CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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volatile solvent is drop casted and allowed to dry. The samples are expected to be thin
enough to allow the electrons to pass through for image formation. The main imaging system
in a TEM instrument is the objective lens system. This lens is highly converging with a very
small focal length. The objective lens produces an enlarged image on a screen placed several
cm below the lens. Due to small focal length of the lens the image produced at a large
distance is highly magnified. Apart from producing magnified image the objective lens
reduces aberrations by rejecting electrons scattered at large angles. The most common
aberrations involved in lenses are chromatic and spherical aberration. Since these aberrations
limits the resolution of lenses it is necessary to understand these aberrations to reduce it.
Spherical aberration is a defect arising from the lenses. In this case the off axis electrons are
focused at different points other than the focal plane. Aberration produces a disc instead of a
sharp image therefore limiting the resolution. Spherical aberration present in these lenses are
given by,
r
s
3
C
s
(2.8)
In the equation above r s
is radius of disc;
Cs
is the coefficient of spherical aberration, and
is angle of electron beam with respect to axis. This type of aberration arises from the
electron lenses therefore cannot be eliminated completely. However it can be minimized by
using a strongly focusing lens. Additionally an aperture is also used to reduce the angular
deviation of electrons.
The other common aberration is called chromatic aberration; similar to light this type
of aberration arises due to electron kinetic energy variations which determine wavelength of
electrons. Therefore electrons of different kinetic energies will be focused at different spots.