Photo Electron Spectroscopy

X-ray PhotoelectronX-ray Photoelectron
Spectroscopy (XPS)Spectroscopy (XPS)
Dr.S.RADHADr.S.RADHA
Assistant Professor of ChemistryAssistant Professor of Chemistry
SaivaBhanuKshatriya CollegeSaivaBhanuKshatriya College
AruppukottaiAruppukottai

Surface AnalysisSurface Analysis
The Study of the Outer-Most Layers of Materials (<100The Study of the Outer-Most Layers of Materials (<100 ΑΑ).).
ElectronElectron
SpectroscopiesSpectroscopies
XPS: X-rayXPS: X-ray
PhotoelectronPhotoelectron
SpectroscopySpectroscopy
AES: Auger ElectronAES: Auger Electron
EELS: Electron EnergyEELS: Electron Energy
Loss SpectroscopyLoss Spectroscopy
Ion SpectroscopiesIon Spectroscopies
SIMS: Secondary IonSIMS: Secondary Ion
Mass SpectrometryMass Spectrometry
SNMS: SputteredSNMS: Sputtered
Neutral MassNeutral Mass
SpectrometrySpectrometry
ISS: Ion ScatteringISS: Ion Scattering

Introduction toIntroduction to

What is XPS?What is XPS?
X-ray Photoelectron SpectroscopyX-ray Photoelectron Spectroscopy
(XPS), also known as Electron Spectroscopy(XPS), also known as Electron Spectroscopy
for Chemical Analysis (ESCA) is a widelyfor Chemical Analysis (ESCA) is a widely
used technique to investigate the chemicalused technique to investigate the chemical
composition of surfaces.composition of surfaces.

What is XPS?What is XPS?
X-ray Photoelectron spectroscopy,X-ray Photoelectron spectroscopy,
based on the photoelectric effect, wasbased on the photoelectric effect, was
developed in the mid-1960’s by Kaideveloped in the mid-1960’s by Kai
Siegbahn and his research group at theSiegbahn and his research group at the
University of Uppsala, Sweden.University of Uppsala, Sweden.

X-ray Photoelectron SpectroscopyX-ray Photoelectron Spectroscopy
Small Area DetectionSmall Area Detection
X-ray BeamX-ray Beam
X-ray penetrationX-ray penetration
depth ~1depth ~1µµm.m.
Electrons can beElectrons can be
excited in thisexcited in this
entire volume.entire volume.
X-ray excitation area ~1x1 cmX-ray excitation area ~1x1 cm22
. Electrons. Electrons
are emitted from this entire areaare emitted from this entire area
Electrons are extractedElectrons are extracted
only from a narrow solidonly from a narrow solid
angle.angle.
1 mm1 mm22
10 nm10 nm

XPS spectral lines areXPS spectral lines are
identified by the shell fromidentified by the shell from
which the electron waswhich the electron was
ejected (1s, 2s, 2p, etc.).ejected (1s, 2s, 2p, etc.).
The ejected photoelectron hasThe ejected photoelectron has
kinetic energy:kinetic energy:
KE=hv-BE-KE=hv-BE-ΦΦ
Following this process, theFollowing this process, the
atom will release energy byatom will release energy by
the emission of an Augerthe emission of an Auger
Electron.Electron.
Conduction BandConduction Band
Valence BandValence Band
L2,L3L2,L3
L1L1
KK
FermiFermi
LevelLevel
FreeFree
ElectronElectron
LevelLevel
Incident X-rayIncident X-ray
Ejected PhotoelectronEjected Photoelectron
1s1s
2s2s
2p2p
The Photoelectric ProcessThe Photoelectric Process

