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Diffraction and Imaging

Diffraction and Imaging. Topics. Objective aperture selection BF/DF imaging Relationship of DP to image. BFP. stronger lens - shorter focal length. Image vs. Diffraction. Objective aperture selection. The BFP contains the DP (shows reciprocal space) and the objective aperture.

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Diffraction and Imaging

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  1. Diffraction and Imaging

  2. Topics • Objective aperture selection • BF/DF imaging • Relationship of DP to image

  3. BFP stronger lens - shorter focal length Image vs. Diffraction

  4. Objective aperture selection • The BFP contains the DP (shows reciprocal space) and the objective aperture. • Remember, small objects in real space become large in reciprocal space. • Larger objective apertures allow finer detail. • … but by allowing more of the scattered electrons to contribute to the image it has lower contrast.

  5. No scattered electrons from hole Large aperture – lower contrast Small aperture – higher contrast Objective Aperture Size

  6. Obj. aperture selection video

  7. BF/DF Aperture Position Dark Field Bright Field

  8. What you see on screen On-axis Dark Field How do you center DF beam?

  9. Not in Bragg condition! Tilt to +g If you move diffracted spot into center, it will disappear.

  10. Still in Bragg condition! Tilt to -g If you move direct beam to +g spot, -g will light up.

  11. DF imaging Dark Field

  12. BF/DF imaging

  13. BF/DF selector Beam Tilts in DF mode DF controls

  14. DF channel (memory) Beam Tilt Display DF page

  15. Displaced aperture DF video

  16. Centered aperture DF video

  17. Crystallographic Imaging • TEM can give both image and crystallographic (diffraction) information. • To be useful, we need to relate the image to the diffraction pattern. • We can then mark crystallographic directions on our images.

  18. Image Rotation • Lorentz force causes the electron to spiral through the lens. • The amount of spiral varies with the magnetic field. • Changing the field (lens strength) rotates the image.

  19. sample 1st intermediate image DP under/over focus

  20. DP under/over focus

  21. DP under/over focus

  22. DP under/over focus

  23. SA aperture MoO3 Crystal

  24. (100) Underfocus DP

  25. Camera Length Diffraction Focus Diffraction Focus

  26. Under-over focus video

  27. (100) MoO3 DP

  28. 34° CCW Rotation angle

  29. Double Exposure Double Exposure

  30. Rotation Angle • We usually work at fixed CL,  measure from DP to image • Note sense of rotation (CW/CCW) • Work with film emulsion up. Be consistent.

  31. EM400 rotation calibration

  32. Lab 5 • Sample: MoO3 crystals on lacy carbon • Tasks: BF/DF imaging, Rotation calibration • Investigate effects of obj. aperture size, on-axis vs. off-axis DF imaging, rotation of image w.r.t. DP

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