In general, it is known that for most materials, when the XPS peak becomes higher shift, it means strong bonding with oxidation, and conversely, when the electron concentration increases, it is known that it is referred to as lower shift.
However, in the case of many 2D MoS2 papers, on the contrary, it seems to be cited in the papers and talking. That is, when the main peaks of MoS2 become higher shifts, it is said that n-type doping.
A slightly ridiculous quote is that CVD MoS2 generally has more n-type doping, since there is usually more S vacancy than exfoliated MoS2. However, in some papers, when checking the peak shift of XPS, it is said that CVD MoS2 is more p-type doping because CVD MoS2 has a lower shift of XPS peak than exfoliated MoS2.
Really, if the XPS peak is lower shift in 2D MoS2, is p-doping correct? If so, how do you physically understand this bonding strength? As far as I know, normally, when increased electron density, it reduces the bonding strength, which induces lower shift of XPS peak. And this understanding is well fitted to many materials.
I did not understand the issue of MoS2 in detail, but the described dependence of the binding energy on the electron concentration is quite typical. Yes, you are right, an increase in the concentration of conduction electrons can provide an increase in the binding energy of core levels. This is because the binding energy is measured relative to the Fermi level. Therefore, an increase in the concentration of conduction electrons leads to an increase in the Fermi energy, which, in turn, leads to an increase in the energy interval between the core levels and EF. This is what XPS records as an increase of binding energy.
Thank you for your comment. Your comment make me have deep thinking. I could find the reason after studying of chemical shift in XPS. (1) If oxidation occurs, then oxygen take electrons from target materials, which reduces the electrostatic shielding effect, therefore inducing the increased binding energy. (2) In similar condition, also because of oxidation, the Fermi level can be reduced, which can lead to decrease the binding energy. Conclusively, by oxidation, some materials can have a lower shift of binding energy, and the other materials can have a higher shift of binding energy. In other words, it is determined by that oxygen bind to ""which electrons"".
Generally speaking, there may be other effects leading to a shift in the binding energy of core levels. However, the effect of the Fermi energy is easy to detect - when the Fermi level is shifted, the energies of the core levels of all atoms that make up the material (in your case, both molybdenum and sulfur) should shift in the same direction and by the same amount.
For example, for the Cu-ZrSe2 system, this effect is shown in Fig. 2 of the article in the attachment.
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