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Chapter 8
Atomic Resolution -Scanning Transmission Electron Microscopy
(STEM)
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Evolution of resolution in EM
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CTEM STEM
parallel beam
CTEM
convergent beam
sample sample
condenser lens
objective lens
Diffraction
Image
8.1 Geometry of CTEM and STEM
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8.2 Convergent Beam Diffraction
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convergent beam electron diffraction
(CBED)
parallel beam electron diffraction (ED)
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8.3 HAADF- STEM(High Angle Annual Dark Field)
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H
thermal diffuse scattering (TDS)
Bragg scattering(elastic
scattering)
8.4 thermal diffuse scattering (TDS)
Channeling
Elastic
Inelastic
Thermal diffuse scattering (TDS), which is a signal used to form the image inHAADF-STEM and which was previously considered as “background intensity,”became a powerful source of information by using an HAADF detector.
phonon
each atom in the specimen vibrates thermally with a frequency of 1012–1013 Hz.
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8.5 Quantitative interpretation of HAADF-STEM imaging
I(R)=
=
* P2(R)
*1sGa 1sAs
O(R)
[110]
[100]
[110]
1.4ÅAsGa
Z=31 Z=33
I ∝ zα
Intensity IS ∝ CZ2Nt
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The intensity of atom columns in HAADF-STEM imaging depends on the average atomic number Z of individual atom columns (app. proportional to the square). Atom columns with higher average atomic number Z exhibit higher intensity. STEM transfer function has no reversals!
Ø Qualitative interpretation of HAADF-STEM images is relatively straightforward. Ø Quantitative interpretation of HAADF-STEM images, i.e. determination of the chemical composition of atom columns based on intensities, requires extensive image calculations and image matching.
STEM-Image
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Parallel illumination
Reversals in contrast transfer function
No image distortions
Point-to point resolution < 0.1nm
Limited chemical information
Well established simulation procedures
Convergent beam
No reversal in contrast transfer function
Possible image distortions
Point-to-point resolution ∼ 0.13nm (0.1 nm)
Limited structural information
Few available simulation programs
HRTEM HR HAADF-STEM
8.6 Comparison of HRTEM and HAADF
Phase Contrast Amplitude Contrast
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Aberration corrected TEM
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Examples of STEM images
GaN
GaN
InGaN
Quantum Well
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Examples of STEM images
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Collection anglesHR STEM HAADF images of CaTiO3 in the [001] zone axis taken at different detector inner and outer collection angles. The intensity ratio between Ti-O/Ca atomic columns is increasing with detector collection range.
125-250 mrad
0.5 nmintensi
ty
0
75
150
225
300
distance (nm)0.0 3.8 7.7 11.5 15.3
60 - 160 mrad 85 - 215 mrad 100 - 220 mrad
0.5 nm 0.5 nm0.5 nmintensity
0
75
150
225
300
distance (nm)0.0 3.8 7.7 11.5 15.3
inte
nsity
0
75
150
225
300
distance (nm)0.0 3.8 7.7 11.5 15.3
inte
nsity
0
75
150
225
300
distance (nm)0.0 3.8 7.7 11.5 15.3
Jožef Stefan InstituteSLONANO2007
October 10-12, 2007, IJS, Ljubljana
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La M edge Ti L edge
Mn L edge
Atomic resolution compositional and bonding maps
8.7 Atomic Resolution Spectrum Imaging
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NTHU
x
y
Inverse Randon Transformation Tomographic Reconstruction
x’y’
z
ϑ
z
x’
f(x,y,z)
Fourier Projection-Slice Theorem
1. Kinematic diffraction 2. Lens aberration can be ignored
8.8 STEM Tomography
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Micro-electronics
Flash device
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8.8 STEM Tomography
Hydroxypyromorphite nanocrystallite whisker and pseudomorph formed on hydroxyapatite
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