... ranges from 10-1 to 100 monolayer for XPS. XPS is more sensitive to core levels than to valence-band electrons, (Ibach, H. & Roy, D., 1977, p10). Binding energies The core electron of an element has a unique binding energy, which acts as a "fingerprint". Thus almost all elements except for hydrogen and helium can be identified via measuring the binding energy of its core electron. Furthermore, the binding energy of core electron is very sensitive to the chemical environment of element. If the same atom is bonded to the different chemical species, this leads to the change in the binding energy of its core electron. The variation of binding energy results in the shift of the corresponding XPS peak, ranging from 0.1eV to 10eV. This effect is termed as "chemical shift", which can be applied to studying the chemical states of elements at the surface. "Due to dipole selection rules, the binding ...

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