Preparation of a boron nitride single layer on a polycrystalline Rh surface

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Applied Surface Science

j o ur na l ho me p age :w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c

Preparation of a boron nitride single layer on a polycrystalline Rh surface

János Kiss

, Károly Révész

, Gábor Klivényi

, Frigyes Solymosi

aDepartmentofPhysicalChemistryandMaterialsScience,UniversityofSzeged,Aradivértanúkt.1,H-6720Szeged,Hungary

bReactionKineticsandSurfaceChemistry,ResearchGroupofHungarianAcademyofSciences,RerrichB.t.1,H-6720Szeged,Hungary

a r t i c l e i n f o

Articlehistory:

Received30July2012

Receivedinrevisedform26October2012 Accepted28October2012

Available online 3 November 2012

Keywords:

Segregationofboron NOdissociationonRh FormationofB Nbond Boronnitride Augerfinestructures

a b s t r a c t

Thesegregationofboronanditsreactivitytowardnitricoxidehavebeeninvestigatedbymeansofhigh- resolutionAugerspectroscopy(AES),X-rayphotoelectronspectroscopy(XPS),ultravioletphotoelectron spectroscopy(UPS),andthermaldesorptionspectroscopy(TDS).ThesegregationofboronfromaRhfoil startedfrom700K.ItspresencealteredthesurfacebehaviorsofRh;theuptakeofNOincreasedbyabout 30–37%.WhereasthedissociationofNOwasabout3–10%onaclean,boron-freesurface,theextentof dissociation(atsaturation)athighestboronlevelwasalmost98%.Thisfeaturestronglysuggestadirect interactionbetweenNOandborononthesurface.Thepresenceofborongreatlystabilizedtheadsorbed nitrogenandoxygenformedinNOdissociation.Boronoxide(BO,B2O2)sublimatedfromthesurface below1000K.Clean,singleBNlayerformedonthesurfaceclosetoamonolayerregime,presumablein nanomashstructure.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Graphenenanoribbons,atomicallythinstripsofgraphenethat arejustafewnanometerswide,areconsideredtobeexcellentcan- didatesfor futureelectronicsapplicationastheirpropertiescan beadjustedthroughwidthandedgeshape[1,3].Hexagonalboron nitride(h-BN)hasgreatpotentialforuseasthedielectriclayerin functionalheterostructureddevices,whichexploittheremarkable propertiesofgraphene[1,2].Thecombinationofgrapheneandh- BNopensuptheexistingpossibilityofcreatinganewclassofatomi- callythinmultilayeredheterostructures.Grapheneandh-BNshare thesamecrystalstructureandhaveverysimilarlatticeconstant but,unlikegraphene,h-BNisaninsulatorwithalargebandgap [4].Alonggraphene,boronnitridenanoribbonshaveattractedmore andmorefundamentalresearchinterest.Forinstance,researchers havefoundthatmagnetismcouldbeinducedinsuchribbonsby replacementofBorNwithBe,C,Al,Si,orwithvacancydefects.

Theyalsofoundthattheribbonscouldhaveanarrowedbandgap andimprovedelectricalconductivitytunedbyatransverseelectric fieldorspecialedgestructure.Thesefindingpromiseabrightfuture inoptoelectronicsandspintronicsforatomicallythinboronnitride nanoribbons.However,amajorchallengeinprovidingexperimen- talevidenceisthatthepreparationofatomicallythinboronnitride isverydifficult[5–7].SincefirstapplicationsofBNinfundamental researchthattheinterestinsinglelayerboronnitride(sBN)has

∗Correspondingauthorat:DepartmentofPhysicalChemistryandMaterialsSci- ence,UniversityofSzeged,Hungary.Tel.:+3662544803;fax:+3662546482.

