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SupportingInformation
UltrafastBidirectionalChargeTransportandElectronDecoherenceAt
Molecule/SurfacesInterfaces:AComparisonofGold,Grapheneand
GrapheneNanoribbonSurfaces
OlgunAdak1,GregorKladnik2,3,GregorBavdek4,AlbanoCossaro5,AlbertoMorgante*3,5,Dean
Cvetko2,5*,LathaVenkataraman1,6*
1DepartmentofAppliedPhysicsandAppliedMathematics,ColumbiaUniversity,NewYork,NY
2FacultyofMathematicsandPhysics,UniversityofLjubljana,Ljubljana,Slovenia
3DepartmentofPhysics,UniversityofTrieste,Trieste,Italy
4FacultyofEducationUniversityofLjubljana,Ljubljana,Slovenia
5CNR-IOMLaboratorioNazionaleTASC,BasovizzaSS-14,km163.5,I-34012Trieste,Italy
6DepartmentofChemistry,ColumbiaUniversity
email:cvetko@iom.cnr.it,lv2117@columbia.edu,morgante@iom.cnr.it
Contents:
1. MeasurementDetailsandSamplePreparation
2. XPSandNEXAFSMeasurements
3. DFTCalculationsforNEXAFSspectra
4. TheRPESMeasurements
5. TheRPESCore-Hole-ClockMethodwithChargeTransferfromtheSubstrate
6. References
2
1.MeasurementDetailsandSamplePreparation
TheexperimentswereconductedattheALOISAbeamlineoftheElettraSynchrotroninTrieste.
Themeasurementchamberwasmaintainedatanultrahigh-vacuumatpressuresof10−10−10−11
mbar.1-3The4,4’-bipyridine (BP)moleculeswerepurchased fromSigma-Aldrich (purity>99%)
andusedwithoutfurtherpurification.ThemoleculeswereplacedinaPyrexcellandconnected
tothepre-chamberthroughaleakvalve.
TheAu(111)surfacewaspreparedbycyclesofAr+sputteringandannealingat400K.X-
rayphotoemissionspectroscopy(XPS)wasusedtocheckforanychemicalimpurities(O,N,and
C). The BP cellwas heated to several tens of degrees above room temperature and BPwas
leakedintoapre-chambertomaintainapartialBPpressureof10−7mbarfor4-5minutes.The
samplewascooledto-65°Cforthemultilayerphase,anditwaskeptatroomtemperaturefor
themonolayerphase.
TheepitaxialgrapheneonNi(111)surfacewaspreparedbyrepeatedlysputtering(Ar+,
2keV)andannealing(900K)theNi(111)surface.XPSandHeliumatomscatteringwereusedto
check for the presence of any chemical impurities and to probe the surface order. Epitaxial
graphenewaspreparedvia catalyticdissociationofethyleneonNi(111) surface.Anethylene
partialpressureof10-6mbarwasmaintainedfor60minuteswhilekeepingthesurfaceat850K.4
ThegraphenefilmwasprobedusingXPSandultravioletphotoemissionspectroscopywithHeII
lineat40.8eV.Examinationofthegrapheneπ-bandbottom,closetoΓ,wasusedtoidentifythe
graphenelayerasepitaxialgraphene.4Theepitaxialgraphene/Ni(111)samplewasexposedto
ambientconditionsbeforebeingtransferredtotheALOISAchamber.Thesamplewasannealed
3
in the ALOISA chamber at 500 K to recover the pristine graphene. XPS, valance band
photoemission spectroscopy and near edge X-ray absorption fine structure (NEXAFS)
spectroscopy measurements were further performed to characterize the epitaxial
graphene/Ni(111)sample.TocreatetheBPself-limitingmonolayerfilm,apartialBPpressureof
10−7mbarwasmaintainedinthepre-chamberfor3minutes,whilethesamplewascooledto-
50°C.
Graphene nanoribbons (GNR) were formed depositing 10,10’-dibromo-9,9’-biantryl
(AOKBIO,98+%purity)onthecleanedAu(111)samplemaintainedat200Ctoensuresaturating
theAu(111)surfacewiththeGNRprecursor.Thissurfacewasthenannealedto400Ctocreate
GNRsasdescribedpreviously.5XPSandNEXAFSmeasurementswereperformedtocharacterize
theGNRfilmontheAu(111)surface.TocreatetheBPmonolayerfilm,apartialBPpressureof
10−7mbarwasmaintainedinthechamberfor5minuteswhilethesamplewascooledto-45°C.
2. X-ray Photoemission Spectroscopy and Near Edge X-ray Absorption Fine Structure
SpectroscopyMeasurements
X-ray photoemission spectroscopy (XPS) measurements were performed at the ALOISA
beamlinewiththeX-raybeamatgrazing-incidence(4°)tothesample.Photoelectronsfromthe
samplewere collected at the normal to the surface using a hemispherical electron analyzer
withanacceptanceangleof2°,andanoverallenergyresolutionof~0.2eV.Theenergyscale
forXPSspectrawascalibratedbyaligningtheAu4f7/2peaktoabindingenergyof84.0eVfor
theAu(111) andGNRonAu(111)measurements. For the epitaxial grapheneonNi(111), XPS
spectrawerealignedtotheFermilevel.
