1. X-ray photoelectron spectroscopy (XPS)
Bergdís Björk Bæringsdóttir
Daníel Arnar Tómasson
Kristján Einar Guðmundsson

2. Theory of xps Based on the photoelectric effect
Emitted x-rays from the machine are focused towards the sample
The x-rays have sufficient energy (hv) to ionize core electrons (BE) and give them excess kinetic energy (KE):
Ke is the response with hv provided from the machine. BE can be determined
Identification of elements possible as binding energies for the core electrons are unique for every element

3. Internal factors affecting the xps spectral data
Spin orbit splitting
Chemical shifts
Oxidation
Noise signal from inner layer electrons losing energy in collisions with upper layer electrons (inelastic scattering)
Auger electrons peaks
And more

4. Spin orbit splitting of peaks
All core electrons with angular momentum (n=2,3,4..) experience spin orbit interactions
Coupling of the orbital magnetic moment and magnetic moment due to the intrinsic spin possession of the electron causes a change in energy for electrons which is observed as splitting of peaks in the xps

5. Chemical shifts and oxidation
Further induce peak splitting
Subtracted electron density due to oxidation means less efficient screening of the core electrons and causes a shift towards higher binding energy
Carbon chemical shift in a molecule
Different bond energies

6. XPS Instrument
XPS is also known as ESCA (Electron Spectroscopy for Chemical Analysis).
XPS works by irradiating atoms of a surface of any solid material with X-Ray photons, causing the ejection of electrons.
It is controlled by using a computer system which controls the X-Ray type and prepares the instrument for analysis.

7. XPS Instrument XPS requires ultra high vacuum (UHV).
The Ultra High Vacuum environment will prevent contamination of the surface and aid an accurate analysis of the sample.
Ultra high vacuum environment to eliminate excessive surface contamination. Mostly water which covers all surfaces, sits in pours, until a very high vacuum is acquired.

8. XPS Technology Applications in the industry:
Consider as non-destructive
because it produces soft x-rays to induce photoelectron emission from the sample surface
Provide information about surface layers or thin film structures
Applications in the industry:
Polymer surface
Catalyst
Corrosion
Adhesion
Semiconductors
Dielectric materials
Electronics packaging
Magnetic media
Thin film coatings

9. XPS Instrument X-Ray Source Ion Source SIMS Analyzer
Sample introduction
Chamber

10. Diagram of the Side View of XPS System
X-Ray source
Ion source
Axial Electron Gun
Detector
CMA
sample
SIMS Analyzer
Sample introduction Chamber
Sample Holder
Ion Pump
Roughing Pump
Slits

11. Kratos Axis-165 XPS system in RRC-East

12. XPS Instrument The Atom and the X-Ray
Concentric Hemispherical Analyzer (CHA) or spherical sector analyzer
The Atom and the X-Ray
X-Ray
Free electron
Valence electrons
proton
neutron
electron
electron vacancy
Core electrons
The core electrons respond very well to the X-Ray energy

13. Instrumentation
Most common type of electrostatic deflection-type analyzer:
Concentric Hemispherical Analyzer (CHA) or spherical sector analyzer
Energy resolution dependant on radius.
Capable of collecting photoelectrons of larger angular distribution.
Photoelectrons of a specific energy are focused by the lens at the slit of the spectrometer. Lens also controls sampling area.
Photoelectrons travel through a circular path and exit into a series of channeltrons (electron multipliers).

14. FUN FACT
Same kind of technology used to curve electron beam in Cern Supercollider and as in the CHA

15. Analytical Capabilities of XPS
Limitations:
Does not detect H or He
Radiation damage possible (worse for achromatic sources)
Charge neutralization needed for insulating material
Chemical analysis can be limited to functional groups and in some cases chemical shifts are not resolvable.
It can:
Identify elements or compounds (except H and He)
Determine oxidation states (for example Ti3+ or Ti4+)
Identify types of chemical bonds (like Si-O or Si-C)
Semi-quantitative analysis (with about 10-15% error)
Determine film thickness

16. Analysis Capabilities continued
Elemental vs. quantitative analysis:
X-rays vs. e- Beam:
Elemental Analysis: atoms have valence and core electrons so core-level binding energies provide a unique signature of elements.
X-Rays:
Hit all sample area simultaneously permitting data acquisition that will give an idea of the average composition of the whole surface.
Electron Beam:
It can be focused on a particular area of the sample to determine the composition of selected areas of the sample surface.
Quantitative analysis: measures intensities, use standards or tables of sensitivity factor

17. XPS Spectrum
The plot has characteristic peaks, which are sharp, for each element found in the surface of the sample
In a XPS graph it is possible to see Auger electron peaks which are usually wider peaks in a XPS spectrum.
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.

18. Identification of XPS Peaks
“Splitting” of 2s, 2p etc. due to spin-orbit splitting
Electron shells
KINETIC ENERGY, eV