E-mailaddress:jkiss@chem.u-szeged.hu(J.Kiss).

beenincreasingsteadily.Inthepastseveralexperimentalstudies havebeenperformedincludingTEM[8–10],AFM[11]andoptical andRamanspectroscopy[9]forcharacterizationofthematerial’s surface.ThemainexperimentalapproachesforproducingsBNhave beenbothmechanical[9,11]andchemical[10]exfoliation.

Oneofthesurfacesciencetoolsproducingboronsourceisthe dehydrogenationofdecaboraneonmetalsurfaces[12].Hexagonal boronnitride(h-BN)waspreparedbythermaldecompositionof borazine,B3H6N3onNi(111)[13],Cu(111)[14]andRh(111)[15].

Surfaceboroncanbepreparedbysegregationprocess,too.Boron segregatesfromthebulktothesurfacebyannealingthepolycrys- tallineRhsurfaceat750–1200K[16,17].Weearlierdemonstrated thatboronimpuritysegregatingtoanRhsurfacedramaticallyalters thereactivityoftheRhsurfacetowardN-andO-containingmoi- eties,suchas(CN)2[18],H2O[17],CO2[19,20].Apossiblereasonfor thisphenomenonisthatboronformsverystrongbondswithNand O,whichcanpromotetheprocessesofsurfacedissociationofthe adsorbedmolecules.Followingsurfacedissociation,wedetected theformationofdissociationproductsviathermaldesorption,and anewfeatureat9.4and7.4eVintheelectronenergylossspectrum duetoB OandB Nbond,respectively.Thesegregationofboron ontoaRhsurfacehasalsobeenobservedbyotherauthors[21–23].

Inpresentworkweexaminethesegregationofboronandinter- actionofsegregatedboronwithNObymeansofdifferentelectron spectroscopicmethods,suchashighresolutionAES,XPS,UPSand thermal desorptionmassspectrometry(TDS).Theprimaryaims weretodeterminethemonolayerBNformedduringthereaction betweenthedissociation productand segregated boron.Earlier theinteractionofNOwithcleanRh(111)surfacewasinvestigated widelybydifferentsurfacesciencetechniques[24–31].Theinitial 0169-4332/$–seefrontmatter© 2012 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.apsusc.2012.10.157

J.Kissetal./AppliedSurfaceScience264 (2013) 838–844 839 adsorptionofNOoncleanRhsurfaceisdissociativeat300K.At

higherNOcoveragetheadsorptionismolecular.

2. Experimental

Theexperimentswereperformedinanultra-highvacuumsys- tem,withabackgroundpressureinthelow-tomiddle10⁻¹⁰mbar range,producedbyturbomolecularandtitaniumgetterpumps.

Thesystemwasequippedwithahemisphericalanalyser(Leybold- HeraeusLHS-10)forUPS,XPSandhighresolutionAES,anArion gunforcleaningandaquadrupolemassspectrometerforTDS.

UPSwasperformedbyusingHeI(21.22eV)andHeII(40.81eV) radiation.TheanglesbetweenthesurfacenormalandtheUVlamp andbetweenthesurfacenormalandtheanalyserwere75^◦ and 16^◦,respectively.ThephotoelectronswereexcitedbyAlK␣radia- tion(1486.7eV)intheXPSregime.TheenergiesoftheXPSpeaks werecalibratedrelativetotheFermilevelofRh,metallicRh3d_3/2 ispositionedat307.1eV.Thephotoemissionsweremeasuredby usingapassenergyof50eVinordertogettheoptimumresolution bymaintaininganacceptablesignal-to-noiseratioatmeasuring timeof60min.HighresolutionAugerelectronspectraweretaken indN(E)/dEmodeusingalock-inamplifier(Ithacho,Dinatrac391 A)with0.5–2eVpeak-to-peakmodulation.Attheseparameters theestimatedresolutionwas1.6–1.9eV.Inordertominimizethe electron-inducedchangesinourexperiments,weused0.2–1␮Aof incidentcurrentand2.5kVofincidentenergy.

