Post on 06-Mar-2018
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XX--ray Photoelectron Spectroscopyray Photoelectron Spectroscopy
� XPS� Auger emission� XPS – Some characteristics� XPS Energy� Quantitative analysis of XPS� XPS Instrumentation� UTEP Instrument� Applications of XPS
•• XPS is a technique used to investigate elemental XPS is a technique used to investigate elemental
composition of surfaces.composition of surfaces.
•• XX--ray Photoelectron Spectroscopy (XPS), also known as ray Photoelectron Spectroscopy (XPS), also known as
Electron Spectroscopy for Chemical Analysis (ESCA) Electron Spectroscopy for Chemical Analysis (ESCA)
•• XPS is based on the photoelectric effect,XPS is based on the photoelectric effect,1,21,2 was was
developed in the middeveloped in the mid--19601960’’s by Siegbahn in Sweden.s by Siegbahn in Sweden.33
1. H. Hertz, Ann. Physik 31,983 (1887).
2. A. Einstein, Ann. Physik 17,132 (1905). 1921 Nobel Prize in Physics.
3. K. Siegbahn, Et. Al.,Nova Acta Regiae Soc.Sci., Ser. IV, Vol. 20 (1967). 1981 Nobel Prize in Physics.
XX--ray photoelectron spectroscopyray photoelectron spectroscopy
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XX--ray photoelectron spectroscopyray photoelectron spectroscopy
XX--ray photoelectron spectroscopyray photoelectron spectroscopy
XX--ray Beamray Beam
XX--ray penetration ray penetration
depth ~1 depth ~1 µµµµµµµµm.m.
Electrons can be Electrons can be
excited in this excited in this
entire volume.entire volume.
XX--ray excitation area ~1x1 cmray excitation area ~1x1 cm22. Electrons . Electrons
are emitted from this entire areaare emitted from this entire area
Electrons are extracted Electrons are extracted
only from a narrow solid only from a narrow solid
angle.angle.
1 mm1 mm22
10 nm10 nm
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�� XPS spectral lines are identified by XPS spectral lines are identified by
the shell from which the electron the shell from which the electron
was ejected (1s, 2s, 2p, etc.).was ejected (1s, 2s, 2p, etc.).
�� The ejected photoelectron has The ejected photoelectron has
kinetic energy:kinetic energy:
KE = KE = hvhv –– BE BE -- ΦΦ
�� Following this process, the atom Following this process, the atom
will release energy by the emission will release energy by the emission
of an Auger Electron.of an Auger Electron.
Conduction BandConduction Band
Valence BandValence Band
L2,L3L2,L3
L1L1
KK
FermiFermi
LevelLevel
Free Free
Electron Electron
LevelLevel
Incident XIncident X--rayrayEjected PhotoelectronEjected Photoelectron
1s1s
2s2s
2p2p
The photoelectric effectThe photoelectric effect
�� L electron falls to fill core level L electron falls to fill core level
vacancy (step 1).vacancy (step 1).
�� KLL Auger electron emitted to KLL Auger electron emitted to
conserve energy released in conserve energy released in
step 1.step 1.
�� The kinetic energy of the The kinetic energy of the
emitted Auger electron is: emitted Auger electron is:
KE=E(K)KE=E(K)--E(L2)E(L2)--E(L3).E(L3).
Conduction BandConduction Band
Valence BandValence Band
L2,L3L2,L3
L1L1
KK
FermiFermi
LevelLevel
Free Free
Electron Electron
LevelLevel
Emitted Auger ElectronEmitted Auger Electron
1s1s
2s2s
2p2p
Auger emissionAuger emission
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• XPS is a quantitative technique (like AES).
• Extracts information from top 1 to 12 nm of material.
• Uses ultra high vacuum.
• XPS detects elements with Z ≥ 3 (lithium); cannot detect H or He as these atoms are too small.
• Detects in the parts per thousand limit; parts per million is possible from top surfaces and long collection time.
• XPS can be used to analyze inorganic compounds, metal alloys, semiconductors, polymers, glasses, ceramics, paints, paper, inks, wood, plant parts, bones, bio-materials, oils, glues, surface contamination, etc
• XPS uses narrow beams of 20 - 200 micrometers of monochromatic Al Kα X-rays or broad 10 – 30 mm beam of non-monochromatic Mg X-rays.
