Electron microscopy 2

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Electron Microscopy IITransmission - , Scanning Transmission - and

Analytical Electron Microscopy

[email protected]

www.microscopy.ethz.ch

Electron Microscopy MethodsTransmission Electron Microscopy (TEM)

Bright / Dark Field (BF/DF)

High-Resolution Transition Electron Microscopy (HRTEM)

Energy-Filtered (EFTEM)

Electron Diffraction (ED)

Scanning Transmission Electron Microscopy (STEM)Bright / Dark Field (BF/DF-STEM)

High-Angle Annular Dark Field (HAADF-STEM)

Analytical Electron Microscopy (AEM)X-ray Spectroscopy

Electron Energy-Loss Spectroscopy (EELS)

Electron Spectroscopic Imaging (ESI)

Scanning Electron Microscopy (SEM)Secondary Electrons (SE)

Back-Scattered Electrons (BSE)

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Development of the First Transmission Electron Microscope

Knoll, Ruska, Z. Phys. 78 (1932) 318

1927 Hans Busch: Electron beams can be focused in an inhomogeneous magnetic field.

1931 Max Knoll and Ernst Ruska built the first TEM.

1986 Nobel prize for Ruska

History of Electron Microscopy

1938 First Siemens electron microscope (resolution ca. 13 nm)


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1939: first TEM serially produced by Siemens

resolution ca. 7 nm


Transmission Electron Microscopes

∼1970: HRTEMPhilips EM400, V = 120 kV

resolution ca. 0.35 nm

∼1990Philips CM30, V = 300 kV

resolution ca. 0.2 nm

Transmission Electron Microscopy

Magnetic LensAn electron in a magnetic field (here: inhomogeneous, but axially symmetric) experiences the Lorentz force F:

F = -e (E + v x B)

|F| = evBsin(v,B)

E: strength of electric field

B: strength of magnetic field

e/v: charge/velocity of electrons

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Magnetic Lens

Lens

Back focal plane

Image plane

Object plane

Light optical analogue

Lens equation: 1/u + 1/v = 1/fMagnification M = v/u

Lens problems:spherical aberation Cschromatic aberation Ccastigmatism


Cross-Section of the Column of a CM30 Microscope

acceleratorelectron gun

condenser systemobjective lenses

samplediffraction lens

intermediate lensprojector lensesviewing screen

photo camera

TV, SlowScanCCD Camera

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Electron GunsThermoionic Guns

Electron emission by heating


Field Emission Guns (FEG)

Electron emission by applying an extraction voltage

W

W

LaB6

>1000500100Lifetime / h

10135x1010109Brightness / A/m2sr

(30-)3005001000Maximum current / nA

3-20500030000Source size / nm

0.4-1.51.5-33-4Energy spread / eV

(300-)180020002700Temperature / K

4.52.44.5Work function / eV

FEGLaB6WProperties

TEM – Imaging and DiffractionOptic axis

Objective lens

Back focal plane

Image plane

Object plane

↓Diffraction pattern


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Image plane

Diffraction pattern


Diffraction and Imaging Mode


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Diffraction pattern

Image

Bright field image

- mass-thickness contrast

- Bragg contrastTransmission Electron Microscopy


Diffraction pattern

Image Dark field image


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Elastic Scattering of Electrons by an Atom


Weak Coulomb interaction within the electron cloud

⇒ low-angle scattering

Strong Coulomb interaction with the nucleus

⇒ scattering into high angles or even backwards (Rutherford scattering)

⇒ atomic-number (Z) contrast

Types of Image Contrast

Mass-Thickness contrast


Bragg contrast

Coherent scattering

Diffracted beams do not pass through the objective aperture leading to a decreased intensity of crystalline areas

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BF Images


Amorphous SiO2 on C foil

Mainly thickness contrast

Au particles (black) on TiO2

Mainly mass contrast

Increasing thickness

BF Images


ZrO2 micro crystals; crystals orientated close to zone axis appear dark

Mainly Bragg contrast

Increasing thickness

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BF and DF imagingPt-Particles in SiO2 nanotubes


