DTIC ADA482476: Alkanethiols on Platinum: Multicomponent Self-Assembled Monolayers

0.4 MB·English

by  Defense Technical Information Center

2578 Langmuir2006,22,2578-2587
Alkanethiols on Platinum:
Multicomponent Self-Assembled Monolayers
Dmitri Y. Petrovykh,*,†,‡ Hiromi Kimura-Suda,§,| Aric Opdahl,§,^ Lee J. Richter,§
Michael J. Tarlov,§ and Lloyd J. Whitman‡
Physics Department, UniVersity of Maryland, College Park, Maryland 20742, NaVal Research Laboratory,
Washington, D.C. 20375, and National Institute of Standards and Technology,
Gaithersburg, Maryland 20899
ReceiVedApril 7, 2005. In Final Form:NoVember 23, 2005
Wehavestudiedtheformationofself-assembledmonolayers(SAMs)ofn-alkanethiolsonplatinumthinfilmsusing
X-ray photoelectron spectroscopy (XPS), reflection-absorption infrared spectroscopy (RAIRS), spectroscopic
ellipsometry(SE),andcontactangle(CA)measurements.Specifically,SAMsof1-hexanethiol,1-dodecanethiol,and
1-octadecanethiolweregrownonpolycrystallinePtfilms,andtheeffectsofPtsurfacepreparation,depositionconditions,
andsolventtreatmentsontheinitialqualityandstabilityofthemonolayerinairwereinvestigated.TheSAMsprepared
underambientconditionsonpiranha-cleanedandUV/ozone-cleanedsubstrateswerecomparedtomonolayersformed
ontemplate-strippedPtinaninertatmosphere.Wefoundthatalkanethiolsdepositedfrom1mMethanolicsolutions
onpiranha-cleanedPtformeddenselypackedmonolayersinwhichalkylchainswereorientedclosetothesurface
normal. Stored in the laboratory ambient, these monolayers were unchanged over about 1 week but were largely
oxidizedinabout1month.Noevidencewasfoundofmoleculesbeingweaklyboundwithinthemonolayerorhaving
undergoneC-Sbondscission;however,threedistinctsulfurstateswereobservedforallsamplesintheXPSofthe
S2pregion.Thelowest-andhighest-binding-energycomponentsareassignedtoalkylthiolateandpartiallyoxidized
alkylthiolate species, respectively. The remaining S 2p component (approximately one-third of the sulfur layer),
intermediateinbindingenergybetweentheothertwocomponents,isattributedtoachemisorbedspecieswithaS
bindingconfigurationdistinctfromthemajorityalkylthiolate: forexample,SboundtoPtboundtoO,Swithadifferent
Ptcoordinationnumber,orSinanadsorbeddisulfide.
1. Introduction byminimizingexposureofsamplestooxygen: deoxygenating
solvents, handling samples in inert atmosphere, etc.4-6 In the
Recent developments in molecular electronic devices have
second approach, no explicit attempt is made to control the
stimulatedinterestintheformationofself-assembledmonolayers
oxidation,andthedepositioniscarriedoutundermoretypical
(SAMs) of organic thiols on various metal surfaces. Because
laboratoryconditionssPtcleaningbymechanicalpolishing7or
SAMsofalkanethiolsongoldhavebeenextensivelycharacter-
inapiranhasolution,8followedbyalkanethioldepositionfrom
ized, most researchers have studied the electrical activity of
anethanolicsolution.Inthisreport,weusestandardtechniques
molecules on gold substrates. However, both theoretical and
tocharacterizetheformationandlongevityofalkanethiolSAMs
practical considerations suggest that other metals should be
consideredformolecularelectronicsapplications.1Forexample, on Pt under ambient conditions, in part to provide a direct
comparison between SAMs on Au and those on Pt. Several
areductionofcontactresistancebynearly2ordersofmagnitude
samplespreparedonPtusingoxygen-freedepositionconditions
hasbeenachievedbyinterfacingSAMswithplatinumvsgold
electrodes.2Fromafabricationpointofview,goldisincompatible arecharacterizedascontrols.Afactorthatinfluencesthequality
andstabilityofathiol-basedSAMisthebondingatthesulfur-
with silicon processing because of its high surface and bulk
diffusivity, reactivity, and ability to form electronic defects.3 substrateinterface;9therefore,wespecificallyfocusonthenature
of the three distinct sulfur states observed in SAMs on Pt by
Several groups have begun to investigate the structure and
X-rayphotoelectronspectroscopy(XPS).Althoughanumberof
stabilityofmonolayersonmetalsotherthanAu,notablyonPd
systematicstructureandstabilitystudieshavebeencarriedout
andPt,whichareconsideredtobegoodthiolatecontactmaterials
for SAMs on Au,8-11 to our knowledge, this is the first such
andtobecompatiblewiththefabricationofsiliconmicroelec-
study for SAMs on Pt.
tronics.TwoapproacheshaveemergedforformingSAMsonPt
surfaces.Thefirstapproachattemptstoavoidsurfaceoxidation
2. Materials and Methods
*Towhomcorrespondenceshouldbeaddressed.DmitriY.Petrovykh, 2.1.Materials.Commerciallyavailable1-hexanethiol,1-dode-
Code 6177, Naval Research Laboratory, Washington, D.C. 20375-5342. canethiol,and1-octadecanethiolwereusedwithoutfurtherpurifica-
E-mail: [email protected].
†UniversityofMaryland. (4)Vilar,M.R.;Bouali,Y.;Kitakatsu,N.;Lang,P.;Michalitsch,R.;Garnier,
‡NavalResearchLaboratory. F.;Dubot,P.ThinSolidFilms1998,329,236-240.
(5)Li,Z.Y.;Chang,S.C.;Williams,R.S.Langmuir2003,19,6744-6749.
§NationalInstituteofStandardsandTechnology.
(6)Brito,R.;Tremont,R.;Feliciano,O.;Cabrera,C.R.J.Electroanal.Chem.
|Currentaddress: PerkinElmerJapanCo.,Ltd.,Yokohama,Japan. 2003,540,53-59.
^ Currentaddress: DepartmentofChemistry,UniversityofWisconsin, (7)Laiho,T.;Leiro,J.A.;Lukkari,J.Appl.Surf.Sci.2003,212,525-529.
LaCrosse,WI54601. (8)Schlenoff,J.B.;Li,M.;Ly,H.J.Am.Chem.Soc.1995,117,12528-
(1)Chen,Y.;Ohlberg,D.A.A.;Li,X.M.;Stewart,D.R.;Williams,R.S.; 12536.
Jeppesen,J.O.;Nielsen,K.A.;Stoddart,J.F.;Olynick,D.L.;Anderson,E.Appl. (9)Laibinis,P.E.;Whitesides,G.M.;Allara,D.L.;Tao,Y.T.;Parikh,A.
Phys.Lett.2003,82,1610-1612. N.;Nuzzo,R.G.J.Am.Chem.Soc.1991,113,7152-7167.
(2)Beebe,J.M.;Engelkes,V.B.;Miller,L.L.;Frisbie,C.D.J.Am.Chem. (10)Rieley,H.;Kendall,G.K.;Zemicael,F.W.;Smith,T.L.;Yang,S.H.
Soc.2002,124,11268-11269. Langmuir1998,14,5147-5153.
(3)Graff, K. Metal Impurities in Silicon DeVice Fabrication; Springer- (11)Schoenfisch,M.H.;Pemberton,J.E.J.Am.Chem.Soc.1998,120,4502-
Verlag: Berlin,1995;Vol.24. 4513.
10.1021/la050928aCCC:$33.50 ©2006AmericanChemicalSociety
PublishedonWeb02/17/2006
Report Documentation Page Form Approved
OMB No. 0704-0188
Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and
maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information,
including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington
VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it

does not display a currently valid OMB control number.
