transmission electron microscopy skills:overview of high-resolution tem & scanning tem lecture 11

8/8/2019 Transmission Electron Microscopy Skills:Overview of High-Resolution TEM & Scanning TEM Lecture 11

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Overview of Highverview of High-ResolutionesolutionTEM & Scanning TEMEM & Scanning TEMLecture 11ecture 11


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Highigh-resolution EMesolution EMgeneral ideaeneral idea

Sampleample(very thin!)very thin!)

Incidentncidentelectronlectronwaveave

Transmittedransmitted&Diffractediffractedwavesaves

Transmitted & diffracted waves each have a differentransmitted & diffracted waves each have a differentphasehaseResult is an interference patternesult is an interference pattern - ourur phase contrasthase contrast orrHREM imageREM image


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Image courtesy U. Dahmen, NCEM, LBNL


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Why are the phases different?hy are the phases different?Transmitted & diffracted waves travel differentransmitted & diffracted waves travel differentdistances in the crystalistances in the crystalEach diffracted wave will have its own phaseach diffracted wave will have its own phaseHighly simplified explanation: there isighly simplified explanation: there is muchuchmoreore to this, but this conveys the ideao this, but this conveys the idea

In fact, it is because each diffracted waven fact, it is because each diffracted waverepresents a different solution to the Schrepresents a different solution to the SchrdingeringerEqn. for the electron in the crystalqn. for the electron in the crystal

Resulting phase depends on theResulting phase depends on the strength & spacing of the

periodic potential of the lattice along a given direction inthe crystal


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So, appearso, appears simpleimple enoughnough (1)1) Calculate the phase differences for the differentalculate the phase differences for the differentdiffracted waves (not easy, but doiffracted waves (not easy, but do-able)ble)(2)2) Create an interference pattern from the overlap ofreate an interference pattern from the overlap ofthese phases in twohese phases in two-dimensionsimensions


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Not even thisot even this simpleimpleThe TEM has very poor lenseshe TEM has very poor lenses

Spherical aberration inpherical aberration inparticulararticularThis aberration causeshis aberration causesdiffracted waves to beiffracted waves to be phasehaseshiftedhifted by the objective lensy the objective lens Complex dependence onomplex dependence onwavelength, Cavelength, Cs,, diffractioniffractionvector and defocusector and defocus Magnitude of phase shift variesagnitude of phase shift varieswith distance from optic axisith distance from optic axis

And thus diffraction anglend thus diffraction angle Thus each diffracted wavehus each diffracted waveundergoes a different phasendergoes a different phase

shifthiftComplicates imageomplicates imageinterpretationnterpretationSpherical aberration


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Not even thisot even this simpleimpleThe TEM has very poor lenseshe TEM has very poor lenses

Spherical aberration inpherical aberration inparticulararticularThis aberration causeshis aberration causesdiffracted waves to beiffracted waves to be phasehaseshiftedhifted by the objective lensy the objective lens Complex dependence onomplex dependence onwavelength, Cavelength, Cs,, diffractioniffractionvector and defocusector and defocus Magnitude of phase shift variesagnitude of phase shift varieswith distance from optic axisith distance from optic axis

And thus diffraction anglend thus diffraction angle Thus each diffracted wavehus each diffracted waveundergoes a different phasendergoes a different phase

shifthiftComplicates imageomplicates imageinterpretationnterpretationSpherical aberration


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Returning to this pictureeturning to this pictureThis means that the phases of the diffracted waves arehis means that the phases of the diffracted waves arechanged by the objective lens focushanged by the objective lens focus


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Image courtesy C. Kisielowski, NCEM,

Thus, the image you get STRONGLY DEPENDS ON THE FOCUShus, the image you get STRONGLY DEPENDS ON THE FOCUSCONDITIONONDITIONA single HREM image The unscrambled exit wave


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So, the lens effectivelyo, the lens effectivelyscramblescrambles theheinformation embedded innformation embedded inthe exit wavehe exit waveThe amount of scramblehe amount of scrambledepends on the defocus &epends on the defocus &CsEmbodied in thembodied in the ContrastontrastTransfer Functionransfer FunctionDifferent diffracted wavesifferent diffracted wavesundergo differentndergo differentmodifications of theirodifications of theirspatial frequenciespatial frequencies

relativeinformation

transfer

Inverse d-spacing of thereflection


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Scherzercherzer defocusefocusBalance effect ofalance effect ofspherical aberrationpherical aberrationwith a specific value ofith a specific value ofnegative defocus.egative defocus.Scherzercherzer defocus:efocus:Scherzercherzer resolution:esolution:

f = 43C

s

12

RScherzer

= 11.51

Cs

1 43 4


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ExamplexampleSii3N4 (0001)0001)

From E.J. Kirkland, Advanced Computing in Electron Microscopy

t = -100 t = -150

Images depend on sample thicknessmages depend on sample thickness(different phase shift as electron wave travels a different distdifferent phase shift as electron wave travels a different distance)nce)


