Structure and nanotribology of thermally deposited gold nanoparticles on graphite

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ContentslistsavailableatScienceDirect

Applied

Surface

Science

j o ur na l ho me pa g e :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

Structure

and

nanotribology

of

thermally

deposited

gold

nanoparticles

on

graphite

Ebru

Cihan

a

_,

_Alper

_Özo˘gul

b

_,

_Mehmet

_Z.

_Baykara

a,b,∗

a_UNAM_-_Institute_of_Materials_Science_and_{Nanotechnology,}_Bilkent_University,_Ankara_06800,_Turkey b_Department_of_Mechanical_Engineering,_Bilkent_University,_Ankara_06800,_Turkey

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received22December2014

Receivedinrevisedform18February2015 Accepted14April2015

Availableonline24April2015 Keywords:

Atomicforcemicroscopy Frictionforcemicroscopy Friction

Nanotribology Nanoparticle

a

b

s

t

r

a

c

t

Wepresentexperimentsinvolvingthestructuralandfrictionalcharacterizationofgoldnanoparticles (AuNP)thermallydepositedonhighlyorientedpyrolyticgraphite(HOPG).Theeffectofthermal depo-sitionamount,aswellaspost-depositionannealingonthemorphologyanddistributionofgold on HOPGisstudiedviascanningelectronmicroscopy(SEM)measurements,whiletransmissionelectron microscopy(TEM)isutilizedtoconﬁrmthecrystallinecharacterofthenanoparticles.Lateralforce mea-surementsconductedviaatomicforcemicroscopy(AFM)underambientconditionsareemployedto investigatethenanotribologicalpropertiesofthegoldnanoparticlesasafunctionofnormalload.Finally, theincreaseinlateralforceexperiencedattheedgesofthenanoparticlesisstudiedasafunctionofnormal load,aswellasnanoparticleheight.Asawhole,ourresultsconstituteacomprehensivestructuraland frictionalcharacterizationoftheAuNP/HOPGmaterialsystem,formingthebasisfornanotribology exper-imentsinvolvingthelateralmanipulationofthermallydepositedAuNPsonHOPGviaAFMunderambient conditions.

1. Introduction

Despite the fact that friction is ubiquitous in nature, the

fundamentalphysicalprinciplesthatgovernthisinteresting

phe-nomenonarestillnotwellunderstood.Whilethemacroscopiclaws

offrictioninvolvingalinearlyproportionalrelationshipbetween

thenormalload(Fn)andthefrictionforce(Ff)arisingbetweentwo

objectsincontacthavebeenwellestablishedsincehundredsof

years thankstopioneeringexperimentsbyAmontons,Coulomb

andothers,themacroscopicallyobservedproportionalityconstant

(theso-called frictioncoefﬁcient)cannotbederived fromﬁrst

principlesasitconstitutesacomplexfunctionofinterface

struc-ture,chemistryandenvironmentalfactorsincludingtemperature

andhumidity[1].Moreover,theunavoidablemulti-asperitynature

of interfacesformed bytwo macroscopicobjects in contact[2]

complicates the physical interpretation of macroscopic friction

experiments,leadingtosubstantialdifﬁcultiesinthe

determina-tionoftheactualcontactarea(A)betweenthetwoobjects,among

others.

Toovercometheabove-mentioneddifﬁcultiesassociatedwith

macroscopictribology(thescienceoffriction,wearandlubrication)

∗ Correspondingauthor.Tel.:+903122903428.

E-mailaddress:mehmet.baykara@bilkent.edu.tr(M.Z.Baykara).

experiments,theresearchﬁeldofnanotribologyhasbeen

intro-duced relatively soon after the invention of the atomic force

microscope(AFM)[3,4].TheAFM,whichcanbethoughtofasavery

high-resolution mechanical microscope, allows therecording of

(sub-)nanometer-scaletopographyaswellasnormalandlateral

forces experiencedbyaverysharpsingleasperity(radiiof

cur-vature usually onthe order of<10nm)in theform of a tip at

theendofamicro-machinedSi/SiO2/Si3N4cantileverduringthe

raster-scanning of a given sample surface under slight contact

(normalforcesontheorderofafewtotensofnN)[5].Thanksto

thesingle-asperitynatureofthecontactformedbetweentheAFM

tip and the sample surface, various nanotribology experiments

conducted on a largenumber of sample surfacesover thelast

coupleofdecadeshaveresultedintheprecisedeterminationof

the effect of normal load, sliding velocity and temperature on

frictionalbehavioratthenanoscale[6–8].Moreover,phenomena

suchasstick-slip[9]andsuperlubricity[10]havebeenobserved

and largely explained, in many cases with substantial support

fromtheoryandcomputationalwork[11].

