Surface Analysis - Universitetet i oslo · Ion Spectroscopies SIMS: Secondary Ion Mass Spectrometry. ... MEF 3100 Spring 2008. Introduction. X-ray Photoelectron Spectroscopy (XPS)

Surface Analysis The Study of the Outer-Most Layers of Materials (

MEF 3100 Spring 2008

X-ray Photoelectron Spectroscopy XPS, ESCA

Photon inn – electron out

Introduction to X-ray Photoelectron Spectroscopy (XPS)

• What is XPS?- General Theory• How can we identify elements

and compounds?• Instrumentation for XPS• Examples of materials analysis with

XPS

What is XPS?

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

What is XPS?

XX--ray Photoelectron spectroscopy, ray Photoelectron spectroscopy, based on the photoelectric effect,based on the photoelectric effect,1,21,2 was was developed in the middeveloped in the mid--19601960’’s by Kai s by Kai Siegbahn and his research group at the Siegbahn and his research group at the University of Uppsala, Sweden.University of Uppsala, 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.


IntroductionX-ray Photoelectron Spectroscopy (XPS)X-ray photoelectron spectroscopy works by irradiating a sample material with mono energetic soft x-rays causing electrons to be ejected.

Identification of the elements in the sample can be made directly from the kinetic energies of these ejected photoelectrons.

The relative concentrations of elements can be determined from the photoelectron intensities.


XPS - ESCA

• Surface sensitive analysis technique based on photoelectric effect. Depth of analysis ~4-40 nm.

• All elements except Hydrogen.• Wide range of materials: Polymers, Ceramics,

metals …. (vacuum compatible)• Applications: corrosion, catalysis, thin films,

surface coatings, segregation …• Gives information on chemical composition and

chemical state.

XPS spectral lines are XPS spectral lines are identified by the shell from identified by the shell from which the electron was which the electron was ejected (1s, 2s, 2p, etc.).ejected (1s, 2s, 2p, etc.).

The ejected photoelectron has The ejected photoelectron has kinetic energy:kinetic energy:

KE=KE=hvhv--BEBE--

Following this process, the Following this process, the atom will release energy by atom will release energy by the emission of an Auger the emission of an Auger Electron.Electron.

Conduction BandConduction Band

Valence BandValence Band

L2,L3L2,L3

L1L1

KK

FermiFermiLevelLevel

Free Free Electron Electron LevelLevel

Incident XIncident X--rayrayEjected PhotoelectronEjected Photoelectron

1s1s

2s2s

2p2p

The Photoelectric ProcessThe Photoelectric Process

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

FermiFermiLevelLevel

Free Free Electron Electron LevelLevel

Emitted Auger ElectronEmitted Auger Electron

1s1s

2s2s

2p2p

Auger Relation of Core HoleAuger Relation of Core Hole

X-ray Photoelectron Spectroscopy

Small Area DetectionXX--ray Beamray Beam

XX--ray penetration ray penetration depth ~1depth ~1m.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


Surface Sensitivity


Surface sensitivity


XPS – ESCA

XPS Energy Scale- Binding energy

BEBE = = hvhv -- KEKE --

specspec

Where: Where: BEBE= Electron Binding Energy= Electron Binding EnergyKEKE= Electron Kinetic Energy= Electron Kinetic Energy

specspec = Spectrometer Work Function= Spectrometer Work Function

Photoelectron line energies: Photoelectron line energies: Not DependentNot Dependent on photon on 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 uniform The binding energy scale was derived to make uniform comparisons of chemical states straight forward.comparisons of chemical states straight forward.


Photo emission process is often envisaged as three steps

•Absorption and ionisation - (initial state effect)

•Response from atom and creation of photoelectron -(final state effect)

•Transport of electron to surface and escape - (extrinsic loss)


Absorbing X-rays – emitting Photoelectrons


A closer look


XPS Peaks


XPS example: Gold


Energy resolution ~1 eV


Electronic structure and spectroscopy


Binding energies?


Chemical shiftFunctional

GroupBinding 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


Chemical shift – binding energies in C 1s peak


965 955 945 935 925

19.8

Binding Energy (eV)

Cu 2p

2p1/2

2p3/2

Peak Area 1 : 2

Orbital=p

ls=1/2,3/2

l=1s=+/-1/2

Spin-Orbit Coupling


Instrumentation

Surface analysis by XPS requires irradiating Surface analysis by XPS requires irradiating a solid in an Ultraa solid in an Ultra--high Vacuum (UHV) high Vacuum (UHV) chamber with chamber with monoenergeticmonoenergetic soft Xsoft X--rays rays and analysing the energies of the emitted and analysing the energies of the emitted electrons.electrons.


XPS instrument at UIO?

Kratos Axis Ultra – new in 2007

People at UiO/Sintef

Martin Spyros


Instrument

1. Chambers 2. Vacuum and Pumps 3. Sources 4. Monochromator5. Analysers

Specimen handlingPlanning experiments


Dual anode X-ray source


Alternative X-ray sources

Surface Analysis�The Study of the Outer-Most Layers of Materials (

Documents

Surface Analysis - Universitetet i oslo · Ion Spectroscopies SIMS: Secondary Ion Mass Spectrometry. ... MEF 3100 Spring 2008. Introduction. X-ray Photoelectron Spectroscopy (XPS)