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Energy dispersive X-ray spectroscopy (EDS)
• Introduction and basic physics [W&C chap. 4.1-4.2]
• Instrumentation [W&C chap. 32]
• Quantification and spectrum imaging [W&C chap. 33-35]
• Spatial resolution [W&C chap. 36]
• Fultz & Howe chapter 5.6 + 5.7
Energy transfer processes
The main energy transfer processes are:
• Brehmsstrahlung
• Single electron excitations
• Collective excitations (plasmons)
The first two processes are observed both in energy dispersive X-
ray spectroscopy (EDS) and in electron energy loss spectrscopy
(EELS).
The last process is only observed in EELS.
Brehmsstrahlung
The change in trajectory is caused by an acceleration of the electron. The
spectrum of the photons generated is given by the Fourier transform of a(t) for
the electron.
F&H
Brehmsstrahlung
Single bremsstrahlung event
Several events
Many events
Kramer’s cross section for production
of bremsstrahlung
F&H W&C
1: Incident
electron transfers
energy to a
«core» electron
of the atom,
which exits the
sample.
2: The atom is left
in an excited state
with a core hole.
3a: The atom can
relax to the ground
state either by
emitting a photon
(characteristic X-
ray)…
3b:…or by
emitting an
Auger-electron
Single electron excitations
Nomenclature
Atomic shell Main quantum
number n
Orbital quantum
number l
Total spin
quantum number
j
Spectroscopic
notation
1s 1 0 +1/2, -1/2 K
2s 2 0 +1/2, -1/2 L1
2p1/2 2 1 1/2 L2
2p3/2 2 1 3/2 L3
3s 3 0 +1/2, -1/2 M1
3p1/2 3 1 1/2 M2
3p3/2 3 1 3/2 M3
3d3/2 3 2 3/2 M4
3d5/2 3 2 5/2 M5
More on nomenclature
W&C
Siegbahn notation is most
commonly used to name the
transitions generating X-rays.
However, this can get quite
complicated as seen in the figure
on the left.
The International Union of Pure
and Applied Chemistry (IUPAC)
recommends an alternative
system that is simpler, but
unfortunately not widely in use.
Copper K lines
Eb(K) = -8979 eV
Eb(L3) = -933 eV
Eb(L2) = -952 eV
Eb(M2,3) = -76 eV
L3 -> K transition (Kα1)
E=hn= Eb(L3)- Eb(K) = 8.046 keV
L2 -> K transition (Kα2)
E=hn= Eb(L2)- Eb(K) = 8.027 keV
M2,3 -> K transition (Kb)
E=hn= Eb(M2,3)- Eb(K) = 8.903 keV
Threshold/critical energy
In order to generate X-rays, the electron beam must have an
energy E0 larger than the critical energy Ec of the process.
Usually not a problem in TEM
E0> 100 keV; Ec< 20 keV
BUT: with Cs correctors, low voltage operation has become more
common. 60 keV, 40 keV, even down to 30 keV.
For heavy elements this may limit which characteristic X-rays are
generated
Ionization cross section
Non-relativistic (Bethe) cross section for ionization
• E0: energy of electrons
• Ec: critical energy of
excitation
• E0/ Ec: Overvoltage
The fluorescence yield
The probability for generating a
characteristic X-ray is given by the
fluorescence yield w
The probability of generating an
Auger electron is the 1- w.
F&H
The detector and electron/hole-pair
generation
• Characteristic X-rays are generated in the specimen and enter
the detector
• There, they generate a number of electron/hole-pairs
depending on their energy
• The electron/hole-pairs are separated by an applied bias, and
the current measured W&C
The detector and electron/hole-pair
generation
• Historically, the most common detectors have been the Si(Li)
detectors
• The so-called Silicon-drift detectors (SDD) have been used for
SEM for quite a while, and are becoming more common also for
TEM
• The detectors are usually cooled to avoid
– diffusion of dopants in the strong applied bias
– thermal noise
• Si(Li) detectors are usually cooled to
LN2 temperatures (-196 C)
• SDD detectors can make do with only
Peltier cooling (~-30 C)
W&C
Windows
• Detectors that are cooled to LN2 temperature «need» to be
protected from contamination
• Water, hydrocarbons
• A «window» is used for protection
• Beryllium, thin polymer, thin polymer with support
• But is the window transpararent «enough»?