L electron falls to fill core levelL electron falls to fill core level
vacancy (step 1).vacancy (step 1).
KLL Auger electron emitted toKLL Auger electron emitted to
conserve energy released inconserve energy released in
step 1.step 1.
The kinetic energy of theThe kinetic energy of the
emitted Auger electron is:emitted Auger electron is:
KE=E(K)-E(L2)-E(L3).KE=E(K)-E(L2)-E(L3).
L2,L3L2,L3
L1L1
KK
FermiFermi
LevelLevel
FreeFree
ElectronElectron
LevelLevel
Emitted Auger ElectronEmitted Auger Electron
1s1s
2s2s
2p2p
Auger Relation of Core HoleAuger Relation of Core Hole

XPS Energy ScaleXPS Energy Scale
The XPS instrument measures theThe XPS instrument measures the
kinetic energy of all collectedkinetic energy of all collected
electrons. The electron signal includeselectrons. The electron signal includes
contributions from both photoelectroncontributions from both photoelectron
and Auger electron lines.and Auger electron lines.

KEKE = hv -= hv - BEBE -- ΦΦspecspec
Where:Where: BEBE= Electron Binding Energy= Electron Binding Energy
KEKE= Electron Kinetic Energy= Electron Kinetic Energy
ΦΦspecspec= Spectrometer Work Function= Spectrometer Work Function
Photoelectron line energies:Photoelectron line energies: DependentDependent on photon energy.on photon energy.
Auger electron line energies:Auger electron line energies: Not DependentNot Dependent on photon energy.on photon energy.
If XPS spectra were presented on a kinetic energy scale,If XPS spectra were presented on a kinetic energy scale,
one would need to know the X-ray source energy used to collectone would need to know the X-ray source energy used to collect
the data in order to compare the chemical states in the samplethe data in order to compare the chemical states in the sample
with data collected using another source.with data collected using another source.
XPS Energy Scale- Kinetic energyXPS Energy Scale- Kinetic energy

XPS Energy Scale- BindingXPS Energy Scale- Binding
energyenergy
BEBE = hv -= hv - KEKE -- ΦΦspecspec
Where:Where: BEBE= Electron Binding Energy= Electron Binding Energy
KEKE= Electron Kinetic Energy= Electron Kinetic Energy
ΦΦspecspec= Spectrometer Work Function= Spectrometer Work Function
Photoelectron line energies:Photoelectron line energies: Not DependentNot Dependent on photonon photon
energy.energy.
Auger electron line energies:Auger electron line energies: DependentDependent on photon energy.on photon energy.
The binding energy scale was derived to make uniformThe binding energy scale was derived to make uniform
comparisons of chemical states straight forward.comparisons of chemical states straight forward.

Free electrons (those giving rise to conductivity) findFree electrons (those giving rise to conductivity) find
an equal potential which is constant throughout the material.an equal potential which is constant throughout the material.
Fermi-Dirac Statistics:Fermi-Dirac Statistics:
f(E) = 1f(E) = 1
exp[(E-Eexp[(E-Eff)/kT] + 1)/kT] + 1
1.01.0
f(E)f(E)
00
0.50.5
EEff1. At T=0 K:1. At T=0 K: f(E)=1 for E<Ef(E)=1 for E<Eff
f(E)=0 for E>Ef(E)=0 for E>Eff
2. At kT<<E2. At kT<<Eff (at room temperature kT=0.025 eV)(at room temperature kT=0.025 eV)
f(E)=0.5 for E=Ef(E)=0.5 for E=Eff
T=0 KT=0 K
kT<<EkT<<Eff
Fermi Level ReferencingFermi Level Referencing

Fermi Level ReferencingFermi Level Referencing
1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8 -1.0
Ef
Fermi Edge of
TiN, room temperture
Binding energy (eV)
N(E)/E

hv
Because the Fermi levels of the sample and spectrometer areBecause the Fermi levels of the sample and spectrometer are
aligned, we only need to know the spectrometer work function,aligned, we only need to know the spectrometer work function,
ΦΦspecspec, to calculate BE(1s)., to calculate BE(1s).
EE1s1s
SampleSample SpectrometerSpectrometer
ee--
Free Electron EnergyFree Electron Energy
Fermi Level, EFermi Level, Eff
Vacuum Level, EVacuum Level, Evv
Φsample
KE(1s) KE(1s)
Φspec
BE(1s)
Sample/Spectrometer Energy LevelSample/Spectrometer Energy Level
Diagram- Conducting SampleDiagram- Conducting Sample

