X ray photoelectron Spectroscopy

1. X-RAY PHOTOELECTON SPECTROSCOPY
PRESENTED BY Y.GANDHI
CA17M008

2. OUTLINE:
• INTRODUCTION
• X-RAYS
• WHY THE CORE ELECTRON?
• BINDING ENERGY
• DEFINITION OF XPS
• XPS FEATURES
• XPS INSTRUMENT
• HOW XPS TECHNOLOGY WORKS
• WHY DOES XPS NEED UHV?
• X-RAYS AND AUGER ELECTRONS
• XPS SPECTRUM AND ANALYSIS
• IDENTIFICATION OF XPS
• SATELLITES
• AUGER FEATURES
• CHEMICAL SHIFTS
• ADVANTAGES AND DISADVANTAGES OF XPS

3. INTRODUCTION:
• XPS technique is based on Einstein’s idea
about the photoelectric effect, developed
around 1905.
• Photoelectric effect : The concept of
photons was used for the ejection of
electrons from a surface when photons
were impinged upon it.
SLIDE - 1

4. X-RAYS:
• Irradiate the sample surface, hitting the core electrons (e-) of the atoms.
• The X-Rays penetrate the sample to a depth on the order of a micrometer.
• Useful e- signal is obtained only from a depth of around 10 to 100 Å on the
surface.
• The X-Ray source produces photons with certain energies:
• MgK photon with an energy of 1253.6 eV
• AlK photon with an energy of 1486.6 eV
• Normally, the sample will be radiated with photons of a single energy
(MgK or AlK). This is known as a monoenergetic X-Ray beam.
SLIDE-2

5. WHY THE CORE ELECTRONS?
• An electron near the Fermi level is far from the nucleus, moving in
different directions all over the place, and will not carry information
about any single atom.
• Fermi level is the highest energy level occupied by an electron in a
neutral solid at absolute 0 temperature.
• Electron binding energy (BE) is calculated with respect to the Fermi
level.
• The core e-s are local close to the nucleus and have binding
energies characteristic of their particular element.
• The core e-s have a higher probability of matching the energies of
AlK and MgK.
Core e-
Valence e-
Atom
SLIDE-3

6. BINDING ENERGY (BE)
These electrons are
attracted to the
proton with certain
binding energy x
This is the point with 0
energy of attraction
between the electron and
the nucleus. At this point
the electron is free from the
atom.
The Binding Energy (BE) is characteristic of the core electrons for each
element. The BE is determined by the attraction of the electrons to the
nucleus. If an electron with energy x is pulled away from the nucleus,
the attraction between the electron and the nucleus decreases and the
BE decreases. Eventually, there will be a point when the electron will be
free of the nucleus.
0
x
p+
B.E.
SLIDE-4

7. • DEFINITION OF XPS :
XPS, also known as ESCA(Electron Spectroscopy for
Chemical Analysis ), is the most widely used surface analysis technique
because of its relative simplicity in use and data interpretation.
SLIDE-5

8. • The 3 –step model:
• 1.Optical excitation
• 2.Transport of electron to the surface
(diffusion energy loss)
• 3.Escape into the Vacuum
SLIDE-6

9. SIX FEATURES SEEN IN A TYPICAL XPS SPECTRUM
• 1.Sharp peaks due to photoelectrons created within the first few atomic layers(elastically
scattered).
• 2.Multiplet splitting(occurs when unfilled shells contain unpaired electrons).
• 3.Abroad structure due to electrons from deeper in the solid which are inelastically
scattered(reduced KE) forms the background.
• 4.Satellites(shake-of fand shake-up)are due to a sudden change in Coulombic potential as
the photoejected electron passes through the valence band.
• 5. Plasmons which are created by collective excitations of the valence band •Extrinsic
Plasmon: excited as the energetic PE propagates through the solid after the photoelectric
process.
• 6. Auger peaks produced by x-rays (transitions from L to K shell: O KLL or C KLL).
SLIDE-7

10. XPS INSTRUMENT
X-Ray Source
Ion Source
Analyzer
Sample introduction
Chamber
SLIDE-8

11. SAMPLE INTRODUCTION CHAMBER
• The sample will be introduced
through a chamber that is in
contact with the outside
environment
• It will be closed and pumped to
low vacuum.
• After the first chamber is at low
vacuum the sample will be
introduced into the second
chamber in which a UHV
environment exists.
First Chamber
Second Chamber UHV
SLIDE-9

12. DIAGRAM OF THE SIDE VIEW
OF XPS SYSTEM
X-Ray source
Ion source
Axial Electron Gun
Detector
CMA
sample
SIMS
Analyzer
Sample introduction
Chamber
Sample
Holder
Ion Pump
Roughing Pump Slits
Instrumentation:
•Electron energy analyzer
•X-ray source
•Ar ion gun
•Neutralizer
•Vacuum system
•Ultrahigh vacuum system < 10-
9Torr(< 10-7 Pa)
•Detection of electrons
•Avoid surface reactions/
contaminations
SLIDE-10

