Transmission Electron Microscopy Skills:Diffraction Contrast Imaging Lecture 10

8/8/2019 Transmission Electron Microscopy Skills:Diffraction Contrast Imaging Lecture 10

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(an embarrassingly short description ofan embarrassingly short description of )Diffraction Contrast Imagingiffraction Contrast ImagingLecture 10ecture 10


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Categories of imagingategories of imagingMassass-thickness contrasthickness contrast Contrast appears in the image due to differences in the inherentontrast appears in the image due to differences in the inherent

scattering from the samplecattering from the sample Z-contrast imaging (a highontrast imaging (a high-resolution version of same)esolution version of same)

Diffraction contrastiffraction contrast In crystalline materials, diffraction occurs.n crystalline materials, diffraction occurs. Utilizing the objective aperture, either the direct (tilizing the objective aperture, either the direct (unn-diffractediffracted)beam or one of the diffract beams is selected to form the imageeam or one of the diffract beams is selected to form the image

Results in crystallographic information being conveyed in the imesults in crystallographic information being conveyed in the imagegePhase contrast (hase contrast (highigh-resolutionesolution imaging)maging) Direct and diffracted beams undergo phase shifts in the materialirect and diffracted beams undergo phase shifts in the material An image if formed by recombining all beams and observing then image if formed by recombining all beams and observing the

resulting interference patternesulting interference pattern


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BF image formedfrom the direct beam

Displaced apertureDF image formed

with off-axis

scattered beam

CTF image where theincident beam is

tilted so that the

scattered beam

remain on axis

Bright Field / Dark Field Imagingright Field / Dark Field Imaging


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Two beam imagingwo beam imaging

Primary imaging mode is therimary imaging mode is the twowo-beameam conditiononditionAllows readily interpretable imagesllows readily interpretable images

Bright Field Image Dark Field Image Weak-Beam DarkField Image


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Twowo-beam conditionseam conditionsExamples of twoxamples of two-beameamconditions near the 011onditions near the 011Zone Axisone Axis


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Twowo-beam conditionseam conditionsExamples of twoxamples of two-beameamconditions near the 011onditions near the 011Zone Axisone Axis


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Kinematical Diffractioninematical DiffractionAssumptionsssumptions Only two diffracted beamsnly two diffracted beams

considered, direct and oneonsidered, direct and onediffractediffracted Intensity of diffracted beam isntensity of diffracted beam issmallmall Single scattering eventingle scattering event Noo absorptionbsorption i.e.:.e.: IIdiffractediffracted


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Kinematicinematic vs. Dynamicals. DynamicalCan be used to explain (to first approximation) the originan be used to explain (to first approximation) the originmuch of much what we see in the microscopeuch of much what we see in the microscopeHowever, cannot be used to accurately calculateowever, cannot be used to accurately calculateintensity relationshipsntensity relationshipsUse dynamical theory:se dynamical theory: Considers multiple beamsonsiders multiple beams Intensity in one or more diffracted beams can be large inntensity in one or more diffracted beams can be large incomparison with transmitted beamomparison with transmitted beam Multiple scattering allowedultiple scattering allowed Absorptionbsorption (loss of electrons) allowedloss of electrons) allowed Can explain intensity accuratelyan explain intensity accurately Very complicated to utilize practicallyery complicated to utilize practically


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Kinematicinematic diffraction in perfectiffraction in perfectcrystalsrystalsThickness contours:hickness contours: At exact Bragg condition (s = 0),

the intensity of the direct anddiffracted beam oscillate in acomplementary way.

For a wedge specimen, theseparation of the fringes in the

image is determined by the angle

of the wedge and the extinction

distance, g.

Thus, the image may appear to

be black or white depending on

the thickness of the specimenwhere you are observing it


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111111

hole

aa

Representative bright field (a),

and dark field (b) micrograph

showing thickness fringes

100 nm100 nm

bb 111111

Kinematicinematic diffraction in perfectiffraction in perfectcrystalsrystals

Courtesy V.R. Radmilovic


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Kinematicinematic diffraction in perfectiffraction in perfectcrystalsrystalsThickness contours:hickness contours: At exact Bragg condition (s = 0),

the intensity of the direct anddiffracted beam oscillate in acomplementary way.

