X-ray Photoelectron Spectroscopy

In XPS, incident X-rays from a source, typically Al Kα, with photon energy of 1486.7 eV, irradiate a sample and, meeting the threshold binding energies of some (if not all) of the core electrons in the constituent atoms in a material, eject these core electrons past the Fermi energy (E__f). Al Kα is a specific X-ray fluorescence emitted during the relaxation of Al atoms that have been taken to excited states through the ejection of their 1s electrons. If the energy of the source X-rays (Es) is sufficiently high, then the core electron can meet the threshold work function (ϕ) needed to get past the vacuum level (E__vac) and emerge with remnant kinetic energy. These electrons are called photoelectrons, and provided that they are close enough to the surface, they can emerge from the surface of the sample and be picked up by an energy discriminating electron detector. Such a detector measures the kinetic energy of the photoelectron (KE), which can be used to calculate the binding energy (BE) of the electrons:

BE = E_S-Φ - KE

Because the inelastic mean free path (IMFP) of the signal electrons is only a few nanometers (i.e. the average distance that electrons travel between inelastic scattering events is a few nanometers XPS requires ultra-high vacuum (UHV) conditions within the measurement chamber.The detection limits for most of the elements are on the order of parts per thousand range (1,000 PPM). In order to achieve better detection limits of parts per million (ppm), the technique requires either a high concentration of the species being detected at top surface or very long collection time (multiple hours). The resulting data will be in the form of a spectrum where an intensity (representing the counts per second of electrons hitting the detector) versus the binding energy. Provided that the X-ray source is energetic enough to eject electrons from particular electronic states of the atoms in the material, there will be one or more corresponding peaks in the spectrum. The BE of a certain peak of an element in the spectrum can then be compared to those of reference materials, or to tabulated values in databases, in order to determine the "chemical state" of that element in the sample. The intensity of a certain elemental peak is, of course, proportional to the concentration of that element in the sample. However, because the probabilities of ionizing different electron states vary, the conversion of the measured counts under spectral peaks to concentrations values will require the normalization of the counts by "sensitivity factors" that corrects for these differing probabilities.

The XPS system can accommodate thin films, bulk samples up to ~ 1cm thick, and powder samples. The sample stage here is 60 mm by 60 mm and can hold as many samples as will fit into this area. Films can be inorganic or organic/biological as long as they are dry.

Figure 1 shows a survey spectrum from the sample, clearly showing the Pt, Si, C and O emissions. In Figure 2, we see the high resolution scan of the Pt 4f_7/2 and 4f_5/2 peaks from the sample. The binding energies of each of the core level peaks can be compared to those found in databases such as the one maintained by the National Institute of Standards and Technology (NIST) (at https://srdata.nist.gov/xps/Default.aspx). The subtle shifts in binding energy relative to those of the reference compounds in the database can reveal the chemical state of each of the elements in your sample. The intensity ratio of the peaks will reveal the surface composition.

Figure 1: A survey spectrum from the sample, clearly showing the Pt, Si, C and O emissions. 

Figure 2: High resolution scan of the Pt 4f_7/2 and 4f_5/2 peaks from the sample.

XPS is a surface chemical analysis technique that is versatile in the range of samples it can be used to investigate. The technique provides quantification of chemical composition, chemical state and the occupied electronic structure of the atoms within a material.

XPS provides elemental the composition of the surface (within 1-10 nm usually), and can be used to determine the empirical formula of the surface compounds, the identity of elements that contaminate a surface, the chemical or electronic state of each element in the surface, the uniformity of composition across top surface and through the depth (by sequentially milling into the material and taking XPS data of the new exposed surface).

Routinely, XPS is used to analyze a wide range of materials, for example metal alloys, other inorganic compounds such as ceramics, polymers, semiconductors, catalysts, glasses, parts of plants biological materials such as cells, bones and many others.