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1、Excitons–Types,EnergyTransfer•Wannierexciton•Charge-transferexciton•Frenkelexciton•ExcitonDiffusion•ExcitonEnergyTransfer(Förster,Dexter)Handout(forRecitationDiscusssion):J.-S.YangandT.M.Swager,J.Am.Chem.Soc.120,5321(1998)Q.ZhouandT.M.Swager,J.Am.Chem.Soc.117,125
2、93(1995)@MITFebruary27,2003–OrganicOptoelectronics-Lecture7ExcitonInsomeapplicationsitisusefultoconsiderelectronicexcitationasifaquasi-principle,capableofmigrating,wereinvolved.Thisistermedasexciton.Inorganicmaterialstwomodelsareused:thebandorwavemodel(lowtempera
3、ture,highcrystallineorder)andthehoppingmodel(highertemperature,lowcrystallineorderoramorphousstate).Energytransferinthehoppinglimitisidenticalwithenergymigration.CaptionfromIUPACCompendiumofChemicalTerminologycompiledbyAlanD.McNaughtandAndrewWilkinson(RoyalSociet
4、yofChemistry,Cambridge,UK).Wannierexciton(typicalofinorganicsemiconductors)Frenkelexciton(typicaloforganicmaterials)Excitons(boundelectron-holepairs)SEMICONDUCTORPICTUREMOLECULARPICTUREtreatexcitonsaschargelessparticlescapableofdiffusion,alsoviewthemasexcitedstat
5、esofthemoleculeGROUNDSTATEWANNIEREXCITONGROUNDSTATEFRENKELEXCITONbindingenergy~10meVradius~100Åbindingenergy~1eVradius~10ÅElectronicProcessesinOrganicCrystalsandPolymersbyM.PopeandC.E.SwenbergChargeTransfer(CT)Exciton(typicaloforganicmaterials)Wannier-MottExciton
6、sColumbicinteractionbetweentheholeandtheelectronisgivenbyEEX=-e2/∈rTheexcitonenergyisthenE=EION–EEX/n2,n=1,2,…EION–energyrequiredtoionizethemoleculen–excitonenergylevelEEX=13.6eVμ/m∈μ–reducedmass=memh/(me+mh)n=∞----------AdaptedfromElectronicProcessesinOrganicCry
7、stalsandPolymersbyM.PopeandC.E.SwenbergAnExampleofWannier-MottExcitonsexcitonprogressionfitstheexpressionν[cm-1]=17,508–800/n2correspondingtoμ=0.7and∈=10TheabsorptionspectrumofCu2Oat77K,showingtheexcitonlinescorrespondingtoseveralvaluesofthequantumnumbern.(FromBa
8、umeister1961).QuotedfromFigureI.D.28.ElectronicProcessesinOrganicCrystalsandPolymersbyM.PopeandC.E.SwenbergChargeTransferExcitonsThelowestCTexcitonstateintheab
