Electronic properties of gold thin films studied by electron energy loss

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)NTRODUCTION

%LECTRONICPROPERTIESOF GOLDTHIN½LMSSTUDIEDBY ELECTRONENERGYLOSS SPECTROSCOPY !NTONIO0OLITANOA B D 'ENNARO#HIARELLOA C A

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#ORRESPONDINGAUTHOR% MAILANTONIOPOLITANO IMDEAORG

!BSTRACT (IGH RESOLUTIONELECTRONENERGYLOSSSPECTROSCOPY WASUSEDTOINVESTIGATETHEELECTRONICPROPERTIESOF ORIENTED!UULTRATHIN½LMSGROWNON#U 4HELOSSSPECTRUMSHOWEDSEVERALFEATURESWHICH WEREASCRIBEDTOBOTHSINGLEANDCOLLECTIVE EXCITATIONS)NPARTICULARWEDISTINGUISHEDFEATURES ASSIGNABLETODISPERSIONLESSSINGLE PARTICLE TRANSITIONSANDTHEDISPERSINGELECTRON HOLE CONTINUUMASWELLASTHEORDINARYANDS LIKESURFACE PLASMON ANDTHEMULTIPOLESURFACEPLASMON +EYWORDSELECTRONENERGYLOSSSPECTROSCOPY%%,3 GOLD½LMS SURFACEPLASMONS

.ANOSCALETHIN½LMSAREATTRACTINGACONSIDERABLEINTERESTIN RECENTYEARSFORBOTHFUNDAMENTALINTERESTANDTECHNOLOGICAL APPLICATIONS ; = 4HEY ARE A UNIQUE CLASS OF NANOSYSTEMS WHOSE ELECTRONIC PROPERTIES CAN BE EASILY TUNED UPON CONTROLLINGTHETHICKNESSOFTHE½LMATTHEATOMICLEVEL4HIS INTEREST IS MOTIVATED BY THE LACK OF INFORMATION ON SUCH SYSTEMS COMPARED WITH BULK CRYSTALS AND BY THE BROAD POTENTIAL APPLICATIONS IN NANOELECTRONICS AND NANOCATALYSIS 'OLDSYSTEMS SUCHASSEMI IN½NITEMEDIA;= THIN½LMS;= QUANTUM WIRES ; = AND NANOPARTICLES ;= HAVE BEEN EXTENSIVELYSTUDIEDASACONSEQUENCEOFTHEDISCOVERYTHAT !UISASELECTIVECATALYSTFORAVARIETYOFIMPORTANTCHEMICAL REACTIONS ; = (OWEVER LITTLE IS KNOWN ABOUT THE COEXISTENCE AND THE POSSIBLE INTERPLAY BETWEEN SINGLE PARTICLE AND COLLECTIVE EXCITATIONS IN THIN !U ½LMS /N THE OTHERHAND ELECTRONICEXCITATIONSPLAYANIMPORTANTROLEIN THEREACTIVITYOFMETALLICSYSTEMS(ENCE UNDERSTANDINGTHE ELECTRONICPROPERTIESOFGOLD½LMSCOULDLEADTOTHETAILORING OF MORE SELECTIVE CATALYSTS AND TO THE IMPROVEMENT OF THE EXISTINGONES-OREOVER STUDIESONNOBLE METALSYSTEMSARE PARTICULARLY MOTIVATED BY FUNDAMENTAL INTEREST )N FACT THE PRESENCEOFLOCALIZEDDELECTRONSMAKESUNREALISTICTHEJELLIUM MODEL USUALLY APPLIED FOR DESCRIBING DYNAMIC SCREENING OF SIMPLE METALS 3EVERAL ATTEMPTS HAVE BEEN UNDERTAKEN IN ORDER TO INCLUDE BAND STRUCTURE EFFECTS IN THEORETICAL MODELS; = (EREINWEREPORTONHIGH RESOLUTIONELECTRONENERGYLOSS SPECTROSCOPY (2%%,3 MEASUREMENTS ON THE ELECTRONIC PROPERTIESOFGOLDTHIN½LMSDEPOSITEDATROOMTEMPERATURE ON#U 4HELOSSSPECTRUMSHOWEDSEVERALFEATURES"ESIDESTHE!U SURFACE PLASMON 30 AT ABOUT E6 LOW ENERGY SINGLE PARTICLETRANSITIONSWEREREVEALEDTOO!BROADPEAKATABOUT E6WASASSIGNEDTOCONTRIBUTIONSFROMBOTHTHES LIKE!U30 AND THE !U MULTIPOLE SURFACE PLASMON -0 /FF SPECULAR (2%%, SPECTRA REVEALED THE EXISTENCE OF A LOSS TENTATIVELY ASSIGNEDTOTHECONTINUUMOFELECTRON HOLEPAIREXCITATIONS