1. REPORT DATE 3. DATES COVERED
2005 2. REPORT TYPE 00-00-2005 to 00-00-2005
4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER
Alkanethiols on Platinum: Multicomponent Self-Assembled Monolayers
5b. GRANT NUMBER
5c. PROGRAM ELEMENT NUMBER
6. AUTHOR(S) 5d. PROJECT NUMBER
5e. TASK NUMBER
5f. WORK UNIT NUMBER
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION
Naval Research Laboratory,Code 6177,4555 Overlook Avenue REPORT NUMBER
SW,Washington,DC,20375
9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR’S ACRONYM(S)
11. SPONSOR/MONITOR’S REPORT
NUMBER(S)
12. DISTRIBUTION/AVAILABILITY STATEMENT
Approved for public release; distribution unlimited
13. SUPPLEMENTARY NOTES
14. ABSTRACT

15. SUBJECT TERMS
16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF 18. NUMBER 19a. NAME OF
ABSTRACT OF PAGES RESPONSIBLE PERSON
a. REPORT b. ABSTRACT c. THIS PAGE Same as 10
unclassified unclassified unclassified Report (SAR)
Standard Form 298 (Rev. 8-98)
Prescribed by ANSI Std Z39-18
AlkanethiolsonPlatinum: MulticomponentSAMs Langmuir,Vol.22,No.6,2006 2579
tion.Hereafter,anabbreviatednotationforthesealkanethiols,C6SH, resolution).SpectraoftheS2pregionswereaccumulatedfor30-
C12SH,andC18SH,respectively,isusedinthetext,andC6,C12, 45min,toobtainasignal-to-noiseratioadequateforresolvingthe
andC18labelsareusedtoindicatethenumberofalkylchaincarbons multiplecomponents.Typically,spectrawereacquiredfromthree
inthefigures.Ethanol(95%,hereafterEtOH)anddichloromethane separate spots on each sample, primarily to test the monolayer
(HPLCgrade,hereafterCHCl)wereusedasreceived(exceptwhere uniformity. The corresponding calculated coverage values varied
2 2
notedotherwise)forthepreparationof1mMsolutionsofalkanethiols bynomorethan10%foreachofthesamples.ThereferencePtand
andforrinsingorsoakingsamplesaftermonolayerdeposition. AuspectrausedtocalibratetheattenuationoftheXPSsignalswere
2.2. Platinum Film Cleaning and SAM Deposition. Diced measuredfromsubstratescleanedinsitubyArionsputteringuntil
fragmentsfromplatinum-coatedsiliconwaferswereusedassubstrates C1sandO1ssignalswerenolongerdetectable.
(200nmofPtsputter-depositedovera30nmTiadhesionlayer). 2.5. XPS Peak Fitting. The peaks in the elemental core-level
ThePtfilmsexhibitedanrmsroughnessof1.7nmoveranareaof spectra were fit using commercial XPS analysis software.15 A
1 (cid:237)m2 as measured by atomic force microscopy (AFM). Prior to convolutionofLorentzianandGaussianlineshapeswasusedtofit
SAM deposition, the diced substrates (e2 cm2) were cleaned by theindividualpeaks.16AlinearcombinationofShirleyandlinear
immersion in a “piranha” solution consisting of 70% HSO and functionswasusedtomodelthebackground,withthecorresponding
2 4
30%HO (30%HO inwater).(Caution: Thissolutionmustbe coefficientsfitsimultaneouslywiththepeaks.Theonlyexception
2 2 2 2
handledwithcare;itisextremelyoxidizing,reactsViolentlywith wasforfitsintheO1sregion,whereadditionalpolynomialterms
organics,andshouldonlybestoredinlooselytightenedcontainers were added to model the nonlinear background caused by the
to aVoid pressure buildup.) Piranha-cleaned Pt will hereafter be proximity of the Pt 4d peak. Multiple-component fitting in the
3/2
denoted p-Pt. After being cleaned, each p-Pt substrate was im- C1sandS2pregions,alwaysstartedfromthelowestBEcomponent
mediately and thoroughly rinsed with high-resistivity water anditsfull-widthathalf-maximum(fwhm),wasusedtoconstrain
((cid:24)18.2 M¿(cid:226)cm) that had been treated to remove organic and the fwhm’s for the higher-BE components.16 In each case, the
biologicalimpurities.Todepositmonolayers,thecleanedsubstrates minimum number of components that produced unstructured fit
were submerged in 1 mM solutions of alkanethiols in EtOH for residualswaschosen.
20hatroomtemperature.Forcomparisonpurposes,SAMswere 2.6. RAIRS Measurements. Reflection-absorption infrared
preparedfromidenticalsolutionsonpiranha-cleanedAusubstrates spectroscopy(RAIRS)wasperformedusingacommercialFourier
(200nmofAuevaporatedovera20nmCradhesionlayeronsilicon), transform spectrometer equipped with a wire-grid polarizer
hereafterp-Au.Afterdeposition,eachsamplewasrinsedthoroughly (p-polarized)andavariable-anglereflectanceaccessory(reflectance
withEtOHandthendriedinastreamofdrynitrogen.Tomeasure angle75(cid:176) ).RAIRSspectrawerecollectedfrom4000to900cm-1
thestabilityofthedepositedmonolayers,thesampleswereexposed usingacryogenicmercurycadmiumtelluride(MCT)detector(1024
toambientlaboratoryairforperiodsrangingfrom1hto55days. scansat2cm-1resolution).Thespectrawerereferencedtoabare
To examine solvent effects, control samples were soaked in Pt substrate, cleaned by the respective procedure. The RAIRS
CHCl overnight after the above standard EtOH deposition or measurementswereperformedonfreshlypreparedsamplespriorto
2 2
depositedfrom1mMsolutionsofalkanethiolsinCH Cl.Inanother XPScharacterization.
2 2
seriesofcontrols,substrateswerecleanedusingacommercialUV/ 2.7. SE Measurements. Spectroscopic ellipsometry (SE) was
ozonecleaner(hereafterUVO-Pt),ratherthaninpiranhasolution. performed using a commercial multichannel instrument with a
UVOcleaningwasperformedforabout20mininozonegenerated rotatingcompensatorand190-1000nmwavelengthworkingrange.
insitufromatmosphericoxygenbyalow-pressuremercuryquartz ThePtsubstratesforSEmeasurementswere(cid:24)1cm2chipsdiced
lamp(185and254nmUVrange,(cid:24)25mW/cm2power). from a single Pt-covered wafer and cleaned by piranha or UVO
2.3. Template-Stripped Platinum Films and Oxygen-Free treatment.Theanalysiswasperformedwithvendor-suppliedsoftware.
SAMDeposition.Template-strippedPt(TS-Pt)filmswereprepared 2.8.CAMeasurements.Contactangle(CA)measurementswere
followingtheprotocolofBlackstocketal.12Briefly,a220nmPt performedatroomtemperatureandambientrelativehumidityusing
film was sputter-deposited on a piranha-cleaned ultraflat Si(100) high-resistivitywaterastheprobingliquid.Sessilecontactangles
wafer.ThePtfilmwasthenremoved(stripped)fromthistemplate weremeasuredwith(cid:24)5(cid:237)Lofwaterdroppedontothesurfacesfrom
byusinganadhesive-coveredSisubstrate.TheTS-Ptfilmsexhibited asyringeneedleandrecordedimmediatelyafterthedrop.Measure-
anrmsroughnessof0.7nmoveranareaof1(cid:237)m2asmeasuredby ments from three different spots were averaged for each sample.
AFM. The stripping was performed inside an inert-atmosphere
gloveboxtolimitoxidationoftheTS-Ptsubstrate.Monolayerswere 3. Experimental Results
thendepositedintheglovebox,from1mMsolutionsinsolventsthat
3.1.EffectsofSolventsandSubstrateCleaningonMono-
weredistilledundernitrogenandcalciumhydride.Hereafter,these
conditionswillbereferredtoas“oxygen-free”deposition. layer Deposition. We used XPS as the primary method for
2.4. XPS Measurements. XPS measurements were performed quantitativeanalysisofthestructureandstabilityofalkanethiol
usingacommercialsystemequippedwithamonochromaticAlKR monolayers.XPShasbeenwidelyusedtocharacterizeSAMs,
source,ahemisphericalelectronenergyanalyzer(58(cid:176) anglebetween e.g.,onAu,9-11,17-26Ag,8,9,11,23,25,27,28Cu,8,9,25,29-31Ni,32Pd,33
monochromator and analyzer), and a magnetic electron lens. The andPt.5-8,34,35TheXPSspectrumoftheS2pregionisparticularly
nominalXPSspotsizeandanalyzerfieldofviewweree1mm2. usefulforcharacterizingalkanethiolSAMs,becausetheposition
Thebindingenergies(BEs)arereportedwith0.1eVprecisionbased
and intensity of the S 2p peaks can be used to identify and
onatwo-pointBEscalecalibrationtotheBEsofAu4f (84.0eV)
7/2
andAu4f (335.2eV)measuredineachrunforAufilmscleaned
5/2 (15)Hesse,R.;Chasse,T.;Szargan,R.FreseniusJ.Anal.Chem.1999,365,
by Ar ion sputtering.13,14 In each run, we also measured BEs of 48-54.
71.1eVforPt4f7/2and314.6eVforPt4d5/2fromfreshlysputter- (16)ThetotalfwhmisthesumofGaussianandLorentziancontributions.The
cleanedPtfilms,inagreementwiththeacceptedvaluesof71.12and Lorentzianfwhmwas0.1eVforC1sandO1sand0.25eVforS2p.ForC1s
314.61eV,respectively.14Forthethinorganicmonolayersinthis componentsC2andC3,thetotalfwhmwasfixedtothatofC1(1.2-1.3eV).
ForS2pcomponents,weusedaspin-orbitintensityratioof0.5andanenergy
study,chargecompensationwasnotnecessaryandwasnotapplied.
splittingof1.2eV.;thetotalfwhmwas0.98eVforS1andS2,1.48eVforS3.
We present data acquired in normal-emission angle-integrated (17)Castner,D.G.;Hinds,K.;Grainger,D.W.Langmuir1996,12,5083-
scansofthePt4f,Pt4d,S2p,C1s,andO1sregions(15-20eV 5086.
windowswith0.1eVspacing,20eVpassenergy,0.36eVanalyzer (18)Ishida,T.;Hara,M.;Kojima,I.;Tsuneda,S.;Nishida,N.;Sasabe,H.;
Knoll,W.Langmuir1998,14,2092-2096.
