Electron - University of Waterloo

Electron
• 1897: Sir Joseph John Thomson (1856-1940)
discovered “corpuscles” – small particles with a charge-to-mass
ratio over 1000 times greater than that of protons.
“Plum pudding model”: electrons in a sea of positive charge.
Nobel Prize 1906.
• 1927: Sir George Paget Thomson (1892-1975) discovered 10 kV
electrons could give diffraction pattern of a 100 nm thick gold foil.
Nobel Prize 1937, shared with Clinton Davison
(experiment on Ni single crystal).
• 1924: Doctoral thesis of Louis-Victor Pierre
Raymond de Broglie (1892-1987)
hypothesized “matter waves” and the idea of
wave- particle duality (only known fro photon
up to then). Nobel Prize 1929.
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Lecture 24
MNS 102: Techniques for Materials and Nano Sciences
Reference: #1 C. R. Brundle, C. A. Evans, S. Wilson, "Encyclopedia of Materials
Characterization", Butterworth-Heinemann, Toronto (1992), Ch. 2, Ch. 3.
Reference: http://www.microscopy.ethz.ch/methods.htm
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Electron
Electron Microscopy – Overview and History
Comparison with Light Microscopy
Transmission Electron Microscopy (TEM): Instrument and
Electron Optics
Comparison between TEM and LM
Resolution, contributing factors to the resolution
Depth of Focus and Depth of Field
Modes of operation: Imaging vs Diffraction vs STEM
Limitations of TEM
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History of Electron Microscopy
• 1926: Hans Busch (1884-1973) demonstrated
that electric and magnetic fields of axial
symmetry (short magnetic coils) can be used
as lenses for electrons and other charged
particles – father of electron optics?
• 1928: Ernst Ruska (1906-1988) began
serious study of magnetic lenses potentially
for EM applications. PhD thesis in 1929
on magnetic lenses.
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History of EM
• 1931: Max Knoll (1897-1969) and
Ruska realized the first but crude
transmission electron microscope
(TEM).
• 1932: Davisson and Calbrick studied
electrostatic lenses.
• 1934: Driest and Muller showed EM
surpassing LM in resolution.
• 1935: Knoll built the first scanning
electron microscope (SEM) with a 100
micron beam diameter.
• 1938: Manfred von Ardenne built the
first true SEM with a 50-100 nm
resolution. The machine was destroyed
in the Berlin air raid in 1944.
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History of EM
• 1938: Albert Prebus, James Hillier of Professor
Eli Franklin Burton’s group at U of T Physics built
the first TEM in North America. Their design
was later adopted by all TEM manufacturers.
http://lifeasahuman.com/2012/artsculture/history/dr-eli-franklin-burton-and-theelectron-microscope/
• 1945: 1 nm resolution achieved.
• 1961: First commercial SEM instruments after
the invention of the secondary electron
detector by Everhart and Thomley (ET).
• 1965: R.F. Pease and W. Nixon achieved 10 nm
SEM resolution.
• 1986: Nobel Prize for Transmission Electron
Microscopy to Ernst Ruska (TEM), and for
Scanning Tunnelling Microscopy (STM) to Gerd
Binnig and Heinrich Rohrer.
• 1997/98: Aberration correction
• 1999: Below 0.1 nm resolution achieved.
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Everhart Thomley SE Detector
SE = Secondary Electron
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Example: Au (a = 0.408 nm)
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Transmission Electron Microscopy vs LM
Specimen
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TEM: Electron Optics 1
• Double condenser lens: 1st condenser to create demagnified
image of the gun crossover and to minimize the spot size;
2nd condenser to control beam divergence at the sample and
the illumination spot size; condenser lens aperture to control
illumination intensity.
• Objective lens: to form an inverted initial
image that can subsequently be magnified,
and to form a diffraction pattern in the
back focal plane. Plus objective lens aperture
(placed in the back focal plane of image)
to select electrons for building the image,
and to improve contrast of the final image.
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TEM: Electron Optics 2
• Intermediate lens: to magnify initial image formed by the
objective lens, and to focus on initial image or diffraction
pattern formed on the back focal plane.
• Projector lens: to magnify the size of image with various
strengths.
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Electrostatic Lens vs Magnetic Lens
• Electrostatic lenses is used to focus
electrons, e.g. in electron source to
create a highly focussed e beam.
• Focussing is independent of the mass,
i.e. electron and ions follow the same
trajectory, and image is inverted – like
light optics.
• Paraxial ray approximation for
converging lens.
