TEM Technique for nano-Science

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TEM Technique for nano-Science I.
Convergent-Beam Electron Diffraction (CBED)
Nanoscience and nanotehcnology need
nano-probe! CBED is a microanalytical technique
that uses a convergent or focused beam of
electrons to obtain diffraction patterns from
small specimen regions. CBED patterns consist of
discs of intensity (rather than spots) which are
rich in detail and can be exploited to reveal
various aspects of specimen microstructure.
Spatial resolution is determined by the focussed
incident probe size.
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Convergent Beam Electron Diffraction (CBED)

• Advantages
• Small probe
• Rocking curve information

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Comparing to Selected Area Electron Diffraction
(SAED)
SAED use parallel illumination and limits the
sample volume by an aperture in the image plane
of the low objective lens.
A SAED pattern of a crystal.
Selection area is 0.5µm in diameter (in
practice governed by aperture size) for selected
area electron diffraction. We cannot get
diffraction from smaller area using SAED.
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Thus, CBED has higher spatial resolution than
SAED. It is determined by the minimum probe
diameter. More information contained in CBED
pattern than spot pattern obtained with parallel
electron beams 1). specimen thickness 2). more
precise information on lattice parameters 3).
crystal system and true 3D symmetry of the atom
arrangement. The price to pay is (experimental
problems of CBED) 1) Contamination (CBED
requires clean specimens, UHV in the
microscope) 2). local specimen heating, thermal
expansion, thermal stresses If possible, use
double-tilt cooling holder for the specimen.
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Electron Probe parallel or convergent?
A(u)A1(u)A2(u), where A1(u) is a sharp peak
sitting on a broad background A2(u). ?f defocus
value. Cs Spherical aberration coefficient.
e
e
sample
Parallel incident beam
Focused convergent beam
A simulated probe using Cs1 mm and ?f-360 nm.
(Zuo 2003).
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How to obtain CBED patterns? 1. Focus image at
eucentric height. 2. Excite Condenser lens C1 to
yield a small spot size. 3. Focus probe with
Condenser lens C2 on to the area of interest. 4.
Press Diffraction button. 5. Optimize pattern.
Spot pattern
CBED pattern disk
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CBED patterns with different convergent angle
Condenser lens aperture
Condenser lens aperture
e
e
e
Small angle
medium angle
Large angle CBED
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CBED patterns with different camera length
Large
Medium
Small
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CBED patterns with different focus (objective
lens)
overfocus
focus
underfocus
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Viewing the shadow image in CBED mode 1.
Condenser lens C2 heavily under-focussed. 2. C2
approaching focus. 3. C2 at focus (CBED
Pattern). 4. C2 over-focussed.
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CBED patterns with different specimen thickness
thin
thick
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How are Kikuchi lines formed? (1) Electrons
which have been inelastically scattered can
subsequently be diffracted. They are observable
only if they are traveling at the Bragg angle to
a set of planes. (2) Two sets of electrons will
be able to do this - those at ?B and those at
-?B. (3) This diffraction results in intensity
changes in the background.  Because there are
more electrons at B than A  (since electrons
passing through B are closer to the incident
direction than those through A) one bright line
is developed (the excess line) together with one
dark line (the deficit line).
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What is Reciprocal Lattice?
Crystal structure in real space
Diffraction in reciprocal space
Reciprocal Lattice
Crystalline Lattice
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High Order Laue Zones
In reciprocal lattice, all reflections (hkl) in
one plane (with the normal of uvw) obeys zone
law h u k v l w N. N is always an
integer, which is called as the Laue order. uvw
is the direction of the incident electron beam.
What is HOLZ lines? Kikuchi lines also arise from
inelastic scattering of HOLZ planes. They are
called HOLZ lines.
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Determination of crystal periodicity in the beam
direction
The tenfold patterns show a tenfold distribution
of the zero-order Laue zone bands and
higher-order Laue zone rings indicating a
periodicity of (a) 1.24 nm and (b) 3.72 nm
respectively, of the Al-Cr-Cu Quasicrystals.
For FOLZ ring L Camera Length
Illustration formation of high order Laue zone
ring.
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Convergent Beam Electron Diffraction (CBED)
High Voltage Calibration using HOLZ lines
201kV
Si 331 200kV, -180 C, Energy-filtered
(13,-13,3)
(-13,13,3)
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HOLZ line can be used to measure accurately the
lattice parameters
(A) HOLZ-line patterns obtain from several
different Cu-Al alloys in the 114 direction.
The HOLZ-line shifts are due to changes in
lattice parameters. (B) Computer simulation of
the the HOLZ pattern showing the lattice
parameter that corresponds to each experimental
pattern. Williams and Carter (1996).
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Convergent Beam Electron Diffraction (CBED) for
symmetry determination
m
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Phase Intergrowth Small probe can be used to
identify structure from small area!
Intergrowth of two orthorhombic Ga-Mn phases in
form of five fold twins.
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Jiang, Zuo and Spence, Acta Cryst. A 59,
341-350, (2003).
Measure Low-order Structure factors
Ti
Charge density deformation map of TiO2 crystal
along (001) plane, it pass through the center
point.
O
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Effects of Energy-filtering Energy filter is
quite important for CBED quantitative analysis
(to reduce strong background due to the large
amount of thermal diffuse scattering).
Unfiltered
filtered, 10 ev Slit