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

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

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

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.

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.

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].

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

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

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)

EXPERIMENTALTECHNIQUES

7

wherethedecaying

8 ='((

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

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.

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.

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

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.

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

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)

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

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.

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)

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

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

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

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.

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.

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.

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.

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

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