A.E. GunnæsMENA3100 V13 Analytical Transmissions Electron Microscopy (TEM) Part I: Basic principles Operational modes Diffraction Part II: Imaging Sample preparation Part III Spectroscopy

Imaging and contrast A.E. Gunnæs Modern TEMs with Cs correctors have sub Å resolution! Resolution of the eyes:~ mm Resolution in a visible light microscope: ~300 nm

A.E. GunnæsMENA3100 V08 Imaging / microscopy 200 nm Si SiO 2 TiO 2 Pt BiFeO 3 Glue TEM - High resolution (HREM) - Bright field (BF) - Dark field (DF) - Shadow imaging (SAD+DF+BF) STEM - Z-contrast (HAADF) - Elemental mapping (EDS and EELS) GIF - Energy filtering Holography

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 Si Ag and Pb glue (light elements) hole We select imaging conditions so that one of them dominates.

A.E. GunnæsMENA3100 V13 Apertures Selected area aperture Condenser aperture Objective aperture

A.E. GunnæsMENA3100 V13 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. GunnæsMENA3100 V13 Simplified ray diagram Objective lense Diffraction plane (back focal plane) Image plane Sample Parallel incoming electron beam Si 1,1 nm 3,8 Å Objective aperture Selected area aperture

A.E. Gunnæs Objective aperture: Contrast enhancement No aperture used Central beam selected Intensity: Thickness and density dependence Si Ag and Pb glue (light elements) hole Mass-thickness contrast

TEM variables that affect the contrast: -The objective aperture size. -The high tension of the TEM. Areas of greater Z and/or t scatter electrons more strongly (in total). Mass-thickness contrast in TEM Incoherent elastic scattering (Rutherford scattering): peaked in the forward direction, t and Z-dependent Williams and Carter, TEM, Part 3 Springer 2009

The uneven metal shadowing increases the mass contrast and thus accentuates the topography. 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. Effect of inversing the contrast of the image. What is the contrast due to in the image? Williams and Carter, TEM, Part 3 Springer 2009

50 nm Diffraction contrast Objective aperture: Contrast enhancement Intensity: Dependent on grain orientation 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 Mass-thickness contrast Diffraction contrast and Two principall types Both types of contrasts are seen in BF and DF images -Primary contrast source in amorphous materials -In crystaline materials -Incoherent electron scattering -Coherent electron scattering -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.

Size of objective aperture Bright field (BF), dark field (DF) and High resolution EM (HREM) BF image Objective aperture DF image Amplitude/Diffraction contrast HREM image Phase contrast

A.E. GunnæsMENA3100 V13 Amplitude/diffraction contrast: weak-beam (WB) Weak-beam Dissociation of pure screw dislocation In Ni 3 Al, Meng and Preston, J. Mater. Scicence, 35, p , 2000.

Shadow imaging (diffraction mode) Objective lense Diffraction plane (back focal plane) Image plane Sample Parallel incoming electron beam

A.E. GunnæsMENA3100 V13 BF image DF image Obj. aperture Obj. lens sample Bending contours 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 –No double diffraction with XRD, kinematical intensities –Forbidden reflection may be observed t 0 : Extinction thickness –Periodicity of the oscillations –t 0 =πV c /λIF(hkl)I Incident beam Diffracted beam Doubly diffracted beam Transmitted beam Wedge shaped TEM sample t0t0

A.E. GunnæsMENA3100 V13 Thickness fringes/contours Sample (side view) e 000 g t I g =1- I o In the two-beam situation the intensity of the diffracted and direct beam is periodic with thickness (I g =1- I o ) I g =(πt/ξ g ) 2 (sin 2 (πts eff )/(πts eff ) 2 )) t = distance ”traveled” by the diffracted beam. ξ g = extinction distance Sample (top view) Hole Positions with max Intensity in I g

A.E. GunnæsMENA3100 V13 Thickness fringes bright and dark field images Sample DF image BF image

A.E. GunnæsMENA3100 V13 Phase contrast : HREM and Moire’ fringes 2 nm 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). HREM image Long-Wei Yin et al., Materials Letters, 52, p kV TEMs are most commonly used for HREM Interference pattern

A.E. GunnæsMENA3100 V13 Moire’ fringe spacing Parallel Moire’ spacing d moire’ = 1 / IΔgI = 1 / Ig 1 -g 2 I = d 1 d 2 /Id 1 -d 2 I Rotational Moire’ spacing d moire’ = 1 / IΔgI = 1 / Ig 1 -g 2 I ~1/gβ = d/β Parallel and rotational Moire’ spacing d moire’ = d 1 d 2 /((d 1 -d 2 ) 2 + d 1 d 2 β 2 ) 0.5 β g1g1 g2g2 ΔgΔg g1g1 g2g2 ΔgΔg

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, non- magnetic, plastic, vacuum If brittle, consider Cu washer with a slot Grid – Several types – Different materials (Cu, Ni…) – Support brittle materials – Support small particles The grid may contribute to the EDS. 3 mm

Preparation of self-supporting discs Top view Cutting – Ductile material or not? Grinding – μm thick – polish Cut the 3mm disc Dimple ? Final thinning – Ion beam milling – Electropolishing

A.E. GunnæsMENA3100 V13 Grind down/ dimple Cross section TEM sample preparation: Thin films Top view Cross section or Cut out a cylinder and glue it in a Cu-tube Grind down and glue on Cu-rings Cut a slice of the cylinder and grind it down / dimple Ione beam thinning Cut out cylinder Ione beam thinning Cut out slices Glue the interface of interest face to face together with support material Cut off excess material Focused Ion Beam (FIB)