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The Technique

X-ray Photoelectron Spectroscopy (XPS) involves irradiating a sample with X-rays of a characteristic energy and measuring the flux of electrons leaving the surface. The energy spectrum for the ejected electrons is a combination of an overall trend due to transmission characteristics of the spectrometer, energy loss processes within the sample and resonance structures that derive form electronic states of the material under analysis. The instrumental contribution is an unwelcome fact of the measurement process, but the background and resonance peaks offer information about the sample surface [1].

Photoemission results in electrons of a given kinetic energy where the recorded energy is determined from a combination of the binding energy of the electron (with respect to some molecular form for each element) and the characteristic energy of the X-ray source. The dependence of the energy spectrum on the anode used to produce the X-rays means that the photoelectric lines move in position when the same sample is analyzed but using a different anode to produce the X-rays. Since these lines provide the bulk of the chemical information found in the spectra, and it is this chemical information that makes XPS such a powerful technique, it has become common practice to display XPS spectra using binding energy for the abscissa. The kinetic energy scale is reported relative to the photon energy of the excitation source and so photoelectric line positions with respect to a binding energy scale become independent of the X-rays used to excite the sample, while Auger line positions are invariant with respect to the X-ray anode only when plotted against a kinetic energy scale.




                                        XPS process [2]


Physical Principles


Photoelectron spectroscopy is based upon a single photon in/electron out process and from many viewpoints this underlying process is a much simpler phenomenon than the Auger process. [3]

The energy of a photon of all types of electromagnetic radiation is given by the Einstein relation :

E  =  h ?

where

 
h - Planck constant ( 6.62 x 10-34 J s )
? - frequency (Hz) of the radiation

Photoelectron spectroscopy uses monochromatic sources of radiation (i.e. photons of fixed energy).

In XPS the photon is absorbed by an atom in a molecule or solid, leading to ionization and the emission of a core (inner shell) electron. By contrast, in UPS the photon interacts with valence levels of the molecule or solid, leading to ionisation by removal of one of these valence electrons.

The kinetic energy distribution of the emitted photoelectrons (i.e. the number of emitted photoelectrons as a function of their kinetic energy) can be measured using any appropriate electron energy analyser and a photoelectron spectrum can thus be recorded.

The process of photoionization can be considered in several ways : one way is to look at the overall process as follows :

A + h?  ?  A+ + e-

Conservation of energy then requires that :

E(A) + h?  =  E(A+ ) + E(e-)

Since the electron's energy is present solely as kinetic energy (KE) this can be rearranged to give the following expression for the KE of the photoelectron:

KE  =  h? - ( E(A+ ) - E(A) )

The final term in brackets, representing the difference in energy between the ionized and neutral atoms, is generally called the binding energy (BE) of the electron - this then leads to the following commonly quoted equation :

KE  =  h? - BE

                                    Photoelectron by XPS technique [2]

XPS Spectrum

The figure below illustrates an XPS survey spectrum across a wide binding energy range including all the concerned elements C, O, Al and Ti while the regional spectra for their specific XPS peaks are illustrated in (b)-(d), respectively. It can be noted from
the XPS survey spectrum that the XPS peaks attributable to the element Al are extremely weak compared with the peaks for elements Ti, C, and O, suggesting the presence of few alumina particles in the top surface layer from which the photoelectron signs can be detected. The similar results were also found with all other samples but their XPS spectra show the peaks for elements Ti and C varying from one sample to another, depending on their time lengths of thermal treatment.

                                                   
                                 Typical XPS spectra [4]

Applications of XPS

  • Examination for and identification of surface contaminations
  • Evaluation of materials processing steps (cleaning, plasma etching,thermal oxidation, silicide thin-film formation etc.)
  • Evaluation of thin-film coating or lubricants
  • Failure analysis for adhesion between components (air oxidation, corrosion etc.)
  • Tribological (or wear) activity
  • Effectiveness of surface treatment of polymers or plastics
  • Surface composition differences of alloys

Limitations

  • Range of elements: All except H and He
  • Lateral resolution: 5mm - 75µm
  • Sampling depth: 0.5-5nm
  • Detection limits: 0.01-0.3 at.%

Sample Requirements

Vacuum compatible materials,flat samples preferred. no destruction except to X-ray sensitive materials and during depth profiling.

References

1. http://www.casaxps.com/help_manual/XPSInformation/IntroductiontoXPS.htm

2. Roger Smart et al., X-ray Photoelectron Spectroscopy, Department of physics and materials science, City University of Hong Kong

3. http://www.chem.qmul.ac.uk/surfaces/scc/scat5_3.htm

4. Z. Yanwei et al., Microstructural study by XPS and GISAXS of surface layers formed via phase separation and percolation in polystyren/tetrabutyl titanate/alumina composite films, Materials Science and Engineering, 2006, vol. B128, pp.  63-69