Principles of ESCA

1. Background

ESCA (electron spectroscopy for chemical analysis), also known as XPS (x-ray photoelectron spectroscopy) is a method for characterizing the top few atomic layers at the surface of a solid. ESCA can not only give the elemental composition of a surface (C, O, N, etc.) but can provide insights into the chemical bonding at the surface.

In recent years, there has been rapid growth of interest in the use of ESCA in the biomaterials area. ESCA has proven particularly useful for investigations of the interface between biological species and organic or inorganic substrates. Examples range from the use of ESCA in studies of titanium implant surfaces and their compatibility with bone-building properties, to the characterization of plasma deposited surfaces that are critical for blood cell adhesion. In some cases, it is found that a fraction of one atomic layer of impurity atoms can affect adhesion.

2. Principles of ESCA

An x-ray photon incident on a sample can ionize an atom producing an ejected free electron. The kinetic energy K of the photoelectron depends on the energy of the photon hv, as expressed by the Einstein photoelectric relation

K = hv - BE.

Here BE is the binding energy of the electron to the atom concerned. This relation is the basis of ESCA: since hv is known, a measure of K determines BE. The value of BE is specific to the atom concerned; measurement of BE serves as a "fingerprint" to identify the atom.
ESCA can be used to detect all elements except hydrogen and helium, with a sensitivity variation across the periodic table of only about 30. It is most useful for solids, including powders and soft materials. The qualitative and quantitative chemical state analysis capabilities, combined with extreme surface sensitivity (usually a few atomic layers) have made ESCA the most broadly applicable surface analysis technique today.