hv
A relative build-up of electrons at the spectrometerA relative build-up of electrons at the spectrometer
raises the Fermi level of the spectrometer relative to theraises the Fermi level of the spectrometer relative to the
sample. A potential Esample. A potential Echch will develop.will develop.
EE1s1s
SampleSample SpectrometerSpectrometer
ee--
Free Electron EnergyFree Electron Energy
BE(1s)
Fermi Level, EFermi Level, Eff
Vacuum Level, EVacuum Level, Evv
KE(1s)
Φspec
Ech
Sample/Spectrometer EnergySample/Spectrometer Energy
Level Diagram- InsulatingLevel Diagram- Insulating
SampleSample

Binding Energy DeterminationBinding Energy Determination
The photoelectron’s binding energy will be
based on the element’s final-state configuration.
FermiFermi
LevelLevel
FreeFree
ElectonElecton
LevelLevel Conduction BandConduction Band
1s1s
2s2s
2p2p
Initial StateInitial State Final StateFinal State

The Sudden ApproximationThe Sudden Approximation
Assumes the remaining orbitals (often called the passive orbitals) areAssumes the remaining orbitals (often called the passive orbitals) are
the same in the final state as they were in the initial state (also calledthe same in the final state as they were in the initial state (also called
thethe frozen-orbital approximationfrozen-orbital approximation). Under this assumption, the XPS). Under this assumption, the XPS
experiment measures the negative Hartree-Fock orbital energy:experiment measures the negative Hartree-Fock orbital energy:
Koopman’s Binding EnergyKoopman’s Binding Energy
EEB,KB,K ≅≅ --εεB,KB,K
Actual binding energy will represent the readjustment of the N-1Actual binding energy will represent the readjustment of the N-1
charges to minimize energy (relaxation):charges to minimize energy (relaxation):
EEBB = E= Eff
N-1N-1
- E- Eii
NN

Binding Energy ShiftsBinding Energy Shifts
(Chemical Shifts)(Chemical Shifts)
Point Charge Model:Point Charge Model:
EEii = E= Eii
00
+ kq+ kqii ++ ΣΣ qqii/r/rijij
EEBB in atom i in givenin atom i in given
refernce staterefernce state
Weighted charge of iWeighted charge of i Potential at i due toPotential at i due to
surrounding chargessurrounding charges

Carbon-Oxygen BondCarbon-Oxygen Bond
Valence LevelValence Level
C 2pC 2p
Core LevelCore Level
C 1sC 1s
Carbon NucleusCarbon Nucleus
Oxygen AtomOxygen Atom
C 1sC 1s
BindingBinding
EnergyEnergy
Electron-oxygenElectron-oxygen
atom attractionatom attraction
(Oxygen Electro-(Oxygen Electro-
negativity)negativity)
Electron-nucleusElectron-nucleus
attraction (Loss ofattraction (Loss of
Electronic Screening)Electronic Screening)
Shift to higherShift to higher
binding energybinding energy
Chemical Shifts-Chemical Shifts-
Electronegativity EffectsElectronegativity Effects

Chemical Shifts-Chemical Shifts-
Electronegativity EffectsElectronegativity Effects
Functional
Group
Binding Energy
(eV)
hydrocarbon C-H, C-C 285.0
amine C-N 286.0
alcohol, ether C-O-H, C-O-C 286.5
Cl bound to C C-Cl 286.5
F bound to C C-F 287.8
carbonyl C=O 288.0

Electronic EffectsElectronic Effects
Spin-Orbit CouplingSpin-Orbit Coupling
2 8 4 2 8 0 2 7 62 8 82 9 0
B in d in g E n e r g y ( e V )
C 1 s
O r b i t a l = s
l = 0
s = + / - 1 / 2
l s = 1 / 2

