Electron microscope (SEM and TEM)

ELECTRON MICROSCOPE
Dr. Prafulla Katkar
Ph.D (Italy), PDF (Denmark)

In the 1920s it was discovered that accelerated electrons
(parts of the atom) behave in vacuum just like light. They
travel in straight lines and have a wavelength which is about
100 000 times smaller than that of light. Furthermore, it was
found that electric and magnetic fields have the same effect
on electrons as glass lenses and mirrors have on visible light.
Why use electrons instead of light ?

The electron microscope uses a beam of electrons to create an
image of the specimen. It is capable of much higher
magnifications and has a greater resolving power than a light
microscope, allowing it to see much smaller objects in finer
detail. They are large, expensive pieces of equipment,
generally standing alone in a small, specially designed room
and requiring trained personnel to operate them.
What is Electron Microscope…?

Dr. Ernst Ruska at the University of Berlin and Max Knoll
combined built the first electron microscope in 1931.
For this and subsequent work on the subject, Ernst Ruska
was awarded the Nobel Prize for Physics in 1986.
Max Knoll Ernst Ruska
History of Electron Microscope

Type of Electron Microscope
 Transmission Electron Microscope (TEM)
 Scanning Electron Microscope (SEM)
Types of Electron Microscope

Components
Transmission Electron Microscope-TEM

(a) Electron gun:
It consists of a cathode that emits electrons on receiving
high voltage electric current (50,000-100,000 volts).
(c) Condense lens: It is the electromagnetic coil which
focuses the electron beam in the plane of the specimen.
(d) Objective lens: It is the electromagnetic coil which
produces the first magnified image formed by the objective lens
and produces the final image.
(e) Projector lens: It is also an electromagnetic coil
which further magnifies the first image formed by the
objective lens and produces the final image.
(f) Fluorescent Screen or Photographic Film: Since
unaided human eye cannot observe electrons, therefore, a
fluorescent screen is used for viewing the final image of the
specimen
Components

• A heated tungsten filament in the electron gun generates a
beam of electrons that is then focused on the specimen by the
condenser.
• Since electrons cannot pass through a glass lens, magnetic
lenses are used to focus the beam.
• lenses and specimen must be under high vacuum to obtain a
clear image because electrons are deflected by collisions with air
molecules.
• The specimen scatters electron passing through it, and the beam
is focused by magnetic lenses to form an enlarged , visible image
of the specimen on a fluorescent screen.
• A denser region in the specimen scatters more electron and
therefore appears darker in the image.
• In contrast, electron-transparent regions are brighter.

Scanning Electron Microscope-SEM
 SEM image of sample by scanning it with a high energy beam of electrons.
 The electrons interaction with sample atoms produce signals that contain
information about the sample’s surface topography, composition, and other
properties.

Scanning Electron Microscope
 Electron gun consisting of cathode and anode
 The condenser lens controls the amount of electrons
travelling down the column.
 The objective lens focuses the beam into a spot on the
sample.
 Deflection coil helps to deflect the electron beam.
 SED attracts the secondary electrons.
 Additional sensors detect backscattered electrons
and x-rays.
Components of SEM

 Beam of electrons is generated by a suitable
source, typically a tungsten filament or a field
electron gun.
 The electron beam is accelerated through a
high voltage and pass through a system of
electromagnetic lenses to produce a thin
beam of electrons.
 Then the beam scans the surface of the
specimen, secondary electrons are emitted
from the specimen by the action of the
scanning beam and collected by a suitably
positioned detector.
Working Mechanism

Application
 It gives detailed 3d and topographical imaging and the
versatile information.
 This works very fast.
 Modern SEMs allow for the generation data in digital
form.
 Most SEM samples require minimal preparation actions.
 Enable us to view without thinning dehydrating, fixing
the sample

Electron microscope (SEM and TEM)

Electron microscope (SEM and TEM)

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