PNNL

Abstract

X-ray photoelectron spectroscopy (XPS) is widely used for characterization of surfaces of materials. Elements in the sample (with the exception of hydrogen and helium) are identified from comparisons of the binding energies of their core levels, determined from measured photoelectron spectra, with tabulated values of these binding energies for the various elements. Information on the chemical state of the detected elements can frequently be obtained from small variations (typically between 0.1 eV and 10 eV) of the core-level binding energies from the corresponding values for the pure elements. Reliable determination of chemical shifts often requires that the binding-energy scale of the XPS instrument be calibrated with an uncertainty that could be as small as 0.1 eV. The surface potential of an insulating specimen will generally change during an XPS measurement due to surface charging, and it is then difficult to determine binding energies with the accuracy needed for elemental identification or chemical-state determination. There are two steps in dealing with this problem. First, experimental steps can be taken to minimize the amount of surface charging (charge-control methods). Second, corrections for the effects of surface charging can be made after acquisition of the XPS data (charge-correction methods). Although the buildup of surface charge can complicate analysis in some circumstances, it can be creatively used as a tool to gain information about a specimen.