%XPERIMENTAL -EASUREMENTSWERECARRIEDOUTINA5(6CHAMBEROPERATING ATABASEPRESSUREOFÀ 0A4HESAMPLEWASASINGLECRYSTAL OF#U 4HESURFACEWASCLEANEDBYREPEATEDCYCLESOFION SPUTTERINGANDANNEALINGAT +3URFACECLEANLINESS ANDORDERWERECHECKEDUSING!UGERELECTRONSPECTROSCOPY !%3 AND LOW ENERGY ELECTRON DIFFRACTION ,%%$ MEASUREMENTS RESPECTIVELY 'OLD WAS DEPOSITED ONTO THE #U SURFACEBYEVAPORATINGFROMAN!UWIREWRAPPEDON ATUNGSTEN½LAMENT)NORDERTOPREPARE¾AT!U½LMSWITHA LIMITED ROUGHNESS WHICH IS AN IMPORTANT PARAMETER AS IT ALLOWS COUPLING BETWEEN LIGHT AND THE 30 RE¾ECTIVITY MEASUREMENTSUSINGTHEELECTRONIMPINGINGBEAMINSPECULAR

'OLD"ULLETIN6OLUME.O

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GEOMETRY DIPOLE SCATTERING WERE PERFORMED FOR VARIOUS DEPOSITION CONDITIONS 3UCH MEASUREMENTS DEMONSTRATED THATASMOOTHSURFACEWITHANEGLIGIBLEROUGHNESSCOULDBE ACHIEVED UNDER OPTIMIZED DEPOSITION PARAMETERS )N PARTICULAR HIGHLY RE¾ECTIVE AND WELL ORDERED ¾AT !U ½LMS COULD BE OBTAINED AT VERY LOW DEPOSITION RATES ¤ -, MIN 4HEOCCURRENCEOFTHEPX !U,%%$PATTERNWASUSED AS THE CALIBRATION POINT OF Q!U -, 3IMILAR RESULTS WERE OBTAINED BY A CALIBRATION PROCEDURE USING !%3 ! CONSTANT STICKINGCOEF½CIENTWASASSUMEDTOOBTAINOTHERDESIRED!U COVERAGE4HE¾ATNESSOFTHE!U½LMISALSOENSUREDBY,%%$ AND 30 DISPERSION MEASUREMENTS (2%%, EXPERIMENTS WERE PERFORMED BY USING AN ELECTRON ENERGY LOSS SPECTROMETER $ELTA 30%#3 4HE INCIDENT ANGLE WITH RESPECT TO THE SAMPLENORMALWAS½XEDAT4HEACCEPTANCEANGLEOF OUR SPECTROMETER IS 4HE ENERGY RESOLUTION OF THE SPECTROMETERWASDEGRADEDTOME6SOASTOINCREASETHE SIGNAL TO NOISE RATIO OF LOSS PEAKS $ISPERSION OF THE LOSS PEAKS IE %LOSSQ\\ WAS MEASURED BY MOVING THE ANALYZER WHILEKEEPINGTHESAMPLEANDTHEMONOCHROMATORINA½XED POSITION 4O MEASURE THE DISPERSION RELATION VALUES FOR THE PARAMETERS%P IMPINGINGENERGYANDQI THEINCIDENTANGLE WERECHOSENSOASTOOBTAINTHEHIGHESTSIGNAL TO NOISERATIO 4HEPRIMARYBEAMENERGYUSEDFORTHEDISPERSION %PE6 PROVIDED IN FACT THE BEST COMPROMISE AMONG SURFACE SENSITIVITY THE HIGHEST CROSS SECTION FOR MODE EXCITATION ANDQ\\RESOLUTION !S