Among thematerial systems investigated nanotribologically

usinganAFM-basedapproach,nanoparticlesonlayeredsubstrates

suchashighlyorientedpyrolyticgraphite(HOPG)[12,13]areof

particular interest, primarily due to the fact that they readily

presentaheterogeneoussamplesurfacewheretheeffectof

chang-ing experimental parameters suchas normal loadon frictional

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430 E.Cihanetal./AppliedSurfaceScience354(2015)429–436

behavior canbecompared andcontrasted onthe nanoparticles

themselvesandthesubstrates.Moreover,therehasbeenarecent

increaseinnanotribologyexperimentsinvolvingthedeliberate

lat-eral manipulation of nanoparticleson structurallywell-deﬁned

substratessuchasHOPGandthemeasurementoftheassociated

frictionalforces, asamodel approachtostudyfrictional effects

indevicesfeaturingslidingcomponentsonnano-and

microme-terscales[14–19].Consequently,theneedfor acomprehensive

nanometer-scalecharacterizationofthestructuralandfrictional

propertiesofsuchsamplesystemsarisesasaprerequisiteforthe

correctinterpretationoftheabove-mentionednano-manipulation

experiments,amongothers.

Motivatedby the discussionabove, we present in this

con-tributionacomprehensivecharacterizationofthestructuraland

nanotribologicalpropertiesofthermallydepositedgold

nanopar-ticles (AuNP) on HOPG substrates. The particular choice of

this sample system is motivated by recently reported

nano-manipulation experiments involving AuNPs under ultrahigh

vacuum(UHV)conditions[18],aswellasthefactthatthegrowth

mechanismsof AuNPs onHOPG havebeen studiedvia various

approachesinthepast[20–24].Inthefollowingsectionsofthis

work,theeffectsofthermaldepositionamount,aswellas

post-depositionannealingonnanoparticlemorphologyanddistribution

arediscussedviascanningelectronmicroscopy(SEM)

measure-ments.Resultsrevealthatatransformationinmorphologyfrom

small,non-uniformlydispersedgoldislandsthatarecoalescedto

formchanneledthinﬁlmsonHOPGtomuchlarger,well-faceted,

mostly hexagonal AuNPs with much lower substrate coverage

takesplace uponpost-deposition annealing.Furthermore,

high-resolution transmission electron microscopy (TEM) images are

utilizedtoconﬁrmthecrystallinecharacterofthehexagon-shaped

AuNPs.Anextensivenanotribologicalanalysisinvolvinganumber

ofAuNPsviaAFMperformedinambientconditionsallowsthe

char-acterizationofthefrictionforcemeasuredonthenanoparticlesand

theHOPGsubstrateasafunctionofnormalload.Finally,an

anal-ysisoftheincreaseinlateralforceexperiencedatAuNPedgesis

presentedasafunctionofnormalloadand nanoparticleheight,

revealingalinearlyincreasingtrendforbothparameters.

It is important to emphasize at this point that the results

reported in this paper form a much-needed basis in terms of

a comprehensivestructuralandtribological characterizationfor

futurenano-manipulationexperimentstobeconductedusingthe

AuNP/HOPG materialsystemunder ambientconditions.In fact,

it isobserved that many AuNPsare frequentlymanipulated on

theHOPGsubstrateduringAFMmeasurements,openingtheroute

towards the quantitative characterization of interfacial friction

betweencrystallinesurfaces.Ontheotherhand,thepresentfocus

hasbeenplacedonimmobileAuNPsstuckbetweenHOPGstepsand

otherstructuralfeatures,suitableforacomprehensivestructural

andfrictionalcharacterizationviaAFMandothertechniques.

2. Experimental

2.1. Samplepreparation

HOPGsubstrateshavebeenpreparedbymechanicalcleavingin

airviaadhesivetapeandimmediatelytransferredintothevacuum

chamberofathermalevaporationsystem(VaksisPVDVapor–3S).

Thermalevaporationof999.9puritygoldonHOPGsubstratestook

placeat a basepressure ontheorder of 5×10−6Torrand at a

depositionrateof0.1 ˚A/sfortotaldepositedamountsbetween1 ˚A

and40 ˚A.Duringdeposition,theHOPGsubstratewasheldatroom

temperature.Afterdeposition,thegold-coatedHOPG substrates

were removed fromthe evaporation system for optional

post-depositionannealingandfurtheranalysisviaSEM,TEMandAFM.

Post-depositionannealingattemperaturesrangingfrom400◦Cto

650◦Candforannealingtimesontheorderof30minto4htook

placeinaquartztubefurnace(AlserTeknik/ProTherm).

2.2. SamplecharacterizationviaSEMandTEM

PriortostructuralandnanotribologicalcharacterizationbyAFM,

samplespreparedasdetailedintheprevioussectionhave been

analyzedviaSEM(FEIQuanta200FEG,typicallyoperatedat10kV)

tostudythemorphologyandthedistributionofAuNPsonHOPG.

Additionally, high-resolution TEM (FEI Tecnai G2 F30,typically

operatedat 300kV)hasbeenutilizedtoconﬁrmthecrystalline

structureofAuNPsviadirectimagingaswellaselectrondiffraction.