For a wedge specimen, the

separation of the fringes in the

image is determined by the angle

of the wedge and the extinction

distance, g.

Thus, the image may appear to

be black or white depending on

the thickness of the specimenwhere you are observing it


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111111

hole

aa




100 nm100 nm

bb 111111




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Translation / rotationranslation / rotationbetween two crystalsetween two crystalsyields fringes in imagesields fringes in images Stacking faultstacking faults Grain boundariesrain boundaries Translation boundariesranslation boundaries Phase boundarieshase boundaries Surfaces (if stepped)urfaces (if stepped)

Electron wave has alectron wave has adifferent amplitude &ifferent amplitude &phase in each crystalhase in each crystal Results in a fringe patternesults in a fringe pattern

Kinematicinematic diffraction in imperfectiffraction in imperfectcrystalsrystals


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gR causes the contrast and fordislocations, R changes with z.g

b = n; if we know g and we determine n,then we know b.

Kinematicinematic diffraction in imperfectiffraction in imperfectcrystalsrystalsStrain field of dislocation causes localdiffraction differencesSeen as line of contrast in the image (iforiented correctly )


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0.5 m

If gb = 0, then you wont see any contrast because the diffracting

planes are then parallel to R.

This is termed the invisibility criterion.




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Weak-beam dark-field imaging


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0.5 m







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0.5 m







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gR causes the contrast and fordislocations, R changes with z.g

b = n; if we know g and we determine n,then we know b.

Kinematicinematic diffraction in imperfectiffraction in imperfectcrystalsrystalsStrain field of dislocation causes localdiffraction differencesSeen as line of contrast in the image (iforiented correctly )


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100 nm100 nm100 nm100 nm

001Al

110 Al

001001AlAl

110110 AlAl

Dark field images of new Al-2at.%Cu-1at.%Si-1 at.%Ge alloy aged 1h and 3hrs

(right) at 190oC; imaged using 112 reflection.

1h1h 3h3h

Dark-field imaging



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Diffraction condition

g

2

= t

g

2

sin2(tseff )

tseff( )2

seff = s2 + 1

g2

eff = g

w2 + 1



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Bright field image Dark field image

Al2O3

GaN

Weak beamdark field image


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111111

hole

aa




100 nm100 nm

bb 111111




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Bend Contours

The origin of bend contours shown for

a foil symmetrically bent on either side

of the Bragg conditions. When thehkl

planes are in the Bragg condition, the

reflection G is excited.

Bend ContoursBend Contours


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[100]

crystal

orientationEach diffracted

plane producestwo bend

contours

Kinematicinematic diffraction in perfectiffraction in perfectcrystalsrystalsBend ContoursBend Contours

[103]

crystal

orientation


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Categories of imagingategories of imagingMassass-thickness contrasthickness contrast Contrast appears in the image due to differences in the inherentontrast appears in the image due to differences in the inherentscattering from the samplecattering from the sample Z-contrast imaging (a highontrast imaging (a high-resolution version of same)esolution version of same)

Diffraction contrastiffraction contrast In crystalline materials, diffraction occurs.n crystalline materials, diffraction occurs. Utilizing the objective aperture, either the direct (tilizing the objective aperture, either the direct (unn-diffractediffracted)beam or one of the diffract beams is selected to form the imageeam or one of the diffract beams is selected to form the image

Results in crystallographic information being conveyed in the imesults in crystallographic information being conveyed in the imagegePhase contrast (hase contrast (highigh-resolutionesolution imaging)maging) Direct and diffracted beams undergo phase shifts in the materialirect and diffracted beams undergo phase shifts in the material An image if formed by recombining all beams and observing then image if formed by recombining all beams and observing the

resulting interference patternesulting interference pattern

Transmission Electron Microscopy Skills:Diffraction Contrast Imaging Lecture 10

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