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Transmissions electron microscopy

Transmissions electron microscopy. Sample preparation Basic principles Imaging aberrations (Spherical, Chromatic, Astigmatism) contrast (Mass-thickness, Diffraction, Phase). Crushing Cutting saw, diamond pen, ultrasonic drill, FIB Mechanical thinning Grinding, dimpling

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Transmissions electron microscopy

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  1. Transmissions electron microscopy Sample preparation Basic principles Imaging aberrations (Spherical, Chromatic, Astigmatism) contrast (Mass-thickness, Diffraction, Phase) MENA3100 V10

  2. Crushing Cutting saw, diamond pen, ultrasonic drill, FIB Mechanical thinning Grinding, dimpling Electrochemical thinning Ion milling Coating Replica methods FIB Sample preparation for TEM Plane view or cross section sample? Is your material brittle or ductile? Is it a conductor or insulator? Is it a multi layered material? MENA3100 V10

  3. Top view TEM sample preparation: Thin films Grind down/ dimple Cut out cylinder Ione beam thinning Grind down and glue on Cu-rings Cut out a cylinder and glue it in a Cu-tube Cut out slices • Cross section Glue the interface of interest face to face together with support material or • Focused Ion Beam (FIB) Cut a slice of the cylinder and grind it down / dimple Cut off excess material Ione beam thinning MENA3100 V10

  4. Basic principles, first TEM Electrons are deflected by both electrostatic and magnetic fields Force from an electrostatic field (in the gun) F= -e E Force from amagnetic field (in the lenses) F= -e (v x B) Wave length: λ= h/(2meV)0.5 (NB non rel. expr.) λ= h/(2m0eV(1+eV)/2m0c2)0.5(relativistic expression) 200kV: λ= 0.00251 nm (v/c= 0.6953, m/m0= 1.3914) • The first electron microscope built by Knoll • and Ruska in 1933, b) The first commercial • electron Microscope built by Siemens in 1939. Nobel prize lecture: http://ernst.ruska.de/daten_e/library/documents/999.nobellecture/lecture.html MENA3100 V10

  5. Electron gun Basic TEM Electron source: ●Tungsten, W ● LaB6 ● FEG MENA3100 V10

  6. Electron guns Field emission gun (FEG) Thermionic gun MENA3100 V10

  7. Technical data of different sources Monochromator: Energy spread less than 0.15 ev http://dissertations.ub.rug.nl/FILES/faculties/science/1999/h.b.groen/c1.pdf H.B. Groen et al., Phil. Mag. A, 79, p 2083, 1999 MENA3100 V10

  8. Cold trap Sample position Basic TEM Electron gun Electron source: ●Tungsten, W ● LaB6 ● FEG • Vacuum requirements: • Avoid scattering from residual gas in • the column. • - Thermal and chemical stability of the • gun during operation. • - Reduce beam-induced contamination • of the sample. • LaB6: 10-7 torr • FEG: 10-10 torr MENA3100 V10

  9. The lenses in a TEM Filament Anode The diffraction limit on resolution is given by the Raleigh criterion: δd=0.61λ/μsinα, μ=1, sinα~ α 1. and 2. condenser lenses Sample Objective lens Compared to the lenses in an optical microscope they are very poor! The point resolution in a TEM is limited by the aberrations of the lenses. Intermediate lenses Projector lens • Spherical • Chromatic • Astigmatism MENA3100 V10

  10. Spherical aberration coefficient r2 r1 α Disk of least confusion Spherical aberrations Cs corrected TEMs are now available ds = 0.5MCsα3 M: magnification Cs :Spherical aberration coefficient α: angular aperture/ angular deviation from optical axis 2000FX: Cs= 2.3 mm 2010F: Cs= 0.5 nm The diffraction and the spherical aberration limits on resolution have an opposite dependence on the angular aperture of the objective. MENA3100 V10

  11. Aberrations in a nutshell Before Cs correction After Cs correction Core of the M100 galaxy seen through Hubble (source: NASA) Q.M. Ramasse MENA3100 V10

  12. Resolution limit http://www.sfc.fr/Material/hrst.mit.edu/hrs/materials/public/ElecMicr.htm MENA3100 V10

  13. v - Δv v Chromatic aberration Disk of least confusion Chromatic aberration coefficient: dc = Ccα ((ΔU/U)2+ (2ΔI/I)2 + (ΔE/E)2)0.5 Cc: Chromatic aberration coefficient α: angular divergence of the beam U: acceleration voltage I: Current in the windings of the objective lens E: Energy of the electrons 2000FX: Cc= 2.2 mm 2010F: Cc= 1.0 mm Thermally emitted electrons: ΔE/E=KT/eV Force from amagnetic field: F= -e (v x B) MENA3100 V10

  14. Lens astigmatism Loss of axial asymmetry x y-focus y x-focus Lens aberrations This astigmatism can not be prevented, but it can be corrected! MENA3100 V10

  15. Operating modes Convergent beam Parallel beam Can be scanned (STEM mode) Specimen Spectroscopy and mapping (EDS and EELS) Imaging mode or Diffraction mode MENA3100 V10

  16. Filament Anode 1. and 2. condenser lenses Spesimen Objective lens Intermediate lenses Projector lens Image or diffraction mode Objective aperture Diffraction plane Bi-prism Selected area aperture Image plane STEM detectors (BF and HAADF) Image or diffraction pattern Viewing screen MENA3100 V10

