SEM TEM

Report
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Scanning Electron Microscopy (SEM)
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Uses
Sample Preparation
Instrument
Principles
Micrographs
Transmission Electron Microscopy (TEM)
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Uses
Sample Preparation
Instrument
Principles
Micrographs
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Topography
◦ Texture/surface of a sample
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Morphology
◦ Size, shape, order of particles
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Composition
◦ Elemental composition of sample
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Crystalline Structure
◦ Arrangement present within sample
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Samples must be small enough to fit in sample
chamber
◦ Most modern microscopes can safely accommodate
samples up to 15cm in height
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Samples must be electrically conductive
Polymer samples typically need to be sputter
coated to make sample conductive
◦ Ultra-thin metal coating
◦ Usually gold or gold/palladium alloy
◦ Coating helps to improve image resolution
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Once sample is properly prepared, it is placed
inside the sample chamber
Once chamber is under vacuum, a high voltage is
placed across a tungsten filament to generate a
beam of high energy electrons (electron gun) and
serves as the cathode
The position of the anode allows for the
generated electrons to accelerate downward
towards the sample
Condensing lenses “condense” the electrons into
a beam and objective lenses focus the beam to a
fine point on the sample
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Scanning coils move
the focused beam
across the sample in
a raster scan pattern
Same principle used
in televisions
Scan speed is
controllable
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As electron beam strikes sample, secondary
electrons are emitted from the sample
In addition, backscattered electrons are also
emitted from the sample
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Secondary Electrons
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Back-Scattered Electrons
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Electrons strike the sample surface
As a result, some electrons “splash” out from
the sample (secondary electrons)
A detector with a strong positive charge
attracts these electrons, however depending
on the surface topography, not all electrons
will be attracted
◦ Electrons on high “peaks” will be attracted to the
positively charged detector
◦ Electrons in low “valleys” will not be attracted to the
detector
Crystalline Latex Particles
Polymer Hydrogel Surface
SEM Images of PVEA Comb-like
Terpolymers
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Electrons from high-energy beam strike the
sample
Some electrons pass close to a nucleus and
are deflected by the positive charge
◦ These back-scattered electrons return to the
sample surface moving at high speed
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Back-scattered electrons is dependent on
atomic number of sample
◦ Can provide elemental composition information
about a sample
BSE Micrograph Showing Crystalline
Lamellae
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Morphology
◦ Shape, size, order of particles in sample
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Crystalline Structure
◦ Arrangement of atoms in the sample
◦ Imperfections in crystalline structure (defects)
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Composition
◦ Elemental composition of the sample
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Samples need to be extremely thin to be
electron transparent so electron beam can
penetrate
Ultramicrotomy is a method used for slicing
samples
◦ Slices need to be 50-100nm thick for effective TEM
analysis with good resolution
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Instrument setup is similar to SEM
Instead of employing a raster scan across the
sample surface, the electron beam is
“transmitted” through the sample
Material density determines darkening of
micrograph
◦ Darker areas on micrograph indicate a denser packing of
atoms which correlates to less electrons reaching the
fluorescent screen
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Electrons which penetrate the sample are
collected on a screen/detector and converted
into an image
Isotactic Polypropylene
Particle
Large
Ceria
Nanoparticles
Small
Ceria
Nanoparticles
Microcrystalline Cellulose
Pros
 Easier sample
preparation
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Ability to image
larger samples
Ability to view a
larger sample area
Cons
 Maximum
magnification is
lower than TEM
(500,000x)
 Maximum image
resolution is lower
than TEM (0.5nm)
 Sputter coating
process may alter
sample surface
Pros
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Higher
magnifications are
possible
(50,000,000x)
Resolution is higher
(below 0.5Å)
Possible to image
individual atoms
Cons
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Sample preparation
Sample structure
may be altered
during preparation
process
Field of view is very
narrow and may not
be representative of
the entire sample as
a whole
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