SCANNING ELECTRON MICROSCOPE AND TRANSMISSION ELECTRON MICROSCOPE

1. SCANNING ELECTRON MICROCOPE &
TRANSMISSION ELECTRON MICROSCOPE
BY –
SIVANANDITA K L

3. • SEM belongs to the family of electron microscopes which
produce images of an object by scanning its surface with highly
focused electron beam.
• The process involves the interaction of electrons with atoms of
an object, creating signals containing information of object’s
composition and topography.
• Arrangement of constituent atoms is studied by 2D beam
scanning upon the sample surface and image acquisition from
collected secondary electrons.
• Scan pattern is generated by the electron beam and the image
is formed by merging beam's position and the detected signal.

4. SEM
• PRINCIPLE – It works on the principle of applying kinetic
energy to produce signals on the interaction of the electrons.
• These electrons are secondary electrons, backscattered
electrons, and diffracted backscattered electrons which are
used to view crystallized elements and photons.
• Secondary and backscattered electrons are used to produce an
image.

5. • Electron Gun
• The first part of the SEM is the electron gun.
• an electron gun fires electrons at the sample you're magnifying. The electrons can be
created a few different ways, but the most common method heats up a tungsten wire
to produce the electrons.
• Condenser Lens
• The second part of an SEM is the condenser lens.
• This is used to narrow the electron beam given off by the electron gun.
• This lens isn't made of glass like you might expect. Instead, it's a lens made of coils of
wire that create an electromagnetic field which compresses the electrons as they travel
travel through it.
• Apertures
• Next are the apertures.
• These allow you to control the diameter of the electron beam being passed through
them.
• The aperture consists of a metal rod with different size holes cut into it.
• The diameter of the electron beam is controlled by changing which hole it travels
through. The aperture also blocks off any extra electrons.

6. Objective Lens and Sample Chamber
• After the apertures is another electromagnetic lens called the objective lens.

7. • HOW DOES IT WORKS ?
• Electrons are produced at the top of the column, accelerated down, and
passed through a combination of lenses and apertures to produce a
focused beam of electrons which then strikes the surface of the sample.
• The sample itself is mounted on a stage in the chamber area and both the
column and the chamber are evacuated by a combination of pumps.
• The position of the electron beam on the sample is controlled by scan coils
situated above the objective lens.
• These coils allow the beam to be scanned over the surface of the sample.
• This beam rastering (a rectangular pattern of parallel scanning lines
followed by the electron beam on a computer) or scanning enables
information about a defined area on the sample to be collected.
• As a result of the electron-sample interaction, a number of signals are
produced.
• These signals are then detected by appropriate detectors.

8. APPLICATIONS
• It used in spot chemical analysis in energy-Dispersive X-ray
Spectroscopy.
• It used in the analysis of cosmetic components which are very tiny in
size.
• It helps to study the filament structures of microorganisms.
• It also helps to study the topography of elements used in industries.

9. TEM
DEFINITION - In a Transmission electron microscope,
the electron beam is transmitted through a very thin
specimen or object and forms a highly magnified and
detailed image of the sample.

10. PRINCIPLE –
• The working principle of the Transmission Electron Microscope
(TEM) is similar to the light microscope.
• The major difference is that light microscopes use light rays to
focus and produce an image while the TEM uses a beam of
electrons to focus on the specimen, to produce an image.
• Electrons have a shorter wavelength in comparison to light which
has a long wavelength.
• The mechanism of a light microscope is that an increase in
resolution power decreases the wavelength of the light, but in the
TEM, when the electron illuminates the specimen, the resolution
power increases increasing the wavelength of the electron
transmission.
• The wavelength of the electrons is about 0.005nm which is
100,000X shorter than that of light, hence TEM has better
resolution than that of the light microscope, of about 1000times.

11. HOW DOES IT WORKS ?
• First of all, a tungsten filament is heated, which is also called an electron gun.
• The heated tungsten filament or electron gun will start to release electron
beams.
• An electromagnetic coil and high voltage(up to several million volts) applied
to these electron beams to accelerate their speed (extremely high speeds ).
• A condenser lens with a high aperture eliminates all the high angle electrons
and focused all the electron beams into a thin, small beam.
• The high-speed electron beams are now transmitted through the specimen.
• The transmitted electron beams are focused into an image with the help of an
objective lens.
• The vaccine chamber of TEM prevents the collide of electrons with the gas
atoms.
• The electron beams are projected on to a phosphorescent screen, which
creates an image of the specimen, also called a micrograph.
• All the images are captured by a charge-coupled device (CCD) camera, which
is located underneath the screen.

12. • APPLICATIONS –
• TEM is also used in production and the manufacturing of
computers and silicon chips.
• To distinguish between animal and plant cells.
• It is also used in nanotechnology for studying nanoparticles.
• It can be used to identify and detect fractures.