TheTEMsampleshavebeenpreparedbymechanicalcleavageofa

thinlayerofthegold-coveredHOPGsampleandsubsequent

sonica-tioninethanol,followedbydrop-castingonaCugrid(300mesh).

2.3. NanotribologicalmeasurementsperformedviaAFM

ComplementarytoSEMandTEMmeasurements,AFM

experi-mentshavebeenutilizedtocharacterizethestructureaswellas

thenanotribologicalpropertiesoftheAuNP/HOPGsamplesystem.

AcommercialAFMinstrument(PSIAXE-100)hasbeenoperated

under ambientconditionsand in thecontact modeto

simulta-neouslymeasurethetopographyof thesamplesurfaceand the

lateralforcesarisingbetweenthetipandthesampleduring

scan-ning.Asinglesiliconcantilever(NanosensorsPPP-CONTRseries,

radiusofcurvature≈10nm)hasbeenusedforallAFM

measure-ments.Toreliablydeterminethenormalaswellasthelateralforces

detectedduringAFMmeasurements,thecantileverhasbeen

cali-bratedaccordingtothemethodsreportedbySaderetal.[25]and

Varenbergetal.[26],respectively,resultinginanormalspring

con-stantkof0.23N/mandacalibrationconstant˛of15.0nN/Vforthe

lateralforcesignal.Whilethedetailsofbothcalibrationtechniques

aredescribedintherespectivereferences,letusindicateherethat

theSaderetal.methodallowsthepracticalcalculationofnormal

springconstantsofAFMcantileversbasedonameasurementof

resonancefrequency,qualityfactorandplan-viewdimensions,

tak-ingintoaccounttheeffectofthesurroundingﬂuidmedium(air)

oncantileveroscillationviaahydrodynamicfunction[25].Onthe

otherhand,theVarenbergetal.method,sometimesreferred to

astheimprovedwedgemethod,involvesthemeasurementofthe

forwardandbackwardlateralforcesignalsontheslopedandﬂat

facesofacommercialcalibrationgratingwithknownfeaturesizes

anddimensions,effectivelyallowingthecalculationoflateralforce

calibrationconstantsusingforceequilibrium argumentsin

con-junctionwiththedeterminationoffrictionloopwidthandoffset

valuesatﬁxednormalloads[26].Inaccordancewithliterature,the

reportedfrictionforcesinourexperimentshavebeencalculated

byconsideringthehalf-widthofthefrictionloopsformedbythe

recordingoflateralforcesduringforwardandbackwardscans[27].

3. Resultsanddiscussion

3.1. Effectofdepositionamountandpost-depositionannealing

onmorphology

In order to obtain a heterogeneous sample system suitable

fornanotribologicalinvestigationviaAFMconsistingof

individ-ualAuNPsofdeﬁnedshapeandreasonablelateralseparationon

HOPG,theﬁrstpreparationstepinvolvedthethermalevaporation

ofgoldontofreshlycleavedHOPGsubstrates.Thegrowth

kinet-icsandmorphologicalcharacteristicsofthinﬁlmsofgoldonHOPG

havereceivedparticularattentioninthepast,wheretypically

non-uniformsurfacecoverageshavebeenobservedduetotherelatively

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Fig.1. SEMimagesofas-depositedAuthinﬁlmsonHOPGfortotaldeposition amountsof20 ˚A(a),10 ˚A(b),5 ˚A(c)and1 ˚A(d).Thermalevaporationhasbeen performedwiththeHOPGsubstrateatroomtemperature,atarateof0.1 ˚A/s.

facetedandmostlyhexagonalortriangular-shapedAu

nanopar-ticles are obtained when evaporation is performed at elevated

substrate temperatures, deposited ﬁlms consisting of

intercon-nected,elongatedislandsleadingtoachanneledmorphologyare

expectedfordepositionswherethesubstrateisheldatroom

tem-perature.

Fig.1demonstrates theeffectofthermal depositionamount

on the resulting thin ﬁlm morphology for ﬁlm thicknesses of

20 ˚A,10 ˚A,5 ˚Aand1 ˚AviaSEMimagesrecordedpost-deposition.

Starting froma thicknessof20 ˚A, closetofull surfacecoverage

isobserved; whileat increasinglysmallerﬁlmthicknesses,thin

ﬁlmmorphologiescomprisinginter-connected,non-uniformly

dis-persedandirregularly-shapedgoldislandsarevisible.Thesurface

coverageandaverageislandsizegraduallydropwithsmaller

depo-sitionamounts, untiluncoveredregions ofthesubstrateonthe

orderofseveralhundredsofnmbecomeobservableata

deposi-tionamountof1 ˚A.Thefactthatanaccumulationofgoldislands

alongcertainlinearfeaturesonthesubstratesbecomesdetectable

atsmalldepositionamountssuchas1 ˚Asupportstheargumentthat

nucleationandgrowthprimarilyoccursatsurfacedefectssuchas

stepedgesandgrainboundaries,inaccordancewithresultsfrom

theliterature[24].