XPS XPS –– some characteristicssome characteristics
XPS XPS –– some characteristicssome characteristics
•XPS is used to determine - chemical state of the elements in the sample- binding energy of electronic states- thickness of layers of different materials near the surface- density of electronic states- uniformity of elemental composition across the top the surface- uniformity of elemental composition as a function of depth
• Quantitative accuracy depends on peak intensity, etc..
• Quantitative accuracy is 90-95% for major peak, 80-90% for mixtures of contamination and material, and 60-80% for weaker XPS signals withpeak intensities 10-20% of the strongest signal.
• Analysis time is about 10 minutes for a survey scan of all elements, 1–10 minutes for high energy resolution scans for chemical statedifferences, 1– 4 hours for a depth profiles.
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• Detection limit is 0.1 – 1.0 %, i.e. from 1 to 10 part per thousand (or 1,000 ppm to 10,000 ppm), for long exposures (8 – 16 hours) the ultimatedetection limit for most elements is approximately 100 ppm.
• Monochromatic beams cover 1–5 mm. Non-monochromatic beams are 10–50 mm in diameter
• Sample sizes range from 1×1 to 3×3 cm, and modern systems acceptsamples up to 30×30 cm
XPS XPS –– some characteristicssome characteristics
Degradation during analysis• Depends on sensitivity of material to wavelength of X-rays used• total dose of the X-rays• temperature of the surface• level of the vacuum.
Metals, alloys, ceramics and most glasses are not measurably degradedPolymers, catalysts, certain highly oxygenated compounds, variousinorganic compounds and fine organics are degraded
Non-monochromatic X-ray sources produce • high energy Bremsstrahlung X-rays (1– 15 keV of energy)• heat (100 to 200 °C), anode is 1 to 5 cm (2 in) from sample.
Heat and Bremsstrahlung X-rays increase degradation of materials. Monochromatic X-ray sources are far away (~50 cm) and do not produce heat nor Bremsstrahlung X-rays
XPS XPS –– some characteristicssome characteristics
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The XPS instrument measures the kinetic energy of all collected The XPS instrument measures the kinetic energy of all collected electrons electrons
which include both photoelectron and Auger electrons.which include both photoelectron and Auger electrons.
Conservation of energy implies: Conservation of energy implies: KEKE = = hvhv -- 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
Notice that photoelectron kinetic energies Notice that photoelectron kinetic energies dependdepend on photon energy, and on photon energy, and
Auger electron kinetic energies Auger electron kinetic energies do not dependdo not depend on photon energyon photon energy
XPS works by identifying the binding energy of the emitted XPS works by identifying the binding energy of the emitted
electrons and comparing to the energy levels of elements.electrons and comparing to the energy levels of elements.
XPS energy analysisXPS energy analysis
Photoelectron peaks reflect discrete bindingenergies of the electrons present in the solid
Silver excited by Mg-K-alpha (1253.6 eV)
XPS energy analysisXPS energy analysis
Photoelectrons from Photoelectrons from
different energy levelsdifferent energy levels
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•• Use of core electrons eliminates atomic thermal effect; nonUse of core electrons eliminates atomic thermal effect; non--core core electrons have energies that vary with temperatureelectrons have energies that vary with temperature•• Spectrometers also have a work function energySpectrometers also have a work function energy•• Insulating samples can charge and modify eInsulating samples can charge and modify e--levelslevels
XPS energy XPS energy –– Other factorsOther factors
BEBE = = hvhv -- KEKE -- ΦΦspecspec-- EEchch
Where: Where: BEBE= Electron Binding Energy= Electron Binding Energy
KEKE= Electron Kinetic Energy= Electron Kinetic Energy
ΦΦspecspec= Spectrometer Work Function= Spectrometer Work Function
EEchch= Surface Charge Energy= Surface Charge Energy
EEchch can be determined by calibration of instrument to a spectral fecan be determined by calibration of instrument to a spectral feature.ature.
