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UNIVERSITÀ DEGLI STUDI DI TRIESTE XXIX CICLO DEL DOTTORATO DI RICERCA IN
NANOTECNOLOGIE
STRUCTURAL AND ELECTRONIC COUPLING OF ORGANIC
HETEROAROMATIC MOLECULES ON INORGANIC SURFACES OF OXIDES
settore scientifico-disciplinare: FIS/03 – Fisica della Materia
DOTTORANDO
Marcos Domínguez Rivera
COORDINATOR
Prof.ssa Lucia Pasquato
SUPERVISORE
Prof. Alberto Morgante
TUTORE
Dr. Luca Floreano
ANNO ACCADEMICO 2015/2016
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CONTENTS
1. INTRODUCTION...................................................................................................................1
2. EXPERIMENTALTECHNIQUES...............................................................................................3
2.1. THEULTRA-HIGHVACUUM........................................................................................................3
2.2. SCANNINGTUNNELINGMICROSCOPY.........................................................................................4
2.3. ELECTRONSPECTROSCOPIES....................................................................................................10
2.3.1. Photoemission:XPSandUPS....................................................................................10
2.3.2. Absorption:NEXAFS..................................................................................................13
2.4. ELECTRONDIFFRACTION.........................................................................................................16
2.4.1. ReflectionHighEnergyElectronDiffraction.............................................................18
3. EXPERIMENTALAPPARATUS..............................................................................................21
3.1. THEALOISABEAMLINE.........................................................................................................21
3.1.1. ALOISAexperimentalchamber.................................................................................23
3.1.2. HASPESexperimentalchamber................................................................................27
3.2. CFMEXPERIMENTALCHAMBER...............................................................................................29
3.3. SUPPORTODISUPERFICIEXPERIMENTALCHAMBER......................................................................31
4. HYBRIDINTERFACESOFHETEROAROMATICMOLECULESONTIO2......................................35
4.1. INTRODUCTION................................................................................................................35
4.2. THERUTILETITANIUMDIOXIDESURFACE...................................................................................36
4.2.1. Surfacepreparationandcharacterization................................................................39
4.3. TETRAPYRROLEMACROCYCLES:PORPHYRINS.............................................................................45
4.3.1. 2H-TPPSpectroscopycharacterization:XPS.............................................................47
4.3.2. 2H-TPPNear-edgeX-rayphotoemissionspectroscopy.............................................534.3.2.1. COMPARISONWITH2H-OEPand2H-tbTPP.......................................................................................57
4.3.3. 2H-TPPfilmstructuredetermination:STMandRHEED............................................604.3.3.1. COMPARISONWITH2H-tbTPP...........................................................................................................67
4.3.4. 2H-TPPtheoreticalapproach....................................................................................694.3.4.1. Lowcoveragefilms.............................................................................................................................694.3.4.2. Highcoveragefilms............................................................................................................................75
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4.4. PHTHALOCYANINES:2H-PCANDTIO-PC..................................................................................81
4.4.1. Spectroscopycharacterization:XPSandNEXAFS.....................................................82
4.4.2. Filmstructuredetermination:STM...........................................................................86
4.4.3. 2H-Pctheoreticalapproach......................................................................................88
5. INTERMOLECULARCOUPLING.DONOR/ACCEPTORSTACKINGONTIO2..............................90
5.1. FULLERENE:C60...................................................................................................................90
5.2. TIO-PC,2H-TBTPPANDC60INTERACTION..............................................................................92
6. CONCLUSIONS.................................................................................................................100
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1. INTRODUCTION
Organicheteroaromatic compoundspresentnicecapabilities thatareattractive fromthe
scientificandtechnologicalpointsofview.Thetransportingchargeandthephotoabsorption
propertiesinthesolarradiationspectrummaketheminterestingfororganicelectronicsand
photovoltaicsapplications.Severaladvantagesovertheinorganiccompetitorsmustbetaken
in count: i) device fabrication is less complex due to the low vacuum and temperature
deposition or solution processing, with direct benefits into production costs and
environmental impact, ii) functionalitywouldbeenhancedduetotheirchemical tailoring
potential,throwmolecularfunctionalization,iii)theweakintermolecularinteractiondueto
vanderWaals forcesmakesorganic films flexibleandcompatiblewithplastic substrates,
openingthewaytowardsthefabricationofsoft,large-areabio-sensors.
The studyof the structural, electronicandchemicalpropertiesof adsorbedorganic films
becomesanopenissue,wheremostmodernsurfacestudiestriedtounderstandandcontrol
thesepropertiesmainlyfocusingontheinteractionoforganicswithsinglecrystalsurfacesof
coinagemetals (Au,Ag,Cu),sincetheycanbeeasilyobtainedandprepared inultrahigh
vacuum(UHV).
A limitedamountofpublicationsdealwith the interfacebetweenorganicsanddielectric
single crystals, in confrontation with its relevance for organic devices: i) thanks to the
photoexcitationbehaviourofdyemoleculesknownassensitizers,attachedtoananoporous
ornanospheresof titaniumdioxide,organicdye-sensitizedsolarcells collect sunlightand
convert it into electricity, ii) the reversible switching of conformational or electronic
configurationsoninsulatinglayers,liketheisomerization[1]inmoleculesorthechargingof
individualatoms[2],arenicebasetoperformnanomemorieswithhighinformationstorage
capabilities,iii)thearchitectureoffieldeffecttransistors[3],thatarethepillarofamplifiers
and logic circuits, consists of a charge transportingmaterial in contactwith an insulator
wherethevoltagetoswitchonthecurrentflowisapplied.
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Theultra-highvacuum
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From the basic surface science point of view, semiconductors and insulators presents a
depletedelectronicdensityregionbetweenvalencebandsandconductionbandsallowing
themolecularstatestoremainunperturbedbytheinteractionwithelectronspopulatingthe
half-filledbandsofmetalsattheFermiedge.Scanningprobetechniquesonthesesystems
produced images ofmolecular orbitals with sub-molecular resolution, and details of the
intramolecular distribution of charge [4]. In addition, the nanomanipulation with the
scanning probe tip, enabled the observation of orbital hybridizationwhenmetal-organic
bondisformed[5],openingtheinsitureal-timestudyofon-surfacechemistrycatalysedby
activesubstrate.
Despitetheevolutiononthefield,alotofdifficultieslikepoorself-assemblypropertiesfor
bi-dimensionalandthree-dimensionalstructures,andtheimpuritiesonthesurface,mustbe
overhaultotherealtechnologicalapplicationinthedesignandproductionofsystematicand
reliablenanodevices.
Thepresentworkisfocusedintheself-assemblyandelectronicandchemicalpropertiesof
differentorganicunits,andtheinfluenceandinteractionwiththerutile-TiO2surfacealong
the(110)plane,thatisoneofthemostfamoustransitionmetaloxides.Theworkisdivided
intwoparts:i)thestudyofchemicalandstructuralpropertiesofheteroaromaticmolecules:
phtalocyanine (2H-Pc), tetraphenyl porphyrin (2H-TPP), tert-butyl tetraphenyl porphyrin
(2H-tbTPP) and octaethyl porphyrin (2H-OEP) through microscopic and spectroscopic
characterizationandtheoreticsimulations,anchoringtheirmainchemicalreactionsunder
therutile-TiO2(110)presenceassourceofatomsandascatalyticsurface,focusinginto2H-
TPP that has demonstrated an exceptionally high thermal stability for itmonolayer that
presentphasetransitions,keepingthecoordinationofthemacrocyclecentralpockettothe
oxygenatomsbeneaththroughouttheself-metalationandflatteningreactionsandii)the
interfacestudyofsomeofthem(i.e.2H-tbTPPandTiO-Pc)takingadvantageoftheirdonor
behaviourwithacceptorunits(fullereneC60),bymeansofvalencebandmeasurementsto
understandtheinfluenceofthemolecule-moleculeandmolecule-substrateinteractionsin
the molecular HOMOs and their capabilities as another way to tune the energy levels
alignment.
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2. EXPERIMENTALTECHNIQUES2.1. THEULTRA-HIGHVACUUM
Theultra-highvacuum(UHV)isdefinedasthevacuumregionbelow10-9mbar,anditisthe
only that allow to prepare and keep atomically clean surfaces long enough to carry out
experimentsonthem.
Fig. 2.1. Relationship between gas pressure, surface contamination and mean free path
lengths. In the rightdowncorner is shown the legend for time contamination (tm), using
stickingfactor0.1and1,whereλXisthemeanfreepathofthestandardXmolecules[6].
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ScanningTunnelingMicroscopy
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The experiments and techniques performed during this thesis were done under UHV
conditions. Techniques basedonparticle beams, requireUHV that allows them to travel
undisturbedtointeractwiththesamplesurface,orthedetector[6].
Thekinetictheoryofgasesgivesusanestimatecontaminationtime,dependingonmolecular
weight,temperature,pressureandstickingcoefficient(probabilitythatonemoleculearrived
onthesurfaceremainsonit,0≤s≤1).Assimplifiedrule,attypicalUHVpressure10-10mbar
ifeverygasmoleculehittingthesurfacesticksonit,thecompletecoveragewilltakeplacein
105secs,enoughtimetoperformanexperimentincleanconditions.IntheFig2.1.isshown
the relationship between gas pressure, surface contamination time andmean free path
length.
Thechamberisconstructedmainlywithaμ-metalshieldedstainlesssteel,glassandceramic
forelectricalcontactandinsulation.Itneedstoremainleakfree,forthisreasontheflanges
presentsteelknifeedgesinbothsides(chamberandflange)thatcompresssoftercopper
gaskets,creatingaleakproofseal[6].
TheUHVregimeisreachedbymultistagepumpingsystems.Firstascrollpumpgivesusa
backgroundpressureof10-2mbarinsidethechamber,thenaturbomolecularpumpgivesus
10-6mbarpressure.Atthispressure,theairandwateradsorbedonthewallsofthechamber
actasvirtualleak,slowlydesorbing.Toimprovethevacuumtoreach10-10mbarandsoon,
thechamberisheatedto150ºCforatleast24hours,removingtheadsorbatesoverthewalls
thatcanbeatthispointeasilypumpedout.Oncecooleddownthechamberreaches10-10
mbarandstillneedstobepumpedtokeepUHVconditions.Theresidualgasremainingas
thispointisbasicallycomposedbyH2O,N2,CO.
2.2. SCANNINGTUNNELINGMICROSCOPY
TheScanningprobemethods’principleofoperationisconceptuallysimple:ametallicsharp
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EXPERIMENTALTECHNIQUES
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tipisplacedintheproximityofasamplesurface,closeenoughtogenerateafinitetunneling
conductance.Ifavoltage!isapplied,anelectrontunnelingcurrent"isgeneratedandonce
amplified can be easily measured. This current depends exponentially on the distance
betweensampleandtip.Thetipisatthispointmadetoscanthesurface,applyingafeedback
looptokeeptunnelingcurrentorheightconstant.
The Scanning Tunneling Microscope (STM) samples in real space the atomic geometry
throughthelocaldensityofstates.Fig.2.2.showstheschematicSTM’soperationprinciple.
Fig.2.2.SchematicdiagramofanSTM,showingthefeedbackcontrolledpiezoscanning
thetipoverthesurface,andthe#valuesobtainedasatopographicalimageonthecomputer
[7].
Since the development ascribed to Binnig et al. [8], it has become a well-established
technique,improvingthevibrationisolationoftheprobeandthesample,fromtheprimitive
versionofsuperconductinglevitationsystemtotheactualsuspensionspringsanddamping
mechanisms using eddy currents; the scan speed with an appropriate sinusoidal signal
appliedtothetipmovement,canachievetherangeofhundredsofframespersecond.
