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Diffraction in the Transmission Electron Microscope Vidhya Sagar Jayaseelan

Diffraction in the Transmission Electron Microscope Vidhya Sagar Jayaseelan. TEM- What is it?. Advantages of TEM. High Resolution images 1,000,000 X Sub-micron level diffraction Defect imaging – dislocations, twins, stacking faults, point defects, antiphase boundaries

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Diffraction in the Transmission Electron Microscope Vidhya Sagar Jayaseelan

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  1. Diffraction in the Transmission Electron MicroscopeVidhya Sagar Jayaseelan

  2. TEM- What is it?

  3. Advantages of TEM • High Resolution images 1,000,000 X • Sub-micron level diffraction • Defect imaging – dislocations, twins, stacking faults, point defects, antiphase boundaries • Simultaneous imaging, structural and compositional analysis • Orientation studies possible • Identifying the phases and crystal structures • Site occupancy of atoms/ions

  4. Disadvantages of TEM • Sample size • Preparation • Instrument issues • Operation • Interpretation

  5. Wavelength of electrons Wavelength – magnification q*U = ½ mv2,λ = h/ (m * v ) where: λ = wavelength h = Planck's constant (6.6 X 10-27) m = mass of the electron (9.1 X 10-28) v = velocity of the electron U = Potential drop λ = (1.23 nm)/ U1/2

  6. Electron Sample Interaction Incident electron beam Backscattered electrons-Elastic Inelastic- Secondary electrons Characteristic X rays Auger Electrons 2θ Inelastic scattering Elastic scattering-Diffraction Transmitted beam

  7. Optics of the TEM • Gun • Lenses • Focal length • Apertures • Screen

  8. Image Contrast in TEM • Thickness • Composition • Atomic number/phases • Diffraction (orientation) • Strain fields • Fringe Effects

  9. Diffraction in TEM Reciprocal lattice –What is it? Equations- Bragg, Laue, Fourier Transform Real Reciprocal Ewald Sphere

  10. Zone axis for SC <100> <100> <110> <111>

  11. Real to Reciprocal lattice equations The relationship • Bragg’s law nλ = 2d sinθ • Laue equation r = (S-S0) / λ • Fourier Transform F(s) = ∫ dx f(x) exp(–i2πsx) Structure factor Fhkl = ∑ fn exp { 2πi (hx1 +ky1+ lz1)}

  12. Single vs Polycrystal

  13. Electron vs XRay Diffraction 2θ

  14. TEM Diffraction- What can we infer? • Phases and crystal structure types • Crystal symmetry and space group • Orientation relationships between phases • Determining growth directions, interface coherency • Identifying defects , i.e. twinning, SFs, Dislocations • Ordering behavior of crystal structures and the site occupancy preferences

  15. Dark field imaging

  16. Dark field imaging

  17. Kikuchi lines Inelastic scattering without significant wavelength change leads to formation of Kikuchi lines by diffraction. Incoming beam has every possible direction. Outgoing strong beam is at Bragg angle. This is particularly seen in thick samples θ

  18. Kikuchi Maps

  19. More…. • Transmission Electron Microscopy- Dr. Vasudevan’s Graduate Course • Transmission Electron Microscopy – David B. Williams and C. Barry Carter • Transmission Electron Microscopy – Ludwig Reimer • WWW

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