C1s at 285.0 C1s at 285.0 eVeV
Au4fAu4f7/2 7/2 at 84.0 at 84.0 eVeV
ElectronElectron--electron electron
repulsionrepulsion
ElectronElectron--nucleus nucleus
attractionattraction
ElectronElectron
NucleusNucleus
BindingBinding
EnergyEnergy
Pure ElementPure Element
ElectronElectron--
Nucleus Nucleus
SeparationSeparation
Fermi LevelFermi Level
Look for changes here Look for changes here
by observing electron by observing electron
binding energiesbinding energies
XPS energy XPS energy –– Energy levelsEnergy levels
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Binding Energy (eV)
Element 2p3/2 3p ∆
Fe 707 53 654
Co 778 60 718
Ni 853 67 786
Cu 933 75 858
Zn 1022 89 933
Electron-nucleus attraction helps us identify theelements
XPS energy XPS energy –– EnergyEnergy levelslevels
The photoelectron’s binding energy will be
based on the element’s final-state configuration.
Conduction BandConduction Band
Valence BandValence Band
FermiFermi
LevelLevel
Free Free
ElectonElecton
LevelLevel Conduction BandConduction Band
Valence BandValence Band
1s1s
2s2s
2p2p
Initial StateInitial State Final StateFinal State
XPS energy XPS energy –– EnergyEnergy levelslevels
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When the charge of valence shell increases or decreases due to chemical bond formation, the electrostatic potential felt by the electron inside the atom changes.
• Atom loses valence charge (Si0 -> Si4+ ) BE increases.
• Atom gains valence charge (O0 -> O2- ) BE decreases.
Kim et al. Appl. Phys. Lett. 103, 103113 (2007)
XPS energy XPS energy –– Chemical shiftsChemical shifts
Point Charge Model:Point Charge Model:
EEii = E= Eii00 + + kqkqii + + ΣΣ qqii/r/rijij
EEBB in atom i in in atom i in
given reference given reference
state state
Weighted Weighted
charge of icharge of i Potential at i due to Potential at i due to
surrounding charges surrounding charges
XPS energy XPS energy –– Chemical shiftsChemical shifts
CarbonCarbon--Oxygen BondOxygen Bond
ValenceValence
LevelLevel
C 2pC 2p
Core Core
LevelLevel
C 1sC 1s
Carbon NucleusCarbon Nucleus
Oxygen AtomOxygen Atom
C 1s C 1s
BindingBinding
EnergyEnergy
ElectronElectron--oxygen oxygen
atom attractionatom attraction
(Oxygen Electro(Oxygen Electro--
negativity)negativity)
ElectronElectron--nucleus nucleus
attraction (Loss of attraction (Loss of
Electronic Screening)Electronic Screening)
Shift to higher Shift to higher
binding energybinding energy
FunctionalGroup
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
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Brow and Pantano. J. Am. Ceram. Soc. 69 (4) pp. 314 (1986)
XPS energy XPS energy –– Chemical shiftsChemical shifts
Hoffmann et al. J. Mol. Str.: THEOCHEM. 725, pp. 5-8 (2005)
XPS energy XPS energy –– Chemical shifts of C in polymersChemical shifts of C in polymers
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Quantitative analysis by XPSQuantitative analysis by XPS
For a Homogeneous sample: I = For a Homogeneous sample: I = NNσσDJLDJLλλATAT
N = atoms/cmN = atoms/cm33 σ σ = photoelectric cross= photoelectric cross--section, cmsection, cm22
D = detector efficiencyD = detector efficiency J = XJ = X--ray flux, photon/cmray flux, photon/cm22--secsec
L = orbital symmetry factorL = orbital symmetry factor T = analyzer transmission efficiencyT = analyzer transmission efficiency
λλ = inelastic electron mean= inelastic electron mean--free path, cmfree path, cm A = analysis area, cmA = analysis area, cm22
Concentration: N = I / Concentration: N = I / σσDJLDJLλλATAT
Let denominator be S= Let denominator be S= σσDJLDJLλλATAT �� N = I / SN = I / S
Can describe Relative Concentration of observed elements as a Can describe Relative Concentration of observed elements as a
number fraction by: number fraction by: CCxx = = NNxx / / ΣΣNNii, , or or
CCxx = I= Ixx//SSxx / / Σ Σ IIii/S/Sii
The values of S are based on empirical data.The values of S are based on empirical data.
