Analytical
Transmissions Electron Microscopy (TEM)
Part I:
The microscope
Sample preparation
Imaging
Part II:
Diffraction
Defects
Part III
Spectroscopy
MENA 3100: Diff
Repetision:
Electron Diffraction:
Small wave length
Large Ewald sphere
-- Perfect crystals plane
Very sensitive to changes in the crystal
structure
Small diffraction
Strong intensity – short exposure time
Directly observed on the viewing
screen
Obtained from very small crystals
Powder X-ray diffraction:
Larger wave length
Small Ewald sphere
Ring pattern
Larger driffrationangle
Lower intensity
Easier to interpret
Electron matter interactions
Coherent incident
high-kV beam
sample
Incoherent elastic
backscattered electrons (SEM)
Second electrons
From within the specimen (SEM)
Characteristic
X-rays (EDS)
Auger electrons (XPS)
Visible light
Sample
Incoherent inelastic
Bremsstrahlung
scattered electrons
X-rays (EDS)
(EELS)
Direct beam
Incoherent elastic
(imaging, diffraction,
forward scattered
Coherent elastic
EELS)
Electrons (STEM,
scattered electrons (STEM,
diffraction,EELS)
Diffraction, EELS)
The characteristic energy transitions
Observable with EELS and EDS
MENA 3100: Spectroscopy
Empty states
Valence electrons
M shell
L shell
K shell
MENA 3100: Spectroscopy
Empty states
3d
M shell
L shell
K shell
3p
3s
Lα1
L2,3
2p
L1
2s
1s
Kα1
EDS
MENA 3100: Spectroscopy
Kβ1
K
EELS
Energy dispersive X-ray spectroscopy
M
L
Lα
kα
K
kβ
hν
MENA 3100: Spectroscopy
EELS
Eo
Eo
K-edge (Si) – 1s orbital
L-edge (Si)
– 2s and 2p orbital
M
L
K
Filled bands
Conduction band
Eb(L)=Eo-Eb(cond.band-L)
Eb(K)=Eo-E(cond.band-K)
Energy Dispersive X-ray Spectroscopy
(EDS)
MENA 3100: Spectroscopy
EDS spectrum
MENA 3100: Spectroscopy
X-ray Spectroscopy
EDS:
EDXS:
X-EDS:
EDX:
MENA 3100: Spectroscopy
Energy Dispersive Spectroscopy
Energy Dispersive X-ray Spectroscopy
X-ray Energy Dispersive Spectroscopy
Energy Dispersive X-ray analysis
Quantification
Peak intensities are proportional to concentration and specimen thickness.
They removed the effects of variable specimen thickness by taking ratios of intensities
for elemental peaks and introduced a “k-factor” to relate the intensity ratio to
concentration ratio:
𝐶𝐴
𝐼𝐴
= 𝐾𝐴𝐵
𝐶𝐵
𝐼𝐵
Each pair of elements requires a different k-factor, which depends on detector efficiency,
ionization cross-section and fluorescence yield of the two elements concerned.
MENA 3100: Spectroscopy
The Detector
MENA 3100: Spectroscopy
Detection Mechanism
MENA 3100: Spectroscopy
Oxford
MENA 3100: Spectroscopy
EDS
SEM
MENA 3100: Spectroscopy
TEM
EDS mapping
MENA 3100: Spectroscopy
Recent advances
MENA 3100: Spectroscopy
Comparison Low Z element
MENA 3100: Spectroscopy
Comparison on resolution
MENA 3100: Spectroscopy
Artefacts in EDS
1. Si escape peak:
A small fraction of the energy is lost and not transformed into
electronhole pairs
2. Sum peak:
Two photons will enter the detector at exactly the same time. The analyzer then
registers an energy corresponding to the sum of the two photons.
Likely to occur if:
- The input count rate is high.
- The dead times are > 60%.
- There are major characteristic peaks in the spectrum.
MENA 3100: Spectroscopy
3. Fluorescence:
This is a characteristic peak from the Si (or Ge) in the detector dead layer.
4.
