THE JOURNAL OF CHEMICAL PHYSICS Study of a hydroge - PDF document

074704-8Liuetal.J.Chem.Phys.,074704(2011)systemstoextendtheirlifecycles.Whereintheenhancedstiffnesshasalsobeendemonstratedonotherpolymericsys-temssuchaspolylacticacidÞlms,theYoungÕsmoduliofwhichwerepromptedby8timesthrough30minbombard-ment(unpublished),respectively.ACKNOWLEDGMENTSWegratefullyacknowledgetheÞnancialsupportfromOntarioCentersofExcellence(OCE)andLANXESSInc.J.YangisalsogratefulforthesupportfromNaturalSci-enceandEngineeringResearchCouncilofCanada(NSERC)andCanadaFoundationforInnovation(CFI).AndY.LiuwouldappreciatethefellowshipsupportfromOntarioGradu-ateScholarship(OGS).A.Centrone,E.Penzo,M.Sharma,J.W.Myerson,A.M.Jackson,N.Marzari,andF.Stellacci,Proc.Natl.Acad.Sci.U.S.A.,9886(2008).F.CarusoandH.Mohwald,,8276(1999).M.A.Reed,J.Chen,A.M.Rawlett,D.W.Price,J.M.Tour,Appl.Phys.,3735(2001).Y.Chen,G.Y.Jung,D.A.A.Ohlberg,X.M.Li,D.R.Stewart,J.O.Jeppesen,K.A.Nielsen,J.F.Stoddart,andR.S.Williams,,462(2003).H.S.NalwaandA.Kakuta,Appl.Org.Chem.,645(1992).D.R.Talham,T.Yamamoto,andM.W.Meisel,J.Phys.Condens.Matter184006(2008).S.Yoshida,T.Ono,andM.Esashi,,475302(2008).S.Yoshida,T.Ono,S.Oi,andM.Esashi,,2516L.J.Kepley,R.M.Crooks,andA.J.Ricco,Anal.Chem.,3191(1992).T.Kang,S.R.Hong,J.Moon,S.Oh,andJ.Yi,Chem.Commun.,3721(2005).A.J.Parnell,S.J.Martin,C.C.Dang,M.Geoghegan,R.A.L.Jones,C.J.Crook,J.R.Howse,andA.J.Ryan,,1005(2009).E.RuckensteinandZ.F.Li,Adv.ColloidInterfaceSci.,43(2005).J.HyunandA.Chilkoti,,5644(2001).B.ZhaoandW.J.Brittain,J.Am.Chem.Soc.,3557(1999).M.Lemieux,S.Minko,D.Usov,M.Stamm,andV.V.Tsukruk,,6126(2003).W.M.Lau,Nucl.Instrum.MethodsPhys.Res.B,341(1997).W.M.Lau,Z.Zheng,Y.H.Wang,Y.Luo,L.Xi,K.W.Wong,andK.Y.Wong,Can.J.Chem.,859(2007).Z.Zheng,X.D.Xu,X.L.Fan,W.M.Lau,andR.W.M.Kwok,J.Am.Chem.Soc.,12336(2004).C.A.Andersen,H.J.Roden,andC.F.Robinson,J.Appl.Phys.,3419J.C.ArnoldandH.H.Sawin,J.Appl.Phys.,5314(1991).J.A.KenneyandG.S.Hwang,J.Appl.Phys.,044307(2007).J.P.ChangandJ.W.Coburn,J.Vac.Sci.Technol.A,S145(2003).M.K.AbatchevandS.K.Murali,Electrochem.Solid-StateLett.,F1B.P.LinderandN.W.Cheung,IEEETrans.PlasmaSci.,1383G.A.Seßer,Q.Du,P.B.Miranda,andY.R.Shen,Chem.Phys.Lett.347(1995).X.Z.Wu,E.B.Sirota,S.K.Sinha,B.M.Ocko,andM.Deutsch,Rev.Lett.,958(1993).H.Schollmeyer,B.Struth,andH.Riegler,,5042(2003).C.Merkl,T.Pfohl,andH.Riegler,Phys.Rev.Lett.,4625(1997).T.Yamamoto,K.Nozaki,A.Yamaguchi,andN.Urakami,J.Chem.Phys.154704(2007).K.W.Herwig,B.Matthies,andH.Taub,Phys.Rev.Lett.,3154H.Mo,H.Taub,U.G.Volkmann,M.Pino,S.N.Ehrlich,F.Y.Hansen,E.Lu,andP.Miceli,Chem.Phys.Lett.,6(2003).F.W.DelRio,K.L.Steffens,C.Jaye,D.A.Fischer,andR.F.Cook,,1688(2010).O.Marti,H.O.Ribi,B.Drake,T.R.Albrecht,C.F.Quate,andP.K.,50(1988).K.Morigaki,H.Schonherr,C.W.Frank,andW.Knoll,,6994G.Oncins,C.Vericat,andF.Sanz,J.Chem.Phys.,044701(2008).Q.Zhong,D.Inniss,K.Kjoller,andV.B.Elings,Surf.Sci.,L