(19)Heister,K.;Frey,S.;Golzhauser,A.;Ulman,A.;Zharnikov,M.J.Phys.
(12)Blackstock,J.J.;Li,Z.Y.;Freeman,M.R.;Stewart,D.R.Surf.Sci.2003, Chem.B1999,103,11098-11104.
546,87-96. (20)Heister,K.;Allara,D.L.;Bahnck,K.;Frey,S.;Zharnikov,M.;Grunze,
(13)Seah,M.P.;Gilmore,L.S.;Beamson,G.Surf.InterfaceAnal.1998,26, M.Langmuir1999,15,5440-5443.
642-649. (21)Yan,C.;Golzhauser,A.;Grunze,M.;Woll,C.Langmuir1999,15,2414-
(14)Powell,C.J.Appl.Surf.Sci.1995,89,141-149. 2419.
2580 Langmuir,Vol.22,No.6,2006 PetroVykhetal.
Figure2. XPSS2pcomponentsforalkanethiolSAMsdeposited
onPtunderdifferentconditions.IntensityisnormalizedtotheS2p
peakfortheC6SH/p-AuSAMinFigure1.
C6SH/p-AuspectrumisalsoscaledbytheratioofScofieldfactors
for Au 4f and Pt 4f .36,37
7/2 7/2
The C6SH/p-Au reference spectrum in Figure 1a is clearly
consistentwithasingleS2pdoublet;theBEof162.0andfwhm
of0.84eV16areinexcellentagreementwithpreviousstudiesof
alkylthiolatesonAu.10,17,18,20-24,38,39Incontrast,alloftheS2p
spectraforSAMsonPtinFigure1aaredistinctlymulticomponent,
Figure 1. XPS of the S 2p region for as-deposited alkanethiol withatleastthreecomponentsrequiredtoobtainunstructured
SAMsonPt.(a)Depositionfrom1mMsolutioninEtOH,rinsein residuals.40Hereafter,thesethreecomponentswillbereferred
EtOH: C6SHonp-Au(O);C6SH,C12SH,andC18SHonp-Pt(b). toasS1,S2,andS3inorderofincreasingBE.TheBEofthe
(b) C18SH on p-Pt: deposition from 1 mM solution in EtOH, S1componentis162.3eVforC6SHandC12SHand162.4eV
overnightsoakinCH2Cl2(top);depositionfrom1mMsolutionin for C18SH.16 The S2-to-S1 relative BE shift is 0.85-0.95 eV
CHCl (middle);depositionfrom1mMsolutioninEtOHonUVO-
2 2 inunrestrictedfitsandhereafterisfixedat0.9eVforconsistency.
Pt substrate, overnight soak in CHCl (bottom). (c) C18SH and
2 2 The S3-to-S1 relative BE shifts are between 2.4 and 3.1 eV.
C6SHdepositiononTS-Ptinaninert-atmospheregloveboxfrom
1mMsolutionsindeoxygenatedEtOH.Openandsolidsymbols) Controlexperiments,resultsofwhicharepresentedinFigure
data, thick lines ) total fits, thin lines ) peak components and 1b and c, were performed to explore the effect of deposition
backgrounds.TheresidualfortheC18SH/p-Ptfitisshownatthe conditions on initial SAM quality and corresponding S 2p
bottomofpanela.
components.InSAMsonAu,themostcommonassignmentof
S2pcomponentswithBEsofaround163.5eV(i.e.,S2inour
quantify various monolayer components (chemisorbed, phys-
case)istounboundthiolsphysisorbedortrappedinthemonolayer.
isorbed,oxidized).Figure1ashowsS2pspectraformonolayers
Existenceofasimilarunboundcomponenthasalsobeenproposed
deposited on p-Pt from 1 mM ethanolic solutions of C6SH,
for SAMs on Pd33 and Pt.5 To investigate this possibility, we
C12SH, and C18SH (solid symbols) as well as a reference
carriedouttwotypesofcontrolexperiments: depositionfrom
spectrum for C6SH deposited on p-Au (open symbols, top of
1 mM ethanolic solutions followed by an overnight soak in
Figure 1a). The spectra are shown normalized to the intensity
CH Cl and deposition from 1 mM solutions in CH Cl . An
2 2 2 2
of the respective substrate signals (Pt 4f or Au 4f ); the
7/2 7/2 overnight soak in CH Cl , a good solvent for alkanethiols, is
2 2
expectedtoremoveanyweaklyboundalkanethiolsleftaftera
(22)Kawasaki,M.;Sato,T.;Tanaka,T.;Takao,K.Langmuir2000,16,1719-
standard deposition from EtOH. Similarly, deposition from
1728.
(23)Heister,K.;Zharnikov,M.;Grunze,M.;Johansson,L.S.O.J.Phys. CH Cl ,althoughpotentiallyslower,shouldsignificantlysuppress
Chem.B2001,105,4058-4061. any2we2aklyboundcomponents.Figure1aandbshowslittle,if
(24)Yang,Y.W.;Fan,L.J.Langmuir2002,18,1157-1164.
(25)Laibinis,P.E.;Bain,C.D.;Whitesides,G.M.J.Phys.Chem.1991,95, any,changeintheS2andS3componentsfollowingtheCH2Cl2
7017-7021. treatments.Onlythemonolayersgrowninaninertatmosphere
(26)Hansen,H.S.;Tougaard,S.;Biebuyck,H.J.ElectronSpectrosc.Relat.
Phenom.1992,58,141-158. onTS-Ptsubstrateswerenoticeablydifferent: theS3component
(27)Himmelhaus,M.;Gauss,I.;Buck,M.;Eisert,F.;Woll,C.;Grunze,M. waseliminatedandS2significantlyreduced(Figure1c).These
J.ElectronSpectrosc.Relat.Phenom.1998,92,139-149. trendscanbemoreclearlyseeninFigure2,whichshowsrelative
(28)Ohgi,T.;Fujita,D.;Deng,W.;Dong,Z.C.;Nejoh,H.Surf.Sci.2001,
493,453-459.
(29)Laibinis,P.E.;Whitesides,G.M.J.Am.Chem.Soc.1992,114,9022- (36)Scofield,J.H.J.ElectronSpectrosc.Relat.Phenom.1976,8,129-137.
9028. (37)NormalizingelementalXPSsignalsbythecorrespondingScofieldfactors
(30)Ron,H.;Cohen,H.;Matlis,S.;Rappaport,M.;Rubinstein,I.J.Phys. (ref36)isastandardwaytocorrectforelement-dependentphotoelectriccross
Chem.B1998,102,9861-9869. sections,themajorfactorthatdetermineselement-specificXPSintensities.This
(31)Sung,M.M.;Sung,K.;Kim,C.G.;Lee,S.S.;Kim,Y.J.Phys.Chem. normalizationignoresanyspatialdistributionoftheelements,butprovidesa
B2000,104,2273-2277. practicalwaytocompareelementalintensities,assuchScofield-adjustedintensity
(32)Mekhalif,Z.;Laffineur,F.;Couturier,N.;Delhalle,J.Langmuir2003, ratiosoftenappearinquantitativeXPSanalysismodels(seeAppendix).
19,637-645. (38)Vericat,C.;Vela,M.E.;Andreasen,G.;Salvarezza,R.C.;Vazquez,L.;
(33)Love,J.C.;Wolfe,D.B.;Haasch,R.;Chabinyc,M.L.;Paul,K.E.; Martin-Gago,J.A.Langmuir2001,17,4919-4924.
Whitesides,G.M.;Nuzzo,R.G.J.Am.Chem.Soc.2003,125,2597-2609. (39)Zerulla,D.;Chasse,T.Langmuir1999,15,5285-5294.
(34)Brito,R.;Rodriguez,V.A.;Figueroa,J.;Cabrera,C.R.J.Electroanal. (40)TheTougaardmodelwithparametersestablishedforSAMs/Au(ref26)
Chem.2002,520,47-52. predictsintensitiesofinelasticbackgroundstobemuchlowerthantheobserved
(35)Long,Y.T.;Herrwerth,S.;Eck,W.;Grunze,M.Phys.Chem.Chem. high-BEshouldersofS2ppeaks.TheasymmetricS2ppeakshapesthuscorrespond
Phys.2002,4,522-526. tomultipleS2pcomponents.
AlkanethiolsonPlatinum: MulticomponentSAMs Langmuir,Vol.22,No.6,2006 2581
Figure 4. Comparison of ellipsometric thickness of alkanethiol
monolayersonvariouslycleanedPtsubstrates.Linearfittothedata
forp-Pt: t )(0.12(0.01)(cid:226)n -(0.14(0.2)(Pearson’srfactor
SE C
)0.978).
monolayers.Infact,theyareverysimilartothosereportedfor
SAMs on platinum oxide.5 By contrast, monolayers deposited
fromEtOHonp-Pthavepeakfrequenciesandwidthssimilarto
thoseofSAMsonp-Au(Table1)andSAMsonplasma-cleaned
Pt.5TheintensitiesoftheCH features,relativetotheCH features,
2 3
arelowerforourp-Ptsamplesthanforp-Auandplasma-cleaned
Pt,5indicatingorientationofthealkylchainsclosertothesurface
Figure 3. RAIRS spectra of C-H region for alkanethiol SAMs normal (see section 4.1).
prepared on Pt substrates cleaned by two different methods.