9 6 5 9 5 5 9 4 5 9 3 5 9 2 5
1 9 . 8
C u 2 p
2 p 1 / 2
2 p 3 / 2
P e a k A r e a 1 : 2
O r b i t a l = p
l s = 1 / 2 , 3 / 2
l = 1
s = + / - 1 / 2

3 7 03 7 43 7 8 3 6 6 3 6 2
6 . 0
P e a k A r e a 2 : 3
A g 3 d
3 d 3 / 2
3 d 5 / 2
O r b i t a l = d
l s = 3 / 2 ,5 / 2
l = 2
s = + / - 1 / 2

Spin-OrbitCouplingSpin-OrbitCoupling
3 . 6 5
8 79 1 8 3 7 9
P e a k A r e a 3 : 4
A u 4 f
4 f 5 / 2
4 f 7 / 2
O r b i t a l = f
l = 3
s = + / - 1 / 2
l s = 5 / 2 ,7 / 2

Electronic Effects- Spin-Orbit CouplingElectronic Effects- Spin-Orbit Coupling
Ti MetalTi Metal Ti OxideTi Oxide

Final State Effects-Final State Effects-
Shake-up/ Shake-offShake-up/ Shake-off
Monopole transition: Only the principleMonopole transition: Only the principle
quantum number changes. Spin andquantum number changes. Spin and
angular momentum cannot change.angular momentum cannot change.
Shake-up: Relaxation energy used toShake-up: Relaxation energy used to
excite electrons in valence levels toexcite electrons in valence levels to
bound states (monopole excitation).bound states (monopole excitation).
Shake-off: Relaxation energy used toShake-off: Relaxation energy used to
excite electrons in valence levels toexcite electrons in valence levels to
unbound states (monopole ionization).unbound states (monopole ionization).
Results from energy made available in the relaxation of the finalResults from energy made available in the relaxation of the final
state configuration (due to a loss of the screening effect of thestate configuration (due to a loss of the screening effect of the
core level electron which underwent photoemission).core level electron which underwent photoemission).
L(2p) -> Cu(3d)L(2p) -> Cu(3d)

Comparison of SensitivitiesComparison of Sensitivities
A T O M I C N U M B E R
2 0 4 0 6 0 8 0 1 0 0
5 E 1 3
5 E 1 6
5 E 1 9
H N e C o Z n Z r S n N d Y b H g T h
1 %
1 p p m
1 p p b
0
R BS
A E S a n d X PS
S IM S
P IX EPIX E

Instrumentation for X-rayInstrumentation for X-ray
PhotoelectronPhotoelectron

Instrumentation for XPSInstrumentation for XPS
Surface analysis by XPS requiresSurface analysis by XPS requires
irradiating a solid in an Ultra-high Vacuumirradiating a solid in an Ultra-high Vacuum
(UHV) chamber with monoenergetic soft X-(UHV) chamber with monoenergetic soft X-
rays and analyzing the energies of therays and analyzing the energies of the
emitted electrons.emitted electrons.

Remove adsorbed gases fromRemove adsorbed gases from
the sample.the sample.
Eliminate adsorption ofEliminate adsorption of
contaminants on the sample.contaminants on the sample.
Prevent arcing and high voltagePrevent arcing and high voltage
breakdown.breakdown.
Increase the mean free path forIncrease the mean free path for
electrons, ions and photons.electrons, ions and photons.
Degree of VacuumDegree of Vacuum
1010
1010
1010
1010
1010
22
-1-1
-4-4
-8-8
-11-11
Low VacuumLow Vacuum
Medium VacuumMedium Vacuum
High VacuumHigh Vacuum
Ultra-High VacuumUltra-High Vacuum
PressurePressure
TorrTorr
Why UHV for Surface Analysis?Why UHV for Surface Analysis?

