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1、AgeneralintroductiontoSTEMdetector1. BFdetectorItisplacedatthesamesiteastheaperturein BF-TEM anddetectstheintensityinthedirectbeamfromapointonthespecimen.2. ADFdetectorTheannulardarkfield(ADF)detectorisadiskwithaholeinitscenterwheretheBFdetectorisinstalled.TheADFdetectorusesscatteredelectr
2、onsforimageformation,similartothe DF modeinTEM. ThemeasuredcontrastmainlyresultsfromelectronsdiffractedincrystallineareasbutissuperimposedbyincoherentRutherfordscattering.3. HAADFdetectorThehigh-angleannulardarkfielddetectorisalsoadiskwithahole,butthediskdiameterandtheholearemuchlargerthan
3、intheADFdetector.Thus,itdetectselectronsthatarescatteredtohigheranglesandalmostonlyincoherentRutherfordscatteringcontributestotheimage.Thereby, Zcontrast isachieved.Inaddition,thereistheoptiontoinstallaSecondaryElectronDetector abovethesamplelikeinaSEMandtherebytoobtainadditionalmorphologi
4、calinformation.4.ThecentralDFandBFmodeinnormalTEMInnormalTEM,wealsomentionedtheDFandBRmode.Actually,theyaredifferentwithADFandBFmentionedherecompletely.InnormalTEM,thedirecttransmittedbeamisblockedbytheapertureandthediffractedbeamisallowedpassing.Whenthediffractedbeamhitsthescreenandformsa
5、imagewhichiscalledasDF.Otherwise,diffractedbeamisblockedwithapertureandtransmittedbeamformsBRimage.However,inSTEMmode,theyaretotallydifferentbecausethebeamonsampleiscone-shapedbeamratherthanparallellight.STEM模式和TEM模式的对比图Z-contrastactuallyusesanannulardetectorwhichcanonlycollectthescattered
6、electronsoutsidethebeamcone.Thatistosay,thetransmittedelectronsinsidebeamconeareinvolved.Thisisgoodthingforotherimagingmodebecausediffractioncontrastwasavoidedandtheimagingonlyhasdependenceonatomicstructure.ThisimagingmodeiscalledasSTEM-HAADF.Here,twoissuesarenotaddressedyet.OneiswhetherST
7、EM-BFandSTEM-ADFimaginginvolvesdiffractioncontrast.Personally,IthinkSTEM-ADFshouldinvolvephasecontrastandZcontrast.ForSTEM-BR,phasecontrastisgreatlyinvolved.However,incaseofHAADF,onlyZ-contrastworks.CameraLengthaffectstheInnerCollectionangel.InourTEM,whencamer
