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Transmission Electron Microscopy
Wacek Swiech, Honghui Zhou, Jim Mabon,
Amish Shah (Intel) and Jian-Guo Wen (ANL)
Frederick Seitz Materials Research Laboratory
University of Illinois at Urbana-Champaign
© 2013 University of Illinois Board of Trustees. All rights reserved.
Advanced Materials Characterization Workshop
June 3rd and 4th, 2013
University of Illinois at Urbana-Champaign | Materials Research Laboratory
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Why Use Transmission Electron Microscopy?
AN is 0.95 with air
up to 1.5 with oil
Transmission Electron Microscope
(TEM) Optical Microscope
200 keV electrons l: 0.027 Å
• Sample thickness requirement:
• Thinner than 500 nm
• High quality image: <20 nm
High Resolution TEM
TEM is ideal for
investigating thin foil, thin
edge, and nanoparticles
< 500 nm
Resolution limit: ~200 nm
R = 0.66(Csλ3)1/4
Resolution
Resolution limit: ~0.25 nm
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Basic Concepts – Electron-Sample Interaction
magnetic
prism e- x 10^3e- x 10^3
Specimen
Incident high-kV beam
Transmitted
beam
Diffracted
beam
Incoherent
beam
(STEM)
Coherent beam
1. Transmitted electrons (beam)
2. Diffracted electrons (beams) (Elastically scattered)
3. Coherent beams
4. Incoherent beams
5. Inelastically scattered electrons
6. Characteristic X-rays
X-rays
Inelastically
scattered
electrons
TEM can acquire images, diffraction
patterns, spectroscopy and chemically
sensitive images at sub-nanometer to
nanometer resolution.
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Structure of a TEM
Gun and
Illumination part Gun: LaB6, FEG
80 – 300keV
Mode selection and
Magnification part View screen
TEM Sample
Objective lens part
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How does a TEM Obtain Image and Diffraction?
Conjugate planes
Object Image
Sample
Objective Lens
Back Focal Plane
Incident Electrons
< 500 nm
First Image Plane
Back Focal Plane
View Screen
First Image Plane
u v
u v f _ _ _ 1 1 1
+ =
Structural info
Morphology
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Electron Diffraction I
Ewald Sphere
l is small, Ewald sphere (1/l) is almost flat
d
Bragg’s Law
2 d sin q = nl
q Zero-order Laue Zone (ZOLZ)
First-order Laue Zone (FOLZ)
….
High-order Laue Zone (HOLZ)
ZOLZ
FOLZ
SOLZ
X-ray: about 1A
Wavelength
Electrons: 0.037A,
@ 100kV
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Electron Diffraction II
50 nm
Polycrystal
Diffraction patterns from single
grain or multiple grains
002
022
Single crystal Amorphous
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Selected Area Electron Diffraction (SAED)
Example of SAED
Selected-area aperture
1) Selected-Area Electron Diffraction
2) NanoArea Electron Diffraction
3) Convergent Beam Electron diffraction
Major Diffraction Techniques
SAED
aperture
J.G. Wen, A. Ehiasrian, I. Petrov
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Selected Area Electron Diffraction (SAED)
L. Reimer and H. Kohl, Transmission Electron
Microscopy, Physics of Image Formation, 2008.
1. Illuminate a large area of the specimen with a
parallel beam.
2. Insert an aperture in the first image plane to select
an area of the image.
