1. Contrast in TEM and STEM
Amplitude and Phase contrast

4. TEM
STEM
STEM:
We do not use lenses to form the image (cromatic aberration is absent)
Now in a TEM we can’t physically put the detector in the BFP of the objective lens to form a STEM image, because it would interfere with the objective aperture. Therefore, we usually insert the detector into a conjugate plane to the stationary DP (Figure 9.19B). So when you form a STEM image in a TEM, you operate the TEM in diffraction mode and insert a detector into the viewing chamber of the TEM, either above or below (in which case you raise) the screen.
STEM images are not magnefied by lenses (by scan dimensions)

5. TEM
STEM

6. Amplitude contrast and Phase-contrast images
The electron wave can change both its amplitude and phase as it traverses the specimen Gives rise to contrast Si
SiO2
Al2O3 Ag
We select imaging conditions so
that one of them dominates.

7. Size of objective aperture Bright Field (BF), Dark Field (DF) and High Resolution EM (HREM)
HREM image
BF image
Objective
aperture
DF image
Why is it called diffraction contrast?
The contrast in the images are due to the fact that some regions in the specimen scatter the incident electron beam more than other parts of the specimen.
- Where is the objective apperture located? Back focal plane
Amplitude/Diffraction contrast
Phase contrast

8. This is the most important aperture in the TEM
This is the most important aperture in the TEM. When inserted, its size controls the collection angle (b); hence, it determines the effect of the aberrations of the most important lens in the instrument and thus directly influences the resolution.

9. BF–DF in STEM BF-DF in TEM mode A B Collects only electrons scattered
more than 50 mrad (~3o).
Intensity: Z-dependent.
Diameters of the Fischione HAADF detector: mm.
Collects Bragg electrons

10. The effect of varying the camera length (L) in STEM mode
corresponds to varying the size of
an objective aperture in the
TEM mode!
STEM DF images are formed by collecting most of the scattered electrons on the ADF detector
The collection angle of the
detector depend on L

11. The effect of varying the camera length (L) in STEM mode
correspond to varying the size of
an objective aperture in the
TEM mode!
STEM DF images are formed by collecting most of the scattered electrons on the ADF deteector
As mentioned already, we describe the magnification of the DP by the camera length (L)
If we increase the magnification of the lenses between the specimen and the viewing screen, we increase the effective distance L between the specimen and the screen.
The collection angle of the
detector depend on L

13. Amplitude Contrast

14. Amplitude Contrast Difference in intensity of two adjacent areas:
The eyes can not see intensity differences < 5-10%.
However, contrast in images can be enhanced digitally.
NB! It is correct to talk about strong and weak contrast,
but not bright and dark contrast
(dark and bright refers to density (number/unit area) of electrons hitting the screen/detector)

15. Amplitude Contrast Difference in intensity of two adjacent areas:
The eyes can not see intensity differences < 5-10%.
However, contrast in images can be enhanced digitally. Si
ITO
EFTEM (16 eV): Bright = Si

16. Amplitude Contrast Two principal types Mass-thickness contrast
Diffraction contrast
-Primary contrast source in amorphous
materials
-In crystaline materials
-Coherent electron scattering
(atoms does not scatter independently)
-Assume incoherent electron scattering
(atoms scatter independently)
Both types of contrasts are seen in BF and DF images
-Can use any scattered electrons to form DF images showing mass-thickness contrast
-Two beam to get strong contrast
in both BF and DF images.
In TEM and STEM mode

17. Mass-Thickness Contrast

18. Rutherford Scattering
Mass thickness contrast arises from incoherent elastic scattering
(Rutherford scattering)
Peaked in the forward direction in thin samples
t and Z-dependent
Differential cross-section for high angle scattering:
Cross-section for elastic scattering is a function of Z
As t increases, more elastic scattering because the mean elastic free
path is fixed

19. TEM variables that affect the contrast:
-The objective aperture size (large -- bad).
-The high tension of the TEM (small -- good)

20. TEM variables that affect the contrast:
-The objective aperture size (large -- bad).
-The high tension of the TEM (small -- good)

21. Objective aperture size effect on mass-thickness contrast in TEM mode
d=70 μm
d=10 μm
Similar effect as reducing the HT of the microscope

22. Calculate the mass thickness contrast:kV
Atom. Nr.
Collection semi-angle
(angle of collection of the obj. app.)
Characteristic screening angle
(by the electron cloud)
Derived from:
To determine the probability that an e- will be scattered farther
than a given angle.

23. Imaging electron scattering at:
Low angles (<~5o) : - Mass-thickness contrast + Bragg diffraction
2. Higher angles: - Only mass-thickness contrast
(low intensity scattered beams)
- Intensity only depends on Z (and on thickness)
Only mass-thickness contrast – How?
A) Amorphous samples: All contrast is mass contrast
B) Crystalline samples: ADF/HAADF mode

24. Imaging electron scattering at:
Low angles (<~5o) : - Mass-thickness contrast + Bragg diffraction
2. Higher angles: - Only mass-thickness contrast
(low intensity scattered beams)
- Intensity only depends on Z (and on thickness)
Only mass-thickness contrast:
A) Amorphous samples: All contrast is mass contrast
B) Crystalline samples: ADF/HAADF mode

25. STEM- ADF
1. Less noisy than DF-TEM: Gathers all scattered electrons, not just electrons scattered from one atomic plane.
2. The lenses are not used to make the ADF image – less chromatic aberration.
3. More contrast than TEM-DF: Adjust L to optimize the ratio of the number of scattered e- hitting the detector.
4. Lower resolution than TEM-DF (probe size + intensity)
5. Alignment, alignment, alignment
When use STEM:
Thick specimen (aberrations)
Beam-sensitive specimens (…)
Specimen has low contrast in TEM
Z-contrast
e- ( BF)
e- ( DF)

26. DF: TEM - STEM STEM ADF contrast is greater than DF TEM
Optimal L
TEM-DF
STEM-ADF
The TEM image of the two phase polymer is noisier, but has better resolution.

