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$: Imaging with Diffraction Contrast - CIME | EPFL Alexander: Imaging with Diffraction Contrast • Possible interactions of beam with specimen • Only some lead to contrast in TEM image$
Duncan Alexander: Imaging with Diffraction Contrast

Imaging with Diffraction Contrast

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Duncan AlexanderEPFL-CIME


Introduction

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When you study crystalline samples TEM image contrast is dominated by diffraction contrast.

An objective aperture to select either the direct beam for a bright-field image,or diffracted beam(s) for a dark-field image.

The TEM image effectively maps spatially the intensity in this beam.

Changes in intensity are given by:- general crystal orientation relative to the electron beam, and crystal phase

- local changes in crystal orientation from crystal defects and bending- sample thickness changes.

These effects combine to give the diffraction contrast that are your TEM data.

Inherent connection to diffraction pattern of your specimen.


• Possible interactions of beam with specimen

• Only some lead to contrast in TEM image

Specimen

Inc

ide

nt

be

am

Auger electrons

Backscattered electronsBSE

secondary electronsSE Characteristic

X-rays

visible light

“absorbed” electrons electron-hole pairs

elastically scatteredelectrons

dire

ct

be

am

inelasticallyscattered electrons

BremsstrahlungX-rays

1-100 nm

Diffraction contrast?



Specimen

Inc

ide

nt

be

amBackscattered electrons

BSE

“absorbed” electrons


1-100 nm

dire

ct

be

am


• Biological specimen with heavy metal stain: majority of contrast from “absorption of electrons.

• Sample of interest biologically, but physics of scattering are boring.


• Materials science specimen: CVD-grown ZnO thin film. Elastic scattering of the electrons (i.e. diffraction) by crystal is major contribution to contrast.

• Changes in crystal orientation from grain to grain or within grains at defects change the diffraction condition and hence the contrast.


Specimen

Inc

ide

nt

be

amBackscattered electrons

BSE

“absorbed” electrons


1-100 nm

dire

ct

be

am



TEM imaging/diffraction modes recap

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Diffractionmode

Imagemode


Bright-field image vs dark-field image

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Bright-field image=

select direct beamwith objective aperture

Dark-field image=

select diffracted beamwith objective aperture


Nanocrystalline sample image/diffraction

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Image modeDiffraction mode

Bright field image setup - select direct beam with objective aperture

Contrast from different crystals according to diffraction condition



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Image mode

Dark field image setup - select some transmitted beams with objective aperture

Diffraction mode

Only crystals diffracting strongly into objective aperture give bright contrast in image



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Image mode

Dark field image setup - select some transmitted beams with objective aperture

Diffraction mode

Only crystals diffracting strongly into objective aperture give bright contrast in image



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BF

DF A

DF B

DF images allow us to pick out individual grains


Diffraction contrast on/off zone axis

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In bright-field imaging, zone axis condition => more scattering to diffracted beamsTherefore intensity in direct beam goes down and bright-field image has strong contrast


Diffraction contrast on/off zone axis

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In bright-field imaging, zone axis condition => more scattering to diffracted beamsTherefore intensity in direct beam goes down and bright-field image has strong contrast

Example: GaN nanowire

Off zone axis

On zone axis

CBED SADP BF

CBED SADP BF

Duncan Alexander: Imaging with Diffraction Contrast 14

Diffraction contrast on/off zone axisIn bright-field imaging, zone axis condition => more scattering to diffracted beams

Therefore intensity in direct beam goes down and bright-field image has strong contrast

Example: GaN nanowire

BF off zone axis BF on zone axis


Image delocalization

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TEM image with no objective aperture. Image formed from direct beam and diffracted beams. Dark-field images from diffracted beams delocalize from bright-field image of

direct beam. Gives shadow images that move with objective focus (mostly result from Cs).


Image delocalization

Image of same nanowires but with objective aperture to make bright-field image. No diffracted beams => no shadow images.This is how you should take your TEM data!


