Electron Microscopy - Wikis09-10]_DOWNLOAD/4 tem ii.… · Electron Microscopy 4. TEM Basics:...

$: Electron Microscopy - Wikis09-10]_DOWNLOAD/4 tem ii.… · Electron Microscopy 4. TEM Basics: interactions, basic modes, sample preparation, Diffraction: elastic scattering theory,$
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,


Insert selected area aperture to choose region of interest

BaTiO3 nanocrystals (Psaltis lab)

Image formation


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


• Laue zones

J.-P. Morniroli


Ewald Sphere : Laue Zones (ZOLZ+FOLZ)

α=2.0mrads=0.2

ZOLZ6-fold

FOLZ3-fold

Source: P.A. Buffat


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


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


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:



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:



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:


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!


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


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


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!


Ring diffraction patternsLarger crystals => more “spotty” patterns

Example: ZnO nanocrystals ~20 nm in diameter


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


Kikuchi lines

Position of the Kikuchi line pairs of (excess and deficient) verysensitive to specimen orientation

Lower-index lattice planes => narrower pairs of lines


Convergent beam electron diffraction


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:


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


Bright Field ImagingDiffraction Contrast

Au (nano-) particles on C film


Bright Field / Dark Field


Bend Contours /Bragg Contours

Bragg conditions change across the bent sample


Twin lamellae in PbTiO3


Ni based Superalloy

BF image DF image


Thickness Fringesextinction contours

Wedge shaped crystal GaAs/AlxGa1-xAs


TEM dark field image g=(200)dyn

HRTEM zone axis [001] HRTEM zone axis [001]


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


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 !


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.


ScherzerScherzer--defocus: defocus: ““blackblack--atomatom”” contrastcontrast


Cu

OC

uO

22

HgHg HgHg

““WhiteWhite--atomatom”” contrastcontrast

Electron Microscopy - Wikis09-10]_DOWNLOAD/4 tem ii.… · Electron Microscopy 4. TEM Basics:...

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