Specific Process Knowledge/Characterization/XPS/XPS technique: Difference between revisions
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=XPS technique= | |||
XPS is a surface sensitive and non destructive technique used for analysis of the elemental composition of a sample. It relies on the photoelectric effect; X-ray photons irradiated onto a sample will cause electrons bound in the sample atoms to become free electrons inside the the sample. The kinetic energy of these photoelectrons depend on the energy of the X-ray photon and their original binding energy. Escaping into the vacuum and counted in the electron spectrometer as function of their kinetic energy, the photoelectrons make a spectrum if represented in terms of numbers as function of their binding energy. | |||
There are two reasons why the technique is extremely useful: | |||
* '''Chemical sensitivity:''' The kinetic energy of the photoelectrons may be determined with such high accuracy (for instance, the resolution of the spectrometer is roughly 1 eV compared to the ~130 eV resolution of the spectrometer in an EDX X-ray detector ) that the small changes in binding energy of the sample electrons caused by the binding to other atoms may be detected. It is therefore possible to obtain information about the chemical environment of the elements - for instance in terms of: | |||
** Oxidation state | |||
** Double or triple bonds | |||
** Surface states | |||
* '''Surface sensitivity:''' The inelastic mean free path of the photoelectrons is very short - as a result, photoelectrons from depths larger than some 10 monolayers will not contribute to the peaks as they have lost part of their kinetic energy. This makes XPS extremely surface sensitive. | |||
The basic principle is shown below. The image is taken from Wikipedia - click on it to go to the wikipedia page on X-ray photoelectron spectroscopy. | |||
[[File:800px-XPS_PHYSICS.jpg|700px| frameless|link=https://en.wikipedia.org/wiki/X-ray_photoelectron_spectroscopy|XPS principle. | [[File:800px-XPS_PHYSICS.jpg|700px| frameless|link=https://en.wikipedia.org/wiki/X-ray_photoelectron_spectroscopy|XPS principle. The image is taken from wikipedia - click on image to access wikipedia directly.]] | ||
The analysis relies on the sequence: | The analysis relies on the sequence: | ||
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; Detection in the analyzer | ; Detection in the analyzer | ||
: In the analyzer the electrons are separated according to the kinetic energy by two concentric spheres held at some potential. By playing on the acceptance angle of the entrance/exit slits of the analyzer one can tune the energy resolution with the parameter called the pass energy. Subsequently detected by the spectrometer, the number of electrons may be plotted as a function of energy thus making up an XPS spectrum. | : In the analyzer the electrons are separated according to the kinetic energy by two concentric spheres held at some potential. By playing on the acceptance angle of the entrance/exit slits of the analyzer one can tune the energy resolution with the parameter called the pass energy. Subsequently detected by the spectrometer, the number of electrons may be plotted as a function of energy thus making up an XPS spectrum. | ||
= More information on XPS = | |||
There is a vast number of textbooks on XPS and the internet/Youtube has plenty information as well. For the training sessions we have chosen a document entitled 'Electron Spectroscopy of surfaces' by Peter S. Deimel and Francesco Allegretti from the Technical University of Munich. It relatively short, yet thorough and takes you through theory and hardware. Skip sections 6-8. Click [[Specific_Process_Knowledge/Characterization/XPS/ExtDocs|here]] to go the page with external material. | |||
For the practical part of the training, some videos have been made. They can only be accessed using this link: | |||
*[https://www.youtube.com/playlist?list=PLjWVU97LayHBtOkPCSv8QA-sD4Dbz1jrb Youtube playlist] | |||