Materials for Biomedical Engineering. Mohamed N. Rahaman

      Auger Electron Spectroscopy (AES)

AES is based on a two‐step mechanism (Figure 5.10a). Collision of an incident electron beam with the surface of a material leads to the emission of an electron from an inner atomic orbital (binding energy = E1) leaving a vacant site that is soon filled by an electron from an outer orbital (binding energy = E2). The energy released in this transition may appear as an X‐ray photon or may be transferred to another electron in an outer orbital (binding energy = E3) which is ejected with kinetic energy E_k given by

      (5.12)

      The ejected electron is called an Auger electron. The electron ejected from the outer orbital moves through the solid and soon loses its energy through inelastic collisions with bound electrons. On the other hand, if the Auger electron is emitted sufficiently close to the surface of the material, it may escape and be detected using a spectrometer. The number of emitted electrons (or the derivative of the counting rate) is measured and plotted as a function of the kinetic energy of the electron. Since each type of atom has its own characteristic energy levels, the peaks in the observed Auger spectrum can be used to determine the elemental composition of the surface by comparison with standard Auger spectra for the elements.

Figure 5.10 Interaction of incident beam (electrons or X‐rays) with a solid, producing atomic excitation with (a) emission of electrons followed by de‐excitation and emission of Auger electrons or (b) emission of photoelectrons.

      X‐ray Photoelectron Spectroscopy (XPS)

      In XPS, the specimen is irradiated with a low‐energy X‐ray beam that leads to the emission of electrons from the inner atomic orbitals by the well‐known Einstein photoelectric effect (Figure 5.10b). The kinetic energy of the emitted photoelectrons E_k is given by

(5.13)

where h is the Planck constant, ν is the frequency of the incident X‐ray photons, E_b is the binding energy of the photoelectron, and W is the work function of the spectrometer, a factor that corrects for the electrostatic environment in the instrument in which the photoelectron is produced and measured. By measuring E_k in a spectrometer of known work function, E_b can be determined from Eq. (5.13). The data are plotted as the number of emitted electrons versus their binding energy. As the binding energy of an electron is characteristic of the atom and the orbital from which it is emitted, the spectrum can be used to provide information on the surface composition by comparing with standard spectra.

      Because the photoelectrons typically have a higher energy than the Auger electrons, XPS is not as surface sensitive as AES, that is, AES can probe a shallower depth than XPS. However, by varying the emission angle at which the photoelectrons are collected, a technique known as angle‐resolved XPS, information from smaller and varying depths can be obtained. A benefit of XPS is that it can provide not just qualitative and quantitative information about the elemental surface composition but information about the chemical bonding (or oxidation state) of the surface atoms as well. As the incident beam is composed of X‐rays, XPS suffers from fewer problems related to electrostatic charging of the specimen and damage to the sample surface when compared to AES that relies on the use of an incident electron beam. Consequently, XPS is often a preferred technique for surface characterization of ceramics that are typically electrically insulating and polymers that are typically insulating and have a low hardness as well. However, as they provide similar information about elemental composition, XPS and AES tend to be used in a complementary manner.

      A common mode of using XPS is to perform a survey scan over a wide binding energy range (typically 0–1000 eV) to provide a qualitative analysis of the surface composition. The fractional concentration of the elements present at the surface can be determined from the area of the major peaks of each element using software. Then, information about the chemical bonding or oxidation state of the relevant atoms can be determined from higher resolution scans of the relevant peaks and measuring their chemical shift, that is, the change in their binding energy. A variation in the number of valence electrons or the types of bonds that they form results in a change in the binding energy of the innermost electrons and, thus, to a chemical shift.

As an example, Figure 5.11 shows an XPS survey spectrum of commercial purity titanium that, prior to analysis, was subjected to treatments commonly used for titanium implants, such as machining, cleaning with various solvents, and steam autoclaving (at 135 °C for 20 minutes) (Lausmaa 1996). The major peaks corresponding to Ti and O reveal an oxidized surface. The minor peak corresponding to C is often due to contamination and, in many cases, to carbon present adventitiously in the spectrometer. Except for Ti and O, the peaks corresponding to C and the minor elements were found to disappear or to decrease significantly after sputtering off a few nanometers of the surface, indicating that these elements were present mainly as impurities on the surface. A peak at ~459 eV, which, from reference spectra, corresponds to Ti in the compound TiO₂, dominates the high‐resolution spectrum of the Ti 2p peak (Figure 5.12). A smaller peak at ~454 eV corresponds to Ti in the underlying metal. Overall, then, the surface oxide layer on this titanium specimen corresponds to TiO₂.

Figure 5.11 XPS survey spectrum for an autoclaved titanium dental implant.

      Source: From Lausmaa (1996) / with permission of Elsevier.

Figure 5.12 XPS high‐resolution spectrum of the Ti 2p peak for a machined titanium implant.

      Source: From Lausmaa (1996) / with permission of Elsevier.

      The alloy Ti6Al4V also sees considerable use as a biomaterial. When subjected to the same treatments, its XPS spectrum is similar to that of commercial purity titanium but, in addition, it often shows a small amount of Al₂O₃. Typically, the concentration of Al in the surface oxide layer is approximately the same as that in the interior of the alloy.

      Secondary Ion Mass Spectroscopy (SIMS)

      SIMS consists of bombarding a surface with a primary beam of Ar, Ne, or He ions and analyzing the emitted ions and ion clusters in a mass spectrometer. As the emitted ions and ion clusters are characteristic of the surface, SIMS provides information about the chemical composition of the surface. Some information on the chemical bonding of the atoms can also be extracted by analyzing the composition of emitted ions and ion clusters.

SIMS