestructureswithcommensuraterelationstothesubstrate

205326-7

ALREVIEWB74,205326͑2006͒

lattice;andtheAFMimagesillustratedrasticchangesinthe

filmmorphologyandaunifiedlengthforthemonolayer

eve

thattheseresultsfromdifferenttechniquesmustbeorigi-

-

tributethesignificantincreaseofmoleculetiltingangleat3

MLasduetotheformationoftheextendedepitaxialdo-

mains͑Fig.4andtheanisotropicislandsonandabovethe

thirdlayerinFig.5͒.EventhoughtheSTMimagingmostly

showsthelatticesymmetryofthetoplayerofthefilm,judg-

ingfromtheabsenceofanyadditionalcontrastmodulation

intheSTMimaging,thedifferenceinlatticesymmetrybe-

addition,theessentiallysamelayerheightforeachlayerin

themultilayerislandsshownbyAFM͑Fig.5͒indicatesa

verticalcrystallineuniformityamongthelayers,including

thefirstmonolayerattheinterface,whichisconsistentwith

ourconjecturethattheherringbonelikecrystallinesymmetry

inFig.4hasbeenconfiguredthroughallthelayersforming

ingthelatticeconstantsbe-

tweenabulktetracenecrystal͑a=7.90Å͒andthesilicon

substrate͑dimerrowspacing=7.68Å͒,alaterallycompres-

sivefi-

ently,inalaterallycompressedepitaxialdomainthemol-

eculeswouldstandmoreuprightthaninthebulkto

t

thattheaveragetitlingangleforthewholefilmfrom

NEXAFSisaboutthebulkvaluewouldrequiresomemol-

eculesinthefiects

maybeaccountable:͑1͒Theareaswhereonlytwomonolay-

erspresentinFig.5arelikelythemodesttiltinganglere-

gions,astheorderedstructureswerenotobtainedfromour

STMmeasurementsontopofasecondlayer;͑2͒fromthe

filmmorphologyshowninFigs.5and6thereareterraced

islandsuptosevenlayersabovetheinterfacewherethetilt-

inganglefortheupperlayermoleculesareexpected,lower,

asthefilmlatticeisanticipatedtobegraduallyrelaxedalong

thedistanceawayfromtheinterface.

Itappearsthatthereisastructuralphasetransitionasso-

ciatedwiththefitiontothepresented

data,wenoticethatthehighlycrystallizedislands͑i.e.,the

apparentepitaxialdomains͒emergewiththethirdlayerfor-

mationonthesurfacefromourAFMmeasurements.

34

The

speculatethatwiththeincreasingcoveragemoremolecules

maysqueezeintothelessdenselypackedfirstandsecond

layers;thedensificationofthelayerswouldenhancethein-

tralayermolecularinteractionandleadtoarearrangementon

thepackingandorientationtowardsthebulkconfiguration,

andtheinfluenceofthesubstratelatticeremainssomewhat

effective,thereforeresultingintheapparentepitaxialdo-

rfilmcoverage-dependentrearrange-

mentofmoleculeswasreportedforphysisorbedbenzeneon

Ru͑001͒surface,

3,35

wheretheinitiallyparallelorientedfirst

physisorbedlayer͑abovetheparallelorientedfirstchemi-

sorbedlayeronthecleanRusurface͒wasfoundtorearrange

intoamorecrowdedlayerwithahighmoleculartiltangleat

highercoverages͑ജ1ML͒.

35

Inthegrowthoftetraceneon

H/Si͑001͒inthiswork,theeffectoftheproposedmolecule

rearrangementandreorientationissignificantatabout3ML,

andwebelievethattheapparentepitaxialdomains͑Fig.4͒

eticsand

thedrivingforceofthestructuretransitionrequirefurther

investigations

Afterthedrastictiltinganglechangeat3ML͑Fig.9͒,a

newtypeofgrowthtemplateappearsonthefilmsurface.

Comparingtothefastincreaseofthemoleculetiltingangle

from1.2MLto3ML,thefurthergrowthupto15ML

introducesaninsignificantincreaseinthemoleculetilting

angle͑Fig.9͒.Thesmallaveragetiltingangleincrease,if

meaningful,islikelyrelatedtotheexistenceofnonepitaxial

regionsat3MLwhicharetransformedtotheepitaxialdo-

atboth3MLand15ML

theaveragetiltingangles͑TableI͒arebasicallyinagreement

withthatoftheaveragebulkvalueof76°.

SIONS

ThestructureoftetracenefilmsonH-terminatedSi͑001͒,

obtainedundertheoptimizedlayer-by-layergrowth

process,

14

hasbeeninvestigatedbydifferentstructural

probes͑STM,AFM,NEXAFS͒.Allthefilmsinvestigated

exhibittetracenemoleculesinuprightstandinggeometryon

hefilmgrowthcondition,thelateral

packingforthemoleculesintheinitialfilmislargelydisor-

creasingthefilmcoveragetoaround3ML,a

structuralphasetransitionoccurswhichleadstotheforma-

tionofapparentepitaxialdomainswithacommensurate

abruptincreaseinthemoleculartiltingangleinthefilm

whenapproaching3MLcoverage͑NEXAFS͒andfromthe

compressivenatureofthelateralfilmlattice͑STM͒,itis

suggestedthatthemoleculesintheepitaxialdomainscould

betiltingmoreuprightthaninthecorrespondingbulkcase.

Forthegrowthbeyond3ML,theaveragemoleculetilting

anglelevelsaroundthebulkvalue.

ACKNOWLEDGMENTS

entruberfor

valuablediscussionsandhelpinsettingupourSTMcontrol

rforpermittingouraccesstohis

iketothank

TomRegierandTomKoztzerforassistanceinNEXAFS

rkwassup-

portedbytheNaturalScienceandEngineeringResearch

CouncilofCanada͑NSERC͒,andbytheCanadaFoundation

forInnovation͑CFI͒andtheOntarioInnovationTrust͑OIT͒.

TheoperationoftheCLSissupportedbyanNSERCMFA

grant.

205326-8

STRUCTUREOFTETRACENEFILMSONHYDROGEN-…PHYSICALREVIEWB74,205326͑2006͒

20

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thesameatomichydrogendosingconditionsbutatlower

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31

Note:Forthe1.2MLgrazingangledatainFig.8,thepeak

featureneartheonsetoftheresonances͑ϳ283.5eV͒islikely

duetotheimperfectionofthebackgroundcorrectionscheme,

notarealspectralfeature.

32

S.Söhnchen,,,.121,525

͑2004͒.

33

Itshouldbenotedthatfortetracenetheaveragetiltinganglein

NEXAFSisrelatedtobutdifferentfromtheanglebetweenthe

moleculelongaxisandthesubstratesurfaceplane,thelatterfor

thebulktetracenewouldbeϳ68°.

34

,ni,͑unpublished͒.

35

,.105,3838͑1996͒.

205326-9