TheSTMisaquantummechanicsempiricaldemonstrationbyitself:anincidentparticleupon
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ScanningTunnelingMicroscopy
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a potential barrier higher than the particle’s kinetic energy has no zero-probability of
traversing the forbidden region and reappearing on the other barrier side. This is the
phenomenonoftunnelingandisaconsequenceofthewavelikepropertiesofelectrons,and
itswavefunction$ # satisfiestheSchrödingerequation
−ħ
'(
)*
)+*$ # + - # $ # = /$ # (2.1)
where0istheelectronmass,#and/areitspositionandenergy.Consideringthecaseofa
piecewise-constant potential- in the classical allowed region/ > - , generates a plane
wavesolution
$ # = $ 0 4±67+(2.2)
where8isthewavevector
8 ='((:;+ = ħ8 = 20(/ − -)Howeverthereis
asolutiontoeq.2.2.inthe/ < -region
$ # = $ 0 4;7+(2.4)
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EXPERIMENTALTECHNIQUES
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wherethedecaying
8 ='((
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ScanningTunnelingMicroscopy
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thatdependsonthematerialsandthecrystallographicorientationofthesurface.Whena
voltage!isapplied,theelectronsinthesamplelyingbetween/Cand/C − 4!haveafinite
probability totunnelingthetip. If thevoltage issmaller4! ≪ A, theenergy levelsof the
tunnelingelectronsareverycloseto/C,andtheprobabilityFofanelectroninthestate$G
occupyingthenthsamplestatetotunneltothetipsurface # = B is
F ∝ $G 0'4;'7I(2.6)
considering$G 0 thewavefunctiongoesthenthsamplestateatthesurface(# = 0)and�
thedecayconstantofasamplestateneartheFermilevelinthebarrierregion
8 ='(J
ħ(2.7)
Aslongastheconditionofthetipisstableintime,theelectronflowisstationary.Thenthe
tunnelingcurrentisproportionaltothesampledensityofelectronicstatesinsidetheenergy
interval4!belowtheFermienergy.Wecanwrite
" ∝ $G 0'4;'7I
:K:LM:K;NO
(2.8)
Ifthevoltageissmallenough,thenwecanrepresenteq.2.8.intermsofthelocaldensityof
states(LDOS),correspondingtothenumberofelectronsperunitvolumeperunitenergyat
agivenpointinspaceandatagivenenergy,attheFermilevelofthesamplesurface
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EXPERIMENTALTECHNIQUES
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" ∝ !PQ 0, /C 4;'7I(2.9)
andthatvalueofthesurfaceLDOSneartheFermiisanindicatorofthesurfaceconductivity
(metallicorinsulating).
AmoreaccuratetreatmentoftunnelingjunctionwasgivenbyBardeenin1961[9],starting
fromtwofreesubsystems(tipandsample)andcalculatingthetunnelingcurrentfromthe
overlapoftheirwavefunctionsusingtime-dependentfirst-orderperturbationtheory[10].
Onthisway,thetunnelingcurrentcanbeobtainedbysummingallthestatesintheinterval
e!involvedintheprocess
" =4T4
ħU /C +
1
24! + W − U /C −
1
24! + W
XY
;Y×
PQ /C +[
'4! + W P\ /C −
[
'4! + W ] W '^W(2.10)
at low temperature, i.e. when limit _`a ≪ 4! , the Fermi distribution U / can be
approximatedbyastepfunction
" =bcN
ħPQ /C +
[
'4! + W P\ /C −
[
'4! + W ] W '^W
Xd*NO
;d*NO
(2.11)
demonstrating that an STM image is a convolution of sample and tip DOS, and as
consequenceofBardeen’stheorythereciprocityprinciple:iftheelectronicstateofthetip
andthesampleareinterchanged,theimageshouldremainthesame.
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Electronspectroscopies
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2.3. ELECTRONSPECTROSCOPIES
Thephenomenainwhichanenergeticenough,incidentlightejectselectronsfrommatteris
known as the photoelectric effect andwas discovered byHertz (1887) and theorized by
Einstein(1905):anelectroninastatewithbindingenergy/`absorbsaphotonwithenergy
ℎfandcangetsoverthematerial’sworkfunctionA,escapingwithakineticenergy/g6G
/g6G = ℎf −/` − A(2.12)
Due to the attractiveness of the electrons as experimental probes (i.e.with electrostatic
fields,theelectrons’energyandmomentumcanbeeasilyfocusedandanalysed;electrons
areeasytocountandvanishafterbeingdetected;asshownintheFig.2.4.,theescapedepth
of electrons is small enough to keep the surface sensitivity of the techniques; electrons
providedirectinformationontheelectronicstructureofthematter,etc.),thiseffectisthe
baseofmanyspectroscopictechniques,inparticularforthisthesis:x-ray(XPS)andultraviolet
(UPS) photoemission spectroscopy to study occupied electronic states by direct
photoionization [11] and near-edge x-ray absorption fine structure (NEXAFS) that gives
informationonunoccupiedstatesinthepresenceofacorehole[12].
2.3.1. PHOTOEMISSION:XPSANDUPS
Dependingonwherearetheelectronsintheatomtheycandisplaycore-(i.e.closedtothe
atomcores)orvalence-(i.e.delocalizedtoparticipatetointeratomicbounds)likecharacter.
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EXPERIMENTALTECHNIQUES
11
Fig2.4.Meanfreepathofelectronsinsolidsasafunctionoftheirenergyanditstheoretical
approach.[12]
InXPS,usuallysoftx-rayphotons(ℎf = 100eV − 1keV)areusedtoprobethesample.This
energyisenoughtoejecttheelectronsmosttightlyboundfromthecore-levels,showingin
the energy vs. photoemission intensity spectra sharp peaks at well define energies
corresponding with the electron binding energies that are characteristic of each atomic
species,givingusanXPSfingerprinttoidentifytheelementspresentinthesample.Several
factorsinfluencetheexactlocationofcorelevelpeaks,thatareusuallyclassifiedasinitial
stateeffects,originatingfromtheenvironmentbeforetheexcitation,i.e.thechangesinthe
chemical environment can lead to variations in the position of the core level, known as
chemicalshifts. Itsorigincouldbeduetoeithertheformationofchemicalbondsthatare
involvedintheelectrontransferandchangethechargedensityoftheatomortheelectron
charge transfer to a given atom that enhances the electron screening of the nucleus,
decreasingtheelectronbindingenergy,ortheelectronchargetransferfromagivenatom
thatweakensthescreening,thusincreasingtheelectronbindingenergies[12].Finalstate
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Electronspectroscopies
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effects take place after the creation of a core hole, (i.e. core-hole screening effects in
conductorsandpolarizationofdielectrics).
Thepeaksshapealsogivesusinformationaboutthelifetimeofthecoreholestatecreated
inthephotoemissionprocess.ItsLorentzianwidthisintrinsicanditsGaussianbroadeningis
due to the resolution limit of the analyser.Moreover, the integrated peak area gives us
informationabouttheamountofdepositedmaterialorthesurfacereactions’rate.
Fig.2.5.Diagramofaphotonabsorptionprocessresultinginaphotoelectronandacore-hole.
Theholeisfilledbyanelectronradiativelybytheemissionofafluorescentphotonornon-
radiativelybyemissionofanAugerelectron[13].
In UPS, ultraviolet light (ℎf = 10eV − 100eV) is used to probe states near the Fermi
energy,asthesubstrates’valenceband,surfacestatesandlowenergyoccupiedmolecular
orbitals(HOMO)ofadsorbates.
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EXPERIMENTALTECHNIQUES
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2.3.2. ABSORPTION:NEXAFS
In the previous techniques, we described the photoemission by direct excitation of the
photo-emitted electrons from the occupied states ofmatter to the vacuum level, but a
photo-emittedelectronencounters empty statesbefore the continuum states above the
vacuumlevel.Thenear-edgeX-rayabsorptionfinestructure(NEXAFS),isusedtoprobethese
unoccupiedstates.Inthistwo-stagesprocess,whenthephotonenergymatchestheenergy
betweenacorestateandanemptystate,theelectronisexcitedtothisunoccupiedstate,
generation a hole, and the consequent decay would be by the emission of a photon
(florescence)ortheemissionofanAugerelectron(Fig.2.5.).So,ifthesampleisirradiated
withmonochromaticx-raysofvariableenergyaroundan ionizationedge,thesubsequent
relaxationofthesystemeitherbyAugerorflorescencechannelsisgoingtobeameasureof
theabsorptioncross-section.Thedetectionofelectronsismorecommonfororganicthin
filmsdueto its largersurfacesensitivity,andthestrongpredominanceofAugeremission
overflorescenceforlightelements(Z
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Electronspectroscopies
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InFig.2.6.fordiatomicmolecules,thereareinadditiontotheemptyatomicstates,empty
molecular orbitals (MOs) that are named as σ and π symmetries andwith * if they are
unfilled.Inπ-conjugatedmoleculesthelowerunoccupiedmolecularorbitalisusuallyaπ*-
orbital,withtheσ*-orbitalsathigherenergies.Thesestatesareusuallyabovethevacuum
level in neutralmolecules but pulled below by electron-hole Coulomb interaction in the
ionizedmolecules.TheNEXAFSdoesn’tionizetheatomormolecule,butcreatesaholeand
excitesanelectronthatwillthereforeinteract.
The natural linewidth of the resonances (as dictated by the corresponding lifetime) is
broadened by the instrumental resolution which eventually swears the splitting of
resonancesduetothemolecularvibrationalstates.Sinceσ*orbitalsarefoundabovethe
vacuum level and at higher photon energies they can overlap with the continuum that
providesalargernumberofdecaychannels,sonoresonancesareexpectedandtheyhave
lowerlife-timeandappearsignificantlybroader.
NEXAFSspectracangiveusinformationaboutthestructuralorganizationofanoverlayer,its
magneticpropertiesandbonding.Bondlengthsinamoleculecanbeestimatedanalysingthe
energypositionofσ*resonances,andsincethislengthissensitivewiththeoxidationstate
ofanatom,chemicalreactioncanbemonitored.Moreover,thechemisorptionofamolecule
isoftenaccompaniedbyachargetransferbetweenmoleculeandsubstrate,thatquenches
the NEXAFS peak associated with the lowest unoccupied molecular orbital (LUMO) or
generatesanewstateclosetoit.
Theintensityofanabsorptionisproportionaltotheprobabilitythatanelectroninaninitial
state,occupiesahigherenergyfinalstatewhenthesampleisilluminatedbyaphotonbeam.
This transition probability is described by the Fermi’s golden rule [14] that links the
resonanceintensityItothematrixelement
" ∝ < U 4 ∙ > l > '(2.13)
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EXPERIMENTALTECHNIQUES
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where4istheelectricfielddirectionand>themomentumoperator.Forlinearlypolarized
light(astheproducedinasynchrotron)anda1sinitialstate,wecanobtainthesimpleform
" ∝ 4 U > l ' ∝ cos' p(2.14)
withptheanglebetweentheelectricfieldvectorandthedirectionofthefinalstateorbital,
thatpresentsamaximumwhen theelectric fielddirection is alignedalongadirectionof
maximumelectrondensity.Thispolarizationdependenceoftheresonancesintensityallows
the molecular orientation determination. Because of the spatial localization of the 1s
electrons,onlyp-likefinalstatesfromtheatomswhoseabsorptionisprobedbytheselected
photonenergywillappearinthespectrum.Thisisknownasthedipoleselectionrule.
