X-ray photoelectron spectroscopy (XPS),
alternatively named electron spectroscopy for chemical analysis (ESCA), is now
a well-established surface analysis technique capable of providing elemental
and chemical state information from the outer 5 to 10 nm of a solid surface. It
is the subject of several well-known texts[1,2,3].
There is also an excellent recent review[4] of some of the latest technical advances.
The
technique is based on the photoelectric effect, discovered by Hertz in 1887,
where the emission of electrons from a material occurs under photon
irradiation. While early XPS experiments showed promise it was not until much
later that the true power of the technique would be revealed. In the 1950s, Kai
Siegbahn (Uppsala University, Sweden), using a high-resolution spectrometer and
a Cu Kα X-ray source, for the first time resolved the sharp peak at the high
kinetic “edge” seen by previous studies. This enabled, for the first time,
accurate determination of photoelectron kinetic energies and thus core level
binding energies. In 1981 Siegbahn was awarded the Nobel Prize for Physics for
this pioneering work[5].
When
an X-ray of known energy (hν), generally, with laboratory-based equipment
either Al Kα at 1486.7eV or Mg Kα at 1253.6eV, interacts with an atom, a
photoelectron can be emitted via the photoelectric effect (Figure 1). The
emitted electron’s kinetic energy (Ek) can be measured and the
atomic core level binding energy (Eb) relative to the Fermi level (EF)
of the sample can be determined using the following equation:
Eb
= hν – Ek – Φsp
where Φsp is the work
function of the spectrometer (typically 4 to 5 eV). The various core level
binding energies observed in a spectrum can be used to identify all the elements
of the periodic table except for hydrogen and helium. Chemical state
information can also be extracted because binding energies are sensitive to the
chemical environment of the atom. Chemical environments that deshield the atom
of interest (i.e. are bound to strongly electron withdrawing groups) will cause
the core electrons of that atom to have increased binding energies. Conversely,
decreased binding energies will be measured for core electrons of atoms that
withdraw electrons from their neighbouring atoms. Essentially, binding energy
will generally increase as chemical state number increases. As an example,
niobium metal has a 3d5/2
binding energy of 202.2 eV, while niobium 2+, 4+ and 5+
oxides have binding energies of 203.7 eV, 206.2 eV and 207.4 eV. Tabulations of
binding energies can be found in a variety of databases[6,7].
Figure
1. Schematic of the
photoemission of a Ni 2p3/2
electron from a nickel atom.
References:
[1] S. Hufner, Photoelectron Spectroscopy, Solid State Science Series, vol. 82 Springer, Berlin, 1995.
[2] D. Briggs, M.P. Seah (Eds.), Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, John Wiley & Sons, Chichester, 1983.
[3] D. Briggs, J.T. Grant (Eds.),Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, IM Publications, Chichester, 2003.
[4] C.S. Fadley, J. Electron Spectrosc. Relat. Phenom. 178/179 (2010) 2.
[5] D. Briggs, J.T. Grant, Perspectives on XPS and AES, in: D. Briggs, J.T. Grant (Eds.), Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, IM Publications, Chichester, 2003, pp. 1-12.
[6] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corp, Eden Prairie, MN, 1992.
[7] C.D. Wagner, A.V. Naumkin, A. Kraut-Vass, J.W. Allison, C.J. Powell, J.R. Jr. Rumble, NIST Standard Reference Database 20, Version 3.4 (Web Version) (http:/srdata.nist.gov/xps/) 2003.
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