2. Why We Need Electron
Microscope?
Light Microscopes are limited by the physics of light to
500x or 1000x magnification and a resolution of 0.2
micrometers.
In the early 1930's there was a scientific desire to see the
fine details of the interior structures of organic cells
(nucleus, mitochondria...etc.).
This required 10,000x plus magnification which was just
not possible using Light Microscopes.
3. HISTORY OF ELECTRON
MICROSCOPE
1897
J.J. Thompson Discovered electron
1924
Louis DE Broglie Identified wavelength of
electron(≈hν)
1926
Knoll & Ruska built Ist electron microscope
1938
First practical Microscope built by Siemens
1940
Commercial Microscope with 2.4 nm resolution
1945
1.0 nm resolution
5. Optical Microscope
Electron Microscope
1.
Uses optical glass lens.
Uses magnetic lens.
2.
Have low magnification (500X or
1000X appx.)
Have high magnification
(10000X appx.)
3.
Does not require vaccum for
operation.
Require vaccum for operation.
4.
Small depth of field.
Large depth of field.
5.
Low price.
High price.
6. Introduction :
Electron microscopes are scientific instruments that use a
beam of energetic electron to examine objects on a very
fine scale.
Electron microscopes are develop due to the limitations of
light microscopes which are limited by physics of light.
In early 1930 theoretical limit has been reaches and there
was a scientific desire to see the fine details of interior
structure of organic cells.
This require 10000X plus magnification which was not
possible using current optical microscope.
7. SCANNING ELECTRON MICROSCOPE
(SEM)
A scanning electron microscope (SEM) is a
type of electron microscope that images a
sample by scanning it with a high-energy beam
of electrons in a raster scan pattern. The
electrons interact with the atoms that make up
the sample producing signals that contain
information about the sample's surface
topography, composition, and other properties.
8. Characteristics that can be viewed on
SEM :
Topography
The surface features of an object or "how it looks", its texture; direct
relation between these features and materials properties.
Morphology
The shape and size of the particles making up the object; direct
relation between these structures and materials properties
Composition
The elements and compounds that the object is composed of and the
relative amounts of them; direct relationship between composition and
materials properties
Crystallographic Information
How the atoms are arranged in the object; direct relation between
these arrangements and material properties
13. Transmission Electron Microscope :
The transmission electron microscope was the
first type of electron microscope to be developed
and is patterned exactly on the light transmission
microscope except that a focused beam of
electrons is used instead of light to "see through"
the specimen. It was developed by Max Knoll
and Ernst Ruska in Germany in 1931.
TEMs find application in cancer
research, virology, material science as well as
pollution, nanotechnology and semiconductor
research.
14. Transmission Electron Microscopy
In a conventional transmission electron microscope, a thin
specimen is irradiated with an electron beam of uniform
current density.
Electrons are emitted from the electron gun and illuminate
the specimen through a two or three stage condenser lens
system.
Objective lens provides the formation of either image or
diffraction pattern of the specimen.
The electron intensity distribution behind the specimen is
magnified with a three or four stage lens system and viewed
on a fluorescent screen. The image can be recorded by
direct exposure of a photographic emulsion or an image
plate or digitally by a CCD camera.
15. Design Of Transmission
Electron Microscope
A simplified ray
diagram of a TEM
consists of an
electron source,
condenser lens
with aperture,
specimen,
objective lens
with aperture,
projector lens and
fluorescent screen.
17. EXAMPLE OF DIFFRACTION PATTERN
In this case incident beam direction B [100] in an Aluminum (f,c.c),
single crystal specimen. Transmitted beam is marked as T and the
arrangement of the diffracted beams D around the transmitted beam
is the characteristic of the four fold symmetry of the [100] cube axis
of Aluminum.
18. Single Crystal Diffraction Pattern
Single crystal are most ordered (lattice type such as f.c.c, b.c.c, s.c etc.)
among the three structures.
Electron beam passing through a single crystal will produce a pattern of
spots.
Type of crystal structure (f.c.c., b.c.c.) and the "lattice parameter" (i.e., the
distance between adjacent planes) can be determined.
Also, the orientation of the single crystal can be determined: if the single
crystal is turned or flipped, the spot diffraction pattern will rotate around
the centre beam spot in a predictable way.
20. Difference Between SEM & TEM :
SEM is based on scattered electrons while TEM is based on
transmitted electrons.
The sample in TEM has to be cut thinner whereas there is no
such need with SEM sample.
SEM allows for large amount of sample to be analysed at a time
whereas with TEM only small amount of sample can be analysed
at a time.
SEM is used for surfaces, powders, polished & etched
microstructures, IC chips, chemical segregation whereas TEM is
used for imaging of dislocations, tiny precipitates, grain
boundaries and other defect structures in solids
TEM has much higher resolution than SEM.
