This document discusses different types of electron microscopy techniques. It begins by explaining the need for high resolution in electron microscopy due to the small scale of samples. Different electron microscopy techniques are then described, including transmission electron microscopy (TEM), scanning electron microscopy (SEM), and scanning tunneling microscopy (STM). The document focuses on SEM, explaining how it works by scanning a sample with a beam of electrons to produce signals containing information about the sample's surface topography and composition. Sample preparation methods and interactions between the electron beam and sample are also outlined.

1. ELECTRON MICROSCOPY
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2. Need Of EM
• Resolution.
• Depth of field
• Microanalysis
• Chemical analysis
• Guidable Medium
• Wave-particle duality
Need Of Electrons
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3. Resolution
where
λ is the wavelength of the radiation, μ is the refractive index of the view medium, 𝝱 is the semi-angle of
collection of the magnifying lens, 𝑚0 mass of electron, c light speed, e charge of electron, V applied electrical
potential difference, h plank’s constant
Wavelength of visible light is in range 390 to 700mm and can theoretically resolve up
to 0.2𝝁m
Wave length of electron is about 0.00370nm can theoretically resolve upto
0.012nm
Abbe’s Equation
R =
0.612ࣅ (Wave𝑙𝑒𝑛𝑔𝑡ℎ)
μ sin 𝝱 𝑛𝑢𝑚𝑒𝑟𝑖𝑐𝑎𝑙 𝑎𝑝𝑒𝑟𝑡𝑢𝑟𝑒
De Broglie’s wave equation
𝝀 =
ℎ
𝑚𝑣
=
ℎ
2𝑚0 𝑒𝑉(1+
𝑒𝑉
2𝑚0
𝑐2)
≈
1.5
𝑉
1
2
𝑛𝑚
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4. Types of EM
• TEM
• SEM
• STEM
• STM
• AFM
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5. • Topography
• Texture/surface of a sample
• Morphology
• Size, shape, order of particles
• Composition
• Elemental composition of sample
• Crystalline Structure
• Arrangement present within sample
What can you see with an SEM?

6. Typical Application Of Sem In Polymer
Analysis
• Examine pull-out of fibers in composite materials upon fracture for
evidence of poor wetting and bonding
• Examine microcracking in surfaces, thin films, and coatings
• Find pinholes in coatings
• Examine grain shape and orientation, often important in extruded
and rolled materials
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7. • Samples must be small enough to fit in sample chamber
• Most modern microscopes can safely accommodate samples up to 15cm in height
• Samples should be carefully prepared for high vacuum compatibility –
excessively high outgassing and liquid evaporation is to be avoided
• 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
SEM Sample Preparation

8.  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
Scanning Electron Microscopy

9. • Scanning coils move the focused
beam across the sample in a raster
scan pattern
• Same principle used in televisions
• Scan speed is controllable
Scanning Electron Microscopy

10. SEM Sample Interactions

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Electron beam-sample interactions
• The incident electron beam is scattered in the sample, both elastically and
inelastically
• This gives rise to various signals that we can detect (more on that on next slide)
• Interaction volume increases with increasing acceleration voltage and decreases
with increasing atomic number
11

12. Where does the signals come from?
• Diameter of the interaction
volume is larger than the electron
spot
 resolution is poorer than the
size of the electron spot
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14. SE1
 The secondary electrons that are generated by the incoming
electron beam as they enter the surface
 High resolution signal with a resolution which is only limited by
the electron beam diameter
SE2
 The secondary electrons that are
generated by the backscattered electrons
that have returned to the surface after
several inelastic scattering events
 SE2 come from a surface area that is bigger
than the spot from the incoming electrons
 resolution is poorer than for SE1
exclusively
Sample
surface
Incoming electrons
SE2
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Factors that affect SE emission
3. Atomic number (Z)
• More SE2 are created with increasing Z
• The Z-dependence is more pronounced at lower beam energies
4. The local curvature of the surface (the most important factor)
15

