Confusion, confusion, confusion, confusion, confusion............................................
I would like to see what your opinion is on how one decides if a C 1s peak or O 1s peak are fitted correctly, i.e. with the correct choice of backgrounds and line shapes. I know that with experience one can better decide on what to choose however, for beginners it is quite a struggle. Most of the peer-reviewed papers do not mention the type of background and line-shape/s that are used in their published work and when different papers (of different authors) are compared (i.e. comparing a C 1s peak of the same material), the choice of background and line-shapes is completely different.
Could it be that there is more than one correct choice of background and line shape for the same material and peak concerned?
Regarding the background: a good discussion on that can be found in the CasaXPS manual, even if you're not using CasaXPS: http://www.casaxps.com/help_manual/manual_updates/peak_fitting_in_xps.pdf
I'd say nobody will throw you under the bus if you use Shirley, it's probably the safest option as long as you don't crop your peaks.
Regarding the fitting: O1s and the C1s of sp3 species are usually rather straightforward since a self-consistent fit with Gaussian or Voigt functions will usually be accepted. I think it is more importent that you report accurately what you did with your data than which of these functions you use for the fitting. The component widths shouldn't vary too much since technically a component width is an apparative constant.
The C1s spectrum of sp2 carbon is indeed a "bad boy" since it is systematically asymmetric and accounting for changes in that is still an open point of discussions. For a while it seemed that the community was converging towards the Doniach-Sunjic peak shape, but this convergence has faded away and the discussion seems to be fully open again.
Your question is too broad and to answer that you may need to read like some books. But, I would give some general hints which you might find them useful:
1. XPS is a fast-growing surface technique that unfortunately, opposite to its growing use, the knowledge and experience about that is not well mature or at least is not as it should have. So, I do recommend not relying on its peer-review papers. There are many publications in this field which give wrong data and information, even papers published in high-ranked journals.
2. I agree that peak fitting for some cases may be needed, but what I see these days is that some researchers sometimes fit XPS peaks without considering the physics behind their data, unfortunately, just as it makes their papers more colorful and probably interesting. So, please think twice if it is really needed. Sometimes, you may simply compare XPS spectra and get lovely, meaningful information.
3. Whatever backgrounds and line shapes if you use, most probably no body would argue against that due to two main reasons: most reviewers are not expert in XPS (the biggest current problem that has let to thousands of not-correct publications, from my point of view) or they may not be expert in your material system. As an example, Shirley has become a common background, but basically, it can also highly depend on the material and its XPS peaks.
4. Two big sources of errors: sputter-etching to remove surface contaminants and using C 1s signal for calibrating spectra. Although the use of these two has become common unfortunately, it has been alarmingly discussed lately that for most materials systems sputter-etching have significant negative impacts and also the choice of C 1s is not correct as there is a broad range for the position of its signal. see these papers from our lab:
Article X-ray photoelectron spectroscopy: Towards reliable binding e...
https://www.nature.com/articles/s41598-021-90780-9
Article Towards reliable X-ray photoelectron spectroscopy: Sputter-d...
Hello Saddam,
Indeed, XPS peak fitting can be quite the challenge and there is a lot of sub-standard fitting in the literature which can lead one astray. Tread carefully!
Thankfully there are some great resources that will help. I would thoroughly recommend that you read at the articles in this Special Topic Collection from JVSTA, which addresses some reproducibility issues in XPS and offers some practical advice for XPS data analysis and acquisition. Of particular interest regarding peak fitting is Major's practical guide to peak fitting, and Gengenbach's paper on carbon 1s interpretation. I've linked the collection and aforementioned papers below. I hope that you find them useful. Cheers.
Article Introduction to topical collection: Reproducibility challeng...
https://avs.scitation.org/toc/jva/collection/10.1116/jva.2020.REPROD2020.issue-1
Article Practical guide for curve fitting in X-ray photoelectron spectroscopy
Article Practical guides for x-ray photoelectron spectroscopy (XPS):...
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