Relative Sensitivities of the ElementsRelative Sensitivities of the Elements
0
2
4
6
8
10
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Elemental Symbol
Re
lative
Se
nsitiv
ity
Li
Be
B
C
N
O
F
Ne
Na
M
Al
Si
P
S
Cl
Ar
K
Ca
Sc
Ti
V
Cr
M
Fe
Co
Ni
Cu
Zn
G
G
As
Se
Br
Kr
Rb
Sr
Y
Zr
Nb
M
Tc
Ru
Rh
Pd
Ag
Cd
In
Sn
Sb
Te
I
Xe
Cs
Ba
La
Ce
Pr
Nd
P
S
Eu
G
Tb
Dy
Ho
Er
T
Yb
Lu
Hf
Ta
W
Re
Os
Ir
Pt
Au
Hg
Tl
Pb
Bi
1s
2p
3d
4d
4f
Quantitative analysis by XPSQuantitative analysis by XPS
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XPS: Comparison of sensitivitiesXPS: Comparison of sensitivities
ATOMIC NUMBER
20 40 60 80 1005E13
5E16
5E19
H Ne Co Zn Zr Sn Nd Yb Hg Th
1%
1ppm
1ppb0
RBS
AES and XPS
SIMS
PIXEPIXE
Quantitative analysis by XPSQuantitative analysis by XPS
XPS: Comparison of sensitivitiesXPS: Comparison of sensitivities
Note: The light Note: The light
elements have a elements have a
low cross section low cross section
for Xfor X--ray emission.ray emission.
Auger Electron Auger Electron
EmissionEmission
XX--ray Photon ray Photon
EmissionEmission
5
B Ne P Ca Mn Zn Br Zr
10 15 20 25 30 35 40 Atomic Number
Elemental Symbol
0
0.2
0.4
0.6
0.8
1.0
Pro
bab
ility
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XPS SpectrumXPS Spectrum
�� The XPS peaks are sharp.The XPS peaks are sharp.
�� In a XPS graph it is possible to see Auger In a XPS graph it is possible to see Auger electron peaks.electron peaks.
�� The Auger peaks are usually wider peaks in a The Auger peaks are usually wider peaks in a XPS spectrum.XPS spectrum.
�� Aluminum foil is used as an example on the Aluminum foil is used as an example on the next slide.next slide.
XPS Spectrum
O 1s
O becauseof Mg source
C
AlAl
O 2s
O Auger
Sample and graphic provided by William Durrer, Ph.D.Department of Physics at the Univertsity of Texas at El Paso
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Auger Spectrum
Characteristic of Auger graphsThe graph goes up as KE increases.
Sample and graphic provided by William Durrer, Ph.D.Department of Physics at the Univertsity of Texas at El Paso
XPS InstrumentationXPS Instrumentation
•• Vacuum chamberVacuum chamber
•• XX--ray sourceray source
•• Spectrometer (in CMA)Spectrometer (in CMA)
•• UTEP InstrumentUTEP Instrument
••Photoelectron multiplierPhotoelectron multiplier
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�� Contamination of surface Contamination of surface –– XPS is a surface sensitive technique.XPS is a surface sensitive technique.
�� Contaminates will produce an XPS signal and Contaminates will produce an XPS signal and lead to incorrect analysis of the surface of lead to incorrect analysis of the surface of composition.composition.
�� The pressure of the vacuum system is The pressure of the vacuum system is < 10< 10--99 TorrTorr
�� Removing contamination Removing contamination –– To remove the contamination the sample surface To remove the contamination the sample surface
is bombarded with argon ions (is bombarded with argon ions (ArAr++ = 3KeV).= 3KeV).–– heat and oxygen can be used to remove heat and oxygen can be used to remove
hydrocarbonshydrocarbons
�� The XPS technique could cause damage to The XPS technique could cause damage to the surface, but it is negligible.the surface, but it is negligible.
