Other Methods

One problem of the techniques presented so far is the lack of chemical information, except in TEM with additional EDX or energy filter characteristics. Because the surface chemistry is a very important parameter determining the properties of biomaterials, often additional methods are applied. They have usually a high surface sensitivity, but their lateral resolution is limited. These complementary methods are in particular:

• X-ray photoelectron spectroscopy (XPS) in which the kinetic energy of photoemitted core electrons gives information on the elemental composition and the binding state of these elements. Thus metals can be distinguished from their oxides because they present another oxidation state. The surface sensitivity is several tens of nanometers; the lateral resolution is poor (between 100 /m and 5 mm). Quantification is possible within an accuracy of 5-10% .

• Auger electron spectroscopy (AES) in which also the kinetic energy of electrons is measured. These electrons are electrons which arise from a special intra-atomic relaxation process, the Auger process. After a core electron hole is produced in an atom (by X-rays, primary electrons, or ions), an electron with higher binding energy relaxes into the deep hole. This energy can then be transferred to an electron which has now enough energy to overcome the binding energy and leave the material. The energy of Auger electrons is also characteristic for the elemental composition, and the surface sensitivity is similar to XPS. There are two main differences: Because in the Auger process more electrons are involved, there is usually no easy interpretation of the chemical state of the electrons. However, because in Auger electron spectroscopy the primary hole-producing process can be by a focused electron beam, the lateral resolution can be in the tens of nanometers range.

• Secondary ion mass spectrometry (SIMS), the third important technique, in which, through ion bombardment, secondary ions are sputtered from the surface which are further analyzed in a mass spectrometer. Because there are also molecular ionic fragments, a huge amount of chemical information can be obtained. If a time-of-flight detector is used, also very large and heavy molecules, for example, biomolecules and polymers, can be detected. The surface sensitivity can be tuned by the ion bombardment and can be restricted to the first monolayer. By harsh bombardment also depth profiles can be measured. The lateral resolution is usually several hundred nanometers. However, the current limit is therefore well below 100 nm.

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