Analytical Transmissions Electron Microscopy TEM Part I Basic
Analytical Transmissions Electron Microscopy (TEM) Part I: Basic principles Operational modes Diffraction Part II: Imaging Sample preparation Part III Spectroscopy A. E. Gunnæs MENA 3100 V 13
Imaging and contrast Resolution of the eyes: ~ 0. 1 -0. 2 mm Resolution in a visible light microscope: ~300 nm Modern TEMs with Cs correctors have sub Å resolution! A. E. Gunnæs
Imaging / microscopy TEM Bi. Fe. O - High resolution (HREM) - Bright field (BF) - Dark field (DF) - Shadow imaging (SAD+DF+BF) STEM - Z-contrast (HAADF) - Elemental mapping (EDS and EELS) Pt Ti. O Si. O 2 Si 200 nm GIF - Energy filtering Holography A. E. Gunnæs MENA 3100 V 08 2 3 Glue
Contrast • Difference in intensity of to adjacent areas: The eyes can not see intensity chanes that is less then 5 -10%, however, contrast in images can be enhanced digitally. NB! It is correct to talk about strong and week contrast but not bright and dark contrast
Amplitude contrast and Phase-contrast images The elctron wave can change both its amplitude and phase as it traverses the specimen Give rise to contrast hole Ag and Pb Si glue (light elements) We select imaging conditions so that one of them dominates.
Apertures Condenser aperture Objective aperture Selected area aperture A. E. Gunnæs MENA 3100 V 13
Use of apertures Condenser aperture: Limits the number of electrons hitting the sample (reducing the intensity), Affecting the diameter of the discs in the convergent electron diffraction pattern. Selected area aperture: Allows only electrons going through an area on the sample that is limited by the SAD aperture to contribute to the diffraction pattern (SAD pattern). Objective aperture: Allows certain reflections to contribute to the image. Increases the contrast in the image. Bright field imaging (central beam, 000), Dark field imaging (one reflection, g), High resolution Images (several reflections from a zone axis). A. E. Gunnæs MENA 3100 V 13
Simplified ray diagram Parallel incoming electron beam 3, 8 Å Si Sample 1, 1 nm Objective lense Diffraction plane Objective aperture (back focal plane) Image plane A. E. Gunnæs MENA 3100 V 13 Selected area aperture
Objective aperture: Contrast enhancement Si Ag and Pb hole glue (light elements) No aperture used Central beam selected Intensity: Thickness and density dependence Mass-thickness contrast A. E. Gunnæs
Mass-thickness contrast in TEM Incoherent elastic scattering (Rutherford scattering): peaked in the forward direction, t and Z-dependent Areas of greater Z and/or t scatter electrons more strongly (in total). TEM variables that affect the contrast: -The objective aperture size. -The high tension of the TEM. Williams and Carter, TEM, Part 3 Springer 2009
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. Williams and Carter, TEM, Part 3 Springer 2009
Objective aperture: Contrast enhancement Intensity: Dependent on grain orientation Diffraction contrast 50 nm Try to make an illustration to explain why we get this enhanced contrast when only the central beam is selected by the optical aperture.
Amplitude contrast Two principall types Mass-thickness contrast and Diffraction contrast -Primary contrast source in amorphous materials -In crystaline materials -Incoherent electron scattering -Coherent electron scattering Both types of contrasts are seen in BF and DF images -Can use any scattered electrons to form DF images showing massthickness contrast -Two beam to get strong contrast in both BF and DF images.