XPS Instrumentation: VacuumXPS Instrumentation: Vacuum
Degree of VacuumDegree of Vacuum
1010
1010
1010
1010
1010
22
--11
--44
--88
--1111
Low VacuumLow Vacuum
Medium VacuumMedium Vacuum
High VacuumHigh Vacuum
UltraUltra--High VacuumHigh Vacuum
PressurePressure
TorrTorr
Dual Anode XDual Anode X--ray Sourceray 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
XPS Instrumentation: XXPS Instrumentation: X--ray sourceray source
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Schematic of XSchematic of X--ray ray MonochromatorMonochromator
SampleSample
XX--ray Anoderay Anode
Energy Energy
AnalyzerAnalyzer Quartz Quartz
Crystal DisperserCrystal Disperser
Rowland CircleRowland Circle
ee--
XPS Instrumentation: XXPS Instrumentation: X--ray sourceray source
5 4 . 7
XX--rayray
SourceSource
ElectronElectron
OpticsOptics
Hemispherical Energy AnalyzerHemispherical Energy Analyzer
Position Sensitive Position Sensitive
Detector (PSD)Detector (PSD)
Magnetic ShieldShieldOuter SphereOuter Sphere
Inner SphereInner Sphere
SampleSample
Computer Computer
SystemSystem
Analyzer ControlAnalyzer Control
MultiMulti--Channel Plate Channel Plate
Electron MultiplierElectron Multiplier
Resistive Anode Resistive Anode
EncoderEncoder
Lenses for Energy Lenses for Energy
Adjustment Adjustment
(Retardation)(Retardation)
Lenses for Analysis Lenses for Analysis
Area DefinitionArea Definition
Position ComputerPosition Computer
Position Address Position Address
ConverterConverter
XPS Instrumentation: SpectrometerXPS Instrumentation: Spectrometer
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XPS InstrumentXPS Instrument
UTEP’sPhi 560 XPS/AES/SIMS UHV
Notes taken from presentation of David EchevarrNotes taken from presentation of David Echevarríía Torres, 2006a Torres, 2006
XPS InstrumentXPS InstrumentThe XPS is controlled by The XPS is controlled by
using a computer system.using a computer system.
The computer system will The computer system will control the Xcontrol the X--Ray type Ray type and prepare the and prepare the instrument for analysis.instrument for analysis.
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XPS InstrumentXPS Instrument
�� The instrument uses different The instrument uses different pump systems to reach the pump systems to reach the goal of an Ultra High Vacuum goal of an Ultra High Vacuum (UHV) environment.(UHV) environment.
�� The Ultra High Vacuum The Ultra High Vacuum environment will prevent environment will prevent contamination of the surface contamination of the surface and aid an accurate analysis and aid an accurate analysis of the sample.of the sample.
University of Texas at El Paso, Physics DepartmentSide view of the Phi 560 XPS/AES/SIMS UHV System
XPS InstrumentXPS Instrument
X-Ray Source
Ion Source
SIMS Analyzer
Sample introductionChamber
The XThe X--Rays either of two energies:Rays either of two energies:
Al Ka (1486.6eV)Al Ka (1486.6eV)
Mg Ka (1253.6 Mg Ka (1253.6 eVeV))
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Sample Introduction ChamberSample Introduction Chamber�� The sample will be introduced The sample will be introduced
through a chamber that is in through a chamber that is in contact with the outside contact with the outside environmentenvironment
�� It will be closed and pumped to It will be closed and pumped to low vacuum.low vacuum.
�� After the first chamber is at low After the first chamber is at low vacuum the sample will be vacuum the sample will be introduced into the second introduced into the second chamber in which a UHV chamber in which a UHV environment exists.environment exists.
First Chamber
Second Chamber UHV
XPS InstrumentXPS Instrument
X-Ray source
Ion source
Axial Electron Gun
Detector
CMAsample
SIMS Analyzer
Sample introduction Chamber
Sample Holder
Ion PumpRoughing Pump Slits
XPS InstrumentXPS Instrument
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Slit
Detector
Electron Pathway through the CMA
0 V
+V
0 V 0 V
0 V
+V
+V
+V
X-RaysSource
SampleHolder
XPS Instrument: XPS Instrument: Cylindrical mirror analyzerCylindrical mirror analyzer
�� The electrons ejected will pass through a device called The electrons ejected will pass through a device called a CMA.a CMA.
�� The CMA has two concentric metal cylinders at The CMA has two concentric metal cylinders at different voltages.different voltages.
�� One of the metal cylinders will have a positive voltage One of the metal cylinders will have a positive voltage and the other will have a 0 voltage. This will create an and the other will have a 0 voltage. This will create an electric field between the two cylinders.electric field between the two cylinders.
�� The voltages on the CMA for XPS and Auger The voltages on the CMA for XPS and Auger ee--ss are are different.different.
