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Dr. Shahid Hussain Abro
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Course Outlines
Electron microscopy of materials. Specimen preparation
techniques, Image focusing techniques, Image forming
techniques
and
interpretation
and
crystallographic
characterizations
information,
of
Image
microstructures,
Convergent beam, weak beam and microanalysis of thin foils,
Working principles of different types of electron microscopes;
TEM,
SEM,
STEM,
HREM.
microscopy in materials engineering.
Examples
of
Electron
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What will we learn
 What
is electron microscopy?
 How
are electrons generated?
 How
are electrons focused?
 How
do electrons interact with matter?
 How
are the electron/matter interactions used to generate
images?
 What
analytical Information we can obtain?
 Sample
Preparation?
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An electron microscope is an instrument that uses
electrons instead of light for the imaging of objects.
The development of the transmission electron
microscope was based on theoretical work done by
Louis de Broglie, who found that wavelength is
inversely proportional to momentum.
In 1926, Hans Busch discovered that magnetic fields
could act as lenses by causing electron beams to
converge to a focus. A few years later, Max Knoll and
Ernst Ruska made the first modern prototype of an
electron microscope.
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 Electron
microscopy is an imaging
e- Source
technology that uses the properties Anode
of electrons rather than light.
1st lens
 uses a beam of electrons to create
2nd lens
an image of the specimen.
Final lens
 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.
Detectors
Backscatter eX-ray
Secondary e-
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The resolution is 1,000 times greater than a light
microscope and about 500,000 times greater than that
of a human eye.
The STM is similar to the TEM except for the fact that
it causes an electron beam to scan rapidly over the
surface of the sample and yields an image of the
topography of the surface. The resolution of a STM is
about 10 nm.
The resolution is limited by the width of the exciting
electron beam and by the interaction volume of
electrons in a solid.
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Resolution is the finest detail that can be distinguished
in an image. The resolving power of a microscope is
quite different from its magnification.
You can enlarge a photograph indefinitely using more
powerful lenses, but the image will blur together and
be unreadable. Therefore, increasing the magnification
will not improve resolution.
The minimum separation (d) that can be resolved by
any kind of a microscope is given by the following
formula:
d = λ/(2n sinθ)
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where n is the refractive index (which is 1 in the
vacuum of an electron microscope) and λ is the
wavelength.
Since resolution and d are inversely proportional, this
formula suggests that they way to improve resolution is
to use shorter wavelengths and media with larger
indices of refraction.
The electron microscope exploits these principles by
using extremely short wavelengths of accelerated
electrons to form high-resolution images.
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Today, electron microscopy is widely used in
metallurgy, biology, material science, physics,
chemistry, and many other technological fields.
It has been an integral part in the understanding of the
complexities of cellular structure, the fine structure of
metals and crystalline materials as well as numerous
other areas of the microscopic world.
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 Electron
Probe Microanalyzer (EPMA)
An electron probe microanalyzer utilizes X-rays emitted due
to electron bombardment to obtain qualitative and
quantitative microanalysis.
 Electron Microprobe (same as EPMA)
 Transmission Electron Microscope (TEM)
Uses transmitted electrons instead of emitted electrons.
 Scanning Transmission Electron Microscope (STEM)
Combines aspects of both SEM and TEM.
 Environmental Scanning Electron Microscope (ESEM)
Similar to a SEM, but does not require the high vacuum.
 Scanning Auger Microscope (SAM)
Similar to an SEM only it uses Auger electron emissions
instead of secondary electron emissions for imaging and
compositional analysis.
Scaning Electron Microscopy
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How are electrons generate?
 Thermionic
emission
 Tungsten (W) filament
 Lanthanum hexaboride (LaB6) filament
 Field emission
The amount of electrons (flux or current density)
determines resolution.
The size of the electron beam (spot size) determines
resolution.
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Thermionic emission is
the emission of electrons from
a heated metal (cathode).
Thermionic emission is the
thermally induced flow of charge
carers from a surface or over a
potential-energy barrier
emission of electrons
induced by an
electrostatic field.