688M.D.Dong,S.Husale,andO.Sahin,Nat.Nanotechnol.,514(2009).O.Sahin,S.Magonov,C.Su,C.F.Quate,andO.Solgaard,Nat.Nanotech-,507(2007).M.Radmacher,R.W.Tilmann,andH.E.Gaub,Biophys.J.,735W.M.LauandR.W.M.Kwok,Int.J.Mass.Spectrom.,245(1998).L.Xi,Z.Zheng,N.S.Lam,O.Grizzi,andW.M.Lau,Appl.Surf.Sci.113(2007).S.Alexander,L.Hellemans,O.Marti,J.Schneir,V.Elings,P.K.Hansma,M.Longmire,andJ.Gurley,J.Appl.Phys.,164(1989).G.MeyerandN.M.Amer,Appl.Phys.Lett.,1045(1988).J.L.HutterandJ.Bechhoefer,Rev.Sci.Instrum.,3342(1993).U.G.Volkmann,M.Pino,L.A.Altamirano,H.Taub,andF.Y.Hansen,Chem.Phys.,2107(2002).A.Holzwarth,S.Leporatti,andH.Riegler,Europhys.Lett.,653H.Mo,S.Trogisch,H.Taub,S.N.Ehrlich,U.G.Volkmann,F.Y.Hansen,andM.Pino,Phys.StatusSolidiA,2375(2004).M.Bai,S.Trogisch,S.Magonov,andH.Taub,Ultramicroscopy,946H.Sirringhaus,N.Tessler,andR.H.Friend,Synth.Met.,857(1999).S.Trogisch,M.J.Simpson,H.Taub,U.G.Volkmann,M.Pino,andF.Y.J.Chem.Phys.,154703(2005).B.Bhushan,HandbookofMicro/NanoTribology,2nded.(CRCPress,BocaRaton,1999).M.Hartig,L.F.Chi,X.D.Liu,andH.Fuchs,ThinSolidFilms,262G.J.Simpson,D.L.Sedin,andK.L.Rowlen,,1429V.V.Tsukruk,I.Luzinov,andD.Julthongpiput,,3029S.N.Magonov,V.Elings,andM.H.Whangbo,Surf.Sci.,L385S.N.MagonovandD.H.Reneker,Annu.Rev.Mater.Sci.,175J.TamayoandR.Garcia,Appl.Phys.Lett.,2394(1997).W.J.Price,S.A.Leigh,S.M.Hsu,T.E.Patten,andG.Y.Liu,J.Phys.Chem.A,1382(2006).R.M.Overney,E.Meyer,J.Frommer,H.J.Guntherodt,M.Fujihira,H.Takano,andY.Gotoh,,1281(1994).O.SahinandN.Erina,,445717(2008).S.D.SolaresandH.Hšlscher,F.W.DelRio,C.Jaye,D.A.Fischer,andR.F.Cook,Appl.Phys.Lett.131909(2009). 074704-7Molecularcross-linkbyhydrogen-bombardmentJ.Chem.Phys.,074704(2011) FIG.9.NanomechanicalmappingofparallelandperpendicularlayersofvirginCthinÞlmintheHarmoniXmode.TheupperÞgureshowsthecontrastofstiffnessovertheentiresurface;thelowerÞgureshowstheYoungÕsmodulusproÞlealongasectionline,wheretheYoungÕsmodulusofperpendicularlayeris1.2GPaandthatofparallellayeris0.2GPa. istheexcitationamplitudeofthecantileverandistheamplitudeofthecantileverexertingforceonasamplewithÞnitestiffness.Then,theYoungÕsmoduluscanÞnallybedeterminedby isrelatedtoapparatuscoefÞcient.TheAFMforcemodulationmodeusesaseparatepiezoelectricactuatortoindependentlyactivatetheprobe(VeecoApplication;forcemodulationimagingwithatomicforcemicroscopy).Theexcitationamplitudeofthecantilevermeasuredonarigidsapphire(435GPa)is10.51mV,whichcorrespondsto2.39nmamplitudeofthecantileverwithcalibratedsensi-tivityof227.52.0nm/V.Thediamondtipofcalibratedspringconstant0.23N/mwasmodulatedatthefrequencyof10.86kHzandcontactdeßectionwithsamplewassetat5.23nN.Representativeamplitudecurvesalongthescandi- FIG.10.AmplitudesoftheAFMcantileverinforcemodulationonvirginperpendicularlayer(dots)andperpendicularlayerbombardedfor700s(dashes)are9.14and10.17mV,respectively.rectionwiththescanningsizeof3mareshowninFig.Theaveragedamplitudesare9.17,10.17,and10.48mVforperpendicularmonolayerwithoutbombardment,theperpen-dicularmonolayerafter700sbombardment,andSiOsur-face,respectively.BecausetheYoungÕsmodulusoftheSiOisknownas70GPa,wecansuccessfullyobtaintheconstantvalueforinEq..ContinuouslyusingEq.,wecaneasilycalculatetheYoungÕsmoduliforperpendicularmono-layerwithoutbombardmentandtheperpendicularmono-layerwith700sbombardmenttobe1.5and6.2GPa,respectively.Themeasuredvalueforthevirginperpendicu-larmonolayer(withoutbombardment)fallsintherangeasre-portedinliteratureandisalsoinagoodagreementwiththevaluemeasuredbyHarmoniX1.2GPaasshownin.Throughtheproposedbombardment,stiffnessofthethinÞlm(forbothperpendicularandparallellayers)canbeenhanceduptoÞvetimesasshowninTableIV.CONCLUSIONSAsurfacemodiÞcationmethodofmolecularthinÞlmsbyhyperthermalhydrogenprojectilebombardmenttech-nologyhasbeenimplementedthroughselectivelybreakingCÐHbondsofhydrocarbonchainstomakecarbonrad-icalsthatleadtoformationofCÐCbondsbetweenhy-drocarbonchains.ThechemicalpropertyofthethinÞlmwaspreservedintermsoftheunchangedcarbonconcen-trationconÞrmedbyXPS.Throughstudyingmorphologi-calandmechanicalpropertiesofthethinÞlms,thebom-bardedmonolayers(theperpendicularlayers)havebeenprovedtopossesscross-linkedmolecularnetworksandasmoothsurfacecharacterizedbyAFM.Themechani-calstrengthofthethinÞlmshasbeenenhancedbyÞvetimes.Wethinkacarbon-rich(e.g.,amorphous)layerwasformedinthehydrogenbombardmentprocess.Theabove-mentionedadvantagesofthistechniqueareveryimportantinimprovingtribologicalpropertiesofmodernminiaturized 074704-6Liuetal.J.Chem.Phys.,074704(2011) FIG.7.Topographic(top)andphase(bottom)imagesofCersafter700sbombardment,wherethephaseimagecannoteffectivelydis-tinguishperpendicularlayersfromparallellayersbecauseoftheincreasedYoungÕsmoduliafterhydrogenbombardment.canmeasurealargedynamicrangeofmechanicalpropertiesfrom1MPato10GPa.Itisatappingmodeandwellsuit-ableformeasuringtheparallellayersoflowstiffness,muchlowerthantheperpendicularlayers.InFig.,tovalidatemode,wetestedtheserepresentativematerials:100MPa,PS2GPa,polypropylene(PP)1.2GPa,andmica50GPa.ThemeasuredresultspresentedinFig.77MPa,1.9GPa,1.5GPa,and10.9GPa,forabovematerials,respectively.Astothemeasurementofmica,losesitsaccuracyformaterialsofstiffnesslargerTABLEIII.YoungÕsmodulusofCthinÞlmsbeforeandafter ParallellayerPerpendicularlayer 0s700s0s700s YoungÕsmodulusby(GPa)YoungÕsmodulusbyforcemodulation(GPa) than10GPa.Inthiswork,weareapplyingbothofforcemodulationandHarmoniXtomeasuretheYoungÕsmodu-lusofthinÞlms.hasbeendevelopedasanad-vancedtappingmodecarryingthecapabilitytoquantitativelyobtainstiffnesscontrastonheterogeneoussurfacewithhighlateralresolution.ThismoderesolvesthetipÐsamplecontactintimedomainthroughanalyzingthedifferenceathigherharmonicsonthebroaderfastFouriertransform(FFT)spec-trumoftipÐsampleinteraction,whichcarriesrichinformationaboutthemechanicalpropertyofthesurface.WeappliedtomeasuretheYoungÕsmoduliofvirginper-pendicularlayer,virginparallellayer,andparallellayerafter700sbombardment.AsshowninFig.,HarmoniXcaneffectivelydistinguishthesetwodifferentlyorientedlayersbystiffness,theparallellayerof0.2GPastiffnesssofterthantheperpendicularlayerof1.2GPastiffness.After700sbombardment,theparallellayerwasstiffenedto1.0GPaandtheperpendicularlayerwasstiffenedto6.5GPa(TableInAFMforcemodulationmode,thecantileverbasisislowfrequencymodulatedwhilethetipisincontactwiththesurface.Astifferareaonthesurfacedeformslessthanasofterareaandleadstoahigheramplitudeofcantileverdeßection.AccordingtoHertziantheoryofelastic-circular-point-contactapplied,theYoungÕsmodulusofthesamplecanbecorrelatedwithcantileveramplitudesasfollows.Thecompressionra-tioisgivenas FIG.8.LargedynamicrangeofnanomechanicalmeasurementsonseveralreferencesamplesbytheHarmoniXmode:LDPE100MPa,PS2GPa,1.2GPaandmica50GPa. 074704-5Molecularcross-linkbyhydrogen-bombardmentJ.Chem.Phys.,074704(2011) FIG.4.AFMmeasurementsshowthattheheightofCmonolayerdecreasesastheexposuretimeincreases.valueof0.70nm,nearlyÞvetimeslargerthantheroughnessofthevirginsurface.Thiscorrespondstoacaseofpartiallybombardedsurface(alsoshowninFig.)forwhichthede-greeofcross-linkingvariesmostsigniÞcantlyovertheentiresurface.Thisphenomenoncanbefurtherexplainedinsuchaway:inourexperiments,Cmoleculesinitiallystoodperpendicularlyonthesubstratewithanall-translength;forapartialbombardmentattheexposuretime200s,somemoleculeswerecross-linkedwitheachotherandthenbentdownbutotherintactmoleculeswerestillstandingperpen-dicularlywithitsfulllength.Asaresult,thepartialbombard-mentproducedaruggedsurfaceandtheroughnesswashigh.Therefore,theroughnessofthebombardedsurfacecanbeausefulparameterthatindicatesthedegreeofthemolecularcross-linkingduetothebombardment.Theresultofrough-nessmeasurementfortheexposuretime200sisalsoconsis-tentwiththeresultofheightmeasurementasshowninFig.wherethestandarddeviationofthemeasuredheightislargestcorrespondingtothemaximumvalueofroughnessherein. FIG.5.RoughnessvarianceofCperpendicularmonolayerwithre-specttotheexposuretime(measuredbyAFM).Theerrorbarsrepresentthestandarddeviationsofroughnessofsurfaceofsevenbombardedsamples. FIG.6.PhasecontrastimagingoftappingAFMmodedistinguishesvirginperpendicularlayer(exposuretime0s)fromparallellayer:thebrightregioncorrespondstoCperpendicularlayerandthedarkregioncorrespondstotheparallellayer.D.EnhancedstiffnessofalkanethinÞlmsbybombardmentInAFMtappingmode,thephaseshiftcanbeapproxi-matelyproportionaltothereducedYoungÕsmodulusoftheWiththetappingconditionsuchthatthesettingpointis60%ofthefreelyvibratingamplitude,inthephaseimageofFig.,wecanvisiblydistinguishtheperpen-dicularlayer(brighterregion)fromtheparallellayer(darkerregion).However,thephaseshiftinthetappingmodecouldonlybequalitativelyassociatedwithmechanicalpropertyvariationsinthepresenceofviscoelasticityand/oradhesionhysteresis.TomayoandGarciatheoreticallyandexperi-mentallydemonstratedthatthephaseshiftisnotsensitivetostiffmaterialsofrelativelylargeYoungÕsmodulus(approxi-matelywhentheYoungÕsmodulus1GPa)evenwithenergydissipationinvolved.Atpresent,asshowninFig.weonlycanqualitativelymeasuretheYoungÕsmoduluswhentheYoungÕsmodulusorthestiffnessofbothlayersissmallerthan2GPabeforebombardment.After700sbombardmentasshowninFig.,suchphasecontrastcanbarelybedifferenti-ated.AndbothofthesetwotypesoflayershadbeenstiffenedwiththeirYoungÕsmodulus1GPa.Toquantifytheenhancedstiffnessofbombardedlayers,AFMHamoniX)andforcemodulationwereappliedtomeasuretheYoungÕsmoduliofbothper-pendicularandparallellayers.Thesetwomodesarewellcomplementarytoeachother.Althoughtheforcemod-ulationmodecanbeusedformeasuringalargerangeofYoungÕsmodulus,itoperatesincontactmodeandmaydam-agesoftsamplesoflowstiffness.Itsresolutionisdegradedatahighscanningspeed.Recently,HamoniXmode,whichworksastorsionaltappingmodeusingaT-shapedcantilever, 074704-4Liuetal.J.Chem.Phys.,074704(2011) FIG.3.Topographicimages(top)andheight(bottom)ofCperpendicularmonolayerwithoutbombardment(0s)andafter700sbombardment.C.RoughnessmeasurementsandcriticalexposureAsshowninFig.,400smightbethecriticalexposuretimethatcorrespondstothenecessaryhydrogenßuencere-quiredforthoroughbombardmentontheCthinÞlm.Inthisbombardmentprocess,abundleofhydrogenmoleculesareprojectedonthetarget.ItisdifÞculttocontrolthedistri-butionofprojectilesevenlyonthesamplesurfaceandthusthecollisionsbetweenhydrogenandCÐHbondsarerandom.However,theeffectofcollisionscouldbequantitativelyde-terminedbasedontheroughnessmeasurementsthroughtworunsofexperimentsinourstudy.ThereferredroughnessisdeÞnedas averageTheroughnessmeasurementswerecarriedontheper-pendicularlyorientedmonolayerswithaconstantscanareaof200nm200nmforallofsamples.Tappingmodewasadoptedasthecharacteristicmeansbecauseitgentlytunesthetappingforceexertedonthemonolayerandownsstatisticaladvantagestoprovidemorereliableresults.showninFig.,virginCperpendicularmonolayerhasaverysmoothsurfaceof0.02nm,whichisveryclosetotheroughnessofthesupportingsiliconsubstrate,0.1nm(Ref.).Wenoticedthattheroughnessonthebom-bardedsurfaceincreaseswiththeincreasingexposuretimes;andat200s,theroughnessincreasedtoapeakvalueat0.04nm.Afterfurtherincreaseoftheexpo-suretimebeyond200s,theroughnessstartedtodecreaseandeventuallythesurfacebecomesverysmoothagainwithastable0.01nmaftertheexposuretimeof400s.Theresultsshownabovesuggestthat,Þrst,thethor-oughlybombardedalkanethinÞlmcanhavethesamesmoothnessasthevirginÞlm.Second,theexposuretime400sspeciÞesthesufÞcientionßuencerequiredforachiev-ingthoroughbombardmentoftheCthinÞlm.Finally,at200sexposuretime,theroughnessreachesthemaximum 