13. HOW DOES XPS TECHNOLOGY WORK?
• A monoenergetic x-ray beam emits
photoelectrons from the from the
surface of the sample.
• The X-Rays either of two energies:
• Al Ka (1486.6eV)
• Mg Ka (1253.6 eV)
• The x-ray photons The penetration
about a micrometer of the sample
• The XPS spectrum contains
information only about the top 10
- 100 Ǻ of the sample.
• Ultrahigh vacuum environment to
eliminate excessive surface
contamination.
• Cylindrical Mirror Analyzer (CMA)
measures the KE of emitted e-s.
• The spectrum plotted by the
computer from the analyzer signal.
• The binding energies can be
determined from the peak positions
and the elements present in the
sample identified.
SLIDE-12

14. WHY DOES XPS NEED UHV?
• Contamination of surface
• XPS is a surface sensitive technique.
• Contaminates will produce an XPS signal and lead to incorrect analysis
of the surface of composition.
• The pressure of the vacuum system is < 10-9 Torr
• Removing contamination
• To remove the contamination the sample surface is bombarded with argon
ions (Ar+ = 3KeV).
• heat and oxygen can be used to remove hydrocarbons
• The XPS technique could cause damage to the surface, but it is
negligible.
SLIDE-13

15. X-RAYS ON THE SURFACE
Atoms layers
e- top layer
e- lower layer
with collisions
e- lower layer
but no collisions
X-Rays
Outer surface
Inner surface
SLIDE-14

16. X-RAYS ON THE SURFACE
• The X-Rays will penetrate to the core e- of the atoms in the sample.
• Some e-s are going to be released without any problem giving the Kinetic Energies (KE)
characteristic of their elements.
• Other e-s will come from inner layers and collide with other e-s of upper layers
• These e- will be lower in lower energy.
• They will contribute to the noise signal of the spectrum.
SLIDE-15

17. X-RAYS AND AUGER ELECTRONS
• When the core electron leaves a vacancy an electron of
higher energy will move down to occupy the vacancy
while releasing energy by:
• photons
• Auger electrons
• Each Auger electron carries a characteristic energy that
can be measured.
SLIDE-17

18. TWO WAYS TO PRODUCE AUGER ELECTRONS
1. The X-Ray source can irradiate and remove the e- from
the core level causing the e- to leave the atom
2. A higher level e- will occupy the vacancy.
3. The energy released is given to a third higher level e-.
4. This is the Auger electron that leaves the atom.
The axial e- gun can irradiate and remove the core e- by
collision. Once the core vacancy is created, the Auger
electron process occurs the same way.
SLIDE-18

19. AUGER ELECTRON
Free e-
e- Vacancy
e- of high energy that
will occupy the vacancy
of the core level
e- released to
analyze
1
1, 2, 3 and 4 are the order of steps in which the e-s will move in
the atom when hit by the e- gun.
e- gun
2
3
4
SLIDE-19

20. CYLINDRICAL MIRROR ANALYZER (CMA)
• The electrons ejected will pass through a device called a CMA.
• The CMA has two concentric metal cylinders at different
voltages.
• One of the metal cylinders will have a positive voltage and the
other will have a 0 voltage. This will create an electric field
between the two cylinders.
• The voltages on the CMA for XPS and Auger e-s are different.
SLIDE-20

21. • When the e-s pass through the metal cylinders, they will
collide with one of the cylinders or they will just pass
through.
• If the e-’s velocity is too high it will collide with the outer
cylinder
• If is going too slow then will collide with the inner cylinder.
• Only the e- with the right velocity will go through the cylinders
to reach the detector.
• With a change in cylinder voltage the acceptable kinetic
energy will change and then you can count how many e-s
have that KE to reach the detector.
SLIDE-21

22. CYLINDRICAL MIRROR ANALYZER (CMA)
Slit
Detector
Electron Pathway through the CMA
0 V
+V
0 V 0 V
0 V
+V
+V
+V
X-Rays
Source
Sample
Holder
SLIDE-22

23. EQUATION
KE=hv-BE-Ø
KE Kinetic Energy (measure in the XPS spectrometer)
hv photon energy from the X-Ray source (controlled)
Ø spectrometer work function. It is a few eV, it gets more
complicated because the materials in the instrument will affect it.
Found by calibration.
BE is the unknown variable
SLIDE-23

24. • The equation will calculate the energy needed to get an e-
out from the surface of the solid.
• Knowing KE, hv and Ø the BE can be calculated.
KE=hv-BE-Ø
SLIDE-24

25. TRANSMISSION FUNCTION
• Transmission function is the detection efficiency of the electron energy analyzer,
which is a function of electron energies. Transmission function also depends on the
parameters of the electron energy analyzer, such as pass energy.
SLIDE-25

26. KE VERSUS BE
E E E
KE can be plotted depending on
BE
Each peak represents the amount
of e-s at a certain energy that is
characteristic of some element.
1000 eV 0 eV
BE increase from right to left
KE increase from left to right
Binding energy
#ofelectrons
(eV)
SLIDE-26

