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.

Advanced Materials Characterization Workshop

Transmission Electron Microscopy

J.G. Wen, C.H. Lei, M. Marshall W. Swiech, J. Mabon, I. Petrov

1. Introduction to TEM1. Introduction to TEM2. Basic Concepts2. Basic Concepts3. Basic TEM techniques3. Basic TEM techniques

Diffraction contrast imagingDiffraction contrast imagingHighHigh--resolution TEM imaging resolution TEM imaging DiffractionDiffraction

4. Scanning4. Scanning--TEM techniquesTEM techniques5. Spectroscopy5. Spectroscopy

XX--ray Energy Dispersive Spectroscopy ray Energy Dispersive Spectroscopy Electron EnergyElectron Energy--Loss SpectroscopyLoss Spectroscopy

6. Advanced TEM techniques6. Advanced TEM techniquesSpectrum imagingSpectrum imagingEnergyEnergy--filtered TEM filtered TEM

7. Other TEM techniques7. Other TEM techniquesLowLow--dose; indose; in--situ; etc.situ; etc.

8. Summary8. Summary

Outline

Why Use Transmission Electron Microscope?

Resolution limit: 200 nm

AN is 0.95 with air up to 1.5 with oil

Transmission Electron Microscope(TEM)

Optical Microscope

100k eV electrons λ: 0.037 Å

Sample thickness requirement:

Thinner than 500 nm

High quality image: <20 nm

Resolution record by TEM

0.63 Å

GaN [211]

Thin foil, thin edge, or nanoparticles

< 500 nm

γ= 0.66(Csλ3)1/4

Basic Structure of a TEM

Gun and Illumination partGun: LaB6, FEG

100, 200, 300keV

Mode selection and Magnification part

View screen

TEM Sample

Objective lens part

How does a TEM get Image and Diffraction?TEM

Conjugated 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

magnetic prism

Basic Concepts High energy electron – sample interaction

Specimen

Incident high-kV beam

Transmitted beam

Diffracted beam

Inelastically scattered electrons

Incoherent beam

Coherent beam

1. Transmitted electron (beam)2. Diffracted electrons (beams)

Bragg Diffraction3. Coherent beams4. Incoherent beams5. Elastically scattered electrons6. Inelastically scattered electrons

TEM

X-rays

Electron Diffraction IDiffraction

Ewald Sphere

λ is small, Ewald sphere (1/λ) is almost flat

d

Bragg’s Law

2 d sin θ = nλ

θ

Zero-order Laue Zone (ZOLZ)First-order Laue Zone (FOLZ)….

High-order Laue Zone (HOLZ)

ZOLZ

FOLZ

SOLZ

X-ray: about 1AWavelength

Electrons: 0.037A

Electron Diffraction IIDiffraction

002

022

50 nm Polycrystal

Tilting sample to obtain 3-D structure of a crystal

Lattice parameter, space group, orientation relationship

Diffraction patterns from single grain and multiple grains

To identify new phases, TEM has advantages: 1) Small amount of materials2) No need to be single phases 3) determining composition by EDS or EELSAmorphous

1) Imaging techniques in TEM modea) Bright-Field TEM (Diff. contrast) b) Dark-Field TEM (Diff. contrast)

Weak-beam imaginghollow-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) mode1) 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 TechniquesMajor Imaging Contrast Mechanisms:

1. Mass-thickness contrast

2. Diffraction contrast

3. Phase contrast

4. Z-contrast

Mass-thickness contrast

Imaging

TEM Imaging TechniquesI. Diffraction Contrast Image:

Contrast related to crystal orientation

Diffraction Contrast Image

Kikuchi Map[111]

[110]

[112]

[001]

Application:Morphology, defects, grain boundary, strain field, precipitates

Two-beam condition

[001]

Many-beam condition

Transmitted beam

Diffracted beam

Objective Lens

Aperture Back Focal PlaneDiffraction Pattern

Bright-field ImageFirst Image Plane

Dark-field Image

Sample Sample

T D

TEM Imaging TechniquesII. Diffraction Contrast Image: Bright-field & Dark-field Imaging