Consideringafinalπ*-planemolecularorbital,linearlypolarizedphotonbeamandthetwo-
foldsymmetryofthe(110)faceofrutile-TiO2theratiobetweentheintensityofa1s→π*
transitioninS-andP-polarizedlightbecomes
"q "r = 1 − cos' s cos' t − sin' s sin' t(2.15)
andduetothesmallincidentangleθabout4º
"q "r = tan' t(2.16)
The resonances seen at the NEXAFS spectra for large molecules belongs to different
submolecularunitsasthebuildingblockprincipleapproaches.Ifthesesubunitsmayadapt
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Electrondiffraction
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different azimuthal orientations respect to the molecular plane, the tilt angle must be
calculatedwithaformulaforthreefoldorhighersubstratesymmetry
"q "r = 3 2 tan' t(2.17)
2.4. ELECTRONDIFFRACTION
As shown in Fig. 2.7.when any incident beam (i.e. photons, ions, electrons, neutrons or
protons)withawavelengthcomparabletotheperiodicalspacingbetweenthestructuresof
interest is scattered from these structure planes it produces an interference. This
interferencewillbeconstructiveifthepathdifferencebetweentwowavesisequaltothe
multipleoftheirwavelength,soiftheplanesdistanceisd,andthebeamcollideswithan
angleθtheinterferencewillbeconstructiveif
z{= 2dsin s(2.18)
thisequationisknownastheBragg’slaw.
Fig.2.7.Wavesreflectingfromtwoparallellatticeplanes.
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EXPERIMENTALTECHNIQUES
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Ifwedescribethewavesbytheincident(ki)anddiffracted(ko)vectors,assumingtheprocess
iselastic,thelengthsofthewavevectorsarethesameduringalltheprocess
_6 = _} ='c
~(2.19)
Bytakinginaccountthetrigonometricrelationbetweenthesevectors,wecanfinallyobtain,
thatforaconstructiveinterferencethedifferencebetweenthewavevectorsoftheincident
anddiffractedwavesmustbeequaltothevectorfromthereciprocallattice|G|
='c
)(2.20)
orinvectorformasLaueinterpretedit
_6 − _} = (2.21)
It’samoregeneralinterpretationthatdoesn’trequirestheBraggassumptionsthatreflection
ismirror-likeandcomingfromtheparallelplanesofatoms.
InageometricapproachtheEwaldsphereshowninFig.2.8.isaconstructionthatcombines
thewavelength of the incident and diffractedwaves, the diffraction angle for a specific
reflectionandthereciprocallatticeofthesample.Consideringthesphereradius
Ä ='c
~= _ (2.22)
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Electrondiffraction
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whenthesphere interceptswithareciprocal latticepoint, theLauecondition issatisfied,
givingaconstructiveinterferenceofthediffractedwaves.
Fig.2.8.EwaldspherewherethedarkspotsarethereciprocallatticepointsthatfulfiltheLaue
condition.
2.4.1. REFLECTIONHIGHENERGYELECTRONDIFFRACTION
Ifwedoreflectionwithhighenergyelectronsbeam(5–100keV)(i.e.providelargeelastic
scatteringcross-sectionforforward-scatteredelectrons),weneedtokeepthepenetration
depthofthetechniquesmallusingareflectiongeometryinwhichthebeamisincidentat
verygrazingangle(1-4º)asseenonFig.2.9.,thusremainingasurfacesensitivetechnique.
Only the electrons which interact with the most superficial layers experiment elastic
collisions originating a diffraction pattern. Other electrons suffer inelastic scattering
transformingtheparallelandmonocineticbeaminadivergent,quasi-monocineticbeamand
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EXPERIMENTALTECHNIQUES
19
increasingtheprobeddepth.ThescatteringoftheseelectronsgiverisetothelinesofKikuchi
ifthematerialiswell-crystallized(i.e.singlecrystal).
InthecaseofLEED,therelationbetweenadiffractionpatternandthereciprocallatticeis
easilyunderstood,butRHEEDpatternsarenotsointuitive.Thistechniqueenableustoknow
the intensity distributions along the reciprocal lattice rods only changing the crystal
orientation. In the LEED, the acceleration voltage is changed to record the intensity
distributionsalongthem,butthischangeinthewavelengthgiveschangeinthemagnitude
ofthestrongdynamicaleffectessentiallyaccompaniedwithLEED,makingdifficulttoanalyse
thesurfacestructure.
Fig.2.93DviewoftheEwaldsphereforreflectionhighenergyelectrondiffraction[15].
Theoretically, the intersection of streaks with the Ewald Sphere should form points.
However, the radius of this one is too large for the energy considered during RHEED
measurements comparedwith the reverseof the interatomicdistances,doing theEwald
spheretocrossalongmanylatticerodsbutonlyfewneargrazingexitangles,andlookingthe
surfaceasacontinuousalongtheincidencedirection(withoutdiffraction),thusdetecting
onlytheperpendicularperiodicities(i.e.usuallyweonlysee(00)and(10)components)that
combinedwiththedispersionangleandenergyof theelectronsbeamandthe imperfect
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Electrondiffraction
20
crystallinequalityofthesurface,makethediffractionpatterntoappearalsointheformof
streaks.
This peculiar geometry allows to performmeasurements during growth of surface films,
been possible to monitor the layer-by-layer growth of epitaxial films by monitoring the
oscillations in intensity of the diffracted beams in the RHEED pattern, doing possible to
controlthegrowthrateinMolecularBeamEpitaxy(MBE)[16].
TheRHEEDpatternalsocanbeusedtomeasureperiodicitiestransversetothe incidence
planemeasuringtheseparationbetweenstreaks[17,18],andcomparingtheheightsofthe
peaks(i.e.Intensity)intherockingcurvestoextractdetailedatomicspacingbyquantitative
analysis[19],thatdoesn’tconcerntothisthesis.
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3. EXPERIMENTALAPPARATUS
3.1. THEALOISABEAMLINE
ALOISA(AdvancedLineforOverlayer,InterfaceandSurfaceAnalysis)isamultitaskbeamline
forsurfacescienceexperiments,itisdesignedtoworkinawidespectralrange(100-8000eV)
and hosts three experimental chambers; the main setup ALOISA dedicated to X-Ray
diffraction and spectroscopy experiments, the end-station ANCHOR (AmiNo –Carboxyl
Hetero-OrganicaRchitectures)equippedwithanindependentmonochromaticX-raysource
andaheliumlamptoperformoff-lineexperimentsandtheend-stationHASPESforhelium
atomscatteringthatcanperformrealtimeHediffractionandXPS.
ELETTRASynchrotronisa3rdgenerationlightsource,operatingastorageringenergyof2or
2.4GeVintop-upmode,andprovidingphotonbeamsintherangeof10-30000eVwithhigh
spectralbrilliance.
TheALOISAphotonbeamisproducedbyaU7.2wiggler/undulatorinsertiondevice(ID)of
theELETTRASynchrotron,thatconsistsinaspatiallyperiodicmagneticfieldproducedbytwo
alternativeorientedmagnetssuperimposedinaface-to-faceconfigurationandseparatedby
auser-tuneablegapthatallowstwooperationmodes.Whenthegapislargeincomparison
withthedistanceoftwomagnetseries( 40-80mm)withlowcriticalbeamenergies(130-
2000eV)theIDoperatesintheundulatorregime.Whenthegapislower( 20mm),i.e.the
magneticfieldisstronger,theamplitudeofthesinusoidalpathincreasesandtheIDoperates
in thewiggler regime. In the Fig. 3.1.we can see the intensityof thephotonbeamas a
functionofthephotonenergyfordifferentIDgapvalues.
Theplanarundulatoriscomposebyanarrayof19periodsofpermanentmagnets80.36mm
long,withatotallengthof1527mm.Thelightonceproduceisselectedinanglebyapinhole
-
TheALOISAbeamline
22
andcollimatedbyaparaboloidalmirrorinsagittalconfiguration.Thisparallellightbeamcan
impinge on two different dispersion devices; a Plane Mirror/Grating Monochromator
(PMGM),forthe120-2000EVrange,andaSi(111)channel-cutcrystalforthe2.8-8keVrange
withaphotonfluxatthesubstrateofabout1x1012inthelowenergyrangeand1-2x1011
inthehigherrange.Thespotsizeinthecentreoftheexperimentalchamberisabout40x200
μm2withanenergyresolvingpower(/ ∆/)between2000and7500.Thelightislinearly
polarizedintheplaneoftheelectronbeamorbit.
Fig.3.1. Intensityof thephotonbeamattheexitof theALOISAwiggler/undulator IDasa
functionofthegap.[20]
AsshownintheFig.3.2.thelightiscollectedfromtheentrypinholebyaparaboloidalmirror
(Parabol-1)andcollimatedtothedispersingsystem.Themonochromaticbeamisfocusedat
theexit slits (ES)byasecondparaboloidalmirror (Parabol-2).Thisexitdivergentbeam is
-
EXPERIMENTALAPPARATUS
23
finallyfocusedonthesampleatthecentreofthechamberbyatoroidalmirror(cylindrical).
Even if the system hasn’t an entrance slit, the optics are used in the sagittal focusing
configurationtominimizetheaberrationsinthedispersiveplane.Alltheopticshaveagold
coatingto increasethereflectivityofX-raysthanksto its largecriticalangle,andtheyare
operatedgrazingincidencewithadeflectionangleof1º.
Fig.3.2.OpticalsystemoftheALOISAbeamlineandHASPESbranchline.
ThebeamisswitchedtotheHASPESbranchlinebyathirdmirror(Toroidalshape)anddue
tothelargedistanceofthechamber(14m),noadditionalrefocusingmirrorsareused.
Theopticalelementsmovement isautomatizedviaTCP-IPconnectionasa serviceof the
BeamlineControlSystem(BCS),developedbyELETTRAtechnicalteam.
3.1.1. ALOISAEXPERIMENTALCHAMBER
TheALOISAexperimentalchamber,shownintheFig.3.3.,iscomposedbytwodifferentiated
parts: ahemispherical chamber,dedicated to the samplepreparation (calledpreparation
-
TheALOISAbeamline
24
chamber)andacylindrical chamberhosting theelectronanalysersandphotondetectors
(calledmainchamber).
Fig. 3.3. Cross-section of the ALOISA chamber. The main experimental apparatus are
highlighted.
ThepreparationandmainchamberarecoupledwithlargebronzeballbearingandslidingO-
rings’ system that allows the complete rotation of the main chamber (including the
detectors)aroundthesynchrotronradiation(SR)beamaxis.
Thesystemispumpedbytwopumpingstagesthatmaintainaconstantbasepressureof10-
11mbarinsidethemainchamber.
Thepreparationchamberisequippedwithamolecularbeamepitaxy(MBE)cryopanelthat
hostsfourevaporationcells,usuallyKnudsencellsorelectronbombardmentevaporators,
andcancalibratethedepositionfluxwithtwomicrobalances.Therearealsogaslinesthat
allowshighpuritygasestobebledintothechamber.TheiongunfortheAr+bombardment
allowsanionenergyof3keVandanemissioncurrentover25μA.AReflectionHighEnergy
ElectronDiffraction(RHEED)apparatuswithelectronenergyof15keVandabletoimpinge
on the surface at grazing angle is available for checking the surface in-situ during the
deposition. Inthepreparationchamber,there isalsothesampletransfersystemandfast
-
EXPERIMENTALAPPARATUS
25
entry-lockallowingquicksampleexchange.Thereisalsoaquadrupolemassspectrometer
(QMS)tochecktheevaporationandthecleanlinessofourvacuumandforleaktestporpoise.
Rotatingelement Rotationaxis Extension Resolution
Experimental
chamber
SRbeam ±120º 0.00015º
Axialframe SRbeam ±120º 0.0002º
Bi-modalframe PerpendiculartoSR
beam
±100º 0.0002º
SampleHolder SRbeam -90º;+185º 0.001º
SampleHolder Grazingangle -2º;+15º 0.001º
SampleHolder Surfacenormal ±95º 0.001º
Table.3.1.RotationsoftheALOISAmainchamberandmanipulatorholder.
