Electron Microscope
Electron microscope is a microscope in which the object is illuminated by highly
accelerated fast moving electron beams.
It has very high magnification of about 1,00,000 X and very high resolving power.
Working Principle
The working principle of the electron microscope is similar to that of the optical
microscope. The given object is illuminated by highly accelerated fast - moving electron. The
focusing of electron beam is done by magnetic fields (magnetic lenses).
The shorter wavelength of electron permits the detailed examination of tiny objects due
to reduction of diffraction effects.
Types of electron microscope
There are three types of electron microscopes. They are:
(i). Transmission Electron Microscope (TEM)
(ii). Scanning Electron Microscope (SEM)
(iii). Scanning Transmission Electron Microscope (STEM)
The scope of this chapter is limited to the first two types only.
Transmission Electron Microscope (TEM)
The essential components of a Transmission Electron microscope are
(i). Electron gun
(ii) Specimen holder
(iii) Electron lenses
(Condenser lens, objective lens, intermediate lens, projector lens)
(iv) Vacuum Pumps and
(v) Viewing screen or photographic plate.
Electron gun consists of a tungsten ‘Hair Pin’ filament, which when heated emits
electrons due to thermionic emission. These electrons are then accelerated and the collimated
stream of electrons of energy 100 KeV and the electron beam of size 50 to 100 m is produced
from the gun.
The electrons from the electron gun are focused by a set of magnetic condenser lenses to
produce the desired illumination of the specimen.All the lenses used in electron microscope are
of magnetic type. These are energized by highly stabilized direct current sources. The fig shows
a magnetic lens. A non - uniform magnetic field of electromagnet behaves like a lens.
In such a field, the electron beam undergoes divergence or convergence depending on the
flux density at different points of the magnetic field. The focal length of the magnetic lens can be
varied by changing the current in the coil.
Working of TEM
The essential parts of TEM are shown in Fig.3.
An electron beam emitted from the electron gun is made to pass through the centre of the
doughnut magnet shaped electromagnet called condenser lens. The electrons get deflected to
form parallel beam which strike the object to be magnified.
Fig .3. Transmission Electron Microscope (TEM)
Second electromagnet called objective lens causes the electron beam to diverge to
produce enlarged image of the object. The third electromagnet called projector lens focuses the
electron beam from part of the enlarged image on the fluorescent screen producing still greater
magnification.
The image obtained on the fluorescent screen is made visible scintillation for direct view.
It can also be obtained on a suitable photographic plate for a permanent record. Sharp focusing is
obtained by adjusting the intensity of the magnetic fields produced by electromagnets.
Uses of TEM
1. It is used in the investigation of atomic structures and structure of crystals in detail.
2. It is used in biology, medicine, physics, chemistry and metallurgy and so on.
3. It is used in the study of structure of textile fibers, purification of lubricating oils,
composition of paper and paints and surfaces of metals and plastics.
4. In biology, it is used to study the virus and bacteria (disease causing agent)
Scanning Electron Microscope (SEM)
Scanning Electron Microscope (SEM) is the latest developed electron microscope. Here,
the image is built by using an electron probe of very small diameter which scans the specimen
surface in parallel straight lines as does a television camera.
This electron microscope is conveniently used to study the surface structure directly.
Construction of SEM
The main part of the scanning electron microscope is shown in Fig.4.
Fig.4. Scanning Electron Microscope (SEM)
It consists of an electron gun and an assembly of magnetic lenses enclosed in an
evacuated column. There are several stages of magnification in the electron microscope. The
magnetic lenses are current carrying coils and the focal length can be controlled by regulating the
current through the coil of the lens.
Working of SEM
The electron beam is focused to a spot of 10nm in diameter and is made to scan the
surface of the specimen.
The interaction of these primary electrons with the specimen surface provides back
scattered and induced emission of electrons called secondary electrons.
The secondary electrons emitted from the specimen are collected by the detector and
converted into current which is amplified as signal voltage. The signal is passed on to a cathode ray - tube (CRT) and the image is formed on the screen of the CRT which may be viewed or
photographed.
In addition to the image formed by secondary electrons, images may also be formed by
collecting back scattered electrons and transmitted electron.
Advantages of SEM
Thicker specimens can also be examined than in the normal transmission electron
microscope.
Application of SEM
Because of the high resolving power, large magnification and greater depth of focusing, it
has wide applications.
1.
In medicine, it is used to study viruses which are the disease causing agents. A
knowledge of their structure helps to find out the methods of their destruction.
2. The electron microscope is used to study about bacteria in detail
3. Evaluation of textile finishes on fabrics and fibers, and the study of surface change
resulting from various fiber treatments.
4. Characterization of optical fiber couplers and connectors; failure mode determination
of optical fibers.
5..Correlations between processing conditions of ceramic materials and their ultimate
micro structure.