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Electron Generation





Thermionic Electron Gun
Heated filament produces electrons
Typically made of Tungsten or
Lanthanum hexaboride
Electrons drawn towards an anode
An aperture in the anode creates a
beam



Field Emission Gun
A very strong electric field is used
to extract electrons from a metal
filament
Filament typically a single
tungsten crystal requires a
vacuum Similar anode setup
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The requirement of high vacuum
 Electrons have extremely low mass
(~1/1000 that of a proton) and easily give
up their energy in collisions with gas atoms
and molecules.
 SEM
technology is not possible without a
high vacuum in at least the source and
focusing column of the machine.
 Column vacuum ~10-7 torr
 Sample chamber vacuum ~<10-5 torr
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How does the e- beam interact with matter?
 Incident
electrons interact
with matter in two ways
 elastic collisions
 inelastic collisions
From these interactions,
information regarding shape,
composition, crystal
structure, electronic
structure, internal electric or
magnetic fields, …
Eo
N
Q=
,
nt ni
λ=
A
N o ρQ
Q=collision cross-section
(probability)
N=num of collisions/unit
volume
nt=number of targets/unit
volume
ni=number of incident
particles/unit area (flux)
Ei
φe
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Inelastic emissions
 Inelastic
interactions result in a wide variety of
emissions:
 Secondary
electrons
 Characteristic
X-rays
 Bremsstarahlung
(continuum) X-rays
 Cathodluminescence
light)
radiation (IR, UV and visible
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Secondary electrons are electrons generated as
ionization products. They are called 'secondary'
because they are generated by other radiation (the
primary radiation). This radiation can be in the form
of ions, electrons, or photons with sufficiently high
energy, i.e. exceeding the ionization potential.
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Characteristic X-rays are emitted when outer-shell
electrons fill a vacancy in the inner shell of an atom,
releasing X-rays in a pattern that is "characteristic" to
each element.
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How is a secondary image generated
 Secondary
electrons are generated by the interaction
of the incident electron beam and the sample. The
secondary electrons emerge at all angles.
 These electrons gathered by electrostatically attracting
them to the detector. Knowing both the intensity of
secondary electrons emitted and position of the beam,
an image is constructed electronically.
incident e- beam
emitted e-
beam location
secondary edetector
~+12,000 V
signal intensity
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How is a secondary image generated?
Emitted electrons are not assembled by the electron
microscope in the way that light (visible photons) are
assembled by the human eye. Light reflecting from a
given spot enters the eye. Many points of such
reflected light are assembled in a pattern on the eye that
exactly mimics the reflecting source.
incident e- beam
incident
light
emitted e-
secondary edetector
eye
~+12,000 V
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Elastic collisions
 Elastic
collisions produce backscattered electrons
(BS).
Ei
φe
Eo
Q(> φo ) = 1.62 ×10
− 20
Z=the atomic number
E=electron energy (keV)
fo=scattering angle
Z
2 φo
cot
2
2
E
2
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Detecting BS electrons
 There
are many types of detectors, only the solid
state type is discussed here.
incident e- beam
solid state BS detector
BS e-
sample
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Microscope Setup
 Transmission
Electron
Microscope Phase contrast
 Image is formed by the
interference between electrons
that passed through the sample
and ones that did not
 Scanning
Electron Microscope
 Electron beam is scanned across the sample
 The reemitted electrons are measured in order to form
 the image.
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Focusing
 When
the image is in focus, there is very low contrast
due to the electron loss around the objective
 By imaging underfocus or overfocus, a phase shift
and amplitude contrast are created.
 This creates a dark image with a white ring around or
a white image with a dark ring (respectively).
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How electron microscopes work
1.The source of light.
2.The specimen.
3.The lenses that makes the specimen seem bigger.
4.The magnified image of the specimen that you see.
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In an electron microscope, these four things are
slightly different.
1.The light source is replaced by a beam of very fast
moving electrons.