We characterized the optical thickness of freshly deposited
Depositionisfrom1mMethanolicsolutionsfollowedbyrinsein
SAMsbySEforallthreesubstrates(p-Pt,UVO-Pt,andTS-Pt).
EtOH.
The optical constants of Pt films are expected to vary with Pt
Table1. VibrationalLinePositionsforC18SHMonolayerson deposition and cleaning conditions; therefore, it is crucial to
AuandPt develop a proper dielectric model for the metal substrate. The
C18SH/p-Au C18SH/p-Pt C18SH/UVO-Pt SEdataforaseriesofsamples,namely,asubstratefreshlycleaned
by the respective procedure and C6SH, C12SH, and C18SH
peak (cid:238) fwhm (cid:238) fwhm (cid:238) fwhm
assignmenta (cm-1) (cm-1) (cm-1) (cm-1) (cm-1) (cm-1) SAMs,weresimultaneouslyfittoathree-phaseopticalmodel
consistingofacommonPtsubstrate,anorganiclayerofvariable
(cid:238)(cid:238)ss((CCHH23))ssyymmssttrr 22885739 1130 22885708 118 22885718 1151 thickness,andair.Theindexofrefraction(nˆ )n+ik)ofthe
(cid:238)a(CH2)asystr 2918 12 2917 17 2923 24 organiclayerwasheldat1.50,consistentwithearliertreatments
(cid:238)s(CH3)FRsymstr 2936 20 2937 12 2936 10 ofalkanethiolSAMsonAu.43Opticalthickness,tSE,asafunction
(cid:238)a(CH3)opasystr 2958 8 nab nab ofthenumberofalkylchaincarbons,n ,isshowninFigure4.
(cid:238)a(CH3)ipasystr 2964 8 2965 10 2965 13 TheerrorbarsreflectmultiplemeasuremCents(2or3)formultiple
aThefollowingabbreviationsareused: str,stretch;sym,symmetric; sampleseries(2or3).Theopticalthicknessesofthemonolayers
asym,antisymmetric;ip,inplane;op,outofplane;FR,Fermiresonance
are similar for all surface treatments.
component.bNotobserved.
WeusedwaterCAmeasurementsasastandardcharacterization
method for hydrophobic methyl-terminated SAMs. For as-
intensitiesofthetotalS2psignalandtheindividualcomponents
depositedmonolayersthesessilecontactangleswere91(cid:176) (C6SH),
for C6SH and C18SH monolayers prepared using all of the
101(cid:176) (C12SH),and104(cid:176) (C18SH)onp-Ptand88(cid:176) (C6SH),99(cid:176)
methodsshowninFigure1.Therelativeintensitiescorrespond
(C12SH),and101(cid:176) (C18SH)onUVO-Pt.Overall,theCAvalues
topeakareasinFigure1normalizedtotheareaoftheS2ppeak
areabout10(cid:176) lowerthanthosereportedforSAMsonAu,Ag,
for the reference C6SH/p-Au SAM. Notably, both the S1 and
andCu.9Ourvaluesarealsoabout5(cid:176) lowerthanpreviousresults
S2componentsareessentiallyindependentofdepositioncondi-
reportedforSAMs/Pt.5ThetrendofincreasingCAwithalkyl
tions in both absolute and relative terms. The corresponding
chainlengthagreeswithresultsforSAMsonAu,Ag,Cu,9and
monolayercoveragesarequantifiedinsection4.2,andthenature
Pt.5SmallerCAvaluesforoxidizedUVO-Ptarealsoinagreement
of the S 2p components is further discussed in section 4.4.
with previous results.5
Figure3showsRAIRSspectraforfreshlydepositedC6SH,
3.2.SAMStabilityinAir.Thephotoemissionspectraofthe
C12SH,andC18SHmonolayersusingPtsubstratescleanedby
S 2p region in Figure 5 show the changes in oxidation and
twodifferentmethods: p-PtandUVO-Pt.Table1liststhepeak
coverage of SAMs/p-Pt exposed to air. We show the data for
parameters for the C18SH films.41 The RAIRS data for
C6SHforthefirstfewdays(topthreecurvesinFigure5)because
monolayersonTS-PtsubstratesarepresentedintheSupporting
thesethinnermonolayersresultinlowerXPSsignalattenuation
Information(FigureSI1,TableSI1).TheRAIRSpeakassign-
and,thus,highersignal-to-noiseratiosforS2andS3components.
mentsfollowref42.ThespectraforSAMsonUVO-Ptexhibit
Over the long term, the thickest SAMs are typically the most
wide and unresolved peaks, indicating relatively disordered
stable;thus,weshowdataforC18SHforthebottomtwocurves
in Figure 5.
(41)Thespectrawereanalyzedbynonlinearleast-squaresfittingtomultiple
Lorentzianlines,exceptinthecaseoftheUVO-Ptsubstrate,inwhichaGaussian
lineshapewasrequired. (43)Shi,J.;Hong,B.;Parikh,A.N.;Collins,R.W.;Allara,D.L.Chem.Phys.
(42)Parikh,A.N.;Allara,D.L.J.Chem.Phys.1992,96,927-945. Lett.1995,246,90-94.
2582 Langmuir,Vol.22,No.6,2006 PetroVykhetal.
Figure6. XPSofO1sandC1sregionsforSAMs/p-Ptexposed
toair.DepositionandrinseinEtOH(toptobottom): C18SHSAMs
Figure5. XPSoftheS2pregionforSAMs/p-Ptexposedtoambient exposedtoairfor1.5hand29days(C1sspectraareshownscaled
air.Exposuretimeandalkylchainlengthasshown.Depositionand by1/2);C6SHSAMsexposedtoairfor1.5h,47h,and5days.Solid
rinseinEtOH.Bottomspectrum(O)isforaC18SHSAMsoaked circles ) raw data, thick lines ) total fits, thin lines ) peak
overnightinCHCl beforeexposuretoair.Openandsolidcircles componentsandbackgrounds.
2 2
)data,thicklines)totalfits,thinlines)peakcomponentsand
backgrounds. tothehydrocarbonchains.Atleasttwoadditionalcomponents
(C2andC3)hadtobeaddedtothefitstoproduceunstructured
Forthefirst5days,themainchangeintheS2pspectraisa residuals.16,45
gradualshiftoftheS3componenttohigherBE: theaverageBE
ToquantitativelycomparetheoxidationdatafromS2pand
oftheS3componentchangesfrom165.0to165.5to165.8eV
O1sregions(Figures5and6),weusedScofield-adjustedS2p
(Figure5).ThisBEshiftisconsistentwithpartialoxidationof and O 1s intensity ratios to the Pt 4f substrate peak,36,37 as
10-15%ofthesulfurheadgroups.Atthesametime,theintensities 7/2
shownforC18SHmonolayersonp-PtinFigure7(TableSI2).
ofallthreecomponentsremainnearlyconstant,indicatingthat
ThereislittlechangeintheS2pcomponentsoverthefirst5days
desorptionorredistributionofmoleculesinthemonolayerdoes
ofexposuretoair(Figures5and7).Between5and29daysin
notoccurforthefirst5days.Incontrast,after29days,about
air, the intensity of the S3 component increases, whereas the
one-halfoftheoriginalintensityofS1-S3componentshasbeen
totalS2pandtheS1intensitiesdecrease(Figure7).Thesetrends
lost for the C18SH monolayer, and additional high-BE com-
areconsistentwithincreasedoxidationofalkylthiolateSgroups
ponents have appeared.
andassociateddesorptionofalkylthiols.Anothernotabletrend
The bottom spectrum in Figure 5 corresponds to a C18SH isthattheintensityoftheS2componentdoesnotincreasewith
monolayerdepositedusingtheEtOH/CH2Cl2procedure.After airexposure;thus,unliketheS3component,theS2component
55daysinair,itshowsthegreatestoxidationofthesampleswe isnotassociatedwithafinalproductofoxidation.Thestability
examined.Bycomparison,inarecentstudyofSAMsonPd,a andoxidationtrendsaresimilarforallofthealkylchainlengths
similarly severe degree of oxidation was observed for a that we have examined (Table SI2, Supporting Information).