• Magnetic lenses is used for condenser
lenses and objective lenses
• Focussing is dependent of the charge-tomass ratio, i.e. 103-105 times less
effective at focussing ions, and image is
inverted and rotated. Focal length
depends on the strength of the magnetic
field.
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How sharp is the image? Contrast Conditions
• Contrast = (Max Signal – Min Signal)/Max Signal
• Based on electron scattering theory – quantum
mechanics…
• Inelastic scattering: occurs in all materials and
leads to absorption
• Incoherent elastic scattering: particularly
important for amorphous materials
• Coherent elastic scattering: leads to diffraction
from single crystal regions
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Light Microscopy vs Electron Microscopy
Light Microscope
Electron Microscope
Wavelength = 
500 nm
(150/V0) =
0.0055 nm at 50 kV
Refraction Index = n
1.5 (glass)
1.0 (vacuum)
Half-angle = 
70 deg
1 deg
Resolution = 0.61  / NA
where NA = n sin 
200 nm
0.16 nm*
Depth of Focus (DOF) =
distance parallel to the
optical axis that a feature on
the specimen can be
displaced without loss of
resolution.
DOF =
λ
𝑛2 −𝑁𝐴2
𝑁𝐴2
+
250 DOF = 0.1 𝑚𝑚
𝑀𝜃
𝑀2
M = 10, DOF = 60 m
M = 100, DOF = 8 m
M = 1,000, DOF = 200 nm
M = 10,
DOF = 1,000 m
M = 100, DOF = 100 m
M = 1,000, DOF = 10 m
M = 10,000, DOF = 1 m
*Less than 0.05 nm possible with Cs (spherical aberration) and Cc (chromatic aberration)
lens correctors. Most TEM specimens are not thin enough to produce images with
resolution that could benefit from Cs correction. For thicker specimens, Cc correction via
energy filtering is much more useful.
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Resolution – Diffraction Limit
• Diffraction limit gives:
• Nonrelativistic electron wavelength vs relativistic electron wavelength
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Resolution
Homework 6A: Calculate the nonrelativistic wavelength, relativistic wavelength,
relativistic mass, and speed for an 1, 10, 20, and 80 keV electron. Relativistic mass is
the rest mass (m0) multiplied by the highlighted term. For electron, m0 is 9.1x10-31
Kg.
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Aberration and Diffraction Effects
• Spherical aberration:
For a good magnetic lens design, KS ~ 1 and for an electrostatic lens, KS > 1,  must be
very small (~ 0.01 rad).
• Chromatic aberration:
d = CC (E/E) 
• Diffraction effects:
For  = 0.0037 nm (for a 100 keV electron beam), and CS = 1 mm (for TEM objective
lens), dmin = 0.474 nm.
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Resolution: EM vs LM
With higher kV (300
kV to 500 kV) and
proper CS correction,
resolution of 0.05 nm
may be realized.
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Light Microscopy vs Electron Microscopy
Light Microscope
Electron Microscope
Wavelength = 
500 nm
(150/V0) =
0.0055 nm at 50 kV
Refraction Index = n
1.5 (glass)
1.0 (vacuum)
Half-angle = 
70 deg
1 deg
Resolution = 0.61  / NA
where NA = n sin 
200 nm
0.16 nm*
Depth of Focus (DOF) =
distance parallel to the
optical axis that a feature on
the specimen can be
displaced without loss of
resolution.
DOF =
λ
𝑛2 −𝑁𝐴2
𝑁𝐴2
+
250 DOF = 0.1 𝑚𝑚
𝑀𝜃
𝑀2
M = 10, DOF = 60 m
M = 100, DOF = 8 m
M = 1,000, DOF = 200 nm
M = 10,
DOF = 1,000 m
M = 100, DOF = 100 m
M = 1,000, DOF = 10 m
M = 10,000, DOF = 1 m
*Less than 0.05 nm possible with Cs (spherical aberration) and Cc (chromatic aberration)
lens correctors. Most TEM specimens are not thin enough to produce images with
resolution that could benefit from Cs correction. For thicker specimens, Cc correction via
energy filtering is much more useful.
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DOF
• Depth of field corresponds to how much of the 3D object remains in focus at the same
time.
• Depth of focus corresponds to the distance over which the image can move relative to
the object and still remain in focus.
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Modes of Operation: Imaging vs Diffraction
• Usually, we do a low resolution imaging
scan to get a quick survey image, then
we proceed to obtain a high-resolution
image (HRTEM) to resolve the fringes.
The fringe spacings (between planes of
columns of atoms) will tell us directly
the interplanar separation between
specific planes.
