more use full to basic science students

1. TRANSMISSION ELECTRON
MICROSCOPE
K. SENTHILARASAN
Edayathangudy G.S.Pillay Arts And Science College.
Nagapattinam-02

2. OVERVIEW
 Introduction
 Principle
 Components
 Construction
 Working
 Advantage
 Disadvantage
 application

3. TRANSMISSION ELECTRON
MICROSCOPE
Introduction
Scanning electron microscope the resolution of the
image is limited only up to 10-20nm.
This will not be useful to view the internal features
of an atom or the morphology of a sample of size say
0.2nm.
Thus to examine the sample of size up to 0.2nm,
the transmission electron microscope can be used.
In this microscope the image is obtained by
transmitting the electrons through the specimen.

4. PRINCIPLE
The electrons are allowed to pass through the specimen
and the image is formed on the fluorescent screen either by
using transmitted electron beam (bright field image) or by
using diffracted electron beam (dark field image) from the
specimen.

5. Components
• Electron Gun
• Magnetic Condensing Lenses
• Fluorescent Screen Or Charge Coupled Device.

6. Electron gun
– A high energy electron beam is produced by thermionic
emission from directly heated tungsten filament.
– This electron beam is accelerated by the anode towards
the specimen.
Magnetic condensing lenses
The electron beam can be converged or diverged by
the lenses.
Theses lenses are usually of magnetic type ie., current
carrying coils. The focal length can be controlled by the
current through the coil of the magnetic lens.

7. CONSTRUCTION
It consist of an electron gun to produce electrons.
Magnetic condensing lens is used to condense the
electron and is also used to adjust the size of the electron
that falls onto the specimen.
The specimen is placed in between the condensing
lens and objective lens.
The magnetic objective lens is used to block the
high angle diffracted beams and the aperture is used to
eliminate the diffracted beam and in turn it increases the
contrast of the image.
The magnetic projector lens is placed above the
fluorescent screen in order to achieve higher
magnification. The image can be recorded by using a
fluorescent screen or CCD.

9. Working
Stream of electrons are produced by the electron
gun and is made to fall over the specimen using the
magnetic condensing lens.
Based on the angle of incidence, the beam is partly
transmitted and partly diffracted. Both the transmitted
beam and the diffracted beams are recombined at the E-
Wald sphere(sphere of reflection which encloses all
possible reflections from the crystal, satisfying Bragg’s
law), to form the image. The combined image is called the
phase contrast image.

11. In order to increase the intensity and the
contrast of the image, an amplitude contrast image has
to be obtained. This can be achieved only by using
the transmitting beam, and thus the diffracted beam
has to be eliminated.
Now in order to eliminate the diffracted beam,
the resultant beam is passed through the magnetic
objective lens and the aperture.
The aperture is adjusted in such a way that the
diffracted image is eliminated. Thus the final image
obtained due to the transmitted beam alone is passed
through the projector lens for further magnification

12. The bright filed image obtained is purely
due to transmitted beam alone.
The dark filed image obtained due to
diffracted image.

13. Advantage
 It can be used to examine the specimen of size upto
0.2nm.
 The magnification is 1,000000 times greater than the
size of the object.
 It has high resolution.
 The resolving power 1 to 2

14. Disadvantage
 The specimen should be very thin
 It is not suitable for thick samples
 3-d image cannot be obtained
 There are changes for the structural change,
during the sample preparation.
 Incase of biological samples, the electrons
may interact with the samples, which may
even damage the samples.

15. Applications
The main application of TEM is in nano-science used
to find the internal structure of nano materials.
It is used to find the 2 D image of very small biological
cells, virus, bacteria.

16. Thank you