Transmission Electron Microscopy (TEM)

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Transmission Electron Microscopy (TEM)

• Hendrik O. Colijn
• OSU Campus Electron Optics Facility
• colijn.1_at_osu.edu
• 292-0674
• www.ceof.ohio-state.edu/classes/MSE605.ppt

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TEM

• CM12 TEM/STEM
• CM200 LaB6 TEM
• Tecnai F20 (S)TEM
• Titan 80-300 (S)TEM

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Why EM?

• Rayleigh Criterion

Let msinb 1, then d ? 300nm for light
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Why EM?

• Rayleigh Criterion for electrons (small b)

l ? 0.003nm for electrons compared to 500nm for
light over 100,000x smaller! However, b is
smaller too.
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EM vs. LM

• Electrons vs. photons different interaction
• Strong e- interactions require a thin sample.
• Possible radiation damage.
• Vacuum environment.
• Magnetic lenses -- charged particles wont go
  through glass.

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Sample constraints

• Vacuum compatible
• Electron-sample interaction different than for
  light. Biological samples (and polymers) often
  need stained.
• Able to withstand radiation dose, needs to be
  somewhat conductive.

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TEM is a projection device
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Similarity of LM and TEM
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SEM
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Illumination sources

• Thermionic
• Tungsten
• LaB6
• Field Emission
• Cold FEG
• Schottky FEG

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Probe Currents vs. E0
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TEM
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TEM Schematic (CM12/EM400)
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TEM Imaging
Remember -- you are looking at a 2D projection of
a 3D object.
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Image vs. Diffraction
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TEM Imaging

• In most cases, you are using amplitude contrast
  rather than phase contrast.
• Like light microscopy, you can do BF and DF
  imaging.
• You can also do diffraction from sub micron areas
  to examine crystal structure.

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STEM in a TEM
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STEM detectors
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STEM
TEM
STEM
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HAADF STEM NiPt Catalyst
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HAADF STEM NiPt catalyst
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Ni3Al Z-contrast STEM
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EDS unit
Oxford Instruments EDSHardwareExplained.pdf
Image courtesy of Oxford Instruments
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EDS detector
Image courtesy of Oxford Instruments
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TEM detector
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EDS detector
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Relative resolution
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SEM EDS
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TEM EDS - NiAl
Al Ka
Ni La
Ni Ka
Ni Kb
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Ionization Cross-section, Q

• Probability of an electron ionizing an atom.
• Overvoltage U E0/Ec
• Bethe cross-section model

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Cross-section vs. overvoltage
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Fluorescence yield, w

• Probability of generating an X-Ray from an
  ionized atom.
• w a 1
• For k shell ionizations
• C 0.0013, S 0.06, Fe 0.3

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Fluorescence Yield
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UTW vs. Be Window
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Cliff-Lorimer equation
Ca Cb are in weight percent (a tradition
started by Castaing)
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Cliff-Lorimer equation

• C-L equation describes relative concentrations
  and gives only n-1 equations for n elements
• The nth equation is to require the sum of the
  concentrations 1. If you neglect an element,
  the calculated concentrations will vary from the
  true concentrations though the relative amounts
  are OK.
• The C-L equation neglects absorption and
  fluorescence. It is exactly valid only in the
  thin film limit (i.e. at t 0).

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k-factors

• kab-factors are relative sensitivity factors.
  Always reported with respect to another (usually
  common) element.
• The original reference element was Si because
  Cliff and Lorimer were geologists. Metallurgists
  will often use Fe.
• We can use theoretical k-factors, but

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Theoretical k-factors
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Electron Energy Loss

• GIF (Gatan Image Filter)
• In-column filter
• EELS spectrum
• Energy filtered imaging

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EELS Post-Column Spectrometer
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GIF spectrometer
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In-Column Spectrometer
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In-column energy filter
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EELS Spectrum
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Zero-loss image
Unfiltered
Filtered
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EELS Spectrum
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N background 1
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N background 2
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N background extrapolation
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N gross peak intensity
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N net EELS image
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TEM techniques

• BF/DF
• Stain specificity
• Low dose
• Diffraction
• Selected Area
• Convergent Beam
• HREM
• Weak phase object
• Aberration correctors
• STEM
• Tomography
• Elemental Info
• EDS
• EELS
• Electron Spectroscopic Imaging
• Holography

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References

• TEM (materials) Williams, D.B. and Carter,
  C.B., Transmission Electron Microscopy A
  Textbook for Materials Science, Kluwer/Plenum,
  1996
• SEM Goldstein, J., Newbury, D., et al, Scanning
  Electron Microscopy and X-Ray Microanalysis, 3rd
  ed., Kluwer/Plenum, 2003