Insidethemainchamberthedetectorsaremountedintwoindependentframes.Theaxial
one is hosted at the end of the main chamber and rotates around the SR beam axis
independentlywiththechamberandthesample.Ithostsfive35mmelectronanalyserswith
lowresolutionof500meVandacceptanceangleof5º.TheyaremainlydedicatedtoAuger
PhotoelectronCoincidenceSpectroscopy(APECS).Thebimodalframeismountedontheside
ofthecylindricalmainchamberandrotatesperpendiculartotheSRbeamandaroundthe
SRbeamtogetherwiththemainchamber.Ithostsa66mmhemisphericalelectronanalyser
with2ºofangularacceptanceforX-rayPhotoemission(XPS)andphotoelectrondiffraction
(PED).ThereisalsooneenergyresolvedphotodiodeformeasuringthetotalcurrentforX-
raydiffraction(XRD)andreflectivity(XRR).Thebimodalframeadditionalhoststwoenergy
resolved(Peltier-cooled)photodiodesoperating insingle-photoncountingmodeforX-ray
diffraction.ThereisalsoaphosphorousplatewithaCCDcameramountedontheaxialframe
-
TheALOISAbeamline
26
for 2D X-ray reflectivity measurements that allows the alignment of the beam and the
sample.
Fig. 3.4. ALOISAmanipulator armwithout sample holder. The light flux arrives from the
cylindricalhoseontheleftsideoverthesamplethatfacesupinsidethecircularholder.
Awide-angleacceptancechanneltronismountedontheaxisofthebimodalframe,usedto
measurethepartialelectronyieldinNEXAFSexperiments.Thischanneltronismountedin
frontofitsapexwithanadditionalgridtorepelthelow-energymultiple-scatteredelectrons,
acting as a high-pass filter, where only higher-energy Auger electrons contribute to the
partial-yieldsignal.
ThemanipulatorshowninFig.3.4.thathoststhesamplehassix-degreeoffreedomandis
mountedhorizontallyintothepreparationchamberandcantranslatethesamplebetween
preparationandmainchamber.SincetheSRbeampassesthroughthewholemanipulator
armandimpingesatgrazingincidenceintothesamplesurface,threerotationswithsome
limitationsasshowninFig3.5.areallowed;aroundthesynchrotronbeam(R1)toselectthe
surfaceorientationwithrespecttothepolarization,thegrazingangle(R3)andtheazimuthal
orientationforthesurfacesymmetryaxis(R2).Also,thedisplacementinthethreeaxes(X,
Y,Z)areallowed.TheirlimitsareexhibitintheTable3.1.
-
EXPERIMENTALAPPARATUS
27
Thesampleholderisavariabletemperaturesystemequippedwithtwotungstenfilaments
andelectrical insulation,whichenables applyinghigh voltage theelectronbombardment
heatingofthesampleover1100K,andagaspipelinewithacold-fingercontactonthesample
holderwhichenablesthecoolingdownwithliquidNitrogenuntil150K.
Fig. 3.5. Sketch of the angular movements available by the manipulator (left) and the
experimentalchamber(right).
ThedataacquisitionandmovementsareautomatizedwithaLabVIEWhomemadeprogram
developedbytheALOISAteam.
3.1.2. HASPESEXPERIMENTALCHAMBER
The Helium Atom Scattering and Photoelectron Spectroscopy (HASPES) chamber is
composed,as seen inFig.3.6.,byamainupstandingcylindrical vacuumchamber,apre-
chamberwithaheliumatomsourceandachamberforheliumdetectionwithaQMS.
-
TheALOISAbeamline
28
HASPEStakeadvantageofthelow-energymonochromaticSRbeamthatentersthrowthe
HedetectionsystemcoincidentwiththeHescatteredbeambutintheoppositedirection.
The sample holder compatible with ALOISA chamber is mounted in a vertical VG CTPO
manipulatorwithsixdegreesoffreedomandahigh-precisionmovementsetbyharmonic
drives.Thesystemisalsoequippedwithathermallinktoacryostatforliquidnitrogenor
liquidheliumcoolingandtungstenfilaments,thatallowsavariabletemperaturebetween
100-1100Krange.
Thedetectorsangleisfixedat110ºforHASand55ºforXPS.Therearethreepossiblesample
rotations; changing the incident angle of helium atoms and SR beam (R1), changing the
surfacesymmetryaxisrespecttothescatteringplane(R2)andtiltrotation(R3).
Fig.3.6.HASPESchamberanditsequipment.
ThechamberdisplaysapumpingsystemthatcontrolsthesmoothtransitionfortheHeto
passfromcontinuumtofree-molecularflowinsideanozzlewhichpressureratiobetween
thestagnationpressureandthebackgroundpressureinthebeamchamberisabout107,so
-
EXPERIMENTALAPPARATUS
29
noshockstructuresappearsintheexpansionregion,allowingaHepressureinthestagnation
chamberof10-100barrangetotunethefluxandmonochromaticityoftheHebeam.
Thisnozzlehasaskimmerthatdefinestheangulardivergenceofthebeamandachopper
thatselectthepulsesforinelasticscatteringmeasurements,andanothercollimatortoenter
intheexperimentalchamberwithacrosssectionofabout0.7mm.Thebeamenergycanbe
setbetween18.6and100meVwiththetemperatureinsidethestagnationchamber.
At this point the neutral atoms are ionized by an electronic beam (the electrons have a
/g6G = 100eVthat is themaximumcross-section to ionizeHeatoms)and filteredwitha
QMSwithachargetomassratioaccuracyof0.05e.m.u.
Thesupplementaryequipmentincludesaheliumlampthatprovidesultravioletradiation,a
150mmhighresolutionhemisphericalelectronanalyser,anelectronguninthescattering
planethatoffersangularlywellresolvedelectrondetection,achanneltronatanangleof50º
forthepartialelectronyielddetectioninNEXAFSexperiments,aniongunforAr+sputtering,
a cryopanelwith three Knudsen evaporation cells and a fast entry lock for quick sample
exchange.
3.2. CFMEXPERIMENTALCHAMBER
ThemainSTMexperimentalchambershownintheFig.3.7.usedduringthisthesisbelongs
totheNanoPhysicsLabatthe“CentrodeFísicadeMateriales”inSanSebastián,Spain,and
ishandledbyDr.CeliaRogero.
This chamber is composed by two parts: a cylindrical chamber dedicated to the sample
preparationandthespectroscopicanalysis(chamber1),andasphericalchamberhostingthe
STMheadandwithsample’spreparationcapabilities(chamber2).
-
CFMexperimentalchamber
30
Fig.3.7.ExperimentalsetupforpreparationandSTMmeasurements(courtesyofC.Rogero,
NanophysicsLab,CentrodeFísicadeMateriales,Donostia/SanSebastián,Spain).
Bothchambersarecommunicating invacuumbya transferarmandseparatedwithgate
valvestopreservethevacuumcleanlinessandisolationduringtheexperiments.Thevacuum
reaches10-10mbarinbothchambersthankstothescrollandturbopumpingsystems,and
the ionpumpof the chamber 1 and the getter pumpof the chamber 2 during the STM
operationtoavoidtothemaximumthemechanicalnoises.
Thechamber1 isequippedwith:anhorizontalfourdegreesoffreedommanipulatorthat
hoststhesample,allowingitsmovementinx,y,zcoordinatesandtheaxialrotationinthe
sample’s plane, R1, and hosts a filament that allows the sample heating by electron
bombardmentupto1100K;oneX-raysourceofAlK-αradiation,withanenergyof1486.6
eVandoneHeIUVlamp(21.2eV)associatedwithasphericalelectronenergyanalyserfor
photoemissionspectroscopy;afastentrysystemthatallowsthesampleexchangeinafew
hours;oneiongunforsputteringthesample;aquartzmicrobalancetotunethemolecules
-
EXPERIMENTALAPPARATUS
31
depositionandasetofcommercialfilament-heatedevaporatorspointingtothecentreof
thechamber.
Thechamber2hoststhecommercialvariabletemperatureSTMhead(SPM150Aarhusfrom
SPECSSurfaceNanoAnalysisGmbH),thatincludesaLiquidNitrogencoolingsystemanda
Zenerdiodesheatingsystem,allowingtoexperimentintherangeof150K-400K.TheSPM
150 Aarhus performs scanning tunnel microscopy in both modes constant height and
constantcurrent,andatomicforcemicroscopy(AFM)thankstothepatentedKolibriSensor®.
ItsnoiseisolationisduetoaspringsandVitonsystemthatallowsdifferentlevelsofdamping
andworkingwithoutdisconnectingtheturbopumpingsystem.
On this chamber there is also an Ion gun for tip cleaning and normal incidence sample
sputtering;amanipulatorwithafilamentforthesampleelectronbombardment;aparking
systemtokeepsamplesunderUHVconditionsforalongperiod;afastentrywithagatevalve
thatallowsthe installationofanadditionalevaporationcellpointing to thecentreof the
sampleduringthescanning,toperformin-situexperiments;alongtransferarmtotranslate
thesamplefromchamber1tochamber2andawobble-sticktomanipulatethesampleplate
insidethechamber.
3.3. SUPPORTODISUPERFICIEXPERIMENTALCHAMBER
DuetothegoodSTMresultsobtainedatthe“CentrodeFísicadeMateriales”, thegroup
decidedtobuyanewSTMheadfromSPECSincollaborationwiththegroupofthemicro-
nano-carbonlaboratory(MNC-lab)managedbyDr.AndreaGoldoni.
AfteritscommissioningtheexperimentalchamberiscomposedbythreepartsasseeninFig.
3.8.: a spherical chamber dedicated to the preparation and alignment of the sample
(preparation chamber), a spherical chamber hosting mainly the STM and availability for
-
CFMexperimentalchamber
32
samplepreparationin-situ(STMchamber)andalargeverticalcylindricalchamberhostinga
completeARPESsystemforspectroscopyexperiments(ARPESchamber).
Fig.3.8.Experimentalsetupforpreparation,STMandARPESmeasurements.
Thepreparationchamberiscomposedbyafastentrytoinsertandextractnewsamplesina
fewhoursthatincludesalongtransferarmtoshiftthesamplefromthepreparationchamber
to the ARPES chamber, a wobble stick tomanipulate the sample inside the preparation
chamber,afourdegreesoffreedommanipulatorthatholdstwofilamentsforheatingand
electron bombardment, a LEED (light energy electron diffraction) for alignment and
detectionofthesymmetryofthesurfacewithandwithoutadsorbates,aniongunforAr+
sputtering,twoevaporator’sentriesthatpointdirectlytothecentreofthechamberandthe
samplewhenit’sinthemanipulator,aquartzmicrobalancetotunethedepositionmounted
directlyonthemanipulatoronthebackofthesampleholder.
TheSTMchamberholdstheSTMhead(SPM150Aarhus)describedinthepreviouschapter,
aniongunforAr+sputteringtocleanthetipandthesampleinnormalincidenceifnecessary,
amanipulatorthatholdsaheatercomposedbytwofilamentsandelectronbombardment
capabilities,onewobblesticktotransferandmanipulatethesampleinsidethechamber,one
-
EXPERIMENTALAPPARATUS
33
transferarmtomovethesamplefromtheSTMchambertothepreparationchamberthat,
likewisetheothertransferarm,includesahomemadesampleholdershownintheFig.3.9.
Thissampleholdersmartlysolvestheproblemwiththeangularlimitationstotransferthe
samplebetweenchambersduetothe“L”configurationofthechambers.
Fig.3.9.SampleholderforSPECSandOmicronsymmetriessampleplatesthatallowsthe
sampleplanetranslationinsevencircularpositions(from-135ºto135ºby45º).
TheARPESchamberissupplybyaVUV5kseriesUVsourcefromGAMMADATA.Thisphoton
sourceisbasedonaHeplasmageneratedbyelectroncyclotronresonance(ECR)technique.