3. Focus the first imaging lens on the back focal plane
Al
matrix
Al-Cu
matrix
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Selected Area Electron Diffraction
1. Can observe a large area of
specimen with a bright beam
2. Since the image plane is
magnified, can easily select an
area with an aperture A 50
mm aperture can select a 2 mm
area
3. Useful to study thick films, bulk
samples, and in-situ phase
transformations
1. Cannot select an area less
than ~1 mm due to spherical
aberrations and precision in
aperture position
2. Difficult to record diffraction
from individual nanoparticles
or thin films
Advantages
Disadvantages
Spherical Aberrations
Selected
Area
Aperture
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Nanoarea Electron Diffraction (NAED)
1. Very weak beam – difficult to see
and tilt the sample
2. More complex alignment than
conventional TEM
3. High resolution images are difficult
to obtain Need to switch
between NAED and TEM modes)
Disadvantages
1. Focus the beam on the front
focal plane of the objective
lens and use a small
condenser aperture to limit
beam size
2. A parallel beam without
selection errors A 10 mm
aperture can form a ~ 50 nm
probe size
3. Useful to investigate individual
nanocrystals and superlattices
Technique
55 nm nanoprobe
Diffraction from a single Au rod
A. B. Shah, S. Sivapalan, and C. Murphy
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Nanoarea Electron Diffraction
M. Gao, J.M. Zuo, R.D. Twesten, I. Petrov, L.A. Nagahara & R. Zhang,
Appl. Phys. Lett. 82, 2703 (2003)
J.M. Zuo, I. Vartanyants, M. Gao, R. Zhang and L.A. Nagahara,
Science, 300, 1419 (2003)
5 mm condenser aperture 30 nm
This technique was developed by CMM
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Aperture-Beam Nanoarea Electron Diffraction
20 mm condenser aperture 20 nm probe size
Conjugate planes
Condenser
Aperture Sample
5 n m5 n m
SrTiO3
CeO2
a) b)
5 n m5 n m
2 nm
020S 011S
002S
002C 220C 111C
u v
This technique was developed by CMM
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Applications of Electron Diffraction
Tilting sample to obtain 3-D structure of a crystal
Lattice parameter, space group, orientation relationship
To identify new phases, TEM has advantages:
1) Small amount of materials
2) No need to be single phases
3) Determining composition by EDS or EELS
Disadvantage: needs experience
1, 2, 3, 4, 6-fold symmetry
No 5-fold for a crystal
5-fold symmetry
More advanced electron diffraction techniques
New materials discovered by TEM
Quasi-crystal Carbon Nanotube
Helical graphene
sheet
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Convergent Beam Electron Diffraction (CBED)
Back Focal Plane
Parallel beam
Sample
Convergent-beam
Sample
1. Point and space group
2. Lattice parameter (3-D) strain field
Large-angle bright-field CBED
Bright-disk Dark-disk Whole-pattern
3. Thickness
4. Defects
CBED SAED
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Major Imaging Techniques
1) Imaging techniques in TEM mode
a) Bright-Field TEM (Diff. contrast)
b) Dark-Field TEM (Diff. contrast)
c) Weak-beam imaging
hollow-cone dark-field imaging
d) Lattice image (Phase)
e) High-resolution Electron Microscopy
(Phase)
Simulation and interpretation
2) Imaging techniques in scanning
transmission electron microscope
(STEM) mode
1) Z-contrast imaging (Dark-Field)
2) Bright-Field STEM imaging
3) High-resolution Z-contrast imaging
(Bright- & Dark-Field)
3) Spectrum imaging
1) Energy-Filtered TEM (TEM mode)
2) EELS mapping (STEM mode)
3) EDS mapping (STEM mode)
Major Imaging Contrast Mechanisms:
1. Mass-thickness contrast
2. Diffraction contrast
3. Phase contrast
4. Z-contrast
Mass-thickness contrast
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TEM Imaging Techniques
I. Diffraction Contrast Image:
Contrast related to crystal
orientation
Kikuchi Map [111]
[110]
[112]
[001]
Application:
Morphology, defects, grain boundary, strain field, precipitates
Two-beam condition
[001]
Transmitted beam
Diffracted beam
Phase Contrast Image
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TEM Imaging Techniques
Objective Lens
Aperture Back Focal Plane
Diffraction Pattern
Bright-field Image
First Image Plane
Dark-field Image
Sample Sample
T D
II. Diffraction Contrast Image: Bright-field & Dark-field Imaging
Two-beam condition
Bright-field Dark-field
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TEM Imaging Techniques
Dislocations & Stacking Faults
Bright Field Image
At edge dislocation, strain from extra half
plane of atoms causes atomic planes to
bend. The angle between the incident beam
and a few atomic planes becomes equal to
the Bragg Angle ΘB.
II. Diffraction Contrast Imaging
R. F. Egerton, Physical
Principles of Electron
Microscopy, 2007.
