Transition metal oxides are a class of materials that are vitally important for developing new materials with functionality and smartness. The unique properties of these materials are related to the presence of elements with mixed valences. The measurement of cation valence states can be performed using X-ray photoelectron spectroscopy and other chemical techniques, but these techniques are suitable only for a large quantity of specimens and the measured results are an average over all of the surface layer or the entire volume. For today's research in the nanoera, it is important to determine the valence states from a region as small as a nanoparticle. In this Letter, we report the success of using electron energy-loss spectroscopy, attached to a transmission electron microscope, to measure the valence states in the Co−O and Mn−O systems at a spatial resolution of 20−70 nm. The reliability and sensitivity of the technique are demonstrated in reference to the composition curves and the electron diffraction data recorded in-situ during the reductions of Co3O4 and MnO2 and during the phase transformation in a solid solution of Fe2O3 and Mn2O3 as well.
In a conventional TEM, electrons are emitted from an electron gun and focused by the condenser lens as a beam, which illuminates the specimen. The electron beam interacts with the specimen and is scattered (or diffracted) by the crystal atoms. Thus, the electron wave at the exit face of the specimen contains information about the potential distribution in the specimen. Since electrons are charged particles, their interaction with a solid is rather strong in comparison with either X-rays or neutrons, so that multiple scattering effects are always present in electron diffraction. This means that electron diffraction must be described by dynamical scattering theory, especially when quantitative structural analysis is necessary. However, many characteristics of electron diffraction can be qualitatively treated based on the kinematical scattering theory, which is actually a single-scattering theory. The purpose of this chapter is to outline some basic concepts of kinematical electron diffraction theory and to introduce the imaging theory of TEM, which will be applied in the future chapters for REM imaging. A systematic kinematical treatment of electron diffraction for perfect and imperfect crystals has been given by Cowley (1981). A complete description of dynamical electron diffraction theories has been given by Wang (1995).