W&C
It may be necessary to tilt the sample towards the detector to avoid
shadowing. But this also has drawbacks.
Spurious and system X-rays
• Spurious X-rays from the
specimen, but not the region of
interest
• System X-rays from the sample
holder, specimen support grid,
microscope itself (Cu, Fe)
W&C
Absorption and fluorescence in the
sample • The X-rays generated in the
primary event must travel
through the specimen to reach
the detector
• The longer the path, the greater
the likelihood of absorbtion…
• …and fluorescence
• Might skew the ratio of element
A and B X-rays detected
• TEM samples are thin
• Absorption is mainly a problem
for low Z elements (e.g. O)
• Fluorescence is rarely a
problem at all
• But still: be aware of these
effects! W&C
Escape peaks
• The detector determines the
photon energy by measuring the
charge pulse from the electron-
hole pairs generated
• Some times, fluorescence occurs
in the detector, and a Si K
photon is generated
• This photon can leave the
detector, taking with it some of the
energy that «should» have gone
into making electron-hole pairs
• The detector then sees a smaller
charge pulse, which is interpreted
as a lower energy of the incoming
photon giving a peak at E-1.74
keV
• Usually a small effect, but be
aware when counting for a long
time
W&C
Internal fluorescence
• The reverse of the previous
problem
• Incoming X-rays can excite the Si
atoms in the detector (dead layer)
making a Si K
• If this photon enters the «active»
region of the detector, it will be
detected
• A small Si K peak appears in the
spectrum
• Usually a small effect, but be
aware when counting for a long
time
W&C
Sum peaks
Detector «sees» one photon with E=E1+E2
E=E1
E=E2
EDS detector
Two photons enter
detector with small t
Mainly a problem when count rates are very
high (>> 10 kcps).
May be mistaken for another element Some
K sum
peaks
Peak overlap, energy resolution
• Typical energy
resolution is
~130 eV
• Measured as
FWHM of Mn
K
• You may easily
see only one
peak where in
reality there are
many
WDS vs EDS
• Uses a diffraction grating
(crystal) to select X-rays with
particular energy (wavelength)
• Braggs law
• Wavelength dispersive, in
stead of energy dispersive
• Excellent energy resolution
• Low background, no detector
artefacts
• Good for light elements
• Serial detection
• Movable parts
• Low effective detection angle
Quantification from EDS spectra
• How to get from a spectrum to composition
• Assumptions usually made in EDS in TEM
• Cliff-Lorimer k-factor method
• Limits of the CL-method and the assumptions
made
• Statistical errors
Williams & Carter chapter 35
30
The Cliff-Lorimer equations
F&H
32
𝑐𝐴
𝑐𝐵= 𝑘𝐴𝐵
𝐼𝐴
𝐼𝐵
𝑘𝐴𝐵 =𝑐𝐴𝐼𝐵𝑐𝐵𝐼𝐴
1
𝑘𝐴𝐵=𝑐𝐵𝐼𝐴𝑐𝐴𝐼𝐵= 𝑘𝐵𝐴
𝑐𝐴 + 𝑐𝐵 = 1
𝑐𝐵
𝑐𝐶= 𝑘𝐵𝐶
𝐼𝐵
𝐼𝐶
𝑐𝐴 + 𝑐𝐵 + 𝑐𝐶 = 1
𝑘𝐴𝐶 = 𝑘𝐴𝐵𝑘𝐵𝐶
Binary system
Ternary system
k-factors are not constants
Depend on:
• Acceleration voltage
• Detector
• Analysis conditions
• Background subtraction
• Peak-integration
k-factors are a sensitivity factor for the particular system. For the best accuracy you the k-factors must be determined for the particular experimental setup.
Usually not done today, calculated k-facors used in stead. Less reliable (+/- 20%)
35
Peak integration vs modeling
• Simply integrating the peak
intensity and subtracting the
background works well in many
cases
• But what about peak overlap?
• The integration would add the
two peaks together
• Inaccurate results
W&C
40
Peak integration vs modeling
• In stead, model the known
peaks e.g. with Gaussian
funtions
• Look at the residual
• Are there unexplained
discrepancies?