While theSEM studypresentedinFig.1regarding the

mor-phology ofgoldthinﬁlmsonHOPGasafunctionof deposition

amountprovidesusefulinformation,theresultingmaterial

sys-tem is unsuitable for precise nanotribological investigationvia

AFMduetothelackofstructurallywell-deﬁned(faceted),large

(hundredsofnmacross)AuNPsatreasonableseparations(atleast

severalhundredsofnm)fromeachother.Inordertotransform

theas-deposited goldﬁlmsinto facetedAuNPs, post-deposition

annealing in a quartz furnacehas beenperformedat

tempera-tures rangingfrom400◦C to650◦Cand forannealing timeson

theorderof30minto4h.Pleasenotethatduetotherelatively

lowsurfacecoverage,samplescomprising1 ˚Aofdepositedgoldon

HOPG(Fig.1(d))havebeenemployedforannealing.Theresults

indicatethatpost-depositionannealinghasadrasticeffectonthe

morphologyanddistributionofgoldonHOPG,leadingtoasevere

Fig.2. SEMimagesoftheAuNP/HOPGsamplesystemafterpost-deposition anneal-ing at(a)500◦_C_for ₃₀_min,_(b,_c) ₆₀₀◦_C_for₂_h_and _(d)₆₅₀◦_C_for ₂_h. _The SEMimagepresentedin(c)isazoomed-inviewoftheregionofthesample surfacedesignatedbythedashedwhiterectanglein(b).Theformationof well-deﬁned,hexagonal/elongated-hexagonalAuNPsatelevatedtemperaturesisreadily observed.

reductionofsurfacecoverageandsigniﬁcantcoalescence.While

annealingtemperaturesupto500◦Conlyresultintheobservation

ofnon-facetedgoldislandsinelongatedshapes(Fig.2(a))upto4h

of annealingtime;well-faceted, hexagonal/elongated-hexagonal

AuNPsof50–180nmheightandupto500nminlateraldimensions

areobtainedatannealingtemperaturesof600–650◦C(Fig.2(b–d)),

inaccordancewithrecentstudiesperformedongoldthinﬁlmson

multilayergraphene[28].Annealingathighertemperatures(700◦C

and above)hasbeenobservedtoresultinminorbutdetectable

morphologicaldamageontheHOPGsurfaceandisthusprevented.

Duetothefactthatthinﬁlmsandclustersofmetalsdeposited

onvarioussubstrates(including,butnotlimitedtometaloxides

andlayeredmaterialssuchasmicaandHOPG)present

opportu-nitiesfordeviceswithelectronic,opticalandmagneticproperties

thatcanbeﬁne-tunedonthenanometerscale,thegrowthkinetics

ofsuchsystemshavebeenstudiedasafunctionofﬁlmthickness

andannealingtemperatureinvariousstudiesinthepast[29–32].

Inparticular,thegrowthofAuﬁlmsonSi(111)couldbedescribed

bythedynamicscalingtheoryofgrowinginterfaces,involvingfour

stagesthatcanbesummarizedasnucleation,lateralgrowth,

coales-cenceandverticalgrowth;leadingtoanAvrami-typerelationship

betweensurfacecoverageandﬁlmthickness,whichis

character-izedbyrapidlateralgrowthatlowcoverages,followedbyadamped

coverage ratewithincreasingﬁlm thickness[29]. On theother

hand,severalcarefulpost-depositionannealingexperimentsofAu

ondifferentsubstrateswereutilizedtointerpretclusterformation

withintheframeworkofstandardripeningmodelsandresultedin

thedeterminationofsurfacediffusioncoefﬁcientsandgrain/cluster

sizedistributions[29–32].

While thefocus of ourcurrent workis ona comprehensive

structural and nanotribologicalcharacterization ofwell-faceted,

hexagonalAuNPsonHOPGandnotonadetailedinvestigationof

growthkineticsassociatedwiththismaterialsystem,wehave

nev-erthelessinvestigatedthecoverageoftheHOPGsubstratebyAuasa

functionofﬁlmthickness(Fig.3(a)),aswellasthelateralsize

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Fig.3. (a)SurfacecoverageofAuonHOPGasafunctionoffilmthickness.An Avrami-typefitrepresentedbytheredsolidcurvereasonablydescribesfilmgrowth[29]. (b)AuNPsizedistributionafterpost-depositionannealingat600–650◦_C._Lateral particlesizesupto500nmareobserved.Thesolidredcurveisafitbythelog-normal function[29].

annealing(Fig.3(b)).Ourresultsregardingsurfacecoverageasa

functionof ﬁlmthickness arein linewiththepreviously

men-tioneddynamic scalingtheoryof growinginterfaces,favoring a

lateralgrowthandsurface-diffusion-drivencoalescenceprocess,

owingtotherelativelyweakchemicalinteractionsbetweenthe

adsorbedAuatomsandtheHOPGsurface(Fig.3(a))[29].

