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Use of the Transmission Electron Microscope
(TEM)
Electron Diffraction Imaging
(N.B. Important: Please see Julian before
proceeding with any diffraction work as there is a danger of damaging the
digital camera with a very bright central beam spot!!)
Many thanks to Ray Whitby for providing me with the details
for this section
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With
the microscope set-up correctly, obtain a bright field image of the
sample
in question, ensure it is in focus. Keep the magnification below 20K
otherwise
the centre spot drifts away from the centre of the screen making
image
recording impossible. This does not affect the diffraction pattern
obtained
as all information comes from an area within the Selected Area
aperture
- so it is not necessary to have a highly magnified image anyway.
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(N.B. from Jules)
Diffraction spot may be centred by the INT ALIGN X and Y shifts (under left
hand pull-down panel).
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Move
the sample so that the area (desired for diffraction) is at the
centre
of the screen and place the Selected Area aperature around that area.
Usually
the smallest aperature to block out the maximum number of stray
reflections.
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Remove
the Objective aperture (if present), then hit the Selected Area
button
(next to the Diffraction button).
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Using
the binoculars, focus the centre spot to its minimum size using the
Diffraction
Spot knob. You may need to increase or decrease the brigthness
to
obtain a decent balance of diffraction spot intensities to that of the
centre
beam, as no beam stopper is present.
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This
should generate a Selected Area Diffraction pattern on the area of a
desired sample.
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Notes
on recording the image:
| N.B.
NEVER USE 'AUTO EXPOSURE' ON THE DIGITAL CAMERA ACQUISITION AS THERE IS A
DANGER OF DAMAGING THE SENSOR WITH A VERY INTENSE CENTRAL BEAM SPOT! (Please
see Julian before proceeding with acquiring digital images from a
diffraction pattern) |
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I
have found that keeping the magnification at 9K, a fairly decent image can
be
obtained from using the "Focus window" acquistion rather than the
"Image
window"
acquistion button (the window on the upper right hand side - not the
middle
one on the right). Take the Exposure off from "Auto" and put in a
range
of values from 0.5 seconds to say 5 seconds. The final image will show
streaking
of the diffracted spots if the Exposure is too high.
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Like
most things, need to try varying the magnification, brightness,
exposure
etc in order to obtain a good diffracted image. If you want to do
Dark-Field
imaging I can provide some details on that too. This can be a
really
helpful technique.
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