full introduction about Transmission electrons microscopy

1. Transmission
electron
microscopy
Abhishek
Mtech (Mechanical engineering)

2. Electron Microscopy Techniques
Introduction
• Electron Microscopes are scientific instruments that use a
beam of highly energetic electrons to examine objects on a
very fine scale.
• The main advantage of Electron Microscopy is the unusual
short wavelength of the electron beams, substituted for light
energy.
• The wavelengths of about 0.005 nm increases the resolving
power of the instrument to fractions

3. Microscopy
• Main branches: optical, electron and scanning probe
microscopy.
• Optical and electron microscopy involves the
diffraction, reflection, or refraction of radiation
incident upon the subject of study, and the subsequent
collection of this scattered radiation in order to build
up an image.
• Scanning probe microscopy involves the interaction
of a scanning probe with the surface or object of
interest.

4. Optical microscopy - definition
• Optical or light microscopy involves passing visible
light transmitted through or reflected from the sample
through a single or multiple lenses to allow a
magnified view of the sample.
• The resulting image can be detected directly by the
eye, imaged on a photographic plate or captured
digitally.
• The single lens with its attachments, or the system of
lenses and imaging equipment, along with the
appropriate lighting equipment, sample stage and
support, makes up the basic light microscope.

5. Optical microscopy - scheme

6. Optical microscopy - magnification

7. Electron Microscopy
• Beams of
electrons are used
to produce images
• Wavelength of
electron beam is
much shorter than
light, resulting in
much higher
resolution

8. Types of Electron Microscope
• Transmission Electron Microscope (TEM) uses a
wide beam of electrons passing through a thin sliced
specimen to form an image. This microscope is
analogous to a standard upright or inverted light
microscope
• Scanning Electron Microscope (SEM) uses focused
beam of electrons scanning over the surface of thick
or thin specimens. Images are produced one spot at a
time in a grid-like raster pattern.

9. Introduction
• Transmission electron microscopy (TEM, also
sometimes conventional transmission electron
microscopy or CTEM) is a microscopy technique in which a
beam of electrons is transmitted through a specimen to form an
image.
• The specimen is most often an ultrathin section less than 100
nm thick or a suspension on a grid.
• An image is formed from the interaction of the electrons with
the sample as the beam is transmitted through the specimen.
• The image is then magnified and focused onto an imaging
device, such as a fluorescent screen, a layer of photographic
film, or a sensor such as a charge-coupled device.

10. • Transmission electron microscopes are capable of imaging at a
significantly higher resolution than light microscopes, owing
to the smaller de Broglie wavelength of electrons.
• This enables the instrument to capture fine detail—even as
small as a single column of atoms, which is thousands of times
smaller than a resolvable object seen in a light microscope.
• Transmission electron microscopy is a major analytical
method in the physical, chemical and biological sciences.
• TEMs find application in cancer research, virology,
and materials science as well as polluton, nanotechnology and
semiconductor research.

12. Transmission Electron Microscopy (TEM)
• Ultrathin sections of
specimens
• Light passes through
specimen, then an
electromagnetic lens, to
a screen or film
• Specimens may be
stained with heavy
metal salts

13. Transmission Electron Microscopy (TEM)
• 10,000–100,000×; resolution 2.5 nm

14. The principles of TEM
• Transmission electron microscopy uses high energy electrons
(up to 300 kV accelerating voltage) which are accelerated to
nearly the speed of light.
• The electron beam behaves like a wavefront with wavelength
about a million times shorter than lightwaves.
• When an electron beam passes through a thin-section
specimen of a material, electrons are scattered.
• A sophisticated system of electromagnetic lenses focuses the
scattered electrons into an image or a diffraction pattern, or a
nano-analytical spectrum, depending on the mode of
operation.

15. • Each of these modes offers a different insight about the
specimen. The imaging mode provides a highly magnified
view of the micro- and nanostructure and ultimately, in the
high resolution imaging mode a direct map of atomic
arrangements can be obtained (high resolution EM = HREM).
• The diffraction mode (electron diffraction) displays accurate
information about the local crystal structure. The
nanoanalytical modes (x-ray and electron spectrometry) tell
researchers which elements are present in the tiny volume of
material.
• These modes of operation provide valuable information for
scientists and engineers in search of stronger materials, faster
microchips, or smaller nanocrystals.

16. Parts of the machine
• The typical transmission electron microscope
laboratory contains a machine with these
components:
I. Electron gun
II. Electron column
III. Electro-magnetic lens system
IV. Detectors
V. Water chilling system
VI. Specimen/sample chamber
VII. Main control panel and operational controls
VIII. Image capture

18. Electron gun
• The electron gun
generates the electron
beam. It is usually
positioned in the top of
the instrument column.
• The emitter is seated
within a cone-shaped
Wehnelt cylinder and the
beam travels out of the
small central hole shown
in the apex of the cone.

19. Electron column
• The electron column is made up of the gun assembly at the
top, a column filled with a set of electromagnetic lenses, the
sample port and airlock, and a set of apertures that can be
moved in and out of the path of the beam. The contents of the
column are under vacuum.