27. INTERPRETING XPS SPECTRUM:
BACKGROUND
• The X-Ray will hit the e-s in the bulk (inner e-
layers) of the sample
• e- will collide with other e- from top layers,
decreasing its energy to contribute to the
noise, at lower kinetic energy than the peak .
• The background noise increases with BE
because the SUM of all noise is taken from
the beginning of the analysis.
Binding energy
#ofelectrons
N1
N2
N3
N4
Ntot= N1 + N2 + N3 + N4
N = noise
SLIDE-27

28. XPS SPECTRUM
• The XPS peaks are sharp.
• In a XPS graph it is possible to see Auger
electron peaks.
• The Auger peaks are usually wider peaks in a
XPS spectrum.
SLIDE-28

29. XPS Spectrum
O 1s
O because
of Mg source
C
Al
Al
O 2s
O Auger
Sample and graphic provided by William Durrer, Ph.D.
Department of Physics at the Univertsity of Texas at El Paso
SLIDE-29

30. XPS OF AU
XPS spectra of Pd/RGO
composite: (a) XPS full spectrum
of Pd/RGO and (b) Pd spectrum.
SLIDE-30

31. ANALYSIS OF XPS SPECTRA:
-> Traditional XPS quantification assumes
•Outer surface of sample is homogeneous
•Outer surface concentration is directly proportional to the peak intensity
->More accurate quantification should include peak intensity, peak shape and background
energy
->In photoelectron spectroscopy electrons detected result from two processes
•the intrinsic electrons –from photoelectron process
•the extrinsic electrons –from scattering of photoelectrons passing through surrounding atoms
->Depending on the depth of the emitting atom within the surface, as well as its lateral
distribution, the extrinsic portion will change dramatically
SLIDE-31

32. IDENTIFICATION OF XPS PEAKS
• The plot has characteristic peaks for each element found
in the surface of the sample.
• There are tables with the KE and BE already assigned to
each element.
• After the spectrum is plotted you can look for the
designated value of the peak energy from the graph and
find the element present on the surface.
SLIDE-32

33. • For p, d and f peaks, two peaks are observed.
• The separation between the two peaks are
named spin orbital splitting. The values of spin
orbital splitting of a core level of an element in
different compounds are nearly the same.
• The peak area ratiosof a core level of an
element in different compounds are also nearly
the same
SLIDE-33

34. • Multiplet splitting occurs when the system has unpaired electrons in the Valence
levels. Example:Mn
• Also the total electronic angular momentum (j) is a combination of the orbital
angular (l) and spin (s) momenta. The j-j coupling is equal to |L ±S| where L and S
are the total orbital angular and spin momenta, respectively.
• For angular quantum number l ≠0 the line is a doublet. (p1/2, p3/2). Splitting: Final
states are given by: j += l + sand j -= l –s
• Examples:For p orbitals the doublet will be p 1/2and p 3/2 because l = 1 and s =
±1/2 therefore j -= 1/2 and j +=3/2. For d orbitals, the doublet will be dx1and d
x2because l = and s = ±1/2 therefore j -= x1and j += x2
SLIDE-34

35. SATELLITES:
• Arise when a core electron is removed by photo ionisation.There is a sudden change
in the effective charge due to loss of shielding electron.
• 2 types of satellites are detected
SHAKE UP:
Outgoing electron interacts with a valence
electron and excites it to a higher energy
level.As a consequence the energy of core
electron is reduced and a satellite structure
appears a few electron volts below the core
position.
SHAKE-OFF:
The valence electron is ejected from ion
completely.Appears as a broadening of
core level peak or contribute to the
inelastic background.
SLIDE-35

36. SLIDE-36

37. CHEMICAL EFFECTS IN XPS
• Chemical shift: change in binding energy of a core electron of an element due to a
change in the chemical bonding of that element.
• Qualitative view: Core binding energies are determined by: •electrostatic interaction
between it and the nucleus, and reduced by: •the electrostatic shielding of the
nuclear charge from all other electrons in the atom (including valence electrons)
•removal or addition of electronic charge as a result of changes in bonding will alter
the shielding
• Withdrawal of valence electron charge -> increase in BE (oxidation) Addition of
valence electron charge -> decrease in BE
SLIDE-39

38. SLIDE-40

39. ADVANTAGES AND DISADVANTAGES OF XPS:
ADVANTAGES DISADVANTAGES
Relatively non-destructive method of sample
characterization and surface sensitive
Very expensive
Element traces analysis – concentration sensitivity
limit at about 1% of atomic fraction
High vacuum required
identifying the chemical state on surfaces, and is
good with quantitative analysis
Slow processing
differentiate between oxidations states of
molecules
Hydrogen and Helium cant be detected
detecting the difference in chemical state between
samples
Relatively complicated sample preparation for
precise XPS study
SLIDE-41