• Perhaps another element is
present?
W&C
42
Introduction
• There is uncertainty in all measurements
• We simply do not have direct access to exact measures of physical
quantities
43
Physical quantity Measuring device Analysis/calculation Result
• All of these steps introduce uncertainties
• A measured quantity x should alway be given as follows:
x = your best estimate ± some measure of the uncertainty
• «Error analysis» is the process of finding this best estimate and
deciding on a measure of uncertainty
Measuring many times
44
200 210 220 230 240 250 260 270 280 290 3000
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Measured value
Num
ber
of
measure
ments
n = 5
mean = 247.3
spread = 7.6
200 210 220 230 240 250 260 270 280 290 3000
5
10
15
20
25
30
Measured value
Num
ber
of
measure
ments
n = 150
mean = 249.1
spread = 9.8
True value= 250
𝜇 = 𝑥 =1
𝑛 𝑥𝑖
𝑛
𝑖=1
𝜎 =
1
𝑛 − 1 𝑥𝑖 − 𝑥
2
𝑛
𝑖=1
The Gaussian distribution
45
Question: If I make one more
mesurement, where is it
likely to appear?
Answer:
• 1 : 68%
• 2 : 95%
• 3 : 99%
We say that the 95%
confidence interval for the
measurement is [-2, +2]
We can treat as the error in
a single measurement.
But shouldn’t many measurements
give a more reliable result?
47
200 210 220 230 240 250 260 270 280 290 3000
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Measured value
Num
ber
of
measure
ments
200 210 220 230 240 250 260 270 280 290 3000
5
10
15
20
25
30
Measured value
Num
ber
of
measure
ments
True value= 250
n = 5
mean = 247.3
spread = 7.6
n = 150
mean = 249.1
spread = 9.8
But shouldn’t many measurements
give a more reliable result?
• Sigma is a measure of the spread of the
measurements.
• If the spread is caused by e.g. the
measurement system you use, this spread
will not improve with more measurements.
• But we are not really interested in the spread
of the measurements, but how reliable the
mean of the measurements is.
• Standard error of the mean
• This quantity improved with the number of
measurements. 48
Error propagation
• Often we are not interested in the property
that is measured directly, but some quantity
which we calcuculate from one or more
measured values.
• For example the force F = m*a, where m and
a are measured with some uncertainty.
• What is then the uncertainty in in F?
• The mean is another such example
• What is the uncertainty in the mean?
49
𝜇 = 𝑥 =1
𝑛 𝑥𝑖
𝑛
𝑖=1
Propagating the errors in the mean
51
𝜇 = 𝑥 =1
𝑛 𝑥𝑖
𝑛
𝑖=1
=𝑥1𝑛+𝑥2𝑛+⋯+
𝑥𝑛𝑛
Assume same uncertainty in all measured xi
This is called the Standard Error in the Mean (SE)
52
200 210 220 230 240 250 260 270 280 290 3000
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Measured value
Num
ber
of
measure
ments
200 210 220 230 240 250 260 270 280 290 3000
5
10
15
20
25
30
Measured value
Num
ber
of
measure
ments
𝜇 ± 1𝑆𝐸 = 247.3 ± 3.4
n = 5
mean = 247.3
spread = 7.6
𝑆𝐸 =𝜎
𝑛=7.6
5= 3.4
n = 150
mean = 249.1
spread = 9.8
𝑆𝐸 =𝜎
𝑛=9.8
150= 0.8
𝜇 ± 1𝑆𝐸 = 249.1 ± 0.8
Introduction to counting statistics
• Generation/emission of X-ray photons from an irradiated
sample is a random process with a probability p per unit time.
• For an large (infinite) counting time t, we would expect to see
N=<N>=t*p
• But what does that mean for set of a real measurements (small
t)?
• We would find N1, N2, N3, N4,..
• These values will not be equal, but will be clustered around an
average 𝑁
• For a large number of measurements, 𝑁 → 𝑁
• Counting experiments of this sort follow the Poisson distribution
• Here, the uncertainty (standard deviation) of a measurement
counting N events is estmated as 𝜎 = √(𝑁) 53
What does this mean for our
interpretation of our experiments? • The intensity of each characteristic peak is a counting of the
number of X-ray photons
• If we count N photons, the uncertainty is 𝜎 = √(𝑁)
• If we reapeat the experiment, there is a 68% likelyhood that the
new measurement will give 𝑁 ± 1𝜎 , 95% likelyhood of 𝑁 ± 2𝜎, 99% of 𝑁 ± 3𝜎
• This uncertainty has to be accounted for in our quantification,
and when we report composition measurements.