20. Electron lens
• Electron lenses are designed to act in a manner emulating that
of an optical lens, by focusing parallel electrons at some
constant focal distance.
• Electron lenses may operate electrostatically or magnetically.
The majority of electron lenses for TEM use electromagnetic
coils to generate a convex lens.
• The field produced for the lens must be radially symmetrical,
as deviation from the radial symmetry of the magnetic lens
causes aberrations such as astigmatism, and
worsens spherical and chromatic aberration.
• Electron lenses are manufactured from iron, iron-cobalt or
nickel cobalt alloys, such as permalloy.

21. These are selected for their magnetic properties, such
as magnetic saturation, hysteresis and permeability.

22. Detectors
• One of the most common detectors seen on a
transmission electron microscope is the x-ray energy
dispersive spectroscopy (EDS or EDX) system. This
typically involves a large dewar for liquid nitrogen
(to keep the detector cold), an arm on which the
equipment sits, and a solid state detector that
penetrates the column (arrow) so it is located near the
sample.

23. Apertures
• Apertures are annular metallic plates, through which electrons
that are further than a fixed distance from the optic axis may
be excluded. These consist of a small metallic disc that is
sufficiently thick to prevent electrons from passing through the
disc, whilst permitting axial electrons. This permission of
central electrons in a TEM causes two effects simultaneously:
firstly, apertures decrease the beam intensity as electrons are
filtered from the beam, which may be desired in the case of
beam sensitive samples. Secondly, this filtering removes
electrons that are scattered to high angles, which may be due
to unwanted processes such as spherical or chromatic
aberration, or due to diffraction from interaction within the
sample.

24. Fluorescent screen

25. A ray diagram for the diffraction mechanism in
TEM

27. Transmission Electron Microscope (TEM)
Working Concept
• TEM works much like a slide projector.
• A projector shines a beam of light through (transmits) the
slide, as the light passes through it is affected by the structures
and objects on the slide.
• These effects result in only certain parts of the light beam
being transmitted through certain parts of the slide.
• This transmitted beam is then projected onto the viewing
screen, forming an enlarged image of the slide.
• TEMs work the same way except that they shine a beam of
electrons (like the light) through the specimen (like the slide).
• Whatever part is transmitted is projected onto a phosphor
screen for the user to see.
• A more technical explanation of typical TEMs workings is as
follows.

28. Working concept of TEM

29. • The "Virtual Source" at the top represents the electron gun,
producing a stream of monochromatic electrons.
• This stream is focused to a small, thin, coherent beam by the
use of condenser lenses 1 and 2. The first lens (usually
controlled by the "spot size knob") largely determines the
"spot size"; the general size range of the final spot that
strikes the sample.
• The second lens (usually controlled by the "intensity or
brightness knob" actually changes the size of the spot on the
sample; changing it from a wide dispersed spot to a pinpoint
beam.
• The beam is restricted by the condenser aperture (usually
user selectable), knocking out high angle electrons (those far
from the optic axis, the dotted line down the center)

30. • The beam strikes the specimen and parts of it
are transmitted.
• This transmitted portion is focused by the
objective lens into an image
• The image is passed down the column
through the projector lenses, being enlarged
all the way.
• The image strikes the phosphor image screen
and light is generated, allowing the user to
see the image

31. Comparison b/w SEM and TEM

32. Sample preparation

39. Image Modes
• In TEM, absorption of electrons plays a very minor
role in image formation. TEM contrast relies instead
on deflection of electrons from their primary
transmission direction when they pass through the
specimen. The contrast is generated when there is a
difference in the number of electrons being scattered
away from the transmitted beam. There are two
mechanisms by which electron scattering creates
images: mass-density contrast and diffraction
contrast.

40. Mass-Density Contrast
• The deflection of electron scan result from interaction
between electrons and an atomic nucleus. Deflection
of an electron by an atomic nucleus, which has much
more mass than an electron, is like a collision
between a particle and a wall. The particle (electron)
changes its path after collision. The amount of
electron scattering at any specific point in a specimen
depends on the mass-density (product of density and
thickness) at that point. Thus, difference in thickness
and density in a specimen will generate variation in
electron intensity received by an image screen in the
TEM.

41. • The deflected electron with scattering angle larger than the α
angle (in the order of 0.01 radians) controlled by the objective
aperture will be blocked by the aperture ring. Thus, the
aperture reduces the intensity of the transmission beam as the
beam passes through it. The brightness of the image is
determined by the intensity of the electron beam leaving the
lower surface of the specimen and passing through the
objective aperture. The intensity of the transmitted beam (It) is
the intensity of primary beam (Io) less the intensity of beam
deflected by object (Id) in a specimen.
Specimen.

