electron microscopy 2
DESCRIPTION
TRANSCRIPT
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Electron Microscopy IITransmission - , Scanning Transmission - and
Analytical Electron Microscopy
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)
History of Electron Microscopy
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1939: first TEM serially produced by Siemens
resolution ca. 7 nm
History of Electron Microscopy
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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Transmission Electron Microscopy
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
Transmission Electron Microscopy
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
Transmission Electron Microscopy
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
Transmission Electron Microscopy
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TEM – Imaging and DiffractionOptic axis
Image plane
Diffraction pattern
Transmission Electron Microscopy
Diffraction and Imaging Mode
Transmission Electron Microscopy
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TEM – Imaging and DiffractionOptic axis
Diffraction pattern
Image
Bright field image
- mass-thickness contrast
- Bragg contrastTransmission Electron Microscopy
TEM – Imaging and DiffractionOptic axis
Diffraction pattern
Image Dark field image
Transmission Electron Microscopy
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Elastic Scattering of Electrons by an Atom
Transmission Electron Microscopy
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
Transmission Electron Microscopy
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
Transmission Electron Microscopy
Amorphous SiO2 on C foil
Mainly thickness contrast
Au particles (black) on TiO2
Mainly mass contrast
Increasing thickness
BF Images
Transmission Electron Microscopy
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
Transmission Electron Microscopy
BF DF conical DF
TEM – Imaging and DiffractionOptic axis
Image plane
Diffraction pattern
Transmission Electron Microscopy
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TEM – Imaging and DiffractionOptic axis
Diffraction pattern
Image
High-resolution imageInterference of several diffracted beams with the direct beam
Transmission Electron Microscopy
TEM – Imaging and DiffractionOptic axis
Diffraction pattern
Image
Mathematics
+ -2i qx
-{f(x)} ( ) e dx
∞ π
∞= ∫F f x
Fourier Transform
Fourier analysis
Fourier synthesis
Transmission Electron Microscopy
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TEM – Imaging and DiffractionOptic axis
Diffraction pattern
HRTEM image
Image Processing
Fourier Transform
Fourier Transform
Filtered image
Transmission Electron Microscopy
ED + HRTEM
Nb8W9O47 – threefold TTB superstructure
Transmission Electron Microscopy
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HRTEM: Detection of Defects
Planar defect in ZnNb14O35F2
Transmission Electron Microscopy
HRTEM: Detection of Defects
Grain boundary between differently ordered areas in a Nb-W oxide
Transmission Electron Microscopy
10 nm
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HRTEM and ED of Aperiodic Structures
Dodecagonal quasicrystal in the system Ta-Te
Transmission Electron Microscopy
HRTEM: Imaging Single Atoms
Gd@C82 in SWCNT
Suenaga et al, Science 290 (2000) 2280
3 nm
3 nm
Transmission Electron Microscopy
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Transmission Electron Microscopy
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
Transmission Electron Microscopy
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
Scanning Transmission Electron Microscopy
From: Williams, Carter: Transmission Electron Microscopy
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°
From: Williams, Carter: Transmission Electron Microscopy
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BF and DF Imaging in STEM
Scanning Transmission Electron Microscopy
Pt Particles in SiO2 nanotubes
BF ADF HAADF
BF DF conical DF TEM
HAADF-STEM or Z contrast Images of Au Particles on Titania
Scanning Transmission Electron Microscopy
50 nm
10 nm
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High-Resolution HAADF-STEM
WO3 segregations in a bronze-type Nb-W oxide
Scanning Transmission Electron Microscopy
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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Scanning Transmission Electron Microscopy
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
Scanning Transmission Electron Microscopy
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)
Analytical Transmission Electron Microscopy
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
Analytical Transmission Electron Microscopy
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
Analytical Transmission Electron Microscopy
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X-ray Spectrometer
Analytical Transmission Electron Microscopy
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
Analytical Transmission Electron Microscopy
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
Analytical Transmission Electron Microscopy
(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)
Analytical Transmission Electron Microscopy
Electron Energy Loss Spectroscopy
Analytical Transmission Electron Microscopy
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
Analytical Transmission Electron Microscopy
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
Transmission Electron Microscopy
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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
Analytical Transmission Electron Microscopy
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.
Analytical Transmission Electron Microscopy
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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
Analytical Transmission Electron Microscopy
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
Analytical Transmission Electron Microscopy
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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 Transmission Electron Microscopy
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
Analytical Transmission Electron Microscopy
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Analytical Electron Microscopy- Restrictions
signal/noise measuring time
spatial resolution
Analytical Transmission Electron Microscopy