XPS is often used to identify chemical bondings in a material. However, before doing that the instrument must be calibrated and binding energy values must be correctly referenced. Here lies the problem, at least in some cases, says Grzegorz Greczynski.
"There is no ultimate method for referencing XPS spectra recorded from insulating samples. XPS was used in more than 13 000 papers published in 2024 and a lot of them neglect this problem", he says.
Already established
Energy referencing in XPS works well for metals, but it's tricky for insulating materials. Already in 2017, Grzegorz Greczynski and Lars Hultman published an article in ChemPhysChem, where they established that the most common method of calibration, using adventitious carbon that accumulates on all specimens during air exposure, has a huge problem.
"Since the beginning of XPS people have used signals from adventitious carbon for referencing. Mostly because it is very convenient", says Grzegorz Greczynski.
It is commonly assumed that the adventitious carbon used for referencing has the same chemical identity. However, Greczynskis and Hultmans research shows that this is not the case.
"We have done systematic studies with carefully prepared samples and presented solid evidence that the layer is accumulating on top of the XPS samples is not always the same."
The storage environment and the storage time seem to matter a lot. Moreover, the sample type also has an influence on the adventitious carbon composition
"You need to be very careful while using this signal for referencing, or you will risk that the entire analysis of your materials chemical bonding will be off", says Grzegorz Greczynski.
Second part of the problem
In the new article Greczynski and Hultman critically review all XPS referencing methods and point out that the problem is not limited to the adventitious carbon technique. They conclude that no reliable referencing method exists for XPS spectra recorded from insulating samples.
"Basically it means that calibrations might be off by the same amount as the effect you are looking for. Our idea for this paper is to stimulate scientists to find better ways of referencing XPS spectra", says Grzegorz Greczynski.
