Introduction to Electron Microscopy Fundamental concepts in

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Introduction to Electron Microscopy

• Fundamental concepts in electron microscopy
• The construction of transmission and scanning
  electron microscopes
• Sample examples of application

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Electron Microscope vs. Optical Microscope
(first one built in 1931 by Ruska and Knoll)
(Leeuwenhoek in 17th century)

• Electron vs. Photon
• Electron charged, has rest mass, not visible
• Photon neutral, has no rest mass, visible at the
  wavelength 400 nm-760 nm.

Because of these differences, the microscope
construction will also be different
What is the common property?
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Comparison of EM and LM

• a. Similarities (Arrangement and function of
  components are similar) 1) Illumination system
  produces required radiation and directs it onto
  the specimen. Consists of a source, which emits
  the radiation, and a condenser lens, which
  focuses the illuminating beam (allowing
  variations of intensity to be made) on the
  specimen. 2) Specimen stage situated between
  the illumination and imaging systems. 3)
  Imaging system Lenses which together produce the
  final magnified image of the specimen. Consists
  of i) an objective lens which focuses the beam
  after it passes through the specimen and forms an
  intermediate image of the specimen and ii) the
  projector lens(es) which magnifies a portion of
  the intermediate image to form the final image.
  4) Image recording system Converts the
  radiation into a permanent image (typically on a
  photographic emulsion) that can be viewed.

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Comparison of EM and LM

• b. Differences 1) Optical lenses are generally
  made of glass with fixed focal lengths whereas
  magnetic lenses are constructed with
  ferromagnetic materials and windings of copper
  wire producing a focal length which can be
  changed by varying the current through the coil.
  2) Magnification in the LM is generally changed
  by switching between different power objective
  lenses mounted on a rotating turret above the
  specimen. It can also be changed if oculars
  (eyepieces) of different power are used. In the
  TEM the magnification (focal length) of the
  objective remains fixed while the focal length of
  the projector lens is changed to vary
  magnification. 3) The LM has a small depth of
  field, thus different focal levels can be seen in
  the specimen. The large (relative) depth of field
  in the TEM means that the entire (thin) specimen
  is in focus simultaneously. 4) Mechanisms of
  image formation vary (phase and amplitude
  contrast). 5) TEMs are generally constructed
  with the radiation source at the top of the
  instrument the source is generally situated at
  the bottom of LMs. 6) TEM is operated at high
  vacuum (since the mean free path of electrons in
  air is very small) so most specimens (biological)
  must be dehydrated (i.e. dead !!). 7) TEM
  specimens (biological) are rapidly damaged by the
  electron beam. 8) TEMs can achieve higher
  magnification and better resolution than LMs.
  9) Price tag!!! (100x more than LM)

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Resolution of a microscope
Where N.A. is the numerical aperture n(sina)
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The resolution is proportional to the
wavelength!
Electron equivalent wavelength and accelerating
voltage
The dualism wave/particle is quantified by the De
Broglie equation ? h/p h/mv ? wavelength
h Planck constant p momentum
The energy of accelerate electrons is equal to
their kinetic energy E eV m0v2/2 V
acceleration voltage e / m0 / v charge / rest
mass / velocity of the electron These equations
can be combined to calculate the wave length of
an electron with a certain energy p m0v
(2m0eV)1/2 ? h / (2m0eV)1/2 ( 1.22 / V1/2 nm)
At the acceleration voltages used in TEM,
relativistic effects have to be taken into
account (e.g. Egt100 keV) ? h / 2m0eV (1
eV/2m0/c2)1/2
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Wavelength and accelerating voltage
There are other factors that limit the resolution!
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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.
  (will be discussed in a later lecture)
• Scanning Transmission Electron Microscope (STEM)
  uses a focused beam of electrons scanning through
  a thin sliced specimen to form an image. The STEM
  looks like a TEM but produces images as does an
  SEM (one spot at a time). It is most commonly
  used for elemental analysis of samples.