Diffraction Contrast Image

Two-beam condition

Bright-field Dark-field

TEM Imaging TechniquesIII. Thickness fringes and bending contour

Diffraction Contrast Image

Bright-field

Electron Wave

thickness

ξgt /

θB

Increasing S S=0Increasing S

θB

S=0S<0

g-g 0

ξgExtinction distance

θB

0 g

S

Excitation errorS = g Δθ

Thickness fringes Bending contour

To distinguish them from intrinsic defects inside sample:

Tilting sample or beam slightly

dφ0 πidz ξg

= φgexp2πisz

Howie-Whelan equation

S>0

S>0

s

I

Dislocation loop

TEM Imaging TechniquesII. Diffraction Contrast Image

Diffraction contrast images of typical defects

Phase = 2 π g • R

Dislocations Stacking faults

Diffraction Contrast Image

dφ0 πidz ξg

= φgexp2πi(sz+g.R)

Howie-Whelan equation

g 1 2 3

23-πEach staking fault changes phase

Two-beam condition for defectsUse g.b = 0 to determine Burgers vector b

Sample

Dislocations

Stacking faults

Weak-beam Dark-field imagingDiffraction Contrast Image

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 gS

Weak-beam meansLarge excitation error

Exact Bragg conditionTaken by I. Petrov

“Near Bragg Condition”

“Away from Bragg Condition”

Experimental weak-beam

Lattice imagingTwo-beam condition Many-beam condition

Phase Contrast Image

C.H. LeiM. Marshall

[001]

Lattice imagingPhase Contrast Image

Lattice image of film on substrates

From a LaB6 Gun Field-Emission GunDelocalization effect from a field-emission gun (FEG)

High-resolution Electron Microscopy (HREM)Weak-phase-object approximation (WPOA)

Δfsch = - 1.2 (Csλ)21

Scherzer defocus

Resolution limit

rsch = 0.66 Cs λ14

34

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 Δfsch 2 Δfsch

f(x,y) = exp(iσVt(x,y))

~1 + i σ Vt(x,y)Vt(x,y): projected potential

Phase Contrast Image

Simulation of images Software: Web-EMAPS (UIUC)MacTempas

Indirect imaging Depends on defocus

J.G. WenContrast transfer function

Selected-area electron diffraction (SAD)Example of SAD and

dark-field imaging

Selected-area apertureHigh-contrast aperture

Diffraction

1) Selected-area Diffraction2) Nanobeam Diffraction3) Convergent-beam electron diffraction

Major Diffraction Techniques

Objective aperture

SADaperture

A. Ehiasrian, J.G. Wen, I. Petrov

M. Gao, J.M. Zuo, R.D. Twesten, I. Petrov, L.A. Nagahara & R. Zhang, Appl. Phys. Lett. 82, 2703 (2003)

Electron Nanodiffraction

J.M. Zuo, I. Vartanyants, M. Gao, R. Zhang and L.A. Nagahara, Science, 300, 1419 (2003)

5 μm condenser aperture 30 nm

Diffraction

This technique was developed by CMM

Convergent-beam electron diffraction (CBED)

Back Focal Plane

Parallel beam

Sample

Convergent-beam

Sample

1. Point and space group2. Lattice parameter (3-D) strain field3. Thickness

Large-angle bright-field CBED

Bright-disk Dark-disk Whole-pattern

4. Defects5. Chemical bonding

CBEDSAD

Diffraction

Convergent-beam electron diffractionDiffraction

Quantitative Analysis of Local Strain Relaxation

a b c

d e f

g h i

C. W. Lim, C.-S. Shin, D. Gall, M. Sardela,R. D. Twesten, J. M. Zuo, I. Petrov and J. E. Greene

CoSi2

Use High-order Laue zone (HOLZ) lines to measure strain field

1O primary e-beam0.5-30 keV

backscattered electrons

secondary electrons<50 eV

Auger electrons

characteristic &Bremsstrahlungx-rays

1 μm

Scanning electron microscopy (SEM)

primary e-beam100-300 keV

characteristic &Bremsstrahlungx-rays

Scanning transmissionelectron microscopy (STEM)

Probe size 0.18 nm

SEM vs STEM

Thickness<100 nm

STEM

“Coherent”Scattering

(i.e. Interference)“Incoherent”

Scatteringi.e.