27. HAADF Z-contrast images ↔ HAADF images.
At high angles (> 5o) Bragg scattering is negligible:
Z-contrast images ↔ HAADF images.
TEM
Z-contrast
(Bi in Si)
HAADF has the advantage that the contrast is generally unaffected by small changes in objective lens defocus (Df) and specimen thickness.

28. Noise reduction of Z-contrast images
HAADF images can be processed
and refined via a maximum entropy
approach to reduce noise.
Direct map of f(θ) variation in the specimen
(similar to an X-ray map).
Grain boundary in SrTiO3

29. Example of HR Z-contrast with the HAADF detector
Atomic structures are visible in both HREM and HAADF images.
HAADF image:
noisier but Z-contrast.
Relate the intensity differences to an absolute measure of the Bi concentration:
Cross-section for elastic scattered by matrix A
Contrast
Fraction of the alloying element that sub.
For matrix atoms
HREM-TEM HR Z-contrast STEM
Atomic concentration of alloying element
Alloying/dopant B

30. Example of mass-thickness contrast in TEM mode-
Metal shadowing
BF-TEM image of latex particles on an amorphous C-film.
The contrast is t-dependent.
What is the shape of the particles?
Effect of evaporation of a heavy metal
(Au or Au-Pd) thin coating at an oblique angle.
What is the contrast due to in the image?
Effect of inversing the contrast of the image.
The uneven metal shadowing increases the mass contrast and thus accentuates the topography.

31. Enhancement of contrast -
Staining of polymers and biological specimens
Heavy metal stains: Os, Pd and U
The heavy metals are left in spesific regions of the structures:
Unsaturated C=C bonds in polymers and cell walls in biological tissue.
Stained two phase polymer
(BF-TEM mode)
(BF-STEM mode)
Increased contrast, but
lower image quality (unless FEG).

32. Diffraction Contrast

33. TEM diffraction contrast
Bragg diffraction contrast: - Coherent elastic scattering
(atoms do not scatter independently)
- Crystal structure
- Orientation of the sample

34. Two-beam condition S: small and positive
The excess hkl Kikuchi line, just outside the hkl spot;
1. What is S?
2. And how to
determine S ?
Diff. rotation

35. Two-beam condition S: small and positive
The excess (bright) hkl Kikuchi line, just outside the hkl spot;
S is..?

36. TEM always shows better contrast than STEM images
Variation in the diffraction contrast when s is varied from
zero small larger +
TEM always shows better contrast than STEM images

37. How to make a CDF - TEM

38. Contrast Examples

39. Examples
Two types of steel (Martensite + Austenite)

40. Examples
Two types of steel (Martensite + Austenite)

41. Examples Two types of steel (Martensite + Austenite)
Erbium-oxide nanoclusters in silicon oxide

42. Examples Two types of steel (Martensite + Austenite)
Erbium-oxide nanoclusters in silicon oxide

43. Summary Mass-thickness contrast
-Areas of greater Z and or t scatter more strongly
-TEM images are better quality (lower noise and higher resolution) than STEM images, but digital STEM images can be processed to show higher contrast than analog TEM images.
STEM mass-thickness contrast images are most useful for thick and/or beam-sensitive specimens.
Z-contrast (HAADF) images can show atomic-level resolution.
Diffraction contrast
-Arises when the electrons are Bragg scattered.
-In TEM, the objective aperture selects one Bragg scattered beam.
-Often, the STEM detectors gather several Bragg beams which reduce
diffraction contrast.
-TEM always shows better contrast than STEM images (noisier and almost never used)

44. Phase-Contrast Imaging

46. Size of objective aperture Bright Field (BF), Dark Field (DF) and High Resolution EM (HREM)
HREM image
BF image
Objective
aperture
DF image
Why is it called diffraction contrast?
The contrast in the images are due to the fact that some regions in the specimen scatter the incident electron beam more than other parts of the specimen.
Amplitude/Diffraction contrast
Phase contrast

47. Phase Contrast Imaging
Exploits differences in the refractive index of different materials to differentiate between structures under analysis

48. Why different phases?
Transmitted and diffracted waves travel through different distances in the crystal
Each diffracted wave will have its own phase
Each diffracted wave represents a different solution to the Schrödinger equation
When several waves are allowed to interact, the phase differences manifest
themselves in the 2-D interference pattern in the image plane
--- Phase contrast image
Factors that contribute to the phase shift:
Thickness, orientation, scattering factor, focus and astigmatism.
Be careful when interpreting them

49. Be careful when interpreting the images

50. The origin of the lattice fringes (2-D interference pattern)
Rewrite the wave equation for two beams:
KD = K + KI
K is the difference vector : K = KD – KI
K = Sg + g

52. The intensity in the image: I = ψ*ψ
Now g0 is effectively perpendicular to the beam so we’ll set it parallel to x and replace d giving

56. Summary Phase contrast – Many beam imaging – High resolution imaging
Interference pattern
- Be careful when interpret the images – Image simulations