Amicrowavegeneratoriscoupledtoadischargecavityinsideamagneticfieldtunedtothe
microwavefrequencytofoundtheECRcondition,providingconcentratedradiationat21,23
eV(HeI)and41eV(HeII).Thefluxdensityiscomparablewiththeobtainedfromabeamline
undulatorandabout500timeshigherthantheonefromotherconventionaldischargeVUV
sources.Itsstabilityandbandwidth(1meV)makeitexcellentforstudiesandmeasurements
thatrequirehighintensity,longmeasurementtimesandhighresolution(i.e.gasphase).It’s
supply with amonochromator that achieved complete separation of Heα and Heβ. The
manipulatorthatholdsthesampleisaJohnsenUltravacmodel3000,withanXYstage,aZ
driveof0.01mmofprecisionandapolarrotationof360degreesattachedtoacryostatfrom
AdvancedResearchSystems(ARS)withabasetemperaturelowerthan9Kandincredible
coolingpower(10W@77K).TheelectronspectrometerisaSCIENTAR3000thatprovides
fastbandmapping,alensacceptanceangleof±15ºandangularresolvedrangeof±10ºand
variabledispersion.Theenergyresolutionis3.0meV,theangularresolutionisof0.1ºfora
-
CFMexperimentalchamber
34
0.1mmemissionspotandthekineticenergyrangeisfrom0.5to1500eVwithapassenergy
between2and200eV.AQMScompletesthechamber.
Theentiresystemispumpedinseveralstagesmainlybyscroll,anddiaphragmpumpforthe
pre-pumpingandturbopumpstoreachandpreservetheUHVinthe10-10mbarrange.
-
4. HYBRIDINTERFACESOFHETEROAROMATICMOLECULESONTIO2
4.1. INTRODUCTION
Tetrapyrrole macrocycles, specifically the porphyrins, are very stable and versatile
molecules.Duetotheircapabilities,theyareinvolvedinthemainlifeprocessesandpresent
largenumberoftechnologicalapplications[21]fromphotocatalysistodye-sensitizedsolar
cells[22].Basically,theirrigidcorebecomesabuildingblockunitthatallowsthemolecular
assemblingincomplexarchitectures,bytuningthestrengthandnatureoftheadsorption
interactionandmolecule-moleculeinteractionbyfunctionalizationwithperipheralligands.
Free base porphyrins or metalloporphyrins can be used, where the optical, electronic,
magnetic and catalytic properties depend on the specificmetallic nucleus. This tuneable
assembling over solid surfaces allows the designing and fabrication of hybrid surfaces
engineeredatthenanoscale[23].Byassemblingporphyrinsatsolidsurfaces,it’spossibleto
designandfabricatehydridsystemswithpropertiesengineeredatthenanoscale.Onsurface
modificationofmetal-freeporphyrins,isaviableroutetoachievechemicalandstructural
controlofmolecularoverlayers.
ThankstotheRutile-TiO2(110)characteristics,adsorbedonsuchasitslargeandanisotropic
surface corrugation, where molecular films are expected to grow with the macrocycle
paralleltothesubstrate,[24]andthepossibilityofchargeinjectionintotheoxide,[25]this
substratebecomesasuitableplaygroundforthecharacterizationofarchetypalorganicdye
molecules.
-
Therutiletitaniumdioxidesurface
36
4.2. THERUTILETITANIUMDIOXIDESURFACE
Theinterestonsurfacescienceofmetaloxideshasbeenrapidlyincreasinginthelastyears,
since oxide surfaces have a lot of applications and are present inmostmetals that are
oxidizedimmediatelyunderambientconditions.Duetoitsproperties,titaniumdioxide,TiO2,
hasalotoftechnologicalapplicationsasaphotocatalyst,asgassensor,aswhitepigment,as
acorrosion-protectivecoating,inceramics,invaristors,asopticalcoating,andinsolarcells
toproducehydrogenandelectricenergy.
Innature,thetitaniumdioxideisapolymorphandpresentsthreecrystalstructures:Anatase,
RutileandBrookite,beingfromthescientificandtechnologicalpointofviewtheAnatase
andRutilestructuresthemostinterestingones.Theyaresemiconductorswithenergygap
~3eV[26],thatmatchesthegapofmanylight-harvestingcompounds.Applyingsynthetic
chemistry these compounds known as organic dyes that are photosensitive can be
functionalizedwithanchoringgroupstoattachtoTiO2surfaces,thataddedtothecapabilities
ofTiO2aselectronacceptortobegrowthintransparentnanostructureswithlargesurface
tovolume ratio, is fundamental inoneof themostextendedapplications,dye-sensitized
solar cells [27]. Theseprototypaldye-sensitized solar cells are composedbyamixtureof
mostlyAnatasethatfavourstheefficiencyandminorityRutilecrystals.Theaveragesurface
energyofanequilibrium-shapecrystal forAnatase is less than for rutile [28], that iswhy
nanoscopicparticlesarelessstableinrutilephase.Inotherhand,thephotocatalystactivity
of the chemically active faces as Anatase-(101) or Rutile-(110) could be induced with
ultravioletlight,convertingtoxiccompoundsandmoleculesintobasicconstituents[29].In
itsstoichiometricformthetitaniumdioxidecanbeuseasgatelayerintransistorsduetoits
highdielectricconstant.Atthenanoscale,thehighsurfacecorrugationofRutile-(110)could
beexploitedasa template for thegrowthof theorganicmolecule layers inanorganized
structureincreasingtheirelectronmobility.
-
HybridinterfacesofheteroaromaticmoleculesonTiO2
37
Sincelargeandpurerutilecrystalsarecommerciallyavailablewiththedesiredfacet,andthe
(110)surfaceisthemoststableface,therutile-(110)canbeconsideredanarchetypalmetal
oxidesurfacetomodeltheabsorptionoforganicmolecules.
Bulkrutile-TiO2asshownintheFig4.1.hasabody-centredtetragonalunitcellwiththeTi
atomsatthebody-centreandcornerpositionsineachone.EachTiatom,withformalcharge
+4, is coordinated to six O ions with formal charge -2 at the vertices of a distorted
octahedron.Atthesametime,eachOatomiscoordinatedwiththreeTiatomswithallthe
O-Tibondslyinginoneplane.
Fig.4.1.BulkstructureofRutile.ThetetragonalbulkunitcellofRutilehasthedimensionsÉ =
Ñ = 4.587Å, ä = 2.953Å.Slightlydistortedoctahedraarethebasicbuildingunits.Thebond
lengthsandanglesoftheoctahedrallycoordinatedTiatomsareindicatedandthestacking
oftheoctahedralinbothstructuresisshownontherightimage[30].
Theunreconstructed(110)surfaceisasimpletruncationacrossthe(110)planeasshownin
Fig4.2.andtheonewithlowestformationenergy[30].TwokindsofTiatomsarepresenton
the surface along the[110]where six-fold coordinated Ti atoms alternatewith five-fold
coordinated atomswith one dangling bond perpendicular to the surface. Two kind ofO
atomsarecreatedaswell.Theso-called“bridgingOatoms” formingrowsparallel to the
-
Therutiletitaniumdioxidesurface
38
[001]directionofOatomstwo-foldcoordinatedprotrudingapproximately1.2Åandrowsof
three-foldcoordinatedatomslyingintheplaneoftheTiatomsandconnectingthesix-fold
and five-fold coordinated Ti atoms’ rows. The dangling bonds of the bridgingO ions are
compensatedbythefive-foldTiones,thataccordingwiththeautocompensationcriterion
explainsthesurfacestability[31].
Fig. 4.2. a) Ball-and-stickmodel of the Rutile crystal structure emphasizing two distorted
octahedronsthatcomposetheunitcell.AandBlinessurroundachargeneutralrepeatunit
withoutdipolemomentumperpendiculartothe[001]direction.b)Thecrystal istruncated
alonglineAandtheresulting(110)surfacesarestableandtheexperimentalevidenceforthe
(1x1)surfacereconstruction.[30]
Only minor relaxations occur mainly perpendicular to the surface [32] with an outward
relaxation relative to the bulk position of the bridging O rows ( 0.2 Å) and an inward
displacementof the five-fold Ti ( 0.1Å) in thehollows. The resultant surfaceunit cell is
characterizedbyarectangularunitcellmeasuring2.959x6.495Åalongthehigh-symmetry
directions[001]and[110]respectively.
-
HybridinterfacesofheteroaromaticmoleculesonTiO2
39
4.2.1. SURFACEPREPARATIONANDCHARACTERIZATION
Inourexperiments,wehaveusedseveral samplesof rutileTiO2(110) fromMateckwitch
thicknessrangingabout1mm.ThepreparationproceduretogetacleansurfaceunderUHV
conditionsiscomposedbytwostepsthatcanberepeateduntilgettingthedesiredsurface;
20minutesofgrazingangleAr+sputteringattypically1.0keVionenergyand10-6mbarAr
pressuredependingonthechambergeometryandthedistanceandfocusbetweentheion
gunandthesamplesurfaceandanannealingcyclebyelectronbombardmenttorestorethe
(1x1)structure,applyingavoltageof800Vtothesampleandstabilizingtheemissioncurrent
at10mAfor4min,20mAfor2minand40mAfor1min,notexceeding10-8mbarofpressure
insidethechamber.
Since the annealing temperature during the last step reachesmore than 750ºC, the lost
oxygenmakes the sample conductiveenough tobeprobedwithSTMandphotoelectron
spectroscopiesavoidingchargingeffects,butitalsoincreasesthedefectsdensitythatmay
conditionthemolecules’adsorption.Particularlytheoxygenvacanciesandevenaperfect
surface[33]undergoodvacuumconditionswouldbeexposedtoresidualwatermolecules,
takingplaceawater-splittingreactionandconvertingthemintohydroxylgroupsduetotheir
dissociation.
-
Therutiletitaniumdioxidesurface
40
Fig. 4.3. STM constant current image of clean rutile-TiO2(110) surface with a (1x1)
reconstruction. The different defects aremarked by blue arrows (i.e. vacancies, hydroxyl-
groupsandwater).
AsexplainedwiththeSTMwecaneasilyprobefilledandemptysurfacestatesdependingif
the applied bias voltage to the sample is negative or positive respectively. For TiO2(110)
surfacetheoptimalbiasvoltagetosampleemptystatesareusuallyintherangeof1.0-1.5V,
and low tunneling current values (80-100 pA). This empty states electronic picture of a
stoichiometricsurfacewithaTi-Oinaperfect1:2ratioshowstwofeatures:darkandbright
rows. The consensus is that thedark rowsare thebridgingO ionswhile thebright rows
correspondtothefive-foldTi ions[34],consideringthatthe largedensityofstates inthe
unfilledbandspatiallyconfinedonthefive-foldTiionsprevailoverthesurfacetopography
anddefects,oxygenvacanciesandhydroxylgroupsareeasilyrecognizableinanempty-state
STMimage,asshowninFig4.3.Theoxygenvacanciesappearasscatteredbright“bridges”,
-
HybridinterfacesofheteroaromaticmoleculesonTiO2
41
occupyingonebridgingOsiteandconnectingtwofive-foldTirows;OHgroupsinsteadshare
thesameappearanceandpositionbuttherelativeheightisapproximatelydouble.
) b) c)
Fig.4.4.Colourcentresassociatedwithbulkdefectsformeduponreductionofthetitanium
dioxidesinglecrystalscauseachangeincolour.a)Singlecrystalreoxidizedinairat1450K,
b)1h10minat1350Kandc)4h55minat1450K.[30]
AsseenintheFig.4.4.pristineTiO2crystaldisplayafainttransparentyellowcolourwhich
exchangestodarktransparentblueandfinallyintoareflective,metallic-likeappearance,the
morethecrystalisheatedinUHVconditions.Thereasonistheprogressivereductionofthe
material,duethelowerenergybarrierfortheirthermaldesorptionofthebridgingOions.