Size of objective aperture Bright field (BF), dark field (DF) and High resolution EM (HREM) Objective aperture BF image DF image Amplitude/Diffraction contrast HREM image Phase contrast
Amplitude/diffraction contrast: weak-beam (WB) Dissociation of pure screw dislocation In Ni 3 Al, Meng and Preston, J. Mater. Scicence, 35, p. 821828, 2000. Weak-beam A. E. Gunnæs MENA 3100 V 13
Shadow imaging (diffraction mode) Parallel incoming electron beam Sample Objective lense Diffraction plane (back focal plane) Image plane
Bending contours sample Obj. lens Obj. aperture BF image DF image A. E. Gunnæs MENA 3100 V 13 Solberg, Jan Ketil & Hansen, Vidar (2001). Innføring i transmisjon elektronmikroskopi
Double diffraction, extinction thickness • Double electron diffraction leads to oscillations in the diffracted intensity with increasing thickness of the sample Incident beam – No double diffraction with XRD, kinematical intensities – Forbidden reflection may be observed • t 0: Extinction thickness – Periodicity of the oscillations – t 0=πVc/λIF(hkl)I Wedge shaped TEM sample t 0 Transmitted Diffracted beam. Doubly diffracted beam
Thickness fringes/contours In the two-beam situation the intensity of the diffracted and direct beam is periodic with thickness (Ig=1 - Io) 000 e g Ig=1 - Io Sample (side view) t Hole Sample (top view) Ig=(πt/ξg)2(sin 2(πtseff)/(πtseff)2)) t = distance ”traveled” by the diffracted beam. ξg = extinction distance A. E. Gunnæs Positions with max Intensity in Ig MENA 3100 V 13
Thickness fringes bright and dark field images Sample BF image A. E. Gunnæs DF image MENA 3100 V 13
Phase contrast: HREM and Moire’ fringes Long-Wei Yin et al. , Materials Letters, 52, p. 187 -191 HREM image Interference pattern 2 nm 200 -400 k. V TEMs are most commonly used for HREM A Moiré pattern is an interference pattern created, for example, when two grids are overlaid at an angle, or when they have slightly different mesh sizes (rotational and parallel Moire’ patterns). http: //www. mathematik. com/Moire/ A. E. Gunnæs MENA 3100 V 13
Moire’ fringe spacing Parallel Moire’ spacing dmoire’= 1 / IΔg. I = 1 / Ig 1 -g 2 I = d 1 d 2/Id 1 -d 2 I Rotational Moire’ spacing dmoire’= 1 / IΔg. I = 1 / Ig 1 -g 2 I ~1/gβ = d/β Parallel and rotational Moire’ spacing dmoire’= d 1 d 2/((d 1 -d 2)2 + d 1 d 2β 2)0. 5 A. E. Gunnæs MENA 3100 V 13 g 2 g 1 Δg g 1 β g 2 Δg
Sample preparation and what to consider before you start
SAFETY!!!! • Know what you handling. – MSDS • Protect your self and others around you. – Follow instructions • If an accident occurs, know how to respond.
Work in the Stucture Physics lab • Get the local HMS instructions from Ole Bjørn Karlsen Sign a form confirming that you have got the information Ask
What to considder before preparing a TEM specimen • • • Ductile/fragile Bulk/surface/powder Insulating/conducting Heat resistant Single phase/multi phase Etc, etc……. What is the objectiv of the TEM work?
Specimen preparation for TEM • Crushing • Cutting – saw, “diamond” pen, ultrasonic drill, FIB • Mechanical thinning – Grinding, dimpling, – Tripod polishing • • • Electrochemical thinning Ion milling Coating Replica methods Etc.
Self-supporting disk or grid • Self supporting disk – Consists of one material • Can be a composite – Can be handled with a tweezers • Metallic, magnetic, nonmagnetic, plastic, vacuum If brittle, consider Cu washer with a slot 3 mm • Grid – Several types – Different materials (Cu, Ni…) – Support brittle materials – Support small particles The grid may contribute to the EDS.
Preparation of self-supporting discs Top view • Cutting – Ductile material or not? • Grinding – 100 -200 μm thick – polish • Cut the 3 mm disc • Dimple ? • Final thinning – Ion beam milling – Electropolishing
Cross section TEM sample preparation: Thin films Cut out cylinder • Top view Cut out a cylinder and glue it in a Cu-tube Cut out slices • • Cross section Glue the interface of interest face together with support material Focused Ion Beam (FIB) Grind down/ dimple Ione beam thinning Grind down and glue on Cu-rings or Cut a slice of the cylinder and grind it down / dimple Cut off excess material Ione beam thinning A. E. Gunnæs MENA 3100 V 13
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