Transcript Imaging and contrast

Analytical
Transmissions Electron Microscopy
(TEM)
Part I:
Basic principles
Operational modes
Diffraction
Part II:
Imaging
Sample preparation
Part III
Spectroscopy
A.E. Gunnæs
MENA3100 V13
Imaging and contrast
Resolution of the eyes:~ 0.1-0.2 mm
Resolution in a visible light microscope: ~300 nm
Modern TEMs with Cs correctors have sub Å resolution!
A.E. Gunnæs
Imaging / microscopy
TEM
BiFeO
- High resolution (HREM)
- Bright field (BF)
- Dark field (DF)
- Shadow imaging
(SAD+DF+BF)
Pt
TiO
SiO2
Si
STEM
- Z-contrast (HAADF)
- Elemental mapping
(EDS and EELS)
200 nm
GIF
- Energy filtering
Holography
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MENA3100 V08
2
3
Glue
Contrast
• Difference in intensity of to adjacent areas:
( I 2  I1 ) I
C

I1
I1
The eyes can not see intensity chanes that is less then 5-10%,
however, contrast in images can be enhanced digitally.
NB! It is correct to talk about strong and week contrast
but not bright and dark contrast
Amplitude contrast and
Phase-contrast images
The elctron wave can change both its
amplitude and phase as it traverses the specimen
Give rise to contrast
hole
Ag and Pb
Si
glue
(light elements)
We select imaging conditions so
that one of them dominates.
Apertures
Condenser aperture
Objective aperture
Selected area aperture
A.E. Gunnæs
MENA3100 V13
Use of apertures
Condenser aperture:
Limits the number of electrons hitting the sample (reducing the intensity),
Affecting the diameter of the discs in the convergent electron diffraction pattern.
Selected area aperture:
Allows only electrons going through an area on the sample that is limited by the SAD
aperture to contribute to the diffraction pattern (SAD pattern).
Objective aperture:
Allows certain reflections to contribute to the image. Increases the contrast in the image.
Bright field imaging (central beam, 000), Dark field imaging (one reflection, g), High resolution
Images (several reflections from a zone axis).
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MENA3100 V13
c
Simplified ray diagram
b
a
Parallel incoming electron beam
3,8 Å
Si
Sample
1,1 nm
PowderCell 2.0
Objective lense
Diffraction plane Objective aperture
(back focal plane)
Image plane
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MENA3100 V13
Selected area
aperture
Objective aperture: Contrast enhancement
Si
Ag and Pb
hole
glue
(light elements)
No aperture used
Central beam selected
Intensity:
Thickness and density dependence
Mass-thickness contrast
A.E. Gunnæs
Mass-thickness contrast in TEM
Incoherent elastic scattering (Rutherford scattering):
peaked in the forward direction, t and Z-dependent
Areas of greater Z and/or t scatter electrons
more strongly (in total).
TEM variables that affect the contrast:
-The objective aperture size .
-The high tension of the TEM.
Williams and Carter, TEM, Part 3
Springer 2009
Example of mass-thickness contrast in TEM modeMetal shadowing
BF-TEM image of latex particles on an
amorphous C-film.
The contrast is t-dependent.
What is the shape of the particles?
Effect of evaporation of a heavy metal
(Au or Au-Pd) thin coating at an oblique angle.
What is the contrast due to in the image?
Effect of inversing the contrast of the image.
The uneven metal shadowing increases
the mass contrast and thus accentuates
the topography.
Williams and Carter, TEM, Part 3
Springer 2009
Objective aperture: Contrast enhancement
Intensity:
Dependent on grain orientation
Diffraction contrast
50 nm
Try to make an illustration to explain why we get
this enhanced contrast when only the central beam
is selected by the optical aperture.
Amplitude contrast
Two principall types
and
Mass-thickness contrast
Diffraction contrast
-Primary contrast source in amorphous
materials
-In crystaline materials
-Incoherent electron scattering
-Coherent electron scattering
Both types of contrasts are seen in BF and DF images
-Can use any scattered electrons to
form DF images showing massthickness contrast
-Two beam to get strong contrast
in both BF and DF images.
Size of objective aperture
Bright field (BF), dark field (DF) and High resolution EM (HREM)
Objective
aperture
BF image
DF image
Amplitude/Diffraction contrast
HREM image
Phase contrast
Amplitude/diffraction contrast:
weak-beam (WB)
Dissociation of pure screw dislocation In Ni3Al,
Meng and Preston, J. Mater. Scicence, 35, p. 821828, 2000.