hexadecanethiolmonolayerafteronly5daysinair(cf.Figure
TheCA,RAIRS,andSEtechniquesaresensitivetotheorder-
6cinref33);thus,itappearsthatmonolayersonPtaresignificantly
ingofalkylchains,butonlyminorchangeswereobservedafter
more stable against exposure to air than those on Pd. afewdaysinair.TheCAvalueschangedby<5%fortheC6SH
Complementary information about the oxidation of SAMs
underambientconditionsisprovidedbyphotoemissionspectra
(44)Theoxygen-freedepositiononTS-Ptsubstratesresultedinthelowest
of the O 1s and C 1s regions (Figure 6). For all samples, the overallO1ssignalsinourstudy.ForC6SH/TS-Pt,theScofield-adjustedintensity
intensityintheO1sregionisspreadoverabout4eV.Giventhe ratios(sameunitsasinTableSI2)are227fortheC2andC3components,220
forO1s,and101forS2.ThetotalintensityoftheoxidizedCcomponentsisequal
typicalfwhmofabout1.5eVforO1sinorganicmaterials,this
tothetotalOsignal;thus,thedataareconsistentwithessentiallynoPtOxonthe
indicatesatleastthreedifferentchemicalstatesforoxygen.The surfacewhileS2isstillpresent.
broadO1senvelopeandnonstoichiometriccompositionofsurface (45)Eachofthealkanethiolmoleculescontainstwochemicallydistinctcarbon
atoms: oneboundtothesulfurheadgroupandoneinthemethylgroup.The
oxides do not allow us to make corresponding assignments of
formerislikelytohaveaBEhigherthanthealkylcarbonsofthemainC1peak.
the observed O 1s components with any certainty. The total TheC2/C1intensityratiosinfitstoourdata(Figure6),however,areabout
O1sintensityincreasesmonotonicallywithexposuretoair,as one-halfofwhatwouldbeexpectedforidealSAMsofrespectivethicknesses.
AnadditionalcomponentoftheappropriateintensityandBEshiftof<1eVcan
expected.ThelowestO1ssignalinthep-Ptserieswasobserved beaddedwithoutsacrificingthequalityofthefitstoaccountfortheheadgroup-
foranas-depositedC18SHmonolayer(topspectruminFigure boundcarbons.TheC2andC3componentsthenmustbeinterpretedasdueto
6).44 The C 1s spectra in all cases are dominated by the main acdovmepnotintieonutsswaintdh/BorEssohlivfetsnotfm1.o2l-ec1u.l6ese.VIn(Ct2h)eanlidte2r.a7tu-r3e,.6aesVsig(Cnm3)evnatroiefs,Ce.g1.s,
peak(C1)withBEbetween284.5and284.9eVthatcorresponds refs6and27.
AlkanethiolsonPlatinum: MulticomponentSAMs Langmuir,Vol.22,No.6,2006 2583
Table2. EvolutionofMonolayerThicknessandStoichiometry
withExposuretoAir
calculatedelemental
thicknessa attenuationandratio experimental
sample (nm) C*b S*c C*/S* C/Sratiod
1.5h
C6SH 0.80 5.47 0.81 6.8 7.5
C12SH 1.60 9.84 0.66 14.9 16.1
C18SH 2.40 13.3 0.53 25.1 24.8
47h
C6SH 1.01 5.35 0.77 6.9 7.7
C18SH 2.23 13.6 0.56 24.3 26
5days
C6SH 0.78 5.48 0.82 6.7 7.5
C12SH 1.50 9.96 0.68 14.6 12.5
C18SH 2.28 13.5 0.55 24.6 25.8
Figure 7. Evolution of O 1s and S 2p intensities for C18SH
29days
SAMs/p-Ptexposedtoair.Componentlabelsandintensitiesarethe
C12SH 1.30 10.2 0.71 14.4 12.5
sameasinFigures5and6.NotethattheinitialtotalOsignalisless
C18SH 1.97 14.0 0.60 23.4 21.7
thanthetotalSsignal.
aThicknesst calculatedfromtheattenuationofthePt4fand4d
XPS
andC18SHmonolayers(EtOH/CH2Cl2deposition)after7days photoelectrons, EAL(Pt 4f7/2) ) 4.01 nm and EAL(Pt 4d5/2) )
3.48nm.25ValuescalculatedfromPt4fand4dattenuationdifferedby
inair.InRAIRSspectratakenafter7daysinair,thepeakshapes
<3%;theiraverageisreported.bPredictedcarbonsignalC*iscalculated
andintensitiesremainedessentiallyunchanged,exceptforasmall
asasumofcontributionsfromn )6,12,or18layers(asapplicable)
shift of (cid:238)(CH ) from 2917 cm-1 to higher wavenumbers. SE C
a 2 attenuatedbythethicknessoftheoverlayinglayers(assumingathickness
showedaslight0.1(0.1nmincreaseinfilmthicknessoverthe oft /n foreachcarbonlayer),EAL(C1s))3.54nm.25 cPredicted
XPS C
first 8 days, possibly due to adsorption of adventitious hydro- sulfursignalS*calculatedassumingattenuationbythetotalmonolayer
carbons.Theseresultsareconsistentwiththeretentionofnearly thickness,EAL(S2p))3.82nm.25 dExperimentalScofield-adjusted
thefullmonolayer,indicatingthatthestructurechangesinduced C1s/S2pintensityratio.36,37
byairexposureoverthefirstfewdaysarelimitedtotheS-metal
interface. QuantitativeanalysisoftheXPSintensitiesprovidesinsight
intoboththefilmthicknessandthestoichiometry.Reportedin
4. Discussion Table2isthemonolayerphotoelectronthickness,t ,calculated
XPS
4.1.MonolayerThickness,Stoichiometry,andMolecular fromtheattenuationofPt4f7/2andPt4d5/2substratepeaks(see
Orientation.TheIR-activeCH stretchfrequenciesaresensitive AppendixA1).25Thesephotoelectronthicknessesforas-deposited
2
to the local intermolecular interactions and thus serve as the monolayers are linear as a function of the number of carbon
primary indicator of chain order.46,47 For C18SH on p-Pt, the atomsnC[tXPS)(0.13(cid:226)nC)nm]andareveryclosetotheextended
observed(cid:238)a(CH2)frequency(2917cm-1)ischaracteristicofan lengthsoftherespectivemolecules[t)(0.127(cid:226)nC)nmexpected
ordered, all-trans chain and is essentially the same as those for upright chains]. The absolute values of photoelectron
reported in a previous study of SAMs on plasma-cleaned Pt,5 thicknesses support the nearly upright alkyl chain orientation
indicating a similarly high degree of crystalline order. For all established from the quantitative analysis of the RAIRS data.
monolayersonp-Ptthe(cid:238)(CH )and(cid:238)(CH )featuresarestrongly This is further corroborated by the linear regression to the SE
s 2 a 2
suppressedcomparedtotheCH features(Figure3).Thetransition data(Figure4).Notably,ourXPSandSEthicknessmeasurements
3
dipolemomentsoftheCH modesareperpendiculartotheall- are consistent with absence of an oxide layer on p-Pt, as both
2
transchain,whereasboththe(cid:238)(CH )and(cid:238)(CH )modeshave yieldoffsetsclosetozero,incontrastwithpreviouslyreported
s 3 a 3
transitiondipolemomentprojectionsalongthechainaxis.The ellipsometric results for SAMs on Pt5 and Pd.33
suppressedintensityoftheCH stretchesindicatesanorientation Thestoichiometry(C/Sratio)canbeestimatedfromtheCand
2
almost perpendicular to the surface normal. S signals, properly accounting for their respective attenuation
To quantitatively determine the monolayer orientation, we duetotheirpositioninthefilm(seeAppendixA1).TheSsignal
performedadetailedanalysisoftheRAIRSdataforC18SHon is attenuated by the full monolayer thickness (S* in Table 2).
p-Pt films.42 Determination of the tilt angle for the presumed TheCcontributionsfromeachatomiclayerareaddedfromtop
all-transmethylenechainisverysensitivetothepreparationof to bottom with increasing attenuation (C* in Table 2). These
thebulkreferencespectrum.48WethereforeusedtheC18SH/Au predictedC*/S*ratiosarecomparedinTable2tomeasuredC/S
spectrum and the known orientation of the monolayers on Au ratios; the good agreement between the two columns leads us
asareference.AssumingthattheC18SH/Autiltangleliesbetween totwoconclusions.First,then-alkanethiolmoleculesareoriented
20(cid:176) and30(cid:176) ,thetiltangleforC18SH/p-Ptliesbetween6(cid:176) and withsulfurgroupsboundtothePtsurfaceandhydrocarbontails
16(cid:176) .Thisisconsistentwiththequalitativeobservationthatthe extendedawayfromtheS/Ptinterface.Second,littleornoC-S
C18SH/p-PtspectrumisverysimilartothatreportedforC18SH bond scission occurs when alkanethiols self-assemble on Pt
onAg,wherethetiltanglewasdeterminedtobe12(cid:176) .9Thereis surfaces. In fact, the measured C/S ratio remains above the
very little difference between the intensities of the CH peaks stoichiometric value even for monolayers either deposited or
intheRAIRSspectraofC12SHandC18SH,implyingt2hatthe soakedinCH2Cl2sthetreatmentsthatshouldatleastpartially
tilt angle must decrease with chain length. removealkylchainsproducedbysuchhypotheticalC-Sbond
scission.
