X-ray Photoelectron Spectroscopy XPS, ESCA · X-ray Photoelectron Spectroscopy XPS, ESCA Photon inn – electron out MEF 3100 Spring 2007

X-ray Photoelectron Spectroscopy XPS, ESCA

Photon inn – electron outMEF 3100 Spring 2007

X-ray Photoelectron Spectroscopy

MEF 3100 Spring 2007

XPS and AES are surface sensitive techniques


Surface Sensitivity


Surface sensitivity


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)


XPS – ESCA


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.


History

• 1887 Heinrich Hertz /1888 Wilhelm Hallwachesilluminating metal surfaces resulted in electronic emission

• 1900 Max Planck -black body radiation• 1905 Einstein - light is quantized

E=hν• 1950 Kai Siegbahn 1981 : Nobel Prizestudied photoejection of electrons with x-rays

• 1950: Instrumentation 1960: Chemical Applications


Absorbing X-rays – emitting Photoelectrons


A closer look


Relaxation - Auger


XPS Peaks


Basic equations:

The relationship governing the interaction of a photon with a core level is:

KE= hν - BE – Φ

• KE = kinetic energy of ejected photoelectron• hν = characteristic energy of X-ray photon• BE = binding energy of the atomic orbital• from which the electron originates. • Φ = spectrometer work function


XPS example: Gold


Electronic structure and spectroscopy


Binding energies?




Spin-Orbit Coupling

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


XPS spectra of Palladium

BE= hν - KE – Φ


WHAT IS THIS?Survey scan : Most elements have major photoelectron peaks below1100 eV, so a range from 1100 - 0 eV is usually sufficient.

?Mg anodeAl anode




Energy resolution ~1 eV


Chemical shift – binding energies in C 1s peak


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


Inelastic Background


Plasmon loss


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 monoenergetic soft Xchamber with monoenergetic soft 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



Instrument

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

Specimen handlingPlanning experiments


Dual anode X-ray source


Alternative X-ray sources

Monochromator

Nλ=2dsinΘFor Al Kα λ=8.3 Å

_

1010 planes in quartz:d= 4.25 Å


Monochromated X-rays


Flooded or focused X-ray beam


CHA with standard input lens


Pass Energy and Resolution


Pass Energy


Ion Etching




Survey scan : Most elements have major photoelectron peaks below1100 eV, so a range from 1100 - 0 eV is usually sufficient.

Mg anodeAl anode

?


Detail ScansFor purposes of chemical state identification, for quantitative analysis of minor components and peak deconvolution or othermathematical manipulation of data.

•Scan should be wide enough to encompass the background on both sides of the region of interest - yet with small enough step size - within a reasonable time.•Radiation-sensitive peaks should be run first.•Sufficient Signal / noise .•Pass Energy ? ∆E same for all scans !






Oxidation of TitaniumOxidation of Ti:Titanium has a big shift. Ti-metal(Tio) to TiO2 (Ti4+).

Ti 2p-scan of Ti-metal and TiO2Binding energies:metal:Ti 2p3/2=451.4 Ti 2p1/2=457.57oxide:Ti 2p3/2=458.8 Ti 2p1/2=464.34

1.Not the same shift for the two spin doublets.2. Metals have more asymmetric peaks than insulators/oxides.3. Weak peak at 450.7eV - “ghost” -

(X-ray with higher energy = Kβ)MEF 3100 Spring 2007

Density of state DOS

DOS Ti-metal

Degree of asymmetry proportional to DOS at EFMEF 3100 Spring 2007


Depth information from XPS


Depth information from Ag-anode

• Greater analysis depthThe Al 2p spectra shown (Figure 3) were acquired with

• (a) Al and • (b) Ag sources. Spectrum • (b) is a clear illustration of the greater

analysis depth of the Ag source as the element : oxide ratio is greater than that acquired with the Al source. An oxide thickness of 1.8nm indicated here was confirmed by independent angle resolved measurements.


Binding Energy Referencing


BEBE = hv = hv -- KEKE -- ΦΦspecspec-- EEchch

Where: Where: BEBE= Electron Binding Energy= Electron Binding EnergyKEKE= Electron Kinetic Energy= Electron Kinetic EnergyΦΦspecspec= Spectrometer Work Function= Spectrometer Work FunctionEEchch= Surface Charge Energy= Surface Charge Energy

EEchch can be determined by calibrating the instrument can be determined by calibrating the instrument to a spectral feature.to a spectral feature.

C1s at 285.0 eVC1s at 285.0 eVAu4fAu4f7/2 7/2 at 84.0 eVat 84.0 eV

Binding Energy Referencing

When analysing insulating samples more care is requiredbecause of sample charging and the uncertainty in the location of the Fermi Level within the band gap.

The term Binding energy is often used without specifyingthe reference level.

Take care when evaluating spectra !


Charge neutralization for insulating samples

• Low-energy electrons from a cold cathode flood gun alleviates positive charging

• Low-energy source of positive ions alleviates the surrounding negative chargeMEF 3100 Spring 2007

Auger parameterThe Auger parameter is defined as:

A.P. = K.E(Auger) - K.E.(photoelectron) + Photon Excitation Energy = K.E.(Auger) + B.E.(photoemission)

Where K.E.(Auger) and B.E.(photoemission) are normally measured for the most intense photoemission and sharpest Auger lines available.

Its advantage as a probe of charges in screening energy is that (unlike photoemission or Auger line measurements taken separately) it is independent of energy referencing problems whilst still being measurable to a high precision. Hence small shifts in its value can be measured and interpreted.


• http://www.uksaf.org/data/table.htmlMEF 3100 Spring 2007


Specimen preparation and handling


Analysis of Multi-Layer Paint Cross Section


Analysis of Materials for Solar Energy Collection by XPS Depth Profiling-

The amorphous-SiC/SnO2 InterfaceThe profile indicates a reduction of the SnOThe profile indicates a reduction of the SnO22occurred at the interface during deposition. occurred at the interface during deposition. Such a reduction would effect the collector’s Such a reduction would effect the collector’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 IllinoisMEF 3100 Spring 2007

Probing Glass Coatings

Windows are coated with complexMulti-layer thin films to meet demands:1) Energy conservation 2) Appearance3) Durability



XPS Analysis of Pigment from MummyArtwork

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

NaCl

XPS analysis showed XPS analysis showed that the pigment used that the pigment used on the mummy on the mummy wrapping was Pbwrapping was Pb33OO44rather than Ferather than Fe22OO33

Egyptian Mummy Egyptian Mummy 2nd Century AD2nd Century ADWorld Heritage MuseumWorld Heritage MuseumUniversity of IllinoisUniversity of Illinois

Databases


Documents

X-ray Photoelectron Spectroscopy XPS, ESCA · X-ray Photoelectron Spectroscopy XPS, ESCA Photon inn – electron out MEF 3100 Spring 2007