Contrast in TEM and STEM Amplitude and Phase contrast

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)

TEM STEM

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.

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

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.

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: 4- 28 mm. Collects Bragg electrons

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

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

Amplitude Contrast

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)

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

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

Mass-Thickness Contrast

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

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

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

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

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.

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

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

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)

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.

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.

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

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

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.

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).

Diffraction Contrast

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

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

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

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

How to make a CDF - TEM

Contrast Examples

Examples Two types of steel (Martensite + Austenite)

Examples Two types of steel (Martensite + Austenite)

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

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

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)

Phase-Contrast Imaging

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

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

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

Be careful when interpreting the images

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

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

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