Advanced Materials Characterization
Workshop
Transmission Electron Microscopy
J.G. Wen, C.H. Lei, M. Marshall
W. Swiech, J. Mabon, I. Petrov
Supported by the U.S. Department of Energy under grants DEFG02-07-ER46453 and DEFG02-07-ER46471
© 2008 University of Illinois Board of Trustees. All rights reserved.
Outline
1. Introduction to TEM
2. Basic Concepts
3. Basic TEM techniques
Diffraction contrast imaging
HighHigh-resolution TEM imaging
Diffraction
4. ScanningScanning-TEM techniques
5. Spectroscopy
X-ray Energy Dispersive Spectroscopy
Electron EnergyEnergy-Loss Spectroscopy
6. Advanced TEM techniques
Spectrum imaging
EnergyEnergy-filtered TEM
7. Other TEM techniques
LowLow-dose; inin-situ; etc.
8. Summary
Why Use Transmission Electron Microscope?
Transmission Electron Microscope
(TEM)
Optical Microscope
Resolution record by TEM
GaN [211]
0.63 Å
Resolution limit: 200 nm
100k eV electrons λ: 0.037 Å
Sample thickness requirement:
AN is 0.95 with air
up to 1.5 with oil
γ = 0.66(C λ3)1/4
s
< 500 nm
Thinner than 500 nm
High quality image: <20 nm
Thin foil, thin edge, or
nanoparticles
Basic Structure of a TEM
Gun: LaB6, FEG
100, 200, 300keV
Gun and
Illumination part
TEM Sample
Objective lens part
View screen
Mode selection and
Magnification part
How does a TEM get Image and Diffraction?
TEM
Incident Electrons
< 500 nm
Sample
Objective Lens
Back Focal Plane
Back Focal Plane
Structural info
First Image Plane
u
Object
1_ 1_ _1
+ =
u v
f
First Image Plane
v
Morphology
Image
Conjugated planes
View Screen
TEM
Basic Concepts
High energy electron – sample interaction
1. Transmitted electron (beam)
2. Diffracted electrons (beams)
Incident high-kV beam
Bragg Diffraction
3.
4.
5.
6.
X-rays
Coherent beams
Incoherent beams
Elastically scattered electrons
Inelastically scattered electrons
Specimen
Diffracted
beam
Coherent beam
Transmitted
beam
Incoherent
beam
magnetic
prism
Inelastically scattered electrons
Diffraction
Electron Diffraction I
SOLZ
FOLZ
d
ZOLZ
θ
Zero-order Laue Zone (ZOLZ)
First-order Laue Zone (FOLZ)
….
Bragg’s Law
Wavelength
X-ray: about 1A
Electrons: 0.037A
2 d sin θ = nλ
High-order Laue Zone (HOLZ)
λ is small, Ewald sphere (1/λ) is almost flat
Ewald Sphere
Electron Diffraction II
Diffraction patterns from single grain and multiple grains
022
Diffraction
Tilting sample to obtain 3-D structure of a crystal
Lattice parameter, space group, orientation relationship
002
50 nm
Polycrystal
Amorphous
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
Imaging
Major Imaging Techniques
Major Imaging Contrast Mechanisms:
1. Mass-thickness contrast
2. Diffraction contrast
3. Phase contrast
4. Z-contrast
Mass-thickness contrast
1) Imaging techniques in TEM mode
a) Bright-Field TEM (Diff. contrast)
b) Dark-Field TEM (Diff. contrast)
Weak-beam imaging
hollow-cone dark-field imaging
a) Lattice image (Phase)
b) High-resolution Electron Microscopy
(Phase)
Simulation and interpretation
2) Imaging techniques in scanning transmission
electron microscopic (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)
Diffraction Contrast Image
TEM Imaging Techniques
I.
