Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 215

In analytical (S)TEM, an additional useful source of information concerns the nature, valence, and coordination of atoms, which affect specifically the energy loss undergone by the primary beam through inelastic scattering by electron clouds within the sample. As in X‐ray absorption spectroscopy (XAS) or, more precisely, X‐ray near‐edge structure spectroscopy (XANES) (see Chapter 2.2), the interaction of a given atom with its local environment, i.e. the influence of the nearest neighbors on its electronic structure, is thus probed with an appropriate spectrometer mounted below the sample, which filters electrons according to their energy loss. This energy loss near‐edge structure spectroscopy (ELNES) has certain advantages, such as a superior spatial resolution in comparison with XANES. In addition, it is applicable to light elements such as Li, Be, or B, whereas appropriate cross sections for X‐ray generation typically restricts EDXS to elements heavier than B. On the other hand, quantification of electron energy loss (EEL) spectra is not as straightforward as in EDXS, and one is restricted to energy losses lower than ≈2 keV with EELS. Thus, it complements nicely with EDXS, whose typical spectral domain for useful application starts at approximately 2 keV.

Figure 8 Al‐L_2,3 edge EELS spectra of MAS sample areas that represent either the residual glassy part or the spinel therein.

As an example, the excitation of aluminum electrons from the 2p level to empty states above the band‐gap energy gives rise to Al‐L_2,3 edge EEL spectra where the presence, relative intensity, and energy position of specific features (A–F) bring information on Al coordination (Figure 8). For spinel, the spectrum is, for instance, typical of Al in octahedral coordination [26], which is not observed in the residual glass matrix whose detailed interpretation is actually more complicated because the features in the 100–110 eV energy range are convoluted with the Si‐L edge signal. Although they are beyond the scope of this chapter, elaborate computation techniques nonetheless exist with which information on coordination can be derived with an increasingly high precision from the intensity and positions of the peaks in EEL spectra [5, 27]. Not all ionization edges are well suited for in‐depth analyses, however, the so‐called white lines of transition metals being generally favorable cases.