How thick can XPS detect? one-layer-atom thickness or more thick?
I used XPS to detect MoS2 and found that the ratio of S vs. Mo is more than 2.
If so, XPS should detect one-layer-atom thickness materials.
Is is right?
95% of the XPS signal originates from the layer with the thickness 3x"lambda", where "lambda" is the electron mean free path (typically 10-20 Å depending on the material)
Adding to the above, the detection depth is also defined by the take-off angle of the electrons. Glancing angles (electrons leave nearly parallel to the surface) are more surface sensitive than perpendicular. The analysis depth is therefore approximately 3*lambda*sin(theta) with theta measured relative to the surface plane.
Professor Jefferey, your answer is very good and easy to understand. Thank you very much!
..and finally it also depends on the kinetic energy (KE) of the electron you are measuring, which since the binding energy for a particular element is fixed the KE can be varied if you use a tunable X-ray source, such as at a synchrotron. The minimum mean free path length is for electrons with a kinetic energy energy of 50 - 100 eV. You can download a useful database from NIST here https://www.nist.gov/srd/nist-standard-reference-database-71. It is free and allows you to calculate mean free path lengths and attenuation lengths for a range of materials based on density, theoretical elemental composition etc.
The detected depth of XPS is involved many factors including the energy of X-ray, the kinetic energy of core-level electrons, the secondary electron escape depth from materials, and so on. One more way, changer the incident angle of the x-ray also changes the detect depth, as shown in the attached picture. For more information, you can read this article:
[1] F.A. Stevie, C.L. Donley, Introduction to x-ray photoelectron spectroscopy, J Vac Sci Technol A, 38 (2020).
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