2.The specimen usually has to be specially prepared and
held inside a vacuum chamber from which the air has
been pumped out (because electrons do not travel very
far in air).
3.The lenses are replaced by a series of coil
shaped electromagnets through which the electron beam
travels.
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1.In an ordinary microscope, the glass lenses bend (or
refract) the light beams passing through them to
produce magnification. In an electron microscope, the
coils bend the electron beams the same way.
2.The image is formed as a photograph (called
an electron micrograph) or as an image on a TV
screen.
TEM - transmission electron microscopy
Typical accel. volt. = 100-400 kV
(some instruments - 1-3 MV)
Spread broad probe across
specimen - form image from
transmitted electrons
Diffraction data can be obtained
from image area
Many image types possible (BF, DF,
HR, ...) - use aperture to select
signal sources
Main limitation on resolution aberrations in main imaging lens
Basis for magnification - strength
of post- specimen lenses
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TEM - transmission electron microscopy
Instrument components
Electron gun (described previously)
Condenser system (lenses &
apertures for controlling
illumination on specimen)
Specimen chamber assembly
Objective lens system (imageforming lens - limits resolution;
aperture - controls imaging
conditions)
Projector lens system (magnifies
image or diffraction pattern onto
final screen)
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TEM - transmission electron microscopy
Examples
Matrix - β'-Ni2AlTi
Precipitates - twinned L12 type γ'-Ni3Al
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TEM - transmission electron microscopy
Examples
Precipitation in an
Al-Cu alloy
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TEM - transmission electron microscopy
Examples
dislocations
in superalloy
SiO2 precipitate
particle in Si
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TEM - transmission electron microscopy
Examples
lamellar Cr2N
precipitates in
stainless steel
electron
diffraction
pattern
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TEM - transmission electron microscopy
Specimen preparation
Types
replicas
films
slices
powders, fragments
foils
as is, if thin enough
ultramicrotomy
crush and/or disperse on carbon film
Foils
3 mm diam. disk
very thin (<0.1 - 1 micron - depends on material, voltage)
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TEM - transmission electron microscopy
Specimen preparation
Foils
3 mm diam. disk
very thin (<0.1 - 1 micron - depends on material, voltage)
mechanical thinning (grind)
chemical thinning (etch)
ion milling (sputter)
examine region
around perforation
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TEM - transmission electron microscopy
Diffraction
Use Bragg's law - λ = 2d sin θ
But λ much smaller
(0.0251Å at 200kV)
if d = 2.5Å, θ = 0.288°
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TEM - transmission electron microscopy
Diffraction
2θ ≈ sin 2θ = R/L
λ = 2d sin θ ≈ d (2θ)
specimen
R/L = λ/d
Rd = λL
L is "camera length"
image plane
λL is "camera constant"
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TEM - transmission electron microscopy
Diffraction
Get pattern of spots around transmitted beam from one grain (crystal)
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TEM - transmission electron microscopy
Diffraction
Symmetry of diffraction pattern reflects
symmetry of crystal around beam direction
Example:
6-fold in hexagonal, 3-fold in cubic
[111] in cubic
[001] in hexagonal
Why does 3-fold diffraction pattern look hexagonal?
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TEM - transmission electron microscopy
Diffraction
Note: all diffraction
patterns are
centrosymmetric,
even if crystal structure
is not centrosymmetric
(Friedel's law)
Some 0-level patterns
thus exhibit higher
rotational symmetry than
structure has
P cubic reciprocal lattice
layers along [111] direction
l = +1 level
0-level
l = -1 level
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TEM - transmission electron microscopy
Diffraction
Cr23C6 - F cubic
a = 10.659 Å
Ni2AlTi - P cubic
a = 2.92 Å
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TEM - transmission electron microscopy
Diffraction - Ewald construction
Remember crystallite size?