SpectrometerSpectrometer

X-ray Photoelectron SpectrometerX-ray Photoelectron Spectrometer
5 4 . 7
X-rayX-ray
SourceSource
ElectronElectron
OpticsOptics
Hemispherical Energy AnalyzerHemispherical Energy Analyzer
Position SensitivePosition Sensitive
Detector (PSD)Detector (PSD)
Magnetic ShieldShieldOuter SphereOuter Sphere
Inner SphereInner Sphere
SampleSample
ComputerComputer
SystemSystem
Analyzer ControlAnalyzer Control
Multi-ChannelMulti-Channel
Plate ElectronPlate Electron
MultiplierMultiplier
Resistive AnodeResistive Anode
EncoderEncoder
Lenses for EnergyLenses for Energy
AdjustmentAdjustment
(Retardation)(Retardation)
Lenses for AnalysisLenses for Analysis
Area DefinitionArea Definition
Position ComputerPosition Computer
Position AddressPosition Address
ConverterConverter

XPS at the ‘Magic Angle’XPS at the ‘Magic Angle’
Orbital Angular Symmetry FactorOrbital Angular Symmetry Factor
LLAA ((γγ) = 1 +) = 1 + ββAA (3sin(3sin22
γγ/2 - 1)/2/2 - 1)/2
where:where: γγ = source-detector angle= source-detector angle
ββ = constant for a given sub-shell and X-ray photon= constant for a given sub-shell and X-ray photon
At 54.7º the ‘magic angle’At 54.7º the ‘magic angle’
LLAA = 1= 1

Electron DetectionElectron Detection
Single Channel DetectorSingle Channel Detector
Electron distribution on analyzer detection planeElectron distribution on analyzer detection plane
Counts in spectral memoryCounts in spectral memory
Step 1Step 1 22 33
Step 1Step 1 22 33
EE11 EE22 EE33 EE11 EE22 EE33 EE11 EE22 EE33

Electron DetectionElectron Detection
Multi-channel Position Sensitive Detector (PSD)Multi-channel Position Sensitive Detector (PSD)
Electron distribution on analyzer detection planeElectron distribution on analyzer detection plane
Counts in spectral memoryCounts in spectral memory
EE11 EE22 EE33 EE11 EE22 EE33 EE11 EE22 EE33 EE11 EE22 EE33EE11 EE22 EE33
Step 1Step 1 22 33 44 55
Step 1Step 1 22 33 44 55

X-ray GenerationX-ray Generation
1s1s
2s2s
2p2p
L2,L3L2,L3
L1L1
KK
FermiFermi
LevelLevel
FreeFree
ElectronElectron
LevelLevel
1s1s
2s2s
2p2p
SecondarySecondary
electronelectron
IncidentIncident
electronelectron
X-rayX-ray
PhotonPhoton

Relative Probabilities of Relaxation of a KRelative Probabilities of Relaxation of a K
Shell Core HoleShell Core Hole
5
B N e P C a M n Z n B r Z r
1 0 1 5 2 0 2 5 3 0 3 5 4 0 A t o m ic N u m b e r
E le m e n t a l S y m b o l
0
0 . 2
0 . 4
0 . 6
0 . 8
1 . 0
Probability
Note: The lightNote: The light
elements have aelements have a
low cross sectionlow cross section
for X-ray emission.for X-ray emission.
Auger ElectronAuger Electron
EmissionEmission
X-ray PhotonX-ray Photon
EmissionEmission

Schematic of Dual Anode X-ray SourceSchematic of Dual Anode X-ray Source
AnodeAnode
FenceFence
Anode 1Anode 1 Anode 2Anode 2
Filament 1Filament 1 Filament 2Filament 2
FenceFence
Cooling WaterCooling Water
Cooling WaterCooling Water
Water OutletWater Outlet
Water InletWater Inlet
Anode AssemblyAnode Assembly
Filament 1Filament 1
Anode 1Anode 1
FenceFence
Filament 2Filament 2
Anode 2Anode 2