43. Diffraction Contrast
• We can also generate contrast in the TEM by a diffraction
method.
• Diffraction contrast is the primary mechanism of TEM image
formation in crystalline specimens. Diffraction can be
regarded as collective deflection of electrons.
• Electrons can be scattered collaboratively by parallel crystal
planes similar to X-rays. Bragg’s Law, which applies to X-ray
diffraction, also applies to electron diffraction.
• When the Bragg conditions are satisfied at certain angles
between electron beams and crystal orientation, constructive
diffraction occurs and strong electron deflection in a specimen
results.
• Thus, the intensity of the transmitted beam is reduced when
the objective aperture blocks the diffraction beams, similar to
the situation of mass-density contrast.

45. • Note that the main difference between the two contrasts is that the
diffraction contrast is very sensitive to specimen tilting in the
specimen holder but mass-density contrast is only sensitive to total
mass in thickness per surface area.
• The diffraction angle (2θ) in a TEM is very small (≤1◦) and the
diffracted beam from a crystallographic plane (hkl) can be focused
as a single spot on the back-focal plane of the objective lens. The
Ewald sphere is particularly useful for interpreting electron
diffraction in the TEM.
• When the transmitted beam is parallel to a crystallographic axis, all
the diffraction points from the same crystal zone will form a
diffraction pattern (a reciprocal lattice) on the back-focal plane. The
diffraction contrast can generate bright-field and dark-field TEM
images.
• In order to understand the formation of bright-field and dark-field
images, the diffraction mode in a TEM must be mentioned. A TEM
can be operated in two modes : the image mode and the diffraction
mode.

47. Phase Contrast
• Both mass-density and diffraction contrasts are amplitude
contrast because they use only the amplitude change of
transmitted electron waves.
• A TEM can also use phase difference in electron waves to
generate contrast. The phase contrast mechanism, however, is
much more complicated than that of light microscopy. T
• he TEM phase contrast produces the highest resolution of
lattice and structure images for crystalline materials.
• Thus, phase contrast is often referred as to high resolution
transmission electron microscopy (HRTEM).
• Phase contrast must involve at least two electron waves that
are different in wave phase.
• Thus, we should allow at least two beams (the transmitted
beam and a diffraction beam) to participate in image formation
in a TEM.

48. • A crystalline specimen with a periodic lattice structure
generates a phase difference between the transmitted and
diffracted beams.
• The objective lens generates additional phase difference
among the beams.
• Recombination of transmitted and diffracted beams will
generate an interference pattern with periodic dark–bright
changes on the image plane because of beam interferences.
• An interference pattern is a fringe type that reveals the
periodic nature of a crystal.
• Theoretical interpretation of phase contrast images is
complicated, particularly when more than one diffraction spot
participates in image formation.

51. Magnification in TEM

52. TEM Imaging
• A Transmission Electron Microscope produces a high-resolution, black and
white image from the interaction that takes place between prepared samples
and energetic electrons in the vacuum chamber.
• Air needs to be pumped out of the vacuum chamber, creating a space where
electrons are able to move.
• The electrons then pass through multiple electromagnetic lenses. These
solenoids are tubes with coil wrapped around them.
• The beam passes through the solenoids, down the column, makes contact
with the screen where the electrons are converted to light and form an
image.
• The image can be manipulated by adjusting the voltage of the gun to
accelerate or decrease the speed of electrons as well as changing the
electromagnetic wavelength via the solenoids.
• The coils focus images onto a screen or photographic plate.

53. • During transmission, the speed of electrons directly correlates to electron
wavelength; the faster electrons move, the shorter wavelength and the
greater the quality and detail of the image.
• The lighter areas of the image represent the places where a greater number
of electrons were able to pass through the sample and the darker areas
reflect the dense areas of the object.
• These differences provide information on the structure, texture, shape and
size of the sample.
• To obtain a TEM analysis, samples need to have certain properties. They
need to be sliced thin enough for electrons to pass through, a property
known as electron transparency.
• Samples need to be able to withstand the vacuum chamber and often
require special preparation before viewing.
• Types of preparation include dehydration, sputter coating of non-
conductive materials, cryofixation, sectioning and staining.

54. Applications of TEM

55. Advantages of TEM
• A Transmission Electron Microscope is an impressive
instrument with a number of advantages such as:
• TEMs offer the most powerful magnification, potentially over
one million times or more
• TEMs have a wide-range of applications and can be utilized in a
variety of different scientific, educational and industrial fields
• TEMs provide information on element and compound structure
• Images are high-quality and detailed
• TEMs are able to yield information of surface features, shape,
size and structure
• They are easy to operate with proper training

56. Disadvantages of TEM
• TEMs are large and very expensive
• Laborious sample preparation
• Potential artifacts from sample preparation
• Operation and analysis requires special training
• Samples are limited to those that are electron transparent, able to
tolerate the vacuum chamber and small enough to fit in the chamber
• TEMs require special housing and maintenance
• Images are black and white
Electron microscopes are sensitive to vibration and electromagnetic
fields and must be housed in an area that isolates them from possible
exposure.
A Transmission Electron Microscope requires constant upkeep including
maintaining voltage, currents to the electromagnetic coils and cooling
water.