EachbridgingOlostgeneratestwoexcesselectronsthatmustgointotheconductionband,
thebottomofwhichisformedbyTi3dstatesthatareratherlocalized[35],changingthe
formaloxidationstatefromTi+4toTi+3withtwoconsequencesshowninFig4.5.;ashiftin
the2pcorelevelbindingenergyoftheTiatoms,andthepresenceofanewelectronicstate
slightlybelowtheconductionbandminimum,inthebandgapregion.Infact,sincetheexcess
chargeinRutileformpolarons,theTiionsreducedbythisexcessofelectronsriseanextra
2pcorelevelpeakatlowerbindingenergy.
-
Therutiletitaniumdioxidesurface
42
Fig4.5.ExperimentalevidenceofthechangeinoxidationstateofTiO2uponremovalofO
atoms.(Right)Ti2p3/2corelevelphotoemissionshowsashouldercorrespondingtoTi3+.(Left)
Bandgapregionwherenewelectronicstateat 0.9eVbelowtheFermilevelisseen.
Fig.4.6.XPSvalencebandspectraofrutile-TiO2(110)1x1.Twofeaturesassociatedwiththe
hydroxylpresence(11eV)andtheoxygenvacancies(0.9eV)arehighlighted.Thespectrais
alignedaccordingwithTi3pbindingenergy.
-
HybridinterfacesofheteroaromaticmoleculesonTiO2
43
TheOH defects can be easily recognized in the STM images, butmore important in the
spectroscopyspectres.Thevalencebandspectrumpresenttwoelectronicfeaturesthatcan
helptodiscriminatebetweenoxygenvacanciesandhydroxylgroups.Thefirstpeakcloseto
theFermilevelisassociatedwithdefectelectrons.AsseenintheFig.4.6.,ifsomechargeis
staticallytransfertothesurfacebyanoxygenvacancycreationortheadsorptionofelectron-
donatingmolecules,theexcessofelectronsfillthistrapstate.Thereplacementofoxygen
vacancieswithhydroxylgroupstakeplacenaturallyinabout10-20minutesassoonasthe
surfaceiscoolingdown.Thisprocessdoesn’tquenchthisdefectpeak[36],butnewfeature
isobservedathigherbindingenergies.Thispeakcanatthispointbeeasilyquenchedbya
calibrateexposuretoO2atroomtemperature,avoidingthedissociationofO2moleculeson
theTi5-foldrowsthatwouldchangethesurfaceproperties.
Athermaltreatmentinairfordryingthesilverpasteusedtoattachthesamples,mightfavour
eithernativehydrogendiffusionfromthebulktothesubsurfacelayersorTiO2hydrogenation
inambientconditions(especiallyatdefectsandsampleedges)[37].
Fig.4.7.TheTiO2(110)-(1x2)surface“added-rowmodel”[25].
TheTiO2crystalcanbestronglyreducebyalongannealing,untilthesamplebecomesdark
blueandtheoxygenvacanciesdensityovertakesthelimitof10%.Thissituationcauseanew
-
Therutiletitaniumdioxidesurface
44
surfacereconstruction(1x2),wheretheperiodicityofthepristineTiO2(110)doublesacross
thesurfacerows.Thisreconstructionisformedbytheoxygen-deficientstrandsthatgrow
overtheTifive-foldrowsuntiltheyformacomplete(1x2)terrace[21].Thesurfacegeometry
forasimple,defect-freeTiO2(110)-(1x2)reconstructionispredictedbyOnishiandIwasawa
[33]:Ti2O3rowsmadeofthreeOandtwoTiionsperunitcelllengthalongthe[001]direction
addedontopofaTifive-foldroweachtwobridgingOrowsinFig4.7..
TheTi5frowsareeasilyidentifiedonthe(1x1)surfaceasbrightrowsasexplained.IntheFig.
4.8. one can appreciate the 1x2 reconstruction, and the Ti5f rows of the (1x1) surface
correspondingalternativelytothemiddleofthe(1x2)darkrowsandbrightpairedrows,that
becomesawaytodistinguishthemolecularalignmentwhenmoleculesdoesn’tadsorbon
the(1x2)reconstruction.Thesurfacealsopresentsdefectsthattendtoaggregateintolinear
chainsassingleorcross linksthatarecommonlyobservedinexcessivelyreducedcrystals
[38].
Fig. 4.8. STM detail imagen from clean rutile-TiO2(110) surface with (1x1) and (1x2)
reconstructions.TheextrapolationoftheTi5frowsfromthe(1x1)regiontothe(1x2)region
isshown.
-
HybridinterfacesofheteroaromaticmoleculesonTiO2
45
4.3. TETRAPYRROLEMACROCYCLES:PORPHYRINS
Onemoleculehasbeenselectedasrepresentativeofporphyrins:tetraphenyl-porphyrin,2H-
TPP.Othertwounits,octaethyl-porphyrin,2H-OEPandtert-butyltetraphenyl-porphyrin,2H-
tbTPP, have been probed along the experiments, to contrast results and obtain a larger
picturewhennecessaryaboutthemolecule-substrateinteractionsunderneath.Allofthem,
illustratedintheFig.4.9,areconstitutedbycentralsp2hybridizedtetra-pyrrolicstructure
withdifferentperipheralfunctionalization.Theirmacrocyclescanincorporatealmostevery
elementof theperiodic table for tuning themoleculeopticalproperties.Their functional
groups represent a nice playground with different anchoring possibilities between the
moleculesandtheoxidesurfacethateventuallycanfavouredthechargeinjection,themain
bottleneckinthelightharvestingprocessindye-sensitizedsolarcells[39].The2H-TPPand
2H-tbTPPpresentfourphenylringsthatarenotco-planarwiththemacrocycleduetothe
interactionwith the hydrogen atoms in the ortho- positions of the phenyl rings and the
adjacentatomsattheporphyrincore.Inaddition,2H-tbTPPcarriesadditionaltertiarybutyl
groupsatthetwometa-positionsofeachphenylleg.The2H-OEPpresentsveryflexibleethyl
substituents attached to the β- positions of the pyrrole moieties that doesn’t induce
significantadsorption-induceddeformations.
Fromthechemicalpointofview,aninterestingpointisthatporphyrinscanbemetalated,in
situbyallowingtoreactwithdepositedmetalatoms(i.e.alreadyonthesurfaceordirectly
evaporating over themolecular layer, or predepositedmetal clusters) [38] but also self-
metalatedby incorporationofsubstratemetalatomsunderthermaltreatment[40]. This
impliesas thirdchannel, thepossibilityof trans-metalation,where themetalloporphyrins
exchangeitscoremetalatombyamorereactivesubstrateatom.Self-metalationhasbeen
successfully achieved onmetal substrates as Cu,Ni or Fe butmuch less is known about
metalation reactions of tetrapyrrole macrocycles at metal oxide surfaces. Despite the
expectedlowerflexibilityofcovalentbondingcomparedtometalbonding,thereisevidence
suggestingthatmetalationonmetaloxidesurfacescouldbeevenfasterthanthatonmetal
-
Tetrapyrrolemacrocycles:Porphyrins
46
surfaces. First, the kinetics of metalation on metal surfaces is strongly influenced by
porphyrin-surfaceHexchange [41]where ithasbeendemonstrated thatporphyrinswith
variousfunctionalterminationsareabletopickupHatomsfromthesurfacealreadyatroom
temperature[42].Second,self-metalationatmetalsurfacesarefavouredinthepresenceof
oxygen[43,44].Schneideretal.demonstratedthat2H-TPPisconvertedintoMg-TPPwhen
adsorbedonMgOsubstrates.Anotherinterestingpointisthathydrogenbyitselfcanalsobe
exploited in freebaseporphyrins tomodify the localmolecular conductanceby selective
cleavage of one pyrrolic NH bond [45], onmetal surfaces to influence in the kinetics of
metalation[46]aswellasinmetalloporphyrinstotunethechiralitybyselectiveabsorption
[47].
Fig. 4.9. Ball-and-Stick molecular structure of free-base octaethyl porphyrin (2H-OEP),
tetraphenyl porphyrin (2H-TPP) and tert-butyl tetraphenyl porphyrin (2H-tbTPP). These
moleculesarecharacterizedby theircentral ringofatomsknownasmacrocyclewith two
typesofNatoms,iminicandpyrrolic.Gray=carbon,blue=Nitrogen,white=hydrogen.
Afewinvestigationshavebeenperformedtostudythemetalationoftetrapyrrolemolecules
atmetaloxidesurfaces,liketheinvestigationabouttheconversionof2H-TPPtoNiTPPatthe
TiO2(110)surface[38]thatwasreportedtotakeplaceatroomtemperaturewhen2H-TPPis
depositedfirst,whereasatemperatureof550KisrequiredwhenNiispredeposited,orthe
-
HybridinterfacesofheteroaromaticmoleculesonTiO2
47
2H-TPPmetalationtoMgTPPadsorbedonMgOnanocubeswherethedrivingforceofthe
process hasbeen attributed to thehigh affinity of step and corneroxygens at theoxide
surfacewith the hydrogens of themolecule, indicating the importance of themolecule-
surfacehydrogenexchange[49].
4.3.1. 2H-TPPSPECTROSCOPYCHARACTERIZATION:XPS
Asstartingpointforthecharacterizationoftheinsituevaporated2H-TPPmoleculeslayer
on a surface at room temperature, the ligand state of nitrogen was probed by X-ray
photoemissionoftheN1scorelevel.Althoughmetal-freeporphyrinsarecharacterizedby
twowell definedN1s peaks of equal intensity (separation of ~2 eV) stemming from the
molecules’ pair of hydrogenated (pyrrolic) nitrogen atoms and the pair of aza- (iminic)
nitrogenatoms[50],thefirstlayerofmoleculesalwaysdisplaysadominantcomponentin
theN1sphotoemissionspectracorrespondingtoanenergyof 400.2–400.5eVasshown
in the Fig. 4.10. 2H-OEP and 2H-tbTPP (Fig. 4.10) were also evaporated in the same
experimentalconditionsonrutile-TiO2(110)-(1x1)obtainingthesamebehavior.
This binding energy corresponds to the expected one for pyrrolic nitrogen. This
“pyrrolization” has been reported in free-base phtalocyanine on rutile-TiO2(110) with
contradictoryexplanations[51].Increasingthedepositionbeyondthemonolayertothethick
film formation, a new peak grows at 398 eV binding energy, that corresponds to the
expectediminicnitrogencomponent,thatreachesandpreservestheintensityparitywith
thepyrroliconeafter4-5layers.ThecomparisonofN1sXPSbetweenthemonolayerand
multilayerisalsoshownforallthethreespeciesasreference.
Takingintoaccountthatthesethreemoleculesareessentiallydifferentintotheirmacrocycle
heightwithrespecttothesubstrate(thatisincreased 0.5Åfromonevarietytothenext
duetosimplestericargumentswithoutconsideringminorrelaxationeffects),theabsenceof
theiminiccomponentcannotbesimplyattributedtoastrongcorelevelshiftofaza-nitrogen
originatedbylocalscreening.
-
Tetrapyrrolemacrocycles:Porphyrins
48
Fig.4.10.XPSfromN1scomparisonbetweenmonolayerandmultilayerformbydeposition
for2H-OEP,2H-TPPand2H-tbTPP.Theabsenceofiminic(rightpeak)componentisclearly
observedinthemonolayerregime.
The2H-TPPmonolayerpresentsalsoalargerbindingenergy(shiftof0.2-0.3eV)withrespect
tothe2H-OEPand2H-tbTPP,whichwetentativelyassigntothelocalrelaxationofthephenyl
ringsthatfitthesubstratecorrugation,thankstotheirangularflexibility(i.e.dihedralangle
-
HybridinterfacesofheteroaromaticmoleculesonTiO2
49
betweenthephenylplaneandthemacrocycleplane).Wheningroundstateconformation
ofcrystalline2H-TPP,thephenylsremaintiltedby60ºoutofplaneforstericrepulsioneffects
betweenthephenylandpyrroleC-Hterminationsfacingoneeachother,whereasNEXAFS
dataindicatea 30˚tiltoffthesurfaceforthefirstlayer2H-TPP.Therefore,theonlyplausible
remaininghypothesisisthehydrogenuptake.
