Transcript No Slide Title

The TEM system and components:
Vacuum Subsystem
Electron Gun Subsystem
Electron Lens Subsystem
Sample Stage
More Electron Lenses
Viewing Screen w/scintillator
Camera Chamber
TEM Illumination control
• Filament saturation
• Filament centering
• Spot size (Condenser
Lens Current)
• Condenser aperture
Focusing the TEM
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Control objective lens current
Adjust astigmatism correction coils too
Use large screen at low mags
Use small screen at high mags
Beware of lingering on an area too long
Iterate focus and stigmators
Can take through-focus-series
Focusing on a hole in a thin carbon film
(Fresnel Fringes)
(a) underfocused objective lens (bright fringe);
(b) at focus (no fringe);
(c) overfocused objective lens (dark fringe).
Note also the change in appearance of the
carbon fine grain.
Magnification ~750,000X.
(From Agar,p.137).
Astigmatism Correction
• Sort of like the SEM in that astigmatism shows up as a
directional defocus
• Must correct condenser lens, objective lens and projector
lens separately
• Use both objective stigmator selectors
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Use the first one
Use the second one
Refocus
Repeat
Contrast Considerations
• Resolving power is good (why?), but…
– To see image features contrast is needed
• How to increase contrast…?
• (SEM contrast is derived from local topography
and/or differential interactions of beam with
sample)
Contrast Considerations
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Mass-thickness Contrast (absorption)
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Density x thickness
More dense or thicker areas look darker due
to absorption of beam electrons.
Thickness fringes due to destructive
interference as beam traverses the sample
Use stains to highlight specific areas
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Uranium, manganese, osmium
Coat and/or shadow areas to generate contrast
Contrast Considerations
• Other “deficiency” contrast mechanism
– Electron scattering
• Random
– Amorphous materials change electron
trajectories
• Regular
– Crystalline materials change trajectories
uniformly
Contrast Considerations
• Most of the beam is NFS
– Goes right through the sample unperturbed
– Other elastic interactions change the direction
of NFS electrons which can be selected for or
eliminated from the image forming beam.
More on this later…
Brightness Considerations
• Type of source (W, LaB6, FE)
• Higher mags means a strong Int-Proj lens,
which means lower intensity
• Hard to see on fluorescent screen
• Ways to mitigate
– use lower mags
– converge beam with condenser lens
– align beam as needed
Beam Energy Considerations
• Higher voltages produces
– shorter wavelength
– better resolution
– greater depth penetration of sample
Beam Energy Considerations
• Lower voltages produces
– greater contrast due to larger scattering angles
for slow(er) moving electrons
– less depth penetration
– larger proportion of electrons involved in
inelastic collision events
Electron Beam-Sample Interaction
Magnification Considerations
• Higher mag-> features look larger (duh)
• But intensity drops off and ability to
properly focus and stigmate do too
• Use LOWEST practical magnification
Most photographic emulsions used in electron microscopy
can resolve image details of ~20µm, thus the resolution
of object details will depend on the image magnification
as shown in the table (resolution = 20µm/magnification):
Magnification
2,000
20,000
50,000
100,000
Resolution at Object (nm)
10.0
1.0
0.4
0.2
Exposure Considerations
• Low intensity situations lead to longer
exposure times
• Vibration will make edges blurry
• High intensity situations lead to short
exposure times (and concomitant error)
Sample Stability Considerations
• High intensity or long exposure situations
may cause sample to degrade (bonds break,
polymer chain-scission, etc.)
• Remember the contamination square in the
SEM??? Same thing happens in the TEMyou will grow a nice carbon bump on
samples as you look at them.
TEM Sample Prep for Materials
TEM Sample Prep for
Biologicals
• Almost always a microtome is involved
Imaging Modes in the TEM
Bright Field Mode
Dark Field Mode
Diffraction Mode
Bright Field Imaging
• If the main portion of the near-forward
scattered beam is used to form the image
– transmitted beam
– 000 beam
– zero-order beam
Dark Field Imaging
• If the transmitted beam is
excluded from the image
formation process
– off-axis imaging
– tilted beam imaging
TEM Imaging:
Ray Paths
Electron Diffraction
• Elastic Scattering Events
– Bragg diffraction
• nl=2d sinq
Electron Diffraction
• Four conditions in Back Focal Plane (BFP) of
the objective lens:
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No sample
Amorphous
Polycrystal
Single crystal
No reflections (only transmitted beam)
Transmitted beam + random scattering
Transmitted beam + rings
Transmitted beam + spots
Electron Diffraction
Angle of incidence ~1/20 to even come close to
satisfying the Bragg condition.
Therefore only the lattice planes close to parallel to the
beam are involved in diffraction.
Electron Diffraction
• Think of TEM as a
diffraction camera
Rd=lL
R is measured
d is the unknown
l is the electron wavelength
L is the camera length
(lL is the camera constant)
Reciprocal relationship between
lattice spacing and distance from
the transmitted spot.
Transmitted Beam
L
Diffracted Beam
R
Electron Diffraction
• Au (111) ring [2.35 Å d-spacing]
With 200KV and L=65cm the (111)
ring should be at about 7.5mm from
the transmitted beam
Rd=lL
R=0.027A*650mm/2.35A
Ray Paths in the TEM
Electron Diffraction
TEM Imaging Modes: Diffraction vs BF
Metal particles
Polymer mix
TEM Images
Electron Diffraction