XPS Instrument: XPS Instrument: Cylindrical mirror analyzerCylindrical mirror analyzer
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�� When the When the ee--ss pass through the metal cylinders, they will pass through the metal cylinders, they will collide with one of the cylinders or they will just pass collide with one of the cylinders or they will just pass through. through. –– If the If the ee--’’ss velocity is too high it will collide with the outer velocity is too high it will collide with the outer
cylinder cylinder
–– If is going too slow then will collide with the inner cylinder. If is going too slow then will collide with the inner cylinder.
–– Only the eOnly the e-- with the right velocity will go through the with the right velocity will go through the cylinders to reach the detector.cylinders to reach the detector.
�� With a change in cylinder voltage the acceptable kinetic With a change in cylinder voltage the acceptable kinetic energy will change and then you can count how many energy will change and then you can count how many ee--sshave that KE to reach the detector.have that KE to reach the detector.
XPS Instrument: XPS Instrument: Cylindrical mirror analyzerCylindrical mirror analyzer
XPS Analysis of Pigment from Mummy ArtworkXPS Analysis of Pigment from Mummy Artwork
150 145 140 135 130
Binding Energy (Binding Energy (eVeV))
PbO2
Pb3O4
500 400 300 200 100 0Binding Energy (Binding Energy (eVeV))
O
Pb Pb
Pb
N
Ca
C
Na
Cl
XPS analysis showed XPS analysis showed
that the pigment used that the pigment used
on the mummy on the mummy
wrapping was Pbwrapping was Pb33OO44
rather than Ferather than Fe22OO33
Egyptian Mummy Egyptian Mummy
2nd Century AD2nd Century AD
World Heritage MuseumWorld Heritage Museum
University of IllinoisUniversity of Illinois
Applications of XPS
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Analysis of Carbon FiberAnalysis of Carbon Fiber-- Polymer Polymer Composite Material by XPSComposite Material by XPS
Woven carbon Woven carbon
fiber compositefiber composite
XPS analysis identifies the functional XPS analysis identifies the functional
groups present on composite surface. groups present on composite surface.
Chemical nature of fiberChemical nature of fiber--polymer polymer
interface will influence its properties.interface will influence its properties.
--CC--CC--
--CC--OO
--C=OC=O
-300 -295 -290 -285 -280
Binding energy (eV)
N(E
)/E
Analysis of Materials for Solar Energy Collection Analysis of Materials for Solar Energy Collection by XPS Depth Profilingby XPS Depth Profiling--The amorphousThe amorphous--SiC/SnOSiC/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 collectorSuch a reduction would effect the collector’’s s
efficiency.efficiency.
PhotoPhoto--voltaic Collectorvoltaic Collector
Conductive OxideConductive Oxide-- SnOSnO22
pp--type atype a--SiCSiC
aa--SiSi
Solar EnergySolar Energy
SnOSnO22
SnSn
Depth500 496 492 488 484 480
Binding Energy, eV
Data courtesy A. Nurrudin and J. Abelson, University of Illinois
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AngleAngle--resolved XPSresolved XPS
θ =15° θ = 90°
More Surface More Surface
SensitiveSensitiveLess Surface Less Surface
SensitiveSensitive
Information depth = Information depth = dsindsinθθθθθθθθd = Escape depth ~ 3 d = Escape depth ~ 3 λλλλλλλλθ θ θ θ θ θ θ θ = Emission angle relative to surface= Emission angle relative to surface
λ λ λ λ λ λ λ λ == Inelastic Mean Free PathInelastic Mean Free Path
θ
θ
AngleAngle--resolved XPS Analysis of Selfresolved XPS Analysis of Self--Assembling Assembling MonolayersMonolayers
Angle Resolved XPS Can Angle Resolved XPS Can
DetermineDetermine
��OverOver--layer Thicknesslayer Thickness
��OverOver--layer Coveragelayer Coverage
Data courtesy L. Data courtesy L. GeGe, R. Haasch and A. , R. Haasch and A. GewirthGewirth, University of Illinois, University of Illinois
0 20 40 60 80 1000.1
0.2
0.3
0.4
0.5
0.6
C(W)
C(Au)
Au
SiW O12 40 d
Electron Emission Angle, degrees
Expt. Data
Model