ThepolycrystallineRhfoil(10mm×10mmand0.127mmthick 99.9%purity)waspurchasedfromHicolCo.Theinitialcleaningpro- cedurehasbeendescribedpreviously[19,20].Itisconsistedofion bombardmentandannealingat1270K.Themajorcontaminants ofRhfoilwereB,P,S,andC.TheP,CandSwereeasilyremoved, butnocompleteeliminationofboronwasachievedevenaftersev- eralcleaningcycles.Thefinalthermaltreatmentwasperformedat 700K.

Segregationof boronwasachieved byannealing the Rhfoil at750–1270K.Thelevelofsurfaceboronischaracterizedbythe relativeintensityoftheboronAugersignal,RB=B178/Rh302.B178

andRh₃₀₂ representpeak-to-peakintensity ratiosof boronKLL andrhodiumMNNAugertransitionsdeterminedindifferentiated mode.Thepeakpositionsaretakenfromthenegativegoingofthe spectra.

3. Resultsanddiscussion

3.1. Generalfeaturesofsegregatedborononrhodiumsurfaces The segregation of boron was pointed out in earlier works [17–24,32].Ithasbeenobservedthatboronisamajorbulkimpurity (17ppm)inRh.Whenthesurfaceisheatedtheboronsegregates tothesurface.Fig.1showsatypicalsegregationcurve,R_Bvalues (relativeintensityofboronAugersignal)areplottedagainsttem- perature.Whenthefreshly sputteredsurfacewasheatedabove 700K boronsignalshowedup,it reacheda maximum valueat 1000K.(Experimentscannotbeextendedtohighertemperatures becauseoftheconstructionofoursampleholder.)Boroncoverages werecalculatedroughlybyusingtheratioofthetheoreticalXPS photoionizationcross-section[33]foradsorbedboronandoxygen andcomparingtotheXPSsignalfromtheknownoxygencoverage onpolycrystallineRhatsaturation(5.8×10¹⁴Oatoms/cm²)[34].

In thisway we found7.96×10¹⁴Batoms/cm² athighest boron concentration,which abouthalfofa monolayer.Thisvaluecor- respondstoa relative Augerpeak ratio ofR_B=0.103.Using the Langmuir–McLeanequation,assumingthatthesegregationprocess isinequilibrium,thesegregationenergyis−71.5kJ/mol.

Fig.1.PlotoftheB/RhAugerpeakratio(RB)versustemperature.

Thesegregationexhibits significantspacespecificity. Onthe Rh(100)face,thesegregatedboronformsa(3×1)LEEDpattern andasboronisdepletedfromthebulk,itthensegregatestoalesser extentandformsa(3×3)orderedoverlayer[23].Thesegregation onthe(755)faceismanytimeslargerthanthatontheRh(331) face[21].

AnanalysisofthewidthoftheHeIUPSuponsegregationof borondidnotrevealanychangesintheworkfunction,indicating thatthedipolemomentproducedbysurfaceboronisverysmall.

ThisisconsistentwiththesimilarPaulingelectronegativitiesof B(2.0)andRh(2.2),whichsuggestthatchargetransferbetween theseelementsisminimal.Independentlyofthecoverage,theB 1slevelappearedat187.8eVinXPS(Fig.2).Thepeakexhibited 3.5eVFWHMatsaturation.Thisvaluesignificantlylargerthanthat observedforB1sonMo(100)surfaceafteradsorptionofdiborane [35].WeassumethattheB1sphotoemissionsignaliscomposedof

186 188 190 192 194

BE (eV)

9900 10000 10100 10200 10300 10400 10500

Intensity (cps)

XPS Spectra

191.5 187.8

192.4

RB~0.07

6L O2

300 K

580 K

720 K

850 K

950 K

1150 187.8

Fig.2. XPspectraforB1slevelafterheatingthesample(RB≈0.075)exposedto6L O2at300Ktodifferenttemperatures.

