SERS物理增强机理精.docx

SERS物理增强机理精.docx

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SERS物理增强机理精

SimulationofRamanEnhancementinSERS-ActiveSubstrateswithAuLayerConsideringDifferentGeometryofNanoparticles

Hui-WenCheng1andYimingLi1,2,3,*

InstituteofCommunicationsEngineering,NationalChiaoTungUniversity,1001Ta-HsuehRoad,Hsinchu300,Taiwan2

DepartmentofElectricalEngineering,NationalChiaoTungUniversity,1001Ta-HsuehRoad,Hsinchu300,Taiwan

3

NationalNanoDeviceLaboratories,Hsinchu300,Taiwan

*

Tel:

+88635712121ext52974;Fax:

+88635726639;E-mail:

*****************.edu.tw

Abstract-Inthiswork,westudysurfaceenhancedRamanidentificationofRhdamine6G（R6G）areexamined.Thispaperspectroscopy（SERS）activesubstratesforthedetectionofisorganizedasfollows.InSec.II,weintroducethefabricationRhodamine6G.Toexaminetheelectromagneticenhancement,processandcomputationaltechniquefortheSERS-activewithdifferentshapeofnanoparticle,weapplythefinite-substrates.InSec.III,thelocalfieldenhancementsof

differencetimedomain（FDTD）algorithmtoanalyzethenanoparticlewithdifferentshapesarecalculatedbythree-structuresbysolvingasetofcoupledMaxwell’sequationsindimensional（3D）finite-differencetime-domain（FDTD）differentialform.Thefieldenhancementsareinvestigatedinthenumericalsimulation.Finally,wedrawtheconclusionsandvisibleregimewiththewavelengthof633nm.Inthesuggestthefuturework.

experimentalmeasurement,thesurfaceenhancedRamanscatteringsignalsfromthesurfaceofsubstrateswith12-hour

II.FABRICATIONANDCOMPUTATIONALTECHNIQUEhydrothermaltreatmentandwithouttreatmentareperformed

andcompared.Throughthethree-dimensional（3D）FDTDFortheflowoffabrication,asshowninFig.1,first,calculation,theenhancementswithdifferentshapeofbufferedoxideetchant（BOE）andstandardRCAcleaningare

nanoparticlearetestedandobtainedwhicharenanoparticle,carriedouttopreparecleansiliconsubstrates（Boron-doped

goldnanocageandgold/silveralloyforspherical,cubicandpyramidicalshapes.Theresultsshowthattheenhancementof

（i）sphericalandcubicshapescanbemuchimprovedbynanocage

andgold/siliveralloystructures.

Keywords-Surface-EnhancedRamanspectroscopy（SERS）,electromagneticenhancement,nanoparticle,goldnanocage,gold/silveralloy,finite-differencetime-domain,hydrothermallytreatedsubstrate.

（ii）

I.INTRODUCTION

Surface-enhancedRamanScattering（SERS）isoneofthecharacterizationtechniques,whichissensitivetotheenhancedelectromagneticfields[1-6].SERS-activesubstrateshaverecentlyattractedagreatdealofattentionforrapididentificationofchemicalandbacterialsamples[5-7].Thefabricatednanostructuresforbothbottom-upandtop-downapproacheshavebeenreported.And,thedegreeofRamanenhancementisstronglydependentonthemorphologyofformulatednanostructures[8].Recently,atop-downapproachforthefabricationofSERS-activesubstratewasproposed[9-12].However,theexpensivesubstrate,equipmentsandcomplicatedprocessareneeded.Therefore,alowcost,environmentfriendlyandsimplefabricationforSERS-activesubstrateswillbeofgreatinterestforbasicandclinicalresearchersaswellasforbiotechnologies.Inthisstudy,weexperimentallyandcomputationallystudythelocalfieldenhancementsofnanoparticlesonhydrothermallyroughenedSERS-activesubstrates,wheretheeffectsofshapeandsizeofAuparticlesandapplicationofthefabricatedsamplesin

（iii）

（iv）

Figure1.SchematicrepresentationforthefabricationofSERS-activesubstrate.First,siliconwaferswerecleanedbyBOEandstandardRCAcleaningprocedures.Then,Tifilmsweredepositedonthepre-cleanedsiliconwafersusingreactiveDCmagnetronsputteringsystem.Theasdepositedsampleswerecleavedandtreatedunderhydrothermalconditionsforvariousdurations.Subsequently,Auwasthermalevaporatedontothehydrothermallyroughenedsubstratesforsensing..

Figure.2（a）TheAFMimageoftitaniumthinfilmstreatedunderhydrothermalconditionfor12hourstreatmentduration.（b）Theplotofsimulatedsubstratewhichispartofrealsubstrate,wherethematrixofnanoparticlesis3x5duetoperiodicalpropertyofthesimulatedstructure.

p<100>）.Then,100-nm-thicktitaniumfilmsaredepositedonthepre-cleanedsiliconwafersusingreactiveDCmagnetronsputteringsystem.Theas-depositedsampleiscleavedinto0.5cmx1cmsquaresandrinsedwithethanol,andde-ionizedwater.Subsequently,thesampleisputintoa23mLTeflon-linedstainlesssteelautoclavefilledwith20mLdistilledwater,whichissealed,andheatedat200oCfor2,4,6,8,10,and12hours,respectively.Thenthetreatedsampleiscooledtoroomtemperaturenaturally,washedwithdistilledwaterforseveraltimes,anddriedwithastreamofcylinderair.Forexample,theimageofFig.2（a）showstheAFMimagesrepresenttitaniumthinfilmstreatedunderhydrothermalconditionsfor12hourstreatmentduration.

