A surface core level shift (SCS) is a kind of
core-level shift that often emerges in
X-ray photoelectron spectroscopy
X-ray photoelectron spectroscopy (XPS) is a surface-sensitive quantitative spectroscopic technique based on the photoelectric effect that can identify the elements that exist within a material (elemental composition) or are covering its surface, ...
spectra of surface
atom
Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons.
Every solid, liquid, gas ...
s.
Because surface atoms have different chemical environments from bulk atoms, small shifts of binding energies are observed by X-ray photoelectron spectroscopy. SCS is ascribed mainly to the lower
coordination number
In chemistry, crystallography, and materials science, the coordination number, also called ligancy, of a central atom in a molecule or crystal is the number of atoms, molecules or ions bonded to it. The ion/molecule/atom surrounding the central i ...
s of surface atoms than bulk atoms. Reduced coordination leads to narrower
valence
Valence or valency may refer to:
Science
* Valence (chemistry), a measure of an element's combining power with other atoms
* Degree (graph theory), also called the valency of a vertex in graph theory
* Valency (linguistics), aspect of verbs rel ...
bandwidth. Such narrowing of the bandwidth increases the density of states, and if more than half of the valence band is filled, the band center is lower than bulk and the binding energy increases. In contrast, if less than half of the valence band is filled, the band center is higher than bulk, and the binding energy decreases.
Because the binding energy in X-ray photoelectron spectroscopy is affected by the final state and other chemical environments, this simple explanation cannot always be applied to the interpretation of X-ray photoelectron spectra. In spite of such complexity, the SCS gives important information about the chemical nature of surface atoms.
Spectroscopy
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