Fig.3.UPspectraofadsorbedNOoncleansurfaceatdifferentexposuresat300Kandatdifferenttemperatures.

twoormoreoverlappingpeaks,indicatingdimersortheformation ofislands.

ThesegregationofborononRhfoilproducedaphotoemission peakat8.6eVindifferenceUPSspectra(notshown).TheB2plevel wouldbeexpectedataround4.0–5.0eV.Thedetectionofthispeak isdoubtful,presumableduetoitsverylowcross-section.Theinten- sityofthisphotoemissionat8.6eVincreasedanditshiftedslightly tohigherbindingenergywithincreasingofthesurfaceconcen- trationofboron[32].InaUPSstudyonironborides,peakdueto emissionfromtheboron2p,2sp²and2sstateswereidentified[36].

TheB2sp²levelislocatedat7.0eVforFe2Bandat10.0eVforFeB.

AccordingtothestudybyJoynerandWillis[36],the2sp² emis- sionisduetoaB–Binteraction.AstheintensityoftheUPSpeakat 8.6–9.0eVincreasedwithboronlevelonRh,wemayassumethat B–Binteractionalsoexistinourcase,too.Stairetal.[35,37]reached asimilarconclusionfortheB/Mo(100)system.

3.2. AdsorptionanddecompositionofNOonboroncontainingRh The chemical nature of adsorbed NO on boron-free poly- crystallineRh wasalmostidenticaltoRh(111) [30,38–41].The appearanceoftheO2pUPSsignalat5.6–5.8eVindicatesthedis- sociationofNOatlowcoverageat300K.Athigherexposuresthe adsorptionispreferentiallymolecular.Newphotoemissionswere producedat∼2.3,9.3and14.5eVbelowFermilevel(Fig.3).The NO-inducedemissionscanbeattributedtothe2␲*,1␲/5␴and4␴ molecularlevels,respectively.WhentheNO-saturatedsurfacewas heated,theintensitiesofthepeaksat9.3and14.5eVdecreased above354Kandthesepeaksdisappearedat 400–432K(Fig.3).

Theemissionat5.6–5.8eVwasmorestable;itwasdetectedup to997K.Thisemissioncanbeattributedveryprobablytothefor- mationofB Ospeciesathightemperatureswherethesegregation ofboronisdominating.MolecularNOdesorbsbetween350and 420KwithTp=381K.Nitrogendesorbsinthreestages(Tp=410, 550and640K).Somecharacteristicdatafordesorptionarecol- lectedinTable1.Theresultsobtainedsuggestthatthedissociation ofNOis limited.Theextentofdissociation issomewhathigher thanonRh(111)duetothehighernumber ofdefectsites.The calculationsofDeLouiseandWinograd[29],basedonXPSmea- surements,showedthatonly3%ofthesaturatedlayerdissociates at300KonsinglecrystalofRh.Itshouldbealsotakenintoaccount thatthedissociationofNOinthesaturatedlayerishampered,as

itrequiresfree adsorptioncentersinthevicinityofmolecularly adsorbedNO.

ThepresenceofboronalteredthesurfacebehaviorofRh;the uptake of NO onboron containing surface increased by about 30–37%.Theextentofdissociation(atsaturation)alsoincreased;

athigherboronimpuritylevelitwasalmost98%.Thepeaktem- peraturesfordesorptionofmolecularlyadsorbedNOfromboron containingRhwereunalteredsuggestingthatthisNOadsorbsand desorbsfromRhsitesnotinfluencedbyboronadatoms(Table1).

However,thepresenceofboron greatlystabilizedtheadsorbed nitrogenandoxygen.Thisstabilizingeffectwassogreatthatthe nitrogenformed in theboronpromoted dissociation ofNO did notdesorbbelow1200K.Thisfeaturestronglysuggestsadirect interactionbetweenNandborononthesurface.

