Chemical and spectroscopic characterization from nm to sub-nm scale with eftem and stem-eels
Advanced materials require control of the structural properties at the sub-nanometer or atomic scale. To fully characterize these systems, the chemical information at high spatial resolution must be achieved. This is possible by using electron energy loss spectroscopy in a scanning transmission microscope (STEM-EELS), or in filtered mode with a parallel beam (EFTEM). The latter can be used, for instance, to reveal the oxidized shell after treatments in assembled nanocrystals. Taking advantage of probe aberration correction in STEM and high-resolution spectrometers, sub-nm and atomic resolution is possible. Here I will show some examples of the use of this technique for mapping the chemical composition and the oxidation state (valence) of atoms in different nanosystems. An example is a CeO2/Pt epitaxial heterostructure, in which EELS from the Ce-M ionization edge reveals a one atomic thick layer with partly reduced Ce 3+ atoms at the interface in agreement with theory predicting 1 reduced Ce 3+ atom per cell to be stable at the interface. Other examples focus on nanoparticles and functionalized nanostructures. For instance, EELS mapping can be used to resolve the oxidation state of the metal atoms in core/shell particles. Moreover, EELS can be used to determine the bandgap in ultrathin perovskite nanosheets, or to map the extension of the plasmons and excitons states in a coupled system such as Au functionalized ZnO nanostructures.