Diffraction Contrast Image:
Contrast related to crystal orientation
[111]
Many-beam condition
[001]
Kikuchi Map
[112]
[001]
[110]
Two-beam condition
Transmitted beam
Diffracted beam
Application:
Morphology, defects, grain boundary, strain field, precipitates
Diffraction Contrast Image
TEM Imaging Techniques
II. Diffraction Contrast Image: Bright-field & Dark-field Imaging
Sample
Sample
Objective Lens
Aperture
T
Back Focal Plane
Diffraction Pattern
D
First Image Plane
Bright-field Image
Dark-field Image
Two-beam condition
Bright-field
Dark-field
Diffraction Contrast Image
TEM Imaging Techniques
III. Thickness fringes and bending contour
Electron Wave
thickness
t / ξg
ξg
Extinction distance
θB
Howie-Whelan equation
dφ0 πi
= φ exp{2πisz}
dz ξg g
S
Excitation error
S = g Δθ
Tilting sample or beam slightly
Thickness fringes
g
0
To distinguish them from
intrinsic defects inside sample:
Bending contour
S<0
Increasing S
S=0
S=0
Increasing S
S>0
S>0
Bright-field
θB
θB
I
s
-g
0
g
Diffraction Contrast Image
TEM Imaging Techniques
II. Diffraction Contrast Image
Two-beam condition for defects
Howie-Whelan equation
dφ0 πi
= φ exp{2πi(sz+g.R)}
dz ξg g
Dislocations
Use g.b = 0 to determine Burgers vector b
Stacking faults
Phase = 2 π g • R
Each staking fault changes phase 2
-π
3
Diffraction contrast images of typical defects
g
Dislocations
Sample
1
2
3
Stacking faults
Dislocation loop
Diffraction Contrast Image
Weak-beam Dark-field imaging
High-resolution dark-field imaging
“Near Bragg Condition”
S
g
g
Exact Bragg condition
1g
2g
Weak-beam means
Large excitation error
Planes do not satisfy Possible planes satisfy
Bragg diffraction
Bragg diffraction
“Away from Bragg Condition”
Taken by I. Petrov
3g
C.H. Lei
Experimental weak-beam
Bright-field
Weak-beam
Dislocations can be imaged
as 1.5 nm narrow lines
Phase Contrast Image
Lattice imaging
Two-beam condition
Many-beam condition
[001]
M. Marshall
C.H. Lei
Phase Contrast Image
Lattice imaging
Delocalization effect from a field-emission gun (FEG)
From a LaB6 Gun
Lattice image of film on substrates
Field-Emission Gun
Phase Contrast Image
High-resolution Electron Microscopy (HREM)
1 Δfsch
Weak-phase-object approximation (WPOA)
2 Δfsch
f(x,y) = exp(iσVt(x,y))
~1 + i σ Vt(x,y)
Vt(x,y): projected potential
Indirect imaging
Depends on defocus
Scherzer defocus
1
Δfsch = - 1.2 (Csλ)2
Resolution limit
1
3
rsch = 0.66 Cs4 λ 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")
Simulation of images
Contrast transfer function
J.G. Wen
Software: Web-EMAPS (UIUC)
MacTempas
Selected-area electron diffraction (SAD)
Example of SAD and
dark-field imaging
Major Diffraction Techniques
1)
2)
3)
Diffraction
Selected-area Diffraction
Nanobeam Diffraction
Convergent-beam electron diffraction
SAD
aperture
High-contrast
aperture
Selected-area aperture
Objective
aperture
A. Ehiasrian, J.G. Wen, I. Petrov
Electron Nanodiffraction
Diffraction
5 μm condenser aperture Æ 30 nm
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)
This technique was developed by CMM
Diffraction
Convergent-beam electron diffraction (CBED)
Parallel beam
Convergent-beam
Sample
Sample
Back Focal Plane
SAD
1.
2.
3.
Large-angle bright-field CBED
CBED
Point and space group
Lattice parameter (3-D) strain field
Thickness
Bright-disk
4.
5.
Dark-disk Whole-pattern
Defects
Chemical bonding
Diffraction
Convergent-beam electron diffraction
Quantitative Analysis of Local Strain Relaxation
a
b
c
d
e
f
g
h
i
CoSi2
C. W. Lim, C.-S. Shin, D. Gall, M. Sardela,
R. D. Twesten, J. M. Zuo, I. Petrov and J. E. Greene
Use High-order Laue zone (HOLZ) lines
to measure strain field
STEM
SEM vs STEM
Scanning transmission
electron microscopy (STEM)
Scanning electron microscopy (SEM)
primary e-beam
100-300 keV
characteristic &
Bremsstrahlung
x-rays
1O primary e-beam
0.5-30 keV
backscattered
electrons
secondary
electrons
<50 eV
Auger
electrons
characteristic &
Bremsstrahlung
x-rays
Probe size
0.18 nm
“Coherent”
Scattering
(i.e. Interference)
Thickness
<100 nm
“Incoherent”
Scattering
i.e.
Rutherford
Dark-field Detector
1 μm
Bright-field
STEM
TEM vs STEM
Ge quantum dots on Si substrate
1.
2.
Ir nanoparticles
TEM
STEM imaging gives better
contrast
STEM images show Zcontrast
5 nm
Z
10nm
ADF-STEM
θ
Annular dark-field (ADF) detector
I ∝ Z2
Z-contrast image
J.G. Wen
Z-contrast imaging
5 nm
L. Long
STEM
HRTEM vs STEM
1. Contrast
•
•
High-resolution TEM (HRTEM) image is a
phase contrast image (indirect image). The
contrast depends on defocus.
