Valence electron energy-loss spectroscopy in monochromated scanning transmission electron microscopy

With the development of monochromators for (scanning) transmission electron microscopes, valence electron energy-loss spectroscopy (VEELS) is developing into a unique technique to study the band structure and optical properties of nanoscale materials. This article discusses practical aspects of spatially resolved VEELS performed in scanning transmission mode and the alignments necessary to achieve the current optimum performance of ∼0.15 eV energy resolution with an electron probe size of ∼1 nm. In particular, a collection of basic concepts concerning the acquisition process, the optimization of the energy resolution, the spatial resolution and the data processing are provided. A brief study of planar defects in a Y1Ba2Cu3O7−δ high-temperature superconductor illustrates these concepts and shows what kind of information can be accessed by VEELS.