Diffraction and imaging
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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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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.

  • 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.


No scattered electrons from hole

Large aperture – lower contrast

Small aperture – higher contrast

Objective Aperture Size


Obj. aperture selection video


BF/DF Aperture Position

Dark Field

Bright Field


What you see on screen

On-axis Dark Field

How do you center DF beam?


Not in Bragg condition!

Tilt to +g

If you move diffracted spot into center, it will disappear.


Still in Bragg condition!

Tilt to -g

If you move direct beam to +g spot, -g will light up.


DF imaging

Dark Field


BF/DF imaging


BF/DF selector

Beam Tilts in DF mode

DF controls


DF channel (memory)

Beam Tilt Display

DF page


Displaced aperture DF video


Centered aperture DF video


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.


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.


sample

1st intermediate image

DP under/over focus


DP under/over focus


DP under/over focus


DP under/over focus


SA aperture

MoO3 Crystal


(100)

Underfocus DP


Camera Length

Diffraction Focus

Diffraction Focus


Under-over focus video


(100)

MoO3 DP


34° CCW

Rotation angle


Double Exposure

Double Exposure


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.


EM400 rotation calibration


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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