Weak-beam
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MENA3100 V13
Shadow imaging
(diffraction mode)
Parallel incoming electron beam
Sample
Objective lense
Diffraction plane
(back focal plane)
Image plane
Bending contours
sample
Obj. lens
Obj. aperture
BF image
DF image
DF image
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MENA3100 V13
Solberg, Jan Ketil & Hansen, Vidar (2001).
Innføring i transmisjon elektronmikroskopi
Double diffraction,
extinction thickness
•
Double electron diffraction leads to
oscillations in the diffracted intensity
with increasing thickness of the
sample
–
–
•
Incident beam
No double diffraction with XRD,
kinematical intensities
Forbidden reflection may be observed
t0: Extinction thickness
–
–
Periodicity of the oscillations
t0=πVc/λIF(hkl)I
Wedge shaped TEM sample
t0
Transmitted
Diffracted beamDoubly
diffracted beam
beam
Thickness fringes/contours
In the two-beam situation the intensity
of the diffracted and direct beam
is periodic with thickness (Ig=1- Io)
000
e
g
Ig=1- Io
Sample (side view)
t
Hole
Sample (top view)
Ig=(πt/ξg)2(sin2(πtseff)/(πtseff)2))
t = distance ”traveled” by the diffracted beam.
ξg = extinction distance
Positions with max
Intensity in Ig
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MENA3100 V13
Thickness fringes
bright and dark field images
Sample
Sample
BF image
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DF image
MENA3100 V13
Phase contrast:
HREM and Moire’ fringes
Long-Wei Yin et al., Materials Letters, 52, p.187-191
HREM image
Interference pattern
2
nm
200-400 kV TEMs are most
commonly used for HREM
A Moiré pattern is an
interference pattern created,
for example, when two grids
are overlaid at an angle, or
when they have slightly
different mesh sizes
(rotational and parallel Moire’
patterns).
http://www.mathematik.com/Moire/
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MENA3100 V13
Moire’ fringe spacing
Parallel Moire’ spacing
dmoire’= 1 / IΔgI = 1 / Ig1-g2I = d1d2/Id1-d2I
Rotational Moire’ spacing
dmoire’= 1 / IΔgI = 1 / Ig1-g2I ~1/gβ = d/β
Parallel and rotational Moire’ spacing
dmoire’= d1d2/((d1-d2)2 + d1d2β2)0.5
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MENA3100 V13
g2
g1 Δg
g1
β
g2
Δg
Sample preparation
and
what to consider before you start
SAFETY!!!!
• Know what you
handling.
– MSDS
• Protect your self and
others around you.
– Follow instructions
• If an accident occurs,
know how to respond.
Work in the Stucture Physics lab
• Get the local HMS
instructions from
Ole Bjørn Karlsen
Sign a form confirming
that you have got the
information
Ask
What to considder before preparing
a TEM specimen
•
•
•
•
•
•
Ductile/fragile
Bulk/surface/powder
Insulating/conducting
Heat resistant
Single phase/multi phase
Etc, etc…….
What is the objectiv of
the TEM work?
Specimen preparation for TEM
• Crushing
• Cutting
– saw, “diamond” pen, ultrasonic
drill, FIB
• Mechanical thinning
– Grinding, dimpling,
– Tripod polishing
•
•
•
•
•
Electrochemical thinning
Ion milling
Coating
Replica methods
Etc.
Self-supporting disk or grid
• Self supporting disk
– Consists of one material
• Can be a composite
– Can be handled with a
tweezers
• Metallic, magnetic, nonmagnetic, plastic, vacuum
If brittle, consider Cu washer
with a slot
3 mm
• Grid
– Several types
– Different materials (Cu,
Ni…)
– Support brittle materials
– Support small particles
The grid may contribute to
the EDS.
Preparation of self-supporting discs
Top view
• Cutting
– Ductile material or not?
• Grinding
– 100-200 μm thick
– polish
• Cut the 3mm disc
• Dimple ?
• Final thinning
– Ion beam milling
– Electropolishing
Cross section TEM sample preparation: Thin films
Cut out cylinder
•
Top view
Cut out a cylinder
and glue it in a Cu-tube
Cut out slices
•
•
Grind down/
dimple
Cross section
Glue the interface
of interest face to
face together with
support material
Focused Ion Beam
(FIB)
Ione beam thinning
Grind down and
glue on Cu-rings
or
Cut a slice of the
cylinder and grind
it down / dimple
Cut off excess
material
Ione beam thinning
A.E. Gunnæs
MENA3100 V13