TheimageofFig.2（b）showstheplaneviewofthegold-coatednanoparticularstructure,wherethematrixofnanoparticlesis3x5duetoperiodicalpropertyofthesimulatedstructure.Numericalsimulationusinga3DFDTDmethodisconductedtoinvestigatethelocalfieldenhancementofsubstrate[13-15].TheMaxwell’scurlequationsinlinear,isotropic,nondispersive,lossymaterialsare

∂B

K

KK∂=−∇×E,

（1）∂EKKt∂t=−J1KK

ε+με

∇×B,

（2）∇⋅BK

=0,（3）

Figure3.ThesimulationprocedureofsolvingtheMaxwell’sequations.

∇K⋅EK=ρ

ε

（4）

whereEKandBK

arethevectorsofelectricandmagneticfields,respectively,ϵandμarepermeabilityandpermittivityandJKandρarethecurrentdensityvectorandchargedensity.Foragloballydefinedcurvilinearspace,Maxwell’sequationsareeasilyimplementedintheirdifferentialform,whereFaraday’slawisEq.

（1）andAmpere’slawisEq.

（2）.

TheFDTDmethodsolvesMaxwell’sequationsbyfirstdiscretizingallequationsviacentraldifferencesintimeandspace.Then,basedupona3DYee’smeshandcomponentsoftheelectricandmagneticfieldsatpoints,thediscretizedspacinginthex,y,andzdirectionsadoptedinoursimulationare|x|=0.01um,|y|=0.01umand|z|=0.01um,wherethetimestepΔtis0.0004andthetimedurationTis3inunitsoffemtoseconds.Thediscretizedequationsareiterativelysolvedinaleapfrogmanner,alternatingbetweencomputingtheEand

HfieldsatsubsequentΔt/2intervals,asshowninFig.3.Notably,weemploytheperfectlymatchedlayerasthesimulationdomainboundariesinwhichbothelectricandmagneticconductivitiesareintroducedinsuchawaythatwaveimpedanceremainsconstant,absorbingtheenergywithout

inducingreflections.III.RESULTSANDDISCUSSION

Inordertohavelesslightabsorption,thelargerscatteringofsubstrateisbettertoachievelargerfieldenhancement.Forchemicalsensing,thehydrothermallyroughenedsubstratesaretreatedwithaqueoussolutionsof10-4MR6G.TheThechemicalstructureofR6GisshowninFig.4（a）.Fig.4（b）showsthatthecharacteristicRamanvibrationalmodesofR6Gimmobilizedonthesubstratewithorwithouthydrothermaltreatment.Thesubstratewithhydrothermaltreatmentshows

Figure4.（a）ChemicalstructureofRhodamine6G（R6G）.ThemoleculeiswidelyusedforSERSmeasurements.（b）TheRamanspectraforR6G（10-4M）immobilizedonhydrothermallyuntreated（blue）andtreated（orange）

substrates.

AuAg

Figure5.Goldnanoparticle,goldnanocageandgold/silveralloy（fromlefttoright）forspherical,cubicandpyramidicalshapes,respectively.

largerintensitythanthatwithouthydrothermaltreatmentduetotheroughnessonthesurface[16].AccordingtotheBeckmann-Kirchhofftheory,theroughenedsurfacehaslargerscatteringonthesurfaceofsubstratesothattheintensitycanbeenhanced.ThroughusingtheFDTDsimulation,theevaluationofelectricfieldonthesubstratesiscarriedoutbythedirectinglightwithawavelengthof633nm.

Notablythenanosensoralsocanbefabricatedbyothersynthesismethodstoachievedifferentshapeofnanoparticles.

Figure6.Theplotofelectricfieldenhancementfactorversusdifferentsamples.

Figure.7.Thetopviewofelectricfielddistributionwithsphericalshapeof（a）Aunanoparticle,（b）Aunanocageand（c）Au/Agalloy,respectively.

Here,weconsidergoldnanoparticle,goldnanocageandgold/silveralloy（fromlefttoright）forspherical,cubicandpyramidicalshapes,asshowninFig5.Thesimulationresultsshowthattheelectricfield（Ex）enhancementofnanoparticlewithcubicshapeislargerthanthatwithsphericalandpyramidshapes,asshowninFig.6.Toimprovetheenhancement,thestructureisconsideredtofabricatebydifferentsynthesizedstructuresforspherical,cubicandpyramidicalshapes,respectively.ThesynthesizedstructuresareillustratedinFig.5,whicharethegoldnanocage（middleone）andgold/silveralloywithemptyandsilverinside,respectively.FromtheresultsofFig.6,theAu/Agalloyandgoldnanocageareadoptedforsphericalandcubicshapesbecausetheenhancementismuchimproved.Forpyramid,theEmetalalloyisalmostthesame.Theseresultscanbeexplainedxenhancementofnanocageorbydistributionofelectricfield.ThecorrespondingdistributionsofelectricfieldareshowninFig.7,8and9,respectively.Forsphericalshape,theenhancementofAunanoparticleislocally

Figure.8.Topviewsofelectricfielddistributionwiththecubicshapeof（a）

Aunanoparticle,（b）Aunanocage,（c）andAu/Agalloy,respectively.

Figure.9.Topviewsofelectricfielddistributionwiththepyramidicalshapeof（a）Aunanoparticle,（b）Aunanocage,and（c