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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
2
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
4
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
5
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
6
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
7
wherethedecaying
8 ='((
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ScanningTunnelingMicroscopy
8
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
9
" ∝ !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
10
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
12
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
13
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
14
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
15
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
16
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
17
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
18
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.
-
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.
-
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
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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