Fig. 4 shows the UPS spectra of NO as a function of NO exposureandthethermallytreatedNOcoveredsurfacedonboron- contaminatedsurfaces.Atlowcoverages,molecularlyadsorbedNO wasnotdetected;strongUPsignalwasobtainedat5.2–5.5eVdue totheadsorbedoxygenfromNOdissociation (Fig.4).Athigher coverages(from0.4L)aphotoemissionsdueto1␲/5␴orbitalsof adsorbedNOmoleculeweredetected.DuringheatingtheNOcov- eredsurface (Fig. 5)the emissionsfor molecular orbitalof NO disappearedfrom374K,theO2pintensified,andanewphotoemi- ssionappearedfrom546Kat9.0–9.5eV.Itcouldnotbeeliminated upto1270K.Thisphotoemissioncanbeattributedtotheformation of␴bondbetweenBandN[42].

Inthenextstepswefollowedthesurfacechemicalchangesdur- ingadsorptionand thermaltreatmentbyhighresolutionAuger electron spectroscopy.In certain cases due totheloss features originated fromdifferentelectron-induced interband,intraband Table1

CharacteristicdataofdesorptionofNOfromcleanandboroncontainingRhsurfaces (RB≈0.075).

Surface Products Tp(K) Kineticsorder Ed(kJ/mol)

Clean

NO/NO 381 1 111

N2(␤1)/NO 410 1 120

N2(␤2)/NO 550 1 160

N2(␤3)/NO 640 2 –

Boroncontaining RB≈0.103

NO/NO 381 1 111

N2(␤1)/NO 402 1 117

N2(␤2)/NO 580 2 –

J.Kissetal./AppliedSurfaceScience264 (2013) 838–844 841

-2 0 2 4 6 8 10 12 14

Binding Energy (eV)

Intensity (Arb. unit)

UPS Spectra

EF=

0.2 L

RB~0.07 0.0 L 0.4 L 6.8 L

Fig.4. UPspectraofadsorbedNOonboroncontainingRhsurfacesatdifferentNO exposuresat300K.

transitionsandplasmonlosses,theAugerspectroscopycouldbe verysensitivetothechemicalnatureofadsorbedmolecules.Fig.6A showstheoxygenAESsignalswithincreasingNO exposuresat highest boron concentration. At lowest NO coverage two AES

-2 0 2 4 6 8 10 12

Binding Energy (eV)

Intensity (Arb. unit)

UPS Spectra

EF=

997 K 837 K 667 K 546 K

447 K 347 K 6.8 L NO 300 K

Fig.5.UPspectraofNOsaturatedboroncontainingRhsurfacesatdifferenttem- peratures.

transitionsweredetectedat513and519eV.Withincreasingexpo- suresbothintensitiesincreased.It isimportanttomentionthat onclean,boron-freesurfaceonestrongpeakappearedat519eV besidessomelossfeaturesatlow-energysideofthespectra.We

480 490 500 510 520 530

eV

dN(E)/dE

AES Spectra

490 500 510 520 530

dN(E)/dE

AES Spectra

0.2 L

0.4 L

0.8 L

6.8 L

Background

300 K

1020 K 374 K

547 K

740 K

880 K

B A

eV

Fig.6.O(KVV)AugerlineshapeafteradsorptionofNOonboroncontainingRhsurfaceatdifferentNOexposuresat300K(A).EffectofheatingonO(KVV)finestructuresof NOsaturatedboroncontainingRhsurfaces(B).Theintensitiesaredividedbythefactorof2.

