Materials Research Lab - Transmission Electron Microscopy · 2019-06-26 · Selected Area Electron...
Transcript of Materials Research Lab - Transmission Electron Microscopy · 2019-06-26 · Selected Area Electron...
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Transmission Electron Microscopy
Wacek Swiech, CQ Chen, Jim Mabon,
Honghui Zhou and Matt Bresin
Frederick Seitz Materials Research Laboratory
University of Illinois at Urbana-Champaign
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Outline
1. Introduction to TEM
2. Basic Concepts
3. Basic TEM techniques Diffraction Bright Field & Dark Field TEM imaging High Resolution TEM Imaging
4. Scanning Transmission Electron Microscopy (STEM)
5. Aberration-corrected STEM / TEM & Applications
6. Microspectroscopy & spectromicroscopy X-ray Energy Dispersive Spectroscopy Electron Energy-Loss Spectroscopy
7. In-Situ TEM
8. Summary
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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: ~0.15 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 8
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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 9
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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 12
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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 © 2015 University of Illinois Board of Trustees. All rights reserved.
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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 28
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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. 32
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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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Specimen
Incident
High-kV Beam
Bremsstrahlung X-rays Auger
Electrons
Secondary
electrons (SE)
Characteristic X-rays
X-ray Energy Dispersive Spectroscopy
Back scattered
electrons (BSE)
Direct Beam
Inelastically
scattered electrons
Electron Energy Loss Spectroscopy
Elastically scattered
electrons
Cathodoluminescence
(CL: Visible light …)
“Absorbed”
Electrons
Electron-Hole
pairs
Analytical (Scanning) Transmission Electron Microscopy
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The emission of Auger electrons is an alternative to X-ray emission as an ionized atom returns to its ground state
(EELS)
(EDS)
Incoming photon: h
Energy Loss
Photons
(XAS)
XPS
Characteristic Signals: EELS, EDS, XPS Signals
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AES
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Energy resolution, Spatial resolution, Elements resolving
700 800 900 1000 1100
Energy (eV)
EDS
XPS
EELS
Copper L shell
L3
L2
400 600 800 1000
Energy (eV)
EDS
EELS
NiO: O K-shell and Ni L shell
Ni L3
L2
L3
Experimental XEDS, XPS, & EELS
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EELS: Electron Optical System
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Post-Column Filter (GIF)
http://img17.imageshack.us/img17/1686/spectprism.jpg
Williams and Carter 2009
V0 = (V1R1 + V2R2)/2R0
Compare: Electron Energy Analyzer
Casaxps.com
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• 4 Dispersive elements • Integrated into column
In-Column (Omega) Filter
EELS: Electron Optical System
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47
X100
Conduction/V
alence bands
Experimental EELS
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NiO
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48 Sub nanometer in Cs-corrected STEM !
Spectral Imaging: 3D/4D data cube
x-y image, z spectrum (EDS/EELS)
Image at DE1
Image at DE2
Image at DE3
Image at DEn
e- x 10^3e- x 10^3
DE
x
y
EDS
STEM-EELS/EDS Spectrum Imaging: Spatially Resolved Analysis
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0
10
20
30
40
50
e- x
10^
3
445 450 455 460 465
eV
bkgd
Spectrum
Bkgd
Edge
2000
3000
4000
5000
e-
420 440 460 480 500 520
eV
460
480
500
520
540
560
580
600
620
640
x 10^4
12
nm
nm
4000
4050
4100
4150
4200
4250
e-
02
4
eV
460
480
500
520
540
560
580
600
620
640
x 10^4
12
nmnm
STEM-EELS Spectrum Imaging: Line Scan
STO
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Dark–Field TEM
• Use the slit to select electrons of a specific energy
• Allow only those electrons to fall on the screen or CCD
• Analogy to dark-field imaging
EF-TEM Imaging
EF-TEM (Energy-Filtered TEM): Parallel Beam
Gatan Review of EFTEM Fundamentals
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Three-window method
Jump ratio Three Window Mapping
Jump Ratio Imaging
Pre-edge image
Post-edge image
5 0 n m5 0 n m
Post-edge 1 Post-edge 2 Post-edge
EF-TEM Imaging: Using Core Loss
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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 EF-TEM Imaging: Using Zero-loss
Gatan Review of EFTEM Fundamentals
Low-loss
Unstained/osmicated cebellar cortex
CBED Pattern
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Changes in the Cu L2,3
edge with oxidation White lines in transition metals
ELLS Near Edge Fine Structure (ELNES): White Lines
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Williams and Carter 2009
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HREM image and EELS spectrum showing the graphitic nature of a thin surface layer
Science, 334, 1687, 2011
Graphitic Tribological Layers in M-on-M Hip Replacements
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54
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• EDS
Cliff-Lorimer equation for
thin film EDS
• EELS (Ratio)
• Absolute quantification (atoms / nm²)
Ik: sum of counts in edge k,
IL: total intensity (including zero loss),
σk: partial cross-section for ionization.
