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

Diffraction and Imaging


Topics

Topics

  • Objective aperture selection

  • BF/DF imaging

  • Relationship of DP to image


Image vs diffraction

BFP

stronger lens - shorter focal length

Image vs. Diffraction


Objective aperture selection

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.


Objective aperture size

No scattered electrons from hole

Large aperture – lower contrast

Small aperture – higher contrast

Objective Aperture Size


Obj aperture selection video

Obj. aperture selection video


Bf df aperture position

BF/DF Aperture Position

Dark Field

Bright Field


On axis dark field

What you see on screen

On-axis Dark Field

How do you center DF beam?


Tilt to g

Not in Bragg condition!

Tilt to +g

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


Tilt to g1

Still in Bragg condition!

Tilt to -g

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


Df imaging

DF imaging

Dark Field


Bf df imaging

BF/DF imaging


Df controls

BF/DF selector

Beam Tilts in DF mode

DF controls


Df page

DF channel (memory)

Beam Tilt Display

DF page


Displaced aperture df video

Displaced aperture DF video


Centered aperture df video

Centered aperture DF video


Crystallographic imaging

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

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.


Dp under over focus

sample

1st intermediate image

DP under/over focus


Dp under over focus1

DP under/over focus


Dp under over focus2

DP under/over focus


Dp under over focus3

DP under/over focus


Moo 3 crystal

SA aperture

MoO3 Crystal


Underfocus dp

(100)

Underfocus DP


Diffraction focus

Camera Length

Diffraction Focus

Diffraction Focus


Under over focus video

Under-over focus video


Moo 3 dp

(100)

MoO3 DP


Rotation angle

34° CCW

Rotation angle


Double exposure

Double Exposure

Double Exposure


Rotation angle1

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

EM400 rotation calibration


Lab 5

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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