• We can also obtain a diffraction pattern
of a selected area (called SAED =
Selected Area Electron Diffraction). Just
like XRD (except for the extremely small
sampling area), these ED patterns
contain detailed info about the
crystallography.
• The third mode is the STEM mode.
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Imaging vs Diffraction
• An objective lens is used to form a
diffraction pattern in the back focal plane
with electrons scattered by the sample and
combine them to generate an image in the
image plane (1. intermediate image).
• Diffraction pattern and image are
simultaneously present in the TEM. By
controlling the strength of the intermediate
lens, they can be made to appear in the
plane of the second intermediate image
and magnified by the projective lens on the
viewing screen.
• In imaging mode, an objective aperture can
be inserted in the back focal plane to select
one or more beams that contribute to the
final image (BF, DF, HRTEM).
• For selected area electron diffraction
(SAED), an aperture in the plane of the first
intermediate image defines the region of
which the diffraction is obtained.
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Bright Field vs Dark Field Imaging
• BF: An aperture is placed in the back focal plane of the
objective lens which allows only the direct beam to pass.
Image results from a weakening of the direct beam by its
interaction with the sample. Therefore, mass-thickness
and diffraction contrast contribute to image formation:
thick areas, areas in which heavy atoms are enriched,
and crystalline areas appear with dark contrast.
• DF: The direct beam is blocked by the aperture while
one or more diffracted beams are allowed to pass the
objective aperture. Since diffracted beams have strongly
interacted with the specimen, very useful information is
present in DF images, e.g., about planar defects, stacking
faults or particle size.
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BF vs DF
• TEM BF and DF images of the same area of microcrystalline ZrO2.
• In the BF image (left), some crystals appear with dark contrast since they are oriented
(almost) parallel to a zone axis (Bragg contrast). Thickness contrast also occurs: areas
close to the edge are thinner and thus appear brighter (lower right side) than those far
of the edge (upper left side).
• In the DF image (right), some of the microcrystals appear with bright contrast, namely
such whose diffracted beams partly pass the objective aperture.
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High-Resolution TEM Imaging
• HRTEM: A large objective aperture has to be selected
that allows many beams including the direct beam to
pass. The image is formed by the interference of the
diffracted beams with the direct beam (phase
contrast). If the point resolution of the microscope is
sufficiently high and a suitable crystalline sample
oriented along a zone axis, then high-resolution TEM
(HRTEM) images are obtained. In many cases, the
atomic structure of a specimen can directly be
investigated by HRTEM. This corresponds to
columns of atoms along the zone axis.
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TEM vs STEM (Scanning TEM)
Specimen
• STEM mode: The electron beam is rastered across the
specimen and the transmitted electrons are detected
by various (annular) detectors. Undiffracted beam is
detected by the Bright Field (BF) detector. Diffracted
beams are detected either by the Annular Dark Field
(ADF) detector or High Angle ADF (HAADF) detector.
• Signals from BF, ADF, HAADF can be used to provide
info about material type (composition and structure),
orientation (diffraction) and topography.
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FePt Alloy Nanoparticles for Biosensing:
Enhancement of Vitamin C Sensor
Performance and Selectivity by
Nanoalloying
Nafiseh Moghimi, K.T. Leung*
WATLab, and Department of Chemistry
University of Waterloo
Waterloo, Ontario, Canada N2L3G1
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Limitations of TEM
• Sampling: The higher the resolution, the smaller the amount of materials that the TEM
examines. The total amount of materials sampled by TEM in the past 15 years is no
more than 0.3 mm3 – very tiny!
• 2D projection of a 3D specimen: The image contains columns of atoms from the top
surface, middle section, and the bottom surface. Not really true 2D and definitely not
3D – Tomography is being developed to overcome this problem but this requires large
specimen rotation (> 120 deg) to get full 3D info.
• Electron beam damage: The energy of the e-beam is large enough to displace atoms in
the specimen. Most materials (organic, biological, polymer, many ceramics) will suffer
radiation damage above 80 keV.
• Specimen preparation: The quality of the info obtained greatly depends on how and
how good the specimen is prepared. Specimen thickness should be less than 100 nm
generally, but the thinner the specimen the better the quality of the image. Methods
include slicing, fracturing, ion milling, electrochemical polishing, focussed ion beam
(FIB) preparation…
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WATLab TEM: Zeiss Libra 200MC
http://microscopy.zeiss.com/microscopy/en_de/products/transmission-electron-microscopy/libra-200for-materials.html
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