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