Near dislocations,
electrons are strongly
diffracted outside the
objective aperture
Specimen
Principal
Plane
Objective
Aperture
Transmitted
Beam
Diffracted
Beam
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Weak-beam Dark Field Imaging
High-resolution dark-field imaging
1g 2g 3g
Planes do not satisfy
Bragg diffraction
Possible planes satisfy
Bragg diffraction
Dislocations can be imaged
as 1.5 nm narrow lines Bright-field Weak-beam
C.H. Lei
g g
S
Weak-beam means
Large excitation error
Exact Bragg condition Taken by I. Petrov
“Near Bragg Condition”
Experimental weak-beam
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TEM Imaging Techniques
Dislocation loop
Diffraction contrast images of typical defects
Phase = 2 p g • R
Dislocations Stacking faults
df0 pi
dz xg
= fgexp{2pi(sz+g.R)}
Howie-Whelan equation
g 1 2 3
2 3 -p Each stacking fault changes phase
Two-beam condition for defects Use g.b = 0 to determine Burgers vector b
Sample
Dislocations
Stacking faults
II. Diffraction Contrast Image
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Lattice Beam Imaging
Two-beam condition Many-beam condition
C.H. Lei M. Marshall
[001]
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Lattice Imaging
From a LaB6 Gun Field-Emission Gun
Delocalization effect from a Schottky-emission gun (S-FEG)
Lattice image of film on substrates
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High Resolution Transmission Electron Microscopy (HRTEM)
Dfsch = - 1.2 (Csl)2
1
Scherzer defocus
Resolution limit
rsch = 0.66 Cs l
1
4
3
4
1 Scherzer Defocus:
Positive phase contrast “black atoms”
2 Scherzer Defocus: ("2nd Passband" defocus).
Contrast Transfer Function is positive
Negative phase contrast ("white atoms")
1 Dfsch 2 Dfsch
f(x,y) = exp(isVt(x,y))
~1 + i s Vt(x,y)
Vt(x,y): projected potential
Simulation of images Software: Web-EMAPS (UIUC)
MacTempas
J.G. Wen Contrast transfer function
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Scanning Transmission Electron Microscopy
magnetic
prism e- x 10^3e- x 10^3
Inelastically scattered electrons
Focused e-beam
STEM
Probe size
0.2 – 0.5 nm
Thickness
<100 nm
“Coherent” Scattering
(i.e. Interference)
“Incoherent”Scattering
i.e. Rutherford
Dark-field
Bright-field
x-rays 1. Raster a converged probe across
and collect the integrated signal
on an annular detector (dark field)
or a circular detector (bright field).
2. An incoherent image is chemically
sensitive (Z-contrast) under
certain collection angles
3. Annular dark field (ADF) STEM is
directly interpretable and does not
have contrast reversals or
delocalization effects like HRTEM
4. STEM resolution is determined by
the probe size, which is typically
0.2 to 0.5 nm for a modern FEG
STEM.
5. Since STEM images are collected
serially, the resolution is typically
limited by vibrations and stray
fields
Technique
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SEM vs STEM
magnetic
prism e- x 10^3e- x 10^3
Inelastically scattered electrons
Primary e-beam
0.5-30 keV backscattered electrons
secondary electrons <50 eV
Auger electrons
x-rays
1 mm
Primary e-beam
60-300 keV x-rays
STEM
Probe size
0.1 – 0.5 nm
Thickness
<100 nm
“Coherent” Scattering
(i.e. Interference)
“Incoherent”Scattering
i.e. Rutherford
Dark-field
Bright-field
SEM
STEM technique is similar to SEM, except the
specimen is much thinner and we collect the
transmitted electrons rather than the reflected
electrons
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TEM vs ADF-STEM
5 nm
5 nm
TEM
ADF-STEM
Ir nanoparticles
10nm
Z-contrast image
Ge quantum dots on Si substrate
1. STEM imaging gives better
contrast
2. STEM images show Z-
contrast
q
I Z2
L. Long J.G. Wen
Annular dark-field (ADF) detector
Z-contrast imaging
Z
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ADF STEM Applications
A. B. Shah et al., 2008
Superlattice of LaMnO3-SrMnO3-SrTiO3 Dopant atoms in Si
P. M. Voyles et al., 2002
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Bright Field STEM vs ADF STEM
40kX 1Mx 15Mx
40kX 1Mx 15Mx
Si GexSi1-x
Si GexSi1-x
Si – Ge Superlattice
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Spherical Aberration
1.3
mm
± 1
0 n
m
2.4 m
Since 1936, Scherzer proved that spherical and chromatic
aberrations would ultimately limit the resolution of the electron
microscope. The method to correct aberrations was well known, but
experimental aberration correctors were not successful until ~1998
due to complexities in alignment and lack of computing power.