Fig.7.N(KVV)AugerlineshapeafteradsorptionofNOonboron-freeRhsurfaceatdifferenttemperatures(A).EffectofheatingonN(KVV)finestructuresofNOsaturated boroncontainingRhsurfaces(B).

believethatthepeakat519eVbelongstoRh NObondwhilethe transitionat513eVcorrespondstotheB NObond.WhentheNO coveredsamplewasheatedthepeakat519eVdisappearedwhen themolecularNOdesorbedabove 374K.The intensityofother oxygenpeakdecreasedfrom740K,disappearedbetween880and 1020K(Fig.6B).TheformationofaB Obondisalsojustifiedby thefinestructureofB(KVV)andO(KVV)AESsignalobtainedinthe interactionofoxygenwithboroncontainingRhat300K[32].The AEStransitionofadsorbedoxygenonaB/Rhsurfaceappearedat 513eVatlowexposure.Withincreasingoftheoxygencoverage, theintensityofthispeakincreasedandatransitionat518eVfor adsorbedoxygenonRhsitesalsodeveloped.

TheAugerfinestructureofnitrogenafteradsorptionanddecom- positionofNOonacleanandboroncontainingRhsurfaceswas somewhat more complex. The adsorbed nitrogen gives a very intense “four-peak”structure in AES[43,44]. Thisstrongstruc- tureoverlapswiththatofotherN-containingmolecules.Onclean RhafterNOadsorptionat300Kfourstructurefinestructurewas observed (Fig.7A).During heating, independentlyfrom theNO dissociationthisstructureremainedupto547–628K,abovethis temperaturetheAESpeakat383eVgained,above740K,where theboronsegregationstarttobedominant,practicallyonepeak remainedat383eV.OnepartofnitrogendesorbsfromRhatoms, other partof nitrogen migrates from Rh sites to boron, which startstosegregatefrom550K.TheformedB Nspeciesexhibits mainlyonepeakbetween380and400eVinAugerspectrum.Sim- ilarstructurewasreportedforhexagonalphaseofboronnitride andBNingraphitizedsteelsheet[45,46].Onboroncontainingsur- face(R_B=0.075)atsaturatedNOcoveragethefourpeakstructure ispresentat300K.WhenthemolecularNOdesorbsat434Kand abovetheAESpeakat384–386eVwasonlypresentwithsome satellitesatlowerbindingside(Fig.7B).Thispeakremainedwith sameintensityupto1270K,indicatingthatBNstablesurfacecom- pound.

Fig.8 shows theAugertransitions in boronrange from 150 to 200eV. Without boron (so-called clean surface, R_B≈0.003) this regime contains some weektransitions, which are always presentineveryAESspectraofRh.Whenthesurfacewasflashed to1050K, boronsignalappearedat179–183eV.WhenNOwas addedtosurfaceat300K,theintensityofelementalboronsignal

significantlydecreased,ashoulder at172eVgainedremarkably.

Whenthe surface washeated tohigher temperature this peak increased further. In addition, the oxygen Auger feature was shiftedfrom520to513.7eV.Thesame changeswereobserved when oxygenwas added to the boron-containing surface, and followingtheoxidationofboronformingB₂O₃[47,48].Nodoubt

Fig.8.EffectofheatingonB(KVV)AugerfinestructuresofNOsaturatedboron containingRhsurfaces.

J.Kissetal./AppliedSurfaceScience264 (2013) 838–844 843 theAEStransitionat172eVbelongs totheboronoxide,which

canbeeliminatedabove880K.AnewAESfeaturedevelopedat 176eVfrom300K. Inharmonywiththeliteraturedata[46,49], weattributedittotheB Nbond.Similarlytothecorresponding NAESsignal,itcannotbeeliminatedbelow1270K.Anextensive argonionbombardmentwasusedtogetbackthecleansurface.