BF DF conical DF


Image plane

Diffraction pattern


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Diffraction pattern

Image

High-resolution imageInterference of several diffracted beams with the direct beam



Diffraction pattern

Image

Mathematics

+ -2i qx

-{f(x)} ( ) e dx

∞ π

∞= ∫F f x

Fourier Transform

Fourier analysis

Fourier synthesis


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Diffraction pattern

HRTEM image

Image Processing

Fourier Transform

Fourier Transform

Filtered image


ED + HRTEM

Nb8W9O47 – threefold TTB superstructure


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HRTEM: Detection of Defects

Planar defect in ZnNb14O35F2


HRTEM: Detection of Defects

Grain boundary between differently ordered areas in a Nb-W oxide


10 nm

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HRTEM and ED of Aperiodic Structures

Dodecagonal quasicrystal in the system Ta-Te


HRTEM: Imaging Single Atoms

Gd@C82 in SWCNT

Suenaga et al, Science 290 (2000) 2280

3 nm

3 nm


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Contrast: • Mass-thickness (BF/DF)• Bragg (BF/DF)• Phase (HRTEM)

Determination of• Structure: HRTEM• Defects: HRTEM, TEM• Lattice constants and symmetry: ED• Particle size: TEM, HRTEM


Scanning Transmission Electron Microscopy (STEM)

Scanning Transmission Electron Microscopy

From: Williams, Carter: Transmission Electron Microscopy

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BF and DF Imaging in STEMAu islands on a C film DF image

recorded by the annular dark field detector

BF image recorded by the bright field detector



STEM Detectors

BF: Bright Field detector

ADF: Annular Dark Field detector

HAADF: High Angle Annular Dark Field detectorScanning Transmission Electron Microscopy

10 mrad ≈ 0.57°


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BF and DF Imaging in STEM


Pt Particles in SiO2 nanotubes

BF ADF HAADF

BF DF conical DF TEM

HAADF-STEM or Z contrast Images of Au Particles on Titania


50 nm

10 nm

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High-Resolution HAADF-STEM

WO3 segregations in a bronze-type Nb-W oxide


HRTEM

2 nm

HAADF-STEM of Nb8W9O47

Information about Elemental Distribution in HAADF-STEM or Z contrast images

Single-crystal X-ray structureof Nb8W9O47

ca. 80% Nb 100% W

P21212 a=12.26, b=36.63, c=3.95 Å

ca. 80% Nb100% W

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Contrast: • Mass-thickness (BF/DF)• Bragg (BF/DF)• Z2 (HAADF)

Determination of• Particles on support: HAADF • Structure and defects : HR

Important: Combination with EDXS or EELS


Interactions of Electrons with Matter

XEDS (EDX, EDS), WDS, EPMA

SEM

SEM

AES

elastic scattering:TEM, HRTEM, STEM, SAED, CBED

inelastic scattering:EELS, ESI

specimen

electron beam

scatteredelectrons

direct beam

X raysbackscattered

electrons

Augerelectrons

secondaryelectrons

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Generation of X-rays

1. Ionization2. X-ray emission

Analytical Transmission Electron Microscopy

Generation of X-rays

1. Ionization2. X-ray

emission

Inelastic interactions

α1 α2

β1

α1

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Electron Configuration of Ti: 1s2 2s2 2p6 3s2 3p6 4s2 3d2

ener

gy

1s

2s

3s

4s

2p

3p

4p3d

K

L

M

N

Kα

Kβ

Inelastic interactions

Energy-Dispersive X-ray Spectroscopy (EDXS)


Au-Particles on TiO2

Inte

nsity

/ co

unts

Energy / keV

Deceleration of incident electrons by interaction with Coulomb field ofnucleus leads touncharacteristic spectrum background(bremsstrahlung).

Ti L

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Possible Transitions Between Electron Shells causing characteristic K, L, M X-rays


Quantitative EDXS

Cliff-Lorimer Equation:

NA/NB = kAB IA/IB

NA,NB: atomic % of element A,BIA/IB: measured intensity of element A,BkAB: Cliff-Lorimer factor


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X-ray Spectrometer


Schematic representation of a energy-dispersive spectrometer with electronic units. Spectrometers can be attached to TEMs and SEMs.