THE PARALLEL MOMENTUM TRANSFER Q\\ DEPENDS ON %P %LOSS QI AND QS ACCORDINGTO

WHERE%LOSS ISTHEENERGYLOSSAND QS ISTHEELECTRONSCATTERING ANGLE;= !CCORDINGLY THE INTEGRATION WINDOW IN RECIPROCAL SPACE ; =IS

WHERE A IS THE ANGULAR ACCEPTANCE OF THE APPARATUS 4HE INTEGRATIONWINDOWINTHERECIPROCALSPACEFOR30DISPERSION MEASUREMENTSWASESTIMATEDTOBE` MUCHLESSTHAN THESCANNEDRANGEINTHERECIPROCALSPACE !LL DEPOSITIONSANDMEASUREMENTSWEREMADEATROOM TEMPERATURE ATWHICHNOTRACEOF!U#UALLOYWASREVEALEDIN BOTH,%%$AND!%3MEASUREMENTS


2ESULTS 4HE(2%%,SPECTRUMACQUIREDINSPECULARGEOMETRYFORATHIN !U½LMLAYERS ON#U ISDOMINATEDBYTHE30AT E6&IGURE INAGREEMENTWITHOPTICAL; =ANDSCANNING TUNNELINGSPECTROSCOPY; =MEASUREMENTS )NTERESTINGLY FORGOLDE WHICHISTHECONDITIONFORTHE EXISTENCEOFA30 OCCURSATE6;= BUTINTHE!ULOSS SPECTRUMTHEREISNOPEAKATSUCHENERGY-OREOVER FOR!U THE INTERBAND TRANSITION ONSET AT E6 ;= LIES BELOW THE ENERGYOF30 THUSTHE30PEAKISEXPECTEDTOBELESSDISTINCT COMPAREDWITH!G FORWHICHTHEOPPOSITEOCCURS;=!SA MATTEROFFACT THE!U30HASAFULL WIDTHATHALFMAXIMUM &7(- OFABOUTE6ATSMALLMOMENTAANDITDISPERSED UP TO E6 AS A FUNCTION OF PARALLEL MOMENTUM TRANSFER ;= )N COMPARISON THE &7(- OF THE 30 IN !G THIN ½LMS GROWNON#U ISABOUTE6;= "ESIDESTHE!U30 ALSOASHARPPEAKATE6TOPPANEL OF&IGURE ANDABROADFEATUREATABOUTE6BOTTOMPANEL WERE REVEALED #ONCERNING THE LATTER PEAK IT COULD BE ASCRIBEDTOANOTHER30)NFACT E ALSOFORENERGIESAROUND E6 ;= SO AS TO SUGGEST THE OCCURRENCE OF PLASMONIC MODESCON½NEDATTHESURFACE)TISWORTHMENTIONINGTHAT FOR A JELLIUM SURFACE WITH A DENSITY RS IE THE EFFECTIVE RADIUSFOR!U THE30SHOULDHAVEANENERGYOFE6;= 4HE PRESENCE OF D ELECTRONS SHIFTS THE ORDINARY 30 ENERGY DOWNTOE6; =(ENCE WEASSIGNTHEPEAKAT E6TOTHES LIKE30OF!U!SIMILAREXCITATIONWASFOUNDAT ABOUT E6 IN !G ½LMS ON #U ;= !CCORDING TO THEORETICAL½NDINGS;= SUCHPEAKHASBOTHS LIKEMONOPOLE ANDMULTIPOLE30COMPONENTSWHICHCOULDNOTBESEPARATED 7EARGUETHATASIMILARMECHANISMOCCURSFOR!U 4HEBEHAVIOROFTHEPEAKATE6DESERVESAREMARKABLE CONSIDERATIONAS BECAUSEOFITSLOWENERGY ITSHOULDIN¾UENCE THE DECAY RATE OF ALL ELECTRONIC EXCITATIONS AT THE METAL SURFACEANDTHUSCHEMICALREACTIONS;= "ESIDETHEFEATUREATE6 ANOTHERPEAKATE6WAS RECORDED(OWEVER THEPEAKATE6ISACCESSIBLEONLYIN SCATTERING GEOMETRIES NEAR TO THE SPECULAR CONDITION THUS ENSURING OF ITS DIPOLAR NATURE ;= /N THE OTHER HAND THE FEATURE AT E6 IS RECORDED ONLY IN OFF SPECULAR SPECTRA VERYLIKELYDUETOITSIMPACTCHARACTER#HANGINGTHESCATTERING GEOMETRYCOULDOPENNEWEXCITATIONCHANNELS4HIS½NDING INDICATES THE INTERPLAY BETWEEN DIFFERENT SINGLE PARTICLE EXCITATIONSIN!UTHIN½LMS ! CAREFUL INSPECTION OF THE (2%%, SPECTRUM OF THE TOP PANELOF&IGUREREVEALEDALSOTHEEXISTENCEOFADISPERSIONLESS FEATUREATE6)TSENERGYISTOOHIGHTOBEASSIGNEDTO PHONONSORVIBRATIONS4HEABSENCEOFANYDISPERSIONPREVENTS FROM ITS ASSIGNMENT TO A PLASMONIC EXCITATION (ENCE WE ASCRIBEITTOANOTHERSINGLE PARTICLETRANSITION 4HE!U30INSTEADEXHIBITSACLEARDISPERSIONOFITSFREQUENCY WITH THE SCATTERING ANGLE TOP PANEL OF &IGURE &IGURE SHOWS THE 30 DISPERSION FOR A THIN ½LM OF !U ON #U -, COMPARED WITH THE CASE OF A THICK !U ½LM ON THE