More-over,weobservearelativelywidedistributionofAuNPsizesup

to500nm(Fig.3(b)),withameanparticlesizeof168_±106nm.

Themaindifferencebetweenthepost-deposition-annealedAuNPs

inourexperimentsandthosethathavebeenobtainedon

differ-entsubstratesis thattheAuNPs inourexperimentsaremostly

hexagonal,whereasAuNPsobtainedonsubstratessuchasmica,

Si(111)and SiO2 are mostly sphericalin nature[29–31].

Post-depositionannealingexperimentsofAuﬁlmsongraphenesamples

of different layernumbers have alsoled to theobservation of

hexagonal/triangularAuNPs,validatingthepropositionthatweak

interactionsbetweentheadsorbedAuatomsandthesubstratelead

tothepreferentialgrowthofcrystalline,facetedAuNPs[28].Let

usﬁnallyindicatethatthewidedistributionofAuNPlateralsize

isfavorablefornanotribologicalinvestigations,asitleadstothe

opportunityofperformingfuturenano-manipulationexperiments

whereinterfacialfrictionforcesactingonAuNPsofdifferentsize

shallbequantiﬁed.

Tosumup,theexperimentalresultspresentedinthissection

constituteanalternativemethodforobtainingwell-facetedAuNPs

onHOPGsubstratesintheabsenceof thecapabilitytoincrease

substratetemperaturesduringthermalevaporation.

Fig.4.TEMmeasurementsperformedonasingle,well-facetedAuNP(a).While high-resolutionimagingattheedgeoftheAuNPrevealscrystallineorder char-acterizedbyatomicstripes(b),diffractiondatapresentedin(c)providesfurther conﬁrmationforthecrystallinityoftheAuNP.

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3.2. ConﬁrmationofnanoparticlecrystallinityviaTEM

It has been shown via a number of experiments and ﬁrst

principlescalculationsthatfrictionoccurringbetweentwo

bod-ies in contact is a function of the physical properties of the

interface- mainly itsstructure [17,18,33].As such, in order to

studycarefullytheeffectofinterfacestructureonfrictionatthe

nanoscale,structurallywell-deﬁned,i.e.crystalline,surfacesare

a prerequisite.While theaccuratedeterminationofthe

atomic-scalestructureandchemistryassociatedwithtypicalSicantilever

apices employed in AFM measurements remains a challenge,

nanotribologyexperimentsinvolvingthelateralmanipulationof

nanoparticlesviaAFMhaverecentlyemergedasaviableapproach

where, e.g., the real contact area during sliding (the size of

thenanoparticle–substrateinterface) canbereadilydetermined

[19].Pioneeringnano-manipulationexperimentsconductedinthis

fashionprimarilyfocusedonSbnanoparticles,whichundergoa

size-dependentphasetransitionfromamorphoustocrystallineat

a particlesize of∼15,000nm2 _and _are _unavoidably_covered_by

anamorphousantimonyoxideshellwhenexposedtothe

ambi-ent [17]. In contrast, AuNPs on substratessuch as HOPG have

veryrecentlyemergedasanalternativematerialsystemfor

nano-manipulationexperiments,wherethecrystallinityoftheinterface

should be conserved even under ambient conditions, allowing

investigationsregarding,e.g.,structurallubricitytobeperformed

underwell-deﬁnedconditions[18].

Basedontheideathattheresultswepresentinthisworkshould

constitutea comprehensive structuraland frictional

characteri-zationof theAuNP/HOPG material system,we have performed

TEMmeasurement toconﬁrmthe crystallinecharacterof

ther-mallydeposited andpost-deposition annealedAuNPs onHOPG.

TheresultsofTEMinvestigationsinvolvingsamplespreparedas

detailed in Section 2.2 are reported in Fig. 4. While the

high-resolutionTEMimagerecordedattheedgeofahexagonalAuNP

revealsthe crystallineorder of Au atoms (Fig. 4(a and b)), the

associatedcharacteristicdiffractionpattern(Fig.4(c))provides

fur-therconﬁrmationforthecrystallinityofAuNPs.Basedonthefact

thattheAuNPsinvestigatedintheTEMstudyreportedherehave

beenexposedtoambientconditionsforseveralweeks,the

possi-bilityfortheformationofanoxidelayeronAuNPsduringrelevant

timeframesforpost-depositionAFMexperimentscanbereadily

excluded.

