扫描透射电子显微镜模式分析Word格式文档下载.docx

扫描透射电子显微镜模式分析Word格式文档下载.docx

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BFdetector

Itisplacedatthesamesiteastheaperturein 

BF-TEM 

anddetectstheintensityinthedirectbeamfromapointonthespecimen.

2. 

ADFdetector

Theannulardarkfield（ADF）detectorisadiskwithaholeinitscenterwheretheBFdetectorisinstalled.TheADFdetectorusesscatteredelectronsforimageformation,similartothe 

DF 

modeinTEM. 

ThemeasuredcontrastmainlyresultsfromelectronsdiffractedincrystallineareasbutissuperimposedbyincoherentRutherfordscattering.

3. 

HAADFdetector

Thehigh-angleannulardarkfielddetectorisalsoadiskwithahole,butthediskdiameterandtheholearemuchlargerthanintheADFdetector.Thus,itdetectselectronsthatarescatteredtohigheranglesandalmostonlyincoherentRutherfordscatteringcontributestotheimage.Thereby, 

Zcontrast 

isachieved.

Inaddition,thereistheoptiontoinstallaSecondaryElectronDetector 

abovethesamplelikeinaSEMandtherebytoobtainadditionalmorphologicalinformation.

4.ThecentralDFandBFmodeinnormalTEM

InnormalTEM,wealsomentionedtheDFandBRmode.Actually,theyaredifferentwithADFandBFmentionedherecompletely.InnormalTEM,thedirecttransmittedbeamisblockedbytheapertureandthediffractedbeamisallowedpassing.WhenthediffractedbeamhitsthescreenandformsaimagewhichiscalledasDF.Otherwise,diffractedbeamisblockedwithapertureandtransmittedbeamformsBRimage.

However,inSTEMmode,theyaretotallydifferentbecausethebeamonsampleiscone-shapedbeamratherthanparallellight.

STEM模式和TEM模式的对比图

Z-contrastactuallyusesanannulardetectorwhichcanonlycollectthescatteredelectronsoutsidethebeamcone.Thatistosay,thetransmittedelectronsinsidebeamconeareinvolved.Thisisgoodthingforotherimagingmodebecausediffractioncontrastwasavoidedandtheimagingonlyhasdependenceonatomicstructure.ThisimagingmodeiscalledasSTEM-HAADF.Here,twoissuesarenotaddressedyet.OneiswhetherSTEM-BFandSTEM-ADFimaginginvolvesdiffractioncontrast.Personally,IthinkSTEM-ADFshouldinvolvephasecontrastandZcontrast.ForSTEM-BR,phasecontrastisgreatlyinvolved.However,incaseofHAADF,onlyZ-contrastworks.

CameraLengthaffectstheInnerCollectionangel.InourTEM,whencameralengthis11cm,theinnercollectionangelis60mrad.Oncethecameralengthreducingto5cm,theinnercollectionangelis110mrad.Therefore,thecollectionangleiscontrolledbythecameralength.Whoaffectsthecameralength?

Accordingtoequation（L=f*Mp*Ml）wherefisthefocaldistanceofobjectivelens（fisrelatedwiththeexcitedcurrent,accelerationvoltage,thenumberofcoil）,MpandMlarethemagnificationofprojectivelensandintermediatelensrespectively.Experimentally,thefcanbechangedsignificantlythroughadjustingtheexcitedcurrent.Besides,inordertochangethecameralength,you

alsocanchangetheexcitedcurrentofprojectivelensorintermediatelens.Wheninnercollectionangleisbiggerthan100mrad,theRutherfordscatteringdominatestheimaging.

TheinneranglesforHAADFdetectorsareatleastthreetimestheangleofelectron-probe-formingaperture.InmanycasesofHAADFimaging,theannulardetectionanglewaspracticallysettobe60-170mradduetothelimitationofcombinationsbetweenaperturesizesandcameralengths.AboveimageshowsthepositionsofthedetectorswhichcanbeinstalledinaSTEMsystem.Dependingonthescatteringangleofthetransmittedelectrons,varioussignalscanbedetectedasafunctionofthepositionofthescanningprobe:

BF（bright-field）-STEM,DF（dark-field）-STEMorHAADF（highangleannulardarkfield）-STEM.

TheDFdetectorsareannularlyshapedtomaximizethecollectionefficiencyandtherangeofthecollectedscatteringanglescanbeadjustedthroughthemagnificationoftheintermediatelenses.

FEIHAADFdetector

Chapter1.IntroductiontoSTEM

SomeTEMshavescanningcoilswhichallowthemtobeusedasascanningtransmissionelectronmicroscope（STEM）.InSTEM,aconicalelectronbeamisfocusedthroughthespecimenbyalensinfrontofthespecimen（C2andtheobjectivelenspre-fieldwhichisalsocalledminilens）,asshownbelow.

TheopticalconfigurationisroughlythesameassimplyforminganimageofthefilamentbyfocussingC2thevery 

firstexperimentwedidintheelectronmicroscope.Infact,ifyourmicroscopehasatwinobjectivelensincludingpre-fieldandbackfield,theopticsarealittlebitmorecomplicated,butletsstartthinkingaboutthingsassimplyaspossible.

Thesmallbeamcross-overatthespecimenplaneisusuallycalledanElectron 

Probe.Imagescanbeformedbymovingtheprobeacrossthespecimen（bythedouble-deflectionshiftcoils）anddetectingthetransmittedelectrons,whichareeit