X-ray Photo Electron Spectroscopy (Xps)

8/2/2019 X-ray Photo Electron Spectroscopy (Xps)

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X-Ray PhotoelectronSpectroscopy (XPS)

David Echevarra TorresUniversity of Texas at El PasoCollege of Science

Chemistry Department


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Outline XPS Background XPS Instrument How Does XPS Technology Work? Auger Electron Cylindrical Mirror Analyzer (CMA) Equation KE versus BE Spectrum Background Identification of XPS Peaks X-rays vs. e- Beam XPS Technology


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XPS Background XPS technique is based on Einsteins idea about the

photoelectric effect, developed around 1905

The concept of photons was used to describe the ejection ofelectrons from a surface when photons were impinged upon it

During the mid 1960s Dr. Siegbahn and his researchgroup developed the XPS technique.

In 1981, Dr. Siegbahn was awarded the Nobel Prize inPhysics for the development of the XPS technique


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X-Rays Irradiate the sample surface, hitting the core electrons (e-) of the

atoms.

The X-Rays penetrate the sample to a depth on the order of amicrometer.

Useful e- signal is obtained only from a depth of around 10 to100 on the surface.

The X-Ray source produces photons with certain energies: MgK photon with an energy of 1253.6 eV AlK photon with an energy of 1486.6 eV

Normally, the sample will be radiated with photons of a singleenergy (MgK or AlK). This is known as a monoenergetic X-Ray beam.


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Why the Core Electrons? An electron near the Fermi level is far from the nucleus,

moving in different directions all over the place, and willnot carry information about any single atom. Fermi level is the highest energy level occupied by an

electron in a neutral solid at absolute 0 temperature. Electron binding energy (BE) is calculated with respect to the

Fermi level.

The core e-s are local close to the nucleus and havebinding energies characteristic of their particular element.

The core e-s have a higher probability of matching theenergies of AlK and MgK.

Core e-

Valence e-

Atom


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Binding Energy (BE)

These electrons areattracted to the

proton with certainbinding energy x

This is the point with 0energy of attraction

between the electron andthe nucleus. At this point

the electron is free from theatom.

The Binding Energy (BE) is characteristic of the core electrons for each element. The BE isdetermined by the attraction of the electrons to the nucleus. If an electron with energy xis pulled away from the nucleus, the attraction between the electron and the nucleusdecreases and the BE decreases. Eventually, there will be a point when the electron willbe free of the nucleus.

0

x

p+

B.E.


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Energy Levels

Vacumm Level

Fermi Level

Lowest state ofenergy

BE

, which is the work function

At absolute 0 Kelvin theelectrons fill from thelowest energy states up.When the electrons occupyup to this level the neutral

solid is in its groundstate.


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XPS Instrument

XPS is also known as ESCA(Electron Spectroscopy forChemical Analysis).

The technique is widely usedbecause it is very simple touse and the data is easilyanalyzed.

XPS works by irradiatingatoms of a surface of anysolid material with X-Rayphotons, causing the ejectionof electrons.

University of Texas at El Paso, Physics Department

Front view of the Phi 560 XPS/AES/SIMS UHV System


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XPS Instrument

The XPS is controlled byusing a computer

system.

The computer system will

control the X-Ray typeand prepare theinstrument for analysis.

University of Texas at El Paso, Physics Department

Front view of the Phi 560 XPS/AES/SIMS UHV System andthe computer system that controls the XPS.


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XPS Instrument

The instrument usesdifferent pump systems toreach the goal of an Ultra

High Vacuum (UHV)environment.

The Ultra High Vacuumenvironment will prevent

contamination of thesurface and aid anaccurate analysis of thesample.University of Texas at El Paso, Physics Department

Side view of the Phi 560 XPS/AES/SIMS UHV System


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XPS Instrument

X-Ray Source

Ion Source

SIMS Analyzer

Sample introductionChamber


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Sample Introduction Chamber

The sample will be introducedthrough a chamber that is incontact with the outsideenvironment

It will be closed and pumpedto low vacuum.