• The background is also Poisson distributed
• This means that the background will show fluctuations with
intensity 𝜎 = √(𝑁)
• How do we then distinguish a “real” signal from “noise”?
54
Let’s quantify the ratio of Cu to Zn
• Cliff-Lorimer equations
• 𝑘𝐶𝑢,𝑆𝑖 = 1.51 ± 0.40 2𝜎
• 𝑘𝑍𝑛,𝑆𝑖 = 1.63 ± 0.28 (2𝜎)
• 𝑘𝐶𝑢,𝑍𝑛 =?
• 𝑘𝐶𝑢,𝑍𝑛 = 𝑘𝐶𝑢,𝑆𝑖 𝑘𝑆𝑖,𝑍𝑛 = 𝑘𝐶𝑢,𝑆𝑖1
𝑘𝑍𝑛,𝑆𝑖 = 0.93
• What is 𝜎?
• 𝜎 = 0.93 0.20
1.51
2+0.14
1.63
2= 0. 15
• 𝑘𝐶𝑢,𝑍𝑛 = 0.93 ± 0.15 (1𝜎)
58
𝑐𝐴
𝑐𝐵= 𝑘𝐴𝐵
𝐼𝐴
𝐼𝐵
1
𝑘𝐴𝐵= 𝑘𝐵𝐴
𝑘𝐴𝐶 = 𝑘𝐴𝐵𝑘𝐵𝐶
59
𝑰𝒁𝒏 𝑰𝑪𝒖 𝝈𝒁𝒏 = 𝑰𝒁𝒏 𝝈𝑪𝒖 = 𝑰𝑪𝒖 𝒄𝑪𝒖𝒄𝒁𝒏
Sample A 9782 269 99 16
Sample B 10063 297 100 17
Sample A: 𝑐𝐶𝑢
𝑐𝑍𝑛= 𝑘𝐶𝑢,𝑍𝑛
𝐼𝐶𝑢
𝐼𝑍𝑛= 0.93 ×
269
9782= 0.026
𝜎 =𝑐𝐶𝑢
𝑐𝑍𝑛
0.015
0.93
2+ (16
269)2+(
99
9782)2= 0.004
Sample B: 𝑐𝐶𝑢
𝑐𝑍𝑛= 0.027
𝜎 = 0.005
Is the difference significant?
𝐷𝑖𝑓𝑓 = 0.001 ± 0.0042 + 0.0052 = 0.001 ± 0.006
60
Measurement #i 𝑰𝒁𝒏 𝑰𝑪𝒖 𝝈𝒁𝒏 = 𝑰𝒁𝒏 𝝈𝑪𝒖 = 𝑰𝑪𝒖 𝒄𝑪𝒖𝒄𝒁𝒏
𝜎𝑖
1 9782 269 99 16 0.026 0.004
2 9261 279 96 17 0.028 0.005
3 1660 26 41 5 0.015 0.006
4 35175 977 188 31 0.026 0.004
5 23370 641 153 25 0.025 0.004
6 11938 343 109 19 0.027 0.004
7 16015 436 127 21 0.025 0.004
8 100026 2757 316 53 0.026 0.004
Measurement #i 𝑰𝒁𝒏 𝑰𝑪𝒖 𝝈𝒁𝒏 = 𝑰𝒁𝒏 𝝈𝑪𝒖 = 𝑰𝑪𝒖 𝒄𝑪𝒖𝒄𝒁𝒏
𝜎𝑖
1 10063 297 100 17 0.027 0.005
2 3668 99 61 10 0.025 0.005
3 9711 199 99 14 0.019 0.003
4 28191 606 168 25 0.020 0.003
5 35148 761 187 28 0.020 0.003
6 13589 294 117 17 0.020 0.003
7 7720 186 88 14 0.022 0.004
8 100245 2222 317 47 0.021 0.003
Sample A
Sample B