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Leo 982 SEM
For SEM a fine probe (beam spot) is formed by
condenser lens and its size determines the
resolution (this differs from the TEM which is
diffraction limited)
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Electron Gun
FEG
W hairpin
LaB6 crystal
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Thermionic Sources
Increasing the filament current will increase the
beam current but only to the point of saturation
at which point an increase in the filament
current will only shorten the life of the emitter
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Beam spot image at different stage of heating
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Electromagnetic lens
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Condenser-lens system
C1 controls the spot size
The condenser aperture must be centered
C2 changes the convergence of the beam
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TEM imaging modes
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STEM image
Bright and dark field STEM image of Au particles
on a carbon film
What is the differences between this dark field
and the previous one?
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Magnification in TEM
Mob Mint Mproj Total Mag
Depends on the magnification, some lens may not
be used
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Electron-specimen interaction
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• Backscattered Electrons
• Formation
• Caused by an incident electron colliding with
  an atom in the specimen which is nearly normal to
  the incident's path. The incident electron is
  then scattered "backward" 180 degrees.
  Utilization
• The production of backscattered electrons
  varies directly with the specimen's atomic
  number. This differing production rates causes
  higher atomic number elements to appear brighter
  than lower atomic number elements. This
  interaction is utilized to differentiate parts of
  the specimen that have different average atomic
  number.

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Backscatter Detector
The most common design is a four quadrant solid
state detector that is positioned directly above
the specimen
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Gold particles on E. coli appear as bright white
dots due to the higher percentage of
backscattered electrons compared to the low
atomic weight elements in the specimen
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Backscatter image of Nickel in a leaf
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• Secondary Electrons
• Source
• Caused by an incident electron passing "near" an
  atom in the specimen, near enough to impart some
  of its energy to a lower energy electron (usually
  in the K-shell). This causes a slight energy loss
  and path change in the incident electron and the
  ionization of the electron in the specimen atom.
  This ionized electron then leaves the atom with a
  very small kinetic energy (5eV) and is then
  termed a "secondary electron". Each incident
  electron can produce several secondary electrons.
• Utilization
• Production of secondary electrons is very
  topography related. Due to their low energy, 5eV,
  only secondaries that are very near the surface
  (lt 10 nm) can exit the sample and be examined.
  Any changes in topography in the sample that are
  larger than this sampling depth will change the
  yield of secondaries due to collection
  efficiencies. Collection of these electrons is
  aided by using a "collector" in conjunction with
  the secondary electron detector. The collector is
  a grid or mesh with a 100V potential applied to
  it which is placed in front of the detector,
  attracting the negatively charged secondary
  electrons to it which then pass through the
  grid-holes and into the detector to be counted.

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A conventional secondary electron detector is
positioned off to the side of the specimen. A
faraday cage (kept at a positive bias) draws in
the low energy secondary electrons. The electrons
are then accelerated towards a scintillator which
is kept at a very high bias in order to
accelerate them into the phosphor.
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The position of the secondary electron detector
also affects signal collection and shadow. An
in-lens detector within the column is more
efficient at collecting secondary electrons that
are generated close to the final lens (i.e. short
working distance).
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Secondary Electron Detector
Side Mounted In-Lens
What are the differences between these two images?
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• Auger Electrons
• Source
• Caused by the de-energization of the specimen
  atom after a secondary electron is produced.
  Since a lower (usually K-shell) electron was
  emitted from the atom during the secondary
  electron process an inner (lower energy) shell
  now has a vacancy. A higher energy electron from
  the same atom can "fall" to a lower energy,
  filling the vacancy. This creates and energy
  surplus in the atom which can be corrected by
  emitting an outer (lower energy) electron an
  Auger Electron.
• Utilization
• Auger Electrons have a characteristic energy,
  unique to each element from which it was emitted
  from. These electrons are collected and sorted
  according to energy to give compositional
  information about the specimen. Since Auger
  Electrons have relatively low energy they are
  only emitted from the bulk specimen from a depth
  of lt 3 nm

