Transcript Transmission Electron Microscopy (TEM)

Transmission Electron
Microscopy (TEM)
By Austin Avery
Overview
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What is Transmission Electron Microscopy?
History of TEM
Theory of TEM
The instrumentation
How is TEM useful?
Pros/Cons
Summary
Transmission Electron Microscopy
• TEM is an instrumental technique that uses a
tiny focused beam of electrons.
• These electrons interact with a very thin and tiny
sample, usually only a couple atoms thick.
• After passing through the sample the electrons
have changed course slightly and are detected by
a photographic film or a CCD camera.
History of TEM
• In 1931 Max Knoll and Ernst Ruska developed
the first TEM microscope, considered the first
‘electron microscopy’.
• The group was first interested in further
developing the resolution of Cathode Ray
Oscilloscopes.
• After WW2 Ruska was finally able to produce
the first TEM with 100,000x resolution power.
TEM Theory
• The reason a TEM can have such a large
resolution factor at such a small size is due to the
de Broglie λ of electrons.
• The wavelength λe= h/√[2moE(1+(E/2moc2))] of
electrons where h is planck’s constant,
mo=9.11x10-31kg, E is the energy of the
accelerated electron, and c is speed of light.
Theory
• Because electrons have wave-particle duality
properties, they can be produced at a specific
energy and wavelength and then analyzed after
being affected by a sample
• Max resolution d=λ/(2n sin α)
• Wavelegth λ
• Numerical Aperture
Instrumentation
• The TEM consists of an Electron gun made from
a Tungsten filament or a Lanthanum hexaboride
crystal
• The e-gun is charged with about 100-300 kV
before any electrons of reasonable energy will be
released from the source.
• Then a series of electromagnetic and
electrostatic lenses direct the electrons into a
beam
Optics
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There are 3 sets of lens on a typical TEM
Condenser lens: Electron beam formation
Objector lens: focus of beam onto sample
Projector lens: expands electron beam into
analytical form onto analysis screen or CCD
• Magnification adjustments are made by varying
the current through the quadrupole or hexapole
lenses
Vacuum Components
• Vacuum system: ~10-4 to
10-9 Pa
• Very important to
prevent arc between
cathode and ground
• Mean free path of
electrons
• Better beam focusing,
less interaction with gas
molecules
Sample Components
• Sample grids: Usually 3mm diameter mesh ring
with 1-100μm size squares made of Cu, Mb, Au,
or Pt
• Samples inserted into section with air locks to
prevent large decreases in vacuum pressure
• Stages are designed to adjust the samples
orientation when inside the TEM for more
accurate readings
Electron Lens Components
• Electron lens: Focus
parallel rays at a constant
length
• TEM lenses are usually
electromagnetic
• Made of Fe, Fe-Co, or
Ni-Co because of their
magnetic properties like
magnetic saturation,
hysteresis, and
permeability
Aperture Components
• Filter electrons that can stray from the beam
path and affect image quality
• Decrease beam intensity, helpful with beam
sensitive samples
• Can be fixed or moveable, depending on the
quality of the instrument or manufacturer
• Made of metallic substances thick enough to
stop stray electrons but allow axial electrons
through
Why use a TEM?
• A TEM is able to form images of sample
molecules and atoms 10’s of thousands times
smaller than any visible light microscopes
• Using the stage tuning a “tilt series” can be
developed to resolve 3D images of samples
Scan Types and Uses
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Bright Field
Diffraction Contrast
Dark Field Image
Crystallography-lattice defects
Biological specimen-many
Sub-atomic ratios
Pros/Cons
• Pros:
• Very high resolution
• Requires very little
sample to test
• Quantitative and
Qualitative
• Can be modified in many
ways to account for
different substances and
requirements
• Cons:
• Tough sample prep.
• Hour consuming runs to
get a few images
• Small field of view, may
take several runs to find
what is being studied
• Sample destruction,
especially biological
samples
Summary
• TEM is useful for small,
nanoscale analytes
• TEM can create 3D
images of samples
• TEM can be modified for
different types of
molecules and atoms
• TEM is not cheap
• TEM is GOOD!
• And that’s the way the
cookie crumbles…
References
• Kirkland, E (1998). Advanced computing in
Electron Microscopy. Springer.
• Hubbard, A (1995). The Handbook of surface
imaging and visualization. CRC Press
• Joachim Frank, editor (2006). Frank, J.
ed. Electron tomography: methods for threedimensional visualization of structures in the
cell. Springer