Schematic of X-ray MonochromatorSchematic of X-ray Monochromator
SampleSample
X-ray AnodeX-ray Anode
EnergyEnergy
AnalyzerAnalyzer QuartzQuartz
Crystal DisperserCrystal Disperser
Rowland CircleRowland Circle
ee--

Applications ofApplications of

XPS Analysis of Pigment from MummyXPS Analysis of Pigment from Mummy
ArtworkArtwork
150 145 140 135 130
Binding Energy (eV)Binding Energy (eV)
PbO2
Pb3O4
500 400 300 200 100 0
Binding Energy (eV)Binding Energy (eV)
O
Pb Pb
Pb
N
Ca
C
Na
Cl
XPS analysis showedXPS analysis showed
that the pigment usedthat the pigment used
on the mummyon the mummy
wrapping was Pbwrapping was Pb33OO44
rather than Ferather than Fe22OO33
Egyptian MummyEgyptian Mummy
2nd Century AD2nd Century AD
World Heritage MuseumWorld Heritage Museum
University of IllinoisUniversity of Illinois

Analysis of Carbon Fiber- PolymerAnalysis of Carbon Fiber- Polymer
Composite Material by XPSComposite Material by XPS
Woven carbonWoven carbon
fiber compositefiber composite
XPS analysis identifies the functionalXPS analysis identifies the functional
groups present on composite surface.groups present on composite surface.
Chemical nature of fiber-polymerChemical nature of fiber-polymer
interface will influence its properties.interface will influence its properties.
-C-C--C-C-
-C-O-C-O
-C=O-C=O
- 3 0 0 - 2 9 5 - 2 9 0 - 2 8 5 -2 8 0
B in d i n g e n e rg y ( e V )
N(E)/E

Analysis of Materials for Solar Energy CollectionAnalysis of Materials for Solar Energy Collection
by XPS Depth Profiling-by XPS Depth Profiling-
The amorphous-SiC/SnOThe amorphous-SiC/SnO22 InterfaceInterface
The profile indicates a reduction of the SnOThe profile indicates a reduction of the SnO22
occurred at the interface during deposition.occurred at the interface during deposition.
Such a reduction would effect the collector’sSuch a reduction would effect the collector’s
efficiency.efficiency.
Photo-voltaic CollectorPhoto-voltaic Collector
Conductive Oxide- SnOConductive Oxide- SnO22
p-type a-SiCp-type a-SiC
a-Sia-Si
Solar EnergySolar Energy
SnOSnO22
SnSn
Depth
500 496 492 488 484 480
Binding Energy, eV
Data courtesy A. Nurrudin and J. Abelson, University of Illinois

Angle-resolved XPSAngle-resolved XPS
θ =15° θ = 90°
More SurfaceMore Surface
SensitiveSensitive
Less SurfaceLess Surface
SensitiveSensitive
Information depth = dsinInformation depth = dsinθθ
d = Escape depth ~ 3d = Escape depth ~ 3 λλ
θθ = Emission angle relative to surface= Emission angle relative to surface
λλ == Inelastic Mean Free PathInelastic Mean Free Path
θ
θ

Angle-resolved XPS Analysis of Self-Angle-resolved XPS Analysis of Self-
Assembling MonolayersAssembling Monolayers
Angle Resolved XPS CanAngle Resolved XPS Can
DetermineDetermine
Over-layer ThicknessOver-layer Thickness
Over-layer CoverageOver-layer Coverage
Data courtesy L. Ge, R. Haasch and A. Gewirth, University of IllinoisData courtesy L. Ge, R. Haasch and A. Gewirth, University of Illinois
0 2 0 4 0 6 0 8 0 1 0 0
0 . 1
0 . 2
0 . 3
0 . 4
0 . 5
0 . 6
C ( W )
C ( A u )
A u
S iW O1 2 4 0 d
E le c tr o n E m is s io n A n g le , d e g r e e s
E x p t. D a ta
M o d e l

Photo Electron Spectroscopy

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