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Spherical Aberration Correction
Before correction After correction
Cs + (–Cs) = 0
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Spherical Aberration Corrector for TEM & STEM
Harold Rose
Univ. of Tech. Darmstadt
Max Haider
CEOS, Germany N
Cs< 0
N
Hexapole 1
Hexapole 2
Round Lens 1
Round Lens 2
Optical
Axis
e-Beam
Uli Dahmen
NCEM
Hexapole Cs Corrector
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Spherical Aberration Corrector for STEM
Ondrej Krivanek
Nion Company
Quadrupole-Octupole C3-C5 corrector
First sub-Å image resolved in a STEM
P. E. Batson et
al., Nature, 2002.
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JEOL JEM2200FS with Probe Corrector @ UIUC
Diffuser
Multilayer wall
The STEM can obtain
1 Å spatial resolution
Only if the instabilities
of the room are
controlled
Piezo
Stage
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Spherical Aberration Correction
JEOL 2010F
Cs = 1 mm
Probe size
0.3 nm
JEOL 2200FS
with probe corrector
Probe size
0.1 nm
Image corrector Probe corrector
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Improved Resolution and Contrast with Cs Corrector
63 pm
Aberration correction combined with a high stability environment and high
quality specimens allow for atomic resolution imaging over a large area.
4k x 4k image shown.
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Sub-Å test using GaN film along [211] zone axis
Annular dark field STEM image
of hexagonal GaN [211]
Fourier transform of the image;
image Fourier components
extend to below 50 pm.
63 pm
TEAM Microscope at LBNL
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New Applications Possible Only with Aberration Correction
1. Better Contrast for STEM Imaging
with smaller probe
2. Reduced delocalization for HRTEM
imaging
3. Sub-2Å resolution imaging at low
voltage (60 – 100 kV) for TEM and
STEM
4. 10-20 X more probe current in the
STEM for EELS and EDS
spectroscopy
Ronchigram
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43
Specimen
Analytical (Scanning) Transmission Electron Microscopy
Incident
High-kV Beam
Bremsstrahlung
X-rays
Auger
Electrons
Secondary
electrons (SE)
Characteristic
X-rays
X-ray Energy Dispersive
Spectroscopy
Back scattered
electrons (BSE)
Forward
Scattered
electrons
Inelastically
scattered
electrons
Electron Energy Loss Spectroscopy
Elastically
scattered
electrons
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EDS and EELS in STEM Mode 44
Spatially Resolved Analysis Sub nanometer in aberration-corrected STEM !
Further refined probe
• Down to ~ 0.1 – 0.2 nm
• Carries more current
~ 1-10 nm
Aberration-corrected
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Some relevant transition events explored in EELS and EDS
• EELS: named by the initial state, e.g., L3: 2p3/2 → [3S1/2, 3d3/2, 3d5/2]
• EDS: named by the initial core-hole state, e.g., Kα: 2p → 1s
David Muller 2006: http://www.ccmr.cornell.edu/igert/modular/docs/4_Chemical_Identification_at_Nanoscale.pdf
Energy Loss
X-ray
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X-ray Energy Dispersive Spectroscopy in STEM
• High spatial resolution (~ nm compared with ~ μm in SEM) — refined incident probe — significantly reduced interaction volume (directly related to the specimen thickness)
Masashi Watanabe: X-ray Energy-Dispersive Spectrometry in Scanning Transmission Electron Microscopes
in Scanning Transmission Electron Microscopy Imaging and Analysis
reproduced from Williams and Carter: Transmission electron microscopy (2009)
• Degraded analytical sensitivity — significantly reduced interaction volume (directly related to the specimen thickness)
— collection limited by microscope column detector configuration (0.3 sr out of 4 π sr)
SEM STEM
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• TEM mode spot, area
• STEM mode spot, line-scan and 2-D mapping
Ti0.85Nb0.15 metal ion etch
Spatial resolution ~ 1 nm
Line scan
A. Ehiasrian, I. Petrov
X-ray Energy Dispersive Spectroscopy Applications
Area Mapping
Al
Mo
Si
Au