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SAMESUBSTRATE WHOSE30DISPERSIONASBEENSTUDIEDIN2EF 4HEDISPERSIONOFTHE!U30WASFOUNDTOBEPOSITIVEIN BOTHCASES!SIMILARRESULTWASREPORTEDFOR!GSYSTEMS; =!CCORDINGTOTHE&EIBELMAN´SMODEL; =OF THE30DISPERSION THISIMPLIESTHATTHEPOSITIONOFTHECENTROID OFTHEINDUCEDCHARGE D> LIESINSIDETHEGEOMETRICALSURFACE Z AS FOR !G ;= /N THE OTHER HAND ACCORDING TO ,IEBSCH´S MODEL ;= THE POSITION OF THE CENTROID OF THE INDUCEDCHARGEISALWAYSEXTERNALWITHRESPECTTOTHEJELLIUM EDGE(ENCE THEPOSITIVEDISPERSIONOFTHE30ISDUETOTHE

&IGURE

Ep=92 eV

s

Intensity (Arb. Units)

i=55.0°

52.5° 1.53

0.37

53.1° 54.5°

0.67

55.0° 55.5° 1.0

3.0 2.0 Energy loss (eV)

4.0

STRONGER IN¾UENCE OF THE POLARIZABLE MEDIUM WHEN THE PENETRATIONDEPTHOFTHE30ISLARGER 4HE DISPERSION RELATION OF !U 30 COULD BE IN PRINCIPLE IN¾UENCEDBYBAND STRUCTUREEFFECTSASACONSEQUENCEOFTHE PRESENCEOFD ELECTRONS; = )TISWORTHNOTICINGTHATTHE30FREQUENCYINTHELONGWAVE LENGTHLIMITSMALLMOMENTA ISINCREASEDBYABOUTME6 INTHIN!U½LMSWITHRESPECTTOTHICKADLAYERS;=4HEBLUE SHIFT OF THE 30 FREQUENCY WAS PREVIOUSLY OBSERVED IN THIN !G ½LMS ON 3I ;= AND #U ;= AND IT IS FULLY DESCRIBED WITHIN THE FRAMEWORK OF S D POLARIZATION MODEL ;=!STHETHICKNESSOFTHE!U½LMISREDUCED THEOVERALL SCREENING OF THE CHARGE ASSOCIATED TO THE 30 VIA THE POLARIZABLEDELECTRONICMEDIUMDIMINISHES GIVINGAHIGHER PLASMON ENERGY &URTHERMORE THE SPILL OUT REGION NOT AFFECTED BY S D POLARIZATION BECOMES MORE IMPORTANT CAUSING A FURTHER BLUE SHIFT OF THE PLASMON FREQUENCY (OWEVER AT HIGHER MOMENTA THE 30 FREQUENCY IS SIMILAR IN THINANDTHICK!U½LM 4HELOWERSLOPEOFTHE!U30DISPERSIONRELATIONINTHIN!U ½LMCOMPAREDWITHTHETHICK½LMCOULDBEASCRIBEDTOTHE MINOR PENETRATION OF THE CENTROID OF THE INDUCED CHARGE INSIDETHEBULK; = -OREOVER THE ABSENCE OF A PLATEAU IN THE 30 DISPERSION ; =FURTHERENSURESOFTHE¾ATNESSOFTHE!U½LM)NFACT THE EXISTENCE OF ISLANDS WITHIN THE ½LM SHOULD IMPLY THE CON½NEMENTOFTHE30 INDICATEDBYTHELACKOFANYDISPERSION FOR30FREQUENCYUPTOACRITICALWAVE VECTORRELATEDTOTHE ISLANDSIZE

