X-ray Photoelectron Spectroscopy (XPS)


  

[Description] [Instrument] [Sample Requirement] [Application]


Description of the XPS technique

X-ray photoelectron spectroscopy, which is also known as electron spectroscopy for chemical analysis (ESCA), is probably the most widely used surface analysis technique. This technique is based on the photoelectric effect and was initially developed by Professor Kai Siegbahn at the University of Uppsala. When a beam of X-rays is directed to a sample, the interaction between an X-ray photon and the core level electron of an atom causes a complete transfer of the photon energy to the electron. The electron then has enough energy to escape from the surface of the sample. This electron is referred to as the photoelectron. The basic principle of XPS is schematically shown in Figure 1. For an electrically conductive solid, the binding energy of the core level electron ( ) can be calculated using the following equation:

                                             (1)

where  is the X-ray photon energy,  is the kinetic energy of the photoelectron and  is the work function of the spectrometer which is about 4-5 eV. The photon energy is known from the X-ray source employed (1486.6 eV for Al Ka and 1253.6 eV for Mg Ka, the two most commonly used sources) and  can be measured by the XPS spectrometer. For insulating materials such as polymers, however, surface charging has to be considered and Eq. 1 should be rewritten as:

                                       (2)

where C  is a charge constant which is unknown and varies from sample to sample. Therefore, for insulating materials, the electron binding energy is usually determined by using an internal reference peak. For example, the C 1s peak of aliphatic carbon at 285.0 eV is often used as the internal reference for polymers. 

Elements have unique electron binding energies.  Therefore, knowing the electron binding energy allows the identification of various elements. XPS is thus able to detect all elements except hydrogen.  Furthermore, the electron binding energy is also sensitive to the electronic environment of the atom. When an atom is bonded to another atom of an element having a different electronegativity, the electron binding energy may increase or decrease. This change in binding energy is called the chemical shift, which can be used to provide chemical information of a molecule or compound. 

 

 


 

Figure 1: Schematic diagram showing the basic principle of XPS [1]

 

Although X-rays penetrate deeply into the sample, the photoelectrons can only escape from a region near the surface. The sampling depth of XPS is given by the following equation:

                                                  (3)

where  is the attenuation length of the photoelectron and  (take-off angle) is the angle between the sample surface and the analyser.  By changing the take-off angle, the chemical information at various depths of the near surface region (from 10 to 100) can be obtained (non destructive depth profiling). In conjunction with ion beam sputtering, composition at deeper depths can also be obtained (depth profiling).

          XPS is also a quantitative technique. The XPS peak intensities after normalized by the sensitivity factors can be used to calculate the surface chemical composition by using the following equation:

                                                                             (4)

where is the concentration of an element i, m is the number of elements  in the sample,  and  are the peak area and the sensitivity factor of the element i, respectively.

Reference

[1]  C.-M. Chan and L.T. Weng, Reviews in Chemical Engineering, 16 (2000) 341-408    

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Instrument at MCPF

MCPF has a Physical Electronics 5600 multi-technique system. This system is capable of providing high-energy resolution and high sensitivity data for a wide range of samples. Some important features of the system are:

          Monochromatic Al Ka X-ray source for superior energy resolution analysis

          Achromatic Mg Ka X-ray source

          Hemispherical electron energy analyser with variable pass energies (6, 12, 23, 59, 117, 185 eV)

          Variable analysis spot sizes (200, 400, 800 mm).

          Electron flood gun for analysis of insulating materials.

          Tiltable stage for angle dependent analysis

          Sputter ion gun for depth profiling

          Computer controlled stage for automated data acquisition  

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Sample requirement

As XPS experiments are carried out under ultra-high vacuum, the samples to be analysed must be vacuum compatible. Both conductive and insulating materials can be analysed.