CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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will appear bright and region with samples will appear black due to scattering of electrons
leading to reduced intensity. Image produced with scattered electrons (after interaction with
the sample) gives rise to a dark field image. The contrast produced in a dark field is much
higher; however the intensities are lower. Therefore dark field imaging requires larger
exposure times. The dark field imaging is carried out by moving objective aperture to
monitor un-scattered beam. However this is of poor quality as we intercept electrons in
region where aberrations are dominant. In crystalline samples the diffraction spots or rings
are monitored therefore specific planes are monitored.
2.6. 2. Layout of TEM
The schematic of a typical TEM is given in the figure 2.2. It consists of an electron
source based on thermionic emission for e.g. tungsten wire. One of an important requirement
to get a sharp image is presence of good temporal and spatial coherence of electrons.
Tungsten guns suffer from the problem of energy spread of emitted electrons and brightness.
These properties contribute to limitation in terms of temporal and spatial coherence.
Therefore in recent times field emission guns have replaced thermionic emission guns which
have better coherence properties. Field emission guns have a small probe size. However cost
of field emission guns is very high as compared to thermionic emission guns. Therefore
thermionic electron guns are still in use in most TEMs around the world. To reduce the
problem arising due to heating the filament to high temperature, low work function materials
like LaB 6 is used. However this type of material requires much higher vacuum due to
poisoning of LaB 6 in presence of oxygen. Apart from coherences, brightness of the source is
also another factor determining the choice of electron guns.