STEM image is a direct atomic column
image (average Z-contrast in the column).
BaTiO3/SrTiO3/CaTiO3 superlattice
J.G. Wen
2. Delocalization Effect
•
•
High-resolution TEM image from FEG has
delocalization effect.
STEM image has no such an effect.
From Pennycook’s group
X-ray Energy-Dispersive Spectroscopy (EDS)
1) TEM mode Æ spot, area
2) STEM mode Æ spot, line-scan and 2-D mapping
Spectroscopy
2-D mapping
Au
HAADF
Spatial resolution ~1 nm
voids Area Mapping
Line scan
Ti
Mo
Si
Al
B
B
A
A
A. Ehiasrian, I. Petrov
Au
Ti
Mo
Ga
Ti0.85Nb0.15 metal ion etch
Creates a mixed amorphised surface layer ~ 6 nm
Liang Wang
Electron Energy-loss Spectroscopy (EELS)
Spectroscopy
ZLP
Low-loss
Post-column
In-column
ZLP
EELS spectrum:
1. Zero-loss Peak (ZLP)
2. Low-loss spectrum (<50eV)
Interacted with weakly bound outer-shell
electrons
Plasmon peaks
Inter- & Intra-Band transition
Application:
Thickness measurement
Elemental mapping
Iℓ
I0
I
t = λ ln( ℓ )
I0
λ : Mean free path
Low-loss
Spectroscopy
Edge Peaks in EELS
3. High-loss spectrum
ZLP
Interacted with tightly bound inn-shell
electrons
Edge peaks
Application:
Elements identification
Chemistry
Ti
O
Mn
La
Edge peak shape
x100
Low-loss
Edge peak position
Amorphous
carbon
Diamond
Nanoparticle
π bonding
π bonding
J.G. Wen
Spectrum imaging
Spectrum imaging
TEM mode
Energy-filtered TEM
1. ZLP imaging
2. Plasmon imaging
3. Edge imaging
ZLP
Edges
Ti
STEM mode
O
1. Plasmon imaging
2. Edge imaging
Mn
Low-loss
La
y
x
Energy-filtered TEM
•Fill in data cube by taking one image at each energy
Image at ΔE1
Image at ΔE2
STEM mode
Image at ΔE3
•Fill in data cube by taking one spectrum at each location
ΔE
Image at ΔEn
Spectrum imaging
Energy-Filtered TEM (EFTEM)
EFTEM - Zero-Loss Peak imaging
Only elastic electrons contribute to image – remove the “inelastic fog”
1. Improve contrast (especially good for medium thick samples)
2. Z<12, the inelastic cross-section is larger than elastic cross-section
ZLP
Low-loss
Spectrum imaging
EFTEM – Plasmon Peak imaging
Spectrum image (20 images)
A
B
Al mapping image
W mapping image
Al
W
30 nm
A
B
J.G. Wen
Spectrum imaging
EFTEM – Edge Peak imaging
Image
Ti
Three-window method
Ti EELS spectrum
Si
Jump ratio
J.G. Wen
Spectrum imaging
STEM + EELS Spectroscopy
convergence
angle ~ 10 mrad
LaMnO3
scan
coils
incident probe
probe size φ ~ 0.2 nm
SrTiO3
LaMnO3
SrTiO3
specimen
Z-contrast image
Z-contrast
image
HAADF detector
Ti
O
magnetic
prism
Mn
EELS spectrum
La
Spectrum imaging
STEM + EELS Spectroscopy
Ti L2,3
STO
SMO
STO
3LMO
STO
2LMO
STO
2LMO
STO
LMO
STO
STMO
STO
STMO
Z-contrast Image
Mn L2,3
La
OK
Scan direction
2nm
O K edge
M4,5
La M4,5
ΔE
Electron Energy-Loss Spectrum
Z-contrast image shows
where columns of atoms are
and EELS spectrum identify
chemical components
Ti
O
Mn
La
Electronic structure
changes are observed in the
fine structure of O K-edge
J.G. Wen, Amish, J.M. Zuo
New TEM: Cs-corrected Analytical STEM/TEM
Sound Isolation
JEOL 2010F, Cs = 1.0 mm
JEOL 2200FS, Cs < 5 μm
Low Airflow
CEOS Corrector
Ω-Filter
Remote operation
Vibration decoupling
With this setup we can achieve
a probe-size of <0.1nm
Cs-corrected Ronchigram
Small probe size for highhigh-resolution scanning
transmission electron microscopic images
1.36Å
Si [110] Zone Axis
2 x 2 LaMnO3-SrTiO3 superlattice
Dec. 2006
June 2007
Dec. 2007
Thick specimen
Same specimen
Thin Specimen
Sr atom
Ti atom
Sr atom
La atom
Mn atom
La atom
JEOL 2010F, Cs = 1 mm
(2nd smallest Probe)
JEOL 2200FS
with probe forming
Cs Corrector
Epitaxial Oxide Films Grown on SrTiO3
Polarized neutron scattering shows
interfacial ferromagnetic moment
measured at 10 Kelvin is enhanced in
LaMnO3 at the sharp LaMnO3-SrMnO3
interface and reduced at the rough
SrMnO3-LaMnO3 interface.