  17. Advanced nanotool JEOL 2010F FEGTEM Ultra high resolution version with analytical possibilities MENA3100 V10

  18. BiFeO3 Pt TiO2 Glue SiO2 Si 200 nm Imaging / microscopy 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 MENA3100 V10

  19. 3,8 Å 1,1 nm Simplified ray diagram Parallel incoming electron beam Si Sample Objective lense Diffraction plane (back focal plane) Objective aperture Selected area aperture Image plane MENA3100 V10

  20. Apertures Condenser aperture Objective aperture Selected area aperture MENA3100 V10

  21. Use of apertures Condenser aperture: Limits the number of electrons hitting the sample (reducing the intensity), Reducing 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). MENA3100 V10

  22. 50 nm Objective aperture: Contrast enhancement Ag and Pb Si hole glue (light elements) A small aperture allows only electrons in the central spot in the back focal plane to contribute to the image. All electrons contributes to the image. Intensity: Thickness and density dependence Mass-thickness contrast Diffraction contrast (Amplitude contrast) One grain seen along a low index zone axis. MENA3100 V10

  23. Objective aperture BF image Weak-beam DF image Diffraction contrast: Bright field (BF), dark field (DF) and weak-beam (WB) Dissociation of pure screw dislocation In Ni3Al, Meng and Preston, J. Mater. Scicence, 35, p. 821-828, 2000. MENA3100 V10

  24. sample Obj. lens Obj. aperture BF image DF image DF image Bending contours MENA3100 V10

  25. g 000 Ig=1- Io t Thickness fringes/contours e In the two-beam situation the intensity of the diffracted and direct beam is periodic with thickness (Ig=1- Io) Sample (side view) Sample (top view) Hole Ig=(πt/ξg)2(sin2(πtseff)/(πtseff)2)) t = distance ”traveled” by the diffracted beam. ξg = extinction distance Positions with max Intensity in Ig MENA3100 V10

  26. BF image DF image Thickness fringes, bright and dark field images Sample Sample MENA3100 V10

  27. HREM image 2 nm Phase contrast: HREM and Moire’ fringes Long-Wei Yin et al., Materials Letters, 52, p.187-191 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). Interference pattern http://www.mathematik.com/Moire/ 200-400 kV TEMs are most commonly used for HREM MENA3100 V10

  28. Moire’ fringe spacing Parallel Moire’ spacing dmoire’= 1 / IΔgI = 1 / Ig1-g2I = d1d2/Id1-d2I Rotational Moire’ spacing dmoire’= 1 / IΔgI = 1 / Ig1-g2I ~1/gβ = d/β Parallel and rotational Moire’ spacing dmoire’= d1d2/((d1-d2)2 + d1d2β2)0.5 g1 g2 Δg g1 Δg β g2 MENA3100 V10

  29. Simulating HREM imagesContrast transfer function (CTF) • CTF (Contrast Transfer Function) is the function which modulates the amplitudes and phases of the electron diffraction pattern formed in the back focal plane of the objective lens. It can be represented as: • k = u • The curve depend on: • Cs (the quality of objective lens)l (wave-length defined by accelerating voltage)Df (the defocus value)u (spatial frequency) In order to take into account the effect of the objective lens when calculating HREM images, the wave function Ψ(u) in reciprocal space has to be multiplied by a transfer function T(u). In general we have: Ψ(r)= Σ Ψ(u) T(u) exp (2πiu.r) T(u)= A(u) exp(iχ), A(u): aperture function 1 or 0 Χ(u)= πΔfλu2+1/2πCsλ3u4 : coherent transfer function MENA3100 V10

  30. Simulating HREM imagesContrast transfer function (CTF) Effect of the envelope functions can be represented as: where Ec is the temporal coherency envelope (caused by chromatic aberrations, focal and energy spread,instabilities in the high tension and objective lens current), and Ea is spatial coherency envelope (caused by the finite incident beam convergence). http://www.maxsidorov.com/ctfexplorer/webhelp/background.htm MENA3100 V10

  31. Scherzer defocus Δ f= - (Csλ)1/2 Δ f= -1.2(Csλ)1/2 Scherzer condition Extended Scherzer condition http://www.maxsidorov.com/ctfexplorer/webhelp/effect_of_defocus.htm MENA3100 V10

  32. HREM simulations One possible model for which the simulated HREM images match rectangular region I HREM simulation along [0 0 1] based on the above structures. The numbers before and after the slash symbol “/” represent the defocus and thickness (nm), respectively ”The assessment of GPB2/S′′ structures in Al–Cu–Mg alloys ” Wang and Starink, Mater. Sci. and Eng. A, 386, p 156-163, 2004. MENA3100 V10

  33. Combined HAADF and EELS HAADF image of an icosahedral FePt particle (false colors): thanks to the small probe size, it is possible to probe precisely the chemical structure of samples at the atomic level, revealing here a small crystalline layer of iron oxide surrounding the outermost shell of the particle. MENA3100 V10

  34. Energy filtering A. Thøgersen et al., Collaboration with Prof. T. Finnstad, UiO, S. Diplas, SINTEF and UniS, UK and NIMS, Japan MENA3100 V10

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