D integration window, collection angle
Quantification
Williams and Carter 2009
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56
T=1.89
0
50
100
150
e- x
10^3
-40 -20 0 20 40 60 80 100 120 140 160 180 200 220
eV
0
5000
10000
15000
20000
e-
-40 -20 0 20 40 60 80 100 120 140 160 180 200 220
eV
0
50
100
150
e- x
10^3
-40 -20 0 20 40 60 80 100 120 140 160 180 200 220
eV
2Ep
Thin
specimen
Thick specimen
122
02 2P P
h h neE
m
p p
Ep
t = thickness
λP= Plasmon MFP,
Io= zero-loss peak
IT= total spectrum
ln( )p o Tt I Il
Log-ratio method
Relative thickness:
Reliable,
Fast and online,
High spatial resolution
STEM-EELS Measurement of Local Thickness
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Combine STEM-EDS & STEM-EELS
0.0 0.5 1.0 1.5 2.0 2.5
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Na/Te(PbxS
1-x)
Ato
mic
ra
tio
Relative thickness (l)
Quantitative Composition Analysis: Combining EDS-EELS
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Comparison of EELS and EDS
EELS (primary) EDS (Secondary)
• 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%) (tilting helps to some degree)
Sensitive to lighter elements Signal weak in high loss region
Sensitive to heavier elements Low yield for light elements
58
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• EELS: Events causing ionization
• EDS: Events after Ionization
59
Comparison of EELS and EDS: Primary vs Secondary
One Ionization Event (~EELS)
Several Relaxation Events (~EDS/AES)
Eloss > Ec >EEDS
• EDS: low x-ray yield for lower Z
• EDS: Low energy x-ray below detection limit
• EDS: higher energy favorable (>1KV most favorable)
• EELS: Lower loss favorable (<1KV most favorable)
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Resolution and Sensitivity (Detection Limit)
Beam diameter & spreading
E0 energy, t thickness, Z
Typical MMF: 0.1–1%, goes inversely with R
Bulk Thin Ultra-thin
MMF (~0.01%) correspond to
~ 107, 300, and <1 atom
Thin foil: better R, worse MMF
FEG higher current/ energy
CS correction improves both
XEDS-TEM: approaching atomic-level
detection and sub-nm spatial resolution
0.01% 0.01% 0.01%
3x106 atoms 300 atoms 1 atom
Williams and Carter 2009
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Ag-Au Core-Shell : Elemental (EDS) Mapping
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Science 337, 954 (2012)
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List of TEMs and Functions at UIUC JEOL
2010 LaB6
JEOL
2100 Cryo
JEOL
2010F
JEOL
2200FS
• 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. Hitachi H-9500 TEM (LaB6, 300 kV)
• Environmental TEM (in-situ heating
and gas reaction)
• Gatan K2 camera for fast recording at
400 frames per second or 1600 frames
per second using partial frame.
• Dynamic TEM (expected)
3. JEOL JEM 2010 F Analytical (S)TEM
• TEM, BF, DF, NAED, CBED, EDS,
STEM, EELS, EFTEM, Z-contrast
imaging
4. JEOL JEM 2200 FS Cs corrected (S)TEM
• Ultra high resolution Z-contrast STEM,
BF STEM, NBD, CBED, EDS, EELS,
EFTEM
6. Hitachi H-8600 TEM (W)
• Biological samples, staff assisted only
Hitachi
H-9500
62
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