-
Sample preparation artefacts (ion milling , grids, reaction to solvent)
Cu/Ni slot
Thickness variations due to milling
Contaminants and reaction products
MENA 3100: Spectroscopy
Electron Energy Loss Spectroscopy
(EELS)
MENA 3100: Spectroscopy
Omega filter
MENA 3100: Spectroscopy
Gatan Imaging Filter (GIF)
Post column energy filter
MENA 3100: Spectroscopy
Microscope outline
Electron gun
Condenser aperture
Sample holder
Intermediate aperture
Projector lenses
Objective aperture
Objective lens
Diffraction lens
Intermediate lens
Fluorescent screen
Gatan Imaging Filter
For EELS
MENA 3100: Spectroscopy
Projector crossover
Viewing screen
Slit
Detector
Multipole lenses
90o magnetic
prism
Beam trap aperture
Energy Losses
• Zero Loss (includes quasi-elastic scattering)
• Intra-/Inter-band transitions (band gap)
• Cherenkov losses
• Bremsstrahlung
• Plasmon losses
• Core losses
MENA 3100: Spectroscopy
EEL Spectral background
MENA 3100: Spectroscopy
Low-Loss EELS
Core-Loss EELS
Low-Loss EELS
Single electron outer shell
excitation
Zero Loss Peak
Elastic scattering:
Coulomb attraction by nucleus
Inelastic scattering:
Coulomb repulsion (outer shell electrons)
The Zero Loss Peak (ZLP)
MENA 3100: Spectroscopy
Low-Loss EELS: Bulk plasmons
Outer-shell inelastic scattering involving many atoms of the
solid.
Collective effect is known as a plasma resonance
An oscillation of the valence electron density
𝐸𝑝 =
ℎ
2𝜋
𝑁𝑒 2
𝑚𝑒 𝜀0
50xalh2_low.dat
h:
Plasmon peak
50
40
30
20
10
Binding Energy (eV)
Planck constant
N: n/V : Valence electron density
0
-10
e:
Elementary charge
me :
Electron mass
εO:
Permittivity of free space
Low-Loss EELS: Surface plasmons
Surface plasmon (Es):
Vacuum/metal interface:
𝐸𝑠 =
𝐸𝑝
2
ZLP
Dielectric/metal boundary:
𝐸𝑠 =
𝐸𝑝
1−ε
2100
Slice1
2000
1900
1800
1700
1600
1500
1400
1300
Counts
1200
Interface between two metals:
𝐸𝑠 =
𝐸𝑎2
+ 𝐸𝑏2
2
1100
1000
900
800
700
600
500
400
300
200
100
0
-5
0
5
10
15
20
25
Energy-Loss (eV)
30
35
40
45
50
55
A. Thøgersen,et al. Journal of Applied Physics 109, 084329 (2011).
Low-Loss EELS: Energy filtering
Kundmann M., Introduction to EELS in TEM, EELS course 2005 San Francisco
Low-Loss EELS: Energy filtering
Kundmann M., Introduction to EELS in TEM, EELS course 2005 San Francisco
Low-Loss EELS: Energy filtering
EFTEM imaging of Si/aSi/ITO (Indium Tin Oxide)
stack sample for REC
Si
ITO
TEM image
Low-Loss EELS: Energy filtering
EFTEM imaging of Si/aSi/ITO (Indium Tin Oxide)
stack sample for REC
TEM image
EFTEM (16 eV)
EFTEM (23 eV)
Low-Loss EELS: Thickness
𝐼𝑝
𝑡 = λ𝑝
𝐼𝑜
t = thickness
λp = plasmon mean free path
Ip = Intensity of the plasmon peak
Io = Intensity of the zero loss peak
Core-Loss EELS (Energy-Loss Near-Edge Structure)
Eo
Eo
K-edge (Si) – 1s orbital
L-edge (Si)
– 2s and 2p orbital
M
L
K
Filled bands
Conduction band
Eb(L)=Eo-Eb(cond.band-L)
Eb(K)=Eo-E(cond.band-K)
Core-Loss EELS: Peak shape
Shape of the edge is a signature of the
transition:
• K-edges: 1s states -- typical sawtooth
profile
• L2,3-edges -- have a delayed
maximum but can contain intense
narrow peaks at the onset, known as
“white lines”, corresponding to
transitions to narrow d bands.
MENA 3100: Spectroscopy
Microanalysis
MENA 3100: Spectroscopy
EFTEM:
MENA 3100: Spectroscopy
MENA 3100: Spectroscopy
Spectral Imaging (SI)
STEM-SI
EFTEM-SI
B.Chaffer et al. Analytical and Bioanalytical Chemistry
(2008) 390, Issue 6, pp 1439-1445
MENA 3100: Spectroscopy
Spectral Imaging (SI)
STEM-SI
MENA 3100: Spectroscopy
EFTEM-SI
MENA 3100: Spectroscopy
MENA 3100: Spectroscopy
EDS vs. EELS
MENA 3100: Spectroscopy
Applications
MENA 3100: Spectroscopy