Thehydrogenmaybepresent intwomaincoordination,asmolecularhydrogenfromthe
residualgas insidethevacuumorasatomichydrogenfromthesample.Accordingtothe
literature,thehydrogendissolutionintheTiO2bulkwouldbefavouredbythermaltreatment,
metalcontaminants(acceptornature)orelectronirradiation.[52]
XPSbyitselfdemonstratesnottobeaconvenientprobetechniqueforbulk-dilutedspecies
due to its surface sensitivity (i.e. the escape depth of photoelectrons is about 5-10 Å),
whereas it is a suitable probe of the small concentration of hydrogen confined into the
surfacebywaterdissociationatreducedTiO2(110)surfaces[53]Thesehydrogenatomsare
presentintheformofhydroxylspeciesandtheycanbemonitoredbyphotoemissioninthe
valencebandenergyrangewhereanewcharacteristicsatelliteoftheO2pbandsurgesat
11eV.[54]
In the Fig. 4.11.we show the valence band spectra of a clean but slightly hydrogenated
surface with an estimated OH concentration of 4-5%), and the same surface after the
depositionofamonolayerof2H-TPPanditsanalogousfor2H-OEP,correspondingwiththe
twomoleculeswiththemacrocycleclosetothesurface.
Ascanbeseen,thepeakassociatedwiththeOHcontributionisquenchedinagreementwith
the hypothesis of the hydrogen uptake. We must take into account that the complete
hydrogenationofthenitrogenpresentinthefirstmoleculeslayercannotbeobtainedonly
with the hydrogen present in the surface, and more important if possible, that the
hydrogenationisaccomplishedevenonsurfaceswithoutOHcontribution(i.e.freeofdefects
surface). We are then left with the hypothesis that additional hydrogen atoms may be
extractedfromthesubsurfacelayers.
-
Tetrapyrrolemacrocycles:Porphyrins
50
Fig.4.11.(left)Valencebandspectratakeat140eVphotonenergy.TheOHpeakat11eVis
quenchedupondepositionofamoleculesmonolayerof(upleft)2H-TPPand(downleft)2H-
OEP.(Right)Detailfromthedefectstates(x20)thatremainsinvariableinbothdepositions.
Isexpectedthattheannealingofthemonolayeratsomepointshouldpresentsomechemical
andconformationalchangesbeforethermaldesorptionormolecularfragmentation.TheN1s
photoemissionspectrameasuredduringatemperaturerampisshowninFig.4.12.Themain
issueisthegradualswitchofthepyrroliccomponenttoanewenergyslightlyhigherthanthe
iminic one, that starts below 100ºC for 2H-TPP as referencemolecule for the porphyrin
repertory.
-
HybridinterfacesofheteroaromaticmoleculesonTiO2
51
Thisnewenergycorrespondstotheonefoundinfilmsofmetalporphyrins,andiscoincident
inB.E.withtheN1speakobtainedbyevaporationofTiatomsoveraporphyrinlayer,with
theadvantagethatthefullyconversionofthepyrroliccomponentintothemetalliconecan
beachievedbyannealingatabout200ºC,whereasthemetalatomsdepositioncannotfully
convertacompactlayerduetothelimitedmetaldiffusionwhenthesurfaceiscompletely
coveredandTiatomsaggregateintoclusters.
It’simportanttoremarkthatthelowonsettemperatureofthechemicalreactioncaneasily
be reach by Dye-sensitized solar cells operative, and due to this low self-metalation
temperatureandthehighTiatomsreactivity,thetrans-metalationcantakeplaceinthedye
metalloporphyrins.
Fig.4.12(Left)2DrepresentationoftheintensityoftheN1speakduringacontinuousscan
undercontrolledannealingofa2H-TPPmonolayer.Thebindingenergyandthetemperature
areshownintheXandYaxisrespectively.(Right)twocutsatRTand80ºCrespectivelyfrom
the left panel, and a last scan taken at the maximum temperature reached during the
experimentwheretheNitrogenpermutesintoitsmetalatedenergy.
-
Tetrapyrrolemacrocycles:Porphyrins
52
ComparingtheC1sanN1sintensitynormalizingtotheTi2pintensityofthespectrasunder
further annealing beyond 400ºC, a decrease about 20% is noticed, corresponding to a
decreaseofthemoleculardensity(Fig.4.13).
TheperfectoverlapoftheTi2pspectra(inparticular,thelowenergytail)seeninFig.4.13,
with andwithoutmolecular overlayer, and before and after annealing, indicates that no
change of the amount of Ti3+ and Ti2+ ions is observed upon self metalation.We can
concludethatTiatomincorporatedinthemacrocyclemusthavethesameoxidationstateof
thesubstrateatoms.
Fig.4.13 (Left)PhotoemissionofN1sandC1speaks from room temperature toannealed
moleculesmonolayermeasuredwithphotonenergyof650eVandnormalizedinintensityto
the Ti2p peak. (Right) Photoemission of Ti2p peaks from clean substrate to annealed
moleculesmonolayermeasuredwithphotonenergyof650eVandnormalizedinintensity.
Performingtheannealingfortheothertwospecies(2H-OEPand2H-tbTPP),theresultsare
consistentwiththeself-metalationofthemolecule,withaminordifferencewithrespectto
-
HybridinterfacesofheteroaromaticmoleculesonTiO2
53
theperipheralsubstituents,reachingthecompleteswitchofthepyrroliccomponentintothe
metalliconeat200ºCforthe2H-tbTPPandat250ºCforthe2H-OEPasseeninFig4.14.
Fig.4.14. (Left)N1sXPSspectrafrom0.8monolayerof2H-tbTPPasdeposited,andunder
thermal treatment.Themetalationof themolecule is reachedand theN1speak shifts to
398.8eV.(Right)XPSforN1sduringthemetalationbythermaltreatmentof0.5monolayer
of 2H-OEP. The behaviour is similar to the other porphyrins, andwhen themetalation is
completedtheN1siscompletelyshiftedtoabindingenergyabout398.8eV.
4.3.2. 2H-TPPNEAR-EDGEX-RAYPHOTOEMISSIONSPECTROSCOPY
Theproximityofthemacrocycletothesubstrategovernstheconformationofthemolecule,
mainlytheconcavityorconvexityofthefourpyrrolemoietiesthatcanbestronglydistorted
by the incorporation of ametal atom, and the tilt angle and rotations of the peripheral
substituents.
-
Tetrapyrrolemacrocycles:Porphyrins
54
Tostudythecorrelationofthedifferentmolecularconformationsandthedifferentthermal
treatmentswhichundergothechemicalreaction(i.e.metalationofthemacrocycle)near-
edgeX-rayabsorptionspectroscopy(NEXAFS)isoneofthebestcomplementarytechniques.
NEXAFS probes the unoccupied electronic states upon adsorption, providing also an
estimation of the charge transfer in the bonding between adsorbate and substrate. The
molecularorientationwith respect to thesurfacecanalsobeestimatedbyanalysing the
dichroism in the spectra when using linearly polarized photon source [55]. To this aim
measurementsattheC1sK-edgeforthethreeporphyrins,atroomtemperatureandafter
annealingarepresentedinthischapter,withspecialconsiderationinthe2H-TPPstudy.All
themeasurementshavebeenperformedwithlinearlypolarizedX-rayswithitselectricfield
orthogonal(Ppolarization)orparallel(Spolarization)tothesurfaceplane,andthescattering
planealongthe[001]symmetrydirection.
IntheFig.4.15.the2H-TPPcarbonK-edgeNEXAFSspectradisplaysanearedgestructure
withapeakassignedtoaLUMOdominatedby1s→π*transitionsintothemacrocycleand
anintenseresonancecorrespondingtotransitionslocalizedmainlyonthephenylrings[56].
Followingtheprocedurediscussedin[57]andexposedin2.3.2.themacrocycleandphenyl
contributionscanbeseparatedtoextractthetiltanglewithrespecttothesurface.Thepeaks
were fitted with low accuracy applying a Fermi step and a Gaussian/Lorentzian fit
approximationusingXPSmania.
Thephenylstiltanglevalueis33.98ºforthemoleculesintheas-grownmonolayeratroom
temperature,presentinga rotationmuch lower thanthe2H-TPPcondensedcrystal’sone
[58].Thisphenyltiltingisaccompaniedbyasmallout-of-planedistortionofthemacrocycle,
asrevealedbythedichroisminthemacrocyclepeak,withacalculatedangleof14.29ºwhich
isconsistentwiththesaddle-shapedistortionofthepristinepyrrolicmoiety.Afterannealing
to themetalation (250ºC), thedichroismof themacrocyclepeak isonly slightlyaffected,
whilethephenylpeakdichroismdecreases,givingacalculatedtiltangleof42.2º,asdueto
an increased rigidity acquired by the entire macrocycle. Further annealing below the
decompositionlimit(400ºC)exhibitsanalmostcompletedichroismofboth,macrocycleand
-
HybridinterfacesofheteroaromaticmoleculesonTiO2
55
phenyl peaks, corresponding to a strong planarity of the molecule usually seen in the
aromatizationofcarboncompounds,withatiltanglecalculatedforthemacrocyclepyrroles
of11.9ºandatiltangle for thephenylsabout10.4º, thatwouldbeadscribedtoacyclo-
dehydrogenationofthemetalated-porphyrin.
Fig.4.15.2H-TPPmonolayeroverrutile-TiO2(110)1x1CarbonK-edgepolarizedNEXAFS.The
spectrahavebeennormalizedtotheabsorptionmeasuredonthecleanTiO2(110)surface.
Displaying(up)1MLatRTand(middle)formationofmetalatedmonolayerafterannealingat
250ºC and (down) the strong dichroism after annealing over 400ºC. The two black bars
indicatetheLUMOandLUMO+11s→π*transitionsascribedto(1)C-macrocycleand(2)C-
phenyl.
-
Tetrapyrrolemacrocycles:Porphyrins
56
NEXAFS fromNitrogenK-edge at RT shown in the Fig. 4.16. insteadpresent threepeaks
correspondingtotheπ-symmetryunoccupiedstateslocalizedontheiminic(LUMO)at397.7
eVandthepyrrolic(LUMO+1)nitrogenat400.1eVandanotherresonanceat402.0eV.The
residualiminiccontributioncanbeassociatedwiththeresidualiminicintensityobservedin
thephotoemissionspectra,sincethesurfaceismostlycoveredbyacidic4H-TPPmolecules
atRT,andthethirdonewouldbeascribedtothemetalloporphyrinphase.Both,theiminic
andthepyrrolicpeaksdisplayastrongdichroism,wheretheresidualintensityofthepyrrolic
peakinSpolarizationmaybeequallycontributedbyabendingofthemacrocycleandbythe
rehybridizationofMOfollowingtheNHbondingtotheObratoms.Adipwouldbeseendue
tothenormalization.
Fig.4.16.2H-TPPmonolayeroverrutile-TiO2(110)(1x1)NitrogenK-edgepolarizedNEXAFS.
ThespectrahavebeennormalizedtotheabsorptionmeasuredonthecleanTiO2(110)surface.
FromlefttorightwecanfollowtheevolutionoftheLUMOsafterandbeforemetalation,and
theirstrongdichroism.
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HybridinterfacesofheteroaromaticmoleculesonTiO2
57
Underannealingbothcomponents,iminicandpyrrolic,convergeintoasingleπ*symmetry
LUMO corresponding to the metalated macrocycle (NTi). This peak presents a large
dichroism,inagreementwiththemetalationofthemacrocycleunits.