Ti
Mo
Ga
Ti
Au HAADF
voids
2-D mapping
L. Wang
A B
A B
Fe
Cr
Nb
Al
Ti
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Electron Energy Loss Spectroscopy – Electron Spectrometer
Post-column In-column
Gatan Imaging Filter (GIF)
JEOL Omega-type Energy filter
Gatan Imaging Filter (GIF)
e- x 10^3e- x 10^3
http://img17.imageshack.us/img17/1686/spectprism.jpg
Analogy: Optical prism
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Electron Energy Loss Spectrum
ZLP Low-loss
• Zero-loss (ZLP)
• Low-loss spectrum (< 50 eV)
due to interactions with weakly bound outer-shell electrons
— Plasmon peak (excitation of oscillations of weakly bound electrons)
— Inter- and Intra-Band transitions
• Core-loss spectrum (> 50 eV)
due to interactions with tightly bound inner-shell electrons
— Inner shell ionization
Ti-L
O-K
Mn-L
La-M
x100
Core-loss
~ 50 eV
Energy Loss (eV)
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Energy (KeV)
Spectrum Comparison between EELS and EDS
x 1
0^3
eV
-10
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
x 1
0^3
450 500 550 600 650 700 750 800 850 900
eV
Fe2O3
x 1
0^3
eV
-30
-20
-10
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
x 1
0^3
450 500 550 600 650 700 750 800 850 900
eV
bkgd
Spectrum
Bkgd
Edge
Fe2O3
Energy Loss (eV)
Fe-L3,2 Fe-L3,2 O-K O-K
Fe-L
Energy Loss (eV)
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Correlations of Core-loss EELS Features With Electronic Structures
Empty States Conduction/valence bands Core-shell energy levels
Neighboring atoms
O-K
Ni-L3,2 Ni-M3,2
Gatan: Review of EELS Fundamentals
O-1s O-2p
Ni-2p
Ni-3d
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Electron Energy Loss Spectrum Applications
Gatan: Review of EELS Fundamentals
• Elemental composition: the core-level binding energy is unique
• Near edge fine structure (ELNES): transition is only possible to local unoccupied states
— information about electronic structures
• Extended fine structures (EXELFS): atom-specific radical distribution of near neighbors
• Measuring the collective excitations, e.g., the combined response of the valence electrons and the electron magnetic field.
• Specimen thickness
• Optical and electronic properties
— Polarization response (complex dielectric function)
— Band structure, e.g., band gap
Thickness measurement
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EELS Near Edge Structure (ELNES) — Chemical Bonding
V J Keast, J. Microscopy, 203, 135 (2001)
C-C Sp3 bonding
C-C Sp3 bonding
C-C Sp2 bonding
C-C Sp2 bonding
π* - Sp2 bonded
C-H bonding
C-H bonding
C-H* bonding
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• Chemical Shift of L3 and L2 peak
• L3/L2 ratio (white line ratio)
• O-K edge near edge fine structure (ELNES)
EELS Near Edge Structure (ELNES) — Oxidation State of Transition Metal Oxide
M. Varela, et.al., PHYSICAL REVIEW B 79, 085117 (2009) H. Zhou
Energy Loss (eV)
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Spectr
um
Image
Counts
eV
0
2000
4000
6000
8000
Counts
500 600 700 800 900
eVC
ounts
eV
0
1000
2000
3000
4000
5000C
ounts
500 600 700 800 900
eV
Counts
eV
0
2000
4000
6000
8000
Counts
500 600 700 800 900
eV
Survey Image Spectrum Image Extracted Spectrum V-L mapping Ni-L mapping
NiO
VO1+x
VO2 V-L3,2
Ni-L3,2
STEM EELS Spectrum Imaging
H. Zhou
V-L3,2
Each image pixel carries one EELS spectrum — spatial and spectroscopy information are stored together
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Atomic-column resolution Spectrum Imaging
Ti-L O-K Mn-L La-M
Sca
n D
irectio
n
DE Electron Energy Loss
Ti L
O K Mn L
La M
Substrate Side
LaMnO3 SrTiO3
Atomic-column Resolution Spectrum Imaging
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Atomic-column Resolution Spectrum Imaging
D. A. Muller et al., Science, 2008.
La Ti
Mn RGB Image
La0.7Sr0.3MnO3 – SrTiO3 Superlattice
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Mapping of the interface chemistry and bonding States
A. B. Shah et al., Phys. Rev. B, 2010
10 x 5 LaMnO3 – SrMnO3 Superlattice
Extra states are found at sharp interfaces.