074704-3Molecularcross-linkbyhydrogen-bombardmentJ.Chem.Phys.,074704(2011) FIG.2.XPSsurveyspectraof(a)virginCmonolayer;(b)thesamplemeasuredafterimmersioninhexanefor5min;(c)Cmonolayeraf-ter600sbombardment;(d)thebombardedsampleimmersioninhexanefor5min.III.RESULTSANDDISCUSSIONSA.X-rayphotoelectronspectroscopyresultsXPSspectraofthinÞlmsaspreparedaredisplayedin,whichwereusedfordeterminingwhetherthechemicalcomponentretainedafterbombardment.ThespectrumofvirginthinÞlms[Fig.]showsvirtualC1signalfromtheCandSi2,Si2,andO1signalsdetectedfromthesubstrate.Withouthydrogenbombardment,thethinÞlmsaresolubleinhexaneafterimmersionfor5min,whichisindicatedbythesigniÞcantreductionoftheC1signalinXPSspectrumasshowninFig..However,forthethinÞlmsbombardedbya10eVhydrogenbeamupto600s,novisiblechangeinC1signalcanbefoundcomparingXPSspectrumsbeforeandafterimmersioninhexaneasshownin.Therefore,itisreasonabletopointouttheoccurrenceofthecross-linkingonthemonolayersafterbeingbombardedfor600s.Suchcross-linkingisthecausethatsigniÞcantlyenhancestheinsolubilityofalkanethinÞlminhexane.ThisindicatespotentialapplicationsofthistechniqueformakingorganicthinÞlmsascoatingsinsevereenvironment.Inaddition,theXPSresult(C1signals)showsthat10eVhydrogenprojectilesforbombardmentcannotbreakCÐCbondsandcausenomainchainscissiononthethinlayersasmentionedbefore.SoonlybreakingofCÐHbondsisinvolvedinthepresentprocess.WewillpresentAFMresultstofurtherillustratephysicaleffectsofthehydrogenbombardmentonmorphologicalandmechanicalpropertiesofthetreatedthinÞlms.B.EffectofbombardmentonthinÞlmheightandmoleculardensityAtthemicroscopicviewpointofsurfacescience,thegrowthmechanismof-alkanelayersonSiOsurfacehasattractedincreasinginterest.Bothoftheoreticalandexperimentalmeasurementssuchastructuralmodel:oneortwolayersofCimmediatelyadsorbedontheSiOsurfacewiththelongaxisofthemoleculeparalleltotheinterface;andthenad-ditionallayersofmoleculesarestandinguprightwiththemoleculesÕlongaxisorientedperpendicularlyandall-translength.Aswementionedbefore,thesetwophases,repre-sentingtwocasesofmonolayerformation,needthoroughDuetodifferentinteractionsofparallelandperpendicu-larlayersexertingontheAFMtip,theamplitude-modulationtappingAFMmodemeasuredÒfalsestepÓheightsoftheall-transandperpendicularmonolayersformedontheSiOstrate.ThepreviousstudyusingcontactAFMmeasuredtheheightoftheall-transconformationofthe-alkanemolecu-larmonolayer.HereweconÞrmedtheresultbyalsoutiliz-ingthecontactAFMmodeformeasuringtheheightofvirginperpendicularmonolayerbeforeandafterbombard-Throughappropriatesamplepreparation,i.e.,0.3wt.%dotriacontaneand5000rpmspin-coatingspeedinambient,perpendicularmonolayerresemblingfractal-likeislandwasasshowninFig..Theheightofperpendicularlayerafterbombardmentwasmeasuredasafunctionofexpo-sureorbombardingtime.