16. Backscattered electrons (BSE)
 A fraction of the incident electrons is retarded by the
electro-magnetic field of the nucleus and if the scattering
angle is greater than 180° the electron can escape from
the surface
 High energy electrons (elastic scattering)
 Fewer BSE than SE
 We differentiate between BSE1 and BSE2
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17. BSE vs SE
SE produces higher resolution
images than BSE
By placing the secondary
electron detector inside the
lens, mainly SE1 are detected
Resolution of 1 – 2 nm is
possible
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18. X-rays
 Photons not electrons
 Each element has a fingerprint X-
ray signal
 Poorer spatial resolution than BSE
and SE
 Relatively few X-ray signals are
emitted and the detector is
inefficient
 relatively long signal
collecting times are needed
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19. SEM Instruments
• Electron Gun
• Electron Column
• Sample Chamber
• Detectors
• Analyzer (computer)
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20. Electron Gun
• Thermionic gun
• Field emission gun
Emission Thermionic Field Emission
W LaB6 FE
Size (angstroms) 1 x 10
6
2 x 10
5
<1 x 10
2
Brightness
(A/cm2.steradian)
104 – 10
5
105 – 10
6
107 – 10
9
Energy Spread (eV) 1 – 5 0.5 – 3.0 0.2 – 0.3
Operating Lifetime
(hrs)
>20 >100 >300
Vacuum (torr) 10-4 – 10
-5
10-6 – 10
-7
10-9 – 10
-10
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21. Electron guns
 We want many electrons per
time unit per area (high current
density) and as small electron
spot as possible
 Traditional guns: thermionic
electron gun (electrons are
emitted when a solid is heated)
 W-wire, LaB6-crystal
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22. Filament saturation
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23. Field emission electron source:
High electric field at very sharp tip causes electrons to
"tunnel"
cool tip ——> smaller E in beam
improved coherence
many electrons from small tip ——> finer
probe size, higher current densities (100X >)

24. Schottky field emission
• A hot field emission gun has some advantages compared to cold
field emitters. The major advantages are better beam current stability,
less stringent vacuum requirements and the fact that there is no need
for periodic emitter flashing (heating the cold filament for a short
time each day) to restore the emission current.
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25. magnetic lens
p
q
Magnetic lens
(solenoid)
Lens formula: 1/f = 1/p + 1/q
M = q/pDemagnification:
(Beam diameter)
F = -e(v x B)
f  Bo
2
f can be adjusted by changing Bo, i.e., changing the
current through coil.
Why suitable for high
energy e
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27. SEM over look
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28. Specimen
What comes from specimen?
Backscattered electrons
Secondary electrons
Fluorescent X-rays
high energy
compositional contrast
low energy
topographic contrast
composition - EDS
Brightness of regions in image increases as
atomic number increases
(less penetration gives more backscattered
electrons)

29. Backscattered electron detector - solid state detector
electron energy up to 30-50 keV
annular around incident beam
repel secondary electrons with
— biased mesh
images are more sensitive to
chemical composition (electron
yield depends on atomic number)
line of sight necessary

30. Back-Scattered Electron Detector
BSE Micrograph Showing Crystalline
Lamellae

31. Secondary electron detector - scintillation detector
+ bias mesh needed in front of
detector to attract low energy
electrons
line of sight unnecessary

32. SEM Micrographs
Crystalline Latex Particles Polymer Hydrogel Surface

33. SEM Micrographs
SEM Images of PVEA Comb-like
Terpolymers

34. • Morphology
• Shape, size, order of particles in sample
• Crystalline Structure
• Arrangement of atoms in the sample
• Imperfections in crystalline structure (defects)
• Composition
• Elemental composition of the sample
What can we see with a TEM?

35. • 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
TEM Sample Preparation

36.  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
 Electrons which penetrate the sample are collected on a
screen/detector and converted into an image
Transmission Electron Microscopy

37. TEM
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38. TEM Micrographs
Isotactic Polypropylene
Particle

39. TEM Micrographs
Large
Ceria
Nanoparticles
Small
Ceria
Nanoparticles

40. TEM Micrographs
Microcrystalline Cellulose

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42. Pros
• Easier sample
preparation
• Ability to image larger
samples
• Ability to view a larger
sample area
SEM Pros and Cons
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

43. Pros
• Higher magnifications are
possible (50,000,000x)
• Resolution is higher
(below 0.5Å)
• Possible to image
individual atoms
TEM Pros and Cons
Cons
 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

44. Electrons Used
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45. STM
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46. STM Types
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47. STM principle
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48. AFM
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49. The Force that is experience by the tip is mainly due to Van der Waals
force, typically 10-11 to 10-6 N at separation of ~ 1 Å.
Can work with insulator
Soft, flexible
(low spring constant)
Cantilever
Tip
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50. More Scanning Probe techniques
Lateral Force Microscopy (LFM): measures frictional forces between
the probe tip and the sample surface
Magnetic Force Microscopy (MFM): measures magnetic gradient and
distribution above the sample surface;
Electric Force Microscopy (EFM): measures electric field gradient and
distribution above the sample surface;
Scanning Thermal Microscopy (SThM): measures temperature
distribution on the sample surface.
Scanning Capacitance Microscopy (SCM): measures carrier (dopant)
concentration profiles on semiconductor surfaces.
Spin-resolved STM: atom-resolved magnetic microscopy.
AFM MFM
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52. Thank You For Listening!
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