3.3. NanotribologicalpropertiesoftheAuNP/HOPGsystem

investigatedviaAFM

Despite thefactthat thenanotribologicalproperties of

vari-ousmaterialsinvestigated viaAFMhavebeendiscussed widely

intheliterature,therearesurprisinglyfewexperimentalreports

on the nanotribology of metallic surfaces [34–37]. In addition

toAFM experimentsperformed underultrahigh vacuum(UHV)

conditionsonAu(111)surfacespointingtowardsaverylow

fric-tionregimeundersmallappliedloadsfollowedbya substantial

increase infrictionaccompaniedbywear[34],thecapabilityto

controlthenanoscalefrictionalpropertiesoftheAu(111)surface

immersedinanionicliquidviatheapplicationofanelectric

poten-tialhasbeendemonstrated[37].Ontheotherhand,anextensive

analysisof nanoscalefrictional properties ofgold nanoparticles

thermally evaporated on HOPGhas not been performedso far

underambientconditions.Consideringthatmicro-andnano-scale

functionaldevicesfeaturingslidingcomponentswouldbemostly

usedunderambientconditionsforpracticalpurposes,theneedto

performananotribologicalstudyofrelatedmaterialsundersuch

conditions–albeitattheexpenseofinterfacecleanliness–arises.

Inordertoperformacomparativestudyoftheloaddependence

ofnanoscalefrictiononAuNPsand HOPGsubstrates,individual

Fig.5.Topographicalmapofa∼75nmhighAuNPtrappedbetweenHOPGsteps acquiredviacontact-modeAFM(a)andits3Drepresentation(b).Smallmounds (<10nminlateraldimensions)ontopoftheAuNPsurfacearehighlighted.

nanoparticleshavebeenimagedviacontactmodeAFMusingthe

experimentalmethodologyandequipmentdetailedinSection2.3.

DuetothefactthatAuNPsontheHOPGsubstratesarefrequently

manipulatedlaterallyduringscanningbasedontheplowingaction

exhibitedbytheAFMtip,effortshavebeendirectedatfocusing

onindividualAuNPstrappedbetweenthestepedgesoftheHOPG

substrate.Infact,manysuchnanoparticlescanbeobserved

dur-ingAFMexperiments,presumablyowingtotheincreasedmobility

exhibitedbythecoalescinggoldclustersonthelow-surface-energy

HOPGsubstrateatelevatedtemperaturesreachedduring

anneal-ing, resulting in lateral motionof AuNPs on thesubstrate and

eventualtrappingbetweenstepedges.Fig.5depicts a

topogra-phyimageassociatedwithsuchahexagon-shapednanoparticle,

togetherwithathree-dimensionalrepresentation.Whilethe

spe-ciﬁcnanoparticledepictedhereexhibitsaheightof∼75nm,AuNPs

of50–180nmheighthavebeenobservedduringtheexperiments.

BeforeswitchingtoadiscussionoflateralforcesrecordedonAuNPs,

it shouldbeindicatedthat topographyimagesofcertainAuNPs

revealtheexistenceofsmallroundmoundsonthetopsurface

(usu-ally∼10nmacrossand<10nminheight),suchasthosehighlighted

inFig.5(a).Whiledeterminingtheexactnatureofthesefeaturesis

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Fig.6.(a)Frictionforce(Ff)maprecordedataregionofthesamplesurfacecontainingahexagonalAuNPataconstantnormalloadofFn=2.3nN.StudyingtheFfproﬁle recordedalongthedashedblackarrowin(a)revealsvanishinglysmall(tensofpN)frictionvaluesfortheHOPGsubstratewhereassubstantiallyhigherfrictionvalues(0.60nN) areobservedontheAuNP(b).Pleasenotetheincreasedlateralforcesexperiencedbythecantileverduetoadditionaltwistingattheedgeofthe∼50nmhighparticle.

mechanismisrelatedtothecoalescenceandgrowthcharacteristics

exhibitedbytheAuNPsduringpost-depositionannealing.

ToinvestigatethenanotribologicalpropertiesoftheAu/HOPG

samplesystem,lateralforcesactingbetweenthetipandthesample

surfaceduringcontactmodeimaginghavebeenrecordedaccording

toestablishedproceduresintheliteratureasindicatedinSection

2.3.Atypicallateralforcemaprecordedonaregionofthesample

surfacecontainingananoparticleispresentedinFig.6(a).Astudy

ofthecalibratedlateralforcesignalalongthedashedsection

pre-sentedinFig.6(b)demonstratesasubstantialdifferenceinfriction

forceontheorderof0.60nNmeasuredontopoftheAuNPas

com-paredtotheHOPGsubstrate,whichexhibitsvanishingfriction.As

expected,thelateralforcevaluedetectedattheedgeoftheAuNP

exhibitsasuddenincrease(∼2.5nN)duetothe

height-difference-inducedadditionaltwistingofthecantilever[38].