Rutherford

Dark-field Detector

Bright-field

TEM vs 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

θ

I ∝ Z2

STEM

L. LongJ.G. Wen

Annular dark-field (ADF) detector

Z-contrast imaging

Z

HRTEM vs STEMSTEM

BaTiO3/SrTiO3/CaTiO3 superlattice

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).

2. Delocalization Effect • High-resolution TEM image from FEG has

delocalization effect.• STEM image has no such an effect.

From Pennycook’s group

J.G. Wen

X-ray Energy-Dispersive Spectroscopy (EDS)

1) TEM mode spot, area2) STEM mode spot, line-scan and 2-D mapping

Ti0.85Nb0.15 metal ion etch Creates a mixed amorphised surface layer ~ 6 nm

Spatial resolution ~1 nmArea Mapping

Al

Mo

Si

AuTiMo

Ga

Ti

AuHAADF

voids

A B

A

B

Line scan

2-D mapping

Spectroscopy

Liang Wang

A. Ehiasrian, I. Petrov

Electron Energy-loss Spectroscopy (EELS)

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 measurementElemental mapping

ZLP

Low-loss

Spectroscopy

ZLP

Low-loss

t = λ ln( )IℓI0

I0

Iℓ

λ : Mean free path

Post-column In-column

Diamond Nanoparticle

π bonding

Amorphous carbon

π bonding

Edge Peaks in EELS Spectroscopy

ZLP

Low-loss

Ti

O

Mn

Lax100

3. High-loss spectrumInteracted with tightly bound inn-shell electrons

Edge peaks Application:

Elements identification Chemistry

Edge peak position

Edge peak shape

J.G. Wen

Spectrum imaging

Energy-filtered TEM1. ZLP imaging2. Plasmon imaging3. Edge imaging

TEM mode

ZLP

Low-loss

TiO

Mn

La

Edges

Image at ΔE1

Image at ΔE2

Image at ΔE3

Image at ΔEn

ΔE

xy

Energy-filtered TEM•Fill in data cube by taking one image at each energy

STEM mode•Fill in data cube by taking one spectrum at each location

1. Plasmon imaging2. Edge imaging

STEM mode

Spectrum imaging

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

Energy-Filtered TEM (EFTEM)

A B

EFTEM – Plasmon Peak imaging

A B

WAl

Spectrum imaging

Spectrum image (20 images) Al mapping image W mapping image

J.G. Wen

30 nm

EFTEM – Edge Peak imaging

Three-window method

Jump ratio

Image Ti

SiTi EELS spectrum

Spectrum imaging

J.G. Wen

scan coils

convergence angle ~ 10 mrad

specimen

incident probeprobe size φ ~ 0.2 nm

HAADF detector

Z-contrast image

magnetic prism

STEM + EELS Spectroscopy

Z-contrast image

Ti

OMn

La

EELS spectrum

Spectrum imaging

SrTiO3

LaMnO3

SrTiO3

LaMnO3

STEM + EELS SpectroscopySpectrum imaging

Z-contrast image shows where columns of atoms are and EELS spectrum identify chemical components

Electronic structure changes are observed in the fine structure of O K-edge

Electron Energy-Loss Spectrum

Scan

dire

ctio

n

Z-contrast Image ΔE

O K edge La M4,5

O KMn L2,3

La M4,5Ti L2,3

2nm

STOSMOSTO3LMOSTO2LMOSTO2LMOSTOLMOSTOSTMOSTOSTMO

J.G. Wen, Amish, J.M. Zuo

TiO Mn La

New TEM: Cs-corrected Analytical STEM/TEM

Sound Isolation

Low Airflow

Vibration decoupling

CEOS Corrector

Ω-Filter

Remote operation

With this setup we can achieve a probe-size of <0.1nm

JEOL 2010F, Cs = 1.0 mmJEOL 2200FS, Cs < 5 μm

Cs-corrected Ronchigram

Small probe size for highSmall probe size for high--resolution scanning resolution scanning transmission electron microscopic imagestransmission electron microscopic images

Si [110] Zone Axis

1.36Å

2 x 2 LaMnO2 x 2 LaMnO33--SrTiOSrTiO33 superlatticesuperlattice

JEOL 2010F, Cs = 1 mm(2nd smallest Probe)

JEOL 2200FSwith probe formingCs Corrector

La atomMn atom

La atom

Sr atomTi atom

Sr atom

June 2007Dec. 2006 Dec. 2007Thick specimen Same specimen Thin Specimen

Polarized neutron scattering shows interfacial ferromagnetic moment measured at 10 Kelvin is enhanced in LaMnO3 at the sharp LaMnO3-SrMnO3interface and reduced at the rough SrMnO3-LaMnO3 interface.