Ep=92 eV i=55.0°

&IGURE s 2.66 Intensity (Arb. Units)

50.0°

thin Au film on Cu(111) thick Au film on Cu(111)

52.0° 53.0°

Energy Loss (eV)

2.64 2.62 2.60 2.58 2.56 2.54 53.8° 55.0° 4

8

12 16 20 Energy loss (eV)

24

0.00

0.04

0.08

0.12

0.16

0.20

28

(2%%,SPECTRAASAFUNCTIONOFTHESCATTERINGANGLEINARESTRICTEDTOP PANEL ANDWIDERRANGEBOTTOMPANEL


2.52

qII (A-1)

30DISPERSIONFORATHINPRESENTRESULTS ANDATHICKDATATAKENFROM 2EF ½LMOF!UON#U

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&IGURE

&IGURE

7x10-3 i=s=55.0°


dN/dE (arb. units)

6

10 ML Au/Cu(111)

(a)

Ep=92 eV

annealing 450 K

5

4 annealing 450 K 3

2 10 ML Au/Cu(111) clean Cu(111)

1

4

LMM

8

400

600

800

1000

Kinetic Energy (eV)

!%3SPECTRUMFORTHE#U SUBSTRATE -,!U#U ANDTHE !U #UALLOY


MVV

200

12

16

20

24

28

Energy loss (eV)

1.0

(b)

0.5 0.0 -1.5 -1.0

-1.5x10-3

/NTHECONTRARY THEPEAKATABOUTE6DOESNOTEXHIBIT ANY DISPERSION BOTTOM PANEL OF &IGURE IT REACHES THE MAXIMUM INTENSITY AT OFF SPECULAR AND SUDDENLY DISAPPEARED BY INCREASING THE OFF SPECULAR ANGLE AS A CONSEQUENCEOFTHEOPENINGOFNEWEXCITATIONCHANNELSVIA INTERBANDTRANSITIONS4HUS ITHASASTRICTDIPOLARNATURE;= !NOTHERFEATUREWHICHDISPERSESFROMTOE6WASRECORDED INOFF SPECULAR(2%%,SPECTRA4HEBROADNESSOFITSLINE SHAPE SUGGESTSTHATITSHOULDBEASSIGNEDTOTHECONTINUUMOFTHE ELECTRON HOLE PAIR EXCITATIONS )N FACT THIS MODE HAS BEEN DEMONSTRATEDTOBEDISPERSIVE; = (ENCE THES LIKE30ANDTHE-0OVERLAPINABROADRANGEIN ENERGY¯MOMENTUMSPACEWITHTHEELECTRON¯HOLECONTINUUM !CCORDINGLY A COMPETITION IN THE EXCITATION BETWEEN THE INCOHERENT ELECTRON¯HOLE EXCITATIONS AND THE COHERENT PLASMONICMODESISEXPECTED(OWEVER THEOVERALLWEIGHTOF THE ELECTRON HOLE CONTINUUM IS HIGHER THAN THAT OF THE COLLECTIVE EXCITATIONS AND THIS EXPLAINS THE SUDDEN DISAPPEARANCEOFTHECOLLECTIVEMODEINOFF SPECULARSPECTRA BOTTOMPANELOF&IGURE &INALLY ITISWORTHSTUDYINGTHEFORMATIONOF!U#UALLOY)N FACT WHILEMANYEXPERIMENTALSTUDIESHAVEBEENPERFORMED ON !U ALLOYING WITH COPPER SUBSTRATES ; = COLLECTIVE