After the first chamber is atlow vacuum the sample will

be introduced into the secondchamber in which a UHVenvironment exists.

First ChamberSecond Chamber UHV


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Diagram of the Side Viewof XPS System

X-Ray source

Ion source

Axial Electron Gun

Detector

CMAsample

SIMSAnalyzer

Sample introductionChamber

SampleHolder

Ion PumpRoughing Pump Slits


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How Does XPS Technology Work?

A monoenergetic x-ray beamemits photoelectrons fromthe from the surface of thesample.

The X-Rays either of twoenergies: Al Ka (1486.6eV) Mg Ka (1253.6 eV)

The x-ray photons The

penetration about amicrometer of the sample The XPS spectrum contains

information only about thetop 10 - 100 of the sample.

Ultrahigh vacuumenvironment to eliminateexcessive surfacecontamination.

Cylindrical Mirror Analyzer(CMA) measures the KE ofemitted e-s.

The spectrum plotted by thecomputer from the analyzer

signal.

The binding energies can bedetermined from the peakpositions and the elementspresent in the sampleidentified.


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Why Does XPS Need UHV? Contamination of surface

XPS is a surface sensitive technique.

Contaminates will produce an XPS signal and lead to incorrectanalysis of the surface of composition.

The pressure of the vacuum system is < 10-9 Torr

Removing contamination To remove the contamination the sample surface is bombarded

with argon ions (Ar+ = 3KeV). heat and oxygen can be used to remove hydrocarbons

The XPS technique could cause damage to the surface,but it is negligible.


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The Atom and the X-Ray

Core electrons

Valence electrons

X-RayFree electron

proton

neutron

electron

electron vacancy

The core electronsrespond very well tothe X-Ray energy


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X-Rays on the Surface

Atoms layers

e- top layer

e- lower layerwith collisions

e- lower layerbut no collisions

X-Rays

Outer surface

Inner surface


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X-Rays on the Surface The X-Rays will penetrate to the core e- of the atoms in

the sample.

Some e-

s are going to be released without any problemgiving the Kinetic Energies (KE) characteristic of theirelements.

Other e-s will come from inner layers and collide with other

e-

s of upper layers These e- will be lower in lower energy. They will contribute to the noise signal of the spectrum.


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X-Rays and the ElectronsX-RayElectron without collision

Electron with collision

The noise signalcomes from theelectrons that collidewith other electronsof different layers.The collisions cause adecrease in energy ofthe electron and it no

longer will contributeto the characteristicenergy of theelement.


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What e-s can the Cylindrical MirrorAnalyzer Detect?

The CMA not only can detect electrons from theirradiation of X-Rays, it can also detect electronsfrom irradiation by the e- gun.

The e- gun it is located inside the CMA while theX-Ray source is located on top of theinstrument.

The only electrons normally used in a spectrum

from irradiation by the e- gun are known asAuger e-s. Auger electrons are also produced byX-ray irradiation.


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X-Rays and Auger Electrons

When the core electron leaves a vacancy anelectron of higher energy will move down tooccupy the vacancy while releasing energy by: photons Auger electrons

Each Auger electron carries a characteristic

energy that can be measured.


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Two Ways to Produce Auger Electrons

1. The X-Ray source can irradiate and remove the e-from the core level causing the e- to leave theatom

2. A higher level e- will occupy the vacancy.3. The energy released is given to a third higher

level e-.4. This is the Auger electron that leaves the atom.

The axial e- gun can irradiate and remove the core e-

by collision. Once the core vacancy is created,the Auger electron process occurs the same way.


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Auger Electron

Free e-

e- Vacancy

e- of high energythat will occupy thevacancy of the corelevele- released to

analyze 1

1, 2, 3 and 4 are the order of steps in which the e-s willmove in the atom when hit by the e- gun.

e- gun

23

4


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Auger Electron Spectroscopy (AES)

Atom layers

e- released fromthe top layer

Outer surface

Inner surfaceElectron beamfrom the e- gun


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Cylindrical Mirror Analyzer (CMA)

The electrons ejected will pass through a devicecalled a CMA.