STEM @ EDXS: Point Analyses

HAADF-STEM of Pd/Pt particles on alumina


PtPd

PtPt

Cu

Al

O

C

PtPd

Pt Pt

Cu

Al

O

C

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STEM @ EDXS: Point Analyses

500 nm

1

HAADF Detector

Energy (keV)

Cou

nts

20151050

60

40

20

0

W

WW WW

W

Mo

MoM

Mo

Mo MoMo

Cu

Cu

EDX HAADF Detector Point 1


(Mo,W)Ox or MoOx and WOx?

Interactions of Electrons with Matter

XEDS (EDX, EDS), WDS, EPMA

SEM

SEM

AES

elastic scattering:TEM, HRTEM, STEM, SAED, CBED

inelastic scattering:EELS, ESI

specimen

electron beam

scatteredelectrons

direct beam

X raysbackscattered

electrons

Augerelectrons

secondaryelectrons

Phonons: lattice vibrations (sample heating)

Plasmons: electron gas oscillations

Cathodoluminescence: electron-holes in semiconductors

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Electron Energy Loss Spectroscopy (EELS)


Electron Energy Loss Spectroscopy


Relationship between the EEL Spectrum and a Core-Loss Exication within the Band Structure

ELNES: Electron Energy Loss Near Edge Structure reflects local density of unoccupied states corresponds to XANES

EXELFS:EXtended Energy Loss Fine Structure information on local atomic arrangements corresponds to EXAFS

Filled states Empty states

EF E

N(E)

Extended fine structure

Near edge fine structure

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Fine structures in EELS


Characteristics of the C_K edge depending on hybridization

Characteristics of the Cu_L2,3edges depending on oxidation

state of Cu

HRTEM and EELS: Imaging and Detection of Single Atoms

Gd@C82 in SWCNT

Suenaga et al, Science 290 (2000) 2280

3 nm

3 nm


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Elemental Maps by Electron Spectroscopic Imaging (ESI)

Post-column filter

gun

round lens

sample holder

aperture (objective and sel. area)

aperture (filter entrance)

prism (with straight edges)

prism (with curved edges)

energy-selecting slit

quadrupole lens

sextupole lens

CCD camera

Energy-filtered image

EELS (section) of VOx-NTs

3 window method


Elemental Maps by Electron Spectroscopic Imaging (ESI)

TEM image Vanadium map Carbon map

V and C, respectively, are located at the sites appearing with bright contrast.


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Imaging of Elemental Distribution

TEM image Ag_M mapM edge: 367 eV

Au_M mapM edge: 2206 eV

Ag/Au Particles on Silica


EELS (EFTEM) versus EDXS (x-ray mapping)

Slow technique; only simple processing required

Fast technique; but complex processing required

Inefficient signal generation, collection and detection

=> inefficient x-ray mapping

Very efficient and higher sensitivity to most elements

=> very efficient mapping technique

Energy resolution > 100 eV causes frequent overlaps

Energy resolution 0.3-2 eV results in far fewer overlaps; fine structures

X-rays provide elemental information only

Elemental, chemical and dielectric information

High detection efficiency for high Z elements

High detection efficiency for low Z elements

EDXSEELS


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Analytical Electron Microscopy

• Qualitative and quantitative information about the composition: EDXS, EELS

• Bonding, coordination, interatomic distances: Fine structure in EELS (ELNES, EXEFS)

• Locally resolved information about composition1. STEM + EDXS and/or EELS2. ESI


Analytical Electron Microscopy- Restrictions

• Electron-matter interactions are mostly elastic⇒ high electron dose necessary

• Long recording times ⇒ high sample stability and absence of drift

• Ionization edges occur at different energies and are of different shape

⇒ not all methods are equally suitable for all elements


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Analytical Electron Microscopy- Restrictions

signal/noise measuring time

spatial resolution


Electron microscopy 2

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