Xps (x ray photoelectron spectroscopy)

Presented by:

Zaahir salam

M.Tech NS &T

University of Texas at El Paso, Physics DepartmentFront view of the Phi 560 XPS/AES/SIMS UHV System

Background

Photoelectric effect discovered by Albert Einstein

Nobel Prize

1905

Photoemission as an analytical tool demonstrated by Kai Siegbahn (Electron Spectroscopy for Chemical Analysis –ESCA)

Nobel Prize

1961

X-Rays Irradiate the sample surface, hitting

the core electrons (e-) of the atoms.

The X-Rays penetrate the sample to a

depth of the order of a micrometer.

Useful e- signal is obtained only from

a depth of around 10 to 100 Å 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

The core e-s are local close to the nucleus andhave binding energies characteristic of theirparticular element.

The core e-s have a higher probability ofmatching the energies of AlK and MgK.

Core e-

Valence e-

Atom

X-Rays Irradiate the sample surface, hitting

the core electrons (e-) of the atoms.

The X-Rays penetrate the sample to a

depth of the order of a micrometer.

Useful e- signal is obtained only from

a depth of around 10 to 100 Å 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

The core e-s are local close to the nucleus andhave binding energies characteristic of theirparticular element.

The core e-s have a higher probability ofmatching the energies of AlK and MgK.

Core e-

Valence e-

Atom

Spectroscopy

Spectroscopy- the study of the light from an object.

Spectrometer- an instrument which spreads out light making a spectra.

Spectra- range of electromagnetic energy separated by wavelength.

Because the energy of an X-ray with particular wavelength is known, the electron binding energy of each of the emitted electrons can be determined by using an equation that is based on the work of Ernest Rutherford (1914):

where BE is the binding energy of the electron, hv is the energy of the X-ray photons being used, KEis the kinetic energy of the electron as measured by the instrument and φ is the work function of the spectrometer (not the material).

KE=hv-BE-Ø

Working Equation

XPS Instrument

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

It is a quantitativespectroscopic technique thatmeasures the

Elemental composition

Empirical formula

Chemical state

Electronic state

The technique is widely usedbecause it is very simple to useand the data is easily analyzed.

University of Texas at El Paso, Physics DepartmentFront view of the Phi 560 XPS/AES/SIMS UHV System

XPS works by irradiating atoms of a surface ofany solid material with X-Ray whilesimultaneously measuring the kinetic energyand number of electrons that escape from thetop 1 to 10 nm of the material being analyzed

The XPS is controlled by using a computersystem.

The instrument uses different pump systemsto reach the goal of an Ultra High Vacuum(UHV) environment.

The Ultra High Vacuum environment willprevent contamination of the surface and aidan accurate analysis of the sample.

University of Texas at El Paso, Physics DepartmentFront view of the Phi 560 XPS/AES/SIMS UHV System and the computer system that controls the XPS.

X-Ray Source

Ion Source

SIMS Analyzer

Sample introductionChamber

XPS Instrument

University of Texas at El Paso, Physics DepartmentSide view of the Phi 560 XPS/AES/SIMS UHV System

Sample Introduction Chamber

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

It will be closed and pumped to lowvacuum.

After the first chamber is at lowvacuum the sample will beintroduced into the secondchamber in which a UHVenvironment exists.

First ChamberSecond Chamber UHV

X-Ray source

Ion source

Axial Electron Gun

Detector

CMAsample

SIMS Analyzer

Sample introduction Chamber

Sample Holder

Ion PumpRoughing Pump Slits

Diagram of the Side View of XPS System

How Does XPS Technology Work?

A monoenergetic x-ray beam emits photoelectrons from the surface of the sample.

The X-Rays either of two energies: Al Kα (1486.6eV) Mg Kα(1253.6 eV)

The x-ray photons The penetration about a micrometer of the sample

The XPS spectrum contains information only about the top 10 - 100 Ǻ of the sample.