4
NearedgeX-rayabsorptionfinestructure(NEXAFS)measurementswereperformedon
thenitrogenK-edgebysweepingtheincidentphotonenergyfrom396eVto420eVinstepsof
0.1eV.Thephoton incidenceanglewasset to6°.Spectrawereacquiredusingachanneltron
detectorwithawideacceptanceangleinthepartialelectronyieldmodeandahighpassfilter
withcutoffenergyset to370eV.Thephotonfluxwasmonitoredonthe lastopticalelement
alongthebeampath.Thesamplenormalwasorientedeitherparallel(p-pol)orperpendicular
(s-pol)tothelightpolarization(s-pol).
The relative intensity of the NEXAFS signal in s-pol and p-pol for the N1s to LUMO
transitionwasusedtoobtaintheorientationofthearomaticringrelativetothesurface.The
angleθof the ring to thesurface isdeterminedas tan ! = 2!!/!!where Is and Ip are the
intensitiesoftheLUMONEXAFSpeakfromthes-polandp-polspectrarespectively.
3.DFTCalculationsforNEXAFSSpectra
We calculate the nitrogen K-edge spectra using GPAW, a grid-based real-space projector-
augmented-wave (PAW) code, with the BLYP exchange-correlation functional6, 7. For these
simulations, isolatedmoleculeswere first relaxed to theiroptimizedgeometries.Default grid
spacingsandconvergencethresholdswereemployed.AllNEXAFScalculationswereperformed
using the half-core-hole approximation8. The absolute energy scale was determined by
performing a delta Kohn-Sham calculation and shifting the calculated spectrum using the
calculatedtotalenergydifferencebetweenthegroundstateandthefirstcoreexcitedstate.
5
SI Figure 1. The measured magic-angle N K-edge NEXAFS spectrum (blue) for bipyridine onAu(111) compared with the calculated absorption spectra (dashed red) in the transitionpotential approximation. The calculated peaks with the highest transition probabilities areindicatedandassignedaccordingly.
4.TheRPESMeasurements
TheRPESspectraatthenitrogenK-edgewereobtainedbytakingXPSscansspanning0-60eV
bindingenergyrangewiththephotonenergytunedbetween394eVand423eVinstepsof0.2
eVand0.5eVforallsurfacesstudied.RPESmeasurementsforchargetransfertimecalculations
wereperformedinmagicangleconditions,withthelightpolarizationandtheelectronanalyzer
at54.7°withrespecttothesurfacenormal.ThisyieldedaRPESsignalthatwasindependentof
themolecular orientation on the sample. For the angle-dependent RPESmeasurements, the
anglebetweenthephotonpolarizationandsurfacenormalsettothevaluesstatedinthemain
textwhiletheelectronanalyzerwasalwayscollinearwiththephotonpolarization.TheseRPES
datawerethennormalizedandanalyzedfollowingpreviouslypublishedprocedures.9,10Briefly,
6
thephoton fluxon the lastopticalelementalong thebeampathwasmeasuredandused to
normalizeandcalibratetheRPESscansforanyfluctuationsinphotonintensityandenergy.The
energy scale for XPS spectrum was calibrated as detailed above. The non-resonant
photoemissionspectrumwasobtained fromtheXPSscanat395eVandsubtracted fromthe
entire RPES spectra. The RPES line scanswere then further normalized by the overall Auger
intensitywhichscaleswiththeabsorptioncross-sectionatagivenphotonenergy.
SIFigure2.NitrogenK-edgeRPESmapsof(A)BPmultilayeronAu(111)(B)BPmonolayeronAu(111),(C)BPmonolayeronepitaxialgrapheneand(D)BPmonolayeronGNR.TheLUMO*resonancescansaretakenalongthedashedblacklines.Thedashedorangelinesindicatetheenergyusedforthescansabovetheionizationedge.
7
InSIFigure2,weshownitrogenK-edgeRPESmapsofaBPmultilayeronAu(111)anda
BPmonolayer onAu(111), epitaxial graphene andGNR.We see theN1s to LUMO transition
signaturearoundphotonenergyof399eV.Fromthesemaps,weobtainthelinescansthatare
showninFigure3ofthemaintext.Thecore-holeclockmethoddescribedbelowisthenusedto
determinethechargetransfertimeandthefractionoftheLUMO*thatdropsbelowtheFermi
levelinthemonolayersystems.
5.TheRPESCore-Hole-ClockMethodwithChargeTransferfromtheSubstrate
Inthissection,wepresenttheextensionofthestandardcore-holeclockmethod11toobtainthe
bidirectionalchargetransfer timeandthe fractionof theLUMO*thatdropsbelowtheFermi
levelfromtheRPESmeasurements.
