Transmission Electron Microscopy Skills:Analytical electron microscopy Lecture 12

8/8/2019 Transmission Electron Microscopy Skills:Analytical electron microscopy Lecture 12

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Analytical electronnalytical electronmicroscopyicroscopyLecture 12ecture 12


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OutlineutlineElastic and inelastic scattering of electronslastic and inelastic scattering of electronsEnergy dispersive spectroscopy (EDS)nergy dispersive spectroscopy (EDS)Electron energy loss spectroscopy (EELS)lectron energy loss spectroscopy (EELS)Energy filtered imaging (EFTEM)nergy filtered imaging (EFTEM)


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Scattering in the TEMcattering in the TEM

EDSEDS

EELSEELS

SEMSEMSEMSEM

AESAES

TEDTED TEMTEM

CLCLEBICEBIC


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Energy Dispersive Spectroscopynergy Dispersive Spectroscopy(EDS)EDS)


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Ionization cross sectionsonization cross sectionsWhat are the chances ofhat are the chances ofeach type of interaction?ach type of interaction?Plasmonslasmons most common,ost common,elastic common, ionizationlastic common, ionizationinteractions, less so.nteractions, less so.O, N, M, L, K, N, M, L, K

Makes senseMakes sense

But generally, we use K, L,ut generally, we use K, L,& MM


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X-ray productionay production

High energy electron scatters inner shell electron (E >igh energy electron scatters inner shell electron (E > Ec)Atom ionized. Can return to ground state by outer shell electrotom ionized. Can return to ground state by outer shell electron filling emptyfilling emptystate.tate.Accompanied by either xccompanied by either x-ray emission or Auger electron emission.ay emission or Auger electron emission.

Characteristic of the particular energy states of the atom!Characteristic of the particular energy states of the atom!


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Possible electron transitionsossible electron transitionsthat generate Xhat generate X-raysaysVery complex process withery complex process withmany possible transitionsany possible transitionsNot all these can beot all these can beobserved in TEMbserved in TEMCommonly observe:ommonly observe:

K, L, MK, L, M

Resolution ofesolution ofspectrometer, nature ofpectrometer, nature ofedges determines ifdges determines if , , ,, , , can be resolved.an be resolved.


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Fluorescence yieldluorescence yield

Key pointey point EDS cannot be used for elements below boronEDS cannot be used for elements below boron

Can be used for C, N, O but itan be used for C, N, O but its difficultdifficult Probably best to do EELS insteadProbably best to do EELS instead


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Continuous Backgroundontinuous BackgroundBremsstrahlungremsstrahlungElectron interaction withlectron interaction withnucleus results inucleus results incontinuous backgroundontinuous backgroundradiationadiationAt low energies,t low energies,attenuated by absorptionttenuated by absorptionby specimen & detectory specimen & detector


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Excited volumexcited volumeWhere do the xhere do the x-rays comeays comefrom in the specimen?rom in the specimen?Dependsepends TEM:EM:

200 to 300 kV normally200 to 300 kV normally

Scattering quite localized toScattering quite localized toelectron probeelectron probe

Not so in SEM of bulkot so in SEM of bulkmaterialsaterials Comparatively poor spatialComparatively poor spatial

resolutionresolution


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Example xxample x-ray spectra (EDS)ay spectra (EDS)Several examples ofeveral examples ofEDS spectraDS spectraNote:ote:

Relative # of countsRelative # of counts

Energy range of eachEnergy range of eachspectrumspectrum

Pure Geureure Gee Silica glassilica glassilica glass

Al film on Sil film on Sil film on Si

Organicrganicrganic

Graphiteraphiteraphite

Pure Alure Alure Al


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EDS ExampleDS Example


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Example xxample x-ray spectra (EDS)ay spectra (EDS)Several examples ofeveral examples ofEDS spectraDS spectraNote:ote:

Relative # of countsRelative # of counts

Energy range of eachEnergy range of eachspectrumspectrum

Pure Geureure Gee Silica glassilica glassilica glass

Al film on Sil film on Sil film on Si

Organicrganicrganic

Graphiteraphiteraphite

Pure Alure Alure Al


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EDS ExampleDS Example


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EDS geometryDS geometryX-rays produced from allays produced from allangles.ngles.The detector is a smallhe detector is a smallcrystal, and subtends onlyrystal, and subtends onlya small angle.small angle.EDS is not efficient.DS is not efficient.Specimen tilted to thepecimen tilted to theappropriate angle toppropriate angle tomaximize collectionaximize collectionefficiency.fficiency.


