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7/27/2019 TRANSMISSION ELECTRON MICROSCOPE.pptx
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Transmission Electron Microscope
(TEM)
Presented ByPrashant KumarM.Tech.1st Year
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Introduction
TEM was first built by Max Knoll and Ernst Ruska in 1931.
1000X more magnification than light microscope.
Used to reveal ultra structure of plant and animal cells aswell as viruses and macromolecules.
Electrons pass through a (very thin) sample (i.e. are
transmitted) to form an image.
Simplistically, In its operation a TEM can be thought of
as analogous to a slide projector.
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1. Electron Gun
2. The condenser
system
3. The sample
4. Image formation5. Projection
System
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Electron Guns
There are 2 types :-
1. Thermionic Electron Gun
2. Field Emission Gun (FEG)
Thermionic sources produce electrons when heated.
Field emission sources produce electrons when exposed to
an intense electric field.
FEGs give much more brightness than thermionic systems.
FEGs give a more monochromatic electron source and
finer probe (i.e. better resolution).
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The Condenser System
The Wehnelt (or 2nd anode in a
FEG) focuses the beam to a
crossover which is accelerated
down the column.
The first condenser de-magnifies
the crossover to give a smaller
point source this is referred to as
C1 or spot size.
The second condenser lens C2 is
used to either converge or spread
the beam of illumination on thesam le intensit or bri htness .
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The Condenser System
A condenser aperture is placed in the beam path to remove
electrons far from the optic axis which would reduce
resolution.
The smaller the aperture the better the resolution, but
there is an associated decrease in brightness need to
compromise.
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Sample preparation
For a metallic sample:
Cut or slice a section of
material less than 1 mm thick.
Produce 3mm diameter blanksby either Punching, or spark
erosion.
Grind and polish blanks to
less than 80m thick and
0.25m or better finish.
For a non metallic sample:
1. Cut or slice a section of material less
than 1mm thick.
2. Mount on glass slide and Grind to lessthan 80m thick with a 0.25m or
better finish.
3. Mount on support grids if necessary.
4. Dimple to leave ~10 m of material
remaining.
5. Ion beam mill to perforation.
6. Some samples require coating to
prevent charging effects.
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For biological samples
Biological samples require fixing and embedding before
being stained with heavy metals (e.g. OsO4) for contrast
prior to ultra microtome sectioning. Very time
consuming.
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Image Formation
Objective
lens
Sample
All rays from a point in theobject are gathered by the lens
and converge to a point in the
image.
All parallel rays are focused in
the focal plane.
The back focal plane of the
objective lens contains
groupings of rays that have left
the object at the same angle.
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The back focal plane contains the diffraction
pattern of the sample.
Diffraction pattern and image are both formed inthe imaging process
The intermediate lens is then focused on either the
image plane (for the image), or the back focal plane
(for the diffraction pattern).
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Projection - Magnification
A series of projector lenses areused to magnify the imageformed by the intermediatelens onto a viewing screen.
Electron micro lenses areelectromagnetic in nature.
They consists of cylindrical softmetal core (pole piece) with ahole drilled through it (bore)
wound with copper wire.
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When a current is passed through the coil amagnetic field is created in the bore.
Changing current in the windings changes themagnetic field and effectively changes the focallength of the lens.
Increase the current and focal length f of the lensdecreases so weaker lens f1 gives a highermagnification than stronger lens f2 as imagedistance v increases but the, object distance isunchanged.
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Applications of TEM
Nanotechnology
Medical Sciences
Metallurgy Material Sciences
Life Sciences
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References1. www.google.com
2. Introduction to nanotechnology By Charles Poole
3. http://www.tcd.ie/CMA/
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