AnalysingtheUPS,XPSandAESdataitturnedoutthatduring heattreatmenttheboronoxidedisappearedfromthesurfaceabove 800K.Furtherprocesses areindicatedby thedecrease intheO Augersignalabove850Kandbythereappearanceoftheelemental boronfeatureat187.8eVinXPS(Fig.2).Afteroxygenadsorptionat 300KtheB1sphotoemissionpeakmovedto191.5eV.Duringheat- ingashoulderdevelopedat192.4eV.Thesebindingenergieswere observedwhenB₂O₃wastreatedinreducingatmosphere,where transientlytwo-valenceboronwasobserved[50].Whentheoxygen coveredRhsurfacewasheatedabove850Kthepeakshiftedbackto 187.8eV.AswedonotexpectthedecompositionofthestableB₂O₃ species(thedissociationenergyofBOmoleculeis787kJ/mol[51]

andB Osinglebonddissociationenergyisconsideredtobelower, around540kJ/mol),weassumeafurthersegregationofboronand areactionbetweenB2O3andelementalboron

B_(S)+B₂O_3(S)−B₂O_2(g)+BO_(g),

theheatofreactionis141.1kJ/mol[51].Theoccurrenceofthisreac- tioninthepresentcaseissupportedbytheobservationthatupon heatingtheRhfoilcontainingtheB Ocomplextohightemper- aturewedetecteddesorbingspeciesat54and27amu(B₂O₂and BO,respectively);thepeaktemperaturewas1050K.Notethatpar- tialoxidationofbulkboronleadstotheformationof(BO)xwhich vaporizesat1300K,asB₂O₂,anddisproportionatesoncondensa- tion[52].

In the interaction of NO with boron-containing Rh surface above900Kwe prepareda cleansingle BNlayerinmonolayer orclosetomonolayerregime.Anh-BNlayerwasfoundtobind a quitelargenumber ofsubstrates,even when thelatticecon- stantofh-BNand thesubstratedonot match.Theprototypical example is the h-BN/Ni(111) interface,which has nice lattice matchingandformsasimpleepitaxial1×1structure[13,53].In thecaseofincommensurateinterfaces,themismatchcanleadto someextraordinarystructures[54].Corsoetal.[15]havereported the formation of a self-assembled nanostructure, the so-called nanomesh,onthelatticemismatchedRh(111)surfacewithaperi- odicityofabout3.2nm.Thiscorrespondstoasituationwherethe latticeof 12×12 Rh(111)matches 13×13 h-BN cells forming periodic nanomesh structure, which is caused by the lattice- mismatchbetweenRh(111)andh-BN.AlthoughourRhsample ispolycrystallinewecannotexcludethattheformedBNconstitute nanomeshonthesurface.Sincethefirstdiscoveryofnanomesh [15]onRh(111)surface,similarstructureshavealsobeenfound onRu(0001)[55],Pd(111)[56]andPd(110)[57].

4. Conclusions

ThesegregationofboronfromaRhfoilstartedat700K.Theseg- regatedboronproducedapeakfortheB1slevelonXPSat187.8eV andemissioninUPSat8.6–9.0eV,respectively.Thepresenceof borondrasticallyalteredthesurfacebehaviorofRh;theuptake ofNOincreasedbyabout30–37%.TheextentofNOdissociation (atsaturation)athighestboronlevelwasalmost98%.Thisfeature stronglysuggestadirectinteractionbetweenNOandborononthe surface.Thepresenceofborongreatlystabilizedtheadsorbedoxy- genandnitrogenformedduringNOdissociation.Boronoxides(BO, B₂O₂)sublimatedfromthesurfacebelow1000K.Thestabilizing effectwassogreatthatthenitrogenbondedwasnotreleasedbelow 1200K.Theformationofboronnitridewasdetectedat9.0–9.5eV

inUPS.Thisphotoemissioncanbeattributedtotheformationof␴ bondbetweenBandN.Above400KnewintenseAugertransitions developedat176eV(B)and384eV(N),whicharetypicalforB N species.Aclean,singleBNlayerformedonthesurface,presumable innanomashstructure.

Acknowledgement

ThefinancialsupportoftheHungarianScientificResearchFund (OTKA)throughprojectsK81517,K81660andTámop-4.2.2/B-10/1- 2010-0012areacknowledged.

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