X-ray Photoelectron Spectroscopy · PDF file1 X-ray Photoelectron Spectroscopy XPS Auger...

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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 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).



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 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


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


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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.



FermiFermi

LevelLevel

Free Free

ElectonElecton

LevelLevel Conduction BandConduction 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


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)


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


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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


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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�� 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


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


X-Ray source

Ion source

Axial Electron Gun

Detector

CMAsample

SIMS Analyzer

Sample introduction Chamber

Sample Holder

Ion PumpRoughing Pump Slits


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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.


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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 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

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