STEM Image of Superlattice
Sharp Interface
Rough Interface
LaMnO3
Sharp Interface
Rough Interface
SrMnO3
LaMnO3
High Mag. Image
SrMnO3
R
Å
SrTiO3 Substrate
S
CdSe nanoparticles
View along [110] zone axis
Polarity of CdSe
100
100
Cd
Se
Cd
Se
Hetero-structure nanoparticle
ZnTe
CdSe
Zn
Te
Cd
Se
Cd
Se
Zn
Te
Quantitative STEM Imaging
1 nm
Stacking
fault
Grain
boundary
1.5 Å
Se Cd
Quantitative STEM Imaging
Projected potential
1 nm
9
11
10
6
3
Monometallic Pt
300
(1
11
)
(11
x 10^4
250
(111
)
200
150
100
0)
50
(1
11
)
0)
(11
0
(111
)
0
50
100
150
200
250
The intensity at each atomic column is
proportional to numbers of atoms
No defects in Pt nanoparticles
Monometallic Pd
Pd nanoparticles contain
many defects such as twin
boundaries
Bimetallic Pd(core)-Pt(shell)
200
x 10^4
150
100
50
0
100
0
50
100
150
200
250
Pd (core) – Pt (shell) structure
Core-shell structure of FePd nanoparticles
Twin in the
nanoparticle
1 nm
Core: Ordered Fe/Pd structure
Shell: non-ordered structure
Pd columns are shown as brighter spots in the core
Amorphous
nanoparticle
Study Nanoparticles by TEM
Size distribution: STEM will give better contrast
> 5nm
Au-nanoparticles
Bi-nanoparticles
Both TEM & STEM
STEM better contrast
2 nm < Size < 5nm
Ultra thin carbon grid
STEM better contrast
< 2 nm
Ultra thin carbon grid
Only STEM
Composition study: EDS counts are low
2200FS EDS system
detector area 2 times bigger
beam 4 times brighter
Special TEM technique
Minimum-Dose for beam sensitive samples
Minimum-dose in TEM mode
Z-contrast tilt-series
Search in low magnification
Focus at another area
Photo with minimum dose
Minimum-dose in STEM mode
HAADF-STEM image
Long exposure time
L. Menard, J.G. Wen
200 nm
BF-STEM image
Short exposure time
200 nm
J.G. Wen
MDS + tomography
will be available on
2100 Cryo-TEM soon
+/- 80 degree tilt
In-situ
In-situ capabilities
1.
2.
3.
4.
5.
6.
7.
8.
9.
Heating (hot stage 1000°C)
Cooling (liquid N2)
Tensile-stage
MEMS tensile stage
Universal MEMS holder
Wet-cell
Nanomanipulator
Environmental holder
Applied voltage to sample
In-situ holders
CNT in water
universal MEMS holder
Water
front
MEMS straining stage
30 nm
10. Cryo transfer holder
nanomanipulation
liquid cell
N. Schmit
J.G. Wen
All developed at CMM
List of TEMs and functions
1. CM12 (120 KeV) (S)TEM
• TEM, BF, DF, CBED (good), EDS,
large tilt angle, etc
2. 2010 LaB6 TEM
• TEM, low dose, NBD, good for
HREM, video function
3. 2100 LaB6 (Cryo-TEM)
• TEM, Low dose, special cryoshielding; high-tilt angle (+/-80)
(using special retainer).
4. 2010F (S)TEM
• TEM, BF, DF, NBD, CBED, EDS,
STEM, EELS, EFTEM, Spectrum
imaging, etc.
5. HB501 STEM
• STEM, BF, DF, EDS, EELS (cold
FEG), ultra-high vacuum
6. JEOL 2200FS (S)TEM
• Cs-corrected probe, TEM, BF, DF,
NBD, CBED, EDS, STEM, EELS,
EFTEM, Spectrum imaging, etc.
CM 12
2010LaB6
2010F
HB501
2200FS
2100 Cryo