Mn-L3 O-K
LMO
SMO
LMO
SMO
Sharp
Rough
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Comparison of EELS and EDS
EELS EDS
• Atomic composition • Chemical bonding • Electronic properties • Surface properties ….
• Atomic composition only
Spatial resolution: 0.1 - 1 nm Spatial resolution: 1 nm – 10 nm
Energy resolution: ~ 1 eV Energy resolution: ~ 130 eV (Mn Kα)
Relatively difficult to use The interpretation sometimes involves theoretical calculation
Easy to use The interpretation is straightforward
High collection efficiency (close to 100 %)
Low collection efficiency (1-3%)
Sensitive to lighter elements Signal weak in high loss region
Sensitive to heavier elements Low yield for light elements
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Energy Filtered Transmission Electron Microscopy
Gatan: Review of EFTEM Fundamentals
Forming images with electrons of selected energy loss
1. Unfiltered image or diffraction pattern (DP) is formed
2. Image or DP is transformed into the spectrum
3. Part of the spectrum is selected by energy-selecting slit
4. Selected part of spectrum is transformed back into an energy-filtered image or DP
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Spectrum imaging
• ZLP imaging
• Plasmon imaging
• Edge imaging
ZLP
Low-loss
Ti
O
Mn
La
Edges
Image at DE1
Image at DE2
Image at DE3
Image at DEn
e- x 10^3e- x 10^3
DE
x
y
Spectrum imaging
Energy Filtered Transmission Electron Microscopy
EFTEM Spectrum Imaging: • Energy filtered broad beam technique • Fills data cube taking one image at each energy at a time • Less time STEM Spectrum Imaging • Focused probe method • Fills data cube by taking one spectrum at each location at a
time • Less does
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• Elastic electrons only
“Inelastic fog” removed
• Contrast enhanced
(Good for medium
thick samples)
• For Z < 12, the
inelastic cross-section
is larger than elastic
cross-section
ZLP
Spectrum imaging EFTEM – Zero-loss Peak imaging
Images from Gatan Review of EFTEM Fundamentals
Low-loss
Unstained/osmicated cebellar cortex
CBED Pattern
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A B
EFTEM – Plasmon Peak imaging
A B
W Al
Spectrum imaging
Spectrum image (20 images) Al mapping image W mapping image
J.G. Wen
30 nm
EFTEM – Plasmon Peak imaging
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EFTEM – Edge Peak imaging
Ti
Si
J.G. Wen
Image
Ti EELS spectrum
Three-window method
Jump ratio Three Window Mapping
Jump Ratio Imaging
Pre-edge image Post-edge image
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List of TEMs and Functions at UIUC
2010 LaB6 2100 Cryo
2010F 2200FS
Thank you !
• Heating stage (up to 1000°C)
• Cooling stage (with liquid N2 )
1. JEOL JEM 2010 LaB6 TEM
• TEM, low dose, NBD, HRTEM, in-situ
experiments
2. JEOL JEM 2100 LaB6 (Cryo-TEM)
• TEM, Low dose, special cryo-shielding
3. JEOL JEM 2010F Analytical (S)TEM
• TEM, BF, DF, NAED, CBED, EDS,
STEM, EELS, EFTEM
4. JEOL JEM 2200 FS Cs corrected (S)TEM
• Ultra high resolution Z-contrast STEM,
BF STEM, NBD, CBED, EDS, EELS,
EFTEM
5. Hitachi H-9500 TEM (expected 09/2013)
• In-situ heating and gas reaction,
HREM, NAED, 300 kV, LaB6 electron
source, Gatan K2 camera for fast
recording at 400 frames per second or
1000 frames per second using partial
fame.
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Acknowledgments
66