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• X-rays
• Source
• Caused by the de-energization of the specimen
  atom after a secondary electron is produced.
  Since a lower (usually K-shell) electron was
  emitted from the atom during the secondary
  electron process an inner (lower energy) shell
  now has a vacancy. A higher energy electron can
  "fall" into the lower energy shell, filling the
  vacancy. As the electron "falls" it emits energy,
  usually X-rays to balance the total energy of the
  atom so it.
• Utilization
• X-rays or Light emitted from the atom will have
  a characteristic energy which is unique to the
  element from which it originated.
• (will be discussed in a separate lecture)

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• Unscattered Electrons
• Source
• Incident electrons which are transmitted through
  the thin specimen without any interaction
  occurring inside the specimen.
• Utilization
• The transmission of unscattered electrons is
  inversely proportional to the specimen thickness.
  Areas of the specimen that are thicker will have
  fewer transmitted unscattered electrons and so
  will appear darker, conversely the thinner areas
  will have more transmitted and thus will appear
  lighter.

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The overlapping areas appear darker
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• Elasticity Scattered electrons
• Source
• Incident electrons that are scattered (deflected
  from their original path) by atoms in the
  specimen in an elastic fashion (no loss of
  energy). These scattered electrons are then
  transmitted through the remaining portions of the
  specimen.
• Utilization
• All electrons follow Bragg's Law and thus are
  scattered according to ml2dsin q (angle of
  scattering). All incident electrons have the same
  energy(thus wavelength) and enter the specimen
  normal to its surface. All incidents that are
  scattered by the same atomic spacing will be
  scattered by the same angle. These "similar
  angle" scattered electrons can be collated using
  magnetic lenses to form a pattern of spots each
  spot corresponding to a specific atomic spacing
  (a plane). This pattern can then yield
  information about the orientation, atomic
  arrangements and phases present in the area being
  examined.

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The diffraction pattern is highly dependable on
the structure of the specimen
Dr. Schroeder will have a lecture dedicated to
diffraction
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• Inelastically Scattered Electrons
• Source
• Incident electrons that interact with specimen
  atoms in a inelastic fashion, loosing energy
  during the interaction. These electrons are then
  transmitted trough the rest of the specimen
• Utilization
• Inelastically scattered electrons can be
  utilized two ways
• Electron Energy Loss Spectroscopy (EELS) The
  inelastic loss of energy by the incident
  electrons is characteristic of the elements that
  were interacted with. These energies are unique
  to each bonding state of each element and thus
  can be used to extract both compositional and
  bonding (i.e. oxidation state) information on the
  specimen region being examined.
• Kikuchi Bands Bands of alternating light and
  dark lines that are formed by inelastic
  scattering interactions that are related to
  atomic spacings in the specimen. These bands can
  be either measured (their width is inversely
  proportional to atomic spacing) or "followed"
  like a roadmap to the "real" elasticity scattered
  electron pattern.

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EELS of NiO
Combine with STEM, can do element mapping in TEM
compare to EDX, EELS is better for lighter
elements
Unlike diffraction pattern blinks on and off, the
Kikuchi line pattern rotates when one tilts the
crystal. Thus is helpful in orientating a crystal
to certain zone axis.
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Specimen interaction volume

• Atomic number of the material being examined
  higher atomic number materials absorb or stop
  more electrons and so have a smaller interaction
  volume.
• Accelerating voltage being used higher
  voltages penetrate farther into the sample and
  generate larger interaction volumes
• Angle of incidence for the electron beam the
  greater the angle (further from normal) the
  smaller the volume

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Monte Carlo simulation of the interaction volume
Try the online simulation at
http//www.matter.org.uk/tem/electron_scattering.h
tm
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High-Resolution Electron Microscopy
Wave functions for elastically-scattered, forward
electrons

• For a weak phase object, the observable image
  intensity is
• I1 - 2??(r) ? FFTT(H).
• represents convolution.
• This shows a pure phase-contrast image.

Illustration of electron wave passing through
under phase object approximation. The phase
changes (contrast) is imaged.
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High resolution Images
Discovery of the carbon nanotube
Pt nanoparticles
S. Iijima, Nature 354, 56 (1991).