Material Surface Analysis with X-Ray Photoelectron Spectroscopy (XPS)

       The XPS technique is used to investigate the chemistry at the surface of a sample. The basic mechanism behind an XPS instrument is illustrated in below picture, where photons of a specific energy are used to excite the electronic states of atoms below the surface of the sample. Electrons ejected from the surface are energy filtered via a hemispherical analyzer (HSA) before the intensity for a defined energy is recorded by a detector. Since core level electrons in solid-state atoms are quantized, the resulting energy spectra exhibit resonance peaks characteristic of the electronic structure for atoms at the sample surface. While the x-rays may penetrate deep into the sample, the escape depth of the ejected electrons is limited. That is, for energies around 1400 eV, ejected electrons from depths greater than 10nm have a low probability of leaving the surface without undergoing an energy loss event, and therefore contribute to the background signal rather than well defined primary photoelectric peaks.

 
       In principle, the energies of the photoelectric lines are well defined in terms of the binding energy of the electronic states of atoms. Further, the chemical environment of the atoms at the surface result in well defined energy shifts to the peak energies. In the case of conducting samples, for which the detected electron energies can be referenced to the Fermi energy of the spectrometer, an absolute energy scale can be established, thus aiding the identification of species.

        However, for non-conducting samples the problem of energy calibration is significant. Electrons leaving the sample surface cause a potential difference to exist between the sample and the spectrometer resulting in a retarding field acting on the electrons escaping the surface. Without redress, the consequence can be peaks shifted in energy by as much as 150 eV. Charge compensation designed to replace the electrons emitted from the sample is used to reduce the influence of sample charging on insulating materials, but nevertheless identification of chemical state based on peak positions requires careful analysis.

       XPS is a quantitative technique in the sense that the number of electrons recorded for a given transition is proportional to the number of atoms at the surface. The chemical shifts seen in XPS data are a valuable source of information about the sample. Tilting the sample with respect to the axis of the analyzer results in changing the sampling depth for a given transition and therefore data collected at different angles vary due to the differing composition with depth.

Chemical shift in X-ray spectroscopy

       The chemical shift reflects the influence of the chemical bonds with neighboring atoms. The chemical shift in the X-ray spectra is caused by changes in the electron binding energies. The more specific reason is about the electro-negativity effect in the presence of another atom. For example, in the C-O bond, due to the electro-negativity of oxygen, C 2p electron in valence level will be attracted by atom O, and C 1s electron in core level will tend to be attracted by carbon nucleus so that the binding energy of C 1s electron will shift to higher binding energy. Therefore we shall here consider different ways of calculating the electron binding energy of an atom or a molecule, methods that can also be used to evaluate chemical shifts.