(:otheroperatingparametersofbombardmentwerekeptthesameinthisstudysothatthedifferentexposuretimescorrespondedtodifferenthydrogenßuences.)TheresultsofheightmeasurementareshowninFig..TheheightofthevirginCmonolayerwasestimatedtobe4.560.19nm,consistentwiththatofanall-transconformationofthemolecularmonolayer44ascalibratedbyhigh-resolutionellipsometryinliterature.Throughthehydrogenbombardment,themonolayerheightdecreaseswiththeincreaseofexposuretimeasshownin.Atandafter600sexposuretime,themolecularlayerheightbecomes2.580.08nm,onlyabout57%oftheheightofthevirginlayer.However,ourXPSresultonthesamesampleshowsthattheestimatedcarbonconcentrationdoesnotchangewiththeexposuretime.ThiscomparisonbetweenAFMandXPSresultssuggestthatthebombardedthinÞlmbecomescross-linkedandformsdensermolecularnetworksasaresultofcross-linkedCÐCbondsbetweenmolecularchains.Itwasfurthernoticedthatthoroughbombardment(theexposuretimelargerthan600s)broughtlessvariationintheheightovertheentireperpendicularlayer.Nevertheless,forexposuretimelessthan600s,e.g.,between50sand400s,cross-linkingwasnotfullysaturated.Asaresult,theheightoftheentirethinÞlmwasnonuniform. 074704-2Liuetal.J.Chem.Phys.,074704(2011)TABLEI.Maximumenergytransferredfromprojectilestotargetatoms. AtomÐatominteraction10eVHH10eVHC5eVHH5eVHC15eVHH15eVH Transferredenergy(eV)102.851.4154.3 haveonlysaturatedCÐC,theyaregoodsamplemoleculesforcharacterizingourhydrogenbombardmentprocess.Inaddition,wehavelearnedinliteraturethatdotriacon-tanemoleculesasgrowingonSiOcaneitherperpendicularlystanduporlaydowninparallel.Theyrepresenttwophasesofself-assembledmonolayers(SAMs)withdifferentmolecu-larorientations:0and90.Ithasbeenknownthatalkanethiolmonolayers,asgrowingongoldsubstrates,tiltwithacertainangleintherangeof0dependingonthechainlengths.Therefore,thetwo-phasenatureofdoctriacontaneitselfisat-tractiveenoughasagoodcandidateforresearchofSAMsaswell,especiallythemechanicalproperties.Twocharacterizationtoolsareused:x-rayphotoelectronspectroscopy(XPS)andatomicforcemicroscopy(AFM).XPSwasÞrstusedtodeterminechemicalcontent,i.e.,carboninthisstudy.AFMwasusedasthemajornanoscopictooltointerpretthephysicaleffectsofthelow-energyhydrogenbombardmentontheCthinÞlms,i.e.,morphologicalandmechanicalpropertiesatthenanoscale.BothcontactandtappingAFMmodesaregenerallyappliedtostudythinlayersfromdifferentaspects.InthispapertorsionalandforcemodulationAFMmodeswereemployedtocharacterizethemechanicalstrengthofthedotriacontanethinÞlms,beforeandafterthehydrogenII.EXPERIMENTALDESIGNANDMETHODA0.3wt.%dotriacontane,CH,solutioninhexanewasspincoatedontoasilicon(100)wafercoveredbyanativeoxidelayer.Whenthecoatingprocesswasperformedonaspincoateratarotationalspeed5000rpmfor1min,thisconcentrationofdoctriacontanesolutioncanformÒfractal-likeislandÓmonolayerasshowninFig..Wehadcarriedouttopographyandphaseimagesatdifferentsitesonthesam-plesandconÞrmedthatunderthisconditiontherewasonlyoneperpendicularmonolayergrowingaboveparallellayers.ThiscanalsobeconcludedbasedonFigs..Sevensam-pleswithdifferentexposuretimestohydrogenbombardmentwerepreparedforexperiment.Totaltworunsofexperimentswerecarriedforstatisticalanalysis.Thelow-energyhydrogenbombardmentwasperformedwithahome-builtandlow-costmass-separatedlowenergyionbeamsystem,whichdeliversahydrogenbeamtothetargetsubstrateinahighvacuumchamber.Hydrogengaswiththepurityof99.8%wasusedinthebombardments.TABLEII.Bondenergyfortypicalchemicalbonds. ChemicalbondCÐHCÐCC Bondenergy(eV)4.33.62.7 TheÞnalbeamenergywascontrolledat10eVandthefullwidthathalfmaximumoftheenergydistributionofhydrogenmoleculeswaslessthan0.6eV.Theßuencewasvariedfromto1withtheexposuretimesvaryingfrom5to700s,andotherparametersofbombardmentwerekeptsame.Themorphologyandmechanicalpropertiesofthemono-layerswereinvestigatedinacommerciallyDimensionVwithNanoscopeVcontrollerfromVeecoInc.,whichwasequippedwithaquadrantphotodetectorfordetectingthecantileverAllAFMexperimentswereperformedinanambientenvironment.RectangularsiliconcantileversfromNanoscienceInstrumentswithnominalspringconstant0.03and3.0N/mwereusedincontactmodeforheightmeasure-mentsandtappingmodeforroughnessmeasurements,respec-tively.Adiamond-coatedcantileverwithcalibratedtipradiusof52nmandspringconstantof0.23N/mfromNanoscienceInstrumentswasusedinforcemodulation.ThecantileverusedintorsionalharmonichasaT-shapedtipandthetipra-diusis8nmascalibratedbythebluntmethodinSPIPsoft-ware(ImageMetrology);anditsspringconstantof2.76N/mwascalibratedbythermalßuctuationmethod.WecalibratedtheYoungÕsmodulusonpolystyreneÐlow-densitypolyethy-lene(PSÐLDPE)calibrationsample(Veeco,Inc.)before,dur-ing,andafterexperiments.TherewasnosigniÞcantchangeintheYoungÕsmodulusforPS(2GPa)andLDPE(0.1GPa).IthasprovedthatourAFMexperimentalresultsarereliableandconsistent. FIG.1.Schematicillustrationofthemechanismofhydrogenbombardmentprocessformolecularcross-linking:generationofcarbonradicalsduetothelow-energyhydrogenbombardment,formationofcross-linkamongmolec-ularchains,andchangeoftheheightofthethinÞlmduetothemolecular THEJOURNALOFCHEMICALPHYSICS,074704(2011)Studyofahydrogen-bombardmentprocessformolecularcross-linkingwithinthinÞlmsY.Liu,D.Q.Yang,H.-Y.Nie,W.M.Lau,andJ.YangDepartmentofMechanicalandMaterialsEngineering,UniversityofWesternOntario,London,Ontario,CanadaSurfaceScienceWestern,UniversityofWesternOntario,London,Ontario,Canada(Received16November2010;accepted24January2011;publishedonline16February2011) Electronicmail:jyang@eng.uwo.ca.)aslistedinTable.Theoretically,themax-imumenergytransferredfroma10eVhydrogenprojectiletoahydrogenatomofamolecularchainis10eV.Fromtheatomicpointofview,theeffectiveenergyleftaftercollision,betweenthehydrogenprojectileandtheHatom,canbreakaCÐHbondwhosebondenergyis4.3eVaslistedinTableHowever,ifthe10eVhydrogenprojectilecolloidswithacarbonatomofthemolecularchain,themaximumenergytransferredisonly2.8eV,whichisnotenoughtobreak