(46)MacPhail,R.A.;Strauss,H.L.;Snyder,R.G.;Elliger,C.A.J.Phys. 4.2. Coverage. In Table 3, the intensities of the S 2p
Chem.1984,88,334-341. componentsandthetotalScoveragearequantifiedforSAMs/Pt
(47)Snyder,R.G.;Strauss,H.L.;Elliger,C.A.J.Phys.Chem.1982,86,
5145-5150. depositedunderthedifferentconditionspresentedinFigure1.
(48)Arnold,R.;Terfort,A.;Woll,C.Langmuir2001,17,4980-4989. TherelativeintensitiesinTable3correspondtopeakareasin
2584 Langmuir,Vol.22,No.6,2006 PetroVykhetal.
Table3. S2pComponentsandSulfurCoverageforSAMs/Pt 4.3.PlatinumOxide.Asapracticalmatter,itisimportantto
totalsulfurcoverage notethatsubmonolayeramountsofplatinumoxidesreadilyform
sample S2pcomponentsa Aureferenceb oncleanPtsurfaces: experimentsinultrahighvacuum(UHV)
description S3 S2 S1 total 1014atoms/cm2 show that molecular oxygen dissociates and chemisorbs on Pt
p-Pt,EtOHc evenatcryogenictemperatures.50-53Therefore,thepresenceof
C6SH 0.24 0.42 1.19 1.85 5.9 such surface platinum oxides can be assumed in most cases
C12SH 0.22 0.32 1.17 1.71 5.5 (especially when exposure to ambient is involved), and the
C18SH 0.19 0.45 1.24 1.88 6.0 interactionofthiolswiththeseoxideswilllikelyplayarolein
p-Pt,EtOH/CH2Cl2d SAM formation and stability.
C6SH 0.13 0.43 1.14 1.70 5.5
XPSpotentiallyhasthesensitivitytodetectasubmonolayer
C18SH 0.12 0.43 1.36 1.91 6.1
of platinum oxide through examination of the Pt 4f or O 1s
p-Pt,CH2Cl2e peaks.Unfortunately,thePt4fpeaksproduceahighlyasymmetric
C6SH 0.13 0.43 1.14 1.70 5.5
inelasticbackground,whichmakesitdifficulttoreliablydetect
C18SH 0.13 0.41 1.40 1.94 6.2
a small platinum oxide component shifted by only about
UVO-Pt,EtOH/CHClf
2 2 0.9 eV.54 For example, we observe no consistent difference
C6SH 0.15 0.45 1.13 1.73 5.5
betweentheshapeofthePt4fspectraforfreshlysputteredPt
C12SH 0.21 0.42 1.19 1.82 5.8
C18SH 0.31 0.42 1.09 1.82 5.8 substratesandthoseoxidizedbyUVOtreatment.Therefore,the
absenceofplatinumoxidecomponentsinthePt4fregion,that
TS-Pt,EtOH,Arg
C6SH 0 0.23 1.16 1.39 4.5 theauthorsofrefs5and55interpretasevidenceofoxide-free
C18SH 0 0.34 1.18 1.52 4.9 Pt surfaces, in fact, only rules out the presence of (several)
monolayersbutnotofasubmonolayerofplatinumoxide.The
aIntensitiesofS2pcomponentscorrespondtothoseinFigure1and
totalO1ssignalgivesanupperlimittotheamountofplatinum
arereportedrelativetotheS2ppeakforC6SH/p-Ausample,i.e.,as
Scofield-adjusted36,37S2p/Pt4f intensityratiosdividedby0.0455. oxideasone-halfthatoftheSforas-depositedsamplesonp-Pt
7/2
Formostsamples,intensityvariationwithinasamplewas<5%;afew substrates (Figure 7). The association of some of the oxygen
outliersshowedupto10%variability.bAbsolutecoveragesarecalculated withoxidizedcomponentsofCandSpreventsamoreaccurate
usingacleanAuabsolutereferenceandanempiricalsensitivityfactor estimate for platinum oxide.
toaccountforthedifferencebetweenPtandAusubstrates(seeAppendix
WehaveevidencethatUVOcleaningoxidizesPt.ForUVO-
A2andSupportingInformationfordetails).cDepositionfrom1mM
solutioninEtOH,rinseinEtOH.dDepositionfrom1mMsolutionin Ptsamples,theO1ssignalwasconsistentlyabouttwiceashigh
EtOH,overnightsoakinCHCl.eDepositionfrom1mMsolutionin asthatforsamplesonp-PtorTS-Pt.RAIRSandSEindirectly
2 2
CHCl, rinse in CHCl.fDeposition from 1 mM solution in EtOH, corroborate the oxidation of UVO-Pt substrates. The RAIRS
2 2 2 2
overnight soak in CH2Cl2, UVO-Pt substrate.gDeposition in inert- dataformonolayersonUVO-PtinFigure3showbroaderpeaks
atmospheregloveboxfrom1mMsolutionindeoxygenatedEtOH,rinse similar to those reported for SAMs on platinum oxide.5 The
inEtOH,TS-Ptsubstrate.
substratedielectricconstants,determinedfromtheSEmultisample
analysis, were significantly different for the UVO-Pt and p-Pt
Figure1normalizedbytheS2ppeakfortheC6SH/p-AuSAM. surfaces (Figure SI2, Supporting Information). If we assume
Quantitatively, these Scofield-adjusted S 2p/Pt 4f intensity thatthep-Ptis“clean”andtheUVO-Pthasathinoxide,then
7/2
ratios36,37areconvertedintoabsoluteScoveragesinthefollowing thebareUVO-Ptsubstratedatacanbewellfittoathree-phase
opticalmodel(substrate,oxide,air)withanoxidethicknessof
way: ForthereferenceC6SH/p-AuSAM,wehaveanabsolute
calibrationthatgivesacoverageof4.5(cid:2)1014cm-2(Appendix 0.53 ( 0.03 nm and a constant nˆ ) 1.76 ( 0.02 + (0.34 (
0.11)i.56,57Theoxidefilmindexofrefractionisverysimilarto
A2).Wethenuseanempiricalsensitivityfactorestablishedfrom
thatobtainedbySE(350-700nm)forthehydrousformofPtO
measurementsofcleanAuandPtsubstratestoobtainabsolute 2
formed by anodic oxidation in H SO (nˆ (cid:25) 1.70 + 0.15i).58
coveragesfromS2p/Pt4f intensityratiosandtheabsoluteAu 2 4
7/2
Can alkanethiols reduce these oxides on p-Pt and UVO-Pt?
reference.TheresultingcoveragevaluesarereportedinTable
A positive claim of a complete reduction of thermal platinum
3; for details and extensive discussion of this quantification
oxide by alkanethiols has been reported on the basis of
approachseeAppendixA2andtheSupportingInformation.The
electrochemicalmeasurements.59However,alkanethioldeposition
estimated experimental uncertainty of the coverage values in
Table3isabout10%;potentialsourcesofsystematicuncertainties
(49)Yang,Y.-C.;Yen,Y.-P.;Yang,L.-Y.O.;Yau,S.-L.;Itaya,K.Langmuir
aredifficulttoquantify,butsomearediscussedintheSupporting
2004,20,10030-10037.
Information. With the exception of the TS-Pt substrates, the (50)Puglia,C.;Nilsson,A.;Hernnas,B.;Karis,O.;Bennich,P.;Martensson,
coverage of freshly deposited films is about 5.8 (cid:2) 1014 cm-2. N.Surf.Sci.1995,342,119-133.
(51)Stipe,B.C.;Rezaei,M.A.;Ho,W.J.Chem.Phys.1997,107,6443-
The high areal density (relative to thiols on Au) is consistent
6447.
withthenear-normalchainorientation.SEmeasurementsdonot (52)Saliba,N.;Tsai,Y.L.;Panja,C.;Koel,B.E.Surf.Sci.1999,419,79-88.
(53)Gambardella,P.;Sljivancanin,Z.;Hammer,B.;Blanc,M.;Kuhnke,K.;
estimatemonolayerthicknessandcoverageindependently,but
Kern,K.Phys.ReV.Lett.2001,8705.
ifthedensityandindexofrefractionofhigh-densitypolyethylene (54)Bancroft,G.M.;Adams,I.;Coatsworth,L.L.;Bennewitz,C.D.;Brown,
(HDPE)areusedtoestimatethecoverageofC18SH/p-Pt,the J.D.;Westwood,W.D.Anal.Chem.1975,47,586-588.
result is (4-5) (cid:2) 1014 cm-2. For comparison, previous radio- Sur(f5.5S)cLi.i2,0Z0.3Y,.5;2B9e,c4k1,0P-.;4O18h.lberg,D.A.A.;Stewart,D.R.;Williams,R.S.
labelingmeasurementsshowed15%highercoverageforSAMs (56)Uncertaintiesareonestandarddeviationaveragedoverfivemeasurement
series.
on Pt vs Au, which, assuming negligible roughness, is
(57)Theuniqueextractionofdielectricconstantsfromultrathin(e10nm)
5.3(cid:2)1014cm-2forSAMsonPt.8Arecentscanningtunneling films is problematic as the film thickness and index become correlated. The
microscopy (STM) study reported a high-coverage phase for correlationcoefficientbetweenthicknessandtherealpartoftheindex(n)was
alkanethiols on Pt(111) with a (x3 (cid:2) x3)R30(cid:176) structure that (cid:24)0.93.However,theestimatedparametersareconsideredunique,asthestatistical
estimateofthefituncertaintiesonthicknessandn,accountingfortheoff-diagonal
corresponds to a 5.1 (cid:2) 1014 cm-2 coverage.49 These literature elementsoftheerrormatrix,arecomparableto((cid:24)50%smallerthan)thereported
uncertaintiesbasedonmultipledatasets.
coveragevaluesarethussomewherebetweentheXPS-determined
(58)Gottesfeld,S.;Maia,G.;Floriano,J.B.;Tremiliosi,G.;Ticianelli,E.A.;
coverages for SAMs on p-Pt and TS-Pt. Gonzalez,E.R.J.Electrochem.Soc.1991,138,3219-3224.