5

10

15

20

25

Energy loss (eV)

A (2%%,SPECTRAFOR-,!U#U ANDTHE!U #UALLOY B $IFFERENCESPECTRUM

ELECTRONIC EXCITATIONS OF THE !U #U ALLOY HAVE NOT BEEN INVESTIGATED YET 4HE FORMATION OF AN !U #U ALLOY UPON ANNEALINGAT+WASDEMONSTRATEDBY!%3MEASUREMENTS &IGURE !S A MATTER OF FACT BOTH #U AND !U CONTRIBUTIONS ARE PRESENT IN THE !%3 SPECTRUM .O TRACE OF ALLOYING WAS REVEALEDATROOMTEMPERATURE INAGREEMENTWITHPREVIOUS ½NDINGS;= !SREGARDSTHEELECTRONICSPECTRUM&IGUREA THE!U30 RECORDED AT E6 IN THE AS DEPOSITED ½LM DECREASED IN INTENSITYUPONALLOYING3EVERALOTHERFEATURESAROSEUPONTHE FORMATIONOFTHE!U #UALLOY ASEVIDENCEDBYTHEANALYSISOF THE DIFFERENCE SPECTRUM &IGURE B 4HEY ARE ASCRIBED TO INTERBANDTRANSITIONSINTHEALLOYLOCATEDATABOUTE6WITH RESPECTTOTHE&ERMILEVEL-OREOVER ANINTERBANDTRANSITION ATE6WASOBSERVEDINTHEALLOY)TSHOULDBENOTICEDTHATAN

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EXCITATION AT SIMILAR FREQUENCIES WAS RECORDED FOR THE CLEAN #U SURFACE ;= BUT IT WAS ASCRIBED TO A COLLECTIVEEXCITATIONOFELECTRONSIN3HOCKLEYSURFACESTATESOF THE BARE SUBSTRATE )T DISAPPEARED UPON DEPOSITION OF ADSORBATES ;= AS EXPECTED FOR MODES RELATED TO SURFACE STATES;=

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7E HAVE SHOWN THAT THE LOSS SPECTRUM OF !U THIN ½LMS ON #U PRESENTS BOTH SINGLE PARTICLE AND COLLECTIVE EXCITATIONS$IFFERENTEXCITATIONCHANNELSAREINCOMPETITION EACH OTHERS IF THEY ARE IN THE SAME FREQUENCY RANGE 4HE ANALYSIS OF THE 30 DISPERSION OF A THIN !U ½LM REVEALED THAT THE CENTROID OF THE INDUCED CHARGE LIES LESS INSIDE THE BULK COMPARED WITH A THICK ½LM -OREOVER ABROADPEAKATABOUTE6WASOBSERVEDANDASSIGNEDTOA COLLECTIVEEXCITATIONWHICHHASBOTHS LIKE30AND-0CHARACTER !NOTHER BROAD FEATURE DISPERSING FROM TO E6 WAS ASSIGNED TO THE ELECTRON HOLE CONTINUUM 4HE !U #U ALLOY WAS FORMED UPON ANNEALING THE !U ½LM AT + 4HE LOSS SPECTRUM OF THE !U #U ALLOY IS DOMINATED BY SINGLE PARTICLETRANSITIONS

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