I have performed XPS of SiC sample doped with N. I have intentionally introduced defects into SiC lattice. I have taken the XPS spectra before and after treatment. Both the spectra is calibrated to the standard gold (84.0 eV).
In the before treatment spectra, I see SiC binding energy (B.E) state at 101 eV (in Si 2p) and 283.5 eV (in C 1s), but in the after treatment spectra I see SiC B.E state at 101.5 eV in (Si 2p) and 284.0 eV (in C 1s). It can be noted that the difference in B.E between Si 2p and C 1s spectra is 182.5 eV, which is same in the before and after treatment. I have following doubts or questions:
1. Is this a "true" chemical shift or is it due to surface charging effect or something else?
2. Does surface charging always shifts B.E to higher scale irrespective of elements present in the compound?
3. How much of a "maximum chemical shift" in B.E can be expected in the wide band gap or insulating materials?
Thanks.
It seems like what you describe is sample charging, do you have a flood gun in use? If the only change is peak position(peak width remains the same) then you should calibrate the energy scale so that the peaks show at the same binding energy, typically using carbon as a one point calibration.
Charging effects are very hard to predict since they depend on the conduction of electrons to the sample which includes from the substrate, sample holder, and even residual gases in the vacuum. That said charging means the surface builds a positive potential, as if you applied a bias to the sample, therefore all peaks move the same amount and in the same direction, to higher binding energy.
I personally have seen charging move peaks by 30 eV, it was a very nonconductive salt, so a few tenths of an eV is not out of the question. Other charging effects are peak doublet formation or peak intensity dropping.
Thanks for your reply.
Yes, it is about sample charging.
1. I have not used an electron flood gun to compensate sample charging effects (as it is not attached to the set up)
2. Does the sample charging always causes shifts to higher B.E scale?
(I have a question or doubt, I presume there is a sample charging effect, I also presume there is a compound material with three chemical species. After XPS scans, two of which are shifting in one direction and the other one is shifting in the opposite direction. How to go about such a spectra then? Is that a true chemical shift or do we need to consider something else?
Charging is caused by electrons leaving the sample without being replenished, you can imagine it like a capacitor plate. Since electrons are not being replenished the surface become more positive causing a voltage bias to build up shifting the spectra strictly to higher binding energy.
X-rays are able to damage material, and since there is no atmosphere, the surface can become reduced. The classic example is organic chlorides, when exposed to x-rays the chlorine sublimates reducing the sample in vacuum. Typically sample reduction is caused by x-ray damage.
@Christopher Goodwin please give a reference regarding to your statement.
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