Chapter 8
Atomic Resolution -Scanning Transmission Electron Microscopy
(STEM)
Evolution of resolution in EM
CTEM STEM
parallel beam
CTEM
convergent beam
sample sample
condenser lens
objective lens
Diffraction
Image
8.1 Geometry of CTEM and STEM
8.2 Convergent Beam Diffraction
convergent beam electron diffraction
(CBED)
parallel beam electron diffraction (ED)
8.3 HAADF- STEM(High Angle Annual Dark Field)
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.
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
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
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
Aberration corrected TEM
Examples of STEM images
GaN
GaN
InGaN
Quantum Well
Examples of STEM images
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
La M edge Ti L edge
Mn L edge
Atomic resolution compositional and bonding maps
8.7 Atomic Resolution Spectrum Imaging
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
Micro-electronics
Flash device
8.8 STEM Tomography
Hydroxypyromorphite nanocrystallite whisker and pseudomorph formed on hydroxyapatite