Ultrahigh vacuum environment to eliminate excessive surface contamination.

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

The spectrum plotted by the computer from the analyzer signal.

The binding energies can be determined from the peak positions and the elements present in the sample identified.

Why Does XPS Need UHV?

Contamination of surface

XPS is a surface sensitive technique.

Contaminates will produce an XPS signal and lead to incorrect analysis 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

X-Rays on the Surface

X-RayElectron without collision

Electron with collision

The noise signal comes from the electrons that collide with other electrons of different layers. The collisions cause a decrease in energy of the electron and it no longer will contribute to the characteristic energy of the element.

What e-s can the Cylindrical Mirror Analyzer Detect?

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

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

The only electrons normally used in a spectrum from irradiation by the e- gun are known as Auger e-s. Auger electrons are also produced by X-ray irradiation.

X-Rays and Auger Electrons

When the core electron leaves a vacancy an electron of higher energy will move down to occupy the vacancy while releasing energy by:

photons

Auger electrons

Each Auger electron carries a characteristic energy that can be measured.

Free e-

e- Vacancy

e- of high energy that will occupy the vacancy of the core level

e- released to analyze

1

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

e- gun

2

3

4

Cylindrical Mirror Analyzer (CMA)

The electrons ejected will passthrough a device called a CMA.

The CMA has two concentric metalcylinders at different voltages.

One of the metal cylinders willhave a positive voltage and theother will have a 0 voltage. This willcreate an electric field between thetwo cylinders.

The voltages on the CMA for XPSand Auger e-s are different.

0 V

+V

0 V 0 V

0 V

+V

+V

+V

X-RaysSource

SampleHolder

Slit

Electron Pathway through the CMA

KE versus BE

Binding energy

No

. o

f e

lect

ron

s

(eV)

KE can be plotted depending on BE

Each peak represents the amount of e-s at a certain energy that is characteristic of some element.

1000 eV 0 eV

BE increase from right to left

KE increase from left to right

Binding energy

No

. o

f e

lect

ron

s

N1

N2

N3

N4

Ntot= N1 + N2 + N3 + N4

N = noise

e- will collide with other e- from top layers, decreasing its energy to contribute to the noise, at lower kinetic energy than the peak .

The background noise increases with BE because the SUM of all noise is taken from the beginning of the analysis.

Noise

XPS Spectrum The XPS peaks are sharp.

In a XPS graph it is possible to see Auger electron peaks.

The Auger peaks are usually wider peaks in a XPS spectrum.

Auger Spectrum

Characteristic of Auger graphsThe graph goes up as KE increases.

XPS Spectrum

O Auger

O 1s

O becauseof Mg source

CAl

AlO 2s

Aluminum foil

Identification of XPS Peaks

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

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

After the spectrum is plotted you can look for the designated value of the peak energy from the graph and find the element present on the surface.

Use of XPS Technology Elements and the quantity of those elements that are present within the top 1-12 nm of the sample surface.

Detects all elements with an atomic number (Z) of 3 (lithium) and above. It cannot detect hydrogen (Z =

1) or helium (Z = 2) because the diameter of these orbitals is so small, reducing the catch probability to

almost zero.

Chemical state analysis of the surface of polymers readily reveals the presence or absence of the chemical

states of carbon known as: carbide (C 2-), hydrocarbon (C-C), alcohol (C-OH), ketone (C=O), organic

ester (COOR), carbonate (CO3), fluoro-hydrocarbon (CF2-CH2), trifluorocarbon (CF3).

Is routinely used to analyze

Inorganic compounds.

Metal alloys.

Semiconductors.

Polymers.

Catalysts, glasses, ceramics, paints, papers, inks, woods, plant parts, make-up, teeth, bones,

medical implants, bio-materials, viscous oils, glues, ion modified materials and many others.

Organic chemicals are not routinely analyzed by XPS because they are readily degraded by either the

energy of the X-rays or the heat from non-monochromatic X-ray sources.

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.

References

Thank You