when size is small, x-ray reflection is broad
To show this using Ewald construction, reciprocal lattice points
must have a size
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TEM - transmission electron microscopy
Diffraction - Ewald construction
Many TEM specimens are thin in one direction - thus, reciprocal
lattice points elongated in one direction to rods - "relrods"
Also, λ very small, 1/λ very large
Only zero level in
position to reflect
Ewald
sphere
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TEM - transmission electron microscopy
Indexing electron diffraction patterns
Measure R-values for at least 3 reflections
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TEM - transmission electron microscopy
Indexing electron diffraction patterns
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TEM - transmission electron microscopy
Indexing electron diffraction patterns
Index other reflections by vector sums, differences
Next find zone axis from cross product of any two (hkl)s
(202) x (220) ——> [444] ——> [111]
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TEM - transmission electron microscopy
Indexing electron diffraction patterns
Find crystal system, lattice parameters, index pattern, find zone axis
ACTF!!!
Note symmetry - if cubic, what
direction has this symmetry (mm2)?
Reciprocal lattice unit cell
for cubic lattice is a cube
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TEM - transmission electron microscopy
Why index?
Detect epitaxy
Orientation relationships at grain boundaries
Orientation relationships between matrix & precipitates
Determine directions of rapid growth
Other reasons
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TEM - transmission electron microscopy
Polycrystalline regions
polycrystalline BaTiO3
spotty Debye rings
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TEM - transmission electron microscopy
Indexing electron diffraction patterns - polycrystalline regions
Same as X-rays – smallest ring - lowest θ - largest d
Hafnium (铪)
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TEM - transmission electron microscopy
Indexing electron diffraction patterns - comments
Helps to have some idea what phases present
d-values not as precise as those from X-ray data
Systematic absences for lattice centering and
other translational symmetry same as for X-rays
Intensity information difficult to interpret
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TEM - transmission electron microscopy
Sources of contrast
Diffraction contrast - some grains diffract more strongly than
others; defects may affect diffraction
Mass-thickness contrast - absorption/
scattering. Thicker areas or mat'ls w/
higher Z are dark
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TEM - transmission electron microscopy
Bright field imaging
Only main beam is used. Aperture in back focal plane blocks
diffracted beams
Image contrast mainly due to subtraction of intensity from the
main beam by diffraction
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TEM - transmission electron microscopy
Bright field imaging
Only main beam is used. Aperture in back focal plane blocks
diffracted beams
Image contrast mainly due to subtraction of intensity from the
main beam by diffraction
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TEM - transmission electron microscopy
Bright field imaging
Only main beam is used. Aperture in back focal plane blocks
diffracted beams
Image contrast mainly due to subtraction of intensity from the
main beam by diffraction
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TEM - transmission electron microscopy
Bright field imaging
Only main beam is used. Aperture in back focal plane blocks
diffracted beams
Image contrast mainly due to subtraction of intensity from the
main beam by diffraction
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TEM - transmission electron microscopy
What else is in the image?
Many artifacts
surface films
local contamination
differential thinning
others
Also get changes in image because of
annealing due to heating by beam
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TEM - transmission electron microscopy
Dark field imaging
Instead of main
beam, use a
diffracted beam
Move aperture to
diffracted beam
or tilt incident
beam
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TEM - transmission electron microscopy
Dark field imaging
Instead of main beam, use a diffracted beam
Move aperture to diffracted beam or tilt incident beam
strain field contrast
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TEM - transmission electron microscopy
Dark field imaging
Instead of main beam, use a diffracted beam
Move aperture to diffracted beam or tilt incident beam
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TEM - transmission electron microscopy
Lattice imaging
Use many diffracted beams
Slightly off-focus
Need very thin specimen region
Need precise specimen alignment
See channels through foil
Channels may be light or dark in image
Usually do image simulation to
determine features of structure
铝 钌 铜 合金
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TEM - transmission electron microscopy
Examples
M23X6 (figure at top
left).
L21 type β'-Ni2AlTi
(figure at top center).
L12 type twinned γ'Ni3Al (figure at bottom
center).
L10 type twinned NiAl
martensite (figure at
bottom right).
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