4.3.2.1. COMPARISONWITH2H-OEPAND2H-TBTPP
NEXAFSfromCarbonK-edgeofthe2H-OEParereportedintheFig.4.17.thereisstillpresent
themacrocycleLUMO(284.3eV)withahighdichroismatroomtemperatureandattheself-
metalatedmonolayerandasecondcomponentthatcanbeattributedtotheLUMO+1(285.1
eV)ofthepyrrolicringspresentingadichroismfollowingthatfromLUMOcounterparts.The
tiltangleofthepyrroleunitsinthemacrocycleisabout8.3º.
Fig.4.17.2H-OEP1.3monolayeroverrutile-TiO2(110)1x1CarbonK-edgepolarizedNEXAFS.
ThespectrahavebeennormalizedtotheabsorptionmeasuredonthecleanTiO2(110)surface.
(up)1MLatRTand(down)formationofmetalatedmonolayerannealingat200ºC.
-
Tetrapyrrolemacrocycles:Porphyrins
58
Afterannealingonlyminorchangesinthepeaksratioaredetectedduetothemetalation
relaxationoftheporphyrinswithanincrementofthemacrocycletiltangleto12.4º.
Fig 4.18. 2H-OEP 1.3 monolayer over rutile-TiO2(110) 1x1 N K-edge polarized spectra
normalizedtotheabsorptionmeasuredonacleanrutile-TiO2(110)surface.Displaying(up)
1MLatRTand(down)formationofmetalatedmonolayerannealingat200ºC.Theiminicand
pyrrolicNcontributioncanbeeasilyascribedtotheLUMOandLUMO+1.
NEXAFS from Nitrongen K-edge shown in the Fig. 4.18. instead presents two peaks
correspondingtotheπ-symmetryunoccupiedstateslocalizedontheiminic(LUMO)at397.8
eVandthepyrrolic(LUMO+1)nitrogenat399.9eV.Theresidualiminiccontributioncanbe
againoriginatedbytheresidualiminicintensityobservedinthephotoemissionspectraand
-
HybridinterfacesofheteroaromaticmoleculesonTiO2
59
thefact that themeasurement isperformedforathicknessof1.3monolayer.The iminic
componentpresentsalargerdichroismwithrespecttothepyrrolicone,suggestingthatthe
macrocycle of the doubly hydrogenated 4H-OEP porphyrin has a larger saddle-shape
distortionwithrespecttothe2H-OEPbareporphyrin.Afterannealing,theresonancesfrom
theinequivalentNitrogenconvergeintoasinglepeakat398.6eVthatisingoodagreement
with that reported formetalloporphyrins (i.e.primarily Fe-OEP) [59]. Thispeakpresenta
largedichroismandthereisnocontributionfromthepristinenitrogenresonances,according
withacompletemetalationofthelayer.
Fig. 4.19. 2H-tbTPP monolayer over rutile-TiO2(110) (1x1), C k-edge polarized NEXAFS
normalizedtotheabsorptionmeasuredonacleanrutile-TiO2(110)surface.Displaying(up)
1ML at RT and (down) formation of metalated monolayer by 2 layers’ desorption after
annealingat200ºC.
FortheCarbonK-edgeof2H-tbTPP,againthetwomainresonancesLUMO(284.1eV)and
LUMO+1(285.3eV)stemfromthemacrocycleandthephenylringsasseenonFig.4.19.The
-
Tetrapyrrolemacrocycles:Porphyrins
60
dichroismatroomtemperaturegiveusatiltangle22ºforthephenylrings,and16.9ºforthe
macrocycle,indicatinganevenlargerflexibilityandrelaxationthanforTPP.Afterannealing
thephenylringsLUMO+1presentasmallerdichroismthatcanbeassignedtoanincreased
tiltingoffthesurfaceby31˚ofthephenylsaftermetalation,inqualitativeagreementwith
thecaseofTPP.
Therotationofthepyrroleandthephenylcomponentsfromeachspeciesaresummarized
intheTable4.1.
MOLECULE/TEMPERATURE PYRROLE PHENYL
2H-TPP/RT 14.29º 33.98˚
2H-TPP/250ºC 13.6º 42.2º
2H-TPP/400ºC 11.9º 10.4º
2H-TBTPP/RT 16.9º 22.0º
2H-TBTPP/200ºC 17.1º 31.1º
2H-OEP/RT 8.3º(1stpeak) ***
2H-OEP/200ºC 12.4º(1stpeak) ***
Table4.1.Calculatedtiltangleswithrespecttothesubstrateplanebyfittingthepeaksfrom
the C K-edge polarized NEXAFS associated with excitations localized in the macrocycle
pyrrolesorinthephenylrings,afterdifferentthermaltreatments.
4.3.3. 2H-TPPFILMSTRUCTUREDETERMINATION:STMANDRHEED
Atroomtemperature,individual2H-TPPmolecules,asshownintheFig.4.20.appearasan
isolatedmonomer displaying six lobes, with the typical saddle-shape observed onmetal
-
HybridinterfacesofheteroaromaticmoleculesonTiO2
61
surfaces[60].ThetwomainlobesareascribedtothepristinepyrrolesovertheO(2c)rows
thatpointdown,whiletheotherfourlobescorrespondtothetiltedphenylrings.
Fig. 4.20. (left) STM image 24.7x24.7nm froma 2H-TPP sub-monolayer as deposited and
(right)acutshowingthesinglemoleculeprofileandrelativeheightrespecttothesubstrate.
(Bias=-1.4VTunnelingCurrent=6pA)
Allthemoleculesarealignedalongthe[001]directionabovetheoxygenrows.Theobserved
presence of spikes and scratches in the scan direction in correspondence of isolated
moleculesisindicativeofrelativelyhighmobilityalongtherowsatroomtemperature.
Afterthermaltreatmentabovethecompletemetalationtemperature>100ºC,thesituation
ismorecomplexwithseveralnewspecies.IntheFig.4.21.wecandistinguishatleastfour
kindofmonomers:i)mostofthemdisplaythesamesaddle-shapestructurealignedtothe
O(2c)rows,likethoseobservedatroomtemperature,ii)someofthemstillhaveadominant
saddle-shape,butdisplayanuniaxialasymmetryalongtherowdirection,iii)afewofthem
displayarectangularstructure,centredonTi(5c)rowsandanazimuthalrotationby60º,and
iv)thelastone.
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Tetrapyrrolemacrocycles:Porphyrins
62
Fig. 4.21. Different molecular conformation of 2H-TPP after thermal treatment over
metalationtemperature.(Upleft)Imageshowingmoleculeswithsaddle-shapealignedover
O(2c)rows,andmoleculeswithuniaxialasymmetryalongtherowsdirectionanditsprofiles.
Bias=1.2VTunnelingCurrent=5pA15x15nm.(Upright)Moleculeswithsaddle-shapealigned
overtheO(2c)rowsandmoleculesrotatedby45ºalignedovertheTi(5f)rows.Bias=1.48V
tunnelingcurrent=4.5pA13x13nm. (Down left)Moleculewithcharacteristic saddleshape
overO(2c) rowand itsprofile,andrectangularshapemoleculeoverTi(5f) rowand(down
right) its profile along the short axis, the 60º rotated axis is marked in blue. Bias=1.4V
Tunnelingcurrent=5pA7x7nm.
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HybridinterfacesofheteroaromaticmoleculesonTiO2
63
Raising the temperature to 400ºC, where the NEXAFS presents the larger dichroism the
monomersagainadoptapredominantconfiguration,withaflatrectangularshapecentred
onTi(5c)rowsandmajorityrotatedby30ºwithrespecttothe[001]directionasseeninFig.
4.22.Thiscanbeascribedtoacyclodehydrogenationreaction[61]liketheoneseeninthe
Fig.4.23,wherethemoleculeloseseightHydrogenscorrespondingtotheortho-positions
ofthephenylringsandtheadjacenthydrogensattheporphyrincore.
Fig. 4.22. (Left) STM image from self-metalated sub-monolayer of 2H-TPP over rutile-
TiO2(110)withpredominanceofrectangularshapemolecules.Bias=1.4V,Tunnelingcurrent=
4pA,35x35nm.(Right)SamephaseSTMimageatnegativebiasshowingsimilarbehaviour.
Bias=-1.6V, tunneling current=4pA, 60x60nm. (Down right) STM image with (down left)
detailed molecule topography where its characteristic DOS distribution is recognizable.
Bias=0.6V,tunnelingcurrent=4pA.
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Tetrapyrrolemacrocycles:Porphyrins
64
Fig.4.23.Cyclodehydrogenationsketchedinball-and-stickmodel.Green=Carbon,Purple=
Nitrogen,White=Hydrogen.Metalatedmoleculewithagreencoreatom.
The high coverage range presents a different behaviour due to the molecule-molecule
interaction.Initiallythemoleculesdon’taggregateintoislandsduetosmallvanderWaal’s
intermolecularinteraction.AsthecoverageincreasesandmoleculesalongO(2c)rowsenter
in contact, their phenyl rings start to arrange the molecules into an ordered scheme
correspondingtoacommensurateoblique-(2x4)phasedisplayingslantingrowsofmolecules
with respect to the [001]directionas seen in theFig. 4.24. Thisphase is recognizable in
RHEEDpatternstakenalongthe[111]tohighlightthefractionalspotsalongthisdirection,
asseeninFig4.25.
Fig. 4.24. (Left) Growing of the high density phase at RT. Bias= -1.299V, Tunneling
Current=4pA, 30x30 nm. (Right) STM image from high coverage 2H-TPP obtained by
multilayerdesorptionwitha thermal treatmentover themetalationtemperature (300ºC).
Bias=2.5V,tunnelingcurrent=10pA.
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HybridinterfacesofheteroaromaticmoleculesonTiO2
65
Fig.4.25.TheRHEEDimagesaretakenfroma0.9MLinthe[1-11]directiontohighlightthe
characteristicfive-folddiffractionpattern,underdifferenttemperatures(upleft)RT,(upright)
140ºC and (down left) 300ºC, for thermal treatment. (Down right) The calculated LEED
patternoftheoblique-(2x4)phasewithasingledomainisshownforcomparison.
Underthermal treatmentthephaseremainsunchangedasseen intheuntil reachingthe
cyclodehydrogenation temperature (about 400ºC), when a new phase r-(2x6) is seen
accordingwithRHEEDpatternandtheimageanalysisasseenonFig.4.26.Infact,further
annealingto350ºCyieldsthecoexistenceofextendeddomainswithoblique-(2x4)andrect-
(2x6)symmetry.Thisnewphasestartsinthemiddleoftheo-(2x4)domainsbyformationof
molecularrowsalongthe[001]direction,beingfullyconvertedatabout400ºC,wherethe
molecules line up in perfectly parallel linear rowswith a rectangular superlattice that is
collinear to the substrateprimitive lattice (Fig.4.26).Upon furtherheating to450ºCand
higher, themolecular rows still preserve their sharp straightness, but local domains are
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Tetrapyrrolemacrocycles:Porphyrins
66
formedwith 1 26 0
symmetry,hereaftercalledoblique-(2x6),togetherwiththerect-(2x6)
(Fig.4.27).
Fig. 4.26. (Left) Annealing above 400ºC. Bias= -1.5V Tunneling Current = 8pA, 80x80nm.
(Right)TheRHEEDwas taken ina0.9MLannealedat450ºCalong the [001]direction, to
highlightther-(2x6)phase.
Themoleculespreserveaclearnodalplanetransversetothemolecularrows,suggestingthat
the macrocycle is no significantly changed (Fig. 4.27). Finding the exact registry of the
moleculeswiththesubstratealongtherows,theyarestilladsorbedatopthesubstrateObr
rows, like 2HTPP, 4HTPP and TiO2-TPP. Fullmolecular desorptionwould be observedon
terraceswherethesubstratedisplaysthecharacteristic(2x1)reconstruction,asoriginated
byveryhighoxygenreduction.ThecorrespondenceofthemolecularrowswiththeObrrows
canbedeterminedaposteriori from thecomparisonwith imagesobtained for the clean
surfaceonadjacentterracesdisplayingeith