Chapter 3: Electron energy-loss spectroscopy and energy-filtered transmission electron microscopy: Nanoscale determination of Fe3+/ΣFe ratios and valence-state mapping
Ute Golla-Schindler and Peter A. van Aken
Iron, the fourth most abundant element in the Earth, commonly occurs in two valence states, Fe2+ and Fe3+, even within a single mineral. Determination of Fe3+/SFe ratios in minerals at sub-micrometre scale has been a long-standing objective in geosciences. One of the most important characteristics of iron is that the charge on the metal is extremely sensitive to its surrounding reduction-oxidation (redox) conditions, which often results in changes in iron valence state reflecting these conditions. The quantification of ferrous/ferric ratios in minerals can therefore provide great insights into physico-chemical conditions of rock formation such as temperature and oxygen fugacity, and allows the determination of redox states for mineral crystallization and the interpretation of geological and geochemical processes. The high spatial resolution available on a (scanning) transmission electron microscope ((S)TEM) combined with the benefits of electron energy-loss spectroscopy (EELS) allows detailed analysis of multivalent element ratios (e.g. Fe2+ and Fe3+) on the scale of nanometres.
Electron energy-loss spectroscopy is a powerful technique for analyzing the interactions of fast probe electrons with matter, and the energy transferred for a certain excitation process can be measured as an energy loss of the incident electron which reduces its kinetic energy. The probability of inelastic scattering over energy loss is called energy-loss spectrum which results from the excitation of inner-shell, valence or conduction electrons. Excitations are only possible from occupied states below the Fermi level to allowed un-occupied states beyond it. Maxima in the energy loss spectrum correspond to strong electron-specimen interactions. Apart from the qualitative and quantitative determination of elements, it is also possible to determine quantitative concentration ratios of elements. A further benefit of EELS is the ability to determine local electronic structures for the extraction of local chemical bonding by analyzing near-edge and extended fine structures of ionization edges. Therefore, a transmission electron microscope equipped with an energy-filter opens an avenue of new spectroscopic methods and imaging techniques for the characterization of materials in the geosciences. This makes it possible to carry out spatially resolved characterization of microscopic regions within inhomogeneous solids on a nanometre scale. The basic principles and opportunities using analytical EELS methods and imaging techniques of energy-filtered TEM (EFTEM) are described here. Typical examples from the authors’ laboratories and from other published work are used to demonstrate applications of EELS and EFTEM in terms of quantification of Fe3+/SFe ratios, imaging quantitative compositional maps and Fe3+/Fe2+ valence-state distribution maps at the nanometre scale. The limits of the accuracy for the quantitative determination of Fe3+/SFe ratios and imaging Fe3+/Fe2+ valence state distribution maps using EELS and EFTEM, respectively are emphasized. Finally, a survey of the review literature and supplementary useful resource material in this rapidly growing area is presented, which will provide a basis for further research in this subject.
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