Inthecore-holeclockmethod,acoreelectron isphotoexcitedtotheLUMO, leavinga
core-holeonthemolecule;andtheenergydistributionofthesubsequentelectronemissionis
measured.Theprocessrelevanttothecore-holeclockmethodisthefillingofthecore-holeand
the subsequent emission of an electron, leaving the LUMO empty; this is denoted as the
participator decay. In a coupled system, the participator decay gets quenched if the charge
transfer to the substrate occurs within the core-hole life-time. Therefore, by comparing the
participatordecayintensityintheisolatedandthecoupledsystem,thechargetransfertimein
the coupled system can be determined as has been done before.11 In the measurements
presentedhere,whenacore-holeispresent,theLUMO*ispartiallybelowtheFermileveland
canthusgetoccupiedduetoachargetransferfromthesubstrate.
8
Todeterminethebi-directionalchargetransfertime,oneneedstoconsidertwotypesof
decay process that will contribute to the participator decay intensity. First, there is a
participatordecay thatoccurs fromtheLUMO*electron.Additionally, there isanAuger type
participatordecay (fromthe fractionof theLUMO*that isbelowFermi) thatcanoccurafter
chargetransferfromtheLUMO*tothesubstrate.Denotingtheparticipatordecayintensityas
IointheisolatedsystemandasIcinthecoupledsystem,wecangetarelationbetweenIoandIc
as:
CT CHc o o
CH CT CH CT
I I I xτ ττ τ τ τ
= ++ +
(1)
where τCH is the core-hole lifetime, τCT is the charge transfer time, x is the fraction of the
LUMO*belowtheFermilevel.ThefirstterminEq.1isthestandardexpressionthathasbeen
usedbeforeandisattributedtothephotoemissiontypeparticipatordecay.11Thesecondterm
isdue to theAuger typeparticipatordecay thatcanoccuronly if theLUMO* ispartlybelow
FermiandchargetransferfromtheLUMO*tothesubstratehasoccurred.Theprocessgiving
risetothesecondterminEq.1isidenticaltotheAugertypeparticipatordecayoccurringafter
the charge transfer from the substrate to LUMO* following an excitation of a core electron
above the ionization edge. We denote this process as the super-participator decay and its
intensity(Is)isgivenby
CHs o
CH BT
I I x ττ τ
=+
. (2)
Here,τBTisthebi-directionalchargetransfertime.
9
Sincethechargetransfertimeinbothdirectionsisequal,Eq.1and2canbesolvedto
findxandτCTwhichyields
(1 )1 (1 )CT CHff
βτ τ
β−
=− −
. (3)
1 (1 )
fx
fββ
=− −
(4)
wherefisIc/IoandβisIs/Ic.
Theequationsabovearevalidwhenthelightpolarizationisperpendiculartothenodal
planeofthenodalplaneofthemolecularπ-system.Therefore,weneedtoaccountforthefact
that the molecular π-system makes a small angle with the substrate, as determined from
NEXAFS measurements presented in the main text. We also need to consider the light
polarizationwhencomparingtheAugertypesuper-participatorintensitieswithphotoemission
typeparticipatorintensitysincethereisaclearangulardependenceinthelatter.
Thephotoemissiontypeparticipatordecayintensityisproportionalto(ε·n)2whereεis
thepolarizationunitvectorandn isthevectornormaltothenodalplaneofthemolecularπ-
system.Wecalculate the (ε·n)2 as a functionofε andn assuming that there isnoazimuthal
dependenceonthemolecularorientationonthesurface.Weuse
sin( ) cos( )x zε θ θ∧ ∧
= + (5)
n = sinα cos(φ −π / 2) x∧
+ sinα sin(φ −π / 2) y∧
+ cosα z∧
(6)
10
whereθisthepolaranglebetweenthesurfacenormalandthepolarizationunitvector,αisthe
polaranglebetweenthesurfacenormalandthenormaltothemolecularπ-system,andφisthe
azimuthalangleofthemolecularπ-system.
Averaging(ε·n)2overφ,andusingEq.5andEq.6weget:
22 2
2 2sin sin( ) co· s cos
2n θ α
θε α= + (7)
Since measurements are made at the magic angle (θ =54.7°), cos2θ=sin2θ/2, and thus the
average(ε·n)2isindependentofα.Weobtain:
Icm = Io
m τCTτCH +τCT
+Iom
cos2θx
τCHτCH +τCT
(8)
Ism =
Iom
cos2θx
τCHτCH +τ BT
(9)
where mcI , m
sI and moI arethemeasuredintensitiesatthemagicangle.SolvingEq.8andEq.9
yields,
(1 )
1 (1 )CT CHff
βτ τ
β−
=− −
(10)
x = cos2θ f β1− f (1−β)
. (11)
wherefis mcI / m
oI andβis msI / m
cI .
11
6.References
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