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Summary HC.2 - electron microscopy
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Summary HC.2 Electron Microscopy - Course: Light and Electron Microscopy UU
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Samenvatting EM hoorcollege 2What do we need to build an EM?
1. Vacuum
2. Emission source
3. Lenses
4. Apertures
5. Detectors
1. Vacuum
A TEM is kept permantaly under vacuum, unless it’s being repaired or serviced. There is
always a vacuum because:
electrons interacted with the atoms in air which causes scattering which causes a short
path length in air (20 cm in air and 2000 km in vacuum, 300 kV).
Next to that the vacuum keeps the specimen clean.
Access to the inside of the column via an airlock system, which can be pumped separately.
The airlock prevents degrading the vacuum of the column when inserting a sample. The
sample is initially pumped from atmospheric pressure to an intermediate vacuum, and then
that intermediate vacuum is mixed with the high column vacuum when the sample is fully
inserted. This reduces the amount that the column must be pumped to maintain high vacuum.
Different parts of the microscope are under different vacuum. The column and electron source
are under a very ultra-high vacuum.
, There are different types of pumps to creates different vacuums.
Exhaust pumps
Trapping pumps
With a IGP you remove air by emitting electrons from a cathode and then by ionizing air
molecules. The ion is going to move to the grounded cathode plate with a high velocity.
When it hits that plate, the ions will become buried within the cathode or they will sputter
cathode material onto the walls of the pump. This sputtered material then removes the gas
molecules by absorbing. You can only use a IGP when you already have a high vacuum.
2. Electron sources
The electron gun needs to
1. extract electrons from a tip
2. accelerate the electrons
There are different ways for extracting electrons from a material:
Thermionic sources produces electrons when heated
Gives the electrons sufficient energy to overcome the natural barrier that prevents
them from leaking out from the surface. The barrier is called the work function .
You can only use materials with:
high melting point: Tungsten (W) filament
cheap butbreaks very fast
low work function : lanthanum hexaboride
(LaB6) crystals.
thermionic sources are less monochromatic (energy
different, different)
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