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X-ray spectrometersay spectrometersDetector: Li doped (drifted)etector: Li doped (drifted)Si reversed bias pi reversed bias p-i-ndeviceeviceX-rays enter, create eays enter, create e-hpairsairsNumber of eumber of e-h pairspairsgenerated directlyenerated directlyproportional to xroportional to x-rayayenergynergyThese are swept to thehese are swept to thecollector contactsollector contactsDone in series, but quickly.one in series, but quickly.Appears parallelppears parallel


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Spurious xpurious x-raysaysRecall, xecall, x-rays generated atays generated atall angles.ll angles.They are high energy.hey are high energy.Can excite further xan excite further x-rayayproduction from:roduction from:

Microscope partsMicroscope parts

Sample holderSample holder

Other regions of sampleOther regions of sample

Be careful!e careful! Do not modify microscopeDo not modify microscope

without working w/ radiationwithout working w/ radiationsafety technicianssafety technicians


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Quantificationuantificationbackground subtractionackground subtractionEDS is readilyDS is readily quantifiableuantifiable .

i.e. can determine thei.e. can determine the

relative proportion of onerelative proportion of oneelement to another.element to another.

Need to subtract theeed to subtract theBremsstrahlungremsstrahlungbackground to find realackground to find realnumber of counts.umber of counts.Most modern softwareost modern softwaredoes this for you.oes this for you.


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QuantificationuantificationCliffliff-Lorimerorimer methodethodRelatively simple ideaelatively simple idea - it works!t works!

But this means you have to have aut this means you have to have a standard.tandard.Standards:tandards:

Ideally, well characterized single phaseIdeally, well characterized single phase Thin sample that is representative, stableThin sample that is representative, stable

Want very high counts: 10Want very high counts: 1044 above backgroundabove background

kABB is a function of the EDS / TEM combination.s a function of the EDS / TEM combination. It isIt is NOTOT a constant (but can be theoretically estimated)a constant (but can be theoretically estimated) Best if experimentally determined for a given systemBest if experimentally determined for a given system

TEM + Detector + GeometryTEM + Detector + Geometry

CA

CB= kAB

IA

IB


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Electron Energy Loss Spectroscopylectron Energy Loss Spectroscopy(EELS)EELS)


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EELS SpectrometerELS SpectrometerElectrons lose energy duelectrons lose energy dueto inelastic scattering.o inelastic scattering.Use a magneticse a magneticspectrometer to bendpectrometer to bendelectrons.lectrons.Essentially acts as a prism,ssentially acts as a prism,therebyhereby coloringoloringelectrons by energy loss.lectrons by energy loss.


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What an EELS edge looks likehat an EELS edge looks likeWell, it kind of dependsell, it kind of dependsHow thick is your sample?ow thick is your sample?Plural scattering in thicklural scattering in thicksamples changesamples changesbackground, whichackground, whichchanges the edge.hanges the edge.Plasmon peaks can belasmon peaks can beconvolutedonvolutedSoo use ase a thinhin sampleample

< 1< 1 mfpmfp,, 500500 is greatis great


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Example EELS Spectraxample EELS SpectraThis is from boron nitridehis is from boron nitride(with some carbonwith some carboncontamination)ontamination)Note sharp onset of the Kote sharp onset of the K-edges.dges.Note theote the white lineshite lines in then theboron edge.oron edge.Broad range of energyroad range of energyHuge number of counts vs.uge number of counts vs.those in EDShose in EDS


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K and L Edgesand L Edges


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M and N edgesand N edges


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EELS vs. Band structureELS vs. Band structureEELS maps empty states aboveELS maps empty states abovethe Fermi level.he Fermi level.Lowow-loss region is fromoss region is from plasmonlasmonexcitation. Lies at energy levelxcitation. Lies at energy levelof conduction / valence bandsf conduction / valence bandsIonization results in electronsonization results in electronswhich are ejected from corehich are ejected from corestates into empty states abovetates into empty states abovethe Fermi level.he Fermi level.This is why these arehis is why these are edgesdges,and notnd not peakseaks - there is anhere is anonset.nset.


