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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.117,12593(1995)@MITFebruary27,2003–Organic
2、Optoelectronics-Lecture7ExcitonInsomeapplicationsitisusefultoconsiderelectronicexcitationasifaquasi-principle,capableofmigrating,wereinvolved.Thisistermedasexciton.Inorganicmaterialstwomodelsareused:thebandorwavemodel(lowtemperature,highcrystallineorder)andthehoppingmodel(highertemperature,lowcrys
3、tallineorderoramorphousstate).Energytransferinthehoppinglimitisidenticalwithenergymigration.CaptionfromIUPACCompendiumofChemicalTerminologycompiledbyAlanD.McNaughtandAndrewWilkinson(RoyalSocietyofChemistry,Cambridge,UK).Wannierexciton(typicalofinorganicsemiconductors)Frenkelexciton(typicaloforgani
4、cmaterials)Excitons(boundelectron-holepairs)SEMICONDUCTORPICTUREMOLECULARPICTUREtreatexcitonsaschargelessparticlescapableofdiffusion,alsoviewthemasexcitedstatesofthemoleculeGROUNDSTATEWANNIEREXCITONGROUNDSTATEFRENKELEXCITONbindingenergy~10meVradius~100Åbindingenergy~1eVradius~10ÅElectronicProcesse
5、sinOrganicCrystalsandPolymersbyM.PopeandC.E.SwenbergChargeTransfer(CT)Exciton(typicaloforganicmaterials)Wannier-MottExcitonsColumbicinteractionbetweentheholeandtheelectronisgivenbyEEX=-e2/∈rTheexcitonenergyisthenE=EION–EEX/n2,n=1,2,…EION–energyrequiredtoionizethemoleculen–excitonenergylevelEEX=13.
6、6eVμ/m∈μ–reducedmass=memh/(me+mh)n=∞----------AdaptedfromElectronicProcessesinOrganicCrystalsandPolymersbyM.PopeandC.E.SwenbergAnExampleofWannier-MottExcitonsexcitonprogressionfitstheexpressionν[cm-1]=17,508–800/n2correspondingtoμ=0.7and∈=10TheabsorptionspectrumofCu2Oat77K,showingtheexcitonlinesco
7、rrespondingtoseveralvaluesofthequantumnumbern.(FromBaumeister1961).QuotedfromFigureI.D.28.ElectronicProcessesinOrganicCrystalsandPolymersbyM.PopeandC.E.SwenbergChargeTransferExcitonsThelowestCTexcitonstateintheab