The CMA has two concentric metal cylinders atdifferent voltages.

One of the metal cylinders will have a positivevoltage and the other will have a 0 voltage. This

will create an electric field between the twocylinders.

The voltages on the CMA for XPS and Auger e-s aredifferent.


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When the e-s pass through the metal cylinders,they will collide with one of the cylinders or theywill just pass through. If the e-s velocity is too high it will collide with the

outer cylinder If is going too slow then will collide with the inner

cylinder. Only the e- with the right velocity will go through the

cylinders to reach the detector.

With a change in cylinder voltage the acceptablekinetic energy will change and then you can counthow many e-s have that KE to reach the detector.


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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


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Equation

KE=hv-BE-

KE Kinetic Energy (measure in the XPS spectrometer)hv photon energy from the X-Ray source (controlled)

spectrometerwork function. It is a few eV, it getsmore complicated because the materials in the

instrument will affect it. Found by calibration.

BE is the unknown variable


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Equation

The equation will calculate the energy needed toget an e- out from the surface of the solid.

Knowing KE, hv and the BE can be calculated.

KE=hv-BE-


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KE versus BE

E E E

KE can be plotted dependingon BE

Each peak represents the

amount of e-s at a certainenergy that is characteristicof some element.

1000 eV 0 eV

BE increase from right to left

KE increase from left to rightBinding energy

#o

felectro

ns

(eV)


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Interpreting XPS Spectrum:Background The X-Ray will hit the e-s

in the bulk (inner e-layers) of the sample

e- will collide with other e-

from top layers,decreasing its energy tocontribute to the noise, atlower kinetic energy thanthe peak .

The background noise

increases with BE becausethe SUM of all noise istaken from the beginningof the analysis. Binding energy

#o

felectrons

N1

N2N3

N4

Ntot=N1 + N2 + N3+ N4

N = noise


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XPS Spectrum The XPS peaks are sharp.

In a XPS graph it is possible to see Augerelectron peaks.

The Auger peaks are usually wider peaksin a XPS spectrum.

Aluminum foil is used as an example onthe next slide.


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XPS

SpectrumO 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 KEincreases.

Sample and graphic provided by William Durrer, Ph.D.Department of Physics at the Univertsity of Texas at El Paso


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Identification of XPS Peaks

The plot has characteristic peaks for eachelement found in the surface of the sample.

There are tables with the KE and BE alreadyassigned to each element.

After the spectrum is plotted you can look for thedesignated value of the peak energy from the

graph and find the element present on thesurface.


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X-rays vs. e- Beam

X-Rays Hit all sample area simultaneously

permitting data acquisition that will

give an idea of the averagecomposition of the whole surface.

Electron Beam It can be focused on a particular area

of the sample to determine thecomposition of selected areas of thesample surface.


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XPS Technology Consider as non-

destructive because it produces soft

x-rays to inducephotoelectron emissionfrom the sample surface

Provide information

about surface layersor thin film structures

Applications in theindustry: Polymer surface

Catalyst Corrosion Adhesion Semiconductors Dielectric materials Electronics packaging Magnetic media Thin film coatings


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References Dr.William Durrer for explanations on XPS

technique, Department of Physics at UTEP. www.uksaf.com www.casaxps.com www.nwsl.net XPS instrument from the Physics

Department.
http://www.uksaf.com/http://www.casaxps.com/http://www.nwsl.net/http://www.nwsl.net/http://www.casaxps.com/http://www.uksaf.com/


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Acknowledgements

Elizabeth Gardner, Ph.D.from the Department of Chemistry at theUniversity of Texas at El Paso

William Durrer, Ph.D.from the Department of Physics at theUniversity of Texas at El Paso

Roberto De La Torre Roche

Lynn Marie Santiago

Documents

X-ray Photo Electron Spectroscopy (Xps)