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![Page 1: Electron Microscopy - Wikis09-10]_DOWNLOAD/4 tem ii.… · Electron Microscopy 4. TEM Basics: interactions, basic modes, sample preparation, Diffraction: elastic scattering theory,](https://reader033.vdocuments.site/reader033/viewer/2022060303/5f08e5537e708231d4243ed5/html5/thumbnails/1.jpg)
Autumn 2009 Experimental Methods in Physics Marco Cantoni
Electron Microscopy
4. TEM
Basics: interactions, basic modes,sample preparation,
Diffraction: elastic scattering theory, reciprocal space, diffraction pattern,
Laue zonesDiffraction phenomena
Image formation: contrasts,
Autumn 2009 Experimental Methods in Physics Marco Cantoni
Insert selected area aperture to choose region of interest
BaTiO3 nanocrystals (Psaltis lab)
Image formation
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Autumn 2009 Experimental Methods in Physics Marco Cantoni
Press “D” for diffraction on microscope console - alter strength of intermediate lens and focus diffraction pattern on to screen
Find cubic BaTiO3 aligned on [0 0 1] zone axis
Take selected-area diffraction pattern
Autumn 2009 Experimental Methods in Physics Marco Cantoni
• Laue zones
J.-P. Morniroli
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Autumn 2009 Experimental Methods in Physics Marco Cantoni
Ewald Sphere : Laue Zones (ZOLZ+FOLZ)
α=2.0mrads=0.2
ZOLZ6-fold
FOLZ3-fold
Source: P.A. Buffat
Autumn 2009 Experimental Methods in Physics Marco Cantoni
Amplitude of a diffracted beam:
ri: position of each atom => ri: = xi a + yi b + zi c
K = g: K = h a* + k b* + l c*
Define structure factor:
Intensity of reflection:
Intensity in the electron diffraction patternStructure factor
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Autumn 2009 Experimental Methods in Physics Marco Cantoni
Consider FCC lattice with lattice point coordinates:0,0,0; ½,½,0; ½,0,½; 0,½,½
Calculate structure factor for (0 1 0) plane (assume single atom motif):
=>
Forbidden reflections
Autumn 2009 Experimental Methods in Physics Marco Cantoni
Cu3Au - like FCC Au but with Cu atoms on face-centred sites. What happens to SADP if we gradually increase Z of Cu sites until that of Au (to obtain FCC Au)?
Patterns simulated using JEMS
Diffraction pattern on [0 0 1] zone axis:
Forbidden reflections
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Autumn 2009 Experimental Methods in Physics Marco Cantoni
Cu3Au - like FCC Au but with Cu atoms on face-centred sites. What happens to SADP if we gradually increase Z of Cu sites until that of Au (to obtain FCC Au)?
Patterns simulated using JEMS
Diffraction pattern on [0 0 1] zone axis:
Forbidden reflections
Autumn 2009 Experimental Methods in Physics Marco Cantoni
Reciprocal lattice of FCC is BCC and vice-versa
Extinction rulesFace-centred cubic: reflections with mixed odd, even h, k, l absent:
Body-centred cubic: reflections with mixed odd, even h, k, l absent:
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Autumn 2009 Experimental Methods in Physics Marco Cantoni
Symmetry informationZone axis SADPs have symmetry closely related to symmetry of crystal lattice
Example: FCC aluminium[0 0 1]
[1 1 0]
[1 1 1]
4-fold rotation axis
2-fold rotation axis
6-fold rotation axis - but [1 1 1] actually 3-fold axisNeed third dimension for true symmetry!
Autumn 2009 Experimental Methods in Physics Marco Cantoni
Twinning in diffractionExample: Co-Ni-Al shape memory FCC twins observed on [1 1 0] zone axis
Images provided by Barbora Bartová, CIME
(1 1 1) close-packed twin planes overlap in SADP
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Autumn 2009 Experimental Methods in Physics Marco Cantoni
Epitaxy and orientation relationshipsSADP excellent tool for studying
orientation relationships across interfaces
Example: Mn-doped ZnO on sapphire
Sapphire substrate Sapphire + film
Zone axes:[1 -1 0]ZnO // [0 -1 0]sapphire
Planes:c-planeZnO // c-planesapphire
Autumn 2009 Experimental Methods in Physics Marco Cantoni
Ring diffraction patternsIf selected area aperture selects numerous, randomly-oriented nanocrystals,
SADP consists of rings sampling all possible diffracting planes- like powder X-ray diffraction
Example: “needles” of contaminant cubic MnZnO3 - which XRD failed to observe!