AlkanethiolsonPlatinum: MulticomponentSAMs Langmuir,Vol.22,No.6,2006 2585
Table4. M-SandM-OBondStrengthsforfccMetals are the most oxidized (section 4.3), and the S3 component is
correspondinglythehighestforSAMsonUVO-Pt(Figure1and
metal Au Ag Cu Ni Pt
Table 3). This observation is in agreement with the data for
M-Sa 418 217 276 344 234b
M-Oa 222 220 269 382 392 SAMs on plasma-oxidized Pt, which showed considerable
intensityabove164eVBE.5Conversely,forSAMspreparedon
aValuesinkJ/molfromBondStrengthsinDiatomicMolecules.In TS-Pt under oxygen-free conditions, the S3 component is
Handbook of Chemistry and Physics; CRC Press: Boca Raton, FL,
undetectable (Figure 1c). The correlation between the S3 and
2003;pp9-52-9-64.Thesebondstrengths,oftenknownasthebond
surfaceoxidationleadsustoconcludethattheinitialoxidation
dissociationenergies,aredefinedasthestandardenthalpychangeofthe
dissociationreactionasdeterminedat298K.bValueconvertedfrom of thiols is mediated or facilitated by Pt surface oxide.
56kcal/molgiveinTable3ofref60,whichdefinesbondstrengthas S1 Component. This is the predominant S 2p component in
“thecohesiveenergypermetal-sulfurbond”. spectraforallsamplesinourstudy(Figures1,5,and7;Table
3)withtheBEbetween162.3and162.4eV.Onthebasisofthis
cansuppressplatinumoxidereductionfeaturesinvoltammograms BE,weassigntheS1componentasalkylthiolatesboundtothe
byeitherremovingtheoxideorbyadsorbingontopoftheoxide; Ptsubstrate,inagreementwiththereportedS2pBEvaluesfor
thus, a submonolayer amount of residual oxide could remain alkylthiolates on other fcc metals: 161.9-162.1 eV on
undetectedelectrochemically.Ingeneral,thestabilityofthiolates Au,9,10,17,18,20-24,28,38,39,62161.8-162.3eVonAg,9,23,28,62162.1-
againstoxidationandtheabilityofthiolstoreducemetaloxides 162.5 eV on Cu,9,29,31 and 161.8 eV on Ni.32
depend in part on the respective metal-oxygen (M-O) and An alternative assignment for S 2p components in this BE
metal-sulfur(M-S)bondenergies(Table4).Ofthemetalsin range was suggested in a recent study of SAMs on Pd: A
Table4,onlyforAuistheM-SbondstrongerthanM-O,and 162.3 eV BE component was assigned as “sulfur present in a
the bond strength hierarchy is reversed for Pt. It is practically metal sulfide interphase”, and a 163.2 eV BE component was
impossibletoprepareoxide-freesurfacesofAg,Cu,orNiinan assignedasalkylthiolates.33SuchanassignmentrequiredC-S
ambientenvironment,andthus,SAMformationforallthreeof bondscission,33aprocessbelievedpossibleoncatalyticmetals.
these metals is affected by the presence of such residual GiventhestrikingsimilarityoftheSAMs/PdS2pdatatoours
oxides.27,30-32Overall,thecomparisonwithotherfccmetalsand andthecatalyticnatureofPt,wecarefullyexaminedthepossibility
thebondstrengthargumentssuggestthatacompletereduction ofthisalternativeassignmentforSAMs/Pt.Wefoundthreepieces
of surface platinum oxides by alkanethiols is unlikely. ofevidencethatruleoutsuchapossibility.First,theS1component
4.4. Assignment of S 2p Components in XPS Data. The fallswithintherangeobservedforalkylthiolatesonfccmetals,
interpretation of the three S 2p components in the XPS data whereastheS2component(163.2eV)clearlydoesnot.63Second,
(Figures1and5)iskeytounderstandingthestructureofSAMs in all as-deposited monolayers on Pt, the S 2p doublet spin-
onPt.SuchassignmentsforSAMsonmetalsotherthanAuare orbitsplittingoftheS1componentiswell-resolved,anditsfwhm
atbesttentativeintheliterature,andoftenareinconflictwith of0.98eVisonly17%largerthanthe0.84eVfwhmmeasured
each other. Here, we attempt to combine our high-resolution forthereferenceC6SH/Aumonolayer.16Thenarrowwidthand
spectraoftheS2pregionwithresultspreviouslyobtainedfor absence of systematic trends in residuals indicate that it is a
alkanethiolSAMsonabroadrangeofmetalsubstrates(Au,Ag, singlespectralcomponent,incontrasttoacombinationofapeak
Cu, Ni, Pd, and Pt) in order to provide the most consistent andalow-BEshouldertypicallyobservedafterC-Sbondscission
interpretation. We first briefly discuss the S3 component, and onfccmetals.Finally,theXPSstoichiometrydatainsection4.1
thenwefocusontheS1(162.3eVBE)andS2(163.2eVBE) indicatethatthereisnolossofCfromas-depositedorsolvent-
components. treatedmonolayersand,hence,thatnoC-Sbondscissionoccurs.
S3Component.TheassignmentoftheS3(BEabout165eV) This stoichiometry analysis rules out the putative mechanism
andhigher-BEcomponents(e.g.,seethebottomtwospectrain required for the alternative assignment of S1.
Figure5)isratherstraightforward,astheyaregenerallybelieved S2Component.Thereareseveralpossibleassignmentsforthe
toresultfromincreasinglyhigheroxidationstatesofsulfur,with S2componentat163.2eVBE.7Twointerpretationsthatwerule
S 2p doublets at 165 eV assigned as S4+ and those above out are radiation (or electron) damage19,39,64 and unbound
167 eV as sulfates.61 Two trends in our data support this thiols.10,17,18OnPt,theradiation-damageinterpretationappears
assignment. First, with exposure to air, the intensity of the S3 unlikely,giventhatarecentstudyshowedthattheS2component
componentincreases(bothinabsolutevalueandrelativetothe didnotchangeafter10hofirradiationbyaMgKRsource.7In
S1), whereas the S1 intensity decreases (Figures 5 and 7), our experiments designed to test the unbound thiol interpreta-
indicating that the primary mechanism of forming the S3 tion,5,7theintensityoftheS2componentremainedessentially
component in the ambient is oxidation of the S1 component. unchanged (Figure 1 and Table 3) after extensive rinsing or
Second, the S3 component gradually shifts to higher BE with overnight soaking in CH Cl , a good solvent that should
2 2
exposure to air, e.g., the average BE of the S3 component for significantlyreduceanunboundcomponent.17Concurringwith
C6SHmonolayersincreasesfrom165.0to165.8eV(Figure5). these XPS results, our RAIRS data also did not show S-H
This shift indicates slow conversion of S from thiolates into vibrational modes expected for unbound thiols.
higheroxidationstates.Arelatedobservationconcerningallof For S2, the consistent 0.9 eV BE shift, narrow fwhm, and
thehigh-BES2pcomponentsisthattheoxidationofthiolgroups insensitivitytodepositionconditionssuggestthatitcorresponds
in SAMs on Pt appears to proceed in a multistep fashion, i.e.,
throughavarietyofstateswithincreasingoxidation,ratherthan (61)Polcik,M.;Wilde,L.;Haase,J.;Brena,B.;Comelli,G.;Paolucci,G.Surf.
Sci.1997,381,L568-L572.
directly into well-defined sulfates as reported for Cu.29
(62)Zharnikov,M.;Frey,S.;Heister,K.;Grunze,M.Langmuir2000,16,
The initial amount of the S3 component in the monolayers 2697-2705.
correlateswithPtoxidation.Inourstudy,theUVO-Ptsubstrates (63)TheBEreportedinref65forS2pinbulkPtSis162.9eViscloserto
S2ratherthanS1.OnlyforbulkPtS2istheBEcomparableat162.4eV(ref66),
butcreatingastoichiometrysimilartoPtS2atthesurfacewouldrequireC-S
(59)Lang,P.;Mekhalif,Z.;Rat,B.;Garnier,F.J.Electroanal.Chem.1998, bondscissionforthemajorityofalkylthiolates,i.e.,suchanassignmentforS1
441,83-93. isincompatiblewiththerestofthedata.
