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The charge neutrality condition requires that the loss of negative charge from the surface region (the consequence of the photoelectric effect) is compensated with sufficiently high rate by electrons from the sample bulk, the substrate, or the surrounding environment. The primary reason for this is the possibility of positive charge accumulation in the sample surface region ( surface charging) 9. This does not, however, guarantee that the BE of energy level of the sample of interest for study (different from the calibration set) is correctly reproduced. For the latter to be reliable, the spectrometer has to be correctly calibrated 8. The chemical state identification is conventionally done by comparing the extracted binding energy (BE) values to compound reference data bases such as the NIST XPS 7. However, significant fraction of these numerous XPS papers contains data that have been wrongly interpreted due to the lack of skills, experience, or knowledge 5, but also because an improper referencing method was employed 6. The tremendous growth of the XPS technique is driven by the possibility of chemical state identification 2, 3, enabled nearly 60 years ago by the first observation of S 2p peak splitting in the XPS spectrum of sodium thiosulfate 4, caused by the fact that S atoms in Na 2S 2O 3 are present in two distinctly different chemical environments.
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With more than 12,000 papers published annually, the value of XPS in materials science can hardly be overestimated 1.