X-ray Photoelectron Spectroscopy XPS is established surface spectroscopy method to provide chemical analysis of specimen surfaces. The method is particularly effective for low atomic number elements and surface impurities. The method requires ultra high vacuum UHV conditions. Chemical analysis is obtained by identification of the characteristic peaks for each element present and analysis of the peak areas. This may be automated or assisted by data analysis software.
Surface condition and composition is highly important in adhesive processes. Surface analysis methods such as XPS provide an important forensic tool to understand causes and location of failure.
XPS is used in forensic analysis of adhesive joints to obtain compositional information on the surfaces. This might help identify any contaminants that may have contributed to failure, unbonded areas and lack of adhesive, and whether failure has occurred in the adhesive, in a parent material or at an interface
X-Ray Photoelectron Spectroscopy (XPS), also known as Electron Spectroscopy for Chemical Analysis (ESCA) determines the chemical composition of a surface using the photoelectic effect.
The sample is irradiated with X-ray photons and electrons are emitted from the sample if the photon is of sufficient energy. The kinetic energy of these photoelectrons is measured by the analyser and the binding energy of the photoelectron is calculated from the equation:-
KE = hn -BE -fs
where KE is the kinetic energy of the emitted photoelectron, hn is the X-Ray photon energy and BE is the binding energy of the photoelectron which is the energy required to remove the electron from the atom. A work function fs is required for photoemission from solids as extra energy is required to transfer the electron from the surface to the vacuum. This is a predetermined value for each spectrometer.
XPS intrumentation consists of an X-ray source, a series of lenses to focus the photoelectrons and an energy analyser and detector. Photoelectrons are counted at each kinetic energy value and a spectrum of intensity vs binding energy is generated from the above equation. The binding energy of an electron is characteristic of the element, orbital and chemical environment therefore XPS can determine the bonding state and/or oxidation states of materials and surface concentrations.
The inherent weakness in XPS has been the lack of spatial resolution. A recent innovation is imaging XPS known as iXPS. Modern spectrometers such as ESCALAB imaging XPS to obtain spatially-resolved chemical maps across the surface of very small features. iXPS the offers the capability of parallel imaging, which obtains positional information from dispersion characteristics of the hemispherical analyser and produces photoelectron images with spatial resolution better than 5 mm. Also, the introduction of a magnetic objective lens has improved the sensitivity for a given spatial resolution enabling the new technique of imaging XPS to be realized.
XPS is based on the photoelectric effect outlined by Einstein in 1905 where the concept of the photon was used to describe the ejection of electrons from a surface when photons impinge upon it. XPS was developed in the mid 1960s by K. Siegbahn and his research group. K. Siegbahn was awarded the Nobel Prize for Physics in 1981 for his work in XPS.
The photon energies of choice for XPS are often Al Kalpha (1486.6eV) or Mg Kalpha (1253.6eV). Other X-ray lines can also be chosen such as Ti Kalpha (2040eV). The XPS technique is highly surface specific due to the short range of the photoelectrons that are excited from the solid.
The energy of the photoelectrons leaving the sample are determined using a Concentric Hemispherical Analyser CHA and this gives a spectrum with a series of photoelectron peaks. The binding energy of the peaks are characteristic of each element. The peak areas can be used (with appropriate sensitivity factors) to determine the composition of the materials surface. The shape of each peak and the binding energy can be slightly altered by the chemical state of the emitting atom. Hence XPS can provide chemical bonding information as well.
XPS is not sensitive to hydrogen or helium, but can detect all other elements. XPS must be carried out in UHV conditions.
A complementary technique used for higher atomic number elements is Auger Electron Spectroscopy AES.
Forensic analysis following peel testing of shoe samples:
E11 Long Term performance in footwear
Introduction to XPS http://www.chem.ucl.ac.uk/people/williams/xps/xpsintro.html
UK Surface analysis Forum http://www.uksaf.org/ http://www.uksaf.org/tech/xps.html
XPS database of peak locations http://www.lasurface.com/database/element.php
Images courtesy University College London UCL