(60)Toulhoat,H.;Raybaud,P.;Kasztelan,S.;Kresse,G.;Hafner,J.Catal. (64)Heister,K.;Zharnikov,M.;Grunze,M.;Johansson,L.S.O.;Ulman,A.
Today1999,50,629-636. Langmuir2001,17,8-11.
2586 Langmuir,Vol.22,No.6,2006 PetroVykhetal.
to a specific binding geometry rather than simply a collection thesitesbeparticularlyhigh,indisagreementwithexistingdata
of random surface sites. Three possible interpretations are for methanethiol on Pt(111).74 The strongest support for the
suggested by the literature and chemical intuition: sulfur alternativebindingsiteinterpretationinourdatacomesfromthe
headgroupsboundtoplatinumoxide,alternativesurfacebinding results for monolayers on TS-Pt, the only case where S2 was
sites, or the presence of disulfides. significantly reduced. TS-Pt is considerably smoother than
SupportfortheargumentthattheS2componentarisesfrom polycrystalline Pt and thus potentially has fewer alternative
sulfur functional groups bound to platinum oxide comes from binding sites.
asystematicstudyofSAMsonanoxidizedAusurface,21which The third potential interpretation for S2 is the formation of
showedtwoS2pcomponentswithBEsof162.1and163.3eV. disulfide moieties when alkanethiols self-assemble on Pt. If
The intensity of the 163.3 eV component was correlated with disulfidesexistonthePtsurface,itispossiblethatS-Ptbonding
oxygen exposure for monolayers prepared in UHV, and it did occursthroughonlyoneoftheSatoms(S1),whilethesecond
notchangeafter4hofirradiationbyaMgKRsource.Alkanethiols S atom (S2) remains unattached to the surface. The major
adsorb on gold oxide with higher density and lower tilt angle advantage of this interpretation is that the relative fraction of
than on clean Au21sproperties that closely parallel the results molecules adsorbed in disulfide form might depend on both
forSAMsonPt.AdditionalevidencethattheS2componentis surfaceoxidationandroughness.Disulfidesmight,infact,form
relatedtoplatinumoxideispresentedinref5.TheS2pspectrum asaresultofreducingsurfaceoxides.Onediscrepancyinthis
isnoticeablyshiftedforaC18SHmonolayeronplatinumoxide model is the clearly missing S1 component in the data for
compared to that on clean Pt. Although individual S 2p monolayersonplatinumoxide,5whereanenhancementofthe
componentsarenotresolvedinthespectrumshowninref5,the disulfideformation(andattachment)canbeexpected.Considering
overall shift of the S 2p envelope appears to be at least the disulfide interpretation of S2 underscores the inherent
0.7-0.8eVtohigherBE,i.e.,veryclosetothe0.9eVshiftof difficultyofdefinitivelyassigningsuchfeaturessthedebateabout
S2. The data in ref 5 confirm that alkanethiols can adsorb on theadsorptionofdisulfidesongoldsurfaceshasbeengoingon
platinum oxide and that the BE for a large fraction of such fordecades.Experimentally,itisextremelydifficulttounam-
monolayersisconsistentwithS2.Apossiblebindinggeometry biguouslydeterminethestructureofsurfacespeciespresentat
istoaPtatomthathasanOnearestneighborandthusshares submonolayer coverages.
asmallerfractionofitselectrondensitywiththeScomparedto Onemightexpectthatstudiesonsingle-crystalsurfacescould
aregularPtsurfaceatom.59BEshiftsreportedforplatinumoxide providemoredefinitivestructuralinformation.However,recent
and bulk platinum sulfides are consistent with this binding limitedstudiesofalkanethioladsorptiononcleansingle-crystal
geometry.54,65,66 The only significant inconsistency of this Pt(111) surfaces in UHV suggest the absence of long-range
interpretationisthat,whereastheoxygen-freedepositiononTS- orderingandamuchmorecomplicatedadsorptionprocessthan
Ptreduced44S2anddepositiononheavilyoxidizedPtproduced forprototypicalSAMsonAuorAg.75,76Similarly,arecentSTM
primarilyS2,5depositiononslightlyoxidizedUVO-Ptdidnot studyprovidedevidenceoflocal(x3(cid:2)x3)R30(cid:176) orderingfor
affect S2. In other words, the positive correlation between S2 alkanethiolsonsingle-crystalPt(111),butalsoreporteddifficulties
andtheamountofsurfaceoxide(suchasobservedformonolayers with imaging any long-range ordering.49 The presence of a
onoxidizedAu21)isnotstrictlyfollowedformonolayersonPt. disordered component such as S2 could naturally account for
ThereisalsonopositivecorrelationbetweenS2andpostdepo- these observations.
sition oxidation in air (Figures 5 and 7).
Thealternativebindingsiteinterpretation,i.e.,thebindingof 5. Conclusions
sulfurtoaPtsurfacesitedifferentfromthatgivingrisetothe WestudiedtheformationofalkanethiolSAMsonPtsurfaces
S1component,haslimitedsupportfromtheavailabledatafor asafunctionofsubstratecleaning,solventsusedfordeposition
otherfccmetals.OnAu,nosignificantBEdifferenceshavebeen and post-treatment, and alkyl chain length. We also examined
observed between Au(111) single crystals and various poly- thelong-termstabilityofsuchSAMsinairandthecorresponding
crystalline Au surfaces (including oxidized ones); however, evolutionofthemonolayerstructurewithexposuretoair.We
oxidationdependenceonthestructureofpolycrystallineAufilms findthatastandarddepositionusingp-Ptsubstratesand1mM
has been reported.21,22,67 Whereas multiple binding sites and ethanolicsolutionsofalkanethiolsfor20hproducedmonolayers
incommensurate monolayers have been reported on other fcc of equal or better initial quality compared with other reported
metals,68-71thetypicalS2pBEdifferencesarerathersmallsthe methods.As-depositedSAMsonPtaredenseandhaveanearly
largest reported value we found was (cid:24)1 eV difference for S uprightchainorientation.Thereisnoevidenceforeitherweakly
chemisorbedatdifferentbindingsitesonNi(111)vsNi(001).72,73 adheringspeciesorC-Sbondscission.TheseSAMsarestable
Therefore, S2 assignment to alternative binding sites would against short-term (about a week) exposure to ambient air but
requirethat,onpolycrystallinePt,theenergydifferencebetween oxidize and degrade significantly after about a month.
XPSspectraoftheS2pregionindicatethatthesemonolayers
(65)Dembowski,J.;Marosi,L.;Essig,M.Surf.Sci.Spectra1993,2,104-
108. consistofatleastthreecomponents.Themajorcomponent,S1,
(66)Dembowski,J.;Marosi,L.;Essig,M.Surf.Sci.Spectra1993,2,133- isassignedtoalkylthiolatestypicalforSAMsonothermetals.
137.
Aminorcomponent,S3,isassociatedwiththiolsinintermediate
(67)Lee,M.T.;Hsueh,C.C.;Freund,M.S.;Ferguson,G.S.Langmuir1998,
14,6419-6423. oxidationstates.Theremainingcomponent,S2,correspondsto
(68)Fisher,C.J.;Woodruff,D.P.;Jones,R.G.;Cowie,B.C.C.;Formoso, approximatelyone-thirdoftheSlayer.Onlycontrolexperiments
V.Surf.Sci.2002,496,73-86.
onTS-Ptsubstratesunderoxygen-freeconditionseliminatedS3
(69)Kariapper,M.S.;Fisher,C.;Woodruff,D.P.;Cowie,B.C.C.;Jones,
R.G.J.Phys.: Condens.Matter2000,12,2153-2161. and reduced S2; these two minor components were largely
(70)Jackson,G.J.;Woodruff,D.P.;Jones,R.G.;Singh,N.K.;Chan,A.S. unaffectedbyotherchangesindepositionconditions.Wecannot
Y.;Cowie,B.C.C.;Formoso,V.Phys.ReV.Lett.2000,84,119-122.
(71)Kariapper,M.S.;Grom,G.F.;Jackson,G.J.;McConville,C.F.;Woodruff,
D.P.J.Phys.: Condens.Matter1998,10,8661-8670. (74)Lee,J.J.;Fisher,C.J.;Bittencourt,C.;Woodruff,D.P.;Chan,A.S.Y.;
(72)Mullins,D.R.;Huntley,D.R.;Overbury,S.H.Surf.Sci.1995,323, Jones,R.G.Surf.Sci.2002,516,1-15.
L287-L292. (75)Sweeney,T.M.M.S.Thesis,UniversityofNewOrleans,NewOrleans,
(73)Mullins,D.R.;Tang,T.;Chen,X.;Shneerson,V.;Saldin,D.K.;Tysoe, LA,2004
W.T.Surf.Sci.1997,372,193-201. (76)Yang,M.;Laracuente,A.R.;Whitman,L.J.Unpublishedresults,2005.