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Autumn 2009 Experimental Methods in Physics Marco Cantoni
Ring diffraction patternsLarger crystals => more “spotty” patterns
Example: ZnO nanocrystals ~20 nm in diameter
Autumn 2009 Experimental Methods in Physics Marco Cantoni
Inelastic scattering event scatters electrons in all directions inside crystal
Cones have very large diameters => intersect diffraction plane as ~straight lines
Kikuchi linesSome scattered electrons in correct orientation for Bragg scattering => cone of scattering
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Autumn 2009 Experimental Methods in Physics Marco Cantoni
Kikuchi lines
Position of the Kikuchi line pairs of (excess and deficient) verysensitive to specimen orientation
Lower-index lattice planes => narrower pairs of lines
Autumn 2009 Experimental Methods in Physics Marco Cantoni
Convergent beam electron diffraction
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Autumn 2009 Experimental Methods in Physics Marco Cantoni
Instead of parallel illumination with selected-area aperture, CBED useshighly converged illumination to select a much smaller specimen region
Convergent beam electron diffraction
Small illuminated area => no thickness and orientation variations
There is dynamical scattering, but it is useful!
Can obtain disc and line patterns“packed” with information:
Autumn 2009 Experimental Methods in Physics Marco Cantoni
Shape, lattice parameters, defects, lattice planes
(K,Nb)O3Nano-rods
Bright field image
dark field image
Diffraction pattern
High-resolution image
Diffraction induced image contrastBF/DF Imaging, high-resolution TEM
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Autumn 2009 Experimental Methods in Physics Marco Cantoni
Bright Field ImagingDiffraction Contrast
Au (nano-) particles on C film
Autumn 2009 Experimental Methods in Physics Marco Cantoni
Bright Field / Dark Field
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Autumn 2009 Experimental Methods in Physics Marco Cantoni
Bend Contours /Bragg Contours
Bragg conditions change across the bent sample
Autumn 2009 Experimental Methods in Physics Marco Cantoni
Twin lamellae in PbTiO3
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Autumn 2009 Experimental Methods in Physics Marco Cantoni
Ni based Superalloy
BF image DF image
Autumn 2009 Experimental Methods in Physics Marco Cantoni
Thickness Fringesextinction contours
Wedge shaped crystal GaAs/AlxGa1-xAs
![Page 14: Electron Microscopy - Wikis09-10]_DOWNLOAD/4 tem ii.… · Electron Microscopy 4. TEM Basics: interactions, basic modes, sample preparation, Diffraction: elastic scattering theory,](https://reader033.vdocuments.site/reader033/viewer/2022060303/5f08e5537e708231d4243ed5/html5/thumbnails/14.jpg)
Autumn 2009 Experimental Methods in Physics Marco Cantoni
TEM dark field image g=(200)dyn
HRTEM zone axis [001] HRTEM zone axis [001]
Autumn 2009 Experimental Methods in Physics Marco Cantoni
the TEM in “high-resolution” mode
A high-resolution image is an interference image of the transmitted and the diffracted beams!
Diffracted electrons: coherent elastic scattering(the electrons have seen the crystal lattice )
The quality of the image depends on the optical system that makes the beams interfere
Bright Field High-Resolution
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Autumn 2009 Experimental Methods in Physics Marco Cantoni
High resolution
• The image should resemble the atomic structure of the sample !
• Atoms…?Thin samples: atom columns: sampleorientation (beam // atom columns)
• Contrast varies with samplethickness and defocus…!
• Comparison with simulation necessary !
Autumn 2009 Experimental Methods in Physics Marco Cantoni
High-resolution TEM
Pb
TiTi OO
PbPb
TiTi
• K. Ishizuka (1980) “Contrast Transfer of Crystal Images in TEM”, Ultramicroscopy 5,pages 55-65.
• L. Reimer (1993) “Transmission Electron Microscopy”, Springer Verlag, Berlin.• J.C.H. Spence (1988), “Experimental High Resolution Electron Microscopy”, Oxford
University Press, New York.
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Autumn 2009 Experimental Methods in Physics Marco Cantoni
ScherzerScherzer--defocus: defocus: ““blackblack--atomatom”” contrastcontrast
Autumn 2009 Experimental Methods in Physics Marco Cantoni
Cu
OC
uO
22
HgHg HgHg
““WhiteWhite--atomatom”” contrastcontrast