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ExamplexampleSii3N4 (0001)0001)

f = -700 f = -900

f = -1300 f = -1100

Images depend onmages depend onfocusocusDifferent relativeifferent relativephases shifts of thehases shifts of the

diffracted waves withiffracted waves withrespect to theespect to thetransmitted waveransmitted wave

Plane Ray Function Display


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Plane RayDiagram

Function Display

SampleSample

Objective lensObjective lens

Back focal planeBack focal plane

Image planeImage plane

Objective apertureObjective aperture

Scaledstructurefactors

Projectedpotential

Object

transmissionfunction

Diffractionpattern

Imageamplitude

Modelstructure

Projectedpotential

DiffractionIntensity

ImageIntensity

V(x,y)

(x,y)

(x,y)

Multislice

calculation

ContrastTransferFunction

Imagesimulation


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Summaryummary - HRTEMRTEMImage strongly depends on defocusmage strongly depends on defocusRelationship between image andelationship between image and atomictomicpositionsositions is not straightforwards not straightforwardUnderstanding of imaging conditions (vianderstanding of imaging conditions (viadefocus, sample thickness and microscopeefocus, sample thickness and microscopecalibrations) necessaryalibrations) necessaryThese provide inputs for image simulationhese provide inputs for image simulationProper match of the image with the calculationroper match of the image with the calculationrequired for true understanding of the imageequired for true understanding of the image


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Z-contrastImage

Spectrometer

AnnularDetector

Incident Probe

CCD-EELS

Detector 530 535 540 545 550 555

0

1000

2000

3000

4000

Counts

Energy (eV)

b2

a1

a2 d

2b

3d

1

b1

d3

c

Bulk

Boundary

1 nm

Resolution

Image

Properties

STEMTEMgeneral ideaeneral idea

Courtesy Nigel Browning


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Types of STEM imagesypes of STEM imagesBrightright-fieldield

Collect central beam with aollect central beam with asmall collection anglemall collection angle Contains elastic (Rutherford),ontains elastic (Rutherford),phonon,honon, plasmonlasmon and Comptonnd Compton

Lowow-angle annular dark fieldngle annular dark field Collection angle of 25ollection angle of 25 - 500milliradiansilliradians (mradrad) Mostly phonon scatterostly phonon scatter

Highigh-angle annular dark fieldngle annular dark field Collection angle of 50ollection angle of 50 - 25050mradrad Largely phonon scatter (TDS)argely phonon scatter (TDS)

Coherent BF-STEM image ofSrTiO3

Images from S.J. Pennycook

HAADF-STEM


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BFF-STEMTEM

Reciprocityeciprocity with HRTEM imagesith HRTEM imagesImages courtesy Dave Muller


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Lowow-angle annular dark fieldngle annular dark field(LAADF)LAADF)

Strain fields cause detrain fields cause de-channeling and scattering to small angleshanneling and scattering to small angles

500 mradrad 255 mradrad

Images courtesy Dave Muller


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Lowow-angle annular dark fieldngle annular dark field(LAADF)LAADF)Here contrast isere contrast iscorrelated with oxygenorrelated with oxygenvacanciesacancies

Images courtesy Dave Muller


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High angle annular dark fieldigh angle annular dark fieldNo contrast reversalso contrast reversalswith thicknessith thicknessDirectlyirectly interpretablenterpretableimagesmages

If you see a whitef you see a whiteblob, therelob, theres an atoman atomcolumn thereolumn there Caveat: the personaveat: the persontaking (& processing)aking (& processing)the image knew whathe image knew what

they were doinghey were doing Screw dislocation core in GaNImage courtesy Ilke Arlsan, Nigel Browning


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HAADF of dislocation coresAADF of dislocation cores

1

3 113[ ] 1

3 110[ ]+ 001[ ]

or

1

31123[ ]

1

31120[ ]+ 0001[ ]

0

400

800

1200

1600

2000

2400

2800

3200

400 410 420 430 440 450

Energy (eV)

Bulk

7-atom ring

8-4 atom rings

9-atom ring

Arslan, Bleloch, Stach and Browning, PRL 2005


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Summaryummary - STEMTEMScatter to high angle is depended on atomiccatter to high angle is depended on atomicweighteightScanning a focused probe, combined withcanning a focused probe, combined withcapturing this intensity can lead to an image thatapturing this intensity can lead to an image thatcaries sensitivity to atomic weight differencesaries sensitivity to atomic weight differencesCan form atomic resolution imagesan form atomic resolution images

Directly interpretableirectly interpretable Show atomically sensitive contrasthow atomically sensitive contrastCan be combined with EELS to givean be combined with EELS to givespectroscopic information atomic columnpectroscopic information atomic column-byy-atomic columntomic column

transmission electron microscopy skills:overview of high-resolution tem & scanning tem lecture 11

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