X-ray photoelectron spectroscopy (XPS) is an analytical technique that studies the surface of structures, based upon the photoelectric effect. Each atom in the surface has core electron with the characteristic binding energy that is theoretically equal to the ionization energy of that electron. When an X-ray beam directs to the sample surface, the energy of the X-ray photon is adsorbed by the core electron of an atom. If there is sufficient photon energy, h?, the core electron will then escape from the atom and emit out of the surface with kinetic energy, Ekreferred to as the photoelectron[1].
For insulating samples, once the photoelectrons are emitted out of the sample surface, a positive charge zone will establish quickly in the sample surface. As a result, the sample surface acquires a positive potential and the kinetic energies of core electrons are reduced by the same amount.
The surface charging results in the shift of the XPS peaks to higher binding energy. In this case, the binding energy has to be calibrated with an internal reference peak. The C 1s peak from the adventitious carbon-based contaminant, with the binding energy of 284.8eV, is commonly used as the reference for calibration. In order to neutralize the surface charge during data acquisition, a low-energy electron flood gun is used to deliver the electrons to the sample surface. The electron flood gun can be tuned to provide the right current to push the XPS peaks back to the real position[1].
The core electron of an element has a unique binding energy. Thus almost all elements except for hydrogen and helium can be identified via measuring the binding energy of its core electron. Furthermore, the binding energy of core electron is very sensitive to the chemical environment of element. The same atom is bonded to the different chemical species, leading to the change in the binding energy of its core electron. The variation of binding energy results in the shift of the corresponding XPS peak, ranging from 0.1eV to 10eV. This effect is termed as “chemical shift”, which can be applied to studying the chemical status of element in the surface.
The number of photoelectrons of an element is dependent upon the atomic concentration of that element in the sample, therefore XPS is used to not only identify the elements but also quantify the chemical composition.
XPS can be used to measure elemental composition of a surface, empirical formula of pure materials, elements that contaminate a surface and determine the state of oxidation on a selected region of a surface among others[2].
References:
[1] http://www.nuance.northwestern.edu/keckii/xps1.asp (accessed May 10, 2010)
[2] Feng-Yuan Z.; Suresh G. A.; Ajay K. P.; Mary E.B.; Shawn P.S.; Thomas P.B. Quantitative Characterization of Catalyst Layer Degradation in PEM Fuel Cells by X-Ray Photoelectron Spectroscopy Electrochimica Acta 54 2009 4025-4030.