Image delocalizationTEM (multi-beam) image Bright-field image

Direction of delocalization depends on direction of diffracted beam in diffraction pattern


Dark-field imaging:displaced aperture vs beam tilt

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Bend contours

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From Williams & Carter Transmission Electron Microscopy


Bend contoursExample: electropolished Ni3Al superalloy

Sample at lower magnification.


Bend contoursExample: electropolished Ni3Al superalloy

Bright-field images. Same region at different sample tilts.See bend contours interacting with dislocations.


Bend contoursExample: mechanically polished and ion beam milled silicon


Thickness fringes

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Thickness fringesDynamical scattering in the dark-field image –

=> Intensity zero for thicknesses t = nξg (integer n)

Composition changes in quantum wells=> extinction at different thickness compared to substrate

See effect as dark “thickness fringes” on wedge-shaped sample:


Phase contrast

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Example: electropolished Ni3Al superalloySimple cubic γ’ precipitates in FCC γ matrix

SADP simple cubic γ’ SADP FCC γ

0 0 01 0 0

2 0 0

0 0 0

2 0 0



SADP simple cubic γ’ Bright-field image

0 0 01 0 0

2 0 0

Phase contrast



SADP simple cubic γ’ Dark-field image g = 1 0 0

0 0 01 0 0

2 0 0

Phase contrast



SADP simple cubic γ’ Dark-field image g = 2 0 0

0 0 01 0 0

2 0 0

Phase contrast


Crystal defects: dislocations

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Burgers vector for edge (top right) and screw (bottom right) dislocations


Crystal defects: dislocationsLocal bending of crystal planes around the dislocation change their diffraction condition

This produces a contrast in the image => g.b analysis for Burgers vector

From Williams & Carter Transmission Electron Microscopy


Crystal defects: dislocations - g.b analysis

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Invisibility criterion!


Crystal defects: dislocations - g.b analysis

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Sakai et al. APL 71 (1997) 2259

Example: threading dislocations in GaN

From visibility in for images a) to c) and near invisibility in d), dislocation B found to have:

b = ±1/3[2 -1 -1 3]or b = ±1/3[1 -2 -1 3]


Planar defects – stacking faults

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Similarly to Burgers vector analysis, stacking faults with displacement vector Rare invisible for g.R = 0

Example: analysis of basal plane stacking faults in ZnO

SADP on [1 1 00] zone axis Bright-field g = 1 -1 0 0 Dark-field g = 1 -1 0 0

g g

g parallel to R: stacking faults visible


Planar defects – stacking faults

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Similarly to Burgers vector analysis, stacking faults with displacement vector Rare invisible for g.R = 0

Example: analysis of basal plane stacking faults in ZnO

SADP on [1 1 00] zone axis Bright-field g = 0 0 0 2 Dark-field g = 0 0 0 2

g perpendicular to R: stacking faults invisible

g g


Amorphous? Or low diffraction contrast?

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Bright-field image of Al-Cr-N-O thin film

• Some regions show clear diffraction contrast, so obviously crystalline.

• Other regions did not. Combined with contrast from probable amorphous surface layer, looked amorphous.

• However CBED proved crystalline, just in low diffraction contrast condition.


Phase contrast, grain boundaries, bend contours, dislocations in one image

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Summary

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When imaging crystalline samples in the TEM, any change in the crystal will change the elastic scattering diffraction condition and hence intensity and contrast in the resulting image. This can be

from:- crystal orientation relative to the electron beam and deviations from bending

- defects in the crystal- changes in phase (e.g. precipitates)

- changes in thickness (dynamical scattering).

Using an objective aperture to select the direct beam (bright-field image) or diffracted beam (dark-field image) combined with carefully selected diffraction conditions gives many possibilities for

analysing and understanding your specimen.

Even without pursuing this you still have to learn to live with diffraction contrast and how it can unhelpfully affect your TEM data (e.g. bend contours). In this way TEM is not like other types of

microscopy!

No objective aperture => ill-defined diffraction condition and image delocalization!

imaging with diffraction contrast - cime | epfl alexander: imaging with diffraction contrast •...

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