Å

LaMnO3SrMnO3

LaMnO3SrMnO3

SrTiO3 Substrate

Sharp Interface

Sharp InterfaceRough Interface

Rough Interface

R

S

High Mag. Image

Epitaxial Oxide Films Grown on SrTiO3

STEM Image of Superlattice

SeCd

SeCd

CdSe nanoparticlesView along [110] zone axisPolarity of CdSe

100

100

TeZn

TeZn

SeCd

SeCd

ZnTe CdSe

Hetero-structure nanoparticle

Stackingfault

Grainboundary

1 nm

1.5 ÅCdSe

Quantitative STEM Imaging

1 nmProjected potential

3

6

9 1011

Quantitative STEM Imaging

Monometallic PtMonometallic Pt

No defects in Pt nanoparticles

x 10

^4

0

5 0

1 0 0

1 5 0

2 0 0

2 5 0

3 0 0

x 10

^4

0 5 0 1 0 0 1 5 0 2 0 0 2 5 0

The intensity at each atomic column is proportional to numbers of atoms

(111

)(1

10)

(111)

(111

)

(110)

(111)

Monometallic Pd Monometallic Pd Pd nanoparticles contain many defects such as twin boundaries

Bimetallic Bimetallic Pd(core)Pd(core)--Pt(shellPt(shell))x

10^4

0

50

100

150

200

x 10

^4

0 50 100 150 200 250

Pd (core) – Pt (shell) structure

100

CoreCore--shell structure of shell structure of FePdFePd nanoparticlesnanoparticles

Core: Ordered Fe/Pd structureShell: non-ordered structure

Twin in the nanoparticle

Amorphous nanoparticle

1 nm

Pd columns are shown as brighter spots in the core

Study Study NanoparticlesNanoparticles by TEMby TEM

Bi-nanoparticles> 5nm

2 nm < Size < 5nm

< 2 nm

Both TEM & STEMSTEM better contrast

Ultra thin carbon gridSTEM better contrast

Ultra thin carbon gridOnly STEM

Au-nanoparticles

Size distribution: STEM will give better contrast

Composition study: EDS counts are low

2200FS EDS system detector area 2 times bigger beam 4 times brighter

Minimum-Dose for beam sensitive samples

200 nm

200 nm

Minimum-dose in STEM mode

HAADF-STEM imageLong exposure time

Minimum-dose in TEM mode

Search in low magnificationFocus at another areaPhoto with minimum dose

MDS + tomography will be available on 2100 Cryo-TEM soon +/- 80 degree tilt

Special TEM technique

BF-STEM imageShort exposure time

Z-contrast tilt-series

J.G. Wen

L. Menard, J.G. Wen

In-situ capabilities1. Heating (hot stage 1000°C)2. Cooling (liquid N2)3. Tensile-stage 4. MEMS tensile stage5. Universal MEMS holder6. Wet-cell7. Nanomanipulator8. Environmental holder9. Applied voltage to sample

10.Cryo transfer holder

In-situ holders

MEMS straining stage

nanomanipulation

liquid cell

universal MEMS holder

In-situ

N. Schmit

All developed at CMM

Water front

30 nm

CNT in water

J.G. Wen

List of TEMs and functions1. 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 function3. 2100 LaB6 (Cryo-TEM)

• TEM, Low dose, special cryo-shielding; 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 vacuum6. JEOL 2200FS (S)TEM

• Cs-corrected probe, TEM, BF, DF, NBD, CBED, EDS, STEM, EELS, EFTEM, Spectrum imaging, etc.

CM 12 2010LaB6 2100 Cryo

2010F HB501

2200FS