The dependence of thefriction force onthe normal loadat

nanoscalecontactsinvolvingasingleasperity(suchastheAFM

tip) onvarious substrateshas beena topic of intenseresearch

sincetheestablishmentofthenanotribologyﬁeld,mostlydueto

themainargumentthatunderstandingthefrictionalbehaviorof

extendedmacro-scale, multi-asperitycontactscouldbeenabled

bytheproperunderstandingofthephysicalmechanisms

responsi-bleforfrictionatsingleasperities.WhileseveralAFMexperiments

haverevealedaspeciﬁc“2/3”powerlawdependence(Ff ∼Fn2/3),

arisingfromtheassumptionofa constantshearstress(=Ff/A)

withinaHertziancontactmechanicsapproach[39–43],therehave

been reports of linear dependence on certain sample surfaces,

aswell[44]. Inorder tofurthercontribute totheexperimental

investigationoffrictionmechanismsatnanoscaleasperitiesand

tospeciﬁcallycharacterizethenanotribologicalbehaviorofAuNPs

in ambientconditions, thefriction forces detectedon20

sepa-rateAuNPshavebeenstudiedasafunctionofnormalloadinour

experiments(Fig.7).Resultsrevealthattheloaddependenceof

frictionforcesonallAuNPsinvestigatedunderambientconditions

followsa “2/3”powerlawwithintheerrorbarsassociatedwith

ourmeasurements,verifyingtheassumptionofaconstantshear

stressanda Hertz-typedeformationmodelfor thissample

sys-tem.Animportantpointtoindicateistheexistenceofanon-zero

Ff(0.37nN±0.11nN)atzeronormalloadasaresultoftheﬁnite

adhesionforceactingbetweenthetipandAuNPsurfacesunder

ambientconditionsduetotheexistenceofawatermeniscusat

thetip-samplecontact[44].Consequently,Ff becomeszeroonly

ataneffectivelynegativenormalload(−1.80nN),corresponding

tothepull-offforcethatneedstobeappliedtothecantileverto

overcomeadhesionandseparateitfromthesamplesurface.Letus

notethatforce-distancespectroscopyexperimentsperformedon

AuNPsonseparatedayshaveresultedinpull-offforcesof

magni-tude2.80±1.30nN,whicharein-linewithourestimatedvalueof

1.80nN.Itshouldbeemphasizedthattheobservationofa“2/3”

powerlawstemmingfromHertziancontactmechanicsinanAFM

experimentrequiresthetipapextocloselyresembleaspherical

geometry(i.e.,single-asperity)[42].Ontheotherhand,tipapices

ofmulti-asperitycharacterareexpectedtoresultinalinear

rela-tionshipbetweenFfandFn,especiallyatFnvaluesseveralorders

ofmagnitudehigherthan1nN[42].Basedonthefactthatourdata

validatesa “2/3”powerlawrelationshipbetweenFfand Fnand

hasbeenacquiredatloads<25nN,itisconcludedthatthespeciﬁc

tipapexgeometryinourexperimentscanbeconsideredtobeof

spherical,single-asperitycharacter.

The absence of an ultralow friction regime and an abrupt

increaseinfrictionaccompaniedbysubstantialwearobservedon

AusurfacesunderUHVconditions[34]isofparticularscientiﬁc

Fig.7.Experimentaldatademonstratingthedependenceoffrictionforce(Ff)on normalload(Fn)forAuNPsaswellastheHOPGsubstrate.WhiletherecordedFf valuesfortheHOPGsubstratearevanishinglysmall,AuNPsexhibitcomparatively signiﬁcantfrictionforces,whichevolveaccordingtoa“2/3”powerlawwith increas-ingnormalload,correspondingtotheassumptionofaconstantshearstressandthe validityofHertziancontact.ErrorbarsfortheFfvaluesonAuNPsarecalculated basedonmeasurementsperformedon20separatenanoparticleswiththesame cantileveronseparatedays.Pleasenotethepull-offforceof−1.80nN,representing theeffectofadhesionbetweenthetipapexandtheAuNPs.

(7)

Fig.8. Experimentaldatademonstratingthedependenceofincreasedfriction forcesatAuNPedges(Ff,edge)onnormalload(Fn)forarepresentativeAuNPof 128nmheight.Asexpectedfromanalyticalcalculationsregardingtheratcheteffect occurringduringAFMmeasurementsonslopedsurfaces[38],Ff,edgeincreases sub-stantiallywithincreasingFn,followingareasonablylineartrend.

interest.Inthementionedexperiments,thesepeculiar

character-istics regardingfriction onAusurfaceshave beenexplainedby

theformationofaneckbetweentheAFMtipandthecleangold

surfaceduetothesigniﬁcantmobilityexhibitedbyAuatomsat

roomtemperature,and associatedplastic deformations[35]. As

themostprobablemechanismforthelackofsimilarobservations

inourexperimentsperformedinambientconditions,the

exist-enceofacontaminationlayeradsorbedontheAuNPsurfacesisof

importance,anargumentwhichisfurthersupportedbynanoscale

frictionexperimentsperformedonAusurfacesmodiﬁedwith,e.g.,

self-assembledmonolayers of alkanethiols which are devoidof

effectsassociatedwithneckformation[45].Finally,itcanbereadily

inferredfromFig.7thatthefrictionforcevaluesmeasuredonHOPG

aresigniﬁcantlylowerthanonAuNPs,owingtotheoutstanding

lubricationpropertiesofthislayeredmaterial[39].Whilea

discus-sionofthefundamentalprinciplesassociatedwiththeexceptional

lubricationpropertiesofHOPG[46] isbeyondthescopeofthis

work,itshouldbeindicatedthatthefunctionalformofthe

depend-enceofFfonFnonHOPGisnotstraight-forwardtodeterminedue

tothevanishinglysmallfrictionvaluesmeasured[39].