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Allowed transitionsllowed transitionsLike EDS, the observableike EDS, the observableedges can be directlydges can be directlyrelated to the electronelated to the electronshells.hells.You will observe K, L, Mou will observe K, L, MedgesdgesDepends on the atomicepends on the atomicweight of your material,eight of your material,which you see.hich you see.


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ELNES / EXELFSLNES / EXELFS

Near edge structure probes DOS in conduction band (ELNES)ear edge structure probes DOS in conduction band (ELNES) Increasing well understood, experiments becoming more routineIncreasing well understood, experiments becoming more routine

Extended structure probes chemical bonding (EXELFS)xtended structure probes chemical bonding (EXELFS) This is still largely a research topic in and among itselfThis is still largely a research topic in and among itself


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ELNESLNES - Fingerprintingingerprinting

450 455 460 465 470 475 480 485 490

Energy Loss (eV)

Ti Li L2,3,3 edge from trivalent Tidge from trivalent Ti2O3differs markedly from tetravalentiffers markedly from tetravalentcompounds TiOompounds TiO2 and CaTiOnd CaTiO3.

Ti2O3

TiO2

CaTiO3

Ti Li L2,3,3 edge from twist boundarydge from twist boundaryclosely matches edge structure oflosely matches edge structure ofTiOiO2 standard (Titandard (Ti4++)..

450 455 460 465 470 475 480 485 490

Energy Loss (eV)

TiO2

Twist Boundary

Ti L2,3 edge

Ti3+

Ti4+

Ti4+

Ti L2,3 edge

Data courtesy Seth Taylor, GE Central Research

Ti valence determination using EELSi valence determination using EELS

omparison of collection EDS /


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Comparison of collection EDS /EELSELSAs yous youve seen the numbere seen the numberof counts in EELS >> EDSf counts in EELS >> EDSFunction of collectionunction of collectionangle.ngle.EDS collects small angularDS collects small angulardistribution of all emitted xistribution of all emitted x-raysaysInelastic scattering ofnelastic scattering ofelectrons is largelylectrons is largelyforwardorward most collected.ost collected.


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Spatial resolutionpatial resolutionFor best performance:or best performance:

Thin sampleThin sample

FEG: high brightness,FEG: high brightness,coherentcoherent small probesmall probe

Probe size:robe size: Thermionic: 2 nmThermionic: 2 nm FEG: < 1FEG: < 1 demonstrated, 2demonstrated, 2

soon at NCEMsoon at NCEM

EDS: need to worry aboutDS: need to worry aboutbeam spreading, spuriouseam spreading, spuriousx-raysaysEELS: can be same size asELS: can be same size asproberobe


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Energy filtered imaging (EFTEM)nergy filtered imaging (EFTEM)

Additionally, can imagedditionally, can image inelasticallynelasticallyscattered electronscattered electronsSpatial distribution of chemistrypatial distribution of chemistrypossibleossible

High spatial resolutionHigh spatial resolution

Short collection timeShort collection time


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EFTEMFTEM - examplexampleFrom an Omega filteredrom an Omega filteredmachineachineGradedraded SiGeiGe multilayer,ultilayer,not Si layer in betweenot Si layer in between -only twonly two monolayersonolayers.Singleingle monlayeronlayer detectionetectionhas been demonstrated.as been demonstrated.Rapid acquisition, highapid acquisition, highcounts, quantifiable withounts, quantifiable withworkork

Pure Si


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Transmission Electron Microscopy Skills:Analytical electron microscopy Lecture 12

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