Toward reliable x-ray photoelectron spectroscopy: Understanding fundamental differences between analyses performed on samples with and without electrical contact to spectrometer

X-ray photoelectron spectroscopy (XPS) analyses of insulators or inhomogeneous samples containing insulating phases are often done with specimens electrically isolated from ground and the surface potential controlled by the flood gun. While such an approach offers the possibility of delivering nondistorted spectra, the consequences for the energy level alignment between the sample and the spectrometer are often not considered. The present contribution highlights these aspects by discussing several, carefully selected, case studies that visualize fundamental differences between XPS analyses performed on samples with and without electrical contact to ground. Consequences for core level shifts, energy splitting between peaks from different samples, and spectra referencing are discussed in detail. It is shown that the choice of the measurement configuration (samples with vs. without contact to the spectrometer) determines to what extent binding energies of core level peaks from conducting and insulating samples (or conducting and insulating phases of the same sample) depend on the specimens’ work function. Recognizing these aspects is crucial for the correct spectra referencing and interpretation of core level shifts for a series of samples with varying work functions.