Abstract
Chemical state analysis in X-ray photoelectron spectroscopy (XPS) relies on assigning well-defined binding energy values to core level electrons originating from atoms in particular bonding configurations. Here, we present direct evidence for the violation of this paradigm. It is shown that the C 1s peak due to C–C/C–H bonded atoms from adventitious carbon (AdC) layers accumulating on Al and Au foils splits into two distinctly different contributions, as a result of vacuum level alignment at the AdC/foil interface. The phenomenon is observed while simultaneously recording the spectrum from two metal foils in electric contact with each other. This finding exposes fundamental problems with the reliability of reported XPS data as C 1s peak of AdC is routinely used for binding energy scale referencing. The use of adventitious carbon in XPS should thus be discontinued as it leads to nonsense results. Consequently, ISO and ASTM charge referencing guides need to be rewritten.
Highlights
Chemical state analysis in X-ray photoelectron spectroscopy (XPS) relies on assigning welldefined binding energy values to core level electrons originating from atoms in particular bonding configurations
Available Al and Au foils with adventitious carbon (AdC) layers resulting from prolonged air exposure are set in contact and mounted together on the sample holder
Peaks due to two other chemical states of C atoms in AdC, O=C–O and C–O, shift in the same manner from higher to lower binding energy (BE), as the probe is moved from Al to Au foil
Summary
Chemical state analysis in X-ray photoelectron spectroscopy (XPS) relies on assigning welldefined binding energy values to core level electrons originating from atoms in particular bonding configurations. The phenomenon is observed while simultaneously recording the spectrum from two metal foils in electric contact with each other This finding exposes fundamental problems with the reliability of reported XPS data as C 1s peak of AdC is routinely used for binding energy scale referencing. The chemical state identification is conventionally done by comparing the extracted binding energy (BE) values to compound reference data bases such as the NIST X PS7 For the latter to be reliable, the spectrometer has to be correctly calibrated[8]. The availability of an internal energy reference, in general, does not present a big challenge for conducting materials in electrical contact to the spectrometer Such samples typically exhibit a clear cut-off in the density of states at the Fermi level (so-called Fermi edge, FE), which serves as a natural zero on the BE scale[11]
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