It is well-known in the nanotribology community that

topography-induced effects on the friction signal recorded

viaAFMarecommonlyencountered,especiallyonmaterial

sys-tems thatfeaturesuddenchangesin topographyin thevertical

direction. The AuNP/HOPG sample system investigated in our

experimentspresentsanidealopportunitytostudyrelatedeffects,

asitcomprisesnanoparticlesof50–180nmheightsituatedona

substratewithwide,atomicallyﬂatterraces.Thesharpincreasein

lateralforcesencounteredat,e.g.,theedgeofaAuNPasdepictedin

Fig.6(b),canbeattributedtotheso-calledratcheteffect,involving

thepartialcontributionofappliednormalloadtothelateralforce

signalonaslopedsurface,aswellasthecollision/impact

under-goneby thetipupon encounteringa slopedsurface,leading to

additionaltwistingofthecantilever[38].Utilizingthelateralforce

mapsrecordedontheAuNP/HOPGsysteminourexperiments,we

haveperformedastudyoftheincreasedlateralforcesencountered

at theedges of AuNPs (Ff,edge)as a function of normal loadFn

and particleheighth.Measurementsperformedon20 separate

AuNPswithdifferentheightspointtowardsalinearlyproportional

relationshipbetweenFnandFf,edge−therelateddataforatypical

AuNPof128nmheightisdepictedinFig.8.Finally,Fig.9presents

Fig.9.Experimentaldatademonstratingthedependenceofincreasedfrictionforces atAuNPedges(Ff,edge)onnanoparticleheight(h),acquiredataﬁxednormalloadFnof 13.8nN.Asitcanbereadilyinferredfromthedatapresentedfor20separateAuNPs withheightsrangingfrom50nmto166nm,Ff,edgedisplaysalinearlyproportional relationshipwithh.TheerrorbarsforFf,edgehavebeencalculatedbyconsideringa largenumberoffrictionforceproﬁles(suchastheonepresentedinFig.6(b))along thescanningdirectionofthecantileverforeachAuNPatthegivenconstantload.

an investigation towards the dependence of increased lateral

forcesdetectedatnanoparticleedgesFf,edgeonnanoparticleheight

h.Asonecaninferfromtheexperimentallyobtaineddata,Ff,edge

increasessubstantiallywithincreasingh,followingalineartrend

withintheerrorbarsofourmeasurements.

4. Conclusions

Wehavepresentedtheresultsofanexperimentalinvestigation

aimedata comprehensivestructuraland nanotribological

char-acterizationofthermallydepositedgoldnanoparticlesonHOPG.

Well-faceted,hexagonal/elongated-hexagonalAuNPswereformed

uponpost-depositionannealinginaquartzfurnaceat600–650◦C,

instrongcontrasttoas-depositedthinﬁlmsexhibitingachanneled,

irregularappearancewithnon-uniformcoverage.Thecrystalline

characterofthenanoparticleswasconﬁrmedviahigh-resolution

TEM imaging and diffraction measurements.To investigate the

nanotribological properties of the nanoparticles as well as the

substrate,AFMmeasurementsperformedviaasingle,calibrated

cantileverhavebeenutilized.IncontrasttoUHVmeasurementsof

nanoscalefrictionongoldsurfaces,ourmeasurementsperformed

underambientconditionspointtowardsa“2/3”powerlaw

depend-enceoffrictiononnormalloadforAuNPs,inaccordancewiththe

assumptionofaconstantshearstressduringslidingandHertzian

contactmechanicsinthepresenceofadhesion.Finally,astudyof

increasedlateralforcesatAuNPedgesasafunctionofnormalload

andparticleheighthasbeenperformed,revealingalinearly

propor-tionaldependenceforbothparameters.Ourexperimentsprovide

acomprehensivecharacterizationofthestructuralandfrictional

propertiesoftheAuNP/HOPGsamplesystem,whichemergesasan

idealcandidateforfuturenano-manipulationexperimentsaimed

atstudyingthefundamentalmechanismsoffrictionunderambient

conditions.

Acknowledgements

FinancialsupportfromtheMarieCurieActionsoftheEuropean

Commission’s FP7 Program in the form of a Career Integration

Grant (grant agreement no.PCIG12-GA-2012-333843), the

Out-standing Young Scientist program of the Turkish Academy of

(8)

(TheScientiﬁc andTechnologicalResearchCouncilof Turkey)is

gratefullyacknowledged.

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