近代分析实验原理：第五课-透射电镜-1

五、透射电子显微镜-1

（TransmissionElectronMicroscope）近代分析实验原理（Introductionofmodernanalyticalmethods）12透射电子显微镜照片：Carbonnanotube31.电镜的结构（Instrumentation）1.1 光学显微镜（Opticalmicroscope）光的折射（Refraction）：反射定律折射定律n12相对折射率(refractionindex)n1（n2）绝对折射率（相对于真空）法线分界面入射光L折射光反射光4透镜（Lens）凸透镜成像图h0hipp´FF´f´111222333hi/h0=p’/p5RealimageFocallengththedistancebetweentheimageandlens6分辨率（Resolution）Resolutionreferstotheminimumdistancebetweentwopointsatwhichtheycanbevisiblydistinguishedastwopoints.thediffractionoflight艾里斑（Airydisk）refractiveindex折射率half-angleofthecone圆锥体oflightenteringtheobjectivelensnumericalaperture数值孔径R~λμ7?没有X射线透镜可以聚焦81.2 电子的波长（Thewavelengthofelectron）波粒二象性粒子性波动性路易·维克多·德布罗意1929年获诺贝尔物理学奖在贝尔实验室ClintonJosephDavisson和LesterHalbertGermer以低速电子束射向镍单晶获得电子经单晶衍射，测得电子的波长与德布罗意公式一致。在阿伯丁大学，G.P汤姆孙以高速电子穿过多晶金属箔获得类似X射线在多晶上产生的衍射花纹，确凿证实了电子的波动性1937年诺贝尔物理学奖9电子的波长（Thewavelengthofelectron）101.3 电子显微镜（Transmissionelectronmicroscope）11thermionicsource1.4 电子源（Electronsource）field-emissionsourceYourTEMwilluseathermionicsource(WorLaB6)orafield-emission(W)sourceandthetwocannotbeinterchanged.Field-emissionsourcesgivemoremonochromaticelectrons;thermionicsourcesarelessmonochromaticandgive‘whiter’electrons.currentdensityworkfunctionRefractory(highmeltingpoint)materialsorthosewithanexceptionallylowΦButthereisalimit,becausehighertemperaturesshortenthesourcelifethroughevaporationand/oroxidation.（ThermionicGuns）Fieldemissionguns（FEGs）anelectricfieldEisconsiderablyincreasedatsharppointslowersthework-functionbarrierElectronstotunnelout‘cold’FE.‘thermal’FEultra-highvacuum(UHV)conditions(<10-9Pa)atapoorervacuumbyheatingthetipWLaB612BrightnessBrightnessisthecurrentdensityperunitsolidangleofthesource.TruesourceTemporalCoherencyAmeasureofhowsimilarthewavepacketsare.theenergyspreadofthebeamelectrostaticlensesHowwelltheelectronwavesare‘instep’withoneanother.13SpatialCoherencyPerfectspatialcoherenceimplythattheelectronswereallemanatingfromthesamepointatthesource.smallersourcesbettercoherencytheeffectivesourcesizetheanglesubtendedbythesourceatthespecimenMakethesourcesize,d,smaller,e.g.,byusingaFEsource.Thisexplainswhyresearchintousingnanotubesaselectronsourcesisongoing.Useasmallerilluminationaperture,thusreducing.Ifyoursourcesizeislarge(e.g.,aWhairpin)decreasetheacceleratingvoltageandthusincrease.relatedtothesizeofthesource.14151.5 电子透镜（Electronlens）magneticlensesnotsusceptible易受影响tohigh-voltagebreakdown16ElectronRayPathsThroughMagneticFields17APERTURESANDDIAPHRAGMS光圈heavymetalsuchasMoorPtand25–50μmthick,butiftheirjobisalsotopreventX-raysfromhittingthespecimen样品theymaybeseveralmillimetersthick18REALLENSESANDTHEIRPROBLEMSImperfectionsofthelenslimittheresolutionofthemicroscope球差：Thefurtheroffaxistheelectronis,themorestronglyitisbentbacktowardtheaxis.apointobjectadiskoffinitesizeimageMostimportantObjectivelensSphericalAberrationDegrades降级thedetailthatwecanresolveinTEMimagesThemainlimitationfortheresolutionofTEM.sphericalaberrationcoefficientthemaximumangleofcollectionofthe(objective-lens)apertureManufacturerslike19InmodernTEM,Cscanbecorrected.theCEOSsystemtheNioncorrectorRoseandcolleaguesinGermanyKrivaneketal.20色差：‘colour’frequency,wavelength,orenergyoftheelectronsEnergylostwhentheelectronspassthroughthespecimen.TheobjectivelensbendselectronsoflowerenergymorestronglyandthuselectronsfromapointintheobjectonceagainareblurredtoformadiskintheGaussianimageplane.21像散：Astigmatismoccurswhentheelectronssenseanon-uniformmagneticfieldastheyspiralroundtheopticaxis.Notperfectlycylindricallysymmetricalsoft-ironpolepiecesofobjectivelensMachineerrorMicrostructuralinhomogeneitiesAperturesnotpreciselycenteredaroundtheaxisContaminationonapertureschargesupanddeflectsthebeamthemaximumdifferenceinfocusinducedbytheastigmatism22分辨率：TheoreticalResolution(Diffraction-LimitedResolution)Rayleigh’scriterionThePracticalResolutionDuetoSphericalAberrationTypically,thevalueforrminis0.25–0.3nmandthebesthigh-resolutioninstrumentshavermin0.1–15nm;1-ÅTEMsareaboutthebestavailablewithoutCscorrectionandabout0.07nmis(atthetimeofwriting)thebestreportedresolutionwithCscorrection.231.6 样品杆（Samplestage）CuCanhaveamorphousCthinfilmtosupportmuchsmallersample(forexample,Cnanotube)Single-tiltordoubletiltCuThereasonthespecimenholderissoimportantinTEMisthatyourspecimenmustinvariablybelocatedwithintheobjectivelensandtheaberrationsassociatedwiththeobjectivelensdeterminetheresolutionoftheTEM.2.3or3.05mmdiameter241.7 电镜系统（THETEMIMAGINGSYSTEM）25Diffractionmode:toseetheDP(diffractionpattern)youhavetoadjusttheimaging-systemlensessothattheBFP(back-focalplane)oftheobjectivelensactsastheobjectplanefortheintermediatelens.ThentheDPisprojectedontotheviewingscreen/CCDasshowninA.Imagemode:ifyouwanttolookatanimageinstead,youreadjusttheintermediatelenssothatitsobjectplaneistheimageplaneoftheobjectivelens.Thenanimageisprojectedontotheviewingscreen/CCD,asshowninB.261.8 电镜样品的制备（Specimenpreparation）TEMspecimenelectrontransparentrepresentativeofthematerialyouwanttostudy27Powdersample28Pre-ThinningHand-grindingjigforTEMspecimenpreparation.Adisc(S)isgluedtothecenterofthejigandtheguidering(G)controlstheamountofgrinding.reducingthespecimenthicknesstoabout0.1mmbeforefinalthinningto100nmthickness.guardringdiscGrindingBulksample29Dimpling30IonMilling31CROSS-SECTIONSPECIMENSEspeciallyfilm32focused-ionbeam(FIB)thecoatingofPtIonpolishing333435BetterspecimenBetterqualityofthedata361.9 电镜的局限性（LIMITATIONSOFTHETEM）high-resolutionimagingtechniquesmallpartofyourspecimenThehighertheresolutiontheworsethesamplingabilitiesComplementarytechniques:likeXRD,SEM,et